Electrocatalyst Performance at the Gas/Electrolyte Interface under High-Mass-Transport Conditions: Optimization of the “Floating Electrode” Method

The thin-film rotating disk electrode (TF-RDE) is a well-developed, conventional ex situ electrochemical method that is limited by poor mass transport in the dissolved phase and hence can only measure the kinetic response for Pt-based catalysts in a narrow overpotential range. Thus, the applicability of TF-RDE results in assessing how catalysts perform in fuel cells has been questioned. To address this problem, we use the floating electrode (FE) technique, which can facilitate high-mass transport to a catalyst layer composed of an ultralow loading of catalyst (1–15 μg_Pt cm_geo^–2) at the gas/electrolyte interface. In this paper, the aspects that have critical effects on the performance of the FE system are measured and parametrized. We find that, in order to obtain reproducible results with high performance, the following factors need to be taken into account: system cleanliness, break-in procedure, hydrophobic agent, ionomer type, and the measurements of catalyst surface area and loading. For some of these parameters, we examined a range of different approaches/materials and determined the optimum configuration. We find that the gas permeability of the hydrophobic agent is an important factor for improving the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) performance. We provide evidence that the suppression of the HOR and ORR introduced by the Nafion ionomers is more than a local mass transport barrier but that a mechanism involving the adsorption of the sulfonate on Pt also plays a significant role. The work provides intriguing insights into how to manufacture and optimize electrocatalyst systems that must function at the gas/electrolyte interface.