Sonochemical Catalyst Synthesis for HER and OER
Authors: Hansen Henrik Erring , Sunde Svein , Burheim Odne Stokke , Seland Frode , Pollet Bruno G. 
Published: December 2020

Improvements on catalysts for electrochemical energy conversion is highly important if technologies such as fuel cells and electrolyzers are to become economically viable in the future. These improvements typically lie in the optimization of the catalyst’s particle size and available surface area, which means that nucleation and growth conditions during the synthesis must be controlled precisely. Common ways of producing such catalysts today involve chemical reduction methods were reducing agents like NaBH4 or ethylene glycol are used to reduce metal salts to their metallic state [1]. Even though such methods are fast and scalable, few ways are available for careful control over the nucleation and growth of the nanoparticles.
To allow for better control over the nucleation and growth of nanoparticles, a sonochemical synthesis method can be employed instead [1]. In sonochemistry, high power ultrasound is applied to an aqueous metal precursor solution resulting in the formation of H-radicals and OH-radicals in the cavitation bubbles [1]. The highly reactive radicals will diffuse into the solution and reduce the metal ions to metallic nanoparticles. The nucleation and growth conditions can therefore simply be manipulated by controlling the rate of radical formation which is directly related to the ultrasound parameters. Several studies have found that the radical formation rate (or the metal reduction rate) is affected by the ultrasound frequency [2][3], ultrasound power [3], solution temperature [3], types of dissolved gasses [3], types of alcohols used [4], and types of surfactants used [5]. The ease of changing these parameters therefore ensures that nanoparticles of optimal sizes can be produced.
In this project we have started to investigate exactly how ultrasound parameters influence the radical formation rate in aqueous systems. This information can therefore be used to synthesize electrocatalysts optimized for HER and OER. Initial investigations of how Ni- and Fe -precursors are affected by ultrasound have also been performed. Sonochemical assisted synthesis of NiFe nanoparticles with NaBH4 was also assessed as a potential hybrid solution.
The results from the investigations of the ultrasound parameters suggested that the addition of ethanol effectively leads to a scavenging of the primary radicals in the solution. Consequently, more radicals become available for reducing metal ions, and the reduction rate increases. Higher ultrasonic power and lower ultrasonic frequency (580 kHz was the lowest frequency used in the experiment) increase the radical formation, which is supported by previous findings [2][3]. The effect of dissolved gasses showed that O2 saturation gave the highest radical yield. However, this can be traced to the sonolysis of O2 itself which lead to additional radical formation from the dissolved gas, and not just from the water vapor in the cavitation bubbles [3]. The ideal solution temperature was found to be dependent on the type of dissolved gas, with Ar exhibiting a higher ideal temperature than O2.
The initial investigations of Ni- and Fe-precursors showed that the sonochemical reduction was not appreciable in the short time frame which was used (30 min). Longer sonication times will be attempted in future works.
For the sonochemical assisted synthesis, NaBH4 was added as a reducing agent to aid the reduction, and an ultrasonic bath was used as the ultrasound source (42 kHz). Physical characterization of the resulting particles showed a significant decrease in the particle size due to the application of ultrasound. However, these changes were attributed to the mechanical effects most dominant at lower ultrasonic frequencies. Similar experiments will be attempted at higher frequencies to observe if the chemical effects of ultrasound act similarly.
References

Bruno G. Pollet. “The use of ultrasound for the fabrication of fuel cell materials”. International Journal of Hydrogen Energy 35.21 (2010). VIII symposium of the Mexican Hydrogen Society, pp. 11986–12004

K Okitsu et al. “Sonochemical synthesis of gold nanoparticles: effects of ultrasound frequency”. The journal of physical chemistry. B109.44 (2005), p. 20673

S Merouani et al. “Sensitivity of free radicals production in acoustically driven bubble to the ultrasonic frequency and nature of dissolved gases”. Ultrasonics – Sonochemistry 22.2 (2015), pp. 41–50

Rachel A Caruso et al. “Sonochemical formation of colloidal platinum”. Colloids and Surfaces A: Physicochemical and Engineering Aspects 169.1-3 (2000), pp. 219–225

K Okitsu et al. “Sonochemical preparation of ultrafine palladium particles”. Chemistry of Materials (USA) 8.2 (1996), pp. 315–317