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

Measurement of hydroxide ion conductivity is paramount to understanding anion exchange membranes (AEM) operated under alkaline conditions, but the measurement is complicated by dissolved CO2 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.