Investigations of Anion Exchange Polymer Interfacial Interactions
Abstract
While there is the exciting possibility of device ready anion exchange membranes (AEMs) on the near-term horizon, much more basic science needs to be done. A critical knowledge base not yet developed are the studies needed to develop anion exchange polymers (AEPs) that can interface the AEM with the catalyst particles in the electrodes providing both high ionic conductivity and transport of reactant and product chemical specie s. For electrochemical devices employing AEMs, it is already becoming apparent that simply making electrodes via an ink process that employs low EW AEPs is a fundamentally flawed approach. First, many AEPs are generally insoluble. Secondly, it is already known that the organic cations in these polymers, which have strong dipoles, have a tendency to cluster. So simply lowering the EW (increasing the ion exchange capacity (IEC)) will not result in a more ionically conducting polymer. Thirdly, because hydrocarbon backbones are stable in base, and are inexpensive, most AEPs are being made from hydrocarbon polymers. AEPs designed as AEMs do not have sufficient reactant and product transport to be used as the electrode layer. Fourthly, in AEM systems water is always a reactant, and in AEM fuel cell the water produced at the anode should be transported back to the cathode via diffusion to form new hydroxide anions. This is contrary to a PEM fuel cell where hydrophobic fillers are used to reject water out of the cell; here we want to effectively transport water back into the membrane. It is well known that in most ion-conducting polymers, surface reorganization occurs both as a function of thickness as well as a function of external environment. As ionomers come into contact with various interfaces including defect structures, rearrangement of the phase-separated polymer moieties occurs. Such confinement impacts can include dramatic changes in mechanical and transport properties due to (re)ordering of the structure. The process and time scales for this self-organization have a tremendous impact on the ability of the polymer to transport reactants and solvents. Interface-induced morphological modifications can result from interactions between different polymer moieties and the external environment including gases (e.g., water vapor), liquids (e.g., fuels), and solids (e.g., electrocatalyst particles). Reorganizations can also be brought about by application of electric fields or by shear forces. Due to the existence of the long polymer chains, surface effects and alignment of the nanostructural units typically propagate through the polymer impacting around 5 or 6 domain layers, which may cause significant changes in related transport properties and structure. For the mesostructured amphiphilic copolymers to be studied in this proposal, surface reorganization is more far reaching. Surface interaction influences, such as when the copolymer mesophase meets the surface, set the alignment of the periodic mesostructure in the interior. In an ion-transport polymer, this restructuring could easily block transport or produce uncontrolled anisotropic distortion. Thus it is essential to understand and control this restructuring. / / Diblock copolymers will be developed with stable cations utilizing hydrophilic blocks that maximize hydroxide and water transport and hydrophobic blocks designed to transport reactant gases. These materials will be studied as thin films on electro-active surfaces of Ag or in constrained environments with well-defined Ag colloids. We will use ionic conductivity, water uptake, species diffusion, morphological investigations and vibrational spectroscopy to study these materials. Learning from the interfacial studies will guide future polymer syntheses. As we understand the polymer-catalyst interfacial interactions in greater detail we will be able to design polymers and nano-particle/polymer interfaces that will maximize desired transport properties for electrochemical catalysis.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Oct 06, 2018
- Source ID
- W911NF1710568
Entities
People
- Andrew M Herring
Organizations
- Army Contracting Command
- Colorado School of Mines
- United States Army