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Hydrogen Economy

  1. Stephen A. Wells1,
  2. Asel Sartbaeva2,
  3. Vladimir L. Kuznetsov2,
  4. Peter P. Edwards2

Published Online: 15 DEC 2011

DOI: 10.1002/9781119951438.eibc0452

Encyclopedia of Inorganic and Bioinorganic Chemistry

Encyclopedia of Inorganic and Bioinorganic Chemistry

How to Cite

Wells, S. A., Sartbaeva, A., Kuznetsov, V. L. and Edwards, P. P. 2011. Hydrogen Economy. Encyclopedia of Inorganic and Bioinorganic Chemistry. .

Author Information

  1. 1

    University of Warwick, Coventry, UK

  2. 2

    University of Oxford, Oxford, UK

Publication History

  1. Published Online: 15 DEC 2011

1 Introduction: Basic Concepts

  1. Top of page
  2. Introduction: Basic Concepts
  3. Key Technologies
  4. Economic, Social, and Political Issues
  5. Future Prospects for a Hydrogen Energy Economy
  6. Concluding Remarks
  7. Acknowledgments
  8. Related Articles
  9. References

A future “hydrogen energy economy” is one in which the dominant role currently played by fossil fuels as an energy vector, especially in personal transportation, would instead be fulfilled by hydrogen, almost invariably associated with the extensive use of fuel cells. The term is often associated with a cleaner future energy economy, which increasingly relies more heavily on renewable or sustainable rather than fossil energy sources, and in which hydrogen, as a secondary energy carrier, will be the principal currency for the transfer of chemical potential energy.1-5 One can therefore distinguish between a sustainable hydrogen economy, in which fossil fuels can play no part, and a transitional hydrogen economy, in which hydrogen will almost certainly be generated using fossil fuels (for example, by the steam reforming of methane) while an infrastructure for hydrogen production, distribution, and end utilization, for vehicular transportation especially, is established.

The development of such a future hydrogen energy economy is motivated by a vision to move away from the fossil fuel or carbon economy, because of the finite available reserves of fossil fuel, especially petroleum, and the damaging environmental consequences of carbon dioxide emissions from fossil fuel combustion, such as anthropogenic climate change and ocean acidification. The vision is assisted by specific properties of hydrogen that make it attractive as a future energy carrier. One of the most attractive of such properties is that hydrogen can be produced from a variety of primary energy sources, for example, by electrolysis using sustainable electricity, and the latent chemical potential energy of molecular hydrogen can be converted back into electricity with very high efficiency in a fuel cell. Hydrogen fuel cell electric vehicles (FCEVs) will be the central transportation technology of any future hydrogen economy and, in principle, offer significant advantages over the internal combustion engine (ICE), namely, higher thermodynamic efficiency and zero emissions except water at the point of use.5

The future use of hydrogen is therefore linked to the transportation sector's embrace of fuel cells. With the assistance of hydrogen technology, sustainable energy can be “harvested” (Figure 1) and channeled to those parts of the transport and power sectors that are currently (exclusively) dependent upon oil. For example, wind, solar, wave, and nuclear power can be used to generate hydrogen, which can subsequently be used in combination with low-temperature fuel cells in cars, buses, etc. Similarly, large-scale applications are centered on “combined heat and power” from hydrogen high-temperature fuel cell combinations.6

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Figure 1. An idealization of a future hydrogen energy economy in which hydrogen, in association with fuel cells, becomes the principal energy vector connecting production to consumption. (Reproduced from Ref. 6. © Elsevier, 2008.)

Significant scientific, technological, and socioeconomic challenges must be overcome before any transition to a hydrogen economy becomes feasible, and these include the sustainable, high-efficiency production of hydrogen, the development of a safe and practical on-board hydrogen store for vehicles, and the development of fuel cells with a longer lifespan and much lower unit cost than is currently available. Various roadmaps have been proposed for the development of transitional hydrogen economies in, for example, Japan, the United States, and the European Union, essentially setting out a timeline for the necessary technological and socioeconomic developments to be achieved. The solutions to these challenges represent real opportunities for the physical sciences and engineering and inorganic chemistry in particular. In this work, we briefly examine the principal technologies underpinning the hydrogen energy economy which require such breakthrough solutions—a substantial number of which require significant input from chemistry, in close multidisciplinary collaborations with our sister disciplines of chemical engineering, materials science, physics, and indeed the social sciences.

The first vision of a hydrogen economy was initially developed during the energy crises of the 1970s to describe a national or international energy infrastructure based on hydrogen produced from nonfossil primary energy sources. It is now widely taken as a term for a future energy economy that relies on renewable rather than fossil energy sources, and in which hydrogen will play a major role as a suitable storage and transmission vector for energy.1-5 It is helpful to review the factors that motivate the development of a hydrogen economy; we can divide these into general factors that motivate a move away from fossil fuels and toward renewable or sustainable energy sources and specific factors that make hydrogen an attractive energy vector. It is also necessary to consider factors that militate against a hydrogen economy; these, in turn, can be divided into negative factors regarding the use of hydrogen, constituting difficulties that must be overcome if a hydrogen economy is to be achieved, and positive features of certain alternatives, which could, in principle, make a hydrogen economy unnecessary.

Of course, in a truly sustainable hydrogen economy, the use of fossil fuels would be abandoned entirely; all energy inputs would come from renewable, sustainable sources, and the resulting economy would be viable and self-sustaining. This, however, is clearly a long-term goal. In a short- or medium-term transitional hydrogen economy, it would be expected that hydrogen would be generated from fossil fuel resources (in particular, the steam reforming of methane), ideally with carbon capture and storage to mitigate CO2 emissions, so that an infrastructure of hydrogen production and distribution, and a large user base for hydrogen consumption can be steadily built up (especially the deployment of hydrogen FCEVs). It is this transitional hydrogen economy that must be the goal of policymakers in the short term.7

1.1 Energy from Fossil Fuels: The Carbon Economy

Our global energy economy is at present dominated by fossil fuels—coal, petroleum, and natural gas (methane)—which represent the primary energy sources for industry, transport, heat, and electricity generation (Figure 2).7-9 The importance of fossil fuels to our present human society cannot be overstated, but the usage of fossil fuels also comes at a cost.

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Figure 2. Our current energy infrastructure. The overwhelming majority of our current energy needs are met by the combustion of fossil hydrocarbon fuels. (Adapted from Ref. 10. © Royal Society of Chemistry, 2008, and from Ref. 11. © Elsevier, 2007.)

Fossil fuel reserves represent energy harvested from the sun, stored by living organisms, and deposited in sedimentary strata over geological timescales, and in our current exploitation of these reserves, we are depleting them at roughly a million times their original rate of deposition. Reserves of even the most abundant fossil fuel, coal, will be exhausted at current usage on a timescale of a few centuries at most. While this may seem like a long time relative to an individual human life or political career, it is short compared to the duration of human civilization (several millennia). Reserves of petroleum, which are especially vital to the modern transport economy, are under even more pressure and face exhaustion on a timescale of decades. The exploitation of unconventional fossil fuel sources, such as tar sands and methane clathrates, can only postpone but not prevent the eventual exhaustion of global fossil fuel reserves.12 The problem is only exacerbated by the increasing global population (which has more than doubled in the past 50 years, now standing at almost 7 billion, and projected to reach 9 billion by 2040) and by generally rising standards of living, with a concomitant increase in energy usage per person. Even before the actual exhaustion of reserves, the declining availability of dwindling fossil fuel resources will create serious geopolitical problems of energy security and competition for resources. A move to renewable and sustainable resources is therefore vital if modern civilization is to be sustained in a form that supports the global human population and provides a tolerable standard of living.

Apart from the issue of medium- and long-term availability, there is a second serious problem with our ever-increasing demand for fossil fuels, that of “carbon emissions” in the form of carbon dioxide, which are strongly implicated as the main driver of climate change.13 The combustion of fossil carbon is putting the global carbon cycle out of balance and increasing the atmospheric concentration of carbon dioxide, which has risen from about 280 ppm to about 380 ppm over the past 150 years; the current rate of increase is about 1–2 ppm annually. The proposition that increased atmospheric CO2 must lead to an overall warming of the globe is scientifically uncontroversial; remarkably, Svante Arrhenius estimated the consequences of a doubling of atmospheric CO2 in the 1890s.14 The reports of the Intergovernmental Panel on Climate Change (IPCC) summarize the issue of anthropogenic global warming (AGW) and make it clear that climate change induced by human activity is a serious and urgent issue; AGW is not the only problem caused by our CO2 emissions.15 The ocean is the major sink for CO2, whose dissolution acidifies the ocean and thus affects the formation of calcareous mineral structures, e.g., shells, by corals and marine larvae. Continued CO2 emissions and increased ocean acidification could have cascading detrimental consequences for the entire oceanic food chain.

These problems with our virtually complete reliance on fossil fuels require that in the long term the global energy economy must surely move away from the use of fossil fuels.16-18 In the short term, and during the transition to a hydrogen economy, the development of efficient methods for carbon capture and storage (CCS) is seen as a possibility.19, 20 In CCS, carbon dioxide is scrubbed from the exhaust gases produced by combustion of carbon compounds and stored, for example, in underground reservoirs such as exhausted oil or natural gas fields.21 CCS is far from an accepted, mature commercial technology; significant scientific and technical challenges remain in the development of better solvents, adsorbers, and membranes for the removal of CO2 from exhaust gas, and equally daunting engineering and socioenvironmental challenges remain in the identification of suitable sites for safe, long-term storage. CCS is likely to be an important factor in the transitional hydrogen economy, especially with regard to the generation of hydrogen by steam reforming of methane. It is advantageous, in terms of both greenhouse gas emissions and thermodynamic efficiency, to generate hydrogen from methane in centralized facilities with CCS and to use the hydrogen in FCEVs rather than to run internal combustion engine vehicles on methane directly.

1.2 The Pivotal Role of Renewable and Sustainable Primary Energy Sources in a Hydrogen Economy

Hydrogen is not a primary energy source, but a secondary energy vector—an energy store. There is no reservoir of “fossil hydrogen” comparable to the fossil carbon reserves that have fueled the global economy since the Industrial Revolution. In a sustainable energy future, energy must be obtained from renewable resources (see Hydrogen Production from Renewables)22; it can then be stored as chemical potential energy in molecular hydrogen (or hydrogen storage materials) and this energy is liberated by subsequent oxidation, emitting only water as a by-product at the point of use. It is as well to review the renewable sources of energy that are potentially available, including solar, wind, hydroelectric, ocean, geothermal, and biomass.23, 24

Solar power generation options include solar photoelectric, in which electrical power is generated using solar panels; solar thermal, where the heat of the sun is concentrated using mirrors and used to run a turbine generator; and solar photochemical, where sunlight incident on a photocatalyst splits water into hydrogen and oxygen directly. The total solar energy flux incident on the earth's surface exceeds our global energy consumption by a factor of around 10 000; therefore, our civilization's energy needs could, in theory, be met by intercepting only a small fraction of the incident solar radiation.25 In practice, however, this is a considerable scientific, technological, and socioeconomic challenge. Ambitious plans have been proposed for large-scale solar power projects in well-insolated regions. The design of new functional materials with significantly improved efficiency and reduced cost, which can be used in solar panels or as photocatalysts, is a major research topic for the area of energy chemistry.

The harvesting of wind energy using large turbines is one of the fastest-growing forms of renewable energy generation. Both wind and solar energy generation involve gathering energy over large areas using arrays of relatively small dispersed generating units. Since, moreover, the areas that are ideal for wind and solar generation are often distant from the major population centers where the demand for energy is greatest, we have the concept of “stranded renewables”, where there is a serious difficulty in conveying the energy from where it is generated to where it is needed. The options are either to transmit electrical energy directly via an improved electrical distribution network, or to convert the energy to a chemical form, e.g., hydrogen or other chemical fuels, and then to transport it via pipeline or tanker.

Hydroelectric power, where flowing water is held back by dams and released through turbines so that we obtain electrical power from its gravitational potential energy, is currently the largest renewable energy contribution to electricity generation, though it still amounts to only a small per cent of global energy use. Ocean energy covers the harvesting of energy from the tides or from the waves and is not yet a major player in electricity generation, although pilot plants are in operation. Geothermal energy is in theory available almost anywhere on Earth if deep drilling is possible, but in practice is most accessible in regions of young volcanism where very hot rock lies close to the surface. Iceland in particular, lying on the Mid-Atlantic Rift, has been a pioneer in both geothermal energy and in the introduction of hydrogen vehicles, with a policy aimed at phasing out fossil fuels by 2030.

Biomass as an energy source covers primary production of biomass intended as energy fuel from forestry or agriculture; the use of biomass waste from agriculture, wood processing in industry, and municipal waste or sewage; and new proposals such as large-scale algaculture for the production of biofuels (see Fuel Cells: Enzymes and Microbes for Energy Production). Perhaps the greatest advantage of biomass is its higher “controllability” in the provision of sustainable energy compared to solar, wind, and hydroelectric power generation, which are subject to natural fluctuations. Moreover, hydrogen is produced directly from biogas without the intermediate stages of electricity generation and electrolysis. The comparatively low energy density of biomass does, however, make its transport inefficient—certainly over long distances.

Renewable resources collectively contribute about 13% of world total primary energy supply, whereas coal, oil, and gas collectively contribute about 80%. In electricity production, renewable resources contribute 18% of global production, almost all of which comes from large hydroelectric projects. In Table 1, we summarize the proportional contribution of different energy sources in 2004 to total primary energy supply and to electricity production23, 26; Table 2 summarizes the current energy production from renewables, the theoretical potential available, and the potential technically available, given reasonable technology assumptions.23, 27, 28

Table 1. Fuel Shares, Including Renewable Energy Sources (RES) of World Total Primary Energy Supply (above) and Electricity Production (below) in 200423, 26
     Renewables
 OilCoalNuclearGasCombustible renewables Hydro Other renewablesa
  1. a

    Other renewables include tide, solar, wind, and geothermal sources.

Fuel shares of world total primary energy supply in 2004 34.2% 25.0% 6.5% 20.8%

 13.1%

      10.6% 2.2% 0.5%
Renewable energy sources in electricity production in 2004 6.7% 39.8% 15.7% 19.6% 17.9%
   1.0% 16.1% 0.8%
Table 2. Annual Global Primary Technical and Theoretical Potentials for Various Renewable Energy Sources in 200423, 27, 28
ResourcesCurrent use (2004) (EJ)Technical potential (EJ)Theoretical potential (EJ)
  1. EJ, Exajoule; 1 EJ = 1018; 1 TJ = 1015 J.

Biomass energy50.02502900
Geothermal energy2.05000140 000 000
Hydropower10.050150
Ocean energy7400
Solar energy0.216003 900 000
Wind energy0.26006000
Total62.47500143 916 450

A potentially important application for hydrogen as an energy vector, in the context of sustainable energy generation, is in energy load balancing, or buffering, and storage. The process of harvesting energy from the environment is susceptible to variations in the natural environment, both predictable and unpredictable; solar power will not work at night or under heavy cloud, while wind power cannot be generated during a calm. Some form of load balancing will therefore be necessary in order for renewable energy to deliver reliable, continuous power to the electricity distribution grid. The production of hydrogen from water by electrolysis is a process that copes very well with variations in load. Therefore a potential load-balancing mechanism is to apply the variable or intermittent power obtained from renewables to the production of hydrogen by electrolysis, and either to make use of the hydrogen in vehicles or to reconvert the stored hydrogen to electrical power using fuel cells at a steady rate suitable for delivery to the electricity grid; a third alternative would be the synthesis of chemical fuels in such “stranded locations” and subsequent transportation of the fuel.

Currently, production of hydrogen by electrolysis (using grid electricity) is about three times more expensive than chemical production from methane. In a comprehensive analysis relative to the UK energy scene, Dutton29 finds that, for a scenario in which transport is fueled entirely from hydrogen (i.e., ignoring biofuels), and if that hydrogen is to be supplied solely by renewable electricity, the additional electricity demand is likely to be roughly equivalent to the present conventional electricity demand. He further notes that in the short term, the link between the intermittent nature of renewable electricity generation and the inherent storage capacity of the hydrogen economy will be best suited for (stranded) remote areas as possible niche application. In the longer term, beyond 2025, a growing hydrogen economy could indeed start to fulfill the role of energy buffering to facilitate a penetration of renewable electricity toward 100%.

Nuclear fission is currently the largest nonfossil fuel contributor to electricity generation, and an expansion of nuclear power is one proposal for meeting the world's growing energy needs without increased combustion of fossil fuels and the attendant emissions of carbon dioxide. The nuclear industry has always had an uneasy relationship with the environmental movement, because of its links with the military production of plutonium, and due to the still unsolved problem of the disposal of “hot” (highly radioactive) nuclear waste from spent reactor fuel, which requires safe storage, sequestered from the environment, over geological timescales. Nevertheless, nuclear power is currently undergoing something of a renaissance, especially in the emerging economies of China and India.30, 31

Fission power is not a genuinely sustainable energy source as it relies on the “burning” of uranium, a nonrenewable mineral resource. Indeed, uranium production has been well below demand since around 1990, with the gap being made up from stockpiles and from weapons-grade uranium and plutonium from decommissioned nuclear weapons. It can, however, be sustained at least over several decades and so may be a valuable energy contributor to the transitional hydrogen economy. Current reactor designs are very inefficient in their use of uranium, extracting only around 1% of the theoretically available fission energy. In theory, far more energy could be obtained from both uranium and thorium in “breeder” reactors. Fission power could indeed contribute to the transitional hydrogen economy both through electricity production and, in theory, through thermochemical cycles of water splitting such as the sulfur–iodine process, powered by waste heat from fission; this, however, is still a subject of basic research and development.

The generation of energy from hydrogen fusion is, ironically, not usually considered an aspect of the “hydrogen economy” per se; it involves the energy released during the fusion of hydrogen isotopes, especially deuterium and tritium, whereas the hydrogen economy is based on the chemistry of protium. Despite decades of intensive research,32 practical energy generation from hydrogen fusion is still at least decades away and thus cannot contribute to a transitional hydrogen economy in the near future. In the long term, hydrogen fusion, if achieved, would be a sustainable source of heat and electrical power.

1.3 Why Hydrogen? Specific Properties and Advantages

Clearly, the desire to transform a fossil-fuel economy into a sustainable one does not in itself motivate the development of any future hydrogen energy economy. The focus on hydrogen is because of properties of the element itself—in particular, its ready availability by dissociation of water, its very high specific energy (the highest of any chemical substance) of 33.3 kWh kg−1, the fact that it oxidizes to form only water, and its ready applicability in fuel cells, which convert its latent chemical energy into electrical energy with very high efficiency.

Hydrogen is the most abundant element in the universe (making up about 75% by mass of all baryonic matter). Although it is not found in free molecular form on Earth in any significant quantity, it is abundant in a myriad of chemical compounds, particularly in water. The electrolytic dissociation of water to form hydrogen and oxygen can be achieved with high thermodynamic efficiency, up to 85% in a conventional alkaline water electrolyzer. Given sufficient electrical power, therefore, hydrogen could be produced in abundance; of course, as noted earlier, such power would have to be produced from sustainable sources in order for hydrogen itself to be correctly labeled as a sustainable or renewable energy carrier.

Other chemical and thermochemical methods for hydrogen production also exist, as discussed in Section 2. At present, the dominant method for commercial hydrogen production is steam reforming of methane.

Hydrogen is, of course, the lightest element in the Periodic Table. The amount of energy produced during hydrogen combustion is higher than that released by any other fuel on a mass basis; hence, molecular hydrogen has the highest energy-to-mass ratio of any chemical fuel. Petrol-fueled car engines can be (and have been) converted to use hydrogen, and sophisticated technology for such combustion engines have been optimized over decades of experience (for example, BMW first carried out research into engines and vehicles operating on liquefied hydrogen since 1978). Far more efficient are combined hydrogen-fuel-cell-electric-motor (Hy-FC-EM) systems, known generally as a power train. The automotive industry—or at least part of it—sees fuel cells as its inevitable, desired future.

The hydrogen fuel cell is therefore the primary motivator for the use of hydrogen as an energy vector in transport. It is possible to simply burn hydrogen in an ICE with only slight modifications from the petrol-burning norm—indeed, the first successful ICE of de Rivaz in 1806 was a hydrogen-burning engine. This application, however, does not offer any advantage in efficiency over current ICEs. The efficiency of ICEs is thermodynamically limited by the Carnot cycle, which constrains the theoretical maximum efficiency of any heat engine, and reduced still further by typical driving and traffic conditions; this leaves engines operating for long periods inefficiently at partial load. In the fuel cell, on the other hand, the chemical energy of hydrogen and oxygen is converted directly into electrical energy with very high efficiency. This means that an FCEV could operate at efficiencies of around 45% overall, whereas current ICE vehicles typically do no better than 25%.

Multiple types of fuel cells exist for the production of electrical energy from hydrogen or hydrogen-containing fuels—for example, the solid-oxide fuel cell (SOFC), molten carbonate fuel cell (MCFC), phosphoric acid fuel cell (PAFC), alkaline fuel cell (AFC), and “polymer electrolyte membrane” or “proton-exchange membrane” fuel cell (PEMFC) (see Fuel Cells: Proton Exchange Membranes). The SOFC and MCFC operate at high temperatures and are more suitable for stationary power generation and heat and power cogeneration; since they take several hours to start up from cold, they are not suitable for road vehicle use. Development of hydrogen FCEVs is focused on the PEMFCs, as these can operate at low temperature (below 100 °C) and offer immediate startup—a vital characteristic for vehicular applications. Of course, it is difficult to predict the world's long-term energy vision, but one scenario is shown in Figure 3, where we illustrate the concept of a hydrogen economy in which hydrogen generation and its use in fuel cells are a critical, integral part of a future energy economy. Here, oil will not be used as the dominant energy source, but used instead for chemical products. A wide range of primary energy sources contributes to the energy mix.

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Figure 3. A possible model for a transitional hydrogen energy economy. Note that in this view, renewable energies are considerably intensified and hydrogen fuel cells are employed throughout as critical technologies. (Adapted from Ref. 10. © Royal Society of Chemistry, 2008, and from Ref. 11. © Elsevier, 2007.)

1.4 Potential Drawbacks with Hydrogen

The advantages of hydrogen might seem so overwhelming that one might wonder why hydrogen FCEV is not already the norm. There are a number of key issues which have so far prevented the widespread use of hydrogen. As noted, hydrogen is not a primary energy source, but an energy carrier, and consequently it will be as clean as the method used in its production. Another chief problems is, of course, that gaseous hydrogen at room temperature and pressure takes up an impractically large amount of space. The currently available technologies of hydrogen storage, as highly compressed gas or as cryogenic liquid, have certain disadvantages for vehicular applications. The weight of a compressed-gas storage system is such that only around a few weight percent hydrogen is achieved, and the work done in compressing or liquefying the hydrogen is a significant fraction of its energy content. Volumetric density is also a significant consideration as there is limited space for fuel storage within a small vehicle. It is widely believed that a really practical hydrogen FCEV will require a different form of hydrogen storage, either by physisorption in a highly porous material or chemisorption in some form of hydride; it must be easily and repeatedly reversible, rapidly taking up hydrogen at a fueling station and releasing it to the fuel cell when needed, with high enough mass and volumetric densities when fully loaded that the mass and volume of the fuel storage system are practical. The achievement of such a hydrogen store is widely regarded as one of the greatest challenges in the transition to a hydrogen economy, as discussed in Section 2.

Hydrogen storage has distinctive safety issues. Hydrogen gas will burn in air over a wide range of concentrations (4–75% by volume), will detonate over a wide range (18.3–59.0% by volume), and has an extremely low minimum ignition energy of only 0.017 mJ. For comparison, methane has a minimum ignition energy of 0.29 mJ, and methane–air mixtures can be ignited by the spark of a static discharge. New types of ventilation and safety precautions are therefore required for hydrogen vehicles, especially for hydrogen storage facilities and hydrogen filling stations. A gradual loss of hydrogen from storage is inevitable for cryogenic storage of liquid hydrogen, due to evaporation.

The establishment of a hydrogen infrastructure also has its own challenges. Compared to natural gas (for which pipeline distribution over thousands of kilometers is quite practical), hydrogen is more difficult to work with as the small, light molecule diffuses through metals, causing not only loss of hydrogen but also dangerous embrittlement of the pipeline structure. However, pipeline distribution of hydrogen over distances of hundreds of miles has been commonplace in the chemical industry for several decades. Distribution of liquid hydrogen or compressed gas by tanker trailers suffers from the issue of the energy input to achieve liquefaction or high compression, and is only practical over short distances and small amounts, for example, the supply of hydrogen to local filling stations from a centralized production or delivery facility.

Another fundamental barrier to the widespread use of hydrogen, however, is cost. Hydrogen production, storage, distribution, and use have not had the century of large-scale, mass-market investment, and development that has made petroleum so dominant in the transport sector, and hydrogen vehicle technologies are still at the prototype stage and have not yet achieved the economies of scale that are possible with mass-market products. Current PEMFCs rely on precious metal catalysts containing rare and expensive elements, especially platinum in a nanoparticulate form; this raises the cost of the fuel cell and, since the catalysts are very sensitive to poisoning by impurities such as CO, places high demands on the purity of the fuel supply. The unit cost of fuel cells must be reduced dramatically (from $2000–4000 per kilowatt to below $100 per kilowatt for PEMFCs) and their lifetimes extended (from below 2000 h to around 5000 h) before they are economically practical for widespread use in personal vehicles. The development of new catalyst and electrolyte materials for fuel cells is another major challenge for chemistry in the development of the hydrogen economy.

1.5 Alternatives and Complements to Hydrogen

The centerpiece of any vision of the hydrogen economy is the transition in the transport sector from ICE vehicles powered by fossil fuels to hydrogen FCEVs. Any other non-fossil-fuel-ICE vehicle technology is therefore an alternative to the hydrogen FCEV and could be viewed as a competitor or (more positively) as a complementary technology. We should note that our discussion applies mostly to the automotive sector—passenger cars and buses, which are responsible for more than 80% of total energy use in the transport sector—rather than to heavy-goods vehicles, where the traditional diesel engine is likely to retain its dominance for the foreseeable future.

Electric vehicles already exist in the form of both battery electric vehicles (BEVs) and hybrid ICE/battery vehicles such as the well-known Toyota Prius. BEVs can be practical for short-range commuter driving, but suffer from limited operational range and long recharging times. Typical hybrids can only operate for a range of around 20 km on batteries alone. The energy density of even a modern Li-ion battery (around 120 Wh kg−1) is only around 1% of the energy density of gasoline, so unreasonable large battery weights (on the order of 1000 kg) would be required to give BEVs the ∼500-km range of a typical modern small car. Battery recharging is also very slow compared to the rapid refueling of a gasoline or hydrogen vehicle—typically an overnight process from a household circuit for a plug-in BEV. A proposed method for overcoming these issues is to make the battery pack itself an interchangeable component, which can be replaced at a battery-swapping station equivalent to a filling station. While this system may well be practical for urban driving within a restricted area, it ties vehicles closely to the battery-swapping system and so is unsuitable for long-range or free-roaming driving.

Batteries do possess attractive features for electric vehicles, especially for their energy efficiency; about 90% of a battery's stored energy can be extracted as useful electricity, and electric motors are about 95% efficient, giving BEVs a net efficiency of around 85%. This is three to four times the efficiency of an ICE and at least twice the efficiency of a hydrogen FCEV. In principle, a dramatic improvement in battery technology could make the range, power, and charging/discharging kinetics of an electric vehicle fully competitive with conventional ICEs. In this scenario, the sustainable energy economy would be based on electricity generated from renewable resources, distributed by power lines, and used in fully electric vehicles, making the hydrogen fuel-cell vehicle unnecessary or obsolete. The required improvement in energy density and cost of electric batteries is beyond what currently seems attainable, however, and hydrogen remains the front-runner for an electric vehicle technology that is truly competitive with the conventional ICE car. For some vehicular applications, especially urban buses that travel only short distance between regular stops, supercapacitors (see Supercapacitors: Electrode Materials Aspects) are a potential alternative to either battery or fuel-cell vehicles, as they can be charged rapidly at stops with enough energy to propel the vehicle to the next recharging point.

Another approach is to continue the use of ICEs running on “biofuels” generated from biomass or with synthetic fuels produced by CO2 conversion. Since the carbon in the fuels has recently been fixed from the atmosphere by vegetation, the use of biofuels is, in principle, carbon-neutral. Bioethanol, from the fermentation of plant sugars, is usable in flex-fuel engines (for example, in E85 blended fuel: 85% ethanol, 15% gasoline) and is currently the most widely used biofuel. Biodiesel, produced by processing of vegetable oils, is a potentially carbon-neutral direct replacement for diesel in ICEs. The environmental benefits of current approaches to biofuels is questionable, however, as their production competes strongly with agricultural food production and planting for biofuels is associated with deforestation. Moreover, any use of fertilizers in crop production carries with it a latent carbon penalty derived from the use of hydrogen in fertilizer production. The continued use of ICEs also lacks the efficiency benefits and reduction in emissions associated with BEVs and FCEVs.

Yet another potential alternative to biodiesel is synthetic fuel (methane, methanol, or diesel) generated by chemical means, e.g., the Fischer–Tropsch process, from water, sustainable hydrogen and carbon dioxide captured in CCS; essentially the reverse of steam reforming (see Section 2.1). The development of efficient and cost-competitive methods of converting CO2 into fuels could address the issue of both carbon sequestration and sustainable and economic production of carbon-neutral fuels, and may be of use in producing synthetic fuel for gasoline ICEs, ideally without using fossil-fuel resources. Such carbon-neutral or sustainable organic fuels offer the possibility of decarbonizing our transport system without the undoubted paradigm shifts required by conversion to a hydrogen energy economy, or indeed by the electrification of the vehicle fleet. Olah et al.33 have pioneered and advanced the concept, and potential widespread adoption, of the methanol economy, emphasizing the production of CH3OH (or dimethyl ether) by the chemical recycling of CO2 (Figure 4). A real attraction of such an approach is that one could envisage the catalytic hydrogenation of CO2 focused at small, delocalized production sites as an alternative to the current large-scale, localized sites producing methanol by steam reforming of CH4.

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Figure 4. A cycle for sustainable methanol production. (Reproduced from Ref. 33. © American Chemical Society, 2009.)

2 Key Technologies

  1. Top of page
  2. Introduction: Basic Concepts
  3. Key Technologies
  4. Economic, Social, and Political Issues
  5. Future Prospects for a Hydrogen Energy Economy
  6. Concluding Remarks
  7. Acknowledgments
  8. Related Articles
  9. References

2.1 Production

Hydrogen is not found in free molecular form in any significant quantity on Earth. However, compounds of hydrogen are plentiful and hydrogen can be produced in large quantities by a variety of chemical and electrochemical means.34, 35 Hydrogen has been utilized in a variety of ways for more than 100 years. Total global hydrogen production is currently around 60 Mt per year, which could be sufficient to fuel at least 600 million fuel cell vehicles, which would be around 80% of the world vehicle fleet. However, such “merchant hydrogen” is presently used almost exclusively as an industrial chemical for a wide variety of processes, including in ammonia production for fertilizers, in refineries for desulfurization and other important processes, in the food industry, and in methanol production. An enormous increase in hydrogen production would be required in order to provide fuel for hydrogen vehicles and realize the hydrogen economy. Hydrogen is generated as a by-product of many industrial processes, for example, the manufacture of chlorine gas by chlorine–alkali electrolysis, and from coke oven gas from steel making. This by-product hydrogen is, in principle, available as a contribution to a transitional hydrogen economy, though the quantities involved are hard to estimate accurately.

At present, the overwhelming majority of hydrogen production, about 96%, comes from fossil fuels. The single largest method of production is the steam reforming of natural gas, producing about 48% of the total. Refinery processes from petroleum, particularly the partial oxidation of heavy hydrocarbons and recovery of by-product hydrogen, contributes about 30%, and the gasification of coal about 18%. Electrolysis, which is assumed to be a dominant production method in a sustainable hydrogen economy, currently accounts for only 4% of total production. Electrolysis is only economically viable where cheap hydroelectric power is available, for example, in Canada or Norway.

2.1.1 Steam Reforming

Steam reforming of methane (or propane) over a catalyst, followed by elimination of trace carbon monoxide and subsequent separation (usually via a membrane) yields high-purity hydrogen. The process requires heat (temperatures of 800–900 °C are typical), pressure (typically 20–40 bars), and a nickel catalyst. Steam methane reforming has the benefit of high efficiency and the economics favors large installations. The thermal efficiency of a large steam-reforming plant, producing around 100 tons of hydrogen per day, is around 70%. Steam reforming is a fully developed commercial technology and is the most economical method for production of hydrogen, at a cost of around £1 per kg. In order to avoid greenhouse gas emissions, CCS will have to be applied to remove the CO2 and sequester it. The additional cost of CCS would add a premium of up to 20% to this cost. We should expect that it will continue to be a major method of hydrogen production in the transitional hydrogen economy.

2.1.2 Coal Gasification

Coal gasification generates hydrogen from the reaction of steam and oxygen with pulverized coal. Pyrolysis of the coal produces a raw syngas contaminated by impurities, such as sulfur compounds, and ash particles. Once cleaned and cooled, the syngas can be passed to a shift reactor and to pressure-swing adsorption (PSA), as for steam reforming. Like steam reforming, coal gasification is a well-established commercial technology, in use worldwide, and an economic method of hydrogen production, and we should expect coal gasification with the addition of CCS to be an important method of hydrogen production in the transitional hydrogen economy.

Following Ball et al.,34 we distinguish three main groups of coal gasification process based on the flow regime by which the fuel and oxidant pass through the system. “Moving bed” gasifiers operate at the lowest temperatures (370–600 °C) and at pressures of 20–25 bars, using either air or oxygen as the oxidant. The syngas produced has a relatively high methane content and so is less suitable for hydrogen production, and the efficiency of carbon conversion is up to 90%. “Fluidized bed” gasifiers operate at higher temperatures of 800–1000 °C, using oxygen and air, and produce a syngas with lower CH4 content with a carbon conversion efficiency of up to 95%.

For hydrogen production the ideal process is the “entrained flow” gasifier, which operates at pressures of 20–85 bars and temperatures of 1400–1600 °C using very finely pulverized coal, with grain sizes below 0.1 mm, and oxygen rather than air as the oxidant. This system can achieve carbon conversion in excess of 95% and produces a synthesis gas with low methane content, consisting mostly of hydrogen and carbon monoxide.

It is of historical interest to note that from the 1800s until the 1960s, municipal gas supplies in the United Kingdom typically consisted of gases produced from coal, with a high hydrogen content; “town gas” for street lighting and domestic supply typically consisted of 50% hydrogen with the balance made up of methane and carbon monoxide. Thus, hydrogen gas was a major component in domestic energy supply until the rise of natural gas in the later twentieth century displaced the older form of gas supply.

2.1.3 Electrolysis

Electrolytic production of hydrogen from water is an efficient, yet costly process due to the comparatively high price of electricity; at present, less than 1% of worldwide hydrogen production originates from electrolysis of water. In simple terms, reverse operation of a PEMFC can be used to generate hydrogen and oxygen from water with an overall efficiency of about 90%. Apart from the PEM cell, all that is required is a source of DC electrical power, which can be supplied either by a conventional grid, or generated on-site from renewable sources such as wind and photovoltaics. In the long term, it is assumed that electrolysis using sustainably generated electricity will supply the needs of the hydrogen economy. At present, however, electrolysis is only a minor player in hydrogen generation, since the associated cost per unit of hydrogen is three to four times higher than in the case of steam reforming; the cost of the electricity supply is the dominant factor. This balance of costs will undoubtedly change in the future, as fossil fuels become increasingly expensive and sustainable electricity generation becomes more widespread, but economic incentives and subsidies will be needed to encourage the development of increased electrolytic hydrogen production capacity in the transitional hydrogen economy.

The alkaline water electrolyzer is the oldest and most widespread form of electrolysis technology. The feedwater is mixed with potassium hydroxide in a 20–40% solution, making a strong alkali. The electrodes are usually nickel or chromium–nickel steel for their corrosion resistance and catalytic properties. Electrolyzers typically operate at temperatures of 70–90 °C with a cell voltage around 2 V, achieving energy efficiencies of around 70% (compared to a theoretical maximum of 85%) and producing hydrogen purities (after drying and deoxidizing of the output hydrogen stream) in excess of 99.8%. They may be operated either at ambient pressure or under pressures of 10–30 bars. The latter case has advantages if the hydrogen is to be distributed by pipeline or stored under compression, but causes difficulties for gas purity as the pressurized gases permeate through the diaphragms separating the cells of the electrolyzer.

High-temperature electrolysis, proceeding at temperatures around 800 °C, is potentially more efficient than low-temperature alkaline electrolysis if the heat required is obtained from the waste heat of some other process. However, this technology is not yet established commercially, although laboratory-scale prototypes have been demonstrated.

Particularly attractive features of electrolyzers are that they are intrinsically modular, so that they can be scaled up by the addition of more cells, and they cope very well with intermittent power supply; the reaction proceeds when current is available and stops when it is not. This is a clear example of linking hydrogen to clean energy production from renewable, intermittent resources; the electrolytic separation of hydrogen from water can be carried out independently of any fossil energy source.

2.1.4 Innovative Methods

Biomass gasification is proposed as a method for generating hydrogen from solid (woody) biomass, and is strongly analogous to coal gasification. Various demonstration units exist and the method is the subject of ongoing research and development. Since biomass is the fuel, its combustion is, in principle, carbon-neutral, so CCS would not be absolutely necessary. Similar considerations apply to biomass pyrolysis. Hydrogen can be generated by some microorganisms in the course of their metabolic processes. Research into possibilities including photoproduction of hydrogen by algae or bacteria, and hydrogen generation from biomass fermentation is ongoing (see Fuel Cells: Enzymes and Microbes for Energy Production). Issues of limited arable land area for biomass crop production and high costs of collection are major concerns with biomass approaches.

Partial oxidation is a process used in the oil industry to obtain hydrogen from heavy hydrocarbons. Steam reforming is neither usable on long-chain hydrocarbons, as the deposition of soot would rapidly inactivate the catalyst and block the reactor, nor on high-sulfur oils because of catalyst poisoning. Partial oxidation, in which the hydrocarbons are reacted with pure oxygen and steam in an exothermic process, which produces a syngas with a high carbon monoxide content, proceeds at a higher temperature (1300–1500 °C) and pressure (30–100 bars) and does not require a catalyst. This makes the process more tolerant of low-quality feedstock. Autothermal reforming combines partial oxidation with steam reforming to convert both lighter and heavier hydrocarbons.

The Kvaerner process uses a plasma arc to split natural gas directly into hydrogen gas and carbon soot, using about one-third of the electricity required for water electrolysis. This process has recently entered the market and could develop as a potential competitor to steam reforming for the generation of hydrogen from natural gas, and as such may be a contributor to the transitional hydrogen economy.

Water splits into oxygen and hydrogen through thermal activation at temperatures in excess of 2200 °C, which is too high for the process to be technologically practical. This energy barrier can be reduced significantly using a so-called thermochemical pathway (see Water Splitting: Thermochemical). For example, in the sulfur–iodine process, iodine, sulfur dioxide, and water react together at 120 °C, forming hydrogen iodide and sulfuric acid. Both of these molecules thermally dissociate at higher temperatures (300 °C for hydrogen iodide, 850 °C for sulfuric acid), releasing hydrogen and oxygen and regenerating iodine and sulfur dioxide. This would be an efficient way to generate hydrogen from the heat of nuclear reactors or from concentrated solar radiation, and the engineering and materials challenges in handling the highly reactive components of the cycle are currently under active investigation.

Photocatalytic dissociation or splitting of water is a highly attractive mechanism for hydrogen production. In general, water can be cleaved directly by a photocatalyst or by use of a “tandem” cell. Here, light is absorbed in two cells, valence band “holes” oxidize water to oxygen, and conduction band electrons reduce hydronium ions to hydrogen (Figure 5).36 The process of photocatalytic splitting water is summarized as follows:

  • equation image(1)
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Figure 5. H2 production via H2O splitting. (Adapted from Ref. 36. © American Chemical Society, 2008.)

This reaction is catalyzed by means of inorganic semiconductors. The three important processes are (i) absorption of photons with energy greater than the electronic band gap; (ii) conversion of absorbed photons into electrical charges; and (iii) the utilization of the photo-induced electrical charges for water splitting. In Figure 6, a plot of quantum efficiency (QE) against wavelength of a wide range of inorganic materials is shown.37 For any prospective photocatalyst to be considered commercially viable, it has to display a QE of >10% in the visible region of the electromagnetic spectrum. To date, no materials have been discovered with that criterion (as seen in the shaded area). Current record holders are ZnS with a QE of ca 90% in UV light with a Pt cocatalyst and for splitting water with visible light GaN : ZnO with a Cr/Rh cocatalyst at 2.5%. Worldwide, research is now actively targeted toward photocatalysts that have an electronic energy band gap that allows the efficient use of the visible spectrum of sunlight. If such materials can be developed with sufficient stability and activity, photocatalytic production of hydrogen and oxygen could be competitive with the generation of solar photoelectricity and subsequent electrolysis.

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Figure 6. Quantum efficiency of different materials for photocatalytic water splitting in the UV and visible part of spectrum. (Adapted by D. Payne from Ref. 37. © American Chemical Society, 2008.)

It is clear that there are a variety of production pathways to hydrogen and these encompass fossil fuels, solar and nuclear energy, as well as a range of renewable sources (Figure 3); similarly, additional strategies for carbon sequestration are integral components for any route to hydrogen. Ewan and Allen38 have carried out an important assessment of four key measures to yield comparative figures of merit for the various routes to large-scale and additional hydrogen production. The measures considered were the level of CO2 emission reduction, primary energy availability, land use implications, and hydrogen production costs. This important overall comparison of routes shows a clear division between those using renewable energies and those associated with the traditional “high energy density” primary energy sources. These authors also point to desired research advances that would make a significant improvement in the position of hydrogen production routes.

Recent advances in the catalytic production of hydrogen from renewable sources are also highlighted in a special issue of the journal Catalysis Today39; here, renewable sources such as biomass and biomass-derived oxygenates including methanol, ethanol, and glycerol are outlined, as well as the photocatalytic decomposition of water. This direct water dissociation is regarded by many as a potential “show-stopper” if conversion efficiencies could be increased by a factor of 2 to 3.

2.2 Hydrogen Distribution

Clearly, current energy supply systems and infrastructure focus on fossil energy sources. It is challenging to plan for the future possibility of an alternative energy source such as hydrogen meeting society's large-scale energy demands in the future (Figures 1 and 3). What is the most suitable supply and infrastructure model—central or delocalized production and storage?

Current prototype hydrogen FCEVs make use of either liquid hydrogen or highly compressed gas (350 bars or more) for on-board storage of the 4–5 kg of hydrogen required to give a hydrogen FCEV a range comparable to a modern gasoline ICE vehicle.11, 35 This is workable for technology development but the weights and, more importantly, the volumes of such storage systems are considered excessive for mass-market vehicles. The cost of the fuel system is also a significant factor; gasoline storage tanks represent a negligible fraction of the cost of production of a gasoline ICE vehicle, but hydrogen storage systems are currently far more expensive, and costs must be reduced in order for mass production of hydrogen vehicles to be economically practical. Storage systems based on physisorption or on chemisorption in novel hydride materials are the subject of intensive research.40, 41

How could an alternative energy vector such as hydrogen meet society's large-scale and ever-increasing energy demand in the future? Pipeline transport of compressed hydrogen under pressures around 20 bars is already common in chemical industries. When hydrogen is produced at such pressures from PSA or from high-pressure electrolysis, this is advantageous for pipeline transport as less work need be done in compressing the hydrogen. The existing natural gas distribution network includes long-distance pipelines, usually made of steel, and local networks of plastic pipes, made from materials such as high-density polyethylene (HDPE) or polyvinyl chloride (PVC), which distribute gas at lower pressures (around 4 bars). There are difficulties in adapting the natural gas network for hydrogen distribution, as the plastic pipes are much too porous to hydrogen, and steel natural gas pipelines are vulnerable to hydrogen embrittlement. The construction of a new pipeline system, using sufficiently nonporous materials such as stainless steel, would be necessary for widespread transportation of hydrogen on the same scale as current natural gas distribution. The scale of pipeline construction is minimized if hydrogen is produced as close to the point of use as possible. For small quantities of hydrogen and transportation over short distances, trailer transport is possible for either compressed gas or liquid hydrogen; transport of liquid hydrogen is more efficient because of its much greater volumetric density. Since the production of liquid hydrogen is uneconomic in small quantities, trailer transport of liquid hydrogen will be the normal method of supplying liquid hydrogen to fueling stations.42, 43

Transportation of large quantities of liquid hydrogen by ship, analogous to the current bulk transportation of oil in tanker vessels, is still a theoretical idea at present. If it could be achieved, however, the economy of maritime transport would make it highly competitive with gas pipeline transport and might allow for the transport of hydrogen generated from renewable sources—geothermal energy in Iceland, for example, or solar power in the Sahara or the Middle East—over thousands of miles to European, North American, or Asian energy markets.

The term ‘hydrogen corridor’ has been used to describe the ensemble of large-scale production of hydrogen in one region or country and its long-distance transport in bulk by ship or pipeline to consumers in another region or country.

Apart from meeting economic, environmental, and safety criteria, the new hydrogen technology must be usable without undue constraints. Operability, everyday suitability, and acceptance of hydrogen energy technology mandate the testing of systems in actual practice. In fact, there are currently around 300 hydrogen fueling stations in operation worldwide, mostly in the United States, Japan, and Germany, as part of various hydrogen-transport projects.42 The first public filling station for liquid hydrogen was taken into service in Munich airport in 1999. A large fueling station needs to provide hydrogen to 300–400 cars per day to be comparable to a conventional petrol station, which, at around 5 kg of hydrogen to fully refuel a car, would require more than a ton of hydrogen per day. Fueling stations may be divided into compressed-gas hydrogen (CGH2) and liquid hydrogen (LH2) types.

A CGH2 station would maintain a store of compressed hydrogen at relatively low pressure, comparable to the 20 bars at which hydrogen can be produced and distributed by pipeline. On-site booster compression is required to dispense hydrogen to vehicles at the pressures of up to 800 bars required to refuel vehicles with on-board 700-bar compressed-gas storage.

An LH2 station would maintain an underground cryogenic tank of liquid hydrogen, supplied to the site by trailer. This aspect of the design is similar to current gasoline filling stations, although, of course, the handling of cryogenic liquids is more challenging; careful attention must be paid to the venting of excess boil-off gas from such a station to avoid any risk of combustion or detonation. LH2 stations have the advantage that they can dispense both liquid hydrogen and boil-off gas as CGH2. This flexibility will be valuable if both LH2 and CGH2 hydrogen FCEVs reach the marketplace.

It is clear that the development of industry standards for the dispensers for LH2 and CGH2 to vehicles, and for the vehicle interface to the dispenser, will be vitally necessary for hydrogen FCEVs to become widespread; a situation in which vehicles from a given supplier were physically unable to refuel from fueling stations associated with a competing supplier would not be tolerable to consumers.

2.3 Storage

Storage of hydrogen as a cryogenic liquid or a compressed gas are the best-developed methods at present. Reversible storage in solid or liquid materials is less well developed but is a subject of intense research effort. Figure 7 gives an overview of hydrogen storage methods and materials,44 drawing attention to the system's materials weight percent hydrogen, the operating temperature, and the energy required for hydrogen release.

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Figure 7. Overview of hydrogen storage methods and materials. (Reproduced from Ref. 44. © Wiley-VCH, 2009.)

2.3.1 Liquefied, Cryogenic Hydrogen

Hydrogen has been liquefied on an industrial scale for more than 70 years, and used in a variety of applications. Hydrogen can be transformed from a gas to a liquid by cooling to temperatures below 20 K; the very low temperature required is natural, given the low molecular weight of hydrogen. The phase diagram of hydrogen is such that the liquid phase exists in only a relatively narrow change of temperatures and pressures; there is a critical point at 33 K, above which the liquid phase does not exist, and it also does not exist at pressures above a few bars. This causes difficulty with liquid hydrogen storage; the evaporation of some of the liquid is inevitable as heat is absorbed from the environment, and the evaporated gas cannot be reliquefied by increased pressure. Venting of boil-off gas is therefore vital for safe LH2 storage and involves a loss on the order of 1% per day of stored hydrogen.

The cooling process for hydrogen involves multiple stages, including a step for the catalytic conversion of orthohydrogen to parahydrogen (a transition in the spin states of the nuclei in the molecule), and is energy intensive. The process is more efficient in larger plants, but generally requires energy input on the order of 20–40% of the chemical energy content of the hydrogen. This figure affects the effective energy efficiency of hydrogen applications, and the avoidance of this large energy loss is one of the drivers in the search for reversible solid-state hydrogen storage materials. Liquid hydrogen storage can be effective for stationary applications, such as the LH2 fueling station, but the weight of the well-insulated tank required to store small amounts of hydrogen safely is a negative feature for on-board vehicular hydrogen storage; such tanks offer a gravimetric hydrogen density of only a few weight percent (up to 10 wt % in some cases), and the venting of boil-off hydrogen from parked cars is a safety issue. Current LH2 prototype vehicles, for example, cannot be parked in enclosed spaces in case of gas buildup. The cost of on-board cryogenic storage tanks must also be reduced greatly for LH2 to be commercially practical. Until recently, liquid hydrogen was evaluated as a viable on-board storage option for transportation. However, virtually all of the world's major car manufacturers are concentrating their efforts on compressed-gas storage (which we now focus on) or liquid hydrogen storage in combination with cryoadsorption on tailored sorbents.

2.3.2 Hydrogen Compressed-Gas Storage

A priority development goal is a vehicle fuel tank with maximum energy storage capacity and minimum weight, volume, and cost. One solution is a pressure vessel made of composite materials in which hydrogen is stored at high pressure. Hydrogen gas compression from ambient pressure to 800 bars—a pressure suitable to refuel a storage system operating at around 700 bars—requires work equivalent to about 15% of the energy content of the hydrogen, and thus is slightly less energetically costly than liquefaction. The energy cost is reduced somewhat if compression begins with gas at a higher pressure, for example, as produced from PSA or high-pressure electrolysis. The weight of a steel tank capable of containing such high pressures safely is considerable. Current 350-bar CGH2 storage tanks for vehicles achieve about 6 wt % of hydrogen, while 700-bar tanks have slightly poorer gravimetric performance at around 4.5 wt % of hydrogen, though with a gain in volumetric performance. The development of improved high-pressure tanks using lightweight composite materials does improve the gravimetric performance. These generally consist of a thin-walled, seamless aluminum liner, completely enveloped by a high-strength carbon-fiber laminate. The cost of the tank is, if anything, a greater barrier than the weight; current tanks cost thousands of dollars, whereas gasoline tanks cost on the order of $50 apiece. To achieve the goal of 600 km, the operating pressure will have to be increased to 35–70 MPa: the solution will surely involve composite materials with high-strength metal liners.

2.3.3 Hydrogen Storage in Materials

It is clear that the safe, compact storage of hydrogen is one of the major obstacles to the widespread adoption of hydrogen as a fuel. Hydrogen storage materials (in essence, “solid” hydrogen storage) can provide a high energy density, compact, and low-pressure route to storing hydrogen. However, Figure 8 clearly indicates the storage challenge for hydrogen (and indeed other energy sources, e.g., lithium batteries) when compared with fossil fuels. The latter clearly have exceptional volumetric and gravimetric energy densities. Although liquid hydrogen on its own would massively outperform them on specific energy, current LH2 and CGH2 storage systems have much poorer performance because of the weight and volume of the ancillary material required to contain the hydrogen. Research is therefore directed to the area for a storage system that will make on-board hydrogen storage a practical reality. This is now a major focus of significant international activity.

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Figure 8. Energy density of various energy storage materials and technologies, illustrating the respective volumetric and gravimetric densities with temperature data for some materials under the composition indicating equilibrium temperature at a hydrogen pressure 1 bar. (Reproduced from Ref. 36. © Wiley-VCH, 2010.)

Hydrogen can be bound within a solid material by two mechanisms, physisorption or chemisorption. In physisorption, molecular hydrogen is bound by van der Waals' type forces to the surfaces of a porous material. As these interactions are very weak (on the order of 5–8 kJ mol−1 of H2), physisorption requires cryogenic temperatures, and research is often directed at systems operating at liquid nitrogen temperature (77 K). However, this is far less challenging than working with liquid hydrogen and avoids the high energy cost of liquefaction. Hydrogen release is, in turn, achieved by the introduction of a small amount of heat energy. The storage material must offer extremely high internal surface area on which to absorb the hydrogen, so nanostructured materials such as zeolites, metal–organic frameworks (MOFs), and nanocarbons have been investigated. Light weight and stability are also required; the latter is critical as the material must function for thousands of cycles of hydrogen loading and unloading. The intrinsic advantage of hydrogen physisorbed in such materials is their rapid charge/discharge times—here hydrogen maintains its molecular identity at all times. Important strategies are now emerging for the optimization of pore sizes and hydrogen adsorption energy in both MOFs and zeolites and there is a real optimism that significant advances are possible toward achieving the Department of Energy (DOE) targets for on-board hydrogen storage.

In chemisorption, by contrast, the hydrogen molecule is dissociated into atoms which are stored either in chemical compounds or within the lattice of a metallic host material. The advantage of this approach is that it is possible to achieve extremely high volumetric and gravimetric hydrogen densities in some cases, even exceeding that of liquid hydrogen (67.8 kg m−3); for example, the material Mg2FeH6 stores 150 kg m−3 of H2, and more complex alloys like (Ti, Cr, V)H1.9 store 160 kg m−3 of H2. Since chemisorption involves the breaking and formation of chemical bonds, the hydrogen-binding energies are much higher than in physisorption. Such materials offer the prospect of a high energy density, compact, and low-pressure means of storing hydrogen. Thermal management of the system is therefore critical. During fueling, heat is released from the reaction of molecular hydrogen with the dehydrogenated storage material. In operation, heat must be introduced to provoke the release of hydrogen from the store. It is therefore desirable that the temperature required for thermal release should not be greater than the temperature of the exhaust gases from the hydrogen fuel cell (less than 100 °C for PEMFCs) so that heat energy can be recycled and no additional energy input is required for hydrogen release. Low temperature of dehydrogenation is also required for a quick start of a fuel-cell system. The decomposition temperature for binary metal hydrides (MHx) is found to correlate strongly with the structural electrode potential, E0. In particular, the easier it is to reduce the metal (i.e., a larger reduction potential), the lower the temperature that is required to decompose the solid into the metal and hydrogen gas.

  • equation image(2)

Figure 9 shows the relationship between the reduction potential and decomposition temperature for binary hydrides.36 It also “calibrates” such considerations against the operating temperatures of the various hydrogen fuel cells. Given that only light elements of the periodic table are able to meet the gravimetric weight density, considerable emphasis is now being placed on complex ternary hydrides, NaBH4 and LiBH4. These can achieve impressive gravimetric densities. With respect to materials design, the decomposition temperature of ternary hydrides can also be altered through the choice of metal constituent, and the trends in decomposition temperature can be rationalized by the relative difference in electronegativities of the metal and anion components.

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Figure 9. Correlation between temperature, Tdec, at which thermal decomposition of binary hydride MHn to the constituent elements proceeds, and the corresponding redox potential of the redox pair Mn+/M0 in acidic aqueous conditions. (Reproduced from Ref. 36. © Wiley-VCH, 2010.)

Arguably, the best-developed methods for solid-state hydrogen storage are based on metal alloy or interstitial hydrides.40, 41, 45 Several types of interstitial hydrides exist which operate at different temperatures and pressures from below zero up to 300 °C and above (Figure 7). Transition metal hydrides are too heavy for vehicular applications but have potential applications in stationary hydrogen storage (e.g., for hydrogen generated from intermittent sources, as mentioned in Section 1.2) or in maritime applications, where heavy hydrogen storage materials can be used in ballast. Research for vehicular solid-state hydrogen storage is now concentrated on hydrides of light elements such as lithium, boron, sodium, magnesium, and aluminum.2 These can achieve impressive gravimetric hydrogen densities (up to 18 wt % in the case of LiBH4). The bonding in these hydrides is intermediate between ionic and covalent; reversibility and the kinetics and temperature requirements of hydrogen release and uptake are still major unresolved issues.40, 41

It seems likely that CGH2 and LH2 systems will continue to play their part in the transitional hydrogen economy. If a breakthrough in hydrogen storage materials were forthcoming, a favorable impetus would then be applied to resolving many of the other challenges facing the hydrogen economy. Solid-state materials do indeed have the potential to outperform other storage technologies; the issue here—as in many new areas—is that of materials discovery.

A possible solution to the reversibility issue is off-board regeneration, where the spent storage material is removed from the vehicle and reprocessed at a chemical facility. This is very similar to the battery-swapping concept for BEVs, but with a much more competitive range between reloads, and allows for hydrogen production on-board by hydrolysis reactions. Attractive, new-generation hybrid storage systems offer a combination of chemisorption and storage under pressure; essentially a pressurized storage tank is filled with an interstitial metal hydride, resulting in a system in which working pressures are lowered considerably.

The storage of hydrogen in liquid-phase organic heterocycles is also a highly promising option for storing hydrogen because of simplicity, safety, scalability, low cost, and easy heat management compared to solid-state hydrogen storage materials.46 The well-known cyclohexane–benzene system has a very high dehydrogenation temperature (>300 °C) but nitrogen-containing organic ring compounds (for example, N-ethyl carbazole and related compounds) have demonstrated reversible hydrogen storage capacities of >7 wt % at 150–200 °C due to an optimal heat of dehydrogenation of 10–13 kcal mol−1 of H2.46 Selective catalysts enable highly reversible catalytic hydrogenation and dehydrogenation of these materials with no significant degradation of the molecules.

Organic liquid hydrogen storage materials have a number of significant engineering advantages since they can be easily pumped during distribution, delivery, and car refueling process. They also have low volatility (b.p. >300 °C) and a high flash point, and can employ a simple and light fuel tank that would not require handling high pressures and temperatures. In addition, the dehydrogenation can be done in a small catalytic dehydrogenation chamber, which would require heating only a small amount of liquid material instead of heating the whole hydrogen storage tank. Organic hydrogen storage materials also consist of abundant and cheap elements (carbon, nitrogen) and only require a small amount of precious metal catalyst for dehydrogenation and hydrogenation steps. The remaining issues for introduction of organic hydrogen storage materials include the understanding and optimization of dehydrogenation mechanism against undesired decomposition pathways, improving kinetic facility for the hydrogenation step and decreasing the toxicity.

2.4 Hydrogen Use: The Fuel Cell

Fuel cells are electrochemical converters, transforming chemical energy from a controlled reaction directly into direct-current electrical power.47 The important distinction between a battery and a fuel cell is that a battery contains its own reaction substances, which are consumed until the battery is discharged, whereas a fuel cell is provided with a continuous flow of reactants from an external source. The basic layout of a fuel cell involves the provision of a fuel to the anode; the transfer of a mobile ion across an electrolyte barrier separating the anode from the cathode; and the provision of oxygen or air at the cathode. Electrons are liberated at the anode and consumed at the cathode; electrical power is extracted from the device by connecting a load across the terminals allowing electrons to flow from the anode to the cathode. This makes the cathode the positive terminal, as is normal for a galvanic cell supplying current. The electrolyte must permit the passage of the mobile ions but be impermeable to electrons, as electron transfer across the electrolyte would constitute a short circuit in the device. In different forms of fuel cell, the mobile ions may either be negative (oxide, hydroxide, or carbonate) and move from cathode to anode, or positive (protons) moving from anode to cathode. The first fuel cell was reported in 1839 by Sir William Grove, consuming hydrogen and oxygen with platinum electrodes and a sulfuric acid electrolyte—an electrolyzer operated in reverse.

The theoretical maximum efficiency of a fuel cell is given by the ratio of the free-energy change to the enthalpy change during the reaction; for a hydrogen fuel cell operating at temperatures around 100 °C, this efficiency is around 90%. The theoretical voltage of a hydrogen cell is 1.23 V, though in practice cells operate at around 0.7 V, and multiple cells must be stacked together, connected by bipolar plates, to achieve higher voltages. The lower cell voltage and resistance losses in the system reduce the efficiency considerably below the theoretical maximum, but even so a hydrogen FCEV can operate with a net efficiency including the drive train of up to 45%. This greatly exceeds the performance of ICEs, which at partial load (typical of driving conditions) routinely operate at around 20% efficiency; the theoretical maximum efficiency of ICEs is limited by the Carnot cycle. Although the efficiency advantages of fuel cells have been appreciated since the nineteenth century, they have repeatedly failed to break into the mass market, largely due to the materials problems involved in developing effective, durable, and inexpensive materials for the anode and cathode catalysts and for the electrolyte.

We briefly summarize the main types of fuel cells in terms of their operating temperatures and the nature of the electrolyte. The SOFC (ceramic oxide electrolyte) operates in the range of 600–1000 °C and the molten carbonate fuel cell at around 650 °C. These types are considered more suitable for stationary electricity generation applications than for transport because of their slow start-up from cold (see Fuel Cells: Intermediate Temperature Solid Oxides). We note that at these high temperatures of operation, the cells can be fueled either with hydrogen or with a hydrogen- and CO-rich syngas generated from the reforming of a purified hydrocarbon gas.

Lower temperature fuel cells fueled by hydrogen include the alkaline fuel cell, operating at around 80 °C with hydroxide ions as the charge carrier; the phosphoric acid fuel cell, operating at around 200 °C with protons mobile in concentrated phosphoric acid; and the PEMFC, operating at around 80 °C. The PEMFC is the leading candidate for the power source in hydrogen FCEVs (see Fuel Cells: Proton Exchange Membranes) (Table 3). The low-temperature fuel cells are less tolerant of fuel variations than the high-temperature cells, as carbon monoxide is a catalyst poison, and must be provided with high-purity hydrogen as fuel.

Table 3. Types of Fuel Cells, Their Temperature of Operation, Electrical Efficiency, and Applications.
Type of cellElectrolyteMobile ion Temperature of operation Electrical power range (kW) Electrical efficiency (%)Application
Adapted from Ref. 6
Alkaline (AFC)Liquid KOHOH50–200 °C0.1–5050–70Space vehicles
Phosphoric acid (PAFC)Liquid phosphoric acidH+∼220 °C50–100040–45Stationary applications
Proton exchange membrane (PEM)Polymeric filmH+30–100 °CTransport applications
Molten carbonate (MCFC)Molten alkali metal carbonatesCO32−∼650 °C200–100 00050–60Stationary application
Solid oxides (SOFC)Ceramic YSZO2−500–1000 °C0.5–200040–72Stationary applications

A variant of the PEMFC is the direct methanol fuel cell, powered not by hydrogen but by a dilute methanol solution. This cell is proposed as a power supply for portable applications such as laptop computers, where it would offer much longer operating times than batteries can; the ease of handling of the methanol solution makes it more suitable than hydrogen for such small-scale applications. The alkaline fuel cell can also be fueled by ammonia.

Perhaps the biggest challenge facing the utilization of the PEMFC for hydrogen vehicles are lifetime and cost. At present, the electrodes are made with a nanostructured platinum catalyst, an expensive material and one which is sensitive to poisoning by carbon monoxide, ammonia, and by sulfur compounds. Materials costs and the costs of manual production of components give a current price of $2000–$4000 per kilowatt, and the lifetime of the cell is below 2000 h.48 For hydrogen FCEVs to be economically competitive with gasoline vehicles, and thus be an attractive option for consumers in the transitional hydrogen economy, costs must be reduced below $100 per kilowatt and lifetime must be increased to above 5000 h. While mass production can help in cost reduction, cheaper and more durable catalyst and electrolyte materials are urgently needed—another fascinating challenge for chemistry and materials in the development of the hydrogen economy (see Fuel Cells: Molecular Catalysis). A considerable contribution to these challenges is indeed being made by chemists, in association, in multidisciplinary projects, in developing new generation, high-performance catalysts, membranes, electrodes, and materials for the new fuel cell technologies.

Conceptually, and indeed operationally, hydrogen fuel and the hydrogen fuel cell form the “hydrogen nexus,” connecting diverse methods of hydrogen generation and possible applications, as illustrated in Figure 1.10

3 Economic, Social, and Political Issues

  1. Top of page
  2. Introduction: Basic Concepts
  3. Key Technologies
  4. Economic, Social, and Political Issues
  5. Future Prospects for a Hydrogen Energy Economy
  6. Concluding Remarks
  7. Acknowledgments
  8. Related Articles
  9. References

The desirability of hydrogen relates mainly to the challenges of pollution (most notably, climate change and local air pollution) and energy security that have arisen from the current reliance on fossil fuels as the primary energy source.49 However, for the hydrogen economy to ever come about, there will need to be a fundamental transition away from the fossil fuel economy and its associated energy system and all-pervasive infrastructure. The development of a hydrogen economy is not only an engineering and scientific challenge; it will have fundamental consequences on the socioeconomic and geopolitical scales; patterns of employment will shift as the newer technology makes different demands on the workforce and supply chain, and regional and global trade patterns will alter as the sources of energy supply shift from fossil-fuel-rich to renewable-resource-rich areas. It is said that “prediction is very hard, especially if it's about the future.” Nonetheless, we briefly sketch the likely nature of these changes. As noted recently by Ekins,49 “…a technological challenge of the kind envisaged is far more than a change in the physical technologies employed in the energy system, important though this change is.” He further notes that “…change cannot and will not come about unless it is accompanied by parallel changes in the economic and social systems.”

The energy sector may expect an increasing emphasis on renewable energy generation; more solar panel construction and installation; more wind turbines and tidal or wave generators; greatly increased production of biofuels; the construction of a new generation of nuclear fission reactors; and the development of an enhanced electrical grid, incorporating new features such as high-temperature superconducting transmission lines and long-distance high-tension DC transmission. We must also expect increasing emphasis, in all technology sectors and in the construction industry, on energy efficiency; reductions in demand by efficiency gains reduce both our current use of fossil fuels and the load on the developing renewable energy supply.50 Geopolitically, the changes required for the development of a hydrogen economy may be perceived as a threat by countries whose economies are heavily dependent on the production of fossil fuels or otherwise heavily invested in the fossil fuel economy.

There are substantial differences worldwide in the cultural expectations that drivers have regarding their vehicles and their use. For example, in the United States, in recent years, there has been a shift toward heavy and inefficient “sports utility vehicles” (SUVs), and there is a long-standing cultural bias toward long-distance free-roaming driving. The development of prototype hydrogen vehicles is biased more toward smaller, lighter vehicles which are suited to short-range urban driving and may be more welcome in the European and Japanese markets.

The transition from gasoline ICEs to hydrogen FCEVs will have fundamental socioeconomic consequences on employment patterns and the skills required of the automotive industry workforce. Essentially, the costs of the drive train of a gasoline ICE include substantial contributions from the mechanical elements of the engine itself and the mechanical transmission system. FCEVs are simpler, mechanically, as they have far fewer moving parts, and so the production and assembly of the power supply and transmission will constitute a smaller fraction of the cost of the drive train. The contribution of electrical and electronic engineering to vehicle construction, by contrast, will increase because of the increased use of electric motors and complex electronic control systems. Different demands will be made on the chemical industry, as there will be an increased need for the components of the fuel cell system (especially the catalyst electrodes and the electrolyte membrane), and a decreased need for products such as the catalytic convertor—currently a major component of the ICE exhaust system.

The widespread use of hydrogen could potentially have environmental effects due to increased anthropogenic emissions of molecular hydrogen to the atmosphere.51 Hydrogen participates in stratospheric chemical cycles involving H2O and various greenhouse gases, and a substantial increase in its concentration might lead to changes in the equilibrium composition of the stratosphere. More accurate modeling of these stratospheric processes and better understanding of other factors such as hydrogen uptake in soil and its effect on microbial communities are required to assess the potential environmental impact of the hydrogen economy. We have 10–20 years, before hydrogen is widely used as an energy carrier, to understand, and take any necessary actions to prevent, any harmful environmental impact. We must avoid a repetition of the mistakes of the past, when the harmful effects of chemicals such as chlorofluorocarbons, lead in petrol, and indeed the use of fossil fuels in general, were understood only after significant damage to the environment had been done. Any potential concern over the use of hydrogen, however, is outweighed by the very real dangers associated with our escalating CO2 emissions.

The safety of hydrogen production, storage, and use is not only a technological issue but also the major psychological and sociological issue facing the adoption of the hydrogen economy. To be accepted by the public, hydrogen must be considered safe. Consumers will undoubtedly have concerns about the safety and dependability of fuel-cell-powered equipment, new dispensing technology, etc., just as they had about other modern devices when they were introduced. Confidence-building will be necessary for transportation, stationary residential and portable applications where customers will interact directly with hydrogen and fuel-cell technology. An important factor in promoting public confidence will be the development and adoption of internationally accepted codes and standards. Education projects, product exposure, and marketing should be developed in order to facilitate the successful introduction of hydrogen as an alternative fuel.52

Although the investments required for the development of a hydrogen economy are substantial, they must be viewed not as a cost but as a long-term investment with a large expected return. In the next few decades, we must expect that countries and regions that can successfully make the transition to sustainability, cutting their dependence on fossil fuels and improving their energy security by efficiency gains and diversification of energy supply, will reap substantial economic and political benefits.

4 Future Prospects for a Hydrogen Energy Economy

  1. Top of page
  2. Introduction: Basic Concepts
  3. Key Technologies
  4. Economic, Social, and Political Issues
  5. Future Prospects for a Hydrogen Energy Economy
  6. Concluding Remarks
  7. Acknowledgments
  8. Related Articles
  9. References

At present, the hydrogen economy is at best embryonic.53 Hydrogen vehicles exist in prototype form, with around 1000 FCEVs in operation worldwide at the end of 2007; typically these development models use 350-bar or 700-bar CGH2 storage and have ranges of 200–300 km. In the public transport sector, around 100 hydrogen buses are in operation. Around 200 hydrogen fueling stations are active. Hydrogen-transport projects are concentrated in cities, as the prototype vehicles are most suitable for short-range urban driving in the vicinity of hydrogen fueling stations. In terms of global geography, hydrogen transport is concentrated in North America, Europe (especially Germany) and Japan. A small number of hydrogen buses were used as part of transportation service at the 2008 Beijing Olympic Games.

The timescale and evolution of a potential transition are the focus of many “road maps” emanating from the United States, Japan, Canada, the European Union, and many others. The key components of these hydrogen roadmaps relate to hydrogen production, storage, distribution, and utilization, and encompass many of the scientific and technological issues and challenges outlined earlier. In January 2004, the European Commission initiated the European Hydrogen and Fuel Cell Platform (HFP) with the expenditure of €2.8 billion over a period of 10 years, with the aim to prepare and direct an effective strategy for developing and exploiting a hydrogen-oriented economy for the period up to 2050. Key highlights and scenarios challenging European hydrogen vision are summarized in Figure 10.54

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Figure 10. The European hydrogen and fuel-cell road map. (Reproduced with permission from Ref. 54. © European Commission, 2003.)

This long-term vision regarding the potential of hydrogen and fuel cells illustrates the final goal of a hydrogen economy that relies mainly on renewable energy production pathways. A target scenario for 2020 was developed for guiding the transition toward introducing hydrogen and fuel cells to the market. In March 2005, HFP published a Strategic Research Agenda and Deployment Strategy,55 followed by an Implementation Plan in January 2007, which combined these two documents into a long-term road map for Europe. The primary objective of this roadmap is to achieve by 2020 EU-wide availability of hydrogen and fuel-cell vehicles with an appropriate coverage of refueling infrastructure. Table 4 summarizes the forecasts of several roadmaps for deployment status and targets for hydrogen technologies and fuel-cell applications.55, 56

Table 4. Key Assumptions on Hydrogen and Fuel Cell Applications.
TechnologyToday20202050
  1. CHP, combined heat and power; IEA, International Energy Agency; n.a., not applicable.

Adapted from Ref. 55, 56
Hydrogen produced from coal with CCS (€ GJ−1)8–107–93–5
Hydrogen transportation/storage cost (pipeline, 5000 kg h−1, 800 km) (€ GJ−1)10–1532
PEM fuel-cell cost (€ kW−1)6000–800040040
High-temperature fuel-cell cost (€ kW−1)8000–10000800200
European Union: portable fuel cells, sold per yearn.a.250 millionn.a.
European Union: fuel-cell vehicles, sold per yearn.a.0.4–1.8 millionn.a.
European Union: stationary fuel cells (CHP), sold per yearn.a.100 000–200 000 (2–4 GW)n.a.
United States: number of fuel-cell vehiclesn.a.2 millionn.a.
Japan: fuel-cell vehicles, cumulative sale targetn.a.5 millionn.a.
IEA forecast: global fleet of fuel-cell vehiclesn.a.n.a.700 million

In 2002, the US DOE, in conjunction with the auto industry, established the US FreedomCAR and Fuel Partnership. This partnership set technology goals and demonstration targets for 2015 that were considered sufficient to enable the industry to move toward the commercialization of the hydrogen technologies as illustrated in Figure 11.48 Some of the most important R&D goals for hydrogen and fuel cell technology are listed in Table 5.

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Figure 11. US hydrogen economy timeline with manufacturing R&D shown. (Reproduced from Ref. 48. © U.S. Department of Energy, 2004.)

Table 5. DOE's 2015 Goals for Hydrogen and Fuel-Cell Technology
Targeted factorGoal for 2015
Hydrogen production$2 per kilogram (by distributed methane reforming)
Hydrogen storage: specific energy density10.8 MJ kg−1 (9.0 wt %)
Hydrogen storage: volumetric energy density9.7 MJ L−1 (81 g of H2 per liter)
Hydrogen storage: system cost$0.56 per megajoule($66 per kilogram of H2 stored)
Hydrogen storage: delivery pressure2.5 bars
Hydrogen storage: operating temperatureFrom −40 to +85 °C
Hydrogen storage: cycle life (1/4 tank to full)1500
Hydrogen storage: refueling rate2.0 kg of H2 per minute
Hydrogen storage: loss of usable H20.05 g h−1 per kilogram of H2 stored
Fuel-cell cost$30 per kilowatt at a volume of 500 000 units per year
Fuel-cell lifetime5000 h
Fuel-cell efficiency (80 mpg equivalent)60%
Vehicle driving range300 miles
Vehicle efficiency80 miles/gasoline-gallon-equivalent

In 2008, the US National Research Council's Committee concluded that on the basis of the substantial financial commitments and technical progress, the hydrogen production technologies and hydrogen and fuel cell vehicles could be ready for commercialization in the period 2015–2020. It was also estimated that, while fuel cell vehicles would not become competitive with gasoline-powered vehicles by 2020, they could account for more than 80 % of new vehicles entering the fleet by 2050.57

These efforts will require considerable resources, especially government and private sector funding. The committee estimated that in addition to the current US government hydrogen R&D expenditures of about $300 million per year, additional $55 billion from 2008 to 2023 will be needed to support a transition to hydrogen fuel cell vehicles. This funding includes an extensive R&D program ($5 billion), support for the production of hydrogen ($10 billion) and support for the demonstration and deployment of the hydrogen vehicles at the earlier stages of commercialization ($40 billion). It is also estimated that private industry would need to invest about $145 billion for R&D, vehicle manufacturing, and hydrogen infrastructure over the same period.57

Across the full range of energy use, hydrogen and fuel cells provide a major opportunity to shift our carbon-based global energy economy ultimately to a clean, renewable and sustainable economy based on hydrogen. To achieve a significant penetration of hydrogen into future energy systems, the methods of hydrogen production, distribution, storage, and utilization must be significantly improved beyond their present performance, reliability, and cost. In addition to technological developments, significant new energy security and environmental policy actions will be also required. To enable competitive and self-sustaining hydrogen and fuel cell systems in the long term, hydrogen-specific policies adopted in the nearer term will be required in order to assure industry that the necessary long-term investments can be made safely. This will allow for the development of sufficient capacity for hydrogen production, distribution, and use to make a hydrogen economy feasible.

Any assessment of the feasibility of a sustainable hydrogen energy economy will involve an appraisal of the many steps, not only in sciences and technology but also social and economic considerations, which will have to be taken on the road to that future. The “systems approach” of looking at the future of hydrogen energy, outlined in the Hydrogen Strategic Framework for the UK58 concludes that there is not one single route to a hydrogen economy, but rather that many factors/variables are involved in determining its direction. It may, therefore, not only be rather difficult—but indeed limiting—to establish one single path to the hydrogen economy at this juncture. A complementary analysis of how a transition to a hydrogen energy economy may enter the UK energy system is the focus of an important new volume by Ekins.49

5 Concluding Remarks

  1. Top of page
  2. Introduction: Basic Concepts
  3. Key Technologies
  4. Economic, Social, and Political Issues
  5. Future Prospects for a Hydrogen Energy Economy
  6. Concluding Remarks
  7. Acknowledgments
  8. Related Articles
  9. References

Any transition to a future hydrogen energy economy will derive largely from a desire to reduce the pollution and energy insecurity that are associated with fossil fuels. At present, such pressures do not appear sufficient to drive the transition—any transition—to a hydrogen energy economy, although, of course, it is possible that this could become so.49 In the case of hydrogen, one can therefore categorize both the physical and socioeconomic dimensions for their successful coevolution toward a hydrogen energy economy. The physical dimension deals with the physical issues (and attendant scientific and technological breakthroughs) involved in the production/storage/distribution/end use of hydrogen, which have been reviewed here. The socioeconomic dimension deals with interests and drivers that push technical change along; a cursory review of these issues has been outlined here.

For a hydrogen economy, the physical dimension encompasses the following48:

  • Science of hydrogen production, storage, and utilization—the physically possible in advances.

  • Technology—the physical realization of the physically possible.

  • Infrastructure—the physical and technical support of the realization of the new energy system.

The socioeconomic dimension centers on the following:

  • Economics—competition and distribution.

  • Institutions—regulatory, financial, and planning.

  • Political drivers—social perceptions driving political priorities and the planning system.

  • Culture—social perceptions driving acceptability and demand.

In this article, we have illustrated the scale and extent of the scientific, technical, and socioeconomic challenges for hydrogen energy technologies, in order to try to understand the kind of transformation of our energy system to which they might ultimately give rise.

Acknowledgments

  1. Top of page
  2. Introduction: Basic Concepts
  3. Key Technologies
  4. Economic, Social, and Political Issues
  5. Future Prospects for a Hydrogen Energy Economy
  6. Concluding Remarks
  7. Acknowledgments
  8. Related Articles
  9. References

We thank the EPSRC for support. Asel Sartbaeva thanks the Samuel and Violet Glasstone Research Fellowship and Stephen Wells thanks Leverhulme Trust for funding. We thank D. Payne for adapting the figure 6.

Abbreviations
AGW

anthropogenic global warming

BEV

battery electric vehicle

CCS

carbon capture and storage

CGH2

compressed gas hydrogen (storage method)

EV

electric vehicle

FC

fuel cell

FCEV

fuel-cell electric vehicle

HDPE

high-density polyethylene

ICE

internal combustion engine

IPCC

intergovernmental panel on climate change

kWh

kilowatt-hour; measure of energy equal to 3.6 million joules

LH2

liquid hydrogen (storage method)

MOF

metal–organic framework

PHEV

plug-in hybrid electric vehicle

PSA

pressure-swing adsorption (gas purification method)

PEMFC

polymer electrode membrane (or proton exchange membrane) fuel cell

ppm

parts per million (gas concentration)

PVC

polyvinyl chloride

RD & D

research development and demonstration

SOFC

solid oxide fuel cell

wt %

weight percent (composition)

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  3. Key Technologies
  4. Economic, Social, and Political Issues
  5. Future Prospects for a Hydrogen Energy Economy
  6. Concluding Remarks
  7. Acknowledgments
  8. Related Articles
  9. References
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