Kiri, P; (2015) Keggin-type heteropolyacids as electrocatalysts in hydrogen fuel cells. Doctoral thesis , UCL (University College London).

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Abstract

Keggin-type heteropolyacids (HPAs) exhibit potential application as electrocatalysts. HPAs, notably phosphomolybdovanadates, could provide platinum-free technology for innovative, low cost hydrogen fuel cell (HFC) systems. Renewable energy company ACAL Energy Ltd have developed the Flowcath® system, which substitutes expensive non-durable platinum catalyst with an aqueous HPA solution. This liquid catalyst flows through the cathode part of the fuel cell, acting as the oxygen reduction catalyst, whilst simultaneously promoting electron transfer. The HPA catalyst is fully regenerated simply by exposure to air after being pumped around the fuel cell stack. Hence, HPAs exhibits fast, reversible, multi-electron redox behavior under ambient conditions (100-150 oC) – most ideal for HFCs. However, development of the HPA catalyst is vital to improve fuel cell performance. This is important to meet the power output demands for automotive applications, thus providing a commercially viable source of renewable, sustainable energy. A series of phosphomolybdovanadates [PMo12-xVxO40](3+x)- (where x = 1-6) and [PMo11- xVxMO40](3+x)- (where x = 1,4; M = first row transition metal) Keggin-type HPAs were synthesized and extensively characterized via twelve different analytical, spectroscopic and diffraction techniques. The data obtained from these structural elucidation techniques have been analyzed in parallel with cyclic voltammetry experiments, to understand the location/effect of the dopant metals within the Keggin structure and on the HPA properties. Further, the structural and electrochemical effects of substituting the central heteroatom with other p-block elements (silicon, germanium, arsenic, sulfur) are reported using the various analytical techniques. Electrochemical data of certain novel transition metal and heteroatom-substituted synthesized HPA samples demonstrate a positive shift in reduction potentials. The stoichiometry, mechanism and kinetics of electron transfer, and diffusion coefficients of [PMo12-xVxO40](3+x)- (where x = 1-6) are discussed. These exhibited enhanced electrochemical properties and potential for improved efficiency of oxygen reduction reaction at the cathode in Flowcath®.

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