Study Of Lowing The Working Temperature Of Proton Conducted Solid Oxide Fuel Cells

As a green energy conversion equipment, solid oxide fuel cells (SOFCs) can directly convert the chemical energy from fuels into electrical energy with high efficiency. Driven by the energy crisis and enviromental deterioration, SOFCs have drawn special attention. Yet, the commercialization of SOFCs is not so desirable. This mainly results from the high operation temperatures of SOFCs (800 to 1000℃), which brings forth long starting time, high manufacturing costs, and sever ionic diffusion between cell components. Lowering the working temperatures of SOFCs to intermediate temperatures (500-700℃) can effectively alleviate these problems. Compared to tranditional oxygen ion conducting SOFCs (O-SOFCs), proton conducting SOFCs (P-SOFCs) have a lower activation energy for ions transferring and electrode reaction, and thus are more suitable to operate at low temperatures. It has been suggested that the main obstacle of P-SOFCs is 1) the contradiction between the chemical stability and the proton conductivity of electrolytes and 2) the relatively low electrode reaction kinetics at the depressed temperatures. In this work, several studies have been conducted to improve the electro-performance and the operating stablity of P-SOFCs, and the main results are listed as follows:(1) In Chapter 1, the present situation and the challenges for SOFCs, especially for P-SOFCs were introduced. Based on these analysis, and the theme of this thesis was made as:1) To explore suitable proton conducting electrolyte materials with good chemical stability and high proton conductivity; 2)To improve the electro-performance of P-SOFCs at the depressed operating temperatures.(2) In Chapter 2, Novel fluoride doping was firstly applied to modified the properties of BCS (BaCeo.sSmo.2O3-δ) electrolytes. The new compound of F-doped BCS has demonstrated significantly improved chemical stability in CO2 containing atmosphere with a better proton transfer, which benefits electrode reactions. Besides, inner marked method was used to explore the ionic transportation during the CO2-BCS reaction. It’s found that both surface reaction and bulk reaction are involved in this reaction, and that the former owns a higher reacting rate than the later. This finding may suggest that BaCeO3 locating in the porous cathode layer showed a faster degrade rate in CO2 containing atmosphere because of the higher surface area of cathode.(3) In chapter 3, SSC impregnation was implemented to optimize the microstructure of SSC-SDC composite cathode, and successfully reduces the polarization resistance from 1.14Ω cm2 to 0.23Ω cm2 at 700℃. The activation energy for oxygen reduction also decrease from 1.14 to 0.78 eV. Impedance spectra analysis showed that the resistance reduction is mainly due to the decreasing of the low-frequency contribution, which is related to the surface diffusion step.(4) In chapter 4, anode substrates with fingerlike pore structure and a functional layer are successfully made by a phase-inversion tape casting method. Based on these substrates, single cells with BZCY35-LSCF/BZCY35/BZCY35-Ni structure are fabricated. Great electrochemical performance is achieved with such cells, of which peak power densities of 748 and 192mW/cm2 are obtained measured at 700 and 500℃, respectively. Fueled with CH3OH, the cell also demonstrate good electrochemical performance, with a peak power density of 500mW/cm2 at 700℃, suggesting that the special microstructure of anode substrates benefit the fuel gas diffusion and effectively catalyze the oxidation reaction of hydrocarbon fuels..(5) In Chapter 5, Application of Ba2Co9O14 as a potential cathode of P-SOFCs is evaluated. The properties of cells change with the content of BCO in BCO-BZCY composite cathodes, and reach maximum with a 70 wt%BCO-30% BZCY cathode. Operating at 750℃, peak power density of 675mW cm-2 was achieved for the cell using70wt%BCO-30% BZCY cathode.(6) Chapter 6 summerizes the works in this thesis, describes the achievements of each works and observes their realistic meaning, finally, looking forword to the future works based on the reaserch we have done.