Date of Award
Doctor of Philosophy
Paul D. Frymier, Robert M. Counce, Nicole Labbé
Polymer electrolyte membrane fuel cells(PEMFCs) are energy conversion devices with significant potential. The factors preventing them from becoming widespread concern production and distribution of hydrogen. Developing an efficient hydrogen infrastructure with an approachable rollout plan is an essential step towards the future of fuel cells. Water electrolysis is limited by the thermodynamics of the process, which leads to high electrical consumption and significant materials challenges. Alternative methods for cleanly generating hydrogen while using a lower cell voltage are required. PEM based electrolyzers can operate with a "depolarized anode", whereby they become significantly less power hungry.
This thesis explores two techniques for chemically depolarized electrolyzer anodes. These include a methanol anode and a phosphomolybdic acid anode. To improve the phosphomolybdic acid anode we have characterized the basic electrochemical behavior of phosphomolybdic acid, the anode behavior in a zerogap electrochemical cell, and the biomass oxidation characteristics of several Keggin ions and potential oxidation promoters.
The methanol cathode was evaluated using a dynamic hydrogen electrode and shown to be significantly more sensitive to crossover induced voltage losses than was previously reported.
Phosphomolybdic acid oxidation kinetics were examined and found to be facile, despite a change in mechanism which occurs after bulk reduction. The temperature dependent diffusion coefficient was found to be on the same order as other likely small, redox active molecules. A previously unreported crossover phenomena was noted and the diffusion coefficient through NAFION was calculated as on the same order as vanadium.
The whole cell performance of the phosphomolybdic acid mediated electrolyzer was examined and found to be highly dependent on supporting electrolyte, temperature, and electrode materials. The optimized condition of 5 M HCl and 80 Celsius showed significant improvement in exchange current density, versus the standard conditions of room temperature and no supporting acid, used in the literature. The electrode kinetics have now been removed as a major problem in the system design.
While the electrochemical performance of the POM mediated electrolyzer was sufficient, the glycerol oxidation rates were found to be lacking. Vanadium, iron, and hydrochloric acid were the most effective additives; while sulfuric acid decreased reaction rates.
Fane, Brian, "An Exploration of Basic Processes for Aqueous Electrochemical Production of Hydrogen from Biomass Derived Molecules. " PhD diss., University of Tennessee, 2017.