Novel Electrode Materials For Solid Oxide Electrolyzer And Their Electrochemical Performance

Solid oxide electrolyzer (SOE) can produce H2 and CO by electrolyzing H2O and CO2 using electricity generated from the solar, wind power and other clean energy. The development of SOE is of great significance for the exploitation of clear energy and the easement of the energy crisis. Currently, the development of SOE is still in its fledgling stage. The cathode of oxygen-ion conducting solid oxide electrolyzers and the anode of proton conducting electrolyzers work in rigorous environments. They play key roles in determing the performance of the SOE. These elelctrode materials should have good electrical, catalytic properties and high stability in the electrode atmospheres. In this dissertation, novel cathode materials for oxygen-ion conducting SOE and anode materials for proton conducting SOE were explored and the electrochemical properties of the materials were investigated. Modification of the electrodes were also made and evaluated.Ni cermet is now widely used as anode materials of solid oxide fuel cells (SOFC). However, Ni cermet is easy to be oxidized by water steam in SOE, which leads to the failure of the electrodes. Therefore, it is necessary to develop novel electrode materials for solid oxide electrolyzers. In the chapter 2 and chapter 3 of this dissertation, Nb2TiO7 and NbTi0.5M0.5O4 (M=Ni, Cu) composite cathodes for direct steam electrolysis in an oxide-ion-conducting solid oxide electrolyzer were investigated, respectively. It has been shown that solid oxide electrolyzer based on Nb2TiO7 composite cathode can operate stably in non-reduction atmospheres. At 2.0V, the current density of the electrolyzer reaches 91 mA·cm-2 and the current efficiency is about 86.4%. For SOE with NbTi0.5M0.5O4 (M=Ni, Cu) cathode, Ni and Cu metallic nanoparticles with high catalytic activity exsolve in reduction atmosphere and anchor on the surface of the high electronically conducting material Nb1.33Ti0.67O4, forming an enhanced composite cathode. The multiplex effects of the metallic catalyst and the ceramic electrode lead to the high stability as well as the superior performance of electrolyzer. For the SOE based on NbTi0.5Ni0.5O4 and NbTi0.5Cu0.5O4 composite electrode, the Faradic efficiencies reach 95% and 93%, respectively, and the polarization resistance of electrolyzer is about 4Ω·cm2 and 6Ω·cm2.In a proton-conducting solid oxide electrolyzer, the anode endures strong oxidation atmosphere. It needs the anode having high stability and efficient catalysis. In the chapter 4 of this dissertation, proton-conducting solid oxide electrolyzer based on La0.6Sr0.4Co0.2Fe0.803-δ (LSCF) and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) composite anode materials for water steam electrolysis were evaluated. The results show that LSCF and BSCF composite electrode materials are stable and efficient for water steam electrolysis. Maximum current efficiency of the electrolyzers based on LSCF and BSCF electrode reaches 58% and 64% with 3%H2O/Ar fed into anode, respectively.The anode reactions in both oxygen-ion conducting and proton-conducting solid oxide electrolyzers are oxygen evolution reactions (OER). OER needs to overcome some energy barrier, which represents relatively high polarization resistance of the anodes. In this dissertation, Co3O4 catalyst was loaded onto the anode of the SOE to improve their electrode performance. By modifying the La0.8Sr0.2MnO3 (LSM) anode with Co3O4 nanoparticles, the polarization resistance (Rp) of the symmetric cells decreases from 1.2Ω·cm2 to 0.24 Ω·cm2, and the Rp of high frequency process of the electrolyzer decreases significantly froml.0Ω·cm2 to 0.4 Ω·cm2 under an electrolysis voltage of 1.8V. The anode of proton conducting SOE is also improved by loading Co3O4 catalyst onto the anode surface. Because the surface reactions of oxygen permeation membrane are also oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), the ideas of surface modification was extended to oxygen permeation membrane. In 6 chapter of this dissertation, Co3O4 nanorod catalyst is loaded onto the surface of BSCF membranes by a hydrothermal process, and its effects on the oxygen permeation rate were investigated. At 600℃, for BSCFmembrane loaded with Co3O4 nanorods, the oxygen permeation rate increased from 0.041 to 0.47ml·min-1·cm-2, which is 11.5 times higher than that of the bare BSCF membrane under Air/He gradient, and the activation energy of oxygen permeation membrane decreases from 146 kJ·mol-1 to 59kJ·mol-1.