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A research team in China has developed a device to split salty seawater to produce hydrogen directly. The device, a membrane-based seawater electrolyzer, helps address the side-reaction and corrosion problems of traditional methods.
The team led by Zongping Shao, a chemical engineering professor at China's Nanjing Tech University, has published their study in the journal Nature and claimed that their model "ran for over 3,200 hours under practical application conditions without failure".
Why traditional methods are not sustainable
Most hydrogen produced today is from fossil fuel sources which can significantly add to the carbon footprint. "Electrochemical saline water electrolysis using renewable energy as input is a highly desirable and sustainable method for the mass production of green hydrogen, said a release.
However, there is a problem. The properties of salt water results in the corrosion of electrodes used in various systems, often making them unviable. The use of polyanion coatings to resist corrosion by chloride ions or highly selective electrocatalysts has not helped enough for practical applications.
A desalination process can solve the issue, "but it requires additional energy input, making it economically less attractive." The size of the equipment involved in the desalination process also makes such solutions less flexible.
The new way proposed
An electrolyzer typically consists of two electrodes coated with catalysts, and a membrane separates the constituent components - hydrogen and oxygen. The formation of the highly corrosive chlorine gas in the process leads to catalysts and electrodes degrading faster. Magnesium and calcium ions in seawater can also block the membranes. These factors decrease the overall efficiency and life of such devices.
“Our strategy realizes efficient, size-flexible, and scalable direct seawater electrolysis in a way similar to freshwater splitting without a notable increase in operation cost,” Shao told IEEE Spectrum.
The team uses a concentrated potassium hydroxide electrolyte solution to dip the electrodes, and a porous membrane helps to separate the electrolyte solution from seawater. The fluorine-rich membrane blocks the liquid water but lets the water vapor pass through.
During electrolysis, water in the electrolyte solution gets spit into its constituent components. This results in a pressure variation between the electrolyte and the seawater, causing the latter to evaporate. At the same time, the water passes through the membrane into the electrolyte and turns back into liquid water, replenishing the stock for the next cycle.
"Importantly, this configuration and mechanism promise further applications in simultaneous water-based effluent treatment and resource recovery and hydrogen generation in one step."
The researchers are positive that their device, along with producing hydrogen, will also be able to recover lithium from the seawater. Further applications of the device extend to activities like cleaning industrial freshwater.
Abstract
Electrochemical saline water electrolysis using renewable energy as input is a highly desirable and sustainable method for the mass production of green hydrogen; however, its practical viability is seriously challenged by insufficient durability because of the electrode side reactions and corrosion issues arising from the complex components of seawater. Although catalyst engineering using polyanion coatings to suppress corrosion by chloride ions or creating highly selective electrocatalysts has been extensively exploited with modest success, it is still far from satisfactory for practical applications. Indirect seawater splitting by using a pre-desalination process can avoid side-reaction and corrosion problems, but it requires additional energy input, making it economically less attractive. In addition, the independent bulky desalination system makes seawater electrolysis systems less flexible in terms of size. Here we propose a direct seawater electrolysis method for hydrogen production that radically addresses the side-reaction and corrosion problems. A demonstration system was stably operated at a current density of 250 milliamperes per square centimetre for over 3,200 hours under practical application conditions without failure. This strategy realizes efficient, size-flexible and scalable direct seawater electrolysis in a way similar to freshwater splitting without a notable increase in operation cost, and has high potential for practical application. Importantly, this configuration and mechanism promises further applications in simultaneous water-based effluent treatment and resource recovery and hydrogen generation in one step.
