Highly dispersed Co-N-RGO electrocatalyst based on an interconnected hierarchical pore framework for proton exchange membrane fuel cells

Abstract

It is a grand challenge to develop non-noble metal-based oxygen reduction (ORR) catalysts with relatively high activity and stability for proton exchange membrane fuel cells (PEMFCs). Herein, zeolitic imidazolate framework (ZIF) precursors were grown in situ on graphene oxide (GO), and Co-N-RGO catalysts with good stability were successfully prepared. Density functional theory (DFT) calculation results show that Zn and Co are mixed and alternated to form a Zn-Co-ZIF structure instead of forming ZIF-8 or ZIF-67, separately. Such a combination promoted the uniform distribution of Co atoms in the N-doped carbon frameworks during subsequent heat treatment. By means of accurate adjustment of the molar ratio of Co/Zn species in the Zn-Co-ZIF precursors, the hierarchical porous structure connected with uniformly dispersed Co–N_x sites was established after thermal activation, and the utilization rate of active sites was improved. The RGO in catalysts, connecting each derivative carbon particle, formed a connected network and reduced the ohmic impedance of the catalyst layer. This strategy avoided the traditional tedious steps of additional acid-leaching and second heat treatments. What's more, the ORR performance of the Co-N-RGO samples was evaluated. The catalyst exhibited a positive half-wave potential (0.75 V vs. RHE), high limiting diffusion current density, and positive stability at 0.5 M H₂SO₄. Meanwhile, the H₂/O₂ fuel cell test result displayed a relatively high ORR activity with a peak power density of 334 mW cm⁻².