Water electrolysis – a method of producing green hydrogen

The ongoing shift in energy focuses on producing energy with minimal or no emissions. A technology making rapid strides is hydrogen fuel cells. To be truly eco-friendly, both producing and using hydrogen must avoid emitting harmful substances. Making hydrogen through water electrolysis achieves this goal. When this process emits no harmful gases, the resulting hydrogen is termed green hydrogen.

What is water electrolysis?

Electrolysis refers to any process where an electric current causes changes in the chemical structure of a substance. In the electrolysis of water, the water molecules break down into ions due to the application of a minimum voltage of 1.229 V. Subsequently, the reduction reaction occurs at the cathode:

2H ₂ O + 2eˉ → H ₂ + 2OHˉ

^{Hydrogen and OH –} ions are produced in it . Oxidation takes place at the anode:

2H ₂ O → O ₂ + 4H ⁺ +4eˉ

Its result is the formation of molecular oxygen, hydrogen ions and electrons.

OH ^– anions combine with H ⁺ cations . After multiplying the cathode reaction by 2 to agree on the number of electrons and ions, the summary equation of the reaction looks as follows:

2H ₂ O → 2H ₂ + O ₂

History of water electrolysis

In 1800, Nicholson and Carlisle first observed water splitting into ions when exposed to an applied voltage. By the turn of the century, there were already over 400 industrial electrolysers in operation. In 1939, the inaugural large-scale plant commenced operations, generating 10,000 normal cubic meters of hydrogen per hour. Over the ensuing decades, diverse technologies emerged, including solid polymer electrolyte (SPE), solid oxide, alkaline electrolyzer, and proton exchange membranes (PEM). Presently, these technologies undergo refinement, while laboratories continue to explore novel methods for water electrolysis.

Water electrolysis technologies

Alkaline electrolyzer

In an alkaline electrolyzer, both the cathode and anode are immersed in water. Since pure water isn’t a good conductor, acids or bases like H2SO4, KOH, or NaOH are typically added to enhance conductivity. To prevent oxygen and hydrogen from recombining into water, a separator—a porous material soaked with electrolyte and conducting ions—is placed between the electrodes. The electrodes can be configured with a gap of several millimeters from the separator, or closely adhered to it. In the former case, the achievable current density is limited to several hundred milliamps per cm2 due to gas bubbles creating a resistive layer on the electrode surface. However, with electrodes snug against the separator, higher current densities are attainable since gas is produced on the opposite side of the electrodes. The cell’s efficiency hinges on current density, but higher densities escalate operational costs. Therefore, selecting this parameter involves finding a compromise.

Alkaline water electrolysis is a well-established technology, with electrolysers capable of producing up to 60 kg of hydrogen per hour. Their extended lifespan renders them economically viable. However, alkaline electrolysers lack the flexibility to adapt to fluctuating source characteristics—a crucial requirement for collaborating with uncontrollable sources such as renewable energy, heavily reliant on weather conditions.

Electrolysers with polymer electrolytic membrane (PEM)

PEM electrolyzers, which stand for polymer electrolyte membrane or proton exchange membrane, differ from the previously described alkaline electrolyzer technology primarily in the type of electrolyte employed. In PEM electrolyzers, a solid polymer serves as the electrolyte. This type of electrolyzer operates solely with deionized water, without the need for additional electrolyte. The electrodes tightly adhere to the electrolyte, forming a separating membrane. During electrolysis, oxygen and hydrogen ions, or protons, are generated at the anode. These protons then traverse the membrane and combine with electrons at the cathode, producing hydrogen.

PEM technology offers several advantages for water electrolysis, including the capability to achieve high current density and efficiency. Additionally, the use of deionized water enables the production of hydrogen with a high level of purity. However, there are drawbacks to PEM electrolyzers, such as the high cost of the materials they comprise and the requirement for high-purity water, which can be expensive to obtain.

High-temperature electrolysis of water vapor

Performing electrolysis at elevated temperatures proves effective due to the reduced amount of electricity required. The necessary heat can be sourced from renewables or from waste heat, such as that generated by nuclear power plants or other high-temperature processes.

High-temperature electrolysis typically occurs at temperatures ranging from 750 to 950°C, using water vapor. The ample thermal energy involved slashes the electrical energy demand by roughly 35% compared to electrolysis at lower temperatures. Moreover, the efficiency of high-temperature electrolysis can reach remarkably high levels, up to 100%. However, as a relatively recent technology, it necessitates extensive research before it can yield profitable applications.

Prospects for water electrolysis

Water electrolysis offers the advantage of producing hydrogen with exceptionally high purity. When powered by renewable energy, it contributes to decarbonization efforts by yielding green hydrogen. While alkaline electrolysis stands as the oldest and most mature technology, its limitations drive ongoing exploration for superior methods. PEM and high-temperature electrolysis technologies are continuously refined, with novel approaches also emerging. It is anticipated that in the foreseeable future, water electrolysis will gain traction as a prevalent method for decentralized hydrogen production. In the quest for decarbonization, green hydrogen holds promise as a significant energy resource.