Visualization of Hydroxide Ion Formation upon Electrolytic Water Splitting in an Anion Exchange Membrane

Abstract

Measurement of hydroxide ion conductivity is paramount to understanding anion exchange membranes (AEM) operated under alkaline conditions, but the measurement is complicated by dissolved CO 2 and the presence of bicarbonates and carbonates. A technique for accurate measurement has recently been reported that involves measuring the conductivity during water splitting, wherein hydroxide ions are assumed, but not proven, to be produced at the cathode and which purge out other anions. In this preliminary study, we visualize the formation of hydroxide ions and their diffusion from the cathode to anode. We do this by way of an anion exchange membrane cast with an acid/base pH indicator, such that visual confirmation of hydroxide production at the cathode is obtained during application of sustained current load. This proof of concept is demonstrated using the AEM, hexamethyl-p-terphenyl poly(methylbenzamidazolium), and pH indicator, thymolphthalein, included at 0.1 and 0.2 wt % concentration. An additional novelty of this work is that we perform conductivity measurements under potentiostatic load rather than galvanostatic load, which we find substantially increases (6× faster) the time required for the membrane to reach a steady state membrane conductivity.

Overview: State-of-the Art Commercial Membranes for Anion Exchange Membrane Water Electrolysis

“…Another method is to electrochemically purge carbonates from the membrane by applying a voltage in a CO 2 -free atmosphere. Values obtained in this way are significantly higher than those measured with the first method and are more close to the conductivity observed in an operating electrolysis cell [71,72].…”

On the effect of anion exchange ionomer binders in bipolar electrode membrane interface water electrolysis

“…One crucial observation in this study was that at the anode, where alkaline OER (i.e., hydroxide consumption to form O 2 ) occurs, the pH does not rise above 9.3, where the color change is expected for thymolphthalein. 35 It seems that hydroxide reaching the electrode is simultaneously consumed, and the local pH remains unclear. Also, for PEMWE systems, Knöppel et al raised doubts about the highly acidic environment caused by the Naon membrane supported by discrepancies in IrO 2 dissolution data for MEA-and ex situ catalyst investigations with externally dened pH.…”

A review of alkaline solid polymer membrane in the application of AEM electrolyzer: Materials and characterization

“…Besides, the lightweight electrolysis cell in terms of size and weight can be manufactured using the lightweight, thin, and micro‐scale AEM membrane. Thus, the low‐cost AEM electrolyzer can be manufactured and contributed to the development of stable hydrogen production, increased durability, and increased energy efficiency in stationary, portable, and transportation power generation systems 48,49 …”