Freedom of movement: researchers track proton mobility to improve conductivity in new material

Researchers at Manchester and Nottingham Universities have mapped the structure of the material, known as a metal-organic framework, or MOF, and how charge flows through it.

The performance of polymer electrolyte membrane fuel cells, used in transportation, depends on the efficiency of the electrolyte material at their centre, which controls the flow of charge between the positive and negative electrodes.

So researchers are attempting to improve the efficiency of the devices by developing smart electrolyte membranes based on materials that facilitate the charge transfer more smoothly.

MOFs are hybrid materials made up of metals connected by organic ligands. They have a number of advantages that make them potential candidates for use as smart electrolyte materials, according to Dr Sihai Yang, a group leader from Manchester University.

Firstly, the organic ligands can be modified by adding groups of atoms - known as functional groups – such as those containing hydrogen donors, to improve the conductivity of the material, he said.

Secondly, since the materials have a porous structure, different small molecules that act as proton carriers can also be loaded into their pores, to further improve their conductivity.

But perhaps most importantly, the crystalline nature of the materials also makes it possible for researchers to study the structure and conductivity of MOFs in precise detail, to gain a better understanding of how to improve their efficiency even further.

To this end, the research team used the powerful x-rays at Diamond Light Source, the UK’s synchrotron science facility, to study the atomic structure and workings of one type of MOF material, known as MFM-500(Ni).

They then used neutrons at the Science and Technology Facilities Council's ISIS Neutron and Muon Source to map the movement of protons through the material.

They found that, contrary to previous predictions that protons moved through the material by jumping between different sites, they in fact move freely within spheres.

Within the structure of the material are a number of these spheres overlapping each other, which allows the protons to simply hop between them, said Yang. “This gives them a three-dimensional continuous diffusion pathway,” he said.

The researchers now hope to optimise the radius of these spheres, he said. “Then at a later date we hope to optimise the structure [of the MOF] to achieve a better packing of the spheres, and therefore an overall improved proton conductivity,” he said.