Hydrogen - How does an industrial electrolyzer work?

In the intermittent renewable energy sector, terms like “electrolysis,” green hydrogen ” and “hydrogen electrolyzer” are gaining momentum. Considered essential for the ecological transition, hydrogen production is tending towards a process that is more and more environmentally friendly.

While many governments and investors are pinning their hopes on the implementation of green hydrogen, how it is produced is still unknown to the general public.

In this article, SirEnergies explains the industrial electrolyzer and its main uses. The time will also come to question the future for the production ofhydrogen by electrolysis?

Hydrogen electrolyzer: what is it?

THEhydrogen electrolyzer Is simply a fuel cell inverted. Unlike the fuel cell, which produces electricity from oxygen and Hydrogen, this device produces these using electricity. For the less scientific among you, here is a simpler explanation.

An electrolyzer is a device composed of a water container in which there is an electrolyte. It also includes two electrodes: an anode or positive pole and a cathode or negative pole.

Once an electric current passes through it, the electrolyser induces a chemical reaction on both sides of the electrolyte: it is electrolysis. This reaction gradually breaks down compound substances into other compound or singular substances.

Result? Oxygen is released at the anode and hydrogen is created at the cathode. The latter can then be transformed into liquid hydrogen using a liquefier.

To be sustainable in the long term, The electrolysis of water must be based on sustainable energy. Among others, we can mention:

• solar energy,
• Biomass,
• Hydraulic power,
• Wind energy...

You can use other energy sources to produce hydrogen by electrolysis. However, the result cannot be considered as green hydrogen.

What are the main types of electrolysers?

The production of hydrogen by electrolysis is a well-established principle. However, there are several systems to achieve this, the main ones of which are the PEM electrolyzer and the alkaline electrolyzer.

The PEM electrolyzer

THEproton exchange membrane electrolyzer or PEM electrolyzer works like a filter. The polymer membrane that equips this device only allows hydrogen ions to pass through.

During the electrolysis reaction, hydrogen ions and electrons formed at the anode pass through the wall to reach the cathode. They are transformed into hydrogen at the cathode. We can summarize chemical reactions with the following equations:

Anode: H2O → H2 + ½ O2 + 2e

Cathode: 2 h+ + 2e → H2

The PEM electrolyzer has the particularity of be able to operate under pressure. It can therefore be smaller than the alkaline electrolyzer. However, you must take into account the particular requirements it requires to produce hydrogen under very acidic conditions.

The alkaline electrolyzer

This device makes it possible to produce hydrogen from an alkaline liquid electrolyte. You can use sodium hydroxide or potassium hydroxide, as these compounds have long been known for their good results.

During the chemical reaction in an alkaline electrolyzer, the water contained in the electrolyte is subdivided into hydrogen and hydroxide ions at the cathode. They then come into contact with a membrane before being oxidized to oxygen and water at the anode.

We can summarize the results of the reactions to two poles of the alkaline electrolyzer by the following equations:

Cathode: 2 h 2 O + 2e → H2 + 2 OH -.

Anode: 2 OH - → ½ O2 + 2e- + H2O.

Balance: H2O → H2 + ½ O2.

Here are some figures to give you a better overview of the conditions necessary for this reaction to take place. It takes approximately:

• 235 kJ/mol of renewable electricity per mole of water,
• 50 kJ/mol of heat,
• Between 40 and 90°C.

The main advantage of the alkaline electrolyzer is the high availability of electrolytes necessary for its functioning. However, it uses bulky cells whose current density is relatively low.

What are the prospects for the industrial electrolyzer?

THEindustrial electrolyzer is of no interest if it cannot be used over the long term to cover the planet's energy needs. Several companies have committed themselves in this direction to set up infrastructures capable of meeting the challenges of tomorrow.

The production of carbon-free hydrogen with Air Liquide

The largest membrane electrolysis unit was created in Quebec. Inaugurated in January 2021 in Bécancour by Air Liquide, this power plant has 4 distinct units. It works on the principle of a proton exchange membrane. Its total power of 20 MW makes it a global reference.

What is the particularity of this industrial electrolyser? La production of carbon-free hydrogen : it works with renewable energy to 99%! In addition, the installation can produce up to 8.2 tons of low-carbon hydrogen per day. To give you an idea of its potential, you should know that this quantity is enough to power 16,000 forklifts or even 230 large trucks!

McPhy's high-pressure electrolysis

The McPhy CEOG project is an innovation that offers hope to the storage of intermittent renewable energies. This hydrogen production platform will avoid the emission of around 39,000 tons of CO2 each year compared to a fossil power plant.

The ingenious mechanism behind this project? The combination between alkaline electrolysis at a pressure of 30 bar and high current density electrodes. The platform has a capacity to produce around 860 tons of green hydrogen per year for a total capacity of 16 MW.

Genvia high-temperature electrolysis

Created in March 2021, the joint project between CEA and the multinational Schlumberger aims at the production of hydrogen on a large scale. The technology used is under development. However, it is promising because it is based on high-temperature electrolysis.

Genvia extends over 600 m² in Béziers in Hérault. The site will host advanced equipment to manufacture the components that make up power stacks. It is a repeated stacking of the reaction core (electrochemical cell) and a metal sheet. The expected return? 99% higher calorific value (PCS)!

The industrial electrolyzer thus operates on the principle of separating chemical compounds from an electrolyte to produce hydrogen. La need for synthetic fuels, the size constraints of the cells and those related to their current density constitute obstacles to the development of this manufacturing process.