Solar Water Splitting
visible-light-responsive semiconductor materials
Solar-driven water splitting into hydrogen and oxygen over semiconductors is a potential means of directly producing hydrogen as a renewable and clean resource. Photoactive oxide semiconductors such as SrTiO3 (Eg=3.1 eV) have been studied for water splitting under UV light irradiation. Visible and infrared radiation account for approximately 95% of solar energy. In addition, a solar-to-hydrogen (STH) conversion efficiency greater than 10% is necessary to achieve commercial viability for water splitting devices, i.e., photoelectrochemical (PEC) cells composed of a photoanode and a photocathode. Therefore, the visible-light-responsive semiconductors with a small Eg < 2.1 eV have attracted a lot of attention for solar water splitting.
Figure 1. Dependence of STH Energy Conversion Eff. on Band-gap Eg of Semiconductors (Seo et al., Angew. Chem. Int. Ed. 2018, 57, 8396-8415).
We have a great interest in intensively visible-light-responsive semiconductor materials for water splitting. In particular, perovskite-type oxynitrides AB(O,N)3 (A=La, Ca, Sr and Ba, B=Ti, Ta and Nb) are capable of adsorbing a wide range of visible light (Eg < 2.1 eV) and also have suitable band edge potentials to straddle water redox potentials. This indicates that the (oxy)nitrides as a single absorber can drive overall water splitting evolving both hydrogen and oxygen from water molecules and have a potential to produce high STH energy conversion efficiency during the water splitting greater than 10%.
Figure 2. UV/Vis diffuse reflectance spectra of perovskite oxynitrides (Seo et al., Angew. Chem. Int. Ed. 2018, 57, 8396-8415).
We have a lot of experience in various preparations and solar-driven water splitting of n-type Ta3N5 and perovskite-type AB(O,N)3. Our present research has been developing for the solar water splitting activity by introducing nano-structural designs of oxynitrides or by modifying the surface and bulk properties of the semiconductors including annealing treatments or doping engineering.
Figure 3. (a) Ar annealing treatment to improve surface and bulk properties of BaNbO2N. (b) Solar-driven water splitting over a photoanode made of the annealed BaNbO2N or BaTaO2N particles (Seo et al., Adv. Energy Mater., 2018, 1800094; ACS Appl. Energy Mater., 2019, 2, 5777-5784).
Fuel Cells/Metal-Air Batteries
Active and Stable ORR and OER Electrocatalysts
There have been many attempts to apply non-platinum electrocatalysts in polymer electrolyte fuel cells (PEFCs), especially to cathode electrocatalysts with improved oxygen reduction reaction (ORR) activity. Groups IV and V elements in the periodic table, such as Ti, Zr, Ta, and Nb, are chemically stable under acidic conditions. Ultrafine metal oxide particles, TaOx, NbOx and ZrOx of 1~2 nm in size, prepared by the electrodeposition in nonaqueous solutions were very active and stable electrocatalysts for ORR in an acidic condition comparable to that of Pt/CB catalysts. We have been struggling for higher electrocatalytic activities of the oxide nanoparticles in aqueous electrolytes at various pH levels.
Figure 1. Ultrafine TaOx/CB particles (a) and the ORR activity in an acidic aqueous electrolyte (Seo et al., Chem. Commun., 2012, 48, 9074; ACS Catal. 2013, 3, 2181).
We are highly interested in the oxide nanoparticles and the derivatives mixed with other elements for oxygen evolution reaction (OER) in metal-air batteries and water splitting. Also, we have studied the ORR and OER kinetics and mechanisms besides catalytic performance because the reactions are very slow compared with hydrogen oxidation/reduction kinetics .
Figure 2. Proposed ORR mechanism on ultrafine TaOx/CB particles (Seo et al., Electrochimica Acta 2014, 149, 76).