Sealing gaskets, key materials for alkaline hydrogen production

Sealing gasket is one of the key materials of alkaline electrolyzer. As the main component of electrolyzer sealing, the quality of its sealing largely determines the quality of the sealing and insulation performance of the electrolyzer chamber, and directly affects the safety and reliability of the entire alkaline water electrolysis hydrogen production system.

1. Function of sealing gasket

(1) Sealing function

The operating pressure of conventional pressure-type alkaline electrolyzers is generally no more than 1.6 MPa. The electrolyzer generally adopts a filter press structure. The sealing system is a flat gasket seal. The sealing water line is opened on the edge of the plate frame and the performance of the modified sealing gasket material is used to ensure the sealing of the entire electrolyzer. The flowing medium inside the electrolyzer includes the circulating electrolyte and the generated hydrogen and oxygen. The function of the sealing gasket is to prevent the electrolyte solution, hydrogen and oxygen from leaking from the electrolyzer under the operating conditions of 30% KOH solution, 90°C and 1.6 MPa.

(2) Insulation function

In addition to sealing performance, the sealing gasket must also have insulation performance to ensure the insulation between the two pole frames of the electrolysis chamber and avoid short circuits inside the chamber.

2. The structure of the sealing gasket and its position in the electrolytic cell

The sealing gasket is in the shape of a ring. Holes need to be drilled on the gasket to provide space for the flow of electrolyte inside the electrolytic cell. At the same time, the positioning holes of the sealing gasket need to be processed to facilitate the installation of the sealing gasket.

There is only one sealing gasket in an electrolytic cell chamber, which is placed between the plate and the diaphragm in the anode area. The outer edge of the sealing gasket coincides with the outer edge of the pole frame, and the position of the inner ring and the edge generally exceeds the edge of the diaphragm by about 10mm. Therefore, the width of the sealing gasket includes the width of the gasket overlapping with the sealing waterline, the width of the flow channel hole on the gasket, and the width of the overlap between the gasket and the diaphragm.

3. The material of the sealing gasket

Since the sealing gasket needs to play an insulating role, the performance is mainly improved by modifying the insulating sealing material. Modified fluoroplastics are a type of sealing material that has been studied more intensively and deeply in alkaline water electrolysis. At present, most of the domestic alkaline water electrolysis sealing materials are carbon fiber, molybdenum disulfide, etc. as reinforcing fillers filled with polytetrafluoroethylene, which are obtained by molding and sintering. The sealing materials currently used are still prone to cold flow and creep under high temperature and high pressure and intermittent shutdown of alkaline electrolytic cells, resulting in cell leakage, which affects the service life of the electrolytic cell.

When examining the performance of sealing gasket materials, we generally examine three main parameters: Vickers hardness, compression rebound performance and creep relaxation performance.

Vickers hardness: Hardness is one of the comprehensive performance indicators composed of a series of mechanical properties such as elasticity, plasticity, and toughness of sealing materials. It can measure the ability of the sealing material surface to resist mechanical pressure, and can also characterize the degree of bonding between the reinforcing filler and the polytetrafluoroethylene body to a certain extent. The hardness of the sealing material can increase the ability of the material surface to resist mechanical pressure to a certain extent, and is an effective means to enhance the compressive strength of the material.

Compression resilience: Compression resilience reflects the elastic or plastic deformation of the sealing material, fills the defects on the sealing surface, and performs elastic compensation to maintain the sealing ability. It is an important indicator to measure the performance of the sealing material.

Creep relaxation performance: Creep relaxation performance is one of the most important properties to characterize the sealing material, reflecting the ability of the gasket material to resist stress relaxation and deformation. For modified PTFE sealing gaskets, creep relaxation is the main reason for leakage in the bolt flange system during the service life, and it plays a decisive role in the sealing ability of the joint surface and the service life of the gasket. Generally, the smaller the creep relaxation rate, the greater the residual compression load and the better the long-term sealing performance.

4. Sealing gasket manufacturing process

At present, the manufacturing process of sealing gaskets is generally based on carbon fiber, polyphenylene sulfide (PPS) and molybdenum disulfide (MoS2) as reinforcing fillers, and is manufactured by mold compression sintering process.

The pretreated polytetrafluoroethylene (PTFE) and mixed filler are weighed according to the ratio, poured into a high-speed mixer with cooling water for mixing, and then the mixture is placed in a mold and molded by a tablet press. After demolding, a 3 mm thick preformed sample is obtained. The preformed sample is sintered and cooled in a box-type resistance furnace according to a certain procedure to obtain a finished sealing gasket.