Abstract
Lowering the operating temperature of solid oxide fuel cells (SOFCs) to the intermediate range (500–700 ºC) has become one of the main SOFC research goals. High operating temperatures put numerous requirements on materials selection and on secondary units, limiting the commercial development of SOFCs. The present review first focuses on the main effects of reducing the operating temperature in terms of materials stability, thermo-mechanical mismatch, thermal management and efficiency. After a brief survey of the state-of-the-art materials for SOFCs, attention is focused on emerging oxide-ionic conductors with high conductivity in the intermediate range of temperatures with an introductory section on materials technology for reducing the electrolyte thickness. Finally, recent advances in cathode materials based on layered mixed ionic-electronic conductors are highlighted because the decreasing temperature converts the cathode into the major source of electrical losses for the whole SOFC system. It is concluded that the introduction of alternative materials that would enable solid oxide fuel cells to operate in the intermediate range of temperatures would have a major impact on the commercialization of fuel cell technology.
Highlights
Fuel Cells (FCs) are electrochemical devices that convert the chemical energy of a reaction directly into electrical energy
A Solid Oxide Fuel Cell (SOFC) is a particular type of fuel cell consisting of a gas tight solid electrolyte separating two electrodes, cathode and anode, exposed to two independent atmospheres of different oxygen partial pressure [3] (Figure 3)
Majumdar et al reported a detailed study about the stress and fracture behaviour of a three-layer ceramic SOFC developed at the Argonne National Lab [17]
Summary
Fuel Cells (FCs) are electrochemical devices that convert the chemical energy of a reaction directly into electrical energy. Hydrocarbons like CH4 can be directly used in high temperature fuel cells because of the rapid electrode kinetics and lesser-required catalytic activity at higher temperatures. In this type of fuel cells, the reforming reactions (conversion of hydrocarbon into hydrogen) can occur within the cell (internal reforming). The electrolytes for solid oxide fuel cells (SOFCs) require high temperatures to achieve the oxide-ion conductivity necessary to ensure high enough power density, with targets of 1 W/cm2 These high operating temperatures (over 800 oC) involve some problems, especially related to the materials compatibility. The origin of this limitation and possible solutions are the main goal of this review and will be discussed along different sections
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