Combining Experiment and Theory To Unravel the Mechanism of Two-Electron Oxygen Reduction at a Selective and Active Co-catalyst

Abstract

We present a combination of comprehensive experimental and theoretical evidence to unravel the mechanism of two-electron oxygen reduction reaction (ORR) on a catalyst composed of mildly reduced graphene oxide supported on P50 carbon paper (mrGO/P50). This catalyst is unique in that it shows >99% selectivity toward H2O2, the highest mass activity to date, and essentially zero overpotential in base. Furthermore, the mrGO catalytically active site is unambiguously identified and presents a unique opportunity to investigate mechanisms of carbon-based catalysis in atomistic detail. A wide range of experiments at varying pH are reported: ORR onset potential, Tafel slopes, H/D kinetic isotope effects, and O2 reaction order. With DFT reaction energies and known thermodynamic parameters, we calculate the potential and pH-dependent free energies of all possible intermediates in this ORR and propose simple kinetic models that give semiquantitative agreement with all experiments. Our results show that mrGO is semiconducting and cannot support the conventional mechanism of coherently coupled proton–electron transfers. The conducting P50 provides electrons for initiating the ORR via outer sphere electron transfer to O2(aq), while the semiconducting mrGO provides the active catalytic sites for adsorption of O2 –(aq) or HO2(aq), depending upon electrolyte pH. Due to this unique synergistic effect, we describe the mrGO/P50 as a co-catalyst. This concept implies departure from the traditional picture of predicting catalytic activity trends based on a single descriptor, and the co-catalyst design strategy may generally enable other semiconductors to function as electrocatalysts as well.

Energetic Control of Redox‐Active Polymers toward Safe Organic Bioelectronic Materials

“…The four‐electron reaction (O 2 + 2H 2 O + 4e − → 4OH − , E 0 = 1.23 V versus reversible hydrogen electrode (RHE), pH ≥ 7) is thermodynamically favorable over the two‐electron reaction ( E 0 = 0.76 V versus RHE, pH > 7) as shown in Figure . However, for some classes of materials, including organic molecules, [ 23 ] the reduction of oxygen can proceed through different pathways which may terminate at the production of dissolved H 2 O 2 instead of the complete reduction to water. The electrochemical overpotential for the ORR is the additional voltage beyond the equilibrium voltage required to drive the reaction at a given rate.…”

“…When the reaction proceeds via the four‐electron process, a strong pH dependence was observed, [ 22 ] while a low pH dependence is observed for the two‐electron process. [ 23 ] While many studies suggest that nitrogen functionalities are important for four‐electron reduction, we note that the role of heteroatom doping in directing the pathway is still unclear. [ 22 ]…”

“…[ 36 ] For the chemical doping of DPP‐based donor–acceptor copolymers with IP >5.2 eV, specially designed dopants with large electron affinities (EA) are required to achieve a high degree of charging. [ 37 ] To properly quantify the relative likelihood of the oxidation reactions of the polymers, the thermodynamic driving forces could be worked out, for example, using DFT calculations [ 23,38 ] if the precise reaction mechanism were known. While the polyanion PSS − in PEDOT:PSS might affect the rate of oxidation, since it is known to be a good proton conductor, [ 39 ] we believe that the rapid oxidation of PEDOT:PSS arises from its high‐lying HOMO.…”

2D Electrocatalysts for Converting Earth‐Abundant Simple Molecules into Value‐Added Commodity Chemicals: Recent Progress and Perspectives

“…As illustrated in Figure b, a 2e‐ORR pathway through superoxide‐mediated mechanism to form H 2 O 2 is also possible . Although this reaction pathway was applied by Luntz and co‐workers to successfully explain the superior 2e‐ORR activity of an electrocatalyst based on P50 carbon paper supporting rGO, it is still somewhat elusive and under debate. Further research is therefore urgently needed to clarify the 2e‐ORR pathways when different electrocatalysts and reaction conditions are used.…”

Grand canonical rate theory for electrochemical and electrocatalytic systems I: General formulation and proton-coupled electron transfer reactions

“…Recent experiments of ORR on carbon-based materials conclusively demonstrate that ET is the rate-and potential-determining step. [40,41]. Also solution pH can alter the reaction mechanism and e.g.…”