carbon fiber paper with efficient electrocatalytic properties

Progress in Natural Science: Materials International 27 (2017) 452–459

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Original Research

Pt nanocrystals electrodeposited on reduced graphene oxide/carbon fiber paper with efficient electrocatalytic properties ⁎

Zhiling Chena, You Zhoua, Yuxin Lia, Jianguo Liua,b, , Zhigang Zoua,

MARK

⁎

a Jiangsu Key Laboratory for Nano Technology, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, 22 Hankou Road, Nanjing 210093, China b Kunshan Sunlaite New Energy Co., Ltd., Kunshan, 1699# South Zuchongzhi Road, Suzhou 215347, China

A R T I C L E I N F O

A BS T RAC T

Keywords: Square-wave potential Reduced graphene oxide Concave nanocube Composite electrode Electrochemical performance

Carbon fiber paper (CFP) wrapped with reduced graphene oxide (rGO) film as the composite support (rGO/ CFP) of Pt catalysts was studied. It was found that rGO could affect the size and morphology of Pt nanocrystals (NCs). Concave nanocubes (CNC) Pt NCs ~ 20 nm were uniformly electrodeposited on high reduced HrGO/CFP while irregular Pt NCs ~ 62 nm were loaded on low reduced LrGO. Compared with Pt-LrGO/CFP and Pt-MrGO/ CFP, the CNC Pt-HrGO/CFP exhibited a higher electrochemically active surface area (121.7 m2 g−1), as well as enhanced electrooxidation activity of methanol (499 mA mg−1) and formic acid (950 mA mg−1). The results further demonstrated that the CNC Pt-HrGO/CFP could serve as the gas diffusion electrode in fuel cells and yielded a satisfactory performance (1855 mW mg−1). The work can provide an attractive perspective on the convenient preparation of the novel gas diffusion electrode for proton exchange membrane fuel cells.

1. Introduction Fuel cells are widely considered to be an important power source due to their high energy conversion efficiency and low pollution [1–3]. Pt-based catalysts have been widely studied, and continue to be an important aspect of fuel cells catalyst research [4–6]. However, the high cost of most widely used Pt-based catalysts limits the further development. Thus, the preparation of new types of catalysts with high catalytic activity but limited Pt loading is an utmost important issue for fuel cells. It is well-known that the properties of the Pt-based catalysts most depend on their size, shape, and chemical composition [7–9]. Recently, considerable studies have shown that Pt nanocrystals with high-index facets have a high density of step atoms, ledges, and kinks which can serve as active sites in electrochemical reactions [10,11]. Tian et al. [12] have successfully prepared tetrahexahedral Pt NCs by modified square-wave-potential method, which is the first report of the synthesis of Pt NCs enclosed with high-index facets with high catalytic activity for electrooxidation of formic acid and ethanol. Many studies have also reported that Pt NCs with high-index facets presents an extremely high catalytic activity for electrooxidation of small organic molecules, for instance, Pt (210) surface possesses high catalytic activity for electrooxidation of formic acid [11], and Pt (331) has high activity for

electrooxidation of ethylene glycol [13]. Consequently, synthesis of Pt NCs with high-index facets is a potential route for enhancing catalytic activities. To further improve the catalytic activity and stability, Pt NCs are usually dispersed on carbon materials with good conductivity and high surface area [14,15]. Reduced graphene oxide (rGO) and graphene are researched as supporting materials of Pt catalyst due to their properties such as, high surface area, removable surface functional groups and excellent conductivity [16,17], and it was found that most of the synthesized Pt catalysts supported on graphene oxide (rGO) or graphene demonstrate improving electrocatalytic performance and stability [18–21]. Liu et al. [22] prepared tetrahexahedral Pt nanocrystals which were supported on graphene have been proved to improve the catalytic performance for ethanol electrooxidation. There are considerable studies of graphene (or rGO) used as stabile carbon support for noble metal nanoparticles [23–26]. However, an attempt to obtain rGO film to be carbon support and a part of GDL has less been reported. From this respect, carbon fiber paper wrapped with rGO film (rGO/CFP) can serve as super-thin gas diffusion layers with high conductivity for proton exchange membrane fuel cells. Thus, highindex faceted Pt NCs electrodeposited on rGO/CFP can be a new approach to rapidly obtain gas diffusion electrode (GDE) for proton

Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding authors. E-mail addresses: [email protected] (J. Liu), [email protected] (Z. Zou). http://dx.doi.org/10.1016/j.pnsc.2017.08.007 Received 14 March 2017; Received in revised form 27 July 2017 Available online 08 September 2017 1002-0071/ © 2017 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

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The ICP-AES of the samples was performed with Optima 5300DV (PE), and Raman spectra were recorded with a Renishaw Micro-Raman System. Electrochemical experiments were conducted by CHI760e electrochemical analyzer (CHI Instrument, CHN). A conventional threeelectrode cell was used, including saturated calomel electrode (SCE) as reference electrode, a platinum foil as counter electrode, and rGO/ CFP support (1.5 cm2) as working electrode. Cyclic voltammetry (CV) curves measuring the ECSA were taken in N2-saturated 0.1 M HClO4 solution at a scan rate of 50 mV s−1, CV curves measuring electrochemical oxidation for methanol, formic acid and ethanol were taken in N2-saturated solution of 1 M CH2OH with 0.5 M H2SO4, 0.25 M HCOOH with 0.5 M H2SO4, 0.1 M ethanol with 0.1 M HClO4, and the scan rate were 20, 50 and 50 mV s−1, respectively. The amperometric i–t curves were both obtained in HCOOH and CH2OH solution.

exchange membrane fuel cells, especially for direct methanol fuel cells and direct formic acid fuel cells. In this present work, GO aqueous solution was directly added onto the surface of carbon fiber paper(CFP) and dried to form a film in the space between the carbon fibers, and anneal reduction method was employed to obtain rGO/CFP(gas diffusion layer); following that, Pt NCs were deposited on the rGO/CFP by square-wave potential method. To our surprise, we found that the reduction degree of rGO affected the morphology of Pt NCs in Pt electrodeposition, and Pt NCs with unique concave nanocubes (CNC) structures bounded by high index facets were obtained on high reduced HrGO. Moreover, these as-prepared CNC Pt NCs on HrGO presented improved performance for the electrooxidation of formic acid and methanol when compared to that electrodeposited on low reduced LrGO. In addition, rGO film here served as a good linker to bridge the gap between carbon paper and Pt catalyst layer, making those as-prepared Pt-rGO/CFP composite electrodes can be assembled in a single fuel cell for practical application.

2.3. Fuel cell test 2. Experimental Natural graphite powder was obtained from Alfa Aesar, H2PtCl6· 6H2O, and carbon fiber paper (CFP) were purchased from Kunshan Sunlaite. HClO4, H2SO4 (98%), HCOOH were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd, CH3OH and ethanol was purchased from Sinopharm Chemicals Reagent Co., Ltd, all of them were analysis reagent (A.R.).

0.2 mg cm−2 Pt (60% Alfa Aesar) was sprayed onto commercial gas diffusion layer (GDL) as anode electrode, the as-prepared Pt-rGO/CFP as cathode electrode, and they were hotpressed with Nafion 211 membrane. The area of the used single cell was 1 cm2. Fuel cell polarization curve test was carried out at 80 °C with a fuel cell test system (Model 850e, Scribner Associates Inc); H2 and Air flow rates were 100 and 200 sccm under fully humidified conditions.

2.1. Synthesis of graphene oxide (GO), rGO-CFP supports and Pt NCs

3. Results and discussion

GO was made by a modified Hummer method. The graphite powder(3 g) and NaNO3(3 g) were mixed with 66 mL of concentrated sulfuric acid in a 1 L beaker, and stirring in ice bath for 1 h. Then 12 g KMnO4 was gradually added, and meanwhile the temperature was kept around 17 °C. After the mixture was stirred at 46 °C for 2 h, 100 mL deionized water was added to the mixture and keep 97 °C for 1 h. Additional 30% H2O2 (12 mL) and deionized water (400 mL) were added before settled for12h. The obtained precipitation was centrifugated (4000 rpm, 10 min) to remove supernatant, and it was filtered and washed with 15% HCl aqueous solution and ethanol. The obtained solid was dried overnight at 60 °C. 100 mg GO was added into 100 mL deionized water to form 1 mg mL−1 dispersion under ultrasonication for 30 min, and stirring for 24 h to form a homogeneous yellowish brown solution. To synthesize the GO/CFP composite supports, about 1 mL solution of GO (1 mg mL−1) was dropped on the carbon fiber paper (20 cm2), and dry for 30 min in 60 °C. GO/CFP was thermal annealed 1 h at 300 °C in air, 1 h at 300 °C in H2/Ar(5% H2) and 1 h at 800 °C in H2/Ar(5% H2), respectively, and then LrGO/CFP, MrGO/CFP and HrGO/CFP supports were obtained. Pt nanocrystals were electrodeposited on the rGO/CFP support in 0.5 mM H2PtCl6 and 0.1 M H2SO4 solution by square-wave potential (SWP) method. The work electrode (1.5 cm2 rGO/CFP) was subjected to −0.70 V (vs Ag/AgCl), and maintained for 0.5 s to generate crystal nuclei. Then the growth of the nuclei to Pt nanocrystals was achieved by SWP (f = 5 Hz) for 2 min, with the lower potential is −0.1 V and upper potential is 1.4 V (vs Ag/AgCl).

3.1. SEM and TEM of the composite support and electrode The as-prepared GO/CFP and rGO/CFP supports were characterized by scanning electron microscopy (SEM). Fig. 1(a)-(d) reveals that the surface of carbon paper was substantially wrapped by GO and rGO film. As the places between the carbon fibers were almost coated, the flat rGO film with a relatively large surface area could be serviced as a perfect support for loading catalyst. Moreover, rGO/CFP loaded metal catalysts could be conveniently used as composite electrode for electrochemical characterization and single cell test. The digital camera images (Fig. 1e) show a change in the color from yellowish brown to metallic luster as the thermal reduction proceeding of GO. Those color changes indicated that the temperature and atmosphere of thermal annealing affect the degree of GO reduction. Obviously, GO was significantly reduced at 800 °C and in H2 atmosphere As shown in Fig. 2a–c, well-dispersed Pt nanocrystals (NCs) were electrodeposited directly on the rGO/CFP through square-wave potential method. It is found that rGO plays a key role in the size and morphology control of Pt NCs. The SEM images show that the Pt NCs grown on the high reduced HrGO (HrGO) were denser and much smaller than those grown on low reduced rGO (LrGO), and the corresponding TEM images (Fig. 2d–f) further demonstrate that the average apex-to-apex diameter of the Pt NCs decreased from 62 nm to 29 nm, and to 20 nm as the reduction degree of the rGO increased. In addition, when rGO was reduced significantely, four-armed star-like particles with a clear concave nanocubes (CNC) structure were obtained on HrGO (Fig. 2i), and the structural model of the CNC Pt NCs was shown in the inset of Fig. 2i. Fig. 2g shows the Pt NCs grown on LrGO/CFP were irregular flower-like particles. The size and shape changes of Pt NCs along with the reduction degree of GO are shown in Fig. 2j. Those different sizes and shapes indicate that rGO played an important role in the electrodeposition of Pt NCs when rGO can influence the nucleation and/or growth process of the nanocrystals. Fig. 2k illustrates the clear CNC structure of Pt NCs, and the measured apex angles were 54.2°, 55.8°, 60.8° and 64° for the crystals oriented along [100] zone axis, which are in agreement with {310}, {410}and

2.2. Physicochemical and electrochemical characterization The size and morphology of the prepared Pt NCs were obtained by scanning electron microscope (SEM, ULTRA55) and transmission electron microscope (TEM, JEOL JEM3010). The crystal structure was characterized by X-ray diffraction (Rigaku Ultima III)，X-ray photoelectron spectroscopy (XPS) measurements were performed with Thermo Fisher Scientific K-Alpha X-ray Photoelectron Spectrometer.

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Fig. 1. SEM images of as-prepared (a) GO/CFP (b) LrGO/CFP (c) MrGO/CFP (d) HrGO/CFP. (e) Typical optical images of carbon fiber paper wrapped by GO film and rGO film.

by which GO was significantly reduced. It was also found that the peak intensity increment of the sp2 bonded carbon network (C˭C) indicates the increasing graphene structure degree through the high temperature annealed treatment in H2. Compared O1s peaks of Pt-LrGO/CFP with Pt-HrGO/CFP, there is a positive shift in Pt-HrGO, a possible reason to cause such shift may be due to the different degree of electron donation from oxygen of oxygenated group to d-orbital of Pt atom [31]. In this case, the interaction between Pt and rGO might affect nucleation as well as growth process of Pt nanocrystals in SWP method.

{720} facets respectively [27,28]. According to the above results, the square-wave potential method can deposit uniform size distribution and high-index faceted CNC structure Pt NCs on HrGO, which may be attributed to the chemical bonding of oxygen groups on the rGO surface determined by the degree of reduction. 3.2. XPS characterization of Pt-rGO/CFP XPS was employed to confirm the reduction degree of GO, and the results is shown in Fig. 3. Fig. 3a–d show that the oxygen content in rGO/CFP decreased as the annealed reduction degree was increased, i.e., 28.9% in GO, 18.7% in LrGO, 12.2% in MrGO and 6.1% in HrGO. Fig. 3a shows the C1s peaks of GO, which consists of five different functional groups of the carbon atoms: C˭C (284.5 eV), C-H (285.0 eV), C-OH, C-O-C (286.5 eV), C˭O (288.0 eV) and O-C˭O (289.0 eV) [29,30]. As can be seen from C1s peaks of HrGO (Fig. 3d), the peak of C–O/C–O–C almost disappeared, indicating that considerable oxygenated groups were removed at 800 °C and in H2 atmosphere,

3.3. Raman and XRD characterization of Pt-rGO/CFP The structural changes of GO and rGOs are reflected in their Raman spectra. As shown in Fig. 4a, there are two typical features, a G band at 1585 cm−1 which is associated with doubly degenerated E2g mode [30], and a D band at 1346 cm−1 which is associated with the disorder induced in the nanomaterial [30]. The ID/IG ratio is generally used to 454

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Fig. 2. SEM images of as-prepared (a)Pt-LrGO/CFP (b) Pt-MrGO/CFP (c) Pt-HrGO/CFP, (d)-(f) corresponding TEM (and size-distribution histogram), (g)-(i) high magnification TEM (structural model of CNC Pt inset i), (j)illustration of the size and shape changes of Pt NCs, (k) HRTEM image of CNC Pt –HrGO/CFP.

ID/IG increased from 0.918 of GO to 0.956 of LrGO, this may be resulted from the conjugated graphene network (sp2 carbon) reestablishment cause by the removal of some oxygenated groups, and the size of the reestablished graphene network becomes smaller than the original, which consequently leads to the increasing intensity ratio of ID/IG [18]. Fig. 4b shows the X-ray diffraction (XRD) of the Pt–rGO/

evaluate the degree of graphitization of the carbon materials. From Raman spectra, it can be found that the ID/IG of HrGO is 0.917 which is lower than LrGO and MrGO, testifying that HrGO obtained a more ordered graphitic structure. The results prove that the high temperature annealed treatment in H2 atmosphere increasing the graphene structure degree in agreement with XPS results. It's also found that the 455

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Fig. 3. XPS C1s spectra obtained for (a) GO/CFP, (b) LrGO/CFP (c) MrGO/CFP (d) HrGO/CFP, XPS spectrum in O1s region (e) and Pt 4f region (f).

Fig. 4. (a) Raman spectra recorded with A) GO/CFP, B) LrGO/CFP C) MrGO/CFP D) HrGO/CFP. (b) XRD spectra of the as-prepared Pt-rGO/CFP.

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Table 1 Pt deposited loading, ECSA, peak current densities of formic acid and methanol oxidation and power density of composite electrodes. Electrodes

Pt loading ug cm−2

ECSA m2 g−1

Peak current densities of formic acid oxidation * mA mg−1

Peak current densities of methanol oxidation * mA mg−1

Power density mW mg−1

Pt-LrGO/CFP Pt-MrGO/CFP Pt-HrGO/CFP

6.5 5.0 6.0

35.9 53.5 121.7

460 680 950

131 312 499

907 1402 1855

*

the value of peak current densities at high overpotential.

poisonous species COads to form CO2 [38]. The second peak current densities of Pt-HrGO/CFP, Pt-MrGO/CFP and Pt-LrGO/CFP were 950 mA mg−1, 680 mA mg−1 and 460 mA mg−1, respectively. Fig. 6c shows the CVs of methanol oxidation in 0.5 M H2SO4 and 1 M CH2OH solution at a scan rate of 20 mV s−1. The peak in the forward sweep at 0.67 V corresponds to methanol oxidation, while the other peak in the reverse sweep at 0.49 V is due to the oxidation of intermediates formed in the back sweep [33]. The peak current densities at 0.49 V (0.67 V) of methanol oxidation on Pt-HrGO/CFP, Pt-MrGO/CFP and Pt-LrGO/CFP were 354 (499), 199 (312) and 66 (131) mA mg−1. That is, the methanol oxidation current densities of low overpotential peak and high overpotential peak on Pt-HrGO/CFP were 5.4 and 3.8 times that on Pt-LrGO/ CFP, respectively. The ethanol oxidation in 0.1 M HClO4 + 0.1 M ethanol on Pt-HrGO/CFP electrode (Fig. 5e) also shows superior activity than those on Pt-LrGO/CFP and Pt-MrGO/CFP electrode. Obviously, the electrocatalytic activities of CNC Pt-HrGO/CFP composite electrode were much higher than those of Pt-LrGO/CFP. In addition, to evaluate the long-term performance of the as-prepared composite electrodes for formic acid and methanol oxidation, the electrodes are demonstrated at 0.65 V for 1000 s, the i – t curves (Figs. 5b and 5d) present that the Pt-HrGO/ CFP electrode have higher initial and steady state current densities in 1000 s. Those above results demonstrated that the Pt-HrGO/CFP which has smaller and clear concave nanocubes structure presents superior electrocatalytic activity and stability to formic acid and methanol electrooxidation over its counterpart. The as-prepared Pt-rGO/CFP composite electrodes were also tested in a single cell as the cathode electrodes and commercial GDL with 0.2 mg cm−2 Pt catalyst as anode electrode. Fig. 5f demonstrates the polarization curves and power density of PEMFC cells. The maximum power densities (shown in Table 1) of Pt-HrGO/CFP, Pt-MrGO/CFP and Pt-LrGO/CFP were 1855, 1402 and 907 mW mg−1. Based on this result, it is anticipated that the convenient preparation of Pt-rGO/CFP with the large flat surface area would serve as the GDE electrode for proton exchange membrane fuel cells.

Fig. 5. Cyclic voltammograms of the Pt-rGO/CFP composite electrodes in 0.1 M HClO4 solution at a scan rate of 50 mV s−1.

CFP, where the peaks at around 39.9°, 46.6°, 67.7°, 81.7°，86.1° are attributed to the diffraction peaks of crystal faces Pt (111), (200), (220), (311), (222)，respectively. As we can see, the as-prepared Pt NCs on rGO/CFP all yield the highest (111) and (200) peaks intensity, indicating that these Pt NCs have same preferential growth orientation. The carbon fiber paper, rGO film and the low loading of Pt possibly result in the high diffraction peaks of C and inconspicuous diffraction peaks of Pt.

3.4. Electrocatalytic activity and fuel cell performance of the composite electrodes Fig. 5 shows the cyclic voltammetry (CV) curves of the Pt-rGO/CFP composite electrodes in 0.1 M HClO4 solution at a scan rate of 50 mV s−1. The Pt loading on rGO/CFP was investigated by ICP-AES. 10 mg of as-prepared Pt-rGO/CFP was dissolved in 1.25 mL aqua regia, and deionized water was added to obtain a 25 mL solution for ICP-AES test. The results of ICP-AES analysis shows that the amounts of Pt loaded on LrGO/CFP, MrGO/CFP and HrGO/CFP were 6.5 µg cm−2, 5.0 µg cm−2 and 6.0 µg cm−2, respectively. The electrochemically active surface areas (ECSAs) were derived by the calculation of the hydrogen desorption area according to QH equation [32], where 0.21M M represents the platinum loading (g cm−2) on the working electrode, QH represents the charge for hydrogen desorption (mC cm−2), and 0.21 (mC cm−2) represents the maximum surface charge transferred to Pt when adsorption of a monolayer of H. As a result, the ECSAs for PtLrGO/CFP, Pt-MrGO/CFP and Pt-HrGO/CFP were 35.9, 53.5 and121.7 m2 g−1, respectively. Therefore, the as-prepared Pt-HrGO/ CFP electrode has a higher surface area of Pt active sites available for electrochemical reactions over its counterpart. The peak occurs only in Pt-LrGO/CFP around 0.7 V is hydraquinone-quinone peak, which reveals that the high temperature treatment in H2 allows the removal of the most oxygenated groups, which is in agreement with XPS results. The electrocatalytic activity of the as-prepared Pt-rGO/CFP composite electrodes for formic acid, methanol and ethanol electrooxidation was examined. The peak current densities of formic acid and methanol oxidation were summarized in Table 1. Fig. 6a shows the cyclic voltammograms (CVs) for the electrooxidation of formic acid in a solution of 0.5 M H2SO4 with 0.25 M HCOOH at a scan rate of 50 mV s−1. For the formic acid electrooxidation on Pt-based catalysts in the positive scan, the first peak at around 0.38 V is usually the direct oxidation, while the second peak at about 0.69 V may correspond to the oxidation of the

4. Conclusions In summary, this study provides new insight into the synthesis of Pt NCs with concave nanocubes structure electrodeposited on rGO/CFP. It is found that the deposited Pt NCs are smaller and more regular with the increase of rGO reduction degree, which may be attributed to the interaction between Pt and oxygen groups on rGO surface. The obtained CNC Pt-HrGO/CFP composite electrode presents a high catalytic performance and stability for the oxidation of formic acid and methanol. This strategy of Pt NCs deposited on rGO/CFP is also valid in proton exchange membrane fuel cell application, and the maximum power density of Pt-HrGO/CFP is 1855 mW mg−1. This research would likely to raise the possibility of using Pt-rGO/CFP as novel gas diffusion electrode in proton exchange membrane fuel cells.

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Fig. 6. (a) Cyclic voltammograms (CVs) and (b) i – t curves (at 0.65 V vs SCE) of as-prepared Pt-rGO/CFP in 0.5 M H2SO4 + 0.25 M HCOOH at a scan rate of 50 mV s−1. (c) CVs and (d) i – t curves (at 0.65 V vs SCE) in 0.5 M H2SO4 + 1 M CH2OH at a scan rate of 20 mV s−1. (e) CVs in 0.1 M HClO4 + 0.1 M ethanol at a scan rate of 50 mV s−1. (f) Polarization curves and power density plots for H2/Air PEMFCs with Pt-rGO/CFP as cathode electrode at 80 °C.

References

Acknowledgement The authors gratefully acknowledge financial support from National Key R & D Plan of China (2016YFB0101308), Joint Funds of the National Natural Science Foundation and Liaoning of China (U1508202), and Natural Science Foundation for Distinguished Young Scholars of Jiangsu Province (BK20150009). Jianguo Liu also thanks the support of PAPD of Jiangsu Higher Education Institutions, QingLan Project of Jiangsu Province, “Six Talent Peaks Program” of Jiangsu Province, and Fundamental Research Funds for the Central Universities, China.

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