Electrode Reaction And Electrochemical Properties Of La₂NiO_4+?-based Cathodes

Perovskite-like layer-structured?K₂NiF₄-type?La₂NiO_4+?-based mixed ionic-electronic conducting?MIEC?complex oxides have emerged as promising candidate materials for cathode of intermediate temperature solid oxide fuel cell?IT-SOFC?.The electrocatalytic activity of La₂NiO_4+?-based complex oxides towards oxygen reduction reaction?ORR?is critical to the cathode utilization.Improvement of electrocatalytic activity towards ORR remains an issue to be resolved for La₂NiO_4+?-based cathodes.Looking further into the kinetics of ORR on La₂NiO_4+?-based cathodes would pave the way to ultimate realization of this goal.In this work,typical La₂NiO_4+?-based complex oxides,including La₂NiO_4+?,La_2-x-x Sr_xNiO_4±??x=0.2 and 0.8?and La₂Ni_0.8Cu_0.2O_4+?,were selected as the model specimens.Porous electrodes of these complex oxides were prepared by a screen-printing technique.In view of the cathode utilization,the electrode reaction kinetics of ORR on these porous electrodes and the resulting electrochemical properties were investigated based on half and single cells.The investigation was performed as a function of contributing factors concerned with ORR on these porous electrodes,such as temperature,cathodic polarization conditions and electrode thickness.The electrocatalytic activity of these porous electrodes was diagnosed with respect to their oxygen defect chemistry.The emphasis was placed on better understanding of the kinetics of ORR on these porous electrodes.The purpose of these efforts is to seek for clues to guide composition engineering and preparation strategies of La₂NiO_4+?-based cathodes,based on which optimized electrochemical properties could be achieved for these cathodes.The electrode reaction kinetics and electrochemical properties of porous La₂NiO_4+?electrodes were investigated based on half cells using electrochemical impedance spectroscopy?EIS?technique.The electrochemical performance of anode-supported single cells with La₂NiO_4+?cathodes was also examined.The overall electrode polarization was detected to be mainly contributed by the following two reaction processes:?1?electron transfer from the electrodes to atomic oxygen species on the surface followed by bulk transport of the resulting oxygen ions within the electrodes?process 1?and?2?adsorption-disassociation of molecular oxygen on the electrodes followed by surface diffusion of the disassociated oxygen species?process2?,with the contribution of the latter process being more appreciable.Applying cathode polarization on La₂NiO_4+?electrode enabled a significant activation of its electrocatalytic activity,which can be explained in terms of reduction of partial Ni³⁺concomitant with formation of oxygen vacancies in the bulk and on the surface.It was found that the electrocatalytic activity of La₂NiO_4+?electrode was highly dependent on its thickness.The length of catalytically active region along thickness direction was estimated be³.7?m at 800°C.Within the electrode thickness range of5 to 45?m,the electrode with the thickness of²0?m showed the optimal electrochemical properties.At 800°C in air,this electrode displayed a polarization resistance of 0.24?·cm²,an exchange current density of 201 mA·cm^-2 and a cathodic overpotential of 41 mV under current density of 200 mA·cm^-2.An anode-supported single cell with²0?m thick La₂NiO_4+?cathode achieved a maximum power density?MPD?of 500 mW·cm^-2 at 800°C in hydrogen fuel and with oxygen as the oxidizing gas.The oxygen defect chemistry,electrical conducting,thermal expansion and electrochemical properties of La_2-xSr_xNiO_4±??x=0.2 and 0.8,?>0?were examined in comparison with La₂NiO_4+?.The electrochemical examination of the Sr²⁺substituted compositions was performed under open circuit voltage?OCV?and cathodic polarization conditions.Moreover,the dependence of electrocatalytic activity on electrode thickness was inspected for porous La_1.8Sr_0.2NiO_4+??x=0.2?electrode.Interstitial oxygen ions and oxygen vacancies were confirmed to be the major oxygen defect species for La_1.8Sr_0.2NiO_4+??x=0.2?and La_1.2Sr_0.8NiO_4-??x=0.8?,respectively.EIS analysis under OCV conditions indicated the Sr²⁺substitution at the A-site of La2NiO4+?did not alter the nature of involved electrode reaction processes,with the process 2 making the dominant contribution to the overall electrode polarization.For the two Sr²⁺substituted electrodes,a higher electrocatalytic activity was found for La_1.2Sr_0.8NiO_4-??x=0.8?electrode under OCV conditions.Nonetheless,the electrocatalytic activity of La_1.2Sr_0.8NiO_4-??x=0.8?electrode obviously degraded after being subjected to cathodic polarization,due to segregation of Sr²⁺out off the lattice and aggregation onto the electrode surface in the form of strontium oxide.In contrast,no significant crystal structure change occurred for La_1.8Sr_0.2NiO_4+??x=0.2?electrode when being subjected to the identical cathodic polarization.As a result,the electrocatalytic activity of La_1.8Sr_0.2NiO_4+??x=0.2?electrode was insensitive to cathodic polarization history.The electrocatalytic activity of La_1.8Sr_0.2NiO_4+??x=0.2?electrode showed an electrode thickness dependence.The length of catalytically active region along thickness direction was estimated be².3?m at 800°C for La_1.8Sr_0.2NiO_4+??x=0.2?electrode,while the electrode with the thickness of¹5?m showed the optimal electrochemical properties.At 800°C in air,the electrochemical properties of this La_1.8Sr_0.2NiO_4+??x=0.2?electrode were comparable in level with the²0?m thick La₂NiO_4+?electrode.The oxygen content,chemical states of constitute elements on surface,electrical conductivity and thermal expansion coefficient of La₂Ni_0.8Cu_0.2O_4+?were examined in comparison with La₂NiO_4+?.The effects of cathodic polarization and electrode thickness on the electrochemical properties of La₂Ni_0.8Cu_0.2O_4+?were also surveyed.The electrochemical attributes of La₂Ni_0.8Cu_0.2O_4+?electrode showed similarity to La₂NiO_4+?electrode in many aspects,such as the nature of involved electrode reaction processes,the dominant reaction process to the overall electrode polarization,electrochemical activation behaviors,electrode thickness effect and cathodic polarization dependence.It was verified that the Cu substitution at B-site of La₂NiO_4+?led to increase amount of oxygen vacancies in the lattice and on the surface,favoring the kinetics of ORR occurring on the surface.Improved electrocatalytic activity was achieved for porous La₂Ni_0.8Cu_0.2O_4+?electrode by tuning the electrode thickness.At 800°C in air,the electrode with the thickness of¹0?m exhibited the optimal electrochemical properties,showing a polarization resistance of 0.13?·cm²,an exchange current density of 420 mA·cm^-2 and a cathodic overpotential of 40 mV under current density of 200 mA·cm^-2.