Neutralization Short-Circuiting with Weak Electrolytes Erodes the Efficiency of Bipolar Membranes

Abstract

Bipolar membranes (BPMs) are critical components of a variety of electrochemical energy technologies. Many electrochemical applications require the use of buffers to maintain stable, nonextreme pH environments, yet the impact of buffers or weak acids/bases on the electrochemical behavior of BPMs remains poorly understood. Our data for a cell containing weak electrolytes is consistent with internal pH gradients within the anion exchange membrane (AEM) or cation exchange membrane (CEM) component of the BPM that form via ionic short-circuiting processes at open-circuit. Short-circuiting results from the coupling of co-ion crossover and parasitic neutralization and leads to buffering of the bipolar interface. This phenomenon, which we term neutralization short-circuiting, serves to erode BPM efficiency by attenuating the open-circuit membrane voltage and introducing parasitic reverse bias currents associated with weak acid/base dissociation at the interface. These findings establish a mechanistic basis for the operation of BPM cells in the presence of weak acid/ base electrolytes.

Ionic Blockades Control the Efficiency of Energy Recovery in Forward Bias Bipolar Membranes

“…The open-circuit Vmem values for catholytes containing 1 M KOH, 0.75 M KOH + 0.25 M KOAc and 0.25 M KOH + 0.75 M KOAc were found to be close to −59 mV•ΔpH (where ΔpH = pHcatholyte − pHanolyte) or, equivalently, −59 mV•pKa(H2O), whereas that for the catholyte containing 1 M KOAc was close to −59 mV•pKa (AcOH) (Figure 2(a)). The pKapinned open-circuit Vmem value, or VpKa, for the latter case is consistent with our findings in our previous study, 38 in which we showed that ionic short-circuiting processes led to buffering of the bipolar interface by the acid-base couple present (H2O/OH − and AcOH/OAc − , respectively). In the case of 1 M H2SO4 | 0.75 M KOH + 0.25 M KOAc and 1 M H2SO4 | 0.25 M KOH + 0.75 M KOAc, the presence of OH − ensures a high pH in the region of the AEM close to the bipolar interface, resulting in the open-circuit Vmem being dominated by the H + /OH − recombination couple and pinned to ca.…”

“…When an analogous series of cells of the type x M H2SO4 + y M NH4Cl | 1 M KOH (x + y = 1) were polarized in forward bias (Figure S4), we observed identical behavior to the 1 M H2SO4 | x M KOH + y M KOAc cells. Additionally, consistent with previous experimental 38,39 and computational 33 reports, we observe that the presence of polyprotic buffer species in the cell 1 M H2SO4 | 1 M KxHyPO4 (x + y = 3), gave multiple limiting current plateaus with inflection points coinciding with ca. −59 mV•pKa (H3PO4) (−130 mV), ca.…”

“…Hence, changes in Vmem reflect only changes in the bipolar junction voltage (VJ). 38 Here, we will use the term weak electrolytes to refer to charged bases whose conjugate acids have a pKa < 14 (e.g., OAc − ) or charged acids with a pKa > 0 (e.g., NH4 + ), and the term strong electrolytes to refer to H + and OH − . Forward bias polarization is gated by acid-base equilibria.…”

“…70 kDa on average), the PSSco-ion is size-excluded from crossing the AEM, and is able to form a stable bipolar interface, with the PSS-H solution playing the dual role of CEM and acid solution. 38 With this cell type, we were able to collect polarization curves with different flow rates applied to the bipolar interface. Indeed, as the flow rate increased from 0 to 200 mL min −1 , the current density considered at the same Vmem of −600 mV increased nearly fivefold from 3.4 mA cm −2 to 17.4 mA cm −2 (Figure 6(c)).…”

Exploring Bipolar Membranes for Electrochemical Carbon Capture

“…27,56 The 1 M KHCO3 case, however, is more intriguing. While previous studies have suggested that the low applied potential current onset for BPMs in weak buffer electrolytes is entirely driven by titration currents resulting from dissociation of the weak acid buffer (i.e., the HCO3 -anion in this case) in the CL, 27,49,57 the model suggests that current density in the initial linear feature is still primarily dominated by field-enhanced water dissociation. Field-enhanced dissociation of the buffering anion accounts for up to 50% of the observed current density.…”