The Study Of Anode Catalysts For Direct Ammonia Fuel Cell

Solid oxide fuel cells(SOFCs)are electrochemical devices that are able to directly convert chemical energy in the fuels into electricity with high efficiency,low emission and excellent fuel flexibility,and thus have attracting worldwide and intensive attention in distributed power generation and energy storage,hydrogen production and power supply for ships and vehicles.Hydrogen fuel is regarded as an ideal fuel for SOFCs because of the high efficiency and no carbon emissions.However,the technologies of storage and transportation of hydrogen limit its wide application.Ammonia(NH₃)is a carbon-free hydrogen carrier with the advantages of high hydrogen contents(17.8%),low cost,and the well-established storage and transportation that has links to make it an accessible fuel source.When NH₃ fueling with SOFCs,it decomposes into hydrogen and nitrogen over the Ni-based anode.The generated hydrogen then acts as a fuel for subsequent electrochemical reaction for the power generation.High operating temperature of SOFCs also alleviates the cost and requirement of a separate NH₃ cracking unit and increases the ionic conductivity so ohmic loss at the electrolyte can be minimized.Although traditional nickel-based cermet anodes exhibit excellent power output when using NH₃ as fuel,they are susceptible to nitrid and suffer from a severe damage over anode due to the low efficiency of amminia conversion at anode,which severely restricts their commercialization.Thus,the development of highly efficient anode catalyst is an effective strategy to improve the cell performance and durability for NH₃-fueled SOFCs.The improved cell performance and durability are key factors of the practical application of NH₃-SOFC.This thesis mainly focused on the investigations of the NH₃decomposition catalytic activity,electrochemical and durability of NH₃-SOFC anode.The main research work and results are showing as following:(1)In this work,NH₃ power generation was carried out using a commercial double cathode flat-tude solid oxide fuel cell(DSC)with Ni/Y₂O₃-ZrO₂(Ni/YSZ)anode.The influences of type of fuel,temperature,fuel flow rate and operational conditions for cell performance and durability on NH₃-fueled DSC,were systematically investigated.The thick anode support(?0.49 mm)and symmetric structure significantly enhanced the structural strength of the DSC cell,which showed 20 times of the anti-fracture load than that of conventional planar SOFCs.Furthermore,the inner channels of the DSC cell could serve as an efficient micro-reactor for NH₃ decomposition.When NH₃ fuel was supplied to DSC,the power density reached 195 m W cm^-2 at 750?,which was close to that of H₂-fueled one(198 m W cm^-2).With the temperature decreased,ammina conversion in DSC anode decreasd,while the difference of cell performance between NH₃ and H₂ fuel increased.We also studied the influence of NH₃ flow rate on the catalytic activity and cell performance,and the results demonstrated that the ammonia conversion rate and cell performance in DSC cell decreased with the increasing flow rate of ammonia.Meantime,no apparent degradation of the NH₃-fueled DSC cell was observed after stability test for120 h at 750?.More importantly,the DSC cell constantly fed with NH₃ exhibited stable open circuit voltages throughout a thermal cycling test between 550 and 750? for 15cycles,indicating that no microstructural damage was caused by such severe operation condition.In addition,we developed the 3D thermo-electro-chemo-mechanical coupling model based on the actual structure of NH₃-fueled DSC and corresponding experimental results,verifying the structure of DSC was more suitable for NH₃ power generation compared with planner SOFC.(2)The key point of NH₃-fueled SOFC is the development of highly efficient NH₃decomposition catalyst at low temperature(<600?).Here,proton-conducting oxide,Ba(Zr,Y)O_3-?(BZY),was investigated as potential support material to load Ni metal by a facile one-step impregnation method using YSZ as starting material.The influence of including Ni loading,Ba loading,and synthesis temperature,of Ni/BZY catalysts on the catalytic activity for NH₃ decomposition were investigated.The Ni/BZY catalyst with Ba loading of 20 wt%,Ni loading of 30 wt%,and synthesized at 900? attained the highest NH₃conversion of 100%at 560?.The kinetics analysis revealed that for Ni/BZY catalyst,the hydrogen poisoning effect for NH₃ decomposition was significantly suppressed.The reaction order of hydrogen for the optimized Ni/BZY catalyst was estimated as low as-0.07,which was the lowest one to the best of our knowledge,resulting in the improvement in the activity.H₂ temperature programmed reduction and desorption analysis results suggested that the strong interaction between Ni and BZY support as well as the storage capability of hydrogen of the proton-conducting support might be responsible for the promotion of NH₃ decomposition on Ni/BZY.Based on the experimental data,a mechanism of hydrogen spillover from Ni to BZY support was proposed.(3)In order to further improve the NH₃ catalytic activity at DSC anode,we took advantage of the structure of DSC by adding extra catalyst(Ni/BZY and Ni/YSZ catalyst)into the inner channels of the anode support.It was found that adding Ni/BZY catalyst into the inner channels of the anode support greatly promoted the NH₃ conversion rate in DSC cell(83?100%@750?,62%?95%@650?),which was higher than adding Ni/YSZ catalyst(83?95%@750?,62?70%@650?).The cell performance on NH₃-fueled DSC with Ni/BZY catalyst achieved electrochemical performance comparable with H₂-fueled SOFC at 650?750?.In addition,when NH₃ was used as the fuel,DSC with Ni/BZY catalyst exhibited excellent durability than that of DSC without catalyst.By establishing the validated model for NH₃-fueled DSC,we identified the variation of physical filed after adding extra catalyst in DSC cell.Thus,our methodology represented a promising strategy for developing high-performance NH₃-fueled SOFC,which was also available for other application,such as hydrocarbon-fueld SOFC and SOFC electrochemical cell.(4)Based on the unique anode structure of DSC,the BZY phase was synthesized on the surface of the porous Ni/YSZ anode by in-situ impregnation method to enhance its catalytic performance.The effects of incorporating different content of Ba on the phase composition and microstructure were systematically studied.And the cell performance and long-term stability for DSC with Ba-Ni/YSZ anode with supply of NH₃ and dry methane fuel were also investigated.The results showed that BZY was formed on 3%Ba-Ni/YSZ anode surface,it exhibited higher catalytic performance than that of Ni/YSZ anode under NH₃atmosphere(83%?95%@750?).The catalytic performance and cell performance decreased with increasing Ba contents from 3 wt%to 6 wt%,which was due to the high contents of Ba damaged the microstructure of anode.More importantly,DSC with 3%Ba-Ni/YSZ anode maintained superior stability during 350 h,compared with Ni/YSZ anode.On the other hand,when dry methane was used as the fuel,3%Ba-Ni/YSZ anode achieved methane conversion of 100%at 750? and exhibited good stability over40 h under a constant current density of 150 m A cm^-2.In addition,3%Ba-Ni/YSZ anode exhibited excellent carbon-deposition resistance,which was greater than that of Ni/YSZ anode.Density functional calculation(DFT)was used to analyze the adsorption/desorption characteristics of hydrogen on the surface of 3%Ba-Ni/YSZ anode.The calculation results revealed that the adsorption energy of H on the surface of the BZY was significantly higher than that on the surface of YSZ or Ni.Meanwhile,the kinetic analysis of NH₃ decomposition indicated that 3%Ba-Ni/YSZ anode had a lower hydrogen reaction order(-0.08)than Ni/YSZ anode(-1.05).Thus,the above results confirmed that hydrogen spillover effect on 3%Ba-Ni/YSZ anode,which promoted dehydrogenation reaction by facilitating H species migrated from hydrogen-enriched Ni to BZY support and suppressed the adsorption of H species on the surface of Ni.