How important is the membrane material in the electrolyzer

The currently known electrolyzers have different principles, and the naming of the electrolyzer type is closely related to the membrane material.

The membrane material is a key material that determines the reaction mechanism, working efficiency, stability and safety of the electrolyzer, and is also one of the most important components in the electrolyzer equipment. The membrane material plays an important role in providing ion/proton channels and isolating gases. This article takes alkaline (ALK) electrolyzers and proton exchange membrane (PEM) electrolyzers as examples to analyze the working mechanism, main performance, and improvement direction of membrane materials, and analyzes the importance of membrane materials for industry reference.

1 Alkaline electrolyzer (ALK)

- Working mechanism: Hydroxyl ions (OH-) pass through porous membranes

The principle of hydrogen production in alkaline electrolyzers is that at the cathode, water molecules are decomposed into hydrogen ions and hydroxide ions. Hydroxyl ions (OH-) pass through the porous membrane to reach the anode under the action of the electric field between the cathode and the anode, and lose electrons to generate water molecules and oxygen molecules; hydrogen ions remain at the cathode to gain electrons, generate hydrogen atoms, and further generate hydrogen molecules and hydrogen gas;

Figure: Schematic diagram of the principle of alkaline electrolyzer

In the early days, asbestos was used as a diaphragm material, but the swelling of asbestos in alkaline electrolytes and the harm of asbestos to the human body made it gradually eliminated. At present, the diaphragm widely used in the industry is a new composite diaphragm based on polyphenylene sulfide (PPS) fabric.

The diaphragm of the alkaline electrolyzer plays a role in ion conduction and gas isolation in the alkaline electrolyzer. Its thickness, hydrophilicity, porosity and pore size are closely related to the electrolysis performance (including resistance, electric density, unit power consumption of hydrogen production, etc.), and also have an important impact on the purity of hydrogen.

——Material properties: ion conductivity and air tightness are its key characteristics, affecting resistance, purity and safety.

1) Ionic conductivity is related to hydrophilicity, affecting electric density and resistance.

One of the functions of the diaphragm is to allow the free movement of ions. In the circuit of the electrolyzer where the reaction occurs, hydroxide ions exist in the solution. Therefore, the hydrophilicity/hydrophobicity of the diaphragm and the solution will directly affect the ion conductivity, that is, the resistance.

In theory, the better the hydrophilicity, the better the conductivity, the lower the internal resistance, and the lower the power consumption per unit hydrogen output; at the same time, better hydrophilicity can also ensure that ions pass through while isolating hydrogen and oxygen. At present, most research is also focused on how to improve the hydrophilicity of the diaphragm.

2) The diaphragm isolates hydrogen and oxygen, and the air tightness affects the purity.

Another key function of the diaphragm is to isolate the hydrogen and oxygen produced in the electrocatalytic process. The diaphragm separates the cathode chamber from the anode chamber, and flows out of the electrolyzer through their respective flow channels to achieve the separation of hydrogen and oxygen. Due to the pressure difference fluctuations between the cathode and the anode during operation, the air tightness and stability of the diaphragm will affect the purity of the outlet, and it is also the key to ensure the safe operation of the electrolyzer.

——Physical improvement: The composite membrane can improve the relevant performance of the diaphragm by adjusting the porosity and thickness.

For the improvement of membrane material performance, on the one hand, the research of various institutions continues to improve the performance of the material itself; on the other hand, the functional coating is applied to the surface of the PPS fabric to improve the relevant performance, forming a "sandwich" composite diaphragm.

The composite diaphragm is mainly coated with a mixture of polymer and zirconium oxide on its surface evenly. Its composition and ratio, and the choice of coating process are the key to affecting the performance of the diaphragm.

Among them, porosity, pore size, and thickness are some indicators for the evaluation of composite membrane process.

Figure: PPS composite material

1) The balance between pore size and porosity affects resistance and airtightness.

The function of the pore is to provide a channel for the transmission of anions and cations in the electrolyte, reduce the internal resistance of the electrolysis process, but also isolate hydrogen and oxygen. If the pore size is too large, the airtightness of the diaphragm will be affected, and if it is too small, the transmission of ions will be hindered. The same is true for porosity. Therefore, effective design and control of the pores are very important. The pore size and porosity of the diaphragm must reach an optimal value to ensure high airtightness and low internal resistance of the diaphragm at the same time. Therefore, the optimization of pore structure is also the focus of diaphragm research.

Figure: SEM pores of different composite materials

2) The thickness of the diaphragm itself must also meet the balance between low internal resistance and strong support.

For composite diaphragms, thickness is also an important parameter. The thickness affects the physical strength of the diaphragm and the internal resistance of the electrolytic cell. The thicker the thickness, the stronger the support, but the greater the internal resistance of the electrolytic cell. The thickness of the diaphragm currently on the market is generally around 500μm~600μm.

2 Proton Exchange Membrane Electrolyzer (PEM)

- Working Mechanism: Hydrogen Protons Pass Through the Proton Exchange Membrane

The proton exchange membrane electrolyzer itself evolved from the solid polymer electrolyte electrolyzer (SPE). Because of the discovery and breakthrough of the perfluorosulfonic acid membrane discovered by DuPont, it was named after the membrane material and called the proton exchange membrane electrolyzer. To this day, most of them are still used and improved on DuPont's perfluorosulfonic acid membrane technology.

Unlike the principle of alkaline electrolyzers, PEM electrolyzers do not use hydroxide ions to pass through the diaphragm, but hydrogen protons (H+) to pass through the proton exchange membrane. That is, a hydrolysis reaction occurs at the positive electrode to produce hydrogen protons (H+), electrons (e-), and oxygen. Protons pass through the PEM membrane and combine with electrons to become hydrogen atoms, and hydrogen atoms combine with each other to form hydrogen molecules.

Figure: Principle of PEM electrolyzer (Figure from literature) - Material properties: Proton conductivity and airtightness are key properties

1) The proton conductivity of PEM is related to the water content, which affects the resistance and electric density.

The proton exchange membrane is composed of perfluorosulfonic acid (PSA) ion polymer, which is essentially a copolymer of tetrafluoroethylene (TFE) and different perfluorosulfonic acid monomers. Protons are conducted by ion polymers, namely sulfonic acid groups. Sulfonic acid groups are hydrophilic groups and can form hydrophilic areas near them. Protons are more likely to move freely in areas with sufficient water content, making it easier to achieve low resistance and high electric density for the entire electrolyzer, and the power consumption per unit of hydrogen production is also lower.

2) PEM can respond quickly to power changes, so it has high requirements for airtightness.

The proton conduction efficiency of the proton exchange membrane is better than that of the alkaline electrolyzer ion conduction efficiency, and can respond quickly to changes in input power. When the power is low, the gas production of oxygen and hydrogen will also decrease. If the air tightness is not good, the concentration of impurities in oxygen and hydrogen will increase, causing danger.

——Physics and its improvement: The thickness adjustment of PEM membrane and the combination of catalyst and gas diffusion layer will increase its performance advantage.

1) The thickness needs to find a balance between conductivity and stability.

Currently, the thickness of proton exchange membrane is generally between 100~175μm. The thickness of proton exchange membrane directly affects the proton conductivity. The thinner the thickness, the smaller the resistance of proton across the membrane. But at the same time, too thin membrane has poor anti-swelling force, mechanical stability and air tightness. At present, the thickness of PEM membrane is also a key research direction.

2) The porous structure of the catalyst layer and the supporting structure of the gas diffusion layer can affect the function of the membrane. The catalytic layer formed by the catalyst is the real place where the reaction occurs in the membrane electrode of the PEM electrolyzer. The surface of the catalyst particles needs to be closely linked to the proton exchange membrane to transfer protons. The fluffy porous structure of the catalytic layer can increase the proton conduction efficiency. Although the gas diffusion layer does not directly participate in the reaction, it provides channels for water, gas, heat, etc., and plays a protective role. It must have a certain flexibility to protect the catalytic layer and the proton membrane from being damaged, and at the same time it must have a certain rigidity to support the thinner proton membrane, etc.

Figure: Schematic diagram of the membrane electrode architecture of the PEM electrolyzer (Figure from the literature)

Conclusion

Whether from the perspective of the relatively mature alkaline electrolyzer technology or the proton exchange membrane electrolyzer technology that is constantly making breakthroughs, membrane materials play a very important role, and the most important functions are to transfer ions/protons and isolate gases.

For the improvement of membrane performance, research is generally carried out around improving ion/proton conductivity to reduce resistance, while ensuring air tightness and stability. Specifically, on the one hand, research will be conducted around the characteristics of the material itself, including hydrophilicity (water absorption), conductivity, air tightness, chemical stability, etc.; on the other hand, we will continue to find a balance in terms of membrane thickness, porosity, mechanical support, etc. by adjusting our own performance or cooperating with other materials.