REMA, a Swiss cleantech company founded in 2023, develops membrane-less water electrolysis technology that uses advanced fluid mechanics based on porous walls to improve the production rate of hydrogen and maintain high hydrogen purity. REMA aims to reduce the costs of green hydrogen production by up to 40%, which could significantly contribute to the widespread adoption of green hydrogen and support global sustainability goals.
Challenges: hydrogen fuel
Conventional water electrolyzers
Green hydrogen (H₂) is a crucial element of the future net-zero world. Using electricity from renewable sources, such as nuclear, solar, and wind, to split water produces green hydrogen. Green hydrogen can decarbonize hard-to-abate industries, such as steel manufacture, long-distance transportation, shipping, and aviation. It can also be used to store renewable electricity seasonally and as a chemical feedstock.
There are several ways to use electricity to turn water into hydrogen right now. These include alkaline electrolyzers, proton exchange membrane (PEM) electrolyzers, and solid oxide electrolysis cells.
When it comes to commercial water electrolysis technology in the megawatt range, alkaline electrolyzers are the most common type. A strong KOH solution (about 10 M) is fed into the alkaline electrolyzer. The electrode chambers are separated by a thick, porous diaphragm, like the 500 µm-thick AGFA's Zirfon membrane. The porous diaphragm becomes conductive when it takes in feed solution through its pores.
The alkaline electrolyzer can use cheap parts, like steel bipolar plates, inexpensive diaphragm separators, and non-precious catalysts for hydrogen and oxygen evolution reactions. Alkaline water electrolyzers, on the other hand, have some disadvantages. For example, they need to operate at low current densities (0.2 to 0.4 A cm⁻²) and compress the hydrogen they produce.
Proton exchange membrane (PEM) electrolyzers were first introduced by General Electric in the 1960s. Now, PEM electrolyzers are a market-ready technology. The PEM electrolyzer uses a gas-impermeable PEM and a pure water feed. The electrode chambers are separated by a thin PEM with a zero gap configuration. PEM electrolyzers have several advantages over alkaline electrolyzers. For example, they can have a high operational current density (over 2 A cm⁻²) and directly produce high-pressure hydrogen gas (30 bar).
The above-mentioned electrolyzers use a membrane to prevent gas cross-contamination. Membrane-less electrolyzers are an emerging technology that removes the need for membranes. This offers a promising approach to lowering the cost of hydrogen production from water electrolysis. They utilize fluidic forces instead of membranes or separators for the separation of electrolysis gas products, which simplifies the design and increases the lifetime and durability of the device.
Furthermore, a membrane-less electrolyzer is compatible with a wide range of electrolytes at different pH. Thus, it can be used for various electrochemical reactions, such as water electrolysis for hydrogen production and brine electrolysis for chlorine production.
There are two types of membrane-less electrolyzers based on electrode configurations.
The diagram below illustrates the membrane-less electrolyzer with parallel electrodes.
The electrodes are placed on the two opposite sides of a rectangular channel. The electrolyte is flowing between the electrodes. Gas products are flowing between the electrodes until the end of the channel. The electrolyte flow keeps the bubbles at the channel sides to prevent crossover. The volume fraction of bubbles increases as they go downstream. Therefore, the channel length and the electrolyte flow rate should be designed carefully to prevent the formation of gas bubbles larger than half of the channel width.
The diagram below illustrates the membrane-less electrolyzer with mesh electrodes.
Two plane meshes act as electrodes. The electrolyte flow enters the area between the meshes and flows through the pores of the mesh. The bubbles are formed on the surface of the mesh. They move to the outer side of the mesh after the detachment.
The mesh electrode electrolyzer can achieve a higher production rate compared to the parallel electrode electrolyzer because the bubbles go through the mesh pores and leave the interelectrode region faster. Furthermore, the bubbles growing on the inner side of the mesh should detach at sizes smaller than the mesh pore size in order to allow the bubbles to flow through the mesh.
Challenges of membrane-less electrolyzers
However, product separation is a challenge for the membrane-less electrolyzers due to the absence of a membrane or separator.
The bubble coalescence and large bubble detachment from the electrodes can lead to the formation of bubbles larger than half of the channel width, which leads to gas crossover. The large bubble formation is more frequent at high production rates. Consequently, the gas crossover is higher at higher production rates. Enlarging the space between the electrodes leads to higher maximum production rate, but it imposes additional ohmic losses due to the longer interelectrode distance.
Moreover, bubbles moving between electrodes block ionic pathways that add to the overpotential losses. In mesh electrode electrolyzers, bubbles larger than the pores cannot go through the pores easily, and they flow between the electrodes until the end of the channel, which limits the production rate.
REMA develops a membrane-less electrolyzer that has three channels separated by porous walls. The electrolyte enters a middle channel and flows to the outer channels through the porous walls. The gas products are produced in the outer channels. Producing bubbles outside the interelectrode region significantly reduces hydrogen crossover and resolves the gas production rate limitation. Moreover, REMA develops a suitable electrolyte with a surfactant to reduce the bubble size. REMA’s advanced membrane-less electrolyzer technology increases the hydrogen gas production rate without having to increase the interelectrode distance or flow velocity.
The diagram below illustrates the structure of a REMA membrane-less electrolyzer with porous walls.
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