Solid-oxide fuel cells (SOFCs) are unique in their ability to directly convert the chemical energy of a wide variety of fuels to electric power with unmatched energy conversion efficiency. The articles in this issue of MRS Bulletin highlight the enormous potential of, and recent progress toward, operating SOFCs at lower temperatures (<650°C). Lower temperatures dramatically increase the number of potential applications for this technology as well as provide the opportunity to incorporate a wider variety of materials in SOFC power generation systems with greater reliability and lower cost. The articles in this issue describe materials development and processing for low-temperature SOFCs, including the enabling of nanotechnology and microelectromechanical systems-based cell designs, the development of highly active electrodes and their three-dimensional microstructural characterization, as well as the use of novel proton-conducting electrolytes, all of which provide new avenues of research. New fabrication methods are also being applied in the development of micro-SOFCs and microtubular SOFCs with higher power densities. Finally, advances in lowering performance degradation rates, a critical commercialization issue, are described.

We present the first steps of a design of the optimal parameters for a full Bragg X-Ray free electron laser (BX-FEL). Aiming towards a future source of coherent X-ray radiation, operating in the strong Compton regime, we envisage the system to be the seed for an advanced light source or compact medical X-ray source. Here we focus on the design of the accelerator parameters: maximum gradient, optimal accelerated charge, maximum efficiency, and ‘wake coefficient’, which relates to the decelerating electric field generated due to the motion of a charged-line or train of charged-lines. Specifically, we demonstrate that the maximum efficiency has optimal value and given the fluence of the materials, the maximum accelerated charge in the train is constant. These two results might be important in any future design.

There is an urgent need for new energy storage and conversion systems in order to tackle the environmental problems we face today and to make the transition to a fossil fuel-free society. New batteries, supercapacitors, and fuel cells have the potential to be key devices for large-scale energy storage systems for load leveling and electric vehicles. In many cases, the concepts are known, but the right materials solutions are lacking. Ionic liquids (ILs) have been highlighted as suitable materials to be included in new devices, most commonly as electrolytes. Attractive features of ILs such as high ionic conductivity, low vapor pressure, high thermal and electrochemical stability, large temperature range for the liquid phase, and flexibility in molecular design have drawn the attention of researchers from many different fields. In addition, there is the possibility of designing new materials and morphologies using electrochemical synthesis with ILs. In this article, we provide an introduction to ILs and their properties, serving as a base for the topical articles in this issue.

La modélisation et la simulation numérique permettent une exploration des différents paramètres decontrôles afin d’identifier les conditions et géométries optimales de fonctionnement dessystèmes. Cette approche permet entre autre d’obtenir les valeurs instantanées desgrandeurs physiques locales. Ces outils numériques progressent et traitent des systèmes deplus en plus complexes confortés par la progression des puissances des ordinateurs. Nousdécrirons dans le présent travail le cas de moteurs alternatifs de type Stirling, àcombustion externe et régénération. Ces moteurs constituent une solution pour laconversion efficace des énergies renouvelables et des chaleurs perdues en travail. Lasimulation de tels systèmes en régime établi permettra d’en déduire les énergies échangéeset de démontrer que ces moteurs offrent un bon rendement de fonctionnement, tout enprésentant une grande souplesse d’adaptation. L’optimisation des machines est fortementliée à leurs paramètres géométriques et physiques (dimensions, matériaux, coefficient detransfert de chaleur, etc.) ce qui peut engendrer un coût important pour les différentsprototypes. La simulation numérique permettra la prédiction du cycle au cours du temps etdonnera accès aux valeurs du travail ainsi que le rendement de la machine simulée. Cettedémarche identifiera les zones de fonctionnements optimaux et réduira le nombre deprototypes et du même coup le coût du projet. La modélisation s’appuie sur la résolutiondes équations de conservation d’écoulements compressibles anisothermes dans un moteur LTDen domaine bidimensionnel (2D, moteur supposé axisymétrique afin de faciliterl’illustration). En guise de validation l’un des résultats obtenus numériquement estconfronté aux résultats expérimentaux obtenus sur un prototype moteur de démonstration. Lebon accord constaté sur le cycle p-V illustre l’intérêtméthodologique.