(703f) Electrocatalysts Design for 2e? Oxygen Reduction Reaction

Electrochemical production of hydrogen peroxide (H2O2) via 2-electron oxygen reduction reaction (ORR) offers a promising approach for sustainable decentralized production of H2O2. However, ORR can go through either 4-electron pathway to water or 2-electron pathway to H2O2. The need for controllable modulation of ORR pathways, as well as the kinetic challenges, calls for the discovery and optimization of efficient catalysts. Herein, we focus on various design strategies of electrocatalyst towards 2-electron ORR, including (1) develop advanced electrocatalysts with high activity and selectivity for different application media, including alkaline, neutral, and acidic media; (2) elaborate ex-situ and in-situ characterization of electrocatalyst structure and electrochemical investigation; (3) reveal the underlying factors that contribute to the high performance and provide guidance for future design; (4) build up the gap between lab screening and practical application to facilitate its commercial utilization.

Our research starts with the electrocatalyst design for alkaline electrolyte. We demonstrate a promising carbon-based catalyst, consisting of oxygen-rich hollow mesoporous carbon spheres (HMCS). The as-prepared HMCS exhibit high onset potential (0.82 V) and half-wave potential (0.76 V) as well as excellent H2O2 selectivity (above 95%). The outstanding performance arises from a combination of several aspects, such as porous structure-facilitation of mass transport, large surface area, and proper distribution of oxygen-containing functional groups modification on the surface. Furthermore, the proposed ORR mechanism on HMCS surface reveals that -OH functional groups help promote the first electron transfer process while other oxygen modification facilitates the second electron transfer.

We designed a series of HMCS for the investigation of porosity engineering on the performance. The as-prepared HMCS samples present similar surface oxygen modification, structure property, and morphology but different surface pore sizes, making it an ideal research platform for studying the influence of mesoscale mass transport. It is revealed that in low current density conditions large surface area is preferred but the mass transport governs the performance in high current density region. On account of the favorable porous structure, HMCS-8 nm delivers the most excellent practical performance (166 mW cm-2) and performs well in the bifunctional Zn-air battery for the wastewater purification (70% RhB degraded after 2 min and 99% after 32 min).

Then we deepen our research to develop electrocatalysts for 2e− ORR in acidic media. We design a series of Au@Pd core@shell structures to investigate the influence of Pd 4d orbital overlapping degree on 2e– ORR performance. Density Functional Theory (DFT) calculations indicate that enhanced H2O2 selectivity and activity is achieved at Pdn clusters with n ≤ 3, and Pd clusters larger than Pd3 should be active for 4e– ORR. However, experimental results show that Au@Pd NWs with Pd4 as the primary structure exhibit the optimal H2O2 performance in acidic electrolyte with high mass activity (5.71 A mg-1 at 0.4 V) and H2O2 selectivity (nearly 95%). Thus we report that Pd4, instead of Pd3, is the upper threshold of Pd cluster size for an ideal 2e– ORR. It results from the oxygen coverage on catalyst surface under potentials, and such oxygen coverage phenomenon causes electron redistribution and weakened *OOH binding strength on active sites, leading to enhanced activity of Pd4 with only 0.06 V overpotential in acidic media.

Lastly, the influence of local coordination environment (LCE) manipulation was studied. We underscore the influence of LCE on directing the 2e− ORR, utilizing Pd cluster as a model catalyst. DFT calculations illustrate the role of first- and second-coordinated sulfur and oxygen in modulating the binding strength of oxygen intermediates. As guided by the DFT screening, the as-prepared Pdx/HMCS presents exceptional catalytic performance with a high mass activity of 4.06 A mg−1 at 0.45 V and selectivity above 94% over a wide potential range in 0.1 M HClO4 electrolyte. Based on the elaborated in-situ characterizations, we confirm that oxygen migrates from the carbon support (second coordination sphere) to the Pd cluster (first coordination sphere) to achieve oxygen coverage on the catalyst surface under potentials. As evidenced by DFT calculations, such an oxygen migration phenomenon as well as optimized first and second coordination environment give rise to outstanding performance.