The separation of oxygen and hydrogen produced by water electrolysis is accomplished by the diaphragm inside the electrolyzer. Therefore, the performance of the diaphragm directly affects the gas purity and DC power consumption within the electrolyzer. To achieve higher-purity gases while reducing the energy consumption of the water electrolyzer, the diaphragm material is critical. Diaphragms currently used in water electrolysis for hydrogen production include polysulfone diaphragms, ion-exchange membranes, and polyphenylene sulfide (PPS) diaphragms, among others.

Among these, polyphenylene sulfide (PPS) fiber is a high-performance material with excellent thermal stability, chemical stability, flame retardancy, and electrical insulation properties. It also features high porosity, large specific surface area, ease of functional modification, and good structural controllability, making it suitable for diaphragms in water electrolysis hydrogen production.

PPS staple fiber was used as the raw material, and process parameters were designed to weave the staple fiber diaphragm. Under set temperature and pressure, the diaphragm was hot-pressed and shaped on a hot rolling machine, followed by sulfonation treatment to enhance its hydrophilicity and liquid absorption capacity.

PPS filament fiber was used as the raw material, and a similar process was employed to weave the filament diaphragm, followed by the same hot pressing and sulfonation treatments as those applied to the staple fiber diaphragm. The surface of the filament diaphragm is relatively smooth, while the surface of the staple fiber diaphragm is less regular and has more protruding fiber ends. These surface characteristics of the staple fiber diaphragm can lead to several issues during electrolysis: first, the protruding fiber ends are prone to detach during prolonged use, which may affect the properties of the electrolyte and shorten its service life; second, the uneven yarn formation results in partially oversized or undersized polarized pores, which can affect the ion exchange rate during electrolysis, as well as gas purity and electrolysis efficiency.

In contrast, the yarn unevenness formed during the twisting of filament fiber used in the filament diaphragm is relatively minor. The pore size of the filament diaphragm is relatively uniform, resulting in lesser impacts on electrolysis efficiency, ion exchange rate, and gas purity compared to the staple fiber diaphragm.

The warp and weft breaking strengths of the filament diaphragm are greater than those of the staple fiber diaphragm, while its areal resistance and resistivity are lower. This is mainly due to the following reasons:

First, under the same areal density condition, the thickness of the filament diaphragm is less than that of the staple fiber diaphragm. Thickness is an important factor affecting diaphragm resistivity and areal resistance—greater thickness reduces the speed and efficiency of ion passage, resulting in higher resistance.

Second, the yarn evenness of the filament diaphragm is better than that of the staple fiber diaphragm, and the thickness uniformity of the woven filament diaphragm is superior, which is also an important factor affecting diaphragm resistance.

Third, during the use of the staple fiber diaphragm in the electrolyzer, the protruding staple fibers on the surface detach and fall into the electrolyte, also leading to an increase in resistance.

Fourth, the filament diaphragm exhibits better hydrophilicity and superior wettability by the electrolyte, offering less resistance to ion migration during electrolysis, thus resulting in lower resistance.

Fifth, the tested values of areal resistance and resistivity for asbestos diaphragms and PPS diaphragms in 30% KOH solution (30% mass fraction of KOH) are shown in the table below.

Based on the measured resistances of different diaphragms, the areal resistance and resistivity of both PPS staple fiber and filament diaphragms are much lower than those of asbestos diaphragms, which is primarily related to diaphragm thickness. The thickness of the experimentally produced PPS diaphragms is about 0.3 mm, whereas that of asbestos diaphragms is about 2.5 mm. The thicker the diaphragm, the greater the distance ions and electrons must travel through it during electrolysis, manifesting as increased areal resistance and resistivity. Therefore, at the same electrolysis voltage, using PPS diaphragms can significantly improve electrolysis efficiency and reduce energy consumption.

3. Pore Size and Porosity​​

Pore size and porosity are important parameters reflecting the passage performance of ions and electrons through the diaphragm during electrolysis, directly affecting the diaphragm's permeability, conductivity, and ability to block other particles. Generally, larger pore sizes and more uniform porosity result in greater pore volume, higher permeability coefficient, and better overall permeability.