Microporous Electrode Binders for Anion Exchange Membrane Water Electrolyzers.

Limited by the strong oxidation environment and sluggish reconstruction process in oxygen evolution reaction (OER), designing rapid self-reconstruction with high activity and stability electrocatalysts is crucial to promoting anion exchange membrane (AEM) water electrolyzer. Herein, trace Fe/S-modified Ni oxyhydroxide (Fe/S-NiOOH/NF) nanowires are constructed via a simple in situ electrochemical oxidation strategy based on precipitation-dissolution equilibrium. In situ characterization techniques reveal that the successful introduction of Fe and S leads to lattice disorder and boosts favorable hydroxyl capture, accelerating the formation of highly active γ-NiOOH. The Density Functional Theory (DFT) calculations have also verified that the incorporation of Fe and S optimizes the electrons redistribution and the d-band center, decreasing the energy barrier of the rate-determining step (*O→*OOH). Benefited from the unique electronic structure and intermediate adsorption, the Fe/S-NiOOH/NF catalyst only requires the overpotential of 345 mV to reach the industrial current density of 1000mAcm-2 for 120 h. Meanwhile, assembled AEM water electrolyzer (Fe/S-NiOOH//Pt/C-60°C) can deliver 1000mAcm-2 at a cell voltage of 2.24V, operating at the average energy efficiency of 71% for 100 h. In summary, this work presents a rapid self-reconstruction strategy for high-performance AEM electrocatalysts for future hydrogen economy.

Highly durable and efficient anion exchange membrane water electrolyzer using one-step fabrication of the integrated electrode by the hot-press process

One promising way to store and distribute large amounts of renewable energy is water electrolysis, coupled with transport of hydrogen in the gas grid and storage in tanks and caverns. The intermittent availability of renewal energy makes it difficult to integrate it with established alkaline water electrolysis technology. Proton exchange membrane (PEM) water electrolysis (PEMEC) is promising, but limited by the necessity to use expensive platinum and iridium catalysts. The expected solution is anion exchange membrane (AEM) water electrolysis, which combines the use of cheap and abundant catalyst materials with the advantages of PEM water electrolysis, namely, a low foot print, large operational capacity, and fast response to changing operating conditions. The key component for AEM water electrolysis is a cheap, stable, gas tight and highly hydroxide conductive polymeric AEM. Here, we present target values and technical requirements for AEMs, discuss the chemical structures involved and the related degradation pathways, give an overview over the most prominent and promising commercial AEMs (Fumatech Fumasep® FAA3, Tokuyama A201, Ionomr Aemion™, Dioxide materials Sustainion®, and membranes commercialized by Orion Polymer), and review their properties and performances of water electrolyzers using these membranes.

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

Alkaline anion exchange membrane (AEM) water electrolysis is a promising technology for producing hydrogen using renewable energies. However, current AEM electrolyzers still employ noble‐metal‐containing electrocatalysts, or have significant overpotential loss, or both. Here non‐noble‐metal electrocatalysts for both the hydrogen and oxygen evolution reactions (HER and OER) are developed. Both catalysts are made of a same NiMo oxide. Judicious processing of these materials in a mixed NH3/H2 atmosphere results in a NiMo‐NH3/H2 catalyst, which has superior activity in HER, delivering 500 mA cm−2 at an overpotential of 107 mV. Doping Fe ions into the NiMo‐NH3/H2 catalyst yields an Fe‐NiMo‐NH3/H2 catalyst, which is highly active for the OER, delivering 500 mA cm−2 at an overpotential of 244 mV. These catalysts are integrated into an AEM electrolyzer, which delivers 1.0 A cm−2 at 1.57 V at 80 °C in 1 m KOH. The energy conversion efficiency at this current density is as high as 75%. This work demonstrates high‐efficiency AEM electrolysis using earth‐abundant catalytic materials.

Anion Exchange Membrane (AEM) water electrolyzer is expected to be the next generation water electrolyzer that can realize low cost and high performance. Because the ionic conductivity of AEM and catalytic activity cannot be sufficiently maintained if DI water is supplied to the anode in the current AEM water electrolyzer, a dilute solution of KOH or K2CO3 (<pH14) is often used as the supporting electrolyte.We compared the performance of AEM water electrolysis when using K2CO3 and KOH solution as the anode electrolyte, adjusted to a pH of around 12, and confirmed that K2CO3 solution provides higher performance at this pH level [1]. On the other hand, many previous studies have shown that using a high pH electrolyte is useful for improving electrolysis performance. In this study, we conducted an electrolysis test by supplying a higher concentration of KOH solution to the anode, and re-examined the effects of the electrolyte solute and pH on electrolysis behavior.In the AEM water electrolysis tests, 1wt.%-KOH solution (pH=13.2) and 10wt.%-K2CO3 solution (pH=11.8) were used as the anode electrolytes. Two types of cells were used in the AEM water electrolysis tests. One was Cell-1, with an electrode area of 25 cm2, and the other was Cell-2, with an electrode area of 1 cm2. In both cell tests, the electrolyte was supplied only to the anode, and the cathode was maintained in a dry state, and all tests were carried out at a cell temperature of 50ºC. In the test using Cell-1, current-voltage characteristics and dew point of the generated hydrogen were measured, and in the test using Cell-2, a reference electrode was inserted, and in addition to the current-voltage characteristics, overpotential separation of the anode and cathode was performed. The test results for both cells were examined, and the effects of the electrolyte solute and pH on the electrolysis behavior were considered.

Next-generation cost-effective anion exchange membrane (AEM) fuel cells (AEMFCs) and AEM water electrolyzers (AEMWEs) have emerged as promising alternatives to costly proton exchange membrane (PEM) fuel cells and water electrolyzers due to the possibility of utilizing platinum-group-metal (PGM)-free catalysts and phasing out unsustainable perfluorosulfonic acid polymers. Anion exchange polyelectrolytes (AEPs), which can be utilized as AEMs or ionomers, are pivotal materials in AEM devices. Despite extensive exploration in the past decade, the application of AEPs has been significantly impeded by their poor ionic conductivity, insufficient alkaline stability, and unfavorable mechanical properties. Therefore, developing highly conductive and robust AEPs is critical to the success of AEMFCs and AEMWEs. (i) Our group has developed a series of highly conductive and durable poly(aryl-co-aryl piperidinium) (c-PAP) AEPs to address the aforementioned issues. c-PAP AEMs and ionomers enable outstanding OH- conductivity (>160 mS cm-1 at 80 °C), alkaline stability (1 M NaOH at 80 °C > 2000 h), dimensional stability, and mechanical properties (tensile strength > 80 MPa), giving them all the properties required for applications in AEM devices. (ii) Based on c-PAP AEMs and ionomers, we have developed high-performance AEMFCs and AEMWEs, as well as provided insights into the ionomer research and the design of membrane electrode assemblies. Typically, c-PAP AEMFCs reached the topmost peak power densities (PPDs) of 2.7 W cm-2 at 80 °C in H2-O2 along with 1000 h cell durability. Moreover, cathode-dried AEMWEs achieved a record-breaking current density of 17 A cm-2 in 1 M KOH, and the cell can be run stably at a 1.5 A cm-2 current density for over 2000 h. The remarkable performances achieved by this new class of c-PAP AEPs identify them as the most promising candidates for practical applications in AEMFCs and AEMWEs. In this account, we will elaborate on our strategies and methodologies associated with c-PAP AEPs and AEM devices, covering the screening and identification of highly durable cation head groups and molecular-engineering approaches to design c-PAP AEMs and ionomers. Moreover, we underscore our strategy in terms of developing highly efficient and durable AEMFCs and AEMWEs. We also elucidate different approaches for further enhancing the ion conductivity and mechanical stability of c-PAP AEMs, including the design of backbones and side chains, cross-linking, and reinforcement. We firmly believe that our series of studies has made substantial contributions to the fields of AEM, ionomers, AEMFCs, and AEMWEs, which have advanced AEM technology to be on par with PEM technology, opening a new avenue for commercialization of AEMFCs and AEMWEs.

Anion exchange membrane water electrolysis (AEM-WE) stands out as a promising method for hydrogen production from renewable energy sources. Unlike proton exchange membrane water electrolysis (PEM-WE), AEM-WE offers the advantage of nonprecious metal catalysts due to mild alkaline conditions, while still enabling a compact cell design and operation under differential pressure. Remarkable performances of AEM-electrolysis cells have been demonstrated in literature, achieving current densities of up to 7.68 A·cm−2 [1], surpassing the record current density of PEM-WE at 6 A·cm−2 [2], both measured at 80 °C and 2 V. However, challenges related to efficiency and stability persist and demand effective solutions to unlock the full potential of AEM-WE. In order to achieve this, cell components, particularly the membrane, catalysts, and electrode structure have to be improved and advanced measurement techniques have to be applied.Different measurement tools serve to characterize, separate, and quantify the dynamic processes inherent in AEM-WE. Electrochemical impedance spectroscopy (EIS) serves as a valuable non-destructive, in-situ diagnostic tool for electrochemical cells, enabling the differentiation of various phenomena based on their respective relaxation times. Moreover, EIS enables the quantification of loss mechanisms thereby facilitating a targeted optimization of electrolysis components and the identification of weaknesses limiting long-time stability.Despite its utility, the broad use of EIS is hindered by its complex interpretation. The widely applied approach of fitting equivalent circuit models (ECMs) aims to extract quantitative data from the semi-circular shapes observed in Nyquist plots. However, the assumption of a physico-chemical meaningful ECM is challenging without prior knowledge of the number, size, and time constants of appearing loss mechanisms. Furthermore, the potential for multiple ECMs to fit one and the same experimental result introduces ambiguity, leading to potential misinterpretations of data.An alternative approach to analyse EIS spectra involves converting the measured data into the time (τ) domain and characterize the different mechanisms based on their characteristic relaxation times. The distribution of relaxation times (DRT) analysis has already been applied across diverse electrochemical systems, spanning from solid oxide fuel cells (SO-FC) and PEM fuel cells (PEM-FC) to PEM water electrolysis and AEM fuel cells (AEM-FC). However, to the best of the authors’ knowledge, DRT analysis of AEM-WE is still absent in literature.In this study we present a thorough investigation of an AEM-WE single-cell, employing a combination of electrochemical impedance spectroscopy (EIS) and the equivalent circuit model (ECM). Notably, we demonstrate the distribution of relaxation times (DRT) analysis at AEM-WE cells for the first time, utilizing a reversible hydrogen electrode (RHE) as a reference. Through half-cell EIS measurements and subsequent DRT spectra analysis, anodic and cathodic half-cell reactions are clearly identified and quantified. The DRT analysis differentiates five loss mechanisms within the AEM-WE system, encompassing the hydrogen evolution reaction, the oxygen evolution reaction, and ionic transport losses within the catalyst layers. By systematically varying operating parameters, we successfully attribute DRT peaks to their respective physicochemical origins. These findings provide valuable insights into the electrochemical processes within the AEM-WE single-cell, significantly advancing our understanding of underlying mechanisms.In fig. 1, we investigate the degradation of AEM-WE single cells through long-term experiments. Periodic EIS measurements enable an in-operando analysis of catalyst degradation. The impedance data is analysed using ECM and DRT. Ohmic contributions resulting from electric and ionic charge transport are distinguished from polarisation resistances arising from electrochemical reactions. A catalyst degradation is observable in fig 1b & c, where the low frequency resistance (LFR) in the Nyquist plot and the DRT peak area increases over time. However, fig. 1b also reveals the reduction of the high frequency resistance (HFR) resulting in an overall performance increase and voltage decrease.[1] N. Chen, S. Y. Paek, J. Y. Lee, J. H. Park, S. Y. Lee, Y. M. Lee, High-performance anion exchange membrane water electrolyzers with a current density of 7.68 a cm −2 and a durability of 1000 hours, Energy & Environmental Science 14 (12) (2021) 6338–6348. doi:10.1039/D1EE02642A.[2] M. Braig, R. Zeis, Distribution of relaxation times analysis of electrochemical hydrogen pump impedance spectra, Journal of Power Sources 576 (2023) 233203. doi:10.1016/j.jpowsour.2023.233203. Figure 1

To achieve carbon neutrality, extensive research is ongoing to produce green hydrogen via water electrolysis combined with renewable energy sources. Understanding and developing catalysts for membrane electrode assembly (MEA) type water electrolysis systems, such as proton exchange membrane water electrolysis (PEMWE) and anion exchange membrane water electrolysis (AEMWE), are critical steps toward this goal. Therefore, this presentation will focus on catalyst development for PEMWE and AEMWE, with emphasis on cost-effective catalysts.For oxygen evolution reaction (OER) catalysis in PEMWE, the primary challenge is to minimize iridium (Ir) usage for sustainable green hydrogen production. Layered monoclinic iridium nickel oxide (IrNiOx) platelets were synthesized using a molten salt method and employed for the OER. These platelets exhibited high OER activity with reduced Ni dissolution in acidic conditions. When applied in MEA, they demonstrated enhanced interconnectivity within the catalyst layer, promoting electron transfer. Even at low Ir loading (0.2 mgIr cm-2), the platelets showed good performance with an initial cell voltage of 1.70 V at 1 A cm-2 and minimal degradation over 100 hours. This highlights the effectiveness of incorporating transition metals into Ir oxide to reduce Ir usage in PEMWE.For oxygen evolution reaction (OER) catalysis in AEMWE, attention is focused on nickel iron (NiFe) hydroxide catalysts for their high activity and stability in alkaline conditions. However, the lack of initial electrical conductivity of as-prepared NiFe LDH limits its potential as an electrocatalyst. To address this issue, a monolayer structuring approach was proposed. This method improved mass transport to allow a high energy conversion efficiency of 72.6% with exceptional stability over 50 hours at 1 A cm-2. Additionally, lack of initial electrical conductivity hinders determination of electrochemically active surface area (ECSA) of NiFe LDH using conventional double layer capacitance method. Use of electrochemical impedance spectroscopy (EIS) at reactive OER potentials to extract the capacitance that is hypothesized to arise due to reactive OER intermediates (O*, OH*, OOH*) adsorbed on the catalyst surface was thus investigated. This allowed the estimation of ECSA and intrinsic activity of NiFe LDH, validating the methodology through rigorous catalyst loading and support studies.For hydrogen evolution reaction (HER) catalysis in AEMWE, replacing platinum (Pt) with Ni-based alloys containing molybdenum (Mo) species has shown promise. Dynamic active sites of Ni catalysts with Mo species require innovative approaches to maintain initial performance, warranting material innovation or careful manipulation of operating protocol. The approaches in which this is made possible is shortly presented, together with the innovations that allows probing of such phenomenon.

Developing a high-efficiency and stable anode catalyst layer (CL) is crucial for promoting the practical applications of anion exchange membrane (AEM) water electrolyzers. Herein, a hierarchical nanosheet array composed of oxygen vacancy-enriched CoCrOx nanosheets and dispersed FeNi layered double hydroxide (LDH) is proposed to regulate the electronic structure and increase the electrical conductivity for improving the intrinsic activity of the oxygen evolution reaction (OER). The CoCrOx/NiFe LDH electrodes require an overpotential of 205 mV to achieve a current density of 100 mA cm-2, and they exhibit long-term stability at 1000 mA cm-2 over 7000 h. Notably, a breakthrough strategy is introduced in membrane electrode assembly (MEA) fabrication by transferring CoCrOx/NiFe LDH to the surface of an AEM, forming a 3D-interlocked anode CL, significantly reducing the overall cell resistance and enhancing the liquid/gas mass transfer. In AEM water electrolysis, it exhibits an ultralow cell voltage of 1.55 Vcell to achieve a current density of 1.0 A cm-2 in 1 M KOH, outperforming the state-of-the-art Pt/C//IrO2. This work provides a valuable approach to designing high-efficiency electrocatalysts at the single-cell level for advanced alkaline water electrolysis technologies.

The performance of a water electrolyser (WE) depends on several aspects, many of which are located in the powerhouse of the cell, namely, the membrane electrode assembly (MEA). The anion exchange membrane WE (AEMWE) is a promising technology; however, both activity and stability must be further developed to surpass the current dominant WE technologies. Herein, we review aspects related to MEA development for anion exchange membrane water electrolysers, covering materials and techniques from the perspective of stability and activity. The gas diffusion layer (GDL) and the microporous layer (MPL) are often combined into a single MEA component, which places great importance on its composition. This composite layer has the greatest impact of any single component on cell performance, as the physical architecture of the GDL/MPL influences the overpotential related to activation, ohmic, and mass transport. The purpose of this review is to serve as an executive summary of the literature related to MEAs for AEMWEs for researchers and industry professionals who seek to further the state of the art.

Anion exchange membrane water electrolyzer (AEMWE) is a potentially cost‐effective technology for green hydrogen production. Although the normal current densities of AEMWEs are below 3 A ⋅ cm−2, operating them at higher current densities represents an efficient, but little‐explored approach to decrease the total cost of hydrogen production. We show here that a benchmark AEMWE has an operational lifetime of only seconds at an ultrahigh current density of 10 A ⋅ cm−2. By using a more conductive and robust AEM, and judicious choices of ionomers, catalyst, and porous transport layer, we have developed AEMWEs that stably operate at 10 A ⋅ cm−2 with extended lifetimes. The optimized AEMWE has an operational lifetime of more than 800 hours, a 5‐order magnetite improvement over the current benchmark. The cell voltage is only 2.3 V at 10 A ⋅ cm−2, comparable to those of the state‐of‐the‐art devices operating at current densities lower than 3 A ⋅ cm−2. This work demonstrates the potential of ultrahigh current density AEMWEs.

Intermittency issues due to the growth of renewable energy usage lead to an increasing interest in energy storage systems. Thereby, one can use the excess energy for the production of hydrogen from water by anion exchange membrane (AEM) water electrolysis using earth-abundant, non-noble metal catalysts. However, the kinetics of the oxygen evolution reaction (OER) limit the activity of the catalyst and hence, the development of performance stable and active OER catalysts is of great importance for the commercialization of AEM water electrolyzers. In metal organic frameworks (MOFs), a porous structure is created by linking metal atoms/clusters with other metal centres using organic ligands. The resulting structure with a high surface area and dispersed metal centres is a promising catalyst for OER.MOF catalysts with Ni and Co metal centres show impressive OER activity. Both, Co-MOF-74 and Ni-MOF-74 exhibit higher OER activity than their oxide counterparts produced by flame spray synthesis in form of nanoparticles (Fabbri (2017) and Abbott (2018)).1,2 Thereby, an increasing OER activity of Ni-MOF-74 during rotating disk electrode stability tests, indicates that the catalytic species undergoes favorable electronic and structural transformations.Using an in-house-developed spectro-electrochemical flow cell, these transformations were studied by operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). Operando XAS enabled us to monitor the changes occurring in the Ni metal centers while performing cyclic voltammetry (CV) from 1 VRHE up to potentials above the OER onset potential with a time resolution of 5 sec. The operando XAS measurements show that Ni-MOF-74 develops into a highly OER active and stable catalytic species. However, operando XRD measurements prove that the crystalline MOF structure changes into an amorphous structure during OER. Monitoring the electronic and structural transformations due to potential cycling, three different structures were extracted: an initial state, which disappeared within the first eight CV cycles, and two novel electronic/local structures appearing at low and high potential, respectively. The transformations in the electronic and local structure of the Ni metal centers occurring between low and high potential appear to be reversible, as the Ni metal centers return to their initial state during long-term storage in air. However, XRD measurements indicate that this further transformation does not affect the structure of the catalyst: Even though the Ni centers return to their original local structure and oxidation state after long-term storage in air, the long-range structure stays amorphous.The study focuses on elucidating the structure-performance relations of Ni-MOF-74 based OER catalysts providing the key structural parameters that lead to the formation of the highly OER active species. The mechanism of reversible and irreversible transformations occurring in Ni-MOF-74 catalysts allows the extraction of activity descriptors for the development of highly OER active and stable non-noble metal catalysts for AEM water electrolysis.

Despite the promising potential of non-platinum group metal electrocatalysts for anion exchange membrane (AEM) electrolysis, their practical application remains a challenge.1 Metal–organic frameworks (MOFs) and their derivatives have gained significant attention due to their high surface area, 3D periodic porous structure, large porosities, diverse composition, and well-defined metal centers as potential OER electrocatalysts in AEM electrolysis.2 However, there is a limited research report that MOFs could be used in the membrane electrode assembly (MEA) test due to the inherent limitations, such as poor stability in water, especially under basic/acidic conditions and low electrical conductivities.3,4 To address these challenges, we present a novel approach involving the introduction of boron atoms into an aromatic carbon framework. In this study, we have successfully fabricated micro-nano flower-like bimetallic NiFe-MOF-74 doped with boron. These innovative materials exhibit exceptional catalytic activity, boasting an impressively low overpotential of 318 mV for OER at a laboratory-grade current density ranging from 10 to 100 mA/cm2. Moreover, they demonstrate rapid reaction kinetics, as evidenced by a small Tafel slope of 89 mV dec-1 in 1.0 M KOH solution. In addition, our research highlights the outstanding performance of boron-modified NiFe-MOF-74 as a robust anode material for AEM water electrolysis. This significant development paves the way for a promising electrocatalyst in the realm of anion exchange membrane water electrolysis. Our findings hold great potential for advancing the field and overcoming the existing challenges associated with non-platinum group metal electrocatalysts in AEM electrolysis.Reference:(1) Song, W.; Peng, K.; Xu, W.; Liu, X.; Zhang, H.; Liang, X.; Ye, B.; Zhang, H.; Yang, Z.; Wu, L.; Ge, X.; Xu, T. Upscaled Production of an Ultramicroporous Anion-Exchange Membrane Enables Long-Term Operation in Electrochemical Energy Devices. Nat Commun 2023, 14 (1), 2732. https://doi.org/10.1038/s41467-023-38350-7.(2) He, F.; Zheng, Q.; Yang, X.; Wang, L.; Zhao, Z.; Xu, Y.; Hu, L.; Kuang, Y.; Yang, B.; Li, Z.; Lei, L.; Qiu, M.; Lu, J.; Hou, Y. Spin-State Modulation on Metal–Organic Frameworks for Electrocatalytic Oxygen Evolution. Advanced Materials 2023, 35 (41), 2304022. https://doi.org/10.1002/adma.202304022.(3) Meena, A.; Thangavel, P.; Jeong, D. S.; Singh, A. N.; Jana, A.; Im, H.; Nguyen, D. A.; Kim, K. S. Crystalline-Amorphous Interface of Mesoporous Ni2P @ FePOxHy for Oxygen Evolution at High Current Density in Alkaline-Anion-Exchange-Membrane Water-Electrolyzer. Applied Catalysis B: Environmental 2022, 306, 121127. https://doi.org/10.1016/j.apcatb.2022.121127.(4) Zhang, B.; Zheng, Y.; Ma, T.; Yang, C.; Peng, Y.; Zhou, Z.; Zhou, M.; Li, S.; Wang, Y.; Cheng, C. Designing MOF Nanoarchitectures for Electrochemical Water Splitting. Advanced Materials 2021, 33 (17), 2006042. https://doi.org/10.1002/adma.202006042.

While anion exchange membrane water electrolyzers (AEMWEs) have achieved significant performance advances in recent decades, overpotentials remain high relative to their proton exchange membrane water electrolyzer (PEMWE) counterparts, requiring AEMWE-specific catalyst layer design strategies to further advance this technology. In this work, the role of the ionomer in catalyst layer structure and quality, catalyst layer stability, and ion conduction for supporting electrolyte-fed AEMWEs is assessed for catalyst layers composed of NiFe2O4 and PiperION TP85 from Versogen at variable ionomer contents (0-30 wt %) for tests up to 200 h. The results reveal that, for supporting electrolyte-fed AEM devices, the ionomer is not required for ion conduction through the catalyst layer. Instead, the ionomer is found to play a critical role in catalyst layer structure and stability, where intermediate ionomer contents lead to the lowest overpotentials, highest effective surface areas, and lowest catalyst layer resistances. Catalyst layer stability is found to be a function of both catalyst adhesion and ionomer loss. These results show that an ionomer may be selected which is not of the same chemistry as the anion exchange membrane, mitigating ionomer stability concerns throughout the catalyst layer and offering a pathway towards highly active and stable AEMWEs.