SOFC Anode Electrode

The most common anode material is nickel-yttria stabilized zirconia (Ni-YSZ) Cermet or a mixture of nickel and YSZ. While nickel serves for the required catalytic activity and electron conductivity, YSZ lowers the effective coefficient of expansion to march with adjacent YSZ electrolyte. However, use of YSZ extends the active zone for the anode reaction. Depending on the design, Ni to YSZ volume ratio varies in the range from 30% to 50% in the composite mixture. Electronic conductivity varies in the range of 0.1-1000 S cm i depending on the porosity and composition of Ni-YSZ composition. 

The typical thickness of anode varies in the range of 40-100 gm. In recent times, a thicker anode in the range of 350-750 gm is used to develop the so-called anode-supported SOFCs that allow for a thinner electrolyte layer. 

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To meet the requirements for electronic conductivity in both the SOFC anode and cathode, a metallic electronic conductor, usually nickel, is typically used in the anode, and a conductive perovskite, such as lanthanum strontium manganite (LSM), is typically used in the cathode. Because the electrochemical reactions in fuel cell electrodes can only occur at surfaces where electronic and ionically conductive phases and the gas phase are in contact with each other (Figure 6.1), it is common... 

The previous discussion has focused on the properties of perovskite materials rather than on their performance as anodes. The number of actual fuel-cell studies is more limited, but this literature has been reviewed recently by Irvine. Various perovskites have been investigated as potential SOFC anode materials however, these early efforts were hampered by low electrochemical activity toward methane oxidation,poor anode structure,or insufficient electrode conductivity. Most recently, Tao and Irvine demonstrated that an anode based on (Lao.75Sro.25)o.9Cro.5Mno.503 can provide reasonable power densities at 1173 K in 3% humidified CH4. Barnett and co-workers also reported stable power generation with methane and propane fuels on an anode based on LaCr03 however, they reported that the addition of Ni, in levels too small to affect the conductivity, was crucial in providing activity for the electrochemical oxidation reactions. 

This is illustrated in Fig. 15.14, which shows that at the reducing conditions necessary for an SOFC anode only a limiting current density of about 0.1 A/cm (oxidation of H2 from simple cracking of CH ) can be obtained at relevant electrode potentials at I000 C. The current density does not increase significantly... 

Several recent breakthroughs in the design of solid oxide fuel cell (SOFC) anodes and cathodes are described in the Chapter of H. Uchida and M. Watanabe. The authors, who have pioneered several of these developments, provide a lucid presentation describing how careful fundamental investigations of interfacial electrocatalytic anode and cathode phenomena lead to novel electrode compositions and microstructures and to significant practical advances of SOFC anode and cathode stabihty and enhanced electrocatalysis. 

SOFC anodes is typically a complex inter-networks of ionically and electronically conducting phases, and gas-filled porosity. Control of the composition and micro-structure is critical for the activity of electrodes [6]. Percolating networks of three-phase boundaries formed by the electronic phase, ionic phase, and the gas-phase are important for high electrochemical performance of the cell. A three-dimensional reconstruction of a typical state of the art Ni/YSZ anode and its three-phase boundaries reproduced from [7] is shown in Fig. 1.2. There are numerous techniques by which the anodes can be fabricated [8,9]. In all cases the NiO-YSZ active layer as fired is a dense material, and most of the porosity results during the reduction process [7]. Zhu et. al [10] reported that a continuous porosity of more than 30% is required to facilitate the transport of reactants and products to and away from the three-phase boundary (TPB). 

High-Temperature Reactions, Anode, Fig. 3 Enhancement of electrode reactive surface area for fuel oxidation in an SOFC anode by mixed conducting materials as compared to Ni-YSZ anode considering both carbon... 

By considering Eqs. 3.1 and 3.2, three clear requirements for SOFC anode materials can be derived. First, oxygen anion cOTiductivity is required to transport the reactant oxygen anions to the reaction site. Secmid, the anode must selectively facilitate the desired electrocatalytic oxidatimi of the fuel. Third, facile electrical conductivity is required to transport the product electrons from the reaction site to the current collector wire. These material requirements are in additirm to considerations of materials compatibility and stability during cell fabricatirm and operation, porosity in the electrode for fuel and product gas-phase diffusion, structural integrity, tolerance to impurities in the fuel and anode materials, and redox stability in case of accidental oxidation. 

Blennow P, Hansen KK, Reine Wallenberg L, Mogensen M (2006) Effects of SrATi-ratio in SrTi03-based SOFC anodes investigated by the use of cone-shaped electrodes. Electrochim Acta 52 1651-1661... 

Electrode for electrochemical oxidation reactions. In solid oxide fuel cells, hydrogen-containing fuels are oxidized by oxygen ions transported through an electrolyte to form water vapor or CO2 as the reaction products at this electrode. SOFC anodes may also act as fuel reforming catalysts when hydrocarbon-based fuels are supplied to the anodes. Electrode for electrochemical reduction reactions. In solid oxide fuel cells, oxygen in ambient air is reduced to oxygen ions at this electrode. 

Despite the relative success of using Ni-YSZ cermets as the anode and LSM-YSZ composites as the cathode in SOFCs, these electrodes suffer from several drawbacks. Below is a selection of some of these shortcomings ... 

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