Reply to the ‘Comment on “Non-PGM electrocatalysts for PEM fuel cells: effect of fluorination on the activity and stability of a highly active NC_Ar + NH3 catalyst”’ by Xi Yin, Edward F. Holby and Piotr Zelenay, Energy Environ. Sci., 10.1039/D0EE02069A

Abstract

In their Comment (X. Yin, E. F. Holby and P. Zelenay, Energy Environ. Sci., the comment.) entitled “Kinetic degradation models of Fe–N–C electrocatalysts for the oxygen reduction reaction (ORR) and mechanistic implications”, Yin, Holby and Zelenay declared that the INRS model, analyzing and interpreting the instability of our NC_Ar + NH₃ highly active, high performance, but unstable Fe/N/C catalyst in a PEM fuel cell, contains several essential misconceptions that weaken the validity of the conclusions regarding the degradation mechanisms that we might draw from our model. In particular, they address two main issues that they believe are not valid: (i) the inclusion of non-kinetic effects in kinetic modeling, which, according to them, will prevent us from obtaining true kinetic information about the loss of FeN₄ active sites when the decay of the current density of the catalyst is measured at 0.6 V in PEM fuel cells; and (ii) inconsistencies in the thermodynamics underpinning the proposed degradation mechanisms. Here, according to their Comment, the cohesion energy of bulk (metal) iron would be disregarded in our thermodynamic calculations of the demetallation equilibrium constant (K_c) of FeN₄ sites in the acid medium of PEM fuel cells. This would lead us to false conclusions about the K_c values determined for all FeN₄ demetallation reactions that were considered in our model. Four of our recent publications are targeted in this Comment: two main ones, published in Energy Environ. Sci., 2018, 11, 365–382, and 2019, 12, 3015–3037; and two satellite ones, published in J. Electrochemical Society, 2017, 164, F948–F957 and 2019, 166, F3277–F3286. In this reply to their comment, we demonstrate that the conclusions drawn in the four targeted publications remain valid and that the arguments expressed in the Comment are unfounded. The fast decay in PEM fuel cells of a highly active, high performance, but unstable Fe/N/C catalyst like our NC_Ar + NH₃ does not follow an electrochemical but a chemical mechanism which is governed (in conjunction with Le Chatelier's principle) by the thermodynamic equilibrium constant (K_c) of the demetallation reaction of its ORR active FeN₄ sites in the catalyst micropores.