Surface Engineering Of Perovskite-Based Electrode For Enhanced High-Temperature Electrocatalytic Activity Towards Small Molecule Conversion

Solid oxide electrolytic cell（SOEC）,as a kind of all solid-state electrochemical device functioning at high-temperature,has the advantage of high energy conversion efficiency,and received extensive attention in recent years.Driven by renewable energy electricity,the use of SOEC to convert CO₂ into fuels and catalyze the oxidative dehydrogenation（ODH）of low-carbon alkanes to produce high value-added alkenes can simultaneously achieve the utilization of greenhouse gas and obtain high-value alkenes,which has important economic and ecological value.Developing new electrode materials with high activity and long-term stability is a key technical challenge in achieving the industrialization of this technology.Perovskite materials are ideal materials for SOEC electrodes because of their high thermal/chemical stability,the flexibility of composition,and high mixed ion electron conductivity.However,as a key component of SOEC,the catalytic conversion activity of perovskite electrode materials for small molecules（CO₂/low-carbon alkanes）still needs to be improved.Current research has shown that the surface properties of perovskite materials,such as electron transfer ability,the adsorption and desorption ability of small molecules on the material surface,are crucial for the catalytic performance of small molecule conversion.The performance of perovskite materials in catalyzing small molecule conversion reaction can be effectively improved by regulating the oxygen vacancy,oxygen species and oxygen activity of materials.However,in SOEC,the mechanism by which the surface properties of electrode materials regulate the activity of small molecule conversion reactions is still unclear,and the method for regulating the surface properties of perovskite materials is not clear.In this thesis,we regulate the surface properties of perovskite-based electrode materials,especially the properties of oxygen species,through in situ exsolution,applying electrical bias,and the coupling of cation doping and electrical bias.With the combination of in situ characterization technology,advanced spectroscopy technology and theoretical calculation,the structure-activity relationship between the surface properties of materials and catalytic activity was systematically revealed.The main research content and results are as follows:（1）An oxide loaded metal nanoparticle heterostructure cathode material was constructed through in situ exsolution method for effective CO₂ electrolysis reduction to produce CO,and the mechanism of how electrode materials surface properties affected the surface kinetics process of CO₂ reduction was revealed.Ruddlesden-Popper（R-P）phase oxide supported Co-Fe alloy nanoparticle heterostructure electrode material（Co-Fe-STCF）was successfully prepared by reducing stoichiometric double perovskite oxide Sr₂Ti_0.8Co_0.2Fe O_6-δ（STCF）in a H₂ atmosphere at high temperature.The cell with a mixture of Co-Fe-STCF and Sm_0.2Ce_0.8O_2-δ（SDC）as the cathode exhibited outstanding performance for CO₂ electrolysis.A current density of 1.26 A·cm^-2 as acquired at a bias of 1.6 V at 800°C,and the CO production rate reached 8.75 m L·min^-1·cm^-2 and the Faraday efficiency was close to 100%.Such high performance was attributed to oxygen vacancies in the oxide matrix and the exsolved metal nanoparticles,which promoted the adsorption and dissociation of CO₂ molecules on the Co-Fe-STCF-SDC surface.Moreover,the Co-Fe-STCF-SDC electrode also showed good stability for operation at 800°C under 1.2 V for 100 h in pure CO₂ environment.Our results indicate that in situ exsolution is an effective method for constructing high-performance electrode materials for CO₂ conversion.（2）By adjusting the electrical bias applied to the anode,we effectively modulated the oxygen species characteristics on the anode surface,which directly impacted the conversion rate and selectivity of the ODH reaction of ethane,and the mechanism of the influence of oxygen species on the activity of ethane ODH reaction on the anode surface was systematically revealed.We constructed a SOEC symmetric cell using Sr₂Ti_0.8Co_0.6Fe_0.6O_6-δ（STCF）as the electrode material,synchronously achieving the catalytic conversion of ethane to ethylene at the anode and the reduction of CO₂ to CO at the cathode.The optimal yield of ethylene reached66.3%at 800 ^oC,which was among the highest value reported in literature for ethane ODH with CO₂ as the oxidant.Using advanced spectroscopic techniques,especially the near atmospheric pressure X-ray photoelectron spectroscopy and adsorption spectroscopy（AP-XPS/XAS）based on the synchrotron,we observed experimentally that the formation of new oxygen active species(O_active)on the anode surface under the driving of the electrolytic voltage revealed that this ethane ODH activity was due to the activation of surface oxygen on the STCF anode by high oxidation activity.Density functional theory calculation further implied that electrochemically driven formation of active oxygen species on the STCF surface upshift the O2p-band center,and facilitates electron transfer and enhanced surface adsorption,leading to a strongly promoted dehydrogenation process.Our results indicate that the oxygen activity on the surface of electrode materials can be effectively regulated by electrochemical voltage for achieving high small molecule conversion activity.（3）By coupling cation doping and electrochemical bias modulation,we regulated the characteristics of oxygen active species on the surface of anode,the activation process of ethane hydrocarbon bonds was optimized and the deep oxidation was effectively suppressed,achieving efficient ethane ODH to produce ethylene.We conducted a systematic study on the ethane ODH performance of anode materials with three different Fe/Co ratios(Sr₂Ti_0.8Co_1.2O_6-δ（STC）,Sr₂Ti_0.8Co_0.6Fe_0.6O_6-δ（STCF）,and Sr₂Ti_0.8Fe_1.2O_6-δ（STF）).The reaction products analysis results indicate that an increase in Co content in the material is beneficial for improving ethane conversion rate,while an increase in Fe content is beneficial for improving ethylene selectivity.The STF anode material with the highest selectivity achieved a 71%yield of ethane ODH to ethylene at 800 ^oC and 1.0 V.Surface adsorption desorption analysis technology and soft X-ray absorption spectroscopy（XAS）technology shown that materials high Co content have better oxygen mobility and higher electron transfer ability.The density functional theory calculation shows that the increase of Co content makes the materials have higher lattice oxygen activity,which is reflected in the shift of the O 2p-band center relative to the Fermi level and the reduction of the oxygen vacancy formation energy,which is conducive to the dehydrogenation of ethane.On the contrary,with the increase of Fe content,the deep oxidation process was suppressed,leading to high selectivity.While high ethylene selectivity was achieved by using anode materials with high Fe,the ethane conversion rate can be improved by applying electrochemical bias,thereby achieving an increase in ethylene yield.Our results indicated that high ethane ODH performance on the SOEC anode can be achieved through the optimization of the intrinsic oxygen activity of the electrode material and the presence of appropriate electrochemical bias.Our approach can also be applied to guide the use of SOEC for other small molecule conversions at high-temperature.