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1. Quantifying and elucidating the effect of CO ₂ on the thermodynamics, kinetics and charge transport of AEMFCs

   https://doi.org/10.1039/C9EE01334B  

   Zheng, Yiwei; Omasta, Travis J.; Peng, Xiong; Wang, Lianqin; Varcoe, John R.; Pivovar, Bryan S.; Mustain, William E. (September 2019, Energy & Environmental Science)

   It has been long-recognized that carbonation of anion exchange membrane fuel cells (AEMFCs) would be an important practical barrier for their implementation in applications that use ambient air containing atmospheric CO 2 . Most literature discussion around AEMFC carbonation has hypothesized: (1) that the effect of carbonation is limited to an increase in the Ohmic resistance because carbonate has lower mobility than hydroxide; and/or (2) that the so-called “self-purging” mechanism could effectively decarbonate the cell and eliminate CO 2 -related voltage losses during operation at a reasonable operating current density (>1 A cm −2 ). However, this study definitively shows that neither of these assertions are correct. This work, the first experimental examination of its kind, studies the dynamics of cell carbonation and its effect on AEMFC performance over a wide range of operating currents (0.2–2.0 A cm −2 ), operating temperatures (60–80 °C) and CO 2 concentrations in the reactant gases (5–3200 ppm). The resulting data provide for new fundamental relationships to be developed and for the root causes of increased polarization in the presence of CO 2 to be quantitatively probed and deconvoluted into Ohmic, Nernstian and charge transfer components, with the Nernstian and charge transfer components controlling the cell behavior under conditions of practical interest. 

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2. Cyclic Voltammetry as a Probe of Selective Ion Transport within Layered, Electrode-Supported Ion-Exchange Membrane Materials

   https://doi.org/10.1149/1945-7111/ac51fd  

   Xu, Jiahe; Leddy, Johna; Korzeniewski, Carol (February 2022, Journal of The Electrochemical Society)

   Cyclic voltammetry was applied to investigate the permselective properties of electrode-supported ion-exchange polymer films intended for use in future molecular-scale spectroscopic studies of bipolar membranes. The ability of thin ionomer film assemblies to exclude mobile ions charged similarly to the polymer (co-ions) and accumulate ions charged opposite to the polymer (counterions) was scrutinized through use of the diffusible redox probe molecules [Ru(NH 3 ) 6 ] 3+ and [IrCl 6 ] 2− . With the anion exchange membrane (AEM) phase supported on a carbon disk electrode, bipolar junctions formed by addition of a cation exchange membrane (CEM) overlayer demonstrated high selectivity toward redox ion extraction and exclusion. For junctions formed using a Fumion ® AEM phase and a Nafion ® overlayer, [IrCl 6 ] 2− ions exchanged into Fumion ® prior to Nafion ® overcoating remained entrapped and the Fumion ® excluded [Ru(NH 3 ) 6 ] 3+ ions for durability testing periods of more than 20 h under conditions of interest for eventual in situ spectral measurements. Experiments with the Sustainion ® anion exchange ionomer uncovered evidence for [IrCl 6 ] 2− ion coordination to pendant imidazolium groups on the polymer. A cyclic voltammetric method for estimation of the effective diffusion coefficient and equilibrium extraction constant for redox active probe ions within inert, uniform density electrode-supported thin films was applied to examine charge transport mechanisms. 

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3. Ion transport in hydroxide conducting block copolymers

   https://doi.org/10.1039/C9ME00022D  

   Elabd, Yossef A. (June 2019, Molecular Systems Design & Engineering)

   Hydroxide ion conducting block copolymers have the potential to possess the multiple properties required for anion exchange membranes to enable long-lasting alkaline fuel cell performance, and therefore can accelerate the advancement of the alkaline fuel cell, a low-cost alternative to the well-adopted commercial proton exchange membrane fuel cell. In this paper, an overview of hydroxide ion transport (a property that is proportional to fuel cell performance) in block copolymers will be presented and the subsequent impact of block copolymer morphology on ion transport (conductivity), where the careful design of block copolymer chemistry and chain architecture can accelerate hydroxide ion transport. 

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4. Effects of fixed charge group physicochemistry on anion exchange membrane permselectivity and ion transport

   https://doi.org/10.1039/D0CP00018C  

   Ji, Yuanyuan; Luo, Hongxi; Geise, Geoffrey M. (April 2020, Physical Chemistry Chemical Physics)

   Understanding the effects of polymer chemistry on membrane ion transport properties is critical for enabling efforts to design advanced highly permselective ion exchange membranes for water purification and energy applications. Here, the effects of fixed charge group type on anion exchange membrane (AEM) apparent permselectivity and ion transport properties were investigated using two crosslinked AEMs. The two AEMs, containing a similar acrylonitrile, styrene and divinyl benzene-based polymer backbone, had either trimethyl ammonium or 1,4-dimethyl imidazolium fixed charge groups. Membrane deswelling, apparent permselectivity and ion transport properties of the two AEMs were characterized using aqueous solutions of lithium chloride, sodium chloride, ammonium chloride, sodium bromide and sodium nitrate. Apparent permselectivity measurements revealed a minor influence of the fixed charge group type on apparent permselectivity. Further analysis of membrane swelling and ion sorption, however, suggests that less hydrophilic fixed charge groups more effectively exclude co-ions compared to more hydrophilic fixed charge groups. Analysis of ion diffusion properties suggest that ion and fixed charge group enthalpy of hydration properties influence ion transport, likely through a counter-ion condensation, ion pairing or binding mechanism. Interactions between fixed charge groups and counter-ions may be stronger if the enthalpy of hydration properties of the ion and fixed charge group are similar, and suppressed counter-ion diffusion was observed in this situation. In general, the hydration properties of the fixed charge group may be important for understanding how fixed charge group chemistry influences ion transport properties in anion exchange membranes. 

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5. A Control-Oriented Model for Predicting Variations in Membrane Water Content of an Open-Cathode Proton Exchange Membrane Fuel Cell

   https://doi.org/10.3390/en17040831  

   Adunyah, Adwoa S; Gawli, Harshal A; Hall, Carrie M (February 2024, Energies)

   Proton exchange membrane (PEM) fuel cells have emerged as a viable alternative energy production source for stationary and transportation applications. Reliable and sustainable fuel cell operation requires effective water management. Membrane water content can vary along the stack during transients which can lead to losses in fuel cell performance. To control these variations, a model that predicts the internal humidity dynamics of the stack is needed. In this study, a control-oriented model for predicting membrane water content variation was developed and implemented in MATLAB/Simulink. A lumped parameter model was initially developed and then further discretized into smaller control volumes to track humidity distribution along the stack. To validate the model’s predictions, the predicted results were compared to computer simulation results from GT-Suite. The root mean square error (RMSE) between the model’s prediction and GT-Suite’s simulation results was found to be within 1.5 membrane water content for all cases, demonstrating the model’s capability to capture the variation in membrane water content along the stack. The developed model will be useful for real-time control of membrane water content distribution in PEM fuel cells. 

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