Multinuclear Cobalt Schiff Base Complexes and their Activity Towards the Oxygen Reduction Reaction

James, Charles (2022) Multinuclear Cobalt Schiff Base Complexes and their Activity Towards the Oxygen Reduction Reaction. Doctoral thesis, University of East Anglia.

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The cathodic oxygen reduction reaction (ORR) remains to be the most challenging aspect of hydrogen fuel cell (HFC) research. The ORR is a sluggish, multi-electron process which currently uses large loadings of platinum to achieve required current densities. With the high cost and low availability of platinum, much effort has been directed towards developing non-precious metal catalysts for the ORR. While good progress has been made, heterogeneous precious metal-free catalysts often show poor selectivity to the four-electron ORR to water, and commonly generate hydrogen peroxide as a by-product which can damage the sensitive fuel cell components. Understanding the mechanism of heterogeneous catalysts can be difficult and as such manipulating the selectivity of the ORR for these systems can be challenging. Studying the ORR for homogeneous molecular catalysts can allow for closer examination of the specific mechanism of the ORR, and make it possible to determine factors which contribute to generation of the two ORR pathways.
In this work, a dinuclear cobalt complex bearing a new heptadentate Schiff base ligand was designed with the intention of influencing the system towards four-electron pathway selectivity. In addition, the catalyst design was inspired by known CoIIsalen complexes’ ability to reach low overpotentials whilst retaining their catalytic activity.
The catalytic activity of the new complexes was determined spectrophotometrically by the use of a homogeneous chemical reductant, and selectivity was determined by iodometric titration. Overpotentials were estimated by cyclic voltammetry and assessing the standard potential of the ORR pathways using previously established thermodynamic data.
Remarkably, it was found that the selectivity of the ORR catalysed by these complexes was dependent on the identity of the proton source used, where high four-electron pathway selectivity was observed when the proton source was a carboxylic acid. When the non-coordinating NH4PF6 was used, almost quantitative selectivity towards the two-electron pathway was observed. The selectivity was also determined to be a result of the identity of the conjugate base, rather than the strength of the Brønsted acid. To the best of our knowledge, such an anion-based selectivity dependence has not yet been observed for the ORR and presents a new consideration when developing catalysts for the ORR.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Chemistry
Depositing User: Kitty Laine
Date Deposited: 27 Jun 2023 10:59
Last Modified: 27 Jun 2023 10:59

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