(68a) Probing Relationships between Bulk and Local Environments to Understand Impacts on the Electrocatalytic Oxygen Reduction Reaction

When coupled with renewable electricity sources, the electrochemical oxygen reduction reaction (ORR) has become a promising route towards the economical and sustainable production of valuable fuels and chemicals. Overall reaction efficiency and selectivity of the ORR is largely linked to the electrocatalyst’s ability to bind key reaction intermediates. However, the ORR, like many electrocatalytic reactions, is also strongly impacted by the local catalyst environment, which is in turn influenced by bulk reactor properties and operating conditions, including electrolyte composition, concentration, and pH, as well as operating potential and mass transport^[1]. Furthermore, given the lack of experimental local environment probes, it is difficult to monitor relevant length scales to elucidate the local catalyst environment properties and directly relate them to changes in catalyst activity and selectivity. To address these challenges, we first characterize changes in the ORR mechanism by systematically tuning bulk reactor properties, such as cation identity and pH, using Tafel analysis and kinetic isotope experiments (KIE). The changes observed in the Tafel slopes (Fig. 1a) point to a pronounced change in the ORR mechanism and rate determining steps (RDS) as a function of pH. KIE results also indicate strong proton involvement in H₂O₂ selectivity, especially at low pH, with reduced effects in neutral pH (Fig. 1b). Additionally, we investigate the use of a local pH probe to track changes in proton concentration at the catalyst surface during ORR, as this has been identified as a critical property that influences catalyst activity and selectivity. Using the rotating ring-disk electrode (RRDE) equipped with an iridium oxide (IrO_x) ring as the pH sensing material (Fig. 1c), we can detect these pH changes in situ ^[2]. However, we find that stability of the IrO_x pH response is affected by both length of experiment as well as exposure to previous environment conditions indicating possible undesirable changes to the probe material. We are optimizing the IrO_x probe to improve our ability to measure local pH and elucidate these relationships between bulk reactor environment, local catalyst environment, and catalyst performance.

References:

1. Bohra, D.; Chaudhry, J. H.; Burdyny, T.; Pidko, E. A.; Smith, W. A., Energy Environ. Sci. 2019, 12 (11), 3380-3389.

2. Steegstra, P.; Ahlberg, E. J. Electroanal. Chem. 2012, 685, 1-7.