Development of Novel SOEC Cathode Catalysts with Ruddlesden-Popper Structure for Efficient CO2 Electrolysis

Abstract

The concentration of CO2 in the atmosphere has been steadily increasing over the past decades owing to the increased use of fossil fuels, and this has accelerated global warming which is a major threat to the environment. In this scenario, technologies that convert CO2 into value-added compounds have received considerable attention. Solid oxide electrolysis cell (SOEC) is a recently developed technology for conversion of CO2 into CO. The SOEC system has several advantages such as fast reaction kinetics and high Faraday efficiency; however, problems related to the conventional nickel electrode catalyst used in this system, such as partial oxidation, agglomeration, carbon deposition, and sulfur poisoning, degrade the performance of the system. Therefore, in recent years, ceramic-based materials with excellent redox stability have been used as an alternative to nickel. Nevertheless, the low catalytic activity of these materials is a limiting factor for practical applications. The present study is aimed at improving the electrochemical performance of ceramic materials for CO2 electrolysis via metal nanoparticle exsolution and partial substitution of fluorine. Furthermore, for practical application of the SOEC system, the electrochemical performances of single cell with the electrode catalysts developed in this study were evaluated under H2S-containing CO2 conditions. In Chapter 2, I report a highly active Ruddlesden-Popper material with a mechanism of in situ exsolution of Co nanoparticles and its use as an effective catalyst for CO2 reduction to produce CO in a SOEC. This catalyst is simply prepared by transforming a perovskite-derivatives and revealed a good reversibility of structural transition between the Ruddlesden-Popper and the perovskite structure during reaction cycles. A high current density of 630 mA/cm2 can be accomplished at a voltage of 1.3 V and temperature of 850 °C with a very high faraday efficiency of 95% or larger. More importantly, no sign of degradation is indicated as observed by galvanostatic stability test, implying that this Ruddlesden-Popper structure is highly robust as the cathode catalyst for the CO2 electrolysis. In situ exsolved Co nanoparticles and high concentration of oxygen vacancies caused by the structural transition are responsible for its high stability and catalytic activity, as characterized by several physicochemical analyses. Chapter 3 discusses the fabrication of a new and efficient sulfur-tolerant cathode catalyst with in situ exsolved and anchored CoNi alloy nanoparticles on the Ruddlesden-Popper support of La1.2Sr0.8Co0.4Mn0.6O4 (CoNi-R.P.LSCM). This catalyst was also prepared by in situ annealing of perovskite derivatives in a reducing atmosphere at the operating temperature of the SOEC. CoNi alloy nanoparticles with a size of 10–30 nm were formed on the surface of the synthesized CoNi-R.P.LSCM catalyst, and the properties of the catalyst were characterized using various physicochemical analyses. The single cell with the CoNi-R.P.LSCF cathode exhibited a high current density of 703 mA/cm2 at 1.3 V and 850 °C with a maximum Faraday efficiency of 97.8%. The high catalytic activity and efficiency are attributed to the in situ grown CoNi alloy nanoparticles and the high oxygen mobility in the Ruddlesden-Popper support. Importantly, no sign of performance degradation was observed in galvanostatic tests conducted over a period of 90 h under H2S-containing CO2 conditions. The findings of this study indicate the possibility of application of CoNi-R.P.LSCM as a cathode catalyst for the electrolysis of CO2 containing impurities, like H2S, emitted from industrial sites, such as power stations and steel plants. In Chapter 4, I report a highly improved cathode catalyst by doping fluorine anions in oxygen sites of Ruddlesden-Popper material for CO2 electrolysis to produce CO in SOECs. The obtained fluorine-doped catalyst of La0.9Sr0.8Co0.4Mn0.6O3.9-δF0.1 (R.P.LSCoMnF) exhibited the higher electrochemical performance of 499 mA/cm2 at 1.3 V and 850 °C with the smaller polarization resistance of 0.853 Ω∙cm2 than those of undoped catalyst of La0.9Sr0.8Co0.4Mn0.6O4-δ (R.P.LSCoMn). Moreover, high faradaic efficiency of 98.6% and CO production rate of 135 μmol/cm2∙min were achieved for CO2 electrolysis reaction in the single cell with the R.P.LSCoMnF cathode catalyst. These enhanced performances are mainly attributed to the improved properties of surface exchange of oxygen and bulk oxygen diffusion by fluorine doping. More importantly, negligible sign of performance degradation was observed from galvanostatic test under CO2 gas streams containing H2S gas, and its structure was maintained even after exposure to 100 ppm H2S in an N2 gas stream at 850 °C for 10 h, suggesting that the R.P.LSCoMnF is highly robust against poisonous sulfur species. Therefore, this newly developed fluorine-doped Ruddlesden-Popper catalyst could be a promising cathode for the electrolysis of sulfur-containing CO2 gas stream which is emitted from steel making blast furnaces or power plants.