In the context of global efforts to combat climate change and promote energy transition, the development of clean energy is particularly urgent. Hydrogen, as a zero-carbon energy carrier, is widely used in hydrogen fuel cell vehicles and energy storage systems. However, currently only 5% of hydrogen is produced through green electrolysis processes, while over 90% relies on natural gas/biogas reforming, a process that generates significant carbon dioxide emissions and severely hinders the green transition of energy.
Electrocatalytic water splitting technology provides a possibility for green hydrogen production by converting water into hydrogen and oxygen through the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). However, in practical applications, bubble adhesion to the catalyst surface can obscure active sites and increase resistance, significantly reducing electrolysis efficiency and becoming a key bottleneck for large-scale hydrogen production.
Faced with this dilemma, researchers are actively exploring various solutions. In recent years, the application of external fields to assist electrolysis has become a research hotspot, including magnetic fields, microwave, surface plasma excitation, and electric field polarization. Although these external fields have improved electrocatalytic performance to some extent, they have not effectively solved the bubble-related issues.
Recently, Professor Wei Zongsu from Aarhus University in Denmark led a research team to publish an important research finding in Applied Catalysis B: Environment and Energy. They discovered that ultrasonic technology exhibits great potential in alkaline water splitting for hydrogen production, providing new ideas and methods to address the aforementioned problems.
This study focuses on the impact of ultrasound on nickel-based catalysts in alkaline water splitting reactions. The core strategy proposed by the authors lies in utilizing the physical and chemical effects of ultrasound to accelerate the release of bubbles from the catalyst surface while promoting the regeneration of active sites on the catalyst surface, thereby enhancing the overall electrocatalytic performance.
When studying the effect of ultrasound on bubble behavior, the authors found that ultrasound acts like an efficient "bubble remover." In the oxygen evolution reaction (OER), without ultrasound, oxygen microbubbles gradually grow on the electrode surface with long residence times, reaching diameters of up to 90 μm, severely affecting the reaction. In contrast, when ultrasound is applied, the bubbles are released rapidly, reducing the diameter to 58 μm, thereby exposing a large number of active sites. Hydrogen bubbles in the hydrogen evolution reaction (HER) exhibit similar behavior, with their residence time significantly shortened and their transport becoming smoother under ultrasonic action. Quantitative analysis of bubble diameter and residence time fully demonstrates the remarkable effect of ultrasound in promoting bubble detachment.
Figure 1 Schematic diagram showing the half-cell reaction configurations of OER and HER under (a, d) no ultrasound, (b, e) ultrasound irradiation, as well as (c, f) the evolution of bubble diameters over time on nickel foil electrodes (inset shows the contact angle between the electrode and bubbles).
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Building on the above research, the authors further explored the impact of ultrasound on the performance of electrocatalytic water splitting using electrochemical testing methods such as cyclic voltammetry (CV) and linear sweep voltammetry (LSV).
Taking the OER reaction of nickel foil catalysts as an example, under non-ultrasound conditions, after 100 cycles, the current density at 1.80 V showed a slight decrease. In contrast, under ultrasound assistance, the electrocatalytic activity of OER was significantly enhanced, with the maximum current density after 100 cycles increasing by approximately 1.43 times compared to the non-ultrasound condition. The HER reaction exhibited similar results; under non-ultrasound conditions, HER performance gradually deteriorated during the potential cycling, while ultrasound-assisted HER activity significantly improved.
Figure 2 Changes in cyclic voltammetry curves of nickel foil catalysts for oxygen evolution reaction: (a) under non-ultrasound conditions; (b) under pulsed ultrasound (866 kHz, 125 W, 50% amplitude); (c) linear sweep voltammetry curve of OER. Changes in cyclic voltammetry curves of nickel foil catalysts for hydrogen evolution reaction: (d) under non-ultrasound conditions; (e) under ultrasound conditions; (f) linear sweep voltammetry curve of HER. (g) Tafel slope graphs of OER and (h) HER. (i) Changes in electrocatalytic activity with increasing ultrasound power under ultrasound conditions.
In addition to the physical aspects, ultrasonic cavitation generates extreme conditions of high temperature and pressure locally, promoting the decomposition of water molecules to produce hydroxyl radicals (・OH) and hydrogen radicals (・H). The authors successfully confirmed the presence of hydroxyl radicals through electron paramagnetic resonance (EPR) analysis. These radicals, as reactive chemical species, can participate in electrocatalytic reactions and facilitate the process.
Figure 3 (a) Schematic diagram of chemical effects induced by ultrasound. (b) EPR spectra of DMPO-OH under ultrasound irradiation and no ultrasound irradiation. (c) Oxygen production at a current density of 10 mA/cm². (d) Histogram of enhanced oxygen evolution reaction potential in 1 M KOH with and without radical scavengers containing 1% p-benzoquinone. (e) Gas production at a current density of 10 mA/cm². (f) Histogram of enhanced hydrogen evolution reaction potential in 1 M KOH with and without radical scavengers containing 1% isopropanol (IPA). (g) Schematic diagram of the contribution of chemical effects to oxygen and hydrogen evolution reactions.
Due to the universal nature of the ultrasound action mechanism, this technology is not only applicable to nickel-based catalysts but is expected to extend to more types of catalyst systems in the future. Through preliminary explorations with different catalysts, researchers found that ultrasound can also produce similar effects in promoting bubble release and enhancing catalytic activity with other catalysts.
This research achievement opens up new directions in the field of electrocatalysis, and in the future, ultrasonic technology is expected to be widely applied in other electrocatalytic reactions, such as carbon dioxide reduction and nitrogen reduction, promoting rapid development in these fields.
