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Nitrogen-doped graphene supported palladium-nickel nanoparticles with enhanced catalytic performance for formic acid oxidation
Electrochimica Acta 220 (2016) 83–90
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Nitrogen-doped graphene supported palladium-nickel nanoparticles with enhanced catalytic performance for formic acid oxidation Yanxian Jin* , Jie Zhao, Fang Li, Wenping Jia, Danxia Liang, Hao Chen, Rongrong Li, Jiajie Hu, Jiamin Ni, Tingqian Wu, Danping Zhong School of Pharmaceutical and Chemical Engineering, Taizhou University, Taizhou 318000, China
A R T I C L E I N F O
Article history: Received 16 June 2016 Received in revised form 9 October 2016 Accepted 13 October 2016 Available online 13 October 2016 Keywords: Pd nanocatalysts Nickel N-doped Formic acid Electro-oxidation
A B S T R A C T
Nitrogen-doped graphene (NG) supported palladium-nickel nanoparticles with uniform dispersion are synthesized as catalysts for the electro-oxidation of formic acid. The catalysts are characterized by X-ray powder diffraction (XRD), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Our TEM results show that the Pd/Ni nanoparticles on NG are uniform with narrower size distribution than those on native graphene. XPS analysis reveals that Pd/NG and 1Pd1Ni/NG systems have more Pd0 and less Pd2+ content in comparison with the Pd/G and 1Pd1Ni/G counterparts respectively. This difference might be caused by the electron-donating effects of nitrogen species on the graphene surface. In addition, the electrochemical results show significantly enhanced catalytic activity and stability of 1Pd1Ni/NG as the catalyst for the formic acid oxidation reaction. The enhanced activity is not only attributed to the better uniform dispersion of nanoparticles and the enhanced electronic effect between metal particles and the support, but also ascribed to the synergistic effect of Ni incorporation to Pd. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Recently, direct formic acid fuel cells (DFAFCs) have attracted considerable research interests as promising clean energy for portable applications because they exhibit many advantages such as high efficiency, low emission and low toxicity [1–3]. Comparing to Pt, the Pd catalysts are the more popularly used anode catalysts in DFAFCs because of their higher catalytic performance and the less poisoning effect by proceeding primarily through direct pathway where formic acid is directly oxidized into CO2 [4,5]. However, the poor long-term stability on Pd catalysts represents a major problem. Thus, some noble metals and transitional metals such as Au, Pt, Sn, Co, Ni and Pb have been added into Pd catalysts in order to improve the the catalytic performance through alloy effect [1,6–10]. Moreover, the addition of transitional metals can reduce the cost of the catalysts. Particularly, Ni has been applied as one of the promising additive components in Pd-based catalysts [11–13]. For example, Du et al. [11] reported that nanoporous Pd57Ni43 alloy nanowire was a catalyst with the high electrocatalytic
* Correspondence to: No. 1139 Shifu Road, Jiaojiang District of Taizhou City, Zhejiang Province, China. E-mail address: snowfl[email protected] (Y. Jin). http://dx.doi.org/10.1016/j.electacta.2016.10.087 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
performance for formic acid oxidation. Lu et al. [13] reported the electrocatalytic performance of the Pd-Ni/C catalyst for formic acid oxidation was much better than that of the Pd/C catalyst. However, the development of more efficient anode catalyst systems with high catalytic activity and excellent durability still remains a big challenge towards the commercialization of DFAFCs. Graphene has emerged as a promising support material for fuel cell electrodes due to its excellent electrical conductivity and unique physical and chemical properties [14]. The different forms of heteroatomic doping might be able to offer additional functions on the surface of graphene, which have been used in the constructions of fuel cells, chemical battery and super capacitor, etc [15]. Heteroatom doping (nitrogen, sulfur, boron etc.) is particularly an important way to tailor physicochemical property and chemical reactivity of graphene [16–18]. For example, compared to the commercial Pt/C catalyst, the application of Pt/ NG catalysts (nitrogen-doped graphene as a support) led to a much better comprehensive performance including higher activity and better stability in the methanol electro-oxidation reaction [19–21]. In addition, Pd nanoparticles (NPs) supported on N and S dualdoped graphene (NS-G) nanosheets exhibited outstanding electrocatalytic performance toward both formic acid and methanol electro-oxidation than those of Pd/Vulcan XC-72R and Pd/undoped graphene catalysts [18]. Nitrogen doping can not only induce
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anchoring sites for well dispersion of metallic NPs on the surface of graphene, but also modulate electronic property of graphene, thus directly affect the activity and the stability of metallic NPs during the methanol oxidation reaction [5]. Another example is that PdPb/ N-doped graphene catalyst exhibited remarkably enhanced activity and durability for electrocatalytic ethanol oxidation. N-doped graphene can bring about high dispersity of metallic nanoparticles, and the electronic effect between support and Pd nanoparticles can be enhanced [22]. Besides doped graphene, the effect of doping on Pd NPs nucleation and growth is well documentated by several papers reporting Pd NPs deposition by several methods on doped graphene [23], doped glassy carbon [24], doped HOPG [25], doped carbon nanotubes [26] or doped carbon [27,28]. For example, Granozzi and coworkers [28] reported that the presence of nitrogen defects played a significant role in improving the metal particles dimension and dispersion. When nitrogen-doped mesoporous carbon were used, the resulting metal nanoparticles were smaller (24 nm) and less prone to aggregation. Also, according to Gaetano Granozzi’s report [25], a drastic reduction had been observed in the size distribution (2.84 1.13 nm) at 500 Cwhen Pd deposition was performed on N-HOPG. On the other hand, PdNi hollow NPs [29] or PdNi alloyed NPs deposition by several methods on single wall carbon nanotubes [30], graphene [12], and nitrogen-doped graphene [31] have been reported. Similarly, PdNi alloyed NPs nucleation and growth will be affected by nitrogen-doping support [29]. In fact, a uniform dispersion of Ni@Pd nanoparticles on nitrogen-doped reduced graphene oxide had a 2.8 nm average particle size [29]. However, studies on the nitrogen-doped graphene (NG) as the support of Pd catalysts for the formic acid electro-oxidation have been rarely reported. In this work, we present a novel catalyst for formic acid eletro-oxidation using NG as support and Pd/Ni nanoparticles as the active components. The composition, structure and morphology of the catalysts are characterized. The electrochemical properties of the catalysts towards formic acid oxidation and underlying mechanism have also been systemically investigated. 2. Experimental 2.1. Catalyst preparation Graphite oxide (GO) material has been prepared by Hummers method [32] from graphite powder (Aldrich, powder, <20 micron) [33]. NG material was synthesized as followed. Firstly, 300 mg GO material and 50 mL of 25 wt.% aqueous ammonia (NH3H2O) were mixed under ultrasonic stirring until a kind of ink-like suspension was formed. Subsequently, the ink-like suspension solution was poured into a Teflon bottle of 100 mL, which was placed in a stainless steel autoclave. Then it was sealed and held at 200 C for 5 h. After filtered and cooled, the precipitate was washed with excess distilled water by vacuum filtration, and dried under vacuum at 90 C. The 2Pd1Ni/NG catalyst was synthesized as followed. Firstly, 2.18 mL of palladium chloride solution (0.01 mol L1) and 7.01 mg of NiSO46H2O were added into 30 mL of double-distilled water. Secondly, 50 mg of NG material was ultrasonically for 30 min until well dispersed in the solution. The obtained suspension was transferred to a four-neck bottle and heated to 60 C. Then, 210 mg of KBH4 was dissolved in 50 mL of water and this solution was added into the suspension drop by drop. This reaction continued to stay for 6 h under 60 C. Finally, the slurry was filtered and dried under vacuum at 90 C. 1Pd1Ni/NG, 1Pd2Ni/NG and Pd/NG samples were prepared by the same method. For comparison, 1Pd1Ni/G and Pd/G samples were also prepared by the similar method, using
graphene as support. Millipore-Milli Q water and analytical-grade reagents were selected to make up all the solutions. Nickel sulfate hexahydrate (NiSO46H2O), aqueous ammonia (NH3H2O), patassium boron hydride (KBH4) and palladium chloride (PdCl2) were purchased from Shanghai Chemical Reagent Co. 2.2. Catalyst characterization Transmission electron microscopy (TEM) were recorded on the JEOL-2010 microscope at a potential of 200 kV and a current of 103 mA. X-ray diffraction (XRD) measurements were performed on Philips PW3040/60 X-ray diffractometer at the condition of Cu Ka radiation wavelength l = 0.15406 nm. X-ray photoelectron spectroscopy (XPS) measurements and analyses were carried out on a ESCALAB 250Xi apparatus. Metal contents of the catalysts were determined by inductively coupled plasma optical emission spectrometer (ICP-OES) (Agilent 725). The electrochemical performance of the sample was tested using three electrode system on CHI730d electrochemical workstation. A certain amout of ethanol, 5 wt.% Nafion solution and 5 mg of catalyst powders were mixed under ultrasonic stirring until a kind of ink-like suspension was formed, and then 10 uL of ink-like suspension was coated on a mirror-polished glassy carbon disk electrode with 4 mm diameter, thus it was used as working electrode. The final catalyst loading was 0.175 mgcm2. The saturated calomel electrode (SCE) was used as the reference electrode, and a salt bridge had been used to prevent the chloride contamination in the solution. Pt foil was used as the counter electrode. Before all electrochemical measurements, a high purity N2 gas flow for about 30 min was carried out to remove dissolved oxygen from the electrolyte. All tests were carried out in an inert atmosphere, and the experiment was carried out at 25 C. 3. Results and discussion 3.1. Physicochemical characterization The phase structures of different catalysts with different Pd/Ni atomic ratios and support were investigated by XRD and the patterns are shown in Fig. 1. For all the samples, the (002) planes of N-graphene or graphene could be well recognized by the broad diffraction peak located at around 26 . The strong diffraction peaks at around 40.0 , 46.5 , 68.0 and 82.0 are attributed to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) lattice planes of the face-centered cubic structures of palladium, respectively (JCPDS No.46-1043). As shown in the inset of Fig. 1, it’s observed that Pd peaks for all
Fig. 1. XRD patterns of Pd/G, Pd/NG and PdNi/NG with different Pd/Ni atomic ratios.
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the PdNi/NG catalysts with different Pd/Ni atomic ratios shift positively compared to those of Pd/NG. This indicates PdNi alloy are formed and attributed to the same alloy type since xPdyNi alloys have the very same XRD pattern and peak position [34]. The alloying degree (x, at.% Ni incorporated into the Pd lattice) was estimated by Eq. (1) [35], where rNi and rPd should be 1.25 Å and 1.37 Å respectively, whereas aalloy and aPd mean calculated lattice constants for PdNi alloy and Pd, respectively. xð1
aalloy rNi Þ¼1 rPd aPd
ð1Þ
The lattice parameters for PdNi/NG and Pd/NG were evaluated from the Pd (220) diffraction according to Bragg's law [36]. All of the calculations, along with other physicochemical parameters are summarized in Table 1. Fig. 2a–f show the typical TEM images of the Pd/G, Pd/NG and 1Pd1Ni/NG catalysts and Fig. 2g–i display the corresponding particle size distribution histograms. As seen, non-uniform Pd particles with significant aggregation are observed on Pd/G sample (Fig. 2a), while fine and well-dispersed metallic nanoparticles are obtained on Pd/NG (Fig. 2b and e) and 1Pd1Ni/NG (Fig. 2c and f) samples. According to the particle size histograms, particles on Pd/ G catalyst display a distinctly broader size distribution between 5 and 32 nm in diameter with a mean size of 14.95 nm (Fig. 2g). In contrast, Pd nanoparticles are found to have a uniform dispersion on the NG support with an average particle size of 2–12 nm, centered in 4.3 nm at Pd/NG catalyst (Fig. 2h). Whereas, the particle size distribution of 1Pd1Ni/NG catalyst become even narrower in a range from 1 to 4 nm with mean particle size of 1.6 nm (Fig. 2i). The same results can be found on nitrogen-doped mesoporous carbon. The presence of homogeneously distributed nitrogen functional groups can provide nucleation sites and thus promote a higher dispersion of metal NPs [28]. In our case, the same conclusion can be drawn that N-doped graphene is beneficial to anchoring metal nanoparticles. XPS was carried out to analyze the electronic structure of the initial Pd/NG, 1Pd1Ni/NG, Pd/G and 1Pd1Ni/G samples, and the typical XPS spectra are shown in Fig. 3a–g, while the element content and functional groups with peak position, attribution and components of the samples are summarized in Table 2. In the XPS spectra of 3a and 3b, the peaks at binding energies of 398.3, 399.8, 401.1 and 403.5 eV are attributed to four types of nitrogen functionalities: the pyridinic (N1), pyrrolic (N2), graphitic N (N3) and oxidized N (N4) [37–41]. According to Table 2, both of N contents in Pd/NG and 1Pd1Ni/NG are ca. 8.37 and 7.17 at.%, and the proportion of four components N1, N2, N3 and N4 on Pd/NG and 1Pd1Ni/NG catalysts are 34.80: 44.28: 16.11: 4.8 and 29.50: 57.81: 10.66: 2.03 respectively, indicating the pyrrolic (N2) is dominant in the formed nitrogen functional groups. A reasonable amount of N species in the catalysts may help metal nanoparticles disperse uniformly on the support, which are depicted in TEM images. The C 1s spectrum of 1Pd1Ni/NG in Fig. 3h could be reasonably fitted with four peaks, the most intense one at the binding energy of 284.8 eV is ascribed to C-C, and the other three weak peaks at 285.71, 287.0 and 290.66 eV correspond to C-N, C-O and O-C¼O,
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respectively [42], indicating the successful introduction of nitrogen atoms and the existence of oxygen containing functional groups in graphene. Also, the oxygen containing functional groups can be identified from the O 1s spectrum of 1Pd1Ni/NG in Fig. 3i. Ni 2p signal of 1Pd1Ni/NG is shown in Fig. 3g. Ni 2p3/2 and Ni 2p1/2 binding energies at 856.2 eV and 873.9 eV with obvious the shake-up peaks at 861.6 and 879.9 eV can be assigned to Ni(OH)2 and NiOOH [43]. Why does XPS show no trace of Ni? One reason is that there is small quantity of PdNi alloy. The other one is little surface-distributed Ni has been oxidized in ambient conditions [12]. These oxides are beneficial to electro-catalytic oxidation of small organic molecules [12,43]. Pd 3d signals of Pd/NG, 1Pd1Ni/NG, Pd/G and 1Pd1Ni/G samples are divided into three pairs of asymmetric peaks in Fig. 3c–f. The two peaks at 335.45 and 340.76 eV are contributed to Pd(0) 3d5/2 and Pd(0) 3d3/2, other two weak doublets at 335.91 and 340.99 eV belong to PdO species. The third pair of peaks at 337.51 and 342.52 eV correspond to PdCl2, an indication of incomplete reduction of the Pd precursor under this condition [12]. According to the results of Table 2, the total content of Pd(0) combined Pd(0) 3d5/2 and Pd(0) 3d3/2 on Pd/G sample is 9.33%, while that on Pd/NG sample rises up to 24.51%. Similarly, the total content of Pd(0) on 1Pd1Ni/NG catalyst increases to 21.02%, compared to the content of 15.66% on 1Pd1Ni/G catalyst. In addition, Pd2+ contents on Pd/NG, 1Pd1Ni/NG, Pd/G and 1Pd1Ni/G are 41.63%, 34.19%, 68.52% and 61.21%, respectively. It was reported that the increased Pt0 components and decreased Pt2+ components were found on Pt/ NG catalysts. It was postulated that the electronic structure of the graphene can be modulated by N species, then the electronic property of the supported Pt catalysts can subsequently be modulated by the metal-support interaction [20]. Herein, increased Pd0 component and decreased Pd2+ component are found for the Pd/NG catalyst compared to Pd/G catalyst. Likewise, more Pd0 and less Pd2+ are observed on 1Pd1Ni/NG in comparison with 1Pd1Ni/G sample. It should possibly caused by the electron donation effects of nitrogen. Such modulation in electronic properties of the catalysts maybe improve their electrocatalytic performance [20]. 3.2. Electrochemical characterization Fig. 4 depicts the CV results of the different catalysts in a 0.5 M H2SO4 solution. The multiple peaks between 0.2 V and 0.1 V can be identified on all the catalysts, which are attributed to the hydrogen adsorption and desorption process. The double layer region can be found in the potential interval of 0.1-0.3 V. Apparently, the double layer region on Pd/NG catalyst gets larger compared with those on Pd/G catalyst. The same conclusion can be drawn that 1Pd1Ni/NG sample has larger double-layer capacitance compared with 1Pd1Ni/G sample. It maybe primarily caused by the great change of surface area, which is related to metal dispersion on the NG and G. Based on TEM results, 1Pd1Ni/NG possesses smaller average size of the metal particles, leading to largest surface area [36]. Moreover, the electrochemically active surface area (ECSA) values of all the catalysts were evaluated by the integration of the hydrogen adsorption areas [44,45], which are
Table 1 Structural characterization data of the catalysts.
Lattice parameter XRD/nm Alloying degree x/% Atomic ratio of Pd:Ni (ICP) Pd content in the sample/wt.%
2Pd1Ni/NG
1Pd1Ni/NG
1Pd2Ni/NG
Pd/NG
1Pd1Ni/G
Pd/G
0.3966 6.59 2.1:1 15.35
0.3952 10.44 1.2:1 16.57
0.3944 12.80 0.65:1 14.35
0.3977 / / 16.55
/ / 1.3:1 15.91
/ / / 15.65
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Fig. 2. TEM images of Pd/G (a/d), Pd/NG (b/e) and 1Pd1Ni/NG (c/f) and the particle size distribution of Pd/G (g), Pd/NG (h) and 1Pd1Ni/NG (i).
presented in the inset of Fig. 4. As shown, the ECSA value of 1Pd1Ni/ NG reaches 157 m2g1, which is much larger than those of Pd/NG and Pd/G (103 and 72 m2g1, respectively). The cyclic voltammograms (CV) results for electrodes with different Pd/Ni atomic ratios and different supports in 0.5 M H2SO4 containing 1 M HCOOH are shown in Fig. 5a and b. Clearly, a large peak at about 0.2 V and a very small peak at about 0.5 V can be recognized on all catalyst electrodes. As shown in Fig. 4b, Pd/NG and 1Pd1Ni/NG catalyst show bigger formic acid electrooxidation (FAEO) peak current density at 484 and 709 mAmg1, respectively, whereas Pd/G and 1Pd1Ni/G exhibit FAEO peak current density at 373 and 439 mAmg1. So N-doped graphene obviously can promote the activity of the catalysts with roughly the same metal components. In comparison with Pd/NG, the PdNi/NG catalysts with different Pd/Ni atomic ratios display better catalytic activity for FAEO. Among them, 1Pd1Ni/NG achieves a highest current density of 709 mAmg1, which is
1.9 times and 1.5 times as large as that of Pd/G and Pd/NG, respectively. It is inescapably clear that addition of Ni into the catalyst could enhance the catalytic performance for FAEO as well. So, it’s supposed that 1Pd1Ni/NG achieves the highest activity due to the synergistic effects of N-doping and Ni incorporation. In order to further evaluate the stability of all the catalysts, the chronoamperometric curves and the galvanostatic polarization curves on different catalysts are investigated in the solution of 1 M H2SO4 containing 1 M HCOOH at a constant potential of 0.4 V and at current density of 7.5 mAcm2, respectively, as shown in Fig. 6 and Fig. 7. According to Fig. 6, the current decayed continuously rapidly for the Pd/G catalyst, suggesting catalyst are poisoned by the chemisorbed carbonaceous species [46]. The Pd/NG is able to maintain the higher current density for over 1 h than Pd/G, and 1Pd1Ni/NG keeps higher current density than 1Pd1Ni/G during all the tested time.
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Fig. 3. XPS spectra of N 1s region in (a) Pd/NG and (b) 1Pd1Ni/NG, Pd 3d region in (c) Pd/NG, (d) 1Pd1Ni/NG, (e) Pd/G and (f) 1Pd1Ni/G; XPS spectra of (g) Ni 2p region, (h) C 1s region and (i) O 1s regions in 1Pd1Ni/NG catalyst.
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Table 2 Fitting results summary of Pd/NG, 1Pd1Ni/NG, Pd/G and 1Pd1Ni/G from XPS. Sample
Pd/NG 1Pd1Ni/NG Pd/G 1Pd1Ni/G
Element Content/at%
N functional groups/(BE, eV)
C
O
N
Pd
73.21 70.68 82.67 79.43
16.15 16.27 15.25 15.52
8.37 7.17 – –
2.27 2.6 2.08 2.3
Ni
3.28 2.75
N1 398.3
N2 399.8
N3 401.4
N4 403.5
34.80% 29.50% – –
44.28% 57.81% – –
16.11% 10.66% – –
4.80% 2.03% – –
Pd species/(BE, eV)
Pd/NG 1Pd1Ni/NG Pd/G 1Pd1Ni/G
Ni species/(BE, eV)
Pd(0) 3d5/2
PdO 3d5/2
PdCl2 3d5/2
Pd(0) 3d3/2
PdO 3d3/2
PdCl2 3d3/2
Ni(OH)2 2p3/2
335.45
335.91
337.51
340.76
340.99
342.52
856.19
861.62
873.86
879.9
13.9 8.45 8.17 5.32
22.46 32.11 12.49 19.38
21.92 17.22 35.80 34.83
10.61 12.57 1.16 10.34
11.40 12.66 9.66 3.75
19.71 16.97 32.72 26.38
– 35.64 – 34.76
– 31.8 – 39.76
– 15.26 – 12.23
– 17.3 – 13.24
NiOOH 2p1/2
N1: Pyridinic, N2: Pyrrolic,N3: Graphitic N, N4: Oxidized N.
Fig. 4. Cyclic voltammograms in 0.5 M H2SO4 solution for 2Pd1Ni/NG, 1Pd2Ni/NG, 1Pd1Ni/NG, Pd/NG, Pd/G and 1Pd1Ni/G electrodes. Scan rate: 20 mV/s.
Chronopotentiometry is a useful method to study the resistance to poisoning of catalysts during formic acid oxidation [47]. As shown in Fig. 7, the initial potential on Pd/G increases continuously more rapidly than that of Pd/NG during all the time, whereas 1Pd1Ni/G keeps rising more rapidly than that of 1Pd1Ni/NG with the higher steady potential. During the experiment, the poisonous species such as COads rising from the formic acid oxidation, will accumulate on the surface of the electrocatalyst and reduce its electrocatalytic activity. The potential must be increased to satisfy the applied anodic current density [47–50]. The higher steady potential and the shorter sustained time during the tested time suggest the weaker poisoning resistance and more poor stability of the catalysts [47–49,51]. Based on the above results, there is a conclusion that N-doped graphene can particularly improve the resistance to poisoning and the the stability of the catalysts with roughly the same metal components, compared to native graphene. The FAEO reaction could proceed through two parallel pathways [3]. One is the direct path being characteristic of direct oxidation of formic acid into CO2, the other is indirect path where CO species, rising from nonfaradaic dissociation of formic acid, are adsorbed on the particle surface and then CO species are oxidized into CO2 [52]. The mechanism of FAEO on Pd is predominately through the direct path. However, the accumulation of poisoning intermediates such as CO will poison the catalysts during the reaction. Ni species such as Ni(OH)2 are oxophilic, so oxygen species generate on its surface at low potentials, which can react with poisoning intermediates such as CO-like species and help to remove effectively CO species from the surface [12]. Therefore, more active sites will be released and promote direct oxidation of formic acid into CO2, which can result in the enhancement in the catalytic stability and activity. On the other hand, the existence of the nitrogen functional groups in the graphene substrates, which leads to smaller particle size of the deposited Pd/Ni nanoparticles. Additionally, the incorporation of nitrogen might cause the improvement of the electrocatalytic performance by the beneficial metal-support interactions via the electronic structure modification of graphene, as verified from the previous TEM spectra and XPS analysis. 4. Conclusions
Fig. 5. Cyclic voltammograms in 1 M HCOOH + 0.5 M H2SO4 solution for 2Pd1Ni/NG, 1Pd2Ni/NG, 1Pd1Ni/NG, Pd/NG, Pd/G and 1Pd1Ni/G electrodes. Scan rate: 20 mV/s.
This study demonstrates that N-doped graphene supported palladium-nickel nanoparticles catalysts exhibit outstanding catalytic activity and stability for the formic acid electro-oxidation.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21403150 and 21506138), Zhejiang Provincial Natural Science Foundation of China (Nos. LQ12B03003 and LQ15B060001), Science and technology project of Zhejiang Province (Nos. 2014C37052), National Students’ innovation and entrepreneurship training program (Nos. 201510350013), Science and Technology Project of Taizhou City (Nos. 1202ky03). References
Fig. 6. Chronoamperometric curves of 2Pd1Ni/NG, 1Pd2Ni/NG, 1Pd1Ni/NG, Pd/NG, Pd/G and 1Pd1Ni/G electrodes in 0.5 M H2SO4 containing 1 M HCOOH at 0.4 V.
Fig. 7. Galvanostatic polarization curves for 2Pd1Ni/NG, 1Pd2Ni/NG, 1Pd1Ni/NG, Pd/NG, Pd/G and 1Pd1Ni/G electrodes in 0.5 M H2SO4 containing 1 M HCOOH solution at current density of 7.5 mAcm2.
According to TEM and XPS results, metal nanoparticles show the more uniform dispersion on nitrogen doped graphene in comparison with native graphene. Increased Pd0 and decreased Pd2+ component are found for the Pd/NG and 1Pd1Ni/NG catalysts in comparison with the Pd/G and 1Pd1Ni/G counterparts respectively, which is possibly related with the electron donation effects of nitrogen. The results demonstrate that 1Pd1Ni/NG catalyst achieves dramatically improved catalytic stability and remarkably enhanced activity up to 709 mAcm2 for FAEO, which is 1.9 times and 1.5 times as large as that of Pd/G and Pd/NG catalysts. The enhanced electrocatalytic activities of the N-doped graphene supported Pd/Ni particles could be attributed to the following three aspects. Firstly, N-doping can cause modulation in electronic properties of the catalysts by the beneficial metalsupport interactions. The modulation in electronic properties has been shown by XPS results, where more Pd0 and less Pd2+ content are obviously found on the Pd/NG and 1Pd1Ni/NG catalysts. Secondly, N-doping could provide efficient anchor sites for good dispersion of metallic nanoparticles on the surface of support, which can increase catalytic surface area accessible to the formic acid and electrolyte solution. Finally, Ni species such as Ni(OH)2 could help to generate abundant oxygenated species on its surface at low potentials, which may quickly remove COads intermediates and release more Pd active sites, and thus promote the direct oxidation of formic acid.
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Nitrogen-doped graphene supported palladium-nickel nanoparticles with enhanced catalytic performance for formic acid oxidation Yanxian Jin* , Jie Zhao, Fang Li, Wenping Jia, Danxia Liang, Hao Chen, Rongrong Li, Jiajie Hu, Jiamin Ni, Tingqian Wu, Danping Zhong School of Pharmaceutical and Chemical Engineering, Taizhou University, Taizhou 318000, China
A R T I C L E I N F O
Article history: Received 16 June 2016 Received in revised form 9 October 2016 Accepted 13 October 2016 Available online 13 October 2016 Keywords: Pd nanocatalysts Nickel N-doped Formic acid Electro-oxidation
A B S T R A C T
Nitrogen-doped graphene (NG) supported palladium-nickel nanoparticles with uniform dispersion are synthesized as catalysts for the electro-oxidation of formic acid. The catalysts are characterized by X-ray powder diffraction (XRD), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Our TEM results show that the Pd/Ni nanoparticles on NG are uniform with narrower size distribution than those on native graphene. XPS analysis reveals that Pd/NG and 1Pd1Ni/NG systems have more Pd0 and less Pd2+ content in comparison with the Pd/G and 1Pd1Ni/G counterparts respectively. This difference might be caused by the electron-donating effects of nitrogen species on the graphene surface. In addition, the electrochemical results show significantly enhanced catalytic activity and stability of 1Pd1Ni/NG as the catalyst for the formic acid oxidation reaction. The enhanced activity is not only attributed to the better uniform dispersion of nanoparticles and the enhanced electronic effect between metal particles and the support, but also ascribed to the synergistic effect of Ni incorporation to Pd. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Recently, direct formic acid fuel cells (DFAFCs) have attracted considerable research interests as promising clean energy for portable applications because they exhibit many advantages such as high efficiency, low emission and low toxicity [1–3]. Comparing to Pt, the Pd catalysts are the more popularly used anode catalysts in DFAFCs because of their higher catalytic performance and the less poisoning effect by proceeding primarily through direct pathway where formic acid is directly oxidized into CO2 [4,5]. However, the poor long-term stability on Pd catalysts represents a major problem. Thus, some noble metals and transitional metals such as Au, Pt, Sn, Co, Ni and Pb have been added into Pd catalysts in order to improve the the catalytic performance through alloy effect [1,6–10]. Moreover, the addition of transitional metals can reduce the cost of the catalysts. Particularly, Ni has been applied as one of the promising additive components in Pd-based catalysts [11–13]. For example, Du et al. [11] reported that nanoporous Pd57Ni43 alloy nanowire was a catalyst with the high electrocatalytic
* Correspondence to: No. 1139 Shifu Road, Jiaojiang District of Taizhou City, Zhejiang Province, China. E-mail address: snowfl[email protected] (Y. Jin). http://dx.doi.org/10.1016/j.electacta.2016.10.087 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
performance for formic acid oxidation. Lu et al. [13] reported the electrocatalytic performance of the Pd-Ni/C catalyst for formic acid oxidation was much better than that of the Pd/C catalyst. However, the development of more efficient anode catalyst systems with high catalytic activity and excellent durability still remains a big challenge towards the commercialization of DFAFCs. Graphene has emerged as a promising support material for fuel cell electrodes due to its excellent electrical conductivity and unique physical and chemical properties [14]. The different forms of heteroatomic doping might be able to offer additional functions on the surface of graphene, which have been used in the constructions of fuel cells, chemical battery and super capacitor, etc [15]. Heteroatom doping (nitrogen, sulfur, boron etc.) is particularly an important way to tailor physicochemical property and chemical reactivity of graphene [16–18]. For example, compared to the commercial Pt/C catalyst, the application of Pt/ NG catalysts (nitrogen-doped graphene as a support) led to a much better comprehensive performance including higher activity and better stability in the methanol electro-oxidation reaction [19–21]. In addition, Pd nanoparticles (NPs) supported on N and S dualdoped graphene (NS-G) nanosheets exhibited outstanding electrocatalytic performance toward both formic acid and methanol electro-oxidation than those of Pd/Vulcan XC-72R and Pd/undoped graphene catalysts [18]. Nitrogen doping can not only induce
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anchoring sites for well dispersion of metallic NPs on the surface of graphene, but also modulate electronic property of graphene, thus directly affect the activity and the stability of metallic NPs during the methanol oxidation reaction [5]. Another example is that PdPb/ N-doped graphene catalyst exhibited remarkably enhanced activity and durability for electrocatalytic ethanol oxidation. N-doped graphene can bring about high dispersity of metallic nanoparticles, and the electronic effect between support and Pd nanoparticles can be enhanced [22]. Besides doped graphene, the effect of doping on Pd NPs nucleation and growth is well documentated by several papers reporting Pd NPs deposition by several methods on doped graphene [23], doped glassy carbon [24], doped HOPG [25], doped carbon nanotubes [26] or doped carbon [27,28]. For example, Granozzi and coworkers [28] reported that the presence of nitrogen defects played a significant role in improving the metal particles dimension and dispersion. When nitrogen-doped mesoporous carbon were used, the resulting metal nanoparticles were smaller (24 nm) and less prone to aggregation. Also, according to Gaetano Granozzi’s report [25], a drastic reduction had been observed in the size distribution (2.84 1.13 nm) at 500 Cwhen Pd deposition was performed on N-HOPG. On the other hand, PdNi hollow NPs [29] or PdNi alloyed NPs deposition by several methods on single wall carbon nanotubes [30], graphene [12], and nitrogen-doped graphene [31] have been reported. Similarly, PdNi alloyed NPs nucleation and growth will be affected by nitrogen-doping support [29]. In fact, a uniform dispersion of Ni@Pd nanoparticles on nitrogen-doped reduced graphene oxide had a 2.8 nm average particle size [29]. However, studies on the nitrogen-doped graphene (NG) as the support of Pd catalysts for the formic acid electro-oxidation have been rarely reported. In this work, we present a novel catalyst for formic acid eletro-oxidation using NG as support and Pd/Ni nanoparticles as the active components. The composition, structure and morphology of the catalysts are characterized. The electrochemical properties of the catalysts towards formic acid oxidation and underlying mechanism have also been systemically investigated. 2. Experimental 2.1. Catalyst preparation Graphite oxide (GO) material has been prepared by Hummers method [32] from graphite powder (Aldrich, powder, <20 micron) [33]. NG material was synthesized as followed. Firstly, 300 mg GO material and 50 mL of 25 wt.% aqueous ammonia (NH3H2O) were mixed under ultrasonic stirring until a kind of ink-like suspension was formed. Subsequently, the ink-like suspension solution was poured into a Teflon bottle of 100 mL, which was placed in a stainless steel autoclave. Then it was sealed and held at 200 C for 5 h. After filtered and cooled, the precipitate was washed with excess distilled water by vacuum filtration, and dried under vacuum at 90 C. The 2Pd1Ni/NG catalyst was synthesized as followed. Firstly, 2.18 mL of palladium chloride solution (0.01 mol L1) and 7.01 mg of NiSO46H2O were added into 30 mL of double-distilled water. Secondly, 50 mg of NG material was ultrasonically for 30 min until well dispersed in the solution. The obtained suspension was transferred to a four-neck bottle and heated to 60 C. Then, 210 mg of KBH4 was dissolved in 50 mL of water and this solution was added into the suspension drop by drop. This reaction continued to stay for 6 h under 60 C. Finally, the slurry was filtered and dried under vacuum at 90 C. 1Pd1Ni/NG, 1Pd2Ni/NG and Pd/NG samples were prepared by the same method. For comparison, 1Pd1Ni/G and Pd/G samples were also prepared by the similar method, using
graphene as support. Millipore-Milli Q water and analytical-grade reagents were selected to make up all the solutions. Nickel sulfate hexahydrate (NiSO46H2O), aqueous ammonia (NH3H2O), patassium boron hydride (KBH4) and palladium chloride (PdCl2) were purchased from Shanghai Chemical Reagent Co. 2.2. Catalyst characterization Transmission electron microscopy (TEM) were recorded on the JEOL-2010 microscope at a potential of 200 kV and a current of 103 mA. X-ray diffraction (XRD) measurements were performed on Philips PW3040/60 X-ray diffractometer at the condition of Cu Ka radiation wavelength l = 0.15406 nm. X-ray photoelectron spectroscopy (XPS) measurements and analyses were carried out on a ESCALAB 250Xi apparatus. Metal contents of the catalysts were determined by inductively coupled plasma optical emission spectrometer (ICP-OES) (Agilent 725). The electrochemical performance of the sample was tested using three electrode system on CHI730d electrochemical workstation. A certain amout of ethanol, 5 wt.% Nafion solution and 5 mg of catalyst powders were mixed under ultrasonic stirring until a kind of ink-like suspension was formed, and then 10 uL of ink-like suspension was coated on a mirror-polished glassy carbon disk electrode with 4 mm diameter, thus it was used as working electrode. The final catalyst loading was 0.175 mgcm2. The saturated calomel electrode (SCE) was used as the reference electrode, and a salt bridge had been used to prevent the chloride contamination in the solution. Pt foil was used as the counter electrode. Before all electrochemical measurements, a high purity N2 gas flow for about 30 min was carried out to remove dissolved oxygen from the electrolyte. All tests were carried out in an inert atmosphere, and the experiment was carried out at 25 C. 3. Results and discussion 3.1. Physicochemical characterization The phase structures of different catalysts with different Pd/Ni atomic ratios and support were investigated by XRD and the patterns are shown in Fig. 1. For all the samples, the (002) planes of N-graphene or graphene could be well recognized by the broad diffraction peak located at around 26 . The strong diffraction peaks at around 40.0 , 46.5 , 68.0 and 82.0 are attributed to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) lattice planes of the face-centered cubic structures of palladium, respectively (JCPDS No.46-1043). As shown in the inset of Fig. 1, it’s observed that Pd peaks for all
Fig. 1. XRD patterns of Pd/G, Pd/NG and PdNi/NG with different Pd/Ni atomic ratios.
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the PdNi/NG catalysts with different Pd/Ni atomic ratios shift positively compared to those of Pd/NG. This indicates PdNi alloy are formed and attributed to the same alloy type since xPdyNi alloys have the very same XRD pattern and peak position [34]. The alloying degree (x, at.% Ni incorporated into the Pd lattice) was estimated by Eq. (1) [35], where rNi and rPd should be 1.25 Å and 1.37 Å respectively, whereas aalloy and aPd mean calculated lattice constants for PdNi alloy and Pd, respectively. xð1
aalloy rNi Þ¼1 rPd aPd
ð1Þ
The lattice parameters for PdNi/NG and Pd/NG were evaluated from the Pd (220) diffraction according to Bragg's law [36]. All of the calculations, along with other physicochemical parameters are summarized in Table 1. Fig. 2a–f show the typical TEM images of the Pd/G, Pd/NG and 1Pd1Ni/NG catalysts and Fig. 2g–i display the corresponding particle size distribution histograms. As seen, non-uniform Pd particles with significant aggregation are observed on Pd/G sample (Fig. 2a), while fine and well-dispersed metallic nanoparticles are obtained on Pd/NG (Fig. 2b and e) and 1Pd1Ni/NG (Fig. 2c and f) samples. According to the particle size histograms, particles on Pd/ G catalyst display a distinctly broader size distribution between 5 and 32 nm in diameter with a mean size of 14.95 nm (Fig. 2g). In contrast, Pd nanoparticles are found to have a uniform dispersion on the NG support with an average particle size of 2–12 nm, centered in 4.3 nm at Pd/NG catalyst (Fig. 2h). Whereas, the particle size distribution of 1Pd1Ni/NG catalyst become even narrower in a range from 1 to 4 nm with mean particle size of 1.6 nm (Fig. 2i). The same results can be found on nitrogen-doped mesoporous carbon. The presence of homogeneously distributed nitrogen functional groups can provide nucleation sites and thus promote a higher dispersion of metal NPs [28]. In our case, the same conclusion can be drawn that N-doped graphene is beneficial to anchoring metal nanoparticles. XPS was carried out to analyze the electronic structure of the initial Pd/NG, 1Pd1Ni/NG, Pd/G and 1Pd1Ni/G samples, and the typical XPS spectra are shown in Fig. 3a–g, while the element content and functional groups with peak position, attribution and components of the samples are summarized in Table 2. In the XPS spectra of 3a and 3b, the peaks at binding energies of 398.3, 399.8, 401.1 and 403.5 eV are attributed to four types of nitrogen functionalities: the pyridinic (N1), pyrrolic (N2), graphitic N (N3) and oxidized N (N4) [37–41]. According to Table 2, both of N contents in Pd/NG and 1Pd1Ni/NG are ca. 8.37 and 7.17 at.%, and the proportion of four components N1, N2, N3 and N4 on Pd/NG and 1Pd1Ni/NG catalysts are 34.80: 44.28: 16.11: 4.8 and 29.50: 57.81: 10.66: 2.03 respectively, indicating the pyrrolic (N2) is dominant in the formed nitrogen functional groups. A reasonable amount of N species in the catalysts may help metal nanoparticles disperse uniformly on the support, which are depicted in TEM images. The C 1s spectrum of 1Pd1Ni/NG in Fig. 3h could be reasonably fitted with four peaks, the most intense one at the binding energy of 284.8 eV is ascribed to C-C, and the other three weak peaks at 285.71, 287.0 and 290.66 eV correspond to C-N, C-O and O-C¼O,
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respectively [42], indicating the successful introduction of nitrogen atoms and the existence of oxygen containing functional groups in graphene. Also, the oxygen containing functional groups can be identified from the O 1s spectrum of 1Pd1Ni/NG in Fig. 3i. Ni 2p signal of 1Pd1Ni/NG is shown in Fig. 3g. Ni 2p3/2 and Ni 2p1/2 binding energies at 856.2 eV and 873.9 eV with obvious the shake-up peaks at 861.6 and 879.9 eV can be assigned to Ni(OH)2 and NiOOH [43]. Why does XPS show no trace of Ni? One reason is that there is small quantity of PdNi alloy. The other one is little surface-distributed Ni has been oxidized in ambient conditions [12]. These oxides are beneficial to electro-catalytic oxidation of small organic molecules [12,43]. Pd 3d signals of Pd/NG, 1Pd1Ni/NG, Pd/G and 1Pd1Ni/G samples are divided into three pairs of asymmetric peaks in Fig. 3c–f. The two peaks at 335.45 and 340.76 eV are contributed to Pd(0) 3d5/2 and Pd(0) 3d3/2, other two weak doublets at 335.91 and 340.99 eV belong to PdO species. The third pair of peaks at 337.51 and 342.52 eV correspond to PdCl2, an indication of incomplete reduction of the Pd precursor under this condition [12]. According to the results of Table 2, the total content of Pd(0) combined Pd(0) 3d5/2 and Pd(0) 3d3/2 on Pd/G sample is 9.33%, while that on Pd/NG sample rises up to 24.51%. Similarly, the total content of Pd(0) on 1Pd1Ni/NG catalyst increases to 21.02%, compared to the content of 15.66% on 1Pd1Ni/G catalyst. In addition, Pd2+ contents on Pd/NG, 1Pd1Ni/NG, Pd/G and 1Pd1Ni/G are 41.63%, 34.19%, 68.52% and 61.21%, respectively. It was reported that the increased Pt0 components and decreased Pt2+ components were found on Pt/ NG catalysts. It was postulated that the electronic structure of the graphene can be modulated by N species, then the electronic property of the supported Pt catalysts can subsequently be modulated by the metal-support interaction [20]. Herein, increased Pd0 component and decreased Pd2+ component are found for the Pd/NG catalyst compared to Pd/G catalyst. Likewise, more Pd0 and less Pd2+ are observed on 1Pd1Ni/NG in comparison with 1Pd1Ni/G sample. It should possibly caused by the electron donation effects of nitrogen. Such modulation in electronic properties of the catalysts maybe improve their electrocatalytic performance [20]. 3.2. Electrochemical characterization Fig. 4 depicts the CV results of the different catalysts in a 0.5 M H2SO4 solution. The multiple peaks between 0.2 V and 0.1 V can be identified on all the catalysts, which are attributed to the hydrogen adsorption and desorption process. The double layer region can be found in the potential interval of 0.1-0.3 V. Apparently, the double layer region on Pd/NG catalyst gets larger compared with those on Pd/G catalyst. The same conclusion can be drawn that 1Pd1Ni/NG sample has larger double-layer capacitance compared with 1Pd1Ni/G sample. It maybe primarily caused by the great change of surface area, which is related to metal dispersion on the NG and G. Based on TEM results, 1Pd1Ni/NG possesses smaller average size of the metal particles, leading to largest surface area [36]. Moreover, the electrochemically active surface area (ECSA) values of all the catalysts were evaluated by the integration of the hydrogen adsorption areas [44,45], which are
Table 1 Structural characterization data of the catalysts.
Lattice parameter XRD/nm Alloying degree x/% Atomic ratio of Pd:Ni (ICP) Pd content in the sample/wt.%
2Pd1Ni/NG
1Pd1Ni/NG
1Pd2Ni/NG
Pd/NG
1Pd1Ni/G
Pd/G
0.3966 6.59 2.1:1 15.35
0.3952 10.44 1.2:1 16.57
0.3944 12.80 0.65:1 14.35
0.3977 / / 16.55
/ / 1.3:1 15.91
/ / / 15.65
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Fig. 2. TEM images of Pd/G (a/d), Pd/NG (b/e) and 1Pd1Ni/NG (c/f) and the particle size distribution of Pd/G (g), Pd/NG (h) and 1Pd1Ni/NG (i).
presented in the inset of Fig. 4. As shown, the ECSA value of 1Pd1Ni/ NG reaches 157 m2g1, which is much larger than those of Pd/NG and Pd/G (103 and 72 m2g1, respectively). The cyclic voltammograms (CV) results for electrodes with different Pd/Ni atomic ratios and different supports in 0.5 M H2SO4 containing 1 M HCOOH are shown in Fig. 5a and b. Clearly, a large peak at about 0.2 V and a very small peak at about 0.5 V can be recognized on all catalyst electrodes. As shown in Fig. 4b, Pd/NG and 1Pd1Ni/NG catalyst show bigger formic acid electrooxidation (FAEO) peak current density at 484 and 709 mAmg1, respectively, whereas Pd/G and 1Pd1Ni/G exhibit FAEO peak current density at 373 and 439 mAmg1. So N-doped graphene obviously can promote the activity of the catalysts with roughly the same metal components. In comparison with Pd/NG, the PdNi/NG catalysts with different Pd/Ni atomic ratios display better catalytic activity for FAEO. Among them, 1Pd1Ni/NG achieves a highest current density of 709 mAmg1, which is
1.9 times and 1.5 times as large as that of Pd/G and Pd/NG, respectively. It is inescapably clear that addition of Ni into the catalyst could enhance the catalytic performance for FAEO as well. So, it’s supposed that 1Pd1Ni/NG achieves the highest activity due to the synergistic effects of N-doping and Ni incorporation. In order to further evaluate the stability of all the catalysts, the chronoamperometric curves and the galvanostatic polarization curves on different catalysts are investigated in the solution of 1 M H2SO4 containing 1 M HCOOH at a constant potential of 0.4 V and at current density of 7.5 mAcm2, respectively, as shown in Fig. 6 and Fig. 7. According to Fig. 6, the current decayed continuously rapidly for the Pd/G catalyst, suggesting catalyst are poisoned by the chemisorbed carbonaceous species [46]. The Pd/NG is able to maintain the higher current density for over 1 h than Pd/G, and 1Pd1Ni/NG keeps higher current density than 1Pd1Ni/G during all the tested time.
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Fig. 3. XPS spectra of N 1s region in (a) Pd/NG and (b) 1Pd1Ni/NG, Pd 3d region in (c) Pd/NG, (d) 1Pd1Ni/NG, (e) Pd/G and (f) 1Pd1Ni/G; XPS spectra of (g) Ni 2p region, (h) C 1s region and (i) O 1s regions in 1Pd1Ni/NG catalyst.
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Table 2 Fitting results summary of Pd/NG, 1Pd1Ni/NG, Pd/G and 1Pd1Ni/G from XPS. Sample
Pd/NG 1Pd1Ni/NG Pd/G 1Pd1Ni/G
Element Content/at%
N functional groups/(BE, eV)
C
O
N
Pd
73.21 70.68 82.67 79.43
16.15 16.27 15.25 15.52
8.37 7.17 – –
2.27 2.6 2.08 2.3
Ni
3.28 2.75
N1 398.3
N2 399.8
N3 401.4
N4 403.5
34.80% 29.50% – –
44.28% 57.81% – –
16.11% 10.66% – –
4.80% 2.03% – –
Pd species/(BE, eV)
Pd/NG 1Pd1Ni/NG Pd/G 1Pd1Ni/G
Ni species/(BE, eV)
Pd(0) 3d5/2
PdO 3d5/2
PdCl2 3d5/2
Pd(0) 3d3/2
PdO 3d3/2
PdCl2 3d3/2
Ni(OH)2 2p3/2
335.45
335.91
337.51
340.76
340.99
342.52
856.19
861.62
873.86
879.9
13.9 8.45 8.17 5.32
22.46 32.11 12.49 19.38
21.92 17.22 35.80 34.83
10.61 12.57 1.16 10.34
11.40 12.66 9.66 3.75
19.71 16.97 32.72 26.38
– 35.64 – 34.76
– 31.8 – 39.76
– 15.26 – 12.23
– 17.3 – 13.24
NiOOH 2p1/2
N1: Pyridinic, N2: Pyrrolic,N3: Graphitic N, N4: Oxidized N.
Fig. 4. Cyclic voltammograms in 0.5 M H2SO4 solution for 2Pd1Ni/NG, 1Pd2Ni/NG, 1Pd1Ni/NG, Pd/NG, Pd/G and 1Pd1Ni/G electrodes. Scan rate: 20 mV/s.
Chronopotentiometry is a useful method to study the resistance to poisoning of catalysts during formic acid oxidation [47]. As shown in Fig. 7, the initial potential on Pd/G increases continuously more rapidly than that of Pd/NG during all the time, whereas 1Pd1Ni/G keeps rising more rapidly than that of 1Pd1Ni/NG with the higher steady potential. During the experiment, the poisonous species such as COads rising from the formic acid oxidation, will accumulate on the surface of the electrocatalyst and reduce its electrocatalytic activity. The potential must be increased to satisfy the applied anodic current density [47–50]. The higher steady potential and the shorter sustained time during the tested time suggest the weaker poisoning resistance and more poor stability of the catalysts [47–49,51]. Based on the above results, there is a conclusion that N-doped graphene can particularly improve the resistance to poisoning and the the stability of the catalysts with roughly the same metal components, compared to native graphene. The FAEO reaction could proceed through two parallel pathways [3]. One is the direct path being characteristic of direct oxidation of formic acid into CO2, the other is indirect path where CO species, rising from nonfaradaic dissociation of formic acid, are adsorbed on the particle surface and then CO species are oxidized into CO2 [52]. The mechanism of FAEO on Pd is predominately through the direct path. However, the accumulation of poisoning intermediates such as CO will poison the catalysts during the reaction. Ni species such as Ni(OH)2 are oxophilic, so oxygen species generate on its surface at low potentials, which can react with poisoning intermediates such as CO-like species and help to remove effectively CO species from the surface [12]. Therefore, more active sites will be released and promote direct oxidation of formic acid into CO2, which can result in the enhancement in the catalytic stability and activity. On the other hand, the existence of the nitrogen functional groups in the graphene substrates, which leads to smaller particle size of the deposited Pd/Ni nanoparticles. Additionally, the incorporation of nitrogen might cause the improvement of the electrocatalytic performance by the beneficial metal-support interactions via the electronic structure modification of graphene, as verified from the previous TEM spectra and XPS analysis. 4. Conclusions
Fig. 5. Cyclic voltammograms in 1 M HCOOH + 0.5 M H2SO4 solution for 2Pd1Ni/NG, 1Pd2Ni/NG, 1Pd1Ni/NG, Pd/NG, Pd/G and 1Pd1Ni/G electrodes. Scan rate: 20 mV/s.
This study demonstrates that N-doped graphene supported palladium-nickel nanoparticles catalysts exhibit outstanding catalytic activity and stability for the formic acid electro-oxidation.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21403150 and 21506138), Zhejiang Provincial Natural Science Foundation of China (Nos. LQ12B03003 and LQ15B060001), Science and technology project of Zhejiang Province (Nos. 2014C37052), National Students’ innovation and entrepreneurship training program (Nos. 201510350013), Science and Technology Project of Taizhou City (Nos. 1202ky03). References
Fig. 6. Chronoamperometric curves of 2Pd1Ni/NG, 1Pd2Ni/NG, 1Pd1Ni/NG, Pd/NG, Pd/G and 1Pd1Ni/G electrodes in 0.5 M H2SO4 containing 1 M HCOOH at 0.4 V.
Fig. 7. Galvanostatic polarization curves for 2Pd1Ni/NG, 1Pd2Ni/NG, 1Pd1Ni/NG, Pd/NG, Pd/G and 1Pd1Ni/G electrodes in 0.5 M H2SO4 containing 1 M HCOOH solution at current density of 7.5 mAcm2.
According to TEM and XPS results, metal nanoparticles show the more uniform dispersion on nitrogen doped graphene in comparison with native graphene. Increased Pd0 and decreased Pd2+ component are found for the Pd/NG and 1Pd1Ni/NG catalysts in comparison with the Pd/G and 1Pd1Ni/G counterparts respectively, which is possibly related with the electron donation effects of nitrogen. The results demonstrate that 1Pd1Ni/NG catalyst achieves dramatically improved catalytic stability and remarkably enhanced activity up to 709 mAcm2 for FAEO, which is 1.9 times and 1.5 times as large as that of Pd/G and Pd/NG catalysts. The enhanced electrocatalytic activities of the N-doped graphene supported Pd/Ni particles could be attributed to the following three aspects. Firstly, N-doping can cause modulation in electronic properties of the catalysts by the beneficial metalsupport interactions. The modulation in electronic properties has been shown by XPS results, where more Pd0 and less Pd2+ content are obviously found on the Pd/NG and 1Pd1Ni/NG catalysts. Secondly, N-doping could provide efficient anchor sites for good dispersion of metallic nanoparticles on the surface of support, which can increase catalytic surface area accessible to the formic acid and electrolyte solution. Finally, Ni species such as Ni(OH)2 could help to generate abundant oxygenated species on its surface at low potentials, which may quickly remove COads intermediates and release more Pd active sites, and thus promote the direct oxidation of formic acid.
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