Observable Electrochemical Oxidation of Carbon Promoted by

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Observable Electrochemical Oxidation of Carbon Promoted by Platinum Nanoparticles Zongkui Kou, Kun Cheng, Hui Wu, Ronghui Sun, Beibei Guo, and Shichun Mu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11086 • Publication Date (Web): 20 Jan 2016 Downloaded from http://pubs.acs.org on January 27, 2016

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ACS Applied Materials & Interfaces

Observable Electrochemical Oxidation of Carbon Promoted by Platinum Nanoparticles Zongkui Kou, Kun Cheng, Hui Wu, Ronghui Sun, Beibei Guo, Shichun Mu* State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China. *Corresponding author. Tel: +86 27 87651837. *E-mail: [email protected](Shichun Mu)

Abstract: The radical degradation of Pt-based catalysts towards oxygen reduction reaction (ORR), predominantly caused by the oxidation of carbon supports, heavily blocks the commercialization of polymer electrolyte membrane fuel cells (PEMFCs). As reported, the electrochemical oxidation of carbon could be accelerated by Pt catalysts, however, hitherto no direct evidence is present for the promotion of Pt catalysts. Herein, a unique ultrathin carbon layer (approximately 2.9 nm in thickness) covered Pt catalyst (Pt/C-GC) is designed and synthesized by a chemical vapor deposition (CVD) method. This magnifies the catalysis effect of Pt to carbon oxidation due to the greatly increased contact sites between the metal-support, making it easy to investigate the carbon oxidation process by observing the thinning of the carbon layer on Pt nanoparticles from TEM observations. Undoubtedly, this finding can better guide the structural design of the durable metal catalysts for PEMFCs and other applications.

Keywords: electrochemical oxidation, carbon, metal catalyst, observation, stability

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■ Introduction A world-wide effort has been devoted to the commercialization of polymer electrolyte membrane fuel cells (PEMFCs)

1

because of their high energy density, low-temperature

operation, fast start-up, and the potential for automotive and portable electrical applications2-4. However, the poor stability of Pt-based catalysts stifles the growth of the PEMFC market5, 6. The low lifetime of Pt-based catalysts can be attributed to the significant loss of electrochemical active surface area (ECSA) of Pt nanoparticles (NPs) owing to the agglomeration of Pt NPs predominantly aroused by the oxidation of carbon supports7,8 especially in the cathode where it is subjected to the harsh environment, such as low pH (0.8 V at 65 ℃ and >1.0 V at room temperature. Borup et al.16 reported that, under the real PEMFC operation, the oxidation of carbon supports in the catalyst layer increases with elevated potentials and decreased humidities. Stevens et al.17,

18

developed a thermal degradation method to study the corrosion of the support in

carbon-supported Pt electrocatalysts for PEMFCs. Unlike the real PEMFC conditions as


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reported, an enhanced corrosion of carbon supports was found when increasing the humidity. The electrochemical oxidation of highly oriented pyrolytic graphite (HOPG) supported Pt NPs14,16, 19 showed that the surface oxidation of the highly resistant HOPG can be suffered at 1.0 V in acid solution14, and Pt might accelerates this process20. However, in most of the cases, the promotion of Pt to the carbon support oxidation is fairly limited or weak due to the few contact sites between the metal-support in a typical point-contact way where the sphere-like Pt NP is dispersed on carbon nanospheres, making it difficult to present or directly observe the carbon oxidation process. As results, the phenomenon of the carbon oxidation mainly arises by guesswork while not by direct observation in the previous report21-25. Herein, a novel method to track the behavior of carbon oxidation catalyzed by Pt is devised in light of a unique graphitic carbon covering layer on Pt NPs under the electrochemical conditions. The carbon layer can greatly increase the amount of metal-support contact sites, thus the radical oxidation of carbon on Pt can be observed directly by the thinning of the carbon layers presented in HR-TEM images. To our best knowledge, there are hardly any reports on a thorough study of the oxidation of catalyst supports by positively tuning the amount of contact sites between the metal-support. Undoubtly, this work is vital to understand the oxidation behavior of carbon supports as well as to provide a guideline for screening durable candidate support materials for PEMFC catalysts.

■ Experimental Methods Preparation of annealed Pt/C and Pt/C-GC. 37 wt % commercial Pt/C catalyst with a particle size of ca.2-5 nm was purchased from Johnson Matthey Fuel Cells. 50 mg Pt/C catalyst was uniformly tiled in a small porcelain boat with 5 cm in length, 2 cm in width, 1.5



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cm in height. Subsequently, the boat was put into the quartz tube with a diameter of 80mm. The Pt/C catalyst was dried at 80ºC for 2 h in the tube furnace under argon and then heated at 700ºC for 30 min. The sample was cooled to room temperature in argon and then transferred into a weighing bottle for storage under a dry environment. The graphitic carbon deposition process of the Pt/C catalyst was accomplished as follows. Firstly, 50 mg Pt/C was uniformly tiled in the above-mentioned boat, dried at 80 ºC for 2 h in the tube furnace under argon, and then heated at 700 ºC respectively for 15 s under acetylene. Elapsed the reaction time, acetylene gas flow was cut and replaced by argon for removal of the rest of reacting acetylene and other gas products during the reactor cooling to room temperature by natural convection. The product was then stored in a weighing bottle. The loading of Pt on carbon supports was 36.81, 43.12, 36.80 wt% for Pt/C, annealed Pt/C and Pt/C-GC, respectively, by an inductively coupled plasma optical emission analysis.

Preparation of the working electrode. The catalyst ink was prepared by ultrasonically dispersing 3.0 mg catalyst in the mixture of 1000 uL isopropanol, 100 uL deionized water and 20 ul dilute aqueous Nafion solution (5 wt% solution in a mixture of lower aliphatic alcohols and DuPont water), and the dispersion was then ultrasonicated for 5 min. The glassy carbon working electrode with 5 mm diameter and 0.1963 cm2 apparent area was polished with 0.05 mm alumina suspensions to a mirror finish before each experiment and served as an underlying substrate of the working electrode. A quantity of 10 uL of the dispersion was pipetted out on the top of the glassy carbon.

Electrochemical characterization. Electrochemical measurements were carried out in a


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conventional sealed three-electrode electrochemical cell at 25 ºC with the glassy carbon disk electrode made by the above mentioned procedure as the working electrode and the platinum wire as the counter electrode. The saturated calomel (SCE) was used as the reference electrode. The solution of 0.1 mol/L HClO4 was purged with ultrapure argon gas for nearly 30 min before starting the experiment. The cyclic voltammograms (CV) were recorded with CHI660E electrochemical analysis instrument controlled by a PC within a potential range from 0 to 1.2 V vs RHE at a sweep rate of 50 mV/s. In order to get rid of the possible effects caused by the Nafion film, the working electrode was treated by continuous cycling at 100 mV/s until a stable response was obtained before the measurement curves were recorded. Fresh electrolyte solution was used for each electrochemical measurement to ensure reproducible results. The accelerated durability test (ADT) is a fast way to evaluate the stability of a catalyst. In this work, the stability of the catalyst was also considered by the ADT which was conducted within the potential range of 0.3–0.9 V. The electrochemical surface area (ECSA) of Pt was calculated by measuring the charge collected in the hydrogen adsorption region (corresponding 0.04–0.27 V at the negative scanning) after double-layer correction, according to the following equation26: ESCA=

[1]

where is the charge collected in the hydrogen adsorption region, is the amount of Pt loading, and (210 µC cm-2) is the charge required for monolayer adsorption of hydrogen on Pt surface. The polarization curves of the ORR were carried out in 0.1 mol/L HClO4 solution saturated with O2 applying linear potential sweep from 0.3 to 1.2 V (vs. RHE) at a rotating rate of 1600 rpm and a scan rate of 10 mV s-1. The kinetic current can be



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calculated from the ORR polarization curve according to Koutecky–Levich equation26 as follows: × [2] |

= |

where is the kinetic current, is the diffusion-limiting current, and is the experimentally measured current (here, at potential 0.9 vs RHE.). And then, the mass activity was calculated based on the following equation: mass activity =

[3]

where is the amount of Pt loading. Meanwhile, a CV curve and an ORR polarization curve were tested after certain potential cycling numbers, and then a group of CV curves and corresponding ORR polarization curves with different potential cycling numbers were obtained.

Physical characterization. TEM and HRTEM for the catalyst samples were taken by a JEM 2100F field emission transmission electron microscope with a spatial resolution of 0.17 nm in the Materials Analysis Center of Wuhan University of Technology. X-ray photoelectron spectroscopy (XPS) analyses were carried out to determine the surface properties of the catalysts with a VG Scientific ESCALAB 210 electron spectrometer using Mg KR radiation at 14kV. Raman spectroscopy using a RENISHAW Raman microscope with excitation by a Ne-He laser was operated at wavelength 633 nm. XRD analysis of nanomaterials was carried out with the Rigaku diffractometer (Japan) using a Cu Kα X-ray source operating at 45 kV and 100 mA, scanning at a rate of 2 º/min from 10° to 90° to get the XRD pattern.

■ Results and discussion


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Synthesis and characterization of Pt/C-GC catalyst. Microstructures of the Pt/C-GC, pristine Pt/C, and annealed Pt/C catalysts were identified by TEM and HRTEM observations (Figs. 1 and 2). For the pristine Pt/C, Pt NPs with 2.7 nm in diameter are homogeneously dispersed on the carbon support (Fig. 1a), .and a disordered carbon structure in the carbon support (Fig. 1b) can be observed. For Pt/C-GC, the size of Pt NPs only grows from 2.7 to 3.5 nm when covered with the graphitic carbon by the CVD process at 700℃ for 30 min as shown in Fig. 1c. Significantly, an ultrathin graphitic carbon layer with approximately 2.9 nm thickness on Pt NPs for Pt/C-GC can be seen from Fig. 1d. However, for annealed Pt/C, without adding the carbon source, Pt NPs on carbon supports coarsen rapidly from 2.7 to 4.6 nm at 700 °C for 30 min due to the absence of the formed graphitic carbon as the block (Fig. 2a). XRD patterns (Fig. S1) show Pt NPs have higher crystallinity after heat treatments21, consistent with HRTEM observation. A peak at around 26° is attributed to the carbon (002) facet. The carbon formed by CVD has higher degree of graphitization in comparison with carbon black (XC-72). The presence of the more sharp peak corresponding to the new Pt (222) facet shows the crystallization degree of Pt NPs increases at higher annealed temperatures, in line with the previous report18. X-Ray photoelectron spectroscopy (XPS) provides additional information about the surface structure of catalysts. The Pt(4f) XPS of Pt/C, annealed Pt/C, and Pt/C-GC (Fig. S2) were deconvoluted into three components with respective binding energies of 71.5, 72.6, and 74.3 eV, respectively, as shown in Table S1. The 4f7/2 signal at 71.5 eV can be ascribed to zero-valent Pt. The signal at 72.6 and 74.3 eV is assigned to PtO and PtO2 species,



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Figure 1 TEM and HRTEM images of carbon layers on Pt NPs supported with carbon

black: (a, b) Pt/C, (c, d) Pt/C-GC and (e) the corresponding schematic of the formation process of graphitic carbon layers on Pt NPs. The corresponding size distribution of Pt NPs is placed in (a) and (c).


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Figure 2 (a) TEM and HRTEM images of annealed Pt/C; (b) At high temperature (700℃), Pt NPs grow up via aggregation. The corresponding size distribution of Pt NPs is placed in (a). respectively27. In the wake of annealing and graphitic carbon deposition, the intensity of Pt2+ and Pt4+ relative to Pt (0) falls down due to the removal of oxygen-containing group. Within the C1s peak, six features at 284.3, 284.9, 285.8, 287.1, 288.5 and 291.1 eV are discriminated, respectively (Fig. 3a-c and Table 1). Comparing the fits of the C1s spectra, a clear difference in the proportion of graphitic carbon (=sp2-carbon) to amorphous carbon and/or unsaturated bonds (=sp3-carbon) can be found28. The higher percentage of sp2-carbon for Pt/C-GC relative



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to Pt/C and annealed Pt/C can account for forming graphitic carbon on both Pt NPs and carbon supports, replenished by TEM (Fig. 1). Raman spectra (Fig. 3d) shows the carbon layer has higher crystallinity after heat treatment, consistent with the result of XPS (Table 1) and HRTEM observation (Fig. 1). The carbon formed by CVD possesses very similar structures to graphite nanocrystallines. The intensity ratio of D to G bands (ID/IG)

Figure 3 Deconvoluted C1s peaks in XPS spectra for various samples: (a) Pt/C, (b) annealed Pt/C, and (c) Pt/C-GC. Raman spectra (d): Pt/C (black), annealed Pt/C (red), and Pt/C-GC (blue). The results confirm that the carbon from splitting decomposition of C2H2 gas has a typical graphitic carbon structure. increases from Pt/C-GC (1.05), annealed Pt/C (1.06) to pristine Pt/C (1.19), demonstrating an increased graphitization degree29. The presence of 2D peak for Pt/C-GC further shows that the improved graphitization degree of the samples after annealed.


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Table 1. Results of the fits of the C(1s) spectra, values given in % of total intensity. Sample

sp2-C BE=284.3

sp3-C BE=284.9

C-OR BE=285.8

C=O BE=287.1

COOR BE=288.5

Anti πband BE=291.1

Pt/C annealed Pt/C Pt/C-GC

34.85 35.66 36.36

27.43 32.82 30.89

16.28 14.22 16.51

6.97 5.89 6.70

7.30 5.13 8.79

7.18 6.28 0.77

Oxidation behavior of carbon in the presence of Pt NPs. Typical CV and ORR curves in 0.1 mol/L HClO4 of all the samples after the accelerated durability test (ADT) from 0 to 2500 cycles at room temperature are displayed in Fig. S3. As shown in Fig. 4, the relevant electrochemical surface area (ECSA) and mass activity of both Pt/C and annealed Pt/C decrease with cycling, instead, for Pt/C-GC they increase rapidly because of the progressive exposition of Pt NPs with the thinning of the graphitic carbon layer during ADT. This can be caused by increasing contact sites between Pt NPs and carbon layers, leading to rapid oxidation of carbon in light of the catalysis of Pt NPs. With increasing cycle numbers to 1500 cycles (Fig. S4 and Fig. 4), the rate of carbon oxidation for Pt/C-GC decreases to the pristine level of Pt/C prior to the graphitic carbon deposition. The only difference is that Pt/C-GC is more stable than Pt/C and annealed Pt/C due to the presence of more durable graphitic structure on supports30-32. The oxidation of carbon supports as the major degradation mechanism for Pt catalysts33-35 involves the reaction in acidic electrolytes as follows36:

C + H# O → CO# + 4H ' + 4e , E+ = 0.207 V vs. RHE

[4]

This reaction is thermodynamically feasible at the potential where the cathode operates typically at 0.2–1.0 V vs. the reversible hydrogen electrode (RHE). For Pt/C-GC ， the oxidation details can be observed and summarized in light of HRTEM images (Fig. 5 and



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Fig. S5). From the results of the Raman spectrum, higher density of D peak for Pt/C-GC, suggested that GC layers had many defects which contributed to ion transferring to the surface of Pt NPs37. As observed in Fig. 5a, at the beginning, Pt NPs were fully covered by ca. 2.9 nm graphitic carbon (GC) layers. After electrochemical oxidation for 500 cycles, Pt NPs started to catalyze the oxidation of GC layers and then its thickness rapidly decreased to ca. 2.2 nm (Fig. 5b). When potential cycles reach 1000 cycles (Fig. 5c), the thickness is down to 1.8 nm. To the last for 2000 potential cycles (Fig. 5d), the thickness of GC layers almost drop to 0 nm as a logarithmic function (Fig. 5e):

T ≈−1.3 × 1056 + 2.9

[5]

where T represents thickness of GC layers, n is the cycle number of ADT.

In order to describe formation process of GC layers, the deposition process (three steps) of the graphitic carbon on Pt NPs by CVD is depicted in Fig. 1e: (A) A light aggregation of Pt NPs occurs due to the migration of Pt species at elevated temperature up to 700℃ (such as Ostwald ripening) under argon; (B) Simultaneously, with catalysis by the Pt catalyst, acetylene which passes into the tube furnace for 10s is cracked into carbon segments on the surface of Pt NPs, and such carbon segments as well as the subsequently formed disorder carbon can block the Pt NP from aggregating during the pyrolysis of acetylene into carbon; (C) After continuously heated for 30 min, the disordered carbon is rearranged into the graphitic carbon on Pt NPs. The oxidation process can be readily observed by HRTEM (Fig. 5), which is predominantly attributed to two factors: the supplemental catalysis of Pt NPs and the more contact sites between Pt NPs and carbon supports, which accelerate oxidation of carbon.


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Figure 4 Relationship between the ECSA (a, c), mass activity (b, d) and the cycle number during the accelerated potential cycling test. Fig. 5e displays the oxidation rate of GC layers has a linear change with potential cycles, which could be attributed to that the continuous oxidation of GC layers. This is because prior to complete oxidation of GC layers contacting with Pt NPs, the amount of contact sites between GC layers and Pt NPs could be retained. This is available to explain the thickness of



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Figure 5 HRTEM images of GC layer changes in Pt/C-GC during the accelerated potential cycling test: (a) onset, (b) 500 cycles, (c) 1000 cycles, (d) 2000 cycles. The GC layers were marked with white dash lines. (e) The mean thickness variation with the accelerated potential cycling test. (f) Schematic of the carbon oxidation on Pt during the cycling test.


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GC layers on Pt NPs becomes thinner and thinner as the linear function of the potential cycles (Fig. 5e). Furthermore, the oxidation process of GC layers with potential cycles is depicted in Fig. 5f. First, the oxidation reaction of carbon to CO2 (reaction 1) occurs at carbon atoms directly contacting with Pt NPs. Subsequently, the rest of carbon is continuously oxidized by reacting with H2O in the supplemented catalysis of Pt NPs, and then the thickness of GC layers covered on Pt NPs is constantly thinned with potential cycles. This thinned process can be clearly observed from HRTEM images shown in Fig. 5 and Fig. S5. Fig. 5 also shows no obvious change occurs for other surfaces of carbon supports without touching Pt NPs, accounting for the hyperslow oxidation of carbon in the absence of Pt NPs in the electrochemical environment.

■ Conclusions In summary, we designed and synthesized a unique Pt/C-GC nanocomposite with an ultrathin graphitic carbon layer structure on Pt nanoparticles (NPs) by means of a CVD method. The electrochemical results clearly prove the promotion of Pt to carbon oxidation by such unique strategy, which is clearly observed from TEM images. This can be attributed to the remarkably increased contact sites between the Pt NPs and the ultrathin carbon layer veiled on Pt NPs, enlarging the catalysis effect of Pt to carbon oxidation in comparison with the conventional point-contact way between the metal-support (especially for the commercial Pt/C catalyst). Thus, we can draw a very important conclusion that much more contact sites between the catalyst and carbon support lead to faster oxidation of carbon materials in terms of promotion of Pt catalysts in electrochemical environments. Therefore, under the same metal catalyst loading on the surface of supports, tuning the contact sites or contact area



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between the Pt NP and support will be considered to improve the stability of Pt catalyst. For example, the carbon support can be protected by sacrificing the ultrathin carbon layer covered on Pt NPs. This is an important strategy in obtaining the durable metal catalysts not only used in PEM fuel cells, but also in other fields.

■ ASSOCIATED CONTENT Supporting Information

Low magnified TEM images for Pt/C-GC; XRD patterns and Pt(4f) XPS survey scan & the corresponding elemental valence forms for Pt/C, Annealed Pt/C, Pt/C-GC; Cyclic voltammograms and linear sweep voltammetry under various accelerated cycles (0-4000); High magnified TEM images and the corresponding thickness of GC layers for Pt/C-GC (PDF).

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

■ Acknowledgements This work was supported by the National Natural Science Foundation of China (51372186), the National Basic Research Development Program of China (973Program, 2012CB215504), the Natural Science Foundation of Hubei Province of China (2013CFA082). The authors wish to thank Xiaoqing Liu and Tingting Luo for HR-TEM measurement support, in the Materials Analysis Center of Wuhan University of Technology.

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Observable Electrochemical Oxidation of Carbon Promoted by

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