Performance Of Solid Oxide Electrolysis Cell With Pr_0.3Sr_0.7Ti_0.3Fe_0.7O_3-? Cathode

Solid oxide electrolysis cell?SOEC?makes H₂ O and/or CO₂ electrolyzation by high temperature electrochemical reaction.It can environmental-friendly and efficiently convert electrical energy into chemical fuel energy.It has been taken into consideration as one of the most promising energy conversion devices and has been enjoying social public concern.Developing high performance of components materials,optimizing the preparation technologies and operation conditions of single SOEC or SOEC stacks,and promoting its commercial application has great realistic significances for solving environment pollution and energy crisis.Strontium ferrate(SrFeO_3-?),with perovskite structure and high electronic-ion mixed conductivity and excellent catalytic performance,has been comprehensively studied in the fields of oxygen-permeable membranes,oxygen sensors,and oxygen electrodes of solid oxide fuel cell?SOFC,reversible process of SOEC?.In recent years,the doped SrFeO_3-? exhibits excellent electrochemical performance as cathode of SOEC or anode of SOFC?collectively referred to the fuel electrode?.In this work,we co-doped Pr and Ti to strontium ferrite in order to enhance the phase stability in strong reducing condition by Ti doping to the B-site and to improve the catalytic activity by Pr doping to the A-site.Pr_0.3Sr_0.7Ti_0.3Fe_0.7O_3-??PSTF?material has been synthesized and used as SOEC cathode for H₂ O electrolyzation and CO₂ electrolyzation.Electrolytic efficiency,electrolytic reaction mechanism,and operation stability have been investigated.The PSTF powder was synthesized by a conventional solid-state reaction method.To verify its suitability for SOEC cathode,some basic physical properties of PSTF material have been studied and compared with freshly prepared sample and reduced sample by reducing gas,such as phase structure,powder morphology,cation valence and oxygen vacancy concentration,thermal stability,and mixed conductivity,etc.The results show that PSTF can maintain the original phase structure under high temperature and reducing atmosphere for SOEC cathode running.It has a reasonable conductivity and a good thermal stability.The cation valence decreases and oxygen vacancy concentration increases of PSTF material in reducing gas condition can further improve electrolysis reaction by providing more active sites for H₂ O or CO₂ adsorption.Furthermore,in order to confirm preparation and application successful of the electrolyzer using PSTF as cathode,chemical compatibility and thermal expansion matching of PSTF with YSZ and SDC electrolyte materials are inspected.The electrolyzers of PSTF/SDC/YSZ/LSM-YSZ and PSTF/YSZ/LSM-YSZ are assembled and used for pure CO₂ electrolysis,H₂ O electrolysis with reducing gas to prevent the oxidation of the cathode,and directly H₂ O electrolysis,respectively.Electrolytic voltage VS.current density characteristic curves?I-V curves?are used to assess electrolytic efficiency of the different electrolysis modes under a variety of operation temperatures and reactant concentrations.The current density reaches 302,934,656 m A cm–2 under voltage of 1.5,2.0,2.0 V for electrolyzer running for 60%H₂O/H₂/Ar,60%H₂O/Ar or 60%CO₂/Ar electrolysis at 800,850,900 °C,respectively.Through test,fitting and analysis of the impedance spectra of every electrolyzer or PSTF cathode,resistance loss of each part in the electrolyzer and cathode reaction steps are summarized with a series of operation temperatures,reactant concentrations and polarization current.The main resistance consumption of PSTF cathode is catalytic reduction reaction of reactant and dissociative adsorption of reactants/products.To study the operating stability,electrolytic voltage changing over time curves under constant current operation of the electrolyzer are carried out.Electrochemical performance degradations are both less than 2.6% after 20 h in H₂ O electrolysis mode with or without reducing gas.Delamination between the LSM anode and YSZ electrolyte is responsible for the decline.No degradation is found when the electrolyzer used for CO₂ electrolysis.From performance comparison of some SOECs with common LSCM-based or LSTO-based cathode materials,the SOEC with pure PSTF cathode without any composite or decoration shows excellent catalytic activities and lower polarization resistances under similar operation conditions.The electrochemical properties of PSTF cathode are systematically compared when it is exposed into gas atmosphere with or without safe gas.And the results prove that it achieves similar performances in electrolytic efficiency,cathode resistance and long-term stability.All the research fully proves that the PSTF material is a promising cathode candidate of SOEC and it can be used for direct H₂ O or CO₂ electrolysis without reducing gas.