Loading Impact of a PGM-Free Catalyst on the Mass Activity in Proton Exchange Membrane Fuel Cells

Abstract

Platinum-group-metal-free (PGM-free) catalysts are currently considered as potential oxygen-reduction-reaction (ORR) catalysts to replace costly and supply-limited platinum at the cathode side of proton exchange membrane fuel cells (PEMFCs). Extensive research efforts have led to substantial progress with regards to the ORR activity of PGM-free ORR catalysts, but there is uncertainty about the dependence of the mass activity on the catalyst loading. In this study, the effect of catalyst loading on the mass activity is investigated by means of rotating disk electrode measurements as well as single cell PEMFC tests using a commercial PGM-free ORR catalyst. Single cell tests with a wide range of loadings (0.4–4.0 mgcat cm−2 MEA) are compared to rotating disk electrode measurements with low loadings of 40–600 μgcat cm−2 disk. In contrast to indications in the literature that the ORR activity depends on catalyst loading, our results reveal an independence of the ORR mass activity from the catalysts loading in both RDE and PEMFC tests, if corrections for the voltage losses in H2/O2 single cell tests are considered. Moreover, no clear relation of the stability to the catalyst loading was found in H2/O2 PEMFCs.

Review on the Degradation Mechanisms of Metal-N-C Catalysts for the Oxygen Reduction Reaction in Acid Electrolyte: Current Understanding and Mitigation Approaches

“…Indeed, for a fixed geometric current density of 0.5 A cm –2 , a lower catalyst loading implies a lower geometric SD (number of Fe-N 4 sites per cm 2 of geometric electrode area), which in turn implies that each active site has to work more. The higher turnover on each active site in turn implies a higher local production of H 2 O 2 and ROS, which could explain the faster decay observed at a low loading by Banham et al Slightly contrasting this report, Damjanovic et al reported no clear effect of catalyst loading (0.4 and 4.0 mg cm –2 studied) on the performance loss observed over repeated PEMFC polarization curves . The decay was significantly higher with the low loading, when looking at the cell voltage drop at 100 mA cm –2 , but comparable for both the loading of 0.4 and 4.0 mg cm –2 , when looking at the cell voltage drop at 500 mA cm –2 .…”

FeNC Oxygen Reduction Electrocatalyst with High Utilization Penta‐Coordinated Sites

“…[18] This demonstrates improvements in accessible surface FeN x active sites are required to compete with Pt/C site densities.Reported FeNC cathode catalyst layers in PEMFCs often possess thicknesses of ~100 µm, where transport resistances severely limits practical operation. [23,24] Ideally thicknesses should not exceed ~10 µm to minimise debilitating mass transport. [25] This means the total possible FeN x sites in PEMFCs are restricted in terms of both sites g FeNC -1 and sites cm FeNC -3 .…”

FeNC Oxygen Reduction Electrocatalyst with High Utilisation Penta-coordinated sites

“…[18] This demonstrates improvements in accessible surface FeNx active sites are required to compete with Pt/C site densities.Reported FeNC cathode catalyst layers in PEMFCs often possess thicknesses of ~100 µm, where transport resistances severely limits practical operation. [23,24] Ideally thicknesses should not exceed ~10 µm to minimise debilitating mass transport. [25] This means the total possible FeNx sites in PEMFCs are restricted in terms of both sites gFeNC -1 and sites cmFeNC -3 .…”