Mass Transfer in a Co/N/C Catalyst Layer for the Anion Exchange Membrane Fuel Cell.

Developing highly efficient non-noble metal catalysts for the cathode of fuel cells is an urgent requirement for reducing the cost. Although the intrinsic activity of non-noble metal materials has been greatly improved, the fuel cell performance is also determined by the mass transfer within the catalyst layer (CL), particularly at high current density. Electrochemical impedance spectroscopy (EIS) combined with rotating disk electrode (RDE) analysis is a powerful tool to quantitatively analyze the influence of the structural properties on CL performance. Here, Co/N/C CLs with gradient pore structures are constructed based on the controllable synthesis of zeolitic imidazolate framework (ZIF)-derived catalyst. The influences of the carbon support, active site, and catalyst loading are comprehensively studied by EIS in different regions (kinetic and mixed-diffusion). The results indicate that a high micro-/mesopore ratio is beneficial to increasing the density of active sites while reducing the mass-transfer efficiency. Inversely, abundant mesopores promote mass transfer, but they result in low active site density. By carefully adjusting the pore structure and chemical composition of the ZIF-derived catalyst, the Co/N/C CL shows a low mass-transfer resistance (95.5 Ω at 0.75 V vs RHE). This work demonstrates the importance of mass transfer within the fuel cell CL, beyond seeking only high activity.