On Solids with Liquidlike Properties and the Challenge To Develop New Proton-Conducting Separator Materials for Intermediate-Temperature Fuel Cells

摘要

In the search for more efficient and cleaner ways to convert energy, fuel cells have attracted increasing interest. Fuel cells utilise the same net chemical reaction as the burning of fuel whether, for example, in a combustion engine or a heating system; but by spacially separating the reactants, they convert chemical energy directly into electrical energy instead of generating heat as an intermediary. A critical part of a fuel cell is the separator material, an electrolyte conducting preferentially one kind of ion but impervious to electrons (and holes). As with a battery, the electrons must instead flow externally where they can be, so to say, harvested by an electrical load. The different types of fuel cells are even named according to the different electrolytes used as separator materials, namely SOFC (solid oxide fuel cell), MCFC (molten carbonate fuel cell), AFC (alkaline fuel cell), PAFC (phosphoric acid fuel cell) and PEMFC (polymer electrolyte membrane fuel cell). During the last decade, research and development activities have mainly focused on PEMFCs and SOFCs, which actually correspond to the fuel cell types operating at the lowest (40 - 90 ℃) and highest temperatures (800 - 1000 ℃). Indeed, in both cases there has been tremendous progress with respect to achieving both performance and long-term stability. While this success was essentially the result of sophisticated engineering efforts making use of available materials, the inherent properties of the materials employed to date seem to inhibit further progress of this promising technology. When Sossina Haile and her team of material scientists at Caltech demonstrated that CsHSO_4, a material new to fuel cell technology although well known as a solid proton conductor, could operate in a laboratory fuel cell, a lot of attention was therefore attracted even outside the community.