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.