Effect Of Pore Structure Of Cathode Supported Solid Oxide Electrolysis Cell On CO₂ Electrolysis Performance

Solid oxide electrolysis cell?SOEC?electrolysis CO₂ technology shows great potential in the utilization and transformation of CO₂,it can use renewable energy such as wind,tidal and solar energy to provide electric energy and thermal energy for the CO₂ electrolysis.SOEC can utilize greenhouse gas CO₂ as well as produce CO,which a chemical industry raw material and gas fuel.It is considered to be the cleanest and most efficient energy cycle system.Therefore,in recent years,the development of high performance SOEC has paid more and more attention.SOEC is a complex electrochemical device,which consists of three basic components:porous anode,dense electrolyte layer and porous cathode.During cell fabrication process,the cell needs supported electrode to provide mechanical strength.In the initial stage,due to chemical and mechanical stability,electrolyte layer has been used as SOEC supporting electrode.Although the electrolyte material is a good oxygen ion conductor,it is far from the highest conductivity,and thick electrolyte layer causes huge ohmic loss.As a result,SOEC has transitioned from an electrolyte supported cell to an electrode supported cell in order to reduce ohmic loss.Nowadays,the majority of SOEC is cathode supported cell.Ni-YSZ has become the most commonly used cathode material for solid oxide cell due to its advantages of high performance and low price.In the Ni/YSZ cathode,Ni acts as the electronic conductor,YSZ as the ionic conductor and pore as the gas channel,which together constitute the electrochemical reaction interface where is called three-phase boundary?TPB?.During the electrolysis process,the reactant gas needs to be transported to the TPB through the pore channel for electrochemical reaction.Therefore,the cathode needs to have appropriate microstructure,which is very important for the gas diffusion,the removal of reaction products and low polarization loss.Therefore,the pore structure of the cathode support has become the main research focus of this paper.In this paper,the effect of pore structure on the performance of CO₂ electrolysis in the cathode supported solid oxide cell has been studied by optimizing the microstructure of the cathode.Firstly,in order to investigate the effect of pore forming agent content on electrode performance,starch was added as pore forming agent to prepare the porous cathode with porosity and pore size distribution.The gas diffusion in the cathode of SOEC plays an important role in the process of CO₂ electrolysis,and therefore the cathode microstructure needs low tortuosity and high porosity to reduce the gas diffusion resistance.The pore structure of the cathode is adjusted by applying different amount of pore forming agent in the process of cathode preparation.The more pore former added produced the higher porosity of cathode and the higher limiting current density.In practical application,high limiting current density is beneficial to diminish gas diffusion limitation.Based on the results,CO₂ electrolysis performance is greatly affected by gas diffusion,which is determined by the employed cathode pore structure.The pore structure of cathode support is effectively improved by adding pore forming agent.In addition,the advanced electrochemical impedance spectroscopy test technology is used to confirm the limit current density measured according to the current voltage curve.The impedance spectrum measured under the limit current density shows that the cell resistance only slightly variation,but the resistance measured above the limit current density significantly increases,which shows that the gas diffusion limitation greatly increases the polarization loss of CO₂ electrolysis.Secondly,the influence of the microstructure of two different cathodes prepared by mesh-templating phase-inversion method on the performance of CO₂ electrolysis has been studied.The mixed cathode slurry was cast into the mould,and a stainless steel mesh was placed in the middle of the slurry.Different microstructures are obtained on the top and bottom sides of the mesh,one is the microchanneled structure,the other is the dendritic structure,both of which have similar large-scale microchannels,but different microstructures in the functional layer area.By comparing the cells with different functional layers,it is found that the cathode with dendritic structure has higher performance in gas diffusion and polarization resistance than the cathode with microchanneled structure.In addition,the dendritic structure also prevents the cathode particles sintering when the electrolyte layer is densified at high temperature.Finally,the effect of the pore structure of the functional layer on the performance of SOEC has been further studied.Because the functional layer provides the electrochemical reaction site,and the reduction of CO₂ in the cathode is the main limiting step of CO₂ electrolysis,the microstructure of the functional layer greatly affects the electrolysis performance.The influence of the microstructure of the functional layer,including porosity,NiO/YSZ ratio and thickness,was studied by using the dendritic structure cathode with fast gas diffusion channel as the support.By adding different content of pore forming agent to adjust the porosity,cells with more porosity produce higher electrolysis current density.The 3D structure of functional layer with different ratio of NiO/YSZ is reconstructed by distance correlation functions?DCFs?,and the three-phase boundary density is calculated.The results show that the optimal ratio of NiO/YSZ is 60:40.The appropriate thickness of functional layer is 35?m,increasing the thickness of functional layer can provide more reactive sites,but thick functional layer will also hinder the gas diffusion.