Reduced Ion Crossover in Bipolar Membrane Electrolysis via Increased Current Density, Molecular Size, and Valence

Abstract

A bipolar membrane (BPM) can be used to accelerate water dissociation to maintain a pH gradient in electrochemical cells, providing freedom to independently optimize the environments and catalysts used for paired reduction and oxidation reactions. The two physical layers in a BPM, respectively, selective for the exchange of cations and anions, should ideally reject ion crossover and facilitate ionic current via water dissociation in an interfacial layer. However, ions from the electrolyte do cross over in actual BPMs, competing with the water dissociation reaction and negatively affecting the stability of the electrolytes. Here, we explore the mechanisms of ion crossover as a function of pH and current density across a commercial BPM. Our unique series of experiments quantifies the ion crossover for more than 10 electrolyte combinations that cover 10 orders of magnitude in acid dissociation constant (K a ) and current densities spanning over more than 2 orders of magnitude. It was found that the ion crossover is dominated by diffusion for current densities up to a maximum of 10−40 mA cm −2 depending on the electrolyte, while migration is of higher importance at high current densities. The influence of the electrolyte pK a or pH on the ion crossover is not straightforward. However, ions with a higher valence or ion size show significantly lower crossover. Moreover, high current densities are the most favorable for high water dissociation efficiencies for all electrolyte combinations. This operational mode aligns well with practical applications of BPMs in electrolysis at industrial relevant current densities.

Orientation of a bipolar membrane determines the dominant ion and carbonic species transport in membrane electrode assemblies for CO2 reduction

“…The crossover in the reverse bias mode does follow the same crossover rate as when the bipolar membrane is placed in a liquid-liquid interface (at 25 mA cm -2 , 7 µmol hr -1 cm -2 ). 11 As mentioned earlier, the flux of K + through the CEL is significantly higher in the forward bias mode due to the neutralization of the carbonic species. In addition, the K + deposits almost 8 times faster at the Ag catalyst layer in the forward bias compared to the reverse bias, while the K + needs to pass the exact same membrane layers (only in different order).…”

“…[2][3][4][5] A third type of membrane to use in an MEA is a bipolar membrane (BPM), consisting of a cation and anion exchange layer (CEL and AEL, respectively) with an internal interface between the two layers where a catalyst is deposited to enhance the possible water dissociation. [8][9][10] In addition to the catalyst at the internal interface, electrolyte composition 11 , and pH gradient 12 , the two-layer configuration of the BPM allows to choose the orientation of the membrane in an electrochemical cell.…”

Suppressing the liquid product crossover in electrochemical CO2 reduction

“…Until now, BPM-based systems have suffered from a high rate of hydrogen production, the result of the proton flux toward the cathode, as well as high overpotentials required to drive water dissociation. 24 Furthermore, the crossover of liquid products, while reduced, was not completely eliminated with present-day BPMs. To enable future rational design of membrane materials for this application, further research is warranted to optimize the synthetic parameters of these membranes, such as the prepolymerization solvent content, the cross-linker content, and the length of alkyl spacers between charged functional groups on the liquid crossover performance.…”

“…The BPM approach promises to inhibit the crossover of both anionic and neutral products of CO 2 electrolysis. Until now, BPM‐based systems have suffered from a high rate of hydrogen production, the result of the proton flux toward the cathode, as well as high overpotentials required to drive water dissociation 24 . Furthermore, the crossover of liquid products, while reduced, was not completely eliminated with present‐day BPMs.…”

The Acid–Base Flow Battery: Sustainable Energy Storage via Reversible Water Dissociation with Bipolar Membranes

“…As with any electromembrane process, selectivity of the membranes is an important parameter also for the ABFB application. Both bipolar and the monopolar membranes should have high permselectivity to reduce co-ion leakage [ 43 ]. Co-ion leakage of “salt” ions (Na + , Cl − ) causes the formation of neutral salt in the acid/base compartments, while co-ion leakage of water ions (H + and OH − ), i.e., the ion crossover through the entire BPM, causes water recombination in the outer solution (i.e., outside the BPM).…”