Thermodynamic stability of LuRhO3 in a photoelectrochemical cell

Abstract

Long term stability of lutetium rhodite (LuRhO3) as an electrode for photoelectrochemical decomposition of water is investigated. The thermodynamic properties of LuRhO3 are determined in the temperature range from 875 to 1300 K using an electrochemical cell incorporating calcia–stabilized zirconia as the solid electrolyte. The standard Gibbs energy of formation of LuRhO3 from its constituent binary oxides Lu2O3 with C-type rare earth oxide crystal structure and Rh2O3 with orthorhombic structure is obtained:ΔGf,oxo(±135)/Jmol−1=−40106+3.282(T/K)The standard entropy of LuRhO3S298.15Ko(LuRhO3)/JK-1mol-1=89.54(±0.35) and standard enthalpy of formation from elements ΔHfo(298.15K)/kJmol−1=−1181.97(±3.86) are derived. The phase relations and stabilities of phases in the LuRhO system are computed from thermodynamic data. An isothermal section of the ternary phase diagram, a 2–D oxygen potential diagram and a 3–D chemical potential diagram at 1200 K are presented along with temperature–composition phase diagrams at various oxygen partial pressures of oxygen. LuRhO3 (p-type) is thermodynamically unstable as a photocathode in contact with hydrogen gas. However, n-type LuRhO3 can be used as a photoanode in the presence of oxygen.