Copper Nitride Nanostructure for the Electrocatalytic Reduction of Oxygen: Kinetics and Reaction Pathway

Abstract

Synthesis of an inexpensive, electrocatalytically active, nonprecious metal-based catalyst for oxygen reduction reaction (ORR) is of significant importance for the development of energy conversion and storage technologies. Herein we describe a new single-step solvothermal method for the synthesis of nanostructured Cu3N and its electrocatalytic activity toward ORR. Our synthetic approach involves reduction of Cu(II) to Cu(I) and subsequent nitridation of Cu(I) by hexamethylenetetramine in argon atmosphere at 200 °C. At elevated temperatures, hexamethylenetetramine hydrolyzes to formaldehyde and ammonia, and the hydrolyzed products efficiently function as reducing and nitridating agents of the copper precursor. The crystalline Cu3N nanoparticles have a quasi-spherical shape with an average size of 80 nm. The nanoparticles are supported on reduced graphene oxide (rGO) and nitrogen-doped rGO (N-rGO) catalyst support, and the electrocatalytic activity toward ORR is evaluated in terms of onset potential, mass specific activity, Tafel slope, and kinetics and reaction pathway. The N-rGO-supported Cu3N (N-rGO/Cu3N) has superior ORR activity compared to the as-synthesized Cu3N and rGO-supported Cu3N. The rate constant for the reduction of O2 to H2O and the disproportionation of intermediate H2O2 are calculated. The kinetic analysis shows that N-rGO/Cu3N favors four-electron reduction of oxygen to water, and the disproportionation of trace amount of in situ generated HO2– (∼6%) is negligible. N-rGO/Cu3N is durable and has good tolerance toward the anode fuel methanol. The synergistic effect of N-rGO and Cu3N plays an important role in controlling the electrocatalytic activity.