What Is the Rate-Limiting Step of Oxygen Reduction Reaction on Fe-N-C Catalysts?

Oxygen reduction reaction (ORR) is essential to various renewable energy technologies. An important catalyst for ORR is single iron atoms embedded in nitrogen-doped graphene (Fe-N-C). However, the rate-limiting step of the ORR on Fe-N-C is unknown, significantly impeding understanding and improvement. Here, we report the activation energies of all of the steps, calculated by ab initio molecular dynamics simulations under constant electrode potential. In contrast to the common belief that a hydrogenation step limits the reaction rate, we find that the rate-limiting step is oxygen molecule replacing adsorbed water on Fe. This occurs through concerted motion of H2O desorption and O-2 adsorption, without leaving the site bare. Interestingly, despite being an apparent thermal process that is often considered to be potential-independent, the barrier reduces with the electrode potential. This can be explained by stronger Fe-O-2 binding and weaker Fe-H2O binding at a lower potential, due to O-2 gaining electrons and H2O donating electrons to the catalyst. Our study offers new insights into the ORR on Fe-N-C and highlights the importance of kinetic studies in heterogeneous electrochemistry.

Automated summary

This study calculates the activation energies of each step of oxygen reduction reaction (ORR) on Fe-N-C and finds that the rate-limiting step is the replacement of oxygen molecule for adsorbed water on Fe. Interestingly, the barrier reduces with the decrease of electrode potential, which can be explained by the stronger Fe-O-2 binding and weaker Fe-H2O binding.