Characterization of Au and Bimetallic PtAu Nanoparticles on PDDA-Graphene Sheets as Electrocatalysts for Formic Acid Oxidation
© Yung et al. 2015
Received: 19 August 2015
Accepted: 6 September 2015
Published: 16 September 2015
Nanocomposite materials of the Au nanoparticles (Au/PDDA-G) and the bimetallic PtAu nanoparticles on poly-(diallyldimethylammonium chloride) (PDDA)-modified graphene sheets (PtAu/PDDA-G) were prepared with hydrothermal method at 90 °C for 24 h. The composite materials Au/PDDA-G and PtAu/PDDA-G were evaluated by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) for exploring the structural characterization for the electrochemical catalysis. According to TEM results, the diameter of Au and bimetallic PtAu nanoparticles is about 20–50 and 5–10 nm, respectively. X-ray diffraction (XRD) results indicate that both of PtAu and Au nanoparticles exhibit the crystalline plane of (111), (200), (210), and (311). Furthermore, XRD data also show the 2°–3° difference between pristine graphene sheets and the PDDA-modified graphene sheets. For the catalytic activity tests of Au/PDDA-G and PtAu/PDDA-G, the mixture of 0.5 M aqueous H2SO4 and 0.5 M aqueous formic acid was used as model to evaluate the electrochemical characterizations. The catalytic activities of the novel bimetallic PtAu/graphene electrocatalyst would be anticipated to be superior to the previous electrocatalyst of the cubic Pt/graphene.
Formic acid and methanol are potential in the conversions between chemical energy and electric energy as the chemical fuels. During earlier investigations, researchers have dismissed formic acid as a practical fuel because of the high overpotential evidenced by experiments, indicative that the reaction starting from formic acid appeared to be too difficult. The pristine Pt-based electrocatalysts have lowered the chemical barriers for conversion. Therefore, the bimetallic Pt-based nanomaterials were introduced as anode catalysts for methanol and formic acid oxidation [1–7]. Furthermore, Pt-M alloy and bimetallic core-shell nanoparticles promised for electrocatalytic reactions have been developed [5–7], which could enhance the electrocatalytic abilities by high CO-tolerance capability during the electrocatalysis. As a one-carbon-containing molecule similar to methanol, formic acid is a suitable organic molecule to be used directly in fuel cells, removing the need of complicated catalytic reforming. With a boiling point of 100.8 °C, formic acid is a liquid at standard temperature and pressure. Formic acid is accordingly much easier and much safer than hydrogen and methanol in energy storage because it does not need the high pressure or the low temperature operation. The efficiency of formic acid fuel cells can be engineered to a much higher level than that of methanol in addressing the crossover issue with the polymer membrane . An alternative pathway leading to CO intermediate for the methanol or formic acid oxidation results in the poisoning of the active Pt surfaces with covalent CO bonding. To avoid this situation, two ways of improvement are considered usually to enhance the Pt activity with catalytic performance: (1) synthesis of the Pt-M bimetallic electrocatalysts and (2) addition of the carbon materials as support [9–11].
The carbon materials (nanoscale active carbon, carbon nanotubes, carbon fibers, and graphene) are used as support to the metal nanoparticles in order to enhance the electrochemical catalytic activities [12–17]. Graphene is one of the most eye-catching materials for researchers in a variety of fields because of its high conductivity and superior mechanical/electrochemical property [18–23]. However, with different methods in synthesized graphenes, these materials are produced with different properties [24–29]. The drawbacks for graphene are its hydrophobicity and the aggregation behavior, which have limited the wet processes to a great extent. Some research groups were working on modification of the graphene surface by ionic liquids and/or polymeric materials [30, 31]. Poly(diallyldimethylammonium) chloride, abbreviated as PDDA, was introduced in this study because the functional groups and the non-covalent interactions of polymer with the graphene surface maintain the nice property of graphene and in addition enhance the electrocatalytic capability [32, 33]. Herein, we used a hydrothermal method to produce the Au nanoparticles and the bimetallic PtAu nanoparticles on PDDA-modified graphene sheets (Au/PPDA-G and PtAu/PDDA-G, respectively) and would use these novel nanocomposites to make formic acid oxidation processes.
PDDA-modified Graphene (PDDA-G)
The preparation of graphene followed the modified Hummer’s method. Into a 250-mL round-bottomed flask held in iced bath, it was introduced with concentrated sulfuric acid (25 mL) then fumed with nitric acid (10 mL in droplets) for 15 min. With iced bath, graphite powder (1 g) (Sigma-Aldrich, Ltd) was introduced into the round-bottomed flask under vigorous stirring. After being well mixed, the solution was added with potassium permanganate (22 g) for 30 min before the iced bath was removed. The solution was then stirred at room temperature for 96 h. Suitable (300 ~ 500 mL) amounts of doubly ion-exchanged (D.I.) water were added to the product mixture while kept in ice bath then the solution was centrifuged for removal of the supernatant. This procedure was performed for three times. The collected precipitates were rinsed with methanol for three times. The muddy-like solid was dried at 80 °C for 12 h to yield the graphite oxide.
In a 100-mL Teflon-lined autoclave with 65 mL D.I. water, 193.8 mg of graphite oxide and 2.54 mL of PDDA (35 wt. %) were added. Then, the autoclave was heated in an oven at 90 °C for 5 h then cooled to room temperature, before removal of the upper clear solution and the solid materials washed with D.I. water for three times. The muddy-like remains were dried at 80 °C overnight. The PDDA-G was then ready to proceed to the deposition of nanoparticles.
Nanoparticles Deposition on PDDA-G
Fifty milliliters of D.I. water, 100 mg PDDA-G, 131.1 mg H2AuCl4 (Sigma-Aldrich, Ltd), and 8 mL hydrazine monohydrate (98 %, Sigma-Aldrich, Ltd) were added and then the mixture was sonicated for 30 min before the mixture was transferred into a 100-mL Teflon-lined autoclave apparatus and heated at 90 °C for 24 h. The composite of Au nanoparticles on PDDA-modified graphene (Au/PDDA-G) as a solution was centrifuged three times with D.I. water followed with removal of supernatants. The residuals were dried under vacuum at 90 °C for 24 h and then collected.
The composite of bimetallic PtAu nanoparticles on PDDA-modified graphene (PtAu/PDDA-G) was obtained in a similar way by mixing 99 mg PDDA-modified graphene, 215.3 mg K2PtCl6 (~0.456 mmol), and 133 mg H2AuCl4 (~0.456 mmol) with 50 mL D.I. water and sonificated for 30 min, followed by addition of 10 mL hydrazine monohydrate (80 %) and sonificated further for 10 min. After that, the mixture was transferred to a 100-mL Teflon-lined autoclave apparatus and heated at 90 °C for 24 h. The collected PtAu/PDDA-G solutions were centrifuged three times with D.I. water followed with removal of supernatants. The residuals were dried under vacuum at 90 °C for 24 h and then collected. The cubic Pt/G was synthesized in the previous method .
X-ray Diffraction (XRD) Characterization
Transmission Electron Microscopy (TEM) Analysis
The TEM analysis employed a JOEL 2100 LaB6 light source, 200 keV, and equipped with EDX. The TEM samples were prepared with ethanol dispersion dropped on the grid Cu net for analysis. The selected area electron diffraction (SAED) was employed to reveal Au/PDDA-G and PtAu/PDDA-G electronic diffraction pattern and the lattice d-spacing.
Thermal Gravimetric Analysis (TGA)
TGA was examined with the Perkin Elmer Pyris 1 TGA from room temperature to 800 °C at a heating rate of 10 °C min−1 with about 5 mg of sample loading on the platinum plate.
X-ray Photoelectron Spectroscopic (XPS) Analysis
The XPS analysis was measured by Physical Electronics PHI Quantra XPS microprobe equipped with an Al Kα monochromatic X-ray source (1486.6 eV, 45° incident angle).
The electrochemical catalysts (11.3 mg Au/PDDA-G and 13.0 mg PtAu/PDDA-G, separately) were dispersed in 5 mL D.I. water with super sonification for 30 min. The electrochemical electrode was prepared by dropping 5 uL of the above dispersions on the glass-carbon electrode (0.0706 cm2) with constant-volume pipet and dried at room temperature. The procedure was repeated for three times. The electrochemical properties were recorded with an Autolab PGSTAT30 potentiostat/galvanostat. These specimens were investigated with the cycle voltammetry (CV, voltage scan range from −0.25 to 1.0 V, at a scan rate of 50 mV/s) and electrochemical impedance spectroscopy (EIS, scan frequency from 10 kHz to 100 mHz at potential 0.3 V with 5 mV amplified) .
Results and Discussion
XPS spectral fittings of Au/PDDA-G, PtAu/PDDA-G, and PDDA-G electrocatalysts
285.7 eV (%)
284.6 eV (%)
533.8 eV (%)
532.1 eV (%)
531.1 eV (%)
530.2 eV (%)
401.1 eV (%)
399.6 eV (%)
76.2 eV (%)
74.3 eV (%)
71.4 eV (%)
70.8 eV (%)
285.5 eV (%)
284.6 eV (%)
533.4 eV (%)
532.2 eV (%)
531.3 eV (%)
530.5 eV (%)
401.5 eV (%)
399.7 eV (%)
287.5 eV (%)
285.7 eV (%)
284.6 eV (%)
284.1 eV (%)
534.2 eV (%)
532.4 eV (%)
531.8 eV (%)
530.7 eV (%)
402.0 eV (%)
401.6 eV (%)
The Au/PDDA-G and PtAu/PDDA-G were studied electrochemically in aqueous solutions made up of 0.5 M H2SO4 only and made up of 0.5 M H2SO4 + 0.5 M HCOOH, for determination of electrocatalytical activities. Figure 5a exhibits the cyclic voltammetric results of Au, Au/PDDA-G, and bimetallic PtAu/PDDA-G. The PtAu/PDDA-G specimen clearly demonstrates the hydrogen redox behavior but not the Au-based nanocomposites. The Au specimen works similarly to the Au/PDDA-G specimen, with preference to oxygen reduction reaction, however, the hydrogen redox abilities do not work similarly to the Pt-based electrocatalysts.
The formic acid oxidation on Pt usually follows the so-called dual pathway .
The peaks at around 0.5 V (iPI) are attributed to the oxidation of HCOOH to CO2; the peaks at about 0.92 V (iPII) are due to the presence of CO on surface of metal catalysts (denoted as COads, as generated from the dissociative adsorption step, whose intensity indicates the amount of COads on the surface of electrocatalysts) [3–7, 24]. A high iPI/iPII ratio (5.36 vs. 0.84) together with a low onset potential (0.20 V vs. 0.25 V) for PtAu/PDDA-G vs. Au/PDDA-G, respectively, indicates that the PtAu/PDDA-G prefers the direct dehydrogenation branch in formic acid oxidation, see Fig. 5b.
We have successfully used a one-pot hydrothermal method to synthesize the Au/PDDA-G and the bimetallic PtAu/PDDA-G nanocomposites for formic acid oxidation. TEM, XRD, and XPS results illustrate that the Au (20–50 nm) and bimetallic PtAu (5–10 nm) nanoparticle could grow well on the graphene sheets. From the oxidation activity test, PtAu/PDDA-G exhibits the highest iPI/iPII ratio, represents the best capability in anti-CO poisoning, and hence displays the greatest stability in long-term operation. The novel PtAu/PDDA-G is potential to be applied in the electrocatalyst for formic acid oxidation.
This work was financially supported by the Ministry of Science and Technology of Taiwan (MOST 104-2221-E-131-010) and partially supported by the Institute of Nuclear Energy Research and Academia Sinica.
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