Norwegian University of Science and Technology (NTNU) researchers have found that sound can drastically improve hydrogen production through water electrolysis.
Alkaline water electrolysers (AWE) and proton exchange membrane water electrolysers (PEMWE) are currently the most commercially available and used electrolysers for producing hydrogen, as they both offer many advantages such as: well-established technologies, ease of use, compact system design, quick response, high dynamic operations, high current densities, great hydrogen production rate of acceptable purity (99.99+%) and fairly high energy efficiencies (<90%).
However, cost, efficiency, and durability need to be further improved. To add to these, they both suffer from molecular hydrogen (and oxygen) bubble accumulation and adhesion (“stickiness”) at the water electrolyser electrode surfaces, and in the electrolyte (solution) flowing in the flow channels, leading to a high ohmic voltage drop (resistance) and a large reaction overpotential in turn yielding high operational energy consumption and costs.
Professor Pollet, Mr. Islam and Dr. Emberson, researchers at the Hydrogen Energy and Sonochemistry Research Labs, at the Norwegian University of Science and Technology, however, found that sound could improve the process.
By applying ultrasound (a sound wave with a frequency above the upper audible limit of human hearing), in the range of 20 kHz – 1 MHz to water electrolysis, the NTNU researchers found that the overall efficiency for hydrogen production was greatly improved. This is due to the fact that ultrasound, in that range of frequencies, is often used as a powerful tool and a process intensification technique to: (i) clean and activate electrode surfaces, and (ii) increase mass transport in the bulk solution and near the surfaces due to high mixing of the solution (for example, ultrasonicating the water at a frequency of 20 kHz is equivalent to conventionally stirring it at 100,000 revolution per minute!).
By the help of high-speed camera (10,000 frames per second), the NTNU researchers also captured the evolution of the hydrogen gas bubbles on the electrode surface. They studied how the hydrogen gas bubbles reacted with the ultrasonic wave.
The researchers also observed that ultrasound helps to agglomerate the smaller hydrogen gas bubbles into larger ones which are removed easily from the water electrolyser electrode surface. This effect increases the production of hydrogen by continuously cleaning and removing adhered hydrogen bubbles on the electrode surface. They postulated that the main contribution of ultrasound was the highly efficient water stirring and gas bubble removal from the water electrolyser electrode surface in turn reducing significantly the bubble surface coverage, and therefore water electrolysis efficiency. This research work was recently published in Ultrasonics Sonochemistry (IF 6.513, 2020)