With the rapid development of modern society and escalating energy demands, the widespread use of traditional non-renewable fossil fuels has exacerbated environmental pollution and the greenhouse effect worldwide. The 'carbon peak, carbon neutral' goal propels wind power and photovoltaic-based renewable clean energy for power generation capacity in China. However, the intermittent and variable nature of renewable energy, along with a limited grid load regulation capacity, hinders the large-scale conversion or storage of intermittent power. Hydrogen serves as a sustainable fuel option due to its impressive high-energy potential at 140 MJ/kg and its capability for extensive, long-term storage, positioning it as a crucial element in transitioning towards a world underpinned by green, low-carbon energy sources. Generating green hydrogen through the electrolysis of water, powered by the variable outputs of renewable energy sources, becomes the most viable strategy for producing green hydrogen, while also leveraging the surplus energy from these renewables. At present, the widely adopted technique for hydrogen generation through water electrolysis is alkaline water electrolysis (AWE) with significant advantages of large single-unit size and low equipment cost, thus being a preferred technology route for large-scale hydrogen production.

However, alkaline water electrolyzer is difficult to use for the new situation of fluctuating power supply to produce green hydrogen efficiently due to the problems of low current density, massive equipment, and poor dynamic operation capability. Alkaline water electrolysis has a problem of low current density. For electrode materials, the lower intrinsic catalytic activity of non-precious metal electrode materials makes the kinetics of the electrolytic water reaction, compared with precious metal catalysts such as platinum and iridium oxide. For the diaphragm materials, the typical thickness of PPS diaphragms offers a significant resistance to the movement of hydroxide ions and potential risks for gas crossover. Also, frequent stops and starts can induce reverse current, resulting in corroded electrodes and reduced lifespan.

To confront these challenges, this review mainly summarized the research progress on transition metal catalysts and the development of industrial electrode technology. Alkaline water electrolysis electrode materials are mainly classified into three categories, i.e., precious metals, transition metals, and nonmetals. Among these, transition metal-based catalysts (i.e., Ni, Fe, Co, and Mol, and their compounds) have the advantages of low cost, simple preparation, and various structural compositions. They are considered as ideal materials to replace precious metal catalysts. The main types of these catalysts include single transition metals, sulfides, phosphides, and nitrides. They are often modified via electronic environment regulation, nanostructure optimization, and multi-component synergistic effects to enhance their intrinsic catalytic activity.

For an efficient and reliable alkaline water electrolysis system, a high-performance diaphragm is also essential. The diaphragm separates the cathode and anode, prevents the mixing of hydrogen and oxygen, and transfers hydroxide ions. This review also represented the research progress on a new generation of composite diaphragms, and discussed the modifying industrial PPS diaphragms. Simple organic fiber diaphragms have a poor pore structure and a high ionic resistance, and their thickness and gas barrier properties are difficult to fully address. Applying an inorganic functional coating to the surface of PPS fabric to create an organic-inorganic composite diaphragm becomes the main aspect of diaphragm research. The inorganic functional coating on the surface of the composite diaphragm improves hydrophilicity and gas barrier properties and reduces the thickness and ionic resistance of the diaphragm.

Alkaline electrolyzer downtime can produce a reverse current due to the potential difference between the conductive metal bipolar plate formed by the electronic pathway and the ionic pathway constituted by the alkaline circulation. This can cause a significant corrosion to the cathode material in the large volume intermediate compartment of the alkaline electrolyzer and creates a potential safety issue. Therefore, some studies addressing the mechanisms of reverse current formation and mitigation strategies are crucial for alkaline water electrolyzers for frequent shutdown and startup.