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 CO2 and water occurs at the triple-phase boundary of uncatalyzed gas-diffusion electrodes, producing H2 and CO. Both gases, but especially H2, 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 CO2 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 Pt4+ (as H2PtCl4) 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 Pt4+ using 1.0 g L-1 PVP with only H2 (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 H2SO4 are shown in Fig. 1c. All catalysts showed hydrogen adsorption-desorption peaks from -0.2 to 0.1 VAg/AgCl. The calculated electrochemical surface area (ECSA), obtained by integrating the charge in the hydrogen adsorption-desorption region, was 7.6 ± 0.7, 14.1 ± 1.2, 33.6 ± 1.2, and 30.3 ± 1.0 m2 g-1
Pt for PVP 0.0, PVP 0.01, PVP 0.1 and PVP 1.0 respectively. The MeOH electro-oxidation experiments were performed in 0.5 M H2SO4 containing 1.0 M MeOH, and the CV curves are shown in Fig. 1d. The forward anodic peak (If) at about 0.7 VAg/AgCl corresponds to MeOH oxidation, while the backward anodic peak (Ib) at about 0.5 VAg/AgCl corresponds to the oxidation of the incompletely oxidized carbonaceous species formed in the forward sweep. The mass activity (MA), defined by the forward peak current density per unit of catalyst loading, was 71 ± 2, 136 ± 7, 463 ± 28, and 341 ± 25 mA mg-1
Pt for PVP 0.0, PVP 0.01, PVP 0.1, and PVP 1.0 g L-1, respectively. Hence, the MA scale with the ECSA . Besides, ECSA (and hence MA) increases when the PVP concentration during synthesis increases. However, this trend is not followed for a PVP concentration of 1.0 g L-1. Even though all catalysts were cleaned using the same protocol, residual PVP might still be left on the surface of Pt NPs synthesized using 1.0 g L-1 PVP, lowering their electrocatalytic activity.
Overall, the GDEx process was a useful tool for synthesizing unsupported Pt NPs using low concentrations of stabilizer and with high electrocatalytic activity.
This project has received funding from the European Union's Horizon 2020 Research and Innovation program under Grant Agreements n° 730224 (PLATIRUS) and n° 958302 (PEACOC)
1. E. Antolini, J. Perez, J. Mat. Sci, 2011, 46, 4435–4457.
2. X. Dominguez-Benetton, O. Martinez-Mora, J. Fransaer, EP21165681, 2021.
3. A. Zalineeva, S. Baranton, C. Coutanceau, G. Jerkiewicz, Langmuir, 2015, 31, 1605–1609.
Figure 1
