Enhanced Water Management and Oxygen Transport in PGM-Free Cathode-Catalyst-Layers for Fuel Cell Applications

Abstract

During the last couple of years intensive research has been conducted on the development of Platinum-group-metal (PGM) free catalysts for the employment in fuel cell applications and more specific for the oxygen reduction reaction (ORR) on the cathode side of PEM-FCs. Tremendous progress has been made in the synthesis of PGM-free catalysts, resulting in competitive catalytic activities of PGM-free catalysts in RDE experiments compared with commercial Pt/C-catalysts and the release of the first commercial available PGM-free fuel cell system from Ballard Power Systems. But nevertheless membrane-electrode-assemblies (MEA) made from novel PGM-free catalysts still exhibit major drawbacks. One of those drawbacks is the relatively low number of active sites of PGM-free catalysts resulting in cathode catalyst layer (CCL) thicknesses of ~ 100 µm, which is a thickness increase of approximately 1000 % compared to commercial Pt/C-CCLs. The thickness of the PGM-free CCLs introduces a dramatically increased oxygen diffusion resistance compared to Pt/C-CCLs. Moreover, the water management in CCLs with thicknesses around 100 µm becomes extremely challenging and flooding is observed frequently at higher current densities. In total the performance of state-of-the-art MEAs enabling PGM-free CCLs is markedly below its theoretical potential. [1][2] Our work is focused on novel MEA-architectures and fabrication techniques for PGM-free CCLs introducing strategies to enhance the reactant transport and the water removal in the CCL. One approach is focused on an improved interfacial contact between the CCL and the ion exchange membrane. As demonstrated in previous publications this interface is a crucial setscrew for lowering the charge-transfer resistance and permitting a significant reduction of the catalyst-loading without reducing the performance in PGM-CCLs. With regards to PGM-free CCLs the reduced layer thickness should additionally be followed by a reduced oxygen diffusion resistance and an improved performance. [3] The second approach involves the insertion of a variety of different additives into the CCL to create an optimized CCL morphology with specific pathways for oxygen-transport and water removal. The additives are composed of nanostructures including fibres and spherical particles, made from different materials and different wettability to tune the hydrophobic-hydrophilic balance, the pore size distribution and charge transport within the catalyst layer. In addition to the fuel cell performances tomographic FIB-SEM studies of the CCLs are conducted, to correlate the different electrode morphologies and the resulting fuel cell performance. The reactant gas transport within the different CCLs is analysed in-situ with limiting current measurements.