Study of cobalt-doped lithium–nickel oxides as cathodes for MCFC Prabhu Ganesan, Hector Colon, Bala Haran, Ralph White, Branko N. Popov * Department of Chemical Engineering, Center for Electrochemical Engineering, University of South Carolina, Columbia, SC 29208, USA Received 23 April 2002; accepted 8 May 2002 Abstract Cobalt substituted lithium–nickel oxides were synthesized by a solid-state reaction procedure using lithium nitrate, nickel hydroxide and cobalt oxalate precursor and were characterized as cathodes for molten carbonate fuel cell (MCFC). LiNi 0.8 Co 0.2 O 2 cathodes were prepared using non-aqueous tape casting technique followed by sintering in air. The X-ray diffraction (XRD) analysis of sintered LiNi 1x Co x O 2 indicated that lithium evaporation occurs during heating. The lithium loss decreases with an increase of the cobalt content in the mixed oxides. The stability studies showed that dissolution of nickel into the molten carbonate melt is smaller in the case of LiNi 1x Co x O 2 cathodes compared to the dissolution values reported in the literature for state-of-the-art NiO. Pore volume analysis of the sintered electrode indicated a mean pore size of 3 mm and a porosity of 40%. A current density of 160 mA/cm 2 was observed when LiNi 0.8 Co 0.2 O 2 cathodes were polarized at 140 mV. The electrochemical impedance spectroscopy (EIS) studies done on LiNi 0.8 Co 0.2 O 2 cathodes under different gas conditions indicated that the rate of the cathodic discharge reaction depends on the O 2 and CO 2 partial pressures. # 2002 Published by Elsevier Science B.V. Keywords: Cobalt substituted nickel oxide; Molten carbonate; Fuel cell; Dissolution 1. Introduction Molten carbonate fuel cell (MCFC) technology is expected to be one of the most promising power generation systems for the coming century owing to its high efficiency and flexibility to a variety of fuels. These are high tempera- ture fuel cells operating at a temperature of 650 8C, and have been under intensive development for the last few decades as a second-generation fuel cell [1,2]. Significant advances have been made in addressing design issues resulting in the development of prototype MCFC power generators. However, several problems remain before commercializa- tion of MCFCs can be realized. The primary challenge remains in the proper selection of materials for the cathode and the current collector. In situ lithiated nickel oxide is in common use as a cathode material in the state-of-the-art MCFC [3]. Nickel oxide has a small degree of solubility in the molten carbonate eutectic melt used as an electrolyte in the MCFC [4] and does not satisfy long-term stability criteria [5]. Nickel oxide reacts with CO 2 present in the electrolyte according to an acidic dissolution mechanism: NiO þ CO 2 ! Ni 2þ þ CO 3 2 The dissolved nickel remains in equilibrium with the NiO cathode. Simultaneously, dissolved nickel ions diffuse from the cathode toward the anode under a concentration gradient. The dissolved nickel ions precipitate in the sections of matrix, where it encounters a reducing atmosphere due to the anode gas. The continuous diffusion of Ni 2þ cation fuels more dissolution of nickel from the cathode [6,7]. Continued deposition of Ni in the anode region eventually leads to a short circuit between the anode and cathode. The cathode dissolution also results in loss of active material and a decrease of the active surface area available for the oxygen reduction reaction (cathodic reaction) leading to degradation in fuel cell performance. Several materials like LiFeO 2 and LiCoO 2 were studied as replacement materials for NiO cathodes [8–10]. However, the exchange current density for the oxygen reduction reaction on LiFeO 2 is about two orders of magnitude lower than that on NiO. Thus, the slow kinetics of the oxygen reduction limits further improvement of cathodes based on this material. LiCoO 2 is more stable than NiO in alkaline environment [8]. However, LiCoO 2 is less electronically conductive and is more expensive than NiO. Stoichiometric lithium–nickel oxides have good electronic conductivity when compared to other ceramic oxides like LiFeO 2 and LiCoO 2 . However, a significant lithium loss occurs on heating these lithiated nickel oxides. Due to this loss, Journal of Power Sources 111 (2002) 109–120 * Corresponding author. Tel.: þ1-803-777-7314; fax: þ1-803-777-8265. E-mail address: popov@engr.sc.edu (B.N. Popov). 0378-7753/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII:S0378-7753(02)00301-4