Diaphragm for Alkaline Electrolysis of Water for Hydrogen Production

The history of electrolytic hydrogen production (WE) as an industrial hydrogen production process can be traced back to the 1920s, when the early technology was mainly based on alkaline electrolysis systems. This type of system uses inexpensive nickel based materials as electrodes, employs porous membranes for electrode separation, and uses potassium hydroxide (KOH) solution with a mass fraction exceeding 20% as the electrolyte. However, such systems suffer from issues such as low productivity and narrow current ranges suitable for operation. The hydrogen crossover problem shows significant differences with changes in current density: the hydrogen crossover phenomenon is prominent at low current densities, and to ensure safety, the hydrogen oxygen mixture content needs to be controlled below 2% (far below the explosive limit of 4%); When the current density is high, the cross hydrogen gas is diluted by the generated oxygen, which improves the safety of the system; When the current density is too high, the electrolytic cell will experience corrosion problems due to a sudden increase in cell voltage, resulting in reduced energy efficiency and shortened equipment life.

In the 1960s, the commercial application of perfluorinated membranes with excellent chemical stability promoted the development of proton exchange membrane (PEM) electrolysis technology for water. This type of membrane material has a dense structure and low battery resistance, allowing PEM electrolysis systems to operate at higher voltage differentials. The current density is significantly increased compared to traditional alkaline systems, effectively reducing the footprint of electrolysis cells and becoming an important technical branch in the field of hydrogen production through water electrolysis.

Despite the obvious advantages of PEM electrolysis technology, there are still two core bottlenecks: first, the acidic working environment is highly corrosive, and hydrogen evolution reaction relies on platinum based catalysts, while oxygen evolution reaction requires iridium based catalysts. Iridium resources are extremely scarce – the global annual supply is only 5-7 tons (as a companion mineral of platinum), which limits the large-scale application of technology. At present, the anode iridium loading capacity of commercial PEM electrolysis cells has been reduced from a few milligrams per square centimeter to 0.05 milligrams (with the goal of meeting the global demand for 5 gigawatts of installed capacity by 2040). Although a low loading capacity of 0.036 milligrams per square centimeter has been achieved in the scientific research field, the long-term stability of the system still needs to be verified. The second is the controversy over the environmental safety of perfluorinated membranes, whose persistent characteristics in the environment and human body may face future regulatory restrictions, driving the industry to seek alternative solutions.

Anion exchange membrane (AEM) electrolysis of water technology is considered an ideal alternative direction. The system uses thin AEM as the membrane and can be fed with pure water or low concentration alkaline solution (≤ 1 mol/L KOH) without relying on precious metal catalysts, reducing the cost threshold. At the same time, the dense membrane structure allows for differential pressure operation, and the low resistance characteristics brought by the thin design enable it to operate at current densities higher than traditional alkaline systems. The main challenge of current AEM technology is insufficient alkaline stability, but breakthrough progress has been made in related research, laying the foundation for technological maturity.

The future technological roadmap of the electrolysis water system largely depends on the development direction of the electrode separator (diaphragm). This judgment stems from the fact that although the research on alkaline system catalysts is more popular, the diaphragm material directly determines whether the final form of the system will approach traditional alkaline electrolysis (AWE) or new AEM electrolysis. At present, research on AEM electrolysis is mainly focused on pure water and 1 mol/L KOH systems, with the core reason being that an increase in hydroxide ion concentration accelerates AEM degradation – for example, some commercial AEM systems recommend using a KOH feed solution of about 0.2 mol/L (1%).

The effect of KOH concentration on AEM electrolysis system presents a dual nature: on the one hand, an increase in concentration accelerates the degradation of AEM, which is not only the result of the direct action of hydroxide ions, but also related to the reduction of protective water molecules around the quaternary ammonium salt groups on the surface of AEM; On the other hand, increasing the concentration of KOH can improve system performance, mainly through three ways: firstly, improving the conductivity of the electrolyte (the conductivity is optimal when the concentration reaches 5-7 mol/L); The second is to optimize the performance balance of ion polymer adhesives (finding the critical point between high mechanical strength brought by low expansion rate and efficient hydroxide ion transport); The third is to slow down the anodic oxidation rate of anionic conductive polymers (which is the main cause of AEM degradation) – when KOH participates in the formation of the double layer, it can hinder the direct contact between the polymer and catalyst particles, thereby reducing the rate of oxidative degradation.

Based on the above characteristics, with the continuous improvement of AEM stability and the performance advantages brought by KOH containing feed solutions, the research focus in the field of electrolyzed water is shifting from pure water systems to low concentration KOH systems. The KOH concentration of future AEM electrolysis systems may be between the current mainstream upper limit of 1 mol/L and the commonly used 5-7 mol/L in traditional alkaline systems. It is worth noting that recent studies have found that ion swelling membranes (ISMs) based on sulfonated para polybenzimidazole (without quaternary ammonium groups) can have a conductivity of over 100 milliSiemens per centimeter in a 1 mol/L KOH solution, demonstrating good potential for application. Therefore, this review will comprehensively cover porous membranes and ISM materials used in traditional alkaline electrolyzed water (AWE), while systematically reviewing the latest research progress in the field of AEM.

The traditional AWE system uses a 5-7 mol/L KOH solution as the electrolyte and relies on a separator to achieve anode cathode separation. After asbestos diaphragms were banned, polymer diaphragms and composite diaphragms became mainstream alternative products, and ISM is a new type of diaphragm material with great development prospects. In contrast, AEM electrolysis technology uses a 0-1 mol/L KOH feed solution to ensure electrolyte conductivity while effectively extending the service life of AEM in alkaline environments, forming a complementary technical route to traditional AWE.

This review will elaborate on membrane materials for alkaline electrolysis of water for hydrogen production from the following aspects: firstly, key membrane materials in traditional alkaline electrolysis technology, including polyphenylene sulfide (PPS) cloth, Zirfon type composite membranes, ISM and cation exchange membranes, etc., with a focus on analyzing their application characteristics in high concentration KOH systems; The second is the application law of AEM in alkaline solutions, which deeply explores the influence mechanism of KOH concentration, and introduces the research and development progress of new AEM such as imidazoline film, polyhydroxyalkylated polymer base film, and polystyrene base film; The third is the key performance indicators and evaluation methods of membrane materials, covering core parameters such as in-plane conductivity, hydrogen permeation and cross characteristics, electrolyte permeability, dimensional stability, bubble point, mechanical strength, alkaline stability, and wettability, clarifying the evaluation criteria and industrial significance of each indicator; The fourth is the design strategy for future diaphragm materials, proposing direction suggestions for structural optimization of Zirfon type diaphragms, AEM, and ISM.

Ultrasonic spraying technology has significant advantages in the preparation of hydrogen producing membrane materials through electrolysis of water, providing key support for optimizing the performance of membrane electrodes. This technology utilizes ultrasonic vibration to atomize catalyst slurry or membrane material precursor into micrometer sized droplets, achieving uniform coating and solving the problems of uneven coating thickness and particle agglomeration in traditional spray coatings. In PEM electrolysis cells, it can accurately control the loading of iridium based catalysts, helping to achieve low loading targets below 0.05mg Ir/cm ², while improving the interfacial adhesion between the catalyst and the membrane and reducing reaction resistance. For the AEM system, ultrasonic spraying can adapt to the characteristics of non precious metal catalyst slurry, ensuring uniform coverage of the coating on the thin AEM surface and avoiding the risk of hydrogen oxygen crossover caused by coating defects. In addition, this technology also has the characteristics of high coating efficiency and high material utilization rate (up to 90% or more), which can reduce the preparation cost of membrane electrodes. Its low-temperature spraying characteristics can also protect the structural integrity of membrane materials, providing guarantees for the improvement of AEM alkaline stability and the high conductivity advantages of ISM, and promoting the large-scale implementation of electrolytic water hydrogen production technology.

Finally, the review points out that AWE and AEM electrolysis technologies may show a trend of integrated development in the future: the lifespan of AEM has been significantly improved in the past decade, and the application of KOH feed solution can alleviate electrode related technical difficulties. This technological integration will promote the development of membrane materials towards high stability, high conductivity, and wide concentration adaptability, providing core support for the large-scale application of alkaline electrolysis hydrogen production.

About Cheersonic

Cheersonic is the leading developer and manufacturer of ultrasonic coating systems for applying precise, thin film coatings to protect, strengthen or smooth surfaces on parts and components for the microelectronics/electronics, alternative energy, medical and industrial markets, including specialized glass applications in construction and automotive.

Our coating solutions are environmentally-friendly, efficient and highly reliable, and enable dramatic reductions in overspray, savings in raw material, water and energy usage and provide improved process repeatability, transfer efficiency, high uniformity and reduced emissions.

Chinese Website: Cheersonic Provides Professional Coating Solutions