LAUSR

The scope of any metal oxide as catalyst for driving electrocatalytic reactions depends on its electronic structure which is correlated to its oxygen defect density. Likewise to transform a spinel oxide such as cobalt ferrite (CoFe2O4) into a worthy pH-universal bifunctional electrocatalyst for hydrogen and oxygen evolution reactions (HER and OER), oxygen defects need to be regulated. Prepared by co-precipitation and inert calcination at 650oC, CoFe2O4 nanoparticles (NPs) require 253 and 300 mV OER overpotentials to reach current densities of 10 and 100 mA/cm2 when Ni foam (NF) is used as substrate. With cost-effective carbon fibre paper (CFP), OER overpotential increases to 372 mV at 10 mA/cm2 at pH 14. The 550oC prepared NPs need HER overpotentials of 218, 245 and 314 mV at -10 mA/cm2 in alkaline, acidic and neutral pH, respectively. The intrinsic activity is reflected from turnover frequencies (TOF) of >3 O2 s-1 and >5 H2 s-1 at 398 and 259 mV overpotentials, respectively. When coupled for overall water splitting, the extremely durable 2-electrode electrolyzer requires cell potential of only 1.63V to reach 10 mA/cm2 at pH 14. The homologous couple also splits sea water splitting at an impressively low cell voltage of 1.72 and 1.47 V at room temperature and 80°C, respectively.