Preparation And Evaluation Of Composite Cathode Based On Ceramic Materials For Solid Oxide Electrolyzer

Conventional Ni-based cathode is easy to be oxidized by H2O or CO2at hightemperatures which indicates that a significant concentration of reducing gas isrequired to flow over the cathode. This thesis aims to develop redox-stable ceramiccathode with decorated catalysts towards direct conversion of H2O or CO2into fuels.The redox-stable ceramic electrode is expected to achieve the direct electrolysis whilethe loaded catalysts are expected to enhance the electrocatalytic activity of thecomposite cathodes.Firstly, we demonstrate the direct conversion of CO2/H2O into fuel in aproton-conducting solid oxide electrolyser with the Ni-based cathode. AC impedancespectroscopy and I-V testing demonstrate two main processes in the electrochemicalprocess from0to2V:(1) the reoxidation of the anode below1.2V (iR-correctedvoltage) and (2) the oxidation of H2O and electrochemical reduction of CO2above1.2V (iR-corrected voltage). It is found that the selectivity to CO reaches100%with~90%current efficiency. However, the carbon deposition, poisoning and oxidation of Nimetal in the cathode degrade the cell performance.Secondly, this thesis investigates a composite cathode La0.2Sr0.8TiO3+(LSTO) forthe direct steam electrolysis in an oxide-ion-conducting solid oxide electrolyzer. Thedependences of electrical conductivity of the reduced LSTO on temperature andoxygen partial pressure are studied and further correlated to the electrochemicalproperties of the cathode in symmetric cell LSTO-SDC/YSZ/LSTO-SDC and solidoxide electrolyzer LSTO-SDC/YSZ/LSM-SDC, respectively. Current efficiencies ofthe solid oxide electrolyzer with LSTO-SDC cathode were found to be92.38%and91.17%with or without reducing gas flowing over them under the applied voltage of1.8V at800℃, respectively.Thirdly, this thesis aims to improve the electro-catalytic activity of the perovskitetitanate that inhibits the electrode performance.This study reports an investigation ofiron doped titanate (La0.2Sr0.8)0.9Ti0.9Fe0.1O3-(LSTFO), which can be reduced toin-situ grown iron nano-catalyst on substrate, as a potential composite cathode forsteam electrolysis in a solid oxide electrolyser. The exsolution and dissolution of theiron nanoparticles is completely reversible in redox treatment cycles. The presence ofexsolved iron nanocatalyst significantly improved the polarizations of thesymmetrical and electrolysis cells of the LSTFO composite electrode. The synergetic effect of catalytical-active iron and redox-stable LSTO produced excellent short-termstability of the cathode with attendant efficient electrochemical reduction of steam andremarkable enhancement of Faraday efficiency by about100%.Finally, thesis also investigates redox-reversible ceramic NbTi0.5Ni0.5O4cathodewhich can be reversibly transformed into Ni metal anchoring on Nb1.33Ti0.67O4surfaceat intermediate temperature in a reducing atmosphere. The electronic conductingNb1.33Ti0.67O4with anchored Ni nanocatalyst combines the redox stability oftheNb1.33Ti0.67O4ceramic and the electrocatalytic activity of the Ni nanoparticles andtherefore demonstrates the potential use for the cathode of solid oxide electrolyzerhigh temperature CO2electrolysis. The direct electrolysis of100%CO2is thenperformed and the maximum Faraday current efficiency reaches as high as65%. Itshould be noted that the reversible transformation between NbTi0.5Ni0.5O4andNb1.33Ti0.67O4+Ni significantly contributes to thethermal and redox cyclingperformance while the anchored interface between Ni and Nb1.33Ti0.67O4improves thestability of the composite cathode for the direct CO2electrolysis.