Nitrogen-Doped Carbon Nanotube-Supported Pd Catalyst for Improved Electrocatalytic Performance toward Ethanol Electrooxidation
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In this study, hydrothermal carbonization (HTC) was applied for surface functionalization of carbon nanotubes (CNTs) in the presence of glucose and urea. The HTC process allowed the deposition of thin nitrogen-doped carbon layers on the surface of the CNTs. By controlling the ratio of glucose to urea, nitrogen contents of up to 1.7 wt% were achieved. The nitrogen-doped carbon nanotube-supported Pd catalysts exhibited superior electrochemical activity for ethanol oxidation relative to the pristine CNTs. Importantly, a 1.5-fold increase in the specific activity was observed for the Pd/HTC-N1.67%CNTs relative to the catalyst without nitrogen doping (Pd/HTC-CNTs). Further experiments indicated that the introduction of nitrogen species on the surface of the CNTs improved the Pd(0) loading and increased the binding energy.
KeywordsDirect alcohol fuel cells Hydrothermal carbonization Nitrogen-doped carbon nanotubes Pd-based catalyst Ethanol electrocatalyst
Hydrothermal carbonization (HTC) enabled the deposition of an N-doped carbon layer on the surface of carbon nanotubes (CNTs).
Nitrogen-doped CNTs facilitated the uniform distribution of Pd nanoparticles.
The interaction between nitrogen in the CNTs and Pd favored the existence of metallic Pd in the catalysts.
Pd/HTC-N1.67%CNTs showed the highest specific activity toward ethanol oxidation.
Direct alcohol fuel cells (DAFCs) have attracted much attention recently due to their high efficiency for energy conversion as well as their low environmental impact [1, 2, 3]. Among small organic molecules such as methanol, ethanol, and ethylene that yield energy upon oxidation, ethanol is the most ideal fuel because of its abundant reserves, low toxicity, and facile storage and transport. For commercial application, anode catalysts with high activity and stability, such as the widely accepted Pt-based catalysts, are critical for high-performance DAFCs [4, 5]. To address the issues of the high price and low reserves of Pt-based catalysts, many studies have been devoted to developing Pd-based catalysts by regulating the active phases and adding various promoters and varying catalyst supports. For example, certain studies have focused on the addition of secondary elements such as Sn, Cu, Ni, and Au to carbon-supported Pd to improve the electrocatalytic activity and stability for ethanol oxidation due to the bimetallic synergetic effect [6, 7, 8]. Moreover, trimetallic counterparts were also found to exhibit greatly improved catalytic performance and provide greater functionality [9, 10, 11].
The support materials also play a significant role in the catalytic reaction. Carbon-based support materials such as active carbon, graphite, and carbon nanotubes (CNTs) have been widely investigated. Among these materials, CNTs have earned distinction based on their high surface area, high length to diameter ratio, and good electrical conductivity [12, 13]. In addition to the common carbon-class supports, some transition metallic carbides, nitrides, phosphides, and corresponding hybrid composites have been employed as efficient supporting materials that positively impact the catalyst activity [14, 15, 16]. Additionally, some studies have shown that doping CNTs with heteroatoms [17, 18, 19, 20, 21, 22] is an effective way to tune their intrinsic properties. The substitute heteroatoms can provide more initial nucleation sites for the formation of noble metal nanoparticles and also enhance the interaction with the nanoparticles, thereby improving the electrocatalytic activity . Nitrogen-doped carbon nanotubes (NCNTs) are particularly promising candidates owing to the strong electron donor behavior of nitrogen, which enhances the π bonding and the basic properties of the NCNTs . A study by Chetty et al.  showed that PtRu nanoparticles supported on nitrogen-doped multiwalled carbon nanotubes exhibited higher activity for methanol oxidation than the same catalysts supported on undoped nanotubes. The introduction of nitrogen-containing functional groups onto a carbon support may improve the catalytic activity by influencing the particle nucleation and growth and changing the electronic structure of the catalyst, as well as increasing the chemical binding energy between the support and catalyst particles, thereby enhancing the durability [21, 22, 23].
Furthermore, the use of inexpensive, sustainable feedstocks to produce N-doped carbon materials conforms to the concept of green chemistry. Generally, NCNTs can be synthesized by two methods, i.e., post-doping treatment or direct synthesis [19, 24]. The post-doping treatment is a multi-step process requiring high-cost apparatus. However, the alternative in situ synthesis requires an expensive precursor, complicated equipment, and is environmentally unfriendly. Therefore, finding a more economic and green strategy to synthesize NCNTs is necessary .
The hydrothermal carbonization (HTC) technique is a sustainable approach for producing NCTNs by using inexpensive feedstocks as carbon precursors. Interestingly, Titiricic et al.  recently reported that the HTC process enables homogeneous coating of nitrogen-doped carbon layers on the surface of CNTs, leading to superior supercapacitor performance. In spite of the paucity of examples, it is expected that nitrogen functionalization of CNTs by the HTC method could afford interesting supports for anchoring Pd for heterogeneous reactions.
In this study, surface functionalization of CNTs is performed by the HTC method using glucose and urea. The HTC process allows the deposition of thin nitrogen-doped carbon layers on the surface of the CNTs. By controlling the ratio of glucose to urea, nitrogen contents of up to 1.7 wt% could be achieved.
Elemental analysis data for supporting materials
Elemental compositions (wt%)
The Pd-based catalyst was synthesized using PdCl2 as the metal source, NaBH4 as the reducing agent, and the different composites as supporting materials. Thus, 48 mg of the pretreated HTC-CNTs and 1.3 mL of 18.9 mM H2PdCl4 were mixed with 30 mL of deionized water under ultrasonic stirring. The pH of the solution was adjusted to 9 by using 1 M NaOH. An appropriate amount of NaBH4 (Pd/NaBH4 = 1:8 mol%) was then added dropwise to the aforementioned solution (with a nominal Pd loading of 5 wt%), and the mixture was stirred vigorously for 4 h. The resulting precipitate was filtered and washed several times with ultrapure water before drying overnight at 60 °C. The weight percentage of Pd in the catalysts, determined by inductively coupled plasma–atomic emission spectroscopy (ICP–AES), was 4.9 ± 0.1 wt%.
X-ray diffraction (XRD, Rigaku D/max-IIIC) was performed using a copper Kα source (λ = 1.5406 Å). The microstructure was analyzed using high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F20 S-TWIN) at 200 kV. X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo Scientific, Inc.) was performed using a monochromatic Al Kα source at 10 mA and 15 kV. The electrochemical measurements were conducted by using a CHI660D electrochemical workstation (Chenhua Inc., Shanghai, China) in a conventional, sealed, three-electrode system. Raman spectra were collected between 500 and 3100 cm−1. The light source was a 532 nm argon laser, and the data were collected with 50-s exposure.
A Pt wire and a Hg/HgO electrode were used as the counter electrode and reference electrode, respectively. A glassy carbon electrode (GCE, Ø3 mm) was employed as the working electrode; the working electrode was prepared as follows: 5 mg of the catalyst was ultrasonically mixed with 25 µL of 20 wt% Nafion solution and 975 µL of isopropyl alcohol to prepare a homogeneous catalyst ink. A 4 µL portion of the catalyst ink was transferred onto the polished glassy carbon electrode surface using a micropipette. The Pd loading on the surface of the glassy carbon electrode surface was 0.014 mg cm−2. Cyclic voltammograms were acquired in the potential range of −0.9 to 0.4 V with a sweep rate of 50 mV s−1, in 1 M KOH and 1 M KOH containing 1 M ethanol, respectively. Chronoamperometry was carried out at a potential of −0.3 V using 1 M KOH solution containing 1 M ethanol.
All solutions were deoxygenated by bubbling with N2 before the tests, and all electrochemical measurements were carried out in a water bath at 25 ± 1 °C.
4 Results and Discussion
According to Fig. 3a–c, the integral area (S) of Pd/HTC-CNTs, Pd/HTC-N1.67%CNTs, Pd/HTC-N0.86%CNTs are 6.43, 5.54, 7.17 mA mg−1 V, respectively. Based on Eq. 3, Q of Pd/HTC-CNTs, Pd/HTC-N1.67%CNTs, Pd/HTC-N0.86%CNTs are 12.86 × 10−5, 11.08 × 10−5 and 14.34 × 10−5 C, respectively. Based on Eq. 2, the EAS of Pd/HTC-CNTs, Pd/HTC-N1.67%CNTs, Pd/HTC-N0.86%CNTs is 30.6, 36.0, 34.2 m2 g−1, respectively. Finally, the calculated EAS values for the Pd/HTC-CNTs, Pd/HTC-N1.67%CNTs, and Pd/HTC-N0.86%CNTs were 30.6, 36.0, and 34.2 m2 g−1, respectively.
The cyclic voltammograms of the three catalysts were acquired in 1 M KOH containing 1 M ethanol (Fig. 2); the current normalized relative to the EAS is shown in Fig. 3d. The specific activity is often used to represent the intrinsic performance of catalysts. The anodic peak observed in the forward scan corresponds to ethanol oxidation, which is of great significance for evaluating the activity of the catalysts. It is clear that the forward anodic peak current density of the Pd/HTC-N1.67%CNTs (8.3 mA cm−2) is higher than that of the Pd/HTC-N0.86%CNTs (6.0 mA cm−2) and Pd/HTC-CNTs (5.6 mA cm−2). Chen et al.  reported that the specific activities of Pd/C and Pd/50CaSiO3/C were 2.7 and 4.4 mA cm−2, respectively. Therefore, the specific activity of the Pd/HTC-N1.67%CNTs catalyst in the present work is about three times higher than that of Pd-based catalysts in the literature.
The Pd 3d5/2 peak was also negatively shifted for the Pd/HTC-N1.67%CNTs catalyst (340.95 eV) relative to that of the Pd/HTC-CNTs (341.05 eV) catalyst. Moreover, Fig. 7b shows a positive shift of the N1s binding energy from 400.75 eV for the Pd/HTC-N0.86%CNTs to 401 eV for the Pd/HTC-N1.67%CNTs. This confirms the interaction between Pd and N [37, 38, 39]. Therefore, due to the electron-donating effects of nitrogen, the electron cloud density of Pd may increase, which can stabilize Pd0, and the nitrogen groups impart a basic nature to the carbon surface and bind strongly to Pd, enhancing the Pd dispersion and preventing agglomeration of the Pd particles, thereby improving the electrochemical activity and stability of the Pd-based catalysts [21, 25, 34]. Accordingly, the Pd/HTC-N1.67%CNTs catalyst showed the best activity and stability toward ethanol electrooxidation.
Three Pd-based catalysts supported on NCNTs with various nitrogen contents were synthesized via the HTC process. The Pd/HTC-N1.67%CNTs catalyst exhibited the best ethanol electrooxidation activity and stability as well as the highest specific activity. This improvement can be attributed to the interaction between nitrogen (doped in the CNTs) and the Pd nanoparticles, which favors the formation of metallic Pd in the catalysts. The HTC process also supplies a number of defects on the surface of the NCNTs that help to anchor metallic Pd for ethanol electrooxidation. The large proportion of metallic Pd not only facilitates OH− adsorption and the removal of the CO-like intermediates, but also increases the specific activity of the catalyst, resulting in a significant improvement of the activity and stability of the Pd-containing catalysts.
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 51672045 and 11374053), and Key Program of University-industry Collaboration from Science and Technology Department of Fujian Province (No. 2015H6009). They would also like to thank Xinqi Zhang for assistance with TEM and Zhenhuan Zheng for assistance with X-ray diffraction analysis.
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