Two 3D structured Co-Ni bimetallic oxides as cathode catalysts for high-performance alkaline direct methanol fuel cells

Two NiCo₂O₄ bimetallic oxides were synthesized via a facile hydrothermal method. SEM and TEM observations show that these materials have three-dimensional (3D) dandelion-like (DL) and flower-like (FL) morphologies. Their large specific surface areas (90.68 and 19.8 m²·g⁻¹) and porous structures provide many active sites and effective transport pathways for the oxygen reduction reaction (ORR). Electrochemical measurements with a rotating ring-disc electrode (RRDE) indicate that the electron transfer numbers of the NiCo₂O₄-DL and NiCo₂O₄-FL catalysts for ORR in an alkaline solution are 3.97 and 3.91, respectively. Fuel cells were assembled with the bimetallic oxides, PtRu/C and a polymer fiber membrane (PFM) as cathode catalysts, anode catalyst and electrolyte film, respectively. For NiCo₂O₄-DL, the peak power density reaches up to 73.5 mW·cm⁻² at 26 °C, which is the highest room-temperature value reported to date. The high catalytic activity of NiCo₂O₄ is mainly attributed to the presence of many Co³⁺ cations that directly donate electrons to O₂ to reduce it via a more efficient and effective route. Furthermore, the catalytic performance of NiCo₂O₄-DL is superior to that of NiCo₂O₄-FL because it has a higher specific surface area and is less crystalline.