Overview of PEM Hydrogen Production by Water Electrolysis II

Overview of PEM Hydrogen Production by Water Electrolysis II

Electrolzer Stack components

The components used in PEM electrolysis are critical for a stack with good performance and durability. Key components include bipolar plates, current collectors, and membrane electrode assemblies (MEAs). The bipolar plates have the task of evenly distributing water across the current  collectors (or diffusion media) inside the cell. The characteristics of bipolar plates are challenging for any class of materials due to the presence of oxygen, acidic environment, and high overvoltage. The design and material selection of bipolar plates in PEMWE are critical issues for scientists and engineers.

The main functions of the bipolar plates in PEMWE are as follows:
1. Conduct electrons to complete the circuit and connect individual cells in series to form a battery pack
2. Provide a flow path for evenly distributing water on the current collector (or diffusion medium)
3. Separate oxygen and hydrogen
4. Provide a structure to support the membrane electrodes for the battery
5. Provide heat conduction to manage the temperature of the electrolyzer

In order to perform these important functions, the material of the bipolar plate must have special properties, such as high electrical and thermal  conductivity, low gas permeability, high mechanical strength and corrosion resistance.

Currently, a variety of materials are used in bipolar plates, including titanium, graphite, and coated stainless steel. However, graphite does not guarantee high performance in PEMWE due to its high corrosion rate and low mechanical strength.Titanium has excellent properties but forms a passive oxide layer that reduces its contact resistance and thermal conductivity. This phenomenon can degrade stack performance over time. Gold-coated titanium bipolar plates have also been studied, with good results in terms of stability. However, this process increases the cost of the components. Coated stainless steel is a cheaper alternative to titanium, but small defects in the coating expose the metal to corrosion. Corrosion of the metal leads to the destruction of the coating, which increases the ohmic resistance. In addition, dissolved metal ions diffuse into the PEM membrane and reduce the ionic conductivity, resulting in increased membrane degradation.

The bipolar plates and diffusion layers are the more expensive components in a PEMWE. The diffusion layer has the function of transferring electrons from the catalytic layer to the bipolar plates and removing gases (hydrogen and oxygen) from the catalytic layer. Porosity and conductivity are therefore important parameters for the diffusion layer. Pore size and structure are critical factors because gases must be vented from the cell and water is kept near the catalytic layer. Improper pore structure can increase the cell voltage by up to 100 mV at 2 A cm−2.