Towards scaling up the sonochemical synthesis of Pt-nanocatalysts

Abstract

Large scale production of electrocatalysts for electrochemical energy conversion devices such as proton exchange membrane fuel cells must be developed to reduce their cost. The current chemical reduction methods used for this synthesis suffer from problems with achieving similar particle properties such as particle size and catalytic activity when scaling up the volume or the precursor concentration. The continuous production of reducing agents through the sonochemical synthesis method could help maintain the reducing conditions (and also the particle properties) upon increasing the reactor volume. In this work we demonstrate that the reducing conditions of Pt-nanoparticles are indeed maintained when the reactor volume is increased from 200 mL to 800 mL. Similar particle sizes, 2.1(0.3) nm at 200 mL and 2.3(0.4) nm at 800 mL, and catalytic activities towards the oxygen reduction reaction (ORR) are maintained as well. The reducing conditions were assessed through TiOSO₄ dosimetry, sonochemiluminesence imaging, acoustic power measurements, and Pt(II) reduction rate measurements. Cyclic voltammetry, CO-stripping, hydrogen evolution measurements, ORR measurements, and electron microscopy were used to evaluate the catalytic activity and particle size. The similar particle properties displayed from the two reactor volumes suggest that the sonochemical synthesis of Pt-nanoparticles is suitable for large scale production.

Keywords: Electrocatalysts; Fuel cells; Nanoparticle synthesis; ORR; Pt; Scale-up; Sonochemistry.

Fig. 1

The sonochemical reactor used for all experiments. The ultrasonic transducer makes up the bottom of the reactor. Ports at the top allows for introduction of gas and sample extraction. Water can be circulated through the external cooling jacket, where the inlet and outlet are located at the bottom right and upper left, respectively.

Fig. 2

Rate of ·OH radical formation (a) and acoustic power (b) as a function of reactor volume. Mean values for all volumes are indicated by the dotted lines. UV–vis spectra (Fig. S3) and temperature profiles (Fig. S4) are provided in the supporting information.

Fig. 3

Sonochemiluminesence images of a 200 mL (a) and 800 mL (b) sonoreactor. The outer reactor walls and the transducer at the bottom of the reactor are outlined in red. The blue colour represents areas of sonochemical activity.

Fig. 4

Formation of Pt(II) as a function of sonication times at different reaction volumes. The initial PtCl₄ concentration was 1 mmol dm⁻³. UV–vis spectra of the Pt(IV)/Pt(II) samples from the two volumes are given in the supporting information (Fig. S6).

Fig. 5

Cyclic voltammograms of Pt/C (20%) in 0.5 mol dm⁻³ H₂SO₄ under Ar atmosphere with a scan rate of 50 mV s⁻¹ (a) and during CO-stripping with a scan rate of 10 mV s⁻¹ (b). The subsequent cycle after the CO-stripping is indicated by a dotted line.

Fig. 6

Polarization curves of Pt/C (20%) in 0.5 mol dm⁻³ H₂SO₄ under Ar atmosphere for the hydrogen evolution reaction (a), and under O₂ atmosphere for the oxygen reduction reaction (b). The currents are normalized for the electrochemical active surface area from the hydrogen underpotential deposition peaks of the respective catalysts.

Fig. 7

Secondary electron microscopy images of Pt/C sonochemically synthesized with a reactor volume of 200 mL (a) and 800 mL (b).