High-Temperature “Ex-Situ” Cyclic Voltammetry to Study the Oxygen Reduction Reaction on Electrocatalysts in Conditions Mimicking Fuel Cell Operation

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The massive efforts dedicated over the last decades to the development of improved electrocatalysts (ECs) and membrane-electrode assemblies (MEAs) have already yielded a significant reduction of the loading of platinum needed to devise low-temperature fuel cells (LT-FCs) achieving the performance and durability level that is required for applications. However, more research is still needed to further curtail the amount of Pt in a LT-FC; this is particular relevant to prevent supply bottlenecks in the perspective of a large-scale rollout of this technology. In a conventional LT-FC running on direct hydrogen most of the Pt loading is concentrated on the cathode electrode, where it is needed to promote the sluggish oxygen reduction reaction (ORR). It is often very time-consuming to optimize the features of a MEA to maximize its performance in a LT-FC running in operating conditions. This is especially the case if the MEA mounts innovative ORR ECs exhibiting features that are very different from benchmark ECs in terms of chemical composition of the active sites and morphology. Thus, the general practice is to screen developmental ORR ECs through “ex-situ” techniques before focusing MEA optimization only on the most promising systems. The most widespread “ex-situ” approach to screen developmental ORR ECs involves the use of a rotating (ring) disk electrode (R(R)DE) setup. Though very practical and accurate, conventional R(R)DE experiments are able to study the ORR features of an EC only in quite a narrow set of operating conditions (T = 30-60°C; P = 1 bar). These latter are different from those that are most commonly adopted at the cathode of a LT-FC (T = 80-100°C; P = 2 – 3 bar). As a result, the information obtained on the ORR features of an EC by means of conventional R(R)DE studies may not be fully representative of the behavior of the EC in an operating LT-FC. To address this shortcoming herein we present an innovative setup able to elucidate the ORR features of both benchmark and innovative ECs in experimental conditions that are very close to those found in operating low-temperature FCs. Specifically, the setup consists of a homemade channel flow electrode (CFE) cell operated in a rather simple closed system with a controlled oxygen concentration and maintaining a high level of cleanliness. The modeling, materials selection, design, and testing of a versatile CFE cell developed from scratch is presented. The cell is compatible with commercially-available RDE components and special care is devoted to ease the overall assembly of the experimental setup. The cell is characterized by a well-defined hydrodynamics, a low dead volume, a high mass-transfer rate and a high signal/noise ratio. The cell also allows to measure accurately the pressure and the temperature in close proximity of the working electrode. It is demonstrated the possibility to study the ORR features of ECs up to a temperature of 80°C and a pressure of 3 bar. Both benchmark and innovative ECs are taken into consideration. The latter exhibit features that are very different in comparison with the conventional Pt/C ECs, especially in terms of: (i) chemical composition of the active sites, that typically includes more than one element; and (ii) morphology of the support, that is based on different carbon nanostructures. The proposed setup is implemented to determine crucial ORR features of the ECs, including the kinetically-controlled current, the activation energy and the accessibility of O2 to the active sites. Acknowledgements This research has received funding from: (a) the European Union’s Horizon 2020 research and innovation program under grant agreement 881603; (b) the project ‘Advanced Low-Platinum hierarchical Electrocatalysts for low-T fuel cells’ funded by EIT Raw Materials; (c) Alkaline membranes and (platinum group metals)-free catalysts enabling innovative, open electrochemical devices for energy storage and conversion – AMPERE, FISR 2019 project funded by the Italian Ministry of University and Research; and (d) the German Federal Ministry of Education and Research (BMBF) under grant agreement number 03SF0585.

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