Fuel cell technology will be an important part of a future renewable energy mix, and considerable effort is being devoted to the development of these devices. In a hydrogen fuel cell, protons are produced at one electrode by the oxidation of hydrogen and then transported across a selective membrane to another electrode as electrical current, thus converting chemical energy to electrical energy. Materials that can transport protons are therefore in high demand, and it was while studying the proton-conducting properties of certain inorganic molecules that Shin-ichi Ohkoshi and co-workers from the University of Tokyo in Japan1 discovered an interesting magnetic effect on conductivity in vanadium–chromium crystals.

Fig. 1: Proton conductivity in a vanadium–chromium Prussian blue analog is dependent on the arrangement of water molecules within the crystal through interference between magnetic order and ionic conductivity.

The molecules they were studying were variants of ‘Prussian blue’, one of the earliest synthetic dyes. Prussian blue is composed of a pair of iron atoms with mixed oxidation states bridged by cyanide linkers. The dye can be varied by replacing the iron with other transition metals, and some of the variants, called analogs, have been found to display interesting magnetic functionality.

In the course of studying the effect of temperature on proton conductivity in a vanadium–chromium Prussian blue analog, Ohkoshi and his co-workers observed that the proton conductivity increased with temperature, as expected, but with an abrupt jump in the rate of increase at 313 K not seen in related materials. This temperature was found to correspond to the ‘Curie temperature’ of the material — the temperature at which magnetic ordering is lost due to a change in structure. This same change in structure has also been associated with a change in the way protons are conducted through the material. “This is the first observation of an interference effect between ionic conductivity and magnetic ordering,” says Ohkoshi, who believes the effect could form the basis for ‘spin-ionics’.

“Protons are conducted through the material by water molecules arranged in the pores,” Ohkoshi explains. “When the structure is changed, the arrangement of trapped water changes as well, and the proton conductivity changes. Our next step will be to investigate the effect of an applied magnetic field on the proton conductivity.”