The Corrosion of Carbon Black in Phosphoric Acid

Carbon as a material is thermodynamically unstable at phosphoric acid fuel cell cathode operating conditions. However, the kinetics of carbon corrosion are sufficiently slow so that carbon can realistically be considered for various materials applications at fuel cell operating conditions. One common use for carbon blacks in fuel cells is as a support for the (usually) noble metal catalyst required for oxygen reduction at the cathode and hydro-gen oxidation at the anode. The electro-chemical corrosion of various carbon blacks has been studied by several workers (1-4). Kinoshita and Bert (2) studied the corrosion of a high surface area carbon black in 96% H3PO 4 at 135 ~ at several potentials and determined that two processes occurred: the formation of surface oxides and evolution of CO 2. They found that over several thousand minutes the rate of carbon corrosion de-creased and that CO2 evolution eventual ly became the major reaction. They speculated that while the mechanism for these processes was complex, corrosion might be controlled by mass transport of reactive species through the bulk carbon particle structure, with oxidation occurring at disorganized amorphous carbon sites inside the part ic le, much in the same way that Heckman and Harling (5) have observed for the gas phase oxidation of carbon blacks. Experiments done in this laboratory indicate that the corrosion of carbon blacks in a phosphoric acid fuel cell environment does proceed in the same manner as noted by Heckman et al. for the gas phase oxidation of carbon blacks. Subscate fuel cell tests using Teflon-bonded electrodes have been performed at conditions expected to accelerate the cor-rosion of carbon blacks. The catalyst used in these tests was highly dispersed platinum supported on Vulcan XC-72 (Cabot Corp.). Figure 1 is a transmission electron micrograph showing the appearance of the carbon black in the catalyst before testing. Figure 2 is a transmission electron micrograph re-presentative of the cathode catalyst afte