Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Controllable Unzipping of Carbon Nanotubes as Advanced Pt Catalyst Supports for Oxygen Reduction Qingzhu Shu,†,‡ Zhangxun Xia,‡ Wei Wei,‡ Xinlong Xu,‡ Ruili Sun,‡ Ruoyi Deng,‡ Quanhong Yang,§ Hong Zhao,*,† Suli Wang,*,‡ and Gongquan Sun‡
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†
The College of Materials Science and Engineering, Dalian Jiao tong University, 794 Huang he Road, Dalian, Liaoning 116028, China ‡ Division of Fuel Cell & Battery, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China § Nanoyang Group, School of Chemical Engineering and Technology, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China S Supporting Information *
ABSTRACT: Alternative carbon support materials provide great opportunities to fabricate advanced catalysts with enhanced activity and durability, especially for energy conversion and storage technologies based on the oxygen reduction reaction (ORR), such as fuel cells and metal−air batteries. In this work, a series of controllable unzipped carbon nanotubes (CNTs) have been successfully synthesized and used as Pt supports for highly efficient ORR catalysts. Benefiting from the unique structure constructed by the 1-D and 2-D nanomaterials, outstanding catalytic performance with enhanced Pt utilization, ORR activity, and durability is achieved for the catalyst of Pt loaded on the medium-level unzipped CNTs, with a remarkable halfwave potential of 0.915 V (vs RHE). The effective mass transport in the cathodes fabricated with the 3-D nanoarchitecture hybridized with CNTs and graphene ribbons catalyst supports results in a significant increase of peak power density for direct methanol fuel cells (DMFCs) compared to that of a traditional electrode with commercial Pt/C catalyst, which demonstrates a great application potential of this novel catalyst design. KEYWORDS: catalysts, direct methanol fuel cells, electrochemistry, oxygen reduction reaction, unzipped carbon nanotubes decrease in activity.19 The unstable chemical properties of carbon black lead to chemical corrosion easily in an acidic environment, thus reducing the long-term stability of the catalyst.20 In view of the inadequacy of carbon black and the increasing demand for catalytic performance, researchers began to pay more attention to the new carbon-based nanomaterials and structures as catalyst supports which can not only improve the utilization of Pt but also optimize its catalytic performance.21,22 Nanomaterials based on sp2-hybridized carbon atoms, such as carbon nanotubes and graphene, have been regarded as promising support materials for electrocatalysts due to their high thermal and chemical stability, controllable electrical properties, excellent mechanical properties, and dopingcontrolled ORR catalytic properties.23−25 However, because of the small radius of curvature and relatively low specific surface area of CNT, it is difficult to realize high density loading of active materials on CNT, which is not only conducive to the rapid charge transfer rate or mass transfer in
1. INTRODUCTION Because of the rapid consumption of fossil fuels, the energy crisis seriously threatens the economic and social development, forcing researchers to find new energy or develop clean and sustainable energy conversion technologies.1−5 Polymer electrolyte membrane fuel cells (PEMFCs), converting chemical energy into electricity with high theoretical efficiency, have attracted great interest for their low environmental impact and high specific energy.6−8 The state-of-the-art PEMFCs still suffer from the sluggish kinetics of the cathode oxygen reduction reaction (ORR).9−11 Platinum-based nanomaterials exhibit excellent electrocatalytic ORR performance, including high exchange current density and low onset overpotential, especially in an acid electrolyte.12,13 To achieve sufficient catalyst utilization and enhanced mass transport within the electrode, carbon black, including acetylene black, Vulcan XC72, and Ketjen black,14−16 has been widely used as catalyst supports in commerce. However, the electrochemical performance and durability of such carbon supports are still far from satisfactory.17 Structural defects and impurities can lead to the degradation or even deactivation of the catalyst.18 Abundant micropores or deep depressions make the catalyst on the inner surface unable to contact with the reactants, resulting in a © XXXX American Chemical Society
Received: March 11, 2019 Accepted: July 1, 2019 Published: July 1, 2019 A
DOI: 10.1021/acsaem.9b00506 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials
loaded to prepare different ORR catalysts through the ethylene glycol method. Different 3D nanoarchitecture derived from varied degrees of unzipping for MWCNTs is studied on the structural effects toward electrochemical properties. An appropriate level of unzipping process could deliver an optimized surface property to decorate the supporting catalysts and provide accessible porous structures for enhanced mass transport as well. Superior ORR activity over commercial Pt/C catalyst (Johnson Matthey Co.) is achieved in the catalyst with Pt supported on the medium-level unzipped MWCNTs associated with increased stability. The enhanced mass transport of the tailored electrode architecture equipped with the 3D structural catalyst is further verified via its application in direct methanol fuel cells (DMFCs).
electrocatalytic reaction but also limits the maximum utilization of the catalyst.26−28 In addition, single graphene lamellae are easy to restack and agglomerate under van der Waals force, and the vertical charge transfer and electrolyte diffusion efficiency are low, which makes the superiority of graphene not fully demonstrated.29,30 Therefore, to obtain the advantages of the two materials in one and two dimensions, researchers introduced highly conductive CNT into graphene to form a three-dimensional network structure,31,32 which could not only prevent the agglomeration of graphene lamellae but also construct a bridge for the electron transport between graphene lamellae.33 CNTs grown outside the graphene plane can hinder graphene agglomeration, provide a new path for electron transport, and optimize the electrochemical properties of the composites.34 The CNT/graphene composite structure by noncovalent electrostatic force was constructed by modified graphene and CNT to achieve enhanced catalytic performance.35−37 However, the preparation process of these materials is complicated, and the structure lacks designability to optimize the catalytic properties. Graphene nanoribbon (GNR) or CNTs/GNRs composites are relatively new carbon materials with the special structures and morphologies, which are obtained by the longitudinal unzipping multiwalled carbon nanotubes (MWCNT);38,39 the main preparation methods include solution oxidation40 and polymer etching41 for the moment. However, as fuel cell catalytic materials, single-component graphene nanoribbons have poor stability in common solvents, high surface energy, and easy stacking, which make it difficult to form micro-/ nanoporous structures. CNTs/GNRs overcomes its shortcomings due to the good mechanical and electrical conductivity of the carbon nanotubes in the middle; the graphene nanoribbons opened in the periphery have regular planar structure, a high aspect ratio, and a special edge effect, which will have great application prospects in the catalytic field.42,43 Although great efforts have been made, the ORR catalytic activity and stability of the catalysts based on CNTs/ GNRs are still not adequate for the application of PEMFCs, whereas the structural effects toward the catalytic performance are also required for further understanding. Moreover, in recent years, the transition-metal/metal oxide nanoparticles combined with porous carbon nanostructures, heteroatomdoped carbon nanostructures (graphene, carbon nanotubes (CNTs), and carbon fibers), metal−organic framework (MOF) derived metal−N−C structures, and perovskites also have made great progress in the preparation of ORR catalysts due to their attractive features of earth abundance, low preparation cost, remarkable toxicity tolerance, and excellent activity and durability in alkaline solutions.44−46 Nevertheless, the electrochemical performance of these advanced catalysts in acidic media is still unsatisfactory, the research on their mechanisms in reactions is not very mature, and the high requirement of preparation conditions and the complexity of the preparation process make it difficult to precisely control of the structure, function, and performance of these materials.47,48 In fact, they are still far from practical application and have no commercial value at all. Therefore, it is still of great practical significance to find a novel porous carbon material to improve the utilization of Pt to increase the electrocatalytic activity and stability of the catalysts. Herein, we have successfully prepared carbon materials with different levels of unzipping MWCNTs by controlling the degree of oxidation exfoliation, and Pt nanoparticles were
2. EXPERIMENTAL SECTION 2.1. Preparation of LU-MWCNT, MU-MWCNT, and HUMWCNT. All chemicals were of analytic grade and used without further purification. Multiwalled carbon nanotubes (10−30 nm in diameter, 15−25 layers in wall) were purchased from Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, and prepared by chemical vapor deposition (CVD). The specific experimental process is as follows: 0.2 g of multiwalled carbon nanotubes (MWCNTs) was dispersed in 60 mL of concentrated sulfuric acid, following 1 h mechanical stirring for a more uniform dispersion. Then, 0.3, 0.6, and 1.0 g of potassium permanganate were added to the reaction system; the addition rate was maintained at 0.2 g/h until the potassium permanganate was used up. After reacting at 0−4 °C for 2 h and slowly heating to 35 °C for 30 min, which was regarded as a moderate temperature reaction, the system was further heated to 75 °C for high-temperature reaction, and the reaction time was set for 1, 3, and 5 h, corresponding to 0.3, 0.6, and 1.0 g of potassium permanganate. Then the mixture was removed from the heat source, cooled to room temperature, and poured onto 300 mL of ice containing 10 mL of 30% H2O2. The bottom sediment was washed three times by 5 wt % HCl centrifugation, filtered by polytetrafluoroethylene membrane with pore size of 0.22 μm, and then washed with deionized water until the pH of the filtrate was nearly neutral. Finally, the oxidized LU-MWCNT, MU-MWCNT, and HU-MWCNT with three different structures were obtained by freeze-drying for 24 h at −50 °C. 2.2. Synthesis of Pt/MWCNT, Pt/LU-MWCNT, Pt/MUMWCNT, and Pt/HU-MWCNT Catalysts. First, the pure MWCNT was pretreated simply. Small amounts of metal impurities and ash in the carbon carrier were removed by refluxing with 2 M HCl at 110 °C for 12 h, then washed to neutral with deionized water, and dried at 80 °C for 12 h. After that, the pretreated MWCNT was weighed with 160 mg and added into a certain amount of glycol. The homogeneous carbon slurry was obtained by ultrasonic dispersion for 1 h. Then, 40 mg of chloroplatinic acid solution was added drop by drop through a constant-pressure drop funnel. After magnetic stirring for 3 h, the pH value was adjusted to alkalinity with KOH solution. The gel solution was stirred under reflux in an oil bath at 130 °C for 3 h and cooled to room temperature; the pH was then adjusted to weak acidity with HCl solution to uniformly precipitate the Pt nanoparticles. After that, the Pt/MWCNT, Pt/LU-MWCNT, Pt/MUMWCNT, and Pt/HU-MWCNT were obtained by washing with a large amount of deionized water and lyophilization after 24 h. In addition, all catalysts were finally treated at low temperature for 5 h in a 5% H2/Ar mixture, and the Pt loading on different unzipped MWCNT was equal (20 wt %). 2.3. Physical Characterization of Carbon Materials and Catalysts. The FTIR spectra were obtained on a Nicolet 6700 infrared spectrometer (Thermo Fisher). A highly sensitive MCTA detector was used to collect 128 times with a resolution of 8 cm−1. Thermal gravimetric (TG) analysis was performed with a Sta 409 PC/ PG thermogravimetric analyzer (Netzsch Co.). The analysis was performed by placing an appropriate amount of the sample in Al2O3 B
DOI: 10.1021/acsaem.9b00506 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials
Figure 1. (a) Synthesis illustration of the gradual unzipping of MWCNT and loading of Pt NPs as ORR catalysts. (b, f) (c, g) (d, h), and (e, (i) correspond to TEM images of MWCNT, LU-MWCNT, MU-MWCNT, and HU-MWCNT. (j− m) TEM analysis of Pt/MWCNT, Pt/LUMWCNT, Pt/MU-MWCNT, and Pt/HU-MWCNT nanocomposites; the Pt particle size distribution histograms are shown in (n, o, p) and (q), respectively. and raising the temperature to 800 °C at a rate of 10 °C min−1 in an argon atmosphere. The specific surface area and pore structure of carbon materials were obtained by N2 cryogenic adsorption (MICROMERITICS ASAP 2020, USA). XRD patterns were recorded from 6 to 90 by using a Rigaku D/Max 2550 VB/PC diffractometer equipped with a Cu Kα irradiation source (= 0.154056 nm) and operated at 40 kV with a current of 200 mA. The Raman spectrum measurement was performed on powder samples by use of a LabRAM HR spectroscope with a laser excitation wavelength of 514.5 nm (Horiba, Japan). The exfoliation degree of carbon nanotubes, the morphology of the catalyst, and the dispersion of Pt were characterized by a JSEM-6360LV field emission scanning electron microscope (SEM) and a JEM-2011 EM transmission electron
microscope (TEM). The valence states and proportions of elements in the catalysts were measured by an AMTCUS X-ray photoelectron spectroscope (XPS) (Kratos Co.). The data were analyzed by XPS Peaks 4.1 software. The conductivity of the catalyst was measured by a SZ-82 four-probe tester (Suzhou Telecom Instrument Co., Ltd.). 2.4. Electrochemical Measurements. Electrochemical activity was measured in a three-electrode cell system using a potentiostat/ galvanostat. Each sample was prepared as follows: First, 5.00 mg of catalyst was accurately weighed and dispersed in 2 mL of ethanol solution by an ultrasonic bath. After 5 min, 30 μL of a 5 wt % Nafion solution was added, following by an ultrasonic dispersion to form a homogeneous slurry. The 20 μL catalyst slurry was precisely removed by a pipet gun and transferred to a glassy carbon electrode with an C
DOI: 10.1021/acsaem.9b00506 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials
Figure 2. (a) X-ray diffraction (XRD) patterns of MWCNT, LU-MWCNT, MU-MWCNT, and HU-MWCNT. (b) Raman spectra of the four samples; as the level of oxidation increases, the intensity and ratio of the D:G bands increases. (c, d) N2 adsorption−desorption isotherms and pore size distribution of stepwise opening/oxidation of MWCNT. area of 0.19625 cm2. The ethanol solvent evaporated spontaneously at room temperature to form a homogeneous catalyst thin layer. The catalyst load is 50 μgPt cm−2. The completed electrode was used as a working electrode. The saturated calomel is the reference electrode, and Pt is the counter electrode. Perchloric acid (0.1 M) is the electrolyte solution. Cyclic voltammetry (CV) was performed in a N2 atmosphere. The sweep range was 0−1.2 V (vs RHE), and the sweep rate was 10 mV s−1. Before the volt−ampere curve was officially recorded, the electrode was cleaned by a fast 100 mV s−1 cleaning process until the curve was stable. Then a linear sweep voltammetry (LSV) test was performed in an O2 atmosphere to characterize the oxygen reduction performance of the catalyst. Accelerated durability testing (ADT) was performed between 0.6 and 1.1 V (vs RHE) at 50 mV s−1 for 3000 cycles, and changes in the ECSA and mass activity before and after testing were used to evaluate the durability of the catalysts. 2.5. Preparation of Single Cell DMFC with the Catalysts and Electrochemical Evaluation. The membrane electrode assembly (MEA) was prepared by clamping the pretreated Nafion 212 film (DuPont) between the anode and the cathode. Both the anode and cathode layers are composed of a backing layer, gas diffusion layer, and catalyst layer. To prepare the catalyst layer, the calculated amount of electrocatalyst (Pt/C-JM-20%, Pt/MWCNT, and Pt/MUMWCNT) was suspended in a mixture of EG and ultrapure water and treated by ultrasound for 30 min after adding 15 wt % Nafion solution. The catalyst ink was evenly sprayed onto hydrophobic treated carbon paper to obtain Pt loading of 1 mgPt cm−2 for the cathode. The anode was prepared in an identical manner with commercial PtRu with a load of 4 mg cm−2. The prepared electrodes were hot-pressed at 120 °C for 1 min to obtain MEA with active geometrical area of 4 cm2. The MEA was then sandwiched between two metal composite bipolar plates with snake-shaped geometry, which were further sealed with silica gel gasket to complete the preparation of single cell DMFC. The DMFC polarization curve was tested by the G20-HT (Green Light). Before the polarization curve was officially recorded, the battery was activated at 80 °C for 3 h. Then 0.5 M methanol solution was introduced into the anode at a flow rate of 1 mL min−1, and the I−V curve was carried out in an air or oxygen atmosphere; the gas flow rate was 100 mL min−1.
3. RESULTS AND DISCUSSION 3.1. Morphological and Structural Characterization of the Unzipped MWCNTs Supported Pt NPs. The unzipping process of MWCNTs and the following loading of Pt nanoparticles (NPs) are shown in Figure 1a. The specific mechanism of the opening of MWCNTs is described in previous works.38,49,50 In brief, bond angle tension on CNTs surface makes alkenes oxidize to ketones, and the juxtaposition of the buttressing ketones distorts the β- and γ-alkenes, making them more prone to the next attack of permanganate. As the process continues, the MWCNTs are gradually unzipped along the radial or the ends due to the lessening of the buttressinginduced strain on the β- and γ-alkenes, and the ketones can be further converted to carboxylic acids through their Oprotonated forms. Therefore, the resulting nanocomposites are highly soluble in water, ethanol, and other polar organic solvents because of the rich oxygen functional groups on the surface, making them easy to support Pt NPs and to form 3D architecture after ultrasonication and isolation. The morphological details of the unzipped MWCNTs are demonstrated in Figure 1b−i. The original MWCNTs have relatively uniform diameters with an average size of 18 nm (Figure 1b), good aspect ratio, and fewer defective structures, whereas the unzipped MWCNTs possess increased average diameters from 25 to 65 nm with the growing of the oxidized levels (Figure 1c−e). Furthermore, graphene nanoribbons (GNRs) derived from MWCNTs are observed, and their content increases gradually. The GNRs are closely overlapped on carbon nanotubes, forming bridges over the nanotube networks, while the remaining CNTs also weaken the agglomeration of GNRs to construct 3D nanoarchitecture of GNRs/CNTs hybrid materials, as shown in Figure S1. The number of carbon layers for different samples with increased unzipped level can be further observed in Figure 1f−I, decreasing from 15 layers for raw MWCNTs to 10, 5, and 2−3 layers for MWCNTs with low, medium, and high D
DOI: 10.1021/acsaem.9b00506 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials
located at 67.7° ascribed to the face-centered-cubic (fcc) facets of Pt (220). The interactions between the support materials and the active metal particles could be critical for the activity and durability of the catalyst by electron decoration. The elemental analysis and chemical composition of Pt NPs loaded on different supports have been performed by X-ray photoelectron spectroscopy (XPS), as shown in Figure S4 (survey spectra) and Figure 3. Figure 3a shows the C 1s high-resolution spectra
unzipped levels, respectively. The unzipping process of chemical oxidation not only leads to the morphological change of MWCNTs but also results in the increased content of oxygen-containing groups on the surface, as indicated in the increased vibration intensity ascribed to CO, C−O, and COO−H/O−H groups in the FTIR spectra (Figure S2a). In addition, the two-step mass loss in the region of
