Group X Hetero-metal Oxides for Photo/electrocatalytic Oxygen Evolution Reaction

Abstract

Part I: For eco-friendly and sustainable development, it is crucial to design effective electrocatalysts for the oxygen evolution reaction (OER) which is the counterpart reaction to the hydrogen evolution reaction. Precious and rare metal-based oxides such as RuO2 and IrO2 are known as effective electrocatalysts for OER. Therefore, recent studies focus on the development of non-precious and efficient electrocatalysts. Perovskite oxides are in the spotlight due to abundance and structure flexibility. Herein, GFO nanostructures (Gd and Fe-based oxide) including perovskite oxide (GdFeO3) were synthesized by conventional hydrothermal reaction. Pure forms of GdFeO3 and Garnet oxide (Gd3Fe5O12) were separated by modulating of concentration of citric acid and metal reagents. Mixed-phase GFO nanostructure (GFO Mix) was compared with phase-pure GFO nanostructures as well. Afterwards, the electrochemical OER performance of GFO nanostructures was investigated using thin-film electrodes prepared via the spin-coating method. Electrochemical OER activity increased as follows: GFO Mix << Gd3Fe5O12 < GdFeO3. Highly oxidized surface by predominant oxidative oxygen species contributes to higher OER activity of perovskite-type GdFeO3 electrode. For rapid electron transfer between substrate and electrocatalysts, a seed layer was introduced to the FTO substrate before deposition of GFO nanostructures. However, the difference among GFO nanostructures disappeared in seed layer-coated thin-film electrodes because the seed layer actively participates in electrochemical OER. This study shows that perovskite-type GdFeO3 has superior OER activity to the others even though an appropriate seed layer needs to be devised for clear comparative experiments of GFO nanostructures in future studies. Part II: For sustainable development, designing efficient electrocatalysts for oxygen evolution reaction (OER), the anodic reaction of water splitting, is crucial. To improve the sluggish kinetics of OER attributed to multiple reaction steps with four-electron transfer and large overpotential, recent studies focus on electron transfer and charge separation. In addition, since the wide bandgap of the n-type semiconductor used as a photocatalyst in the photoelectrochemical (PEC) reaction limits light absorption, numerous researches have been conducted to modulate band structure and reduce bandgap of the semiconductor. Herein, Group X hetero-metal oxides were prepared by combining Group X elements (Pt and Pd) with NiO (widely used electrocatalyst), and their electrochemical performance was investigated. The OER activity decreased as follows: PtOx + Ni/NiO > PdO + Ni/NiO > Ni/NiO >> PtOx. Furthermore, Group X hetero-metal oxides were introduced on the surface of TiO2 nanorod (TNR) for PEC application. The synergistic effect of Group X hetero-metal oxides significantly improved the PEC performance of TNR electrodes. Notably, with the photoanodic current of 2.62 mA/cm2 at 1.23 V vs. RHE and 70 % IPCE value, the Pt/PtOx + Ni/NiO hetero-metal oxides performed as the most effective cocatalyst on the TNR surface, which is ~ 4 times greater than pristine TNR. These results show that the heterostructure of metal oxides based on Group X elements can effectively enhance oxygen evolution activity.