Direct methanol fuel cell (DMFC) has been the focus of researchers in the past decades, due to its simple structure, high energy density and easy integration [1-2]. As a device that converts chemical energy stored in methanol to electricity, the activity and stability of the catalyst applied in DMFC directly determines the output performance of the fuel cell. Until now, the low activity and high cost of catalyst are still the biggest problems that impede DMFC’s further commercialization. Pt nanoparticles, revealing the highest activity among all catalysts, are usually dispersed on carbon materials to enhance the utilization and decrease the cost [3-4]. The structure and properties of carbon support have a significant influence on the size and distribution of Pt nanoparticles, which plays a key role in the activity and stability of the catalysts. Here in this work, holey graphene aerogel was utilized to support Pt nanoparticles as the catalyst for DMFC. The three-dimensional porous structure of holey graphene aerogel can effectively prevent the stacking due to van der Waals forces between graphene sheets. More importantly, the large number of defects on the graphene sheet induced by hydrogen peroxide etching will anchor Pt nanoparticles, leading to smaller sizes and more uniform distribution.

Graphene oxide (GO) was obtained via a modified Hummers’ method [5]. 50 ml GO aqueous solution was heated to 100 °C, after which 30% H₂O₂ solution was added [6]. The mixture was kept stirring for certain time and then cooled down. After the removal of excess H₂O₂, the etched graphene oxide was mixed with L-ascorbic acid in a glass vial and kept at 40°C for 6h. The obtained hydrogel was then dialyzed and freeze-dried to form the aerogel, on which Pt nanoparticles were later dispersed via a typical microwave-assisted process [7]. The catalysts with 0.5h and 1h etching time were recorded as 0.5h Pt / HGA and 1h Pt / HGA, while the non-etched sample was denoted as 0h Pt / GA for comparison. Transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HRTEM) were performed for surface morphology using FEI Tecnai G2F30. Cyclic voltammetry (CV) was conducted at 0.05Vs^-1 by using a three-electrode cell with a Hg/Hg₂SO₄ reference electrode in the solution of 0.5M H₂SO₄ mixed with or without 0.5M methanol, in order to estimate the electrochemical surface area (ESA) and electrocatalytic activity of the catalysts.

Fig.1 shows the TEM (a) and HRTEM (b) images of 0.5h Pt / HGA. It can be seen that Pt nanoparticles with average size smaller than 2nm were dispersed uniformly on graphene sheets. Fig.2 demonstrates CV curves of three samples in 0.5M H₂SO₄. The ESA is calculated with columbic charges accumulated during hydrogen adsorption and desorption. The values of ESAs for three samples are 55.12 m² g^-1, 88.35 m² g^-1, 69.57 m² g^-1respectively. Among them, 0.5h Pt / HGA exhibits the largest ESA, followed by 1h Pt / HGA and 0h Pt / GA. For 0.5h Pt / HGA, the etching process increases the defects on graphene sheets, which contributes to the anchoring of Pt nanoparticles. The better dispersion and smaller sizes of Pt nanoparticles lead to the increased electrochemical active sites, thus a larger ESA. For 1 h Pt /HGA, due to the longer etching time, the scale of graphene oxide layer becomes smaller, resulting in a loss of specific surface area during the gel formation and the decline of ESA. All three catalysts show much larger ESAs than the commercial Pt/C catalyst with the value of 43 m²g^-1. Fig.3 shows CV curves of three catalysts for methanol oxidation. All curves have two oxidation peaks. One located at 0.2V in the positive scanning direction is for methanol oxidation and the other located at 0V in the negative direction is for the oxidation of intermediate products. It is obvious that 0.5h Pt / HGA reveals the highest catalytic activity for methanol oxidation, followed by the 1h sample. This result is consistent with the ESA results. In addition, the onset potentials of three catalysts shifted negatively as the etching time extended, indicating that methanol oxidation is more likely to occur. In conclusion, holey graphene aerogel supported Pt nanoparticles reveals higher electrocatalytic activity for direct methanol fuel cell.

References

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