Low cost hydrogen production by anion exchange membrane electrolysis: A review

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Low cost hydrogen production by anion exchange membrane electrolysis: A review ⁎

⁎

Immanuel Vincent , Dmitri Bessarabov

DST HySA Infrastructure Centre of Competence, Faculty of Engineering, North-West University, Potchefstroom 2520, South Africa

A R T I C L E I N F O

A BS T RAC T

Keywords: Anion exchange membrane Electrolysis Non-noble metal catalyst Membrane electrode assembly Hydrogen production

Anion exchange membrane (AEM) water electrolysis is a hydrogen production method that is achieved with an AEM, using electricity. One of the major advantages of AEM water electrolysis is the replacement of conventional noble metal electrocatalysts with low cost transition metal catalysts. AEM electrolysis is still a developing technology; therefore, with a view to using it to eventually achieve commercially viable hydrogen production, AEM electrolysis requires further investigation and improvements, specifically regarding its power efficiency, membrane stability, robustness, ease of handling, and cost reduction. This review addresses state of the art technology of AEM electrolysis for hydrogen production. It also provides a summary of important research that has been carried out on membranes, electrocatalysts, and ionomers used in AEM electrolyzers, and the performance of such electrolyzers. The aim of this review is to identify gaps in AEM water electrolysis research and to make recommendations for future directions in AEM water electrolysis research.

1. Introduction Hydrogen has been identified as an alternative energy carrier to generate power for domestic, industrial, and automotive purposes [1– 5]. It has great potential to change the world's energy sector, similar to what the computer and the internet achieved in the modern information technology sector [6]. Hydrogen offers many advantages as an alternative energy carrier, one of which is the energy content of hydrogen (118 MJ kg–1 at 298 K), which is much higher than that of most fuels (e.g., gasoline 44 MJ kg–1 at 298 K) [7]. Furthermore, the use of hydrogen can mitigate the issues associated with the use of fossil fuels: CO2 and other greenhouse gases [8–13]. Hydrogen, in the form of hydrocarbons and water, is one of the most readily available elements on our planet. It can also be produced by various methods, such as thermal, electrolytic, and photolytic processes from fossil fuels, biomass, and water [14–18]. Water is considered a long-term source for the production of hydrogen due to its wide availability. The production of hydrogen from water can significantly reduce the depletion of fossil fuels and CO2 emissions [19–22]. Hydrogen can be produced from water by various methods,

including water splitting and electrolysis. Water splitting can be carried out by a thermochemical process, chemical conversion of biomass, and photocatalytic water splitting [23]. In water splitting, the water is disassociates into hydrogen and oxygen by a series of consecutive chemical reactions. The conversion occurs in the temperature range 800–2000 °C [24]. The heat required to convert water into hydrogen and oxygen may be generated by solar or nuclear power plants [25]. The major advantage of thermochemical method is that issues associated with various separation stages of the components can be avoided. Photoelectrocatalytic hydrogen production is another promising method for the production of hydrogen from water [26]. Here, solar energy is converted into chemical energy, in the form of hydrogen, by using TiO2 as a photocatalyst [27]. In this technology, the oxidation and reduction reactions occur simultaneously. The major disadvantages associated with the use of TiO2 are wide band gap limits in the visible light region, frequent recombination of photogenerated electron-hole pairs, and higher overpotential for evolution [28]. Various steps have been taken to eliminate the disadvantages of TiO2; it has been coupled with a carbon material, surface modified with adsorbents,

Abbreviations: AEM, anion exchange membrane; CCM, catalyst coated membrane; CCS, catalyst coated substrate; CV, cyclic voltammetry; DFT, density functional theory; DI, deionized; EIS, electrochemical impedance spectroscopy; GDL, gas diffusion layer; GDE, gas diffusion electrode; GO, graphene oxide; HER, hydrogen evolution reaction; I2MEA, integrated inorganic membrane electrode assembly; LDH, layered double hydroxide; LDPE, low density polyethylene; MEA, membrane electrode assembly; OER, oxygen evolution reaction; ORR, oxygen reduction reaction; PEM, proton exchange membrane; PGM, platinum group metal; PSF, polysulfone; QAPS, quaternary ammonium functionalized polystyrene; VBC, vinylbenzyl chloride ⁎ Corresponding author. E-mail addresses: [email protected] (I. Vincent), [email protected] (D. Bessarabov). http://dx.doi.org/10.1016/j.rser.2017.05.258 Received 24 August 2016; Received in revised form 24 February 2017; Accepted 29 May 2017 1364-0321/ © 2017 Published by Elsevier Ltd.

Please cite this article as: Vincent, I., Renewable and Sustainable Energy Reviews (2017), http://dx.doi.org/10.1016/j.rser.2017.05.258

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and TiO2 incorporated into nanoparticle [28]. Furthermore, an mesoporous TiO2 nanopowders has been prepared to enhance the catalytic activity, using cetyltrimethylammonium bromide as a surfactantdirecting and pore-forming agent [27]. A metal-free, conjugated, semiconductor-based active polymeric graphitic carbon nitride gC3N4 photocatalyst was developed for the photocatalytic H2 production [29,30]. The performance of g-C3N4 was enhanced by metal doping, metal deposition, and incorporation of a carbon compound [31] The photocatalytic performance was increased by an appropriate band structure formation at the heterojunction interface [32,33]. The modified Pt-loaded g-C3N4 exhibited a considerable improvement in the photoreduction [31]. Water electrolysis is a well-developed technology for the conversion of water into hydrogen and oxygen at low temperatures [34–37]. Electrolysis can be distinguished according to the type of electrolyte used in the electrolyzer. The two main water electrolysis technologies to produce hydrogen are alkaline electrolysis [38–42] and proton exchange membrane (PEM) electrolysis [43–45].

The formation of K2CO3 reduces the performance of alkaline electrolysis. This is due to several factors. 1. This reaction reduces the number of hydroxyl ions present in the anolyte that are needed for the anodic reaction [66]. 2. It also reduces the ionic conductivity of the electrolyte due to modification of the electrolyte composition [62]. 3. The K2CO3 precipitates in the pores of the gas diffusion layer (GDL), blocking ion transfer [67,68]. Therefore, the overall performance of electrolysis is reduced when using KOH liquid as an electrolyte. Furthermore, the hydrogen is stored at high pressure in the cathode compartment, which is not possible with liquid KOH [69–71]. On the other hand, although PEM electrolysis is efficient, the Nafion membrane, which is used in this type of electrolysis, is highly acidic (equivalent to 10–20% H2SO4) [72]; it therefore limits the choice of suitable catalyst material to the noble metals [73–75]. Nafion-based membranes that are commonly used in PEM electrolysis are very expensive. Furthermore, the conductivity of the membrane is reduced by the release of impurities from the feed water in the cathode compartment. Impurities bind to the active sites of the membrane in the membrane electrode assembly (MEA) [76–78]. Furthermore, the stack materials are more expensive than the stack materials used for alkaline electrolysis [79].

1.1. Conventional electrolysis 1.1.1. Alkaline electrolysis In alkaline electrolysis, the most commonly used anode and cathode materials are nickel- and cobalt-based oxides, respectively, and the most commonly used liquid electrolyte is 30–40% KOH [46,47]. The electrolyte is circulated through the electrodes to provide the alkalinity required by the system. The anode and cathode chambers are separated by a porous diaphragm which conducts hydroxyl ions but not hydrogen and oxygen [46,48,49]. The diaphragms are made of ceramic oxides such as asbestos and potassium titanate, or polymers such as polypropylene and polyphenylene sulfide [50–53]. Typical alkaline water electrolysis operates at a current density of about 400 mA cm–2, at moderate temperatures of 70–90 °C, with a cell voltage in the range 1.85–2.2 V, and conversion efficiencies in the range 60–80% [54]. The advantages of alkaline electrolysis are that it does not depend upon a noble metal catalyst for the hydrogen production and handling is easy due to the relatively low temperatures [55].

1.3. Polymeric anion exchange membranes Over the past few years, polymeric anion exchange membranes (AEMs) have been developed for electrochemical system applications. They offer benefits for both PEM and alkaline electrolysis, and for fuel cells [80]. AEMs have been used for alkaline fuel cells but to date not for electrolysis [81–84]. The main difference between alkaline and AEM electrolysis is the replacement of the conventional diaphragm with an AEM in alkaline water electrolysis. AEM electrolysis offers several advantages. 1. A transition metal catalyst is used instead of a noble metal (platinum group metal; PGM) as catalyst. 2. Distilled water or a low concentration of alkaline solution can be used as electrolyte instead of concentrated KOH. 3. The quaternary ammonium ion-exchange-group-containing membrane that is used in AEM electrolysis is less expensive than the Nafion-based membranes. 4. The interaction between CO2 and the AEM is low due to the absence of metal ions in AEMs. 5. Furthermore, the absence of a corrosive liquid electrolyte in this technology offers advantages such as the absence of leaking, volumetric stability, ease of handling, and a reduction in the size and weight of the electrolyzer.

1.1.2. Proton exchange membrane electrolysis In PEM electrolysis, the anode and cathode catalysts are typically IrO 2 and Pt black, respectively [56,57]. An acidic membrane is used as solid electrolyte (Nafion, DuPont) instead of a liquid electrolyte (The membrane conducts H+ ions from anode to cathode, and separates hydrogen and oxygen that are produced in the reaction. The PEM electrolyzer can operate at a current density of 2000 mA cm–2 at 90 °C, at about 2.1 V [58]. The kinetics of the hydrogen and oxygen production reaction in PEM electrolysis are faster than in alkaline electrolysis due to the acidic nature of the electrolyte and the metal surface of the electrodes [59]. PEM electrolysis offers safety due to the absence of caustic electrolyte [60]. One of the advantages of PEM electrolysis is the possibility of using high pressure on the cathode side, while the anode can be operated at atmospheric pressure [61].

Overall, AEM technology is low cost and highly stable for hydrogen production [85]. The specifications, advantages, and disadvantages of the major electrolysis techniques are summarized in Tables 1 and 2.

1.2. Technology and cost problems associated with conventional electrolysis

1.4. Anion exchange membrane electrolysis AEM electrolysis is a developing technology. Many research organizations and universities are actively involved in this research, largely due to its low cost and the high performance it offers [72]. However, compared to the other conventional electrolysis technologies, few research articles ( < 20) have been published on AEM electrolysis. AEM electrolysis still requires further investigations into the following, for example: power efficiency, membrane and catalyst stability, ease of handling, and reduction of cell cost [72,85]. Significant improvements

During electrolysis, the oxygen produced at the anode is frequently in contact with the air. The liquid KOH electrolyte is highly sensitive to CO2 in the ambient air [62–64]; hydroxyl ions react with the CO2 and K2CO3 is formed [62,65]:

CO2 + 2OH− → CO3 + 2H2 O

CO2 + 2KOH → K2 CO3 + H2 O 2

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Table 1 Comparison of main water electrolysis technologies with AEM electrolysis.

Electrolyte Charge carrier Temperature range (°C) Typical Discharge H2 pressure (bar) Separator OER catalyst HER catalyst Typical current collector Cell sealant Anodic reaction Cathodic reaction Conventional current density (mA cm−2) Demonstrated durability (h) Hydrogen purity (vol%) Typical current efficiency Demonstrated rated production(N m3 h−1) Specific energy consumption (kW h N m−3) Demonstrated rated power (kW) System cost (€ kg−1) Technology status

Alkaline [49,50]

PEM [49,50]

AEM [49,74]

20–30% KOH OH− 65–100 25–30 Asbestos, PAMa, ZrO2–PPSb, NiO, Sb2O5–PSc Ni2CoO4, La–Sr–CoO3, Co3O4 Ni Ni Metallic 2OH- → H2O + ½O2+ 2e2H2O + 2e- → H2 + 2OH200 − 500 100 000 99.3–9.99 50–70.8 1–760 4.5–7.5 2.8–3534 1300–800 Mature

PFSA H+ 70–90 30 − 80 PFSA(e.g., Nafion) Ir/Ru oxide Platinum Titanium Synthetic rubber or fluoroeleastomer 2H2O → O2 + 4 H+ +4e4 H+ + 4e- → 2H2 800 − 2500 100 000–50 000 99.9999 48.5–65.5 0.265 − 30 5.8–7.3 1.8–174 2000–1200 Mature for small scale

QAPS OH− 50–70 ~ 30 QAPS(e.g., A−201) Co3O4 CeO2-La2O3 Ni Synthetic rubber or fluoroeleastomer 2 OH- → H2O + ½O2+ 2e2H2O + 2e- → H2 + 2OH200 − 500 NA 99.99 39.7 0.25–1 5.2–4.8 1.3–4.8 NA R&D

Abbreviations: a Polysulphone-bonded polyantimonic acid. b ZrO2 on polyphenylsulphone (PPS). c Polysulphone impregnated with Sb2O5 polyoxide. PFSA – perflurosulfonated acid. QAPS – quaternary ammonia polysulfone. OER – Oxygen evolution reaction. HER – Hydrogen evolution reaction. NA- Not available.

performance of the AEM electrolyzer, as characterized under various operating conditions. Section 5 addresses the modeling of AEM electrolysis. Section 6 presents a summary and outlook of the state of the art of AEM technology. Finally, suggested AEM electrolysis research areas, and suggestions for future research and development are given in Section 7.

Table 2 Advantages and disadvantages of alkaline, PEM and AEM electrolysis [43]. Alkaline

Mature technology Non-PGM catalyst Long term stability Low cost Megawatt range Cost effective Low current densities Crossover of gas Low dynamic Low operating pressure Corrosive liquid electrolyte

PEM Advantages Higher performance Higher voltage efficiencies Good partial load Rapid system response Compact cell design Dynamic operation Disadvantages High cost of components Acidic corrosive components Possible low durability Noble metal catalyst Stack below Megawatt range

AEM

Non-noble metal catalyst Noncorrosive electrolyte Compact cell design Low cost Absence of leaking High operating pressure

2. Basic principle of anion exchange membrane electrolysis AEM electrolysis is the electrochemical splitting of water into hydrogen and oxygen with the assistance of an AEM. Fig. 1 illustrates the AEM process and its components. An external power supply is

Laboratory stage Low current densities Durability Membrane degradation Excessive catalyst loading

are still required in the development of an AEM-based electrolyzer to ultimately contribute to a future hydrogen-based economy. Therefore, a thorough review of topics related to AEM electrolysis is considered necessary, for example: AEM electrolysis performance; operating parameters; and the components used for the electrolyzer, such as membranes, ionomers, electrocatalysts, and, overall, the MEA. This review of currently available literature identifies gaps that exist in the research, and hence areas in which further investigation and development are required. 1.5. Layout This review article is structured as follows. Section 2 describes the theory of AEM electrolysis and the basic principles involved. Section 3 describes the components of AEM electrolyzers, including the membrane, ionomer, electrocatalyst, and the MEA. Section 4 describes the

Fig. 1. Schematic diagram of anion exchange membrane (AEM) water electrolysis. GDL: gas diffusion layer, ACL: anode catalyst layer, CCL: cathode catalyst layer, MEA: membrane electrode assembly.

3

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3.1.1. First-principles DFT calculations A comprehensive theoretical study of the hydroxyl ion (OH–) transport mechanism through an AEM was investigated based on density functional theory (DFT) by Yang et al. [88]. The study was carried out for the quaternary-ammonium-functionalized polystyrene (QAPS), coupled with OH−. The first-principles method based on DFT calculations was performed by using the Gaussian 09 suite of programs [89]. In an AEM electrolyzer, the hydroxyl ions are generated by the HER at the cathode, then diffuse through the AEM to the anode. In the AEM, the OH– ions are fixed on one side of trimethylamine through three bifurcated hydrogen bond between –CH3 and OH–. The binding energy of the hydrogen bond is 47.1 kcal mol–1, which is very high, and it has strong interaction towards OH–. Hence, the OH– moves towards the hydrogen bond spontaneously due to the interaction. The authors explored the relationship between ion transport and rotation of C–CH2 bond linking with the QA groups. The transition state of the rotation minimum is around 180° between the potential energy barrier and QA groups. The calculated energy barriers are given in Fig. 4 of literature [88]. The OH– moved to between two QA groups due to the rotation of the C–CH2 single bond and transition state structure. The rotation energy barrier was 4.1 kcal mol–1 when the system was under vacuum, which is lower than the water solution. However, the reverse rotation energy barrier in water solution (35.5 kcal mol–1) was much lower than the vacuum. The authors concluded the above process is the ratedetermining step of the OH– into the QAPS-AEM. The simulation results were in good agreement with the results of Takaba et al. [90] and Chen et al. [91]. According to Clark and Paddison [92], the side chain connectivity also plays an important role in the ion exchange process in the AEM. Therefore, three –CH2 units were added between the two side chains of –(CH3)3. The binding energy of a hydrogen bond between OH− and – (CH3)3 in the model was 41.9 kcal mol–1, which was higher than the rotation energy barriers (10.9 kcal mol–1). The OH– was traverse across the functional group following the rotation of the QA group by hydrogen bonding. When the length of the adjacent side chain was long, the hydroxyl ion transport was slow. This was due the lower degree of chloromethylation of the polymer matrix and the lower conductivity of the AEM [93,94].

connected to the anode and cathode to provide a DC supply. The overall reaction consists of two half-cell reactions: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Water is circulated through the anode side where it is reduced to form hydrogen and hydroxyl ions by the addition of two electrons. The hydroxyl ions diffuse through the AEM to the anode section by the positive attraction of the anode, while the electrons are transported to the anode through the external circuit. In the anode chamber, the hydroxyl ions recombine as water and oxygen by losing electrons. The oxygen forms as bubbles and is released from the surface of the anode. Both half-cell reactions require catalytic activity to form and release the respective gases from the electrode surfaces. The following half-cell reactions apply [72]:

Anode : Cathode :

Overall :

4OH− → O2 + 2H2 O + 4e− 4H2 O +

4e−

→ 2H2 +

2H2 O → 2H2 + O2

E0 = 0. 401 V

4OH−

E0 = −0. 828 V

E0 = 1. 23 V

The overall reaction requires 1.23 V theoretical thermodynamic cell voltage to split the water into hydrogen and oxygen at 25 °C. However, in practice, the requirement of the cell voltage for efficient hydrogen production is > 1.23 V. The additional voltage is required to overcome both the kinetics and ohmic resistance of the electrolyte and components of the electrolyzer. Typical operational cell voltages for alkaline and PEM electrolysis are 1.85–2.05 V and 1.75 V at 70–90 °C [86]. 3. Anion exchange membrane electrolyzer components Developments pertaining to the main electrolyzer components, such as the membrane, ionomer, electrocatalyst, and MEA, and their effects on electrolysis performance, are now reviewed. Table 3 shows the components of the MEA, the operating conditions, and subsequent maximum current density obtained at a particular voltage. 3.1. Anion exchange membranes The AEM is one of the chief components that determines the performance of an electrolyzer. The function of the AEM is to transport hydroxyl ions from cathode to anode, and to act as a barrier for electrons and gases that are produced by the electrochemical reaction [85]. The AEM is composed of a polymer backbone coupled with anion exchange functional group. Generally, the polymer backbone is polysulfone (PSF) or polystyrene cross linked with divinylbenzene (DVB), and the ion exchange groups are –NH3+, –RNH2+, –RN+, =R2N+, – R3P+, –R2S+, or quaternary ammonium salts [85]. The desirable properties of an efficient AEM include high mechanical, thermal, and chemical stability, ionic conductivity, and barrier action with respect to electrons and gases. The polymer backbone is responsible for mechanical and thermal stability. The functional group is accountable for the ion exchange capacity, ionic conductivity, and transport number. Both the polymer matrix and the functional groups together determine the chemical stability [87]. The synthesis of an AEM with high mechanical stability and high ionic conductivity is challenging. The addition of excess ion exchange groups may increase the ionic conductivity, but it leads to loss of mechanical strength due to excessive water uptake. The AEM then becomes chemically unstable due to hydroxide attack on the fixed ion, which leads to poor ionic conductivity. Another major limitation of an AEM is degradation of quaternary ammonium groups by nucleophilic substitution and the Hoffmann elimination reaction (displacement of CH3 by OH–), which may reduce the conductivity of the membrane [85]. Alkaline polyelectrolyte membranes have recently been used in AEM electrolysis. The first DFT-principles of DFT calculations, development of these membranes, and their properties, such as ionic conductivity, stability, and degradation, are now discussed.

3.1.2. Alkaline polymer electrolyte membranes An alkaline polymer electrolyte membrane is generally composed of a polymer that conducts hydroxyl ions. Cationic functional groups are grafted onto the polymer backbone. Fig. 2(a) shows the quaternary ammonium group grafted onto a PSF backbone [95]. A commercially available polymer electrolyte that is commonly used in AEM electrolysis is A-201 (Tokuyama Corporation, Japan). Fig. 2(b) shows the structure of an A-201 membrane, with associated hydroxyl ions on the functional groups [96]. The backbone of A-201 is a linear hydrocarbon and the functional groups are quaternary ammonium groups. Besides the above, several other polymer backbones and functional groups for AEMs have been developed and tested for AEM electrolysis. The polymer backbones include PSF, chloromethylated low-density polyethylene (LDPE), methylated melamine, and poly(vinylbenzyl chloride). Functional groups such as 1-azabicyclo-[2.2.2]-octane and 1-methylimidazole have been investigated [97,98]. 3.1.2.1. Ionic conductivity. The ionic conductivity of an AEM plays a significant role in the performance of the AEM. Higher levels of hydroxyl ion conductivity allow much higher current densities to be achieved. The ionic conductivity of A-201 in the HCO3– form is 12 mS cm–1 at 25 °C [96]. Parrondo et al. [99] developed and characterized AEMs with a backbone of chloromethylated PSF with various cationic functional groups: quaternary benzyltrimethylammonium (PSF-TMA+ OH–), 4

Carbon paper

Ni foam

Parrondo et al. [99]

Xiao et al. [101]

5

Zeng et al. [129] Velan et al. [119] P. Sivakumar et al. [120] Pandiaranjan et al. [126]

Zeng et al. [125]

Scott et al. [106]

Scott et al. [98]

Ni-Fe Graphene Graphene

Ce0.2MnFe1.8O4

Pt coated Ti

Ni/CeO2-La2O3/C

3.5

40

3

3

Cu0.7CO2.3O4

Cu0.7CO2.3O4

3

0.085

40

2.5

36

2.9

Ni Ni/CeO2-La2O3/C Cu0.7CO2.3O4

Ni-Fe

Pb2Ru2O6.5

Ni/CeO2-La2O3/C

IrO2

Ni foam NiO Ni oxide

Carbon paper Ni foam Stainless steel mesh Stainless steel mesh Stainless steel mesh Ni foam

Ni foam

Pavel et al.[85]

Ayers et al. [103] Ahn et al.[110] Faraj et al. [97] Scott et al. [111]

Ti foam

Pt coated Ti

Carbon cloth NiO Ni

Carbon paper Ni /C Stainless steel mesh Stainless steel mesh Stainless steel mesh Carbon cloth

Stainless steel fiber felt

Carbon paper

Carbon cloth

Ti foam

GDL

Loading mg cm−2

GDL

Catalyst

Cathode

Anode

Membrane electrode assembly

Leng et al. [72]

Reference

Ni

Ni-Mo Graphene Graphene

CuCoO3

Nano Ni

Nano Ni

Ni CuCoO3 Pt

Ni-Mo

Pt black

CuCoO3

Pt black

Catalyst

3.5

40

2

2

1

0.085

40

2.5

7.4

3.2

Loading mg cm−2

Table 3 A review on the materials, components used on AEM water electrolysis research and development in recent years.

FAA-3-PK-130

Mg-Al layered double hydroxide PSF Quaternary ammonium Selemion AMV Selemion AMV

QPDTB

A-201, Tokuyama LDPE-g-VBC Quaternary ammonium radiation grafted membrane mm-qPVBz/Cl-

(xQAPS)

Chloromethylated PSF (CMPSF)

A-201, Tokuyama

A-201, Tokuyama

Membrane

–

1.8

1.9 2

2.2

1.9

1.9

QPVB/ClPoly(DMAEMA-coTFEMA-co-BMA) PTFE

1.8 1.9 2.1 1.8

300

150 90

208

100

100

500 150 460 100

400

400

1.8

1.85

470

399

Current density mA cm−2

1.9

1.8

Voltage V

– AS-4

PSF-TMA+ Cl-, PSFABCO+ Cl- or PSF-1M+ Cl-) (xQAPS)

PTFE

AS-4

Ionomer

Operating condition

70 80 30

70

50

55

50 50 25

70

50

50

50

Temperature °C

0–5.36 KOH Deionized water Deionized water

Deionized water Deionized water 0.1 M NaOH/ Na2CO3

1 M KOH 1% K2CO3 1 M KOH

Ultrapure water

deionized water 1% K2CO3/ KHCO3 Ultra-pure water

Electrolyte

I. Vincent, D. Bessarabov

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O

( O

Faraj et al. [97] investigated the UV-induced grafting of vinylbenzyl chloride (VBC) with Dabco. They determined the stability by running the electrolysis at 460 mA cm–2 for 500 h. The cell potential increased by 6 mV per day and the resistance increased from 0.30 to 0.43 Ω cm2. A possible reason for the increase in cell potential was loss of the quaternary ammonium sites from the backbone due to the strong alkaline environment. Xiao et al. [101,102] also monitored the stability of a quaternary ammonium PSF membrane at 70 °C. They found the cell potential to be stable (1.8–1.85 V) over a period of 8 h. Ayers et al. [103], using the Fenton's reagent test, observed the degradation of hydroxyl ions after 5 h performance. FTIR and Raman spectroscopy were also used to determine the stability of the polymer. The membrane was monitored over periods of 0, 2.5, and 5 h. The membrane remained physically intact, but spectral analysis revealed signal losses associated with SO2, C–O–C, C–N, and HCO3–. This provided an indication of the degradation of the polymer backbone of the membrane. Cracks, observed by optical microscopy, also indicated degradation. Nonetheless, the membrane survived for 5 h in the aggressive alkaline medium. Faraj et al. [97] monitored the stability of the LDPE-g-VBC-Dabco membrane over a period of 500 h and found that it exhibited better stability than some other membranes. The stability and ionic conductivity of the LDPE-g-VBC-Dabco membrane were reduced due to the alkalinity, the degradation of the polymer backbone or the ion exchange functional groups, or both. Parrondo et al. [99] confirmed the degradation of the polymer backbone; however they did not explain the mechanism of the degradation or the interaction between hydroxyl ions in the polymer backbone and the fixed cation exchange groups. Overall, the performance and stability of the LDPE-g-VBC-Dabco membranes were considered satisfactory. Although the hydroxyl ion conductivities achieved in some polymers are adequately high, the stability of the AEM is lower than the level required for the commercialization of AEM electrolysis. Further research is essential for the development of new polymers with more stable functional groups and with high ion exchange properties.

CH3

S

C

O

O

CH3

H2C

)n

Me3N+OH (b)

+ N OH-

N

+

OH-

+ N OH-

N

+

OH

-

+ N OH-

N

+

-

OH

N

OH-

+

Fig. 2. (a): Structure of the quaternary ammonium polysulfone membrane [95]. (b): Structure of the A-201 membrane (Tokuyama Corporation, Japan)[96].

quaternary benzyl quinuclidium 1-azaoniumbicyclo-[2.2.2]-octane (PSF-ABCO+ OH–), and quaternary benzyl-1-methylimidazolium (PSF-1M+ OH–). These cationic ion exchange groups are functionalized on the benzyl position of the PSF polymer matrix in the chloride ion form. The ionic conductivities were calculated and results for the three different exchange groups were compared. The ionic conductivities of the PSF-ABCO+ OH–, PSF-TMA+ OH–, and PSF-1M+ OH– at 60 and 80 °C were 12, 15, and 16, and 5, 14, and 20 mS cm–1, respectively. Ionic conductivity of PSF-TMA+ OH– and PSF-1M+ OH– increased with increasing temperature, but that of PSF-ABCO+ OH– decreased. Membranes containing a PSF backbone with TMA+ OH– exhibited highest conductivity. Cao et al. [98] synthesized a methylated melamine grafted poly(vinylbenzyl chloride) (mm-qPVBz/Cl−) membrane. The melamine was used as an amination reagent. When the temperature was increased from 25 to 60 °C, the ionic conductivity of the membrane increased from 16 to 27 mS cm–1, while the resistance of the membrane decreased from 0.62 to 0.37 Ω cm2. Faraj et al. [97] prepared a LDPE-based AEM by a UV-induced grafting method. The quaternary ammonium sites with 1,4-diazobicyclo-(2.2.2)-octane (Dabco) were grafted on the LDPE backbone. The ionic conductivity recorded at 30, 45, and 60 °C was 14, 19, and 25 mS cm–1 (in the form of HCO3–/CO3–).

3.2. Ionomers Ionomers are binders that assist in creating transport pathways between the membrane and the reaction sites in the catalyst layer. PSF is the most widely used alkaline ionomer due to its thermal and chemical stability [104]. The ionic conductivity of the ionomer can be improved by increasing the number of ion exchange groups in the polymer matrix. The main challenge associated with an increase in the number of ion conductive groups is that the water uptake also increases, which leads to dissolution of the ionomer in the solvent at elevated temperatures. The addition of cross-linkers to the polymer matrix may increase the strength of the polymer chain, but it may also reduce the chain mobility and void volume. However, the addition of an unsuitable cross-linker restricts the hydroxide mobility by forming short-chain cross-linking [105]. The stability and performance of the ionomers used in AEM electrolysis are now discussed. Wu and Scott [106] developed a hydroxide ion conductive ionomer for AEM electrolysis. They synthesized poly 2-dimethylaminoethyl methacrylate; 2,2,2,-trifluoroethyl methacrylate; and butyl methacrylate (DMAEMA-co-TFEMA-coBMA) by co-polymerization followed by quaternization and evaluated performance in AEM electrolysis. The mechanical properties of the MEA present in the ionomer were characterized. The Young's modulus and elongation rupture point were 0.229 GPa and 48.6%, respectively. The conductivity of the quaternized poly(DMAEMA-co-TFEMA-coBMA) membrane containing 26 mol% DMAEMA increased from 15 to 25 mS cm–1 when the temperature was increased from 20 to 50 °C. The membrane containing 41 mol% DMAEMA exhibited higher conductivity than a membrane with 21 mol% DMAEMA; conductivities as high as 59 mS cm–1 at 50 °C were achieved with the former. The

3.1.2.2. Stability. The stability of the membrane is one of the crucial factors that determine whether or not a membrane is suitable for AEM electrolysis. The stability of a membrane is commonly examined by keeping the current density constant for a particular period while the voltage is being monitored. Variation in the voltage reflects whether the membrane is stable or unstable; an increasing voltage indicates membrane instability. A possible reason for the instability of a membrane is degradation of the polymer backbone or the ion exchange groups. The commercially available Tokuyama A201 membrane is reported to be stable for 600 h at 100 mA cm–2 at 50 °C [100]. Parrondo et al. [99] reported PSF AEMs with quaternary benzyl ammonium and imidazolium groups to be stable at a constant current density of 400 mA cm–2. The performance was then monitored over a period of 6 h, during which time the cell voltage was increased from 1.6 to 2.4 V. The increase in voltage could be the reason for the degradation of the polymer backbone or the loss of anion exchange groups. However, in post-mortem analysis, it was observed that no degradation of the ion exchange group had taken place. Degradation of the polymer backbone was subsequently cited as the possible reason for CO2 interruption.

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stability was examined at 100 mA cm–2. The cell voltage was 1.9 V and the voltage was reasonably stable over a period of 5 h. Leng et al. [72] determined the stability of the MEAs with the AS-4 ionomer (Tokuyama Corporation, Japan) and the aminated Radel PSF (A-Radel) ionomer. The MEAs was prepared using the catalyst-coated substrate (CCS) technique. The maximum electrolysis performance achieved was 399 mA cm–2 at 1.80 V with high frequency resistance (HFR) of 0.23 Ω cm2. The current density was constant for the A-Radel ionomer for up to 317 h, whereas the current density of AS-4 ionomer was stable for only 40 h. The lifespan of the MEA with the AS-4 ionomer was only 27 h. Investigations were therefore carried out to determine the reason for the degradation. When the anolyte was changed from distilled water to 1 M KOH, the cell voltage decreased significantly, from ~2.55 to ~1.58 V, and the HFR decreased from 1.24 to 0.27 Ω cm2. The most significant finding, however, was that the performance of the MEA recovered when changing from distilled water to KOH. This indicated that the degradation of the MEA was either due to membrane/ionomer or to the catalyst layer. The reason for the decrease in HFR remain unclear. In summary, the A-Radel ionomer exhibited much higher stability than the AS-4 and DMAEMA. Development of durable ionomers with high ionic conductivity is required. More detailed evaluation of performance and stability are also required, prior to wider application in AEM electrolysis.

Fig. 3. Polarization curves recorded after a 24 h test run for an AEM water electrolysis cell containing MEAs with different HER catalyst loadings [85].

amount of catalyst on the electrode and the surface area of the electrode in order to calculate the catalyst mass and specific activities, which are the accepted measures of true ORR catalytic activity. Koutecky–Levich analysis of data obtained from RDE experiments was also performed in order to determine the kinetic parameters of the ORR number of electrons transferred and the heterogeneous rate constant. The mass activity and specific activity were high in the catalyst loading range 0.05–0.1 mg cm–2. The Koutecky–Levich plot was a straight line, which indicates first-order rate kinetics. The 2e– pathway electron transfer was more predominant than the 4e– at lower catalyst loading. The catalyst was moderately stable in alkaline conditions. The reaction rate constant was high, at 0.05 mg cm–2. Parrondo et al. [99] studied AEM electrolysis with low cost OER electrocatalysts. PGM catalysts such as Pt black and Ru perchlorate were used as HER and OER catalysts. Lead-ruthenium oxide pyrochlore was selected as the OER catalyst instead of the conventional IrO2 because the cost was only 8% of that of the IrO2. The catalyst ink Pt black was applied on the carbon paper (CP). Ru was deposited on the porous media electrode as a gas diffusing electrode (GDE). The loading of both catalysts was around 2.5 mg cm–2. Xiao et al. [101] used electrocatalysts such as Ni-Fe and Ni-Mo complexes for the HER and OER to fabricate MEAs. Ni-Fe complexes are known to be the best non-noble metal catalysts for the OER in alkaline media [109]. The Ni-Fe catalyst was uniformly spread on a hot porous electrode. Specifically, a solution containing Ni and Fe nitrates was sprayed onto the hot porous electrode substrate by the electrochemical codeposition method. A negative current was applied to the substrate to achieve uniform distribution of catalyst. Here, the porous electrode was made entirely from Ni-Mo, instead of coating a Ni-Mo layer for the HER. The reason for this is that a thick Ni-Mo layer can lead to high ohmic potential loss. The Ni-Mo solution was filled in the stainless steel skeleton at 500 °C under a H2 atmosphere. The loading of both the catalysts was 40 mg cm–2. The electrocatalysts were stable for 8 h. Ayers et al. [103] characterized AEM electrolysis technology for low cost electrolysis. One of the main objectives of their work was to understand the possibility of using non-noble metal complexes as catalysts. They studied the ternary catalysts 30% Ni-Fe/C, 30% Ni-FeCo/C, and 30% Ni-Fe-Mo/C. Their results are presented in Fig. 4 [103]. The 30% Ni-Fe/C catalyst showed the best results. They then compared their results to results for the conventional catalyst IrO2 and found that the cell voltage was much higher than with IrO2. The electrolysis performance with non-noble metal catalysts was significantly lower than with noble metal catalysts. Ahn et al. [110] used Ni particles as HER catalyst. The Ni particles were deposited on CP using cathodic electrodeposition at the optimized

3.3. Electrocatalysts The theoretical thermodynamic potential for water electrolysis is 1.23 V [107]. However, both the OER and HER require an electrocatalyst to overcome the kinetics of the reaction. Selection of a suitable electrocatalyst is necessary to reduce the overpotential requirement of the electrolysis. Conventional PEM and alkaline electrolysis require noble metals (Ir, Ru, and Pt black) as catalysts, which increases the cost of electrolysis [43]. Therefore, the development of a non-noble metal catalyst is crucial to reduce the cost of AEM electrolysis. Attempts have been made to find less costly transition metal catalysts for AEM electrolysis [85]. Currently, IrO2, Ni, Ni-Fe alloys, graphene, Pb2Ru2O6.5, and Cu0.7CO2.3O4 are used as OER catalysts, while Pt black, CuCoOx, Ni-Mo, Ni/CeO2-La2O3/C, Ni, and graphene are used as HER catalysts. An electrocatalyst is expected to be stable in oxidative and reductive environments. Various techniques can be used to apply the catalyst to the membrane, gas diffusing layer, or current collector. Recently, Pavel et al. [85] developed and evaluated AEM electrolysis using low cost transition metal catalysts. The commercial catalysts Acta 4030 (Ni/CeO2-La2O3/C) and Acta 3030 (CuCoOx) (Acta SpA, Italy) were used as HER and OER catalysts. The loadings of the HER and OER catalysts were 7.4 and 36 mg cm–2, respectively. These catalysts were designed to withstand relatively mild alkaline conditions (pH 10–11). The authors explained the effect of HER loading on the kinetic contribution and performance of the AEM electrolysis. The catalyst loading range selected for HER was 0.6–7.4 mg cm–2. The OER catalyst loading was kept constant at 36 mg cm–2. The results obtained for different catalyst loadings are presented in Fig. 3 [85]. The resistance recorded for the catalyst range 0.6–7.4 mg cm–2 was 0.218– 0.136 Ω cm2. This study revealed that the HER catalyst loading strongly affects the cell potential. The peak potential varied from 2.1 to 2.4 for the catalyst range 0.6–7.4 mg cm–2. The peak area and the peak time were linearly related to the catalyst loading. In a study carried out by us [108], we made use of the commercially available catalyst Acta 3030 (CuCoO)x for the electrochemical characterization. The electrocatalytic activity was examined by cyclic voltammetry (CV) in an inert atmosphere. Thin film RDE (rotating disc electrode) measurements of the ORR (oxygen reduction reaction) in an O2-saturated electrolyte were performed at different rotation rates for different catalyst loadings. The results were normalized to the 7

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copper-centered metal organic framework. The catalyst showed a superior trifunctional catalytic activity for the OER, ORR, and HER. The power density attained by the catalyst was 76% that of the commercial Pt catalyst. Liang et al. [118], reported a hybrid material constituting Co3O4 nanocrystals grown on reduced GO as a highperformance bifunctional catalyst for the both the ORR and the OER. However, the catalytic activity was surprisingly increased by the nitrogen doping of graphene due to the coupling of CO3O4 and graphene. All of these graphene-based electrocatalysts showed excellent catalytic activity and superior stability in alkaline conditions. Velan and coworkers [119] and Joe et al. [120] also investigated AEM electrolysis; they used the graphene oxide-(GO-)modified platinum-free metal electrode. Graphene was adopted for AEM electrolysis because it had already been used effectively in Li ion batteries and many electrochemical systems [121,122]. The electrochemical activity of the electrode was examined by chronoamperometry for the OER reaction at 30 °C. The activity of the electrode was compared to that of the bare Ni foam, NiO foam, and the GO-coated NiO foam. It was observed that the graphene-coated NiO foam provided a higher current density for the OER reaction due to an increased contact area. Electrochemical impedance spectroscopy (EIS) studies also confirmed that the GO-coated NiO showed better charge transfer and ion diffusion characteristics. It was also found that the contact resistance was a function of temperature; there was a decrease in resistance with an increase in temperature, due to the hinting improvements in electrochemical properties. The stability of the electrode was monitored continuously for 24 h at 80 °C in 5.36 M KOH at 1.9 V; the change in voltage appeared to be insignificant. In conclusion, the GOcoated electrode showed a higher current density and better performance than a non-GO-coated electrode. In summary, both the HER and OER are sluggish under alkaline conditions with the non-PGM catalysts that have been studied to date. Two types of non-PGM catalysts were evaluated, Acta 4030 and Acta 3030, and the Ni alloy catalyst. The Acta 4030 and Acta 3030 exhibit better performance than the Ni alloy catalyst. The 30% Ni-Fe alloy catalyst exhibits better performances than the other Ni alloy compounds. Most of the research to date has focused on the OER electrocatalyst. The activity, stability, and low cost offer promise that this graphene-based catalyst could be alternative to the conventional AEM electrolysis catalysts. Further research into the catalytic activity kinetics, and the stability and use of both the OER and HER electrocatalysts is required.

Fig. 4. Chronoamperometric characterization of advanced non-PGM oxygen evolution catalysts for use in AEM electrolysis cells [103].

conditions of 95 V for 50 s to obtain high electrochemical activity. Extremely small quantities of Ni were used (8.5 μg Ni cm−2). Evaluation of the HER and OER electrocatalytic activity of the Ni/CP showed that the CP without Ni was inactive in the KOH electrolyte. The HER and OER characteristics were then evaluated by CV. It was concluded that the electrochemical activity was affected by the electrochemical surface area and structural defects. Wu and Scott [111] evaluated a CuxCo3O4 nanoparticle OER electrocatalyst for AEM electrolysis. The catalyst was prepared by thermal decomposition. They investigated the catalyst reaction mechanism, stability, and electrolysis performance. The HER and OER catalysts were Pt/C and nano-size CuxCo3O4 with loadings of 3 and 2 mg cm–2. The catalyst ink was prepared and coated on both sides of the membrane. The stability of both the catalysts was evaluated using CV; the initial and 200th cycles were compared for CuxCo3O4. Only insignificant differences were observed after the 200th cycle. The CuxCo3O4 was considered more stable than the other compound (Co3O4) in the catalyst. It met the requirements for an OER catalyst in AEM electrolysis. AEM electrolysis in which CuxCo3O4 was used showed better performance than when Co3O4 was used. The resistance for Co3O4 was 0.23 Ω cm2, whereas the resistance for CuxCo3O4 was 0.3 Ω cm2. The authors suggested that the overall resistance was due to lack of contact of catalyst layer and catalyst membrane interface. Recently, graphene has attracted much attention due to the absence of noble metals in the conventional catalysts. Graphene-based catalysts were developed to replace the conventional noble metal catalysts [112]. The former offer high specific surface area, chemical and electrochemical stability, high conductivity, and low cost [113]. Nitrogen atom doping of graphene, induce the changes in the asymmetry spin density of the carbon lattice and local charge density [114]. Recently, nanoribbon network doped with nitrogen, dual-doped graphene-carbon nanosheets as a sandwiched graphene, combination of a graphene oxide (GO) and copper-centered metal organic framework were found to be active for both OER and HER [114–116]. Yang et al. [115], developed a metal-free three-dimensional graphene nano-ribbon network doped with nitrogen. The fast electron transport between the reaction sites was attributed to interconnected graphene networks. Zhang et al. [117], worked on mesoporous carbon foam co-doped with nitrogen and phosphorus. The activities of the OER and ORR were examined, independently, in a three-electrode configuration. DFT calculation confirmed that the N and P co-doping and graphene edge effects are essential for the bifunctional electrocatalytic activity. Li et al. [114], developed nitrogen and phosphorus dual-doped graphene-carbon nanosheets as a sandwiched graphene, a few layers thick. The excellent bifunctional catalytic performance is attributed to the synergistic effects between the doped N and P atoms and the high conductivity of the incorporated graphene. Johan et al. [116], investigated the combination of a graphene oxide (GO) and

3.4. Membrane electrode assembly and electrolyzer performance The MEA is the core of AEM water electrolysis. The MEA comprises the AEM, ionomer, anode and cathode catalyst layers. The MEA is generally fabricated by the catalyst-coated membrane (CCM) method or the CCS method [123,124]. In the CCM method, the homogeneous ink is a mixture of an electrocatalyst and a binder, such as a PTFE solution and isopropyl alcohol (IPA). The ink is applied to both sides of the AEM (e.g., by spraying) and dried. It is then placed between the GDLs and subjected to mechanical or hot pressing. On the other hand, in the CCS method, the electrocatalyst ink is directly deposited on the GDLs and then sintered to form the electrode (GDE). The AEM is housed between both the GDLs or GDEs and hot pressed to form the MEA [72]. The MEA is the chief component that determines the performance of the electrolyzer. The electrolyzer performances, based on the MEAs used by various researchers, are now discussed. Pavel et al. [85] fabricated the MEA by the CCS method. The A201 was used as AEM. The Acta 4030 and the Acta 3030 were used as the HER and OER catalysts, with loadings of 36 and 7.4 mg cm– 2 , respectively. The membrane was placed between the catalystcoated GDLs and mechanically pressed with a filter press to form the MEA. Electrolysis experiments were carried out at 43 °C and 8

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anode catalyst layer was prepared using homogenous inks composed of Acta 3030, DI water, IPA, and I2 ionomer solution (Acta SpA, Italy) The cathode catalyst ink was a mixture of Acta 4030 and IPA, amorphous fluropolymer (AF) solution (DuPont, USA), and DI water. The welldispersed anode catalyst ink was brush coated on Ni foam. The cathode catalyst ink was sprayed on the microporous CP with a spray gun (Iwata, Japan). Both the GDEs were dried at 80 °C for 2 h. The anode GDE was sintered at 340 °C for 30 min and the cathode GDE was sintered at 300 °C for 2 h, simultaneously. The catalyst loadings for the OER and HER were 30 and 7 mg cm–2, respectively. The AEM used in the electrolyzer was A-201. The membrane was pretreated by soaking overnight in the 1 M KHCO3, then rinsed with water to remove any residue. The membrane was sandwiched between both the anode and cathode GDEs. The MEA was housed between the single-path titanium and graphite serpentine flow channels. The entire cell was mechanically tightened to ensure intimate contact between with the MEA components. The effective area of the MEA was 5 cm2. Experiments were carried out at 60 °C. The performance of the electrolyzer was comparable with results of Pavel et al. [85]; the achieved current density was 500 mA cm–2 at 1.89 V. Recently, Zeng and Zhao [125] carried out pioneering work in AEM electrolysis. They introduced a new method, referred to as I2MEA (integrated inorganic membrane electrode assembly), for AEM water electrolysis. They elected to use an inorganic material containing a MgAl layered double hydroxide (LDH), which contained a combination of inorganic ions and fixed cations. The I2MEA method was chosen for two reasons. The first was to avoid using the conventional AEM preparation process. Conventional AEM synthesis typically includes chloromethylation, bromomethylation, quaternizataion, and alkalization, but these processes are time consuming, chemicals are highly toxic, and cryogenic conditions are required. Second, in conventional AEM, the cationic functional group in the polymer matrix is degraded by nucleophilic substitution and Hoffmann elimination. The LDH offers higher hydroxyl ion conductivity and stability under alkaline conditions. The Acta 4030 and Acta 3030 catalysts were used as HER and OER catalysts respectively with a loading of 40 mg cm–2. The catalyst slurry was coated on the LDH, then sandwiched between Ni foam and CP. This was followed by hot pressing at 120 kg cm–2 for 15 min 70 °C. The flow rate of the electrolyte was maintained at 2 mL min–1. Using this new I2MEA method, the maximum current density achieved was 205 mA cm–2 at 2.2 V and 70 °C. The stability was monitored at 80 mA cm–2 for 600 h. There was a slow decay rate of 100 μV h–1. This means that the hydroxyl ions were moving by a structural diffusion mechanism, assisted by the carbonate ions in the interlayer region. The conductivity was stable for about 200 h. Pandiarajan et al. [126], fabricated MEA with Fumasep® (FAA-3PK-130; Fumatech, Germany). The catalyst loadings of the anode and cathode were 3.5 mg cm–2 of Ce0.2MnFe1.8O4 and Ni powder, respectively. The performance achieved by the system was 300 mA cm–2 at 1.8 V. The stability of the MEA was estimated under a current density of 200 mA cm−2 at room temperature for 100 h. The cell voltage increased with time. The stability of the Ce0.2MnFe1.8O4 and MnFe2O4 was investigated by running chronoamperometric responses at 2 V (vs. RHE) by circulating DI water. There was no significant voltage drop observed in the performance of Ce0.2MnFe1.8O4 electrodes after 100 h of operation. Electrolyzers with the I2MEA feature were studied under various operating conditions, specifically, at different temperatures, using different electrolytes, and employing membranes of different thicknesses. For membrane thicknesses of 300, 500, and 700 µm, the corresponding internal resistances were 3.27, 5.11, and 7.39 Ω cm2, respectively. The increase in thickness reduced the performance of the electrolyzer but increased the mechanical strength. Fig. 5 [125] shows the polarization curves of an AEM water electrolysis cell based on I2MEAs for membranes of various thicknesses.

0.1 MPa. The maximum performance achieved by the cell was 470 mA cm–2 at 40 °C with voltage ranging between 1.9 and 2.01 V. Leng et al. [72] developed and evaluated AEM water electrolysis in a solid state cell. The MEA was fabricated using both the CCS and CCM methods. An A-201 AEM and conventional OER and HER noble metal catalysts such as IrO2 and Pt black were used to improve the reaction kinetics. The OER and HER catalyst loadings were 2.6 and 2.4 mg cm– 2 , respectively. The catalyst-coated AEM was held between Ni foam and carbon cloth and mechanically pressed to form the MEA. MEAs were prepared using two different ionomers (AS-4 and the A-Radel) and the performances evaluated. The flow fields were single serpentine flow channels on graphite end plates on the cathode side and titanium parallel channel end plates on the anode side. Deionized (DI) water was used as electrolyte at a flow rate of 3 mL min–1. The experiments were carried out at 50 °C. The maximum performance of this electrolysis system was 399 mA cm–2 at 1.80 V with a high frequency resistance of 0.23 Ω cm2. Parrondo et al. [99] prepared a MEA using an inexpensive PSF AEM membrane and an inexpensive OER electrocatalyst. A PGM catalysts, such as Pt black and Ru perchlorate, were used as HER and OER catalysts. The catalyst, Pt black ink, was applied on CP. Ru pyrochlore was deposited on the porous media electrode as GDE. The loading of both the catalysts was about 2.5 mg cm–2, with about 30 wt% binder. The membrane was sandwiched between the two electrodes, no mechanical and thermal pressing was applied. Electrolysis experiments were carried out at 50 °C. The absence of CO2 was maintained by bubbling with N2. The ultrapure water was supplied as anolyte and catholyte at 400 mL min–1. Xiao et al. [101] used an alkaline polymer electrolyte (self-crosslinking PSF) for AEM water electrolysis. Electrocatalysts such as Ni-Fe and Ni-Mo complexes for HER and OER were selected for fabrication of the MEA. A Ni-Fe catalyst was uniformly spread on the hot porous electrode by the electrochemical codeposition method. The porous HER electrode was made entirely of Ni-Mo, instead of only coating with the Ni-Mo. The loading of both the OER and HER catalysts was 40 mg cm–2 and the achieved current density was 400 mg cm–2 at 1.7 V. Ahn et al. [110] prepared a MEA using the A-201 AEM with an extremely small amount of Ni catalyst (8.5 μg Ni cm–2). The catalyst was deposited on CP by a direct electrodeposition method and it was sandwiched between CPs. When a 1.0 M KOH solution was supplied to the cathode, the performance of the electrolyzer was 150 mA cm–2 at 1.9 V. Faraj et al. [97] prepared a MEA using the LDPE-g-VBC-Dabco membrane for AEM electrolysis. Acta 4030 and Acta 3030 were used as HER and OER catalysts respectively. The membrane was housed between the anode and cathode GDL. The electrolyzer performance was 460 mA cm–2 at 2.1–2.2 V. The membrane was stable over an observation period of 500 h; there was no significant change in voltage at 460 mA cm–2. Wu and Scott [111] prepared a MEA using a membrane (50 µm) with quaternary ammonium functional groups for the OER, nano-size CuxCo3O4 for the HER, Pt/C, and catalyst. The catalyst loading was 2 and 3 mg cm–2. The catalyst ink was prepared and coated on both sides of the membrane by the CCM method. The performance was 100 mA cm–2 at a voltage of 1.8 V. In another study, Wu and Scott [106] used a catalyst that they had used in previous work (Cu0.7CO2.3O4 and Co3O4) and nano-size Ni. The current collector meshes were covered by the catalyst ink. The loading of the anode and cathode was 2 and 3 mg cm–2, respectively. The MEA was soaked in 1 M KOH solution to convert the chloride form to the hydroxyl form. Experiments were carried out at 20–30 °C. The performance of the electrolyzer was very poor; the current density achieved was 100 mA cm–2 at 1.9 V. In our work, the MEA was prepared by the CCS method. The Acta 3030 and Acta 4030 were employed as OER and HER catalysts. The 9

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Fig. 6. Polarization curves as function of temperature at the current density of 400 mA cm−2. Tests were run using K2CO3 (1 wt%) electrolyte solution on AEM water electrolysis cell containing HER cathode electrode with 7.4 mgcat cm−2 and atmospheric pressure [85].

Fig. 5. Polarization curves of an AEM water electrolysis cell based on I2MEAs with various thicknesses of membrane [125].

literature values. Scott et al. [98] reported the electrolyzer performance for temperatures of 25, 40, and 55 °C. As expected, the performance increased with increasing temperature due to the lower activation overpotential. The starting potential was 1.5 V at a current density of 250 mA cm–2, but it decreased to 100 mA cm–2 at 1.9 V. The large overpotential was due to the membrane and the electrode resistance. Zeng and Zhao [125] also investigated the effect of temperature on the electrolysis performance at 50, 60, and 70 °C. They used the I2MEA. Fig. 7 [125] shows the effect of temperature based on the I2MEA. The performance significantly increased with increasing temperature, as expected, due to the increased hydroxyl ion conductivity and activity of the catalyst.

In summary, the cell performances and stabilities of various MEAs have been discussed. The MEA comprising an A-201 and LDPE-g-VBCDabco membrane with the Acta 4030 and Acta 3030 HER and OER catalysts exhibited the best performance. Introduction of the new I2MEA method by Zeng and Zhao [125] offered a great improvement; the performance of electrolyzer was comparable to that of the A-201 AEM. Nonetheless, further research into the characterization of the MEA is required, including analysis of impedance, determination of the transport mechanism, and degradation analysis, towards, ultimately, the development of new MEA systems. 4. Operating conditions

4.3. Operating pressure

The performance of an AEM electrolyzer is strongly dependent on the operating conditions. For AEM electrolysis, the operating parameters are cell voltage, current density, temperature, pressure, and choice of the electrolyte, all of which are now discussed.

An AEM electrolyzer generally operates at pressures up to 3 MPa [85]. At higher pressure, the ohmic potential is reduced due the gas bubbles generated during the electrochemical reaction. However, the pressure does not significantly affect the performance of the electrolysis cell. Ahn et al. [110] investigated the effect of compression pressure on the cell voltage. The electrolysis cell was compressed by tightening the screws. The applied pressures were then 3.5, 4.1, 4.8, and 5.5 MPa. Chronoamperometric experiments were carried out and the current– voltage data recorded after 1 min, for a constant current density of 20 mA cm–2. The cell voltage initially decreased due to the reduction of the contact resistance and then it increased drastically. The authors suggest that this may have been due to poor transport of the hydrogen

4.1. Current density In electrolysis, the cell voltage and the current density are critical operating parameters. The cell voltage determines the energy requirements and efficiency of the cell. The current density is the result of the hydrogen production. A higher current density implies a faster electrochemical reaction. An AEM electrolyzer operates in the current density range 100–500 mA cm–2. One of the challenges that running at a higher current density presents is bubble formation on the electrode surface. This is because it increases the overpotential required. Therefore, the most suitable operating current density must be between the optimum hydrogen production and electrical energy efficiency. 4.2. Operating temperature The operating temperature is another important operating parameter. The AEM electrolyzer is designed to operate at temperatures between 50 and 80 °C. An increase in temperature increases the rate of the electrochemical reaction and reduces the overpotential. Pavel et al. [85] investigated the effect of temperature on electrolysis performance; studies were carried out at 30, 40, and 50 °C for a current density of 470 mA cm–2. Fig. 6 [85] shows the polarization curves recorded at the different temperatures. As expected, the cell potential decreased with increasing temperature. This data was comparable with the data of Leng et al. [72]. Leng et al. and Ahn et al. [72,110] carried out investigations in the temperature range 50–70 °C. As expected, cell voltages gradually decreased with increasing temperature. Furthermore, the calculated activation energy from the Arrhenius equation compared well with

Fig. 7. Polarization curves of AEM water electrolysis cell based on I2MEAs for different temperatures [125].

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Fig. 9. Polarization curves of the AEM water electrolysis cell based on I2MEAs with various electrolytes [125].

Fig. 8. Polarization curves obtained using different electrolyte solution: 1 wt% K2CO3, 1 wt% K2CO3/KHCO3 (0.67 wt% K2CO3 and 0.33% KHCO3) and 1 M KOH. Tests were run on cell containing HER cathode electrode with 7.4 mgcat cm−2 loading at 316 K and atmospheric pressure [85].

experiments at 90 mA cm–2 and obtained a voltage of 2 V. Parrondo et al. [99] and Xiao et al. [101] used ultrapure water as the electrolyte at 400 mA cm–2 and recorded cell voltages of 1.8 V and 1.85 V. In other studies, Wu and Scott [106] and Ahn et al. [110] used 1 M KOH as the electrolyte at 100 and 150 mA cm–2 and recorded cell voltages of 1.8 V and 1.9 V. In summary, the operating conditions of AEM electrolysis, including current density, temperature, operating pressure, and electrolyte, have been discussed. The lowest voltage was obtained at 400 mA cm–2 when 1 M KOH was used at 80 °C, with a screw pressure of 4.1 MPa. The corrosive nature of 1 M KOH and the chance of interaction between KOH and CO2 in ambient air are very high. The voltage difference between 1 M KOH and 0.1 M K2CO3 was found to be 0.07 V at 40 °C. Indications are therefore that the 1 M K2CO3 electrolyte is the best option for AEM electrolysis. The voltage difference in the 40– 60 °C range is only 0.09 V. Much heat energy is required to run at higher temperatures. Therefore, the optimal operating conditions at 400 mA cm–2 are 0.1 M K2CO3 electrolyte at 50 °C, with screw pressure 4.1 MPa.

and oxygen in the electrochemical reaction. Furthermore, the highly compressed CP might not allow a path for the escape of the hydrogen and oxygen. Both the hydrogen and oxygen might be trapped inside the MEA and block the active sites of the catalyst. This behavior has been reported elsewhere in the literature [127,128]. On the other hand, at the low pressure of 3.5 MPa, the cell showed poor performance due to poor transportation of hydrogen and oxygen. For the Ni/CP system, the authors identified the optimal compression pressure that yielded the highest cell performance to be 4.1 MPa. 4.4. Electrolyte The electrolyte plays a vital role in electrocatalytic activity and largely determines the performance of the electrolyzer. An electrolyte that has a high concentration provides good ionic conductivity. Conventionally, a 30–40% concentration of KOH is used as electrolyte. In AEM electrolysis, DI water, ultrapure water, 1 wt% K2CO3, 1 wt% K2CO3/KHCO3, and 1 M KOH have all been used as electrolytes. Pavel et al. [85] compared the performances of electrolyzers of different electrolytes: 1% K2CO3/KHCO3, 1% K2CO3, and 1 M KOH (pH values 11.2, 10.2, and 14). Fig. 8 [85] shows the polarization curves for the various electrolytes. The cell potential was 1.79 V at a constant current density of 470 mA cm–2, and the resistances were 0.152, 0.132, and 0.087 Ω cm2. The resistance for 1 M KOH was lower than for the other electrolytes. Zeng and Zhao [125] also evaluated the performance of an electrolyzer, based on their I2MEA, with various electrolytes: 0.1 M NaOH, 0.1 M Na2CO3, 0.1 M mixture of 0.08 M Na2CO3 + 0.02 M NaHCO3, 0.1 M NaHCO3, and DI water. The pH values ranged from 12.63 to 8.51. The voltages obtained using the different electrolytes, at 208 mA cm–2, were 1.725 V, 1.845 V, 1.944 V, 2.099 V, and 2.5 V, respectively. Fig. 9 [125] shows the polarization curves for the various electrolytes based on I2MEAs. The internal resistances were 3.27, 4.36, 5.44, 8.39, and 17.92 Ω cm2, respectively. The results suggest that the internal resistances are a strong function of the pH value. It was also observed that the rate constant of the OER decreased with decreasing hydroxyl ion content. Hence, it appears that both these observation reduce the performance of the electrolyzer. Faraj et al. [79] carried out experiments at 460 mA cm–2 current density; the voltages of the electrolytes 1 M KOH, 0.1 M K2CO3, and KHCO3 were 1.78, 1.88 and 1.9 V, respectively. DI water has been used as electrolyte by several researchers. Leng et al. [72] ran experiments at 399 mA cm–2 and obtained a voltage of 1.8 V. Scott and coworkers [111] ran experiments at 100 mA cm–2 and obtained a voltage of 1.9 V. Sivakumar and coworkers [120] ran

5. Modeling of AEM electrolysis Zeng and coworkers [129] modeled AEM electrolysis. Because AEM electrolysis involves many physicochemical processes, including electrochemical reactions, and mass and charge transport, understanding the process through experimentation alone is challenging. Zeng and coworkers [129] therefore developed a mathematical model to investigate the mass transport, charge transport, and electrochemical reactions. They determined the voltage loss and performance of AEM electrolysis by applying the effects of the current density, membrane thickness, and liquid saturation. For their modeling, they made the following assumptions: 1. Experiments are carried out under steady state and isothermal conditions. 2. The cathode layer is treated as a thin interface layer, since it is much thinner than the diffusion layer. The electrochemical kinetics are derived from the Butler–Volmer equation:

⎧ ⎛ (1 − αa ) nF ⎞ ⎫ ⎛ α nF ⎞ OER:ja = io, a⎨exp ⎜ a ηa⎟ − exp ⎜ − ηa⎟ ⎬ ⎠⎭ ⎝ ⎝ RT ⎠ RT ⎩ ⎧ ⎛ (1 − αc ) nF ⎞ ⎫ ⎛ α nF ⎞ HER:jc = io, c⎨exp ⎜ c ηc⎟ − exp ⎜ − ηc⎟ ⎬ ⎠⎭ ⎝ ⎝ RT ⎠ RT ⎩ 11

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Fig. 12. Effect of the saturation of electrolyte on the performance [129].

Fig. 10. Effect of the current density on performance for applied voltage [129].

resistance but it offers low mechanical strength. Therefore, the thickness needs to be optimized to achieve the best balance between good hydroxyl ion conduction and mechanical strength. Another study of Zeng and coworkers [129] addressed the effect of liquid saturation on the electrolysis performance. The liquid saturation strongly influences the physical parameters such as electrode porosity, wettability, permeability, and surface tension. The liquid saturation level was varied in the range 0.95–0.5. Fig. 12 [129] shows the effect of different saturation levels on the current density. The cell efficiency decreased with increasing cell voltage. This is because lower saturation reduces the number of active sites, thereby increasing the loss of catalytic activity. Studies carried out by Zeng and coworkers. [129] were particularly valuable; they modeled and then validated AEM electrolysis, for the first time. The numerical and experimental data matched within 2% error. They recommend that further developments in the areas of active electrocatalysts and highly conductive hydroxyl ion AEMs are required. An increase in membrane thicknesses eventually results in an increase the resistance. Unfortunately, here, the authors did not provide much information of the increase in overall resistance.

where, i0, a is the anode exchange current density and ηa is the anode overpotential (0. 37Am −2), i0, c is the cathode exchange current density and ηc is the cathode overpotential (0. 43Am −2 ), E 0 is the standard voltage (1. 229V ),αa is the anode transfer coefficient (0. 5), αc is the cathode transfer coefficient (0.5), R is the universal gas constant ( 8.314 J mol−1K−1), and F is the Faraday's constant (96, 485. 3A s mol−1). All the physiochemical and operating parameters were carefully selected and the model was validated experimentally. The membrane selected for use in their experiments had a PSF backbone with quaternary ammonium functional groups. It was sandwiched between the Ni-Fe and Ni-Mo electrodes. The experiments were carried out at 70 °C. The effect of current density on the voltage is shown in Fig. 10 [129]. The efficiency decreased with increasing current density. This is because the total voltage loss comprises the activation loss of the two electrodes and the ohmic loss, and it is proportional to the current density. To achieve an optimal rate of hydrogen production and efficiency, the operating current density must be optimal. The higher exchange current density resulted in a reduction in voltage and higher efficiency. The authors suggest that at higher exchange current densities the reaction kinetics are fast and, consequently, the activation loss may be low. Yet another important parameter of AEM electrolysis is the thickness of the membrane because it controls the hydroxyl ion transport. Fig. 11 [129] shows the effect of different membrane thicknesses on the current density (performance). The thickness of the membrane was increased from 35 µm to 140 µm. It is evident that the cell voltage increases with increasing thickness and, therefore, the overall efficiency of the electrolyzer is reduced. The explanation presented is that the thicker membranes has higher transport resistance. On the other hand, the thinner membrane shows lower transport

6. Summary and challenges AEM electrolysis technology is still at an early stage of development. In this article we have presented an overview of the current state of low cost hydrogen production by AEM electrolysis. We have discussed the basic components of the AEM electrolyzer, such as the AEM, electrocatalyst, ionomer, and MEA, and electrolyzer performance. Recent developments (since 2011) have been critically reviewed. To date, the best performance achieved by AEM electrolysis is 500 mA cm–2 at 1.8 V, for the following system: A-201 membrane, and OER and HER catalysts such as Acta 3030 and Acta 4030. The optimum operating conditions of AEM electrolysis were identified as 0.1 M K2CO3 at 45 °C, with a screw pressure of 4.1 MPa. Nevertheless, AEM electrolysis technology still faces some challenges regarding the achievement of improved low cost production of hydrogen; there are still many problems to be overcome in this regard. Specific features/weak points of the current AEM electrolysis technology that require attention/improvement include the following:

• • • • • Fig. 11. Effect of the membrane thickness on performance for applied voltage [129].

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Degradation of the AEM due to nucleophilic attack and Hoffmann elimination on the fixed cationic groups on the polymer matrix Membrane and electrode deformation due to degradation of the quaternary ammonium functional groups Lower conductivity of the hydroxyl ion than the proton conductivity Degradation of the ionomer in the MEA The potential drop, due to the excessive loading of non-noble metal catalyst at the anode

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• • • • •

Lower performance of the transition metal catalysts than the noble metal catalysts, and lower catalyst utilization Sluggish OER reaction when the number of hydroxyl ions is reduced in the electrolyte Blocking of active sites of the catalyst at high pressure by hydrogen and oxygen trapped inside the MEA Performances achieved with AEM electrolysis are still lower than with PEM electrolysis due to the higher internal resistance of the MEA Performances of electrolytes such as distilled water, ultrapure water, and 0.1% K2CO3 are lower than the performance of 1 M KOH.

•

7. Recommendations for future research There are still several development challenges in AEM electrolysis that should be addressed prior to its general acceptance and future large-scale introduction.

•

•

•

•

Development of improved anion conductive polymers (membranes and ionomers) Development of anion conductive polymers (membrane and ionomers) with high conductivity, hydroxyl ion selectivity, and stability (mainly at temperatures > 60 °C), and lower gas crossover is essential. The following novel AEMs are suggested: radiation red – coli grafted AEMs [130], AEM blends with imidazolium-functionalized and sulfonated poly(ether ether ketone) [131], AEMs crosslinked with polar electron-donating linkers [132], and an AEM made of styrenic diblock copolymer with a quaternary-ammonium-functionalized hydrophilic block and a cross-linkable hydrophobic block [133] can also be tried An understanding of the interaction between the hydroxyl ions, polymer backbone, and quaternary ammonium functional groups is necessary in order to avoid undesirable nucleophilic attack and Hoffmann elimination. Furthermore, characterization of the properties of AEMs under AEM water electrolysis conditions, by means of polarization, EIS, chronoamperometry, and gas crossover analysis is required. Development of improved hydroxyl ion conductivity The hydroxyl ion conductivity could be increased by the addition of nanoparticle fillers and blends [134]. However, the development of ionomers may be slow, largely due to the toxic nature of many ionomers. A non-toxic, high ionic conductivity, thermally and chemically stable ionomer should be developed for AEM electrolysis. Development of the OER and HER Most of the OER catalysts are based on the composition of Ni and a rare earth metal. Use of Ag-alloy-based and Ni-Cr-based catalyst may be considered for the OER [135]. New, highly efficient electrocatalysts, such as nanorod-shaped Co3O4 anchored on multiwalled carbon nanotubes should be developed and be used together with diethylenediamine as OER catalyst [136]. Co3O4 nanocrystals grown on reduced GO were found to be a synergic catalyst for OER [118]. This could perhaps also be considered for use in AEM electrolysis. The development of HER catalysts also requires attention. Low cost HER catalysts with enhanced catalytic activity, catalyst utilization, and stability are required for the HER. Physical and electrochemical characterization of the MEA Physical and electrochemical characterization of the MEA should contribute to a better understanding, and later the enhancement of, hydroxyl ion transport through the MEA. The membrane degradation mechanism between the anode and cathode needs to be understood. Furthermore, interaction between the membrane and ionomer and cross-linking at the interface should be investigated. Development of a MEA with improved water management and thermal stability is also required. Development of advanced physical and electrochemical characterization tools, such as current interruption, current mapping, and EIS techniques is essential.

Development of both empirical and physical predictive relations for operating parameters should also be addressed. Industrialization and commercialization of AEM electrolysis AEM electrolysis is a developing technology for small-scale onsite hydrogen production. This small-scale AEM electrolyzer can produce hydrogen up to 1 N m3H2 h−1 [60]. Use of the AEM electrolyzer avoids the requirement of a compressed hydrogen storage cylinder (10 N m3 H2). Acta SpA (Italy) commercially launched a portable electrolyzer for rack-mountable applications. The high-purity hydrogen produced from this AEM electrolyzer can be used for various applications, such as fuel gas for flame applications, e.g., soldering and brazing metals, glass, quartz and crystal. Another major application is inflation of meteorological balloons. However, the electrical energy requirement for the smallscale electrolyzers are higher than the medium- and large-scale electrolyzers. This is because the auxiliary equipment, such as the pump, control panel, valves, etc., consume the major portion of the electrical consumption. The size of the electrochemical cell is less significant as regards electricity consumption. In spite of more efficient and reliable technology now being available, remains subject of further research and development, for commercialization and industrialization.

To conclude, AEM electrolysis with non-noble metal catalysts will reduce the capital cost of the entire system. However, the overall performance of AEM electrolyzer must be increased to further reduce the total cost of hydrogen production. Acknowledgements The work was supported through Department of Science and Technology (DST) HYSA KP5 and NRF 85309 grants. References [1] Crabtree GW, Dresselhaus MS. The hydrogen fuel alternative. MRS Bull 2008;33:421–8. [2] Dinga G. Hydrogen: the ultimate fuel and energy carrier. Int J Hydrog Energy 1989;14:777–84. [3] Edwards PP, Kuznetsov VL, David WIF, Brandon NP. Hydrogen and fuel cells: towards a sustainable energy future. Energy Policy 2008;36:4356–62. [4] EUROPEAN COMMISSION. Hydrogen energy and fuel cells. Brussels; 2003. [5] Ghergheleş C, Ghergheleş V. Hydrogen - the fuel of the future?. J Electr Electron Eng 2008;1:51–3. [6] Jain IP. Hydrogen the fuel for 21st century. Int J Hydrog Energy 2009;34:7368–78. [7] Revankar ST, Brown NR, Kune C Oh S. Development of efficient flowsheet and transient modeling for nuclear heat coupled sulfur iodine cycle for hydrogen production. Project No. 06-060; 2010. [8] Winter CJ. Hydrogen energy - abundant, efficient, clean: a debate over the energysystem-of-change. Int J Hydrog Energy 2009:34. [9] Vezirolu T, Barbir F. Hydrogen: the wonder fuel. Int J Hydrog Energy 1992;17:391–404. [10] Tollefson J. Hydrogen vehicles: fuel of the future?. Nature 2010;464:1262–4. [11] Thomas C. Affordable hydrogen supply pathways for fuel cell vehicles. Int J Hydrog Energy 1998;23:507–16. [12] Shinnar R. The hydrogen economy, fuel cells, and electric cars. Technol Soc 2003;25:455–76. [13] Kopasz JP. Fuel cells and odorants for hydrogen. Int J Hydrog Energy 2007;32:2527–31. [14] Wenguo X, Yingying C. Hydrogen and electricity from coal with carbon dioxide separation using chemical looping reactors. Energy Fuels 2007;21:2272–7. [15] Ahmed S, Krumpelt M. Hydrogen from hydrocarbon fuels for fuel cells. Int J Hydrog Energy 2001;26:291–301. [16] Akutsu Y, Lee DY, Chi YZ, Li YY, Harada H, Yu HQ. Thermophilic fermentative hydrogen production from starch-wastewater with bio-granules. Int J Hydrog Energy 2009;34:5061–71. [17] Butt N, Beyer HL, Bennett JR, Biggs D, Maggini R, Mills M, et al. Biodiversity risks from fossil fuel extraction. Science 2013;342:425–6. [18] Ni M, Leung DYC, Leung MKH, Sumathy K. An overview of hydrogen production from biomass. Fuel Process Technol 2006;87:461–72. [19] Artero V, Chavarot-Kerlidou M, Fontecave M. Splitting water with cobalt. Angew Chemie – Int Ed 2011;50:7238–66. [20] Singh WM, et al. Pegram D, Duan H, Kalita D, Simone P, Emmert GL. Hydrogen production coupled to hydrocarbon oxygenation from photocatalytic water split-

13

Renewable and Sustainable Energy Reviews xxx (xxxx) xxx–xxx

I. Vincent, D. Bessarabov ting. Angew Chemie – Int Ed 2010;51:1653–6. [21] Wang Y, Li H, He P, Zhou H. Controllable hydrogen generation from water. ChemSusChem 2010;3:571–4. [22] Liao CH, Huang CW, Wu JCS. Hydrogen production from semiconductor-based photocatalysis via water splitting. Catalysts 2012;2:490–516. [23] Funk JE. Thermochemical hydrogen production: past and present. Int J Hydrog Energy 2001;26:185–90. [24] Xiao L, Wu SY, Li YR. Advances in solar hydrogen production via two-step watersplitting thermochemical cycles based on metal redox reactions. Renew Energy 2012;41:1–12. [25] Steinfeld A. Solar thermochemical production of hydrogen – a review. Sol Energy 2005;78:603–15. [26] Peter LM. Photoelectrochemical water splitting. a status assessment. Electroanalysis 2015;27:864–71. [27] Darbandi M, Gebre T, Mitchell L, Erwin W, Bardhan R, Levan MD, et al. Nanoporous TiO2 nanoparticle assemblies with mesoscale morphologies: nanocabbage versus sea-anemone. Nanoscale 2014;6:5652–6. [28] Ong W-J, Tan L-L, Chai DS-P, Yong DS-T, Mohamed PAR. Facet-dependent photocatalytic properties of tio2-based composites for energy conversion and environmental remediation. ChemSusChem 2014;7:690–719. [29] Ong Wee-Jun, Tan Lling-Lling, Ng Yun Hau, Yong Siek-Ting, S-PC . Graphitic carbon nitride (g-C3N4)-based photocatalysts for artificial photosynthesis and environmental remediation: are we a step closer to achieving sustainability?. Chem Rev 2016;116:7159–329. [30] Jinshui Zhang YC, Two-dimensional XW. covalent carbon nitride nanosheets: synthesis, functionalization, and applications. Energy Environ Sci 2015;8:3092–108. [31] Wang X, Yun Z, Lihua L, Wang B, Wang X. Graphitic carbon nitride polymers toward sustainable photoredox catalysis. Angew Chem Int Ed Engl 2015;54:12868–12884reviews. [32] Ong W-J, Tan L-L, Chai S-P, Yong S-T. Heterojunction engineering of graphitic carbon nitride (g-C3N 4) via Pt loading with improved daylight-induced photocatalytic reduction of carbon dioxide to methane. Dalton Trans 2015;44:1249. [33] Ong W-J, Tan L-L, Chai S-P, Yong S-T. Graphene oxide as a structure-directing agent for the two-dimensional interface engineering of sandwich-like graphene-gC3N4 hybrid nanostructures with enhanced visible-light photoreduction of CO2 to methane. Chem Commun 2014;51:3–6. [34] Deiman JR. Water electrolysis. Water 2010;2:4–9. [35] Millet P, Grigoriev S. Water electrolysis technologies. Renew Hydrog Technol Prod Purif Storage Appl Saf 2013:19–41. [36] Senftle FE, Grant JR, Senftle FP. Low-voltage DC/AC electrolysis of water using porous graphite electrodes. Electrochim Acta 2010;55:5148–53. [37] Wei ZD, Ji MB, Chen SG, Liu Y, Sun CX, Yin GZ, et al. Water electrolysis on carbon electrodes enhanced by surfactant. Electrochim Acta 2007;52:3323–9. [38] Rashid MM, Mesfer MK Al, Naseem H, Danish M. Hydrogen production by water electrolysis: a review of alkaline water electrolysis, PEM water electrolysis and high temperature water electrolysis. Int J Eng Adv Technol 2015:2249–8958. [39] Manabe A, Kashiwase M, Hashimoto T, Hayashida T, Kato A, Hirao K, et al. Basic study of alkaline water electrolysis. Electrochim Acta 2013;100:249–56. [40] Marini S, Salvi P, Nelli P, Pesenti R, Villa M, Berrettoni M, et al. Advanced alkaline water electrolysis. Electrochim Acta 2012;82:384–91. [41] Santos DMF, Sequeira CAC, Figueiredo JL. Hydrogen production by alkaline water electrolysis. Quim Nova 2013;36:1176–93. [42] Zeng K, Zhang D. Recent progress in alkaline water electrolysis for hydrogen production and applications. Prog Energy Combust Sci 2010;36:307–26. [43] Carmo M, Fritz DL, Mergel J, Stolten D. A comprehensive review on PEM water electrolysis. Int J Hydrog Energy 2013;38:4901–34. [44] Smolinka T. PEM water electrolysis – present status of research and development essen, May 18; 2010. [45] Langemann M, Fritz DL, M??ller M, Stolten D. Validation and characterization of suitable materials for bipolar plates in PEM water electrolysis. Int J Hydrog Energy 2015;40:11385–91. [46] Ganley JC. High temperature and pressure alkaline electrolysis. Int J Hydrog Energy 2009;34:3604–11. [47] Ursua A, Gandia LM, Sanchis P. Hydrogen production from water electrolysis: current status and future trends. Proc IEEE 2012;100:410–26. [48] Appleby AJ, Crepy G, Jacquelin J. High efficiency water electrolysis in alkaline solution. Int J Hydrog Energy 1978;3:21–37. [49] Zeng K, Zhang D. Recent progress in alkaline water electrolysis for hydrogen production and applications. Prog Energy Combust Sci 2010;36:307–26. [50] Wendt H, Hofmann H. Ceramic diaphragms for advanced alkaline water electrolysis. J Appl Electrochem 1989;19:605–10. [51] Rosa V. New materials for water electrolysis diaphragms. Int J Hydrog Energy 1995;20:697–700. [52] Hu W. A novel cathode for alkaline water electrolysis. Int J Hydrog Energy 1997;22:621–3. [53] Kjartansdóttir CK, Nielsen LP, Møller P. Development of durable and efficient electrodes for large-scale alkaline water electrolysis. Int J Hydrog Energy 2013;38:8221–31. [54] J. O Jenson, V. Bandur, Yde L. Pre-Investigation of water electrolysis; 2008. [55] Ferrero D, Lanzini A, Santarelli M, Leone P. A comparative assessment on hydrogen production from low- and high-temperature electrolysis. Int J Hydrog Energy 2013;38:3523–36. [56] Millet P, Ngameni R, Grigoriev SA, Mbemba N, Brisset F, Ranjbari A, et al. PEM water electrolyzers: from electrocatalysis to stack development. Int J Hydrog Energy 2010;35:5043–52.

[57] Sutherland RB. Performance of different proton exchange membrane water electrolyser components [Master thesis (MEng in chem. eng)]. Potchefstroom, South Africa: North-West university; 2012. [58] Millet P, Mbemba N, Grigoriev SA, Fateev VN, Aukauloo A, Etiévant C. Electrochemical performances of PEM water electrolysis cells and perspectives. Int J Hydrog Energy 2011;36:4134–42. [59] Rossmeisl J, Qu ZW, Zhu H, Kroes GJ, Nørskov JK. Electrolysis of water on oxide surfaces. J Electroanal Chem 2007;607:83–9. [60] Godula-Jopek A, editor. Hydrogen production by electrolysis. Weinheim: WileyVCH; 2015. [61] Bessarabov D, Wang H, Zhao N. PEM electrolysis for hydrogen production. Boca Ration: CRC Press; 2015. [62] Gülzow E, Schulze M. Long-term operation of AFC electrodes with CO2 containing gases. J Power Sources 2004;127:243–51. [63] McLean G, Neit T, Richard P, Djilali N. An assessment of alkaline fuel cell technology. Int J Hydrog Energy 2002;27:507–26. [64] Gouérec P, Poletto L, Denizot J, Sanchez-Cortezon E, Miners JH. The evolution of the performance of alkaline fuel cells with circulating electrolyte. J Power Sources 2004;129:193–204. [65] Schulze M, Gülzow E. Degradation of nickel anodes in alkaline fuel cells. J Power Sources 2004;127:252–63. [66] Gulzow E. Alkaline fuel cells: a critical view. J Power Sources 1996;61:99–104. [67] Larminie J, Dicks A. Fuel cell systems explained. New York: Wiley; 2001. [68] Naughton MS, Brushett FR, Kenis PJA. Carbonate resilience of flowing electrolytebased alkaline fuel cells. J Power Sources 2011;196:1762–8. [69] Agel E, Bouet J, Fauvarque JF. Characterization and use of anionic membranes for alkaline fuel cells. J Power Sources 2001;101:267–74. [70] Xu T, Liu Z, Yang W. Fundamental studies of a new series of anion exchange membranes: membrane prepared from poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and triethylamine. J Memb Sci 2005;249:183–91. [71] Wu J, Yuan XZ, Martin JJ, Wang H, Zhang J, Shen J, et al. A review of PEM fuel cell durability: degradation mechanisms and mitigation strategies. J Power Sources 2008;184:104–19. [72] Leng Y, Chen G, Mendoza AJ, Tighe TB, Hickner MA, Wang C-Y. Solid-state water electrolysis with an alkaline membrane. J Am Chem Soc 2012;134:9054–7. [73] Ayers KE, Anderson EB, Capuano CB, Carter BD, Dalton LT, Hanlon G, et al. Research advances towards low cost, high efficiency PEM electrolysis. ECS Trans 2010;33:3–15. [74] Ayers KE, Capuano C, Anderson EB. Recent advances in cell cost and efficiency for PEM-Based water electrolysis. ECS Trans 2012;41:15–22. [75] Immanuel V, Parvatalu D, Bhardwaj A, Prabhu BN, Bhaskarwar AN, Shukla A. Properties of nafion 117 in highly acidic environment of bunsen reaction of I-S cycle. J Memb Sci 2012;409–410:137–44. [76] Ous T, Arcoumanis C. Degradation aspects of water formation and transport in proton exchange membrane fuel Cell: a review. J Power Sources 2013;240:558–82. [77] Sun S, Shao Z, Yu H, Li G, Yi B. Investigations on degradation of the long-term proton exchange membrane water electrolysis stack. J Power Sources 2014;267:515–20. [78] Siracusano S, Baglio V, Briguglio N, Brunaccini G, Di Blasi A, Stassi A, et al. An electrochemical study of a PEM stack for water electrolysis. Int J Hydrog Energy 2012;37:1939–46. [79] Selamet ÖF, Becerikli F, Mat MD, Kaplan Y. Development and testing of a highly efficient proton exchange membrane (PEM) electrolyzer stack. Int J Hydrog Energy 2011;36:11480–7. [80] Varcoe JR, Atanassov P, Dekel DR, Herring AM, Hickner MA, Kohl PA, et al. Anion-exchange membranes in electrochemical energy systems. Energy Environ Sci 2014;7:3135–91. [81] Piana M, Boccia M, Filpi A, Flammia E, Miller HA, Orsini M, et al. H2/air alkaline membrane fuel cell performance and durability, using novel ionomer and nonplatinum group metal cathode catalyst. J Power Sources 2010;195:5875–81. [82] Piana M, Catanorchi S, Gasteiger HA. Kinetics of non-platinum group metal catalysts for the oxygen reduction reaction in alkaline medium. ECS Trans 2008;16:2045–55. [83] Faraj M, Elia E, Boccia M, Filpi A, Pucci A, Ciardelli F. New anion conducting membranes based on functionalized styrene–butadiene–styrene triblock copolymer for fuel cells applications. J Polym Sci Part A Polym Chem 2011;49:3437–47. [84] Filpia A, Boccia M, Gasteiger HA. Pt-free cathode catalyst performance in H2/O2 anion-exchange membrane fuel cells (AMFCs). ECS Trans 2008;16:1835–45. [85] Pavel CC, Cecconi F, Emiliani C, Santiccioli S, Scaffidi A, Catanorchi S, et al. Highly efficient platinum group metal free based membrane-electrode assembly for anion exchange membrane water electrolysis. Angew Chem Int Ed 2014;53:1378–81. [86] Zoulias E, Varkaraki E. A review on water electrolysis. Tcjst 2004;4:41–71. [87] Merle G, Wessling M, Nijmeijer K. Anion exchange membranes for alkaline fuel cells: a review. J Membr Sci 2011;377:1–35. [88] Yang G, Hao J, Cheng J, Zhang N, He G, Zhang F, et al. Hydroxide ion transfer in anion exchange membrane: a density functional theory study. Int J Hydrog Energy 2016;41:6877–84. [89] Frisch M, Trucks G, Schlegel K, Scuseria G, Robb M, Cheeseman J, et al. Gaussian 09, revision C.01; 2010. [90] Takaba H, Shimizu N, Hisabe T, Alam MK. Modeling of transport mechanisms of oh- in electrolyte of alkaline fuel cell. ECS Trans 2014;61:63–9. [91] Chen C, Tse YLS, Lindberg GE, Knight C, Voth GA. Hydroxide Solvation and transport in anion exchange membranes. J Am Chem Soc 2016;138:991–1000. [92] Clark JK, Paddison SJ. The effect of side chain connectivity and local hydration on

14

Renewable and Sustainable Energy Reviews xxx (xxxx) xxx–xxx

I. Vincent, D. Bessarabov

[93]

[94]

[95] [96] [97]

[98]

[99]

[100] [101]

[102] [103]

[104] [105]

[106]

[107] [108]

[109] [110]

[111]

[112]

[113]

[114]

[115]

proton transfer in 3M perfluorosulfonic acid membranes. Solid State Ion 2012;213:83–91. Yan X, Gu S, He G, Wu X, Benziger J. Imidazolium-functionalized poly(ether ether ketone) as membrane and electrode ionomer for low-temperature alkaline membrane direct methanol fuel cell. J Power Sources 2014;250:90–7. Hibbs MR, Hickner MA, Alam TM, Mcntyre SK, Fujimoto CH, Cornelius CJ. Transport properties of hydroxide and proton conducting membranes. Chem Mater 2008;20:2566–73. Lu S, Pan J, Huang A, Zhuang L, Lu J. Alkaline polymer electrolyte fuel cells completely free from noble metal catalysts. Proc Natl Acad Sci 2008;105:20611–4. Fukuta K. Electrolyte materials for AMFCs electrolyte materials for AMFCs and AMFC performanc. AMFC Workshop 8 may 2011, Tokuyama Corporation. Faraj M, Boccia M, Miller H, Martini F, Borsacchi S, Geppi M, et al. New LDPE based anion-exchange membranes for alkaline solid polymeric electrolyte water electrolysis. Int J Hydrog Energy 2012;37:14992–5002. Cao Y-C, Wu X, Scott K. A quaternary ammonium grafted poly vinyl benzyl chloride membrane for alkaline anion exchange membrane water electrolysers with non-noble-metal catalysts. Int J Hydrog Energy 2012;37:9524–8. Parrondo J, Arges CG, Niedzwiecki M, Anderson EB, Ayers KE, Ramani V. Degradation of anion exchange membranes used for hydrogen production by ultrapure water electrolysis. RSC Adv 2014;4:9875–9. Leng Y, Wang L, Hickner MA, Wang CY. Alkaline membrane fuel cells with in-situ cross-linked ionomers. Electrochim Acta 2015;152:93–100. Xiao L, Zhang S, Pan J, Yang C, He M, Zhuang L, et al. First implementation of alkaline polymer electrolyte water electrolysis working only with pure water. Energy Environ Sci 2012;5:7869–71. Pan J, Lu S, Li Y, Huang A, Zhuang L, Lu J. High-performance alkaline polymer electrolyte for fuel cell applications. Adv Funct Mater 2010;20:312–9. Ayers KE, Anderson EB, capuano CB, Niedzwiecki , Hickner MA. Characterization of anion exchange membrane technology for low cost electrolysis. ECS Trans 2013;45:121–30. Zeng L, Zhao TS. High-performance alkaline ionomer for alkaline exchange membrane fuel cells. Electrochem Commun 2013;34:278–81. Komkova EN, Stamatialis DF, Strathmann H, Wessling M. Anion-exchange membranes containing diamines: preparation and stability in alkaline solution. J Memb Sci 2004;244:25–34. Wu X, Scott K. A polymethacrylate-based quaternary ammonium OH− ionomer binder for non-precious metal alkaline anion exchange membrane water electrolysers. J Power Sources 2012;214:124–9. Marini S, Salvi P, Nelli P, Pesenti R, Villa M, Berrettoni M, et al. Advanced alkaline water electrolysis. Electrochim Acta 2012;82:384–91. Vincent I, Bessarabov D. electrochemical characterization and oxygen reduction kinetics of cu-incorporated cobalt oxide catalyst. Int J Electrochem Sci 2016;11:8002–15. Gong M, Dai H. A mini review of NiFe-based materials as highly active oxygen evolution reaction electrocatalysts. Nano Res 2014;8:23–39. Ahn SH, Lee B-S, Choi I, Yoo SJ, Kim H-J, Cho E, et al. Development of a membrane electrode assembly for alkaline water electrolysis by direct electrodeposition of nickel on carbon papers. Appl Catal B Environ 2014;154– 155:197–205. Wu X, Scott K. CuxCo3−xO4 (0 ≤ x < 1) nanoparticles for oxygen evolution in high performance alkaline exchange membrane water electrolysers. J Mater Chem 2011;21:12344–51. Wang S, et al. Zhang L, Xia Z, Roy A, Chang DW, Baek JB. BCN graphene as efficient metal-free electrocatalyst for the oxygen reduction reaction. Angew Chemie – Int Ed 2012;51:4209–12. Jiao Y, Zheng Y, Jaroniec M, Zhang Qiao S. Origin of the electrocatalytic oxygen reduction activity of graphene-based catalysts: a roadmap to achieve the best performance. J Am Chem Soc 2014;19:4394–403. Li R, Wei Z, Gou X. Nitrogen and phosphorus dual-doped graphene/carbon nanosheets as bifunctional electrocatalysts for oxygen reduction and evolution. ACS Catal 2015;5:4133–42. Bin Yang H, Miao J, Hung S-F, Chen J, Tao HB, Wang X, et al. Identification of

[116]

[117] [118]

[119]

[120] [121]

[122]

[123] [124]

[125]

[126]

[127]

[128]

[129]

[130]

[131]

[132]

[133]

[134] [135] [136]

15

catalytic sites for oxygen reduction and oxygen evolution in N-doped graphene materials: development of highly efficient metal-free bifunctional electrocatalyst. Sci Adv 2016;2(e1501122):1–12. Jahan M, Liu Z, Loh KP. A graphene oxide and copper-centered metal organic framework composite as a tri-functional catalyst for HER, OER, and ORR. Adv Funct Mater 2013;23:5363–72. Zhang J, Zhao Z, Xia Z, Dai L. A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. Nat Nanotechnol 2015;10:444–52. Liang Y, Li Y, Wang H, Zhou J, Wang J, Regier T, et al. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat Mater 2011;10:780–6. Seetharaman S, Balaji R, Ramya K, Dhathathreyan KS, Velan M. Graphene oxide modified non-noble metal electrode for alkaline anion exchange membrane water electrolyzers. Int J Hydrog Energy 2013;38:14934–42. Joe JD, Kumar DBS, Sivakumar P. Production of hydrogen by anion exchange membrane using AWE. IJSTR 2014;3:38–42. Lu T, Zhang Y, Li H, Pan L, Li Y, Sun Z. Electrochemical behaviors of grapheneZnO and graphene-SnO2 composite films for supercapacitors. Electrochim Acta 2010;55:4170–3. Li B, Cao H, Shao J, Li G, Qu M, Yin G. Co3O4@graphene composites as anode materials for high-performance lithium ion batteries. Inorg Chem 2011;50:1628–32. Tang H, Wang S, Jiang SP, Pan M. A comparative study of CCM and hot-pressed MEAs for PEM fuel cells. J Power Sources 2007;170:140–4. Song S, Zhang H, Ma X, Shao Z, Baker RT, Yi B. Electrochemical investigation of electrocatalysts for the oxygen evolution reaction in PEM water electrolyzers. Int J Hydrog Energy 2008;33:4955–61. Zeng L, Zhao TS. Integrated inorganic membrane electrode assembly with layered double hydroxides as ionic conductors for anion exchange membrane water electrolysis. Nano Energy 2015;11:110–8. Pandiarajan T, John Berchmans L, Ravichandran S. Fabrication of spinel ferrite based alkaline anion exchange membrane water electrolysers for hydrogen production. RSC Adv 2015;5:34100–8. Ahn SH, Choi I, Park H-Y, Hwang SJ, Yoo SJ, Cho E, et al. Effect of morphology of electrodeposited Ni catalysts on the behavior of bubbles generated during the oxygen evolution reaction in alkaline water electrolysis. Chem Commun (Camb) 2013;49:9323–5. Zhang D, Zeng K. Evaluating the behavior of electrolytic gas bubbles and their effect on the cell voltage in alkaline water electrolysis. Ind Eng Chem Res 2012;51:13825–32. An L, Zhao TS, Chai ZH, Tan P, Zeng L. Mathematical modeling of an anionexchange membrane water electrolyzer for hydrogen production. Int J Hydrog Energy 2014;39:19869–76. Ran J, Wu L, Wei B, Chen Y, Xu T. Simultaneous enhancements of conductivity and stability for anion exchange membranes (AEMs) through precise structure design. Sci Rep 2014;4(6486):1–5. Li Z, Jiang Z, Tian H, Wang S, Zhang B, Cao Y, et al. Preparing alkaline anion exchange membrane with enhanced hydroxide conductivity via blending imidazolium-functionalized and sulfonated poly(ether ether ketone). J Power Sources 2015;288:384–92. Amel A, Smedley SB, Dekel DR, Hickner MA, Ein-Eli Y. Characterization and chemical stability of anion exchange membranes cross-linked with polar electrondonating linkers. J Electrochem Soc 2015;162:F1047–F1055. Ren X, Price SC, Jackson AC, Pomerantz N, Beyer FL. Highly conductive anion exchange membrane for high power density fuel-cell performance. ACS Appl Mater Interfaces 2014:5–8. Movil O, Frank L, Staser JA. Graphene oxide-polymer nanocomposite anionexchange membranes. J Electrochem Soc 2015;162:F419–F426. Zeng K, Zhang D. Evaluating the effect of surface modifications on Ni based electrodes for alkaline water electrolysis. Fuel 2014;116:692–8. Zhang Y-X, Guo X, Zhai X, Yan Y-M, Sun K-N. Diethylenetriamine (DETA)assisted anchoring of Co3O4 nanorods on carbon nanotubes as efficient electrocatalysts for the oxygen evolution reaction. J Mater Chem A 2015;3:1761–8.