High performance of carbon nanowall supported Pt catalyst for methanol electro-oxidation

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High performance of carbon nanowall supported Pt catalyst for methanol electro-oxidation Chengxu Zhang a,b, Jue Hu a,*, Xiangke Wang a, Xiaodong Zhang a, Hirotaka Toyoda c, Masaaki Nagatsu d, Yuedong Meng a a

Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, 230031, Hefei, PR China Department of Modern Physics, University of Science and Technology of China, 230026, Hefei, PR China c Department of Electrical Engineering and Computer Science, Nagoya University, Nagoya, Japan d Nanovision Science Section, Graduate School of Science and Technology, Shizuoka University, Hamamatsu, Japan b

A R T I C L E I N F O

A B S T R A C T

Article history:

The performance of Pt catalyst supported on carbon nanowall (CNW) and vertically aligned

Received 3 December 2011

carbon nanofiber (VACNF) for methanol electro-oxidation has been compared. Pt/CNW and

Accepted 29 March 2012

Pt/VACNF electrodes were fabricated by growing CNW and VACNF on carbon papers with

Available online 5 April 2012

inductively coupled plasma enhanced chemical vapor deposition, followed by sputter deposition of Pt nanoparticles using a radio-frequency magnetron sputtering system. Scanning electron microscopy and transmission electron microscopy results show that the Pt nanoparticles are homogeneously dispersed on the surface of CNW and VACNF. The histograms of Pt nanoparticle diameter for both electrodes reveal that the Pt/CNW electrode shows a broader particle size distribution. Cyclic voltammetric measurements show that the Pt/CNW electrode has a better electrochemical activity and methanol oxidation property than Pt/VACNF electrode. The unique structure of CNW ensures that Pt/CNW electrode has a faster electron transport rate and shorter electron transport path, which lead to an obvious improvement of electro-catalysis activity compared to the Pt/VACNF electrode and show a further potential application in direct alcohol fuel cells.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

The direct alcohol fuel cell (DAFC) is an attractive power source which has the advantages of high energy density, liquid fuel and non-polluting [1–4]. Despite these advantages, some barriers for its commercial are still existing, such as high cost and slow kinetics of the Pt based catalysts, insufficient durability of the carbon supports [2]. The catalytic activity of Pt based catalysts depends on many factors, among which the carbon-supported materials play an important role [5,6]. The carbon supports should provide a high surface area, good electrical conductivity and high electrochemical stability under fuel cell operating conditions [5,6]. Presently, carbon

black (CB) is the most widely used supporting material of Pt based catalysts both in the anode and cathode of DAFC. Generally, the high specific surface area of CB mostly contributes to the micropores less than 1 nm which are difficult to be fully accessible [5]. Moreover, there are two main reasons which leading to a low Pt utilization of CB supported catalysts: (i) some catalyst particles get trapped in the micropores of CB and become inaccessible [6], (ii) the dense structure of CB results in significant mass transfer limitations [7]. Meanwhile, CB is known to be a serious electrochemical oxidation which not only poisons the catalyst but also makes the catalyst nanoparticles detached from the electrode and aggregated to larger particles [7,8].

* Corresponding author: Fax: +86 551 5591310. E-mail address: [email protected] (J. Hu). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.03.047

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In recent years, carbon nanotubes (CNTs) as promising support materials for DAFC catalysts are attracting great interest for their remarkable properties such as high electrical conductivity, high surface area and splendid chemical stability [9–12]. However, it is difficult to prepare uniform catalyst nanoparticles onto the inertness surface of CNTs, which graphene sheets rolled over on themselves to form tubular structure [13]. Carbon nanofibers (CNFs) have a cylindrical nanostructure with extremely small graphene sheets stacked in various directions with respect to the fiber axis [14]. This distinguishing characteristic of CNFs from CNTs makes the surface of CNFs mainly present as graphite edges. Previous studies have shown that the edge plane sites played extremely important roles in the electron-transfer rate and catalytic activity [15]. Meanwhile, the presence of more edge plane defects of CNFs makes positive contribution to the loading of catalyst nanoparticles onto CNFs with both high dispersion and uniform particle size [13,16–18]. The longterm durability of DAFC catalysts is another important issue. According to our previous studies [18], there is a obvious aggregation of Pt nanoparticles after long term electrochemical measurements on the surface of CNFs which leads to a serious loss of electrochemical activity. This result should be attributed to the weak interaction between Pt nanoparticles and CNFs. Carbon nanowalls (CNWs), two dimensional carbon nanomaterials comprising plane graphene layers mounted vertically on a substrate, have been the focus of interest due to their large surface to volume ratio and good electrical conductivity [19–21]. The detailed micro-structure of CNWs directly grown on carbon paper (CP) has been reported by Lisi et al. via transmission electron microscopy (TEM) observation [22]. Combined with Raman spectroscopy, the CNWs were found to be consisted mainly of stacked ‘‘graphenes’’ oriented along the ‘‘wall’’ direction. The number of graphene layer is about 1–2 at the tip up to several 10 s at the base and there are many defects on the surface of a single nanowall. Because of these unique characteristics, CNWs have many potential applications, such as electron emitter and ideal catalyst support. Since these two new kinds of carbon supports (CNFs and CNWs) have different micro-structure, it is very important to investigate the effects of carbon support structure on the electrode electrochemical properties by a comparison of Pt/ VACNF and Pt/CNW electrodes. In this work, we directly grow vertically aligned carbon nanofiber (VACNF) and CNW on CP. Then Pt nanoparticles of a loading about 0.025 mg cm2 were deposited onto these two kinds of carbon nanomaterials to form Pt/VACNF and Pt/CNW electrodes. The electrochemical activities and methanol oxidation performances of both electrodes are studied and the structure characters which maybe influence the electrochemical properties of Pt/VACNF and Pt/CNW electrodes are also discussed.

2.

Experimental

2.1.

Synthesis of Pt/VACNF and Pt/CNW electrodes

VACNFs and CNWs were directly grown on CP (HCP020P) by inductively coupled plasma enhanced vapor deposition

(ICP-PECVD). Before the grown of VACNFs and CNWs, Fe films with a thickness of 5 nm were deposited onto the CP. Pt nanoparticles about a loading of 0.025 mg cm2 were deposited on the surface of VACNFs and CNWs. Both the Fe films and the Pt nanoparticles were deposited by a radio-frequency magnetron sputtering system (RF-MS). The experimental details of ICP-PECVD and RF-MS systems can be found in our previous study [18]. The VACNFs synthesis parameters were fixed: 200 W for the input power, 20 Pa for the reactor total pressure, hydrogen and methane in the partial pressure ratio of 4:1, 50 V for the substrate bias voltage, 400 C for the substrate temperature, 40 min for the reaction time and at least 60 min in argon atmosphere for cooling down. CNWs were first time prepared by the home-made ICP-PECVD apparatus under the VACNFs grown condition nothing but the substrate was grounded.

2.2.

Characterization

The morphology of the VACNFs and CNWs was characterized by scanning electron microscopy (SEM) (Sirion 200, FEI, USA) at the operation voltage of 5.0 kV and tilt angle of 45. The morphology of the Pt/VACNFs and Pt/CNWs was characterized by TEM (JEOL 2010, Japan) at the operation voltage of 200 kV. For TEM, the Pt/VACNFs and Pt/CNWs were scratched off the CP by a blade and dispersed in ethanol under sonication. Then the samples were dropped on the copper grid and imaged. The crystalline structure of Pt nanoparticles was identified by X-ray diffraction (XRD) ˚) (X’Pert Pro. Philips, Holland) using Cu Ka (kKa1 = 1.5418 A as the radiation source. The electrochemical measurements were carried out in a three-electrode test cell using an Autolab potentiosat/galvanostat (IM6e, Zahner, German). A saturated calomel electrode (SCE) was used as the reference electrode and a platinum foil was served as the counter electrode. To eliminate dissolved oxygen, all the solutions used in the electrochemical measurements were purged with high-purity nitrogen (99.999%) for 60 min, and the nitrogen atmosphere was kept by nitrogen bubbles in the solutions throughout the experiments. Before electrochemical testing, the Pt/VACNFs and Pt/CNWs were washed in 6 M hydrochloric acid for more than 24 h to remove the iron catalyst particles [23]. Then, both electrodes were washed by deionized water to remove any trapped hydrochloric acid and dried in a vacuum oven at 80 C for 2 h. The electrodes were carried out electrochemical measurement without casting any Nafion solution. For comparison, a conventional electrode made with commercially available 40 wt.% Pt/C (HISPECTM 4000, JM) with the Pt loading of 0.025 mg cm2 was also evaluated [18]. The JM, Pt/VACNF and Pt/CNW electrodes were cut into 2.0 cm · 0.5 cm and the electrode exposing area for electrochemical measurement was fixed to 0.2 cm2 (0.4 cm · 0.5 cm). For cyclic voltammetry (CV), the potential was cycled at 0.05 V s1 and all experiments were carried out at 20 C. Several activation scans (at least ten) were performed until reproducible voltammograms were obtained, and all the voltammograms showed here were the last circles.

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Results and discussion

3.1. Morphology and crystalline structure of Pt/VACNF and Pt/CNW electrode SEM images of VACNFs and CNWs are shown in Fig. 1. As shown in Fig. 1a, the VACNFs are fairly straight and vertically aligned on the CP. The inset figure of Fig. 1a is the side view of VACNFs directly grown on the CP. The height of the VACNFs is about 1–2 lm, and the diameter is about 20–100 nm. Fig. 1b shows the CNWs directly grown on the smooth area of the CP, and the inset image of Fig. 1b is the enlarged SEM image of CNWs. Compared to the one dimensional carbon nanomaterials such as CNTs and CNFs, the CNWs show a typically two-dimensional wall-like structure. The width of CNWs is about 1–2 lm and the height is less than that of VACNFs. TEM images of Pt/VACNFs and Pt/CNWs are shown in Fig. 2. According to our previous TEM results, the VACNF directly grown on CP show a fishbone (or herribone) structure [18]. The image shown in Fig. 2a is a part of Pt/VACNF, and the Fig. 2b is a part of Pt/CNW. The Pt nanoparticles both have a high and homogeneous dispersion on the surface of CNFs and CNWs. The histograms of Pt nanoparticles diameter for Pt/VACNFs and Pt/CNWs are shown in Fig. 3a and b, respectively. The mean diameters of Pt nanoparticles are about 4.0 nm both for Pt/VACNFs and Pt/CNWs. The similar Pt particle size on CNWs and VACNFs suggests that though the height of CNWs is quite small, the actual surface area for Pt deposition is close to VACNFs.

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XRD patterns for Pt/VACNF and Pt/CNW electrodes were shown in Fig. 4. The diffraction peaks at 26.5, 42.3, 44.5, 54.6, 59.8, 77.4 and 83.5 observed in the diffraction patterns of Pt/VACNF and Pt/CNW electrodes can be attributed to the graphite structures (0 0 2), (1 0 0), (1 0 1), (0 0 4), (1 0 3), (1 1 0), and (1 1 2), respectively. The Pt nanoparticles can be demonstrated by the diffraction peaks at 39.6, 46.4, 67.8 and 81.5, which can be assigned to Pt(1 1 1), Pt(2 0 0), Pt(2 2 0) and Pt(3 1 1), respectively. All the four peaks were similar to the results obtained by Xu and Lin [24] for Pt nanoparticles that crystallize in face centered cubic structure. The average size of Pt nanoparticles can be calculated from Pt(2 2 0) peak according to Scherrers’s formula [25]: d¼

0:9kka1 B2h cos hmax

ð1Þ

where d is the average size of the Pt nanoparticles, kKa1 is the X-ray wavelength, hmax is the maximum angle of the (2 2 0) peak, and B2h is the half-peak width for Pt(2 2 0) in radians. The mean particle sizes are approximately 3.9 nm for Pt/VACNFs and 3.8 nm for Pt/CNWs which are consistent with typical TEM results.

3.2. Electrochemical characterization of Pt/VACNF and Pt/ CNW electrodes The electrochemical properties of Pt/VACNFs and Pt/CNWs were evaluated using CV. Fig. 5 shows cyclic voltammgrams of JM, Pt/VACNF and Pt/ CNW electrodes recorded between 0.25 and 1.0 V vs. SCE in nitrogen saturated 1 M H2SO4.

Fig. 1 – Scanning electron micrographs of (a) VACNFs and (b) CNWs directly grown on CP.

Fig. 2 – Transmission electron micrographs of (a) Pt/VACNF and (b) Pt/CNW electrodes.

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Fig. 3 – Histograms of Pt nanoparticle diameters for (a) Pt/VACNF and (b) Pt/CNW electrodes.

Fig. 5 – Cyclic voltammograms of JM, Pt/VACNF and Pt/CNW electrodes in N2-saturated 1 M H2SO4 at a scan rate of 0.05 V s1. Current densities are normalized with respect to the Pt loading.

adsorption–desorption of Pt, which consist of a sharp peaks and a broad peaks in the potential region of 0.25 and 0.1 V. The Pt/VACNFs exhibit a sharper hydrogen adsorption peak than Pt/CNWs between 0.25 and 0.15 V, which maybe attribute to the narrow particle size distribution of Pt/VACNFs [26], as shown in Fig. 3.Pt utilization efficiency (g) is an essential parameter for evaluating the electrochemical activity of catalysts and can be obtained by Eq. (2) [26]: g¼ Fig. 4 – (a) X-ray diffraction patterns of Pt/VACNF and Pt/ CNW electrodes, (b) detailed Pt(2 2 0) peaks in the XRD patterns of Pt/VACNF and Pt/CNW electrodes.

The current density was normalized based on the geometric electrode area. All catalysts show typical hydrogen

SECSA SCSA

ð2Þ

where SECSA (cm2 mg1) is the electrochemical surface area (ECSA) of Pt catalyst, SCSA is the chemical surface area (SCA, cm2 mg1) which can be calculated from the mean particle diameter which obtained from XRD by using Eq. (3): SCSA ¼

6 qd

ð3Þ

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Table 1 – Comparison of morphological data and ECSA of JM, Pt/VACNF and Pt/CNW electrodes. Catalyst

JM 40% Pt/C Pt/VACNFs Pt/CNWs

Pt loading (mg cm2 Pt) 0.025 0.025 0.025

Pt particle size (nm) XRD

TEM

3.0 3.9 3.8

– 4.0 4.0

Fig. 6 – Cyclic voltammograms of JM, Pt/VACNF and Pt/CNW electrodes in N2-saturated 1 M H2SO4 + 2 M MeOH at a scan rate of 0.05 V s1. Current densities are normalized with respect to the Pt loading.

where d is the mean diameter of the Pt nanoparticles and q is the density of Pt (21.09 g cm3). The area of H-adsorption can be used to estimate the ECSA of Pt catalysts. The ECSA of catalyst can be calculated according to Eq. (4) [27]: SECSA ¼

QH LPt  0:21

ð4Þ

in which LPt represents the Pt loading (mg cm2), QH (mc cm2) is the charge-exchanged during the electro-adsorption of H on Pt surface and 0.21 (mc cm2) is the charge required to oxidize a monolayer of H2 on a smooth Pt. The ECSA, CSA and g of JM, Pt/VACNF and Pt/CNW electrodes are given in Table 1. The analysis of g indicates that the Pt utilization efficiency of Pt/VACNF or Pt/CNW electrodes is significantly higher than that of commercial JM Pt/C electrode with the same Pt loading. This may attribute to the electrode structure of Pt/VACNF or Pt/CNW that most Pt nanoparticles can be exposed to the three-phase boundary. Compared with the Pt/VACNF, the

CSA (cm2 mg1)

ECSA (cm2 mg1)

948.1 729.5 748.7

509.7 499.7 594.9

g (%) 53.8 68.5 79.5

Pt/CNW electrode has a higher ECSA (594.9 cm2 mg1 vs. 499.7 cm2 mg1 for Pt/VACNF), and a higher Pt utilization (79.5% vs. 68.5% for Pt/VACNF). The methanol oxidization of JM, Pt/VACNF and Pt/CNW electrodes were evaluated by using CV between 0 and 1.1 V vs. SCE in nitrogen saturated 1 M H2SO4 + 2 M MeOH. As shown in Fig. 6, two typical oxidation peaks can be seen from each electrode, which represent the oxidation of methanol and their intermediates such as CO [4,23]. The three electrodes have the similar methanol oxidation peaks position (Ef) in the forward scan, as shown in Table 2. It can be seen that the normalized current density of methanol oxidation decreases in the following order: Pt/CNWs (1524 mA cm 2 mg1 Pt) > Pt/VACNFs (1119 mA cm2 mg1 Pt) > JM 40 wt.% Pt/C (548 mA cm2 mg1 Pt). The JM electrode with the smallest Pt nanoparticle sizes exhibits the lowest methanol oxidation current density at the same Pt loading, which may be contributed to the Pt utilization. The Pt/CNW electrode shows a higher methanol oxidation current density than Pt/VACNF electrode. Considering the mean Pt nanoparticle sizes of two electrodes are quite close, the improvement of the methanol oxidation of Pt/CNWs should not be ascribed to the catalyst particle size effect. The ratio of the forward anodic peak current density (If) to the reverse anodic peak current density (Ib), If/Ib, can be used to infer the CO tolerance of the catalyst. Low If/Ib value usually indicates poor oxidation of methanol to CO2 during the forward scan and excessive accumulation of carbonaceous species on the catalyst surface [28]. High If/Ib ratio shows improved CO tolerance. The Pt/CNW electrode shows a better If/Ib ratio than Pt/VACNF electrode (1.2 vs. 1.1). The catalytic activity of Pt based catalysts depends on many factors, such as the components of catalysts, particle size of catalysts, crystalline structure of Pt based catalysts and electron transport between catalyst nanoparticles and carbon supports [2,14]. Among them, the electron transport may be the key factor in this study, since these two kinds of electrodes have the same catalyst components, catalyst crystalline structure and similar catalyst particle sizes, based on TEM and XRD results. As reported by previous works [22,29], CNWs mainly consists of tens to hundreds of parallel graphene layers

Table 2 – Catalytic activities towards the methanol oxidation of JM, Pt/VACNF and Pt/CNW electrodes. Catalyst

JM 40% Pt/C Pt/VACNFs Pt/CNWs a

Onset potential (Va) 0.27 0.23 0.19

Forward peak

Backward peak

Ef (Va)

If (mA mg1 cm2)

Ib (mA mg1 cm2)

0.799 0.772 0.767

548 1119 1524

485 986 1302

These potentials are versus a saturated calomel electrode (SCE).

If/Ib

1.1 1.1 1.2

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Fig. 7 – Reaction and electron transfer schemes of the (a) Pt/ VACNF and (b) Pt/CNW electrodes.

standing almost vertically on the substrate. Meanwhile, the schematic of the herring-bone VACNFs are presented by our previous work [18]. The surface reaction and electron transport of Pt/VACNF and Pt/CNW electrodes were depicted in Fig. 7. The methanol molecule was oxidated into CO2, H+ and electrons in the surface of Pt nanoparticles. Then the electrons were transmitted into the carbon supports. There are mainly two electron transport paths: inside one graphene layer and between the graphene layers. The electron transport between the graphene layers is the predominant electron conductive way in VACNFs. However, electron transport inside one graphene layer is the predominant way in CNWs. Previous studies have shown that the edge-plane sites play extremely important influence of electron-transfer rates, which reported to be 105 times higher than those at basal-plane graphite, indicating a faster electron conduction inside one graphene than that between the graphene layers [30,31]. It can be inferred that the faster electron

Fig. 8 – Chronoamperometric curves of (a) Pt/VACNF and (b) Pt/CNW electrodes in N2-saturated 1 M H2SO4 + 2 M MeOH at a potential of 0.65 V vs. SCE. Current densities are normalized with respect to the Pt loading.

transport inside the one graphene in CNW plays an important role in the improvement of methanol oxidation of Pt/CNW electrode. Typical chronoamperometric curves on Pt/VACNFs and Pt/ CNWs at a constant potential of 0.65 V vs. SCE in 1 M H2SO4 + 2 M MeOH were recorded, as shown in Fig. 8. When the potential is fixed at 0.65 V, the oxidation current are found to decrease rapidly corresponding to accumulation of the intermediates species such as COads. A slower decay of

Fig. 9 – TEM images of the (a) Pt/VACNF and (c) Pt/CNW electrodes and histograms of Pt nanoparticle diameters for (b) Pt/ VACNF and (d) Pt/CNW electrodes after electrochemical measurements.

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current density with time implies that the catalyst has good anti-poisoning ability [32]. As seen from Fig. 8, the decay of the oxidation current density on Pt/CNWs is slower than Pt/ VACNFs. The oxidation current density on Pt/CNWs is larger than that on Pt/VACNFs at the end of the test. This result indicates that the Pt/CNW electrode has good poisoning-resistance/durability ability, which is identical with the results of CV measurements. TEM analysis was carried out on Pt/VACNFs and Pt/CNWs to investigate the morphological changes of the catalyst nanoparticles after the CV and chronoamperometry measurements, as shown in Fig. 9. Fig. 9a and c present the TEM images of Pt/VACNFs and Pt/CNWs and the corresponding histograms of Pt nanoparticle diameters are shown in Fig. 9b and d, respectively. The results of TEM images of Pt/VACNF and Pt/CNW electrodes before and after electrochemical measurements show that: (1) the average diameter of Pt nanoparticles of Pt/VACNF and Pt/ CNW electrodes after electrochemical measurements are about 5.0 and 4.7 nm, respectively, indicating the aggregation of Pt nanoparticles in both electrodes; (2) there is a much more serious migration of Pt nanoparticles of Pt/VACNF electrode than Pt/CNW electrode. These results indicate that the Pt/ CNWs have better longterm stability than Pt/VACNFs. As previous report, each CNW has many graphitic edges and defects, which was confirmed by the Raman spectroscopy [20]. These defects play an important role in the grain growth of Pt nanoparticles, and maybe hinder the migration of Pt nanoparticles, which may decrease the ECSA and cause the degradation of catalytic activity [33].

4.

Conclusion

VACNFs and CNWs have been used as carbon support for Pt based catalyst for methanol oxidation, which directly grown on CP by just adjusting the substrate bias by an ICP-PECVD system. Pt nanoparticles have been deposited onto VACNFs and CNWs to form electrodes by the RF-MS technique. The results show that the Pt/CNW and Pt/VACNF electrodes exhibit significant improvement of Pt utilization and methanol electro-oxidation activity than commercial JM Pt/C electrode. The Pt/CNW electrode is found to exhibit a superior electrocatalytic activity toward methanol oxidation than Pt/VACNF electrode. The improved characteristics are associated with the unique structure of CNWs, which results in a better electron transport property of CNWs than VACNFs. Moreover, the structure defects in CNWs playing an important role in improving the durability of the electrode by hinder the migration of Pt nanoparticles. Compared to Pt/VACNF electrode, Pt/ CNW electrode has a more suitable potential to serve as the highly efficient electrode for direct alcohol fuel cells.

Acknowledgements This research is financially supported by the Institute of Plasma Physics, Chinese Academy of Sciences (Nos. Y05FCQ1128; 095GZ1156Y), the National Nature Science Foundation of China (Nos. 11175214; 10975162) and the Core-University program of Japan-China.

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