In-situ transformation of electrodeposited W–Co oxide to ZnCo2O4 nanoparticles as an effective bifunctional catalysts in Zn-air batteries

Abstract W–Co was cathodically electrodeposited on gas diffusion layer (GDL) electrodes for application as a bifunctional catalyst for Zn-air batteries. The electrodes were synthesized galvanostatically at various pH values (4.5 and 7.5) and deposition times (5, 15 and 25 min). Transmission electron microscopy (TEM) analysis showed that the deposits oxidized in contact with air, forming a conformal layer of W–Co oxide with a thickness of ~10 nm. During battery cycling, most of the W dissolved in the electrolyte. The remaining Co oxide agglomerated, forming nanoparticles less than 100 nm in size. Zinc (present in the electrolyte as ZnO) was incorporated into the Co oxide nanoparticles. As such, the conformal layer of W–Co oxide transformed into cubic spinel ZnCo2O4 nanoparticles which provided excellent bifunctional catalytic activity. The W in the original deposit acted as a conduit to facilitate formation of the ZnCo2O4 nanoparticles. The ZnCo2O4 catalyst demonstrated good performance in two-electrode and three-electrode full-cell Zn-air battery tests. In three-electrode tests at 10 mA cm−2, a round trip efficiency of 62% was achieved, exceeding the efficiency of Pt/Ru–C (61%). The final efficiencies were 59% for the three-electrode configuration (after 100 cycles). A high maximum power density of 216 mW cm−2 was also achieved.