Electrochemically-Assisted Synthesis of Platinum Nanoparticles Using Gas-Diffusion Electrocrystallization (GDEx) and Their Electrocatalytic Activity for Methanol Oxidation

In recent years, interest in using unsupported catalysts, especially Pt-based nanomaterials, has resurged in direct methanol fuel cells (DMFC). Unsupported catalysts eliminate the problems related to the corrosion of carbon catalyst supports, thus improving the long-term stability of DMFC. In addition, the design of catalyst synthesis protocols to tailor nanostructured materials with high surface area and catalytic activity improves catalyst performance. 1 Consequently, we recently reported a method for the synthesis of platinum group metal (PGM, i.e., Pt, Pd, Rh) nanoparticles (NPs), using a process called Gas Diffusion Electrocrystallization (GDEx) (Fig. 1a). 2 The simultaneous electrochemical reduction of CO 2 and water occurs at the triple-phase boundary of uncatalyzed gas-diffusion electrodes, producing H 2 and CO. Both gases, but especially H 2 , are reducing agents for water-soluble noble metal ions leading to the formation of small metal nanoclusters. CO can also act as a capping agent. Furthermore, the presence of CO 2 stabilizes the pH, avoiding the formation of metal (hydr)oxides. In this work, we used the GDEx process to synthesize unsupported Pt NPs using polyvinyl pyrrolidone (PVP, 55000 Mw) as a stabilizer and evaluated their electrocatalytic activity for methanol oxidation. The synthesis was performed at -30 mA cm -2 , using 3.0 mM Pt 4+ (as H 2 PtCl 4 ) as metal precursor and different concentrations of PVP (i.e., 0.0, 0.01, 0.1 and 1.0 g L -1 ). We chose low concentrations of stabilizer to facilitate its removal after synthesis, as clean surface catalysts are required for electrocatalytic applications. The size distribution of the Pt NPs, measured using Scanning Electron Microscopy (SEM), was 64 ± 22, 60 ± 22, 42 ± 18, and 38 ± 12 nm for PVP 0.0, PVP 0.01, PVP 0.1, and PVP 1.0, respectively. For comparison, the reduction of 3.0 mM Pt 4+ using 1.0 g L -1 PVP with only H 2 (either electrogenerated or bubbled) produced bigger and much more polydisperse particles (100 nm–1000 nm), highlighting the importance of the GDEx process and the presence of CO to synthesize small NPs using low concentrations of stabilizer. Furthermore, Transmission Electron Microscopy (TEM) images (Fig. 1b) revealed that the NPs are nanoclusters of single crystals of 2 nm–4 nm in diameter. The synthesized Pt NPs were cleaned using NaOH, 3 and their electrocatalytic activity was evaluated in acidic media. The CV curves of Pt NPs in 0.5 M H 2 SO 4 are shown in Fig. 1c. A