Pressurized alkaline water electrolysis

Water electrolysis, is the chemical process that splits water molecules into their basic components: oxygen and hydrogen. When coupled with renewable sources of energy, this technique is employed to produce green hydrogen, a key vector in the global energy mix with the potential to decarbonize the energy and transport sectors.

Depending on experimental conditions and cell design, there are different variations of water electrolysis, including:

• Water electrolysis (AWE) and anionic exchange membrane water electrolysis (AEM WE), that occur at a basic pH.

• Proton exchange membrane water electrolysis (PEM WE), that operates in an acidic pH environment.

• Solid oxide cells (SOEC), that use minerals to split water steam at temperatures of 500-800 °C.

In particular, AWE uses a diaphragm to separate the anode and cathode, whereas PEM and AEM provide for the use of ion exchange membranes.

Developed over a century ago, AWE is the most mature water electrolysis technology for large-scale green hydrogen production. Since its origin, it has been evolving, moving from traditional AWE to advanced water electrolysis technology, a technique whereby De Nora's proprietary electrocatalytic coatings and thinner diaphragm are adopted.

The adoption of zero-gap electrode configurations has further enhanced its performance.

Another significant improvement of the advanced AWE is that it’s performed under pressurized conditions. This feature poses significant advantages, such as higher efficiency and flexibility as well as easier downstream operations.

Under pressurized conditions, the hydrogen bubble size (Boyle’s law) on the electrodes’ surface is smaller with respect to the atmospheric AWE. This means that there’s more space on the surface of the electrodes to transform water into hydrogen, and therefore, the reduction reaction occurs at higher rates. The smaller bubble size, due to higher pressures, also makes the potash solution more conductive.

These features enable a higher energy density, and this translates into higher cell efficiency.

Using the cell as a pressurized vessel offers another major advantage. In fact, hydrogen needs to be pressurized during the downstream process to be stored or used for other applications.
Working under pressure means hydrogen in the production line is already compressed and doesn’t need further treatment, thus making operations easier.

Pressurization also enhances the system’s dynamism and flexibility, particularly when green hydrogen production relies on intermittent renewable energy sources like solar and wind.
During the power-up and shut-down processes, hydrogen volumes will change drastically due to the variation in the current load (Faraday’s law).

If hydrogen takes less space in the system due to the higher pressure, the system will be more dynamic in responding to the frequent volume highs and lows. Therefore, the gas deficit will be less demanding for the electrolyzer, and damage to the stack will be less frequent.

Water electrolysis for green hydrogen production is a pivotal solution for decarbonization. In particular, pressurized alkaline water electrolysis stands out as a promising technology, offering an effective and innovative response to the challenges of climate change.