Cooler ceramic fuel cells: Ceramic ion conductors can be used as electrolytes in fuel cells using natural gas. One drawback of such solid-oxide fuel cells that conduct oxygen ions is their high operating temperatures (at least 600°C). Duan et al. have made a proton-conducting ceramic fuel cell with a modified cathode material that exhibits high performance on methane fuel at 500°C (see the Perspective by Gorte). Science , this issue p. 1321 ; see also p. 1290

Solid oxide fuel cells (SOFCs) are efficient and fuel flexible electrochemical energy conversion devices that can power the future green society with regards to homes, cars, and even down to portable electronics. They do have the potential to become low cost, since no noble metals are used. Their broad commercialization, however, is hampered by the high operating temperatures of 700–900 °C. Lowering the operating temperature of SOFCs is challenging as both the charge transport in the solid electrolyte and oxygen exchange reactions are thermally activated processes. Herein, the recent progress in the development of anode, electrolyte, and cathode materials to lower the operating temperature of SOFC below 600 °C is summarized and the new opportunities, as well as challenges that remain to be solved, are discussed. The focus of this review is addressed to thin film SOFCs, sub‐micrometer SOFCs (μSOFCs) based on microelectromechanical systems, as well as devices based on proton‐conducting oxide electrolyte (protonic ceramic fuel cells), which are especially promising for powering portable devices.

This review paper focuses on the phenomenon of thermochemical expansion of two specific categories of conducting ceramics: Proton Conducting Ceramics (PCC) and Mixed Ionic-Electronic Conductors (MIEC). The theory of thermal expansion of ceramics is underlined from microscopic to macroscopic points of view while the chemical expansion is explained based on crystallography and defect chemistry. Modelling methods are used to predict the thermochemical expansion of PCCs and MIECs with two examples: hydration of barium zirconate (BaZr_1−xY_xO_3−δ) and oxidation/reduction of La_1−xSr_xCo_0.2Fe_0.8O_3−δ. While it is unusual for a review paper, we conducted experiments to evaluate the influence of the heating rate in determining expansion coefficients experimentally. This was motivated by the discrepancy of some values in literature. The conclusions are that the heating rate has little to no effect on the obtained values. Models for the expansion coefficients of a composite material are presented and include the effect of porosity. A set of data comprising thermal and chemical expansion coefficients has been gathered from the literature and presented here divided into two groups: protonic electrolytes and mixed ionic-electronic conductors. Finally, the methods of mitigation of the thermal mismatch problem are discussed.