Kinetic isotope effect in the oxygen reduction reaction (ORR) over Fe-N/C catalysts under acidic and alkaline conditions

•Kinetic isotope effect measured on non-precious metal (Fe-C,N) catalysts for the oxygen reduction reaction.•Correction made for different equilibrium potentials of oxygen reduction reaction to form D2O and H2O.•A kinetic isotope effect of ~ 3.4 is measured in acid suggesting predominantly inner sphere electron transfer.•A kinetic isotope effect of ~ 2.5 is measured in alkaline suggesting a contribution from outer sphere transfer.

Heat treated Fe-N/C materials which are highly effective oxygen reduction catalysts in alkaline and acid, show a significant kinetic isotope effect (KIE). The values in acid (~ 3.4) and alkaline (~ 2.5) are much larger than the value for the metal free catalyst in acid (~ 1.8) suggesting that the rate determining step (RDS) is a proton coupled electron transfer in acid with a significant pathway involving a proton independent step under an alkaline environment.

Graphical abstractDownload high-res image (218KB)Download full-size image