Bifunctional catalytic activity of Zn1-xFexO toward OER/ORR: Seeking an optimal stoichiometry

Fe-doped zinc oxide (Zn1-xFexO, x=0, 0.05, 0.1, 0.15 and 0.20; ZnO:Fe) nanoparticles were produced using an eco-friendly and rapid microwave processing of a precipitate and tested as catalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in moderately alkaline solution (0.1 M Na2SO4, pH = 8). The phase composition, crystal structure, morphology, textural properties, surface chemistry, optical properties and band structure were examined in detail to comprehend the influence of Zn2+ partial substitution with Fe3+ on the catalytic activity of ZnO:Fe. The linear sweep voltammetry study showed that, compared with the pure ZnO, the ZnO:5Fe exhibits enhanced catalytic activity toward ORR, while the activity decreased with the increase in the amount of Fe-dopant. Improved ORR activity of the ZnO:5Fe is established by more positive onset potential (0.394 V vs. RHE), current density (0.231 mA·cm2 at 0.150 V vs. RHE), and faster kinetics (Tafel slope, b = 248 mV·dec-1) driven by nearly 4-electron transfer reaction. Synergistic effect of (1) sufficient amount of surface oxygen vacancies, and (2) certain amount of plate-like particles composed of crystallites with well developed (0001) and (0001 @#x0305;) facets may be the origin of improved catalytic activity of the ZnO:5Fe toward ORR. Quite the contrary, the OER study showed that the introduction of Fe3+ ions into the ZnO crystal structure leads to enhanced catalytic activity of all the ZnO:Fe samples compared with the pure ZnO, probably due to modified binding energy and optimized band structure. Among the ZnO:Fe samples, ZnO:10Fe gives the maximal current density of 1.066 mA·cm-2 at 2.216 V vs. RHE with the onset potential of 1.856 V vs. RHE. The smallest potential difference between OER and ORR (ΔE=1.58 V) promotes ZnO:10Fe as the promising bifunctional catalysts toward ORR/OER in moderately alkaline solution. This study demonstrates that electrocatalytic activity of Fe-doped ZnO strongly depends on defect chemistry and consequently band structure. The study not only provide fundamental insight into the electrocatalytic activity of Zn1-xFexO but also guides the design of its stoichiometry optimal for high bifunctional activity toward OER/ORR.