Recent Advances and Challenges of Hydrogen Production Technologies via Renewable Energy Sources

Currently, fossil fuels play a major role in meeting the world's energy demand. Fossil fuels, in contrast, threaten the planet's ecosystems and biological processes, contribute to global warming, and result in unfavorable climatic shifts. These energy sources are also finite and will eventually deplete. Thus, energy transition, which is the key from fossil fuels to renewable energy sources, is regarded as an essential course of action for decarbonizing the global economy and reducing the catastrophic and irreversible effects of climate change. Thereby using/consuming green hydrogen energy is a vital solution to meet the world's challenges. Subsequently, the pros and cons of several hydrogen generation methods, such as the conversion of fossil fuels, biomass, water electrolysis, microbial fermentation, and photocatalysis, are then compared and outlined in terms of their technologies, economies, consumption of energy, environmental aspects, and costs. Currently, the chemical industry uses green hydrogen (H2) primarily to produce green emerging fuels methanol and ammonia (NH3), which are regarded as alternate sources of energy. Finally, the current state of energy demands, recent developments in renewable energy sources, and the potential of hydrogen as a future fuel are outlined. Moreover, the discussion concludes with predicted opportunities and challenges.


Introduction
[6] Consuming fossil fuels causes the emission of greenhouse gases into the atmosphere, including volatile compounds, nitrogen oxides, and carbon dioxide, as well as solid particles, which leads to a global change in the climate. [7,8]oreover, 85% of the world's energy needs are met by carbon-based fuels. [9,10]To meet the need for energy, over 36 billion tons of CO 2 are released into the atmosphere annually. [11]Almost 90% of these emissions come from fossil fuels, and in the upcoming years, they are anticipated to rise much more. [12]In addition to their negative effects on the environment, fossil fuels are in short supply and their prices are volatile, which has an effect on both the profitability of the oil production and consumption businesses as well as the ability of consumers to buy goods and services. [13]To achieve a decrease in both energy use and carbon emissions, certain targets have been set. [14]By 2030, the population of the world is anticipated to reach 8 billion, and an increase in energy demand is anticipated. [15]oreover, invasive plants and food waste are inexpensive and easily accessible for the conversion to the production of renewable energy, especially tree clippings and agricultural crop waste. [5,16]Food scraps, agrochemical, mixed plastics, pharmaceutical, municipal waste residue, and animal waste are plentiful and cheap, which are all readily available and inexpensive lignocellulosic feedstocks. [17]It outlines what additional steps would be necessary over the next 10 years to put the world on track for net-zero CO 2 emissions by 2050.In this sense, thorough longterm and integrated planning are necessary for a power system with net-zero emissions. [18]To reduce emissions, the power industry will be crucial, but low-carbon fuels like hydrogen are also required. [19]Accelerated adoption of clean energy technology would be necessary to meet this goal. [20]Numerous studies have concentrated on creating new technologies for renewable energy sources as a replacement for fossil fuels. [21]here are a rising number of nations with laws that actively encourage investment in hydrogen technologies. [22]The simplest and most prevalent element on earth is hydrogen.Since hydrogen is a chemical element that quickly combines with other elements, it is always present in other substances like water, hydrocarbons, and alcohol. [23,24]Moreover, sources include geothermal, hydroelectric, solar, and wind power. [25]Hydrogen is a viable energy carrier and crucial for energy security because of the variety of home energy sources.It is ideal for hydrogen to be created using a range of materials and technological approaches. [26,27]A variety of technologies can be used to produce hydrogen, including electrolytic, thermal (processing of renewable liquid, natural gas reforming, and bio-oil, biomass, and coal gasification), and photolytic.However, its long-term viability depends on how energy efficient the hydrogen generation method is and how much hydrogen is obtained. [28]In this regard, the decarbonization of the current energy system will heavily rely on renewable energy.So, by reducing the carbon footprint, the production of hydrogen from renewable sources can address this issue and eventually result in a sustainable energy system. [29]he number of annual research articles on H 2 production has increased significantly from year to year, as shown in Figure 1.In addition, hydrogen needs to be used in sectors where it is currently underrepresented, like transportation or buildings, in addition to being produced in a more environmentally friendly manner, to significantly contribute to the transition to clean energy. [30]Hence, using data from both renewable resources, this view summarizes and discusses the primary hydrogen generation technologies.Technologies that use renewable resources are also examined in terms of their energy effectiveness and hydrogen production efficiency.Also, the key difficulties and prospects for hydrogen production technologies in the future are assessed.

Renewable Energy Sources
Although there are many renewable energy sources that can be used to produce H 2 , the shift to a hydrogen economy faces significant challenges due to the variable and sporadic character of these resources. [31]As a result, this necessitates technical adjustments, particularly for balancing changeable renewable supply, such as solar, wind, and other sources, and fluctuating energy demand.When moving toward a hydrogen economy, additional study is also needed in the areas of cost-effective production techniques, regulations, R&D, and the development of hydrogen infrastructure.As a result, we have included five different types of renewable energy sources in this analysis, including hydropower, geothermal, solar, biomass, and wind energy [32] (Figure 2).

Geothermal Energy
Geothermal energy is viewed as a viable environmental choice.The reliability of geothermal energy is not anticipated to be significantly harmed by climate change, but geothermal energy use could significantly lower greenhouse gas emissions.Nevertheless, the environment does not replenish the heat as quickly as it is withdrawn, which reduces the life span of geothermal facilities.Such geothermal systems can be operated sustainably thanks to contemporary reservoir management techniques and the constant replenishment of heat from earth processes. [33]

Wind Energy
Another option to fossil fuels is wind energy, which is a plentiful, renewable, and clean source.Wind energy is converted into electrical energy through wind power.According to the data, wind energy is one of the fastest-expanding energy sources globally.The convective effect, which is brought on by solar heating of the earth's surface, is primarily responsible for producing the winds.This generated wind energy is known to be inversely related to velocity cube. [34]This form of energy offers a dualuse technology, meaning that when wind is being used, the land may still be used for solar energy gathering, farming, ranching, and forestry.The proximity to the electric grid, wind power density, connectivity to the road network, topography of the land, elevation above mean sea level, and distance from protected areas are the essential success factors for a wind energy project.Onshore and offshore wind farms are separated based on location into two types. [35]

Biomass Energy
The term "biomass" refers to the biological components of living things or plants.In less developed and industrialized nations, it is one of the most often used energy sources.
Biomass, the most advanced renewable energy source, provides 3% of the primary energy demands of advanced nations and 35% of those of developing countries, respectively. [36]o generate heat, biomass can be burned directly or indirectly by being converted into several types of biofuel.Forest leftover biomass, which includes dead trees, branches, and stems, can be transformed into other useful energy sources like methane gas or fuels for vehicles like ethanol and biodiesel.The way that biomass is used differs by region. [37]

Solar Energy
Investigating the energy that the sun can produce is another possibility for sustainable energy.In fact, one of the most important sources of renewable energy is the sun.Photovoltaics and concentrators can be used to harness the sun's energy to produce electricity.Because of its lower maintenance costs, it is superior to hydropower and wind energy.The drawback of solar energy is that it is very dependent on the geographical setting and time of year that receives the most sun exposure.In isolated areas where installing or obtaining conventional utilities is difficult, solar energy is a good alternative. [12]

Biomass energy
Figure 2. Schematic illustration of various forms of renewable energy sources.

Hydropower
Hydropower appears to be more dependable and clean than any other renewable energy sources.Less greenhouse gas emissions are hydropower's most significant benefit.Hydropower is a method of producing electricity using a plentiful, renewable source of water.By erecting dams close to moving water bodies, hydropower is frequently generated.The concept is around using the kinetic energy of moving water to power mechanical dynamos and create electricity.Hydropower is produced by employing turbines to transform the kinetic energy of moving water into electrical energy. [38]Nonetheless, because of how expensive they are, dams are typically viewed as long-term investments.Also, choosing a location for a hydroelectric plant requires careful planning because it must address delicate concerns including the deterioration of riverine habitats and biodiversity. [39]These are a few of the well-known applications for the previously mentioned available renewable energy sources.Nevertheless, due to either geographic or technological limitations, they do not materialize as envisioned in human life.Hydrogen has been considered as a potential energy carrier in the quest to discover new strategies or alter established ones to squeeze the most energy possible out of renewable energy resources. [40]

General Overview of Hydrogen
In the near future, hydrogen will be crucial to the energy sector.It is a fuel without any emissions that is easily manufactured at home.The most prevalent element in the cosmos, hydrogen only occurs on earth in compounds like water and biological molecules due to its reactivity.It is an odorless, combustible, and colorless gas, which raises safety concerns. [41]At 298 K, hydrogen has a higher heating value of 141.8 MJ kg À1 and a lower heating value (LHV) of 120 MJ kg À1 . [42]Its value exceeds that of the majority of fuels, such as gasoline, which has a value of 44 MJ kg À1 at 298 K.However, liquid hydrogen has a density of 8 MJ L À1 as opposed to 32 MJ L À1 , which is a factor of four less energy per volume than hydrocarbon fuels like gasoline.Compared to hydrocarbons, hydrogen gas has a lower energy density by volume but a higher energy density by weight; hence it needs a larger storage tank. [43]Liquefied hydrogen, for instance, has 2.4 times the energy content but requires 2.8 times the volume to store compared to liquefied natural gas.At the same time, there are a number of threats associated with the low temperature for liquefied hydrogen storage at ambient pressure and a temperature of 253 °C. [2]Hydrogen is preferred as a potential green fuel because of its clean burning properties, potential for domestic production, compatibility for fuel cell cars, and potential for 2-3 times higher efficiency than petrol. [44]iven that the amount of energy produced by 1 kg of hydrogen is equal to the amount of energy produced by 1 gallon of gasoline (4 kg), it has a low volumetric energy density.Although using hydrogen for energy applications does not release greenhouse gases, the sustainability of hydrogen depends on how cleanly the hydrogen is produced and how much energy is utilized in the process. [45]In this way, the generation of green hydrogen must be maintained utilizing renewable energy sources like, nuclear, geothermal, solar, or wind, as well as renewable resources like water and biomass.In addition to intermittent energy production, one issue with renewable energy sources is the inability to economically store the generated electricity. [46]his is because, despite several investigations, there is currently no practical way to store electricity using batteries. [47]By creating green hydrogen that can be stored and used at any moment, this issue could be resolved.Despite the fact that hydrogen may be utilized directly in internal combustion engines and for flameless combustion, fuel cell technology is the main area of attention for the development of hydrogen technology globally. [48]The primary obstacles to achieving the so-called hydrogen economy are the economic viability of hydrogen generation and the security of the energy supply, which is why this topic has been included in political dialogues. [49]The main goal is to replace the current power economy based on fossil fuels by mass-producing hydrogen utilizing easily available energy sources.Addressing hydrogen distribution, transportation, storage, and production, at the same time, is related to establishing the hydrogen economy and supporting strategies (Figure 3).In a nutshell, the hydrogen economy is significant because it opens the door to a society with no carbon emissions, contributes to efforts to combat climate change and global warming, and offers sustainable substitutes. [50]

Hydrogen Production Technologies
The International Energy Agency (IEA) states that if significant obstacles, like safety, cost-effective manufacturing techniques, logistics, and infrastructure, are solved, green hydrogen could help reduce our carbon footprint. [31]There are now more than 100 methods for creating hydrogen, more than 80% of which rely on turning fossil fuels into steam and 70% of which rely on natural gas steam reforming (SR). [51]The energy expended must be present in excess and be continuously available to extract hydrogen from these sources. [52]Hence, sustainable hydrogen production would be made possible by utilizing the potential of renewable energies in hydrogen production methods.The primary methods for producing hydrogen in the future will be determined by the type of raw material used (Figure 4).The former uses a cost-effective approach that involves cracking or reforming fossil fuels, making it the more developed and widely used industrially.In 2016, the world produced roughly 85 million tons of hydrogen, which was utilized in the creation of semiconductors, power plants, fuel, metals, fertilizer, and food. [53]Hydrogencontaining materials can be extracted in a different of ways, such as bioenergy, photonic, chemical, combinations heat, and electric of those processes.These methods can be used to extract hydrogen from hydrocarbon or non-hydrocarbon-containing materials. [54]1.Production of Hydrogen from Fossil Fuels Fossil fuel processing techniques convert hydrogen-containing compounds produced from fossil fuels, such as ethanol, methanol, hydrocarbons, or petrol into a hydrogen-rich gas stream.[55] Methane (natural gas) fuel processing is currently the most widely used commercial method of producing hydrogen.The elimination of sulfur, which is a critical challenge in the development of a hydrogen-based economy, is present in the majority of fossil fuels.The desulfurization procedure will therefore also be covered.[56] Moreover, a recent development in plasma reforming technology will be discussed.From hydrocarbon fuels, hydrogen gas can be created using three fundamental techniques: autothermal reforming (ATR), SR, and partial oxidation (POX).These methods generate a lot of carbon monoxide (CO). The eferential oxidation or methanation processes, which are discussed subsequently, are used in a subsequent step in one or more chemical reactors to mainly convert carbon monoxide (CO) into carbon dioxide (CO 2 ).[57] The world's hydrogen supply is still mostly derived from fossil fuels because to the strong relationship between the cost of production and the relatively low price of fuel.These days, the most widely used processes for generating hydrogen from fossil fuels are pyrolysis and hydrocarbon reforming.[58] These methods virtually make it possible to produce all the hydrogen required.Table 1 compiles the key details of each fossil fuel-based technology discussed in the next subsections, such as the feedstock employed, operating circumstances, and maturity.

SR
Currently, one of the most popular and reasonably priced methods for producing hydrogen is SR.Because of its great operational efficiency and cheap production and operating costs, it  [16] Copyright 2022, Springer.has a distinct advantage.Natural gas, lighter hydrocarbons, methanol, and other oxygenated hydrocarbons are the most often used raw materials. [59]In the SR reaction, hydrogen and carbon dioxide are produced when steam and hydrocarbons combine at a high temperature. [60]Natural gas and, much less frequently, liquefied petroleum gas and naphtha are the two fuels that SR uses to extract hydrogen from. [61]The most common method of hydrocarbon reformation is steam methane reforming from natural gas or light hydrocarbons, as was previously indicated.In this process, hydrogen and carbon monoxide are first mixed to create synthesis gas into carbon dioxide and additional hydrogen.The general reaction of methane SR is depicted in Equation ( 1): Because of how endothermic the reforming reaction is, a lot of heat is needed.Because of this, these reactions are normally conducted between 800 and 1000 °C. [62]They evaluate the requirement for expensive construction materials for the reformer to handle the thermal stresses due to the high temperatures required to convert methane into hydrogen. [63]The development of temperature profiles in the catalyst bed and the production of coke are two additional drawbacks that would need to be considered.Both nonprecious metals (often nickel) and precious metals from Group VIII elements (typically platinum or rhodium) can be used as catalysts.Because of considerable mass-and heattransfer constraints, conventional steam reformers are restricted by the efficacy factor of pelletized catalysts, which is often less than 5%. [64]As a result, kinetics is rarely the limiting issue with ordinary steam reformer reactors, leading to the industrial use of less expensive nickel catalysts. [65]The proportion of H to C atoms in the feedstock material is a significant characteristic of the SR process.The formation of carbon dioxide is decreased when this ratio rises.On an industrial scale, the SR of methane process produces hydrogen with a thermal efficiency of between 70% and 85%. [66]

Coal Gasification
An important resource that may be used to create a wide range of goods is coal.Coal gasification is a thermochemical conversion process that produces gaseous byproducts such as carbon monoxide and hydrogen from coal. [67]This process aims to take the place of burning coal to reduce harmful emissions and increase the energy density of the fuel.There are numerous applications for coal as a fuel source.Coal gasification is one of them and it can produce electricity, liquid fuels, chemicals, and hydrogen. [68] combination of hydrogen and carbon monoxide is created when coal combines with H 2 , O 2 , and steam under high pressure (synthesis gas).Steam and carbon monoxide continue to react via the WGS process to produce more hydrogen and carbon. [69]ollowing this procedure, the system's pure hydrogen is removed from it, and the carbon has been trapped and sequestered (Figure 5).So, this technology requires attention to make it more environmentally friendly so that it won't harm the environment. [70]However, to make the process an environmentally friendly procedure, both time and money are required.The primary drawback of using coal gasification to produce hydrogen rather than other techniques that use different feedstocks is increased CO 2 emissions due to the high carbon content. [71]

POX
POX is an alternative technique to SR processes.This process may use coal, heavy fuel oil, methane, or other feedstocks.The optimum method for creating hydrogen from heavy fuel oil and coal is POX.With oxygen and sometimes steam present, the raw material is gasified during the noncatalytic PO process at temperatures between 1300 and 1500 °C and pressures between 3 and 8 MPa.Compared to SR (H 2 : CO = 3: 1), more CO is produced (H 2 : CO = 1: 1 or 2: 1).To complete the process, CO is converted with steam into H 2 and CO 2 .Many gases, including H 2 O, CH 4 , CO, H 2 , CO 2 , carbon oxysulfide, and hydrogen sulfide (H 2 S), are produced during POX processes.To generate adequate heat for the endothermic reactions, a portion of the gas is burned.As an unwanted intermediate product, the soot produced by the breakdown of acetylene is produced.The amount is determined by the H:C ratio of the original raw fuel. [71]his method has the benefit of not requiring an external energy source due to the strong exothermic nature of reactions involving oxygen.High reaction temperatures (>1000 °C) and the C/O ratio confine the product distribution of POX processes.The POX system can have catalysts added to it to lower the operating temperature.Unfortunately, due to coke and hot spot formation brought on by the exothermic nature of the processes, temperature control is proving challenging.The general summary of the POX reaction is given in Equation ( 2): The most abundant products in this process from a thermodynamic standpoint are H 2 and CO at 550 °C, with CO serving as a precursor to coke that can be eliminated by its oxidation into CO 2 or by the WGS reaction, which increases the generation of H 2 .The benefits of POX include easy operation, reduced energy use, and adaptable feedstock.The unit's short useful life due to high reaction exothermicity, which could result in hot spots and, ultimately, catalyst deactivation by sintering pose obstacles for its industrial deployment. [72]Also, due to thermal management, their usage for practical and compact portable devices may be challenging due to the high operating temperatures (>800 °C) and safety considerations. [73]1.4.ATR The exothermic POX with oxygen (O 2 ) that occurs during ATR supplies the energy required for the endothermic SR activities.ATR blends endothermic SR and exothermic POX processes.Essentially, the reformer is filled with steam and oxygen, causing the oxidation and reforming processes to happen at the same time and resulting in a reaction that is thermodynamically neutral. [74]Similar to POX or SR, the selection of a catalyst is essential to the result; nickel-based catalysts are the most widely used because of their effectiveness and low cost.This process requires less energy than SR or POX thanks to its superior thermal efficiency.In contrast, ATR yields a larger hydrogen yield than POX and a lower yield than SR.Since ATR is simpler, less expensive, and doesn't require external heat, it is preferable to SR of methane.Another significant benefit of ATR over SR process is that it can be easily stopped and restarted and generates more hydrogen than PO alone. [75]1.5.Hydrocarbon Pyrolysis Pyrolysis is the chemical breakdown of organic compounds when heat is applied.The importance of pyrolysis procedures in the conversion of waste and biomass into various usable chemical products has led to their increased popularity in recent years.[76] In the absence of oxygen, pyrolysis decomposes organic molecules to create liquid (bio-oil), gaseous (hydrogen/syngas), and solid (biochar) byproducts.Several industries, including waste management, material synthesis, soil improvement, and energy production, use pyrolysis.Moreover, methane is thermally decomposed during methane pyrolysis.Methane conversion in the percentage range is seen above approximately 500 °C when nickel is used as the catalyst.[77] The breakdown reaction begins at temperatures above 700 °C in the absence of an appropriate catalyst.To achieve technically significant reaction rates and methane conversion rates, the temperature must be substantially higher, such as for plasma torches at up to 2000 °C, thermal processes above 1000 °C, and when using for catalytic processes above 800 °C.Since natural gas, not methane, is used as the feed stock in technical operations, a differentiation between methane and natural gas pyrolysis must be made when evaluating the technological readiness levels of the various processes described in the literature.Typically, methane is used in theoretical analyses and experimental investigations without taking into account other reaction partners.[78] Nevertheless, in addition to methane, real natural gases frequently contain a variety of additional substances (sulfur compounds, higher hydrocarbons, H 2 O, CO 2 , etc.) that also react during pyrolysis and thus significantly affect selectivity, products, and conversion rate.Hence, only a limited amount of methane pyrolysis experimental and theoretical findings may be applied to natural gas.This is especially true for the quality of the product gas, the life span of the catalyst, and the solid deposits inside the reactor. [79]

Hydrogen Production from Renewable Resources
Freshwater, fossil fuels, ocean, biomass, and hydrogen sulfide are just a few examples of the plentiful hydrogen elements that can be found in nature.But, hydrogen must be taken from fossil .Schematic representation for the gasification process that produces hydrogen using a shell reactor.Reprinted in accordance with the International CC-BY Creative Commons Attribution 4.0 license. [139]Copyright 2020, MDPI.fuels to manufacture it with little or minimal environmental impact.In general, chemicals originating from water or biomass can be used to produce hydrogen through the process of extraction from natural resources. [67]

Biomass for Hydrogen Production
In terms of producing hydrogen, biomass is thought to be more promising than fossil fuels because of its abundant supply, simple oxidation, and high annual output. [80]Agricultural waste, lignin, sawdust, forest leftovers, municipal solid waste, microalgae, cellulose, polyols, and animal byproducts are just a few examples of the plant and animal components that can be converted into biomass, which is a renewable source of primary energy (Figure 6). [81]The two methods for producing hydrogen from biomass are thermochemical and biological mechanisms.These topics will be covered in the sections that follow.Given the state of technology and the state of the economy in many developed nations, producing hydrogen from biomass and residual wastes is both technically and economically possible.By 2050, it has been predicted that biomass will supply more than 25% of the world's energy needs.Contrary to fossil fuels, biomass-toenergy processes minimize CO 2 emissions and absorb CO 2 from the atmosphere, resulting in a scenario with net-zero emissions of the greenhouse gas. [82]Table 2 provides an overview of the key technologies for both processes, including the type of biomass used, the operational circumstances, and the technological maturity.
Biological Processes: Since the previous two decades, the biological method of producing hydrogen has exploded in popularity due to the rising percentage of waste materials and the resulting need to reduce them.Microorganisms that dwell in an aqueous environment, at atmospheric pressure, and at room temperature catalyze biological processes. [83]In areas where supplies of biomass or biowaste materials are easily accessible, such techniques can be used.The natural availability of these precursors lowers the initial raw material's transportation and energy expenses.Dark fermentative hydrogen generation and photofermentative processes are the two main biological processes utilized to produce hydrogen.One of the most well-known methods for producing bio-hydrogen is dark fermentation.Anaerobic bacteria that are cultivated in the dark on carbohydrate-rich substrates can create H 2 .It is discovered that the species of Enterobacter, Bacillus,   and Clostridium produce hydrogen.The ideal carbon sources for fermentation processes that result in the production of acetic and butyric acids as well as hydrogen gas are carbohydrates, notably glucose.Based on the microorganism's metabolic pathway and the initial concentration of sugar in the fermentation medium, it is possible to calculate the theoretical output of hydrogen.Depending on the microorganisms and substrate, a mixed gas containing CO 2 and H 2 as well as other trace gases as CO, H 2 S, and CH 4 is formed. [16]By simultaneously producing adenosine triphosphate from adenosine diphosphate and nicotinamide adenine dinucleotide þ hydrogen, bacteria can use glycolytic pathways to transform glucose into pyruvic acid.Pyruvic acid is further transformed into CO 2 and H 2 with the aid of the enzymes pyruvate ferredoxin-oxidoreductase and hydrogenase.The transformation of pyruvate into acetyl-CoA, and then into acetate, butyrate, and ethanol, can be used to gauge the amount of bio-hydrogen produced.The proportion of butyrate to acetate determines how much hydrogen is available from glucose.In contrast, one of the likely mechanisms for the creation of biological hydrogen is thought to be photofermentation.By employing both the reduction power generated by the oxidation of organic molecules, such as low-molecular weight fatty acids, and light energy, anoxygenic photosynthetic bacteria, in particular purple non-sulfur (PNS) bacteria, can decrease H þ ions to gaseous H 2 .Due to the lack of O 2À evolving reactions, the ability to employ a wide spectrum of sunlight, the high substrate conversion yields, and the potential to combine this type of H 2 production methods with waste disposal, this approach is thought to be promising. [1]rganic substrates are fermentatively transformed into hydrogen and carbon dioxide utilizing sunlight as the energy source (Equation (3)).

Biomass feed stocks
The organic acid substrates are oxidized by the tricarboxylic acid cycle, which generates protons, carbon dioxide, and electrons.PNS bacteria are involved; advantages are in terms of removing pollutants from the environment, using industrial waste, and using organic acids made by dark fermentation. [84]hermochemical Fabrication: By using a thermochemical process, biomass is transformed into hydrogen and gases rich in hydrogen.The future of zero greenhouse gas emissions, which is required for sustainable growth, is viewed as the creation of hydrogen-rich gases from synthesis gas obtained from such technologies.Gasification and pyrolysis are the two fundamental components of the thermochemical process.In addition to other gaseous products, the two conversion processes yield CH 4 and CO, which can be further processed to yield extra hydrogen through WGS and SR reaction. [85]Other mechanisms include liquefaction and combustion, which are both less desirable options because they produce little hydrogen and emit byproducts while also needing operating conditions of 5-20 MPa without the presence of air, which are challenging to satisfy.The four distinct thermochemical processes of gasification, pyrolysis, combustion, and liquefaction are the key components of these technologies.
Pyrolysis: Pyrolysis or co-pyrolysis is another promising technology for the synthesis of hydrogen.In this method, raw organic material is heated and gasified between 500 and 900 °C and 0.1 and 0.5 MPa of pressure. [86]By applying heat under anaerobic conditions, the thermochemical breakdown process known as pyrolysis transforms organic materials into solid biochar, biooil, and pyrolytic gas.The efficiency of the many spontaneous reactions that make up pyrolysis depends on a number of variables, including residence time, heating rate, pressure, moisture content, particle size, biomass type, temperature, and pretreatment technique.As neither air nor oxygen is present during the procedure, the possibility of dioxin production is eliminated.As neither air nor water is present, secondary reactors are not necessary for the generation of carbon dioxide (CO 2 ) or carbon monoxide (CO).As a result, this method of producing hydrogen aids in lowering emissions.Nonetheless, there will be significant CO x emission when there is water or air present (i.e., when the materials are not dry).This method has several advantages, such as fuel flexibility, a decrease in CO x emissions, and comparative ease of use, compactness, and clean carbon byproduct.High temperatures (>800 °C), moderate temperatures (500-800 °C), and low temperatures (500 °C) can all result in pyrolysis.Fast pyrolysis (FP) is a technique used to transform organic material into products with a greater energy content.All three phases, i.e., gas, liquid, and solid, produce FP products.One of the challenges with this method is the potential for fouling from the created carbon, although its proponents believe that it can be minimized with the correct design. [77]asification: The range of potential raw materials for gasification includes agricultural, lignocellulosic, forestry, and municipal solid waste organic fractions, provided that the latter has a moisture level under 40%.The gasifying agents can be air, pure oxygen, steam, carbon dioxide, or mixes.At temperatures between 700 and 1200 °C, oxygen, air, steam, or any combination of these can be used to gasify biomass. [83]Steam increases the production of H 2 and provides a high heating value gas without nitrogen.Compared to air gasification, steam gasification has a greater energy cost due to its high endothermic nature, but it eliminates the need for an expensive oxygen separation process.Air is the most affordable and popular agent, but it contains a significant amount of nitrogen, which slows down how quickly the synthesis gas is heated.Higher heating rates are produced using pure oxygen; however, the method is less economically profitable due to the cost of producing oxygen.When steam is employed as the gasifying agent, both the rate of heating and the amount of hydrogen in the synthesis gas can be enhanced.In this situation, the rate of heat is increased to 10-15 MJ Nm À3 , as opposed to the 3-6 MJ Nm À3 achieved from biomass gasification using air as the gasifying agent.The presence of CO 2 in synthesis gas makes processes that use it as an agent promising. [68]t follows that depending on the raw material and gasifying agent used, the chemical makeup of the synthesized gas will vary greatly.Using gasifying means, biomass, a solid carbonaceous feedstock is converted into a gaseous mixture such as H 2 , light hydrocarbons, CO, CH 4 , CO 2 , tar, ash, minor contaminants, and char via a high-temperature POX process. [87]ombustion: A variety of biomass materials, including wood, dry leaves, tough vegetable husks, rice husks, and dried animal manure, can be used in combustion plants.A chemical reaction that produces heat is what causes combustion.Chemical energy is released when biomass is burned in the presence of air.Within combustion chambers, combustion occurs at 800-1000 °C temperatures.It is important to stress that biomass must have a humidity level below 50% to be burned to make biofuels.Due to the high reactivity of the resulting char and fuel as well as the high volatility of the fuel, biomass offers advantages as a combustion feedstock.Burning biomass is often not ideal for producing hydrogen because it produces a significant amount of CO 2 .Hydrogen of 9.56 vol% was produced during the burning of the algal biomass.Other gases that are released include SO x , CH 4 , CO x , NO x , and all of which are influenced by the biomass's composition and source.There is a substantial cost connected with the treatment of these gases, which increases the entire cost of the operation. [88]ydrothermal Liquefaction: The approach that has drawn the most attention recently is the thermochemical liquefaction of biomass since it has a higher energy density, a quicker reaction time, and can be used with a larger variety of materials. [89]oreover, the liquid product created by processes like pyrolysis typically contains a lot of oxygen, which enhances instability and makes it challenging to use as fuel.Conversely, liquefaction has the potential to result in a liquid with far less oxygen.Hydrothermal liquefaction (HTL) is one of the most promising thermochemical liquefaction techniques.HTL is a method that can effectively treat both wet and dry biomass from lignocellulosics to organic waste without limiting the amount of lipid present. [90]The water either remains in a liquid state or an extremely dense supercritical state under these circumstances. [91]This method makes use of the unique qualities of compressed hot water.The biomass goes through a series of depolymerization events during the liquefaction process, including hydrolysis, dehydration, and decarboxylation, producing insoluble compounds like bio-crude oil or bio-carbon. [92]Moreover, various byproducts are produced, such as soluble organic compounds or gases (H 2 , CH 4 , CO, CO 2 , or CO) (mainly acids or phenols). [93]he majority of research done thus far focuses on the reaction mechanism, including how different forms of biomass affect reaction temperatures and heating rates, pressure, the type and nature of catalysts or pH modifiers, the performance of biofuels, and the features of the finished product.The most popular solvent for conducting organic reactions is water since it is regarded as environmentally friendly, secure, affordable, and affordable medium. [94]

Hydrogen Production from Water
Nowadays, hydrogen is utilized as a raw material for chemical synthesis, but other uses, such as energy storage and fuels for vehicles, are now possible.As there are no greenhouse gas emissions when hydrogen is produced using renewable electricity, it can play a significant role in the fight against climate change. [95]Innovators are employing electrochemical water electrolysis to produce hydrogen from two basic ingredients: energy and water, to create a "green hydrogen economy". [96]he simplest method of dividing water into hydrogen and oxygen is electrolysis, which involves an electrical current running across two electrodes. [97]But, it can also be split using many types of energy, including photonic energy (photoelectrolysis), biophotolysis, and thermal energy (thermolysis), employing microorganisms. [98]Table 3 summarizes these features as well as additional aspects of this process.Figure 7 also depicts the global freshwater removal and consumption of three distinct industries, including agriculture, the deployment of a global hydrogen economy, and the production and generation of energy from fossil fuels.
Electrolysis: By applying an electrical current, water undergoes electrolysis, separating into hydrogen and oxygen.KOH electrolyte is commonly dissolved in an aqueous solution using two electrodes (anode and cathode). [20]The name of this device is electrolyzer.Depending on how much hydrogen is produced on a small or large scale, an electrolyzer may be small or enormous.The "distributed hydrogen production" use of this technology is pertinent.Depending on the source used to generate energy to split the water molecule, electrolysis can produce hydrogen with absolutely no emissions of greenhouse gases.Alkaline water electrolysis, solid oxide electrolysis, and protonexchange membrane electrolysis are some of the various water electrolysis technologies. [99]A gas separator is necessary for alkaline water electrolysis to prevent the mixing of the gas byproducts.Concentrated lye is used as the electrolyte, and the electrodes are made of non-noble metals (e.g., nickel).The electrolyte in proton-exchange membrane electrolysis is a humidified polymer membrane, and the electrocatalysts are noble metals like platinum or iridium oxide. [100]For all technologies, the operating temperature ranges from 50 to 80 °C and the operating pressures are adjustable up to 30 bar.In contrast, solid oxide electrolysis increases the need for heat by boiling water at high temperatures (700-900 °C) into hydrogen and oxygen.Alkaline water electrolysis and proton-exchange membrane electrolysis are consequently more appealing technologies for wide-scale adoption due to the lower initial investment and longer unit lifetime.As a result, scientists are focusing on nuclear or wind energy as potential energy replacements for electrolyzers. [101]It is anticipated that using such an energy source will lower the cost of transmitting electrical energy.The main challenge with this method is to increase process efficiency while simultaneously reducing the cost of producing electrolyzers.Another challenge is integrating the compressor into the electrolyzer, which is necessary to prevent the cost of a separate compressor, which is ideal for high-pressure hydrogen storage. [102]hermolysis: Chemical reactions that create hydrogen are part of thermochemical processes that take place at temperatures between 500 and 2000 °C. [103]The source of heat could be nuclear reactor or solar concentrators.Each cycle involves the recycling of chemicals, resulting in a closed-loop chemical reaction that uses only water to produce hydrogen. [104]When water is divided into hydrogen and oxygen using thermochemical water splitting, a sequence of chemical processes is carried out at a high temperature.Using this method, hydrogen may be produced effectively and affordably.Still, it was unable to go to the commercial level.This technology is appropriate for centralized, large-scale hydrogen production. [105]sing the aforementioned approach has the benefit of producing clean, pure hydrogen with no emissions of greenhouse gases.With the development of nuclear reactor technology, it is possible to create necessary heat at low temperatures while also lowering the cost of solar concentrators and heat-transfer medium. [106]hotoelectrolysis: The most abundant renewable resources, water and sunlight, are used to create hydrogen in the photoelectrochemical water splitting process. [107]A two-electrode configuration is frequently used, with one electrode acting as an anode to produce oxygen and the other as a cathode to make hydrogen. [108] semiconducting electrode called the anode produces electrons when exposed to solar light.These electrons are capable of creating hydrogen from water.These cells are referred to as photoelectrochemical cells (PECs) when they are used to produce hydrogen.The commercial potential of this technology is still in its infancy.A technology called the microbial PEC uses synergistic microbial conversion assisted by light to produce sustainable biohydrogen from organic waste.In the PEC, electrons are produced electrochemically from organic substances on the microbial or bio-anode by active microorganisms.A membrane separates semiconductors (electrodes) in an electrolyte to form the PEC.Sunlight and semiconductors are employed directly to split water into hydrogen and oxygen during the photoelectrochemical process. [109]Water molecules oxidize during the photoelectrochemical reaction, producing oxygen and biohydrogen reduction while H 2 production takes place in the opposite electrode.With little impact on the environment, this process has a significant potential for the production of biohydrogen. [110]iophotolysis: Water breaks down into molecular hydrogen and oxygen in biological systems through a process known as biophotolysis when light is present. [111]Oxygenic photosynthesis is a process that photoautotrophic organisms like cyanobacteria and microalgae are capable of. [112]This method has the benefit of producing hydrogen from water in an aqueous environment under ambient settings.From the views of both water and CO 2 utilization, it might be seen as an economically viable and environmentally friendly strategy.Presently, this method needs a large surface area to capture enough sunlight because of the low hydrogen output. [113]The challenges to commercialization are caused by biological regulatory mechanisms that, in anaerobic conditions, inhibit linear electron transfer, compete with hydrogenase for electrons, quench absorbed light energy, and inactivate the H 2 -producing enzyme. [114]As a result, compared to its predicted potential of 12%-13%, the conversion efficiency of absorbed photons into H 2 is much lower (Table 4). [115]

Advances of Hydrogen Storage
There are numerous possible improvements and advantages to hydrogen storage.Because hydrogen has a poor energy density  [140] Copyright 2021, American Chemical Society.
per unit volume, storage is one of the main issues with using it as an energy carrier. [116]The broad use of hydrogen as a sustainable and clean energy source for a range of uses, such as energy storage and transportation, depends on these developments.Hence, better storage methods and technologies that have the potential for higher energy density must thus be created right away.Nowadays, hydrogen is kept for on-board uses in high-pressure tanks as a compressed gas, in cryogenic liquid form (below the critical temperature of 33 K), or in solid-state compounds such as porous materials, complex hydrides, or metal hydrides.The practical methods for storing hydrogen are depicted in Figure 8 and are categorized into four categories: physical adsorption, hydrogen liquefaction, high-pressure gas storage, and chemical absorption.Hydrogen gas has an exceptional energy value per unit mass because of its low molecular weight and high molar combustion heat. [117]owever, hydrogen gas has a far LHV per volume than conventional fuels like coal and hydrocarbon due to its low density (both in liquid and gas form).Hydrogen can be stored in a material either as atoms or as H 2 molecules through two well-known processes such as chemisorption and physisorption. [118]It appears that solid-state storage is an effective, practical, and safe technique to accomplish this.Among the numerous storage modes mentioned earlier (including solid state, gaseous, and liquid mode), metal/intermetallic hydrides were discovered to be the first to be developed as an efficient, secure, and cost-effective alternative.When compared to other methods of storing hydrogen, such as high-pressure and liquid hydrogen storage, these hydrides offer a high volumetric hydrogen-storage capacity. [119]he only commercially available hydrogen-storage approach for mobile systems is one that uses physically compressed storage with high-pressure tanks.By the composition of their cylinders, the four different types of compressed tanks are divided: Table 4. List of many ways for producing hydrogen along with its drawbacks, benefits, levels of costs, and efficiency.Reprinted in accordance with the CC-BY Creative Commons Attribution 4.0 International license. [141]Copyright 2022, Springer.
Type I is made entirely of metal, Type II has a metal liner wrapped with hoop, type three vessels consists of fully warapped composite cylinder with a metal liner and Type IV comprises plastic liner with full composite wrapping.Conventional highpressure hydrogen-storage tanks consist of aluminum and steel, but they are insufficiently robust.Recently, efforts have been made to create composite containers made of carbon fiber and epoxy that have a high mechanical strength. [120]Hydrogen gas is stored as a form of metal hydrides in a solid state via a process called chemisorption; unfortunately, the development of hydrogen-fuel-cell-based technology has been hindered by the high cost and maintenance of high-pressure cylinders.At the high pressure of 700 bar, there is a possibility of hydrogen leakage, and this approach uses a lot of energy. [121]This method has drawn attention because of its excellent stability, high storage density, and short area requirement.There are numerous metal hydride containers that have been created. [122]Metal hydrogenstorage materials include LiBH 4 , LiAlH 4 , AlH 3, NaAlH 4 , and NaBH 4 .Mg 2 NiH 4 is particularly well known due to its benefits, including its large storage capacity, low cost, and light weight.
The drawback is that hydrogen adsorption-desorption is not reversible due to its strong bonding force and slow kinetics, necessitating high temperatures for desorption.By using London dispersion forces, hydrogen molecules are weakly adsorbed onto the surface of the material during the physisorption storage process. [123]The technology that stores hydrogen molecules at room temperature and at relatively low pressures, known as physisorption, is the most affordable.Physical adsorption often uses porous materials since it needs a lot of specific surface area. [124]The benefits of this technique include its superior cycle stability, low weight, fast charging, fast discharging speeds, and high storage density. [125]For storing hydrogen, physisorption minimizes hydrogen boil-off loss and uses less energy when charging and discharging than liquid storage at cryogenic temperatures.The materials that have been studied the most thoroughly include those that are porous, such as metal-organic frameworks, zeolites, carbon-based materials, and covalent organic frameworks.However, as will be covered later, there are certain difficulties with hydrogen storage: [126] high energy required in compressed hydrogen storage, because of its low specific gravity; conditions for temperature and pressure while keeping hydrogen in solid form; design considerations, legal problems, societal challenges, and high expense; potential chemical reactions and the low storage durability of materials (polymers, metals, fiber, etc.) cause safety problems; and hydrogen may become contaminated during bulk storage near geographical features, necessitating additional purification before use.

Hydrogen as Future Energy Generation and Application
As previously mentioned, hydrogen energy has the ability to act as an energy carrier, and both developed and developing nations now recognize the importance of this energy source for achieving global sustainable growth. [127]By 2050, the globe is expected to need between 600 and 1000 EJ of primary power, according to research.In emerging nations, where a high need for power is present for economic development and poverty reduction, energy demands are anticipated to rise even more. [128]Fossil fuels, which, at the current pace of usage, are expected to run out in roughly 50 years, nevertheless make up the majority of the world's primary energy mix. [129]Most people consider hydrogen and fuel cells to be crucial technologies for a future source of sustainable energy.For the total energy demand, it is predicted that RS shares of 36% by 2025 and 69% by 2050 might lead to an increase in hydrogen shares of 11% by 2025 and 34% by 2050. [130]Fuel cells can be used to convert hydrogen into electrical energy, which can then be transferred and stored. [131]epending on the energy source used to produce it, hydrogen is environmentally beneficial; for example, when hydrogen is produced from water, it oxidizes back into water.There are several reasons why hydrogen is a suitable and logical choice as a chemical fuel to replace fossil fuels. [132]The key justification is that it complements electricity as an energy transport.Maybe the most well-known application of hydrogen nowadays is in the field of transportation.Electric vehicle owners sometimes bemoan their limited range and lengthy recharge times.Such worries are unfounded because hydrogen-powered fuel cell electric vehicles have a significantly greater range, require fewer behavioral adjustments, and refill more quickly. [133]Hydrogen can also be used to heat dwellings.It can be burned either by itself or in conjunction with natural gas.To manufacture hydrogen, which can be kept for use in high-pressure tanks or caves, proponents of hydrogen energy have suggested utilizing excess wind farm electricity at night. [119]In addition, raw materials for the industrial sector are currently being employed in the production of hydrogen products. [134]It can, however, play a significant role in a lot more sectors if we are to fully achieve its potential as a full energy carrier.Its primary usage, accounting for 35% of the estimated 50 million metric tons generated annually on a global scale, is as a feedstock for the manufacturing of ammonia. [135]Upon realizing its role as a flexible energy carrier, etc., it could have major applications in the production of electricity and power to gas, as well as in the transport of goods and people (Figure 9). [136]he Hydrogen Council stated in their research that demand and supply for H 2 might reach 10 EJ annually by the end of 2050, and that this demand and supply is further anticipated to rise by roughly 5%-10% year beyond 2050.Consequently, it may be claimed that H 2 is a future strong challenger in the global energy system. [137]

Challenges and Perspectives
A reliable alternative energy source that doesn't release any carbon is hydrogen.A civilization without carbon emissions and hydrogenization are both ideal uses for hydrogen (the use of hydrogen as a major energy source).Each hydrogen-generating technique has its own distinct set of prerequisites and steps to get over these restrictions. [138]For a variety of uses, numerous alternative hydrogen-manufacturing techniques will be used.The goals of optimum, reliable, affordable, clean, and efficient hydrogen generation cannot all be met by a single technique.Thus, some difficulties still require more study, as listed in the following.1) Since hydrogen cannot be found in its natural state and must be created, new manufacturing techniques that use less energy and enable mass production of hydrogen must be developed.In addition, since no CO 2 emissions are produced, using water as a feedstock would be preferable to lessen the environmental impact.2) Because hydrogen is a gas at room temperature, it has a low volumetric density.To create the same amount of energy, it needs a volume that is more than 3000 times larger than that of other liquid fuels.Moreover, the hydrogen volume needs to be decreased to make it simpler to store and transfer.
3) Since hydrogen is more flammable than other fuels, its safety is called into doubt.Moreover, because it is an asphyxiant gas, it can cause suffocating by lowering the oxygen content of the air.As a result, various safety and security considerations must be taken into account when handling or storing it.4) As soon as hydrogen is ready for use, it must be applied as efficiently as possible to produce heat or power.5) Since hydrogen is produced through a manufacturing process rather than from a natural source, its cost is three times more than that of fossil fuels.Also, it must be considered that storage could raise the price even further, particularly if high-pressure technologies are used.The authors provide the following recommendations for future research.1) Consistent, long-term regulations adoption should aim to embrace low-carbon emission and enhanced renewable methods, as well as carbon dioxide capture and sequestration technology, to promote creative technologies and market expansion.2) Modern methods for separation and purification: reduced costs for oxygen plants, a high-cost component of multifuel gasifiers, would boost the overall economic growth of hydrogen production.Compact, inexpensive, and very effective hydrogen purification is required for scattered innovators, like as those found in homes or gas stations for vehicles.Despite the fact that many purification techniques are effective in large commercial facilities, scaling them down to the appropriate level for distributed generation is frequently difficult.3) Important reforms: small reformers that run on natural gas, propane, or methanol can be used to supply hydrogen to a number of initial fleets and significant retailers.The technique may be enhanced in terms of dependability, catalyst lifetime, and interactions with fuel cells and storage devices.4) Better and affordable electrolyzers: as these devices can be used with distributed energy sources and may open up new and expanding market opportunities, there needs to be a strong focus on lowering their cost and increasing their productivity.The cost of electrolysis, which is more expensive than thermal manufacturing, should decrease with more understanding of high-temperature and pressure electrolysis.5) Cutting-edge, carbon-free renewable energy sources: photolytic processes use sunlight to separate water molecules and produce hydrogen, potentially reducing costs and boosting effectiveness.6) Future nuclear plant designs might use better nuclear plant energy for thermochemical water splitting.7) New techniques for CO 2 capture: a cheap technique for storing carbon dioxide might enable widespread hydrogen production with zero emissions.8) Improved hydrogen generation technologies: combining manufacturing technology with other components of the hydrogen framework, such as commercial applications, will be more cost-effective.
Despite the evident environmental advantages of producing electricity from sustainable hydrogen, hydrogen energy solutions need to be implemented to make hydrogen a viable alternative to fossil fuels.To achieve new technological advancements in hydrogen production, storage, and consumption, further research needs be conducted.Each technology's success or failure will depend on a variety of factors, including cost, stability, and environmental concerns, in addition to energy efficiency.Social acceptance is a further barrier to hydrogen energy that needs to be taken into account.In some places, hydrogen is regarded as a hazardous and explosive gas, which harms its reputation, particularly in places where there are no infrastructures for hydrogen use or where no commercial applications have yet been created.Copyright 2022, MDPI.

Conclusion
The goal of recent developments in renewable-energy-based hydrogen production technology has been to increase the process's sustainability and efficiency.Solar-thermal water splitting, biomass gasification, and electrolysis are a few of the principal techniques.The use of cutting-edge catalysts and membrane technology in electrolysis has evolved, lowering energy consumption and increasing cost-effectiveness.The goal of optimizing biomass gasification is to lower emissions and improve conversion efficiency.Research is also being done on solar-thermal water splitting's ability to use solar energy to produce hydrogen.Currently, a significant portion of commercial hydrogen generation is made utilizing natural gas and fossil fuels.It is anticipated that this technology will be used to produce goods on a fairly big scale in the future.The production of greenhouse gases is a significant downside of these processes, and the depletion of fossil fuel reserves is another issue of concern.Natural gas, shown to be a promising technology in comparison to the current scenario of all other hydrogen generation processes, currently supplies 95% of the world's hydrogen needs.Electrolysis is a helpful alternative that uses hydrogen production at a practical level.Furthermore, to secure the energy supply chain for the upcoming energy shift using hydrogen produced by electrolysis, largescale energy storage is essential.To produce renewable energy at the scale needed to attain net-zero by 2050, the Underground Seasonal Hydrogen Storage holds significant promise for overcoming the temporal irregularities that are naturally present in that energy.The most popular methods of producing hydrogen, such as methane pyrolysis, water electrolysis, steam methane reforming, and gasification of coal or biomass with or without carbon capture and storage technologies, all have drawbacks.The necessity for additional cost savings, production scalability, and guaranteeing the availability of renewable energy sources for steady hydrogen generation are among the difficulties in this field.To enable the widespread use of hydrogen as a clean energy carrier, further progress is needed in the areas of hydrogen storage and transportation.
Deva Brinda Deepak is a VP (BD) at Norwegian Solar AS India Private Limited, India.Having 23 years of academic experience, she served for various premier Institutions and Engg universities.She published many papers, book chapters, patents, and consultancy projects in the field of electrical engineering and sustainable energy and served as a scientific, technical, and advisory committee member and reviewer for many international conferences and journals.She steered many workshops, FDPs, and guest lectures.In connection to that, she received two FDP grants.Her main areas of interest encompass deregulated power systems, sustainable energy and smart grid, etc. Mohan Lal Kolhe is a full professor of smart grid, hydrogen energy, and sustainable electrical energy systems at the University of Agder, Faculty of Engineering and Science, Norway.He is a leading renewable energy technologist with three decades of international academic experience, having previously held academic positions at world-renowned universities such as University College London (UK/ Australia), University of Dundee (UK), University of Jyvaskyla (Finland), Hydrogen Research Institute, QC (Canada), and others.He was also a member of South Australia's first Renewable Energy Board (2009-2011) and worked on formulating renewable energy policies.

Figure 1 .
Figure 1.Number of publications searched by the word (hydrogen, renewable energy, hydrogen production, hydrogen storage) in their titles.The data were collected from Dimensions research database on May 16, 2023.

Figure 3 .
Figure 3. Hydrogen fabrication and applications methods.Reprinted in accordance with the International CC-BY Creative Commons Attribution 4.0 license.[16]Copyright 2022, Springer.

Figure 4 .
Figure 4. Schematic illustration of hydrogen production methods.

Figure 5
Figure 5. Schematic representation for the gasification process that produces hydrogen using a shell reactor.Reprinted in accordance with the International CC-BY Creative Commons Attribution 4.0 license.[139]Copyright 2020, MDPI.

Figure 6 .
Figure 6.Kinds of biomass and how they are transformed into products like hydrogen and other useful ones.

Figure 7 .
Figure 7. Assessment of the global freshwater consumption and withdrawal of three different sectors.Reprinted in accordance with the CC-BY Creative Commons Attribution 4.0 International license.[140]Copyright 2021, American Chemical Society.

Figure 8 .
Figure 8. Various hydrogen-storage systems' processes and phenomena.Reprinted in accordance with the CC-BY Creative Commons Attribution 4.0 International license.[141]Copyright 2022, Springer.

Figure 9 .
Figure 9. Hydrogen production effects on other sectors.Reprinted in accordance with the CC-BY Creative Commons Attribution 4.0 International license.[142]Copyright 2022, MDPI.

Abaynesh
Yihdego Gebreyohannes is a senior membrane scientist at Osmoses Inc., working on the development and scale-up of membranes and modules.She obtained her Ph.D. at the University of Calabria, Italy; KU Leuven, Belgium; and University of Paul Sabatier, Toulouse III, France (Erasmus Mundus Doctorate in Membrane Engineering-EUDIME).She has published more than 35 papers and 1 patent.She has received numerous grants and awards including excellence in membrane engineering award by EFCE, best exploitation of research technology and knowledge transfer by KU Leuven, EMJD by EU, FWO research grant, etc.She is executive committee for AMSiC and chair for AMSIC4 and active member of EMS and WA-MS.Segenet Dagmawi Agegnehu received her B.Sc. in materials science and engineering from Adama Science and Technology University, Ethiopia, in 2022.Currently, she is a master's student at Adama Science and Technology University, Ethiopia.Her research interests focus on synthesis and characterization, semiconductor, catalysts, dyes, polluting waterways, and renewable energy.

Table 1 .
An overview of fossil-Fuel-based hydrogen production technologies.

Table 2 .
Overview of biomass-based hydrogen production techniques.

Table 3 .
Hydrogen production techniques summary from water.