Application Study Of K₂NiF₄-type Oxides:Based On Intermediate-temperature Symmetrical Solid Oxide Fuel Cells

Solid Oxide Fuel Cell?SOFC?is all-ceramic structured electrical power generating device with high energy efficiency and low environment impact.SOFC can be applied in many fields due to its flexibility fuels such as hydrogen and hydrocarbons.The SOFC performance is to a large degree determined by the catalytic properties of the electrodes.The traditional SOFC configuration may be replaced by a new strategy,where the same electrode material could be used simultaneously as both anode and cathode to build a symmetrical Solid Oxide Fuel Cell?SSOFC?.There are several advantages using this SSOFC configuration,such as simple preparation,solving the problems of possible sulphur poisoning and coke formation on the anode which can be potentially addressed by simply reversing the gas flows.K₂NiF₄-type oxides have been attracted great attention due to their good structural stability,high electronic conductivity and ionic conductivity.As one type of electrodes for intermediate-temperature SOFC?IT-SOFC?,the related research is on the first step.Currently,most research of K₂NiF₄-type oxides focus on anode or cathode separately.Thus,it is of great importance and interest to develop an active electrode material that possesses a combined property with K₂NiF₄-type phase used as both anode and cathode.In this thesis,to understand the mechanism of ORR for A2BO₄-type oxide,the oxygen interaction mechanism on La2 Ni O₄?100?surfaces,including perfect and defect structure,was studied using first-principles calculations based on DFT at first.Second,the effect of CGO on properties of composite cathode containing CGO and La_0.8Sr_1.2Co O₄-??LSCO₄?K₂NiF₄-type structure oxide were investigated in detail and one possible mechanism of transport path for oxygen ions was imposed.Finally,we studied the K₂NiF₄-type oxides based on A2 Fe O₄ and A2 Mn O₄ and assess the feasibility of their use as symmetrical electrode materials for IT-SSOFC.We presented the structural and energetic results of O2 adsorbed onto the perfect and defective La2 Ni O₄?100?surface to elucidate the interaction mechanism between O2 molecule and cathode using atomistic computer simulation based on density functional theory.The results showed that the surface structure and the adsorbed configurations were vital for O2 adsorption.The adsorbed species on the perfect surface were energetically less favourable than defective surface.The Ni site was preferred with adsorption energy of 1.25 e V?Ni-super?and 1.80 e V?Ni-per?,much higher than these of La site,supporting the fact that transition metal cations are more active than lanthanon metals in K₂NiF₄-typecompounds.Surface oxygen vacancy was found enhanced the adsorption energy of O2 molecule on La2 Ni O₄?100?surface,in addition,oxygen vacancy could be an active site in O2 adsorption.The most stable configuration was Ni-O-Ni mode with the highest adsorption energy being 2.61 e V.This could be confirmed by the analysis of the local density of states?LDOS?and the difference electron density.The original material of LSCO₄ was prepared successfully with a citric nitrate process.A composite cathode material was prepared by mixing LSCO₄ and CGO.The XRD results showed that LSCO₄ is chemical compatibility with CGO and LSGM.Moreover,the Rp of LSCO₄-CGO cathode was smaller than that of without CGO added.The lowest polarization overpotential of 63 m V is measured for a LSCO₄-35% CGO composite cathode at current density of 313 m A cm-2 at 750 oC.The cell?LSCO₄-CGO/LSGM/Ni O?shows good performances with maximum output power densities of 152,291,and 515 m W cm-2 at 650,600 and 750 oC,respectively.The K₂NiF₄-type manganate electrode material,La0.6Sr1.4Mn O₄±??LSMO₄?was also prepared.The results of XRD and TGA showed that LSMO₄ has good stability and chemical compatibility with LSGM under reducing atmosphere.At 800 oC,the conductivities of LSMO₄ were 5.5 and 0.4×10-2 S m-1 in air and 5% H2/Ar,respectively.We explained the reason of huge difference of conductivity of LSMO₄ existing in different atmosphere by combined experiments and ab initio calculations.The LSMO₄ electrode polarization resistances in air and under 5% H2/Ar were 0.87 and 2.07 ? cm2 at 800 oC,respectively.The performance of a LSGM electrolyte-supported symmetrical fuel cell with the configuration of LSMO₄/LSGM/LSMO₄ tested and the power density was 59 m W cm-2 at 800 oC.Lax Sr2-x Fe O₄±??0.6?x?1.4?oxides were prepared successfully by solid state reaction method.Under reducing atmosphere,the structure of Lax Sr2-x Fe O₄±??0.6?x?1.4?oxides was stable and the electrode materials and the electrolyte LSGM have good chemical compatibility.The conductivity of Lax Sr2-x Fe O₄±??0.6?x?1.4?oxides decreased in both air and 5% H2/Ar when the content of La increased.The sample with x=0.6 has the highest conductivity and smallest polarization resistance?Rp=1.21 ?.cm2?in air while the sample with x=0.8 has the highest conductivity and Rp?7.5 ?.cm2?under reducing atmosphere.Meanwhile,the power density of La_0.8Sr_1.2Fe O₄±?/ LSGM/La_0.8Sr_1.2Fe O₄±? was 73 m W cm-2.The layered perovskite La_0.8Sr_1.2Fe_0.9M_0.1O₄±??M=Co,Cu,Mn?were prepared successfully by solid state reaction method.They all have good stability and chemical compatibility with LSGM under reducing atmosphere.After reduced,part of Co ions and Cu ions were reduced to nano-sized Co metal and Cu metal for and dispersed homogeneously on the La_0.8Sr_1.2Fe_0.9Co0.1O₄±? and La_0.8Sr_1.2Fe_0.9Cu0.1O₄±? surface respectively.At 800 oC,the conductivities of La_0.8Sr_1.2Fe_0.9Co0.1O₄±? were 44.8 and 0.14 S m-1 in air and 5% H2/Ar,respectively;for La_0.8Sr_1.2Fe_0.9Mn0.1O₄±?,they are 73.2 and 0.86 S m-1;forLa_0.8Sr_1.2Fe_0.9Cu0.1O₄±?,they are 25.2 and 0.23 S m-1,respectively.La_0.8Sr_1.2Fe_0.9M_0.1O₄±??M=Co,Cu,Mn?have proper catalytic activity for oxygen reduction reaction and hydrogen oxidation reaction.At 800 oC,the cell tests showed that the power densities of La_0.8Sr_1.2Fe_0.9Co0.1O₄±?/LSGM/La_0.8Sr_1.2Fe_0.9Co0.1O₄±?,La_0.8Sr_1.2Fe_0.9Cu0.1O₄±?/LSGM/La_0.8-Sr_1.2Fe_0.9Cu0.1O₄±?,La_0.8Sr_1.2Fe_0.9Mn0.1O₄±?/LSGM/La_0.8Sr_1.2Fe_0.9Mn0.1O₄±? were 275,337,and 183 m W cm-2,respectively.The results implied that La_0.8Sr_1.2Fe_0.9M_0.1O₄±??M=Co,Cu,Mn?oxides can be the potential electrodes for IT-SSOFC.