Understanding the Impact of the Three-Dimensional Junction Thickness of Electrospun Bipolar Membranes on Electrochemical Performance

Abstract

The use of electrospun bipolar membranes (BPMs) with an interfacial three-dimensional (3D) junction of entangled nano-/microfibers has been recently proposed as a promising fabrication strategy to develop high-performance BPMs. In these BPMs, the morphology and physical properties of the 3D junction are of utmost importance to maximize the membrane performance. However, a full understanding of the impact of the junction thickness on the membrane performance is still lacking. In this study, we have developed bipolar membranes with the same composition, only varying the 3D junction thicknesses, by regulating the electrospinning time used to deposit the nano-/microfibers at the junction. In total, four BPMs with 3D junction thicknesses of ∼4, 8, 17, and 35 μm were produced to examine the influence of the junction thickness on the membrane performance. Current–voltage curves for water dissociation of BPMs exhibited lower voltages for BPMs with thicker 3D junctions, as a result of a three-dimensional increase in the interfacial contact area between cation- and anion-exchange fibers and thus a larger water dissociation reaction area. Indeed, increasing the BPM thickness from 4 to 35 μm lowered the BPM water dissociation overpotential by 32%, with a current efficiency toward HCl/NaOH generation higher than 90%. Finally, comparing BPM performance during the water association operation revealed a substantial reduction in the voltage from levels of its supplied open circuit voltage (OCV), owing to excessive hydroxide ion (OH–) and proton (H+) leakage through the relevant layers. Overall, this work provides insights into the role of the junction thickness on electrospun BPM performance as a crucial step toward the development of membranes with optimal entangled junctions.

Combined Electrospinning–Electrospraying for High-Performance Bipolar Membranes with Incorporated MCM-41 as Water Dissociation Catalysts

“…Operating BPM in forward bias mode is important for their application in fuel cells, redox-flow batteries, and CO 2 reduction . BPMs exhibited a lower voltage drop in comparison with pristine BPMs (not including MCM-41) as reported in our previous work. , Moreover, we again see a clear optimum for the membrane with 1 h of catalyst deposition, in line with the earlier observed optimum. Clearly, adding MCM-41 as a catalyst using our unique hybrid electrospinning/electrospraying approach leads to enhanced performance in water splitting and in acid and base recombination.…”

“…We observe that the recorded OCVs were between 0.72 and 0.75 V for all fabricated BPMs in this work except for BPM-1 h-CES, where the OCVs were 0.62 and 0.51 V and were measured for solution concentrations of 0.5 and 1 M, respectively. Average values of OCVs for the fabricated BPMs in this work are higher (closer to theoretical values of 0.79 V for 0.5 M HCl and NaOH and 0.83 V for 1 M HCl and NaOH @ 25 °C) than the values reported in our previous work with similarly structured membranes. , This is mostly an indication of the positive effects of having MCM-41 as the water dissociation catalyst, where it could impact both the ion selectivity of the ion exchange layers and the recombination rate of H + /OH – at the BPM junction. − We also again see that MCM-41 works less well under acidic conditions; hence, the lower voltage for BPM-1 h-CES. Furthermore, the OCV data show an improvement due to utilization of MCM-41 as the water dissociation/formation catalyst, except for BPM-1 h-CES, where addition of MCM-41 to the cation exchange side did not improve the OCV.…”

“…Average values of OCVs for the fabricated BPMs in this work are higher (closer to theoretical values of 0.79 V for 0.5 M HCl and NaOH and 0.83 V for 1 M HCl and NaOH @ 25 °C) than the values reported in our previous work with similarly structured membranes. 7 , 50 This is mostly an indication of the positive effects of having MCM-41 as the water dissociation catalyst, where it could impact both the ion selectivity of the ion exchange layers and the recombination rate of H + /OH – at the BPM junction. 51 − 53 We also again see that MCM-41 works less well under acidic conditions; hence, the lower voltage for BPM-1 h-CES.…”

Separation of Polymeric Charge Enables Efficient Bipolar Membrane Operation

“…Compared to the theoretical V ΔpH of −800 mV predicted by eq 1 (Figure 1b, blue dashed line) based on the measured pH values for H 2 SO 4 and KOH (see Table S1), the measured value of −560 mV is 240 mV less negative, corresponding to voltage loss equivalent to a 4-unit shallower pH gradient. Consistent with previous reports suggesting that deviations of the experimentally measured V mem from V ΔpH are due to co-ion leakage, 22,23 we attribute the predominant cause of the severely eroded V mem to the crossover of the co-ions HSO 4 − and K + through the BPM, which is enabled via coupled parasitic neutralization, a phenomenon that we uncovered in a preceding publication. 27 In order to eliminate the effect of co-ion crossover on the open-circuit V mem , we turned to the use of the polyelectrolytes, poly(4-styrenesulfonic acid) (PSS−H) and poly((pvinylbenzyl)trimethylammonium hydroxide) (PVBTMA− OH) (structures shown in Figure 1).…”

“…Consequently, ion-pairing interactions can serve to depolarize the bipolar interface of a BPM. A poignant example widely invoked in the BPM literature is the attenuation in membrane voltage ( V mem ) due to the crossover of co-ions and their pairing interactions with excess fixed charges of the BPM. , However, ion-pairing interactions are not exclusive to co-ions, and it has not been explicitly considered whether interactions between the polymeric charges embedded within the BPM could have a similar depolarizing effect. In direct relation to this, polymeric charges have been assumed to be fixed in position and rigid without explicit examination of their mobility .…”