GdFeO3-based Electrode Materials and Application to Electrochemical OER and ORR

Abstract

[Part 1] Bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are necessary for the renewable energy systems. However, the kinetically slow and large energy-demanding procedures of the oxygen electrocatalysis make preparation of the bifunctional catalyst difficult. In this work, we report a novel hierarchical GdFeO3 perovskite oxide of a sphere-like nanostructure and the surface modification with the Group X hetero-metal oxides. The nanostructured GdFeO3 layer behaved as a bifunctional electrocatalyst in the oxygen electrocatalysis of OER and ORR. Moreover, the surface decoration with Pt/PtOx-doped NiO nanoparticles enhanced the electrocatalytic performances substantially. Incorporation of the zero-dimensional mesoporous PtOx+NiO nanoparticles into the three-dimensional porous GdFeO3 nanostructure maximizes the electrochemically active surface area (ECSA). The elemental composition and valence states of the electrode materials have been analyzed by X-ray diffraction and X-ray photoelectron spectroscopy. The surface area and pore size of nanoparticles were analyzed from N2 adsorption/desorption isotherms. Tafel analysis indicates that the surface modification effectively improves the kinetics of oxygen reactions and accordingly increases the electrocatalytic efficiency. Finally, the 2 wt% PtOx+NiO|GdFeO3 (x = 0, 1, 2) electrode achieved the OER overpotential of only 0.19 V at 10 mA/cm2 in an alkaline solution and a high turnover frequency (TOF) of 0.28 s-1 at η = 0.32 V. Furthermore, the ORR activity is observed with an onset potential of 0.80 V and a half-wave potential (E1/2) of 0.40 V vs. RHE. X-ray analysis indicates presence of the highly oxidative oxygen (O2 2– /O– ) and coexistence of a metallic Pt and high oxidation states of Pt2+ and Pt4+ ions on the PtOx+NiO|GdFeO3 surface, of which species promotes concertedly the bifunctional oxygen electrocatalysis. [Part 2] The development of electrocatalysts for oxygen evolution reaction in the electrochemical water splitting is an important factor to enable the reaction to occur quickly and efficiently. Efficient OER catalysts have been developed using earth-abundant metal, and one of the promising methods is perovskite system. The performance of the electrocatalysts can be increased by adjusting the A and B sites of ABO3 type perovskite. In this research, perovskite oxide based on gadolinium and iron was synthesized using a hydrothermal reaction. The structure of catalysts was analyzed and the electrochemical activity was evaluated by controlling the Fe ratio of the B-site which was indicated catalytic activity. Using X-ray diffraction (XRD) and Rietveld refinement, it was confirmed that the GFO-based materials were composed of GdFeO3, Gd3Fe5O12 and Gd2O3 together. The oxidative state of metals and the binding energy of the elements were confirmed using X-ray photoelectron spectroscopy (XPS). As the Fe ratio increased, the binding energy and oxidative state of elements were hardly changed and the intensity of the lattice oxygen was increased. The surface morphology was observed to be spherical nanostructure, and the ratio of constituent elements was confirmed using scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) elemental mapping. The electrochemical measurements were conducted to evaluate the activity as an electrocatalyst for the oxygen evolution reaction. Electrochemical studies showed that the efficiency was increased with the ratio of Fe located at the B-site, and a comparison of the amorphous and crystalline GFO-based materials showed that the amorphous has lower overpotential and better current density.