Mechanisms of Two-Electron and Four-Electron Electrochemical Oxygen Reduction Reactions at Nitrogen-Doped Reduced Graphene Oxide

Abstract

Doped carbon-based systems have been extensively studied over the past decade as active electrocatalysts for both the two-electron (2e-) and four-electron (4e-) oxygen reduction reaction (ORR). However, the mechanisms for ORR are generally poorly understood. Here we report an extensive experimental and first-principles theoretical study of the ORR at nitrogen-doped reduced graphene oxides (NrGO). We synthesize three distinct NrGO catalysts and investigate their chemical and structural properties in detail via X-ray photoelectron spectroscopy, infrared and Raman spectroscopy, high-resolution transmission electron microscopy and thin-film electrical conductivity. ORR experiments include the pH dependences of 2e-versus 4e-ORR selectivity, ORR onset potentials, Tafel slopes and H/D kinetic isotope effects. These experiments show very different ORR behavior for the three catalysts, both in terms of selectivity and the underlying mechanism which proceeds either via coupled proton-electron transfers (CPETs) or non-CPETs. Reasonable structural models developed from DFT rationalize this behavior. The key determinant between CPET vs. non-CPET mechanisms is the electron density at the Fermi level under operating ORR conditions. Regardless of the reaction mechanism or electrolyte pH, however, we identify the ORR active sites as sp2 carbons that are located next to oxide regions. This assignment highlights the importance of oxygen functional groups, while details of (modest) N-doping may still affect the overall catalytic activity, and likely also the selectivity, by modifying the general chemical environment around the active site. File list (2) download file view on ChemRxiv NrGO_ACS_Catal_updated.pdf (3.76 MiB) download file view on ChemRxiv NrGO_SI_ACS_Catal_updated.pdf (6.66 MiB)

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

“…The ORR yields either H 2 O 2 (two‐electron process) [ 20 ] or water (H 2 O) (four‐electron process), [ 21,22 ] as well as charging (oxidation) of the OMIEC that acts as the catalyst. 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 .…”

“…We note that the mechanism of the ORR is more complicated than described in the simplified picture and can proceed via several intermediates. [ 22 ]…”

“…In this regard, the rate for the ORR of organic molecules was reported to depend on the non‐carbon heteroatom composition of the polymer and the pH of the electrolyte. 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.…”

“…[ 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 ]…”

Recent Progress of Single‐atom Catalysts in the Electrocatalytic Reduction of Oxygen to Hydrogen Peroxide

“…In general, WOR involves rather simple experimental conditions, without the need of a gas diffusion electrode; yet it requires an ultra-high overpotential, and currently there is a lack of effective electrocatalysts with high selectivity and stability [12][13][14][15]. Therefore, electrocatalytic H 2 O 2 production primarily relies on two-electron ORR [16,17].…”