Electrochemical Measurement of Water Transport Numbers in Anion-Exchange Membranes

Abstract

Anion-exchange membranes (AEMs) are a possible replacement for perfluorosulfonic-acid membranes in energy-conversion devices, primarily due to the hydroxide mobile ion allowing the devices to operate in alkaline conditions with less expensive electrocatalysts. However, the transport properties of AEMs remain understudied, especially electro-osmosis. In this work, an electrochemical technique, where the open-circuit voltage is measured between two ends of a membrane maintained at different relative humidities, is used to determine the water transport number of various ionomers, including Versogen and Sustainion AEMs and Nafion cation-exchange membrane (CEM), as a function of water content and temperature. In addition, the CEMs and AEMs are examined in differing single-ion forms, specifically proton and sodium (CEM) and hydroxide and carbonate (AEM). Carbonate-form AEMs have the highest transport number (∼11), followed by sodium-form CEMs (∼8), hydroxide-form AEMs (∼6), and proton-form CEMs (∼3). Finally, a multicomponent transport model based on the Stefan-Maxwell-Onsager framework of binary interactions is used to develop a link between water transport number and water-transport properties, extracting a range for the unmeasured membrane water permeability of Versogen as a function of water content.

Pure-Water-Fed Forward-Bias Bipolar Membrane CO2 Electrolyzer

“…As indicated in previous studies, , one possible failure mode of the forward-bias BPM CO 2 electrolyzer is the delamination issue between the AEL and PEL, which is projected to result in increase of contact resistance. However, this behavior is not observed for the asymmetrical BPMs in this study, as shown by the descending trend and plateauing behavior of the HFR, which further illustrates the superiority of asymmetrical design of BPM with thinner AEL to allow for efficient CO 2 back diffusion from the bipolar junction to the cathode and possible improved heat and water management at the bipolar junction. , …”

“…However, this behavior is not observed for the asymmetrical BPMs in this study, as shown by the descending trend and plateauing behavior of the HFR, which further illustrates the superiority of asymmetrical design of BPM with thinner AEL to allow for efficient CO 2 back diffusion from the bipolar junction to the cathode and possible improved heat and water management at the bipolar junction. 37 , 38 …”

“…However, this behavior is not observed for the asymmetrical BPMs in this study, as shown by the descending trend and plateauing behavior of the HFR, which further illustrates the superiority of asymmetrical design of BPM with thinner AEL to allow for efficient CO 2 back diffusion from the bipolar junction to the cathode and possible improved heat and water management at the bipolar junction. 37,38 Investigating How Cathode Differential Pressure Impacts Electrolyzer Efficiencies. Operating electrolyzers at elevated pressures to produce pressurized gaseous products can mitigate the need for downstream compression and also reduce the size of piping and system components.…”

Water Crossover in Proton Exchange Membrane Water Electrolysis

“…−1 ) a significantly larger value of 2.5 was obtained. Recently, Petrovick et al 17 investigated the water transport in a similar ex situ setup for Nafion and several anion exchange membranes. It was found that the proton drag coefficient increases from < 2 for 20 °C to ∼3 at 50 °C in a humidification range of 4-10 mol H2O mol SO3 −1 .…”