Electrochemical characterization of Pr₂CuO₄ cathode for IT-SOFC

Abstract

The electrochemical properties of Pr₂CuO₄ (PCO) electrode screen-printed on Ce_0.9Gd_0.1O_1.95 (CGO) electrolyte were investigated. PCO was synthesized by a solid-state route from the stoichiometric mixture of oxides at 1273 K, 20 h. Thermogravimetric analysis (TGA) of PCO both in air and Ar demonstrated its stability up to 1173 K. X-ray powder diffraction study of the PCO–CGO mixture annealed in air at 1173 K for 100 h did not reveal chemical interaction between materials. The oxygen reduction on porous PCO electrodes applied on CGO electrolyte was studied in a symmetrical cell configuration by AC impedance spectroscopy at OCV conditions at 773–1173 K and $p_{{\text{O}}_2}$ = 10⁻⁴–1 atm. Analysis of the data revealed that depending on temperature and oxygen partial pressure different rate-determining steps of the overall oxygen reduction reaction take place. Calculated value of area specific resistance (ASR) of PCO electrode is 1.7 ± 0.2 Ω cm² at 973 K in air and it is constant after 6 subsequent thermocycles. We have found that oxygen reduction on PCO applied on CGO takes mainly place at the triple-phase boundary (TPB) since Adler–Lane–Steele (ALS) model is not valid. Therefore electrochemical characteristics of PCO electrode can be improved by further optimization of both microstructure of the electrode and electrode/electrolyte interface and PCO can be considered as a promising cathode material for intermediate temperature solid oxide fuel cells (IT-SOFC).

Introduction

Mixed electronic and ionic conductors with layered structures are presently regarded as promising cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) operated at 773–973 K [1], [2], [3], [4], [5]. Among them, rare-earth (RE) nickelates and cuprates RE₂MO₄, M = Ni and Cu attract much practical interest [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. They exhibit relatively high electrical conductivity, which depends on the nature of rare-earth cation and varies in the range of 10–100 S/cm at 773–1173 K [12], [13], [14], [15], [16], [17], [18]. Moreover, RE₂MO₄, M = Ni and Cu with K₂NiF₄ structure show high values of oxygen diffusion and surface exchange rate due to the possibility of incorporation of excess interstitial oxygen into the structure [4], [12], [13], [14], [15], [16], [18]. Relatively low polarization resistances of porous nickelates and cuprates electrode layers deposited onto solid electrolyte are acceptable for their use as cathode materials in SOFC [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [16]. Despite reduced thermodynamic stability of Pr₂NiO₄ in comparison with La₂NiO₄ and Nd₂NiO₄ (e.g. Pr₂NiO₄ decomposes at T > 1123 K in oxidizing atmosphere into Pr₄Ni₃O_10−δ and PrO_x [19]), it is widely studied as cathode material in SOFC due to both high electrical conductivity [13], [15], [16], [17], [18] and electrochemical activity for oxygen reduction [7], [15], [16]. Thermal expansion coefficients (TECs) of praseodymium nickelates and cuprates (∼12–13 × 10⁻⁶ K⁻¹) [13], [16], [18] are comparable with ceria solid electrolyte (for Ce_0.9Gd_0.1O_1.95 TEC = 12.4 × 10⁻⁶ K⁻¹) [20]. It should be mentioned here that RE₂NiO₄ and La₂CuO₄ have K₂NiF₄-type structure (T-phase), which can be described as a sequence of perovskite layers alternating with RE₂O₂ slabs having rock-salt structure (Fig. 1a). However, RE₂CuO₄ cuprates with other than RE = La cations, in particular Pr₂CuO₄, possess so-called T′-phase structure with RE₂O₂ slabs having fluorite structure (Fig. 1b) [21]. Recently some of us showed that “compressed” RE₂O₂ slabs in the T′-phase lead to a smaller separation between available empty oxygen octahedral sites and RE cations and hamper oxygen anion diffusion [18]. Despite detailed investigation of the electrochemical behavior of Pr₂NiO₄ electrodes [7], [16], no studies of Pr₂CuO₄ electrodes are available to our knowledge.

In the present work we prepared Pr₂CuO₄ (PCO) electrodes on CGO (Ce_0.9Gd_0.1O_1.95) electrolyte by screen-printing and report on the impedance spectroscopy study of the symmetrical cells PCO/CGO/PCO at 773–1173 K and partial oxygen pressure ($p_{{\text{O}}_2}$) 10⁻⁴–1 atm.