Due to the increasing energy demand and environmental pollution (caused by heavy reliance on fossil fuels), there is an i mminent need for clean and sustainable energy. Technologies such as fuel cells and metal-air batteries offer a way forward. Fuel cells are energy conversion devices with a high energy density (~ 600 Wh/Kg) and high efficiency (~ 60%). Metal-air batteries also have a high energy density, roughly 3–30 times greater than Li-ion batteries. Oxygen reduction reaction (ORR) is at the heart of the fuel cell and metal-air battery systems. However, the sluggish kinetics, high cost associated with catalyst, and catalyst instability make ORR one of the primary bottlenecks associated with mass deployment for these energy technologies.
This work aims to develop novel electrocatalysts for ORR, which are cost-effective, earth-abundant, and durable. Metal oxides-based highly active electrocatalysts have been developed through a nano-compositing approach, which would be useful for zinc-air batteries and anion exchange membrane fuel cells (AEMFC). At the outset, we report that S, N doped graphene quantum dot-TiO2 based heterostructure shows excellent ORR activity with an onset potential of 0.91 V vs. RHE and a half-wave potential of 0.82 V vs. RHE. Further improvements are shown using CuOx-TiO2-based heterostructure catalysts (limiting current density of 5.8 mA cm-2). The stability and durability are shown to be even better with CuO-ZnO-based heterostructures. In the fourth work (involving the CuOx-Co3O4 system), we optimized ORR performance and tested the catalysts for the zinc-air battery. We observed a very good peak power density (~103 mW cm-2) with excellent rate capability. The catalyst also shows excellen t specific capacity (820 mA h gZn-1) and long-term stability (~80 h during charge-discharge cycles). These results outperformed the benchmark Pt/C catalyst. The excellent ORR activity is attributed to the formation of oxygen vacancies on the surface of these catalysts. The unique structure formation also helps to get excellent electronic conductivity, large surface area, appropriate porosity, and desirable interface charge transfer.