Towards the ideal fuel cell
Semipermeable polymeric membranes that selectively allow the passage of negative ions (anions) play a key role in several important technologies including fuel cells. Neutron studies untangle the complex dynamics in an anion exchange membrane when employed in a fuel cell setting.
Fuel cells generate electricity from an electrochemical reaction usually between hydrogen (as the fuel) and oxygen (from the air) with the only waste product being water. They offer a significant green option as a future carbon-free power source. Like all electrochemical cells they consist of an anode and a cathode in contact with a conducting electrolyte through which charge carriers – ions like hydrogen (H+) or hydroxide (OH-) can travel between the electrodes hosting the reaction sites.
A favoured fuel cell design is one where the electrolyte is a synthetic membrane consisting of a solid ionic polymer chemically tailored to mediate the passage of specific ions and water. These ion exchange membranes are extensively used in industry for a wide variety of applications such as waste extraction and desalination. There is therefore a great deal of interest in understanding and optimising the various transport processes across these membranes while maintaining their stability.
In the case of fuel cells, there is increasing focus on designs based on so-called anion exchange membranes (AEMs) that allow OH- ions to pass through. An important reason is that the electrodes in AEM cells do not need to incorporate platinum or other expensive noble-metal or rare-earth catalysts to work, as required by designs using so-called proton-conducting membranes that rely on the transport of H+ ions. However AEM technology is less well-advanced – the membrane stability needs to be improved as does overall cell performance.
An international research team including the ILL therefore decided to study the incredibly intricate behaviour involved in transporting OH- ions between the cathode and anode via an AEM, with the aim of helping to improve design. Such membranes consist of a polymer backbone (generally, chains of carbon rings) to which are attached groups of atoms carrying a positive charge. Associated with these cationic sites are negative anions such as chloride (Cl-) or bromide (Br-) in the commercially available product, which can then be exchanged with the more mobile OH- ions needed for fuel-cell operation. Of great importance is understanding how the OH- ions move across the membrane – which could be either by diffusion or by ‘hopping’ via hydrogen-bonded water molecules, which may also be interacting with the polymer, as revealed by tiny motions in the polymer chain itself. Knowing the hydration level – the number of water molecules around each of the polymer’s cationic sites is critical to understanding its effect on the ion current flow.
A spotlight on nano-scale motions
Quasi-elastic neutron scattering provides the means of unpicking these subtly coupled effects. Neutrons can reveal motions and dynamical changes in molecules like polymers and water because they can interact with these motions at given energies. The resulting characteristic energy changes can then be measured by a spectrometer. Our team employed three complementary spectrometers designed to cover timescales from picoseconds to nanoseconds so as to access dynamical changes associated with the ion flow as well as those within the polymer backbone.
Another advantage of neutrons is that they readily detect hydrogen bound in polymers, water and hydrogen-containing ions. Furthermore, by selectively substituting hydrogen atoms with the heavier isotope deuterium (which scatters differently) in the polymer chain, or the OH- ions and watery solvent, we can highlight specific interactions of interest. We were also able to assess whether the polymer degraded during cell operation by comparing the behaviour of the membrane containing OH- ions with versions substituted with Br- or Cl- ions as in the original form.
The experiments were carried out over a range of operating temperatures and hydration levels, and the resulting changes in dynamical behaviour analysed. We were able to uncover details of how water molecules mediate the OH- hopping process, which is more prevalent at low hydration. At medium hydration, polymer–water interactions affect the ion conduction, but at high hydration diffusion becomes the dominant transport mechanism. These experiments thus provided valuable insights into the conduction mechanisms and the electrolytic environment that are significant in designing the next generation of fuel cells and other devices using AEMs.
IN16B backscattering spectrometer and IN6-Sharp (ILL), the IRIS Time of Flight Near Backscattering Spectrometer at the ISIS Neutron and Muon Spallation Source (UK), and the High Flux Backscattering Spectrometer at the National Institute of Standards and Technology, US.
Fabrizia Foglia et al.,“Disentangling water, ion and polymer dynamics in an anion exchange membrane”, Nature Materials, 2022, 21, 556.
Fabrizia Foglia, Keenan Smith, Adam J. Clancy, Thomas S. Miller, Daniel J. L. Brett, Paul R. Shearing and Paul F. McMillan (University College London, UK), Quentin Berrod, Gérard Gebel and Sandrine Lyonnard (University of Grenoble France), Victoria García Sakai and Najet Mahmoudi (ISIS, UK), Markus Appel (ILL), Jean-Marc Zanotti (University of Paris-Saclay, France), Madhusudan Tyagi (NIST and University of Maryland, US), John R. Varcoe and Arun Prakash Periasamy (University of Surrey, UK).
Contact ILL : Markus Appel