Modeling Electrolyte Composition Effects on Anion-Exchange-Membrane Water Electrolyzer Performance

Abstract

Anion-exchange membrane (AEM) water electrolysis could allow inexpensive and greener hydrogen production than other alternatives, such as steam methane reforming. To increase performance, hydroxide salts are often added to the water feed, with the tradeoff of corrosivity and complexity. Recently, carbonate salts that are less corrosive have shown promise, but their specific functionality remains unknown. In this paper, we use a mathematical model to compare an AEM electrolyzer with added potassium carbonate to an AEM electrolyzer with added potassium hydroxide. We show that the conductivity of the carbonate-form membrane has little impact on the performance of the device, but that carbonate ions replace hydroxide in the ionomer, which creates a Nernstian voltage difference across the membrane. The replacement of hydroxide anions with carbonate also reduces utilization of the catalyst in the anode, resulting in an additional voltage loss. * Estimated based on information from Pavel et al. (5) + Given by Pavel et al. (5) † Estimated based on measured exchange current densities for non-platinum-group-metal catalysts in alkaline environments (14).

Elucidating the Role of Hydroxide Electrolyte on Anion-Exchange-Membrane Water Electrolyzer Performance

“…The ion-exchange between the two phases is governed by Donnan equilibrium. 52 This ion-exchange also consumes energy and results in the ion-exchange overpotential (i.e., a junction potential) as shown in Figs. 7b and 7c.…”

“…Mathematical modeling.-The 1-D, continuum, two-phase AEM electrolyzer model is extended from our previous work, where a detailed description is given. 52 Here, we briefly introduce the physics and the major modifications made to study the KOH concentration effects. A summary of the key parameters is presented in Table SII…”

“…where i rxn is the reaction current and R ex,OH -is the ion-exchange between ionomer and liquid electrolyte, which is given by the Donnan equilibrium 52…”

Method—Practices and Pitfalls in Voltage Breakdown Analysis of Electrochemical Energy-Conversion Systems

“…Anion-exchange membrane electrolysis.-The addition of an electrolyte salt to electrolysis cells has been used to reduce voltage losses, particularly in AEM electrolyzers, in which various salts have been shown to improve performance over that of pure water. [36][37][38] The additional electrolyte salts are thought to improve ionic conductivity within the catalyst layer and enable the reaction at the catalyst/liquid-electrolyte interface, effectively increasing electrochemically active surface area and reducing kinetic voltage loss. The chemical composition of the electrolyte and its concentration may affect the exchange current density of the electrochemical reactions in the electrolyzer as well.…”

“…One source of voltage loss comes from ion exchange between the liquid electrolyte and ionomer. The ion-exchange rate between the ionomer and liquid electrolyte is derived from Donnan equilibrium, 38 Figure 9 shows the power-loss analysis voltage breakdown for a 1-D model of an AEM electrolysis cell using 1 M KOH solution as the liquid electrolyte. The grey section indicates the voltage loss induced by ion-exchange power loss.…”