S. M. Haile
Department of Materials Science and Engineering, Northwestern University
Electrochemistry at the Metal-Electrolyte-Gas interface of Solid-Acid Proton-Conducting Electrolytes
Location: EB1 Room 1011
Friday, January 27th 2017 - 11:00 am
Fuel cells operating at intermediate temperatures are promising alternatives to combustion engines because of their fuel flexibility, high efficiency, and compatibility with inexpensive interconnects. Solid acid fuel cells (SAFCs), in particular, offer the advantage of operation at temperatures near 250 ° C using a truly solid electrolyte, CsH2PO4, that facilitates anhydrous proton transport. The atypical temperature of SAFC operation in combination with an atypical electrolyte suggests atypical electrochemical reaction pathways. In this work we explore the hydrogen electro-oxidation pathway using thin-film Pt electrodes. For all film geometries investigated, hydrogen electro-oxidation (H2(g) → 2H+ + 2e-) is found to occur via a pathway that involves bulk transport of a hydrogen species (presumably the hydride ion) through the Pt film. Triple phase boundaries, at which the electrocatalyst (Pt), the electrolyte (CsH2PO4), and the gas phase are in simultaneous contact, traditionally viewed as the active sites for electrocatalysis, play a negligible role in the global reaction of the thin-film configuration. The measurements also suggest that in typical SAFC electrodes, in which Pt, Pt/C and CsH2PO4 are simply combined in a mechanical mixture, much of the Pt is electrically isolated and cannot contribute to the electrocatalysis. Based on this insight, electrodes in which Pt is decorated on carbon nanotubes, which serve as electronic interconnects, have been prepared. The Pt decoration step is carried out using a simple reduction method, beginning with H2PtCl6, and the composite electrode is fabricated by electrospray deposition of a suspension of Pt-CNTs in an aqueous solution of CsH2PO4 onto carbon paper current collectors. This approach results in a dramatic increase in Pt utilization, implying lower Pt loading requirements and, ultimately, significantly reduced fuel cell costs.