What are electrolysers?
An electrolyser is a device that uses electricity to split water or other chemicals into their constituent elements through electrolysis. Electrolysis is a chemical reaction where an electric current passes through a substance, causing it to decompose into its basic components.
In the case of water electrolysis, an electrolyser uses an electric current to split water molecules (H2O) into hydrogen (H2) and oxygen (O2) gases. The hydrogen gas can be stored as either compressed gas or liquefied. The oxygen created is released back into the air or captured and stored to supply other industrial processes.
Electrolysers, which use water, are a critical technology for producing Green Hydrogen from renewable energy sources like solar energy, wind energy, geothermal energy, etc.
The different kinds of electrolysers
There are 4 main kinds of water electrolysers: Alkaline (ALK), Proton Exchange Membrane (PEM), Solid Oxide Electrolyser Cell (SOEC), and Anion Exchange Membrane (AEM). The operation of each electrolyser varies slightly based on the specific electrolyte material it employs.
Alkaline Electrolyser (ALK)
An alkaline water electrolyser features two electrodes operating within a liquid alkaline electrolyte solution, typically composed of potassium hydroxide (KOH) or sodium hydroxide (NaOH). A diaphragm separates these electrodes, serving to isolate the produced gases and facilitate the transport of hydroxide ions (OH−) from the cathode to the anode.
Advantages of this system include a lower initial capital expenditure (CAPEX), a well-established track record, and reduced degradation, resulting in an extended lifespan of the stack. However, there are some drawbacks, including higher operational expenses (OPEX) due to lower efficiency, a decreased level of purity, and the need for potassium hydroxide (KOH) recirculation.
Proton Exchange Membrane Electrolyser (PEM)
A PEM electrolyser utilizes a polymer membrane for ion conduction in the electrolysis process to generate hydrogen gas. Key components include the proton exchange membrane, electrodes (often platinum-based), gas diffusion layers for uniform gas distribution, and bipolar plates serving as current collectors and providing mechanical support. The membrane selectively allows protons to pass while blocking electrons and other ions.
Advantages of PEM electrolysis include reduced operational expenditure (OPEX), compatibility with dynamic energy sources, and the ability to maintain high product gas purity. However, there are challenges, including higher capital expenditure (CAPEX), sensitivity to impurities, and the requirement for precious metal catalysts, impacting overall cost-effectiveness.
Solid Oxide Electrolyser Cell (SOEC)
A solid oxide electrolyser cell (SOEC) is a solid oxide fuel cell that runs in regenerative mode to achieve the electrolysis of water by using a solid oxide, or ceramic, electrolyte to produce hydrogen gas and oxygen. Functioning at temperatures of 500 to 850 °C, similar to solid oxide fuel cells, SOECs enable high-temperature electrolysis.
This system offers advantages such as superior efficiencies leading to low operational costs and the capability to operate in reverse mode. However, a notable drawback is the elevated operating temperature, contributing to extended start-up and break-in times. Additionally, the technology is still in developmental stages, with limited precedent in large-scale commercial applications.
Anion Exchange Membrane Electctrolyser (AEM)
AEM electrolysis involves water electrolysis using a semipermeable membrane that facilitates the conduction of hydroxide ions (OH−)—known as an anion exchange membrane. Similar to a proton-exchange membrane (PEM), this membrane serves to separate products, provide electrical insulation between electrodes, and conduct ions. However, in contrast to PEM, AEM specifically conducts hydroxide ions.
A significant advantage of AEM electrolysis is the use of cost-effective transition metal catalysts instead of high-cost noble metal catalysts. Additionally, AEM electrolysers can operate on pure water or slightly alkaline solutions, reducing the risk of leakage compared to highly concentrated alkaline solutions in ALK. Some challenges include lower durability and stability compared to PEM counterparts, particularly on the oxygen generation side, lower efficiencies, and scaling up to multi-megawatt levels for heavy industrial applications is unproven.
The Road Ahead
As the world transitions to renewable energy sources, electrolysers provide a critical pathway for generating green hydrogen fuel. While alkaline electrolysis has an established track record of success, emerging technologies like proton exchange membrane (PEM), solid oxide, and anion exchange membrane (AEM) electrolysers offer the promise of improved efficiency and cost reductions necessary for mass adoption.
Through ongoing innovation, water electrolysis is positioned to play a major part in decarbonizing hard-to-abate sectors. However, realizing the full emissions reduction potential will require investment and support today to surmount near-term adoption barriers. If cultivated appropriately, hydrogen from renewables-powered electrolysis can emerge as one foundation of a net zero global energy system.