Facile and Low Temperature Synthesis of Nd1.8Sr0.2NiO4-δ Cathode Nanofibers for Intermediate Temperature Solid Oxide Fuel Cells

Abstract

Solid oxide fuel cells (SOFCs) are usually operated at high temperatures (800-1000 °C). As the overall efficiency of SOFC is governed by thermodynamics and kinetics during operation, reducing the operating temperature to 500-650 °C causes significant increase in the electrode polarization losses especially at cathode due to high activation energy towards oxygen reduction reaction (ORR) at cathode, which eventually reduces the overall cell performance. Addressing increased polarization losses at relatively low temperatures (500-650 °C) has been the key issue and a focus of many groups over past couple of decades. Conventional cathode materials lose their ORR activity and so discarded for intermediate temperature (IT)-SOFC application. In this regard, mixed ionic-electronic conductors (MIECs) offer high thermal and chemical stability along with high oxygen diffusion and good electronic conductivity at IT [1,2]. In addition, Ruddlesden-Popper (RP) type MIECs with A2BO4 structure have been demonstrated as promising cathode for IT- SOFCs due to their unique structure, rapid oxygen surface exchange kinetics, and excellent stability in atmosphere. Particularly Nd2NiO4+δ oxides offer an excellent oxygen diffusion coupled with high surface exchange coefficients and comparable thermal expansion coefficients with those of oxygen-ion conductors (YSZ and CGO) [2]. Enhanced electronic conductivity and electrochemical performance have been demonstrated for partial replacement of Nd with Sr in our previous reports [3].Albeit, the intrinsic material properties such as electrochemical catalytic activity and electrical conductivity are of primary importance, an actual electrode performance strongly depends on its microstructure. Therefore, in addition to using high catalytic and conductive materials, achieving an extremely high-power density, thus, requires a finely controlled electrode microstructure, to expand the electrochemically active triple-phase boundary (TPB). Recently, electrospinning has been considered as an efficient, cost effective and promising method for synthesis of ceramic nanofibers. Pertinent literature suggests that SOFC with cathode nanofiber has superior/better performance (nearly doubled power density and significant reduced polarization resistance) compared to powder type electrodes. Selection of suitable cathode materials with modified microstructure (nanofibers) would facilitate more electrochemically active sites for the oxygen reduction at relatively low temperatures <650 °C.In this paper we synthesized Nd1.8Sr0.2NiO4-δ (NSNO) cathode nanofibers using electrospinning technique and compared its electrochemical performance with NSNO nanopowder. Practically, 10 wt.% Polyvinylpyrrolidone (PVP) solution was prepared by dissolving it in ethanol, to which stoichiometric amounts of Nd-nitrate, Sr-nitrate, Ni-nitrate (total weight percentage of metal salts- 3 wt%) were added, together with some deionized water. The mixture was then stirred for 10 h at room temperature on a magnetic stirrer to obtain a homogeneous sol. The sol was loaded into the electrospinning syringe. Distance between the spinneret and the collector was fixed at 25 cm and the high-voltage supply was maintained at 30 kV. The spinning rate was controlled at 5 ml h- 1 by a syringe pump. The obtained nanofibers were dried at 80 °C for 12 h, then sintered at 900 °C for 2 h in air to obtain single-phase NSNO.The obtained nanofibers were characterized using room temperature XRD, XPS, SEM and dc electrical conductivity (400-650 °C). In addition, electrochemical impedance spectroscopy studies on symmetric cells were carried out as a function of temperature (400-650 °C). Improved electrochemical performance is attributed to enhanced electrochemical active sites for ORR due to nanofibers and optimum porosity.