Green hydrogen:A promising way to the carbon-free society

Ying Zhou,Ruiying Li,Zexuan Lv,Jian Liu,4,Hongjun Zhou2,,*,Chunming Xu

1 College of Science,China University of Petroleum-Beijing,Beijing 102249,China

2 State Key Laboratory of Heavy Oil Processing,China University of Petroleum-Beijing,Beijing 102249,China

3 College of New Energy and Materials,China University of Petroleum-Beijing,Beijing 102249,China

4 State Key Lab of Heavy Oil,China University of Petroleum-Beijing at Karamay,Xinjiang 834000,China

Keywords:Hydrogen Hydrogen production Electrolysis of water Wind energy Solar energy

ABSTRACT With increasing importance attached by the international community to global climate change and the pressing energy revolution,hydrogen energy,as a clean,efficient energy carrier,can serve as an important support for the establishment of a sustainable society.The United States and countries in Europe have already formulated relevant policies and plans for the use and development of hydrogen energy.While in China,aided by the‘‘30.60”goal,the development of the hydrogen energy,production,transmission,and storage industries is steadily advancing.This article comprehensively considers the new energy revolution and the relevant plans of various countries,focuses on the principles,development status and research hot spots,and summarizes the different green hydrogen production technologies and paths.In addition,based on its assessment of current difficulties and bottlenecks in the production of green hydrogen and the overall global hydrogen energy development status,this article discusses the development of green hydrogen technologies.

1.Introduction

With the sudden suppressive impact of COVID-19 pandemic on the world,energy demand has dropped significantly,while relevant supply adjustments have been delayed.Although the supply and demand of the energy market is sluggish,it is still experiencing major changes unseen in the world for a century [1,2].Reviewing the energy transition policies and carbon neutrality goals of various countries,it can be clearly seen that the pace of the global transition to clean and low-carbon energy is significantly accelerating.Notably,supported by the global consensus and efforts of the international community,the United Kingdom,France and Germany have incorporated carbon neutrality into their national legislation;Japan and South Korea have also made strategic commitments to carbon neutrality;and China has also committed to stop increasing carbon dioxide emission before 2030 and achieve carbon neutrality before 2060.As a result,the energy sector is ushering in a fundamental adjustment,the modern coal and chemical industry is accelerating its greening,and energy transformation has become a top priority.As a nation,China has been committed to actively promoting the transition from traditional energy to low-carbon energy,and in the Government Work Report of 2019,a relevant development intention for hydrogen energy was first put forward.Later,a hydrogen energy development route was further described in the energy white paper—The Energy in China’s New Era,released in 2020,and the plan for a green industry centered on hydrogen energy has gradually become clearer [3,4].

Hydrogen has the advantages of being clean,non-polluting,renewable,storable and versatile.Its reserves are quite abundant when accounted for by the hydrogen element,and its unit calorific value is relatively high.It can easily be seen as a potential energy storage media that could be what is known as the ‘‘ultimate energy”of the 21st century,and is important to support the energy transition [5,6].Hydrogen is used in many fields,including construction,industry,electricity,transportation,etc.Its industrial chain can broadly be divided into three segments:upstream hydrogen production,midstream hydrogen storage,and downstream hydrogen use.In view of the different hydrogen production pathways,hydrogen sources can be divided into ‘‘gray hydrogen”,‘‘blue hydrogen”and ‘‘green hydrogen”.‘‘Gray hydrogen”mostly results from industrial by-products.Its energy source is mainly fossil fuels,and the hydrogen production process is accompanied by the production and emission of carbon dioxide.In a modification of the ‘‘gray hydrogen”production method,if the by-product carbon dioxide can be captured,used,or stored,preventing pollution,then the resultant process is termed‘‘blue hydrogen”production.If it is obtained through renewable energy sources,such as wind energy,solar energy,biomass energy and other green sources,then it is classified as ‘‘green hydrogen”.Through the gradual replacement of ‘‘gray hydrogen”technologies,and use of carbon–neutral‘‘blue hydrogen”as a transition technology,it is possible to realize the application and promotion of the carbon-free‘‘green hydrogen”technologies.While,at the 2019 Hydrogen Industry Development and Innovation Summit,Dr.-Ing.Saehoon Kim,Secretary-General of the International Energy Commission,predicted that ‘‘hydrogen will satisfy the demand for 18% of the final energy use in 2050 globally”.Similarly,China expects that in 2050,the proportion occupied by hydrogen energy of the country’s overall energy system will reach 10% of demand,and 70% of the hydrogen sources will come from renewable energy sources.

Based on the development of hydrogen strategy in various countries,the combination of hydrogen with zero-carbon technology and negative-carbon technology is more meaningful[7,8].The cost of hydrogen production from traditional fossil energy will gradually rise with the increase of carbon tax and discouragement of carbon emissions,while the cost of green hydrogen obtained from renewable energy will also gradually decrease with the improvement of technology and industry.At present,the most promising hydrogen production technology involves the electrolysis of water.In addition,hydrogen production fueled by renewable energy(wind and solar)and biomass are also potential approaches in the zero-carbon pathway.Nowadays,existing researches mostly focus on the development status,problems and challenges of related hydrogen production technologies as a singular field.However,the dependence and interrelationship of hydrogen sources and carbon sources,as well as the impact on hydrogen production technologies and industrial development,are not covered.Therefore,this review discusses the research on green hydrogen production technologies and the carbon-free pathway in detail,analyzes the development prospects of green hydrogen technologies based on the current energy situation,national policies and technical difficulties,and further evaluates the challenges faced at the current degree of industrialization and the difficulty of popularization.Finally,reasonable suggestions are put forward,in a bid provide corresponding reference and support for further promotion of the greening of the industrialization process of hydrogen.

2.Methods of Green Hydrogen Production

2.1.Electrolysis of water

The technology of hydrogen production by electrolysis of water is currently mature in industrial applications.The electricity in this process has great potential to be driven by renewable energy and obtained through low-carbon or carbon-free methods.Therefore,this technology is considered to be the most promising green hydrogen production technology.The process of producing hydrogen from water electrolysis is mainly composed of two half reactions [9,10]:one is hydrogen evolution reaction (HER) in the cathode,and the other is oxygen evolution reaction (OER) in the anode.At present,the application and research of this technology mainly includes 4 types:alkaline water electrolysis(AWE),proton exchange membranes (PEM),solid oxide electrolytic cells (SOE)and solid polymer anion exchange membrane (AEM).The schematic diagram of the four technologies [11] are shown in Fig.1,and the relative comparison parameters of each technology are involved in Table 1 [12].The AWE technology [13] covers the earliest industrialization time,reliable technology,low cost,and easy operation,but the device occupied a large area.The PEM technology is easy to integrate and has high conversion efficiency,but the cost is high [14].Although the catalyst cost is one of the big reasons to lead this technology not to be applied on a large scale,while,due to the small size of the electrolytic cell,it is easy to couple with wind energy and photovoltaics,so this technology is considered to be applicable in the future.The SOE technology [15] is subject to the certain restrictions due to the high temperature as the working conditions,while the AEM technology [16,17] is still in the research stage,and the development of both needs to be a breakthrough in related materials science.

Table 1 The operating parameters of technologies—AWE,PEM,SOE and AME

Fig.1.The schematic diagram of the AWE (a),PEM (b),SOE (c) and AEM (d).

At present,the device for AWE technology is mainly composed of electrolyzer,electrolyte,diaphragm,cathode and anode.Generally,with low working temperature,and the diaphragm is generalpurpose asbestos.It is necessary to balance the pressures of the cathode and anode during operation to avoid explosion caused by the interpenetration of hydrogen or oxygen.Since the electrolyzer for alkaline water electrolysis is slow to start and requires long response time,the method of increasing the conversion rate can be achieved by increasing the current intensity or reducing the voltage [18].For PEM technology,the main device is still an electrolytic cell,but the core part becomes a polymer cation exchange membrane.The cathode and anode catalysts are attached to this membrane,and the bipolar plate is responsible for the connection of multiple membrane electrodes.The advantage of PEM technology is that it can achieve single conductivity of hydrogen ions and thus has higher safety.In addition,the construction of the electrolytic cell is compact,and the electrolyte does not corrode the device.The SOE technology [19] requires a hightemperature reaction site,the nickel-based porous cermet is used as the cathode,the anode is perovskite oxide of rare earth elements,and the electrolyte layer is usually an oxygen ion conductor.Due to the high operating temperature,the standard of material for the internal devices are relatively high.The AEM technology[20,21]uses an anion exchange membrane,with low working temperature,fast start-up and non-corrosive reaction medium.

In the aspect of technological development,the researches focus more about on the electrolytic cell devices and the electrode catalysts [22].In the research of water electrolysis device,the choice of electrolyte and the quality of the diaphragm for theAWE technology have the decisive influence on the efficiency of hydrogen production.KOH or NaOH solutions are universal electrolytes[23].The appropriate selection of electrolyte concentration has a certain impact on the number of ions involved in charge transfer and ion migration resistance,at the same time,attention should also be paid to the influence of electrolyte concentration on electrolysis efficiency.Asbestos,as the diaphragm used in the early stage,has been replaced due to its poor corrosion resistance.Not only the nickel-based mesh supports ZrO2,NiTiO3and other asbestos substitutes,but the composite diaphragms formed by the cross-linking of high molecular polymers and specific inorganic materials,have already been developed,however,they still need to be optimized.The proton exchange membrane is the core part of the PEM technology,and the research focuses on the influence of different proton exchange membranes on the performance of the PEM electrolytic cell and the development of new membrane materials [24,25].The relevant researches of the diffusion layer can realize the need to optimize the material and structure,which is helpful for the efficient mass transfer between the end plate and the catalyst.The development of end plates can reduce costs to a certain extent.Toghyani et al.[26] studied the different shaped channels of the end plate and found that the double-channel serpentine runner showed the best performance.Lettenmeier et al.[27]used stainless steel as the end plates to coat Ni and Ti protective layers respectively,and found that the Ni modified Ti coating can reduce the contact resistance and maintain the high stability of 1000 h operation,in order to provide a new way to solve the problem of high cost of based by Ti.For SOE technology,in addition to stability under high temperature conditions,the anode material also needs to have compatibility with the electrolyte material,while the cathode material has excellent conductivity and catalytic activity.The anode material is mainly a composite formed by the mixed oxide of the perovskite structure adding ion conductive materials.For example,the mixed addition of the La-Co-Fe system might improve the ion conductivity.For the electrode catalysts,the‘‘volcano plot”diagram can reflect the relationship between the exchange current density (j0) and the free energy of hydrogen adsorptionwhile the ideal catalyst hasapproaching zero.According to the water electrolysis process,the reaction of HER mainly follows the Volmer-Heyrovsky mechanism or the Volmer-Tafel mechanism,but regardless of the reaction path,is the most important indicator for the evaluation of HER activity.And the noble metals such as Pt,Pd,and Ir are ideal catalyst choices,but due to their high prices,current research focuses on transition metals such as Co,Mo,and Ni.The researches of catalysts are mainly centered on precious metal catalysts (construction of heterojunction,surface modification of bimetallic alloys and interface construction),transition metal catalysts (sulfides,phosphides,nitrides,carbides,borides) and single-atom catalysts (precious metals,non-precious metals).Wang et al.[28]obtained the Pt-modified Ni3N electrocatalyst and the Ni3N could increase the electron conduction rate in this hybrid nanostructure.The results show that the Ni3N/Pt composite with the 15% (mass)Pt shows excellent HER activity under alkaline conditions,with an overpotential of 160 mV (200 mA.cm2,36.5 mV.dec-1) and a current retention rate of 82.5% (24 h rear).In addition to Pt,Ru and Pd have also been used to construct the heterojunction in catalysts [29,30].For bimetallic catalysts,it can be constructed by combining precious metals (Pd,Ru,and Pt et al.) with nonprecious metals (Co,Ni,and Cu et al).Fu et al.[31] reported that,compared with commercial Pt/C and elemental Ir,the HER activity can be improved 5 times and 10 times by the IrMo0.59alloy nanocatalyst,respectively.Density functional theory (DFT) calculations show that the Mo sites on IrMo alloy can effectively optimize the adsorption free energy of H2O and OH,thereby improving the kinetics of the catalytic reaction.For the transition metal catalysts,element doping,conductive substrate,crystal image and adjustment of active sites are the hot topics in the research of transition metal sulfides.The main characteristics of this type of catalyst are abundant reserves and low cost.As a new type of semiconductor material,transition metal phosphide can control the stability and conductivity of the catalyst by controlling the surface morphology and the proportion of phosphorus.Transition metal carbides can achieve high stability and high activity of the catalyst by constructing specific nanostructures or hybridizing with other forms of carbon materials.Transition metal nitrides can realize the unique electronic structure of the catalyst by introducing other metals.And the transition metal borides are similar to transition metal carbides,which can be adjusted to achieve wide pH applicability of the catalyst.For the study of single-atom catalysts,on one hand,the goal is to improve the atomic utilization and reaction activity of noble metal catalysts.However,because single atoms are prone to migration and agglomeration,how to achieve high catalytic activity and stability is a current problem.On the other hand,nonprecious metal catalysts have been difficult to achieve high activity at present,but the construction of single-atom catalysts,using low coordination and unique electronic structure,can provide a solution for non-metal catalysts.

In terms of engineering demonstration,AWE technology has been widely used in industries such as pharmaceuticals,energy storage,and metallurgy.The researches and development institutions of related equipment are represented by the 718th Research Institute of CSSC (PERIC) of China,Lurgi Company of Germany,Delitai Company of American,Hydro Company of Norway,Vandenberg IMET Company of Belgium,Milan NeNora Company of Italy etc.At present,the maximum hydrogen production capacity of a single domestic alkaline water electrolysis equipment is 1000 m3.h-1.The catalysts used in the industry mainly choose Ni-based alloys,which can adapt to the alkaline corrosive environment,but the electrolysis efficiency needs to be improved.Although,currently,the research of catalyst used in AWE technology is the hot topics,how to construct electrode materials with low cost,excellent performance and low energy consumption is the focus of further promoting engineering applications.While,the development of PEM technology [32] originated from the needs of the US military oxygen generators.Because of the problem of the consumption of renewable energy,European and American countries are actively promoting the intervention of PEM process.Relevant agencies,such as the China Astronaut Center,the Dalian Institute of Chemical Physics.Chinese Academy of Sciences,Siemens in Germany,Proton Onsite in the United States,ITM power in the United Kingdom,and Hydrogenics in Canada,can represent the level of this field.Among them,the PEM equipment‘‘HYLYZER 600”developed by Canadian Hydrogenics can achieve the single reactor hydrogen production capacity of 400 m3.h-1(standard).So far,no matter the research of platinum electrodes or the proton exchange membranes,the development of PEM commercialization in China is still poor.SOE technology has the advantage of flexible coupling with renewable energy.The Idaho National Laboratory in the United States,the Juelich Research Center in Germany,the French Atomic Energy Commission (CEA),and the Institute of Nuclear Energy and New Energy Technology (INET) of Tsinghua University in China have carried out the optimization of solid oxide stacks and the design and operation of high-temperature electrolytic cells.After a series of researches,the current problems need to be solved in this technology is mainly about the poisoning of electrode materials and the high internal resistance in its longterm operation.As for AEM technology,it is only in the laboratory research stage now,and related breakthroughs and developments await for the innovation of electrode materials.

2.2.Photocatalysis

Photocatalysis is a process in which a catalyst absorbs photons to generate high-energy electrons and holes and then initiates a redox reaction.In 1972,Fujishima and Honda discovered that photocatalytic water splitting can produce hydrogen on the TiO2electrode.This discovery provides a foundation for realizing solar water splitting to produce hydrogen.Photocatalytic splitting of water for hydrogen production is through the use of semiconductor photocatalyst to generate electron hole pair under the light irradiation,and the H2as the target product could be obtained in the reduction reaction.The mechanism is shown in Fig.2[33].This process can convert and store solar energy in the form of chemical energy,which is regarded as a promising environmental green hydrogen production technology.

Fig.2.The mechanism of photocatalytic hydrogen production.

So far,various compounds,especially alcohols and biomass,have been widely used as photocatalytic raw materials.The recombination of photogenerated charge and poor light absorption are the main bottlenecks in the application of photocatalysis [34–36].Conventional methods to solve these problems can inhibit charge recombination through band gap engineering and light irradiation,thereby improving the photocatalytic performance of the material.In addition,defect engineering is another method that can adjust the optics,charge separation,and surface properties of photocatalytic materials.The design of vacancy in the materials not only improves their optical and charge transfer properties,but also affects their surface properties,which could help the adsorption of reactants on the catalyst surface [37].Elewa et al.[38] researched mono-thiophene (MT),tri-thiophene (TT),dithiophene benzo-thiadiazole (DTBT) and diphenyl benzo-thiadiazole(DPBT) for the first time based on a series of tristyrene (TP) with discontinuous conjugated covalent organic polymers(COP)photocatalyst,among which,DPBT-TP can significantly increase the HER rate.This report proved that a polymer photocatalyst constructed from a discontinuous conjugated system(conjugation length:limited 5 aromatic rings) is sufficient for effective visible light-driven hydrogen evolution.It provides a design strategy of alternative materials for polymer photocatalysts to improve the performance of visible light hydrogen evolution.Li et al.[39]fixed a small noble metal promoter on a relatively large photocatalyst.In this study,the minuteness TiO2quantum dot photocatalyst was loaded on a low-cost NiO/NiS2porous nanosheet micro-co-catalyst,which promoted the rapid charge separation of photogenerated electrons from a small photocatalyst to a large-scale co-catalyst.The hydrogen production of such catalyst is 16 times higher than that of the pure TiO2photocatalyst.Yu et al.[40]used the flash nanoprecipitation(FNP)method to prepare highly efficient nano-level photocatalyst with hydrophilic soluble polymers (PC-PEG5 and PS-PEG5)without using the surfactants.It is worth noting that the hydrogen evolution rate of nano-sized PC-PEG5 could be increased by 70 times.Lee et al.[41]used citric acid as the surfactant to synthesize ZnS nanoparticles which co-doped with In and Cu in deionized water and ethanol solvents.This ZnS microcrystals have the rough surface nanostructure,and can reach a maximum hydrogen production rate of 752.7 μmol.h-1.g-1within 6 hours under simulated sunlight.Song et al.[42]prepared a novel MoP-Cu3P hybrid photocatalyst used the metal organic framework (MOF) by the one-step synthesis method.The hybrid exhibited significant improvements in optical and electrical properties.The MoP-Cu3P hybrid shows enhanced photocatalytic activity,and the hydrogen production rate is 855 μmol.h-1.g-1(3.34 times higher than that of Cu3P).Zhan et al.[43] successfully prepared layered mesoporous NiO@N doped carbon microspheres(HNINC)as a high-performance photocatalyst for hydrogen evolution using ultra-thin nanosheet units.The unique structure and hierarchical mesoporous structure of HNINC’s N-doped carbon layer can effectively promote the separation and transfer of photo-induced electron-hole pairs,and provide abundant active sites for photocatalytic reactions,resulting in a significantly higher yield of hydrogen than used the method of deposited NiO and platinum.In addition,Ali et al.researched the green nano-advanced materials [44,45] and the radiation [46] as the supplementary means in the water splitting,which also showed good results in the hydrogen generation.

In general,photocatalytic production of green hydrogen has certain advantages,but there are a lot of the challenges exited.Due to the low radiation efficiency of the light source,the poor reactivity is still a big reason for the hindrance of large-scale commercial application.In addition,the mechanism of the photocatalytic hydrogen production process is fuzzy,and most studies only focused on the energy band and catalyst research.Even though,the reasonable design of reactors and devices is also very important for improving catalytic activity,but such research is still very limited.

2.3.Biomass

Using biomass as raw material,through gasification,conversion,decarbonization and separation,the pure hydrogen is obtained finally [47–49].Biomass can also be covered in the biochemical processes such as pretreatment,enzymatic hydrolysis,and fermentation to obtain hydrogen-containing compounds such as methane,methanol,ethanol,etc.,and then reformed by catalytic reaction to obtain high-purity hydrogen.The routes are showed in Fig.3 [50] and the researches of the conversion from methane to hydrogen could be found in Table 2.At present,from the perspective of technological development,hydrogen production from biomass has the basic conditions for industrialization.At this stage,the costs in two parts of green hydrogen production—the storage and transportation,occupied for a high proportion,which is the main obstacle to the opening of green hydrogen energy in the energy system.Hydrogen production by biomass is expected to be able to overcome the two major problems of green hydrogen sources and hydrogen storage and transportation costs.It might be an effective way to solve the current hydrogen energy development problems and shows great commercial prospects [60].

Table 2 The conversion methods from methane to hydrogen

Fig.3.The route of hydrogen production by biomass.

Biological hydrogen production and thermochemical hydrogen production are the two pathways for the hydrogen production by biomass.Biological way is through hydrogen-producing microorganisms,such as anaerobic fermentation hydrogen production and photosynthetic hydrogen production,but the yield and stability are poor,and the possibility of large-scale production is limited.Thermochemical way is through the pyrolysis of biomass hydrocarbon components into synthesis gas (CO,H2),etc.,and in the reaction of CO and H2O to produce hydrogen [61].Liu et al.[62]designed a new polymeric carbon nitride-based photocatalyst with edge amino functionalization.The study found that the modified photocatalyst has visible light broad spectrum absorption,and its spectral response activity range can be extended to 600 nm.Compared with the existing biomass hydrogen production photocatalyst (generally it can only absorb light below 475 nm),it is significantly broaden the absorption of visible light.At the same time,the structure of energy band was analyzed,and it was found that the oxidation capacity of the catalyst was improved,and it could promote the adsorption and activation of biomass on the surface of the catalyst,improving the efficiency of photocatalytic biomass hydrogen production.Granados-Fitch et al.[63] first reported the use of Re2C as a catalyst for the production of hydrogen from biomass.Re2C is synthesized from rhenium and graphite in a stoichiometric ratio of 1:1 and 2:1.The highest hydrogen production was obtained using 10% (mass) Re2C catalyst at 800 °C.Giang et al.[64]studied the pretreatment of biomass.The pretreatment parameters and the conditions for hydrogen production of the pretreated biomass are optimized.Under acidic conditions,heating the chlorella in the pretreatment can effectively dissolve the biomass to obtain higher hydrogen yield.Yin’s team[65]studied the potential of microwave pretreatment to improve hydrogen production from macroalgal.The pretreatment of microwave at different temperatures (100–180 °C,30 min) was tested.After microwave treatment,the hydrogen production process is transformed from butyrate type fermentation to acetic acid type fermentation.And hydrogen production can be indicated by dehydrogenase activity.The highest hydrogen production of 15.8 ml.g-1TS was obtained from the microwave-treated algae at 160 °C.Compared with the control tests,the maximum hydrogen production was increased by 1.9 times.Supercritical water gasification (SCWG) uses the dissolving ability of supercritical water to dissolve various organic substances in biomass to produce a high-density,low-viscosity liquid,which could be quickly vaporized under high temperature and high pressure conditions to produce the mixed gas rich in hydrogen.Compared with other biomass thermochemical hydrogen production technologies,SCWG has unique advantages.Without a high-energy-consuming drying process,it can directly gasify wet organisms with high water content and will not cause the secondary pollution.In addition,the heat from the produced high-pressure hydrogen can be directly used for combustion in engines or turbines to obtain electricity.Although SCWG is a promising biomass fuel conversion technology,but the application of this technology on an industrial scale still has some limitations.On the one hand,currently only a few researchers have the opportunity to use short residence time continuous low tube reactors or continuous stirred tank reactors for SCWG experiments.On the other hand,the current research hotspots on SCWG hydrogen production technology mainly focus on the influence of different types of biomass compounds on the hydrogen production yield.Because the SCWG hydrogen production system is a complex process,according to the specific conditions of the reaction,there are a series of competing reactions to a certain extent,such as hydrolysis,oxidation and methanation etc.So,when different reactions dominate,the corresponding hydrogen production efficiency will make a huge difference,so the regulation of the hydrogen production process is the key to efficient hydrogen production [66].The team of Professor García-Jarana [67] studied the rapid supercritical water vaporization of cellulose (a biomass model compound) in a continuous reactor toproduce hydrogen under supercritical conditions.Because the experimental conditions are closer to industrial-scale conditions,and the problems similar to batch reactors can also be avoided,the influence of the operating parameters of the system(temperature,oxygen content,initial concentration of cellulose,residence time etc.) on the efficiency of hydrogen production are tested,so that it will help to improve the process.AlNouss et al.[68]researched the biomass gasification process and the optimization of important parameters (feedstock type and gasification agent(gasifier),so that to maximize hydrogen production.By establishing a model and simulating the gasification process,improved the performance of hydrogen production from mixed biomass gasification in different scenarios.

All in all,in terms of carbon sources,although the technology of producing hydrogen from biomass meets the standards of green hydrogen,it still needs to be improved in accordance with the current carbon footprint calculation of the hydrogen production process,so as to achieve carbon-free energy consumption in the hydrogen production process.

3.Energy Source of Green Hydrogen Production

3.1.Wind power

Wind power is one of the ways of green hydrogen production technology.Wind energy is converted into electrical energy through wind generators,and then the hydrogen is produced by electrolysis of water and passed through the hydrogen transmission system to achieve terminal elimination thereby completing the wind energy converting to hydrogen energy[69–71].According to the way of connecting to the grid,hydrogen production technology can be divided into three types [72].The grid-connection system is the way to connect wind turbines to the grid,obtain electricity from the grid and produce hydrogen by electrolyzing water (for example,electricity is taken from the grid by a 35 kV or 220 kV of wind site).This method is mainly used for wind consumption and energy storage of large-scale wind site,and the route is shown in Fig.4[73].For the off-grid system,the electrical energy is produced by a single fan or multiple fans which is directly provided to electrolysis equipment without going through the grid for hydrogen production.It is mainly used in distributed hydrogen production system or partially used in fuel cell power generation.The grid-connected without transmission system is the way to connect wind turbines with the grid,but the electricity not transmit to other sites,only meet the demand for hydrogen production locally.

The hydrogen production project by wind power was first proposed by the United States,which produced hydrogen through a generator connected to an electrolysis reactor.The National Renewable Energy Laboratory in U.S.and Xcel Energy have launched the ‘‘Wind-to-Hydrogen”demonstration project,which uses wind power and photovoltaic power to produce and store hydrogen,maximizing the use of renewable energy and optimizing the energy Transfer.The ‘‘Wind-to-Hydrogen”project developed by the United States was a joint project of the National Renewable Energy Laboratory(NREL) and Xcel Energy in 2004,hosting by the National Wind Energy Technology Center of NREL.The windhydrogen system output optimal capacity ratio technology,the impact of electrolysis technology on wind-hydrogen system and the analysis of renewable energy output and system cost-benefit are involved in this plan.At this stage,the industrial scale of new energy production systems and the commercialization of community or personal scale systems have been completed,and wind energy can be directly used for energy storage,which shows advantages in technology and cost.After the U.S.,Japan also proposed a series of plans and applications for hydrogen production from wind power.The European Union (E.U.),as a leader in the field of converting wind energy into hydrogen,plans to achieve sustainable development without relying on fossil fuels by the end of 2060,and an important part of achieving this goal is the storage and application of renewable energy in the form of hydrogen.The E.U.has implemented hydrogen production demonstration projects in Greece and Spain,which combine wind energy with hydrogen production technology.It involves hydrogen storage,fuel cell and seawater desalination technologies,and provides‘‘green”hydrogen energy for energy storage,power supply and fresh water supply [74].In 2011,the state of Brandenburg (German) built and operated the world’s first wind-hydrogen hybrid power station.In 2014,Germany proposed the idea of using hydrogen generated by wind power to inject into the natural gas pipeline network and established a demonstration plan.This is an important beginning for wind power production.Under the impetus of the European Commission’s Seventh Framework Program for European Research and Innovation,a hydrogen storage project called INGRID was launched.The purpose of this project is to optimize the power generation quality of intermittent renewable energy power through the hydrogen energy storage system while improving the utilization efficiency of the renewable energy system to ensure the safety and stability of the power grid.The project with a total storage capacity of 39 MW.h,is constructed by a power generation system consisting of 3.5GW of solar,wind and biomass resources,a solid-state hydrogen storage system with a hydrogen storage capacity of more than 1 ton and a set of 1.2 MW hydrogen generator composition.Germany formulated the ‘‘power to gas”plan and implemented it gradually.In 2012,the proportion of renewable energy in the total power generation of Germany reached 22%,and it is planned to increase to 35%in 2020 and reach to 50%in 2030.The basic route is to finally use surplus wind energy and other renewable energy,electrolyze water to generate hydrogen,store and join it in the existing gas pipeline network.Hydrogen is used as an energy carrier or raw material for mixed hydrogen natural gas fuel,or as a chemical raw material and as a fuel for hydrogen fuel cell vehicles [75].For China,in 2008,the China Urban Planning and Design Institute proposed a model for the construction of a seawater desalination hydrogen production base.North China Electric Power University and Shanghai University of Finance and Economics discussed the feasibility and economics of hydrogen production from wind power,as well as the problems of hydrogen production by wind power.In 2013,the State Power Corporation of China proposed a new method of large-scale wind power storage,and pointed out that the effective hydrogen storage and fuel cell technology are the key technical issues of the system.In September 2016,a 70 MPa hydrogen fueling station was completed in Dalian and key equipment achieved independent innovation in China.In 2017,as the China’s first hydrogen production industry application project,the hydrogen production station of the Hebei Guyuan Project was successfully launched.It is the world’s largest hydrogen production project,providing experience and foundation for the realization of the scale and industrialization of wind power generation.In 2019,the‘‘Large-scale Wind/Photovoltaic Complementary Hydrogen Production Key Technology Research and Demonstration”project was approved and entered the start-up stage.This project will fill the gap in megawatt-level wind-solar hydrogen coupled energy storage demonstration project and become a world-leading demonstration project in China.

Fig.4.Schematic diagram of wind power hydrogen production for large-scale wind farms.

In recent years,there are a lot of theoretical and technical researches of hydrogen production technology by wind power generation.Takahashi et al.[76] proposed a coordinated control method for hydrogen production technology by wind power generation,in which variable-speed wind turbines and hydrogen production devices are installed together.The power curve is smoothed to reduce the impact of wind energy fluctuations on the power system and hydrogen production equipment.After that,the system is introduced and simulated in detail,and its performance is evaluated.Pino et al.[77] analyzed the influence of the operating temperature of the electrolyzer on the hydrogen production system of wind power generation.The production efficiency at the actual working temperature is compared with the production efficiency at the rated temperature,and the result shows that the hydrogen production efficiency at the actual temperature is overestimated.Valdes et al.[78] simulated a wind power plant for hydrogen production by electrolysis,and proposed two methods for optimizing the total power of hydrogen production by wind power.Sarrias-Mena et al.[79] studied the coupled operation of electrolyser and wind turbines,and compared the operating performance of four types of electrolysers.Guilbert et al.[80] discussed the two main issues regarding the energy consumption of the electrolyzer (Faraday efficiency and converter).Use a multistack configuration of PEM electrolysers provided by the Wind Turbine Conversion System(WTCS).This method is based on the modeling of wind turbines and electrolysers.The WTCS and the electrolyser are connected through a stacked interleaved stepdown converter to improve the reliability of output current.Step-down converter is designed to be connected in an array configuration (i.e.,parallel architecture) so that each converter operates at its maximum efficiency.Nematollahi et al.[81] evaluates wind and solar data from selected metering stations in Sistan—Baluchistan in Iran to produce hydrogen as a clean fuel.For the use of wind and solar energy,a detailed technical analysis was carried out.According to the evaluation of the hydrogen production of several small wind turbines,it is shown that up to 39.7 tons of H2can be produced each year.Huang et al.[82]analyzed the potential use of the wind and solar energy to produce hydrogen in China.The entropy method was used to simulate the China’s green hydrogen production capacity from the provincial and regional perspectives,and the dynamic changes from 2017 to 2030 were studied.The results show that:Firstly,the efficiency of hydrogen production by wind power is significantly higher than that of solar.Secondly,compared with 2017,the efficiency and potential of green hydrogen production have increased in 2030.Thirdly,according to the potential of green hydrogen production in Northwest and North of China is significantly higher than other regions,so that the policy which could promoted the hydrogen energy industry were proposed.Kim et al.[83] researched the modeling of wind-hydrogen installation and the control coordination scheme(CCS)to optimize the power generation of variable-speed wind turbine generators(WTG) and help balance the supply and demand in the system.Kilkis et al.[84]studied the rational use of energy on a customized hydrogen ship.This ship equipped with 100%wind,wave and solar systems,will built a two-step hydrogen production system in the Black Sea in the future.Yi et al.[85] conducted modeling analysis and simulation research based on the 1.5 MW wind power hydrogen production system,and established the simulation model of the electrolyser at different temperatures and the hydrogen production system simulation model at different wind speeds.The simulation results proved the feasibility of the system.Ning[86]studied the adaptability of the traditional water electrolysis hydrogen production device under the conditions of wind power wide power fluctuations by adjusting various process parameters.Guo et al.[87] analyzed the important role of wind power hydrogen production devices in reducing system operating costs and increasing clean energy consumption.They compared the operation of the system under different hydrogen load requirements.The simulation results showed that reasonable hydrogen load arrangements are beneficial to further Promote the green and economic operation of the system.In a study of Li et al.[88],the problem of using renewable energy(RES)such as wind energy to produce clean hydrogen through water electrolysis in small decentralized factories was solved.This work proposes a method for multi-objective optimization of wind hydrogen production system (WHPS)based on dynamic power and hydrogen management strategy (PHMS),which enhances battery assistance to electrolysers through PHMS adapted to low wind speed conditions.

Table 3 shows the lasts researches of the economic data of hydrogen production by wind power.In addition,the surveys from different countries could be seen that it is economically feasible to use wind energy to generate hydrogen to a certain degree.Gruger et al.[95]proposed a smart operational strategy that takes electricity price,wind energy availability and hydrogen demand into account,which effectively maximizes the use of wind energy and further reduces the cost of hydrogen production.The results show that this method can reduce the cost of hydrogen production up to 9.2%,in the meantime,increased wind energy utilization by as much as 19%.Song et al.[96]discussed hydrogen production from offshore wind power in China.It also involved the factors,including production,storage,conversion,transportation and destination handling,which could affect the final cost of hydrogen.The research shows that Chinese offshore wind power could provide Japan with a cost-competitive source of green hydrogen.Woznicki et al.[97] conducted a technical and economic study on the combined green hydrogen production technology of offshore wind farms and seawater electrolysis.And the results showed that the goal of producing hydrogen offshore is economically feasible.In addition,a study by Moslem et al.[98] investigated the technoeconomic evaluation of a solar and wind hybrid grid system located in the urban area in eastern Iran.With an initial investment of 440,000USD,including four wind turbines and a grid system(optimized system),the renewable energy recovery rate is 84%.This system is economically feasible which could pay back the investment in a period of 4 years.

Table 3 The economic data of hydrogen production by use of wind power as the energy source

In short,Germany,as the representative of E.U.,developed relatively fast in hydrogen production,and has demonstration projects in hydrogen production,storage and utilization.At present,hydrogen energy is mainly used for hydrogen fuel power generation and hydrogen fuel cells.Due to the need for huge infrastructure (such as hydrogen fueling stations,hydrogen transportation networks,etc.),the development of hydrogen vehicles has been slow.The researches of wind power hydrogen production technology in China developed a little late and the progress is relatively slow.At present,there is no mature and commercial operation of wind power hydrogen storage and fuel cell power generation systems.There is insufficient experience in the design of large-scale wind power hydrogen storage demonstration projects.And the key technology,efficiency improvement and economy have not been substantive.

3.2.Solar energy

At present,the solar hydrogen production system by photovoltaic-water electrolysis can be divided into direct coupling and indirect connection according to the connection mode between the solar photovoltaic panel and the water electrolysis cell [99].The direct coupling system is through the optimal structure matching between the photovoltaic array and the water electrolyzer,without the maximum power point tracking (MPPT)controller,direct current-direct current (DC-DC) controller and storage battery etc.,making the system more economical and efficient.Indirect connection is the method used by most photovoltaic-water electrolysis hydrogen production systems,which is composed of photovoltaic modules,control modules,batteries and hydrogen energy storage systems.Because the system requires electronic equipment such as MPPT and DC-DC controllers,so not only increasing the system cost,but also inevitably produces power transmission loss and reducing system efficiency.

In 1839,Becquerel discovered that semiconductor materials produced a potential difference (PD) under light,and named it as the‘‘photovoltaic effect”.With the popularization of clean energy in the 21st century,photovoltaic power generation has developed across the world[100,101].Photovoltaic power generation technology in the European market emerged earlier and has a larger market share.The European Photovoltaic Industry Association announced a set of global photovoltaic market forecasts.In 2019,the overseas photovoltaic market will exceed 80GW,and it is expected that the installed photovoltaic capacity of 16 countries will increase by more than 1 GW.The global total cumulative import volume may reach 700 GW before 2020.The markets of India,Chile,Saudi Arabia and other countries are rapidly starting.Photovoltaic power generation has become widely used in the world,and the photovoltaic industry has gradually evolved into an important industry in many countries.Since 2010,the focus of the global photovoltaic application market has shifted from the European market to China,the U.S.,and Japan.This type of market has accounted for about 70%of the global market in total.While,it only took 5 years for China to account for 5%to 26%of the total global installed capacity.In 2019,China’s newly installed capacity ranked first in the world for five consecutive years [102].Although the Asian market for photovoltaic power generation started late,it is promising.The European and Asian market are occupying 42%of the market share,respectively.In 2013,Germany issued a policy of‘‘self-use,surplus electricity to the grid”to encourage the development of photovoltaic power generation technology.Among them,the distributed installed capacity can reach up to 75%.As early as 1973,the solar power generation plan of the U.S.government kicked off photovoltaic power generation.In 1980,the U.S.government officially planned and funded the photovoltaic power generation plan.Since then,the U.S.has continued to complete policy encouragement and financial support for photovoltaic power generation,and has also introduced many preferential policies (such as direct subsidies for photovoltaic power generation investment,photovoltaic power generation tax refunds,etc.).Traditional photovoltaic power generation markets such as Japan and Europe are gradually shifting to emerging markets such as the U.S.and China.For China,as early as the early the 1950s,photovoltaiccell research projects began to emerge.In the 1970s,the photovoltaic power generation industry came out.In the 1990s,driven by the international environment,photovoltaic power generation industry of China is blooming,because the materials and technologies almost depend on the support of international market,so the progress of domestic photovoltaic power generation innovation technology is extremely slow.In 2002,the introduction of the ‘‘Bright Project”brought the domestic photovoltaic industry to a new level.With the great support of the Chinese government,photovoltaic industry normalized and has become the world’s largest photovoltaic power generation market.As of 2016,the cumulative installed capacity of domestic photovoltaic power generation has reached 7.742 × 107kW,ranking first in the world.In 2020,the world’s first one kiloton solar fuel synthesis demonstration project was successfully tested in Lanzhou New District of China.This project is mainly composed of three systems:solar photovoltaic power generation,hydrogen production by water electrolysis,and methanol production by CO2hydrogenation.The hydrogen production technology by water electrolysis uses alkaline water electrolysis,which greatly reduces the cost of hydrogen production.Currently,it is the most effective alkaline water electrolysis hydrogen production system in the world.Additionally,this project also provides a new way for the development and utilization of solar energy resources in western China[103].

Until now,there are a large number of researches on the materials,operation system and devices of hydrogen production technology by solar energy.Varunaa et al.[104] used Ti4+to replace MgH2,which can be considered as a promising hydrogen storage material and photovoltaic applications.Yi et al.[105] used the ultra-thin cobalt/iron-molybdenum oxide nanosheets on foamed nickel to prepare the high-efficiency HER and OER,respectively.After being integrated with commercial monocrystalline silicon cells,under the illumination of the solar simulator,the efficiency of solar energy to hydrogen is 15.1%and there was no performance degradation in 160 hours.This solar energy conversion technology demonstrates the potential for long-term and cost-effective hydrogen production in large-scale industrialization and provides exploration for new energy conversion systems.Xiao et al.[106] found that high-efficiency hydrogen production in photovoltaic electrolysis cell (PV-EC) system requires the high coupling efficiency of the two modules and the low overpotential of the electrolytic cell.By using stable and low-cost perovskite solar cells (PSC) to drive the reforming electrolyser,matching the maximum power point of the photovoltaic device,a single PSC can drive three reforming in series,so that the electrolyzer produces 1.77 mg.h-1H2for the entire PV-EC system.Gougui et al.[107] carried out a research on a small-scale solar hydrogen production system using PEM technology in the Ouargla region (southeast of Algeria).The performance of the complex commercial electrolyser (HG-60) were tested.Considering that it is more effective to improve the voltage matching,they found that the number of commercial electrolysers increased by four series-connected HG-60 stacks and the power transmission efficiency could reach to 99%.Marino et al.[108]studied an independent photovoltaic system,by optimizing the system component chain (photovoltaic generators,electrolysers,tanks,fuel cells) to avoid the energy overproduction that cannot be stored or converted into hydrogen due to battery or storage tank capacity limitations.The work of Azzolini et al.[109] proposed a load management photovoltaic (PV) system for driving hydrogen production,so that the system can dynamically respond to available photovoltaics based on power changes.The system can obtain more than 99.5% of the available energy from the photovoltaic array,thereby reducing costs and increasing energy production.Cotfas et al.[110] proposed a continuous discretization algorithm,which could optimize the efficiency of photovoltaic cells and panels,predict the generated energy,and calculate accurately the temperature and irradiance parameter functions.Kovac et al.[111]proposed a system which applied to a solar power plant (960 Wpeak) to produce hydrogen through water electrolysis.A new type of alkaline electrolyzer with bipolar design was constructed,in the same time,the hydrogen production rate of this system was about 1.138 g.h-1.Kursun et al.[112] studied the influence of the use of concentrated photovoltaic thermal system (CPV/T)system on Rankine cycle (RC) heat source (flat panel solar collector) on RC efficiency and hydrogen production.Because of introducing this system,the hydrogen production could be increased from 0.02 kg.h-1to 0.3 kg.h-1within limits.Cabezas et al.[113]studied the performance of photovoltaic modules and the entire system,including concentrators,spectroscopes,electrolytic cells,and reactors.Combining solar concentration with spectroscopes,photovoltaic power generation and direct photon energy conversion can be performed simultaneously by the following methods Produce hydrogen electrolysis and photoelectrochemical water splitting.Reuss et al.[114] modeled and debugged the behavior of the integrated system under different equipment and operating conditions.Three generation ways with differences in connection and integration of subsystems,which are composed of photovoltaic (PV) and electrolysis (EL),are studied.Compared with the models in the literature,based on the electrochemical process,it talked about the loss problem and how to improve the overall performance.Senthilraja et al.[115] discussed the influence of different parameters such as the inclination angle of solar collectors,collector design and heat transfer fluid type on the performance of photovoltaic-Thermal (PVT) systems and hydrogen production systems.Finally,using Adaptive Neuro-Fuzzy Inference System(ANFIS) technique to predict the thermal efficiency,electrical efficiency and hydrogen production rate.Fopah-Lele et al.[116]tested by the numerical and experimental and found that the solar hydrogen production efficiency in sub-Saharan Africa can reach up to 19%.As a clean energy source with a wide range of sources,photovoltaic hydrogen production technology is more and more favored by the energy sector due to its environmental friendliness and technical feasibility.

For the hydrogen production cost used the photovoltaic as the energy source,the latest researches are showed in Table 4.Mojtaba Fereidooni et al.studied the feasibility of hydrogen production from photovoltaic power generation systems,evaluated the economic feasibility of a 20 kW photovoltaic power station located in Yazd(Iran),and evaluated its photovoltaic hydrogen production capacity through experimental studies and simulations.The hydrogen production potential is about 373 tons/year,and the economic analysis shows that the project investment payback period is near to 16.36 years [100].Marino et al.evaluated an independent photovoltaic system.According to the commonly used economic indicators to evaluate the competitiveness of the current electrolytic hydrogen energy storage,it can be found that the investment cost of the system is still high.To have an advantage in its payback period,its cost should be reduced by at least three-quarters[108].In a word,at present,there is no cost advantage in the direct use of grid-connected photovoltaic generation to produce hydrogen in large-scale does not have cost advantages,unless the cost of energy or the cost of net have price competitive.

Table 4 The economic data of hydrogen production by use of photovoltaic as the energy source

4.Conclusions and Suggestions

In the current global energy strategy,the green hydrogen industry is crucial in the development of low-carbon energy.In green hydrogen production,avoiding the consumption of traditional fossil energy in obtaining hydrogen,or‘‘carbon-free green hydrogen”,is the current main direction.To achieve this goal,the combination of water electrolysis with power generated from wind energy,solar energy and other renewable energy is the main thrust.However,the application of ‘‘gray hydrogen”and ‘‘blue hydrogen”is being considered to ensure a reasonably smooth energy transition.Based on the relevant green hydrogen strategic plans being issued and implemented by European and North American countries,their various government departments are working on the establishment of a regulatory framework to support and promote the development of the green hydrogen industry chain.Therefore,for China,in view of ensuring the healthy development of the green hydrogen industry,the following three suggestions have been put forward.

(1) Enhance the core competitiveness of the industry and integrate the needs of low-carbon development(transportation,industry and construction etc.).By taking hydrogen energy as one of three terminal energy delivery forms,China should strive to achieve an integration of these three networks of energy (grid,gas network,hydrogen network) to gradually realize clean terminal energy consumption.

(2) Green energy should be regarded as a powerful means to optimize the energy structure,promote energy revolution,and address climate change needing concerted efforts of the national leadership,employers and industry participants.By focusing on green hydrogen for energy consumption,transformation from a low-carbon society to a carbon-free society can be achieved.

(3) As it is difficult both to effectuate large-scale green hydrogen production and to achieve ‘‘low energy consumption”or‘‘high stability”in the short and medium terms,it is necessary to conduct trials in some regions first,permit the coexistence of multiple hydrogen production methods,and build a hydrogen energy industry chain with close connection to the upstream and downstream sectors.To some extent,there is also a need to avoid the occurrence of ‘‘unfinished”projects caused by reckless investment and offer guidance for the orderly and healthy development of the green hydrogen industry.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Natural Science Foundation of China(22108303)and Science Foundation of China University of Petroleum,Beijing (2462021YJRC002).