Solid Oxide Fuel Cells (SOFCs) are one of the most promising technologies for future efficient conversion of the chemical energy stored in fuels to electrical energy. One of the primary advantages of SOFCs is the potential to operate with a wide variety of fuels. While H2 is the fuel of choice for most fuel cells, operation with fossil-derived and bio-derived hydrocarbon fuels would bypass the costly requirement of a new H2 infrastructure, and accelerate adoption of fuel cell technology. This is feasible as SOFCs transport oxygen anions from the air electrode (cathode) to the fuel electrode (anode). The primary barrier to the realization of fuel flexible SOFCs is the anode material set. Traditional SOFC anodes are based on Ni composites. While these are very efficient for H2 and CO fuels, Ni catalyzes graphite formation from dry hydrocarbons, leading to rapid degradation of cell performance and possible mechanical failure. This motivates the development of new anode materials and composites. The principle requirements of an anode are oxygen anion conductivity, electronic conductivity, and electrocatalytic activity toward the desired reaction. This entry reviews the primary issues in direct hydrocarbon anode development and discusses the new materials and composites that have been developed to meet this challenge.

Direct hydrocarbon solid oxide fuel cells.

Dependence of polarization in anode-supported solid oxide fuel cells on various cell parameters

Improved cathodes are required for low-temperature operation of solid-oxide fuel cells (SOFCs). Recent work has shown that electrode fabrication and modification by infiltration of active components into a porous scaffold can result in outstanding electrochemical performance. In this paper we review the literature on this new approach for cathode preparation and discuss the insights that this work has provided for understanding the relationships between the materials properties, electrochemical performance, and electrode stability.

High‐Performance SOFC Cathodes Prepared by Infiltration

Ba(Zr0.1Ce0.7Y0.2)O3- δ as an electrolyte for low-temperature solid-oxide fuel cells

This paper presents a new computational framework for modeling chemically reacting flow in anode-supported solid-oxide fuel cells (SOFC). Depending on materials and operating conditions, SOFC anodes afford a possibility for internal reforming or catalytic partial oxidation of hydrocarbon fuels. An important new element of the model is the capability to represent elementary heterogeneous chemical kinetics in the form of multistep reaction mechanisms. Porous-media transport in the electrodes is represented with a dusty-gas model. Charge-transfer chemistry is represented in a modified Butler-Volmer setting that is derived from elementary reactions, but assuming a single rate-limiting step. The model is discussed in terms of systems with defined flow channels and planar membrane-electrode assemblies. However, the underlying theory is independent of the particular geometry. Examples are given to illustrate the model.

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Modeling Elementary Heterogeneous Chemistry and Electrochemistry in Solid-Oxide Fuel Cells

Anode-supported solid oxide fuel cells (SOFCs) with Ni+yttria-stabilized zirconia (YSZ) anode, YSZ-samaria-doped ceria (SDC) bilayer electrolyte, and Sr-doped LaCoO 3 (LSC)+SDC cathode were fabricated. Fuel used consistedof H 2 diluted with He. N 2 , H 2 O, or CO 2 , mixtures of H 2 and CO, and mixtures of CO and CO 2 . Cell performance was measured at 800°C with the above-mentioned fuel gas mixtures and air as oxidant. For a given concentration of the diluent, cell performance was higher with He as the diluent than with N 2 as the diluent. Mass transport through porous Ni-YSZ anode for H 2 -H 2 O, CO-CO 2 binary systems. and H 2 -H 2 O-diluent gas ternary systems was analyzed using multicomponent gas diffusion theory. At high concentrations of diluent, the maximum achievable current density was limited by the anodic concentration polarization. From this measured limiting current density, the corresponding effective gas diffusivity was estimated. Highest effective diffusivity was estimated for fuel gas mixtures containing H 2 -H 2 O-He mixtures (∼0.55 cm 2 /s), and the lowest for CO-CO 2 mixtures (∼0.07 cm 2 /s). The lowest performance was observed with CO-CO 2 mixture as a fuel, which in part was attributed to the lowest effective diffusivity of the fuels tested and higher activation polarization.

Fuel Composition and Diluent Effect on Gas Transport and Performance of Anode-Supported SOFCs

Anode-supported solid oxide fuel cells with a thin film of yttria-stabilized zirconia (YSZ) as the electrolyte were fabricated. The cells were operated directly on pure methanol and on an equivolume mixture of ethanol and water over a range of temperatures. Power density achieved with methanol was between 0.6 W/cm 2 at 650°C and 1.3 W/cm 2 at 800°C, and with ethanol + water between 0.3 W/cm 2 at 650°C and 0.8 W/cm 2 at 800°C. Results were compared with tests on humidified hydrogen as a fuel. No carbon deposition on the Ni-YSZ anode was observed with either methanol or an equivolume solution of ethanol and water as fuels. Differences in performance with different fuels were attributed to differences in anode polarization.

A High Performance, Anode-Supported Solid Oxide Fuel Cell Operating on Direct Alcohol

A simple, approximate analysis of the effect of different cathode and anode areas on the measurement of cell performance on anode-supported solid oxide fuel cells, wherein the cathode area is smaller than the anode area, is presented. It is shown that the effect of cathode area on cathode polarization, on electrolyte contribution, and on anode resistance, as normalized on the basis of the cathode area, is negligible. There is a small hut measurable effect on anode polarization, which results from concentration polarization. Effectively, it is the result of a greater amount of fuel transported to the anode/electrolyte interface in cases wherein the anode area is larger than the cathode area. Experiments were performed on cells made with different cathode areas and geometries. Cathodic and anodic overpotentials measured using reference electrodes, and the measured ohmic area specific resistances by current interruption, were in good agreement with expectations based on the analysis presented. At 800°C, the maximum power density measured with a cathode area of ∼1.1 cm 2 was ∼1.65 W/cm 2 compared to ∼1.45 W/cm 2 for cathode area of ∼2 cm 2 , for an anode thickness of 1.3 mm, with hydrogen as the fuel and air as the oxidant. At 750°C, the measured maximum power densities were ∼1.3 W/cm 2 for the cell with a cathode area ∼1.1 cm 2 , and ∼1.25 W/cm 2 for the cell with a cathode area ∼2 cm 2 .

Yi Jiang

Papers

Fuel Composition and Diluent Effect on Gas Transport and Performance of Anode-Supported SOFCs

A High Performance, Anode-Supported Solid Oxide Fuel Cell Operating on Direct Alcohol

The Effect of Testing Geometry on the Measurement of Cell Performance in Anode-Supported Solid Oxide Fuel Cells: The Effect of Cathode Area

Synthesis of Nanosize Yttria-Stabilized Zirconia by a Molecular Decomposition Process

A High Performance, Anode-Supported Solid Oxide Fuel Cell Operating on Direct Alcohol.