Key Takeaways & Executive Findings
- •• • 50SOPBI-act membrane achieved record ionic conductivity of 135 mS cm−1 at room temperature and 358 mS cm−1 at 80 °C in 1 M KOH, outperforming all existing AEMWE and ISMWE systems, enabling efficient low-alkali electrolysis. • • The SPBI membranes were synthesized via copolymerization of sulfonated and non-sulfonated monomers, allowing tunable sulfonation degrees to balance conductivity and alkali stability without crosslinking, facilitating scale-up. • • Deprotonated benzimidazole rings increase electron density on ether carbon atoms, enhancing the stability of aromatic ethers in alkaline environments, addressing the degradation issue common in AEMWE. • • The 50SOPBI-act membrane exhibited acceptable swelling behavior and excellent conductivity across a wide range of alkaline concentrations, demonstrating its potential for practical water electrolysis at low alkali concentrations.
Abstract
Water electrolysis is a promising method for producing green hydrogen from renewable energy sources. The mainstream membrane-based water electrolysis technologies include proton exchange membrane water electrolysis (PEMWE), alkaline water electrolysis (AWE), and anion exchange membrane water electrolysis (AEMWE). Each technology has inherent limitations: PEMWE requires expensive platinum group metal catalysts and fluorinated membranes; AWE uses porous diaphragms, limiting operational window and differential pressure capability; AEMWE suffers from quaternary ammonium group degradation under alkaline conditions. Ion-solvating membranes (ISMs), dense polymeric membranes with solvation sites that absorb aqueous alkaline electrolytes, offer an alternative. Most ISMs are based on polybenzimidazole (PBI), but achieving high efficiency at low alkali concentrations remains a bottleneck. Introducing sulfonic acid moieties into PBI enhances water and KOH uptake, improving conductivity, but alkali stability often requires crosslinking, which hampers scale-up. Henkensmeier et al. adapted sulfonated polybenzimidazole (SPBI) membranes originally for PEM fuel cells to ISMWE. By copolymerizing sulfonated and non-sulfonated monomers, they created a series of SPBI membranes with varying sulfonation degrees. The sulfonated segments boost electrolyte uptake and conductivity, while non-sulfonated units ensure alkali resistance and mechanical stability. Deprotonated benzimidazole rings increase electron density on ether carbons, enhancing stability in basic environments. The 50SOPBI-act membrane exhibited promising performance, with record ionic conductivity of 135 mS cm−1 at room temperature and 358 mS cm−1 at 80 °C in 1 M KOH, outperforming existing AEMWE and ISMWE systems.
1. Introduction
Current membrane-based water electrolysis technologies face critical trade-offs. PEMWE offers high performance but relies on expensive platinum group metal catalysts and fluorinated membranes, limiting industrial scalability. AWE uses cost-effective components but suffers from porous diaphragms that restrict operational windows and prevent differential pressure operation. AEMWE aims to combine advantages but is hindered by the intrinsic instability of quaternary ammonium groups under alkaline conditions. These limitations underscore the need for alternative membrane materials that can operate efficiently at low alkali concentrations while maintaining stability and scalability.
Ion-solvating membranes (ISMs) have emerged as a promising solution, with polybenzimidazole (PBI) as the primary polymer family. However, achieving high ionic conductivity at low alkali concentrations has been a persistent challenge. Introducing sulfonic acid groups into PBI enhances water and KOH uptake, but often requires crosslinking to ensure alkali stability, which complicates manufacturing. The recent work by Henkensmeier et al. addresses this bottleneck by designing non-crosslinked sulfonated PBI (SPBI) membranes with tailored sulfonation degrees. By strategically balancing sulfonated and non-sulfonated segments, they achieved high conductivity and stability without crosslinking, as demonstrated by the 50SOPBI-act membrane's record performance in 1 M KOH. This approach offers a viable path toward efficient and scalable low-alkali water electrolysis.
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Simian Fei, Liang Ge, Tongwen Xu (2026). Next-generation PBI-based ISMs: game changer of low-alkali water electrolysis. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3797-2
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Frequently Asked Questions
What is the maximum ionic conductivity achieved by the 50SOPBI-act membrane and under what conditions?
The 50SOPBI-act membrane achieved record ionic conductivity of 135 mS cm−1 at room temperature and 358 mS cm−1 at 80 °C in 1 M KOH, outperforming all existing AEMWE and ISMWE systems.
How does the sulfonation degree affect the membrane's performance and stability?
The sulfonation degree was tuned by adjusting the copolymerization ratio of sulfonated and non-sulfonated monomers. Higher sulfonation enhances electrolyte uptake and ionic conductivity, but may compromise alkali stability. The 50SOPBI-act membrane balanced these properties, showing acceptable swelling and excellent conductivity across a wide range of alkaline concentrations.
What is the mechanism by which the SPBI membranes achieve alkali stability without crosslinking?
The deprotonated, negatively charged benzimidazole ring increases electron density on the ether carbon atom, thereby enhancing the stability of aromatic ethers in basic environments. This intrinsic stabilization allows the membrane to resist alkaline degradation without the need for chemical crosslinking.
What are the implications of this work for industrial scale-up of low-alkali water electrolysis?
The non-crosslinked SPBI membranes are more amenable to scale-up compared to crosslinked systems, as they avoid complex crosslinking steps. The record conductivity at low alkali concentration (1 M KOH) suggests potential for efficient operation with reduced alkali usage, lowering operational costs and environmental impact.
How does the 50SOPBI-act membrane compare to commercial AEMWE membranes in terms of performance and stability?
The 50SOPBI-act membrane outperforms existing AEMWE and ISMWE systems in ionic conductivity, achieving 135 mS cm−1 at room temperature and 358 mS cm−1 at 80 °C in 1 M KOH. This indicates superior performance potential, though long-term stability and durability under electrolysis conditions remain to be fully evaluated.
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