Tubular Solid Oxide Fuel Cells

This paper reviews the design, materials and fabrication processes, and operation of tubular solid oxide fuel cells. A large number of tubular cells have been electrically tested, some to times over 30,000 hours; these cells have shown excellent performance and performance stability. In addition, successively larger size electric generators utilizing these cells have been designed, built and operated since 1984. Two 25 kW power generation field test units were fabricated and delivered for field testing during 1992; these units represent a major milestone in the commercialization of solid oxide fuel cells for power generation. INTRODUCTION High tem perature solid oxide fuel cells utilizing y ttria -stab ilized zirconia electrolyte offer a clean, pollution-free technology to electrochem ically generate electricity at high efficiencies. These fuel cells provide many advantages over traditional energy conversion systems; these include high efficiency, reliability, modularity, fuel adaptability, and very low levels of NOX and SOX emissions (1,2). Furthermore, because of their high temperature of operation (~1000°C), these cells can be operated directly on natural gas eliminating the need for an expensive, external reformer system. These fuel cells also produce high quality exhaust heat which can be used either for process heat or a bottoming electric power cycle to further increase the overall efficiency. This paper review s the current status of the tubular design solid oxide fuel cells fo r power generation. OPERATING PRINCIPLE A solid oxide fuel cell (SOFC) essentially consists o f two porous electrodes separated by a dense, oxygen ion conducting electrolyte. The operating principle of such a cell is illustrated in Figure 1. Oxygen supplied at the cathode (air electrode) reacts with incoming electrons from the external circuit to form oxygen ions, which migrate to the anode (fuel electrode) through the oxygen ion conducting electrolyte. A t the anode, 665 Proceedings of The Electrochemical Society, PV 1993-04, 665-677 (1993) DOI: 10.1149/199304.0665PV © The Electrochemical Society ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 207.241.231.81 Downloaded on 2019-04-27 to IP oxygen ions combine with H2 (and/or CO) in the fuel to form H2O (and/or CO2), liberating electrons. Electrons flow from the anode through the external circuit to the cathode. The reactions at the two electrodes can be written as follows: Cathode: l/2 O 2 + 2 e ' = O2‘ Anode: Hn + O2" = H^O CO + O2’ = co2 The overall cell reaction is simply the oxidation of fuel (H2 and/or CO) and the open circuit voltage, E, of the fuel cell is given by the Nemst equation: E RT 4F In ( o x i d a n t ) P Q ( f u e l ) where R is the gas constant, T is the cell temperature, F is the Faraday constant, and Po2’s are the oxygen partial pressures. When a current passes through the cell, the cell voltage (V) is given by: V = E IR r/A tîf where I is the current passing through the cell, R is the electrical resistance of the cell, and ri a and i?p are the polarization voltage losses associated with irreversibilities in electrode processes on the air side and the fuel side, respectively. To keep the IR loss low, the electrolyte is fabricated in the form of a thin film. MATERIALS AND FABRICATION PROCESSES S olid oxide fuel ce lls of severa l d iffe ren t designs are p resen tly under developm ent; these include planar, monolithic and tubular geometries (3,4). The materials being considered for cell components in these different designs are either same or very similar in nature. The most progress to date has been achieved with the tubular geometry fuel cell. Figure 2 illustrates the design of the Westinghouse tubular geometry cell. In this design, the active cell components are deposited in the form of thin layers on a ceramic support tube. The materials for different cell components have been selected based on the following criteria: (a) Suitable electrical properties required of different cell components to perform their intended cell functions.