Advancing Electrocatalysis in Solid Acid Fuel Cells

Abstract

Solid acid fuel cells are currently performance limited by the electrochemical reaction kinetics at the electrodes. For acceptable power output, precious metal catalysts such as platinum, are required, rendering the technology too expensive for commercialization in all but niche applications. This thesis explores new approaches to solid acid fuel cell electrodes with the aim of reducing the catalyst loading or even eliminating precious metals entirely, without sacrificing performance. Two broad approaches are pursued: nanostructuring for enhanced catalyst utilization and incorporation of carbon-based materials for enhanced electrical transport and even electrocatalysis. Electrospray deposition is shown to be a viable technique to produce nanoparticles of the solid acid fuel cell electrolyte material CsH2PO4. In situ aerosol particle size measurements using a differential mobility analyzer and a condensation particle counter allowed the characterization of the electrospray parameter space, resulting in CsH2PO4 particle size control between 10 and 50 nm. Co-deposition of the CsH2PO4 nanoparticles together with a stabilizing surfactant polyvinylpyrrolidone (PVP) and platinum catalyst nanoparticles allows the creation of highly active, porous, interconnected electrode nanostructures. These nanostructures directly deposited onto fuel cell components, either the carbon paper current collector or the thin film electrolyte layer, serve as electrodes. A 30-fold reduction of platinum loading, without sacrificing electrode performance as compared to mixed powder-electrodes, is demonstrated. The direct deposition of CsH2PO4 nanoparticles with the stabilizing surfactant PVP onto a prefabricated CsH2PO4 electrolyte layer and subsequent magnetron sputtering of a nanometer thin platinum film lead to surprising catalyst-mass normalized electrode activities for solid acid fuel cell anodes. Specifically, a 25-fold increase in the mass normalized activity is shown as compared to the predicted values from analysis of platinum thin films with a controlled geometry. The second part of the thesis deals with the introduction of carbon nanotubes to the solid acid fuel cell electrodes. Three types of carbon nanotubes (CNTs) were grown directly onto the carbon paper current collector, in all cases using a chemical vapor deposition method and nickel catalyst nanoparticles. (i) Conventional CNTs were shown to act as effective current collectors for electrosprayed composite electrode structures, containing platinum nanoparticles. Matching of scales between the current collector and the electrosprayed structure leads to improved interconnectivity of the platinum catalyst nanoparticles and a higher density of electrochemically active triple phase boundaries. (ii) Nitrogen doped carbon nanotubes (NCNTs) were shown to actively catalyze oxygen electroreduction in solid acid fuel cells, with no platinum present. (iii) Undoped but defective carbon nanotubes (dCNTs) were shown to be highly efficient catalysts of the oxygen electroreduction reaction, surpassing the activity of the state of the art, platinum containing electrodes. This is the first time undoped carbon nanotubes have been reported to be catalytically active for electroreduction of oxygen. In addition, catalytically active carbon nanotubes show excellent catalysis of the water splitting reaction, creating the opportunity for new applications of these solid state electrochemical devices.