Evolution of Formation BIMEVOX with Sol Gel Method
One technology that can reduce the dependencies on current energy resources is fuel cells and one of them is solid oxide fuel cells (SOFC). Generally, SOFC have three components, i.e. catode, anode, and electrolyte. Electrolyte material of fuel cell must have a high ionic conductivity. Bi2VO5.5 (Bismuth vanadate, BIVOX) is one of oxide with high ionic conductivity. This oxide has three polymorphs, namely alpha, beta, and gamma. Gamma polymorph is the polymorph with the highest ionic conductivity. However, this polymorph is not stable at low temperatures and will transform to polymorph alpha or beta. To stabilize the gamma phases, one can substitute vanadium with various cation dopants. The result of this subtitution is a new family oxides namely bismuth metal vanadium oxide (BIMEVOX). BIMEVOX is generally synthesized via solid state reaction at high temperature and usually still has impurities. Thus a different synthesis method to obtain the gamma phase of BIMEVOX at low temperatures is needed. BIMEVOX synthesis method used in this study is a sol-gel method. This method can produce a high purity product because all of the metal cations are well mixed at the atomic level. BIMEVOX successfully synthesized using sol gel method with citric acid and ethylene glycol as chelating agent and a mole ratio of citric acid: ethylene glycol: total of cations = 10:4:1, at pH 1-3, and the gel was burned at the final temperature of 600 °C. Formation of BIMEVOX via sol-gel method is through gel to bismuth metal, complex of vanadium(IV), and complex of zinc, then a mixture oxide (BiVO4 and BIMEVOX) and finally BIMEVOX. This γ-BIMEVOX has a tetragonal structure with space group I4/mmm. Coordination of vanadium in this BIMEVOX is octahedral, tetrahedral, and square pyramidal. This BIMEVOX has a direct band gap energy of 2.18 eV. This BIMEVOX has γ-γ’ transition at 450 °C.
Keywords: BIMEVOX, sol gel, polymorph, vanadium coordination, band gap,γ-γ’ transition (more…)