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.

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

The generation of H 2 is possible in different processes like the steam reforming of hydrocarbons, currently used in industrial processes or electrolysis of water. [1,2] In terms of electricity generation and utilization of H 2 as fuel in transportation, the latter approach seems to be opportune to minimize the reliance on fossil fuels and curb CO 2 -emissions. [1,3] With electrolysis, the generated H 2 could help establish a carbon-neutral transportation system. Additionally, in combination with fuel cells it can act as quasi-energy storage, both short-and long-term, to balance temporal disparities in the generation of renewable energy. [4][5][6] One implementation of water electrolysis is its proton exchange membrane (PEM) variant, which utilizes a protonconducting polymer membrane and noble-metal catalyst electrodes. [1] These catalysts are most often made from Pt on the cathode side and Ir on the anode. Noble metals are to some extent necessary here especially for the anode since the chemical environment presents harsh conditions and high oxidizing potentials, making most catalyst materials unstable long-term. Utilization of Ir comes with the downside of high investment costs, due to scarcity of Ir and therefore high price, which hinders the widespread application of PEM electrolysis systems. [1,3,6] In 2020 alone, the price of Ir tripled. [1,3] Reduction of the overall Ir content in PEM electrolysis systems is thus paramount for further industrial implementation. Recently, most employed catalysts consist either of metallic Ir, so-called Ir black or IrO 2 but these offer quite low volumetric activity, due to their nature as bulk catalysts. One strategy to reduce the Ir loading is to attach the catalyst to a conductive support material, quite similar to already applied fuel cell catalysts, with Pt supported on C. [7][8][9] In PEM electrolysis, the support also has to withstand harsh conditions, and thus the choice of material is drastically narrowed. Some metal oxides, like TiO 2 , are stable at the anode; however, their electrical conductivity is several orders of magnitude lower than that of bulk Ir; thus, the overall current efficiency is diminished. [7] Doping of the metal oxide lattice can alleviate the conductivity problem. One of the most promising candidates is antimony-doped tin oxide (ATO), which shows adequate electrical conductivity and stability. [10][11][12][13][14][15] Moreover, ATO might induce positive effects into Ir catalyst species, for instance, 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 als...