Applications Of Molybdenum-based Anodes And Catalysts In Internal Reforming Solid Oxide Fuel Cells

Solid oxide fuel cell?SOFC?is a device with excellent conversion rate to transform chemical energy of fuels into electrical energy,which offer high electric conversion efficiency,high environmental performance and fuel flexibility,making the SOFCs one of the most promising power generation technologes.Compared with other fuel cell counterparts,hydrocarbon fuels can be used in SOFCs directly without the use of a complex and costly external reformer.There are two methods of internal reforming technology:one approach is achieved in the SOFC anode,named direct internal reforming,the other is performed with a catalyst,called indirect internal reforming.Therefore,the investigation and development on highly effective internal reforming anodes and catalysts bring about a great advance in promoting the development of SOFC techniques.Conventional Ni–YSZ composite anode,the most widely used anode material at present,offers high oxygen-ionic and electrionic conductivity,and electrocatalytic activity for fuel oxidation.However,cells with Ni–YSZ as anode always suffer from a severe degradation due to carbon deposition or sulfur poisoning using hydrocarbon fuels.Consequentely,the development of novel SOFC anodes with excellent sulfur and coking tolerance plays a key role of in commercialization of SOFCs.The present work will focus on the development of mixed ion-electronic conductors?MIECs?as SOFC anodes.Perovskite anode materials favor the charge and oxygen-ion transfer,promote the electrochemical oxidation of fuels and serve as the reaction zone because of their defect nature,thus the area of electrochemical reaction expands considerably,and the coking deposition and sulfur poisoning would be suppressed effectively.In the present work,we investigate the application and properties of several molybdenum-based anode materials and catalysts in internal reforming SOFCs.In order to improve the durability,the selection of the B-site element in A₂BB'O₆type double perovskites plays a key role for the electrode performance.Given the improvement on durability of Cr and the influence of Mo on the electrochemical activity,Mo-based double-perovskite A₂CrMoO_6–d?ACM;A=Ca,Sr,Ba?serie as SOFC anodes are investigated and characterized.The electrode powders were synthesized by solid-state reaction,and were confirmed by XRD characterization to be perovskites.The crystalline structure refinement indicates the structures of CCM,SCM and BCM are orthogonal?P bnm?,cubic?Fm 3m?and hexagonal?P 6₃–mmc?,respectively,where SCM possesses the highest symmetry.The ACM anodes demonstrate a pretty high electrical conductivity compared to most of the present anodes,especially the SCM anode.Owing to the higher concentration of the oxygen-vacancy and the Mo⁶⁺carrier,SCM also demonstrate better reducibility.At low oxygen partial pressures,ACM anodes demonstrate superior thermal and chemical compatibilities with the La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85?LSGM?electrolyte.Cells with SCM as anode deliver much more stable sulfur tolerance compared to the Ni–YSZ anode.Meanwhile SCM anode offers compatible power output compared to that of the Ni–YSZ anode.Given the above superior sulfur tolerance of the Cr-rich anode materials,Cr was replaced by Fe to improve the inadequate electrochemical activity,and La was incorporated at the A site to further promote the conductivity.Sr_2–xLa_xFeMoO_6–d?SLFM,0?x?1?was synthesized and investigated as the anode material for SOFCs.At RT,undoped Sr₂FeMoO_6–d?SFM?is crystallized in tetragonal?I 4/mmm?.Slight substitution of La for Sr does not change its structure,but the B-site cation order was found to decrease;when heavily doping the A site with La?0.4?x?1?,the cation order continued to decrease,accompanied by the decrease on symmetry.SLFM tends to transfer from orthorhombic to monoclinic structure.The grain size is effectively decreased by the incorporation of La,which is confirmed by SEM characterization.At low oxygen partial pressures,SLFM anodes demonstrate superior thermal and chemical compatibilities with the Ga_0.1Ce_0.9O_1.95?GDC?electrolyte.In addition,by slightly doping La for Sr,an improvement on conductivity is achieved.Whereas heavily doping would induce the decrease of Mo⁶⁺concentration,which serves as the charge carrier.Sr_1.8La_0.8FeMoO_6–d provides an improvement over the identical single cells with undoped SFM as the anodes on cell performance when operating in hydrogen and methane.Especially the power output of the cell with Ni–YSZ anode in hydrogen is nearly doubled by the Sr_1.8La_0.2FeMoO_6–d anode.When operating in methane,the single cell delivers a MPD of 790 mW cm^-22 at 800°C because of the superior electrocatalytic acitivity of Sr_1.8La_0.2FeMoO_6–d for methane oxidation.Consequencely,the coking and sulfur tolerance of SFM anode is maintained whereas the power output is improved.Based on the above investigations,the Mo-based double-perovskite anodes demonstrate excellent electrical and electrochemical activity,and high compatibility with electrolytes.The MoO_x serves as the redox center,providing conductivity and catalyzing the fuel oxidation.We assume the electrocatalytic activity of the MoO_x might be,in part,restricted by the perovskite constrain.Given the intrinsic MIEC nature of MoO₂ and the superior electrocatalytic activity of Ni–YSZ,we designed a new SOFC with highly active MoO₂ as the anode barrier layer to compare with the design with MoO₂ as the internal conversion catalyst of methane.The power output of single cells was investigated on two designs.For the former,the MoO₂ catalyst is used as the coking resistant barrier covering the surface of the Ni–YSZ anode,allowing the direct electrochemical oxidation of methane to occur on the anode;For the latter,the MoO₂ catalyst is placed at the fuel inlet of the SOFC,serving as the partial oxidation catalyst.Based on our experimental data,the design with MoO₂ as the catalyst at the cell inlet performs better because the design eliminates the extra interface polarization.Good stability was achieved continuously operated in a mixture of methane and air for>150 h under potentiostatic mode without obvious coking deposition on the MoO₂ catalyst and the Ni–YSZ anode.