Understanding the activity origin and mechanisms of the oxygen reduction reaction on the tetramethyl metalloporphyrin/MoS₂ electrocatalyst

The efficiency of the oxygen reduction reaction (ORR) on the cathode plays a crucial role in determining the performance of proton exchange membrane fuel cells. Porphyrin, distinguished by its cost-effectiveness, eco-friendly nature, and efficient utilization of its metal, stands out as a promising candidate for a metal single-atom catalyst in fuel cell cathodes. The metal and support modifications significantly impact the porphyrin's ORR activity. Nevertheless, the effects of Ni, Co, and Fe metals in tetramethyl metalloporphyrin/MoS₂, named MeTMP/MoS₂, catalyst on the mechanisms and activity of the ORR remain unknown. This study elucidates the topic using van der Waals dispersion-corrected density functional theory (DFT) calculations and thermodynamic model. Results showed that the rate-limiting step is located at the first and second hydrogenation steps in the associative mechanisms for Ni and Co (Fe) substitutions, respectively. For the dissociative mechanisms, the dissociation of molecular oxygen to two oxygen atoms is the rate-determining step on all the NiTMP/MoS₂, CoTMP/MoS₂, and FeTMP/MoS₂ catalysts. The presence of the MoS₂ support significantly reduces the thermodynamic activation barrier of the ORR, and hence improves the ORR activity in the dissociative mechanisms. This activation barrier is 3.45, 0.92, and 1.82 eV for NiTMP/MoS₂, CoTMP/MoS₂, and FeTMP/MoS₂, which is much better compared to 4.85, 3.34, and 2.19 eV for NiTMP, CoTMP, and FeTMP, respectively. CoTMP/MoS₂ is the best candidate among the considered catalysts for the ORR. Furthermore, we provide a detailed explanation of the physical insights into the interaction between the ORR intermediates and the catalysts.