ACTIVITY OF PGM-FREE ELECTROCATALYSTS FOR OXYGEN REDUCTION REACTION: PH AND CO-CATALYSIS EFFECTS

Abstract

Fuel cells offer a source to the current and always increasing demand for electric power. But as any new technology, there are challenges that need to be addressed to render it feasible for the market place. One of this challenges is finding the appropriate materials to catalyze the oxygen reduction reaction (ORR) that occurs in the cathode. Oxygen is used as an oxidant in a significant portion of the fuel cells due to its readily availability and high reduction potential. Now, one the bottlenecks that stops the large-scale adoption is the expensive and rare metals that have been used as catalysts for this reaction. One solution to this issue came with the development of platinum-metal group free (PGM-free) catalysts, which are composed of abundant and low cost elements like carbon, nitrogen and transition metals. These PGM-free catalysts have demonstrated their ability to effectively catalyze the ORR in highly alkaline and highly acidic media, as these have been the usual operating conditions for fuel cells that they were developed for. Due to this success, these PGM-free catalysts have attracted the attention for other applications, like the use in physiological devices or in microbial fuel cells, where the pH is far away from acid or alkaline. This has led to the need to understand how to introduce PGM-free catalyst in fuel cells that operate at pHs around neutrality and to learn about the way their activity towards the ORR is affected by changes in the concentration of hydronium and hydroxyl ions. The current study addresses these two issues. To begin with, four different transition metals were used in the synthesis of the PGM-free catalysts and tested at neutral pH. It was found that the iron containing PGM-free catalyst provides the highest current densities and lower hydrogen peroxide production. This same PGM-free catalyst was compared against platinum at neutral pH and demonstrated to have higher ORR performance and stability than the precious metal catalyst. Enzymes like bilirubin oxidase (BOx) catalyze the ORR at pH around neutrality, as they were developed by the biological systems to facilitate this reaction within them. The next achievement in this study was to successfully integrate BOx onto the PGM-free catalyst, obtaining a co-catalytic effect. This led to the next discovery, which consisted in unveiling what are the chemical and morphological characteristics of the PGM-free catalyst that make the integration of the BOx optimal. The closing component of this study is the exploration of the pH effect on the surface chemistry and the electrochemical activity towards the ORR of a PGM-free catalyst. It was found that the pH has an effect in surface chemistry of the PGM-free catalyst and this leads to a change in the kinetic and electron transfer parameters of the catalytic process