Synthesis And Performance Of Nanofiber Electrodes For Solid Oxide Fuel Cells

To develop clean and efficient energy technology is the urgent need of the sustainable development of human society. Solid oxide fuel cells(SOFCs) are energy conversion devices transforming the chemical energy of fuels and oxidants directly into electrical energy at high efficiencies while producing minimized emissions, which have attracted increasing attention. Developing intermediate-to-low temperature SOFC with reduced cost and extended life is the requirement of commercialization, and also the hotspot and trend of research and development. As one of the major components, the pros and cons of electrodes directly affect the electrochemical performance, long-term stability and lifetime of the entire SOFC. Based on electrode triple-phase boundary(TPB) theory, nanofiber-structured composite electrode materials were designed and prepared, and nanofiber-based composite cathodes and anodes with high performance were successfully fabricated. Furthermore, the applicability of these nanofiber-structured composite electrodes were studied via single cells.At first, this paper explored the emerging cathode material, Y0.3Sr0.7Co O3-δ(YSC), which is difficult to form phase pure material by sintering. YSC particles were synthesized by sol-gel method and calcination at 1100 °C for 96 h to form YSC accompanied by a small amount of Y2O3 impurity. Electrospinning technique was first applied to prepare YSC nanofibers. After calcination at 900 °C for 2 h, YSC phase was completely formed but still accompanied by a trace amount of Y2O3. The YSC nanofibers can be easily sintered onto GDC electrolyte at a temperature as low as 1000 °C, and the as-prepared YSC nanofiber cathodes possess high porosity that facilitates the accommodation of large amounts of GDC. The nanofiber-based YSC-GDC composite cathodes show good electrochemical performance. The YSC-GDC composite cathode with an optimal YSC:GDC mass ratio of 1:0.44 gives the interfacial polarization resistance(Rp) value of 0.200 Ω·cm2 at 650 °C, which are lower than that of particle-based YSC-GDC composite cathode. After the nanofiber-based YSC-GDC(1:0.44) composite cathode was polarized galvanostatically at 700 °C for 108 h, the polarization resistance increased obviously, indicating that the cathode performance degraded and its stability was not good enough.Based on the research results of YSC, Sm0.5Sr0.5Co O3-δ(SSC) nanofibers with controllable morphology were first achieved using electrospinning technique. After calcination at 800 °C for 2 h, phase pure SSC can be obtained. The SSC nanofibers were sintered onto GDC electrolyte at 1000 °C. Infiltration of GDC phase produced nanofiber-based SSC-GDC composite cathodes with good electrochemical performance. When the mass ratio of SSC to GDC is 1:0.87, the Rp reaches the optimal value of 0.038 Ω·cm2 at 650 °C, which is superior to the best data reported in literature. After the nanofiber-based SSC-GDC composite cathode was polarized at a constant current of 0.2 A·cm-2 at 650 °C for 107 h, the Rp value of the cathode was not increased. This result suggests that the cathode has good long-term stability. At the same time, the Rp values of the nanofiber-based SSC-GDC composite cathode on La0.8Sr0.2Ga0.8Mg0.2O3-δ(LSGM) electrolyte were also lower than the values reported in literature. After the SSC-GDC(1:0.87) composite cathode was polarized at a constant current of 0.2 A·cm-2 at 800 °C for 100 h, the Rp value of the cathode were changed from 0.0383 Ω·cm2 to 0.0379 Ω·cm2. This result suggests good electrochemical stability, which provides the possibility of the application of the SSC-GDC composite in single cells.La0.2Sr0.8Ti O3+δ(LST) nanofibers with controllable microstructure were first synthesized by electrospinning, and phase pure LST nanofibers were obtained at a very low calcination temperature of 600 °C, which facilitates the implementation of low electrode-electrolyte sintering temperature. The LST nanofibers were successfully sintered onto 1mol%Ce O2-10mol%Sc2O3-89mol%Zr O2(Sc SZ) electrolyte at 1000 °C, which was lower than the formation temperature of LST anode-Sc SZ electrolyte interface secondary phase at 1200 °C. Infiltration of GDC phase in LST nanofibers produced nanofiber-based LST-GDC composite anodes, and the Rp values decreased with the increasing amount of infiltrated GDC till the LST:GDC ratio of 1:0.92 was reached, after which no change was observed until LST:GDC ratio of 1:1.31. The nanofiber-based LST-GDC(1:0.92) composite anode shows the lowest Rp and the Rp values are 0.95, 0.63, 0.38 and 0.27Ω·cm2 at 800, 850, 900 and 950 °C, respectively, which are lower than the previously reported values. Under a constant voltage of 0.557 V, the nanofiber-based LST-GDC(1:0.88) composite anode was heated and cooled repeatedly between 550 and 950 °C at a heating-and-cooling rate of 13 °C·min-1. After 26 thermal cycles, the current density exhibited no degradation, which demonstrates good thermal shock resistance of the LST-GDC composite anode. After 11 redox cycles at 950 °C, the Rp value was still 0.261Ω·cm2. The result clearly shows that the nanofiber-based LST-GDC composite anode has excellent redox stability. Furthermore, the nanofiber-based LST-GDC composite anode sintered on LSGM also shows outstanding electrochemical performance.Based on the research results mentioned above, the nanofiber-based SSC-GDC composite cathode and LST-GDC composite anode were synthesized on each side of a LSGM electrolyte, producing a LSGM-supported single cell. The measurement results show excellent electrochemical performance of the nanofiber-based composite electrodes, which preliminarily affirms the practical applications of the nanofiber-based composite electrodes. The influences of electrolyte ohmic resistance, electrode ohmic resistance and electrode electrochemical reaction resistance on the cell kinetics were preliminarily analyzed using these single cells. The results show that the interfacial polarization resistance, ohmic resistance and electrochemical reaction resistance of the nanofiber-based LST-GDC(1:1.14) composite anode are obviously lower than those of the nanoparticle-based LST-GDC(1:0.5) composite anode, reflecting excellent electrochemical performance. By means of reduction of the thickness of the LSGM electrolyte, the power performance of single cells can be improved. At 800 °C, the single cell gave a maximum power density of 0.149 W·cm-2 using H2 as fuel, while a maximum power density of 0.155 W·cm-2 using 512 ppm H2S-H2 as fuel. After an 8-h operation under a constant voltage of 0.5 V using 512 ppm H2S-H2 as fuel gas at 800 °C, the current did not decline exhibiting good resistance to H2 S poisoning. When the cathode of the single cell was polarized at a constant voltage of-0.5V(electrolysis of water: H2O+2e-â†'H2+O2-) at 800 °C with a partial pressure of water vapor of 0.10 atm and a partial pressure of H2 of 0.90 atm, the current density was-0.332 A·cm-2, showing good performance of electrolysis of water. These studies of the single cells provide a basis for the practical application of the nanofiber-based SSC-GDC composite cathode and LST-GDC composite anode.