Proton Exchange Membrane Water Electrolysis as a Promising Technology for Hydrogen Production and Energy Storage

Proton exchange membrane (PEM) electrolysis is industrially important as a green source of high-purity hydrogen, for chemical applications as well as energy storage. Energy capture as hydrogen via water electrolysis has been gaining tremendous interest in Europe and other parts of the world because of the higher renewable penetration on their energy grid. Hydrogen is an appealing storage medium for excess renewable energy because once stored, it can be used in a variety of applications including power generation in periods of increased demand, supplementation of the natural gas grid for increased efficiency, vehicle fueling, or use as a high-value chemical feedstock for green generation of fertilizer and other chemicals. Today, most of the cost and energy use in PEM electrolyzer manufacturing is contributed by the cell stack manufacturing processes. Current state-of-the-art electrolysis technology involves two options: liquid electrolyte and ion exchange membranes. Membrane-based systems overcome many of the disadvantages of alkaline liquid systems, because the carrier fluid is deionized water, and the membrane-based cell design enables differential pressure operation.

climate change and air pollution. The growing demand for energy requires a rapid shift from fossil fuels to renewable energy sources, such as wind, solar, biomass, hydropower, and geothermal energy [3]. In this context, hydrogen was proposed as a promising candidate for a secondary source of energy as early as 1973 [4]. Being a potential energy carrier in the future, hydrogen plays an important role in the path toward a low-carbon energy structure that is environmentally friendly [5][6][7][8][9][10].

Hydrogen production from renewable energy sources
Currently, the steam reforming process is the most economical way of producing hydrogen. In fact, as much as 96% of hydrogen is made from hydrocarbon fuels [5], which neither address the dependence on finite resources nor reduce the amount of carbon from the energy structure. An alternative way of producing hydrogen is the power-to-gas strategy where intermittent energy resources are transferred and stored as hydrogen ( Figure 1). Here, hydrogen is mainly produced from water electrolysis where water is split into hydrogen and oxygen by supplying electrical energy: In an electrolyzer, the above reaction is separated by an electrolyte (either in liquid or solid form) into two half reactions. The hydrogen evolution reaction (HER) occurs at the cathode: While the oxygen evolution reaction (OER) occurs at the anode: Water electrolysis technologies are classified into three categories based on the applied electrolyte: alkaline water electrolysis, proton exchange membrane (PEM) water electrolysis, and solid oxide water electrolysis [11]. PEM water electrolysis systems provide several advantages over the other two electrolysis technologies, such as higher rate of hydrogen production, more compact design, and greater energy efficiency [12][13][14][15]. Compared to alkaline electrolysis, the solid electrolyte membrane in PEM electrolysis reduces the hydrogen crossover significantly and thus allows for high-pressure operation. In addition, as required by the role of electrolytic hydrogen production in renewable energy storage, dynamic response of PEM water electrolysis is superior to alkaline electrolysis or solid oxide electrolysis. The large quantity of liquid electrolyte in alkaline electrolysis requires the proper temperature to be maintained and could raise issues for a cold start. On the other hand, solid oxide electrolysis operated in a temperature range of 500-700 Ci sm o r e suitable for constant operation than a dynamic response where the heat-up step could be slow.
The produced hydrogen can have several pathways to different applications ( Figure 1). It can be utilized for hydrogen fueling stations to power fuel cell vehicles or feed the combined heat and power (CHP) units for household uses. Moreover, the electrolytic hydrogen can be used as chemical feedstock in methanation after combining with CO 2 stream from biogas or flue gas to produce renewable natural gas. Further, the generated hydrogen can also be consumed as a raw material by hydrogen users such as oil refining and semiconductor industry. Finally, the hydrogen can be transferred to electricity when the grid demand is high.
Hydrogen can also be produced from biomass via pyrolysis or gasification. Wood, agricultural crops and its byproducts, organic waste, animal waste, waste from food processing, and so on are all sources of biomass. Biomass pyrolysis is basically [9]: Employing catalysts, such as Ni-based catalysts, can enhance the yield of hydrogen from biomass pyrolysis. Moreover, hydrogen production can be improved by introducing steam reforming and water-gas shift reaction to the pyrolysis [9]. For the gasification process, biomass is pyrolyzed at higher temperatures producing mostly gaseous products [9]: It is beneficial that biomass pyrolysis and gasification can be operated in small scale and at remote locations, which reduces the cost of hydrogen transportation and storage and improves the availability of hydrogen to end consumers [16]. In addition, pyrolysis and gasification can consume a wide range of biomass feedstocks [16]. Therefore, biomass is recognized as a major renewable and sustainable energy source to replace fossil fuel.

Hydrogen storage
Currently, hydrogen storage in high-pressure vessel is the most widely used method [7]. However, hydrogen is pressurized up to 700 bar for practical purposes such as the refueling time at a hydrogen station or the driving range for a fuel cell vehicle [17]. Hydrogen compression to 700 bar consumes a lot of energy that makes the volumetric energy density decrease Proton Exchange Membrane Water Electrolysis as a Promising Technology for Hydrogen Production and Energy… http://dx.doi.org/10.5772/intechopen.78339 from 10 to 5.6 MJ/L, much lower than gasoline (34 MJ/L) [6,17]. Therefore, solid-state storage is usually coupled with high-pressure hydrogen vessels. For example, hydrogen can be stored in the interstitial sites of metal hydride crystals [17]. This method achieves higher volumetric energy density at room temperature than liquid hydrogen and consumes less operating energy for storage. Thus, metal hydride cartridge is suitable for portable application due to the convenience of refill/replace [7,18]. In addition, with appropriate hydrogen refill and release properties at room temperature, metallic hydrides are good for stationary energy storage [17]. One drawback of solid-state storage is that metallic hydrides contain heavy transition metals, which reduce the gravimetric energy density of the device [19].
In summary, hydrogen offers several advantages as an energy carrier: its combustion produces energy and only water that is carbon-free as a byproduct; it can be produced from renewable and sustainable sources; its energy can be distributed quite easily, in accordance with the end user's requests and with the development of new technologies for transportation and storage; it may be used in both centralized or distributed energy production [7]. In spite of these advantages, hydrogen has failed to be widely used in energy systems due to numerous barriers, including costs of production and storage and the availability of infrastructure [10]. This chapter focuses on the technological challenges of PEM water electrolyzers for hydrogen production.
2. Introduction of PEM water electrolysis 2.1. Fundamentals of PEM water electrolysis Figure 2 shows the schematic diagram of PEM water electrolysis. The electrolysis process is an endothermic process and electricity is applied as the energy source. The water electrolysis reaction is thermodynamically possible at potentials higher than 1.23 V vs. RHE (reversible hydrogen electrode). The thermoneutral potential at which the cell can operate adiabatically is 1.48 V vs. RHE. Typical PEM water electrolysis devices operate at potential well over 1.48 V vs. RHE and heat is generated by the reaction [15]. The PEM water electrolysis system, similar to proton exchange membrane fuel cell (PEMFC), anode and cathode is separated by a solid polymer electrolyte (Nafion) of thickness below 0.2 mm. At the anode, water is oxidized to produce oxygen, electrons, and protons. The protons are transported across the electrolyte membrane to be reduced to hydrogen. The catalyst for water oxidation or oxygen evolution is typically iridium, which can withstand the corrosive environment due to high overpotential on the anode. Water is channeled to the anode by a titanium flow field, and a piece of porous titanium mesh is placed between the anode catalyst layer and the water channel serving as the diffusion layer. The cathode configuration is similar to the PEMFC with Pt-based catalyst and a graphite flow field to transport hydrogen. A piece of carbon paper is used as the gas diffusion layer (GDL) placed between the cathode catalyst and the flow field.
The hydrogen production rate of ideal electrolysis is proportional to the charge transferred, according to Faraday's law. It can be expressed as [20]: 22:41 1000 3600 (6) where N cell is the total number of cells in the system and I cell is the electric current. η F is the Faraday efficiency, or current efficiency, and is defined as the ratio of ideal electric charge and the practical charge consumed by the device when a certain amount of hydrogen is generated. η F is usually about 0.95 [20]. The specific energy consumption E (kWh/Nm 3 ) for a given time interval Δt is: Another important parameter for PEM water electrolysis is the efficiency: where HHV is the higher heating value of hydrogen (39.4 kWh/kg at STP). Since PEM water electrolysis is usually supplied by liquid water, the HHV assumes that all the heat from the water is recovered by restoring the water temperature to the initial ambient state [20].

Challenges for PEM water electrolysis: the OER catalyst
Significant advances are needed in catalyst and membrane materials as well as the laborintensive manufacturing process for PEM water electrolysis to be cost-effective for widespread application in renewable energy systems [21]. The state-of-the-art anode catalyst in  Loading of Ir and Ru combined. *** Degradation rate at 1.0 and 3.0 A cm À2 , respectively. Table 1. Reported degradation rates for iridium-based catalysts in long-term electrolyzer tests.

Nanostructures in Energy Generation, Transmission and Storage
conventional PEMWEs is iridium oxide (IrOx) or mixed oxide with ruthenium [22,23]. Typical catalysts for commercial electrodes have IrOx loading from 1 to 3 mg cm À2 [24]. This level of catalyst loading is too high to meet the long-term cost targets for energy markets [23,25,26]. Furthermore, while using current electrolysis technology, the translation of catalyst development from lab scale to the megawatt scale remains challenging in terms of catalyst cost and stability [25].
For Ir-and Ru-based OER catalyst, a balance between activity and stability has been reported [27,28], which strongly depends on the chemical structure and surface properties of the oxide [23]. The oxide properties, however, are highly sensitive to synthesis conditions. For Ir-based OER catalyst, most synthesis methods involve a calcination step to improve the catalyst stability [29][30][31][32][33][34][35]; however, for this approach, a sacrifice of the OER activity is usually inevitable. Indeed, thermally prepared IrOx shows higher stability but lower activity compared to electrochemically prepared oxide, or hydrous IrOx [30,31,35]. The latter is also frequently referred to as amorphous IrOx due to the presence of lower valence Ir (III) oxide [36][37][38]. According to reference [39], Ir (III) is the major intermediate species for iridium dissolution. Combining both activity and stability remains a challenge for Ir-based catalysts, and it is the major hurdle that limits the reduction of the anode iridium loading in PEM water electrolysis.
The enhancement of catalyst stability is of equal importance as the reduction of catalyst loading. Long-term operation at high current density up to thousands of hours is particularly challenging with an Ir loading less than 1 mg cm À2 . Recent literature has seen a growing number of studies on longer-term electrolysis operation ranging from hundreds to several thousand hours [40][41][42][43][44][45][46][47] ( Table 1). In particular, the best reported stability was on a nanostructured thin film (NSTF) cell, which achieved 5000 hours with a constant current load of 2 A cm À2 and 0.25 mg cm À2 Ir loading [45]. IrOx supported on Ti catalyst with 50 wt% Ir and a low catalyst loading of 0.12 mg cm À2 achieved more than 1000 hours operation at a lower current load of 1 A cm À2 [41]. No supported catalyst, other than titanium supported catalyst has been able to achieve such high catalyst stability. [48] 3. The role of nanostructures in PEM electrolyzer performance 3.1. The structure of iridium oxide (IrOx) The OER performance of IrOx strongly depends on the chemical structure and surface properties of the oxide [23]. Before discussing the influence of oxide structure on the electrolyzer performance, a thorough understanding of the OER mechanism is needed. The OER on IrOx consists of three steps forming a closed circle (Figure 3) [39,49,50]: Reaction (9) is fast and in pre-equilibrium. Reaction (10) is the rate-determining step (RDS), and the theoretical Tafel slope of this mechanism is 2RT/3F = 40 mV/dec. Since it is noted that Eq. (11) involves two iridium sites, a binuclear mechanism of OER on IrOx is proposed for Eq. (11) [51]. With the aid of density functional theory (DFT) calculations, Steesgra et al. [52] show that the first step of the binuclear mechanism is the oxidation of Ir (IV) -Ir (IV) to Ir (IV) -Ir (V), which is Eq. (10). Therefore, IrO 2 •H 2 O is considered to be the precursor for the binuclear mechanism. The coverage of Ir (V) increases with the anodic potential until the conditions for the binuclear mechanism are satisfied [39] at the onset of OER.
In a series of studies of iridium dissolution [31,32,39,53], it is proposed that the precursor for dissolution are the oxygenated Ir (III) species (Figure 3). The formation and reduction of higher valence Ir (IV) species pass through an Ir (III) intermediate, which is shared in both OER and iridium dissolution. It is concluded in [39] that iridium dissolution has no direct link with OER activity and that they are two pathways sharing an intermediate species, Ir (III). Thus, it is possible to suppress one without alternating the other [39]. Therefore, the ratio of Ir (IV) to Ir (III) on the IrOx surface is a crucial parameter that controls the iridium dissolution and the OER.

Nanostructures in Energy Generation, Transmission and Storage
bond [58,59]. Calcination at a high temperature of "dry" IrOx (O IIÀ species) where electrophilic O IÀ becomes strongly bonded O IIÀ species and thus the OER activity is reduced [36], in agreement with other studies [30,31,35,62]. However, the "dry" IrOx is more stable against dissolution as the Ir (III) species on the surface is reduced [32,60].
In addition, the IrOx structure can be modified when a metal oxide support is incorporated with IrOx nanoparticles via the metal/metal oxide support interaction. For example [63], when IrO 2 nanoparticles are supported on antimony-doped tin oxide (ATO), the d band vacancy and iridium oxidation decreased due to the interfacial electron charge donation from ATO to iridium, evidenced by the shift of X-ray absorption white line toward lower energy. This is corroborated with the increased Ir-O bonding distances for ATO-supported IrOx from extended X-ray fine structure (EXAFS) analysis, which is consistent with a lower average iridium oxidation state. Such interaction between IrOx and ATO suppressed the growth of higher valent IrOx layer that leads to iridium dissolution, thereby improved catalyst stability [63]. Similar interaction between metal/metal oxide supports was also found for IrO 2 supported on TiO 2 catalyst where the catalyst stability was enhanced as well [64].

Mixed bimetallic oxide
Mixed bimetallic oxides of iridium and a non-noble metal have been used to optimize the anode catalyst for PEM electrolyzer application [65][66][67][68][69][70][71][72][73]. The most apparent benefit of bimetallic oxide is to reduce the iridium loading in the catalyst if the catalyst activity remains comparable or higher compared to pure IrOx. For example, at >10 mol% iridium content in IrO 2 + SnO 2 mixed oxide the Tafel slope for OER is identical with pure IrO 2 , suggesting that the surface properties of IrO 2 + SnO 2 mixed oxide is dominated by IrO 2 and behave as pure IrO 2 [65]. In particular, iridium content in the 30-90 mol% range shows higher OER performance than pure IrO 2 [65].
More importantly, mixed bimetallic oxides modify the electronic or crystal structures of IrOx, which can significantly enhance the OER activity. The bond forming or breaking during OER is governed by the interaction between the O-2p orbital of intermediates with the d orbitals of surface sites of the transition metals [66][67][68][70][71][72][73]. Thus, the OER activity depends on the dorbital electronic structure of the transitional metals. For example, Sun et al. [73] doped IrOx with copper and obtained enhanced OER activity in acid media at 30-50 mol% concentration. The Cu doping led to an IrO 2 lattice distortion due to the CuO 6 octahedron's Jahn-Teller effect and also generated oxygen defects (Figure 4a), which significantly affected the energy distribution of the d-orbitals of Ir sites. The induced partial oxygen defects and the lifted degeneracy of t 2g and e g orbitals reduced the energy required for the O-O bond formation, thereby enhanced the OER activity. In another case, Reier et al. [69] prepared Ir-Ni mixed oxide thin film and found a 20-fold enhancement of OER activity compared to pure IrOx thin film. As shown in Figure 4b, the surface Ni elements are leached out during OER and weakened the binding energy of the Nidepleted oxygen with the lattice, forming oxygen with lower binding energy, and similar to the electrophilic oxygen. This promotes the formation of weakly bonded surface hydroxyls, which govern the overall OER reaction rate and suppress the formation of unreactive divalent = O species on the surface. Furthermore, Tae et al. [70] reported Ir-Co mixed oxide with 5% iridium loading that exhibits excellent OER activity and stability. At anodic potentials, the oxygen vacancy in Co (III) is transferred to Ir (IV) O 6 center, leading to the formation of coordinatively unsaturated Ir (VI) O 5 structure (Figure 4c), which is highly active for OER.

Effect of particle size
Small particles are usually favorable for higher mass activity due to the high surface area to bulk ratio, which facilitates the reduction of catalyst loading. More importantly, the change of electronic structure is unavoidably accompanied with the change of particle size, as evidenced in the early work on X-ray photoelectron spectroscopy (XPS) [74][75][76]. As particle size decreases, the binding energy is shifted to higher levels due to the increase of lattice strain and the coordination reduction [77,78]. Richter et al. [78] shows that two mechanisms contribute to such an energy shift. The first is the initial state effect caused by the increase of lattice strain as particle size decreases, a result of d-hybridization; the second is the final state relaxation that increases with the decrease of particle size, which results in the stronger screening of the core hole leading to higher binding energy. Small particles show enhanced dhybridization, which shift the d-band center to lower levels, decreasing the bond strength of the adsorbates during an electrochemical reaction [79].
Abbot et al. [34] systematically studied the particle size effect for IrOx in the range of 2 -30 nm. EXAFS analysis reveals that, as the IrOx particle size decreases, the coordination number decreases and the Ir-O bond length increases. The core-level binding energy was thus shifted to higher levels as the particle size decreases, which is in agreement with previous literature [74][75][76]. As a result, the iridium-oxygen bonding is weakened as the particle size decreases, leading to higher content of hydroxide, and lower average oxidation state of iridium from Ir (IV) to a combination of Ir (IV) and Ir (III). Based on previous discussions in Section 3.1, surface hydroxide and mixed Ir (IV) and Ir (III) state are favorable for OER. Indeed, Abbot et al. [34] show that the IrOx particles of 2 nm have 10 fold higher OER mass activity and three fold higher turnover frequency compared to the IrOx particles of 30 nm.

Effect of ionomer content
Perfluorosulfonic acid ionomer provides proton conductivity in the catalyst layer, extends the reaction zone, and improves the catalyst utilization [80,81]. Ma et al. [82] focused on the effect of ionomer content on the ohmic resistance of PEM electrolyzers with iridium supported on titanium carbide (TiC) as an anode catalyst. Ma et al. [82] showed that increasing the ionomer content from 10 to 40 wt% causes a decrease in PEMWE performance. Using a simple non-linear fit: The authors derived the ohmic loss contribution in the performance and concluded that higher ohmic resistance is associated with high ionomer content. It is noted that with this non-linear fit method, the obtained ohmic resistance includes contributions from all sources of the membrane electrode assembly (MEA) components, including the ionic resistance. Xu et al. [83] investigated the effect of ionomer content from the voltammetric charge and interfacial resistance perspective. The amount of voltammetric charges in cyclic voltammetry is proportional to the surface active sites [84]. A wider range of ionomer contents, from 5 to 40 wt%, were studied. The highest total charge was achieved at 25 wt% ionomer content, which is in agreement with the optimum PEMWE performance. Based on Butler-Volmer equation, the authors constructed a model to describe the I-V curves and concluded that the optimum ionomer content minimizes the interfacial resistance between the membrane and the electrode.
In a recent study, Bernt and Gasteiger [85] investigated the effect of anode ionomer content by analyzing the voltage loss contribution to the PEMWE performance. The ohmic resistance was found to increase abruptly when the ionomer content is above 20 wt%. This suggests that an electronically insulating film of residual ionomer forms at the electrode/GDL interface when the ionomer volume exceeds the void volume of the catalyst layer, which results in higher contact resistances. Further, such electronic insulation caused higher OER overpotential with >20 wt% ionomer content and thereby decreased the catalyst utilization. After subtracting the voltage losses due to ohmic and kinetic losses, Bernt and Gasteiger [85] attributed to the remaining losses (red diamond) to mass transport ( Figure 5). The main path for oxygen removal from the electrode is not permeation but convective transport through the void volume of the anode. Thus, higher ionomer content imposes higher transport resistance for oxygen removal.
In sum, similar to PEM fuel cell electrodes, the ionomer content is an important parameter for the optimization of PEM electrolyzers. While low ionomer content provides insufficient proton conduction and less accessible surface active sites, high ionomer content induces high interfacial resistance and mass transport loss due to insufficient void volume for oxygen diffusion. PEM electrolyzers are economically beneficial for a large-scale application operating at high current densities (>2 A cm À2 ) and high pressures (30-45 bar). Thus, optimization of the ionomer content becomes crucial to minimize the cell potential loss at high current and pressure and improve cell efficiency.

PEM water electrolysis for energy storage 4.1. Integration with renewable energy technologies
Power generated from renewable energy sources, such as solar and wind, requires energy storage devices to balance its fluctuation and intermittence because of variable weather conditions [86]. Hydrogen production by water electrolysis has been developed as an alternative technology for energy conversion and storage that can be fitted to renewable energy systems [87,88]. This section will briefly introduce the role of PEM electrolyzers in power-togas, solar, and wind energy systems.
Power-to-gas is emerging as a novel energy storage method that uses the surplus electricity from the grid during off-peak periods and converts it to hydrogen through a water electrolysis process [89]. The key technology for this strategy is the electrolyzer, which bridges the power to utilization by producing hydrogen. Electrolyzers must meet the following requirements in order for power-to-gas to become efficient and economically viable [90]: (1) high efficiency of hydrogen production; (2) fast response to power fluctuation; (3) very low minimal load for stand-by; (4) high-pressure operation to reduce the cost of hydrogen compression; and (5) long durability and lifetime. The major drawback of power-to-gas is the low efficiency and high cost of electrolyzers. While PEM electrolyzers offer fast response, high pressure, and the production of pure hydrogen, scaling up to MW scale has been the major technical challenge. As mentioned previously, the major hurdle of this challenge is the cost of catalysts and other MEA components (e.g., the Ti-based bipolar plates).
Solar energy conversion into hydrogen by water splitting has been long studied by various research groups due to its easy scale-up nature. The state-of-the-art photocatalytic and photoelectrochemical system has an efficiency of 10 and 5%, respectively, in the direct conversion of solar to hydrogen (STH) [91]. To reduce the price of hydrogen and boost up the efficiency of STH, an electrolyzer cell (EC) can be coupled with a photovoltaic cell (PV), called photovol-taicÀwater electrolysis system (PV-EC) [92]. The STH efficiency can be increased up to 30%. Further, a direct current to direct current (DC-DC) converter can be implemented between PV and EC ( Figure 6). The main role of the converter is maximum power point tracking of the sun light to the EC system. Thus, coupling electrolyzers with highly efficient PVs and with Figure 6. Design principle of the PV-Conv-EC system based on an independent PV, the EC performance, and the existence of a converter. (a) Hydrogen power per square centimeter (p H2 ) and kinetic loss per square centimeter (p kin )atagivencu rren t density-voltage (j À V) curve of the PV and EC. The intersection between the PV and EC j À V curve has a lower voltage and a higher current density than the p PV, max point .(b)p H2 and p kin after the DC-DC converter assistance on (a). Reproduced with permission [92]. Copyright (2015) American Chemical Society.
converter assistance, high STH efficiency PV-Conv-EC systems can achieve 20.6% STH efficiency and 78% PV electricity to hydrogen conversion efficiency [92].
Wind power is heavily influenced by meteorological variances and requires balancing power for the load fluctuation. Wind to hydrogen (WTH) strategies by water electrolysis could offer a solution to this problem [9]. Synergy between wind farms and water electrolyzers makes hydrogen a buffer solution to balance the grid power as well as produce hydrogen from surplus wind power [9]. Thus, the wind power can be utilized at its maximum capacity. Table 2 summarizes the commercially available PEM electrolysis systems from 12 major manufacturers worldwide. Note that only the highest capacity from each manufacturer is listed here. Six of the manufacturers are able to achieve a system capacity around 1 MW or higher. The largest system so far is from Hydrogenics, a 15 MW plant with 10 cell stacks capable of producing 3000 Nm 3 hydrogen per hour. Giner currently offers a more powerful cell stack, Kennebec stacks, even though its current systems have not yet incorporated this brand of the cell stack. The Kennebec stacks have the largest capacity among all the stacks available from these manufacturers. It has a capacity of 5 MW and is capable of producing 2200 kg of hydrogen per day (that is, 1020 Nm 3 per hour). Most of the systems offer highpurity hydrogen with the high delivery pressure that is suitable for on-site storage and fueling.

Current status of commercial manufacturers
The system consumption of electricity is very close among all the manufacturers, averaged around 5.5 AE 0.5 kWh per Nm 3 of hydrogen.

Concluding remarks and outlook
The growing demand for energy and the accompanied environmental issues call for a rapid transition to low-carbon/carbon-free energy structure. In this context, hydrogen serves as an ideal secondary energy source for energy storage and transport. The key technology for hydrogen energy is water electrolysis. In particular, PEM electrolysis has been driven strongly by flexible energy storage in recent years as it offers several advantages compared to alkaline and solid oxide electrolysis. Nowadays, more mega-Watt scale PEM electrolysis systems are available on the market and in the field. However, further technological advancement is still demanded in the field of electrocatalysis and material science to obtain a deeper understanding of catalytic reactions and design new catalysts such that PEM electrolysis is more durable and cost-effective. Furthermore, as PEM electrolysis is but one building block for the future hydrogen economy, efforts in R&D should emphasize the compatibility with other technologies and optimize the synergic effects.