Reproducibility and alternative electrochemical methods for testing ORR/OER electrocatalysts

Abstract

Screening potential new electrocatalyst materials for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performance is a crucial component for the advancement of energy storage and conversion devices. Drop casting catalyst inks onto a rotating disc/rotating ring disc electrode (RDE/RRDE) surface is a widely used screening method due to its speed and simplicity, however, it is highly sensitive to electrode preparation and measurement conditions which limits reproducibility and comparability across studies. The first section of this thesis systematically investigates the impact of three modifications to the drop casting procedure: the use of a heat lamp during drying, electrode rotation during drying, and the inclusion of IPA in the ink. Electrochemical performance and catalyst distribution were analysed with RRDE measurements and large area energy dispersive x-ray spectroscopy (EDS) mapping. The electrodes that were dried at rotation or with a heat lamp exhibited lower OER currents and smaller confidence deviations. EDS analysis showed rougher surfaces for IPA containing inks and the formation of concentrated rings of catalyst material on some rotated electrodes, with no electrode achieving uniform catalyst distribution. Contact angle measurements also showed lower angles for the electrodes dried without a heat lamp, rotation, or IPA.
The second section introduces sampled current voltammetry (SCV) as a potential alternative catalyst screening technique due to the correlation between RDE/RRDE rotation rate and SCV sampling time. SCV applies a series of steps from a rest potential to a target potential and back to the rest potential again. Current transients are obtained at each target potential step which are then sampled at a specific time, the currents obtained at each target potential for a specific sampling time are then used to construct a sampled current voltammogram. A conditioning procedure is carried out between each target potential step to reduce the influence of electrode history compared to conventional potential sweeping techniques, this was adjusted to control the surface oxide coverage on platinum disc electrodes. Variations to the conditioning procedure upper potential limit (EUP) and rest potential (EREST) showed changes in SCV onset potential and current magnitude, linked to oxide coverage, and anion adsorption. These findings demonstrate SCV as a highly sensitive technique for studying electrode surface processes.
The final section of this thesis investigates SCV on higher surface electrodes, including platinum black and electrodeposited transition metal oxides (TMOs). The magnitude of the SCV currents observed for the platinum black electrodes were all much higher than those observed in the rotating disc electrode voltammograms, consistent with utilisation of the full electrode surface area, compared to the geometric surface area considered in RDE. The next part of this chapter focused on electrodeposited TMOs on RDEs and stainless steel electrodes. Electrodes dried at 25°C showed better OER activity, whereas the 500°C dried depositions showed better ORR activity, all depositions showed small errors indicating high reproducibility. Comparison between RDE and SCV testing is complicated for larger surface area electrodes due to the differing surface sensitivities between the two techniques.

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Identifiers

ORCID for Amber Watson: orcid.org/0009-0003-9616-1406

ORCID for Andrea Russell: orcid.org/0000-0002-8382-6443

ORCID for Guy Denuault: orcid.org/0000-0002-8630-9492

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