Carbon Nanotube

The synthetic MnO2/MWNT nanocomposite was studied with respect to its electrocatalytic activity toward the reduction of oxygen in alkaline media and w...
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J. Phys. Chem. C 2007, 111, 1882-1887

ARTICLES Polymer-Assisted Synthesis of Manganese Dioxide/Carbon Nanotube Nanocomposite with Excellent Electrocatalytic Activity toward Reduction of Oxygen Kuanping Gong,† Ping Yu,† Lei Su, Shaoxiang Xiong, and Lanqun Mao* Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, the Chinese Academy of Sciences (CAS), Beijing 100080, China ReceiVed: May 10, 2006; In Final Form: NoVember 2, 2006

This study describes a facile and effective polymer-assisted route to synthesis of structurally uniform and electrochemically active manganese dioxide/multiwalled carbon nanotube (MnO2/MWNT) nanocomposite and investigates the electrocatalytic activity of the synthetic MnO2/MWNT nanocomposite toward the reduction of oxygen in alkaline media. Poly(sodium 4-styrene sulfonate) (PSS) used here as the polymer to assist the synthesis of the nanocomposite serves as a bifunctional molecule both for solubilizing MWNTs into an aqueous solution and for tethering Mn2+ precursor onto MWNT surfaces to facilitate the follow-up chemical deposition of MnO2 to eventually on-spot grow MnO2 nanoparticles onto MWNTs. The synthetic MnO2/MWNT nanocomposite has a uniform surface distribution and large coverage of MnO2 nanoparticles onto MWNTs, which was characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). The synthetic MnO2/MWNT nanocomposite was studied with respect to its electrocatalytic activity toward the reduction of oxygen in alkaline media and was found to possess a good electrocatalytic activity toward the four-electron reduction of oxygen. The MnO2/ MWNT nanocomposite synthesized with the polymer-assisted method could be potentially used as air electrode materials for catalytic reduction of O2 in alkaline fuel cells and metal/air batteries.

Introduction Considerable attention has been drawn on the reduction of oxygen in alkaline media because of its great importance in energy conversion technologies, such as alkaline fuel cells and metal/air batteries.1 It has been recognized that materials or composites that are competent to be used as air electrodes for the reduction of oxygen should at least bear an excellent electrocatalytic activity toward a four-electron (4e) reduction of O2, a high structural porosity for ion and gas transport, and a good conductivity.1,2 To date, design and synthesis of such kinds of multifunctional materials have been a long-standing goal in various research fields, such as chemistry, material sciences, and energy conversion technologies.3 Manganese oxides (MnOx) such as MnO2, Mn2O3, and MnOOH represent one kind of low-cost and environmentally benign metal oxides that have been widely used as the electrocatalysts in alkaline air electrodes for O2 reduction in alkaline fuel cells and metal/air batteries.1a,4 Previous attempts on the mechanistic aspects of the MnOx-based alkaline air electrodes have revealed that MnOx, MnO2 in particular, possesses an excellent electrocatalytic activity toward O2 reduction in alkaline media, through a redox-mediation mechanism or sequential disproportionations of the reduction inter* To whom correspondence should be addressed. Fax: 86-10-62559373; e-mail: [email protected]. † Also in Graduate School of the CAS.

mediates of O2 reduction (i.e., superoxide and peroxide ion in alkaline media) to eventually effect an apparent 4e-reduction of O2.5 On the other hand, previous effects on carbon nanotubes (CNTs) have revealed that, as a new kind of carbon nanostructures, CNTs possess unique structural and electronic properties, such as a good conductivity (depending on the preparation of CNTs) and a high surface area to weight ratio as well as the ability to form a three-dimensional conducting matrix.6 These properties have essentially enabled CNTs to be very useful in electrochemical studies and applications, such as electrochemical sensors,7 biosensors,8 and supercapacitors.9 Recent efforts have demonstrated that CNTs can also be used as a support for the electrocatalysts, for example, Pt and Ru, used in fuel cells.10 The striking electrocatalytic properties of MnO2 and the unique structural and electronic features of CNTs mentioned above substantially suggest that a nanocomposite composed of both components could be potentially used as new kinds of air electrode for O2 reduction in alkaline fuel cells and metal/air batteries. However, the synthesis of such kind of nanocomposite with a uniform structure and large loading of MnO2 nanoparticles onto CNTs remains a challenge although the unique physical and chemical properties of metal oxide/CNT nanocomposites reported thus far have greatly motivated extensive interests in developing new strategies for synthesizing such kinds of nanocomposites, with main efforts on achieving a homogeneous dispersion of CNTs for the subsequent attachment of metal oxides because of the aggregation nature inherent in CNTs

10.1021/jp0628636 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007

Polymer-Assisted Synthesis of Nanocomposite in most solvents.11 Even though some creative methods including supercritical flow solution, microemulsion system, oxidation and sonication-aided deposition, and coprecipitation have so far been demonstrated for the preparation of metal oxide/CNT nanocomposites, such as TiO2/CNT,6a Y2O3/CNT,11b Co3O4/ CNT,11c RuO/CNT,11d SnO2/CNT,11e ZnO/CNT, 11f and MnO2/ CNT,11g a facile and effective method for the synthesis of structurally uniform and electrochemically active MnO2/CNT nanocomposite is yet highly desired. This study describes a polymer-assisted method for the synthesis of MnO2/multiwalled CNTs (MWNT) nanocomposite, with view to establishing a facile and effective route to preparation of electrochemically active metal oxide/CNT nanocomposites that could be potentially used as air electrode material for O2 reduction in alkaline fuel cells and metal/air batteries. As far as we know, the studies undertaken here have not been reported so far and the demonstrated polymer-assisted method is facile and effective and could be developed for a large-scale synthesis of structurally uniform and electrochemically active MnO2/MWNT nanocomposites that could be potentially used as alkaline air electrodes for the catalytic reduction of O2 in alkaline fuel cells and metal/air batteries. Experimental Section Chemicals and Materials. MWNTs (diameter, 10-30 nm; length, 5-15 µm) were purchased from Shenzhen Nanotech Port Co. Ltd. (Shenzhen, China) and were purified by refluxing the as-received MWNTs in 2.6 M nitric acid for 36 h before use. Manganese sulfate monohydrate (MnSO4‚H2O), hydrogen peroxide (H2O2, 30% v/v), poly(sodium 4-styrene sulfonate) (PSS, MW, 70 000), and ammonia solution (NH3‚H2O) were obtained from Beijing Chemical Company (Beijing, China). Aqueous solutions were prepared with doubly distilled water. Synthesis and Characterization of MnO2/MWNT Nanocomposite. A 30.0 mg sample of MWNTs and 30.0 mg PSS were mixed into 35.0 mL distilled water under stirring at 90 °C for 5 h, followed by addition of 20 mg of MnSO4‚H2O. After another 1 h of stirring, 100 µL of concentrated NH3‚H2O and 480 µL of concentrated H2O2 were stepwise added into the mixture, and the resulting mixture was refluxed at the boiling point for 6 h. After being cooled to 50 °C, the suspension was filtered with a Millipore filter (pore diameter, 0.45 µm), and the product obtained was washed with distilled water and was dried at 100 °C under vacuum to give MnO2/MWNT nanocomposite. The synthetic MnO2/MWNT nanocomposite was characterized with scanning electron microscopy (SEM) on a Hitachi S4300-F microscope (Tokyo, Japan). To prepare the samples for the SEM experiments, 1.0 mg of the synthetic MnO2/MWNT nanocomposite or MWNTs was dispersed in distilled water (10 mL) with assistance of sonication, and then 2.0 µL of the resulting suspension was dropped onto silica wafer and the samples were allowed to dry under vacuum prior to each experiment. The powder X-ray diffraction (XRD) patterns of the synthetic MnO2/MWNT nanocomposite were recorded on Rigaku Dmax 2000 X-ray diffractometer with Cu KR radiation. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al KR radiation. The base pressure was about 3 × 10-9 mbar. The binding energies were referenced to the C1s line at 284.6 eV from adventitious carbon. Photoelectron spectrum was recorded for Mn2p1/2 and Mn2p3/2 core levels. Electrochemistry. Glassy carbon electrodes (GC, 3-mm diameter, BAS, West Lafayette, IN) were used as the substrate

J. Phys. Chem. C, Vol. 111, No. 5, 2007 1883 to confine the as-synthesized MnO2/MWNT nanocomposite for electrochemical investigations. Prior to use, the electrodes were first polished with emery paper (#2000), 0.3- and 0.05-µm alumina slurry on a polishing cloth, and then were cleaned under bath sonication for 10 min and were finally rinsed with distilled water. Both synthetic MnO2/MWNT nanocomposite and pristine MWNTs (without MnO2) were dispersed into distilled water to give homogeneous dispersions (0.1 mg/mL) upon bath sonication. A 10.0 µL of MnO2/MWNT dispersion or the same amount of pristine MWNT dispersion was coated onto GC electrodes and the electrodes (denoted as MnO2/MWNT-modified and MWNT-modified electrodes) were dried under ambient temperature. For rotating ring-disk electrode (RRDE) voltammetry, 15.0 µL of the MnO2/MWNT dispersion or the same amount of the pristine MWNTs was confined onto the GC disk electrode. Care was taken to ensure that the only GC disk electrode was totally covered by the MnO2/MWNT or the pristine MWNTs. Cyclic voltammetry was performed on a computer-controlled Electrochemical Workstation (CHI 832A, CHI Inc., Austin) in a one-compartment cell with MWNT-modified and MnO2/ MWNT-modified GC electrodes as working electrode, a Ag/ AgCl electrode (saturated with KCl) as reference electrode, and a platinum spiral wire as auxiliary electrode. A 0.10 M KOH solution was used as the supporting electrolyte. The electrolyte was bubbled with N2 gas for at least 30 min to make the solution saturated with N2, and the corresponding electrochemical experiments were carried out under N2 atmosphere. RRDE voltammetry was performed with a modified GC disk electrode and a Pt ring electrode (9-mm outer diameter and 6-mm inner diameter). The collection efficiency of the rotating ring-disk electrode was determined to be 0.30. RRDE experiments were conducted with a motor speed controller and a bipotentiostat (CHI 832A, CHI Inc., Austin). Potential-sweep electrolysis was carried out at the modified GC disk electrode while the Pt ring electrode was polarized at +0.50 V for the oxidation of HO2intermediate (if generated from the disk electrode). Experiments were conducted at room temperature (25 ( 1 °C). Results and Discussion PSS-Assisted Synthesis of MnO2/MWNT Nanocomposite. As demonstrated previously,12 some kinds of polymers, for example, polyethylenimine,12a poly[N-(2-aminoethyl) acrylamide],12b polyethylene glycol,12c poly(acrylic acid), and poly(vinyl alcohol),12d could be used to assist the synthesis of metal oxides in terms of their striking properties, such as tuning the viscosity of the aqueous solutions and stabilizing the synthetic nanoparticles and, to this end, such a polymer-assisted technique has recently been demonstrated to be a facile, cost-effective, and general approach to the synthesis of metal oxides with excellent properties and potential applications.12a This method is envisaged to be particularly attractive for the synthesis of metal oxide CNT nanocomposites through growing metal oxides onto CNTs because, besides the striking features described above, the polymers employed in this method can noncovalently functionalize CNTs through a polymer-wrapping mechanism13 and, as a consequence, the polymer-functionalized CNTs can be well solubilized and thereby dispersed into the aqueous media. Moreover, the strategy through noncovalent sidewall functionalization of CNTs with the polymers, such as negatively charged PSS in this case, essentially creates active sites onto the tube surface that can be used to tether metal ion precursors (i.e., Mn2+ in this case) for subsequent on-spot chemical deposition of metal oxides (i.e., MnO2 in this case) onto CNTs.

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Figure 1. SEM images of the MWNTs functionalized with PSS (A) and the synthetic MnO2/MWNT nanocomposite (B). Scale bar in A and B is 1 µm and 250 nm, respectively.

As expected, PSS was found to be able to well solubilize MWNTs into its aqueous solution, as could be evident from the good stability of the homogeneous dispersion of MWNTs into the aqueous solution of PSS (data not shown). As a consequence, the MWNTs functionalized with PSS and further on-spot decorated with MnO2 nanoparticles are well separated without an obvious aggregation, as could be seen from the SEM images of the PSS-functionalized MWNTs and the synthetic MnO2/MWNT nanocomposite displayed in Figure 1. MnO2 nanoparticles deposited onto MWNTs have a uniform size (ca. 15-nm diameter on average) and a good distribution as well as a high surface coverage onto MWNTs. These properties are believed to be benefited from the advantages of the PSS-assisted method employed here as described above. Moreover, the adsorption of MnO2 onto MWNTs was found to be very stable; the synthetic MnO2/MWNT nanocomposite even could be subjected to bath sonication in ethanol for 30 min without observable loss of MnO2 nanoparticles from MWNTs. These features of the synthetic MnO2/MWNT nanocomposite, along with its electrocatalytic activity toward the reduction of O2 (vide infra), substantially make them well competitive to be used as a new kind of alkaline air electrodes for O2 reduction in alkaline fuel cells and metal/air batteries. Figure 2 displays powder X-ray diffraction (XRD) patterns (A) and the X-ray photoelectron spectrum (XPS) for Mn2p (B) of the synthetic MnO2/MWNT nanocomposite. The diffraction peaks at 2θ values of 26.50, 54.30, and 77.70 were ascribed to the 002, 004, and 110 reflections of the MWNTs, respectively,14 while other peaks at 2θ values of 32.80, 34.40, 38.80, 42.60, 44.30, and 65.50 were well indexed to the reflections of γ-MnO2 as indicated in Figure 2A (Joint Committee on Powder Diffraction Standards (JCPDS) card No. 14-0644, a ) 6.36 Å, b ) 10.15

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Figure 2. (A) XRD patterns of the MnO2/MWNT nanocomposite synthesized with the PSS-assisted method. The stars and numbers in the figure indicate the reflections from MWNTs and MnO2, respectively. (B) XPS spectrum for Mn2p of the synthetic MnO2/MWNT nanocomposite.

Figure 3. CVs obtained at the MWNTs (dotted curve) and the MnO2/ MWNT (solid curve) confined onto GC electrodes in 0.10 M HAcNaAc buffer (pH 4.0). Scan rate, 20 mV s-1. The initial potential was 0.0 V.

Å, c ) 4.09 Å). This demonstration confirms the formation of the MnO2/MWNT nanocomposite with the PSS-assisted method. As shown in Figure 2B, the Mn2p spectrum shows two peaks at 642.2 and 653.5 eV, corresponding to the binding energies of Mn2p3/2 and Mn2p1/2, respectively, consistent with those of MnO2 as reported in the literature,15 indicating that the MWNTs are successfully decorated with MnO2 nanoparticles to form electrochemically active nanocomposite, which could be further confirmed by cyclic voltammetry. Figure 3 depicts typical cyclic voltammograms (CVs) of the MnO2/MWNT nanocomposite confined onto GC electrode in

Polymer-Assisted Synthesis of Nanocomposite

Figure 4. CVs at MWNT-modified (dotted curves 1 and 1′) and MnO2/ MWNT-modified (solid curves 2 and 2′) GC electrodes in 0.10 M KOH solution saturated with N2 (panel A) or ambient air (panel B). Scan rate, 20 mVs-1.

0.10 M HAc-NaAc solution (pH 4.0). A couple of well-defined peaks were observed at ca. 0.81 V (vs Ag/AgCl), which were ascribed to the overall redox process of MnO2 shown below.16

MnO2(s) + 2e- + 4H3O+ a Mn2+ (aq) + 6H2O This voltammetric response was not able to record at the MWNTs (without decoration with MnO2 nanoparticles, dotted curve), suggesting the formation of electrochemically active MnO2/MWNT nanocomposite through the deposition of redoxactive MnO2 nanoparticles onto the MWNTs with the PSSassisted method. O2 Reduction at the MnO2/MWNT Nanocomposite. Figure 4 displays CVs at MWNT-modified (dotted curves 1′and 1) and MnO2/MWNT-modified (solid curves 2′and 2) GC electrodes in 0.10 M KOH solution saturated with N2 (panel A) or ambient air (panel B). In 0.10 M N2-saturated KOH solution (panel A), the MnO2/MWNT-modified electrode exhibits two cathodic peaks, one well-defined at ca. -0.35 V and the other ill-defined at -0.10 V (solid curve 2′). The first reduction peak at -0.10 V was seriously broadened and was attributed to the overlapping of three reduction peaks of energetically different Mn4+ ions located on the MnO2 surface into Mn3+ intermediates (e.g., γ-Mn2O3 and γ-MnOOH in alkaline media), according to the literature.5c The second reduction peak at -0.35 V was attributed to the consequent reduction of Mn3+ intermediates generated at the first reduction steps into Mn(OH)2. McBreen has suggested that the reduction of MnO2 was terminated when Mn(OH)2 was produced as the end product.17 In the reversed positive cycle, two well-defined anodic peaks were recorded at the MnO2/MWNT-modified electrode. The first oxidation peak at -0.33 V was ascribed to the oxidation Mn(OH)2 into Mn3+ intermediates, such as γ-Mn2O3 and γ-MnOOH, while the second one at 0.05 V was attributed to the oxidation of the generated Mn3+ intermediates into MnO2. In 0.10 M KOH solution saturated with ambient air (panel B), the MWNT-modified electrode exhibits two reduction peaks at -0.25 V and -0.75 V (dotted curve 1), which were ascribed to the successive two-electron reduction of O2 with HO2- as the intermediate, that is, the first reduction peak at -0.25 V was attributed to the reduction of O2 to HO2- redox-mediated by the oxygen-containing groups at the MWNTs,18 and the

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Figure 5. CVs obtained at the MnO2/MWNT-confined GC electrode in 0.10 M air-saturated KOH solution at different scan rates of (from inner to outer) 10, 20, 50, 100, 200, 300, 400, and 500 mV s-1. Inset, plot of the peak current against the square root of scan rate.

second one at -0.75 was due to the reduction of HO2- to OH-. Interestingly, the O2 reduction occurs in a relatively different way at the MnO2/MWNT-confined GC electrode as shown in Figure 4 (panel B, solid curve 2). For example, only one reduction peak was obtained at -0.30 V for the O2 reduction and the peak for the reduction of HO2- to OH- at the MWNTmodified electrode at more negative potential was not recorded. Moreover, the peak current at the MnO2/MWNT-confined GC electrode was obviously larger than that of the first reduction peak at the MWNT-modified electrode at -0.25 V. This observation implies that an apparent 4e-reduction of O2 was achieved at the synthetic MnO2/MWNT nanocomposite confined onto GC electrode. Figure 5 depicts CVs obtained at the MnO2/MWNT-modified GC electrode in 0.10 M KOH solution saturated with ambient air at various scan rates. The peak current for O2 reduction was linear with the square root of potential scan rate employed (inset, Figure 5), indicating that the O2 reduction at the MnO2/MWNTmodified GC electrode is a diffusion-controlled process. The number of electron transfer (n) involved in the O2 reduction was estimated with Randles-Sevcik equation19 and was calculated to be 3.7, suggesting that an apparent 4e-reduction of O2 was almost obtained at the synthetic MnO2/MWNT nanocomposite. The O2 reduction at the MnO2/MWNT nanocomposite was further studied with RRDE voltammetry as shown in Figure 6. As shown, at the MWNT-modified GC disk electrode (without decoration with MnO2 nanoparticles) (dotted curve 2), the O2 reduction undergoes two stepwise reduction processes at -0.40 V and -1.0 V, which were attributed to the sequential 2e-reductions of O2 (i.e., O2 to HO2- and HO2- to OH-) with HO2- as the intermediate that was oxidized at the Pt ring electrode (dotted curve 2′). This was consistent with cyclic voltammetry for the O2 reduction at the MWNT-modified electrode (Figure 4, panel B, dotted curve 1). At the MnO2/ MWNT-modified disk electrode (solid curve 1), the disk current recorded for the O2 reduction was obviously larger than that at the MWNT-modified disk electrode (dotted curve 2). In the meantime, a very slight current was recorded at the corresponding Pt ring electrode (solid curve 1′). These observations essentially reveal that an apparent 4e-reduction pathway is involved in O2 reduction at the synthetic MnO2/MWNT nanocomposite, which is consistent with cyclic voltammetric results shown in Figures 4 and 5. The 4e-reduction pathway for the O2 reduction at the MnO2/MWNT nanocomposite was further

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Figure 6. Rotating ring-disk voltammograms for O2 reduction in 0.10 M KOH solution saturated with ambient air. The synthetic MnO2/ MWNT (curve 1) and pristine MWNTs (curve 2) were confined onto GC disk electrodes. Pt ring electrode was polarized at +0.5 V for the oxidation of HO2- produced from the corresponding disk electrodes. Electrode rotation rate, 800 rpm. Scan rate, 1 mV s-1.

transfer effects, F is the faraday constant, DO2 is the diffusion coefficient of O2 in solution (1.73 × 10-5 cm2 s-1), CO2b is the concentration of O2 in solution (2.4 × 10-7 mol cm-3), V is kinetic viscosity (0.01 cm2 s-1), and ω is the angular frequency of the rotation in terms of rad s-1.20 A is the geometric area of the electrode (0.28 cm2). The value of n for the O2 reduction at the MnO2/MWNT nanocomposite was calculated to be 3.5, which is close to the theoretical value for 4e-reduction of O2, confirming that a 4e-reduction of O2 was almost achieved at the synthetic MnO2/MWNT nanocomposite. These demonstrations essentially indicate that the MnO2/MWNT nanocomposite synthesized with the PSS-assisted method possesses a good electrocatalytic activity toward the 4e-reduction of O2, possibly through a redox-mediation mechanism and sequential disproportionations of the reduction intermediates of O2 reduction (i.e., superoxide and peroxide ion in alkaline media) as demonstrated previously.5a-e Although the achieved 4e-reduction of O2 did not result from the MWNTs in the synthetic MnO2/MWNT nanocomposite, the use of MWNTs as the framework to prepare the structurally uniform and electrochemically active MnO2/ MWNT nanocomposite is reasonably believed to be an advance in development of new kinds of alkaline air electrodes in terms of the unique structural and electronic properties of the MWNTs, such as good conductivity and capability to form a threedimensional matrix. It is known that the currently used MnOxbased alkaline air electrodes are generally fabricated by mixing MnOx electrocatalysts into a conducting and porous carbon matrix consisting of carbon black and active carbon, and the performance of such kinds of air electrodes is largely dependent on the homogeneity, porosity, and conductivity of the MnOxcontaining carbon matrix. As described above, the unique properties of the MWNTs essentially enable them to be very potential candidate for the carbon materials, that is, active carbon and carbon black, used in the current alkaline air electrodes, and thus it is reasonable to say that the uses of MWNTs as the support for the MnO2 electrocatalysts in this work would facilitate the fabrication of new kinds of alkaline air electrodes for O2 reduction in alkaline fuel cells and metal/air batteries. Conclusions

Figure 7. Koutecky-Levich plots for O2 reduction at the MWNTmodified (O) and MnO2/MWNT-modified (b) GC electrodes and theoretical curves for n ) 2 and 4 as indicated in the figure. The current was taken from the rotating disk voltammograms for O2 reduction at both electrodes at -0.4 V.

confirmed with rotating disk voltammetry (data not shown). The electron number (n) involved in the O2 reduction was calculated according to Koutecky-Levich plot for O2 reduction displayed in Figure 7 and the equation shown below.19

i-1 ) ik-1 + idl-1 )

1 + nFAkCO2b

In summary, we have demonstrated a polymer-assisted route to synthesis of structurally uniform and electrochemically active MnO2/MWNT nanocomposite. The synthetic nanocomposite has been demonstrated to be very potential as a new type of alkaline air electrodes with an excellent electrocatalytic activity toward 4e-reduction of O2. The method demonstrated here is facile and mild and could be developed for a large-scale synthesis of electrochemically functional MnO2/MWNT nanocomposite. In addition, the polymer-assisted method may be further developed to be versatile for synthesis of other kinds of metal oxide decorated CNT heterostructures with practical prospects. Acknowledgment. This work was financially supported by National Natural Science Foundation of China (20375043, 20435030, 20575071), Chinese Academy of Sciences, and Centre for Molecular Science, Institute of Chemistry. References and Notes

1 2/3 -1/6

0.62nFADO2 ν

b

C O2 ω

1/2

where ik represents the current in the absence of any mass-

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