Lowering or eliminating the noble-metal content in oxygen reduction fuel cell catalysts could propel the large-scale introduction of commercial fuel cell systems. Several noble-metal free catalysts are already under investigation with the metal-nitrogen-carbon (Me-N-C) system being one of the most promising. In this study, a systematic approach to investigate the influence of metal ratios in bimetallic Me-N-C fuel cells oxygen reduction reaction (ORR) catalysts has been taken. Different catalysts with varying ratios of Fe and Co have been synthesized and characterized both physically and electrochemically in terms of activity, selectivity and stability with the addition of an accelerated stress test (AST). The catalysts show different electrochemical properties depending on the metal ratio such as a high electrochemical mass activity with increasing Fe ratio. Properties do not change linearly with the metal ratio, with a Fe/Co ratio of 5:3 showing a higher mass activity with simultaneous higher stability. Selectivity indicators plateau for catalysts with a Co content of 50% metal ratio and less, showing the same values as a monometallic Co catalyst. These findings indicate a deeper relationship between the ratio of different metals and physical and electrochemical properties in bimetallic Me-N-C catalysts.

This study investigates and compares four different deposition methods for an iridium‐based catalyst on antimony‐doped tin oxide support for oxygen evolution reaction in water electrolysis. Different synthesis routes often lead to varying properties of the resulting catalyst and can result in performance disparities. Here, some of the most prominent methods are carried out on the same support material and evaluated with special focus on the deposition yield of Ir and thus cost efficiency along with electrochemical performance. The catalysts are also assessed based on their chemical composition, namely Ir or IrO₂‐based, with an additional thermal treatment to convert Ir to IrO₂ species. The chosen synthesis routes result in different Ir species to obtain tetragonal IrO₂ a modified Adams fusion approach delivers the best controllable and highest Ir loading and thus superior electrochemical performance. As far as metallic Ir catalysts are concerned, a wet‐chemical reduction‐based synthesis results in the most desirable catalyst, which however falls behind the Adams fusion catalyst upon thermal treatment to IrO₂. The work in this study is a comprehensive analysis of different synthesis influences and recommends practices for laboratory‐based syntheses and an outlook on industrial viability.

Water electrolysis is a crucial technology for large-scale hydrogen generation, that is required for the transition to a fossil fuel-free energy system. Even though water electrolysis systems are already deployed in a limited capacity, the technology is largely constrained to liquid alkaline electrolysis. Proton exchange membrane (PEM) electrolysis could pose an alternative but it is still hindered by high investment costs, in-part due to its reliance on scarce noble-metal catalysts. Alternative structural designs of the anode catalyst layer (CL) could reduce Iridium loading of the whole system and thus accelerate its wide-spread application. In fuel cell research, it was already reported that a multi-layer design with varied ionomer content enhances performance of the CL and in-turn lowers required catalyst loading.[1,2,3] In this study, a gradient design for ionomer content is employed for anode CLs for the application in PEM water electrolysis. CLs are coated in a stacked multi-layer design via ultrasonic spray coating of Iridium/Nafion^® suspensions. The target ionomer loadings (10 and 30 wt.%) are confirmed by a combination of thermo-gravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The coating process yields homogenously loaded reference CLs as well as through-plane graded CLs with the desired ionomer loading. Additionally, scanning electron microscopy (SEM) in conjunction with energy dispersive spectroscopy (EDS) shows equal CL thickness across all ionomer loadings and spatially differentiated Nafion^® content throughout the CLs thickness. Electrochemical characterizations are carried out in an electrolysis-adapted gas diffusion electrode (GDE) half-cell setup to allow for quick examination and prototyping of produced layers. Differences in electrochemical performance of these layers can be observed with one gradient design CL showing reduced overpotential across all current densities compared to the homogenous CLs. In-particular a high ionomer loading near the membrane improves performance, most likely due to increased proton conduction to the membrane and higher available pore volume near the porous transport layer. Further analysis is performed on electrochemical studies to deepen the understanding of the role of ionomer in anodic PEM water electrolysis CLs. Particularly, the investigation of protonic and electric conductivity through the CL is of interest, because this parameter is most influenced by ionomer loading.[4] References: [1] Y. Wang, T. Liu, H. Sun, H. Wie, F. Yuanzhi, S. Wang, Electrochimica Acta 2020, 353, 136791. [2] R. Roshandel, F. Ahmadi, Renewable Energy 2013, 50, 921-931. [3] K.-H. Kim, H.-J. Kim, K.-Y. Lee, J.H. Jang, S.-Y. Lee, E. Cho, I.H. Oh, T.H. Lim, International Journal of Hydrogen Energy 2008, 33, 2783-2789. [4] M. Mandal, M. Moore, M. Secanell, ACS Applied Materials & Interfaces 2020, 13, 49549-49562. Figure 1