Study on Fabrication and Performance of Proton Conducting Solid Oxide Fuel Cells with BaCeO3-BaZrO3 Based Electrolyte

Author:Liu Zhi Jun

Supervisor:liu jiang


Degree Year:2019





Solid Oxide Fuel Cells(SOFCs)is an electrochemical energy conversion device which can directly convert chemical energy in fuels into electricity with high efficiency and environmental friendliness.Low-temperature SOFCs based proton conductor have attracted much attention worldwide owing to their potential long-term stability and their low activation energy for proton conduction.Recent years,there have been inspiring research and development on applying the proton conducting oxides in low temperature solid oxide fuel cells and high temperature electrochemical synthesis.Many perovskite-type oxides show high proton conductivity in a reducing atmosphere.In fact,perovskite-type oxides with significant protonic conductivities are found from those with large size of A-cation,such as BaCeO3-based and BaZrO3-based materials.While BaCeO3-based oxides exhibit high protonic conductivity,they are not stable at low temperature and under CO2-containing atmosphere.BaZrO3-based materials are stable but low grain boundary conductivity.Meanwhile,BaCeO3-based and BaZrO3-based materials are rather refractory.So it is imperative to optimize the BaCeO3-BaZrO3 based materials for achieving a good compromise between ionic conductivity and chemical stability for SOFCs application at lower operation temperature.Therefore,in this work,we systematically study the sinterability,chemical stability,electrochemical performace of BaCeO3-BaZrO3 based electrolyte.Moreover,symmetrical fuel cell,electrolyte supported and anode supported proton conducting SOFC is fabricated.The best performance is observed in the anode supported SOFCs with the configuration of Ni-BZCNY/BZCY-2 mol%Ni-Fe/LSCF-BZCY.The main contents are specifically demonstrated as follows:Firstly,proton conductor Ba(Zr0.1Ce0.7Y0.2)O3-δ(BZCY)with perovskite structure have been synthesized by a cost effective solid reaction sintering method.High quality,fully dense,and large-grained BZCY electrolyte material has been developed by addition of nickel oxide.Symmetrical solid oxide fuel cell with the configuration of Ag-BZCY/BZCY/Ag-BZCY has been fabricated and shows high open circuit voltages(OCV),indicating that the electrical conductivity of the BZCY was not adversely influenced by the addition of a small NiO.Secondly,the sintering aid amount,sintering temperature,and sintering time were optimized by analyzing the phase formation,sintering behavior,microstructure,densification and conductivity for a series of as-prepared BZCY electrolyte pellet with different NiO as sintering aid.High sinterability,fully-densified BZCY pellets have successfully fabricated by using 0.5 wt%NiO additive at a sintering temperature of 1400oC for 5 h.The electrochemical performance for a series of single cells with different concentration NiO modified BZCY electrolyte are measured and analyzed.The optimized composition of 0.5 wt%NiO modified BZCY as an electrolyte supporter for solid oxide fuel cell demonstrate a high electrochemical performance.Furthermore,partial NiO might dissolve into the perovskite lattice structure and the other NiO react with BZCY to form BaY2NiO5 secondary phase.Excessive NiO is especially detrimental to the electrical properties of BZCY and thus lower the open circuit voltage.Thirdly,a high electrochemical performance of anode supported proton conducting SOFC at medium-low temperatures have been achieved by assembling a BZCY electrolyte,a LSCF-BZCY composite cathode and Ni-BZCNY anode.The thickness of electrolyte is 25μm and the effective area is 0.2 cm2.Moreover,BZCY membrane ionic conductivity is significantly enhanced and the electrode polarization resistance is as low as 0.05Ωcm2 at 700oC in humidified hydrogen(3 vol%H2O).Simultaneously,the maximum cell power outputs of 912 mW cm-2 is obtained at 700oC.Even at 450oC,peak power density reaches 205 mW cm-2.Fourthly,we report our findings that transition metal additives not only function as sintering aids but also affect the electrical property,stability,and even catalytic activity of proton conducting ABO3-type perovskites.A solid solution of yttrium doped barium cerate and zirconate,BaZr0.1Ce0.7Y0.2O3-δ(BZCY),is selected as the substrate material.2 mol%of Ni1-xFex(x=0,0.1,0.3,0.5,0.7,0.9,and 1.0)oxides and 4 mol.%of FeO1.5 are respectively added into BZCY to prepare electrolytes of Ni-based anode-supported SOFCs.All the electrolytes with additives can be sintered to almost fully dense at 1400oC for 5 h,while BZCY without additive is porous.XRD spectra show that Ni and Fe are doped into the lattice of BZCY.For the first time,we discover positive function of Fe additive.With 2 mol.%of Fe added,the conducting activation energy of BZCY is reduced to 0.35 eV compared to that with Ni additive,0.42 eV.At lower temperatures,the SOFCs with Fe-added electrolytes have better electrochemical performance than that with Ni.Of all the SOFCs prepared with a regular(unoptimized)LSCF-based cathode,the cell with BZCY electrolyte added with 2 mol%Ni0.5Fe0.5 gives the highest power density,which is 973 mW cm-2 at 700oC and 120 mW cm-2at 450oC.The polarization resistance of the SOFC with 4 mol.%Fe-added electrolyte is the lowest among all the prepared SOFCs.Also,the BZCY with addition of Fe reveals a much improved tolerance to CO2.Based on theoretical analysis,we conclude that properties of proton conducting oxides can be tuned by doping proper amount of transition metals into the B-sites of the perovskites.This work provides a new path for exploring and optimizing novel proton conducting ceramics with good sinterability and stability as well as high conductivity and catalytic activity.