Electrolysis is recognised as the most promising method of hydrogen production. Especially when it is based on the use of renewable electricity, guaranteeing the production of clean hydrogen for industry, transport, and energy. What is water electrolysis and how is hydrogen produced? Let’s find out!
Hydrogen is considered one of the vectors of the ongoing energy transition. This is due to the lack of emissions associated with its use in transport, energy, and industry. The products of combustion are energy and water. However, to talk about full decarbonisation, the source of the new fuel must be addressed.
Hydrogen is produced by several processes, but not all of them are fully green. Currently, more than 120 million tonnes of hydrogen are produced worldwide, about two-thirds of which is produced in dedicated facilities (about 74 million tonnes) and the remainder as a by-product of technological processes or as a mixture of other gases.
Interestingly, Poland ranks third among the largest producers in Europe with a volume of about 1 t of hydrogen per year, almost all of which comes from gas and coal sources.
As recently as 2021, hydrogen from fossil fuels accounted for 90.4% of total production in the EU, with an additional 7.1% as a by-product. The most important methods of producing hydrogen from fossil fuels include steam reforming of methane SMR (grey hydrogen) and gasification of solid fuels – lignite (brown hydrogen) and hard coal (black hydrogen). Of the remaining processes, about 1.6% were grid electrolysis, 0.2% RES electrolysis (green hydrogen), and 0.1% blue hydrogen, i.e. fossil fuel hydrogen with CO₂ capture.
One of the goals of the energy transition is to increase the use of green hydrogen and replace grey hydrogen and fossil fuels in the most demanding economic sectors, where electrolysis will play a crucial role.
Electrolysis – what is it?
The term electrolysis was popularised thanks to the discoveries of Michael Faraday, who was the first to describe the laws governing this process, which became the basis for further research by physicists. Electrolysis is defined as the process of breaking down ionic compounds into their elements by passing an electric current. The basic elements required to carry out the process are an electrolyte, electrodes, and an external power source.
Water electrolysis – hydrogen production
Electrolysis is one of the commercially available technologies for hydrogen production. In this process, water (H₂O) is separated into its constituent elements – hydrogen (H₂) and oxygen (O₂) under the influence of an electrical voltage. This process is carried out in devices called hydrogen and oxygen generators or, more commonly, electrolysers. Alkaline electrolysers (AWE) are the oldest and most proven technology. In addition, there are Proton Exchange Membrane Electrolysers (PEM), Solid Oxide Electrolysers (SOE), and Anion Exchange Membrane Electrolysers (AEM).
Alkaline Electrolysers (AWE)
Alkaline electrolysis is the oldest and most recognised technology, and therefore the most widely used commercial method for producing electrolytic hydrogen. One of its main advantages is that it can be scaled up to MW, making it used in industrial applications.
An alkaline electrolyser cell consists of two electrodes (anode and cathode) immersed in an alkaline electrolyte, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), usually at a concentration of about 20-30%. This serves to facilitate the flow of ions between the electrodes.
In an alkaline electrolyser, reaction (I) takes place at the anode, while reaction (II) takes place at the cathode:
- 4OH- –> H₂O + O₂ + 4e-
- 2H₂O + 2e- –> H₂ + 2OH-
Alkaline electrolysers – advantages and challenges
Alkaline electrolysis stands out from other production methods, especially in industrial and large-scale applications. Key advantages of alkaline technology include:
- Maturity and high level of technology readiness,
- Performance and durability – higher tolerance to contamination of components,
- Greater benefits for large installations,
- Economic benefits of lower equipment price.
The challenge currently facing the industry is to increase the efficiency of alkaline electrolysers, especially when working with unstable RES. Until now, projects have mainly used grid energy to power equipment. Consistent operating conditions allowed this technology to operate almost without failure for many years. The goals of increasing the production and use of renewable hydrogen produced from variable renewable energy sources make it crucial to improve and increase the efficiency of electrolysis. This is the main objective of SES Hydrogen Energy in developing its alkaline technology.
Proton Exchange Membrane Electrolysers
Proton exchange membrane (PEM) electrolysis is another promising technology for hydrogen production. Instead of a liquid electrolyte, it uses an acid membrane as a solid electrolyte. The process involves the reaction of water at the anode to produce oxygen and positively charged hydrogen ions. Electrons flow through the external circuit and hydrogen ions selectively move through the PEM to the cathode, where they combine with electrons from the external circuit, resulting in hydrogen gas.
In a PEM electrolyser, reaction (III) occurs at the anode, while reaction (IV) occurs at the cathode:
- 2H₂O –> 4H+ + 4e- + O₂
- 4H+ + 4e- –> 2H₂
The advantages of PEM electrolysers are high efficiency, high flexibility of operation, wide control range, and short start-up time, as well as more effective prevention of hydrogen and oxygen mixing. On the other hand, the challenge is the corrosive operating environment, which determines higher purchase and maintenance costs due to the use of components made of expensive materials such as platinum or iridium.
Solid Oxide Electrolysers
Solid oxide electrolysers are currently at an early stage of technological maturity. They use a ceramic membrane, which selectively conducts negatively charged oxygen ions. In the process, the vapour on the cathode combines with electrons from the external circuit to form hydrogen gas and negatively charged oxygen ions. These, pass through the membrane and react at the anode, forming gaseous oxygen.
In a SOEC electrolyser, the (V) reaction occurs at the anode, while the (VI) reaction occurs at the cathode:
- H₂O (vapour) + 2e- –> O₂- + H₂
- O₂ + 2e- –> O₂
They are characterised by a high level of efficiency (80-90%) and a high electrolysis temperature (850-1200°C), which can result in limited component life, thus affecting costs.
Anion Exchange Membrane Electrolysers (AEM)
Anion exchange membrane electrolysers are an extremely promising technology for hydrogen production, combining the advantages of alkaline and PEM technology and, like them, it is a low-temperature technology. By using cheaper components that do not contain platinum group metals, the technology allows for lower costs and potentially longer equipment life.
With an AEM-based electrolyser, the expected reaction for the anode (VII) and cathode (VIII) are described by the formulas:
- 4OH –> 2H₂O + O₂ + 4e-
- 4H₂O + 4e- –> 2H₂ + 4OH-
Currently, AEMs are not popularised as much as PEMs and alkaline electrolysers. At SES Hydrogen Energy, we are focusing on developing our own AEM technology to increase efficiency and lower the target cost of hydrogen production.
Production of electrolytic hydrogen – applications
Green electrolytic hydrogen is an important tool for energy transition for good reason. Thanks to its properties, it finds several applications in key economic sectors, reducing carbon emissions and increasing the use of renewable energy.
Hydrogen in power and heat generation
Hydrogen will help balance and increase the efficiency of RES by using surplus energy to produce gaseous hydrogen, which can be stored for long periods, used in industrial applications and transportation, or converted back into energy using cells (at the current stage of technology development, this involves the occurrence of certain energy losses).
Hydrogen also represents an opportunity for the development of independent, distributed energy systems, which are particularly important in places where connection to the local district heating network is not possible. Hydrogen could become an important fuel for the heating sector through combustion in dedicated hydrogen boilers and H2Ready boilers.
Hydrogen in industry
Hydrogen is used in many industrial applications, including the chemical industry for ammonia production, the petrochemical industry for hydrotreating, reforming, and hydrocracking, and the glass industry. The key is to replace emissive gray hydrogen in these processes with green electrolytic hydrogen. Great potential for its use is also seen in energy-intensive heavy industry and metallurgy, including to production of green steel.
Hydrogen in transportation
Another important area of transformation is transportation, where hydrogen can be used as a fuel for fuel cell vehicles, buses, and trucks (FCEVs), in trains, as well as to produce e-fuels, including e-ammonia or e-kerosene for shipping and aviation.
Thus, we see that the role of electrolysis and electrolysers is crucial to changing Europe’s energy landscape.
[1] IRENA, Hydrogen Overview
[2] https://www.eea.europa.eu/en/analysis/indicators/greenhouse-gas-emissions-from-energy
