Preparation And Characterization Of Proton Conducting Solid Oxide Fuel Cells

As the fourth generation of new power generation technology after thermal power,hydropower and nuclear power generation,a cutting-edge energy storage strategy based on solid oxide fuel cell（SOFC）is rapidly developed to continuously and efficiently convert chemical energy from fuels into electricity and heat at high temperature working conditions,reducing power consumption and compensating waste heat without the need for precious metals as catalysts.As a consequence,it has gradually attracted world-wide attention due to their ability to reduce the dependence on fossil fuels and meet the“3E”criteria（Economy,Ecology&Efficiency）,in addition,providing further possibilities for the transition to a fully sustainable energy economy.In comparison with the traditional oxygen ion conducting solid oxide fuel cells（O-SOFCs）,the proton-conducting solid oxide fuel cells（P-SOFCs）has a number of merits such as lower operating temperature range（500-700 ^oC）and directly generating H₂O at the cathode side to avoid fuel dilution.However,the development of P-SOFCs lags far behind O-SOFCs,mainly due to technical challenges about the poor stability of the proton-conducting materials in an atmosphere containing H₂O and CO₂ under operating conditions and the difficulties in the fabrication of a thin electrolyte film.In recent years,P-SOFCs using proton conductors as electrolyte materials has expected to improve the fuel cell performance by taking advantages of their high ionic conductivity,high ionic transferring number and low ionic conducting activation energy（0.4-0.6 e V）in an intermediate temperature operation.Two main challenges which restricting the experimental research and the commercialization of P-SOFCs’application are high manufacturing cost and difficulty in scale-up.The easy volatilization of Barium further increases the difficulty in the sintering of electrolyte materials.Typically,Yttrium and Ytterbium co-doped Ba Zr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ（BZCYYb）electrolyte material is selected as the representative of this paper,which reported in the literature with better electrochemical properties.At the same time,this paper focuses on the preparation and related performance of P-SOFCs based on BZCYYb electrolyte material.The specific contents are as follows:（1）The calcination temperature of Ba Ce_0.7Z_r0.1Y_0.1Yb_0.1O_3-δelectrolyte material synthesized by solid state reaction（SSR）method was investigated.The phase structure,thermodynamic properties,chemical stability,conductivity at different atmospheres and mechanical strength of the material were characterized in detail.The powder with high electrical conductivity,good chemical compatibility and thermal matching with anode and cathode materials was obtained.And then,the migration mechanism of internal carriers(H⁺and O^2-)in BZCYYb and the phase transition results deriving from the thermal expansion curve were also explained,which further shows the superiority of this material suitable for P-SOFCs.（2）P-SOFCs based on BZCYYb electrolyte material were prepared by co-pressure combined with co-sintering techniques.The composition proportion and pre-sintering temperature of electrolyte and anode-supported materials were suitably adjusted through analyzing the sintering shrinkage curve of them.Finally,it is determined that the anode powder is the mixture of 15 wt%graphite,Ni O and electrolyte with higher shrinkage activity,which not only ensures the shrinkage matching between the electrolyte layer and the anode-supported layer,but also the densification of the electrolyte can be driven by the active shrinkage of the anode-supported layer,because the shrinkage of the anode layer is slightly higher than that of the electrolyte layer.The performance of the Ni O-BZCYYb/BZCYYb/LSCF-BZCYYb single cell was further tested,and the corresponding maximum power densities at 650,700,750,800 ^oC were 60.14,81.18,102.68 and 117.97 m W·cm^-2,respectively.Further stability test of galvanostatic discharge experiment showed an acceptable stability for～100 h operation under a constant current density load of 80 m A·cm^-2 at 700℃.（3）Under the premise of our previous work experience,the application of a dense proton-conducting electrolyte film via aqueous-based co-tape casting in conjunction with co-sintering technology was studied for the first time.This chapter focuses on the fabrication procedure of the aqueous-based co-tape casting method,including dispersing the optimal casting slurry evenly and stably and optimizing the overall process to reach the best formulation.A large area 80 mm×80 mm anode-supported composite film with good flatness and mechanical strength were successfully designed by co-sintering at 1450℃ for 5 h.Additionally,the effects of different addition ratio of potato starch（5,7.5,10 and 15 wt%）on the micro-structure of anode support were explored.By comparing the electrochemical performance of the full cell at the same test temperature and test atmosphere with the AC impedance spectrum analysis results under open-circuit voltage by Distribution of Relaxation Time Method（DRT）,it was confirmed that the anode-supported layer reached the ideal microstructure when the addition ratio of potato starch（pore former）in the anode slurry was set as 10 wt%.Moreover,the corresponding maximum power densities（MPD）of the single cell reached 134,205,293 and 389 m W·cm^-2 at 650,700,750and 800℃ in condition of humidified hydrogen（3 vol%H₂O）as the fuel gas and air as the oxidant.The smaller ohmic impedance（R_o）was obtained due to the accurate control on the thickness of the electrolyte layer（～12μm）,which is 0.76,0.56,0.43and 0.32Ω·cm²at 650,700,750 and 800℃,respectively.Hence,the electrochemical performance of the P-SOFCs was further optimized.