Sonochemical Synthesis of Nanoparticles for Fuel Cells and Electrolyzers

Abstract

Green hydrogen represents an environmentally friendly alternative to fossil fuels as an energy carrier in many different energy-demanding industries. However, commercialized electrochemical conversion devices utilizing hydrogen are expensive mostly due to the use of noble metals such as platinum and iridium in the catalyst layer. To make these devices a viable alternative to fossil fuels, the catalyst performance needs to be increased while also reducing the amount of noble metals. Large scale production with a high degree of reproducibility is also desired when trying to reduce the cost. In order to achieve this, new nanoparticle synthesis methods must be developed. The current synthesis routes are typically based on batch processes which are difficult to control. They also utilize harmful chemicals which can be a hazard to the chemist and the environment alike. Replacing these with a continuous synthesis process where every parameter is easily controlled could therefore contribute to making hydrogen a viable energy carrier.

The main objective of this thesis work was to develop the sonochemical synthesis method for the production of electrocatalysts in water electrolyzers and fuel cells. The focus has been on improving the slow rate of catalyst production through systematic investigations of different ultrasound parameters like the ultrasonic frequency and radical scavenger composition. Different material systems including platinum-, copper-, and silver-nanoparticles have also been synthesized with the sonochemical method to evaluate how versatile this method is.

The rate of catalyst production has been monitored through UV-visible spectroscopy with various colorimetric techniques. Direct UV-vis measurements of the nanoparticle colloidal solution were performed to assess the development of the nanoparticle colloidal solutions. This was more relevant when monitoring silver and copper due to their pronounced localized surface plasmon resonance peaks. For platinum, the development of Pt(IV) and Pt(II) were used as a measure of the catalyst formation rate. This was achieved through colorimetric enhancements of the Pt(IV) and Pt(II) peaks through the addition of potassium iodide. Rate of radical formation is closely related to the catalyst production rate which was also monitored colorimetrically through detection of H2O2 and I3-.

The performance of the resulting catalysts has mainly been measured by their hydrogen evolution reaction activity in acidic solution. This was used to assess the activity of both Pt-nanoparticles synthesized at different ultrasonic frequencies, and sonochemically synthesized Cu@Pt-nanoparticles. Corresponding particle size measurements through electron microscopy and X-ray diffraction have also been recurring methods for evaluating the effect of changing sonochemical parameters. Our measurements revealed that the sonochemical method allows for very reproducible Pt-nanoparticle sizes and hydrogen evolution performance over a broad frequency range (20 kHz – 488 kHz), and using a wide selection of acoustic powers 11.8 W - 70.0 W.

The choice of radical scavenger and its concentration was found to be one of the most important parameters in the sonochemical method towards achieving higher reduction rates. A scavenger concentration equivalent to complete bubble coverage leads to the highest reduction rates for reactions driven by secondary radicals. These include the reduction of Pt(II) and Ag(I) to their respective metal nanoparticles. However, with appropriate choice of scavenger (ethylene glycol), pyrolytic decomposition is also able to drive the reduction of Ag(I) to Ag-nanoparticles. For reactions where pyrolytic decomposition products are also contributing (Pt(IV), Pd(II) and Au(III)), the optimal scavenger concentration is increased by an order of magnitude towards higher concentrations.

From this work we have found that the sonochemical synthesis method is very well suited for producing Pt-nanoparticles for electrocatalytic purposes. The reproducibility is good, the size range and size distribution attainable by the sonochemical method are within the optimal range for Pt-electrocatalysts, and there is a significant potential for scaling up the sonochemical synthesis method. There is some concern related to the versatility of this method as the first row transition metals are either impossible to synthesize or impractically slow. Continued pursuit of the electron transfer behaviour between secondary radicals and transition metal ions are therefore necessary in order to explain why some transition metals are reduced more easily than others with the sonochemical method. Perhaps identifying these mechanisms also allows for new ways of bypassing them. Developing larger scale reactors to study the scale-up potential of the sonochemical method is also a highly important area of research if the sonochemical method is to be applicable on an industrial scale.