Sputter-Deposited Pt PEM Fuel Cell Electrodes: Particles vs Layers

Platinum catalyst layers with Pt loadings were deposited by magnetron sputtering from a variable deposition angle α onto gas diffusion layer (GDL) substrates and tested as cathode electrodes in proton exchange membrane (PEM) fuel cells using Nafion 1135 membranes and Teflon-bonded Pt-black electrode (TBPBE) anodes. Layers deposited at normal incidence are continuous and approximately replicate the rough surface morphology of the underlying GDL. In contrast, glancing angle deposition (GLAD) with and continuous substrate rotation yields highly porous layers consisting of vertically oriented Pt particles, high and wide, that are separated by . The particle electrodes exhibit a higher (lower) mass-specific performance than the continuous-layer electrodes for a high (low) current density . This is attributed to a higher porosity but lower overall electrochemically active surface area for the particles compared to the continuous layer. Increasing in particle cells from yields increasing potentials, but causes a voltage drop at , associated with the reduced pore density at large . Comparison cells with a TBPBE cathode exhibit comparatively low Tafel slopes but a lower Pt mass specific performance than the sputtered catalysts. Quantitative analyses of kinetic and mass-transport losses in the polarization curves suggest a competing microstructural effect, favoring mass-transport performance and an efficient oxygen reduction reaction for particle and continuous layer electrodes, respectively. The overall results suggest that in addition to the well-known promise of sputter-deposited Pt catalysts as an approach to increase Pt utilization at low loading, GLAD provides the unique ability to control Pt porosity and to achieve efficient reactant flow for high-current-density operation.