Palladium based catalysts for oxygen reduction in polymer electrolyte membrane fuel cells

Abstract

An important issue in low temperature polymer electrolyte membrane fuel cells (PEMFC) is the lack of alternative catalysts to platinum for the oxygen reduction reaction (ORR). The high cost and potential limited availability of platinum restricts its long term use for large scale applications in PEMFC. Consequently, there is a great interest in alternative catalysts to platinum for PEMFC. In this research a systematic study of the synthesis and optimization of carbon-supported palladium and palladium alloy nanoparticle electrocatalysts is reported. The catalysts investigated were Pd, Pd-Au, Pd- Co, Pd-Fe and Pd-Ti supported on carbon black (Vulcan XC-72R). At least two different atomic metal to metal ratios for bimetallic catalysts were investigated. All catalysts were initially evaluated for the ORR by voltammetry in a three-electrode cell.

Different reducing agents, including hydrogen, ethylene glycol (EG), formaldehyde and sodium borohydride were used for the synthesis of Pd nanoparticles. The use of EG led to Pd nanoparticles with the highest ORR activity; this synthetic method was optimised by adjusting the pH of the system. Pd nanoparticles of approximately 6 nm diameter dispersed on carbon black with exchange current densities for the ORR of ca. 1.0 x 10-11 A cm-2 were obtained. Two synthetic procedures were chosen for the preparation of bimetallic catalysts: simultaneous co-deposition of both metals on the carbon support and deposition of the second metal on carbon-supported Pd. Pd-Co alloy with atomic ratio Pd:Co 4:1 exhibited improved ORR activity compared to Pd/C after being heat treated at 300 ºC under H flow. The effect of heat treatment under H flow on 22 the ORR activity and physicochemical properties was also studied. Pure Pd particles exhibited sintering after heat treatment; the presence of Au, Co and Fe decreased the degree of sintering and the presence of Ti did not affect Pd particle growth.

Pd and Pd-Co were evaluated in low temperature hydrogen PEMFC, and Pd was tested as cathode catalysts in hydrogen polybenzimidazole (PBI) based high temperature PEMFC, and in direct methanol fuel cells (DMFC). Optimized Pd and Pd-Co catalysts were tested in a hydrogen low temperature PEMFC and the results were compared to those of the state of the art commercial Pt catalyst. With approximately 1.7 times higher metal loading than Pt (still significantly lower cost) the fuel cell with the Pd cathode gave better performance than that with Pt operating with air at 40 ºC. A comparative study of Pd and Pt was carried out in DMFC using different methanol concentrations and under different operating conditions. At methanol concentrations of 5 M and higher, the Pd cathode based cell performed better than that with Pt at 60 ºC with air.

A pseudo one dimensional model for Nafion® -based low temperature hydrogen PEMFC was developed to simulate the influence of cathode catalyst, metal loading, electrode thickness and different operating conditions on the cell voltage and current density output. The model considered mass transport through a thin film electrolyte and through porous media but not gas flow along the channels of the cell. The model closely predicted experimental results at 20 and 40 ºC. Above 40 ºC cell performance did not improve experimentally as was predicted by the model; this lack of improvement was attributed to

the decrease of oxygen permeability through Nafion® caused by the lower humidity at higher temperatures. Predicted results showed that enhanced fuel cell performance in the whole current density range could be achieved by increasing metal loading in the cathode whilst maintaining the catalyst layer thickness, which could be practically achieved by increasing the metal content of the carbon-supported catalyst.