Hydrogen peroxide (H2O2), a top 100 global chemical, finds extensive use in various chemical, biological, and industrial processes. Current industrial production relies on high-temperature, high-pressure methods, leading to high costs and significant waste byproducts. Therefore, cost-effective and eco-friendly H2O2 production methods are urgently needed. Electrosynthesis of H2O2 via a two-electron pathway oxygen reduction reaction (ORR) presents a promising alternative. While research has focused on enhancing the activity and selectivity of electrocatalysts for this process, sluggish reactivity and low selectivity remain significant obstacles. Carbon-based single-metal-atom catalysts (SACs) are attracting considerable attention due to their low coordination environment and high catalytic potential. However, most SACs utilize N-doped carbon supports, which tend to favor the four-electron ORR pathway, thus being unsuitable for H2O2 electrosynthesis. Oxidized carbon materials, with their oxygen functional groups (COOH and C-O-C), have shown promise in promoting the two-electron ORR pathway. This study explores the potential synergistic effects of anchoring SACs on oxidized carbon supports to enhance H2O2 electrosynthesis. The researchers hypothesized that combining the advantages of single-atom catalysts with the inherent selectivity of oxygen-functionalized carbon materials would lead to a highly efficient and selective electrocatalyst for H2O2 production.

The literature extensively covers the challenges and opportunities in electrochemical H2O2 production. Studies have highlighted the need for catalysts that can efficiently and selectively drive the two-electron ORR pathway, avoiding the less desirable four-electron pathway that produces water. Numerous materials have been explored, including metal nanoparticles and metal-organic frameworks. However, achieving high selectivity and activity simultaneously has proven difficult. The use of single-atom catalysts has emerged as a promising approach, offering precise control over the catalytic site and electronic structure. However, the choice of support material is crucial, as it significantly impacts the catalyst's performance and selectivity. Oxidized carbon materials, particularly graphene oxide with its diverse oxygen functional groups, have demonstrated potential in selectively guiding the ORR towards H2O2 formation. This existing knowledge forms the basis for the current investigation into the development of a highly efficient electrocatalyst by combining the advantages of single-atom cobalt catalysts and oxygen-rich graphene oxide supports.

The researchers synthesized Co1@GO through a multi-step process. Initially, Co nanoparticles (Co NPs/GO) were prepared by reducing CoCl2 on graphene oxide in a 5 vol% H2/N2 atmosphere at 300 °C. Subsequently, acid leaching with 2 M HCl and 2 M HNO3 was employed to etch the Co nanoparticles into single atoms, stabilizing them through bonding with the oxygen functional groups of the graphene oxide. The resulting Co1@GO was characterized using various techniques, including high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). These characterization methods allowed for the confirmation of the atomic dispersion of cobalt atoms on the graphene oxide support, the identification of Co-O-C active centers, and the determination of the oxidation state of the cobalt atoms. Electrochemical measurements were performed using a three-electrode system with a rotating ring-disk electrode (RRDE) in O2-saturated 0.1 M KOH solution. Linear sweep voltammetry (LSV) was used to evaluate the ORR activity and selectivity, with the Pt ring electrode monitoring H2O2 production. The selectivity, yield, and electron transfer number were calculated from the disk and ring currents. Stability tests were conducted using chronoamperometry. Bulk H2O2 production was evaluated in an H-cell electrolyzer using Co1@GO as the cathode and Pt foil as the anode. The H2O2 concentration was determined using a Ce(SO4)2 titration based colorimetric method. Density functional theory (DFT) calculations were performed to investigate the catalytic mechanism, determining free energies, overpotentials, and reaction pathways for ORR on different Co-O structures. The Vienna Ab initio Simulation Package (VASP) was employed for the DFT calculations.

The synthesized Co1@GO electrocatalyst demonstrated exceptional performance in H2O2 production. HAADF-STEM images confirmed the atomic dispersion of cobalt atoms on the graphene oxide support, with no nanoparticles or clusters observed. XPS and XAS analysis revealed the presence of Co-O-C active centers and indicated that the cobalt atoms were in a high oxidation state (+2 to +3). The electrocatalytic measurements revealed a significantly enhanced ORR activity with an onset potential of 0.91 V versus RHE and a H2O2 production rate of 1.0 mg cm−2 h−1. This represents a 19-fold improvement compared to graphene oxide and a 2-fold enhancement compared to Co nanoparticles/graphene oxide. The H2O2 selectivity of Co1@GO was 81.4% at 0.6 V. The Co1@GO catalyst also maintained its high performance and stability in a two-electrode system, exhibiting a steady-state H2O2 production rate of ~5.7 mol g−1 h−1 or 28 mol m−2 h−1. DFT calculations identified the Co-O3-C structure as the most likely active center, exhibiting a low overpotential (0.06 V) for the two-electron ORR pathway. This theoretical finding supports the experimental observation of high reactivity and selectivity for H2O2 electrogeneration. The calculations also revealed that the high oxidation state of the cobalt atoms and the presence of oxygen functional groups in the Co-O-C active center contribute to the superior catalytic activity and selectivity. Electron transfer number measurements indicated that the ORR predominantly followed a two-electron pathway for all samples (Co1@GO, Co NPs/GO, Co clusters/GO, and GO), confirming that the Co-O-C center promotes this preferred pathway for H2O2 production.

The results demonstrate the successful development of a high-performance electrocatalyst for H2O2 production through the rational design of single-atom cobalt catalysts supported on graphene oxide. The synergistic interaction between the high oxidation state of single cobalt atoms and the oxygen functional groups of the graphene oxide creates a unique Co-O-C active center that promotes the two-electron ORR pathway, resulting in significantly enhanced activity and selectivity. The findings address the long-standing challenge of low selectivity and sluggish kinetics in electrochemical H2O2 production. The high activity and selectivity of Co1@GO, surpassing previously reported catalysts, highlight the potential of this strategy for developing advanced electrocatalysts for various applications. The combined experimental and theoretical investigations provide a comprehensive understanding of the structure-activity relationship in this catalyst, offering valuable insights for future catalyst design.

This research successfully demonstrated the synthesis and application of a novel electrocatalyst, Co1@GO, for highly efficient and selective H2O2 electrosynthesis. The catalyst's superior performance is attributed to the unique Co-O-C active centers, showcasing a synergistic effect between single cobalt atoms and oxygen functional groups on the graphene oxide support. Future work could explore the scalability of the synthesis method, investigate other single-atom catalysts and support materials, and investigate the catalyst's performance in different electrolytes and under various operating conditions.

While the Co1@GO catalyst shows remarkable performance, further investigation is warranted to explore its long-term stability under continuous operation. The DFT calculations, while providing valuable insights, rely on certain assumptions and approximations, and future studies could use more sophisticated computational methods to refine the understanding of the catalytic mechanism. Additionally, the study primarily focused on alkaline conditions; the catalyst's performance in acidic or neutral media requires further investigation for broader applicability.