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Keywords:

  • catalysis;
  • energy;
  • fuel cells;
  • photosynthesis;
  • renewable resources

Energy is becoming a big issue in our society, fueled by the growing awareness of the finite resources of fossil liquid fuels and the noticeable changes in climate resulting from their consumption. The need to accelerate the introduction of renewable energies to the energy pool has been fostered by the recent oil price volatility and political tension in the Gulf region. These and other factors have created an intense social pressure for an accelerated transition from fossil to renewable fuels. At the same time, concerns about greenhouse gases have progressively changed from possibility to reality, and the fraction of the world′s population accessing energy on a massive scale is exponentially increasing. The immense consumption of energy, either in fossil form or as electricity, in emerging countries such as China and India has additionally driven the global demand for sustainable development. The accident at Fukushima, also recalling the past accidents of nuclear power stations, may have refocused the social, political, and scientific interests of future energy scenarios on renewable energies based on solar energy.

The global primary energy consumption amounts to about 5×1014 MJ a−1.2 This is a very large number, describing the scale of the energy challenge. Until now the major energy contributors to this large number are still based on fossil fuels. No other energy source, neither renewable nor nuclear, could replace fossil fuel-energy in the short or mid term. This is either due to the non-equivalent development state of such technologies (low efficiency, low energy density) or due to political uncertainty and safety issues (for nuclear energy). The general consensus is that there should be a well-considered roadmap towards a future energy scenario, with the replacement of fossil energy by renewable energies as the final goal (Figure 1). However, on the short term energy from fossil fuels remains the only energy source world-wide, although its efficiency needs to be increased and carbon dioxide emissions must be drastically decreased. A proper portion of biomass should be integrated in the energy pool, if it does not compete with food supply; another even more essential issue for mankind. However, looking for alternative and renewable energies could be the only solution for the sustainable development of our society. Therefore, an increasing use of solar energy will become predominant for a longer-term energy scenario, and remains, at least for the current understanding of science, the only solution to replace fossil fuel energy under the perspective of sustainability. New concepts, new methods, new materials, new processes, and new technologies are required for the new developments,3 and there is still a long way to go before we can replace our current energy supply to a certain extent.

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Figure 1. Centi′s roadmap of a future energy scenario. Reprinted with permission from Ref. [1].

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In a solar-based renewable energy scenario,4 primary energy in the form of electricity can be produced from solar energy (through photovoltaics), from wind turbines, and by concentrating solar power (CSP). The strong dependence of solar and wind energies on time of day and season requires storage of the generated electricity. The chemical storage of energy could involve charging batteries or producing hydrogen, hydrocarbons, ammonia, and other synthesized fuels, such as methanol. Chemical storage of energy offers more flexibility and higher energy densities (materials for high-performance batteries are still under investigation) than, for example, its mechanical or physical storage. In nature, the direct storage of solar energy occurs in biomass, which also collects carbon dioxide. The direct utilization of stored solar energy could be the discharging of batteries. A process for indirect utilization of stored solar energy is through fuel cells, and through combustion of synthesized fuels. It should be mentioned here that electricity covers only one-third of our current energy demand. Liquid fuels appear to be unavoidable for some applications, despite some promising developments achieved for hybrid automobiles.

Both, on the long road towards reaching a solar-based energy pool (Figure 1; for the short and intermediate term, an essential field is the conversion energy from fossil fuel sources and storage of the gained energy), and in the solar-based “Energiemix” (Figure 2), chemistry is indispensable for all efforts to save energy by performing our technical production processes with optimized resource utilization, and is the strategic core discipline for all future energy conversion processes that are based on primary solar energy.

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Figure 2. “Energiemix” by Robert Schlögl: an energy scenario for energy storage and conversion based on hydrogens as storage molecules and nitrogen or carbon dioxides as exhaust product. Batteries, fuel cells, and combustion are the systems with which the primary solar energy will be converted into electricity. The German version of this “Energiemix” has been published in Ref [4].

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• Chemistry can play a relevant role in global primary energy consumption by maximizing the selectivity of molecular transformations and by supporting the minimization of physical loss through adequate material developments.

• Chemistry enables the storage of energy converted by physical process (photovoltaic or wind) in chemical bonds, allowing for long-term strategic energy reserves and long-distance transportation.

• Chemistry can provide solutions for the direct storage of primary electricity from solar and wind sources in batteries for operating on mobile applications, and for buffering via chemical storage (redox-flow batteries as one example) at medium and large scales.

• Chemistry can provide solutions that circumvent the use of precious metals or minimize their application to an absolutely minimal level; a prerequisite for the broad application of fuel-cell technologies.

• Chemistry provides, in addition to improving the efficiency of fossil energy and thus reducing emissions, elegant and safe ways to capture and convert carbon dioxide to fuel, polymers, and so on, closing the carbon cycle.

• Chemistry energy research enables the identification and verification of ingenious chemical conversion concepts and of materials that create new technologies.

An example of how chemistry determines the fate of mankind is reflected by the discovery of the ammonia synthesis and its industrialization; a most important chemical process called the Haber–Bosch process. At the beginning of last century, all civilized nations stood in deadly peril of not having enough to eat. The demand for food is a demand for nitrogen fertilizer. It was Fritz Haber who discovered how ammonia, a chemically reactive, highly usable form of nitrogen, could be synthesized by reacting atmospheric dinitrogen with hydrogen in the presence of iron at high pressures and temperatures, and Carl Bosch who subsequently developed it on an industrial scale. The importance of Haber’s discovery cannot be overestimated—as a result, billions of people have been fed,5 as illustrated in Figure 3. Although a shadow side of the discovery is that millions of people have died in armed conflicts over the past 100 years,6 a secondary contribution to mankind by the large-scale production of ammonia has been the industrial manufacture of a large number of chemical compounds and many synthetic products. Thus the Haber–Bosch process, with its impact on agriculture, industry, and the course of modern history, has changed the world.

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Figure 3. How chemistry changed our life: trends in human population and nitrogen use throughout the twentieth century. Of the total world population (solid line), an estimate is made of the number of people that could be sustained without reactive nitrogen from the Haber–Bosch process (long dashed line), also expressed as a percentage of the global population (short dashed line). The recorded increase in average fertilizer use per hectare of agricultural land (blue symbols) and the increase in per capita meat production (green symbols) is also shown. Reproduced from Ref. [5]. Copyright 2008, Nature Publishing Group.

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The challenge faced by mankind at the beginning of last century was a shortage of food, resolved by the Haber–Bosch process. In a highly abstracted description, the 20th century was a century of ammonia. Without ammonia synthesis and thus the production of nitrogen fertilizer, we may not have had today′s world. Unfortunately this is not realized by many people since food supply remains, at least now, not a problem.7 This century′s challenges to mankind are energy and climate change, and these are cross-linked. Along a similar line of thought as the previous abstraction, the 21st century will be the century of hydrogen and carbon dioxide. Hydrogen may be the core of a solar-based energy scenario, and closing the carbon cycle is the only way to maintain our climate. However, we are still only at the beginning of resolving the energy and climate challenges of this century.

It is worth remembering that the challenge of the 20th century (i.e., ammonia synthesis) is related to three Nobel prizes: to Haber in 1918 for the synthesis of ammonia from its elements, to Bosch in 1931 for his contributions to the invention and development of chemical high-pressure methods, and to Ertl in 2007 for his studies of chemical processes on solid surfaces that disclose the reaction mechanism of ammonia synthesis; work dating back to the 1980s, 70 years after Haber′s discovery in 1908. The challenges of the 21st century hold promising opportunities for scientists, and the contribution of chemistry is indescribable.

This “Chemistry of Energy Conversion and Storage” issue contains papers dealing with all of the aspects mentioned above. The papers are selected presentations from the 1st International Symposium on Chemistry of Energy Conversion, held in Berlin, Germany from February 27–March 2, 2011. This special issue is also the successor issue to “EnerChem” (Energy Chemistry) in ChemSusChem, February 2010, highlighting the most important developments in chemistry of energy conversion and storage of the last two years.

Biographical Information

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  2. Biographical Information

Dang Sheng Su completed his Ph.D. at the Technical University of Vienna (Austria) in 1991, and then moved to the Fritz Haber Institute (FHI) of the Max Planck Society in Berlin (Germany) as a post-doctoral fellow in the Department of Electron Microscopy. After a short stay at the Hahn-Meitner Institut GmbH and the Humboldt Universität zu Berlin (Germany), he joined the FHI in 1999, where he works on nanomaterials in heterogeneous catalysis and energy storage. He is currently Professor at the Institute of Metal Research and head of the Catalysis and Energy Materials Division of the National Laboratory of Materials Science, Chinese Academy of Sciences, Shenyang (PR China).

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