This technology is a direct seawater electrolysis method for producing hydrogen that radically addresses the side-reaction and corrosion problems. The electrolysis system in seawater can operate without failure for over 3,200 hours at a constant current density of 250 mA cm⁻² under practical application conditions. This technology realizes efficient, size-flexible, and scalable direct seawater electrolysis in a manner comparable to freshwater splitting without a significant increase in operation cost, and has high potential for practical application.
Challenges
Hydrogen has the advantages of wide sources, storability, zero carbon emission, no pollution, and high energy density. It is essential to the future energy field. The electrolysis of seawater using renewable energy is a highly desirable and sustainable method for the mass production of green hydrogen. However, direct electrolysis of seawater is hampered by insufficient durability due to electrode side reactions and corrosion issues caused by the complex constituents of seawater.
Seawater contains an abundance of chloride ions (Cl⁻). Cl⁻ can be oxidized during electrolysis to form toxic, environmentally harmful and corrosive ClO⁻ and Cl₂. Calcium and magnesium ions may precipitate as a result of electrolysis; therefore, it is necessary to use acid for precipitation treatment, which incurs additional expenses. Low concentrations of H⁺ and OH⁻ ions in seawater reduce the efficiency of electrolysis, necessitating the use of ion-exchange membranes. However, impurity ions and microorganisms can easily clog and contaminate ion exchange membranes, resulting in a substantial rise in maintenance expenses.
Therefore, desalination/purification of seawater is required. This method, however, requires the establishment of seawater desalination plants on the coast, which greatly increases the cost in terms of construction, operation, manpower, and maintenance.
Solution
Professor Heping Xie’s team at Sichuan University and Shenzhen University has developed a technology that integrates an in-situ seawater purification process with an electrolysis system. The in-situ seawater purification method uses a hydrophobic porous polytetrafluoroethylene (PTFE)-based waterproof breathable membrane between the seawater and a self-dampening electrolyte (such as concentrated potassium hydroxide solution or sulfuric acid solution). The PTFE membrane permits the biased diffusion of water vapor but completely prevents the passage of liquid seawater and impurity ions.
Due to the difference in water vapor pressure between seawater and self-dampening electrolyte (SDE), water vapor migration from the seawater across the membrane to self-dampening electrolyte is self-driven via a liquid–gas–liquid phase transition mechanism. This unique water purification mechanism ensures 100% ion-blocking efficiency. The hydrophobic nature of the membrane provides antifouling capability, while the micrometer-scale gas diffusion path enables a high water migration rate.
The figure below depicts the electrolysis system with the in-situ seawater purification method.
The system consists of the following components:
Energy supply module: providing electric energy for electrolyzer stack;
Electrolyzer stack: consisting of a plurality of electrolyzer cells which are stacked in series or in parallel to perform electrolysis of water. The electrolyzer stack is immersed in the self-dampening electrolyte;
Top lid: containing hydrogen collection tube, oxygen collection tube, and conductive wires of the energy supply module;
Support frame: fixing the electrolyzer stack;
Mesh tank: placing the support frame. The inner wall of the mesh tank is close to the PTFE-based waterproof breathable membrane. The PTFE membrane forms a chamber for storing the electrolyte solution;
Gas storage tanks: connecting with the electrolyzer stack to collect the produced hydrogen and oxygen gas.
The figure below depicts the structure of an electrolyzer cell.
The electrolyzer cell contains an insulation slot, an anode plate, an anode catalyst layer (such as nickel iron foam, nickel molybdenum foam, or nickel-iron layered double hydroxide), a separator (such as polysulfone), a cathode catalyst layer (such as nickel-coated platinum mesh), and a cathode plate. The space between the anode plate, the anode catalyst layer, and the insulating slot forms a chamber that is filled with the self-dampening electrolyte. The space between the cathode plate and the cathode catalytic layer forms a chamber that is filled with the self-dampening electrolyte. During electrolysis, water undergoes a reduction reaction at the cathode catalytic electrode to produce hydrogen, while an oxygen evolution reaction takes place at the anode catalytic electrode. The separator or ion exchange membrane is used to transport OH⁻ or H⁺.
The key to this technology is the use of a hydrophobic porous PTFE-based waterproof breathable membrane for in-situ seawater purification process.
As shown in the figure below, the system is immersed into seawater during operation. The PTFE membrane serves as the interface between seawater and the self-dampening electrolyte. The PTFE membrane allows the biased diffusion of water vapor but fully prevents the penetration of liquid seawater and impurity ions (Cl⁻, SO₄²⁻, Mg²⁺, etc.).
The in-situ seawater purification process lies in the difference in water vapor pressure between the seawater (high) and the self-dampening electrolyte (low) across the PTFE membrane. This water vapor pressure difference drives spontaneous seawater gasification (evaporation) at the seawater side and the diffusion of water vapor through the short gas path inside the membrane to the self-dampening electrolyte side, where it is re-liquified by absorption by the self-dampening electrolyte. This self-driven phase transition migration process allows the in situ generation of pure water for electrolysis from a seawater source with 100% ion-blocking efficiency. The interface pressure difference is successfully maintained by the water in the self-dampening electrolyte consumed by simultaneous electrolysis.
Therefore, when the water migration rate equals the electrolysis rate, a new thermodynamic equilibrium is established between the seawater and the self-dampening electrolyte. A continuous and stable water migration through a ‘liquid–gas–liquid’ mechanism is realized to provide fresh water for electrolysis.
The system has a H₂ generation capability of 386 l h⁻¹. It can be stably operated at 250 mA cm⁻² for over 1,600 h (even 3,200 h) with an attractively low energy consumption of 5.0 kWh Nm⁻³ H₂, and no obvious electrocatalyst corrosion or membrane wetting was observed, as shown in the figure below.
Commercilization
On December 16, 2022, Shenzhen University, Sichuan University Dongfang Electric Co., Ltd., and Dongfang Electric (Fujian) Innovation Research Institute Co., Ltd. signed a four-party cooperation agreement entitled “pilot test and industrialization promotion of the original technology of hydrogen production by in-situ direct electrolysis without seawater desalination”.
Dongfang Electric Group invested 30 million RMB in the preliminary research and development funds for the disruptive technology of in-situ direct electrolysis hydrogen production without desalination of seawater, obtained the intellectual property rights of the original technology patent and related materials reported by Nature, and formed a four-party cooperation alliance for tackling the pilot test and demonstration of the original technology, iterative upgrading, and promoting industrialization.
