Rational Comparison of Anode Material Properties for Anion Exchange Membrane Water Electrolysis

Due to the increasing need for sustainable energy, the hydrogen (H2) economy is a topic of interest where H2 is used for energy storage and as a fuel [1]. In this regard, the utilization of hydrogen as a promising energy source for achieving global decarbonization goals has led to the exploration of water electrolysis (WE) as a viable route for producing green hydrogen [2,3]. Although proton exchange membrane water electrolysis (PEMWE) is established, they suffer from the reliance on expensive noble metals as catalysts. In contrast, recently, anion exchange membrane water electrolysis (AEMWE) is emerging as a low-cost alternative solution that combines the strengths of both alkaline, WE (low capital cost and use of liquid electrolyte), and PEMWE (low ohmic resistance and high gas purity) where cost-effective transition metal oxides can be used [4-5]. Among various materials that are being investigated as AEM anodes, Ni-based systems show high promises. However, a rational comparison between different Ni-based benchmarks as AEM anodes under different stages of electrode production is still lacking. Such a comparison would provide insight into the material properties that are relevant for optimization in each stage thereby producing highly performing catalysts. To achieve this, these materials have to be processed under comparable conditions during the electrode fabrication to draw meaningful correlations between the performance and influencing parameters. Herein, we developed a protocol for rationally comparing different commercial benchmark anode materials in AEMWE. The aim is to understand material properties that influence the final catalyst performance and further enhance their catalytic activity by fine-tuning pivotal material properties. We selected Ni-Co-O, Ni-Fe-O, and Ni-Co-O doped by Fe as potential anode materials for AEMWE and performed a systematic evaluation and comparison of the properties in powder, ink, and electrode layer stages. After the initial characterization of the powders by complimentary techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM), we developed ink formulations from each material. The stability of dispersions in various solvents was assessed using Hanson Solubility Parameters extracted from Analytical Centrifugation (AC) data [6]. AC in conjunction with own developed algorithms were then used to represent long-term stability data as transmittograms. During the ink development, particular care was taken to use solvent matrices that lead to comparable particle size distributions within the materials library together with constant catalyst to binder ratio and ink processing (e.g., ultrasonication) to assure the properties of each material to be the only free parameter in both ink formulation and electrode development. In the subsequent step, electrode layers from each material were obtained by spray deposition with defined deposition parameters (substrate temperature, ink flow and nozzle to substrate distance) that enabled the achievement of comparable electrode layers. These layers were characterized by atomic force microscopy and N2 sorption for understanding the microstructure and specific surface area of each electrode which was used for normalizing the electrochemical data. Specifically, an own developed AFM-based multi-stage data quantification was applied for evaluating the microstructure on larger scales [7]. Finally, the electrochemical performance of these materials as anodes in AEMWE were tested and correlated with the inherent material properties as well as ink and electrode layer characteristics. This work sheds light on a systematic workflow for characterizing AEMWE anode materials at different stages which provides a comprehensive picture about the underlying property-performance relationship. The developed coherent workflow can be expanded to other materials in AEMWE. Staffell, Iain, et al. "The role of hydrogen and fuel cells in the global energy system." Energy & Environmental Science 12.2 (2019): 463-491. Kusoglu, A., Chalkboard 1-the many colors of hydrogen. The Electrochemical Society Interface, 2021. 30(4): p. 44. Harkema, D., G. Palasantzas, and J. Miocic, Hydorgen in the European energy transition: Where will it come from? 2022. Miller, H.A., et al., Green hydrogen from anion exchange membrane water electrolysis: a review of recent developments in critical materials and operating conditions. Sustainable Energy & Fuels, 2020. 4(5): p. 2114-2133. Santoro, C., et al., What is Next in Anion‐Exchange Membrane Water Electrolyzers? Bottlenecks, Benefits, and Future. ChemSusChem, 2022. 15(8): p. e202200027. Bapat, S., et al., Towards a framework for evaluating and reporting Hansen solubility parameters: applications to particle dispersions. Nanoscale Advances, 2021. 3(15): p.4400-4410. Jain, A., et al., "Small-Area Observations to Insight: Surface-Feature-Extrapolation of Anodes via Multistage Data Quantification. " Under review, ChemCatChem