Deciphering the selectivity descriptors of heterogeneous metal phthalocyanine electrocatalysts for hydrogen peroxide production

Abstract

The electrocatalytic 2e⁻ oxygen reduction reaction (2e⁻ ORR) provides an appealing pathway to produce hydrogen peroxide (H₂O₂) in a decentralized and clean manner, which drives the demand for developing high selectivity electrocatalysts. However, current understanding on selectivity descriptors of 2e⁻ ORR electrocatalysts is still insufficient, limiting the optimization of catalyst design. Here we study the catalytic performances of a series of metal phthalocyanines (MPcs, M = Co, Ni, Zn, Cu, Mn) for 2e⁻ ORR by combining density functional theory calculations with electrochemical measurements. Two descriptors (ΔG_*O − ΔG_*OOH and ΔG_*H2O2) are uncovered for manipulating the selectivity of H₂O₂ production. ΔG_*O − ΔG_*OOH reflects the preference of O–O bond breaking of *OOH, affecting the intrinsic selectivities. Due to the high value of ΔG_*O − ΔG_*OOH, the molecularly dispersed electrocatalyst (MDE) of ZnPc on carbon nanotubes exhibits high selectivity, even superior to the previously reported NiPc MDE. ΔG_*H2O2 determines the possibility of further H₂O₂ reduction to affect the measured selectivities. Enhancing the hydrophobicity of the catalytic layer can increase ΔG_*H2O2, leading to selectivity improvement, especially under high H₂O₂ production rates. In the gas diffusion electrode measurements, both ZnPc and CoPc MDEs with polytetrafluoroethylene (PTFE) exhibit low overpotentials, high selectivities, and good stability. This study provides guidelines for rational design of 2e⁻ ORR electrocatalysts.