Mechanism And Characteristics Research On CO₂/H₂O Co-electrolysis By Solid Oxide Electrolysis Cells

The electrochemical conversion of CO₂ and H₂O to fuel in solid oxide electrolysis cells（SOEC） provides a pathway for renewable electricity storage while utilizing greenhouse gas CO₂ at the same time. In order to optimize the SOEC performance, it is crucial to investigate the kinetic mechanisms and characteristics of CO₂/H₂O co-electrolysis. This dissertation presented systematic researches on patterned, porous and tubular SOEC in dif ferent scales.Firstly, button cells with patterned Ni electrode and single crystal YSZ electrolyte were fabricated to investigate the electrochemical performance of H₂O electrolysis, CO₂ electrolysis and co-electrolysis. Patterned electrode is an effective approach to avoid complexities associated with porous electrodes and obtain the intrinsic kinetics of electrochemical reactions due to the well-defined length of TPB. The electrochemistry was positively related to temperature, polarization voltage and gas partial pressure. The possible rate-controlling step was O（Ni）+（YSZ）â†'（Ni）+O2-（YSZ） for CO₂ electrolysis or H₂O（YSZ）+（Ni）+e-â†'H（Ni）+OH-（YSZ） for H₂O electrolysis. The surface diffusion of O（Ni） or H₂O（YSZ） also could respectively control s the reaction rate for CO₂ or H₂O electrolysis when the polarization voltage was not high. The electrochemical reaction rate of patterned Ni electrode for H₂O electrolysis was 12-15 times higher than that for CO₂ electrolysis, thus the reaction rate of co-electrolysis was very close to that of H₂O electrolysis. The distribution and structural feature of carbon deposition on patterned Ni for both SOEC and SOFC were observed to clarify the electrochemical reaction CO（Ni）+（YSZ）+2e-?C（Ni）+ O2-（YSZ） at TPB.Secondly, the electrochemical performance and product compositions of porous button SOEC were studied. CH4 could be detected in the gas products and significantly promoted more than 9-12 times by electricity. C（s）+2H2â†'CH4 was proposed as one of reaction pathways for CH4 generation of co-electrolysis. A one-dimensional elementary reaction model of SOEC was developed, coupled with heterogeneous elementary reactions, electrochemical reactions, electrode microstructure, mass transport and charge transport. The model was u tilized to predict the effect of operating parameter and electrode microstructure on co-electrolysis performance, and analyze the coupling properties of reaction and transfer in electrode. The concept of main z ones for heterogeneous chemical reaction and electrochemical reaction was proposed and successfully explained the different experimental phenomena. The mass transfer flux D?c determined the heterogeneous chemical reaction while the charge transfer flux σ?V determined the electrochemical reaction. Due to the opposite directions of O2 diffusion and electrochemical reaction in the oxygen electrode, the concentration polarization of SOFC could be 7 times higher than SOEC.At last, a tubular SOEC was built to directly convert the CO₂ and H₂O to syngas and CH4. The power and power density of single tube could achieve 4.15 W and 2817 W/m2, the CH4 yield could reach 10% at 550 oC. A two-dimensional tubular SOEC model was developed to study the effects of fluid and heat transfer on performance.