Anode Catalyst Research Articles

AEM Water Electrolysis with Ni-Based Catalysts and Tailored Poly(arylene piperidinium) Materials for Improved Stability

Anion-exchange membrane water electrolysis (AEMWE) is a promising low-temperature technology for hydrogen production. It leverages the advantages of the currently dominant technologies (alkaline water electrolysis and proton-exchange membrane water electrolysis) by using transition metal group based catalysts and a zero-gap configuration, enabling high efficiency. However, current AEMWE systems show a limited stability and efficiency.In this work, commercial and non-commercial polymers are used to fabricate catalyst-coated membranes with a production method oriented towards scalable production (1). A tailored poly(arylene piperidinium) polymer (PAP) shows enhanced stability compared to commercial Aemion+TM during 100 h operation at 1 A/cm2 and 60 °C in 1 M KOH electrolyte (Figure 1) (2). Additionally, different anode catalysts are employed and measured in a reference electrode setup to separate the cell voltage into contributions from the anode and cathode (3). Here, the overpotential of Ni(OH)2 is higher compared to the more active NiFe-layered-double-hydroxide(LDH) and iridium oxide. Employing this setup to measure individual degradation of cathode and anode is part of ongoing research.Figure 1. a) Tailored PAP material compared to Aemion+TM based systems operated at 1 A/cm2 showing a linear degradation of ca. 400 µV/h. b) Polarization curves of AEMWE single cells with different anode catalysts, measured with an integrated reference electrode setup, which allows to distinguish anode and cathode contribution to the cell voltage. Measured at 60 °C in 1 M KOH electrolyte, 5 cm2 cell area.References Koch, S.; Metzler, L.; Kilian, S. K.; Heizmann, P. A.; Lombeck, F.; Breitwieser, M.; Vierrath, S. Toward Scalable Production: Catalyst‐Coated Membranes (CCMs) for Anion‐Exchange Membrane Water Electrolysis via Direct Bar Coating. Advanced Sustainable Systems 2023, 7 (2). DOI: 10.1002/adsu.202200332.Weber, R.; Klingenhof, M.; Koch, S.; Metzler, L.; Merzdorf, T.; Meier-Haack, J.; Strasser, P.; Vierrath, S.; Sommer, M. Meta -kinks are key to binder performance of poly(arylene piperidinium) ionomers for alkaline membrane water electrolysis using non-noble metal catalysts. J. Mater. Chem. A [Online] 2024, 12 (13), 7826–7836.Xu, Q.; Oener, S. Z.; Lindquist, G.; Jiang, H.; Li, C.; Boettcher, S. W. Integrated Reference Electrodes in Anion-Exchange-Membrane Electrolyzers: Impact of Stainless-Steel Gas-Diffusion Layers and Internal Mechanical Pressure. ACS Energy Lett. [Online] 2021, 6 (2), 305–312. Figure 1

Automated Microstructural Analysis of Anodic Catalyst Layers in Proton Exchange Membrane Water Electrolyzers (PEMWEs) Using Python-Based Image Processing Framework

Adverse climatic conditions caused by greenhouse gas (GHG) emissions from non-renewables have led to increased focus on utilization of clean alternative fuels such as hydrogen (H2). Clean H2 can be obtained via electrochemical reaction of water splitting through proton exchange membrane water electrolyzers (PEMWEs). However, challenges to its widespread commercialization lie with the high H2 production cost, a large portion of which comes from use of expensive anodic catalyst materials. To improve anodic catalyst utilization to attain better performance at reduced cost, a thorough understanding of anodic catalyst layer microstructure is needed. In-depth characterization through scanning transmission electron microscopy with energy dispersive spectroscopy (STEM/EDS) can provide a comprehensive dataset, which can be processed further to extract structural parameters and correlate to electrochemical performance. However, extracting relevant information from these datasets can be very challenging and time-consuming.In this study, an automatic python-based image processing framework is implemented to quantify compositional and microstructural parameters for different IrO2 based catalyst inks and their corresponding catalyst layers. The code can provide areal density of catalyst materials, pore/agglomerate size distribution, connectivity of phases, coverage of catalyst particles by ionomer, and its content percentage and distribution. Quantified parameters are then correlated with the electrochemical performance of the PEMWEs. The results provide adequate pointers to not only help capturing microstructural details of catalyst layers but also provide a basis to predict electrochemical behavior and performance of the inks and catalyst layers being studied.

Microstructural Characterization of Porous Transport Layer Coatings

The porous transport layer (PTL) is a key component in polymer electrolyte membrane (PEM) water electrolyzers and serves the role of controlling the transport of reactants and products to and from the anode. Maintaining a low interfacial contact resistance between the titanium (Ti) fiber or sinter PTL and the iridium (Ir) anode catalyst is critical to maintaining the performance of the electrolyzer cell. This requires costly precious metal coatings, typically Pt, to inhibit the formation of native oxide layers during long-term cell operation. Strategies to reducing the thickness of these costly coatings including surface treatments, such as etching and abrasion, alternative low-cost coating materials, and implementation of microporous layers. It is crucial to understanding the morphologies and degradation mechanisms of these new strategies in order to accelerate the development of cheaper, more robust PTL coatings.In this work, correlative ex situ characterization methods, including laser confocal microscopy (LCM), focused ion beam (FIB), scanning electron microscopy (SEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM), are utilized to explore the impact surface treatments, such as etching and abrasion, on the Pt coatings uniformity and surface roughness. The thickness, local topography, and continuity of the Pt coating were quantified and correlated with interfacial contact resistance and electrochemical cell performance measurements. Changes to the thickness and composition of the coatings and oxide layers after single-cell tests will also be reported. Overall, these findings aim to guide the development of PTL treatments and coatings, ensuring the creation of highly durable and efficient PEM water electrolyzers that align with the U.S. Department of Energy's Hydrogen Shot goal of reducing the cost of clean hydrogen production to $1 per kilogram by 2031.Funding for this work was provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office through the H2NEW Consortium. Electron microscopy research conducted as part of a user project at the Center for Nanophase Materials Sciences (CNMS), which is a US Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory.

Investigation of Anion Exchange Membrane Water Electrolysis Efficiencies at Cell Level Using Online Mass Spectrometry

In the future, water electrolysis (WE) will play a decisive role in the energy-efficient and sustainable production of green hydrogen. In this respect, anion exchange membrane water electrolysis (AEMWE) is a promising technology that combines the advantages of the already established alkaline and acidic water electrolysis systems. The alkaline environment makes it possible to replace precious metal catalysts with nickel or iron as cost-effective alternatives. At the same time, using an anion-conducting membrane instead of a porous separator enables higher current densities and flexible operation.1 In direct comparison to proton exchange membrane water electrolysis (PEMWE), durability and stability are still the main problems of AEMWE, but significant progress has been made in recent years. Electrochemical cell tests lasting a year and longer are available for the commercial AEMs Sustainion®2 and Aemion+®3. In addition, piperidinium-based membranes have now also been tested for 1000 hours.4 While the stability improvements of AEMWE are going in the right direction, the question remains to what extent the high efficiencies achieved in PEMWE can be matched. For the latter, the individual contributions to the cell voltage5, 6 and the expected gas permeations7, 8 at different current densities have already been studied in detail – an understanding that is still missing for AEMWE systems.Consequently, in this work, we want to give an insight into AEMWE efficiencies for a standard cell configuration. For this purpose, we used the novel PiperION® membrane and a scalable non-noble NiFe-LDH catalyst from Matteco as the anode catalyst in a self-designed 5 cm² cell setup. Furthermore, we set the focus on implementing product gas analysis using online mass spectrometry to investigate the faradaic efficiency and draw a conclusion on the gas-crossover mechanisms of the novel technology. Tracking the weight of our electrolyte reservoirs during operation additionally allowed us to analyze the water drag. Both phenomena were examined over a wide range of current densities from 0 to 5 A cm-2 at 70 °C and 1 M KOH to provide currently missing information for AEMWE development. References H. A. Miller, K. Bouzek, J. Hnat, S. Loos, C. I. Bernäcker, T. Weißgärber, L. Röntzsch and J. Meier-Haack, Sustainable Energy Fuels, 4(5), 2114–2133 (2020).B. Motealleh, Z. Liu, R. I. Masel, J. P. Sculley, Z. Richard Ni and L. Meroueh, Int. J. Hydrog. Energy, 46(5), 3379–3386 (2021).M. Moreno-González, P. Mardle, S. Zhu, B. Gholamkhass, S. Jones, N. Chen, B. Britton and S. Holdcroft, Journal of Power Sources Advances, 19, 100109 (2023).C. Hu, J. Y. Lee, Y. J. Lee, S. H. Kim, H. Hwang, K. Yoon, C. Park, S. Y. Lee and Y. M. Lee, Next Energy, 1(3), 100044 (2023). M. Suermann, T. J. Schmidt and F. N. Büchi, ECS Trans., 69(17), 1141–1148 (2015). M. Bernt and H. A. Gasteiger, J. Electrochem. Soc., 163(11), F3179-F3189 (2016). M. Bernt, J. Schröter, M. Möckl and H. A. Gasteiger, J. Electrochem. Soc., 167(12), 124502 (2020). P. Trinke, B. Bensmann and R. Hanke-Rauschenbach, Int. J. Hydrog. Energy, 42(21), 14355–14366 (2017).

Iron Effects in Advanced Alkaline Electrolyzer

Global warming, characterized by an alarming rise in the Earth’s temperature, is one of the serious impacts of industrialization. Transition towards a sustainable future requires the diversification of our energy sources by replacing fossil fuels with renewable energy such as green H2, to lower global CO2 emissions. Alkaline water electrolysis is one of the widely used affordable water splitting techniques that produces hydrogen gas. Since Fe contamination in this system is an unavoidable factor, it is important to understand the optimum Fe concentration for maximum performance. In this work, we are investigating the change in the total cell performance by spiking Fe2+ (0 to 140 ppm) solution at industrial standard conditions (6 M KOH, 80° C).In this work, we are investigating the change in the total cell performance by spiking Fe2+ (0 to 140 ppm) solution at industrial standard conditions (6 M KOH, 80° C). The KOH was purified prior to the experiment making sure the Fe limits are below the ICP-MS detection limit. To fully understand how Fe activates/deactivates the electrode performance, we chose NixCoy(O)OH coated on Ni mesh anode, commercial AWE cathode and Zirfon-220 as separator and performed testing in both 3-electrode set up and zero-gap electrolyzer set up. Our initial results show that, Fe effect on the total cell performance follows a bell curve where the maximum contribution comes from the activation of the anode. It must be noted that by spiking 40 -50 ppm Fe in the electrolyte the total cell potential is reduced by over 1V which is a huge gain. As we spike over 60 ppm the performance starts to go down. This can be explained by the deposition of FeOx on the anode which is a bad OER catalyst in comparison with NixCoy(O)OH. Our anode has a high active surface area from the nano sheet composition along with high conductivity from CoOx. At 20 ppm Fe in the electrolyte, the anode over potential reduced by 49 mV at 10 mA and by ~110mV at 250mA at 80 °C. Unlike bare Ni electrode as cathode, commercial cathodes are very resistant to Fe fouling only leading to a few millivolts of change. It is important to understand that Fe has maximum effect at low temperature, where the total cell potential was reduced by 200mV by adding 40 ppm Fe solution. As we increase the temperature to 80 °C, reaction kinetics gets improved and the optimum Fe concentration for maximum performance decreases.The activation in the anode performance at high Fe concentration has been verified by studying the surface morphology SEM imaging and by taking the EDX analysis. It has been seen that the anode gives maximum performance when we have sites of NixCoyFeO(OH) deposited on the anode catalyst. As our next step, we will be evaluating the changes in the surface morphology of the cathode along with the amount of Fe being deposited at each spiking. This will provide more information on how we should engineer the cell components to achieve the maximum output. Figure 1

Research and Development Needs to Enable Renewable-Powered Liquid Alkaline Electrolyzers

Low temperature electrolysis (LTE) is the most promising method for fully renewable hydrogen production. However, to compete with current hydrogen production methods (which produce CO2), substantial cost reductions and efficiency improvements are necessary. Of the various LTE technologies, liquid alkaline (LA) electrolysis is the most mature. It has proven large scalability and can operate with all earth-abundant materials. However, most conventional LA electrolyzers are limited to lower current density operation than newer LTE technologies.Most LA electrolyzers operate with uncatalyzed Ni anodes, despite the existence of many highly active catalysts for oxygen evolution in alkaline conditions. This is because catalyzed systems require constant operation, as the catalyst layers degrade and delaminate rapidly upon exposure to the reverse polarization conditions experienced during system shutdown. This limitation prevents the integration of catalyzed electrodes, as even a single unexpected plant shutdown causes significant efficiency and cost losses. Further, this prevents fully catalyzed LA electrolyzers from directly operating with renewable power sources with large power fluctuations. Therefore, the development of reverse-current tolerant catalyst layers is essential to enable high-efficiency renewable energy powered LA electrolyzers.This talk will detail the mechanisms of electrode degradation during electrolyzer system shutdown. Our findings reveal anode catalyst layers fail through a combined electrochemical and mechanical degradation mechanism. I will discuss key differences in the cathode versus anode operating environments that explain the difference in reverse current tolerance. Considering these results, I will highlight priority research and development directions for developing anode and cathode catalysts for renewable-powered LA electrolyzers.

Development of Pseudo-Homogeneous Three-Dimensional Reaction Anode Structure for Proton Exchange Membrane Water Electrolysis

Solid Polymer Electrolyte (SPE) diaphragm such as Nafion🄬 has been applied to proton exchange membrane water electrolysis (PEMWE) due to the high efficiency and response. The structure of PEMWE consists of a catalyst layer, diffusion layer, current collector, flow-field, etc. In addition to the membrane, these elements and structural parameters were adapted from the proton exchange membrane fuel cell (PEMFC) technology due to the time series factors in the development process and are still strongly influenced by that technology today. Although PEMFC technology is applied to these elements in PEMWE, the material composition, structure, and various parameters are not fully optimized, and the basic structure and technology are diverted from various PEMFC conditions, although the main object of fluid has greatly changed from the gas phase to the liquid phase. As the above background, PEMWE technology needs to go back to the basic level of structural design, mainly from the viewpoint of mass transfer, and reconstruct the component structure.In this study, we focused on the mass transfer characteristics that most affect the performance of PEMWE and investigated a new structure for gas-liquid two-phase fluids for the electrode structure, which is the most important element of PEMWE.Figure 1 shows the two and three-dimensional reaction electrode structures in PEMWE. Figure 1(a) is the electrode structure of a conventional general PEMWE, which is the catalyst reaction in the catalyst layer on the rate-limiting electrode side proceeds in a heterogeneous two-dimensional reaction area. In this current method, sufficient diffusion cannot be maintained from the viewpoint of mass transfer, which leads to performance decrease due to the inability to remove obstacles such as bubbles to mass transport to the catalyst layer. Figure 1 (b) shows the pseudo-homogeneous three-dimensional reaction electrode structure developed in this study. The catalyst is supported on the porous current collector itself. This structure dramatically increases the reaction area from two-dimensional to three-dimensional, and it is expected to greatly improve electrolytic performance by optimizing various parameter conditions.The experimental was performed as following. The anode catalyst IrOx was synthesized from H2IrCl6-nH2O and NaNO3 by the Adams method1). Catalyst Ink, which mixed with the synthesized catalyst and Nafion🄬 dispersion solution, was dropped onto a Ti-web current collector and dried. The anode catalyst loading current collector was evaluated in a 1 cm2 component evaluation electrolyzer2).In this development, the essential issue is the establishment of the conditions to support the catalyst in the three-dimensional reaction area and to maintain the supply path of the proton from the SPE membrane efficiently.References T. Saida, S. Hirano, E. Niwa, F. Sato, and M. Maruyama, ECS Trans., 85, 865 (2018). K. Nagasawa, T. Ishida, H. Kashiwagi, Y. Sano, S. Mitsushima, Int. J. Hydrog. Energy, 46, 36619 (2021). Figure 1

Low Ir-Containing Materials As Anode Catalysts for PEM Water Electrolysis

The PEM electrolysis is based on the application of IrO2 as anode catalyst. Until now, there is no real alternative for this electrocatalyst, since the highly acidic and oxidative conditions at the anode lead to rapid corrosion.[1-3] With steadily increasing energy consumption, the demand for iridium will exceed the available resources in a few decades. As a result, the price of iridium as well as the technologies based on it will continuously increase. An enhancement of PEM electrolysis capacities on a large scale can therefore only be realized in the long term if the application of iridium is significantly reduced.[4] Therefore, our research aims at reducing the iridium content of PEM anodes while maintaining catalytic activity and stability.In this work, two approaches to save iridium were applied. On the one hand the iridium content was reduced by supporting it, on the other hand an additional, readily available metal is incorporated that is protected from corrosion by the presence of iridium and at the same time assists the iridium in catalysis.So, bimetallic included low-loading Ir and different transition metal (Ru, Sn, and Ag) nanoparticles on titanium nitride (TiN) were synthesized by using a chemical reduction method and thermal treatment. Experimentally, a IrSnOx/TiN sample in 0.5 M H2SO4 with an Ir loading of 0.05 mgIr/cm2 achieves a mass activity of over 450 A/gIr at 1.6 V vs RHE and an overpotential of 297 mV at 10 mA/cm2. The investigation includes stability tests at constant current density of 10 mA/cm2 and determination of impedance by electrochemical impedance spectroscopy (EIS).As result, a suitable anode material was successfully achieved, which exhibits a similarly good activity and stability values compared to the IrO2 materials that have been used industrially so far, but with a significantly reduction of iridium to 24% of the usually applied amount.[1] C. Minke, M. Suermann, B. Bensmann, R.H. Rauschenbach, Int. J. Hydrog. Energy, 2021, 46, 23581-23590.[2] F. Scarpelli, N. Godbert, A. Crispini, I. Aiello, Inorganics, 2022, 10, 115.[3] J. Gao, Y. Liu, B. Liu, K.W. Huang, ACS Nano, 2022, 16, 17761-17777.[4] T. Reier, H. N. Nong, D. Teschner, R. Schlögl, P. Strasser, Adv. Energy Mater., 2017, 7, 1601275.

Quantification of O2 Crossover and Concurrent Faradaic Efficiency Loss of H2 Production Via PEM Water Electrolysis

PEM water electrolyzers (PEMWEs) are considered to be a key technology for the decarbonization of major sectors by being able to store fluctuating renewable energy as chemical energy in the form of hydrogen gas. While the scarcity of the necessary iridium catalyst on the anode limits widespread application minding the high loadings still needed today,1 there are additional efficiency losses that are still to be fully understood.By employing a proton exchange membrane, PEMWEs are able to produce hydrogen at elevated pressure on the cathode while keeping the balance of plant costs low as the anode can still be operated with liquid water at close to ambient pressure. Hydrogen crossover to the anode compartment through the membrane is significantly increased at elevated cathode pressures, which is majorly considered a safety issue. As the anode catalyst does not serve as a good hydrogen oxidation reaction catalyst, explosive mixtures of hydrogen and oxygen can form at the anode (and its exhaust) depending on the membrane type and thickness, current density, temperature, hydrogen partial pressure at the cathode, cathode catalyst layer structure etc.2-4 Aside of safety concerns – which can be bypassed by the use of recombination interlayers in the membrane5 – gas crossover also reduces the Faradaic efficiency of the system as parts of the hydrogen produced cannot be used further because it does not reach the cathode exhaust.A so far not majorly considered additional Faradaic efficiency loss stems from oxygen crossover from the anode to the cathode. As the platinum based cathode catalyst is very active for the oxygen reduction reaction, basically all of the oxygen crossing over to the cathode compartment recombines with the hydrogen present, forming liquid water.6 The permeability of oxygen through common PEM materials is significantly lower than that of hydrogen.7 But due to the osmotic drag from the anode to the cathode as well as possibly high bubble pressures of oxygen at the anode catalyst layer / membrane interface,8 oxygen crossover deserves closer attention.In this study we show quantification of the oxygen crossover in operating PEMWE single cells by a full balance of the gases detected at both the anode and cathode exhaust using an online mass spectrometer. A direct correlation of Faradaic efficiency of the oxygen and hydrogen produced matches the expected chemical recombination on the cathode, which serves as a direct measure of the oxygen crossover to the cathode compartment during PEMWE operation. Depending on the exact operating conditions the Faradaic efficiency loss caused by oxygen crossover can easily be in the percentage range, stressing the importance of having a closer look at the oxygen crossover characteristics of PEMWE cells in order to improve the Faradaic efficiency of the system. Taie, Z.; Peng, X.; Kulkarni, D.; Zenyuk, I. V.; Weber, A. Z.; Hagen, C.; Danilovic, N., Pathway to Complete Energy Sector Decarbonization with Available Iridium Resources using Ultralow Loaded Water Electrolyzers. ACS Appl Mater Interfaces 2020, 12, 52701-52712.Bernt, M.; Schröter, J.; Möckl, M.; Gasteiger, H. A., Analysis of Gas Permeation Phenomena in a PEM Water Electrolyzer Operated at High Pressure and High Current Density. J. Electrochem. Soc. 2020, 167.Ito, H.; Miyazaki, N.; Ishida, M.; Nakano, A., Cross-permeation and consumption of hydrogen during proton exchange membrane electrolysis. Int. J. Hydrogen Energy 2016, 41, 20439-20446.Trinke, P.; Keeley, G. P.; Carmo, M.; Bensmann, B.; Hanke-Rauschenbach, R., Elucidating the Effect of Mass Transport Resistances on Hydrogen Crossover and Cell Performance in PEM Water Electrolyzers by Varying the Cathode Ionomer Content. J. Electrochem. Soc. 2019, 166, F465-F471.Zhang, Z.; Han, Z.; Testino, A.; Gubler, L., Platinum and Cerium-Zirconium Oxide Co-Doped Membrane for Mitigated H2 Crossover and Ionomer Degradation in PEWE. J. Electrochem. Soc. 2022, 169.Schalenbach, M.; Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., Pressurized PEM water electrolysis: Efficiency and gas crossover. Int. J. Hydrogen Energy 2013, 38, 14921-14933.Schalenbach, M.; Hoefner, T.; Paciok, P.; Carmo, M.; Lueke, W.; Stolten, D., Gas Permeation through Nafion. Part 1: Measurements. The Journal of Physical Chemistry C 2015, 119, 25145-25155.Weber, C. C.; Wrubel, J. A.; Gubler, L.; Bender, G.; De Angelis, S.; Buchi, F. N., How the Porous Transport Layer Interface Affects Catalyst Utilization and Performance in Polymer Electrolyte Water Electrolysis. ACS Appl Mater Interfaces 2023, 15, 34750-34763.

Nickel-Based Metal-Organic Framework Materials with Mixed Ferrocene-Based Ligands As Anodic Catalysts for Water Electrolysis and Urea Electrolysis

Water electrolysis is a highly promising technology for hydrogen production, generating green hydrogen with no greenhouse gas emissions. The water electrolysis reaction involves two half-reactions: hydrogen evolution and oxygen evolution. While oxygen evolution in water electrolysis involves a four-electron transfer requiring higher overpotential and thus leading to energy consumption. Urea electrolysis offers lower theoretical energy consumption than water electrolysis, making them a key technology in future electrolytic hydrogen production processes. Conventional anodic catalysts, such as Ir and Ru, exhibit good performance in oxygen evolution, but being precious metals, they face limitations due to high cost and limited abundance. Its is crucial to find an active, durable and cost-effective anode catalyst for realistic application.Metal-organic framework (MOF) materials, due to their high porosity, large surface area, and unique structural features, have emerged as promising materials for water electrolysis. However, their relatively poor stability is a drawback that needs to be overcome. In herein, we design the mixed ligand strategy for synthesizing the MOF materials. Ferrocene-based ligands, ferrocenedicarboxylic acid (Fd), combing with terephthalic acid (H2BDC) as the mixed ligands for this study. The MOF catalysts, namely Ni-BDC and Ni-BDC-Fd, was synthesized using a simple one-step hydrothermal method with ferrocenedicarboxylic acid and terephthalic acid.Raman spectra revealed the presence of Fe-O, originating from the iron in Fd, demonstrating a structural transformation during the hydrothermal process. XRD analysis also confirmed the existence of FeO. The SEM images showed nanosheets and nanoneedles morphologies for Ni-BDC, while a dense nanofiber structure for Ni-BDC-Fd demonstrating the effect of 2nd ligand on the MOF structure.Electrochemical tests were conducted in 1 M KOH electrolyte and 1 M KOH + 0.5 M urea electrolyte, including oxygen evolution reaction (OER), urea oxidation reaction (UOR). The overall reaction comprising the same catalysts for the anode and the cathode side is performed to examine the real electrolysis performance.Results showed that the addition of Fd enhanced OER and UOR activity in alkaline and urea electrolytes. Ni-BDC-Fd achieved 1.483 V vs. RHE and 1.386 V vs. RHE at 65 mA/cm-2 driving potential for OER and UOR, respectively, requiring a smaller potential than Ni-BDC to achieve the same current density. To verify that the performance improvement was not solely due to iron, NiFe-BDC was synthesized by adding iron nitrate, showing inferior UOR performance compared to Ni-BDC-Fd but superior OER performance to NiFe-BDC. This indicated that the performance improvement resulted from structural changes rather than just the presence of iron atoms. Ni-BDC-Fd exhibited the lowest Tafel slope in OER, indicating a faster reaction rate due to the increased active surface area during OER. After continuous catalysis for 80 hours in the alkaline water electrolysis cell and urea electrolysis cell, a significant improvement in the loss of potential was observed.These results suggest that the addition of ferrocene dicarboxylic acid altered the structure of Ni-BDC, maintaining an appropriate interlayer distance. This not only enhanced its catalytic performance but also improved its catalytic stability, demonstrating a promising strategy for preparing metal-organic framework materials for electrocatalysis applications. Figure 1

Enhancing Anode Reversal Tolerance by Low-Carbon and Carbon-Free Anode Catalyst Layers for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs), as the main hydrogen utilization device, are considered a powerful tool for accelerating a carbon-neutral society. However, durability and cost are the two main obstacles to the commercialization of PEMFCs. Specifically, the anode reversal has been extensively studied as a severe issue that deteriorates the performance and durability of PEMFCs and leads to cell failure. Commercially, iridium oxide (IrOx) or iridium black is intensively utilized as oxygen evolution reaction catalysts to fabricate reversal tolerant anodes (RTAs) despite the slight performance loss. However, previous studies concentrated on investigating the origin or phenomena during the cell-reversal, the key factors and mechanisms affecting the anti-reversal performance, and the interaction among the anode catalyst layer components are still yet to be explored.Hence, we systematically studied the performance using three typical carbon supports with three metal contents and different metal deposition orders by experiments and model calculations. First, we synthesized Ir outside Ir/Pt/C and Pt outside Pt/It/C with uniform nanoparticle size and distribution and characterized them comprehensively. A new parameter, “relative metal coverage”, is put forward to evaluate and predict the anti-reversal ability, showing that higher relative metal coverage results in a longer reversal time and lower polarization performance degradation rate. In addition, Ir outside Ir/Pt/C shows better reversal tolerance than Pt outside Pt/It/C, which has been rarely mentioned in previous studies. Further, the experimental results show that even though the metal coverage is high, the performance degradation caused by carbon corrosion is still inevitable. On this basis, we synthesized a high specific surface area conductive oxide Ti4O7 as the anode catalyst support to eliminate the adverse effects of carbon corrosion. Previously, the initial performance of Ti4O7-supported catalyst was obstructed by the disadvantaged electrical conductivity of Ti4O7. We obtain a comparable polarization performance after optimizing the Ti4O7-supported anode catalyst layer parameters by shortening the electron transfer pathway and increasing metal coverage. Typically, the Ir@IrOx/Pt/Ti4O7-fabricated RTA with low Ir loading displays approximately ten times longer reversal time (6 hours) and two orders of magnitude lower degradation rate than a conventional carbon-supported counterpart. The degradation origin of the Ti4O7-supported anodes is also studied by postmortem characterizations, pointing to the Pt oxidation caused by the formation of TiOx thin layers on the Pt surface. These two works aim to promote the development of highly durable and reversal tolerance anode and provide a bright perspective for practical high-performance, low-degradation, and cost-friendly RTA fabrication.

Investigating the Effects of Anode Catalyst Conductivity and Loading on Performance for Anion Exchange Membrane Water Electrolysis

Water electrolysis (WE) is a promising route for carbon-free production of green hydrogen, which is increasingly considered to be an important energy carrier. Among the low-temperature WE technologies, anion exchange membrane water electrolysis (AEMWE) has the potential advantages of high operating current densities and low cost due to the use of zero-gap assembly and platinum group metal (PGM)-free components, respectively [1]. However, significant challenges remain for this technology, including low efficiency, insufficient durability, and poor performance in pure water operation. Recent efforts have yielded improvements in the individual components, particularly the oxygen evolution reaction (OER) catalysts and anion exchange polymers; however, to further improve the performance of the AEMWE, it is also necessary to understand and improve the integration of these materials into the membrane electrode assembly (MEA). Within the MEA, the anode and cathode catalyst layers (CLs) incorporate catalyst and ion-conducting and/or binding polymers, which are deposited onto either the membrane or the porous transport layer. The material properties of each component, as well as the loadings of the catalyst and ionomer and methods of dispersion and deposition, have significant impact on the catalyst layer morphology and uniformity [2]. These CL properties affect the transport of electrons, hydroxide ions, and evolved gases, ultimately controlling the accessibility and utilization of catalyst active sites, with significant consequences for both the performance and durability of AEMWEs.Most of the effort in catalyst development for AEMWE has focused on the OER, aiming to improve sluggish kinetics and improve stability at the highly oxidizing operating potentials. A number of PGM-free catalysts have shown promising activity in rotating disk electrode (RDE) tests, particularly oxides with varying ratios of Ni, Fe, and Co. The activity improvements with tuning the composition are often attributed to the increase in surface area and shift in redox potentials for the in-situ conversion to higher valence sites with higher intrinsic OER activity. However, these activity trends do not always translate into device-level performance because of differences in the operating environment, e.g. hydroxide concentration, reaction rates, etc. In the AEMWE, where typically an order of magnitude higher loadings and current densities are required, the challenges of conductivity and mass transport to allow for full site access are exacerbated. Electrochemical impedance spectroscopy (EIS) is a valuable tool for understanding the origin of voltage losses in the AEMWE, typically broken down into ohmic, kinetic, and mass transport losses. Recently, a transmission line model has been applied to non-Faradaic EIS to isolate another contribution to voltage loss: the catalyst layer resistance (CLR), which corresponds to in-plane and through-plane electronic and ionic resistances in the catalyst layer [3]. Using this approach, the roles of intrinsic catalyst activity and catalyst layer properties can be decoupled, helping to guide materials integration efforts.In this study, we investigate two types of OER catalyst materials, NiFe- and Co-based oxides. Within each composition, the catalysts have similar active sites but vary in electronic conductivity, crystal structure, and surface area. By comparing the AEMWE performance at different catalyst loadings, we are able to determine the role of catalyst properties in determining active site utilization and electronic resistance within the catalyst layer. Figure 1 shows the trends in performance and ohmic and catalyst layer resistance as a function of loading for high-conductivity (Co@Co3O4) and low-conductivity (Co3O4) catalysts. Despite nominally having the same surface chemistry and similar particle morphology, they display both different performance and different trends with loading. Using in-situ EIS diagnostics and ex-situ characterization of the catalyst layer morphology, the relationship between catalyst utilization, CLR, and the physical properties of the catalyst and catalyst layer is elucidated. This work provides insight into the catalyst properties that most strongly affect AEMWE performance, as well as how materials integration strategies must be tailored based on these properties.

Influence of Different Operating Conditions on Iridium Dissolution in a Proton Exchange Membrane Water Electrolyzer

Polymer electrolyte membrane (PEM) water electrolysis plays a key role in the global decarbonization scheme by coupling the intermittent renewable energy sources to the production of high-purity hydrogen. Its deployment onto a GW scale, however, is hindered by the cost and availability of the precious metal catalysts utilized in today’s commercial systems, primarily by that of Ir used as the anode catalyst for the oxygen evolution reaction (OER). With the continuous increase in the demand for carbon-free hydrogen, drastic reductions from the current Ir loading of 1 – 2 mgIr cm-2 by at least an order of magnitude, without compromising the overall electrolyzer performance, are of crucial importance.1 In the recent years, significant efforts have been focused on understanding PEM electrolyzer degradation mechanisms and developing the necessary mitigation strategies, with a special focus being placed on the anode catalyst layer (CL).2-4 Recent results from our group showed that there are two main pathways for Ir to be removed from the anode CL: dissolution into the anode water line and transport through the membrane and the cathode CL.5 In the present work, we aim to shed light onto the effects influencing Ir dissolution under intermittent operation by quantitative analysis of the main Ir sinks within an electrolyzer and the determination of the charge of the dissolved Ir species.5, 6 Various accelerated stress tests with square-wave configuration aimed at accelerating Ir-based anode CL degradation were performed and the results were compared to those obtained during steady-state operation of the electrolyzer cell, which was used as a reference point. The influence of electro-diffusion, electroosmotic drag and Ir redeposition back to the ACL onto Ir dissolution were analyzed in detail to obtain deeper understanding on the Ir degradation processes. M. Clapp, C. M. Zalitis, M. Ryan, Perspectives on current and future iridium demand and iridium oxide catalysts for PEM water electrolysis. Catal Today 420, (2023). S. M. Alia et al., Electrolyzer Performance Loss from Accelerated Stress Tests and Corresponding Changes to Catalyst Layers and Interfaces. J Electrochem Soc 169, (2022). Z. Q. Zeng et al., Degradation Mechanisms in Advanced MEAs for PEM Water Electrolyzers Fabricated by Reactive Spray Deposition Technology. J Electrochem Soc 169, (2022). A. Weiss et al., Impact of Intermittent Operation on Lifetime and Performance of a PEM Water Electrolyzer. J Electrochem Soc 166, F487-F497 (2019). M. Milosevic et al., In Search of Lost Iridium: Quantification of Anode Catalyst Layer Dissolution in Proton Exchange Membrane Water Electrolyzers. Acs Energy Lett 8, 2682-2688 (2023). A. P. Dam, B. Y. A. Abuthaher, G. Papakonstantinou, K. Sundmacher, Insights into the Path-Dependent Charge of Iridium Dissolution Products and Stability of Electrocatalytic Water Splitting. J Electrochem Soc 170, (2023).

Sulfur-Phosphorus Doped Metal Composites As Superior Catalyst for Electrolytic Hydrogen Production in Real Seawater

In the face of the increasingly severe global energy crisis and challenges posed by climate change, promoting energy transition has become an international consensus. Among various renewable energy sources, hydrogen is considered an ideal energy carrier due to its clean and efficient characteristics. The electrolysis of seawater for hydrogen production has garnered widespread attention for its ability to fully utilize abundant seawater resources and the advantage of zero carbon dioxide emissions. However, achieving efficient, stable, and economically viable hydrogen production in seawater still faces many challenges, with catalyst design and optimization being recognized as key issues.Firstly, NiFe-LDH was synthesized using a simple one-pot solvothermal method. Subsequently, phosphorization, sulfurization, and sulfur-phosphorus co-doping were conducted to obtain samples namely NiFe-P, NiFe-S and NiFe-SP, respectively. Various electrochemical tests, including hydrogen evolution reaction (HER), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were firstly performed in simulated seawater to evaluate the electrochemical performance of the synthesized catalysts. The overall seawater splitting reaction was conducted with the same catalysts for both anode and cathode sides.After comparing the electrochemical activity of four different catalysts, it was found that NiFe-SP performs superior HER activity in alkaline simulated seawater. NiFe-SP required merely -0.143 V to deliver 100 mA cm-2 current density, which was 95 mV lower than NiFe-LDH to achieve the same current density. In EIS tests, NiFe-SP exhibited a smaller charge transfer resistance than NiFe-LDH, and NiFe-SP had the largest double-layer capacitance (Cdl) value in ECSA measurements, indicating that NiFe-SP possessed and lowest reaction resistance and the maximum electrochemically active surface area.X-ray Diffraction (XRD) and Raman spectroscopy analyses indicate that the catalyst, after sulfur-phosphorus doping, does not exhibit a pure-phase LDH structure but transforms into a heterogeneous structure of Ni3S2 and FeS2. Scanning Electron Microscopy (SEM) observations suggest that sulfur-phosphorus co-doping causes the original spherical structure of NiFe-LDH to aggregate, with the appearance of some lamellar structures on the surface. Energy Dispersive Spectroscopy (EDS) also confirms the successful doping of sulfur and phosphorus elements into NiFe-LDH, with uniform distribution on the material's surface.Finally, the catalyst underwent a 2000-hours continuous stability test at a constant current of 100 mA cm-² in real seawater. The results indicate that the potential of NiFe-SP increased by only 5%, demonstrating that the doping of sulfur and phosphorus not only reduces the catalytic overpotential but also significantly sustains potential retention. This suggests that the metal composite structure obtained after sulfur-phosphorus co-doping is well-formed and stable. The systematic electrochemical tests and stability evaluation in this study highlights the potential of sulfur-phosphorus co-doped metal composite NiFe-SP as an efficient and stable catalyst for real seawater electrolysis. This not only contributes to a deeper understanding of the fundamental principles governing catalyst behavior but also opens new possibilities for the practical application of hydrogen production technology. Figure 1

(Invited) PEM Water Electrolysis Stack Development and Related Characterization Tools

For decades, proton-exchange membrane (PEM) water electrolysis (WE) has been mainly used for oxygen generation in anaerobic environments. Over the past two decades, however, it has been increasingly used for hydrogen generation in the industrial sectors at various scales. The PEMWE technology is also considered a key in the ongoing energy transition, if the process of hydrogen generation by means of WE is linked to renewable energy sources, such as wind, solar, etc.The following key elements enable the operation of a PEM WE plant for hydrogen production: a PEM-based WE stack and the balance of plant. The related system includes, but not limited to such key modules as the water and oxygen management systems, hydrogen gas management system, water input system, safety system, power electronics and electrolysis cell stack power supply, control system, and some other.A WE stack comprises several cells connected in series with electrically conductive bipolar plats and end plates. General design principles for the stack involve reducing efficiency losses, while aiming at lower costs and increase in durability. WE stacks should be designed in such a way, that an even current distribution is maintained, the water feed is optimized, suitable compression ratios are achieved, and preferably high discharge pressure of hydrogen can be enabled. For the stack level, in simple terms, efficiency is benchmarked by the total applied voltage, which includes the Nernst potential, anode and cathode overpotentials, and ohmic overpotentials due to the membrane ionic resistance and interfacial resistance at given current density. Overpotential represents inefficiencies in a cell stack, and some of stack design efforts focus on reducing these overpotential contributions. However, a compromise has to be found between (a) costs reduction challenges (that are often associated with the reduction of PGM-based catalyst loading, attempts to replace Ti as a key material for the bipolar plates, and the less expensive SPE membrane), (b) sufficient durability, (c) performance of the stack, (d) increase in power density of a stack.This talk will focus on the WE stack, and its subcomponents review, as well as characterization methods for both, a single cell and a stack and also challenges of the large-scale stack manufacturing. The key components of the WE stack include ion-conductive solid polyelectrolyte membranes (SPE), anode and cathode catalyst layers (CL), bipolar plates and current collectors/ gas diffusion layers (porous transport layers).In order to gain fundamental understanding of the relationships between electrical loses, degradation of the stack components and strategies to increase power density of the stacks, advanced characterization and modelling tools need to be used. These include, but not limited to electrochemical impedance spectroscopy (EIS), current interrupt (CI), dynamic compression measurements with piezoelectric senor plates, current mapping, gas cross-over analysis, computational fluid dynamics (CFD) simulations modelling, various visualization tools, etc. For example, CFD could assist in understanding and improving fluid flow dynamics in a WE stack as it is a complex challenge that requires a detailed examination of the geometry, channel design, and operating conditions within the stack. As for EIS, the usage of the distribution of relaxation times (DRT) approach would potentially allow to focus on the direct analysis of the data rather focusing on the equivalent circuit development.Approximately 15 years ago the South African Government approved a national program HySA: Hydrogen South Africa, which resulted in the development of the expertise and capacity to conduct research, development, and earlier commercial activities around green hydrogen production by means of water electrolysis. These activities include the development of local IP at the components, stack, and system levels.

Operando X-Ray Scattering for Electrocatalysis and Beyond

The energy transition stands as one of the greatest challenges of today’s society. In this context, electrocatalyst materials at the heart of electrochemical energy conversion devices such as fuel cells and water electrolyzers are expected to play an increasing crucial role in the near future. The urgent bottleneck to be overcome in electrocatalyst materials development to allow the widespread deployment of electrochemical systems is thus reaching combined high activity and long-term stability at low cost. Despite the diversity in electrocatalyst materials, the latter being largely imposed by the various types of electrochemical systems (noble vs. non-noble metals in acidic vs. alkaline media for example) and the prerequisites of the different electrochemical processes (oxidation or reduction reactions of various species at different electrode potential ranges), most activity and stability properties of electrocatalysts directly derive from their (surface) chemistry and structure [1]. Such properties (and their temporal evolution) can thus be directly investigated by means of in situ or operando high energy X-ray scattering (XRD) technique [2].In this presentation, the versatility of in situ and operando XRD technique in addressing key bottlenecks in electrocatalyst materials development, notably by probing adsorption and oxidation trends [3], will be showcased. After an overview of previous contributions from the literature, the talk will focus on our ongoing works which encompass the study of Pt-based nanocatalysts for the oxygen reduction reaction (ORR) at proton-exchange membrane fuel cells (PEMFCs) anode, carbon-capped Ni@C catalyst for the hydrogen oxidation reaction (HOR) at anion-exchange membrane fuel cells (AEMFCs) anode and IrCu unsupported aerogel catalyst for the oxygen evolution reaction (OER) at proton-exchange membrane water electrolyzers (PEMWEs) anode.Finally, the ability of operando XRD to provide device-relevant insights at the macroscale (such as ionomer hydration in PEMFCs or water distribution in AEMFCs) beyond electrocatalysts microstructural properties will be presented.

(Invited) Cost Reduction Strategies in Low Temperature Water Electrolysis with Improved Catalytic Activity

Proton exchange membrane water electrolysis (PEMWE) technology is especially advantageous for green H2 production as a clean energy vector. During the water electrolysis process, the oxygen evolution reaction (OER) requires a large amount of iridium (2-3 mgIr cm−2) as catalyst. This material is scarce and expensive, representing a major bottleneck for large-scale deployment of electrolyzers. Recently, Retuerto et al. have synthesized different Ir double perovskites (Sr2CaIrO6) with the Ir atoms in a high oxidation state (Ir6+/5+) 1 and their catalytic activity was extensively investigated. Specifically, a Ca-based double perovskite (Sr2CaIrO6) has showed the highest OER activity similar to those of Ir or Ir oxides-based catalysts while being stable for the OER in half cell measurements. We report the development of a membrane electrode assembly taking advantage of the high activity of the Sr2CaIrO6 catalyst that has only 0.2 mgIr cm−2, around 10 times lower compared to what it is used in the anodes of commercial PEMWE, showing superior performance and stability. The morphology/structure of the developed anode and Sr2CaIrO6 catalyst, before and after operation in PEMWE, were extensively characterized by X-ray diffraction (XRD, high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge structure (XANES). We discuss in this contribution how the finding of this work can provide a strategy to lower the Ir content in commercial PEMWE.Anion exchange membrane water electrolysis (AEMWE) combines the advantages of proton exchange membrane water electrolysis (PEMWE), i.e., high current density while maintaining high efficiency and fast dynamic response with the advantages of alkaline water electrolysis (AWE), i.e., low-cost materials. However, AEMWE technology is still depends on a supporting electrolyte and its durability is low. In this regard, the German Aerospace Center (DLR) is developing novel approaches and cell components. Raney-nickel electrodes were developed via vacuum plasma spraying, reaching performance and stability comparable to SoA MW size PEMWE. The electrodes were further refined, switching to lower cost atmospheric plasma spraying while maintaining high performance. Studying the effect of supporting electrolyte concentration it was found, that lowering the ohmic resistance is by far the largest challenge for high performance AEMWE, especially at low KOH concentrations and electrode kinetics are less impactful. Moreover, a 17% efficiency uplift was achieved by applying a carefully tuned, thermally sprayed, nickel-based macro porous layer (MPL) onto a traditional mesh porous transport layer. Both ohmic and mass transport resistances were found to be lowered by the MPL. FIB-SEM and µCT analysis revealed an ideal structure, i.e., 30-40% porosity, almost exclusively open pores, and a broad pore size distribution2. In a fundamental study3, the mechanism of the OER activity was investigated as well as the effect of trace amounts of iron in Ni(OH)2. X-ray diffraction (XRD) and Thermogravimetric analysis (TGA) TGA analysis lead to the conclusion, that iron stabilized the α-Ni(OH)2which is the most active. CV analysis showed that an important redox switching step is also accelerated by Fe-doping. M. Retuerto, L. Pascual, J. Torrero, M. A. Salam, Á. Tolosana-Moranchel, D. Gianolio, P. Ferrer, P. Kayser, V. Wilke, S. Stiber, V. Celorrio, M. Mokthar, A. S. Gago, K. A. Friedrich, M. A. Peña, J. A. Alonso, D. G. Sanchez, S. Rojas, Highly Active and Stable OER Electrocatalysts Derived from Sr2MIrO6 for Proton Exchange Membrane Water Electrolyzers. Nat. Commun. 2022, 13, 7935. Razmjooei, F.; Morawietz, T.; Taghizadeh, E.; Hadjixenophontos, E.; Mues, L.; Gerle, M.; Wood, B. D.; Harms, C.; Gago, A. S.; Ansar, S. A.; Friedrich, K. A., Increasing the performance of an anion-exchange membrane electrolyzer operating in pure water with a nickel-based microporous layer. Joule 2021, 5 (7), 1776-1799. Wang, L.; Saveleva, V. A.; Eslamibidgoli, M. J.; Antipin, D.; Bouillet, C.; Biswas, I.; Gago, A. S.; Hosseiny, S. S.; Gazdzicki, P.; Eikerling, M. H.; Savinova, E. R.; Friedrich, K. A., Deciphering the Exceptional Performance of NiFe Hydroxide for the Oxygen Evolution Reaction in an Anion Exchange Membrane Electrolyzer. ACS Applied Energy Materials 2022, 5 (2), 2221-2230.

The Effect of Water Quality on PEM Electrolyzer Performance

There has been growing interest in the use of removable, clean, and sustainable energy sources as a solution to eliminate environmental problems caused by fossil fuels. In particular, the electrolysis of water to produce green hydrogen is one of the most promising technologies. Among various electrolysis techniques, proton exchange membrane water electrolyzer (PEMWE) stands out as the most promising for mobility and large-scale energy-storage applications (1). Hydrogen production by PEMWEs requires approximately 45 kg of high-quality water to produce 5 kg of hydrogen. It has been estimated that about 22% of the total cost of a state-of-the-art PEMWE system is for water deionization treatment (2). This water purity requirement is not only an economic barrier, but also a constraint for the use of this system in areas where high-quality water is not accessible or in areas where water for human consumption is a scarce resource. Therefore, studying the effect of water quality on PEMWE performance is an important first step in identifying key degradation mechanisms on the path to discovering new PEMWE architectures that are more tolerant of poor-quality feed waters.In this work, the effect of common impurities, such as Na+ and Mg2+, in feed water on the performance of common PEMWE architectures is studied. The main goals of the work are: 1) to understand the tolerance range for long term operation, 2) to identify major degradation mechanisms when the failure occurs. Experiments were carried out in an electrolyzer equipped with platinized Ti electrodes, IrO2 and Pt/C as anodic and cathodic catalysts, respectively, and a 115 Nafion® membrane. Durability studies of 100-hour duration and electrochemical characterization of the electrolyzer at different time intervals (polarization curves and impedance spectroscopy) were performed. The membrane/electrode assembly was probed by SEM /EDX before and after the experiments and cation concentrations in the water were monitored by ICP. An electrochemical model is proposed to explain the observed behavior of the electrolyzer with DI water and in the presence of impurities.The results show that the presence of cations affects the performance of the electrolyzer at different levels depending on the concentration of the cation and its valence (Mg2+ > Na+ >DI water). Mg+2 had the same effect as Na+ even at 5 times lower concentration. Cations have higher affinity than protons for sulfonic acid groups (-SO3 -), leading to a displacement of protons and a consequent reduction in the membrane proton conductivity. However. according to the polarization curves, there are effects beyond the ohmic potential due to the increase in membrane resistance. The major effect of cations is on the reaction kinetics, which is the primary reason for the observed increase in cell voltage. The presence of cations within the membrane blocks the transport of protons, which creates a lack of protons at the cathode, resulting in the kinetic change from proton reduction to reduction of water. Moreover, the EDX results showed that Na+ and Mg2+ ions transfer from anode to cathode and deposit on the membrane and catalyst layers, which may cover the active catalyst sites, block the proton transport inside the membrane and degrade the catalyst ionomer layers.References Sazali N. Emerging technologies by hydrogen: A review. International Journal of Hydrogen Energy. 2020 (45): 18753-18771Becker H, Murawski J, Shinde DV, Stephens IEL, Hinds G, Smith G. Impact of impurities on water electrolysis: A review. Sustainable Energy & Fuels. 2023;7(7):1565-603.

Effect of Catalyst and Electrode Structures on the Operating Current Densities of Liquid Alkaline Water Electrolyzers

Liquid Alkaline Water Electrolyzers (LAWEs) stand out as the technology best-suited for cost-effective hydrogen production. They do not require expensive platinum group metal catalysts and have proven durability afforded by nickel-based electrodes and porous separators. While nickel electrodes and porous separators provide chemical stability in highly alkaline conditions, they do suffer from lower catalytic activity for hydrogen and oxygen evolution reactions (HER and OER), and separators possess higher ohmic resistance due to the absence of ion-conducting groups. Consequently, most LAWEs operate at low current densities ranging from 0.2 to 0.6 A/cm2, resulting in a bulky footprint and comparatively lower energy efficiency due to a reduced hydrogen production rate. To address these challenges, considerable efforts have been directed towards developing thin separators and efficient transition metal-based electrocatalysts for HER and OER in alkaline media. However, the majority of studies related to catalysts are limited to half cells, with only a limited number of catalysts evaluated in relevant LAWE configurations.In this work, we introduce a next-generation Zero-gap LAWE testing capability, benchmarked for performance through H2NEW consortia. Employing this setup, we have evaluated hydrogen and oxygen evolution catalysts under LAWE-relevant conditions on different electrode substrates, each offering a unique electrode structure. Our preliminary findings indicate that Zero-gap LAWEs, when coupled with NiMo cathode and NiFe-type anode catalysts, can operate at current densities exceeding 0.75 A/cm2 at approximately 1.8 V without compromising their short-term durability. In contrast, the benchmark cell exhibited a meager current density of around 0.22 A/cm2. In this presentation, effects of electrode structure, catalyst type, and other relevant parameters related to Zero-gap LAWEs will be discussed in detail. Acknowledgement:This research is supported by the U.S. Department of Energy (DOE) Hydrogen and Fuel Cell Technologies Office through the Hydrogen from Next-generation Electrolyzers of Water (H2NEW) consortium.

Enhanced Local Ion Transport in High-Performance Anion Exchange Membrane Water Electrolyzers Via Quaternary Ammonium-Functionalized Metal-Organic Framework Electrocatalysts

Anion-exchange membrane water electrolyzers (AEMWEs) present a multitude of advantages, particularly in the utilization of non-platinum group (non-PGM) catalysts. While diverse catalysts have demonstrated notable electrocatalytic activity in alkaline conditions, the integration of the ionomer with the electrocatalyst at the active catalyst layer is crucial for enhancing overall AEMWE performance in practical device demonstrations by improving mass transport. However, the intrinsic instability of polymeric ionomers, leading to physical detachments or alkaline (chemical) degradation, poses a challenge that can potentially diminish performance. In this study, drawing inspiration from the aforementioned ionomer chemistry, we introduce non-PGM NiFe-based metal-organic frameworks (MOFs) incorporating quaternary ammonium (QA) cation moieties covalently functionalized to benzenedicarboxylic acid (BDC) ligands as promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline environments. Three specifically designed functional groups with varying ion exchange capacities (IEC) and alkaline stability—trimethylamine (TMA), N-methylpyrrolidine (C4N), and N-methylpiperidine (C5N)—were directly tethered to BDC ligands. The alkaline OER activity of Ni2Fe1-QA-MOF was meticulously assessed using rotating disk electrodes and AEMWE devices. These directly tethered QA groups markedly facilitated improved ionic transport of OH- ions at the catalyst/electrolyte interfaces compared to BDC ligands, ultimately leading to a significant enhancement in half-cell activity and AEMWE performance. In-situ IR/RAMAN spectroscopy revealed that QA moieties highly promoted local ionic transport at the interfaces, as evidenced by characteristic peaks indicating a robust interaction between QA and OH- ions. Unexpectedly, QA moieties not only contributed to ionic transport in local environments but also induced oxidation states of Ni metal sites, while Fe sites exhibited no changes during the applied potential in the OER region, as comprehensively revealed by synchrotron X-ray absorption spectroscopy (XAS). In comparison with conventional NiFe layered double hydroxide (LDH) structures, we posit that the substantial structural transformation of Ni moieties can be attributed to an increased degree of M-O bond interaction, enhanced by the local interaction between QA and OH- ions. Density functional theory (DFT) calculations also validate the balanced adsorption of OER intermediates on QA-MOFs. Finally, QA-NiFe-MOF was directly synthesized on a porous transport layer (PTL), and its AEMWE performance was evaluated without the use of any ionomer. At 1.0 M KOH, C4N, C5N, and TMA-based NiFe MOFs exhibited current densities of 9.35 A/cm2, 9.11 A/cm2, and 7.6 A/cm2, respectively. These current densities surpassed those of bare BDC-NiFe MOF and commercial IrO2 anode catalysts, which exhibited only 7.39 A/cm2 and 6.4 A/cm2, respectively. Consequently, we emphasize that the facilitation of local ionic transport could yield remarkable AEMWE performance, even in the absence of conventional polymeric ionomers.