Renewable energy is a crucial development for human society. However, as renewable energy sources (e.g., hydropower, wind power, and solar power) are intermittent by nature, it is not possible to produce renewable energy in a continuous and stable manner over long periods of time. This issue has led to significant curtailment of wind and solar energies. Energy storage technologies allow renewable energy to be stored and released when needed, thus ensuring continuous and stable electrical power output from renewable power generation systems, and improving the capability of the grid to accept intermittent energy from renewable sources.

Traditionally, energy storage technologies may be divided into three categories: physical, chemical, and thermal storage. Physical energy storage includes mechanical energy storage (e.g., pumped water storage, compressed air storage, and flywheel storage) and electromagnetic energy storage (storage via supercapacitors or superconductors). Chemical energy storage refers to electricity storage based on electrochemical principles, via lead-acid batteries, lithium-ion batteries, sodium-sulfur batteries, or flow batteries. Thermal energy storage involves the storage of thermal energy in an insulated container to allow the direct utilization of thermal energy for thermal power generation. The main purpose of these technologies is to store energy, and they are thereby useful for on-site electricity usage within short charge/discharge periods. However, energy storage capacity is a limiting factor for long-term energy storage, e.g., in seasonal energy storage.

In the non-conventional energy system, hydrogen energy is an ideal secondary energy source. In comparison to other energy sources, hydrogen has a high calorific value, and its energy density (140 MJ/kg) is more than twice that of other solid fuels (50 MJ/kg). Moreover, the combustion product of hydrogen is water, which makes hydrogen an environmentally friendly source of energy. Hydrogen can be stored as a gas or liquid in high-pressure tanks, or in solid-state hydrogen storage systems such as metal hydrides, complex hydrides, and porous materials. Therefore, hydrogen is considered to be the most promising energy carrier alternative to conventional fossil fuels, and is an excellent energy storage medium for renewable and sustainable energy systems. The advantages of using hydrogen as an energy carrier are numerous. First, the interconversion of hydrogen and electricity can be performed at high levels of efficiency via water electrolysis and fuel cell technologies. Second, very high energy densities can be achieved in compressed hydrogen. Finally, hydrogen energy has the potential to be upscaled to grid-scale applications.

Furthermore, the conversion of wind and solar power (both of which are strongly volatile by nature) into hydrogen is beneficial for energy storage and transportation; the stored hydrogen can be used for fuel cell power generation or directly as a fuel gas/ chemical feedstock.

There are many ways to produce hydrogen, including fossil fuel reforming, decomposition, photolysis, or water electrolysis. At the global scale, approximately 4×10⁹ t of hydrogen is consumed annually for ammonia production, organic hydrogenation, petroleum refining, metal smelting, electronics manufacturing, generation of high-temperature flames, and cooling of thermoelectric generators. To date, more than 95 % of the hydrogen produced worldwide has been derived from fossil fuel reforming, which is a process that inevitably emits CO₂. On the other hand, water electrolysis technology uses electricity generated from renewable energy sources to produce hydrogen for grid-scale applications, without emitting any CO₂; this method accounts for 4 % to 5 % of the global H₂ production [1]. At present, China is the world’s largest hydrogen producer, and has held this position for seven consecutive years. Due to economic factors, more than 95 % of China’s hydrogen is derived from fossil fuels.

Hydrogen obtained via water electrolysis is extremely pure (at least 99.9 %). Hence, water electrolysis-derived hydrogen can be directly used for the manufacturing of high-precision electronic devices, a process that generally requires hydrogen of high purity.

Hydrogen production via water electrolysis refers to the dissociation of water molecules into hydrogen and oxygen in an electrochemical process under an applied direct current. In this process, hydrogen and oxygen, respectively, are evolved from the cathode and the anode. Water electrolysis technologies can be divided into several distinct categories, depending on the type of electrolyte being used: alkaline water electrolysis, proton exchange membrane (PEM) water electrolysis, and solid oxide electrolysis.

The industrial utilization of water electrolysis technology began in the 1920s, when alkaline water electrolysis was used to produce hydrogen on an industrial scale to meet the needs of ammonia production and petroleum refining. After the 1970s, the demands imposed by energy shortages, environmental pollution, and space exploration led to the development of PEM water electrolysis. This also led to the development of compact high-pressure alkaline electrolyzers, which were required in specialized areas. At present, there are two electrolytic hydrogen production technologies that are applied in practice: alkaline water electrolysis and solid polymer electrolyte (SPE) water electrolysis.

Alkaline water electrolysis refers to the use of a direct current to electrolyze water into hydrogen and oxygen from a KOH or NaOH electrolyte solution, in conjunction with a diaphragm such as an asbestos cloth. However, the gases produced by this process need to be dealkalized. Industrial scale alkaline water electrolysis was first achieved in the mid-20th century. Thus, this technology is mature, and a typical alkaline electrolyzer has a service life of up to 15 years. An alkaline electrolyzer typically consists of a liquid electrolyte and a porous diaphragm [2], as shown in Fig. 1.

Fig. 1. Schematic of alkaline water electrolysis.

Typically, alkaline electrolyzers operate at a current density of approximately 0.25 A/cm² , with an energy efficiency of around 60 %. In a liquid electrolyte system, an alkaline electrolyte (such as KOH) reacts with CO₂ in the air to form a carbonate that is insoluble under alkaline conditions, such as K₂CO₃. The insoluble carbonate clogs the porous catalytic layer, hindering the transfer of products and reactants and greatly reducing the performance of the electrolyzer. Furthermore, it is difficult to quickly initiate or shut down an alkaline electrolyzer, and it is also difficult to adjust the rate of hydrogen production quickly. This is because the pressures at the anode and cathode of the electrolyzer must be kept constant to prevent the passage of hydrogen and oxygen gases through the porous asbestos diaphragm, as the mixing of these gases will result in an explosion. Thus, it is difficult to match alkaline electrolyzers with renewable energy sources that exhibit rapid fluctuations in power output.

The sheer number of problems to be overcome in alkaline electrolyzers indirectly led to the rapid development of SPE water electrolysis technology. Since the first SPE electrolyzers used a PEM, SPE electrolyzers are also known as PEM electrolyzers. In this type of system, a PEM is used in place of the asbestos diaphragm as a proton conductor and to isolate the gases generated at the cathode and anode. In this way, PEM electrolyzers avoid the disadvantages associated with the use of strongly alkaline liquid electrolytes in liquid alkaline electrolyzers. Furthermore, PEM electrolyzers have a zerogap structure and are highly compact, which reduces the ohmic resistance of these electrolyzers and greatly improves their overall performance. The operating current density of PEM electrolyzers is usually greater than 1 A/cm² , which is at least four times that of alkaline electrolyzers. PEM electrolyzers are environmentally friendly, safe, free from alkaline electrolytes, reliable, and highly efficient, and they involve high gas purity, low energy consumption, small device volumes, and high gas pressures. Hence, it is widely recognized that PEM electrolyzers are among the most promising electrolytic hydrogen production technologies in the field of hydrogen production [3].

The main components of a typical PEM water electrolyzer include cathode and anode plates, cathode and anode gasdiffusion layers, cathode and anode catalyst layers, and the PEM. The plates serve to affix the components of the electrolyzer and to conduct electricity and distribute water and gas. The gasdiffusion layers serve to collect current and facilitate the transfer of gases and liquids. The core of the catalyst layer is a threephase interface consisting of the catalyst, an electron-conducting medium, and a proton-conducting medium; the electrochemical reaction takes place at this interface. The PEM is a solid electrolyte, and perfluorosulfonic acid membranes are the most common type of PEM. The role of the PEM is to isolate the hydrogen generated at the cathode from the oxygen generated at the anode, and to prevent electron transfer while facilitating proton transfer. The principles of hydrogen production via PEM water electrolysis are shown in Fig. 2. Currently, commonly used PEMs include Nafion^® (DuPont), Dow membrane (Dow Chemical), Flemion^® (Asahi Glass), Aciplex^® -S (Asahi Chemical Industry), and Neosepta-F^® (Tokuyama). In comparison to alkaline water electrolysis systems, PEM water electrolysis systems do not require dealkalization and have a greater margin for pressure regulation. During the initial stages of the commercialization of PEM technology, the cost of the PEM electrolyzer itself accounted for the bulk of the cost of these systems. In a PEM electrolyzer, the water electrolysis reaction occurs in the membrane electrode assembly (MEA), which consists of the proton exchange membrane, the gas diffusion layers, and the catalyst layers; the MEA thus constitutes the core of a PEM electrolyzer. Increases in the operating current density could reduce the equipment overhead in PEM water electrolysis. Moreover, the suitability with a wide range of operating current densities is beneficial for matching PEM water electrolysis with the fluctuating power output of renewable energy sources.

Fig. 2. Principle of PEM-based electrolytic hydrogen production.

Due to polarization phenomena, the actual electrolysis voltage of the electrolyzer is greater than the theoretical electrolysis voltage predicted by thermodynamics theory, E_rev. The polarization of a PEM electrolyzer includes activation polarization, ohmic polarization, and concentration polarization. In a PEM electrolyzer, the polarization of the anodic oxygen evolution reaction is much higher than that of the cathodic hydrogen evolution reaction; this is an important factor for determining electrolyzer efficiency. As electrochemical polarization is mainly related to the activity of the electrocatalyst, the use of a highly active catalyst and a three-phase interface that is conducive to electrode reactions helps to reduce electrochemical polarization. Hydrogen and oxygen are evolved during water electrolysis; due to the strong oxidative ability of atomic oxygen, the catalyst support and electrolyzer material on the anode must be strongly resistant to both oxidation and corrosion. An ideal oxygen evolution electrocatalyst should have a high specific surface area and porosity, high electron conductivity, good electrocatalytic performance, long-term mechanical and electrochemical stabilities, minimal bubble effect, and high selectivity, while also being affordable and non-toxic. Typical oxygen evolution catalysts that satisfy the above requirements include noble metals like Ir or Ru and their oxides, as well as binary and ternary alloys or mixed oxides with an Ir/Ru substrate. Since Ir and Ru are expensive and scarce resources and the current Ir loading of PEM electrolyzer catalysts often exceeds 2 mg/cm² , there is an urgent need to reduce the IrO₂ loading of PEM water electrolyzers [4]. Commercial Ptbased catalysts can be used directly at the cathode to catalyze the hydrogen evolution reaction. At present, the typical loading of Pt in the cathode of PEM water electrolyzers is about 0.4–0.6 mg/cm.

The main causes of ohmic polarization in PEM water electrolysis are the ohmic resistances of the electrodes, membranes, and current collectors. Membrane resistance increases with increasing membrane thickness, and is the main source of ohmic polarization loss. Thus, to reduce membrane resistance and by extension the ohmic polarization, a thinner membrane can be selected. However, it is also necessary to consider factors such as gas permeation and membrane degradation, as the gases generated will gradually permeate deeper into the membrane with increasing electrolysis time and membrane temperature. Furthermore, permeation depth is inversely proportional to membrane thickness. The use of materials with excellent electrical conductivity to fabricate electrodes and current collectors will increase the proton conductivity of the catalyst layer and of the membrane, reduce the contact resistance of each component, and decrease the thicknesses of the catalyst layers, all of which are beneficial for reducing ohmic polarization. Concentration polarization is directly related to the supply of water and the discharge of evolved gases, and is affected by the hydrophilicity/hydrophobicity of the gas-diffusion layer and the design of the flow field. The diffusion layer in PEM water electrolyzers is often made of a Ti substrate surface and coated with a corrosion-resistant material to improve its corrosion resistance under hydrogen and oxygen evolution conditions. As the materials and structure of the diffusion layer influence ohmic polarization and diffusion polarization, respectively, adequate consideration must be paid to both of these aspects. The cost of the Ti substrate and surface treatment materials account for a large percentage of the total cost of a PEM electrolyzer stack. Due to the high cost of the catalysts and electrolyzer materials, PEM water electrolysis is currently a more expensive process than conventional alkaline water electrolysis. At present, the development of PEM water electrolysis technology is focused on improving the electrolyzer efficiency, in terms of catalyst performance, membrane materials, and diffusion layer materials.

PEM water electrolyzers were used as oxygen generators in the US Navy’s nuclear submarines during the 1970s. In the 1980s, the National Aeronautics and Space Administration (NASA) used PEM water electrolysis technology in space stations to supply the astronauts with life-supporting oxygen and to fuel boosters for controlling orbital attitude. In recent years, many countries have made great achievements toward the development of PEM water electrolysis technology.

The “New Sunlight” and “WE-NET” programs of Japan began in 1993. By 2020, a total of US$ 3 billion will have been invested in the research and development (R&D) of key technologies for hydrogen energy production through these programs. Both programs, which share a common goal of building a global energy network that combines hydrogen production, hydrogen transportation, and hydrogen energy utilization, highlighted hydrogen production via PEM water electrolysis as an important target for development. By 2003, the “WE-NET” project succeeded in developing PEM electrolyzers with electrode areas of 1–3 m² , current density of 25 000 A/m² , single-cell voltage of 1.705 V, and which operate at 120 °C and a pressure of 0.44 MPa [5]. In early 2018, 11 Japanese companies announced the establishment of Japan H2 Mobility (JHyM), whose objective is to deploy hydrogen fuel stations throughout Japan to facilitate the commercialization and propagation of fuel-cell vehicles. The JHyM consortium aims to construct 160 hydrogen stations by 2020.

In Europe, research on PEM water electrolysis began in France in 1985. The Kurchatov Institute in Russia also began work on PEM water electrolysis around the same time, and has constructed a series of PEM electrolysis stacks with different gas production volumes. The GenHyPEM program [6], which is funded by the European Commission and composed of eleven universities and research institutes in Germany, France, the United States, Russia, and other countries, has invested 2.6 million euros into research on PEM water electrolysis technology. The aim of the program is to develop a PEM water electrolyzer with high current density (>1 A/cm), high working pressure (>5 MPa), and high electrolysis efficiency. The GenHy^® series of products achieve electrolysis efficiencies of as high as 90 % and system efficiencies of 70 %–80 %. The NEXPEL project, which is jointly carried out by Sintef, Statoil, Mumatech, and the University of Reading, has invested a total of 3.35 million euros toward the development of novel hydrogen production technologies based on PEM water electrolysis. The aim of the project is to reduce the cost of hydrogen production (to 5 000 euros/Nm³ ) and increase the operating life of PEM electrolyzer stacks to 40 000 h.

In 2014, the European Union (EU) proposed a threestep roadmap toward hydrogen production via PEM water electrolysis. The first step is to meet transport-related demands for hydrogen, and calls for the construction of a distributed PEM water electrolysis system that is suitable for large hydrogen stations. The second step is to meet the hydrogen demands of industry, via the production of 10 MW, 100 MW, and 250 MW PEM electrolyzers. The third step is to meet the needs of largescale energy storage, which includes hydrogen power generation during periods of peak electricity usage, the use of hydrogen as a household gas, and the large-scale use of hydrogen in the transportation industry. A plan has been proposed to gradually replace alkaline electrolyzers with PEM electrolyzers in hydrogen production. The EU has also stipulated that the response time of electrolyzers (in terms of hydrogen production) should be less than 5 s; currently, only PEM water electrolysis technology can meet this requirement.

In 2011, Hydrogenics (Canada) was awarded a project to install a HySTAT^TM 60 electrolyzer in Switzerland, which will deliver electrolysis-derived hydrogen to a hydrogen station. These electrolyzers can produce 130 kg of pure hydrogen on a daily basis. To date, Hydrogenics has built large-scale hydrogen stations capable of handling hydrogen filling pressures of up to 70 MPa in Germany, Belgium, Turkey, Norway, the United States, Switzerland, France, and Sweden. In 2012, the AC Transit company deployed a hydrogen station based on solar powered water electrolysis in Emeryville, California, which uses a 510 kW solar power generation system to produce hydrogen via water electrolysis; this station is able to meet the hydrogen needs of 12 buses or 20 cars. The electrolytic hydrogen generator used in this station was supplied by Proton Onsite (USA), and it is capable of producing 65 kg of hydrogen per day (at a pressure of 5 000 to 10 000 psi). As of 2016, Germany had already constructed 50 hydrogen stations.

In terms of commercial products, American companies like Proton Onsite, Hamilton, Giner Electrochemical Systems, the Schatz Energy Research Center, and Lynntec are world leaders in research on and manufacture of PEM water electrolyzers. Hamilton has produced a PEM electrolyzer that is capable of producing hydrogen with a purity of 99.999 % at a rate of 30 Nm³ /h. Giner Electrochemical Systems has developed a 50 kW water electrolyzer prototype, whose cumulative operational time at high pressures has exceeded 150 000 h. This prototype is capable of operating at high current densities and high working pressures without requiring a high-pressure feeding pump.

At present, Proton Onsite is the world’s leading supplier of PEM water electrolyzers, and its products are widely used in laboratories, hydrogen stations, military applications, and the aviation industry. Proton Onsite has more than 2 000 hydrogen production devices based on PEM water electrolysis distributed throughout 72 countries across the globe, accounting for 70 % of global hydrogen production via PEM water electrolysis. The HOGEN-S and HOGEN-H electrolyzers are capable of producing hydrogen with purity of up to 99.9995 % at rates between 0.5 m³ /h and 6 m³ /h; even without a gas compressor, they can achieve gas pressures of up to 1.5 MPa. The new HOGEN^® C series is mainly used in hydrogen stations. These electrolyzers have an energy consumption of 5.8–6.2 kW·h/Nm³ , and each electrolyzer is capable of producing hydrogen at a rate of 30 Nm3 /h (65 kg/d), which is five times the hydrogen production rate of the H series, despite being only 1.5 times larger than the latter. The first hydrogen station in England began operations in 2016, and is equipped with Proton Onsite’s HOGEN^® H electrolyzers and gas compression units, which achieve a daily hydrogen production capacity of 12 kg. This hydrogen station was used in conjunction with a 65 kW wind turbine. By 2009, the PEM water electrolyzers developed by Proton Onsite have collectively operated for over 18 000 h in high-pressure environments (i.e., operating pressure of approximately 16.5 MPa), and the reported stack life time of these PEM electrolyzers is greater than 60 000 h. In 2015, Proton Onsite launched the M-series of PEM electrolyzers, which are suitable for grid-scale energy storage. These electrolyzers have a hydrogen production capacity of 400 m³ /h, making them the world’s first megawatt PEM water electrolyzers. The daily hydrogen production capacity of these electrolyzers can reach 1 000 kg, and they have the potential to meet the growing demands of large-scale energy storage.

The rapid development of electrolytic hydrogen production technology has led to a boom of pilot projects of renewable electrolysis. Power-to-gas technology, i.e., the production of hydrogen gas through renewable energy, has become an important aspect of renewable energy development around the world. In 2012, the E.ON company (Germany) constructed a power-to-gas facility in the Falkenhagen region to electrolyze water into hydrogen gas during power demand valleys, thus utilizing the redundant power output from renewable energy sources. In 2013, the hydrogen gas produced by this facility was injected into local natural gas pipelines and used to supply power to the grid during power demand peaks; this improved renewable energy utilization and reduced power wastage due to fluctuating demand. In 2014, a power-to-gas facility was constructed in Toronto (Canada), whose hydrogen generators have a total combined capacity of 2 MW. The use of hydrogen energy technology in energy storage allows grid operators to use redundant power to produce hydrogen during power demand valleys, and to generate power (for consumption by the power grid) using this hydrogen during power demand peaks.

Since 2017, the industrial application of PEM-based hydrogen generation has propagated at a breakneck pace across the globe. Nel (Norway) has acquired Proton Onsite (U.S.), while Siemens, Giner, and Hydrogenics have continued to produce megawattlevel hydrogen energy storage products for renewable power sources. H&R Ölwerke Schindler (Germany) purchased a 5 MW Siemens electrolyzer for 10 million euros, which will produce hundreds of tons of hydrogen each year. The hydrogen produced by this hydrogen plant will be used for petroleum refining, thereby pioneering the use of renewable electrolysis technology at the industrial scale.

The storage and transportation of hydrogen are important issues for hydrogen energy storage and utilization. Besides tanker trucks, high-pressure hydrogen pipelines are also being developed for hydrogen transport. In Germany’s power-to-gas project, hydrogen is mixed with natural gas for transport through natural gas pipelines. In addition, pure hydrogen pipelines running for hundreds of kilometers have been successfully established by Air Liquide.

Although PEM water electrolysis is more advanced than alkaline water electrolysis in terms of technological development, the former is presently more expensive. The National Renewable Energy Laboratory (NREL) of the United States released a report on the use of wind energy to provide electricity for hydrogen production via PEM water electrolysis, which includes an estimate of the costs involved in the scalingup of PEM technology. It is expected that the proportion of the total cost incurred by the electrolyzer stack will decrease from the current level of 40 % to 10 % when hydrogen production by PEM water electrolysis increases from 10 kg/d to 1 000 kg/d. Thus, there is significant scope for reducing the cost of largescale hydrogen production via PEM water electrolysis.

In terms of standards and specification for PEM-based hydrogen production, the International Electrotechnical Commission Technical Committee (IEC/TC) has begun to develop standards for hydrogen production by PEM water electrolysis (IEC/TC 105), which are expected to be released in 2019 or 2020.

Currently, renewable energy utilization rates in China are low due to the widespread curtailment of hydro-, wind, and solar power. In 2015 alone, total wind power curtailment in China amounted to 3.39 × 10¹⁰ kW·h. The use of this energy for hydrogen production would have produced 6.78 × 10⁹ Nm³ /a (i.e., 6.1 × 10⁵ t/a) of hydrogen, based on the assumption that 5 kW·h is required to produce 1 Nm³ of hydrogen. The sum of curtailed hydropower, solar power, and wind power in 2016 could have produced 3 × 10⁶ t of hydrogen. Hence, the stability and economic efficiency of power stations, the service life of power generation equipment, and the overall utilization of renewable energy could be significantly improved by using this curtailed energy for hydrogen production (i.e., storing unstorable energy from fluctuating sources as electrolytically produced hydrogen).

In China, hydrogen production by alkaline water electrolysis has reached industrial scale. The total number of water electrolysis devices installed in China is between 1 500 and 2 000, which produce a combined 8 × 10⁴ t/a of hydrogen via water electrolysis. Alkaline water electrolysis is the dominant form of electrolytic hydrogen production in China. The alkaline water electrolyzers can produce hydrogen at a rate of up to 1000 Nm³ /h, and the representative companies of China’s electrolytic hydrogen production industry are Suzhou Jingli Hydrogen Production Equipment Co., Ltd. and Tianjin Mainland Hydrogen Equipment Co., Ltd.. However, as the hydrogen produced by these electrolyzers requires dealkalization, the electrolyzers are very large and polluting.

The development of PEM-based hydrogen production technology in China is still in the early stages of a transition from R&D to industrialization. Research into PEM water electrolysis in China began in the 1990s; the research institutes involved in R&D toward PEM water electrolysis technology (mainly to meet the hydrogen and oxygen production needs of specialized areas) include the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences, the 718th Research Institute of China Shipbuilding Industry Corporation (CSIC), and the 507th Research Institute of China Aerospace Science and Technology Corporation (CASC). At present, the PEM electrolyzers which are currently being sold in small quantities on the Chinese market are mainly foreign products with hydrogen production rates of between 0.3 Nm³ /h and 2.0 Nm³ /h. DICP began R&D toward hydrogen production via PEM water electrolysis in the 1990s; by 2008, they had developed an electrolyzer stack and system with a hydrogen production capacity of 8 Nm3 /h, an output pressure of 4.0 MPa, and resultant hydrogen purity of 99.99 %. In 2010, the PEM hydrogen generator developed by DICP was superior to that of similar international products in terms of energy consumption. Although China’s PEM-based hydrogen generators are superior to foreign products in terms of single-machine power consumption, they are still inferior in terms of hydrogen production rate.

In 2017, a project was initiated in Guyuan (Hebei Province) to construct a 10 MW demonstration project that will use a foreign-made electrolytic hydrogen generator to convert wind power into hydrogen. The plan of the Guyuan project is to use a portion of the hydrogen generated for industrial production, thus reducing the consumption of fossil fuels such as coal and natural gas in industrial hydrogen production. The other portion will be used to establish a network of hydrogen stations once the automobile industry is ready to produce hydrogen-powered automobiles.

Although commercial PEM water electrolysis devices are already in use, the costs of the PEM and the precious metal electrocatalysts used in the acidic electrolyte of PEM electrolyzers are too high. This hinders the popularization of this technology at larger scales. Therefore, there is an urgent need to develop novel, low-cost electrolyzer systems, alongside efforts to reduce the power consumption of water electrolysis in general.

Electrolyzer costs can be greatly reduced by adopting alkaline conditions, since low-cost, non-noble-metal catalysts can be used in these conditions. Alkaline anion-exchange membrane (AEM) water electrolysis technology combines the characteristics of SPE and alkaline electrolyzers by using an alkaline solid electrolyte membrane (instead of a PEM) to conduct hydroxide ions and isolate the gases evolved at the cathode and anode. In this design, the cathode and anode of the electrolyzer are in close contact with a solid polymer AEM, thereby reducing the voltage drop between the two electrodes and combining the advantages of both conventional alkaline water electrolysis and PEM water electrolysis.

The membrane material used in AEM water electrolysis is a solid polymer AEM which is capable of conducting OH– ions. The catalysts used in these systems can be a non-noblemetal catalyst like Ni, Co, or Fe, which are similar to the catalysts used in conventional alkaline water electrolysis. This greatly reduces the cost of the catalyst in comparison to PEM water electrolysis which uses noble-metal catalysts such as Ir or Pt. Furthermore, AEM water electrolysis requires a much lower degree of corrosion resistance of the electrode plate materials in comparison to PEM water electrolysis. Currently, R&D efforts in this area are focused on the development of alkaline solid polymer AEMs and highly active non-noblemetal catalysts. The major R&D institutions involved in this area are the NREL, Proton Onsite, Northeastern University (USA), Penn State University (USA), University of Surrey (UK), DICP, and Wuhan University (China). At present, R&D has mainly focused on AEMs, particularly on extending their operating lives. Once breakthroughs are achieved in terms of key materials, the upscaling of hydrogen production to industrial scale can be initiated by adopting proven PEM water electrolysis and liquid alkaline water electrolysis technologies. In terms of developments related to the enhancement of energy efficiency, solid oxide electrolyzer cell (SOEC) technology, which uses solid oxides as electrolytes, has recently emerged. SOECs can operate at temperatures of 400–1 000 °C, and are able to use heat for the conversion of electricity to hydrogen. Furthermore, SOECs have very high energy-conversion efficiency (up to 100 %) and do not use noble-metal catalysts.

Research teams in Japanese companies like Mitsubishi Heavy Industries, Toshiba, and Kyocera have conducted research on the electrodes, electrolytes, and interconnectors of SOECs. Research into SOEC technology is also being carried out by the Idaho National Laboratory (INL), Bloom Energy, Topsoe Fuel Cell, Korea Energy Research Institute, and RelHy (a project supported by the EU Commission) toward the development and testing of next-generation high-temperature electrolyzers. Furthermore, the focus of research in this area has gradually shifted from electrolyzer materials towards electrolyzer stacks and system integration [7]. The prototype SOEC stack developed by the INL has an output of 15 kW and uses CO₂/H₂O co-electrolysis to produce syngas. The INL has also collaborated with Ceramatec to produce an SOEC that can quantitatively regulate the CO and H₂ produced and which operates at temperatures between 650 °C and 800 °C [8]; the electrolysis products of this SOEC are also passed to an in-line methanation reactor operating at 300 °C that uses a nickel catalyst to produce a 40 %–50 % (vol) methane product [9]. This proves the feasibility of hydrocarbon fuel production via CO₂/H₂O co-electrolysis.

The Sunfire company (Germany) launched its first SOEC products in 2017 and demonstrated their use in hydrogen stations. In China, DICP, Tsinghua University, and the University of Science and Technology of China (USTC) have carried out explorative research on SOECs related to their work on solid oxide fuel cells. SOECs are extremely demanding in terms of the properties of the materials used. Under the hightemperature and high-humidity conditions of electrolysis, the Ni in commonly used Ni/yttria-stabilized zirconia (YSZ) hydrogen electrodes is susceptible to oxidation and decreased reactivity. Hence, there is a need to investigate the performance attenuation and mechanisms of controlling the microstructure of Ni. Oxygen electrodes made of conventional materials are susceptible to severe anode polarization and delamination during water electrolysis. Furthermore, the voltage drop across the oxygen electrode is much higher than those across the hydrogen electrode and the electrolyte; thus, it is necessary to develop new materials and new oxygen electrodes to reduce polarization loss. Secondly, to enable stack integration, it is necessary to improve the operating lives of the glass or glass-ceramic sealing materials in SOECs, which are significantly reduced under high-temperature, high-humidity conditions. If major breakthroughs are achieved in these aspects, SOEC technology could become an important approach for efficient hydrogen production.

In the 2017 Bonn Climate Conference, it was predicted that hydrogen energy consumption will account for one-fifth of total energy consumption by 2050. As a result of this change, annual CO₂ emissions would be reduced by approximately 6 × 10⁹ t from current levels, which would account for 20 % of the total reductions in CO₂ emission needed to restrict global warming to less than 2 °C. The Hydrogen Council estimates that the annual demand globally for hydrogen may increase by a factor of 10 by 2050, approaching 80 EJ (8 × 10¹⁹ J).

With hydrogen energy storage technology, it is possible to achieve seasonal energy storage. Currently existing industrialscale alkaline water electrolysis could be adopted in the short term for the rapid consumption of renewable energy. Nonetheless, there is a trend towards the replacement of alkaline water electrolysis technology with PEM water electrolysis technology. In developed countries, hydrogen production via advanced PEM water electrolysis technology is gradually being upscaled to meet the demands of grid-scale energy storage and to replace alkaline water electrolysis. PEM-based hydrogen production is also becoming increasingly popular in the renewable energy sectors of countries across the world.

Since 2016, the National Development and Reform Commission (NDRC) and National Energy Administration (NEA) of China have continuously issued documents to support the development of renewable energy-based hydrogen production. The opportunity to expand commercial-scale pilot projects for PEM-based hydrogen production, reduce the cost of electrolytic hydrogen production via its commercialization and popularization, and promote the integration of electrolytic hydrogen production with renewable energy sources should be taken advantage of. It is expected that in the next 5 to 10 years, hydrogen produced via PEM water electrolysis will gradually begin to enter the industrial hydrogen market and be used in energy storage and industrial hydrogenation. From a technological perspective, it is necessary to conduct research into the key materials and components of SOECs, as well as electrolyzer testers and testing methods. Basic and applied research should also be bolstered to solve problems in the design of materials and stacks for high-temperature SOEC water electrolysis, and to demonstrate and apply highefficiency SOEC-based hydrogen production and energy storage. In short, electrolytic hydrogen production from renewable energy has the potential to become the technology of choice for power grid operators and for hydrogenproducing and hydrogen-consuming industries.