High-Performance Oxygen Electrode And Electrolyte Materials Enabling Stable Operation Of Proton-Conducting Solid Oxide Cells At High Steam Partial Pressure

As a promising new energy technology,proton-conducting solid oxide cells（SOCs）can achieve the high efficiency conversion between chemical energy and electrical energy due to their fast proton conduction at low temperatures,which may fundamentally redefine the entire energy conversion approach in the future.The commercial development of proton-conducting SOCs is largely dependent on the availability of key component materials,the oxygen electrode and electrolyte.However,with the operating temperature decreases,the catalytic activities of oxygen reduction reaction（ORR）and water oxidation reaction（WOR）of the oxygen electrode decreases significantly,manifested as polarization loss dominating.In addition to the basic properties of high proton conductivity,higher steam partial pressure also requires higher stability of the electrolyte.Moreover,the inconsistent matching of thermal expansion coefficient（TEC）between oxygen electrode and electrolyte can also lead to the accumulation of stress at the interface,resulting in cracking.Therefore,in response to the above issues,this thesis mainly focuses on the design of key component materials,to achieve high performance and stability of proton-conducting SOC.The specific research contents are as follows:（1）Combining oxygen-ion with electron-conducting oxide La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ（LSCF）and proton-conducting oxide BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ（BZCYYb1711）to form an oxygen electrode for researching the electrochemical performance of proton-conducting SOC.The experimental results show that the peak power density of a single cell based on the LSCF-BZCYYb1711 composite oxygen electrode is 0.98 W cm^-2,while the LSCF oxygen electrode is only 0.68 W cm^-2.The difference in ORR activity is mainly related to polarization loss.In the EC mode,the addition of proton-conducting phase leads to the current density of the composite oxygen electrode reach 1.23 A cm^-2 at a working voltage of 1.3 V（20%H₂O）,and the WOR activity is also better than that of the LSCF oxygen electrode.The composite oxygen electrode can effectively expand the three-phase boundary region and increase the number of active sites,thereby accelerating the catalytic reaction.Meanwhile,the combination with electrolyte materials can also reduce the TEC of the oxygen electrode,and realize the stable electrolysis of proton-conducting SOC for 50 h without significant attenuation.（2）Based on NdCoO₃（NC）oxide,a highly active oxygen electrode Nd Ni_0.5Co_0.5O_3-δ（NNC）with triple conductivity(H⁺,e^-and O^2-)characteristics was designed by Ni doping modification,and its availability as a proton-conducting SOC oxygen electrode has been analyzed.Ni doping has less effect on the crystal structure of the material,but can induce changes in the electronic structure,resulting in the formation of more oxygen vacancies.Meanwhile,the TEC value of NNC decreases from 31.03×10^-6 K^-1 of NC to 20.58×10^-6 K^-1,which is more compatible with the electrolyte material.The peak power densities of the NC and NNC oxygen electrodes at 700°C are 0.54 W cm^-2 and 0.85 W cm^-2,respectively,while the electrolysis current densities at 50%H₂O and 1.3 V are 1.16 A cm^-2 and 1.98 A cm^-2,respectively.The superior ORR and WOR properties can be attributed to the increased oxygen vacancy concentration,which is conducive to generating more proton defects and improving proton conductivity.In addition,the NNC oxygen electrode can not only perform long-term reversible output in FC mode and EC mode,but also operate stably under different operating conditions.（3）Co-free oxygen electrode Ba_0.95Fe_0.5Sn_0.2Bi_0.3O_3-δ（BFSBi95）with A-site cationic defect were synthesized,and the effects of non-stoichiometric regulation on phase stability,thermal expansion characteristics,conductivity,proton hydration kinetics,and electrochemical performance were investigated.To maintain charge neutrality,BFSBi95 will form more bulk phase oxygen vacancies without changing the cationic valence state at the B-site.In addition,the number of chemically adsorbed active surface oxygen molecules in BFSBi95 is also higher than the stoichiometric BaFe_0.5Sn_0.2Bi_0.3O_3-δ（BFSBi）.Although the electronic conductivity of BFSBi95 is smaller than that of BFSBi,the ionic conductivity is higher and has a lower activation energy.A-site defect adjustment can also decrease the TEC value of the material.The BFSBi95 oxygen electrode provides a maximum power density of 0.36 W cm^-2 and an electrolysis current density of 0.56 A cm^-2（50%H₂O and 1.3 V）at 600°C,corresponding to polarization resistances of 0.46Ωcm²（open circuit voltage）and 0.10Ωcm²（1.3 V）,respectively.However,excessive A-site cation defects can also have a negative impact on the phase stability of the oxide.（4）BaZr_0.3Ce_0.5Y_0.1Yb_0.1O_3-δ（BZCYYb3511）electrolyte and Pr Ni_0.5Co_0.5O_3-δ（PNC）oxygen electrode were doped with Fe at B-site to further improve the stability and performance of proton-conducting SOC.For BZCYYb3511 oxide,Fe doping can not only increase the average grain size,thus contributing to the densification of the electrolyte layer,but also increase the proton transport number,thus accelerating proton migration.Fe doping can change the elemental valence state of PNC,resulting in a decrease in oxygen vacancy concentration.It is worth noting that one of the most important effects of Fe doping is that it can increase the TEC value of the electrolyte while decreasing the TEC value of the oxygen electrode,achieving a good match between the two and ensuring the durability of proton-conduction SOC.Although the single cell based on BaZr_0.3Ce_0.48Fe_0.02Y_0.1Yb_0.1O_3-δ（BZCYYb F3511）electrolyte and Pr Ni_0.4Co_0.4Fe_0.2O_3-δ（PNCF）oxygen electrodes do not exhibit the most excellent electrochemical performance,but the good thermal matching between key component materials and stable electrolytes enables the superior long-term stable operation of proton-conducting SOC.The above thesis focuses on the typical ABO₃-type perovskite oxides,and reasonably designs the oxygen electrode with high activity and the electrolyte with high stability by the strategies of composite,doping and defect regulation.Different characterization techniques are used to explore the enhancement mechanism of electrochemical performance from various aspects,and illustrate the optimization law of thermal expansion behavior on the interface structure,which provides new research ideas to achieve the high efficiency and stable operation of proton-conducting SOC.