Effect of Carbon Support on the Durability of d-PtCo Catalysts in PEM Fuel Cells

Abstract

Catalyst support materials are an essential component of proton exchange membrane (PEM) fuel cells to decrease catalyst Pt loading. To be classified as a good catalyst support, a material must have high surface area, high electrical conductivity, be durable in harsh environments, and be economically feasible for large scale commercial production. In light of this, carbon is one such material that has widely been used in PEM fuel cells as a catalyst support. Various carbon materials of differing morphologies have been utilized as supports where high surface area is beneficial to greater Pt accessibility and higher performance, while graphitization and lower surface areas are beneficial to durability especially at high potentials. Here, we present a durability analysis of platinum cobalt catalyst supported on five structurally different carbon supports which are as follows: high surface area carbon TEC36E32 (29% Pt / 2.3% Co / 526.6 m2/g-Cat), high surface area graphitized carbon TEC36EA32 (29.3% Pt / 3.3% Co / 108.5 m2/g-Cat), Vulcan carbon TEC36V32 (29.1% Pt / 3.6% Co / 160 m2/g-Cat), graphitized Vulcan carbon TEC36VA32 (29.3% Pt / 3.4% Co / 70 m2/g-Cat), and acetylene black TEC36F32 (29.3% Pt / 3.4% Co / 523.6 m2/g-Cat). Each of the aforementioned carbon supported dealloyed-PtCo catalysts were spray coated onto one side of a DuPont® XL membrane at a loading of approximately 0.1 mgcat/cm2. The other side of the membrane, which served as the anode, was kept the same for all samples and spray coated in the same manner at the same loading with platinum supported on Vulcan carbon TEC10V20E (20% Pt). The membrane electrode assemblies (MEAs) were then incorporated into 5 cm2 differential cells with SGL Sigracet® Gas diffusion layers (GDLs). Subsequent to conditioning, catalyst performance was evaluated via the following characterizations: local oxygen transport resistance measurement, fuel cell polarization curves, sheet resistance measurement, impedance measurement in air and HelOx, mass activity measurement, and cyclic voltammetery measurements. DOE recommended square wave catalyst accelerated stress testing (AST) was utilized to degrade the catalysts with a recovery protocol and characterization performed after 15,000 and 30,000 cycles of the AST.1 Figure 1 illustrates the performance of TEC36V32 and TEC36E32 cathode catalyst MEAs where power densities of 0.643 W/cm2 and 0.716 W/cm2 were obtained respectively at 80oC, 100%RH and 150kPa pressure. In addition to this characterization, evolution of the resistances that arise from oxygen transport during the catalyst durability testing, both pressure dependent and pressure independent, will be presented for the various carbon supports. Further characterization of the catalysts after the testing will also be presented to provide insight on the influence of carbon support materials on catalyst performance with cell degradation. Acknowledgement: This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Fuel Cells Program Manager: Dimitrios Papageorgopoulos and Technical Development Manager: Greg Kleen).