Synthesis of a small-size metal oxide mixture based on MoO and NiO with oxygen vacancies as bifunctional electrocatalyst for oxygen reactions

In this work, a 3d metal (Ni) and a 4d metal (Mo) were mixed using tungstic acid (TA) as particle size/shape controller to develop a MoOx-NiO (labelled as Mo-Ni) bifunctional electrocatalyst for oxygen reduction (ORR) and oxygen evolution (OER) reactions in alkaline medium. The formation of MoOx-NiO was determined by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). XPS also revealed that the Mo-Ni material contained oxygen vacancies, while transmission electron microscopy (TEM) indicated that MoOx-NiO material had sub <20 nm particle sizes. EDX mapping analysis performed at different electrochemical cycles revealed that the composition (4:1 Mo to Ni) was kept. The Mo-Ni material presented a similar current density than Pt/C during the ORR, displaying a half-wave overpotential of 110 mV. For OER, the Mo-Ni material required 60 mV more energy than a commercial IrO2/C to achieve 10 mA cm−2, and 170 mV less energy than Pt/C. Additionally, the Mo-Ni material displayed superior durability for both reactions, keeping 92% of its initial current during the ORR after 8 h of operation, and 100% during the OER, being more stable than commercial Pt/C and IrO2/C.