Currently, electrolyzers used for electrolyzed water are expensive, either requiring expensive catalysts or expensive metal casings. Recently, researchers published a study in "Nature-Energy" that they made an electrolytic cell that only requires cheap materials.
According to the "Science" report, scientists knew how to split water into hydrogen and oxygen as early as 200 years ago. Because the generated gas mixture will explode, the most common setting today is to separate the anode and cathode with a thick layer of porous plastic plates. The researchers also used cheaper metal catalysts, such as nickel and iron, to accelerate the reaction.
To enable water to better conduct ions through the device, today's most common electrolyzers add high levels of potassium hydroxide (KOH) to the water. At the cathode or anode, water molecules decompose into H + and OH-, H + combines with electrons from the cathode to form hydrogen, and OH- diffuses through the membrane to the anode or cathode, where they react to form oxygen and water.
Yu Seung Kim, a chemist at Los Alamos National Laboratory in the United States, said that because KOH is highly corrosive, engineers must use expensive inert metals (such as titanium) to make electrolytic cells.
This defect prompted researchers to develop a cell technology called a proton exchange membrane (PEM) in the 1960s. In this technique, the membrane is designed to selectively allow H + to pass, and the PEM catalyst is not fixed on the electrode, but is fixed on both ends of the membrane. In this device, the catalyst on the anode side decomposes water molecules into H + and OH-, the latter immediately reacts with the catalyst to generate oxygen molecules. The catalyst on the cathode side membrane converts H + to hydrogen.
Because OH- does not migrate in the PEM, the electrolytic cell does not require highly alkaline conditions. The rate of hydrogen production in such devices is usually five times that of alkaline devices. But these membranes also have their own disadvantages: some expensive corrosion-resistant metals are still needed to withstand the acidic conditions produced by proton-conducting membranes. They also require catalysts made of platinum and iridium, which are both expensive and rare. "There are simply not enough precious metals for large-scale hydrogen production," said Xu Hui, an engineer at Giner Electrochemical Company.
Now, Kim and his colleagues in conjunction with researchers at Washington State University said they have combined the advantages of these two methods. The new device developed has created a highly alkaline environment to promote the decomposition of water, but it is through The catalyst is realized on the opposite surface of the ion conductive membrane.
Like KOH, the catalyst on the cathode side decomposes water molecules into H + and OH-, the former is converted into hydrogen, and the latter passes through an anion exchange membrane (AEM). The new device is designed to create a highly alkaline local environment that accelerates the movement of OH- toward the anode and reacts to generate oxygen under the action of the catalyst.
The alkaline conditions near the membrane of the device allow the electrolytic cell to rely on cheap and abundant nickel, iron, and molybdenum-based catalysts to decompose water. Since the alkalinity is local, the electrolytic cell can be made of stainless steel. Kim and colleagues report that this new device produces hydrogen three times faster than traditional alkaline devices, but still slower than commercial PEM electrolyzers. "Combining the old alkali technology and thin-film PEM technology is our way forward." Xu Hui said. (Xin Yu)
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