Hydrogen production by water electrolysis technologies: A review

Results in Engineering 20 (2023) 101426

Available online 19 September 2023

2590-1230/© 2023 The Author. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/).

Hydrogen production by water electrolysis technologies: A review

Mostafa El-Shafie

Institutes of Innovation for Future Society, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan

A R T I C L E I N F O Keywords:

Proton exchange membrane Hydrogen production Impure water

Membrane electrode assembly Water electrolysis

Renewable energy

A B S T R A C T

Hydrogen as an energy source has been identified as an optimal pathway for mitigating climate change by combining renewable electricity with water electrolysis systems. Proton exchange membrane (PEM) technology has received a substantial amount of attention because of its ability to efficiently produce high-purity hydrogen while minimising challenges associated with handling and maintenance. Another hydrogen generation tech- nology, alkaline water electrolysis (AWE), has been widely used in commercial hydrogen production applica- tions. Anion exchange membrane (AEM) technology can produce hydrogen at relatively low costs because the noble metal catalysts used in PEM and AWE systems are replaced with conventional low-cost electrocatalysts.

Solid oxide electrolyzer cell (SOEC) technology is another electrolysis technology for producing hydrogen at relatively high conversion efficiencies, low cost, and with low associated emissions. However, the operating temperatures of SOECs are high which necessitates long startup times. This review addresses the current state of technologies capable of using impure water in water electrolysis systems. Commercially available water elec- trolysis systems were extensively discussed and compared. The technical barriers of hydrogen production by PEM and AEM were also investigated. Furthermore, commercial PEM stack electrolyzer performance was evaluated using artificial river water (soft water). An integrated system approach was recommended for meeting the power and pure water demands using reversible seawater by combining renewable electricity, water electrolysis, and fuel cells. AEM performance was considered to be low, requiring further developments to enhance the mem- brane’s lifetime.

1. Introduction

In recent years, concerns about the unsustainable use of fossil fuels and global warming have grown due to increases in global energy de- mand as a result of continuous population growth and energy-intensive lifestyles. Energy resources are needed to support all human activities [1–3]. Therefore, the development of renewable energy sources has essential in the acceleration of the energy transition toward global carbon neutrality. Hydrogen (H2) is considered one of the most carbon-free energy carriers, as only water is produced when it reacts with oxygen to generate electricity [4]. In addition, hydrogen has beneficial physical and chemical properties as a clean energy carrier such as its high gravimetric energy density (140 MJ/kg) [5] (see Tables 1–7).

For several decades, various hydrogen production technologies have been investigated, such as steam methane reforming, biomass, water electrolysis, etc. [7,65–67]. Table (1) lists the shares of different hydrogen production technologies of the total production capacity.

Currently, hydrogen production from hydrocarbons comprises

approximately 96% of the global market [6]. Carbon dioxide (CO2) emissions into the atmosphere contribute substantially to the green- house gas (GHG) effect, which is the key driver in climate change [66, 68,69]. Several major international meetings have been held to address global warming, including COP 26 (Glasgow-UK, 2021) and COP 27 (Sharm El-Sheikh-Egypt, 2022). The world’s governments are joining forces to confront climate change by transitioning to carbon-free fuels.

The international energy agency (IEA) proposed targets for the primary energy sources of hydrogen production from 2020 to 2050 to achieve net zero emissions, as shown in Figure (1) [70]. Fossil fuels (without carbon capture) as sources of hydrogen are not sustainable and do not contribute to net zero emissions.

CO2 and other GHG emissions must be avoided in the hydrogen production processes. Today, most of the hydrogen produced globally is generated by steam methane reforming and water electrolysis processes.

The global demand for hydrogen in 2020 was 90 Mt—70 Mt of which was pure hydrogen generated with no carbon emissions [72]. Further- more, most of the generated hydrogen was used in oil refining or petrochemical applications, fuel cells, and industrial chemicals [73,74].

E-mail address: mostafaelshafie81@gmail.com.

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Received 1 July 2023; Received in revised form 31 August 2023; Accepted 14 September 2023

Most of the hydrogen has been generated from steam methane reform- ing. Hydrogen can be generated from renewable energy sources such as solar or wind energy or non-renewable energy such as fossil fuels, particularly methane reforming.

In addition, the integration of nuclear energy as a source of elec- tricity for hydrogen production plants has been recently investigated [75]. Hydrogen can be generated from water using different technolo- gies, including water electrolysis and splitting, the latter of which can be achieved through thermochemical processes, biomass conversion, or photocatalysis [76]. The splitting of water into hydrogen and oxygen occurs through a series of chemical reactions. Water conversion occurs at a high temperature range of 800–2000 ᵒC [77]. Solar and nuclear power plants can generate the required heat for the thermochemical water splitting process [78].

The production of hydrogen by photocatalysis is a promising method in which water is dissociated into hydrogen and oxygen using solar energy and TiO2 as a photocatalyst [79]. The main disadvantages of this technology are the use of TiO2 which leads to a wide band gap in the visible light region, and the evolution of over potential [80]. Plasma technology has also received a great deal of attention because of its ability to dissociate gases into their elements. Hydrogen production from water vapour by dielectric barrier discharge (DBD) plasma as well as hydrogen production from water containing impurities by DBD plasma have been recently investigated [81, 82].

Hydrogen is primarily generated by steam reformation of fossil fuels, the process operation of which is complicated by carbon dioxide emis- sions. Water electrolysis is the most effective zero-emission hydrogen production technology when utilizing renewable energy as the elec- tricity source. Polymer electrolyte membrane (PEM) water electrolysis using an ion exchange membrane is a high efficiency technology for

generating high-purity hydrogen. PEM water electrolysis technology development includes improving catalyst and system performance and integrating renewable energy. The fundamentals of PEM water elec- trolysis technology have been investigated [83]. Hydrogen production by water electrolysis offers several advantages, including high-purity H2, no output pollutants, and a wide range of input energy sources. In addition, H2 production from water electrolysis has been used for many years in industrial applications [71].

The costs of green hydrogen production are influenced by the renewable electricity generated from solar, tidal, geothermal and wind energy [71]. Moreover, solar photovoltaic (PV) has become the lowest-cost source of green electricity. An innovative cooling system for a PV solar unit comprised of a thermoelectric layer utilizing hybrid nanomaterial has been investigated [84,85]. It has been shown that the optimal locations for solar energy are the in the Middle East. Further- more, hydrogen production processes involving electrolysis can operate flexibly using various renewable electricity sources. The most significant factor is the stability of the renewable electricity source. Figure (2) il- lustrates the long-term hydrogen production cost of electrolyzers in various regions worldwide using solar PV and wind energy systems [71].

The hydrogen-based fuels are more economical, although they have higher hydrogen production costs and less operating time at entire load operations than other fuels. Therefore, the green hydrogen production pathway is the optimal solution to reduce the environmental impacts of GHGs. However, water electrolysis requires higher energy consumption;

industrial electrolyzer energy consumption is 4.5–5 kWh/m³. High en- ergy consumption is a substantial challenge that should be addressed to minimize the hydrogen production costs. An overview of the global hydrogen production trends and best practices in Saudi Arabia was presented, with national policy recommendations for the energy in- dustry [86]. In addition, industrial-scale hydrogen production in Morocco for sustainable technologies has been evaluated [87].

Furthermore, a study of hydrogen production in Ireland was performed which includes the deployment of power-to-gas (P2G) systems, the transition to renewable energy systems, and industrial heating re- quirements [88].

Water electrolysis can produce high purity hydrogen and can be feasibly combined with renewable energy. Water is a requirement of these systems as the main input to the electrolyzer to produce hydrogen.

Also, water electrolysis energy consumption in conventional industrial Table 1

Global hydrogen production sources [6].

Energy Source Bcm³/yr^a Share (%)

Natural gas 240 48

Oil 150 30

Coal 90 18

Electrolysis 20 4

Total 500 100

aBcm: billion cubic meters.

Table 2

Comparison of the available water electrolyzer technologies.

Specification Alkaline [48, 60] PEM [48, 60] SOEC [7] AEM [8,9]

Technology maturity Mature Commercial R&D R&D

Cell temperature, ^◦C 65–100 70–90 900–1000 50–70

Cell pressure, bar 25–30 30–80 <30 −30

Current density, mA/cm² 200–500 800–2500 300–1000 200–500

Cell voltage, V 1.8–2.4 1.8–2.2 0.95–1.3 1.8–2.2

Voltage efficiency, % 50–70.8 48.5–65.5 81–86 39.7

Specific system energy consumption,

kWh/Nm³ 4.5–7.5 5.8–7.5 2.5–3.5 4.8–5.2

Hydrogen production, Nm³/hr <760 0.265–30 – 0.25–1

Stack lifetime, hr 10, 000 <20,000 <40,000 NA

Electrolyte 20–30% KOH Perflurosulfonated acid

(PFSA) Yttria-stabilized zirconia

(YSZ) DVB polymer support with

KOH or NaHCO3

Separator Asbestos, NiO, ZrO2, stabilized with

PPS mesh PFSA (e.g., Nafion) Solid electrolyte quaternary ammonia

polysulfone (QAPS)

Charge carrier OH⁻ H⁺ O2− OH⁻

OER catalyst Ni-coated perforated stainless steel Ir/Ru oxide Perovskite-type CO3O4

HER catalyst Ni Platinum Ni/YSZ Ni

Hydrogen purity, (vol%) 99.3–99.9 99.999 – 99.99

Capital cost (€_{/kWel) 1000–1200 1860–2320} >2000 NA

Abbreviations.

OER: oxygen evolution reaction.

HER: hydrogen evolution reaction.

NA: not available.

application is relatively high and about 5 kWh m⁻³H2. In addition, availability of fresh water is a serious problem in many regions of the world. Therefore, it is important to further develop water electrolysis technologies and minimize the energy consumption to meet the re- quirements of sustainable hydrogen production. The current study aims to present a detailed analysis and an overview of the progress of con- ventional hydrogen electrolyzers. Thermodynamic analyses of hydrogen production via water electrolysis types are also discussed, along with the technical barriers associated with using impure water in different elec- trolyzer types. Potential technology advancements and future ap- proaches are also highlighted. Finally, hydrogen production costs and challenges in the field are evaluated.

2. Thermodynamic analysis of water electrolysis

Generally, purified or deionized water can be used as the feedstock for PEM electrolyzers. Fresh water is available from rivers, groundwater, and lakes but requires several filtration steps. Furthermore, the water types can vary based on the hardness and salts in the water. Numerous experimental studies of PEM electrolyzer stacks and systems operation have been performed over the last 15 years [66,89]. Renewable elec- tricity can provide continuous and stable electricity for hydrogen elec- trolyzers. The National Renewable Energy Laboratory (NREL) in the US demonstrated the application of storing excess wind energy as hydrogen and later using it to charge cars and power homes with the use of fuel cells [90]. Due to the importance of energy consumption in hydrogen electrolyzers, we will investigate the thermodynamics of the electro- chemical process in the following subsection.

2.1. Energy efficiency and energy consumption of water electrolysis The electrochemical decomposition process of water occurs accord- ing to the following anode and cathode evolution reactions. The anode and cathode reactions and their respective reversible potential in a water electrolytic cell can be expressed as follows:

Anode: H2O →2H⁺+½ O2 +2e⁻ (Urev =1.229 V) (1)

Cathode: 2H⁺+2e⁻ → H2 (Urev =0.00 V) (2) Overall reaction: H2O → H2 +½ O2 (Urev =1.229 V) (3)

The reversible cell voltage can be derived as a function of the Gibbs free energy:

ΔG=ΔH− Q=ΔH− TΔS (4)

ΔG=zFUrev (5)

Urev=ΔGR

zF (6)

The reversible voltage is determined at standard temperature and pressure (298.15 K and 1 atm), ΔGR =237.21 kJ/mol, ΔS =0.1631 kJ/

mol K, and ΔН =285.84 kJ/mol. The number of electrons is z =2, while F is Faraday’s constant F =96485 C/mol. By substituting ΔG_R and ΔН₀, the theoretical potential value is obtained, Urev = 1.229 V [89,91].

Water electrolysis begins operation by connecting the applied voltage to the cell electrodes (cathode and anode). The applied potential decreases due to the cell overpotential. The activation and ohmic overpotential are characterized as the cell overpotential, the cell operating voltage is the summation of the reversible, activation, ohmic overpotential [92], expressed as follows:

Ucell=Urev+Uact+Uohm (7) The proton transfer and kinetic electrochemical behaviour in the water electrolyzer comprise the activation overpotential. Losses in the applied voltage occur during the water decomposition chemical re- actions, and electrons transfer to or from the electrodes. The ohmic overpotential is expressed as the potential losses due to the resistance of the membrane against the electron flow, which depend on the water electrolysis type and the membrane thickness. Therefore, the ohmic overpotential can be calculated as a function of membrane conductivity, thickness, and current density [93]. The numerical simulation results of the ohmic overpotential compared with the reversible potential of the electrolyzer cell are presented in Figure (3). The simulation predicted the overpotential losses in water electrolyzers [94]. The ohmic resis- tance is also estimated as a function of the ionic and electronic

Table 3

Comparison of OER performance with various catalysts.

Catalyst Substrate Electrolyte Stability Loading η₁₀ (mV) Tafel slope (mV dec⁻¹) Ref.

Ultrafine Ir NPs GC 0.5 M HClO4 1,000 cycles or 20,000s@10 mAcm⁻² 90 μgIr cm⁻² 290 65.1 [10]

Ir@HEDP/CNBs GC 0.1 M HClO4 5 h@10 mAcm⁻² – 290 49.1 [11]

Ir-SA@Fe@NCNT GC 0.5 M H2SO4 12 h@10 mAcm⁻² 1.14 μ_{gIr cm}⁻² _{250 58.2 [12]}

Ir0.06Co2.94O4 Au electrode 0.1 M HClO4 200 h@10 mAcm⁻² 5 μ_{gIr cm}⁻² _{292 45 [13]}

3D Ir GC 0.1 M HClO4 8 h@2.5 mAcm⁻² 11.5 μ_{gIr cm}⁻² _{240 40.8 [14]}

Ir/g-C3N4/NG GC 0.5 M H2SO4 2,000 cycles 6.7 μgIr cm⁻² 287 72.8 [15]

Mesoporous Ir nanosheets GC 0.5 M H2SO4 8 h@10 mAcm⁻² – 240 49 [16]

Amorphous Ir nanosheets GC 0.1 M HClO4 5,000 cycles 0.2 mgcm⁻² 255 40 [17]

Ru–N–C GC 0.5 M H2SO4 30 h@1.49 V 0.28 mgcm⁻² 267 52.6 [18]

Ru1–Pt3Cu GC 0.1 M HClO4 28 h@10 mAcm⁻² 1.92 μ_{gRu cm}⁻² ₂₂₀ – [19]

Ru/RuS2 GC 0.5 M H2SO4 3,000 cycles or 24 h@10 mAcm⁻² 0.849 mgcm⁻² 201 47.2 [20]

fcc-Ru octahedral GC 0.05 M H2SO4 – – 168 47.8 [21]

IrNi NPNWs GC 0.1 M HClO4 200min@5 mAcm⁻² 25 μgIr cm⁻² 283 56.7 [22]

Ir3Cu GC 0.1 M HClO4 12 h@5 mAcm⁻² – 298 47.4 [23]

IrNi NCs GC 0.1 M HClO4 2 h@5 mAcm⁻² 12.5 μ_{gIr cm}⁻² ₂₈₀ – [24]

Rh22Ir78 GC 0.5 M H2SO4 2,000 cycles 9.8 μ_{gIr cm}⁻² _{292 101 [25]}

IrRu@Te GC 0.5 M H2SO4 20 h@10 mAcm⁻² 0.6 mgcm⁻² 220 35 [26]

Co–RuIr Au electrode 0.1 M HClO4 25 h@10 mAcm⁻² – 235 66.9 [27]

Pt39Ir10Pd11 GC 0.1 M HClO4 – 16.8 μgPt +Ir +Pd cm⁻² 372 128.7 [28]

IrNiCu DNF GC 0.1 M HClO4 2,500 cycles 20 μgIr cm⁻² 300 48 [29]

Co–IrCu GC 0.1 M HClO4 2,000 cycles 20 μgIr cm⁻² 290 50 [30]

IrCoNi PHNC GC 0.1 M HClO4 200min@5 mAcm⁻² 10 μ_{gIr cm}⁻² _{303 53.8 [31]}

IrOxQD/GDY GC 0.5 M H2SO4 3,000 cycles or 20 h@1.6 V – 236 70 [32]

IrHfxOy GC 0.1 M HClO4 6 h@5 mAcm⁻² – 330 60 [33]

Cr0.6Ru0.4O2 GC 0.5 M H2SO4 10 h@10 mAcm⁻² – 178 58 [34]

W0.2Er0.1Ru0.7O2-δ GC 0.5 M H2SO4 500 h@10 mAcm⁻² 0.33 mgcm⁻² 168 66.8 [35]

a/c-RuO2 GC 0.1 M HClO4 60 h@10 mAcm⁻² 0.404 mgcm⁻² 205 48.6 [36]

RuIrOx GC 0.5 M H2SO4 24 h@10 mAcm⁻² 10 μgIr +Ru cm⁻² 233 42 [37]

resistances [95]. The largest overpotential of 2.6 V was obtained at the Pt electrode on the anode side, and the overpotential increased with the current density. Moreover, the cathode overpotential was lower than that of the anode because of the rapid kinetic reactions at the anode surface.

The energy consumption of a water electrolyzer is an important parameter to evaluate the cell performance over an extended period.

The theoretical decomposition cell voltage of water is 1.229 V. The hydrogen production efficiency from the cell is about 100%. The elec- trolyzer energy consumption can be determined according to the following formula [96]:

CE=

∫^Δt

0

Ncell.Icell.Ucelldt

∫^Δt

0

fH2dt

(8)

Where Ncell is the number of cells that constitute the electrolysis module, fH2 is the hydrogen production rate in Nm³/h, and Icell and Vcell are the cell current in A and voltage in V, respectively, determined for a given time interval of Δt. The cell efficiency and power consumption are determined at the applied current. The electrolyzer efficiency is defined as the ratio between the higher heating value (HHV) of hydrogen and the total energy consumption (CE) in kWh/Nm³, calculated as follows [96]:

η_E=HHV of H2

CE

×100 (9)

Industrial cell voltage is applied at a range of 1.8–2.6 V, meaning that the voltage values higher than the theoretical voltage of 1.229 V are expressed by the greater overpotential and ohmic overpotential losses. A theoretical analysis of the cell voltage versus the energy consumption and energy efficiency is shown in Figure (4). It can be seen that energy consumption increases with increasing cell voltage. In addition, energy efficiency decreases at higher cell potentials, and the higher operating cell voltage influences the electrolyzer performance.

3. Water electrolysis types

Most water electrolysis technologies generate hydrogen and oxygen from water at lower operating temperatures [97–100]. The main types of water electrolysis for generating hydrogen are alkaline electrolysis [8, 101–104], PEM electrolysis [105–107], solid oxide electrolysis (SOE) [46], and polymer anion exchange membrane (AEM) electrolysis [108–111]. These electrolyzer types are discussed in the following subsections.

3.1. Alkaline water electrolyzer

Alkaline water electrolyzers are commonly used to produce

hydrogen in large-scale applications. Nickel (Ni) and cobalt oxides are utilized to form the anode and cathode materials, respectively. More- over, potassium hydroxide (KOH) electrolyte is the most useable with 30–40% and the alkalinity is generated by circulating the electrolyte across both electrodes [112,113]. Figure (5) shows a schematic diagram of an alkaline water electrolyzer cell [114]. The anode and cathode re- actions occur to produce hydrogen and oxygen. The separating porous diaphragm is installed between the two electrodes, and the hydroxyl ions (OH⁻) are generated and passed through the porous diaphragm materials, which are made of ceramic oxide materials such as asbestos and polymers [115,116]. The hydrogen and hydroxide are produced at the cathode at moderate operating temperatures of 65–100 ^◦C; oxygen is then generated at the anode by the hydroxide reaction. The conversion efficiency of alkaline water electrolyzers is 60–80%, and the cell oper- ating voltage is 1.8–2.4 V [117]. The advantages of alkaline electro- lyzers are that they can operate at low temperatures and do not require catalysts to activate and produce hydrogen [118]. In contrast, alkaline electrolyzer electrode corrosion is considered the foremost challenge because of the presence of an alkali solution.

3.2. Proton exchange membrane electrolyzer

PEM water electrolyzers can produce high-purity hydrogen at lower temperature ranges of 70–90 ^◦C. IrO2 and Pt are used as the anode and cathode catalyst materials, respectively [119,120]. Nafion membrane is typically used to conduct the hydrogen ions (H⁺); it is applied as a solid electrolyte, as opposed to the liquid electrolyte in alkaline electrolyzers.

A PEM electrolyzer cell schematic diagram and reaction mechanism are shown in Figure (6). Hydrogen ions migrate through the solid membrane to form hydrogen molecules. Because of the electrolyte’s acidic nature and the electrode’s metallic surfaces, the reaction kinetics of PEM electrolyzers are rapidly induced compared with those of an alkaline cell [121]. As a result, PEM electrolyzers are safer and can operate at at- mospheric pressure on the anode side while the cathode side is exposed to higher pressures [89, 122]. A significant challenge of PEM electro- lyzers is their difficulty in scaling up for large-scale (MW) applications.

Therefore, an extensive analysis should be performed to validate the large-scale unit’s characteristics and the expected lifetime of PEM electrolyzers.

3.3. Solid oxide electrolyzer

In solid oxide electrolyzers, the cell operates at substantially higher temperatures than alkaline and PEM electrolyzers in the range of 900–1000 ^◦C. The cell electrodes are made from nickel, and less elec- tricity is required for the hydrogen separation process, illustrated in Figure (7) [114]. Different heat input sources, such as waste heat and nuclear energy, can be used to reduce the dependence on electricity for heating [123,124]. In addition, the cell efficiency increases with higher operating temperatures. Unfortunately, high temperatures also increase the rate of electrolyte degradation and reduce the SOEC lifetime. In Table 4

Gives the performance of AWE using fluctuating renewable power for different HER and OER catalysts [38].

Reaction Stability test protocol Catalyst Electrolyte Temperature

(^◦C) Current density for η _(mA/

cm²)

η_initial (mV)

η_end

(mV) Ref.

HER − 200 mA/cm, 1 h - OCP, 1 h: 50 cycles MnnCo1enPx 1 M KOH 25 100 105 195 [39]

− 0.4–0.5 V, 0.5 V/s, CV, 40,000 cycles Co-ns/Ni 7 M KOH – 100 68 64 [40]

− 0.25–0.4 V, 5 mV/s, LSV, 3000 scans NiFe/CP 1 M KOH RT 100 245 265 [41]

− 300 mA/cm, 30 mine 0.074 V, 60 s, 33

cycles Raney Ni 30% KOH 70 300 277 557 [42]

OER 0.5–1.8 V, 1 V/s, CV, 60,000 cycles Co-ns/Ni 7 M KOH – 100 350 360 [40]

0.5–1.8 V, 1 V/s, CV, 20,000 cycles NiFe-HyOx/316SS 7 M KOH 20 100 267 262 [43]

0–240 mA/cm2,1 mA/s, CV, 2000 cycles Ni0.9Fe0.1Co2O4/

Ni 1 M KOH 22 240 563 583 [44]

600 mA/cm2 for 3 se0.3 V for 3 s, 2000

cycles NiCoOx/Ni 7 M KOH 25 100 420 600 [45]

addition, existing SOEC demonstrations can only operate at the kilowatt scale.

3.4. Anion exchange membrane

AEM is the most recent water electrolysis technology, primarily driven by electrochemical applications for hydrogen production over the last few years. AEM electrolyzers combine the advantages of alkaline and PEM water electrolyzers [125]. The reaction mechanism of an AEM cell is illustrated in Figure (8). As can be seen, hydrogen and hydroxyl are generated at the cathode side.

Moreover, non-noble catalyst materials can be used for AEM cells, leading to lower hydrogen production costs [47]. However, AEM per- formance is still low due to the poor catalyst activity and the lower conductivity of the AEM membrane. Therefore, AEM water electrolysis requires further investigations, specifically regarding membrane mate- rials, cell cost, and efficiency.

The most important operating conditions for the four water elec- trolyzer types are summarized in Table (2). It is clear that the SOEC and the AEM electrolyzers are still at the research and development (R&D) stage and are less mature compared with alkaline and PEM technologies.

In addition, the benefits of AEM include lower overall cost and high H2

production stability [48]. However, AEM technology requires

significant development to be utilized for industrial-scale H2 production in the near term. In addition, the different water electrolysis technolo- gies have different challenges, such as cell performance, durability, membrane materials, catalysts, and cell cost.

Table 5

AEM water electrolysis component materials [46].

Membrane electrode assembly Operating conditions Electrolyte Ref.

Anode Cathode Membrane Ionomer

GDL Catalyst Loading

(mg cm⁻²⁾

GDL Catalyst Loading (mg cm⁻²⁾

Voltage

(V) Current density (mA cm⁻²⁾

T (^◦C)

Ti foam IrO2 2.9 Ti foam Pt black 3.2 A-201, Tokuyama AS-4 1.8 399 50 deionized

water [47]

Ni foam Ni/

CeO2–La2O3/C 36 Carbon

cloth CuCoO3 7.4 A-201, Tokuyama PTFE 1.9 470 50 1%

K2CO3/ KHCO3

[48]

Carbon

paper Pb2Ru2O6.5 2.5 Carbon

paper Pt black 2.5 Chloromethylated

PSF(CMPSF) PSF-TMA + Cl-, PSF- ABCO +Cl- or PSF-1M +Cl-)

1.8 400 50 Ultra-pure

water [49]

Ni foam Ni–Fe 40 Stainless

steel fiber felt

Ni–Mo 40 (xQAPS) (xQAPS) 1.85 400 70 Ultrapure

water [50]

Carbon

paper Ni 0.085 Carbon

paper Ni 0.085 A-201, Tokuyama – 1.9 150 50 1 M KOH [51]

Ni foam Ni/

CeO2–La2O3/C Ni/C CuCoO3 LDPE-g-VBC AS-4 2.1 460 50 1% K2CO3 [52]

Stainless steel mesh

Cu0.7^CO2.3^O4 3 Stainless steel mesh

Pt 1 Quaternary

ammonium radiation grafted membrane

1.8 100 25 1 M KOH [53]

Stainless steel mesh

Cu0.7CO2.3O4 3 Stainless steel mesh

Nano Ni 2 mm-qPVBz/Cl- QPVB/Cl- 1.9 100 55 Deionized

water [54]

Stainless steel mesh

Cu0.7^CO2.3^O4 3 Stainless steel mesh

Nano Ni 2 QPDTB Poly

(DMAEMA- co-TFEMA- co-BMA)

1.9 100 50 Deionized

water [55]

Ni foam Ni/

CeO2–La2O3/C 40 Carbon

cloth CuCoO3 40 Mg–Al layered

double hydroxide PTFE 2.2 208 70 0.1 M

NaOH/

Na2CO3

[56]

Ni foam Ni–Fe Carbon

cloth Ni–Mo PSF Quaternary

ammonium 70 [57]

NiO Graphene NiO Graphene Selemion AMV – 1.9 150 80 0–5.36

KOH [58]

Ni oxide Graphene Ni Graphene Selemion AMV 2 90 30 Deionized

water [59]

Pt coated

Ti Ce0.2MnFe1.8O4 3.5 Pt coated

Ti Ni 3.5 FAA-3-PK-130 1.8 300 Deionized

water [60]

Table 6

Maximum levels of cationic elements in drinking water [61].

Elements Contaminants Concentration (mg/L)

Primary Barium 2

Chromium 0.1

Selenium 0.05

Arsenic 0.01

Antimony 0.006

Cadmium 0.005

Beryllium 0.004

Tellurium 0.002

Secondary Zinc 5

Copper 1.3

Iron 0.3

Aluminum 0.2

Silver 0.1

Manganese 0.05

4. Electrocatalysts for water electrolysis

4.1. PEM water electrolysis hydrogen evolution reaction (HER) and oxygen evolution reaction (OER)

In this subsection, HER and OER catalysts used in the most common

hydrogen production water electrolyzers, PEM and AWE, are discussed.

HER occurs at the cathode side of the PEM electrolysis cell. Currently, Pt-based materials are the most suitable for the PEM cathodes because they exhibit significant HER activation in acidic electrolytes. Further- more, Pt-based electrocatalysts are considered to be the state–of–the–art electrochemical catalysts for PEM electrolysis [126,127]. However, limited availability and the high cost of Pt are the main constraints limiting its use in large-scale applications. Therefore, the development of highly active catalysts with high stability is needed to increase the applicability of PEM electrolyzers. A substantial amount of research has focused on minimising Pt catalyst loading [128–136]. The high activity and durability of a Pt@N-containing carbon shell on carbon nanofibers (Pt@CS/CNF) have been investigated [137]. Figure (9) shows TEM and XPS images of the improved electrocatalyst as well as Pt/CNTs catalyst activity [130]. An analysis of density functional theory (DFT) and high –angle annular dark field scanning transmission electron microscopy (HAADF-STEM) showed a high interaction and electrocatalytic activity between Pt and CNT atoms.

Transition metals have been recently utilized as cathodic electro- catalysts, including MoS₂, MoSe₂ and WS₂ [138–149]. MoS₂ was fabri- cated according to the Nafion PEM method; the resulting performance of the cathodic HER was an overpotential of 175 mV at a current density of 10 mA [150]. On the electrocatalytic surface, the electronic structure was improved by cation-doping which increased the hydrogen adsorp- tion on the cathode [151,152].

Table 7

Performance comparisons between Alkaline, PEM using pure water, and direct seawater electrolysis [62].

Electrolysis

technology Energy consumption [kWh/kgH2

Efficiency based on

HHV [%] Ref.

PEM electrolysis 47–63 67–82 [63]

Alkaline electrolysis 47–66 62–82 [63]

Direct seawater

electrolysis 50–53 75–79 [64]

Fig. 1. Energy sources for a net-zero emissions scenario of hydrogen produc- tion for 2020–2050 [71].

Fig. 2. Hydrogen production costs from renewable electricity (solar PV and wind energy systems) in 2030 [71].

Fig. 3. Overpotential losses in a water electrolyzer [94].

4.1.1. Oxygen evolution reaction electrocatalysts for PEM water electrolysis The development of OER electrocatalysts has received a significant amount of attention to enhance PEM electrolyzer performance. Iron (Ir)- based and ruthenium (Ru)-based catalysts are the most common OER electrocatalysts suitable for PEM electrolyzer acidic environments. The highest OER electrocatalyst activity is exhibited by Ru-based catalysts, but the stability of Ru catalysts is still lower than Ir-based catalysts [10, 18]. Ir-based electrocatalysts are of significant interest because of their ability to perform in acidic environments, with low overpotential and high stability. The activation of Ir-based electrocatalysts can be enhanced by controlling the following parameters: particle size, crystal structure, morphology, and substrate materials. Utilizing ultrathin or small Ir-based electrocatalyst particles is a method to increase the

surface area and thereby achieve high catalytic activity [150,153,154].

The ultrafine size of iron nanocrystals and their homogeneous distri- bution are shown in Figure (10a–c). The CNBs enhance the conductivity and the ultrafine Ir electrocatalyst has a larger surface area which leads to good stability and a lower overpotential of 290 mV at a current density of 10 mA [11]. Furthermore, the integration between Co oxide and Ir SA gives excellent stability and high OER electrocatalyst activity, as demonstrated in Figure (10d–f) [13]. Moreover, 3D superstructure iron electrocatalyst performance is also presented in Figure (10g-i). A 3D Ir catalyst consisting of ultrathin iron nanosheets increases the surface area and achieves high OER electrocatalyst activation and a low over- potential of 240 mV [14]. The performance of various OER electro- catalysts for PEM electrolyzers is summarized in Table (3).

Fig. 4. Cell performance versus overpotential.

Fig. 5. Alkaline water electrolysis technology [114].

Fig. 6. PEM water electrolyzer schematic diagram and reaction mecha- nism [114].

Fig. 7. SOEC reaction mechanism [114].

4.2. Alkaline water electrolyser catalyst materials for HER and OER The development of new electrode materials is essential for enhancing AWE performance. The selection of a high resistance reverse current is an important parameter for catalyst and electrode materials because of the AWE performance influenced by the reverse current. The performance of various electrode catalyst materials is compared for HER and OER in Table (4). The durability of catalysts is evaluated and considered during the start-up and shut-down operations of AWE elec- trolyzers [39–44]. Chemical and physical degradation as a result of the reverse current is the main challenge requiring the development of improved catalyst materials for AWE electrolyzers. The addition of manganese to Co/phosphate-based catalysts for HER has been shown to

reduce the oxidation caused by degradation [39]. In addition, OER was improved by adding Co(OH)₂ nanosheets which leads to self-repairing of nickel (Ni) electrodes [19]. Furthermore, the use of fluctuating power such as the direct use of renewable energy increased the cell voltage because degrades the cell performance [38].

5. Technical barriers to H2 production from impure water 5.1. Technical barriers to anion exchange membrane electrolyzers

Oxygen evolution reaction selectivity: AEM electrolysis is a promising technology for hydrogen production from water. Primary AEM water electrolysis components and materials are listed in Table (5). The AEM electrolyzer cathode and anode operate under alkaline conditions.

Therefore, lower cost (e.g. steel, Ti) and non-Nobel metals (e.g. Fe, Ni, or Co-based catalysts) can be used to produce hydrogen at lower costs [155]. Because of the shortage of pure or drinking water, AEM elec- trolysis offers a substantial advantage of being able to utilise impure water. Thus, the focus in this section is on the technical barriers to producing hydrogen from impure water via AEM electrolyzers. Water is supplied to the anode, which diffuses through the membrane to the cathode based on the concentration gradient. The membrane conducts the OH⁻ radical generated at the cathode to the anode, and then oxidizes it into H2O and O2 products. The impurities included in the feedstock water affect the cathode fouling rates. The membrane permselectivity should reduce the fouling of cationic impurities [156]. Using seawater in electrolyzers has also been investigated [62,157,158]. The membrane permselectivity is considered a vital parameter to minimize the cation impurities effect, which can deposit and block the active membrane sites.

The electrolysis of wastewater involves different concerns, such as halide reactions and chlorine (Cl) reactivity. The influence of Cl⁻ on the OER has been investigated [159]. Seawater was shown to be unsuitable for PEM water electrolyzers due to co-generation of Cl2 and Br2 [160].

The OER reaction is influenced by the more significant chlorine con- centration to form ClO⁻; it also appears on the surface of the catalyst material [161]. Various species are produced by the chlorine reactions;

Figure (11) presents the chloride and bromide-oxidation reactions for Fig. 8.AEM reaction mechanism [114].

Fig. 9. a, b) Electocatalyst TEM images, c) XPS analysis of Pt@CS/CNF, d) TEM images after a durability test [137], e) Pt/CNTs synthesis process, f) Pt/CNTs HAAD-STEM analysis image, and g) HER calculated ΔGH [130].

the OER [159]. The chlorine reactions exhibit different resulting species.

The chlorate reactions depend on the pH of the feedstock water, mass transport, and current density. The OER reaction kinetics for impure water still require further investigation. The influence on the AEM cell performance of adding sodium chloride (NaCl) to the electrolyte was investigated; the NaCl addition showed a significant decrease in the cell current density and an increase in the electrolyte conductivity [64]. The cell faradaic efficiency operating at a lower current density can reach 100%; however, Cl⁻ oxidation reduced the efficiency to 93.4% [162].

The stability of Na⁺does not influence AEM cells. Impure and nat- ural salt water poisons cathodes by cationic contamination. In contrast, impure and sea water can be used in AEM and PEM water electrolyzers.

The cell’s performance using natural river water and seawater has been investigated [98]. The overpotential of river water was shown to be lower than the resulting overpotential from salt water.

The direct use of natural seawater was investigated, and more extensive potential results were presented [163]. Different catalyst materials were also studied at the anode and cathode using natural seawater buffered with pH (7.4), and the activity exceeded the HER/OER activity at both electrodes (Ir–C/Pt–C) [164–166]. Therefore, appropriate selection of electrocatalyst materials will help minimize cell overpotential. Conventional alkaline and PEM electrolyzers use noble catalyst metals such as Pt, Ru, and Ir, which will increase the electro- lyzer cost [105]. Thus, non-noble metal catalyst development has been investigated to reduce the AEM cell cost [48]. Various OER catalyst materials include IrO₂, Ni–Fe alloys, Cu_0.7CO_2.3O₄, graphene, and Pb2Ru2O6.5, while the HER catalysts include Pt-black, Ni–Mo, Ni/CeO2–La2O3/C, Ni, CuCoOx, and graphene. Low-cost catalyst mate- rials were evaluated and developed for the AEM electrolyzer [48]. The loading of the commercial catalysts for OER and HER are Acta 3030 Fig. 10. a) Synthesis process, b) TEM analysis images of Ir@HEDP/CNBs, c) Performance of OER [11], d) Synthesis process of Ir SA electrocatalyst, e) Performance curve, f) Various catalysts activity versus mass activity [13], g) 3D Ir superstructure synthesis process, h) Performance of OER, i) 3D superstructure performance and Tafel slopes [14].

Fig. 11.Chlorine resulting species at the OER [159].

(CuCoOx) and Acta 4030 (Ni/CeO2–La2O3/C), respectively. Figure (12) illustrates the resulting cell voltage at various catalyst material loadings.

The catalyst was measured over the loading range of 0.6–7.4 mg/cm² [48]; the HER catalyst loading was shown to influence the electrolyzer potential.

The most significant elements that influence the cell’s performance are Ca, Na, and Mg; these elements’ contaminations can be removed by washing. Pure water production from the direct use of a seawater electrolyzer is the most significant aspect of generating hydrogen and oxygen, which is then utilized in fuel cells to generate water and elec- tricity. Seawater can be fed into the electrolyzer directly or indirectly after treatment. The energy consumption by several desalination tech- nologies is shown in Figure (13). The energy consumption of the reversible seawater was 3,370 kWh/Nm³H2O to generate 1 Nm³H2O [167, 168]. The combined use of a seawater electrolyzer and fuel cell in an integrated cycle to generate green hydrogen, electricity, and pure water is investigated. The reversible seawater electrolysis energy consumption is higher than other established water technologies like reverse osmosis (RO), multi-stage flash distillation (MSF), multi-effect distillation (MED), and mechanical vapour compression (MVC).

Reversible seawater electrolysis can produce large volumes of hydrogen, but the electrolysis technologies should be more interesting.

However, the energy consumption of the seawater electrolyzer was higher than that for high purity water or RO water. The entire system is illustrated in Figure (14) including the power demand and production of pure water from the combination of an electrolyzer and fuel cells. The PV devices can produce renewable electricity, or solar and wind energy can supply the power required by households (city), electrolyzers, and the RO desalination of seawater to provide pure water. Hydrogen and oxygen produced by the electrolyzer can be used at night to generate renewable electricity to meet power demands and produce pure water by the fuel cell and desalination process. The water generated from the fuel cell is considered pure water and can be fed into the RO process. The seawater electrolyzer and fuel cell integrated system offers advantages by storing renewable electricity as hydrogen and producing purified water from seawater and fuel cells. The seawater splitting has recently received significant interest; a pre-purification process is also needed to decrease the cation element’s concentrations and remove contaminants.

Consequently, higher energy consumption is required for splitting seawater into hydrogen and oxygen.

Cationic poisoning is a significant challenge regarding seawater electrolyzers. The presence of cationic contamination can lead to elec- trolyzer failure. Anodes have been shown to be influenced by higher cation element concentrations [169]. Cathodes are also affected by

cationic contamination. HER occurs at the cathode, generating negative hydroxyl (OH⁻) ions. The impurities in the water influence the con- ductivity of the AEM. Table (6) lists the maximum levels of cationic contaminants in drinking water. The higher levels of primary and sec- ondary elements also have negative effects on the process of splitting impure water [61].

Furthermore, various chemical degradation pathways of polymer development have been studied [170]. The membrane’s physical degradation is also accelerated by the use of impure water; in particular, metal hydroxides can damage the electrolyzer membrane. Contamina- tion influences mass transport and decreases performance. Ionic contamination affects the AEM and PEM membranes, leading to decreased water transport.

Further investigation is needed to increase the membrane perfor- mance using impure water. The membrane permselectivity is the most significant challenge to mitigate the impacts of overpotential resulting from the impure water [163]. Electrodialysis has been investigated for the purification and desalination of various contaminated feedstock water levels [166,167,171]. Further advancements in membrane design are required to overcome undesirable impurities and increase the life- time. The direct splitting of the dirty water and seawater by conven- tional alkaline and PEM electrolyzers can be performed under certain conditions.

5.2. Technical barriers of PEM stack electrolyzers

PEM electrolyzers split water based on an electrochemical process into hydrogen and oxygen molecules. The PEM electrolyzers can be used to generate green hydrogen from renewable electric sources. PEM electrolyzers have several advantages, including more minor mass vol- ume characteristics, higher hydrogen production purity, and higher safety levels [172]. A PEM electrolyzer is comprised of an electrolytic conductor, anode, and cathode. The integration of the anode and cath- ode with the electrolyte is named the membrane electrode assembly (MEA). In a hydrogen electrolyzer stack, the electric energy is converted into chemical energy, and the electrons and protons recombine to generate hydrogen at the cathode, as shown in the cross-sectional schematic in Figure (15) [95]. The minimum applied voltage to initiate the water electrolysis reaction corresponds to the sum of the reversible potential for each semi-reaction at both electrodes.

The basic hydrogen electrolyzer stack layout is presented in Figure (16). The electrolysis process has been used for many years to generate hydrogen for the ammonia synthesis process. A commercial electrolyzer stack can generate high-purity hydrogen using renewable electricity. The most significant challenge faced by this technology is the availability and cost of pure water as a feedstock for the system. PEM commercial electrolyzer stack performance was investigated for Fig. 12. AEM water electrolysis performance at different HER catalyst

loading [48].

Fig. 13.Energy consumption of various desalination technologies [167,168].

hydrogen production from artificial river water [173]. The commercial PEM electrolyzer, model EHC070 from Enoah Inc., Japan, composed of a Nafion membrane and using a noble metal catalyst layer, has been investigated. The hydrogen electrolyzer stack components are shown in Figure (17). The total active area of electrodes was 11.34 cm². The noble metal catalysts Pt and IrO₂ were utilized for the cathode and anode, respectively. The protons H⁺are passed through the Nafion membrane from the anode to the cathode side. The effects of the cation element on the MEA performance were also measured. The influence of the cation elements (Na⁺, Mg²⁺, K⁺, and Ca²⁺) on the MEA performance can be seen in the cell resulting voltage. Figure (18) depicts the resulting cell voltage utilizing soft and ultrapure water. The impure water or artificial soft water cell voltage show a higher voltage than that for the ultrapure water; the cell voltage substantially increased to 3 V for contaminated water. In contrast, the purified water cell voltage has a constant value of 1.8 V. It was clear that the cation elements decreased the conductivity of the MEA, and these elements are replaced with H⁺protons. The cell voltage rapidly increased because of the hydrogen production reaction

at the cathode [174].

Water contaminants influenced the PEM electrolyzer cell voltage, which appeared in the larger cell voltage compared with the pure water.

The concentration of higher cation elements decreased the cell effi- ciency because the electrolyzer efficiency is inversely proportional to the electrolyzer’s resulting voltage, meaning that the cell voltage increased based on the water contaminants; this leads to lower energy efficiency and higher energy consumption by the electrolyzer [175]. Impure water can be supplied to the PEM electrolyzer, but it will decrease the cell performance and the durability of the electrolyzer. The performance of the cell electrolyzer using different water types with long-term opera- tion (200 h) is shown in Figure (19). The cell voltage was measured for 200 h for each tested sample. It was observed that the steady cell voltage was obtained at ¼ soft or river water concentration mixed with ultrapure water, then the voltage increased with the operation time because of the higher depositing of cations ions on the cell membrane that increased the cell resistance and decreased the membrane conductivity. Moreover, a lower affinity was exhibited from the monovalent cation ions Na⁺and K⁺compared with divalent contaminants that were clearly observed for the resulting cell voltage.

6. Summary and outlook of the hydrogen production from water electrolyzers

Most studies of impure water electrolyzers focus on the anode re- actions. However, the electrolyzer’s design should be included in both electrodes—cathode and anode. AEM and PEM electrolysis cells have inherent challenges due to the cation element’s concentrations and poisoning effects. The continuous improvement of the cell performance is required by using inexpensive materials such as higher catalyst loading and electrode structure, and maintaining the MEA conductivity will enhance the electrolyzer performance. Furthermore, the relation between the catalyst durability and characteristics like morphology, catalyst phase, and composition should be investigated. The deposition Fig. 14.The proposed integrated system for meeting power and pure water demands using seawater.

Fig. 15.PEM electrolyzer cross-sectional view [95].

Fig. 16.Basic layout of H2 and O2 production including the electrolyzer.

Fig. 17.Exploded view of a commercial PEM electrolyzer: a) end plates, b) current collectors, c) gaskets, d) gas diffusion layers, e) MEA [173].

Fig. 18.PEM performance at different feeding water types [173].

Fig. 19.PEM cell performance for various feeding water types [173].