ABSTRACTAnion exchange membrane water electrolysis (AEMWE) is one of the most promising candidates for green hydrogen production needed for the de‐fossilization of the global economy. As AEMWE can operate at high efficiency without expensive Platinum Group Metal (PGM) catalysts or titanium cell components, required in state‐of‐the‐art proton exchange membrane electrolysis (PEMWE), AEMWE has the potential to become a cheaper alternative in large‐scale production of green hydrogen. In AEMWE, the porous transport layer and/or micro porous layer (PTL/MPL) has to balance several important tasks. It is responsible for managing transport of electrolyte and/or liquid water to the catalyst layers (CLs), transport of evolving gas bubbles away from the CLs and establishing thermal and electrical connection between the CLs and bipolar plates (BPPs). Furthermore, especially in case the CL is directly deposited onto the MPL, forming a catalyst‐coated substrate (CCS), the MPL surface properties significantly impact CL stability. Thus, the MPL is one of the key performance‐defining components in AEMWE. In this study, we employed the flexible and easily upscaled technique of atmospheric plasma spraying (APS) to deposit spherical nickel coated graphite directly on a low‐cost mesh PTL. Followed by oxidative carbon removal, a nickel‐based MPL with superior structural parameters compared to a state‐of‐art nickel felt MPL was produced. Due to a higher activity of the nickel APS‐MPL itself, as well as improved catalyst utilization, a reduction in cell voltage of 63 mV at 2 A cm−2 was achieved in an AEMWE operating with 1 M KOH electrolyte. This improvement was enabled by the high internal surface area and the unique pore structure of the APS‐MPL with a broad pore size distribution as well as the finely structured surface providing a large contacting area to the CLs.

Polymer membrane electrolyzers benefit from high-pressure operation conditions and low gas cross-over and can either conduct protons (H+) or hydroxide ions (OH-). Both types of electrolyzers have a similar design, but differ in power density and the choice of catalysts. Despite the significant endeavor of their optimization, to date, there is no well-established impedance model for detailed analysis for either type of these devices. This complicates the in-situ characterization of electrolyzers, hindering the investigation of degradation mechanisms and electrocatalytic processes as a function of applied current density or time. Nevertheless, a detailed understanding of such individual processes and distinguishing the performance-limiting factors are the keystones for sophisticated device optimization. In this work, an impedance model based on electrode processes has been developed for an anion exchange membrane electrolyzer utilizing iridium oxide anode and platinum cathode electrocatalysts. This model allows to deconvolute the measured impedances into constituents related to the individual electrode processes and to estimate actual physico-chemical quantities such as the reaction kinetic parameters and double-layer capacitances. We discuss the meaning of the fitting parameters and show that this model enables, for the first time, the estimation of the electrochemically active surface area of the anode electrocatalysts under reaction conditions. We present an equivalent electrical circuit to analyze the impedance response of anion exchange membrane electrolyzers. It is based on the reaction kinetics at anode and cathode and enables analysis of critical parameters such as double layer capacitances, cell resistance and allows assessment of the electrochemically active surface area of the anode.image

Anion exchange membrane water electrolysis (AEMWE) is an attractive method for green hydrogen production. It allows the use of non-platinum group metal catalysts and can achieve performance comparable to proton exchange membrane water electrolyzers due to recent technological advances. While current systems already show high performances with available materials, research gaps remain in understanding electrode durability and degradation behavior. In this study, the performance and degradation tracking of a Ni3Fe-LDH-based single-cell is implemented and investigated through the correlation of electrochemical data using chemical and physical characterization methods. A performance stability of 1000 h, with a degradation rate of 84 mu V h-1 at 1 A cm-2 is achieved, presenting the Ni3Fe-LDH-based cell as a stable and cost-attractive AEMWE system. The results show that the conductivity of the formed Ni-Fe-phase is one key to obtaining high electrolyzer performance and that, despite Fe leaching, change in anion-conducting binder compound, and morphological changes inside the catalyst bulk, the Ni3Fe-LDH-based single-cells demonstrate high performance and durability. The work reveals the importance of longer stability tests and presents a holistic approach of electrochemical tracking and post-mortem analysis that offers a guideline for investigating electrode degradation behavior over extended measurement periods. Transitioning industrial processes toward renewable energy is vital for decarbonization. Green hydrogen, generated via anion exchange membrane water electrolysis (AEMWE) offers cost-effective, efficient hydrogen production. While recent research has improved AEMWE components, long-term durability and comprehensive electrode studies are lacking. This study investigates a Ni3Fe-LDH-based single-cell's 1000 h operation, tracking anode degradation, and establishing correlations with overall cell stability.image

Anion exchange membrane water electrolysis is an attractive clean energy technology for producing hydrogen for energy storage, transport 1,2 and numerous other applications. Rational choice of highly active and stable catalysts as well as the proper design of catalyst layers are crucial to achieve technical relevance of electrolyser systems. The establishment of clear understanding of optimal catalyst treatment and methods of implementation are key steps towards optimized electrolyzer performance and durability. One aspect of catalyst performance in catalyst layers is the catalyst size distribution. A multimodal size distribution of catalyst particles or agglomerates can jeopardize the layer homogeneity and thus electrode performance. In this work, the effect of high-energy ball tumbling milling on the promising Ni3Fe-LDH OER catalyst followed by catalyst dispersion control was correlated to the microstructure of the catalyst layer, the achieved catalyst activity and utilization, and the resulting single cell performance and stability. Physico-chemical characterization confirmed the stable layered double hydroxide structure of the catalyst. By milling, a 300-fold reduction of catalyst agglomerate size, and an 8.8-fold increase of the geometrical surface was achieved. The optimized solvent compositions effectively increased the catalyst ink stability. We found that a significantly decreased catalyst agglomerate size resulted in very homogeneous mixtures of catalyst and ionomer. By tailoring the electrode structure design, lower internal electronic resistances of the electrodes, decreased charge-transfer resistances (Rct) of the membrane electrode assembly, and stable single cell durability of 1000 h with a minor degradation rate of 57 µV h-1 were accomplished. This work presents a facile and scalable approach of NiFe-LDH catalyst treatment and dispersion control and provides a guideline to follow for further electrode development and increased AEM water electrolyzer performances. References (1) Hydrogen Applications. Hydrogen Europe. 2020. https://hydrogeneurope.eu/hydrogen-applications (accessed Jan 7, 2022). (2) Vincent, I.; Bessarabov, D. Low Cost Hydrogen Production by Anion Exchange Membrane Electrolysis: A Review. Renew. Sustain. Energy Rev. 2018, 81 (August 2016), 1690–1704. https://doi.org/10.1016/j.rser.2017.05.258. This work has been performed in the frame of the CHANNEL project. This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (now Clean Hydrogen Partnership) under grant agreement No 875088. This Joint undertaking receives support from the European Union's Horizon 2020 Research and Innovation program, Hydrogen Europe and Hydrogen Europe Research.

Anion exchange membrane (AEM) water electrolyzers is an attractive alternative approach for green hydrogen production. The alkaline environment allows the use of non-PGM catalysts and, based on recent research in catalysts and membranes, may soon challenge the established proton exchange membrane water electrolyzers 1,2 . Transition metal-based catalysts have attracted much attention because they provide an excellent oxygen evolution reaction in alkaline media 3 . In particular, Ni-Fe-layered double hydroxides (LDH) have been intensively studied in recent years and have shown fast intrinsic electrocatalytic activity for water splitting 4,5 . In this work, we used Ni 3 Fe-LDH as an anode catalyst, DURAION® as ionomer and membrane and investigated the effects of electrode design and cell operation on the performance and stability of AEM electrolyzers. The optimized electrode was operated stably for 1000 hours at 1 A cm -2 with an overall degradation rate of 0.014 V h -1 . At the end of the lifetime, the cell was disassembled and subjected to a series of experiments to investigate the physical and chemical degradation. This work provides a fundamental understanding and specific approach to the use of nickel-iron-based electrodes and promotes the further development of AEM water electrolyzers through highly stabilized, Ni-rich, and low-cost anodic electrocatalysts. This work has been performed in the frame of the CHANNEL project. This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (now Clean Hydrogen Partnership) under grant agreement No 875088. This Joint undertaking receives support from the European Union's Horizon 2020 Research and Innovation program, Hydrogen Europe and Hydrogen Europe Research. Vincent, I. & Bessarabov, D. Low cost hydrogen production by anion exchange membrane electrolysis: A review. Renew. Sustain. Energy Rev. 81 , 1690–1704 (2018). Miller, H. A. et al. Green hydrogen from anion exchange membrane water electrolysis: A review of recent developments in critical materials and operating conditions. Sustain. Energy Fuels 4 , 2114–2133 (2020). Gong, M., Wang, D. Y., Chen, C. C., Hwang, B. J. & Dai, H. A mini review on nickel-based electrocatalysts for alkaline hydrogen evolution reaction. Nano Res. 9 , 28–46 (2016). Zignani, S. C., Faro, M. Lo, Trocino, S. & Aricò, A. S. Investigation of NiFe-based catalysts for oxygen evolution in anion-exchange membrane electrolysis. Energies 13 , (2020). Mohammed-Ibrahim, J. A review on NiFe-based electrocatalysts for efficient alkaline oxygen evolution reaction. J. Power Sources 448 , 227375 (2020).

Water splitting is an environmentally friendly strategy to produce hydrogen but is limited by the oxygen evolution reaction (OER). Therefore, there is an urgent need to develop highly efficient electrocatalysts. Here, NiFe layered double hydroxides (NiFe LDH) with tunable Ni/Fe composition exhibit corresponding dependent morphology, layered structure, and chemical states, leading to higher activity and better stability than that of conventional NiFe LDH-based catalysts. The characterization data show that the low overpotentials (249 mV at 10 mA cm(-2)), ultrasmall Tafel slopes (24 mV dec(-1)), and high current densities of Ni3Fe LDH result from the larger fraction of trivalent Fe3+ and the optimized local chemical environment with more oxygen coordination and ordered atomic structure for the metal site. Owing to the active intermediate species, Ni(Fe)OOH, under OER conditions and a reversible dynamic phase transition during the cycling process, the Ni3Fe LDH achieves a high current density of over 2 A cm(-2) at 2.0 V, and durability of 400 h at 1 A cm(-2) in a single cell test. This work provides insights into the relationship between the composition, electronic structure of the layer, and electrocatalytic performance, and offers a scalable and efficient strategy for developing promising catalysts to support the development of the future hydrogen economy.

Renewable and storable fuel hydrogen can be produced through light-driven water splitting using photovoltaic-biased electrosynthetic (PV-EC) devices. The required voltage to drive the reaction is approximately 1.5 V, which can be provided using multi-junction silicon solar cells. To generate these voltages at the operation point, the solar cells are electrically optimized with respect to the standard test conditions. However, if such devices were to be used outdoors, a wide range of different illumination conditions has to be considered. Herein, we discuss the dependence of the solar-to-hydrogen efficiency on the spectral quality, the incident illumination intensity and the operation temperature. It is found that in the case of high irradiation intensities (e.g. 1 sun), high operation temperatures reduce the PV performance, but the overall PV-EC device performance remains almost unchanged due to improved kinetics in the EC part. In contrast, when the illumination intensity is reduced, the loss in PV performance cannot be compensated by improved EC performances due to higher temperatures.