Abstract
From fundamental studies of cationic membranes we were able to show that very high anionic conductivities could be achieved in thin robust membranes with controlled water contents. Over the last several years we have further developed these advanced anion exchange membranes (AEMSs) to improve both their chemical and mechanical stability. One of these polymers is being commercialized as the TuffBrane material and we are now deploying this membrane in devices for electrochemical energy conversion or water purification. While there has been a large amount of work to develop electrocatalysts for fuel cells, much of this was optimized using rotating disk electrodes in aqueous solvents. In solid state electrochemical energy conversion devices using polymer electrolytes, the electrolyte is a polymer with a local morphology and chemistry. This has led us to conclude that it is the polymer/catalyst interfaces that must be understood and improved to achieve the maximum performance in these devices. We have used a toolbox of random, di-block, and triblock polymers where the hydrophobic component or block is polycyclooctene, poly isoprene, or their hydrogenated analogues, polyethylene or polymethylbutylene. The hydrophilic component or block is derived from polychloromethylstyrene via quaternization with trimethylamine or methyl pyrrolidine to give the more stable MPRD cation. We have begun our studies using Ag as a model system although our plans are to extend these studies to Ni. Our first studies used commercial Ag colloids as catalyst particles; these studies showed that we could actually change the degree of crystallinity of the polyethylene block in triblock anion exchange polymers and so alter the polymers water uptake. We then extended this work to spin coated thin films on atomically flat silicon wafers or the same substrate with a Ag coating, and where able to show a number of these polymers had very strong vertical alignment using grazing Incident Small Angle X-Ray Scattering. This has led us to show that the cation can dramatically effect the activity of the electro catalyst via its interaction with the catalyst surface. Our work using Ni foam electrodes in separated anode experiments is showing that the same polymers can also dramatically affect the onset potential and Tafel slope for the oxygen evolution reaction in carbonate electrolyte with or without the addition of the Ag colloids. Extending these studies to single cell electrolysis we see interesting effects in both demonized water, carbonate and hydroxide electrolytes. The results of these studies will be discussed in this paper.
The work was sponsored by the Army Research Office under grant, W911NF-17-1-0568