Heterogeneous anion conducting membranes based on linear and crosslinked KOH doped polybenzimidazole for alkaline water electrolysis

Separators and Membranes for Advanced Alkaline Water Electrolysis

Traditionally, alkaline water electrolysis (AWE) uses diaphragms to separate anode and cathode and is operated with 5-7 M KOH feed solutions. The ban of asbestos diaphragms led to the development of polymeric diaphragms, which are now the state of the art material. A promising alternative is the ion solvating membrane. Recent developments show that high conductivities can also be obtained in 1 M KOH. A third technology is based on anion exchange membranes (AEM); because these systems use 0-1 M KOH feed solutions to balance the trade-off between conductivity and the AEM's lifetime in alkaline environment, it makes sense to treat them separately as AEM WE. However, the lifetime of AEM increased strongly over the last 10 years, and some electrode-related issues like oxidation of the ionomer binder at the anode can be mitigated by using KOH feed solutions. Therefore, AWE and AEM WE may get more similar in the future, and this review focuses on the developments in polymeric diaphragms, ion solvating membranes, and AEM.

Thin Film Composite Membranes as a New Category of Alkaline Water Electrolysis Membranes

Alkaline water electrolysis (AWE) is considered a promising technology for green hydrogen (H2 ) production. Conventional diaphragm-type porous membranes have a high risk of explosion owing to their high gas crossover, while nonporous anion exchange membranes lack mechanical and thermochemical stability, limiting their practical application. Herein, a thin film composite (TFC) membrane is proposed as a new category of AWE membranes. The TFC membrane consists of an ultrathin quaternary ammonium (QA) selective layer formed via Menshutkin reaction-based interfacial polymerization on a porous polyethylene (PE) support. The dense, alkaline-stable, and highly anion-conductive QA layer prevents gas crossover while promoting anion transport. The PE support reinforces the mechanical and thermochemical properties, while its highly porous and thin structure reduces mass transport resistance across the TFC membrane. Consequently, the TFC membrane exhibits unprecedentedly high AWE performance (1.16 A cm-2 at 1.8 V) using nonprecious group metal electrodes with a potassium hydroxide (25 wt%) aqueous solution at 80 °C, significantly outperforming commercial and other lab-made AWE membranes. Moreover, the TFC membrane demonstrates remarkably low gas crossover, long-term stability, and stack cell operability, thereby ensuring its commercial viability for green H2 production. This strategy provides an advanced material platform for energy and environmental applications.

"Thiol-ene" crosslinked polybenzimidazoles anion exchange membrane with enhanced performance and durability

To address the "trade-off" between conductivity and stability of anion exchange membranes (AEMs), we developed a series of crosslinked AEMs by using polybenzimidazole with norbornene (cPBI-Nb) as backbone and the crosslinked structure was fabricated by adopting click chemical between thiol and vinyl-group. Meanwhile, the hydrophilic properties of the dithiol cross-linker were regulated to explore the effect for micro-phase separation morphology and hydroxide ion conductivity. As result, the AEMs with hydrophilic crosslinked structure (PcPBI-Nb-C2) not only had apparent micro-phase separation morphology and high OH^- conductivity of 105.54 mS/cm at 80 °C, but also exhibited improved mechanical properties, dimensional stability (swelling ratio < 15%) and chemical stability (90.22 % mass maintaining in Fenton's reagent at 80 °C for 24 h, 78.30 % conductivity keeping in 2 M NaOH at 80 °C for 2016 h). In addition, the anion exchange membranes water electrolysis (AEMWEs) using PcPBI-Nb-C2 as AEMs achieved the current density of 368 mA/cm² at 2.1 V and the durability over 500 min operated at 150 mA/cm² under 60 °C. Therefore, this work paves the way for constructing AEMs by introduction of norbornene into polybenzimidazole and formation of hydrophilic crosslinked structure based on "thiol-ene".

Electrode Separators for the Next-Generation Alkaline Water Electrolyzers

Multi-gigawatt-scale hydrogen production by water electrolysis is central in the green transition when it comes to storage of energy and forming the basis for sustainable fuels and materials. Alkaline water electrolysis plays a key role in this context, as the scale of implementation is not limited by the availability of scarce and expensive raw materials. Even though it is a mature technology, the new technological context of the renewable energy system demands more from the systems in terms of higher energy efficiency, enhanced rate capability, as well as dynamic, part-load, and differential pressure operation capability. New electrode separators that can support high currents at small ohmic losses, while effectively suppressing gas crossover, are essential to achieving this. This Focus Review compares the three main development paths that are currently being pursued in the field with the aim to identify the advantages and drawbacks of the different approaches in order to illuminate rational ways forward.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Vanadium Redox Flow Battery Using Aemion™ Anion Exchange Membranes

The vanadium redox flow battery (VRFB) is a promising and commercially available technology that poses advantageous features for stationary energy storage. A key component of the VRFB in terms of cost and system efficiency is the membrane. In recent years, anion exchange membranes (AEMs) have gained interest in VRFB research as they in general exhibit lower vanadium crossover due to a more substantial Donnan exclusion effect. In this study, a low-resistance flow cell was developed and the electrochemical performance of Aemion™ anion exchange membranes AF1-HNN5-50-X, AF1-HNN8-50-X and AF1-ENN8-50-X were compared against commonly used cation exchange membranes, Nafion® 211 and 212. The VRFB using AF1-ENN8-50-X exhibited superior performance versus Nafion® 212 regarding cycling efficiency and rate performance. However, relatively high and comparable capacity losses were observed using both membranes. NMR analysis showed no sign of chemical degradation for AF1-ENN8-50-X by immersion in VO2+ solution for 800 h. Although Aemion™ AEMs showed good chemical and electrochemical performance, considerable electrolyte crossover was observed due to high water uptake.

Anion-exchange membrane water electrolyzers and fuel cells

The key components, working management, and operating techniques of anion-exchange membrane water electrolyzers and fuel cells are reviewed for the first time.

Dimethylimidazolium-Functionalized Polybenzimidazole and Its Organic-Inorganic Hybrid Membranes for Anion Exchange Membrane Fuel Cells

A quaternized polybenzimidazole (PBI) membrane was synthesized by grafting a dimethylimidazolium end-capped side chain onto PBI. The organic-inorganic hybrid membrane of the quaternized PBI was prepared via a silane-induced crosslinking process with triethoxysilylpropyl dimethylimidazolium chloride. The chemical structure and membrane morphology were characterized using NMR, FTIR, TGA, SEM, EDX, AFM, SAXS, and XPS techniques. Compared with the pristine membrane of dimethylimidazolium-functionalized PBI, its hybrid membrane exhibited a lower swelling ratio, higher mechanical strength, and better oxidative stability. However, the morphology of hydrophilic/hydrophobic phase separation, which facilitates the ion transport along hydrophilic channels, only successfully developed in the pristine membrane. As a result, the hydroxide conductivity of the pristine membrane (5.02 × 10-2 S cm-1 at 80 °C) was measured higher than that of the hybrid membrane (2.22 × 10-2 S cm-1 at 80 °C). The hydroxide conductivity and tensile results suggested that both membranes had good alkaline stability in 2M KOH solution at 80 °C. Furthermore, the maximum power densities of the pristine and hybrid membranes of dimethylimidazolium-functionalized PBI reached 241 mW cm-2 and 152 mW cm-2 at 60 °C, respectively. The fuel cell performance result demonstrates that these two membranes are promising as AEMs for fuel cell applications.

Preparation and performance characterization of novel PVA blended with fluorinated polyimide membrane for gas separation

Fluorinated polyimide and PVA blending membranes were prepared by aqueous solution casting. We chose a poly (amic acid) ammonium salt (PAAS) in aqueous solution based on a novel green strategy as the PI precursor. The blending membranes were characterized by ATR-FTIR, DSC, TGA and gas permeation measurement. The ATR-FTIR analysis revealed that the imidization reaction of 6FPI based on aqueous precursor was completed at 180°C and hydrogen bonds formed between PVA and 6FPI. 6FPI showed good compatibility with PVA segment in blending membranes without obvious separated phase structure. The blending membranes showed high separation properties, for blending with 6FPI the gas separation performance stability was improved due to the hydrogen bonds between hydroxyl groups of PVA and carbonyl groups of 6FPI, and the rigid structure of 6FPI. At high operating pressure 10 bar, the CO2 permeability and CO2/N2 selectivity remained rather high. Using water as the solvent in the PAAS synthesis and membrane preparation is more environmentally friendly and less costly.