The Impact of Platinum Loading on Oxygen Transport Resistance

The effect of cathode platinum loading on oxygen transport resistance in a fuel cell electrode was determined by measuring limiting current in fuel cells over a wide range of platinum loadings (0.03 to 0.40 mgPt/cm2). The measurements show that the electrode oxygen transport resistance is inversely proportional to platinum loading or, equivalently, platinum surface area, and is mathematically similar to a 12 s/cm resistive film coating the active platinum sites. At low platinum loading this anomalous resistance significantly reduces the partial pressure of oxygen at the platinum surface and seriously degrades fuel cell performance at high power conditions. The magnitude of this film-like resistance is equivalent to ∼35 nm of bulk-like ionomer but only 4–10 nm, assuming uniform coating, is present in the electrode. Alternatively, 380 nm diameter agglomerates with 20 nm of ionomer coating their external surface would create the same resistance, but such agglomerates are not apparent in SEM micrographs. As a result, the source of this resistance remains unknown, and further investigations are required to understand and mitigate it.