Synthesis of PtM (M=Co, Ni)/Reduced Graphene Oxide Nanocomposites as Electrocatalysts for the Oxygen Reduction Reaction
© Li et al. 2016
Received: 17 November 2015
Accepted: 21 December 2015
Published: 5 January 2016
A series of PtM (M=Co, Ni)/reduced graphene oxide (rG-O) nanocomposites were successfully synthesized through a facile hydrothermal method. The as-synthesized nanocomposites were characterized using transmission electron microscopy and high-resolution transmission electron microscopy, X-ray diffraction, inductively coupled plasma-atomic emission spectrometer, and X-ray photoelectron spectroscopy. The electrochemical performance and oxygen reduction reaction (ORR) activity of PtM/rG-O nanocomposites were evaluated using cyclic voltammetry and the rotating disk electrode method. The results show that the addition of the reductant (1,2-hexadecanediol, HAD) in the reaction system slightly improved the ORR activity of PtM/rG-O nanocomposites with a negligible influence on the size and morphology of alloy NPs. Furthermore, PtNi/rG-O nanocomposites displayed the higher electrochemical stability than PtCo/rG-O nanocomposites. These results provide a facile strategy for the synthesis of Pt-based alloy NPs/rG-O nanocomposites for applications in catalysis and energy-related processes.
Noble metal platinum (Pt) is considered as one of the most important catalysts in modern industry (such as chemical synthesis and fuel cell) due to its rather high catalytic activity [1, 2]. Nevertheless, the extensive application of Pt catalyst is seriously limited because of its high cost and easy deactivation from the surface adsorption of poisonous intermediates or reaction products. One of the solutions to overcome these drawbacks is to explore Pt-based alloys with low-cost 3d transition metals (such as Fe, Co, Ni, and Cu) [3–6]. The presence of 3d transition metals is favorable to minimize the adsorption of poisonous intermediates or reaction products on the surface of the catalyst, resulting in the increase of the active sites for reactant molecules [7, 8].
Graphene is a promising matrix for catalysts because of its electrical and thermal conductivity, mechanical properties, and high specific surface area . Various metal oxides (such as TiO2, Fe3O4, and Co3O4) [10–12] and noble metals (such as Pt and Pd) [13–16] have been loaded on the surface of reduced graphene oxide (rG-O), which displayed the improved catalytic activity on a series of reactions, such as the ORR, oxygen evolution reaction, and degradation of organic dyes.
In this work, PtM (M=Co, Ni)/rG-O nanocomposites were synthesized through a facile hydrothermal route. The influence of the reductant (1,2-hexadecanediol, HAD) on the size and shape of PtM NPs was studied. Furthermore, the electrochemical performance and ORR activity of PtM/rG-O nanocomposites were evaluated using cyclic voltammetry (CV) and the rotating disk electrode (RDE) method.
Platinum acetylacetonate (Pt(acac)2, 97 %) was from Sigma-Aldrich Corp., St Louis, MO. Other chemicals were of analytical grade (Sinopharm Chemical Reagent Co., Ltd) and used without further purification. Deionized water (16 MΩ · cm) was obtained from a Nanopure Water Systems UV (Thomas Scientific, Swedesboro, NJ).
Synthesis of PtM/rG-O Nanocomposites
GO was prepared using natural graphite powder (Sinopharm Chemical Reagent Co., Ltd) according to the modified Hummer’s method. Prior to the synthesis of PtM/rG-O nanocomposites, the as-prepared GO was dispersed in deionized water by ultrasonication (KQ2200E system, Kunshan Ultrasonic Instruments Co., Ltd, 40 KHz, 80 W) for 3 h. PtM/rG-O nanocomposites were synthesized by the solvothermal method using ethylene glycol (EG)-water as the solvent. In a typical synthesis, Pt(acac)2 (0.25 mmol, 0.0985 g) was dissolved in EG (30 mL) under magnetic stirring with a short heating (90–100 °C, 5 min). Co(NO3)2⋅6H2O (0.25 mmol, 0.0728 g) or NiSO4⋅6H2O (0.25 mmol, 0.0657 g) was subsequently dissolved in the solution containing Pt(acac)2. In the presence of the additional reductant, 1,2-hexadecanediol (HAD, 0.5 mmol, 0.129 g) was dissolved in EG (10 mL) and then added dropwise in the EG solution containing the metal salts (20 mL). Then, 10 mL of GO aqueous solution (2 mg/mL) was added dropwise into the EG solution. After 30 min of stirring, the mixture was transferred to, and sealed in, a 50-mL Teflon-lined stainless steel autoclave and heated to 180 °C for 8 h and then cooled to room temperature. The precipitate was collected and washed alternately with ethanol and deionized water by centrifugation (10,000 rpm, 5 min) and then dried at 60 °C in vacuum.
Characterizations of PtM/rG-O Nanocomposites
The phase structure of the samples was characterized by X-ray diffraction (XRD; D/MAX-RB, RIGAKU Corp., Japan) using Cu Kα radiation (λ) 1.5406 Å. The morphology of the samples was observed by transmission electron microscopy (TEM, Tecnai G2 20, FEI Corp., the Netherlands) and high-resolution transmission electron microscopy (HRTEM, JEM-2100 F, JEOL Corp., Japan). X-ray photoelectron spectroscopy (XPS) was done on a VG Multilab 2000 (Thermo Electron Corp., MA) using Al Kα radiation as the excitation source. Raman spectra were recorded using a micro-Raman spectrometer (INVIA, RENISHAW Corp.; 785-nm excitation wavelength). The elemental composition was obtained with an Optima 4300DV inductively coupled plasma-atomic emission spectrometer (ICP-AES) (Optima 4300DV, PerkinElmer Corp.).
The electrochemical performance of PtM/rG-O nanocomposites was measured with a conventional triple electrode system. A saturated calomel electrode (SCE) was used as the reference electrode and a platinum foil as the counter electrode. The sample was mixed with 1500.0 μL of deionized water, 400.0 μL of isopropanol, and 100.0 μL of Nafion solution (0.5 wt.%) to form a 2-mg/mL suspension, then 5 μL of ink was dispersed onto a mirror-polished glassy carbon disk electrode (f = 5 mm) as the working electrode. CV curves were obtained in 0.1 M HClO4 solution at the scan rate of 50 mV/s at room temperature in a potential window of 0–1.2 V versus the reversible hydrogen electrode (RHE). The electrochemical active surface area (ECSA) was calculated based on the formula ECSA = Q H /(m Pt × q H ), where Q H is the charge for H upd adsorption, m Pt is the loading amount of metal, and q H (210 μC cm−2) is the charge required for monolayer adsorption of hydrogen on Pt surfaces . The ORR activity of different samples was evaluated by the rotating disk electrode (RDE) technique in O2-saturated 0.1 M HClO4 solution with a sweep rate of 10 mV s−1 at 1600 rpm, at room temperature.
Results and Discussion
The composition of as-synthesized PtM/rG-O nanocomposites was analyzed by ICP-AES. The results suggested an average composition of Pt67Co33 for PtCo/rG-O, Pt62Co38 for PtCo/rG-O-HAD, Pt74Ni26 for PtNi/rG-O, and Pt69Ni31 for PtNi/rG-O-HAD. These results suggested that the addition of HAD slightly decreased the size of PtM alloy NPs and improved the component of transition metals. The low ratio of Co and Ni in alloy NPs could be attributed to the weak reducing ability of the reaction system, which resulted in the partial reduction of Co2+ and Ni2+ into Co0 and Ni0 .
In summary, PtM (M=Co, Ni)/rG-O nanocomposites were successfully synthesized through a facile hydrothermal route. PtM alloy NPs were homogenously loaded on the surface of rG-O sheets. The results show that the addition of the reductant (HAD) in the reaction system slightly improved the ORR activity of PtM/rG-O, which displayed a negligible influence on the size and shape of alloy NPs. Meanwhile, PtNi/rG-O nanocomposites displayed the higher electrochemical stability than PtCo/rG-O nanocomposites. These results provide a facile strategy for the synthesis of various Pt-based alloy/rG-O nanocomposites for applications in catalysis and energy-related processes.
This work was financially supported by the Natural Science Foundation of China (nos. 30800256 and 31300791) and the basic research project of Wuhan Science and Technology Bureau (no. 2014060101010041).
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