Electrolysis: the technology that turns electricity into hydrogen

Electrolysis is used to transform electricity into hydrogen.

Specifically, electrolysis is a chemical transformation process using electricity.

How hydrogen electrolysis works

The electrolysis of water involves breaking up the water molecule to form 2 moles of Hydrogen and one of Oxygen in the gaseous state.

2H₂O_(aq) → 2H_2(g) + O_2(g)

The incoming water is demineralised upstream of the electrolysis system.

This is done by reverse osmosis and collected in a tank, which serves as a reservoir to feed the electrolyser.

The latter is a special device for performing electrolysis. On average, an electrolyser has an efficiency of 5 kWh/Nm3.

The physical principle of an electrolysis cell is the same for any technology:

• Water is pumped onto the surface of an electrode
• Thanks to the supply of electricity, the molecule is split into ions and electrons
• The electrons flow through an external circuit, the ions through the electrolyte
• On the opposite electrode, a new reaction takes place to form the reducing substance

Let us now look at the differences between the currently available electrolysis technologies.

The different electrolysis technologies

There are four main electrolysis technologies:

• AEL (Alkaline Electrolyser), with alkaline electrolysers
• PEM (Proton Exchange Membrane) with proton exchange membrane
• AEM (Anion Exchange Membrane) with anion exchange membrane
• SOEL (Solid Oxide Electrolyser), with solid oxide electrolyser

Let us look in more detail at the advantages and disadvantages of these technologies.

Alkaline technology

In alkaline electrolysis (Alkaline ELectrolyser, AEL), the electrolyte used is an aqueous solution of usually potassium hydroxide.

This technology has four main advantages:

• Good efficiency, 55-65% (on LHV)
• Does not use noble metals (this allows lower production costs and a lower selling price of the electrolyser)
• Economical materials
• High service life
• Established and reliable technology

However, alkaline technology also has some disadvantages:

• low current density (0.5 A/cm2), larger plant size for the same amount of hydrogen produced compared to another technology
• Lack of membrane, possibility of gas crossover, resulting in limited hydrogen purity
• Limited partial load (40%-100% max.), with too low efficiencies and low reactivity, thus not suitable for coupling with renewables
• Operating pressures limited by the liquid electrolyte; the higher pressure favours crossover effects

PEM technology

Electrolysis with proton exchange membrane electrolysers proton (PEM) uses a polymer membrane instead of an electrolyte.

The advantages of this technology are:

• High efficiency: 60 – 70 % (on LHV)
• Good mass transport, with higher efficiencies due to the solid electrolyte
• High current density, with higher hydrogen production for the same surface area
• High hydrogen quality due to solid membrane preventing crossover
• Ability to work at partial loads (20%-100%) and with high reactivity, making coupling with renewables easier
• Allows working at high pressures

However, this technology has two disadvantages:

• Moderate degradation coefficient
• High investment cost
• Use of rare metals such as platinum and iridium, the extraction of which still has a high environmental impact
• Low recyclability of components

AEM technology

Electrolysis with anion exchange membrane electrolysers (AEM) uses an aqueous solution of potassium hydroxide plus a polymer membrane as the electrolyte.

This type of electrolysis offers the following advantages:

• Good efficiency: 50%-70% (on LHV)
• Less corrosive electrolyte than alkaline technology
• Does not use noble metals: this allows lower production costs, a more affordable price and less environmental impact
• Lower acidity of the electrolyte solution
• Possibility of working at partial loads (20%-100% or higher ranges) with high reactivity, excellent coupling with renewables
• Possibility of working at high pressures and with good purity

On the other hand, it also has some disadvantages:

• Low current density (1 A/cm2 ), higher size and cost for the same amount of hydrogen produced compared to another technology
• Very innovative but underdeveloped technology, few large-scale applications
• High cell degradation coefficient

SOEL technology

Solid oxide electrolysis (SOEL) uses solid ceramic electrolytes as the electrolyte.

This type of electrolysis offers the following advantages:

• Very high efficiencies (80%-90% on LHV)
• Possibility of also working in reverse function, as a fuel cell
• Possibility of also working with fluids other than water

On the other hand, it also has some disadvantages:

• Operation at high temperatures (600-900 °C)
• Little flexibility on load variations
• Short service life
• Mainly academic applications, although the first commercial installations are beginning to appear
• Still very high costs, but with prospects of a sharp drop in the coming years

Why electrolysis is used to produce hydrogen

As just explained, electrolysis is the electrochemical process used to produce hydrogen from water.

This technology allows for zero emissions, provided that the energy supplied to the system comes from renewable sources.

There are many applications that cannot be electrified and that necessarily require the use of a fuel.

For this reason, the production of hydrogen through electrolysis is crucial to achieving the pollutant emission reduction targets set by the European Union.

In fact, hydrogen produced through electrolysis is the only fuel that does not generate any pollutants, no matter how it is used.

At Simplifhy, we offer a range of products capable of producing hydrogen by means of electrolysis.

Contact usfor more information.