The demand for hydrogen peroxide (H2O2) has surged in recent decades due to its widespread applications across various industries. The global market is projected to reach USD 4.0 billion by 2027, growing at an annual rate of 5.0%. Currently, industrial H2O2 production heavily relies on the anthraquinone process, which can yield high concentrations (up to 70%). However, this process is energy-intensive, involving multiple steps (hydrogenation, oxidation, extraction, and post-treatment), leading to significant waste and environmental concerns. Many applications, such as agricultural irrigation and disinfection, prefer lower concentrations (<6%) of H2O2. For instance, a 0.1% H2O2 solution demonstrates excellent bactericidal properties in water disinfection. The limitations of the anthraquinone process, including its energy consumption and environmental impact, motivate the search for sustainable and efficient alternatives. Electrochemical hydrogen peroxide production (EHPP) emerges as a promising solution, offering a greener and potentially more cost-effective approach. This method involves the two-electron oxygen reduction reaction (2e ORR), directly converting oxygen to H2O2. Transition metal-nitrogen-carbon (M-N-C) electrocatalysts have shown promise in EHPP, exhibiting considerable efficiency. However, several challenges hinder the optimization of these catalysts. Many M-N-C catalysts have the majority of the active transition metal (TM) species embedded within the bulk carbon material, limiting active site utilization, increasing costs, and potentially decreasing intrinsic activity. The high prices and potential supply risks associated with some 3d TMs also necessitate strategies for improved active metal utilization. Furthermore, the unclear electronic configuration and structural disorder in M-N-C catalysts make identifying active sites challenging, hindering the precise manipulation of electronic structures. While symmetrical square-planar M-N4 models are often proposed based on synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy, the limitations of fitting Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) data can lead to ambiguities or misinterpretations in distinguishing similar bond lengths. Therefore, a deeper understanding of the electronic structures and coordination environments of M-N-C catalysts is crucial to establish clear structure-performance relationships and improve the efficiency of EHPP. This research addresses these challenges by developing and characterizing a novel electrocatalyst to enhance EHPP in neutral media.

Extensive research has been conducted on the electrochemical production of hydrogen peroxide (H2O2), focusing on various catalyst materials and reaction conditions. Single-atom catalysts (SACs), particularly those based on transition metals coordinated with nitrogen and carbon (M-N-C), have shown remarkable promise due to their high activity and selectivity. Studies have explored various 3d transition metals (Mn, Fe, Co, Ni, Cu) incorporated into M-N-C structures for EHPP. These investigations have often highlighted the importance of the metal's electronic structure and coordination environment in determining catalytic activity and selectivity. The square-planar Co-N4 configuration has been widely studied, but its limitations in achieving high H2O2 selectivity, especially in neutral media, are well-documented. The impact of different ligands and the surrounding environment on the catalytic performance of SACs has also been a focus of research. However, a comprehensive understanding of the structure-activity relationships in neutral media, where many practical applications lie, is lacking. Previous work often relies on model fitting procedures for XAFS data which can be ambiguous when multiple similar coordination schemes are present. Moreover, the precise determination of the active sites and their correlation with the overall performance remains a critical challenge, which necessitates novel approaches in both catalyst design and characterization. Several reports have demonstrated the use of different synthesis strategies to control active sites, improve metal utilization, and enhance the overall performance of M-N-C catalysts. However, there is still a need for more efficient and sustainable methods that can be easily scaled up for practical applications. The present study tackles these challenges by adopting a surface engineering strategy combined with advanced characterization techniques, and comprehensive DFT calculations to uncover the structure-performance relationship and provide a more accurate understanding of EHPP.

This research employed a surface engineering strategy to synthesize a series of M-N-C electrocatalysts with ultralow metal loadings (<0.1 wt%). The process started with the oxidation of Ketjenblack EC300J carbon black (CB) using 35 wt% HNO3 to create oxidized carbon black (OCB). For the synthesis of CoNCB, 100 mg of OCB was dispersed in deionized water and sonicated. Then, a solution of Co(NO3)2·6H2O was added, followed by further sonication and freeze-drying. The resulting powder was annealed in NH3 at 850 °C for 1 h, and the product was treated with 4 M HCl and washed to obtain the final CoNCB catalyst. MnNCB, FeNCB, NiNCB, and CuNCB were prepared similarly using their respective metal nitrate precursors. A nitrogen-doped carbon black (NCB) was synthesized as a control sample without any metal precursor. The materials were extensively characterized using various techniques. Aberration-corrected scanning transmission electron microscopy (STEM) in annular bright-field (ABF) and high-angle annular dark-field (HAADF) modes were employed to visualize the morphology and atomic-level structure of the catalysts. The metal content was determined using microwave plasma atomic emission spectroscopy (MP-AES). Synchrotron radiation-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, in total electron yield (TEY) mode, was utilized to investigate the electronic structure of surface components at the O K-edge, N K-edge, and Co L-edge. Co K-edge X-ray absorption spectroscopy (XAS) measurements were performed in transmission and fluorescence modes to analyze the electronic and geometric structure of Co species at the bulk level. Data analysis included wavelet transform EXAFS (WT-EXAFS) and fitting of the FT-EXAFS spectra to determine coordination environment. The electrochemical performance was evaluated using a rotating ring-disk electrode (RRDE) system in O2-saturated 0.1 M phosphate buffer solution (PBS, pH = 7.0). ORR polarization curves, H2O2 selectivity calculations, and Tafel analysis were conducted to assess the electrocatalytic activity and selectivity. The stability of CoNCB was evaluated through accelerated durability testing (ADT). Finally, the practical EHPP performance of CoNCB was evaluated using a flow cell, with H2O2 production quantified by cerium sulfate titration and UV-vis spectroscopy. Density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) to investigate the electronic structure and predict the catalytic activity of various Co-C/N/O configurations, including the simulation of Co K-edge XANES spectra using the FDMNES program. The adsorption energy of OOH* intermediate was used to correlate with catalytic activity.

The study's key findings center around the superior performance of the CoNCB electrocatalyst with an asymmetric Co-C/N/O configuration for EHPP in neutral media. HAADF-STEM imaging confirmed the atomic dispersion of Co atoms primarily anchored on the distorted surface of the carbon black support, maximizing active site exposure. Analysis of NEXAFS and XAS data, particularly the Co L-edge and Co K-edge spectra, revealed an asymmetric electronic configuration for the Co-C/N/O species, distinguishing it from the more symmetrical Co-N4 configuration. The electrochemical measurements showed CoNCB exhibiting an outstanding mass activity of 6.1 × 10⁵ A gCo⁻¹ at 0.5 V vs. RHE and a high H2O2 production rate of 4.72 molcatalyst⁻¹ h⁻¹ cm⁻². The H2O2 selectivity was over 95% in the potential range of 0.45–0.75 V. The Tafel slope analysis (62 mV dec⁻¹) indicated a fast kinetic activity. Accelerated durability testing demonstrated good stability. Flow cell testing confirmed the practical viability of CoNCB, achieving a H2O2 production rate of 4.72 mol gcatalyst⁻¹ h⁻¹ cm⁻² with a Faradaic efficiency over 60%. DFT calculations supported the experimental findings. The calculations showed a positive correlation between the asymmetry of the Co-C/N/O configuration, the off-center distance of the Co atom, and the adsorption energy of the OOH* intermediate. This stronger OOH* binding energy, particularly in the asymmetric configurations, was linked to enhanced H2O2 selectivity and production rates compared to the square-planar symmetric Co-N4 model.

The results of this study demonstrate the significant advantage of using an asymmetric Co-C/N/O configuration in CoNCB for EHPP in neutral media. The superior performance observed experimentally is strongly supported by the DFT calculations, which reveal a direct correlation between the asymmetry of the Co coordination environment and the adsorption energy of the OOH* intermediate. The stronger binding of OOH* in the asymmetric configurations promotes the two-electron pathway of ORR, leading to higher H2O2 selectivity. The findings challenge the widely accepted view of the square-planar symmetric Co-N4 configuration as the optimal structure for EHPP catalysts and highlight the potential of exploring asymmetric coordination environments for enhancing catalytic activity and selectivity. The achieved mass activity and H2O2 production rate significantly outperform previously reported Co-N-C catalysts, indicating the potential for practical application in various industries. The study's approach, combining advanced characterization techniques with detailed DFT calculations, provides a robust methodology for investigating the structure-activity relationship in M-N-C electrocatalysts, paving the way for future design and optimization of high-performance EHPP catalysts.

This research successfully synthesized and characterized an atomically dispersed cobalt electrocatalyst (CoNCB) with an asymmetric Co-C/N/O configuration that demonstrates exceptional activity and selectivity for electrochemical hydrogen peroxide production in neutral media. The superior performance is attributed to the unique asymmetric coordination environment, which facilitates stronger adsorption of the OOH* intermediate, favoring the two-electron pathway of oxygen reduction. The findings highlight the potential of exploring asymmetric coordination geometries in the design of high-performance EHPP catalysts. Future research could explore other transition metals and ligand combinations to further optimize the catalytic performance and explore different carbon supports to improve the stability and long-term durability of the electrocatalyst. Investigating the effects of various reaction parameters such as pH and temperature will also provide further insights into the mechanism and optimization of EHPP.

While the study provides strong evidence for the superior performance of CoNCB, there are some limitations. The exact determination of the Co-C/N/O coordination environment remains challenging due to the complexity of the system and the limitations of EXAFS fitting in distinguishing C, N, and O atoms with similar bond lengths. Although DFT calculations provided strong supporting evidence, the computational models are approximations of the real catalytic active sites. Long-term stability tests might need to be extended to ensure the long-term stability and prevent any unforeseen degradation under continuous operation. The flow cell experiments, while demonstrating practical viability, were conducted under specific conditions, and further studies are necessary to evaluate the catalyst’s performance under different operational parameters and conditions.