Investigations of Anion Exchange Polymer – Catalyst Interfacial Interactions
Authors: Buggy Nora Catherine , Kuo Mei-Chen , Herring Andrew M. 
Published: February 2020

Anion exchange membrane (AEM) fuel cells offer a promising alternative to proton exchange membrane fuel cells. AEMs utilize more facile electrochemical reaction kinetics by operating in a basic environment, which should theoretically allow them to utilize non-precious metal catalysts such as silver or nickel. This research focuses on the characterization of a novel block copolymer AEM and elucidating the anion exchange polymer-catalyst interface. The block copolymer system has been engineered to combine the features of high ion conductivity of a hydrophilic block with the reinforcing properties of a hydrophobic block. The ABA triblock polymer utilizes a polychloromethylstyrene-b-polypolycyclooctene-b-polychloromethylstyrene (PCMS-PCOE-PCMS) backbone and dimethylpiperidinium (MPRD) cation, which has shown excellent hydroxide stability. This work aims to first understand the bulk properties of the polymer and then subsequently compare them to the properties of the material at the catalyst interface, where potential interactions could lead to restructuring of the polymer and thus alter the transport of water, hydroxide, fuel, and electrical conduction. This will be studied by fabricating well-defined interfaces between the polyelectrolytes and catalyst surface, either as thin films on an electroactive surface of silver (Ag) or in a constrained environment of Ag colloids. If the volume fraction of Ag colloids is small, the polymer properties will be representative of those at the polymer-catalyst interface. In-plane conductivity will be measured by EIS using a four fine Pt wire electrode fixture, from which an activation energy of aqueous ionic movement can also be derived. To further understand how the mechanical properties of the polymer are affected at the catalyst interface, dynamic vapor sorption will be used assess water content as a function of relative humidity (RH) and thermogravimetric analysis will be used to determine thermal properties. Additionally, morphological changes and restructuring at the catalyst interface as a function of RH will be elucidated using AFM and GISAXS. The information obtained from these techniques will determine whether the polymer is aligned with the catalyst surface. Pulse field gradient spin echo (PFGSE) NMR spectroscopy will be used to study the movement of water molecules (1H NMR), carbonate and bicarbonate (13C NMR), and fluoride (19F NMR). When performed at varying temperature and RH, self-diffusion coefficients can be obtained, and the tortuosity of the transport pathway can be estimated. ATR FTIR spectroscopy will also be used to investigate the nature of the water content and compare it between the bulk polymer and polymer-catalyst interface. Cyclic voltammetry using a bipolar membrane fuel cell will also be used to study the adsorption and desorption on the catalyst layer. The characterization of the polymer-catalyst interface and further understanding of any restructuring that occurs will provide a knowledge base for optimizing the design of an AEM.