Microwave-Enabled Incorporation of Single Atomic Cu Catalytic Sites in Holey Graphene: Unifying Structural Requirements of a Carbon Matrix for Simultaneous Achievement of High Activity and Long-Term Durability

This work reports our discoveries from the first exploration in microwave pyrolysis of a metal-organic framework. A time- and energy-efficient approach was developed for direct fabrication of electrochemical single-atom catalysts (E-SACs) without the requirement of post-treatment. The most unique structure of the fabricated E-SAC is that the Cu catalytic sites were not in the amorphous carbon matrix as those achieved via traditional pyrolysis but in the basal planes of pristine holey graphene nanoplatelets. The as-prepared Cu-E-SAC exhibits excellent catalytic activity and selectivity in reducing oxygen to water in both acidic and alkaline media. The desired direct 4e(-) pathway is more favorable in acidic versus alkaline media, which is different from all the Cu-E-SACs reported so far and most transition-metal-based E-SACs. The superior performance is attributed to the unique structure of the catalytic sites. The large graphene domains in the holey graphene materials provide higher delocalized electron-rich pi band and increase the d-orbital energy level of the Cu centers. Consequently, their binding strength for molecular oxygen is largely enhanced, improving the oxygen reduction reaction and likely promoting a direct 4e(-) pathway with minimized generation of a peroxide byproduct. Considering the high conductivity and excellent stability against oxidation of the holey graphene material, this work, for the first time, suggests that the contradictory structural requirement of a carbon matrix for high catalytic activity and long-term durability can be unified and simultaneously satisfied. Combined with the merits of simplicity and rapidness for fabricating both holey graphene and E-SACs, this work provides a possible strategy to address the critical challenges of precious metal-free single-atom catalysts.