898 Chem. Mater. 2010, 22, 898–905 DOI:10.1021/cm901698s
MnO2-Based Nanostructures as Catalysts for Electrochemical Oxygen Reduction in Alkaline Media† Fangyi Cheng, Yi Su, Jing Liang, Zhanliang Tao, and Jun Chen* Institute of New Energy Material Chemistry and Engineering Research Center of Energy Storage & Conversion (Ministry of Education), Chemistry College, Nankai University, Tianjin 300071, People’s Republic of China Received June 18, 2009. Revised Manuscript Received October 27, 2009
This paper reports a systematical study on the electrochemical properties of MnO2-based nanostructures as low-cost catalysts for oxygen reduction reaction (ORR) in alkaline media. The results show that the catalytic activities of MnO2 depend strongly on the crystallographic structures, following an order of R- > β- > γ-MnO2. Meanwhile, morphology is another important influential factor to the electrochemical properties. Among various micro and nanostructures, R-MnO2 nanospheres and nanowires outperform the counterpart microparticles. Furthermore, a new nanocomposite catalyst by depositing Ni nanoparticles on R-MnO2 nanowires (denoted as MnO2-NWs@NiNPs) was prepared and characterized. The as-prepared MnO2-NWs@Ni-NPs nanocomposite exhibits an onset potential of 0.08 V, a specific current of 33.5 mA/mg, and an overall quasi 4-electron transfer involved in oxygen reduction reaction, indicating its potential application as the electrocatalyst of oxygen reduction reaction. Introduction Efficient oxygen reduction reaction (ORR) is of crucial importance in the fields of electrochemical energy storage and conversion including fuel cells and metal-air batteries.1 Until now, the most frequently investigated ORR catalysts have been based on noble metals such as Pt and Pt alloys, which have shown the best overall catalytic performance.2-6 However, the high price and scarcity of precious metals severely limit their widespread applications. Therefore, lowering the amount of noble metals in catalysts and exploiting new catalytic materials have triggered extensive research interests.7,8 Transition metal chalcogenides/carbide and organic macrocycles represent † Accepted as part of the 2010 “Materials Chemistry of Energy Conversion Special Issue”. *Corresponding author. Fax: (86) 22-2350-6808. E-mail: chenabc@nankai. edu.cn.
(1) (a) Kinoshita, K. Electrochemical Oxygen Technology; Wiley: New York, 1992. (b) Chen, J.; Cheng, F. Y. Acc. Chem. Res. 2009, 42, 713. (2) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493. (3) Komanicky, V.; Iddir, H.; Chang, K. C.; Menzel, A.; Karapetrov, G.; Henessy, D.; Zapol, P.; You, H. J. Am. Chem. Soc. 2009, 131, 5732. (4) Xu, Y.; Ruban, A. V.; Mavrikakis, M. J. Am. Chem. Soc. 2004, 126, 4717. (5) (a) Wang, C.; Daimon, H.; Lee, Y.; Kim, J.; Sun, S. H. J. Am. Chem. Soc. 2007, 129, 6974. (b) Wang, C.; Daimon, H.; Onodera, T.; Koda, T.; Sun, S. Angew. Chem., Int. Ed. 2008, 47, 3588. (6) Qian, Y.; Wen, W.; Adcock, P. A.; Jiang, Z.; Hakim, N.; Saha, M. S.; Mukerjee, S. J. Phys. Chem. C 2008, 112, 1146. (7) (a) Wang, B. J. Power Sources 2005, 152, 1. (b) Zhang, L.; Zhang, J.; Wilkinson, D. P.; Wang, H. J. Power Sources 2006, 156, 171. (8) Fern andez, J. L.; Walsh, D. A.; Bard, A. J. J. Am. Chem. Soc. 2005, 127, 357. (9) Anson, F. C.; Shi, C.; Steiger, B. Acc. Chem. Res. 1997, 30, 437. (10) (a) Feng, Y.; He, T.; Alonso-Vante, N. Chem. Mater. 2008, 20, 26. (b) Feng, Y.; Alonso-Vante, N. Phys. Status Solidi 2008, 245, 1792.
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two categories of the most promising alternatives.7,9,10 Recently, manganese oxides (MnOx) have attracted much attention because of their prominent advantages of abundance, low cost, environmental friendliness, and considerable catalytic activity toward electrochemical ORR.11-17 Despite insufficient stability in acidic media, MnOx can be applied as promising catalysts in air electrode for both alkaline fuel cells and metal-air batteries. For example, MnOx particles doped with transition metal exhibited ORR specific activities close to the benchmark Pt/C catalyst.12 Mn3O4 nanoparticles loaded on mesoporous carbon also showed high ORR electrocatalytic performance.18 Previous studies proposed that the catalytic activity toward ORR in alkaline electrolyte results from (11) (a) El-Deab, M. S.; Ohsaka, T. Angew. Chem., Int. Ed. 2006, 45, 5963. (b) Ohsaka, T.; Mao, L.; Arihara, K.; Sotomura, T. Electrochem. Commun. 2004, 6, 273. (12) (a) Cha^inet, I. R.; Chatenet, M.; Vondrak, J. J. Phys. Chem. C 2007, 111, 1434. (b) Klapste, B.; Vondrak, J.; Velicka, J. Electrochim. Acta 2002, 47, 2365. (c) Bezdicka, P.; Grygar, T.; Klapste, B.; Vondrak, J. Electrochim. Acta 1999, 45, 913. (13) (a) Lima, F. H. B.; Calegaro, M. L.; Ticianelli, E. A. J. Electroanal. Chem. 2006, 590, 152. (b) Lima, F. H. B.; Calegaro, M. L.; Ticianelli, E. A. Electrochim. Acta 2007, 52, 3732. (14) (a) Gong, K.; Yu, P.; Su, L.; Xiong, S.; Mao, L. J. Phys. Chem. C 2007, 111, 1882. (b) Mao, L.; Zhang, D.; Sotomura, T.; Nakatsu, K.; Koshiba, N.; Ohsaka, T. Electrochim. Acta 2003, 48, 1015. (c) Mao, L.; Sotomura, T.; Nakatsu, K.; Koshiba, N.; Zhang, D.; Ohsaka, T. J. Electrochem. Soc. 2002, 149, A504. (15) Cao, Y. L.; Yang, H. X.; Ai, X. P.; Xiao, L. F. J. Electroanal. Chem. 2003, 557, 127. (16) (a) Verma, A.; Jha, A. K.; Basu, S. J. Power. Sources 2005, 141, 30. (b) Yang, J.; Xu, J. J. Electrochem. Commun. 2003, 5, 306. (17) (a) Hu, F. P.; Zhang, X. G.; Xiao, F.; Zhang, J. L. Carbon 2005, 43, 2931. (b) Zhang, G. Q.; Zhang, X. G.; Wang, Y. G. Carbon 2004, 42, 3097. (18) Wang, Y. G.; Cheng, L.; Li, F.; Xiong, H. M.; Xia, Y. Y. Chem. Mater. 2007, 19, 2095.
Published on Web 11/17/2009
r 2009 American Chemical Society
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the oxygen-containing groups and the redox reactions.11-16 However, uncertainty about the ORR mechanism on MnOx still remains because of the complex chemistry among Mn species that possess different valence and crystallographic structures. Early reports have shown that the catalytic performance of MnOx follows the sequence of Mn5O8 < Mn3O4 < Mn2O3 < MnOOH and that among MnO2 phases, the performance sequence turns to β- β- > γ-MnO2. This order is different from the previously reported result claiming that β-MnO2 is inferior to γ-MnO2.15 Such discrepancy is probably ascribed to the influence of catalyst morphology. In the previous work, size and shape of the investigated MnO2 is not provided. In the present study, the employed MnO2 nanowires possess similar diameters, therefore convincing the comparison among different MnO2 phases. Effect of Carbon Addition on ORR Catalytic Activity. In practical application, the electrode materials should hold both high electrochemical activity and adequate electronic conductivity.28 For this reason, we have studied the ORR catalytic activity of the electrodes that consist of the mixture of carbon black and R-MnO2 nanowires. Figure 4 shows the effect of R-MnO2 content on the current density and the onset potential of ORR in 0.1 M KOH solution. Each data point is the mean value of three repeated measurements. As seen from Figure 4a, in the (28) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Schalkwijk, W. V. Nat. Mater. 2005, 4, 366.
Figure 4. Effect of R-MnO2 content on (a) the current density of RDE and (b) the ORR onset potential obtained at a rotation rate of 2500 rpm.
absence of R-MnO2, neat carbon shows poor catalytic activity at the applied potential of -0.20 and -0.40 V. Current densities first increase and then decrease with R-MnO2 content, displaying a volcano-shaped curve. The electrode modified by the mixture of 30 wt % R-MnO2 nanowires and 70 wt % carbon powders exhibits the maximum current. Moreover, the onset potential of ORR increases with MnO2 content, as shown in Figure 4b. The neat carbon shows quite negative onset potential, in agreement with reported results.13 The presence of MnO2 can considerably lower the overpotential. However, the effect on the onset potential is not obvious when the content of MnO2 is higher than 20 wt %. Therefore, considering the compromise between the current density and the onset potential, the optimal composition of the catalyst is fixed at 30 wt % MnO2 and 70 wt% carbon in the following investigations. Effect of Morphology on ORR Catalytic Activity. To evaluate the morphological effect, R-MnO2 materials with different morphologies were prepared. Figure 5 shows the typical SEM images of the as-synthesized R-MnO2 bulk particles, nanowires, and nanospheres. The obtained bulk R-MnO2 particles exhibit irregular shape with sizes ranging from several to tens of micrometers (Figure 5a). The diameters of the as-prepared R-MnO2 nanowires vary from 20 to 80 nm with average length up to 1-3 μm (Figure 5b). Figure 5c shows the typical morphology of R-MnO2 nanospheres, revealing the formation of flowerlike spheres that are composed of tightly bound intercrossing nanoflakes. These nanoflakes possess uniform thickness of approximately 20 nm. The preparation of R-MnO2 with micro- and nanostructures allows us to testify the shape and size effect on the electrochemical catalytic performance. Figure 6a shows the LSV curves of R-MnO2 bulk particles, nanowires, and nanospheres, which are mixed
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Figure 5. SEM images of (a) R-MnO2 bulk particles, (b) nanowires, and (c) flowerlike nanospheres.
Figure 6. (a) Comparison of the LSVs of neat R-MnO2 nanowires, bulk particles/carbon, nanowires/carbon, and nanospheres/carbon. (b) The corresponding K-L plots for three R-MnO2 samples at -0.3 V.
with carbon powders. Data for neat nanowires are also shown for comparison. It can be clearly seen that both R-MnO2 nanowires and nanospheres display higher limiting current density compared with that of microsized particles. At the rotation rate of 2500 rpm and the applied potential of -0.60 V, the ORR current of the bulk particles is 3.03 mA cm-2, lower than that of nanowires and nanospheres (3.87 and 4.0 mA cm-2, respectively). Moreover, Figure 6b shows the K-L plots for the three catalysts at -0.3 V. The number of transferred electrons involved in ORR (n) for R-MnO2 nanowires is calculated to be 3.8, which is close to the theoretical value of 4-electron ORR. The K-L plot of the nanospheres is parallel to
that of the nanowires and the n value is determined to be 3.7. Similarly, n is 3.6 for R-MnO2 bulk particles. Therefore, R-MnO2 nanostructures outperform the counterpart microstructures in ORR electrocatalytic activity. Characterization and Property Study of Ni-Coated MnO2 Nanowires. Previous reports have shown that metal (such as Ag, Ni, Mg, Ca) doped manganese oxides exhibit high catalytic activity toward oxygen reduction in alkaline media.12,17,29 Grygar and co-workers prepared Me-MnOx/C (Me = Ni, Ca, Mg) composite by coprecipitation of MnOx and metal ions on carbon black.12c The dopant metal element was proposed to incorporate in the crystal lattice. Hu et al. synthesized Ag-MnO2/SWNT by chemical reduction of silver permanganate on singlewalled carbon nanotubes.17 However, the use of expensive Ag and carbon nanotubes would limit the practical application of such catalyst despites its favorable performance. On the other hand, we have demonstrated that nanocomposites of 1D nanostructures coated with transition metal nanoparticles can be applied as very effective catalyst.30 In this context, we report herein the preparation and properties of MnO2-NWs@Ni-NPs, which combine the low-cost metallic Ni and high-activity R-MnO2 nanostructure. Figure 7 shows the characterization of the as-prepared MnO2-NWs@Ni-NPs. Before Ni deposition, the R-MnO2 sample presents clean and smooth surface (Figure 7a) while the surface of R-MnO2 becomes apparently coarse and rough after Ni plating (Figure 7b). This sharp contrast clearly indicates the efficient deposition of Ni layer on the surface of MnO2 nanowires. Low-magnification TEM image (Figure 7c) reveals that the diameter and length of the nanowires is almost retained. The metal particles are uniformly deposited on the surface of the wire, as can be seen from the high-magnification TEM observation of a single composite nanowire (Figure 7d). The diameter of the deposited Ni particles is around 5 nm, as shown in HRTEM image (inset of Figure 7d). EDS was used to analyze the chemical composition of the MnO2-NWs@Ni-NPs (Figure 7e). The main elements include Mn, O, and Ni. No distinct signals of other elements are detected except for C and Sn, which originate from the carbon support used for SEM measurement and the residual Sn during sensitization process, respectively. (29) Roche, I.; Cha^inet, E.; Chatenet, M.; Vondrak, J. J. Appl. Electrochem. 2008, 38, 1195. (30) Cheng, F. Y.; Chen, J.; Gou, X. L. Adv. Mater. 2006, 18, 2561.
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Figure 8. LSV curves of C, Ni/C, MnO2/C, and MnO2-NWs@Ni-NPs/C obtained at a rotation rate of 900 rpm and a potential scan rate of 1 mV s-1.
Table 1. Summary of the ORR Catalytic Characteristics of C, Ni/C, MnO2/C, and MnO2-NWs@Ni-NPs/C
catalyst
Figure 7. SEM images of (a) undeposited R-MnO2 nanowires and (b) MnO2-NWs@Ni-NPs; (c, d) TEM images, (e) EDS spectrum, and (f) XRD pattern of MnO2-NWs@Ni-NPs. The inset of d shows the HRTEM of a single Ni nanoparticle.
The Ni content is determined to be approximately 5 wt %. XRD pattern of the MnO2-NWs@Ni-NPs (Figure 7f) can be readily indexed to R-MnO2 (identical to Figure 1a), suggesting that the MnO2 phase is essentially maintained and that the coated Ni is in poorly polycrystalline form. The catalytic characterization of the prepared MnO2NWs@Ni-NPs for ORR was performed on RDE. For comparison, neat carbon powders (C), carbon supported nickel powders (Ni/C, 30 wt % Ni), and mixture of carbon and MnO2 nanowires (MnO2/C, 30 wt % MnO2) were also tested. Figure 8 shows the LSV curves of C, Ni/ C, MnO2/C, and MnO2-NWs@Ni-NPs. The corresponding main electrochemical properties are summarized in Table 1. The neat carbon and Ni/C mixture display similar current-potential response, which indicates that in the absence of MnO2, the deposited Ni particles merely contribute slightly to the ORR activity. However, compared to pristine MnO2, the MnO2-NWs@Ni-NPs exhibits obviously enhanced catalytic activity, including increased ORR current and more positive half-wave potential (i.e., the potential where the current attains half of the maximum value). The mass activity measured at rotation rate of 2500 rpm and potential of -0.3 V achieves 33.5 mA mg-1, which is comparable to the reported value for Ni2þ-doped MnOx/C nanoparticles.12a Moreover, the calculated overall number of transferred electrons is 3.84, higher than that of pristine R-MnO2 sample. Figure 9 further compares the corresponding masstransport corrected Tafel plots (log i vs E) of C, MnO2/ C, and MnO2-NWs@Ni-NPs/C catalysts. Obviously, the
C Ni/C MnO2/C MnO2-NWs@Ni-NPs/C
onset potential (V) -0.19 -0.18 0.06 0.08
half-wave potential (V) -0.36 -0.37 -0.15 -0.09
currents (mA) 0.114 0.139 0.327 0.365
n 2.01 2.06 3.80 3.86
nanocomposites outperform the uncoated MnO2 nanowires, and carbon powders. Two Tafel regions with slopes close to 60 and 120 mV dec-1 can be observed at low and high overpotentials, respectively. This slope variation is generally explained with respect to the change in the coverage degree of adsorbed oxygen.13 It is worthy noting that the MnO2-NWs@Ni-NPs displays smaller Tafel slope than that of the other two samples, which is desirable for electrocatalytic applications according to a recent study.31 Therefore, the RDE results indicate that the deposition of Ni nanoparticles (with ∼5 wt % metal loading) can further improve the catalytic characteristics of R-MnO2 nanowires toward ORR, resulting in lowered overpotential and enhanced current density. Discussion. In alkaline media, the oxygen reduction on classic noble metal (e.g., Pt) catalyst mainly undergoes a direct four-electron (4e) pathway (eq 2), which competes with a serial 2 2e pathway.32,33 Recent report has described the ORR mechanism on manganese oxides by a first partial 2-electron reduction of molecular oxygen (O2) to hydrogen peroxide ions (HO2-) (eq 3), which are further electrochemically reduced to OH- (eq 4) or chemically decomposed to OH- and O2 (eq 5).14 Supposing that HO2- reduction and decomposition proceed (31) Banham, D. W.; Soderberg, J. N.; Birss, V. I. J. Phys. Chem. C 2009, 113, 10103. (32) (a) Antoine, O.; Durand, R. J. Appl. Electrochem. 2000, 30, 839. (b) Chatenet, M.; Aurousseau, M.; Durand, R.; Andolfatto, F. J. Electrochem. Soc. 2003, 150, D47. (c) Chatenet, M.; Genies-Bultel, L.; Aurousseau, M.; Durand, R.; Andolfatto, F. J. Appl. Electrochem. 2002, 32, 1131. (33) Schneider, A.; Colmenares, L.; Seidel, Y. E.; Jusys, Z.; Wickman, B.; Kasemo, B.; Behm, R. J. Phys. Chem. Chem. Phys. 2008, 10, 1931.
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Figure 9. Tafel plots of different samples measured at the potential scan rate of 1 mV s-1.
infinitely rapidly and completely, the total reactions of eqs 3-5 is equivalent to eq 2 and thus one can expect an apparent overall 4e ORR. It has been reported that manganese oxides are highly catalytically active toward the peroxide decomposition or disproponation reaction. 11-17 Hence the intermediate HO2- formed on MnO2 at the first step can be consumed almost instantaneously. O2 þ 2H2 O þ 4e - f 4OH -
ð2Þ
O2 þ H2 O þ 2e - f HO2 - þ OH -
ð3Þ
HO2 - þ 2H2 O þ 2e - f 3OH -
ð4Þ
2HO2 - f 2OH - þ O2
ð5Þ
The redox between Mn species is believed to assist the charge transfer involved in oxygen reduction on the basis of the following equations12-15 MnO2 þ H2 O þ e - r f MnOOH þ OH -
ð6Þ
2MnOOH þ O2 r f 2ðMnOOH 3 3 3 OÞ
ð7aÞ
MnOOH þ O2 r f MnOOH 3 3 3 O2
ð7bÞ
ðMnOOH 3 3 3 OÞ þ e - r f MnO2 þ OH -
ð8aÞ
MnOOH 3 3 3 O2 þ e - r f MnO2 þ HO2 -
ð8bÞ
The total reaction of eqs 6, 7a, and 8a equals to eq 2 and results in an apparent 4e reduction process, whereas the summary of eqs 6, 7b, and 8b gives eq 3 with an overall 2e transfer. These two schemes differ in the manner of O2 adsorption. For reaction 7a, each oxygen molecule may adsorb onto two neighboring MnOOH sites. In the case
Figure 10. N2 adsorption/desorption isotherms of (A) R-MnO2 bulk particles, (B) R-MnO2 nanowires, (C) R-MnO2 nanospheres, and (D) MnO2NWs@Ni-NPs.
of reaction 7b, O2 is likely absorbed on a single Mn site through an end-on way without breaking the OdO bond. Our results suggest that both schemes coexist in the catalyzing process because the apparent n is determined to be 3.1, 2.7, and 2.3 for pure R-, β-, and γ-MnO2, respectively. According to the above proposed mechanism, an effective catalyst for ORR in alkaline media should facilitate O2 adsorption, HO2- decomposition, as well as ionic and electronic transportation. Our experimental results show that both n and ORR currents follow the order of R- > γ- > β-MnO2. Among the three MnO2 phases, R-MnO2 possess the largest tunnel size (Figure 1g) and thus may favor the insertion and transfer of ions in the lattice framework. In addition, it is reported that R-MnO2 contains more defects and -OH groups,25 which are beneficial to surface adsorption of O2 and dissociation of O-O bonds. As a result, R-MnO2 displays more positive onset potential. However, despite possessing narrower tunnels that are unfavorable for ionic intercalation (Figure 1h), β-MnO2 exhibits slightly higher activity in comparison with γ-MnO2, which is probably due to the higher electrical conductivity of β-MnO2 than that of γ-MnO2.34 Electrical conductivity plays an important role in affecting the catalytic activity of semiconducting MnO2, which is also confirmed by the improved ORR current density of MnO2 nanowires after adding carbon powders and depositing Ni nanoparticles (Figures 4,8). With respect to the effect of morphology, nanostructures outperform the bulk particles. Generally, nanostructured catalysts could lead to new electronic and catalytic properties because of the small size effect and high surface area.35 As size decreases, the reactivity is enhanced because of larger surface-to-bulk ratio and numerous surface defects. Higher surface areas permit more active sites for the contact between catalyst and electrolyte. The measured BET specific surface areas (34) Kanungo, S. B.; Parida, K. M.; Sant, B. R. Electrochim. Acta 1981, 26, 1157. (35) Britto, P. J.; Santhanam, K. S. V.; Rubio, A.; Alonso, J. A.; Ajayan, P. M. Adv. Mater. 1999, 11, 154.
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(Figure 10) are 7.9, 32.9, 40.1, and 68.3 m2 g-1 for R-MnO2 bulk particles, nanowires, flower-like nanospheres, and MnO2-NWs@Ni-NPs, respectively. The sequence of surface areas is consistent with the order of catalytic activity. Moreover, it should be noted that nanostructures are more porous than the bulk particles, which might facilitate the diffusion, adsorption, and transport of O2 gas. Therefore, the superiority of nanostructured MnO2 over the bulk form is understandable. Finally, the MnO2-NWs@Ni-NPs combines the highly active R-MnO2 nanowires and the electronically conductive Ni nanoparticles. The electrical conductivity is 0.086 and 82.5 S/m for the pristine R-MnO2 nanowires and MnO2-NWs@Ni-NPs, respectively. The deposition of metal nanoparticles enhances the conductivity by nearly 3 orders of magnitude, which should greatly favor fast electronic transfer and reduce electrode polarization during the catalytic ORR process. It should be also mentioned that the doping of MnOx with transition metal is beneficial to improved catalytic activity and selectivity toward 4e ORR even after aging.29 Furthermore, the 3d-transition metals are also known to contribute to enhanced ORR kinetics and catalytic activity because of the shift of d-band center position that affects the adsorption of oxygen-containing species on catalyst surface.2
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Therefore, MnO2-NWs@Ni-NPs exhibited the best overall catalytic performance. Conclusions In conclusion, a systematic study has been carried out on the catalytic properties of MnO2-based nanostructures as cheap electrocatalyst for oxygen reduction reaction in alkaline media. The catalytic performance of MnO2 is found to depend strongly on both the crystallographic structure and the morphology. The catalytic activities follow an order of R- > β- > γ-MnO2 while excluding morphological influence. Moreover, R-MnO2 nanospheres and nanorods are superior to the bulk microparticles because of higher oxygen reduction potential and larger current density. Electrochemical investigations reveal that a quasi-4 electron transfer is attained for R-MnO2 nanostructures. Depositing Ni nanoparticles on R-MnO2 nanowires to form MnO2-NWs@Ni-NPs further improves the overall ORR catalytic activities. Acknowledgment. This work was supported by the Programs of National 973 (2005CB623607), 863 (2007AA05Z124), and Tianjin High-Tech (07ZCGHHZ00700). Y.S. is presently moved to Changchun Institute of Applied Chemistry (CAS), Jilin, China.
