Preparation And Properties Of La-Ni-O Based Composite Bifunctional Oxygen Electrodes

Reversible solid oxide fuel cell?RSOFC?operated at high temperature,as an efficient energy conversion and storage technology,can not only convert chemical energy to electricity in solid oxide fuel cells?SOFC mode?,but also could convert excess electricity to chemical energy in solid oxide electrolysis cells?SOEC mode?,which realized the efficient utilization of energy.Moreover,rechargeable zinc-air batteries operated at room temperature is regarded as potential energy devices due to its high energy density,environmental friendliness and low cost.Two important reactions,oxygen reduction reaction?ORR?and oxygen evolution reaction?OER?,occur on the oxygen electrode of these reversible batteries.However,the slow kinetic process restricted the development of reversible batteries.Therefore,efficient bifunctional oxygen electrodes with both ORR and OER activities are essential to the performance of reversible batteries.The purpose of this paper is to develop efficient bifunctional oxygen electrodes for high temperature RSOFC and room temperature rechargeable zinc-air batteries.Bifunctional properties of La-Ni-O based oxygen electrodes prepared by the co-synthesis method were studied,and the mechanism of enhanced performance was also analyzed.Lanthanum nickel-Ce_1-x-x La_x O_2-?composite was prepared by the co-synthesis method?CS?.The phase formation mechanism during the co-synthesis procedure was analyzed.The obtained composite?L2N1-L3N2-LDC?was compsoed of La₂NiO_4+??L2N1?,La₃Ni₂O_7+??L3N2?and La-doped CeO₂?LDC?.LDC in composite was determined to be Ce_0.56La_0.44O_2-?by calculating the cell parameters,which provided a reference for La-Ni-O composite prepared by the co-synthesis method.The impedance spectra of L2N1-L3N2-LDC under various oxygen partial pressure revealed that the charge transfer process is the rate determining step of electrode reaction at 600-700?.L2N1-L3N2-LDC applied as the oxygen electrode of RSOFC showed referably electrochemical properties both in SOFC and SOEC modes,which mainly due to the small particle size and uniform mixing resulting in the expansion of triple phase boundary?3PB?.RSOFC with L2N1-L3N2-LDC oxygen electrode revealed high reversible cycling stability after SOFC/SOEC cycling for twice cycles.In order to compare the influence of synthesis methods on the properties of composite electrodes,composite?CS-L2N1-LDC?was prepared by the co-synthesis method.RSOFC with CS-L2N1-LDC oxygen electrode revealed higher electrochemical performance than that of physical mixed oxygen electrode both in SOFC and SOEC modes.Furthermore,the mechanism of enhanced properties of CS-L2N1-LDC was also analyzed.This result showed the feasibility and advantages of lanthanum nickel-based composite prepared by the co-synthesis method applied as RSOFC oxygen electrodes.Bifunctional oxygen electrodes played a decisive role in properties of rechargeable zinc-air batteries operated at room temperature.Therefore,Sr,Co co-doped La_1.7Sr_0.3Co_0.5Ni_0.5O_4+??LSCN?was prepared by the sol-gel method.The effects of Sr,Co doping on the structure and bifunctional activities of LSCN were also analyzed.Compared with L2N1,ORR and OER properties of LSCN were improved significantly.Moreover,LSCN exhibited higher intrinsic activities,which mainly due to mixed valence of Ni/Co and higher surface concentration of O₂^2-/O^-.The total discharge time of the primary zinc-air battery with LSCN bifunctional oxygen electrode achieved about 144 h after five replacements of zinc plates.The output performance of the primary zinc-air battery was not significantly reduced during discharge process.The charge-discharge voltage difference of rechargeable zinc-air batteries using LSCN bifunctional oxygen electrode increased from 1.07 V to 1.26 V?increased by 0.19 V?after 100 cycles?100 h?of charge-discharge cycle,indicating the high bifunctional properties and stability of LSCN.For the layered perovskite La_n+1Ni_n O_3n+1?n=1,2,?????oxides,LaNiO₃?n=??exhibited the best bifunctional activities.In order to further improve its bifunctional activities of LaNiO₃,Ag/LaNiO₃ was prepared by the co-synthesis method.The nano-Ag particle adhered tightly on the surface of LaNiO₃ particles.Compared with LaNiO₃,Ag/LaNiO₃ exhibited enhanced bifunctional activities in0.1 M KOH solution.The expanded adsorption area of O₂ and the tight connection between Ag and LaNiO₃ lead to the rapid electron transport during reactions.The open-circuit voltage of the primary zinc-air battery with Ag/LaNiO₃ oxygen electrode was 1.43 V.The maximum power density achieved 60 mW·cm^-22 at 90mA·cm^-2.Moreover,the rechargeable zinc-air battery with Ag/LaNiO₃ exhibited lower charge-discharge voltage difference?1.02 V?and better charge-discharge cycling stability after 150 cycles,indicating its efficient bifunctional activities.In order to solve the problems of large particle size and poor stability of traditional NiFe alloy nanoparticles,composite consisting of La₂O₃,NiM?M=Fe,Co?and nitrogen-doped carbon nanotubes?NCNTs??La₂O₃/NiFe-NCNTs and La₂O₃/NiCo-NCNTs?were prepared by in-situ reduction of the mixture consisting of LaNi_0.5M_0.5O₃?M=Fe,Co?,glucose and urea at high temperature.Nano-alloy particles?NiFe and NiCo?located on the top of NCNTs,and NCNTs interpolated into the composite,which improved the electron transport and reaction rate.All these were conducive to the rapid progress of ORR and OER reactions.In particular,La₂O₃/NiFe-NCNTs displayed the best bifunctional activities.The rechargeable zinc-air battery with La₂O₃/NiFe-NCNTs displayed the low charge-discharge voltage difference?1.04 V?after charge-discharge cycling for100 cycles,which was obviously superior to commercial Pt/C?1.44 V?,indicating the efficient bifunctional activities and stability of La₂O₃/NiFe-NCNTs.