Investigation And Development Of Cone-shaped Tubular Solid Oxide Fuel Cells For Portable Application

Solid oxide fuel cell （SOFC） is a highly efficient, environment friendly and all-solid-state electrochemical power generation device, which uses a solid ceramic material as theelectrolyte and has a wide range of applications. The cone-shaped tubular segmented-in-seriesSOFC design can achieve a relatively higher voltage and power in a limited space, goodthermal shock resistance and thermal cycling capability, so that it is specially suitable forSOFC stack for portable applications, which need light weight and compact volume. Thisthesis centers around the research and development of technique to frabricate cone-shapedtubular SOFC, new anode material and its anti-coking mechanism, a novel direct carbon solidoxide fuel cell （DC-SOFC） device to speed up the industrialization process of cone-shapedtubular SOFC for portable application.Gel-casting technique is developed to fabricate the green cone-shaped anode substratesand the performance of a single cell and stack are investigated. The results show that themicrostructure of the cell fabricated by gel-casting technique is ideal, YSZ electrolyte filmfabricated by co-sintering of the electrolyte and green anode substrates is dense and adhereswell to both the anode and cathode. The maximum power density of the single cell usinghydrogen as fuel is about600and900mW/cm²at750°C and800°C, respectively. The opencircuit voltage （OCV） is about1V. The maximum output power of the three-cell-stack usinghydrogen as fuel is about2.8W. When using methane as fuel, the maximum output power is3.0and4.0W at800°C and850°C. It means that the performance of the cell fabricated bygel-casting technique is fine. The performance of the stack decreases when measured under aconstant current density of10mA/cm²at850°C using humidified methane as fuel. So findingnew anode material with anti-coking ability for SOFC using hydrocarbons as fuel isimperative.A new anode material (Ni_0.75Fe_0.25-5MgO)/YSZ is developed. The X-ray spectrum of thefabricated Ni_0.75Fe_0.25O power indicats that Fe was doped into the crystal structure of NiO,forming a solid Ni1xFexO solution. The results show that the power density increases as aproper amount of Fe and MgO is doped into the anode, while the performance decreases astoo much MgO is added into the anode. A maximum power density of648mW/cm²at0.7Vand800°C for the SOFC with (Ni_0.75Fe_0.25-5MgO)/YSZ anode is achieved using humidifiedmethane as fuel. The stability test results for the cells with different anodes show that the cellwith Ni0.75Fe0.25/YSZ anode fails rapidly in methane. When MgO is added into the bulk anode,the voltage remains stable over the whole period of20h. This may be explaned as follows: on the one hand, the doped stable oxide MgO can suppress carbon fiber growth; on the otherhand, MgO particles located on the grain boundaries of YSZ make the YSZ skeleton strongenough to stand the stress caused by carbon fiber growth. SEM micrographs of the anodesubstrates of the single cells after the output performance and stability test further strengthenthe mechanisms above. This proves that （Ni0.75Fe0.25–5MgO）/YSZ is a promising anodematerial for cone-shaped tubular SOFCs operated on methane fuel.Based on the works above, we have researched on the performance of SOFCs with(Ni_0.75Fe_0.25-5MgO)/YSZ anode using CO as fuel. For the cell with either of the anodes, theperformance on hydrogen is better than that on CO, and that on moist CO is poorer than ondry CO. The durability results show that while the cells with traditional Ni/YSZ anode failafter operating on either dry or moist CO for about10h, the voltage of the cells with(Ni_0.75Fe_0.25-5MgO)/YSZ anode remains stable during the whole test time （40h）. And thepresence of water in CO fuel has no effect on the stability of cell with （Ni0.75Fe0.25-5%MgO）/YSZ anode. SEM images of the surface of the anodes after output performance andstability test shows that cells with Ni/YSZ anodes have more deposition than （Ni0.75Fe0.25-5MgO）/YSZ anode when using CO as fuel. The OCV of the two-cell-stack with （Ni0.75Fe0.25-5MgO）/YSZ anode is2V and its maximum output power is4W at800°C. A durability testof the two-cell-stack on moist CO is performed at a constant current density of83mAcm2. Itindicates that the stability of the as-prepared two-cell-stack with (Ni_0.75Fe_0.25-5MgO)/YSZanode is good during the whole testing time of90h. This character is very significant for theportable application of SOFCs operated on carbon fuel.A portable device has been designed for the application of direct carbon solid oxide fuelcell. The results show that Fe catalyst catalyses the Boudouard reaction of cabon fuel andimproves the cell performance. The OCV of the single DC-SOFC is1.05V and its maximumoutput power density is280mW/cm²at850°C. Stability test results show that the voltage canremain stable when the operating current is small and larger operating current resulted inshorter operation life. EDX analysis shows that the reddish fuel residue is mainly Fe2O3andindicates carbon fuel is depleted. The performance of the three-cell DC-SOFC stack forportable application is poor. For sealing is difficult for this portable device, further work isnecessary to solve the sealing problems of this portable device.