Polymer from the membrane
Nafion membranes for hydrogen cells could be improved with new information on their interaction with water. Ellis Davies reports.
A research team from Russia, in collaboration with the Australian National University in Canberra, have described the growth of polymer fibres from a Nafion membrane on interaction with water, for the first time. The membranes are used in hydrogen fuel cells to separate the anode and cathode, and decrease in efficiency as they swell from interaction with water. This issue could now be tackled using the new information.
Nafion is both a strongly hydrophobic and strongly hydrophilic polyelectrolyte tetrafluoroethylene with terminal sulphonic groups, and is therefore an amphiphilic. It is a type of ion-exchange membrane, which has selective ion permeability. In Nafion’s case, the hydrated protons move almost unhindered through the membrane to the cathode, where they are reduced with the release of gaseous hydrogen. The effectiveness of this process is controlled by the proton conductivity of the membrane itself.
The foundation of the study began with the Professor Jerry Pollack’s theory on the exclusion zone. He showed that near a Nafion surface, a region of several hundred micrometres is formed in water, from which colloidal microspheres are pushed out to create an empty space. From this, the team hypothesised that this zone is a special colloidal system, and that there is something that fills it. In this case, polymer fibres.
In a hydrogen cell, a large volume of water sits close to the membrane surface, causing cylindrical micelles to form due to the Nafion’s amphiphilicity and because of the concentration of sulphonic groups at the Nafion/water interface. The resulting micelles look like extended fibres. Researchers believe that these fibres eventually grow towards the bulk of water, into the exclusion zone, similar to the unraveling of a coil of string.
This causes the membrane to become thicker, reducing its proton conductivity and decreasing the efficiency of the hydrogen cell. Nikolai Bunkin, of Bauman Moscow State Technical University, Russia, and one of the authors of the study, told Materials World, ‘Following the results of our work, this effect can be minimised by a small change in the isotopic composition of water used in the hydrogen cell.’
A Nafion membrane is particularly sensitive to deuterium – an isotope of hydrogen – content in water, the researchers found. This is most pronounced in water with 100 and 1,000 parts per million of deuterium. An alteration in this content could therefore improve the efficiency of hydrogen cells.
To investigate, the team developed a non-destructive testing technique for studying the area where the membrane came closest to the water based on photoluminescence spectroscopy, which can detect near-surface structures. ‘In addition, we conducted experiments with a common Fourier Transform Infrared spectrometer, which showed the presence of a correlation with the data from the first method,’ said Bunkin. Using this device, the team was able to detect the polymer fibres growing into the water.
Bunkin believes the results will improve the performance properties of the membranes, specifically their proton conductivity in hydrogen fuel cells, causing increased energy efficiency. ‘Note that the sort of curious phenomena we observed in our study is encountered in many fields of science. The classic case is bubble-bubble interactions in salty water – this is utterly inexplicable and highly ion specific. The venerable fields of physical, colloid, surface and polymer chemistry and electrochemistry, on which our classical intuition depends, are badly flawed, if not plain wrong, and missing hidden variables like effects of dissolved gas in water, which are huge and often ignored. Understanding phenomena like the exclusion zone will allow progress in many fields,’ he said.
Going forward, researchers will focus on specific ion effects in Nafion. Bunkin says that this is bound to throw light on the basic electrochemical forces. ‘The end goal is to better advance the emerging new physical chemistry,’ he says. ‘The classical theory ignores key players like, in Hofmeister effects, dissolved gas, nanobubbles and quantum forces. Professor Barry Ninham of the Australian National University has been in the vanguard of such developments for many years. Now they are coming to fruition. Our research can also be relevant in biology, as well as applications from sterilising water to desalination.’