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Electrostatic Layer-by-Layer Assembled Carbon Nanotube Multilayer Film and Its Electrocatalytic Activity for O2 Reduction Meining Zhang,† Yiming Yan,† Kuanping Gong,† Lanqun Mao,* Zhixin Guo, and Yi Chen Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received April 30, 2004. In Final Form: July 26, 2004 Multilayer films of shortened multiwalled carbon nanotubes (MWNTs) are homogeneously and stably assembled on glassy carbon electrodes with the layer-by-layer (LBL) method, based on electrostatic interaction of positively charged poly(diallyldimethylammonium chloride) and negatively charged and shortened MWNTs. The film assembly and electrochemical property as well as the electrocatalytic activity toward O2 reduction of the MWNT multilayer film are studied. Scanning electron microscopy, the quartz crystal microbalance technique, ultraviolet-visible-near-infrared spectroscopy, and cyclic voltammetry are used for characterization of film assembly. Experimental results revealed that film growth is uniform, almost with the same coverage of the MWNTs in each layer, and that the assembled MWNTs are mainly in the form of small bundles or single tubes on the electrodes. Electrochemical studies indicate that the LBL assembled MWNT films possess a remarkable electrocatalytic activity toward O2 reduction in alkaline media. This property, combined with the well-dispersed, porous and conductive features of the MWNT film illustrated with the LBL method, suggests the potential application of the MWNT film for constructing an efficient alkaline air electrode for energy conversions.
Introduction The development of novel alkaline air electrodes has been a long-standing goal because of their wide use in metal-air batteries, especially zinc-air batteries, and alkaline fuel cells.1,2 While the empirical utilization of carbon and metal oxides (typically manganese oxides) has enabled the as-constructed alkaline air electrodes to satisfy the practical requirements, such as material cost, stability, and mass production, recent studies have revealed that the performance of such electrodes is mainly limited by their poor activity for O2 reduction.3-6 The use of platinum nanoparticles as the electrocatalyst for O2 reduction would well overcome such a limitation; unfortunately, the platinum-based air electrode yet suffers from the disadvantages of high cost and low tolerance against methanol crossover.4,7 Carbon nanotubes (CNTs) represent a new type of carbon material and have been widely recognized as the quintessential nanomaterial since their discovery in 1991.8 Recent electrochemical studies revealed that the unique properties of the CNTs make them very promising in * To whom correspondence should be addressed. Phone: +8610-62646525. Fax: +86-10-62559373. E-mail:
[email protected]. † Also in the Graduate School of the Chinese Academy of Sciences. (1) Kinoshita, K. Electrochemical Oxygen Technology; John Wiley & Sons Inc.: New York, 1992. (2) Ogumi, Z.; Matsuoka, K.; Chiba, S.; Matsuoka, M.; Iriyama, Y.; Abe, T.; Inaba, M. Electrochemistry 2002, 70, 980. (3) Mao, L.; Arihara, K.; Sotomura, T.; Ohsaka, T. Chem. Commun. 2003, 2818. (4) Ohsaka, T.; Mao, L.; Arihara, K.; Sotomura, T. Electrochem. Commun. 2004, 6, 273. (5) Mao, L.; Sotomura, T.; Nakatsu, K.; Koshiba, N.; Zhang, D.; Ohsaka, T. J. Electrochem. Soc. 2002, 149, A504. (6) Mao L.; Arihara, K.; Sotomura, T.; Ohsaka, T. Electrochim. Acta 2004, 49, 2515. (7) Uchida, H.; Mizuno, Y.; Watanabe, M. J. Electrochem. Soc. 2002, 149, A682. (8) Iijima, S. Nature (London) 1991, 354, 56.
electrochemical applications,9,10 for example, for protein electrochemistry,11 development of novel electrochemical sensors and biosensors,12 construction of new supercapacitors, and as a catalyst support for fuel cells. In addition to their potential electrochemical applications mentioned above, CNTs have several features that make them particularly attractive for development of novel nonplatinum alkaline air electrodes. First, carbon nanotubes have a high electrochemically accessible area of porous tubes. Second, carbon nanotubes have good electronic conductance and strong mechanical property. Third, besides their catalytic activity observed in neutral and acidic media,13 shortened CNTs possess a remarkable electrocatalytic activity toward O2 reduction in alkaline media as will be demonstrated. These properties essentially suggest that the CNTs are a potential candidate for development of effective, low-cost, and environmentally benign non-platinum alkaline air electrodes for energy conversions. However, such a potential application and the electrocatalytic activity of the CNT film for O2 reduction in alkaline media have not been investigated so far. It is known that the poor solubility of the CNTs in most solvents has greatly limited the investigations of the CNTs and their applications for the development of CNT-based electronic devices, including CNT-based alkaline air electrodes in this case, although considerable attention has been paid to covalent and noncovalent functional(9) Zhao, Q.; Gan, Z.; Zhuang, Q. Electroanalysis 2002, 14, 1609. (10) Dai, L.; Soundarrajan, P.; Kim, T. Pure Appl. Chem. 2002, 74, 1753. (11) Gooding, J. J.; Wibowo, R.; Liu, J.; Yang, W.; Losic, D.; Orbons, S.; Mearns, F. J.; Shapter, J. G.; Hibbert, D. B. J. Am. Chem. Soc. 2003, 125, 9006. (12) Wang, J.; Musameh, M.; Lin, Y. J. Am. Chem. Soc. 2003, 125, 2408. (13) Britto, P. J.; Santhanam, K. S. V.; Rubio, A. R.; Alonso, J. A.; Ajayan, P. M. Adv. Mater. 1999, 11, 154.
10.1021/la048923l CCC: $27.50 © 2004 American Chemical Society Published on Web 09/01/2004
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ization of the nanotubes to improve their solubility.14-16 As demonstrated previously, the oxidation procedure used for purification of a carbon nanotube, mainly to remove metal catalyst and amorphous carbon, also results in partial oxidation of the carbon atoms to produce oxygencontaining groups (such as carboxylic acid groups) especially in the open ends of the nanotubes.16,17 These groups produced are negatively charged in aqueous solution and can interact with positively charged polyelectrolytes.18,19 Thus, one may expect to achieve multilayer films of the CNT on a solid substrate by using the so-called layerby-layer (LBL) method.20,21 In the present work, we use the layer-by-layer method to assemble homogeneous and stable CNT multilayer films on glassy carbon (GC) electrodes, based on the electrostatic interaction between positively charged poly(diallyldimethylammonium chloride) (PDDA) and negatively charged and shortened multiwalled carbon nanotubes (MWNTs). The electrocatalytic activity of the as-assembled MWNT film for O2 reduction in alkaline media and the potential application of the MWNT for developing a novel alkaline air electrode are evaluated. Experimental Section Reagents. MWNTs were purchased from Nanoport Co. Ltd. (Shenzhen, China), and the as-received MWNTs were treated via sonication in 1:3 concentrated nitric-sulfuric acid at ca. 50 °C. Such a procedure shortens the nanotubes and produces oxygen-containing moieties mainly on the open ends of the nanotubes. PDDA (200 000-350 000 Mw) was purchased from Aldrich and used as received. All other chemicals were at least of analytical reagent grade and used without further purification. Aqueous solutions were prepared with doubly distilled water. Preparation of {PDDA/MWNT}n Multilayer Films. GC (3 mm diameter, Bioanalytical Systems Inc.) electrodes were used as the substrate to grow the multilayer films. The electrodes were polished first with emery paper and then with aqueous slurries of fine alumina powders (1 and 0.05 µm) on a polishing microcloth, and were finally rinsed with doubly distilled water in an ultrasonic bath for 10 min. A 1.0 mg sample of shortened MWNTs was dispersed in 1.0 mL of borate buffer (pH 9.18), and the suspension was agitated in an ultrasonic bath for 10 min, giving a black dispersion that was stable for 1-2 days. The PDDA/ MWNT multilayer films were grown on the GC electrodes by alternately dipping the electrodes into a 1 wt % aqueous solution of positively charged PDDA containing 0.5 M NaCl and a 1.0 mg/mL negatively charged MWNT dispersion for 30 min. The films were first carefully rinsed with distilled water after each dipping step to remove the excess of assembly materials, and then dried with nitrogen gas. The electrodes modified with five layers of PDDA and MWNT (hereafter denoted as {PDDA/ MWNT}5/GC electrodes) will be used for the electrochemical measurements. The addition of 0.5 M NaCl to PDDA solution was reported to result in a uniform multilayer growth since the presence of salts clearly increases the amount of polyelectrolyte deposition.22 Film Assembly Characterization. Scanning electron microscopy (SEM; Hitachi S4300-F microscope, Hitachi Inc., Tokyo, Japan), the quartz crystal microbalance (QCM) technique, (14) Kooi, S. E.; Schlecht, U.; Burghard, M.; Kern, K. Angew. Chem., Int. Ed. 2002, 41, 1353. (15) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; Mclean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Mater. 2003, 2, 338. (16) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (17) Kuznetsova, A.; Popova, I.; Yates, J. T., Jr.; Bronikowski, M. J.; Huffman, C. B.; Liu, J.; Smalley, R. E.; Hwu, H. H.; Chen, J. G. J. Am. Chem. Soc. 2001, 123, 10699. (18) Bumsu, K.; Park, H.; Sigmund, W. M. Langmuir 2003, 19, 2525. (19) Bumsu, K.; Sigmund, W. M. Langmuir 2003, 19, 4848. (20) Mamedov, A. A.; Kotov, N. A.; Prato, M.; Guldi, D. M.; Wicksted, J. P.; Hirsch, A. A. Nat. Mater. 2002, 1, 190. (21) Rouse, J. H.; Lillehei, P. T. Nano Lett. 2003, 3, 59. (22) Dudas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725.
Zhang et al. ultraviolet-visible-near-infrared (UV-vis-near-IR; 340 spectrophotometry) spectroscopy, and cyclic voltammetry (CV) were used to characterize film assembly. Film assembly was monitored at each step with a quartz crystal microbalance using 9 MHz QCM resonators (AT cut, Beijing 707 Factory). To mimic the GC electrode surface used for film assembly, one layer of negatively charged cysteine was first formed on the gold-coated resonators (geometric area 0.28 cm2) by immersing the resonators into 10 mM cysteine aqueous solution for 24 h. The PDDA/MWNT bilayers were then assembled on the resonators as for the GC electrodes. The resonators were dried with nitrogen gas before the frequency change (∆F) was measured. The adsorbed mass of the MWNT in each layer was calculated with the Sauerbery equation. Under the present experimental conditions, the dry film mass per unit area is calculated by the following equation:23
M/A (g cm-2) ) -∆F (Hz)/(1.83 × 108) {PDDA/MWNT}n multilayer films used for UV-vis-near-IR spectroscopy were prepared on a quartz slide. The quartz slide was first clearned with a piranha solution (a 1:3 mixture of 30% H2O2 and concentrated H2SO4) and then thoroughly rinsed with distilled water. The PDDA/MWNT bilayers were alternatively assembled on the quartz slide as for the GC electrode. The film assembly was also monitored at each assembly step with cyclic voltammetry with the {PDDA/MWNT}n/GC electrodes as the working electrode. The oxygen-containing groups introduced at the open ends of the nanotubes show a pair of redox waves, and thereby, the increase in the peak currents readily reflects the growth of the MWNT on the GC electrode. Apparatus and Measurements. Electrochemical measurements were conducted with a conventional, two-compartment, three-electrode cell with a computer-controlled BAS 100 B/W electrochemical analyzer (BAS). The {PDDA/MWNT}5/GC electrodes were used as the working electrode, a platinum spiral wire was used as the counter electrode, and a Ag/AgCl electrode (saturated with KCl) was used as the reference electrode. The working electrode and the counter electrode were separated with a porous glass. Cyclic voltammetry was used for characterization of film assembly and investigation of electrochemical properties of the MWNT multilayer films assembled and their electrocatalytic activity toward O2 reduction in 0.10 M KOH solution. Rotating ring-disk electrode (RRDE) voltammetry was performed with an RRDE consisting of a GC disk (6 mm diameter) and a Pt ring (9 mm outer diameter and 7.5 mm inner diameter) with a collection efficiency of ca. 0.30. Five layers of PDDA/MWNT were assembled on the GC disk electrodes in the same manner as that for the GC electrodes used in cyclic voltammetry. RRDE experiments were performed with a bipotentiostat (CHI 832A, CHI Inc., Austin). Potential-sweep electrolysis was carried out at the {PDDA/MWNT}5/GC disk electrode, and the platinum ring electrode was maintained at +0.50 V for the oxidation of HO2- produced at the disk electrode. All experiments were conducted at room temperature.
Results and Discussion Film Assembly. The assembly and morphology of the PDDA/MWNT multilayer films were first characterized by SEM. Figure 1 displays typical SEM images of {PDDA/ MWNT}1 (A), {PDDA/MWNT}3 (B), and {PDDA/MWNT}5 (C) assembled on a silicon wafer, which was first cleaned in a piranha solution and then thoroughly rinsed with distilled water. These SEM profiles demonstrate that an obvious increase in nanotube coverage was observed with increasing number of film assembly procedures, and thus, one may expect to obtain a homogeneous, porous, and three-dimensional MWNT multilayer film with a large surface area provided the assembly procedure is repeated many times. As shown in Figure 1C, the substrate was mostly covered with homogeneous MWNT films after five layers of PDDA and MWNT were assembled, and thus, (23) Zhou, L.; Yang, J.; Estavillo, C.; Stuart, J. D.; Schenkman, J. B.; Rusling, J. F. J. Am. Chem. Soc. 2003, 125, 1431-1436.
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Figure 2. QCM frequency shifts for cycles of alternative adsorption during growth of PDDA/MWNT films on gold resonators first coated with a monolayer of cysteine and then with PDDA (2) and a shortened MWNT (b).
Figure 1. Representative SEM images of {PDDA/MWNT}1 (A), {PDDA/MWNT}3 (B), and {PDDA/MWNT}5 (C) assembled on a silicon wafer. The scale bar in A-C was 5 µm. (D) represents the SEM image of {PDDA/MWNT}1 with a high amplification with a scale bar of 1 µm.
the GC electrodes modified with five bilayers of PDDA and MWNT were typically used in the subsequent electrochemical experiments. In addition, a close inspection of the first layer of MWNTs grown on the PDDA-treated substrate (D) revealed that the MWNTs, 0.50-1 µm in length and 3080 nm in diameter, are mostly well distributed on the surface, and that most of the adsorbed MWNTs are in the form of small bundles or single tubes. This is very different from other methods employed for preparing CNT-based electrodes, e.g., cast-coating, mixing the CNTs with carbon paste, and confining the CNTs with polymeric matrixes, in which the CNTs are mainly in the form of big bundles. Such small bundles and single tubes homogeneously assembled on the substrate are believed to be very attractive for air electrode development since most of the well-dispersed nanotubes are electrochemically accessible and thereby can be used for O2 reduction, yielding a high current output. It should be noted that, although the LBL method used here may not be suitable for practical development of air electrodes, the properties of the MWNT film demonstrated by such a method, e.g., porous and three-dimensional nature of the multilayer films consisting of homogeneously dispersed MWNT small bundles and single nanotubes, suggest the promising application of the MWNT films for practical development of novel alkaline air electrodes. Film assembly was also monitored by QCM, and the decrease in frequency (∆F) of each assembly step is depicted in Figure 2. As shown, the adsorption of PDDA results in a very slight decrease in the ∆F presumably due to the loss of the MWNTs, e.g., those with a long tube and few oxygen-containing groups, weakly adsorbed on the PDDA film. Nevertheless, such a decrease was negligible compared with the large increase in ∆F caused by the adsorption of the MWNT. ∆F is almost linear with the layer number of the MWNT films assembled, indicative of uniform loading of the MWNTs in each layer. This can be further confirmed by UV-vis-near-IR and CV as
Figure 3. UV-vis-near-IR absorbance for {PDDA/MWNT}n layer-by-layer assembled on a quartz slide. The inset shows a plot of the absorbance at 267 nm versus the layer number.
demonstrated below. The mass of the MWNTs assembled in each layer was calculated to be 3.09 µg. Figure 3 shows the increase in absorbance upon assembly of MWNTs in each layer on the quartz slide. PDDA does not exhibit any absorbance in the wavelength employed, whereas the shortened MWNTs show a strong adsorption at 267 nm. The absorbance was found to clearly increase with the growth of the film and to be linear with the layer number (inset in Figure 3). This observation again indicates the growth of the PDDA/MWNT film in each assembly step is uniform. Figure 4A shows typical cyclic voltammograms obtained with {PDDA/MWNT}n/GC electrodes in 0.10 M phosphatebuffered solution (PBS, pH 7.0). A well-defined reversible redox wave with a formal potential of -50 mV was observed, which was attributed to the redox process of the oxygen-containing groups produced. As shown, both the charge current, which can be defined at the anodic process at +0.40 V, and the faradic current of the redox wave clearly increase with increasing number of film assembly steps, reflecting the growth of the MWNTs on the electrode. The linearity between the cathodic current of the redox wave and the layer number (Figure 4B) again suggests that almost the same amount of the MWNTs was assembled in each layer by the LBL method. In addition, the as-assembled MWNT multilayer film was found to be very stable; cyclic voltammograms obtained for the above redox wave essentially remained unchanged on a con-
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Figure 5. Typical cyclic voltammograms obtained at {PDDA/ MWNT}5/GC (A) and GC (B) electrodes in 0.10 M KOH solution saturated with N2 (dotted lines) or O2 (solid lines). Scan rate: 100 mV s-1.
tinuous potential cycling and repetitive uses of the electrodes. Such a high stability substantially suggests the MWNTs strongly adsorb on the PDDA film in each layer through the electrostatic interaction, and that each PDDA/MWNT bilayer was well assembled with the LBL method. Electrochemistry of the Multilayer Film and Its Electrocatalytic Activity toward O2 Reduction. As shown in Figure 4, the MWNT multiayer assembled onto a GC electrode exhibits one pair of redox waves that was attributed to the redox process of the oxygen-containing groups. Both the anodic (Ipa) and cathodic (Ipc) peak currents of the present {PDDA/MWNT}5/GC electrodes vary linearly with potential scan rate in the range of 20500 mV s-1, and the ratio of Ipc/Ipa at a given scan rate is nearly unity (not shown), characteristic of a surfaceconfined reversible electrode process of such a redox couple on the shortened MWNTs. In addition, the formal potential of such a redox couple was found to be dependent on electrolyte pH, and the plot of the formal potential against electrolyte pH gives a slope of ca. -60 mV/pH (not shown), indicating that the numbers of protons and electrons involved in such a redox process are identical, which is similar to an earlier report in which the CNTs were simply cast-coated onto the electrode.24 This demonstrates that the present strategy of using the layer-by-layer method to homogeneously assemble the MWNT multilayer films
on the substrate (i.e., GC electrode) well retains the inherent electrochemical properties of CNTs, providing the advantages of the existing form of the adsorbed MWNTs (i.e., small MWNT bundles and single tubes), uniformity and stability of the as-developed MWNT multilayer film. Figure 5 depicts cyclic voltammograms for O2 reduction at the {PDDA/MWNT}5/GC (A) and bare GC (B) electrodes in 0.10 M KOH solution. At the bare GC electrode, O2 reduction undergoes two processes at ca. -0.40 and -0.90 V in the potential window employed. Both processes have been documented to be a two-electron (2e) reduction process of O2 to peroxide (HO2- in 0.10 M KOH), the first one of which was electrochemically mediated by the oxygen-containing groups (so-called quinone-like groups) at the GC electrode surface with superoxide as the intermediate,4 while the second was attributed to a direct 2e reduction process at the GC electrode.7 The lack of sufficient oxygen-containing groups at the bare GC electrode surface is responsible for the observed insufficient mediated 2e reduction of O2 at -0.40 V, thereby resulting in a subsequent direct 2e reduction process at a more negative potential (-0.90 V) at the GC electrode (solid line, B).3,6,25 The {PDDA/MWNT}5/GC electrode exhibits one pair of redox waves at -0.40 V with a peak separation of ca. 60 mV in N2-saturated 0.10 M KOH solution (Figure 5A, dotted line). Such a redox wave was ascribed to the reversible redox process of the oxygen-containing groups produced at the MWNTs in 0.10 M KOH solution. At such an electrode, the O2 reduction experiences two processes at -0.30 and -0.90 V (Figure 5A, solid line). The first reduction process at -0.30 V was essentially redoxmediated by the oxygen-containing groups at the MWNTs.
(24) Luo, H.; Shi, Z.; Li, N.; Gu, Z.; Zhuang, Q. Anal. Chem. 2001, 73, 915.
(25) Tammeveski, K.; Kontturi, K.; Nichols, R. J.; Potter, R. J.; Schiffrin, D. J. J. Electroanal. Chem. 2001, 515, 101.
Figure 4. (A) Typical cyclic voltammograms for {PDDA/ MWNT}n multilayer films assembled on GC electrodes in PBS (pH 7.0). The dotted line represents typical the cyclic voltammogram of a PDDA-modified GC electrode. Scan rate: 50 mV s-1. (B) Cathodic peak current obtained at the {PDDA/MWNT}n/ GC electrode against the number of bilayers (n).
LBL Assembled Carbon Nanotube Multilayer Film
Figure 6. RRDE voltammograms for O2 reduction at the {PDDA/MWNT}5/GC (curve 1) and bare GC (curve 2) disk electrodes in O2-saturated 0.10 M KOH solution. Curves 1′ and 2′ represent the current for the oxidation of HO2- produced at the corresponding disk electrodes. Potential scan rate: 10 mV s-1. Electrode rotation rate: 400 rpm. The Pt ring electrode was polarized at +0.50 V for the oxidation of HO2-.
This was evident by a comparison of the cyclic voltammogram obtained in 0.10 M KOH solution in the absence of O2 (dotted line) with that obtained in the presence of O2 (solid line). As shown, the presence of O2 clearly increases the reduction peak current while decreasing the reversed oxidation peak current of the redox couple at the MWNTs. This observation reveals that the assembled shortened MWNTs bear plenty of oxygencontaining groups (probably so-called quinone-like groups as those at the GC electrode surface). These functional groups are capable of efficiently redox-mediating the 2e reduction of O2 to HO2-. Superior to that obtained at the bare GC electrode, such an intrinsic feature of the shortened MWNTs substantially enables the 2e reduction of O2 to be efficiently redox-mediated at the first reduction step as will be further evident with RRDE voltammetry shown below. The peak current for the 2e reduction of O2 at the {PDDA/MWNT}5/GC electrode varies linearly with the square root of the potential scan rate (not shown), while the potential for the O2 reduction does not change obviously, suggesting that the catalytic reduction of O2 at the {PDDA/MWNT}5/GC electrode is a fast, diffusioncontrolled process. Figure 6 shows typical RRDE voltammograms obtained with {PDDA/MWNT}5/GC (curves 1 and 1′) and bare GC (curves 2 and 2′) disk and platinum ring electrodes. Similar to the results obtained with CV, the two reduction
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processes recorded at the bare GC disk electrode at -0.50 and -1.20 V were ascribed to the 2e reduction of O2 to HO2- that was oxidized at the ring electrode (curve 2′). Very different from those at the bare GC electrode, where the second reduction step was still ascribed to the direct 2e reduction of O2 to HO2-, the second reduction process at the {PDDA/MWNT}5/GC electrode (at -0.90 V) was attributed to the reduction of HO2- to water (OH- in this case), which was supported by the decrease in the ring current for HO2- oxidation (curve 1′). This demonstration again suggests that oxygen-containing groups at the MWNT multilayer film redox-mediate the 2e-reduction of O2 at the first step, which is more efficient than that at the bare GC electrode. Such a catalytic activity of the MWNT multilayer film toward O2 reduction is very useful for air electrode development since the catalytic performance obviously shifts the potential for efficient O2 reduction to a positive direction and subsequently improves the performance of the air electrode.3 This could be compared with the carbon materials, i.e., carbon black and active carbon, presently used in alkaline air electrodes. Although the carbon materials may also bear so-called quinone-like functional groups that are capable of redoxcatalyzing O2 reduction in alkaline media, they should be used in combination to maintain the catalytic activity, conductivity, and porosity, substantially rendering difficulties in construction of air electrodes. As demonstrated above, the use of the shortened MWNTs would mostly meet the requirements for the practical alkaline air electrodes. Thus, it is reasonable to conclude that the use of MWNTs would offer a facile and effective alternative to efficient non-platinum air electrodes. Further processes, including constructing practical MWNT-based alkaline air electrodes, are currently under way. Conclusions Stable multilayer films of carbon nanotubes have been homogeneously assembled on glassy carbon electrodes by the layer-by-layer method, and the assembled MWNT multilayer films were found to bear some striking features, such as the adsorbed form of small MWNT bundles or single tubes on the substrate, uniformity, good stability, and remarkable electrocatalytic activity toward O2 reduction in alkaline media. These intrinsic properties, coupled with the good conductivity and high surface area of the MWNTs, strongly suggest the potential applications of the MWNTs for the development of novel MWNT-based alkaline air electrodes for better energy conversions. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant 20375043) and Chinese Academy of Sciences (Grant KJCX2-SW-H06). We are grateful to Professor Lin Li for many constructive discussions. LA048923L
