Retracted Article: Carbon black hybrid material furnished monodisperse platinum nanoparticles as highly efficient and reusable electrocatalysts for formic acid electro-oxidation

Abstract

Monodisperse platinum nanoparticles (Pt NPs) furnished with a Vulcan Carbon-Activated Carbon (VC-AC) hybrid have been prepared in the presence of tripropylamine (TPrA) and used as novel catalysts (Pt NPs/TPrA@VC-AC) for formic acid oxidation (FAO) reactions at room temperature. The characteristics of the novel catalysts have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), cyclic voltammetry (CV) and chronoamperometry (CA). The X-ray diffractograms of all the prepared electrocatalysts showed the typical face-centered cubic (FCC) structure. Chronoamperometry and cyclic voltammetry measurements showed a strong increase in the performance of Pt NPs/TPrA@VC, Pt NPs/TPrA@AC and Pt NPs/TPrA@VC-AC hybrid material electrocatalysts after formic acid addition, but the Pt NPs@VC-AC electrocatalyst showed the best performance compared to Pt NPs/TPrA@VC and Pt NPs/TPrA@AC.

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Introduction

Formic acid is a liquid at room temperature, which makes it more convenient and less dangerous than gaseous hydrogen when handled, stored, and transported. Direct formic acid fuel cells (DFAFCs) are considered to be one of the promising clean energy sources, with high energy conversion efficiency and low environmental pollution.^1–8 Although DFAFCs offer an exciting potential for use in future micro-electronic devices, more research is needed in developing a better formic acid electrooxidation. For this purpose, platinum has been widely used as electrocatalyst due to its highest catalytic activity among the anode metal catalysts for electro-oxidation of small organic fuels and the cathode catalysts for oxygen reduction.^9–13 However, the surface of Pt is usually heavily poisoned by the strong adsorption of CO intermediates during the oxidation of organic fuels, resulting in the lowering of catalytic performance with platinum as an anode catalyst. Also, the high cost and limited resources of this precious metal prevent the commercialization of the DFAFCs.^14,15 In order to make the most use of this precious metal and reduce the cost, Pt nanoparticles (NPs) are usually loaded on high surface area supporting materials.^16,17 By the way, it is also well known that the performance of a catalyst layer in a fuel cell is influenced by dispersivity and stability of NPs on the surface of support materials.¹⁸ Therefore, homogeneous distribution of NPs on the surface of support materials is a prerequisite to obtain high performance of catalysts.¹⁹ In this context, it is essential to mention some of the recent studies, which have been able to achieve well-dispersed nanoparticles over the supporting materials such as carbon black, carbon nanotubes, carbon nanospheres, and carbon fiber nanocomposites.^16–20 Herein, a facile synthesis of monodisperse ultrafine platinum nanoparticles supported on Vulcan XC-72 (VC), Activated Carbon (AC) and VC-AC hybrid, their excellent electrocatalytic activity toward formic acid oxidation have been reported. The synthesis relies on the usage of VC-AC hybrid material as supporting agent and dimethyl ammine-borane (DMAB) as the reducing agent. The synthesis process is schematically presented in Fig. 1.

Fig. 1 Schematic illustration of the synthesis process of Pt NPs/TPrA@VC-AC in THF.

Material and methods

Chemicals and materials

PtCl₄ (99%) was purchased from Alfa, tetrahydrofuran (THF) (99.5%), formic acid and H₂SO₄ were bought from Merck, DMAB, tripropylamine (TPrA) were purchased from Sigma-Aldrich, Vulcan and activated carbon were obtained from Cabot Europa Ltd. All chemicals were used as received. De-ionized water was purified by Millipore water purification system (18 MΩ) analytical grade. All glassware and Teflon-coated magnetic stir bars were cleaned with aqua regia, followed by washing with distilled water before drying in an oven.

Preparation of Pt NPs/TPrA

Sonochemical double reduction method using tripropylamine (TPrA) as a stabilizer ligand was applied to synthesized Pt(0)/TPrA. Briefly, firstly, 0.25 mmoles of PtCl₄ and TPrA were completely involved in anhydrous tetrahydrofuran (about 10 mL). Then, 1.0 mmole DMAB was added to diminish the amine stabilized platinum complex until a brown color was observed in the solution, which indicated the formation of platinum nanoparticle. All reactions were performed in an inert atmosphere. Lastly, after centrifugation of the Pt(0)/TPrA nanoparticles, they were washed several times with ethanol in order to remove unreacted species and dried under a vacuum at room temperature. The prepared Pt(0)/TPrA were dissolved in ethanol and mixed in a 1 : 1 ratio with Vulcan Carbon (VC), activated carbon (AC) and the mixture of VC and AC (VC-AC) as supporting materials by the help of ultrasonic tip sonicator.

Results and discussion

The monodisperse Pt NPs/TPrA@VC-AC, Pt NPs/TPrA@VC and Pt NPs/TPrA@AC have been characterized by the use of XRD, TEM, HRTEM and XPS techniques. Spectroscopies and applications of the prepared catalysts were conducted by cyclic voltammetry (CV) and chronoamperometry (CA) for formic acid oxidation (FOA) reaction. The structure and morphology of the monodisperse Pt NPs/TPrA@VC, Pt NPs/TPrA@AC and Pt NPs/TPrA@VC-AC were first characterized by TEM as shown in Fig. S1, S2† and 2, respectively. The monodisperse Pt NPs/TPrA@VC-AC (3.63 ± 0.39 nm) were uniformly distributed on the surface of VC-AC hybrid material (Fig. 2) and the average particle size of all prepared catalysts have been summarized in Table S1.†

Fig. 2 TEM image and particle size distributions for Pt NPs/TPrA@VC-AC.

The XRD patterns of all prepared catalysts were presented in Fig. 3 and it can be said that the Pt NPs/TPrA@VC-AC shows a broader Pt (220) peak relative to others, indicating that the Pt nanoparticles have smaller sizes. The diffraction peak located at 2θ of 24.8 in the XRD pattern corresponds to the (002) reflection of carbon black support. Fig. 3 also displays the bulk analysis of the XRD peaks at about 2θ = 39.80, 46.50, 67.50, 81.20 and 86.10 that are associated with the (111), (200), (220), (311) and (222) planes crystallographic plane, respectively in face-centered cubic (fcc) Pt crystallite. The 2θ value of (111) diffraction peak shows a slight negative shift relative to the bulk Pt (2θ = 40.12). This indicates the Pt–Pt interatomic distance increases in the Pt NPs.^21,22

Fig. 3 XRD patterns of the Pt NPs/TPrA@AC (a), Pt NPs/TPrA@VC (b) and Pt NPs/TPrA@VC-AC (c) catalysts.

The average crystallite particle size of all prepared catalysts were found by using the Debye–Scherrer equation²³ as shown in Table S1.†

where k = a coefficient (0.9), λ = the wavelength of X-ray used (1.54056 Å), β = the full width half-maximum of respective diffraction peak (rad), θ = the angle at the position of peak maximum (rad).

X-ray photoelectron spectroscopy (XPS) was performed to see the effect of the surface oxidation state of platinum in monodisperse Pt NPs/TPrA@VC-AC, Pt NPs/TPrA@VC, Pt NPs/TPrA@AC. For this purpose, the Pt 4f region of spectrum was analyzed in all prepared catalysts. The fittings of XPS peaks were performed by Gaussian–Lorentzian method and the relative intensities of the species were estimated by calculating the integral of each peak, after smoothing, subtraction of the Shirley-shaped background. The deconvulation of the high resolution Pt 4f region (Fig. 4) contains two pairs of doublet. The ratio of the 4f_7/2/4f_5/2 signals for all prepared catalysts was found to be 4/3, which agrees with the literature.²⁴ In the entire Pt spectra, the 4f_7/2 signal at around 71.10 eV in all prepared catalyst is assigned to zero-valent Pt.²⁵ The other doublets of Pt are occurred in even higher binding energies (∼72.3 and 74.6 eV).²⁶ They are most likely caused by very small fraction of oxidized Pt⁴⁺ species possibly due to unreduced Pt precursor or PtOx species formed during the catalyst exposure to the atmosphere (Table S2†). By integrating the relative peak areas, the percentage of Pt(0) species in the prepared nanoparticles were calculated to be about ≥80%, which suggested that a large portion of the surface atoms on Pt nanoparticles were not oxidized.^27,28

Fig. 4 XPS spectra for Pt in the Pt NPs/TPrA@AC, Pt NPs/TPrA@VC and Pt NPs/TPrA@VC-AC hybrid material.

The cyclic voltammogram (CV) measurements have been performed in the absence of formic acid as blank CVs (Fig. S3†) of the monodisperse Pt NPs/TPrA@AC (a), Pt NPs/TPrA@VC (b) and Pt NPs/TPrA@VC-AC (c) catalysts in 0.1 M H₂SO₄ solution (see the Experimental section for details) and showed that the well-defined oxygen and hydrogen adsorption/desorption regions in the electrocatalyst reflect the electrochemical surface area (ECSA) of the prepared catalysts. Generally, ECSA is the one of the most important parameters in the catalyst to specify the catalytic properties of nanomaterials for alcohol electro-oxidation, due to being surface-sensitive. The ECSA of the catalysts in m² g⁻¹ could be calculated from the following formula.^29–31

where Q is the amount of charge exchanged during the electroadsorption of hydrogen atoms on Pt and 0.21 mC cm⁻² represents the charge required to oxidize a monolayer of H₂ on platinum.

The chemical surface areas (CSA) of the prepared catalyst was calculated using XRD data as shown following equation, assuming homogenously distributed and spherical particles,³²

where d is the mean Pt crystalline size in Å (from the XRD results) and ρ is the density of Pt metal (21.4 g cm⁻²). Comparing these two areas (ECSA and CSA), it is possible to estimate the catalyst utilization efficiency (%) using,³³

Table S3† summarizes all results of the CSA, ECSA, and % Pt utility for the prepared catalyst and it can be concluded that Pt NPs/TPrA@VC-AC has higher active surface areas, and % Pt utility than the other prepared catalyst.

Fig. 5 compares the electrocatalytic performance of the Pt NPs/TPrA@VC-AC, Pt NPs/TPrA@VC and Pt NPs/TPrA@AC for FAO in 0.5 M H₂SO₄. There were two well-defined oxidation peaks which appear in the forward and reverse potential scans respectively. Formic acid oxidation during forward scan from 0.0 to 1.0 V is accompanied with the formation of oxygenated species on the surface of Pt. As can be seen in Fig. 5, in the positive scan direction (forward scan), the main oxidation peak of formic acid on the monodisperse Pt NPs/TPrA@VC-AC which is located at potential about 0.52 V with oxidation peak current density of 38.90 mA cm⁻², corresponds to the direct oxidation pathway of formic acid (HCOOH → CO₂ + 2H⁺ + 2e⁻).³⁴ The oxidation current on the monodisperse Pt NPs/TPrA@VC-AC decreases apparently at potential about 0.7 V because the Pt oxides are formed at these potentials and they are not active to formic acid oxidation. In the negative scan direction (backward scan), main oxidation peak at potential about 0.29 V due to electrooxidation of formic acid and removal of the incompletely oxidized carbonaceous species formed during the positive scan direction, which takes place on the clean and very active Pt NPs surface, were observed.^35,36 In comparison with Pt NPs/TPrA@VC and Pt NPs/TPrA@AC, the potential of the main peak for the electrooxidation of formic acid on the Pt NPs/TPrA@VC-AC electrocatalyst shifted about 150 mV in the positive direction and the peak current density was increased by 2.72 times. On the other hand, the onset potential which is related to the breaking of C–H bonds and the subsequent removal of intermediates by the oxidation with OH_ad supplied by Pt–OH sites or other sources³⁷ for formic acid electrooxidation on the Pt NPs/TPrA@VC-AC is comparable with other prepared catalysts. The higher peak current density and comparable onset potential of the formic acid electrooxidation on the Pt NPs/TPrA@VC-AC electrocatalyst as compared to the others indicate that the Pt NPs/TPrA@VC-AC is kinetically more active for formic acid electrooxidation. These results demonstrate that the VC-AC hybrid, as a supporter of Pt NPs facilitates the electron transfer kinetics for the electrode reactions. The high electronic and ionic transport capacity of the VC-AC on the electrode surface highly favors the electron transfer for the electrooxidation of formic acid. On the other hand, the enhanced performance of Pt NPs is attributed to the better mass transport inside the VC-AC hybrid, well dispersion of ultrafine Pt NPs on/in the VC-AC hybrid, higher utilization of Pt NPs and finally higher electrochemical activity of the single crystals of Pt NPs, suggesting that Pt NPs/TPrA@VC-AC is a good electrocatalyst for formic acid electrooxidation. The enhanced catalytic activity of the Pt NPs/TPrA@VC-AC for formic acid oxidation was mainly a result of the higher Pt(0) ratio, higher ECSA, % Pt utility, monodispersity of Pt NPs on VC-AC.

Fig. 5 Cyclic voltammetry curves of formic acid oxidation on monodisperse Pt NPs deposited on VC, AC, and VC-AC in 1 M HCOOH + 0.5 M H₂SO₄ solution. Scan rate 50 mV s⁻¹.

Besides, it should be noted that, GC (glassy carbon) did not shows any characteristic response for formic acid electrooxidation in the studied potential ranges. This indicates that the GC is not capable of favoring the electrooxidation of formic acid which signifies that the existence of Pt NPs/TPrA@VC-AC, Pt NPs/TPrA@VC, Pt NPs/TPrA@AC is a necessary factor for formic acid electrooxidation.

To investigate the stability of all prepared catalysts, chronoamperometric measurements were performed for formic acid oxidation as shown in Fig. 6 that shows representative CA curves obtained in 1 M formic acid + 0.5 M H₂SO₄ at a potential of 0.52 V (which is close to peak maxima seen in Fig. 6) for 1500 s. It can be seen that the oxidation currents decreased rapidly at the early stage of the chronoamperometric measurements and gradually became steady with time going on. The dramatic drop of the current at the beginning of the chronoamperometric measurements should be mainly due to two reasons. First, there would be a electric-double layer charging process at catalysts/electrolyte interface upon stepping the potential to a new value, which usually gives a rapidly decaying current with time at the beginning of potential step. Besides, a rapid accumulation of strongly adsorbed species (e.g., CO, hydrocarbons and oxygenates) should occur on electrode surface upon initializing the oxidation of organic molecules^38,39 at the beginning of the potential step, which results in rapid decrease in the numbers of the surface sites. In comparison with the other prepared catalysts, the Pt NPs/TPrA@VC-AC exhibited much lower degradation rate and maintained higher oxidation current along the entire chronoamperometric measurements. The enhanced stability of the Pt NPs/TPrA@VC-AC should be attributed to the relatively monodispersity of Pt nanoparticles on VC-AC hybrid.

Fig. 6 Chronoamperometric curves of all prepared catalysts. The curves were recorded in solutions of 0.5 M H₂SO₄ + 1 M HCOOH at 0.52 V.

Conclusions

In summary, facile, one pot, effective and practical synthetic methods of monodisperse Pt NPs/TPrA@VC-AC, Pt NPs/TPrA@VC and Pt NPs/TPrA@AC in THF were reported by dimethyl ammine-boranes as reducing agent for the electrooxidation of formic acid. Thanks to the ultrasmall sizes, monodispersity and high % Pt(0) surface, the prepared Pt NPs/TPrA@VC-AC exhibited superior electrocatalytic performance for formic acid oxidation compared to the Pt NPs/TPrA@VC and Pt NPs/TPrA@AC, therefore showing great prospect as anode electrocatalysts for direct liquid fuel cells. This facile, straightforward, and controllable method offers a new pathway for the preparation of new electrode materials (VC-AC hybrid) with high catalytical activities, which can find extensive applications in direct liquid fuel cells. The porous structures of VC-AC electrode materials provide a large surface area and distance for the electro-oxidation of formic acid.

Acknowledgements

This research was supported by Dumlupinar University Research Funding Agency (2014-05 and 2015-35). The partial supports by Science Academy and FABED are gratefully acknowledged.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00232c

‡ These author contributed equally this work.

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