Elucidating oxygen reduction reaction over the full-pH range: a synthesis-active sites-performance trilogy of Fe–Nx–Cs

Abstract

Iron–nitrogen–carbons (Fe–N_x–Cs) are among the most studied platinum group metal-free (PGM-free) electrocatalysts for oxygen reduction reaction (ORR). However, detailed and comprehensive studies of ORR activity and selectivity along the whole pH range, considering the possible influence of morphology and surface chemistry, are currently lacking in the literature. Herein, four Fe–N_x–Cs electrocatalysts synthesized with different methodologies and displaying different morphological and physicochemical features were tested for ORR with a rotating ring disk electrode (RRDE) in the whole pH range. The trends of onset potential (E_on), half-wave potential (E_1/2), peroxide yield, number of transferred electrons (n), charge transfer coefficient (α) and logarithm of kinetic current densities (logJ_k) along the pH scale were reported. Among the electrocatalysts, both unique behaviors and common electrochemical trends were identified, each characterized by varying rates of change. The occurrence of Fe agglomeration, the surface area and chemistry were found to influence the trends of these physicochemical quantities, giving rise to differences among the tested electrocatalysts. Therefore, the study concluded that the ORR electrocatalysts investigated possess different morphological and physicochemical properties developed during the distinct synthesis processes. Although similar electrochemical activity patterns were exhibited by the samples under analysis, differences in the rate of variations within such trends were noticed, signifying modulations in the reaction kinetics or mechanistic pathways due to contrasting morphological and physicochemical characteristics. This can eventually suggest the possibility of selecting an appropriate electrocatalyst for operating at a specific pH.