Date of Award

Fall 2023

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

Sirivatch Shimpalee

Second Advisor

Benjamin H. Meekins

Abstract

Polybenzimidazole (PBI) membranes are high temperature proton exchange membranes (PEMs) which have advantages such as being able to operate in high temperatures, strong acid environments, and have no need for external humidity during operation. PBI membranes have been demonstrated to work effectively in multiple electrochemical devices, including fuel cells, hydrogen separation pumps, redox flow batteries, and a hybrid sulfur electrolyzer. In this work, para-PBI and densified para-PBI membrane have been used to improve electrochemical performance in harsh operating conditions that are both high temperature (≥120 ̊C) and high acid concentration (≥ 5M). First, we demonstrated the use of para-PBI membranes as replacements for the state-of-the-art Nafion membrane in an oxygen depolarized cathode (ODC) HCl electrolyzer. Both para-PBI and densified para-PBI membranes reduce the cell voltage more than 100 mV at 0.5 A/cm2, and the cell voltage gap between PBI and Nafion increases with increasing current density. The effect of aqueous HCl flow rate, back pressure, temperature, and phosphoric acid swelling time for densified para-PBI on the electrolyzer performance are shown and discussed. Since PBI membranes do not need water to conduct protons, we also developed a fully anhydrous HCl electrolyzer. This electrolyzer can directly convert pure HCl gas to dry H2 and dry Cl2 at calculated single-pass efficiency >90% at 160 ̊C and 1.8 V with no evidence of corrosion in the system. Swelling the densified para-PBI in PA for 3 hours improved the anhydrous HCl electrolyzer performance compared to both a densified para-PBI swelled for 24 hours in phosphoric acid and a gel para-PBI membrane. To enhance understanding of the transport behavior and physics inside the anhydrous HCl electrolyzer, a computational fluid dynamics (CFD) model of the anhydrous electrolyzer was also developed. The CFD model allows the study of experimental conditions that are difficult to perform safely and aids both optimization of operating parameters and anticipation of operational issues when scaling the system up. The simulation results showed evidence of near cell starvation during the experiments. To improve electrolyzer performance, HCl stoichiometry was increased, and back pressure was applied to the electrolyzer. Applied back pressure and increased flow rate significantly improved anhydrous HCl electrolyzer performance. We also use the CFD model to discuss potential issues that must be addressed when transferring this lab-scale process to industrial scales.

Rights

© 2024, Kris Likit-anurak

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