Fuel cells convert chemical energy directly into electrical energy with high efficiencies and environmental benefits, as compared with traditional heat engines. Yttria-stabilized zirconia is perhaps the material with the most potential as an electrolyte in solid oxide fuel cells (SOFCs), owing to its stability and near-unity ionic transference number. Although there exist materials with superior ionic conductivity, they are often limited by their ability to suppress electronic leakage when exposed to the reducing environment at the fuel interface. Such electronic leakage reduces fuel cell power output and the associated chemo-mechanical stresses can also lead to catastrophic fracture of electrolyte membranes. Here we depart from traditional electrolyte design that relies on cation substitution to sustain ionic conduction. Instead, we use a perovskite nickelate as an electrolyte with high initial ionic and electronic conductivity. Since many such oxides are also correlated electron systems, we can suppress the electronic conduction through a filling-controlled Mott transition induced by spontaneous hydrogen incorporation. Using such a nickelate as the electrolyte in free-standing membrane geometry, we demonstrate a low-temperature micro-fabricated SOFC with high performance. The ionic conductivity of the nickelate perovskite is comparable to the best-performing solid electrolytes in the same temperature range, with a very low activation energy. The results present a design strategy for high-performance materials exhibiting emergent properties arising from strong electron correlations.

The use of oxide fuel cells and other solid-state ionic devices in energy applications is limited by their requirement for elevated operating temperatures, typically above 800°C (ref. 1). Thin-film membranes allow low-temperature operation by reducing the ohmic resistance of the electrolytes. However, although proof-of-concept thin-film devices have been demonstrated, scaling up remains a significant challenge because large-area membranes less than ~ 100 nm thick are susceptible to mechanical failure. Here, we report that nanoscale yttria-stabilized zirconia membranes with lateral dimensions on the scale of millimetres or centimetres can be made thermomechanically stable by depositing metallic grids on them to function as mechanical supports. We combine such a membrane with a nanostructured dense oxide cathode to make a thin-film solid-oxide fuel cell that can achieve a power density of 155 mW cm⁻² at 510 °C. We also report a total power output of more than 20 mW from a single fuel-cell chip. Our large-area membranes could also be relevant to electrochemical energy applications such as gas separation, hydrogen production and permeation membranes.

Some ionic surfactant/water systems undergo the coagel-gel phase transition. Both coagel and gel phases consist of alternating layers of bilayer membranes of surfactants and water. The hydrocarbon chains are in a crystalline state, and thickness of water layer is in the order of 10 A in the coagel phase. Upon the transition from coagel to gel, the hydrocarbon chains become able to rotate around the chain axis, and the thickness of water layer discontinuously increases to the order of 1000 A. We present a statistical mechanical theory to explain this phase transition. The mean field Gibbs free energy of the system is written as a function of three-order parameters which are concerned with the rotational motion of the hydrocarbon chains, the degree of dissociation of counterions of the surfactant molecules, and the distance between neighboring membranes.The area occupied by one hydrophilic head group on the membrane surface in the rotating state of the hydrocarbon chain is assumed to be larger than that in its fixed state. The theory successfully explains the order4isorder phase transition of the hydrocarbon chains and the simultaneous separation of bilayer membranes. The expansion in the area of one molecule leads to a coupling between three-order parameters. The theoretical calculation predicts that the strength of this coupling results in the three types of phase transition: (1) coagel/gel phase transition, (2) pseudogel (the hydrocarbon chains are in crystalline state, and the membranes separate)/gel phase transition, (3) coagel/pseudocoagel (the rotation of hydrocarbon chains around the chain axis is released, and the membranes do not separate) phase transition.

We investigated relations between food texture and thin film properties rather than bulk properties. Such thin film rheological or 'tribological' effects may characterize physical behavior and physical perceptions of food products. Experiments were done on three chocolate samples. The Surface Forces Apparatus (SFA) proved to be effective for studying the very complex chocolate system. Very complex thin film behavior was found to be quite different for the three samples. Fat constitution and average particle size were important in determining the type of friction forces generated between shearing/sliding surfaces separated by thin films of chocolate. Such studies can help characterize specific physical qualities of chocolate.