Atomically Dispersed High-Active Site Density Copper Electrocatalyst for the Reduction of Oxygen.

Enlarging the M-Nx active-site density is an effective route to enhance the ORR performance of M-N-C catalysts. In this work, a single-atom catalyst Cu-N@Cu-N-C with enlarged Cu-N active site density was prepared by the second doping and pyrolysis (SDP) of Cu-N-C derived from Cu-doped zeolite imidazole frameworks. The half-wave potentials of Cu-N@Cu-N-C were measured as 0.85 V in alkaline electrolyte and 0.75 V in acidic media, which was 50 mV and 60 mV higher than that of Cu-N-C, respectively. N adsorption-desorption isotherm curves and corresponding pore distribution analysis were used to verify the successful filling of additional Cu and N in micropores of Cu-N-C after SDP. The obvious increase in Cu contents for Cu-N@Cu-N-C (1.92 wt%) compared with Cu-N-C (0.88 wt%) tested by ICP demonstrated the successful doping of Cu into Cu-N-C. XAFS analysis confirmed the presence of Cu-N single-atom active centers in Cu-N@Cu-N-C. The N 1 s high-resolution XPS results proved a great increase in Cu-N contents from 13.15% for Cu-N-C to 18.36% for Cu-N@Cu-N-C. The enhanced ORR performance of Cu-N@Cu-N-C was attributed to the enlargement of Cu-N active site density, providing an effective route for the preparation of efficient and low-cost ORR catalysts.