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ISSN : 1226-0088(Print)
ISSN : 2288-7253(Online)
Membrane Journal Vol.32 No.5 pp.275-282
DOI : https://doi.org/10.14579/MEMBRANE_JOURNAL.2022.32.5.275

Recent Progress on Proton Exchange Membrane Based Water Electrolysis

Seungmin Yang*, Rajkumar Patel**†
*Nano Science and Engineering, Underwood International College, Yonsei University, Incheon 21983, South Korea
**Energy and Environmental Science and Engineering, Integrated Science and Engineering Division, Underwood International College, Yonsei University, Incheon 21983, South Korea
†Corresponding author(e-mail: rajkumar@yonsei.ac.kr; http://orcid.org/0000-0002-3820-141X)
September 29, 2022 ; October 1, 2022 ; October 11, 2022

Abstract


In contemporary days, hydrogen-based energies including batteries are renowned to be effective. And its effectiveness comes from the fact that it possesses high efficiency as an energy carrier. Eco-friendly and high purity of hydrogens comes out from water electrolysis. And among different types of electrolysis, proton exchange membrane (PEM) water electrolysis is considered the most renewable, cheap, and eco-friendly. It produces oxygen and hydrogens which are feasible in using as energies. Since it has such a number of benefits, increased research is going on in PEM electrolysis. Nafion is widely used as PEM, but high cost and various other disadvantages leads to the exploration of alternative materials. This review is broadly classified into Nafion and non Nafion based PEM for water electrolysis.


Nafion , electrolyzer , proton exchange membrane , solid polymer electrolyte

수소이온 교환막 기반 수전해의 최근 연구 동향

양 승 민*, 라즈쿠마 파텔**†
*연세대학교 언더우드학부 융합과학공학부 나노과학공학
**연세대학교 언더우드학부 융합과학공학부 에너지환경융합전공

초록


현대에는 배터리를 비롯한 수소 기반 에너지가 효율적이라고 널리 알려져 있다.이러한 결과는 수소가 에너지 수 송체로써의 높은 효율을 가지고 있다는 사실로부터 기반한다. 자연 친화적이며 높은 순도를 가진 수소는 수전해로부터 제조 할 수 있다. 다양한 종류의 전기분해 중, 수소이온 교환막 수전해는 가장 재생 가능하며 싸고 자연 친화적이다. 이는 에너지 로써 사용이 되기에 적합한 산소와 수소를 생성한다. 이와 같이 많은 장점이 있기에 활발한 연구가 수소이온 교환막 전기분 해에 대해 진행되고 있다. 나피온은 수소이온 교환막으로 널리 쓰이지만, 비싼 비용과 다양한 다른 단점으로 인해 여러 가지 종류의 대체재가 개발되고 있다. 본 총설에서는 크게 나피온과 비나피온(non-Nafion) 기반 수소이온 교환막 수전해로 나누어 다루었다.


    1. Introduction

    Renewable source of energy is in growing demand due to the global warming mainly caused by fossil fuel. Generation of electrical energy for application in portable as well as stationary sectors are very high. Alternative renewable sources are hydropower, wind power or solar cell. Fuel cell is excellent alternative technology which generate electrical energy from renewable resource[1-5].

    Hydrogen is another source of clean energy which can be generated by reforming of fossil fuels but its quality is lower as compared to generation by water splitting. Although hydrogen evolution reaction (HER) at the cathode is relatively easy but not the oxygen evolution reaction (OER) at the anode. Electrocatalytic splitting of water is well established process. Water electrolyzer based on polymer electrolyte membrane or know as solid electrolyte like Nafion has wide range of advantages then alkaline water electrolysis[6-8].

    Nafion membrane are excellent proton exchange membrane with some limitations such as cost and stability at higher temperature. Another disadvantage is the hydrogen cross over when the membrane is in the form of hydrated state. Nafion membrane gets plasticized by which glass transition temperature of the polymer gets reduced. This leads to the higher permeability for hydrogen through the membrane and reduce in the efficiency of the electrolyzer. This phenomenon was reduced by compositing with nanosized tungsten oxide (WO3)[9]. In order to reduce the hydrogen crossover, thin polyamide layer was generated in-situ on the Nafion membrane by interfacial polymerization [10]. This substantially reduce the passage of hydrogen gas and enhance the electrolyzer efficiency.

    Sulfonated polyether sulfone is an alternative solid polymer electrolyte membrane used in methanol electrolysis[ 11]. There are several non Nafion membrane are researched for the fabrication of water electrolyzer which is reviewed in this work. Classification of the review is presented in Fig. 1.

    2. PEM Electrolyzer

    Alkaline water electrolysis is a well-developed process in which alkaline solution (KOH) is used as an electrolyte and electrode are made up of nickle. PEM based water electrolysis is one of the excellent alternative that mainly use Nafion membrane. A gapless bipolar membrane water electrolyzer features a high-pH environment for oxygen evolution reaction and a low-pH for hydrogen evolution[12] (Fig. 2).

    The advantages provided by anion exchange membrane water electrolysis can be shown at proton exchange membrane water electrolysis by integrating a water-splitting bipolar interface into it. The research showed the result after moving the bipolar interface to the anode. In a cell voltage of 2.2 V environment, it showed current densities of 450 (with water splitting catalyst) and 5 mA/cm2 (without water splitting catalyst). Also, moving the bipolar interface to the place between the acidic membrane and high pH-anode resulted in a further increase in current densities with 9000 mA/cm2. As compared to PEM membrane electrode assembly (MEA), bipolar interface MEA shows two times higher current density even at 2.2 V cell voltage.

    2.1. Nafion

    Nafion, a perflurosulfonic acid (PFSA) polymer is widely used as PEM for different applications but has certain limitations. Semi interpenetrating (Semi-IPN) network of Nafion with other polymer is an alternative to improve its properties in water electrolysis application. Thermal esterification of polyacrylic acid (PAA) and polyvinyl alcohol (PVA) in presence of Nafion to form semi-IPN resulted in higher mechanical firmness and a decrease in hydrogen crossover[13] (Fig. 3).

    Compared to recast Nafion (re-Nafion,) the semi-IPN membrane composed of Nafion, PAA, and PVA (NPP-95) was able to attain hexagonal cylindrical architecture and enhance thermal, mechanical, and dimensional stability. It also holds lower hydrogen permeability (49.6% lower at 100 %RH and 80°C), higher conductivity (1561 mA/cm2 at 0.6 V), peak power density (1179 mW/cm2), proton-exchange membrane fuel cell (PEMFC) performance (4310 mA/cm2 at 2 V), and lower ohmic resistance (at 60~80°C). NPP-95 possesses low water uptake (WU) which resulted in remarkably competent at managing water.

    When using PEM poor gas barriers like perfluorosulfonic acid (PFSA) membrane, they have degradation and safety problems[14] (Fig. 4).

    By compositing monolayer hexagonal boron nitride (hBN) with Nafion improve its properties drastically. The hBN/Nafion is prepared by using chemical vapor deposition on large-area monolayer hBN (~25 cm2) to Nafion 117. Naffion 117’s hydrogen barrier property tremendously increases, and the mechanical stability is also enhanced. While the demerit of this membrane is decreased proton conductivity, increasing the operating temperature led to an increase in the efficiency of water electrolysis. With the help of hBN, hydrogen permeability decreased by 40% while electrolysis efficiency decreased by 19% in comparison to Nafion. In addition, the enhancement resulted in higher long-term (100h) stability than the Nafion 117.

    PFSA is expensive and possesses high gas permeability[ 15]. So, it is important to find a replacement for the PFSA that has low gas permeability and low ohmic resistance. By stretching Nafion 117, increased hydrophilic channel tortuosity, better cell performance (At 1.9 V, current density increased from 1.5 to 3.2 A/cm2), and high persistence at a constant current can be achieved. The increased channel tortuosity achieved by stretching Nafion 117 biaxially up to 6.4 times results in a cut down in hydrogen permeability by a factor of 1.6. This prevents membrane degradation (lowers degradation rate by 2.5 times more) and broadens the operating current density scope.

    2.2. Non Nafion

    Durability and efficiency of PEM in the polymer electrolyte electrolysis membrane are the most important factors. Cost, higher membrane thickness and instability at higher temperature are three major defects of Nafion membrane. In order to overcome these defects, lot of research going on to find alternative materials. Perfluorosulfonic acid (PFSA) membranes can be replaced with Radiation-grafted membranes. Radiation-grafted membranes can be created with styrene, acrylonitrile, and 1,3-diisopropenylbenzene undergoing the process of preirradiated ethylene tetrafluoroethylene (ETFE) base film and sulfonation[16] (Fig. 6).

    In the research, the embodiment of grafts is analyzed with scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Also, the relativity of hydrogen crossover and area resistance of the membranes is analyzed. This shows the indication of the target areas for membrane development for electrolyzer applications. Mechanical properties are measured through tensile tests. The result of the figure of merit that is induced by analyzing hydrogen crossover, area resistance, and mechanical properties shows that radiation- gifted membranes attained in the current research are more promising than Nafions. When the grafting is about 40% then the ion exchange capacity (IEC) is marginally higher than Nafion.

    Bender et al. reported two membranes based on proton exchange membranes (PEM) as a replacement for the PFSA based polyelectrolytes[17] (Fig. 6 & 7).

    One is a controllable covalently bound multiblock-coionomers (MBIs) with a nanophase-separated structure. The membrane is controlled by changing hydrophilic and hydrophobic rates. Second is the ionically cross-linked blend membranes of partially fluorinated polybenzimidazole and pyridine side-chain-modified polysulfones. This membrane has high proton conductivities and high mechanical stability. Proton conductivities of the MBIs membranes are in the range of 50 to 100 mS/cm which is lower than the blend membrane.

    The hydrocarbon-based membrane is a promising membrane that can be used in Polymer Electrolyte Membrane Water Electron (PEMWE) since it extends the operating current range[18]. This is made possible by their characteristic of low gas permeability and high proton conductivity. However, when an increase in the ion exchange capacity occurs, it results in poor mechanical stability and increased gas permeability. To solve this problem, the research suggests a poly(p-phenylene)- based multiblock polymer with an oligomeric chain extender (CE-sPP-PPES). Compared to the Nafion-based PEMWE, CE-sPP-PPES-based PEMWE showed a 2.1 times wider operating range from its less ohmic overvoltage and less gas crossover at the most desirable ion exchange capacity. Increase rate of cell voltage for the multiblock copolymer is 107 μ V/h which is slightly higher than the Nafion membrane.

    Proton conductive PFSA, which are used in electrolyte membranes is pricey and extremely proton permeable[ 19]. The research represents the substitution of PFSA, the hydrogen-based sulfonated poly proton- carrying polymers (BPSH). BPSH is synthesized by changing the ion exchange capacity (IEC) from 1.2 to 2.0 meq/q and employing ion-carrying units into polymers. At 80°C and 100% RH, BPSH showed lower barrer (20-45 barrer) than PFSA (~115 barrer for Nafion). Also, at the same IEC, random BPSH membranes resulted in greater selectivity in proton to hydrogen than the multi-block BPSH. In a 1.9 V environment, the leading performance of random BPSH with the IEC ~1.9 meq/g and ~50 μm thickness showed 5.3 A/cm2 while Nafion212 showed 4.8 A/cm2. However, when given the 360 successive dynamic current densities of 3 and 0.02 A/cm2, the degradation rate of BPSH (951 μV/h) was higher than Nafion 212 (613 μV/h).

    Klose et al., shows the analogy of hydrocarbon MEAs with that of Nafion-MEAs[20]. At a 1.8 V environment, PEMWE-MEA with sPPS shows a current density of 3.5 A/cm2 and frequency resistance of 57 ± 4 mΩcm2 while Nafion-MEA (N115 as a membrane) shows 1.5 A/cm2 and 161 ± 7 mΩcm2. This shows that the efficiency of PEMWE-MEA outweighs Nafion-MEA. Also, in a humidified surrogate test, sPPS-membranes resulted in higher detachment of gas (< 0.3 mA/cm2) than Nafion N115-membranes (> 1.1 mA/cm2).

    The promising performance of Sulfonated polysulfone (SPSf) PEM as an alternative to Nafion was reported[21]. SPSf which is used for the formation of a proton-carrying membrane was obtained by using trimethyl silyl chlorosulfonate as the sulfonating agent. SPSf PEM with IrO2 anode and Pt/C cathode showed similar polarization and hydrogen permeability to that of Nafion 115 membrane. At a 1.8 V environment, after 35 h of potentiostatic operation, SPSf MEA was able to obtain 1.35 A/cm2 of current density.

    Kang et al., proposes phosphonated aromatic polymers as PEMs[22]. This material showed effective ionic clustering with the interdomain distance controlled by the acid content of the polymer. It also possesses high proton conductivity, at 80°C a fully hydrated environment, it reached 11 mS/cm and also showed 4mS/cm at 50 %RH, 80°C. The PEM showed high stability up to 400°C. PEM also showed high radical resistance. Even after 5 h immersion in Fenton’s reagent at 80°C, there was no difference in weight, outer aspect, or molecular structure.

    3. Conclusions

    Demands for clean energy is ever increasing due to the growing environmental pollution and changing weather pattern. Generation of hydrogen from water by electrolysis using proton exchange membrane have huge advantage over water splitting method. Purity of hydrogen gas is higher by using solid electrolyte membrane electrode assembly. Nafion membrane is well established proton exchange membrane for number of applications but has some limitation due to its cost and thermal instability at higher temperature. As a result, alternative material like fluorinated polymer are explored. This review describe the water electrolysis based on proton exchange membrane for Nafion and non Nafion membrane.

    Figures

    MEMBRANE_JOURNAL-32-5-275_F1.gif

    Schematic representation of classification of the review.

    MEMBRANE_JOURNAL-32-5-275_F2.gif

    (a) aPTEac surface structure and schematic overview of PEMWE MEA. (b) aPTEal surface structure with high IEC binder and schematic overview of AEMWE MEA. (c) aPTEal surface structure with low IEC binder and schematic overview of AEMWE MEA. (d) Variation of IrO2 loading in aPTEal with a 9 wt% Aemion binder in CL. All MEAs fabricated with a 36 μm Aemion membrane and cPTEal with 0.5 mg cm–2 Pt. (e) Comparison of performance of different AEMWE setups in PTE design (aPTEal: 2 mg Ir cm–2 with 9 wt% Aemion binder with high and low IEC, cPTEal: 0.5 mg Pt cm–2 with 10 wt% high IEC Aemion). The PEM reference (aPTEac: 2 mg Ir cm–2 with 10 wt% Nafion binder, cPTEac: 0.5 mg Pt cm–2 with 20 wt% Nafion binder) was based on a Nafion 212 membrane Possible permselectivity mechanism of the prepared AEMs (membranes grafted with long alkyl chains) (Reproduced with permission from Thiele et al.[12], Copyright 2020, American Chemical Society).

    MEMBRANE_JOURNAL-32-5-275_F3.gif

    Semi-IPN membrane re-Nafion and N117-based PEMWE performance under different operating conditions (i.e., cell temperature and voltage): (a) cell–polarization curves; (b) EIS curves from a Nyquist plot at 60°C; and (c) EIS curves from a Nyquist plot at 80°C Possible permselectivity mechanism of the prepared AEMs (membranes grafted with long alkyl chains) (Reproduced with permission from Al Munsur et al.[13], Copyright 2021, American Chemical Society).

    MEMBRANE_JOURNAL-32-5-275_F4.gif

    (a) Schematic illustration of the membrane and electrode assembly containing a proton-conductive gas barrier layer (hexagonal boron nitride, hBN) for water electrolysis. (b) Fabrication procedure of the hBN/Nafion composite membrane Possible permselectivity mechanism of the prepared AEMs (membranes grafted with long alkyl chains) (Reproduced with permission from Kim et al.[14], Copyright 2021, American Chemical Society).

    MEMBRANE_JOURNAL-32-5-275_F5.gif

    Results of tensile tests of membranes in the machining direction under (left) ambient and (right) fully hydrated conditions Possible permselectivity mechanism of the prepared AEMs (membranes grafted with long alkyl chains) (Reproduced with permission from Albert et al.[16], Copyright 2015, American Chemical Society).

    MEMBRANE_JOURNAL-32-5-275_F6.gif

    Synthesis scheme for the multiblock-co-ionomers (Reproduced from Wang et al.[17], 2021, MDPI).

    MEMBRANE_JOURNAL-32-5-275_F7.gif

    Photographs of the multiblock-co-ionomer membranes: (a)—MBI-LL; (b)—MBI-LS; (c)—the ternary blend membrane 3CBM-1 (Reproduced from Wang et al.[17], 2021, MDPI).

    Tables

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