The selective electrosynthesis of hydrogen peroxide necessitates oxygen reduction reaction to be halted at the two-electron level, despite the fact that many Co-based catalysts facilitate further oxygen reduction beyond the activation of molecular oxygen. Here we consider oxygen-modified graphene oxide as a ligand-abundant carbon substrate for confinement of single cobalt atoms and regulation of their reactivity to afford peroxide liberation in alkaline media. The main objective is to discern which factor accounts for the improved catalytic performance: catalyst loading, residual cobalt nanoparticles, or atomic-level Co-O-C coordination sites. Catalyst series composed of graphene oxide, cobalt nanoparticles supported on graphene oxide, acid-treated cobalt clusters on graphene oxide, and Co${}_1$@GO are characterized with the help of electron microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, Co K-edge X-ray absorption spectroscopy, rotating ring-disk voltammetry, hydrogen peroxide production in H-cell and density-functional-theory calculations. Sequential acid leaching converts ca. 1.9 nm cobalt nanoparticles to single cobalt atoms immobilized in oxygen-coordination environment, while retaining approximately 1.8 wt\% of cobalt content. The resulting catalyst lacks any detectable Co-Co interaction in the range of 2.0-3.0~\AA{} of EXAFS spectrum, has a Co-O signal at 1.47~\AA{} and features oxygen-rich surface with the composition of C--O--C:C--O--H:C=O equal to 2.3:1.5:1. In oxygen saturated 0.1 M KOH electrolyte, Co${}_1$@GO catalyst reaches an onset potential of 0.91 V versus RHE, a peroxide selectivity of 81.4\% at 0.60 V, and a hydrogen peroxide production rate of \SI{1.0}{\milli\gram\per\centi\meter\squared\per\hour} at 0.50 V. The production rate is almost nineteen times higher compared to graphene oxide and twice as high as that of cobalt nanoparticles on graphene oxide. After 36,000 s of electrocatalytic cycling, Co${}_1$@GO maintains disk and ring currents of 0.32 and 0.024 mA, respectively, whereas peroxide production rate via H-cell electrolysis is estimated to be \SI{5.7}{\mole\per\gram\per\hour} and \SI{28}{\mole\per\meter\squared\per\hour}. Computation predicts Co--O${}_3$--C to be an appropriate active site for formation energy $-2.78$ eV, two-electron reduction potential 0.06 V, and four-electron reduction potential 1.07 V. The results confirm that oxygen-modulated single cobalt atoms, not cobalt clusters, achieve the rate-selectivity control required for peroxide electrosynthesis in an alkaline environment.

hydrogen peroxide,two-electron oxygen reduction,graphene oxide,single-atom catalyst,Co--O--C coordination,RRDE; EXAFS,density-functional theory