Elucidating Proton Involvement in the Rate-Determining Step for Pt/Pd-Based and Non-Precious-Metal Oxygen Reduction Reaction Catalysts Using the Kinetic Isotope Effect

Abstract

The development of non-precious-metal (NPM) catalysts to replace the Pt alloys currently used in fuel cells to facilitate the oxygen reduction reaction (ORR) is a vital step in the widespread utilization of fuel cells. Currently, the ORR mechanism for NPM catalysts is not well understood, prohibiting the design and preparation of improved NPM catalysts. We conducted a kinetic isotope effect (KIE) study to identify the rate-determining step (RDS) of this intricate electrocatalytic reaction involving multiple proton-coupled electron transfer (PCET) processes. We observed a KIE of about 2 for the ORR catalyzed by a NPM catalyst, which demonstrates that for these electrocatalysts protons are involved in the RDS during ORR. These results contribute to a more complete understanding of the ORR mechanism and suggest that the design of future NPM catalysts must include careful consideration of the role of protons during ORR.

Kinetic Isotope Effect as a Tool To Investigate the Oxygen Reduction Reaction on Pt‐based Electrocatalysts – Part I: High‐loading Pt/C and Pt Extended Surface

“…As the ORR rate‐determining step is known to be, on Pt surfaces and nanostructures, the O 2 +H + +e − →OOH ads step, it indicates that the limiting elementary step should either be the oxygen adsorption (O 2 →O 2,ads ) or an electron transfer without a coupled proton transfer (O 2,ads +e − →O 2,ads − ). The result is in agreement with previous studies of ORR kinetic isotope effect on platinum, such as Yeager's group original work in phosphoric acid and Gewirth's group more recent work . While the kinetic isotope effect under these specific conditions is defined, future work will investigate low platinum loading electrocatalysts, where inter‐particle spacing, and support interactions can be relevant.…”

“…similarly measured an appreciable KIE in acidic conditions and a KIE of unity in alkaline conditions for nitridated carbon (NC) catalysts . Another recent study included both platinum and non‐platinum group catalysts and calculated KIE using the Koutecky‐Levich equation with reduction currents limited by mass transport rather than a kinetic model at potentials approaching the standard reduction potential . A KIE of approximately unity was reported for the ORR on platinum in a sulfuric acid electrolyte by this method, however, no measurements as of now have been conducted in perchloric, which poisons Pt to a much lower extent.…”

Quantum-to-Classical Transition of Proton Transfer in Potential-Induced Dioxygen Reduction

“…where j 0 , C 0 , α, η, F , R, and T are exchange current density, oxygen concentration, transfer . Furthermore, in order to avoid unknown liquid junction effects due to the use of reference electrodes such as an Ag/AgCl electrode [23,24], we used a reversible hydrogen or deuterium electrodes by following the protocol of Yeager and his coworkers [25]. Prior to discuss the dioxygen reduc- The C D 0 /C H 0 is known to be 1.101.…”

Voltammetric H/D Isotope Effects on Redox‐Active Small Molecules Conjugated with DNA Self‐Assembled Monolayers

“…[7] Hydrogen tunneling processes have been discussed to explain such large KIEs. [7,8] Relatively small KIEs have been used to study the electrode reaction mechanisms of the reduction of oxygen [9,10] and furfural. [11] In medicinal chemistry and biochemistry, H/D isotope effects have been utilized to investigate the reaction mechanisms of enzymes.…”