(Invited) Controlling Charge Transfer and Ion Transport in Electrodes for the Oxygen Evolution Reaction
Authors: Herring Andrew M. , Buggy Nora Catherine , Wu Ivy , Kuo Mei-Chen , Ezell Morgan , Beiler Kaylee , Johnson Andrew , Dunn Kevin 
Published: November 2022

Anion Exchange Membrane (AEM) based energy conversion devices offer the possibility of utilizing non-precious metals as the catalyst and the advantage of more facile electrochemical conversions. While many AEMs are showing practical performance and durability’s, their potential will only be realized if the catalyst particles are modulated by the cationic ionomer for maximum performance. The catalyst layer in the electrode provides the triple-phase boundary between the anion-conducting polymer, supported catalyst particles, and reactants where the electrochemical reactions take place. In proton-exchange membrane fuel cells (PEMFC), the electrode catalyst layer was developed empirically over many decades. State of the art AEM devices could be developed with new scientific insight and methodology facilitating the opportunity to be much more quickly developed as well as to exhibit maximum performance and durability. Also for PEMFC to improve Pt catalyst utilization, high surface area carbon-supported Pt catalysts were developed, followed by the development of catalyst inks and the implementation of electrode spraying techniques commonly in use today. However, carbon supported catalysts do not have the necessary durability for the oxygen evolution reaction (OER) necessitating the use of Iridium oxide in acid systems.

We hypothesize that the crucial component of an electrode is the ionomer that modulates the transport of ions and neutrals to the catalyst surface and by its interaction with the surface of the catalyst can also modulates charge transfer in solid state electrodes. So, by understanding the ionomer chemistry we can design ionomer catalyst interfaces for maximum performance. For the OER we use porous Ni electrodes that we imbibe with catalyst ionomer inks. In these inks we can not only vary the catalyst to ionomer ratio, but the ionomer chemistry and IEC. To date we have studied nano-structured silver1 or cobalt oxide catalysts. The ionomers are either random or block co-polymers consisting of polyisoprene or polycyclooctene hydrophobic components as the hydrophobic component that can be hydrogenated to polymethylbutylene or polyethylene. The other component is a quaternizable polychlorostyrene that can we functionalize with trimethyl amine or methylpyrolidine. These combinations give a large experimental space in which to probe the effect of ionomer chemistry on electrode performance. We test the electrodes in a separated anode experiment where linear sweep voltammetry is used to get kinetic information and cyclic voltammetry is used to determine the double layer capacity that we use as a measure of electrochemical surface area. Using silver the ionomer chemistry clearly effcts charge transfer and transport of ions and neutrals in the electrode.1 Optimized electrodes are them used to construct 5 cm2 electrolysis cells where the electrolyzer performance and durability is evaluated. In these cells we use a Pt on carbon anode with a cationic ionomer of fixed composition and high performance triblock AEM2 that we developed that is currently being commercialized under license to SparkIonix as Tuffbrane™. Our studies to date use pumped carbonate as the water source, but we are also extending this work to distilled water or hydroxide solutions. Removing the supporting anions from the water feed by ionomer/catalyst interface engineering would dramatically improve these AEM based water electrolysis systems.

“Evaluating the Effect of Ionomer Chemical Composition in Silver-Ionomer Catalyst Inks toward the Oxygen Evolution Reaction by Half-Cell Measurements and Water Electrolysis.” N.C. Buggy, I. Wu, Y. Du, R. Ghosh, M.-C. Kuo, M.S. Ezell, J.M. Crawford, S. Seifert, M.A. Carreon, E.B. Coughlin, A.M. Herring,* Electrochimica Acta., 2022, 412, 140124.

“A Polyethylene-based Triblock Copolymer Anion Exchange Membrane with High Conductivity and Practical Mechanical Properties.” N.C. Buggy, Y. Du, M.-C. Kuo, K.A. Ahrens,S. Wilkinson, S. Seifert, E.B. Coughlin,* and A.M. Herring*, ACS Applied Polymer Materials, 2020, 2, 1294 – 1303.