Structure And Properties Of Electrode Materials For Symmetrical Solid Oxide Fuel Cell

Solid oxide fuel cells(SOFCs)are an energy conversion device that directly converts chemical energy into electrical energy.It presents lots of merits,including high efficiency,low emission,all-solid-state structure,and unsurpassed fuel flexibility,and so has attracted more attention worldwide.In a typical configuration,SOFC contains one dense electrolyte and two porous electrodes.Due to their different chemical environments and electrochemical tasks,these two electrodes usually have different chemical compositions.Recently,symmetrical solid oxide fuel cells(SSOFCs)have attracted wide attention,in which one material is used as both cathode and anode simultaneously.Compared with the traditional SOFC,the manufacturing of the SSOFC only requires one sintering step for two electrodes,which can decrease the cell fabrication cost and minimize the incompatibility problems.More importantly,with this design,any performance loss caused by carbon deposition can be recovered by reversing the airflow.Despite these special advantages,there are still great challenges in finding a suitable electrode material,which can not only withstand the reducing and oxidizing atmospheres,but also can provide excellent electrochemical performance for both cathode and anode.Only a limited number of materials were found to meet these stringent requirements.Among the potential SSOFC electrodes,Mn-based A-site layered perovskite,LnBaMn2O5+?,has drawn much attention.In this work,A-site lanthanides element were screened by first-principles calculation from the viewpoints of charge transportation and catalytic activity.In addition,the chemical-induced expansion and catalytic activity of the material were modulated by B-site partial substitution to improve the electrochemical performance.First-principles calculations were performed on LnBaMn2O5+?(Ln=La,Pr,Nd,Sm,Gd,Y),to study the effect of different lanthanide elements on the lattice parameters,binding energy,and density of electronic states.The electronic structure characteristics of materials are correlated with the conductivity and catalytic activity,which provides a theoretical foundation for the selection of A-site lanthanides.The materials with Ln=Sm and Gd show high thermodynamic stability,while those with Ln=Pr,Nd and Sm have potential advantages in electrical conduction and catalytic activity.Considering the overall performance,Sm was selected as the A-site element for LnBaMn2O5+?.The structural evolution characteristics and oxygen content changes of SmBaMn2O5+? under different atmospheres and temperatures were investigated.The electrical conductivity and the catalytic activity of the materials in different atmospheres were characterized,and the single-cell performance was also studied.The results show that SmBaMn2O5+? keeps A-site layered perovskite structure,owns high conductivity and appropriate thermal expansion coefficient over a wide range of oxygen partial pressure.It shows outstanding catalytic activity toward oxygen reduction and fuel oxidation reactions.The polarization resistance values are 0.066 and 0.314 ? cm2 in air and H2 at 900?,respectively.The maximum power density(MPD)of La0.8Sr0.2Ga0.8Mg0.2O3-?(LSGM,-300 ?m)electrolyte-supported cell with SmBaMn2O5+? as symmetrical electrodes reaches 565 mW cm-2 at 900?.After the anode side is modified by 15 wt%Co-Fe alloy nanoparticles,the MPD can be further increased to 782 mW cm-2.In view of the strong metal-oxygen bond,Mn in SmBaMn2O5+? was partially replaced by Mg and Ti to control the chemical-induced expansion(CIE).By reducing the variation of nonstoichiometric oxygen content against atmosphere change,the CIE was decreased.The structural stability towards oxidation and reduction,as well as the thermal expansion compatibility between electrode and electrolyte were improved.The weight difference between reduced and oxidized SmBaMn1.9Mg0.1O5+? is 28%,which is smaller than that of SmBaMn2O5+?.The CIE of the Mg-doped sample in reduction and oxidation processes are 21%and 39%,respectively,much smaller than those of the undoped samples.Among several redox cycles,the ohmic resistance growth of SmBaMn1.9Ti0.1O5+? symmetrical cell caused by oxygen partial pressure changes is lower than that of SmBaMn2O5+?.The Mg and Ti doping will not seriously deteriorate the catalytic activity.The LSGM(?300 ?m)electrolyte-supported symmetrical cells with SmBaMn1.9Mg0.1O5+? and SmBaMn1.9Ti0.1O5+? as electrode deliver a MPD of 596 and 603 mW cm-2 at 90?,respectively,and show considerable short-term stability and good thermal cycle resistance.To further improve the catalytic activity of symmetrical electrode,the A-site deficient and B-site Co-doped(SmBa)0.9Mn1.8Co0.2O5+? was constructed.Cobalt nanoparticles were exsolved and coated on the particle surface in reducing atmosphere(anode),which were then oxidized to C03O4 in air(cathode).The decorated materials as electrodes show excellent oxygen surface exchange ability and remarkable catalytic activity.The polarization resistances are 0.039 and 0.214? cm2 in air and H2 at 900?,respectively.The reaction mechanism of anode and cathode was systematically studied.The hydrogen dissociation process is the main rate-limiting step of anodic reaction and charge transfer is of cathode.The existence of cobalt mainly promotes the hydrogen dissociation process of the anode,while the Co3O4 boosts the process of oxygen charge transfer and adsorption and dissociation of cathode.The LSGM(-300 ?m)electrolyte-supported cell with the decorated electrodes can achieve the MPD of 712 mW cm-2 at 90?,which is higher than that of other materials.The results demonstrate that the designed(SmBa)0.9Mn1.8Co0.2O5+? with different nanoparticle decorations is a promising electrode material for SSOFCs.