Kevin Huang

Oxide-Ion Conducting Ceramics for Solid Oxide Fuel Cells

Realization of a solid oxide fuel cell (SOFC) operating at 700°C on a hydrocarbon fuel or gaseous H2 is an outstanding technical target. For the past 25 years, efforts to achieve this goal have been based on yttria-stabilized zirconia as the electrolyte, a NiO + electrolyte composite as the anode, a porous La0.85Sr0.15MnO3 (LSM) metallic perovskite as the cathode, and a La1−xSrxCrO3 ceramic as the interconnect material. This paper reviews progress in our laboratory on an alternate approach that would use a Sr- and Mg- doped LaGaO3 perovskite as the electrolyte, a Sm-doped ceria (SDC) as the anode or as a buffer layer with a NiO + SDC composite as the anode, a mixed oxide-ion/electronic conductor (MIEC) as the cathode, and a stainless steel as the metallic interconnect.

Introduction to solid oxide fuel cells (SOFCs)

Technology advancement to address the world’s growing demand for clean and affordable energy will require simultaneous advances in materials science and technology in order to meet the performance demands of new power-generating systems. Fuel cells emerge as highly efficient, fuel flexible, and environmentally friendly electricity producing devices. These unique characteristics advantageously differentiate fuel cells from conventional heat engines for power generation and therefore have attracted worldwide attention – from research and development activities in institutes to commercialization efforts in industries – for the last few decades. In this chapter, the history, advantages, applications, and designs of solid oxide fuel cells (SOFCs) are briefly reviewed.

Steam methane reforming and carbon formation in solid oxide fuel cells (SOFCs)

This chapter discusses the thermodynamic equilibrium and kinetic mechanism of the steam methane reforming reaction. The equilibrium compositions and the cell electromotive force are calculated under various temperatures, steam methane ratios, and pressures. The kinetic mechanism of the steam methane reaction is also briefly mentioned. Carbon formation is discussed from the perspectives of thermodynamics and kinetics.

Current, gas flow, utilization, and energy balance in a solid oxide fuel cell (SOFC)

This chapter describes the basic relationships among current, gas flows, and utilizations of fuel and air, two reactants of an SOFC. Examples are given to calculate the stack compositions and the exhaust temperature from the energy balance in an SOFC system.

Thermodynamics of the solid oxide fuel cell (SOFC)

Thermodynamics is the theoretical foundation for any type of electrochemical concentration (or galvanic) cell. It depicts the fundamental relationship between thermodynamic quantities and electrical quantities; this relationship not only allows determination of thermodynamic properties of materials by accurate electrochemical methods, but also defines the maximum cell voltage of a specific chemical reaction and its dependence on concentration, temperature, and pressure. In addition, the volume changes that occur with variations in temperature, oxygen stoichiometry, and partial pressure of oxygen can also be understood by the principles of thermodynamics. In this chapter, the laws of thermodynamics are applied to elucidate the electrochemical and mechanical behaviors of components of SOFCs.

Electronic properties of solids for solid oxide fuel cells (SOFCs)

The performance of an oxide used as an electrode or as an interconnect between cells of an SOFC depends critically on the electronic properties of the oxide. However, there are very few reports in the literature that discuss how the electronic properties of an oxide effect the electronically conducting electrodes and interconnect in an SOFC. In this chapter, the electronic properties of oxides are discussed from the perspective of solid-state physics, and the results are applied particularly to electrode and interconnect materials commonly used in SOFCs.

Voltage losses in a solid oxide fuel cell (SOFC)

The maximum voltage achievable by a single SOFC is governed by its electromotive force (EMF) under the open-circuit voltage (OCV) condition. Upon delivering electrical current, the components of an SOFC exhibit resistance, resulting in voltage losses. The cell voltage useful for power generation is, therefore, the cell EMF subtracted by the voltage losses of the individual components. The knowledge of these voltage losses as well as their distributions in an SOFC is critically important for maximizing the power output, an ultimate goal of any power-generating device. Based on the nature of the resistance, the voltage losses can generally be classified into three categories: ohmic, activation, and concentration polarizations. In this chapter, the voltage loss from each type of polarization will be discussed extensively.

Poisoning of solid oxide fuel cell (SOFC) electrodes

The degradation of SOFC performance is often found to be associated with various mechanisms of electrode poisoning. Good examples include the presence of trace sulfur in the fuel stream that can lead to deterioration of the anode performance, and gas-phase chromium oxide in the air stream that can degrade cathode performance. A commonly accepted mechanism for these poisoning effects is the loss in number of reactive sites for either oxygen reduction or fuel oxidation by the coverage with nonreactive or insulating species at the triple phase boundaries. The electrode overpotential and ohmic IR loss are therefore gradually increased with time, causing the degradation of SOFC performance. In this chapter, the poisoning mechanisms of sulfur, silica, and phosphorus on the anode and chromium on the cathode are discussed.

Direct current (DC) electrical efficiency and power of a solid oxide fuel cell (SOFC)

One of the unique advantages of SOFCs is the intrinsically high efficiency of producing electrical power. Depending on the power rating and stack configuration, the net AC electrical efficiency of an SOFC generator ranging from a few to hundreds of kilowatts varies from 35 to 55%. When combined with the waste-heat recovery cycle, the overall energy conversion efficiency of an SOFC system falls into the range of 80–90%. The high electrical and system efficiencies of SOFCs are believed to be one of the major drivers for the enduring interest in SOFC development in the last few decades. In this chapter, the DC electrical efficiency and factors that effect it are extensively discussed. The overall electrical efficiency is not the topic of this chapter; it is simply a multiplication of the DC electrical efficiency with the conversion efficiencies of the individual power conditioning system (PCS) components.

Transport of charged particles in a solid oxide fuel cell (SOFC)

One of the important aspects of SOFC components is the transport behavior of charged particles; each type of charged particle contributes a partial electrical current density, either ionic or electronic in nature. In this chapter, the law of irreversible thermodynamics is primarily applied to the transport phenomena of oxide-ions, electron holes, and excess electrons in the electrolyte and interconnect materials. The steady-state P O 2 P O 2 profiles and ionic and leakage (electronic or ionic) current densities across the two dense electrolyte and ceramic interconnection membranes are particularly solved from the phenomenological transport equations.