A review on the status of anode materials for solid oxide fuel cellsIntroductionDevelopment of anode materialsNi-ZrO2(Y2O3) cermetCeO2 (rare-earth doped) anodeOther anode materialsConclusionsAcknowledgementsReferencesMaterials Science and Engineering A362 (2003) 228–239A review on the status of anode materials for solid oxide fuel cellsW.Z. Zhu, S.C. Deevi∗Research and Development Center, Chrysalis Technologies Incorporated, 7801 Whitepine Road, Richmond, VA 23237, USAReceived 23 April 2002; received in revised form 2 July 2003AbstractPresent review is aimed at providing a state-of-art development of anode for solid oxide fuel cell (SOFC) with principal emphasis onthe materials aspect. The criteria for the anode of SOFC are first presented. The prospects and problems of the currently developed anodematerials are elucidated. In particular, the electrochemical properties of the Ni/YSZ cermet anode that is the most commonly employed in theestablishment of SOFC stack is described along with various approaches attempted for their improvements. The advantages and disadvantagesof other anode materials are comparedto offer some insights for the research and development of newgeneration of anode materials for SOFC.© 2003 Elsevier B.V. All rights reserved.Keywords: Solid oxide fuel cell; Anode; Mixed ionic-electronic conductor1. IntroductionSolid oxide fuel cell (SOFC) is an electrochemical de-vice that converts the energy of a chemical reaction di-rectly into electrical energy. Owing to the utilization ofsolid electrolyte and highest operating temperatures (typi-cally at 700–900◦C), it offers many advantages over con-ventional power-generating systems in terms of efficiency,reliability, modularity, fuel flexibility, and environmentalfriendliness [1,2]. In addition, SOFCs offer the possibility ofco-generation with gas turbine power systems to enable fullexploitation of both electricity and heat, thereby enhancingthe efficiency up to approximately 70% [3–7]. Single SOFCcell essentially consists of two porous electrodes separatedby a dense, oxygen-conducting electrolyte. The operatingprinciple of such a cell is schematically illustrated in Fig. 1.On the cathode (air electrode) side, oxygen reacts with in-coming electrons from external load to become oxygen ionsthat migrate through the electrolyte. On the anode (fuel elec-trode) side, fuel is oxidized by incoming oxygen ions to lib-erate the electrons that flow through the external electricalcircuit. The charge flow in the external circuit is balanced byionic current flow within the electrolyte. More specifically,∗Corresponding author. Present address: Research, Development& Engineering, 4201 Commerce Road, Phillip Morris, Richmond,VA 23234, USA. Tel.: +1-804-274-1934; fax: +1-804-274-2891.E-mail address: [email protected] (S.C. Deevi).the oxygen is disassociated and converted to oxygen ionsat cathode/electrolyte interface, whereas the electrochemicaloxidation of fuel takes place at anode/electrolyte interface.The ideal voltage (E◦) from a single cell under open circuitconditions is close to 1.0 V dc as calculated from the Nernstequation. However, the useful voltage output (V) under loadconditions, that is, when a current passes through the cell,is given by [8]V = E◦− IR − ηc− ηa(1)where I is the current passing through the cell, R the elec-trical resistance of the cell, and ηcand ηathe polarizationlosses associated with the cathode and anode, respectively.The voltage loss due to internal electrical resistance encom-passes the contributions from electrodes and electrolyte withoverwhelming contribution coming from electrolyte on ac-count of its ionic conduction in nature. To minimize the IRloss, the increasingly preferred practice is to fabricate dense,gas-tight electrolyte membrane as thin as possible [9,10].Among the SOFC components, the porous anode servesto provide electrochemical reaction sites for oxidation ofthe fuel, allow the fuel and byproducts to be delivered andremoved from the surface sites, and to provide a path forelectrons to be transported from the electrolyte/anode reac-tion sites to the interconnect in SOFC stacks. Interconnectis a component to connect anode of one cell with cath-ode of another so that voltage output can be enhanced forpractical application. The characteristics of high operating0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0921-5093(03)00620-8W.Z. Zhu, S.C. Deevi/Materials Science and Engineering A362 (2003) 228–239 229Air Flow Air inlet P(O2)=10-4atm P(O2)=10-0.7atmPorous Cathode 1/2O2 + 2e- O2- Dense YSZ O2- Porous Anode H2 + O2- H2O + 2e- Load P(O2)=10-18atm P(O2)=10-8atmFuel Flow Fuel inlet e Alternative anode reactions: CO +O2- CO2 + 2e- CH4 + 4O2- 2H2O + CO2 + 8e- Fig. 1. Schematic drawing showing the working principle of a solid oxide fuel cell operating on hydrogen.temperature of SOFC present special challenges related tomaterials degradation, in particular, constitute one of thetoughest criteria for the dimensional and chemical stabilityof anode material itself in reducing atmospheres. Besides,desired anode is supposed to be mixed conductor with pre-dominant electronic conductivity to permit the passage ofelectrons. It should display no reaction with neighboringelectrolyte and interconnect (chemical compatibility withadjacent components). It should also have thermal expan-sion coefficient close to those of adjoining components. Itmust show high electrocatalytic activity toward oxidation offuel gases, and preferably, desired catalytic activity towardthe hydrocarbon reforming. High wettability with respect tothe electrolyte substrate is highly advantageous for compet-itive anode. It is evident that anode should have continuouschannels made of pores to allow rapid transport of fuel andreactant gases. Anode is supposed to exhibit excellent car-burization and sulfidation resistance. Most advantageously,the anode ought to be fuel flexible, ease of fabrication, andlow cost are of tremendous importance for a wide range ofcommercial applications. Unfortunately, no current workinganodes can fulfil all of above requirements.The electrical resistance of anode is essentially com-prised of internal resistance, contact resistance, concen-tration polarization resistance, and activation polarizationresistance. The internal resistance