Facile preparation of ultra-low Pt loading graphene-immobilized electrode for methanol oxidation reaction

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Facile preparation of ultra-low Pt loading graphene-immobilized electrode for methanol oxidation reaction S.Y. Toh a, K.S. Loh a,*, S.K. Kamarudin a,b, W.R.W. Daud a,b a

Fuel Cell Institute, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia

b

article info

abstract

Article history:

Pulse current electrodeposition (PCE) technique was used to prepare graphene-supported

Received 13 November 2017

platinum nanoparticles (GN-PtNPs) electrodes for the methanol electro-oxidation reac-

Received in revised form

tion in acidic media. The influences of the PCE parameters (applied current density, con-

23 June 2018

centration of the Pt precursor, and duty cycle) upon the as-prepared GN-PtNPs electrodes

Accepted 3 July 2018

for the methanol oxidation reaction (MOR) in terms of catalytic activity and tolerance

Available online xxx

against poisoning were studied using the Taguchi design of experiment (DOE). The analysis of variance (ANOVA) and signal-to-noise (S/N) ratio analysis provided prediction of optimal

Keywords:

electrodeposition conditions to yield GN-PtNPs electrode which give the best MOR per-

Graphene

formance. The values of confirmatory experiment were demonstrated close to the values

Pulse current electrodeposition

predicted using the Taguchi method. Transmission electron microscopy images showed

Methanol oxidation reaction

that the Pt crystallites in flower-like structure were deposited evenly on the surface of

Taguchi method

graphene sheet. The Pt crystallites were predominantly in a zero-valent, metallic Pt state based on the X-ray photoelectron spectroscopy analysis. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction In light of advances in portable electronic devices, such as mobile phones, cameras and laptops, the demand for power requirements is also increasing accordingly. Apart from the safety concerns of power sources, the high-power density of the power sources [1] is of the utmost importance to allow devices to work for a long duration prior to recharging. Numerous power sources operated on the principle of electrochemical such as fuel cells [2,3], metal-air batteries [1,4]

and lithium ion batteries [5,6] have been developed to meet the power requirements. Of various power sources, a direct methanol fuel cell (DMFC) which uses methanol as its fuel, has been envisaged as one of the most promising power sources for these portable electronic devices due to its several favourable features such as relative high theoretical energy density and ease of storage and transport [7e9]. Unlike rechargeable batteries that require longer charging times to replenish a depleted power pack, a DMFC can have its fuel replaced in minutes [10] to resume acting as a power supply for electronic devices.

* Corresponding author. E-mail addresses: [email protected] (S.Y. Toh), [email protected] (K.S. Loh), [email protected] (S.K. Kamarudin), wramli@ukm. edu.my (W.R.W. Daud). https://doi.org/10.1016/j.ijhydene.2018.07.016 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Toh SY, et al., Facile preparation of ultra-low Pt loading graphene-immobilized electrode for methanol oxidation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.016

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Despite these advantages, the methanol electro-oxidation reaction at the anode of the DMFC is rather sluggish and requires the use of a considerable amount of expensive noble metal catalysts, particularly platinum (Pt) or its alloys for practical application. Pt or its alloys are the best known oxygen reduction reaction (ORR) catalysts [1,5,11], and generally also demonstrate high catalytic activity towards the methanol oxidation reaction (MOR) at room temperature. A pure platinum catalyst on its own is readily poisoned at low temperatures by carbon monoxide (CO) species [12], which are produced as intermediates by the methanol electro-oxidation reaction [13], leading to low catalytic efficiency and rapid electrocatalytic activity degradation. Thus, numerous nonnoble metals transition metal compounds such as NiZr [14] and tungsten carbide [15] and other noble metals such as rhodium [16,17] have been explored and developed as substitutes to Pt catalyst for MOR. However, the catalytic performances of these non-platinum based materials are far behind that of Pt-based electrocatalysts [10,18]. There are no or limited metal elements, alternative to Pt, that are active toward MOR in acidic medium [18]. Pt remains an indispensable active component in MOR catalyst formulations, particularly in an acidic medium [19]. Due to the limited options for MOR catalyst formulations, the present approaches in MOR catalyst formulations have focused on the creation of new Pt-based catalysts with enhanced catalytic performances (in terms of activity and stability) and lower Pt content by improving the Pt utilization. Several strategies have been adopted to overcome the cost barrier imposed by the use of noble metal catalyst, including nanostructure design, alloying [9] and use of a support. In recent years, the nanostructuring strategy has been widely used to synthesize various cost-effective and highly efficient catalysts. Nanostructure catalysts in a variety of shapes including nanospheres, nanowires, core-shell structure, hollow structure, nanotubes, nanofibers, 3D structure and so on, have been successfully developed via various novel templatebased synthesis methods [2,5,20]. Over the past few decades, there has been great progress in reducing the cost associated with Pt catalyst through nanostructuring [6] and alloying Pt with other oxophilic metals (such as ruthenium) [21] on a support. The catalysts synthesized by these strategies generally exhibit enhanced MOR catalytic activity, but unfortunately, they still suffer from the rapid loss of electrocatalytic activity in fuel cell operations as a result of either aggregation of the catalyst particles [20] owing to the inherent tendency of small particles to minimize their total surface energy [22], corrosion of the catalyst support [8,23] or dissolution of the alloying metals [24,25]. In recent years, a number of novel carbonaceous substrates such as graphene [13,26,27] and carbon nanotubes [28,29] have been developed as catalyst support to overcome these challenges, as well as reducing the cost of electrode materials and obtaining a stable dispersion of electrocatalyst. Graphene oxide (GO), a derivative of graphene, has been extensively studied as Pt and Pt alloy catalyst supports owing to their large surface area [30] and the high electrical conductivity [31,32] of the graphene framework, as well as the abundant oxygen functional groups on their surfaces that act as anchoring sites for metal nanoparticles [13,33]. The oxygen functional groups on the graphene framework have

been reported to assume the role of Ru as an auxiliary catalyst in the case of a PteRu catalyst [13] to remove the intermediate carbonaceous species, which in turn alleviates the poisoning effect on the Pt catalyst. At present, the chemical reduction method remains the most common way to prepare graphene-supported Pt (GNPtNPs) composite catalysts, which involves labourious procedures to reduce the mixture of the Pt precursor and graphene oxide using an excess of the reducing agent, such as ethylene glycol [16,33,34] or hazardous sodium borohydride compounds [35,36]. In such a synthesis method, the Pt nanoparticles are generally dispersed and located on both surfaces of the graphene sheets, and the resulting composite is subsequently applied to the surface of the electrode for catalysis applications. However, it has been widely reported that the electrochemical reaction is confined to the three-phase reaction zone [37e39], i.e., at the interface between the polymer electrolyte and the Pt catalyst that is exposed to the reactants. Inactive catalyst sites are inevitably created in the catalyst layer prepared by this conventional method as one side of the GN-PtNPs composite is basically located outside the threephase reaction zone, which results in lower Pt utilization. Moreover, the preferable orientation and morphology of the graphene-supported Pt composite catalyst for catalysis may no longer the same as it was in the colloidal state because the graphene-supported Pt composite catalyst tends to agglomerate when the composite catalyst is applied to the surface of an electrode. Additionally, there is an unavoidable loss of the composite catalyst during the electrode preparation as a result of the multiple step processes. One promising approach to preparing a graphenesupported Pt composite catalyst that avoids these limitations and drawbacks is to deposit the Pt directly on a graphene-immobilized electrode using an electrochemical deposition technique. In comparison to other preparation approaches, the electrochemical deposition approach allows a facile, environmental-friendly, and economical way to prepare a graphene-supported Pt composite catalyst electrode. The electrochemical deposition can be carried out by direct constant potential, cyclic voltammetry, or pulse electrodeposition technique. Zhou et al. [40] used a constant potential to electrodeposit highly dispersed Pt nanoparticles onto graphene, and the resulting Pt NPs@G exhibits high electrocatalytic activity and long-term stability towards methanol electrooxidation than the Pt NPs@Vulcan. In a work done by Radhakrishnan et al. [41], cyclic voltammetry technique was employed to deposit Pt nanostructures onto reduced graphene oxide (rGO) for methanol electrooxidation application. Their work showed that different Pt nanostructures on rGO can be obtained by varying the number of deposition cycles. Hsieh et al. [42] employed pulsed electrodeposition to deposit a very low Pt loading with flower-like nanoclusters on graphene sheets as electrocatalyst for proton exchange membrane fuel cells. Over the past few years, the pulsed electrodeposition technique has been widely utilized to prepare a variety of Pt nanostructures with low loading on different substrates [43e45] for catalysis application. In general, pulse electrodeposition can be carried out in either a voltage or current mode. Pulse electrodeposition carries out in current mode is generally known as pulse current electrodeposition, and it is

Please cite this article in press as: Toh SY, et al., Facile preparation of ultra-low Pt loading graphene-immobilized electrode for methanol oxidation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.016

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preferable due to its convenient setup-up, tunable experimental parameters [39] and scalability for industrial requirements. Despite several research works utilizing pulse electrodeposition to prepare Pt-based catalysts on various supports in the past, the Pt-based catalysts were prepared using the voltage mode of pulse electrodeposition [43e45]. The use of pulse current electrodeposition to prepare graphenesupported Pt catalysed electrodes for MOR has rarely been explored or reported in detail. Yaldagard et al. [46] employed pulsed current electrodeposition to deposit Pt on graphene as electrocatalyst for ORR application. Liu et al. [47] demonstrated the deposition of Pt nanoparticles on glassy carbon electrode with pulsed current deposition for ammonia oxidation. Recently, Woo et al. [39] reported using a pulsed current electrodeposition technique to deposit Pt on graphene as electrocatalyst for MOR, but their works give no details on the electrodeposition optimization conditions for MOR. To the best of our knowledge, no details are available with respect to the optimization of pulse current electrodeposition conditions to yield a graphene-supported Pt electrode for MOR catalysis applications in an acidic medium. In this work, we prepared a graphene-supported Pt composite catalyst electrode using a pulse current electrodeposition technique. In pulse current electrodeposition, each pulse current consists of an on-time (ton) and off-time (toff) during which a current or zero-current is applied, respectively. The performance of the Pt supported on the grapheneimmobilized electrode can be greatly influenced by the electrodeposition conditions, such as the applied current density, concentration of the Pt precursor and the pulse duty cycle. The Taguchi method was employed to yield the optimal graphene-supported Pt composite catalyst electrode for MOR catalytic performance in an acidic medium. The effect of the pulse current electrodeposition parameters, such as the applied current density, concentration of the Pt precursor and duty cycle (on-time/off-time) were evaluated. The particle distribution of the Pt catalyst on graphene was also characterized.

Experimental Materials Graphite flakes, sulphuric acid (95.0e98.0 wt %), potassium permanganate, potassium hydroxide, hydrochloric acid (37 wt %) and sodium nitrate were purchased from Sigma-Aldrich (Malaysia) for use in the synthesis of graphite oxide. Hexacholoroplatinic acid (H2PtCl6) and methanol (99.9%) were used for the electrodeposition of Pt and the electrochemical characterizations, respectively. Screen-printed carbon (SPC) electrodes (Fig. S1) were purchased from Scrint Technology Sdn Bhd. All chemicals were analytical grades and used as received. Distilled water was used throughout this work.

Preparation of the GO dispersion and graphene-immobilized electrodes Graphene oxide (GO) nanosheets were obtained via liquidphase exfoliation of graphite oxide produced from the

3

modified Hummers' method [48] by ultrasonication in an aqueous solution. The resulting graphene oxide dispersion with a concentration of 0.1 mg/ml was then used for the preparation of the electrodes. The graphene-immobilized electrodes were prepared by immobilizing a thin layer of GO film onto SPC electrodes (Fig. S1). In brief, 10 ml of a graphene oxide dispersion (0.1 mg/ml) was drop-casted onto freshly cleaved SPC electrodes, and the electrodes were left to dry slowly under ambient conditions overnight.

Preparation of the GN-PtNPs electrode by pulse current electrodeposition The pulse current electrodeposition of Pt was conducted in a two-electrode cell with an electrochemical instrument (Autolab PG128N). The electrodeposition was carried out on the previously prepared graphene-immobilized SPC electrodes in a Pt plating solution consisting of H2PtCl6 at room temperature. The graphene-immobilized SPC electrode (as a cathode) and a graphite rod (as an anode) were set 5 mm apart from one another and subsequently immersed into the Pt solution for 2 min prior to the Pt pulse current electrodeposition process. The total charge density imposed on all the electrodes was fixed at 0.01 C cm
2 in this study. The applied current density was varied from 0.5 mA cm
2 to 50 mA cm
2, while the concentration of the Pt precursor solution was in the range of 1 mMe4 mM. The duty cycle was varied at ratios of 0.05, 0.25 and 0.5, and the on-time (ton) was kept constant at 0.1 s. The duty cycle is generally defined as follows: Duty cycle ¼

ton ton þ toff

(1)

Upon electrodeposition, the resulting electrodes were rinsed thoroughly to remove any residue from the Pt precursor and were subsequently dried at room temperature.

Design of experiment (DOE) approach The Taguchi experimental design was employed to determine the optimal electrodeposition parameter settings that produced the GN-PtNPs electrode with the best methanol electrooxidation activity in an acidic medium. Since the amount of methanol electro-oxidized at the electrode is directly proportional to the magnitude of the current density, the peak current density of the forward scan in a cyclic voltammogram was taken as an indicator of the catalytic activity of the asprepared GN-PtNPs electrodes for the methanol electrooxidation in this study. Three electrodeposition parameters (factors), the applied current density (A), concentration of H2PtCl6 (B), and duty cycle (C) at three different levels, were considered, as specified in Table 1. Based on the selected number of factors and levels (three factors at three levels), the experiment was designed by adopting the L9 Taguchi orthogonal array DOE, as illustrated in Table 2. The Taguchi L9 orthogonal array DOE requires a total of 9 experiments to complete the entire study. In this study, each experiment was repeated three times to minimize the experimental error and obtain reliable data. Hence, a total of 27 experiments were performed. The methanol oxidation peak current density of

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Table 1 e The electrodeposition control parameters (factors) and their respective levels. Parameters

Levels

A: applied current density (mA cm
2) B: concentration of Pt precursor (mM) C: duty cycle

1

2

3

0.5 1 0.05

5 2 0.25

50 4 0.50

the forward scan in the cyclic voltammogram was taken as the response variable in this design. The analysis of variance (ANOVA) of the mean methanol oxidation current response and signal-to-noise (S/N) ratio were used to determine the significance of the control factors. The statistical analysis of the results was performed using the Qualitek-4 program.

cyclic voltammogram between 0.05 and
0.245 V versus the Ag/AgCl reference electrode, which was based on the assumption that the charge related to a hydrogen monolayer adsorption on Pt is equivalent to 210 mC cm
2-Pt [45]. The particle size, shape and distribution of the platinum electrocatalyst on graphene were examined using a transmission electron microscope (Hitachi, model H-8000). To determine the amount of Pt electrodeposited on the grapheneimmobilized electrode, the electrode was treated in concentrated hydrochloride acid for 48 h, and the resulting Pt traces in the form of a PtCl2
6 complex were measured using a UV-vis spectroscopy technique reported by Georgieva et al. [49]. The oxidation states of Pt were measured by X-ray photoelectron spectroscopy (XPS), PHI Quantera II.

Electrocatalytic activity measurement

Results and discussion

The electrocatalytic activity of the as-prepared electrodes was measured in a conventional three-electrode cell at room temperature using a potentiostat (Autolab PG128N). A Pt wire and an Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively, while the as prepared Pt-electrodeposited, graphene-immobilized SPC electrode served as the working electrode. The electrocatalytic activity of the as-prepared sample electrodes for methanol oxidation was measured in a solution of 1 M methanol and 0.5 M H2SO4 using a cyclic voltammetry technique. All measurements were performed with a scan rate of 50 mV s
1 over a potential window of 0 Ve1.0 V with respect to Ag/AgCl electrode for 20 cycles. For convenient comparison purpose, the measured potentials vs. Ag/AgCl were converted to the reversible hydrogen electrode (RHE) scale according to simplified Nernst equation (ERHE ¼ EAg/AgCl þ 0.197 V). Prior to each measurement, the electrolyte solution was de-aerated with nitrogen gas for 15 min. All data presented in this paper were taken from the 20-th cycle. The methanol electro-oxidation current density was calculated by normalizing the measured methanol oxidation current (MOC) on the SPC electrode of 4 mm in diameter.

Taguchi design and ANOVA In this work, the effect of the pulse current electrodeposition parameters on the catalytic activity of the GN-PtNPs electrode towards MOR and the tolerance of the catalyst against poisoning by intermediate carbonaceous species was evaluated using cyclic voltammetry (CV) in an electrolyte of 1 M CH3OH and 0.5 M H2SO4 at a scan rate of 50 mV s
1. Fig. 1 shows the cyclic voltammogram of the GN-PtNPs electrode in an electrolyte of 1 M CH3OH and 0.5 M H2SO4 at a scan rate of 50 mV s
1. The voltammogram consists of two strong anodic peaks. The peak observed at approximately 0.85 V (vs RHE) in the forward scan is typically attributed to the methanol electro-oxidation reaction, while the peak in the backward scan at approximately 0.50 V is associated with the removal of the intermediate carbonaceous species formed by the forward scan [50]. Since the magnitude of the peak current density is proportional to the amount of methanol electrooxidized at the electrode, the catalytic activity of the GNPtNPs electrodes was determined utilizing the MOC observed at 0.85 V in the cyclic voltammogram. On the other hand, it is generally accepted that the ratio of the forward anodic peak

Characterization of the optimized GN-PtNPs electrode The electrochemically active surface area (EASA) of Pt was estimated using the hydrogen adsorption-desorption in the

Table 2 e The Taguchi L9 orthogonal array design of experiment. Experiment no.

1 2 3 4 5 6 7 8 9

Control parameters and their levels A

B

C

1 1 1 2 2 2 3 3 3

1 2 3 1 2 3 1 2 3

1 2 3 2 3 1 3 1 2

Fig. 1 e Cyclic voltammogram profiles of GN-PtNPs electrode in 1 M methanol and 0.5 M H2SO4 at 50 mV s¡1.

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(If) to the backward anodic peak current (Ib), If/Ib, is an indicator of the catalyst tolerance to incompletely oxidized species that accumulate on the surface of the electrode [7]. Thus, the If/Ib value was used to indicate the tolerance of the catalyst against poisoning. A higher ratio suggests a more effective removal of the poison species from the catalyst surface [51,52]. The experimental results of the MOC and the If/Ib values for each sample in cyclic voltammograms (Fig. S3) are summarized in Table 3. The results were analysed using the Taguchi S/N ratio analysis. The S/N ratio is a measure of the quality characteristics and deviation from the desired value. The greater the value of the S/N ratio, the smaller the variance around the desired value [53]. Thus, the highest possible value of the S/N ratio for the experimental results is desirable for the optimum performance prediction. As higher MOC and If/Ib values were desired in this work, the S/N ratios were calculated using the “larger is better” quality characteristic equation as follows (Eq. (2)): S=N ratio ðhÞ ¼
10 log10

1X 1 n y2i

(2)

where yi ¼ the observed response value and n ¼ the number of replications. The mean S/N ratio for each level of the factors was summarized, and the S/N response table for the MOC and If/Ib ratios is presented in Table 4. The response table includes the ranks based on Delta statistics, which compare the relative magnitude of effects for each electrodeposition parameter. The Delta statistic is the highest average S/N for each factor minus the lowest average S/N for each factor [54]. Ranks are assigned based on the Delta values, whereby rank 1 is assigned to the highest Delta value, rank 2 to the second highest Delta value, and so forth. As seen, factor A (current density) has the most impact on the S/N ratios of both the MOC and If/Ib, while factor B (concentration) and C (duty cycle) have the least impact on the S/N ratios of the MOC and If/Ib, respectively. Fig. 2 shows the corresponding main effects plots for the S/N of the MOC and If/Ib. The main effects plots show the general trend of the influence of the factors. A higher S/N ratio value defines the optimal level for the corresponding factor. Therefore, the optimal electrodeposition settings to yield GN-PtNPs electrodes that give the highest MOC value are a current density (A) at level 3, concentration (B) at level 2 and

Table 3 e The experimental results and S/N values of methanol oxidation current (MOC) and the If/Ib values for the GN-PtNPs electrodes in Taguchi L9 orthogonal array design of experiment. Exp. No. 1 2 3 4 5 6 7 8 9

Methanol oxidation current

If/Ib ratio

Mean (mA)

S/N

Mean (mA)

S/N

5.483 14.553 13.703 28.836 27.720 19.376 25.436 24.226 34.773

14.775 23.258 22.735 29.198 28.855 25.745 28.108 27.685 30.824

1.91 1.67 1.52 2.03 2.14 1.73 2.84 2.45 2.13

5.616 4.464 3.615 6.128 6.617 4.754 9.065 7.769 6.579

Table 4 e The mean S/N response table for methanol oxidation current (MOC) and If/Ib ratio. Response

Level

Parameter A (current B (concentration) C (duty density) cycle)

MOC

If/Ib

1 2 3 Delta (D) rank 1 2 3 Delta (D) rank

20.26 27.93 28.87 8.61 1 4.57 5.83 7.81 3.24 1

24.03 26.60 26.44 2.57 3 6.94 6.28 4.98 1.96 2

22.74 27.76 26.57 5.02 2 6.05 5.72 6.43 0.71 3

duty cycle (C) at level 2. Meanwhile, the optimal electrodeposition settings to yield a GN-PtNPs electrode with the highest value of If/Ib are a current density (A) at level 3, concentration (B) at level 1 and duty cycle (C) at level 3. ANOVA was subsequently performed to measure the relative percent influence of the factors on the variability of the results as well as to determine the significance of the factors. The results of ANOVA on the S/N ratio of the MOC are depicted in Table 5. Based on the percent contribution (P) term of ANOVA, it is obvious that factor A (applied current density) has the most influence (67.4%) on the variation in the MOC, while factor B (concentration) has the least influence (4.2%) on the variation in the MOC. Factor C (duty cycle) has a moderate influence (19.2%) on the variation in the MOC. Based on the standard F-table at the 0.10 level of significance (90% of the confidence level), F0.10 (2, 2) has a value of 9.00 [55]. The computed values of the F ratio for factor A (30.16) and C (9.30) are greater than that of the limiting values obtained from the F-table. Therefore, factor A (current density) and C (duty cycle) have a significant effect on the variation in the MOC. The computed F ratio for factor B (2.80) is smaller than the limiting values obtained from the F-table, and this factor is considered less significant to the variation in the MOC. Table 5 also presents the results of the ANOVA on the S/N ratio of If/Ib. A review of the percent contribution (P) term of the ANOVA table clearly indicates that factor A (current density) is the most influential factor (68.4%) on the variation of If/Ib, while factor C (duty cycle) has the least influence (2.05%). Factor B (concentration) shows a moderate influence (24.59%) on the variation of If/Ib. Based on the standard F-table at a 0.10 level of significance (90% of the confidence level), F0.10 (2,2) has a value of 9.00. Since the computed values of the F ratio for factors A and B are 56.11 and 20.82, respectively, which are larger than the limiting values obtained from the F-table, these factors are considered significant for the variation of If/Ib. Meanwhile, the computed F ratio for factor C (2.65) was smaller than the limiting values obtained from the F-table, and factor C was considered less significant to the variation of If/Ib.

Prediction and confirmation of the experimental design Based on the outputs from the S/N ratio analysis and ANOVA, the expected values for the MOC and If/Ib ratio at the optimal

Please cite this article in press as: Toh SY, et al., Facile preparation of ultra-low Pt loading graphene-immobilized electrode for methanol oxidation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.016

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Fig. 2 e Main effect plot for S/N ratio of methanol oxidation current (MOC) and If/Ib ratio.

electrodeposition conditions in terms of the S/N ratio were estimated using the following equation (Eq. (3)).  S Np ¼ S=Nm þðS=NAo
S=Nm ÞþðS=NBo
S=Nm ÞþðS=NCo
S=Nm Þ (3) where S/Np is the predicted S/N ratio optimal conditions. S/NoA, S/NoB, and S/NoC denote the mean S/N ratios for factors A, B, and C at their respective optimal levels. The computed S/Np was then converted back to the original units of the experimental results using the following equation (Eq. (4)).  S=Np 1=2 Ypredicted ¼ 10 10

(4)

where Ypredicted is the mean expected result at the optimal conditions. The value of the mean S/N ratio at the optimum level for the MOC and If/Ib responses can be obtained from the respective S/N ratio response tables (Table 4). In the case of the MOC response, the mean S/N ratio was computed as 25.687, whereas the mean S/N ratios for factor A, B, and C at their respective optimal levels were 28.87, 26.60, and 27.76, respectively (Table 4). By substituting these values into Eq. (3), the computed estimate of the mean S/N for the MOC response at the optimal level conditions (A3B1C2) was predicted to be 31.86, corresponding to 39.20 mA in the original unit of measurement. The mean S/N for the If/Ib response at the optimal level conditions was computed in the same manner. The

Table 5 e The analysis of variance (ANOVA) on S/N ratio of methanol oxidation current (MOC) and If/Ib ratio.

MOC

If/Ib

a b c d e

Sources of variance

DFa

SSb

MSc

Fd

P (%)e

A: Current density B: Concentration C: Duty cycle Other/error Total A: Current density B: Concentration C: Duty cycle Other/error Total

2 2 2 2 8 2 2 2 2 8

134.049 12.440 41.354 4.444 192.287 15.991 5.932 0.755 0.284 22.964

67.024 6.220 20.677 2.222

30.156 2.798 9.303 e

7.995 2.966 0.377 0.142

56.116 20.816 2.649 e

67.400 4.157 19.194 9.249 100 68.387 24.591 2.047 4.965 100

Degree of freedom. Sum of square. Mean square (variance). Variance ratio. Percent influence

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expected value of the mean S/N for the If/Ib ratio response at the optimal level conditions (A3B1C2) was calculated to be 9.04, which is equivalent to a mean value of 2.83. A confirmation experiment was performed to verify the assumption used and the expected results of the experiment obtained from the statistical analysis. Three sample electrodes based on the optimal levels of the design factors, A3B1C2 for the MOC and If/Ib responses, were prepared for the confirmatory trials. The experimental results obtained from the trials were compared with the predicted values, as depicted in Table 6. As seen, the mean MOC obtained from the experiment trials was at 37.1 mA, which was close to the predicted value of 39.20 mA. Likewise, the mean If/Ib ratio obtained from the experimental trials was 2.67, which was also close to the expected mean value of 2.83. There was good agreement between the predicted and experimental results in both the MOC and If/Ib responses. Therefore, the experimental design model adopted in this study can be deemed appropriate and used for pulse current electrodeposition optimization.

Characterization of the GN-PtNPs electrodes The amount of Pt deposited onto the graphene-immobilized electrodes was determined using an UV-vis spectroscopy technique. In this approach, the Pt traces on the electrodes complex in a hydrochloride were made into a PtCl2
6 solution, and its concentration was measured using the absorbance at a wavelength of 260 nm (Fig. S1) [49]. Table S1 shows the amount of the Pt electrodeposited on the graphene-immobilized electrodes determined by UV-vis spectroscopy. In general, the amount of Pt electrodeposited on the graphene-immobilized electrodes under all experimental conditions was seen to be in the same order, ranging from 2.59  10
3 to 2.84  10
3 mg/cm2 (Table S1). The consistency in the amount of Pt loading under the same imposed charge density suggests that the pulse current electrodeposition is indeed a promising method to prepare graphenesupported Pt electrodes. In other words, the effect of Pt loading on the electrodes for the MOR and tolerance against poisoning can be rationally excluded from consideration in this work. To better understand the electrochemical properties of the GN-PtNPs electrode prepared under the optimal conditions (A3B1C2; denoted as GN-PtNPs-OPT), the EASA of Pt on the electrodes was estimated by calculating the total charge of the hydrogen desorption process from a cyclic voltammogram (See Fig. S4) in 0.5 M H2SO4 with an assumption that a monolayer adsorption of hydrogen corresponds to 210 mC cm
2 of Pt [56]. The integrated area under the desorption peak in the CV curves represents the total charge (QH) of the desorption process of absorbed hydrogen from the surface of Pt catalyst. The expression for the total charge of hydrogen

desorption (QH) after taking into consideration of double layer capacitive charge (QDL) is computed from Eq. (5). QH ¼

1 Vr

ZE2 I,dE

(5)

E1

where E1 and E2 are the interval of potentials for desorption peak, I is the current generated (A) and Vr is the potential scan rate (V/s) of CV. The EASA is determined according to the following equation (Eq. (6)).   EASA cm2 of Pt ¼

QH ðmCÞ 210 mC cm
2 of Pt

(6)

For comparison, the GN-PtNPs electrode prepared under experimental condition number 1 (A1B1C1; denoted as GNPtNPs-EX1), which exhibited the lowest performance in terms of methanol oxidation activity, was used. As seen in Table S1, the EASA for GN-PtNPs-OPT was 0.987 cm2-Pt compared to that of GN-PtNPs-EX1 (0.047 cm2-Pt). Thus, it can be rationally deduced that the higher MOC observed in the GN-PtNPs-OPT electrode compared to that of the GN-PtNPsEX1 electrode is due to higher EASA of Pt on the grapheneimmobilized electrodes. Fig. 3 shows the TEM images of a plain graphene (prior to deposition of Pt), GN-PtNPs-EX1 and GN-PtNPs-OPT. Under the experimental condition number 1, Pts in the form of spherical nanoparticles (~2e5 nm) and clusters (~120 nm) were observed to be uniformly distributed on the graphene sheet (Fig. 3b). In contrast, dendritic Pt clusters (~150e250 nm) were predominantly formed and uniformly distributed on the graphene sheet for GN-PtNPs-OPT (Fig. 3c). The dendritic shape of the Pt cluster has been referred to as flower-like Pt in some studies [43,44]. Both the methanol oxidation current and If/Ib ratio of the GN-PtNPs-OPT were higher than that of GN-PtNPsEX1 despite having an almost equal amount of Pt loading on the graphene-immobilized electrodes. This observation implies that methanol molecules can be more effectively oxidized on the flower-like Pt on graphene-immobilized electrodes during the forward potential scan, generating less poisonous species compared to that of the spherical-shaped Pt on graphene-immobilized electrodes. The findings are in agreement with several previous works [43,57,58] that indicated the dendritic form of Pt is favourable over other shapes of Pt for the catalysis of the MOR. It has been widely reported that the catalytic activity of Ptbased catalysts for MOR in fuel cells is also highly associated with the oxidation states of the Pt crystallites [59]. The oxidation states of Pt can be determined using XPS. The XPS spectra of GN-PtNPs-OPT and GN-PtNPs-EX1 electrodes are presented in Fig. 4. Under experimental condition number 1 for GN-PtNPs-EX1, the high-resolution Pt 4f spectrum was characterized by a doublet, corresponding to the Pt 4f7/2

Table 6 e Results of the confirmation experiment for methanol oxidation current (MOC) and If/Ib ratio. Response variables MOC If/Ib ratio

Optimal combination of parameters used

Significant parameters (at 90% confidence level)

Predicted optimum value

Experimental value

A3B1C2 A3B1C2

A, C A, B

39.20 mA 2.83

37.1 mA 2.67

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8

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0

Fig. 3 e TEM images of (a) plain graphene, (b) GN-PtNPs-EX1, and (c) GN-PtNPs-OPT.

Fig. 4 e The XPS spectra of (a) GN-PtNPs-EX1 and (b) GN-PtNPs-OPT.

(71.1 eV) and Pt 4f5/2 (74.4 eV) signals (Fig. 4a) [60,61]. The two peaks had a binding energy difference of 3.33 eV and a peak area ratio of 4:3, which are well-recognized as characteristic peaks of Pt 4f. The Pt 4f spectrum can be deconvoluted into two species of Pt denoted as Pt (0) and Pt (II) with binding energies of 71.4 and 72.8 eV, respectively, at the 4f7/2 signal peak. The relative intensities of the Pt (0) and Pt (II) species were determined to be 85.0% and 15.0%, respectively. This indicates that the predominant Pt species in GN-PtNPs-EX1 was the zero-valent metallic Pt state. Likewise, the XPS spectrum of GN-PtNPs-OPT (Fig. 4b) prepared under optimal conditions also showed doublet peaks and could be deconvoluted into Pt (0) and Pt (II) species. The relative intensities of Pt (0) and Pt (II) were 89.6% and 10.4%, respectively. In comparison, the zero-valent metallic state of Pt in GN-PtNPs-OPT was slightly higher than that of GN-PtNPs-EX1. The presence of Pt (II) state in the XPS spectra suggests that not all Pt particles on graphene are in metallic state and some of the Pt particles are anchored and bonded to the oxygen functional groups on the graphene frameworks. The higher methanol oxidation current and If/Ib ratio rendered by the GN-PtNPs-OPT electrode suggest that the metallic Pt state is more electrochemically active for the MOR than the oxidized state of Pt [8,13,39]. This finding is consistent with the EASA results, which indicated

an increase in the Pt active sites in the GN-PtNPs-OPT electrode compared to that of GN-PtNPs-EX1.

Conclusion The Taguchi DOE with an L9 orthogonal array was successfully implemented to optimize the pulse current electrodeposition parameters, which in turn yielded a GN-PtNPs electrode with the best performance for MOR. The applied current density was found to be the most significance factor affecting the performance of the GN-PtNPs electrode in terms of the methanol oxidation activity and tolerance against poisoning. The pulse current electrodeposition method is a promising method to prepare low content and welldispersed Pt particles on graphene. Under the optimal electrodeposition conditions, the dendritic Pt particles were uniformly deposited on graphene sheets with approximately 90 at. % of the Pt species in a zero-valent state. The high performance of the optimized GN-PtNPs electrode in terms of the methanol oxidation current and tolerance against poisoning can be attributed to the dendritic structure and the metallic state of Pt, which give rise to more active sites for the MOR to occur.

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0

Acknowledgement This work is supported by the Universiti Kebangsaan Malaysia through the University Research Grants (Grant No. DIP-2017024 and DIP-2014-002).

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Appendix A. Supplementary data [16]

Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.07.016. [17]

references

[1] Meng F, Zhong H, Bao D, Yan J, Zhang X. In situ coupling of strung Co4N and intertwined NeC fibers toward freestanding bifunctional cathode for robust, efficient, and flexible ZneAir batteries. J Am Chem Soc 2016;138:10226e31. [2] Meng FL, Wang ZL, Zhong HX, Wang J, Yan JM, Zhang XB. Reactive multifunctional template-induced preparation of Fe-N-Doped mesoporous carbon microspheres towards highly efficient electrocatalysts for oxygen reduction. Adv Mater 2016;28:7948e55. [3] Lu J, Li Y, Li S, Jiang SP. Self-assembled platinum nanoparticles on sulfonic acid-grafted graphene as effective electrocatalysts for methanol oxidation in direct methanol fuel cells. Sci Rep 2016;6:21530. [4] Wang Z-L, Xu D, Xu J-J, Zhang X-B. Oxygen electrocatalysts in metal-air batteries: from aqueous to nonaqueous electrolytes. Chem Soc Rev 2014;43:7746e86. [5] Wang Z-L, Xu D, Zhong H-X, Wang J, Meng F-L, Zhang X-B. Gelatin-derived sustainable carbon-based functional materials for energy conversion and storage with controllability of structure and component. Sci Adv 2015;1. [6] Wang Z-L, Yan J-M, Wang H-L, Ping Y, Jiang Q. Au@Pd coreshell nanoclusters growing on nitrogen-doped mildly reduced graphene oxide with enhanced catalytic performance for hydrogen generation from formic acid. J Mater Chem A 2013;1:12721e5. [7] Zhang LY, Zhang W, Zhao Z, Liu Z, Zhou Z, Li CM. Highly poison-resistant Pt nanocrystals on 3D graphene toward efficient methanol oxidation. RSC Adv 2016;6:50726e31. [8] Zhang J, Ma L, Gan M, Fu S, Zhao Y. TiN@nitrogen-doped carbon supported Pt nanoparticles as high-performance anode catalyst for methanol electrooxidation. J Power Sources 2016;324:199e207. [9] Tian XL, Wang L, Deng P, Chen Y, Xia BY. Research advances in unsupported Pt-based catalysts for electrochemical methanol oxidation. J Energy Chem 2017;26:1067e76.  AS, Baglio V, Antonucci V. Direct methanol fuel cells: [10] Arico history, status and perspectives. Electrocatalysis of direct methanol fuel cells. Wiley-VCH Verlag GmbH & Co. KGaA; 2009. p. 1e78. [11] Xue Q, Xu G, Mao R, Liu H, Zeng J, Jiang J, et al. Polyethyleneimine modified AuPd@PdAu alloy nanocrystals as advanced electrocatalysts towards the oxygen reduction reaction. J Energy Chem 2017;26:1153e9. [12] Xu GR, Han CC, Zhu YY, Zeng JH, Jiang JX, Chen Y. PdCo alloy Nanonetworks
Polyallylamine inorganiceorganic nanohybrids toward the oxygen reduction reaction. Adv Mater Interfaces 2018;5:1701322. [13] Hu Y, Wu P, Yin Y, Zhang H, Cai C. Effects of structure, composition, and carbon support properties on the

[18] [19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30] [31]

9

electrocatalytic activity of Pt-Ni-graphene nanocatalysts for the methanol oxidation. Appl Catal B Environ 2012;111e112:208e17. Hays CC, Manoharan R, Goodenough JB. Methanol oxidation and hydrogen reactions on NiZr in acid solution. J Power Sources 1993;45:291e301. Okamoto H, Kawamura G, Ishikawa A, Kudo T. Characterization of oxygen in WC catalysts and its role in electrocatalytic activity for methanol oxidation. J Electrochem Soc 1987;134:1645e9. Kang Y, Li F, Li S, Ji P, Zeng J, Jiang J, et al. Unexpected catalytic activity of rhodium nanodendrites with nanosheet subunits for methanol electrooxidation in an alkaline medium. Nano Res 2016;9:3893e902. Kang Y, Xue Q, Jin P, Jiang J, Zeng J, Chen Y. Rhodium nanosheetsereduced graphene oxide hybrids: a highly active platinum-alternative electrocatalyst for the methanol oxidation reaction in alkaline media. ACS Sustainable Chem Eng 2017;5:10156e62. Bashyam R, Zelenay P. A class of non-precious metal composite catalysts for fuel cells. Nature 2006;443:63. Kakati N, Maiti J, Lee SH, Jee SH, Viswanathan B, Yoon YS. Anode catalysts for direct methanol fuel cells in acidic media: Do we have any alternative for Pt or PteRu? Chem Rev 2014;114:12397e429. Zhong H-x, Li K, Zhang Q, Wang J, Wu Z-j, Meng F-l, et al. In situ anchoring of Co9S8 nanoparticles on N and S co-doped porous carbon tube as bifunctional oxygen electrocatalysts. NPG Asia Mater 2016;8:e308. Huang W, Wang H, Zhou J, Wang J, Duchesne PN, Muir D, et al. Highly active and durable methanol oxidation electrocatalyst based on the synergy of platinumenickel hydroxideegraphene. Nat Commun 2015;6:10035. Chen X, Wu G, Chen J, Chen X, Xie Z, Wang X. Synthesis of “clean” and well-dispersive Pd nanoparticles with excellent electrocatalytic property on graphene oxide. J Am Chem Soc 2011;133:3693e5. Yang M, Guarecuco R, DiSalvo FJ. Mesoporous chromium nitride as high performance catalyst support for methanol electrooxidation. Chem Mater 2013;25:1783e7.    P, Selih evar S, Ruiz-Zepeda F, Jovanovic VS, Sala M, Hoc Hodnik N, et al. Potentiodynamic dissolution study of PtRu/C electrocatalyst in the presence of methanol. Electrochim Acta 2016;211:851e9. Zhao X, Yin M, Ma L, Liang L, Liu C, Liao J, et al. Recent advances in catalysts for direct methanol fuel cells. Energy Environ Sci 2011;4:2736e53. Huang T, Mao S, Zhou G, Zhang Z, Wen Z, Huang X, et al. A high-performance catalyst support for methanol oxidation with graphene and vanadium carbonitride. Nanoscale 2015;7:1301e7. Tiwari JN, Tiwari RN, Singh G, Kim KS. Recent progress in the development of anode and cathode catalysts for direct methanol fuel cells. Nano Energy 2013;2:553e78. Zhao Y, Fan L, Ren J, Hong B. Electrodeposition of PteRu and PteRueNi nanoclusters on multi-walled carbon nanotubes for direct methanol fuel cell. Int J Hydrogen Energy 2014;39:4544e57. Cheng Y, Jiang SP. Highly effective and CO-tolerant PtRu electrocatalysts supported on poly(ethyleneimine) functionalized carbon nanotubes for direct methanol fuel cells. Electrochim Acta 2013;99:124e32. Stoller MD, Park S, Zhu Y, An J, Ruoff RS. Graphene-based ultracapacitors. Nano Lett 2008;8:3498e502. Bolotin KI, Sikes KJ, Jiang Z, Klima M, Fudenberg G, Hone J, et al. Ultrahigh electron mobility in suspended graphene. Solid State Commun 2008;146:351e5.

Please cite this article in press as: Toh SY, et al., Facile preparation of ultra-low Pt loading graphene-immobilized electrode for methanol oxidation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.07.016

10

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0

[32] Park S, Ruoff RS. Chemical methods for the production of graphenes. Nat Nano 2009;4:217e24. [33] Li Y, Gao W, Ci L, Wang C, Ajayan PM. Catalytic performance of Pt nanoparticles on reduced graphene oxide for methanol electro-oxidation. Carbon 2010;48:1124e30. [34] Chen H, Duan J, Zhang X, Zhang Y, Guo C, Nie L, et al. One step synthesis of Pt/CeO2egraphene catalyst by microwaveassisted ethylene glycol process for direct methanol fuel cell. Mater Lett 2014;126:9e12. [35] Zhang Z, Lu Z-H, Chen X. Ultrafine NiePt alloy nanoparticles grown on graphene as highly efficient catalyst for complete hydrogen generation from hydrazine borane. ACS Sustainable Chem Eng 2015;3:1255e61. [36] Zhou Y, Yang J, Zhu C, Du D, Cheng X, Yen CH, et al. Newly designed graphene cellular monolith functionalized with hollow Pt-M (M ¼ Ni, Co) nanoparticles as the electrocatalyst for oxygen reduction reaction. ACS Appl Mater Interfaces 2016;8:25863e74. [37] Lee J, Seo J, Han K, Kim H. Preparation of low Pt loading electrodes on Nafion (Naþ)-bonded carbon layer with galvanostatic pulses for PEMFC application. J Power Sources 2006;163:349e56. [38] Choi KH, Kim HS, Lee TH. Electrode fabrication for proton exchange membrane fuel cells by pulse electrodeposition. J Power Sources 1998;75:230e5. [39] Woo S, Lee J, Park S-K, Kim H, Chung TD, Piao Y. Electrochemical codeposition of Pt/graphene catalyst for improved methanol oxidation. Curr Appl Phys 2015;15:219e25. [40] Zhou Y-G, Chen J-J, Wang F-b, Sheng Z-H, Xia X-H. A facile approach to the synthesis of highly electroactive Pt nanoparticles on graphene as an anode catalyst for direct methanol fuel cells. Chem Commun 2010;46:5951e3. [41] Radhakrishnan T, Sandhyarani N. Three dimensional assembly of electrocatalytic platinum nanostructures on reduced graphene oxide e an electrochemical approach for high performance catalyst for methanol oxidation. Int J Hydrogen Energy 2017;42:7014e22. [42] Hsieh C-T, Wei J-M, Hsiao H-T, Chen W-Y. Fabrication of flower-like platinum clusters onto graphene sheets by pulse electrochemical deposition. Electrochim Acta 2012;64:177e82. [43] Hsu IJ, Esposito DV, Mahoney EG, Black A, Chen JG. Particle shape control using pulse electrodeposition: methanol oxidation as a probe reaction on Pt dendrites and cubes. J Power Sources 2011;196:8307e12. [44] Ye W, Zhang X, Chen Y, Du Y, Zhou F, Wang C. Pulsed electrodeposition of reduced graphene oxide on glass carbon electrode as an effective support of electrodeposited Pt microspherical particles: nucleation studies and the application for methanol electro-oxidation. Int J Electrochem Sci 2013;8. [45] Du H-Y, Yang C-S, Hsu H-C, Huang H-C, Chang S-T, Wang CH, et al. Pulsed electrochemical deposition of Pt NPs on polybenzimidazole-CNT hybrid electrode for hightemperature proton exchange membrane fuel cells. Int J Hydrogen Energy 2015;40:14398e404. [46] Yaldagard M, Seghatoleslami N, Jahanshahi M. Preparation of Pt-Co nanoparticles by galvanostatic pulse

[47]

[48]

[49]

[50]

[51]

[52]

[53] [54]

[55] [56]

[57]

[58]

[59]

[60]

[61]

electrochemical codeposition on in situ electrochemical reduced graphene nanoplates based carbon paper electrode for oxygen reduction reaction in proton exchange membrane fuel cell. Appl Surf Sci 2014;315:222e34. Liu J, Du X, Yang Y, Deng Y, Hu W, Zhong C. A one-step, clean, capping-agent-free electrochemical approach to prepare Pt nanoparticles with preferential (100) orientation and their high electrocatalytic activities. Electrochem Commun 2015;58:6e10. Wang D, Yan W, Vijapur SH, Botte GG. Electrochemically reduced graphene oxideenickel nanocomposites for urea electrolysis. Electrochim Acta 2013;89:732e6. Georgieva M, Andonovski B. Determination of platinum(IV) by UV spectrophotometry. Anal Bioanal Chem 2003;375:836e9. Xu S, Ye L, Li Z, Wang Y, Lei F, Lin S. Facile synthesis of bimetallic Pt-Ag/Graphene composite and its electro-photosynergistic catalytic properties for methanol oxidation. Catalysts 2016;6. Yung T-Y, Lee J-Y, Liu L-K. Nanocomposite for methanol oxidation: synthesis and characterization of cubic Pt nanoparticles on graphene sheets. Sci Technol Adv Mater 2013;14:035001. Yavari Z, Noroozifar M, Khorasani-Motlagh M. The improvement of methanol oxidation using nanoelectrocatalysts. J Exp Nanosci 2016;11:798e815. Roy RK. A primer on the Taguchi method. 2nd ed. Society of manufacturing engineers; 2010. Mughal MA, Newell MJ, Vangilder J, Thapa S, Wood K, Engelken R, et al. Optimization of the electrodeposition parameters to improve the stoichiometry of films for solar applications using the Taguchi method. J Nanomater 2014;2014:10. Ross PJ. Taguchi techniques for quality engineering. 2nd ed. McGraw-Hill; 1995. Xiao Y, Fu Z, Zhan G, Pan Z, Xiao C, Wu S, et al. Increasing Pt methanol oxidation reaction activity and durability with a titanium molybdenum nitride catalyst support. J Power Sources 2015;273:33e40. Xia BY, Wu HB, Wang X, Lou XW. One-pot synthesis of cubic PtCu3 nanocages with enhanced electrocatalytic activity for the methanol oxidation reaction. J Am Chem Soc 2012;134:13934e7. Wang X, Zhang X, He X, Ma A, Le L, Lin S. Facile electrodeposition of flower-like PMo12-Pt/rGO composite with enhanced electrocatalytic activity towards methanol oxidation. Catalysts 2015;5. Wang H, Wang Y, Zhu Z, Sapi A, An K, Kennedy G, et al. Influence of size-induced oxidation state of platinum nanoparticles on selectivity and activity in catalytic methanol oxidation in the gas phase. Nano Lett 2013;13:2976e9. Moulder JF, Chastain J. Handbook of X-ray photoelectron spectroscopy: a reference book of standard spectra for identification and interpretation of XPS data: physical electronics division. Perkin-Elmer Corporation; 1992. Woicik JC. Hard X-ray photoelectron spectroscopy (HAXPES). Springer International Publishing; 2016.

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