Boulif Nadia, Nijmeijer Kitty, Borneman Zandrie
Membrane Materials and Processes, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
ACS Electrochem. 2025 May 14;1(8):1360-1372. doi: 10.1021/acselectrochem.5c00052. eCollection 2025 Aug 7.
Background sodium chloride in hydrochloric acid and sodium hydroxide solutions leads to large overpotentials when a bipolar membrane (BPM) is operated under forward bias (FB). Under FB polarization, the accumulation of salt ions at the junction hinders the transport of H and OH ions, thus increasing the mass transport resistance and lowering the water recombination rate. The "ionic blockade" phenomenon is mainly observed if the base is contaminated with Cl ions due to the poor OH/Cl selectivity of the BPM's anion exchange layer (AEL). This shortcoming is successfully reduced by modifying the AEL with a sub-micrometer thick poly-(benzimidazole) (PBI) coating. Ionic crosslinking between the AEL and PBI leads to a denser interface that enhances the size exclusion of Cl ions. Furthermore, the negative charges of deprotonated benzimidazole units at the basic operating conditions contribute to the Donnan exclusion of Cl ions, while the OH ions can still hop between the alkaline-doped free volumes of the PBI film. The enhanced OH/Cl selectivity prevents the accumulation of Cl ions at the junction and leads to lower overpotentials during the forward bias operation of BPMs in salt-contaminated acid and base. As a result, the PBI-coated BPM has a peak power density 1.6 times higher than that of an uncoated BPM when harvesting electrical energy from a pH gradient. The BPM modification also benefits flow battery applications, as the calculated BPM voltaic efficiency at 100 A/m (dis)-charge current density is increased from -3.7% to 57% with the addition of the PBI coating.
当双极膜（BPM）在正向偏压（FB）下运行时，盐酸和氢氧化钠溶液中的背景氯化钠会导致较大的过电位。在FB极化下，盐离子在交界处的积累阻碍了H⁺和OH⁻离子的传输，从而增加了传质阻力并降低了水复合率。如果由于BPM的阴离子交换层（AEL）的OH⁻/Cl⁻选择性差而使碱被Cl⁻离子污染，则主要会观察到“离子封锁”现象。通过用亚微米厚的聚（苯并咪唑）（PBI）涂层修饰AEL，成功地减少了这一缺点。AEL和PBI之间的离子交联导致更致密的界面，增强了对Cl⁻离子的尺寸排斥。此外，在碱性操作条件下，去质子化苯并咪唑单元的负电荷有助于对Cl⁻离子的唐南排斥，而OH⁻离子仍可在PBI膜的碱性掺杂自由体积之间跳跃。增强的OH⁻/Cl⁻选择性可防止Cl⁻离子在交界处积累，并导致在受盐污染的酸和碱中BPM正向偏压操作期间过电位降低。结果，当从pH梯度收集电能时，涂有PBI的BPM的峰值功率密度比未涂覆的BPM高1.6倍。BPM修饰对液流电池应用也有益，因为在添加PBI涂层后，在100 A/m²（充）放电电流密度下计算的BPM伏打效率从 - 3.7%提高到57%。
