Preparation And Electrochemical Study Of Proton-conducting Solid Oxide Fuel Cells

Energy crisis and environmental pollutions are problems that all country is now facing for the sustainable development.Fuel cells,which have been seen as a keystone for the future energy economy,have received considerable attention for their high energy conversion efficiency and low impact to environment as a mean of generating electricity.Among the fuel cell community,the solid oxide fuel cell (SOFCs) is a currently hot topic.However,the traditional SOFCs work at high temperatures,leading to many problems,such as electrode sintering,diffusion at interface and difficulty in preparation of seals and interconnect.The current trend in SOFC developments is the reduction of their working temperatures.Ceramic proton conductors have received much attention from the SOFC community because proton-conducting SOFCs would permit a reduction of the working temperature, which meet the demand of current trend of reducing the working temperature of SOFCs.This thesis investigates the chemical stability,sinterability,electrochemical performance and the thin-film preparation techniques for the BaCeO₃ based proton-conducting SOFCs.Chapter 1 describes the potential applications of proton-conducting SOFCs as well as its working principle and research progress.The trade-off relation between chemical stability and conductivity of proton-conducting electrolyte materials is intensively discussed.In Chapter 2,an in-situ reaction method is presented for preparing the proton-conducting electrolyte membrane with high quality.The key part of this method is to directly spray well-mixed suspension of metal oxides instead of pre-synthesized ceramic powders on the anode substrate in order to make the membrane dense and form the pure ceramic phase at the same time.Further,the in-situ reaction method promotes the densification of the supported membrane,which is proven to be a facile method for preparing protonic ceramic membranes.In Chapter 3,a thin Ba₃Ca_1.18Nb_1.82O_9-δ(BCN18) membrane electrolyte is prepared by the in-situ reaction method.It is the first time to realize the preparation of a thin-film BCN18 membrane.The obtained BCN18 membrane,which is about 15μm in thickness and showes high chemical stability against H₂O and CO₂,reaches high density after sintering at 1400℃.Furthermore,we study the electrochemical properties of a BCN18-based fuel cell.Although the cell performance of the BCN18-based fuel cell is not as good as the traditional BaCeO₃ based fuel cell, considering its high chemical stability,it still makes this material interesting for SOFCs at elevated temperatures,especially in an aggressive condition.In Chapter 4,Ta is introduced into BaCeO₃ lattice to form a new composition of BaCe_0.7Ta_0.1Y_0.2O_3-δ(BCTY10) for increasing the chemical stability of BaCeO₃-based materials.The research result shows that partially replacement of Ce by Ta can increase the chemical stability of barium cerate with only a little loss of electrical performance.The BCTY10 membrane can remain stable in 100%CO₂ at high temperatures.The BCTY10-based fuel cell generates a maximum power density of about 200 mW/cm² at 700℃.The BCTY10-based fuel cell remains stable in fuel cell working environment for more that 100 h,whereas the Ta-free BaCeO₃ fuel cell decays in a few hours.Chapter 5 describes an all solid state reaction for preparing a proton-conducting SOFC and also investigates the influence of the amounts of the pore forming additives in the anode substrates on the densification of the BCTY10 membranes.The result indicates that the supported BCTY10 membrane becomes denser with the increasing amount of pore forming additive in the anode.The BCTY10 membrane on a NiO-BCTY10 anode containing 30 wt.%starch achieves a high density and meet the requirement for using as an electrolyte for SOFCs.Furthermore,we find that Ta-doping strategy shows even better stability than the traditional Zr-doping strategy for stabilizing BaCeO₃.The single cell shows desirable cell performance,implying the all solid state reaction is a novel and easy way to prepare single ceils.The high stability of BCTY10 and the good cell performance indicates that BCTY10 is a promising material for proton-conducting SOFCs.In Chapter 6,we intensively study the influence of the doping of In on the chemical stability,sinterability and electrical performance for BaCeO₃-based materials.The result shows that indium is quite beneficial to the improvement of the sinterability for BaCeO₃ materials and the sinterability increases with the increasing doping amount of In.The supported BaCe_0.7In_0.3O_3-δ(BCI30) membrane reaches dense after firing at 1150℃,about 200-300℃lower than other rare earth doped-BaCeO₃ materials.The element of In increases the chemical stability of the BaCeO₃ materials.All the different levels of In-doped samples(from 10%to 30%) show much better chemical stability than that of the traditional rare earth doped BaCeO₃.The chemical stability of the In-doped samples is even better than that of BaCe_0.7Zr_0.1Y_0.2O_3-δ.The BCI30 membrane conductivity can compare with that of the traditional rare earth doped-BaCeO₃.Unlike the traditional strategy for stabilizing BaCeO₃,which indeed increases the chemical stability but greatly lowers the sinterability and conductivity of the samples,the In-doping strategy is beneficial to the improvement of the chemical stability of BaCeO₃ samples and enhances their sinterability in the meantime with little loss of electrical performance.A BCI30-based fuel cell generates a maximum power density of about 342 mW/cm² at 700℃and the open circuit voltage of the cell keeps stable for more than 100 h,indicating the In-doped BaCeO₃ materials are quite promising for application.Chapter 7 describes a single step co-firing process to prepare proton-conducting SOFCs,which has not been achieved before.The result proves the proton-conducting SOFC with the BaCe_0.7In_0.3O_3-δ(BCI30) electrolyte can be prepared by a single step co-firing process.Furthermore,we find that the co-firing temperature has great influence on the single cell performance.Although the cell co-fired at 1150℃shows the lowest polarization resistance and the cell co-fired at 1350℃shows the highest membrane conductivity,the cell co-fired at 1250℃seems to reach a proper compromise between the polarization resistance and the membrane conductivity, which leads to the lowest total cell resistance and the best cell performance.In Chapter 8,the researches presented in this dissertation are evaluated and future work concerning the development of proton-conducting SOFCs is discussed.