Oxygen Reduction Reaction Kinetics in Sr-Doped La₂NiO_4+δRuddlesden-Popper Phase as Cathode for Solid Oxide Fuel Cells

Abstract

In this paper, electro-catalytic reduction of oxygen in Sr-doped lanthanum nickelates, La 2-x Sr x NiO 4+δ (0 ≤ x ≤ 0.4) RuddlesdenPopper (R-P) phase, has been investigated. The oxygen reduction reaction (ORR) kinetics is evaluated via electrochemical impedance spectroscopy (EIS) with the symmetric cell configuration. The maximum performance is achieved with the un-doped La 2 NiO 4+δ , ∼0.13 cm 2 at 800 • C. Sr doping decreases the electrode performance progressively. Further detailed analysis indicates that bulk ionic diffusion and surface oxygen exchange co-limit the ORR of those cathodes. Sr substitution leads to both lowered bulk diffusion and surface exchange rates. With high Sr content (x = 0.3, 0.4), oxygen ion transfer resistance between nickelate/electrolyte is observed. As for the surface exchange process, oxygen adsorption is suggested to be the main rate-limiting step (RLS) according to the reaction orders, which is retarded further by the oxidation of Ni 2+ to Ni 3+ as Sr content increases. The possible role of incorporation process in determining the overall reaction rate is also discussed.Intermediate temperature (600∼800 • C) solid oxide fuel cells (ITSOFCs) have attracted increasing interest because of the relaxed requirement for sealing, interconnects and thermal expansion matching. However, the lowered operating temperature usually leads to poorer performance due to the increased electrolyte resistance and the retarded electrode reaction kinetics. In order to improve the cathode performance of IT-SOFCs, mixed ionic and electronic conductors (MIEC) have been brought up as potential electrode materials. The fast ionic conduction in MIEC could transport oxygen vacancies from electrolyte to 3-dimension of the cathode network, thereby enlarging the electrochemically reactive area. 1 Recently, K 2 NiF 4 type La 2 NiO 4+δ (LNO)-based oxides with higher oxygen ionic conductivity than conventional LaCoO 3 -based perovskites such as (LaSr)(CoFe)O 3-δ (LSCF), have also been proposed as possible candidates for IT-SOFCs cathode application. [2][3][4][5][6][7][8][9][10][11] The lattice structure of LNO is composed of perovskite alternating with LaO rock salt layers. Oxygen hyperstoichiometry is confirmed and associated to the incorporation of oxygen atoms into the interstitial sites of LaO layers. [12][13][14][15] The ionic conductivity of such materials mainly results from the in-plane transport of interstitial oxygen in the LaO layers. [16][17][18][19] La and/or Ni site element substitution have been used to tailor the materials in attempt to improving certain physical parameters, such as the surface exchange (k), bulk diffusion (D) coefficients and electrical conductivity. Despite excellent oxygen surface exchange and bulk diffusion properties, un-doped LNO did not show superior ORR performance, which is believed to relate to low electronic conductivity of such material. Sr substitution for La in LNO is confirmed to improve the electronic conductivity beneficially, 20 while the ionic conductivity is decre...

Surface Oxygen Exchange Properties of Sr Doped La2NiO4+δ as SOFC Cathode: Thin-Film Electrical Conductivity Relaxation Investigation

“…Figure 3 shows the ECR for Sr20 and Sr40 obtained with 0.20 atm P O2 at 700 o C. Sr doping impairs the surface exchange kinetics substantially. k for Sr20 and Sr40 is 3.1×10 -6 and 1.8×10 -6 cm/s, which is in good agreement with the degradation in the electrochemical performance of lanthanum nickelate cathodes as Sr doping we reported previously (17). In order to study the possible rate-limiting step(s) in the surface oxygen exchange process and Sr effect on those steps, the reaction orders is derived and discussed.…”

“…La 2-x Sr x NiO 4+δ (x=0, 0.2, 0.4) with R-P phase are first synthesized as reported elsewhere in detail (17). In short, stoichiometric nitrates together with citric acid were dissolved into distilled water.…”

Impact of Oxygen Deficiency on the Electrochemical Performance of K2NiF4‐Type (La1−xSrx)2NiO4−δ Oxygen Electrodes

“…On the contrary, Guan et al . reported that the electrochemical performance of (La 1− x Sr x ) 2 NiO 4+ δ ( x =0–0.2) electrodes applied on yttria‐stabilized zirconia solid electrolyte with a CGO10 buffer layer decreases progressively with increasing Sr content ( R η =0.41 Ω cm 2 for x =0 and 14.4 Ω cm 2 for x =0.2 at 700 °C) and attributed this to a decrease of both the bulk oxygen diffusion and surface exchange rates caused by Sr substitution.…”

Enhanced surface exchange activity and electrode performance of (La2−2xSr2x)(Ni1−xMnx)O4+δ cathode for intermediate temperature solid oxide fuel cells

“…Therefore, the exploration of novel IT cathode materials with reduced polarization resistance is imperative to minimize the overall energy loss. In recent, A 2 BO 4+δ R-P phase lanthanum nickelates have attracted increasing research interests [7][8][9]. Such materials feature a layered structure of alternative stacking LaO rock salt with ABO 3 perovskite.…”