Revisiting the Role of the Triple-Phase Boundary in Promoting the Oxygen Reduction Reaction in Aluminum–Air Batteries

The decomposition, electron transfer, and protonation of oxygen molecules are typically assumed to be the rate-limiting steps of the oxygen reduction reactions (ORR), and the activation energy barriers of these reactions can be surmounted using catalysts. In this study, the physical rate-limiting step of the ORR consists of the adsorption of gaseous oxygen molecules at the liquid–solid phase boundary, indicating that the formation of a gas–liquid–solid triple-phase boundary (TPB) is important for accelerating the ORR kinetics. This is experimentally confirmed by analyzing the ORR in aluminum–air batteries. Moreover, the formation of a TPB using the hierarchical pores of sparked reduced graphene oxide is demonstrated, which serve as the cathode, and the remarkable electrochemical performance of the fabricated battery is presented. These findings can be used to accelerate the ORR kinetics by maximizing the TPB, particularly in aluminum–air batteries.