Improving Oxygen Reduction Reaction Activities on Carbon

4324

J. Phys. Chem. C 2010, 114, 4324–4330

Improving Oxygen Reduction Reaction Activities on Carbon-Supported Ag Nanoparticles in Alkaline Solutions Junsong Guo,† Andrew Hsu,† Deryn Chu,‡ and Rongrong Chen*,† Richard G. Lugar Center for Renewable Energy, Indiana UniVersity-Purdue UniVersity, Indianapolis, Indiana 46204, and Army Research Laboratory, Adelphi, Maryland, Maryland 20783 ReceiVed: NoVember 12, 2009; ReVised Manuscript ReceiVed: January 23, 2010

Carbon-supported Ag (Ag/C) catalysts with four different metal loadings were prepared by a citrate-protecting method. Oxygen reduction reaction (ORR) activities on these carbon-supported Ag-nanocatalysts (Ag/C) in alkaline solutions were studied. Four major findings are reported in this paper: (1) Test results indicate that the Ag/C catalysts promote predominately a four-electron pathway for ORR on electrodes over the swept potentials from 0.2 to -0.8 V vs Hg/HgO in O2-saturated 0.1 M NaOH solutions. (2) A novel marker for predicting ORR activities on the Ag/C catalysts based on the cyclic voltammetry characteristic curves has been identified: the ORR activities have a strong correlation with the intensity of the anodic peak at the potential of 0.230 V vs Hg/HgO in Ar-saturated 0.1 M NaOH solutions. (3) As the metal loading on carbon particles increases from 10 to 60 wt %, the peak intensities increase linearly, and the ORR onset potentials shift positively with maximum shift of 62 mV for 60% Ag on carbon support. (4) A hitherto unnoticed poisoning effect has been discovered: silicate has a significant poisoning effect on the ORR activities of the Ag/C catalysts. 1. Introduction Recently, alkaline fuel cells (AFCs) have regained attention because, compared to the acid environment of a proton exchange membrane fuel cell (PEMFC), an alkaline media provides a less corrosive environment to the catalysts and electrodes.1-4 In an AFC, the chemical energy in fuels (such as hydrogen, hydrazine,5 or alcohol6) and oxidants (oxygen) can be directly and efficiently transferred into electricity, which makes the AFC a promising power supply for portable electronics. Compared to PEMFCs, one of the main advantages of AFCs is the possibility to replace Pt-based electrocatalysts with non-Pt electrocatalysts, including Ag,7,8 Au,9,10 Pd,11,12 Ni,13 manganese oxide,14 prophyrins,15 and phthalocyanines.16 Among these options, the relatively inexpensive and abundant Ag is a top candidate to replace Pt for ORRs in alkaline solutions due to its high activity.17-19 Although several research groups have investigated carbonsupported silver (Ag/C) catalysts for ORRs in alkaline media, the effects of silver particle size and metal loading on ORR activities are still not well understood, and results and findings from various research groups are inconsistent and no definite conclusions can be drawn from them. For instance, it has been reported that catalysis by 20 weight percent (wt %) of Ag/C with 174 nm mean Ag particle size seems to facilitate an ORR pathway with four-electron reactions, while the finer catalysts with 4.1 nm mean particles size appeared to facilitate a twoelectron ORR pathway; however, the catalyst metal loading was only 0.5 wt %.7 F. H. B. Lima et al. reported a 2.3-electron ORR reaction on 20 wt % Ag/C with relatively large 47.7 nm silver particle size,20 while the result of L. Demarconnay et al. showed a 3.6-electron ORR reaction on 20 wt % Ag/C with a particle size close to 15 nm.21 C. Countanceau et al.22 investigated the influence of metal loading on Ag/C catalyst activity † ‡

Indiana University-Purdue University. Army Research Laboratory.

in a half-cell. They found that the optimum loading of silver was around 20 wt %. Further increasing metal loading, the catalyst performance would decrease. However, J. Varcoe et al.17 found the performance of Ag/C electrodes with 60 wt % metal loading to be as good as that on 20 wt % Pt/C electrodes in a solid alkaline membrane fuel cell. In addition, alkaline solution is easily contaminated by carbonate and silicate when it is exposed in air with a glass container during storage and long-term testing. As far as we know, no one has reported the influence of such contamination on ORR. In this work, the Ag/C catalysts with four different metal loadings were prepared by a citrate-protecting method. Catalyst structure, morphology, and electrochemical properties were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and cyclic voltammetry (CV) techniques. The ORR pathways on the prepared Ag/C catalysts were studied. Influences of Ag metal loading on the carbon particle support and carbonate and silicate contamination on the ORR mechanisms were investigated. Key factors for improved ORR activities were identified. A hitherto unnoticed poisoning of catalysts by silicate from glass in aqueous alkaline electrolyte has been discovered, which could explain some of the inconsistencies from previous results reported in the literature. 2. Experimental Section 2.1. Ag/C Catalysts Preparation. Silver loadings on carbon particles of 10, 20, 40, and 60 wt % were prepared separately by a citrate-protecting method as described by J. Zeng et al.23 The procedure to prepare these Ag/C catalysts is the same and briefly introduced as the following. For the preparation of 200 mg of 10 wt % Ag/C catalysts, 177.7 mg of sodium citric (Alfa Aesar) and 111.0 mg of NaOH (JT Baker) were mixed to prepared 18.5 mL of 50 mM sodium citrate solution, and then 18.5 mL of 10 mM AgNO3 was added. Later 25 mL of 7.4 mM NaBH4 (Strem chemicals) solution was added dropwise under vigorous stirring to obtain a yellowish-brown Ag colloid.

10.1021/jp910790u  2010 American Chemical Society Published on Web 02/19/2010

Carbon-Supported Ag Nanoparticles

J. Phys. Chem. C, Vol. 114, No. 10, 2010 4325

Figure 1. TEM images of (a) 10, (b) 20, (c) 40, and (d) 60 wt % Ag/C.

At this time, 180 mg of Vulcan XC-72 carbon black (Cabot Corp.) was taken to disperse into the above Ag colloid. After the suspension was stirred for 12 h, the black suspension was filtered, washed, and dried in a vacuum oven at 80 °C for 12 h, and then the catalyst sample was obtained. 2.2. Physical Characterization. XRD patterns of Ag/C samples were recorded with a Siemens X-ray diffractometer using Cu KR radiation with a Ni filter. The tube current was 30 mA with a tube voltage of 50 kV. The 2θ angular region between 15° and 85° was explored at a scan rate of 5° min-1. Morphologies of catalysts were investigated by using a Tecnai G2 TWIN/BioTWIN transmission electron microscope (FEI Company) operated at 80 kV. 2.3. Electrochemical Characterization. Electrochemical activities of catalysts were measured by a setup consisting of a computer-controlled Pine potentiostat, a radiometer speed control unit from Pine instruments company (MSRX speed control), and a rotating ring disk electrode radiometer (RRDE, glassy carbon with a diameter of 5.0 mm as the disk and with platinum as the ring). Catalyst ink was prepared by ultrasonically mixing 2.0 mg of the Ag/C catalyst with 10 uL of the Nafion solution (5%) and 1 mL of ethanol. Then, 20 uL of the prepared catalyst ink was dropped on the surface of the glassy carbon to form a working electrode. Because the metal loading in catalysts was different, the amount of silver per square centimeter on the

working electrode with 10, 20, 40, and 60 wt % Ag/C catalysts was 0.02, 0.04, 0.08, and 0.12 mg · cm-2, respectively. The electrochemical measurements were conducted in an argon- or oxygen-purged 0.1 M NaOH solution using a standard three-electrode cell with a Pt wire serving as the counter electrode and a Hg/HgO/0.1 M OH- electrode (0.164 V vs NHE) used as the reference electrode, respectively. H2O2 production in O2-saturated 0.1 M NaOH electrolytes was monitored in a RRDE configuration using a polycrystalline Pt ring biased at 0.3 V vs Hg/HgO/0.1 M OH-. The ring current (Iring) was recorded simultaneously with the disk current (Idisk). The collection efficiency (N) of the ring electrode was calibrated by K3Fe(CN)6 redox reaction in an Ar-saturated 0.1 M NaOH solution. The value of the collection efficiency (N ) Iring/ Idisk) determined is 0.23 for the Pt/C electrode. The fractional yields of H2O2 in the ORR were calculated from the RRDE experiments as XH2O2 ) (2Iring/N)/(Idisk + Iring/N). 3. Results and Discussion 3.1. Physical Characterization. Figure 1 shows TEM images of 10, 20, 40, and 60 wt % Ag/C catalysts prepared by the citrate-protecting method. Although the Ag metal loadings vary from 10 to 60 wt %, the distribution and the mean particles sizes of the Ag/C catalysts are almost identical, which implies that the metal loading does not have a significant effect on the Ag metal dispersion in these four catalysts. According to the

4326

J. Phys. Chem. C, Vol. 114, No. 10, 2010

Figure 2. XRD patterns of 10, 20, 40, and 60 wt % Ag/C.

process of preparing catalysts, there are two steps that likely influence the particle size distribution of the Ag/C catalysts. The first is the Ag colloid preparing step, and the second is the anchoring of the silver particles on the surface of the carbon support. From the results of the TEM images, Ag particles sizes do not change much during Ag metal anchoring on the surface of different amounts of carbon supports, which indicates that carbon supports provide enough surface area for dispersing the Ag nanoparticles, even for Ag metal loading up to 60 wt %. From the TEM images, one can see that the mean particle size of the Ag nanoparticle in the colloid is around 15 nm. Several factors, such as the concentration of AgNO3, NaBH4, and sodium citrate solution, are considered to have impacts on the Ag nanoparticle size during the preparation of the Ag colloid. Further optimizing the synthesis process for making the Ag/C catalysts would lead to a reduction of Ag nanoparticle size. From Figure 1c and Figure 1d, one can see that some of Ag nanoparticles tend to aggregate together when metal loading is higher than 40 wt %. The structures of four different metal loadings of the Ag/C catalysts were further characterized by XRD and are shown in Figure 2. The peaks located at about 25° are attributed to the graphite (002) crystalline plane of carbon with a hexagonal structure. Intensities of the peaks obviously become weaker with increasing metal loadings. The peaks located at about 38.3°, 44.2°, 64.4°, and 77.4° should be attributed to the (111), (200), (220), and (311) crystalline planes of Ag with a face-centered cubic (fcc) structure according to the silver powder diffraction file (PDF040783). In the XRD patterns of Ag/C catalysts, there are no obvious characteristic peaks of silver oxide detected, which indicates that most of the silver in the Ag/C catalysts is in metallic form. In addition, the average crystallite sizes of Ag particles can be calculated from the (220) peak using Scherrer’s equation. The mean particle sizes of 16.2, 15.1, 13.9, and 14.0 nm of 10, 20, 40, and 60 wt % Ag/C catalysts, respectively, were calculated, which correlates well with the results obtained by the TEM. 3.2. Electrochemical Characterization. Figure 3 shows the cyclic voltammetry curves of Ag/C catalysts with four different metal loadings in a freshly prepared 0.1 M NaOH solution purged with Ar. In the potential range of 0.200-0.550 V vs Hg/HgO, there are three anodic peaks observed and designated as A1, A2, and A3, which locate at about 0.230, 0.300, and 0.380 V vs Hg/HgO, respectively. Peaks A2 and A3 are associated

Guo et al.

Figure 3. Cyclic voltammetry of four Ag/C catalysts with different metal loadings in Ar-purged 0.1 M NaOH solutions. Inset: curves of A1 anodic peak intensities with metal loading. Scan rate: 20 mV s-1.

with the formation of bulk phases of AgOH and Ag2O, while peak A1 is due to the silver dissolution and the formation of a surface monolayer of Ag2O films.24 Relationships of the peak intensities of A1 with various metal loadings are plotted in the inset of Figure 3. The peak intensities of A1 have a linear correlation with the catalyst metal loadings, while no correlations of peak intensities vs the metal loading were found for peaks A2 and A3. The ORR polarization curves obtained on the Ag/C catalysts at rotation rates from 400 to 2500 rpm in an oxygen-saturated 0.1 M NaOH solution are shown in Figure 4. The limiting currents of the ORR increase prominently with rotation rate, but the ORR onset potential is kept almost constant on the same catalyst. Compared to the ORR onset potential on Vulcan XC72, the ORR onset potential on 10, 20, 40, and 60 wt % Ag/C catalysts shifts positively at about 101, 131, 150, and 173 mV, respectively. Higher metal loadings result in more positive ORR onset potentials. However, the ORR onset potential on the 60% Ag/C catalyst electrode with Ag loading of 0.12 mg · cm-2 was still 58 mV lower than that on the 20% Pt/C catalyst electrode with Pt loading of 0.04 mg · cm-2 (Figure 4f). The electrochemical reduction of O2 is a multielectron reaction that has two main possible pathways: one involving the transfer of two electrons to produce H2O2 and the other involving a direct four-electron pathway to produce water. The limiting currents at different rotation speeds are used to construct the Levich plots for different catalysts as shown in Figure 5, which is derived according to Koutecky-Levich equation25

Ilim ) 0.62nFD2/3ν-1/6c0ω1/2 where Ilim is the limiting current density, n is the number of electrons transferred per oxygen molecule, F is the Faraday constant, D is the O2 diffusion coefficient in 0.1 M NaOH, ν is the kinematic viscosity, c0 is the concentration of oxygen, and ω is the rotation rate in the radian. The number of electrons transferred for the ORR on the Pt/C is around 4 by forming water as the reaction product26 in 0.1 M NaOH solutions saturated with O2. From the slopes shown in Figure 5, if assuming the exchange electron number for Pt/C is equal to 4, the number of electrons transferred on Vulcan XC-72 and 10, 20, 40, and 60 wt % Ag/C were calculated to be 1.93, 3.63, 3.83, 3.83, and 3.77, respectively. Except for the 10 wt % Ag/C



Figure 4. Oxygen reduction polarization curves on (a) 10, (b) 20, (c) 40, and (d) 60 wt % Ag/C, (e) Vulcan XC-72, and (f) 20 wt % Pt/C (BASF) catalysts.

catalyst, all other Ag/C catalysts show the ORR via a fourelectron transfer pathway. The fact that less than four electrons transferred on the 10 wt % Ag/C is probably due to the low Ag content on the electrode where the ORR could be catalyzed by the Vulcan XC-72 carbon via the two-electron pathway. To further verify the ORR mechanism via either a fourelectron or a two-electron pathway on the Ag/C catalysts, rotating ring-disk electrode (RRDE) measurements were carried out to monitor the formation of H2O2 during the ORR process. Figure 6 gives the RRDE polarization curves for O2 reduction on the Ag/C catalysts with a rotation rate of 2500 rpm in a 0.1 M NaOH solution saturated with oxygen. High ring current and low limiting disk current were observed on the Vulcan XC-72

electrode, which indicated a formation of H2O2 as an ORR intermediate. For Pt/C and Ag/C catalysts, the ring currents were much lower, but the disk limiting currents were much higher than those on Vulcan XC-72. The inset of Figure 6 gives the H2O2 yields on Vulcan XC-72, Pt/C, and Ag/C catalysts. On Pt/C or Ag/C with metal loading over 20 wt % electrodes, no significant solution phase H2O2 was detected, and thus the H2O2 yield was negligible. For the 10 wt % Ag/C electrode, a significant ring current was detected and up to 10% of the H2O2 yield was measured, which suggests that the ORR on an electrode with low Ag loadings could proceed via a two-electron reduction process due to the ORR catalytic activity of Vulcan XC-72. Therefore, the low number of electrons transferred for

4328


Guo et al.

Figure 5. Levich plots of O2 reduction on Vulcan XC-72, 20 wt % Pt/C (BASF), and four Ag/C catalysts with different metal loadings.

Figure 7. (a) Cyclic voltammetry and (b) ORR polarization curves of 60 wt % Ag/C catalysts obtained in a 0.1 M NaOH solution aging in glass or plastic bottles vs that in freshly prepared 0.1 M NaOH solution saturated with argon or oxygen.

Figure 6. RRDE measurements of oxygen reduction reactions on four Ag/C catalysts with different metal loadings, Vulcan XC-72, and 20 wt % Pt/C (BASF) with a rotation rate of 2500 rpm in oxygen-purged 0.1 M NaOH. Collection efficiency N ) 0.23; ring potential Er ) 0.3 V vs Hg/HgO; scan rate 20 mV s-1.

ORR on 0.5 wt % Ag/C catalyst with 4.1 nm silver particles observed by J. Han et al.7 is likely because of the low metal loading. Long-term stability of the Ag/C catalyst is very important for its application in fuel cells. In a 0.1 M NaOH aqueous solution, the reversible oxygen reduction potential for the ORR (O2 + 2H2O + 4e- f 4OH-) is about 0.238 V vs Hg/HgO, which is higher than that of the A1 peak potential (0.230 V vs Hg/HgO). Therefore, silver particles are unstable for the dissolution at open circuit via the following equation:27

4Ag + O2 + H2O f 4Ag+ + 4OHWhen the oxygen reduction reaction occurs, the Ag/C electrode potential drops to less than 0.0 V vs Hg/HgO (Figure

4), as a result that silver should be in a form of metallic state based on the CV curves shown in Figure 3. We expect that Ag/C catalysts are stable for the ORR under fuel cell operation conditions, which has been reported by Lee et al.28 However, the problem of Ag dissolution in 0.1 M NaOH aqueous solutions saturated with O2 under an open circuit condition of AFCs needs to be further addressed through developing Ag-alloy catalysts with an increased oxidation potential. By replacing NaOH electrolytes with solid alkaline polymer electrolytes, the stability of the Ag/C catalyst may also be improved. 3.3. Effect of Impurities. During the investigation of ORR activities of Ag/C catalysts in alkaline solutions, we found that the performance of Ag/C greatly depended on the kind of containers used and the aging time of the prepared alkaline solution. In Figure 7, the performances of the 60 wt % Ag/C catalysts were separately studied in freshly prepared 0.1 M NaOH solutions and aging NaOH solutions that were placed in a glass bottle or a plastic bottle for 24 h. The results obtained after 24 h in the glass bottle indicated that the ORR onset potential shifted negatively about 57 mV in the freshly prepared solution when compared to the aging NaOH solution. However, the ORR polarization curves obtained from 24 h in the plastic bottle indicate that the aging NaOH solution was the same as in the freshly prepared solution. From the cyclic voltammetry curves (shown in Figure 7a) obtained in 0.1 M NaOH solutions saturated with Ar, the A1 peak intensity decreased dramatically when the catalyst was tested in the alkaline solution aging in the glass bottle, while the intensities of peaks A2 and A3 increased. The cyclic voltammetry curves obtained in alkaline


Figure 8. (a) Cyclic voltammetry and (b) ORR polarization curves of 60 wt % Ag/C catalysts obtained in a 10-3 M sodium silicate + 0.1 M NaOH solution, a 10-4 M sodium silicate + 0.1 M NaOH solution, and a 0.1 M NaOH solution saturated with argon or oxygen.

solutions aging in the plastic bottle show that the peak intensities of A1, A2, and A3 did not change. These results indicated that the impurities from the dissolution of the glass bottle greatly inhibited the catalyst ORR activity and the formation of peak A1. Most importantly, we discover that the peak A1 intensity can be used as a key parameter to predict ORR activities of Ag/C catalysts. When the mean Ag particles size is identical, increasing the metal loading results in linear increases in the peak A1 intensity and therefore also results in an improvement of ORR catalytic activity. As we know, the main composition of glass is SiO2, which reacts with NaOH solution and produces sodium silicate. Therefore, we performed a study by adding different amounts of sodium silicate into 0.1 M NaOH solutions to verify the effects of silicate on catalyst ORR activities. Figure 8 shows the ORR curves obtained on 60 wt % Ag/C in O2-saturated 0.1 M NaOH solutions with different sodium silicate additions. Comparing the results obtained in the freshly prepared NaOH solution, the ORR onset potential shifted less positive at about 61 and 98 mV, respectively, when the alkaline solution contained 10-4 and 10-3 M sodium silicate, which clearly confirms that silicate is an impurity greatly inhibiting the ORR performance of Ag/C catalysts. The cyclic voltammetry curves (Figure 8a) show decreasing A1 peak intensity and increasing A2 and A3 peak intensities with an increasing sodium silicate addition in the electrolyte.


Figure 9. (a) Cyclic voltammetry and (b) ORR polarization of 60 wt % Ag/C catalysts tested in a 10-2 M Na2CO3 + 0.1 M NaOH, a 10-3 M Na2CO3+ 0.1 M NaOH, and a 0.1 M NaOH solution saturated with argon or oxygen.

The influences of carbonate on the ORR activity and the electrochemical properties were also investigated and are shown in Figure 9. By adding sodium carbonate up to 10-2 M into the freshly prepared NaOH solution, the ORR activities on 60 wt % Ag/C catalyst did not change from that without the sodium carbonate additions. Carbonate is not demonstrated to be a poisonous species for ORRs on Ag/C catalysts. 4. Conclusion In this paper, four different metal loading Ag/C catalysts were prepared by using a citrate-protecting method. The factors which likely influence the Ag/C activity for the ORRs have been investigated in alkaline solutions. From the results of cyclic voltammetry, RDE, and RRDE, the ORRs on Ag/C catalysts are found to proceed mainly through a four-electron pathway. Ag/C catalytic activities for the ORRs greatly depend on the peak A1 intensity, which was found to be a key parameter to predict ORR activities on Ag/C catalysts. With increasing metal loading, peak A1 intensity increases linearly and the ORR onset potential shifts positively about 62 mV with Ag loading increasing from 10 to 60 wt %, as the Ag loading increased 0.02, 0.04, 0.08, and 0.12 mg · cm-2, respectively. Silicate is identified as a poisoning species that greatly inhibits the ORR activity on Ag/C catalysts. Compared with the results obtained in freshly prepared NaOH solution, a trace of 10-4 M silicate can greatly inhibit the formation of peak A1 and cause a negative shift of ORR onset potential up to 60 mV.

4330


Acknowledgment. This work was supported by the U. S. Army Research Lab (Grant No. W911NF-07-2-0036). References and Notes (1) Varcoe, J. R.; Slade, R. C. T. Fuel Cell 2004, 5, 187. (2) Bidault, F.; Brett, D. J. L.; Mifflryon, P. H. J. Power Sources 2009, 187, 39. (3) Lin, B.; Kirk, D.; Thorpe, S. J. Power Sources 2006, 161, 274. (4) Lu, S.; Pan, J.; Huang, A.; Zhuang, L.; Lu, J. Pans 2008, 105, 20611. (5) Asazawa, K.; Yamada, K.; Tanaka, H.; Oka, A.; Taniguchi, M.; Kobayashi, T. Angew. Chem., Int. Ed. 2007, 46, 8024. (6) Modestov, A. D.; Tarasevich, M. R.; Leykin, A. Y.; Filimonov, V. Y. J. Power Sources 2009, 188, 502. (7) Han, J.; Li, N.; Zhang, T. J. Power Sources 2009, 193, 885. (8) Tan, C.; Wang, F.; Liu, J. Mater. Lett. 2009, 63, 969. (9) EI-Deab, M. S.; Sotomura, T.; Ohsaka, T. Electrochem. Commun. 2005, 7, 29. (10) EI-Deab, M. S.; Sotomura, T.; Ohsaka, T. Electrochim. Acta 2006, 52, 1792. (11) Jiang, L.; Hsu, A.; Chu, D.; Chen, R. J. Electrochem. Soc. 2009, 156, B643. (12) Yang, Y.; Zhou, Y.; Cha, C. Electrochim. Acta 1995, 40, 2579. (13) Asazawa, K.; Sakamoto, T.; Yamaguchi, S. J. Electrochem. Soc. 2009, 156, B509. (14) Roche, I.; Chanet, E.; Chatenet, M.; Vondrk, J. J. Phys. Chem. C 2007, 111, 1434.

Guo et al. (15) Gojkovic, S. L.; Gupta, S.; Savinell, R. F. J. Electroanal. Chem. 1999, 462, 63. (16) Chen, R.; Li. H.; Chu. D.; Wang, G. J. Phys. Chem. C 2009, 113, 20689. (17) Varcoe, J.; Slade, R.; Wright, G.; Chen, Y. J. Phys. Chem. B 2006, 110, 21041. (18) Lima, F. H. B.; Zhang, J.; Shao, M. H. J. Phys. Chem. C 2007, 111, 404. (19) Chatenetm, M.; Genies-Bultel, L.; Aurousseau, M. J. Appl. Electrochem. 2002, 32, 1131. (20) Lima, F. H. B.; Sanches, C.; Ticianelli, E. J. Electrochem. Soc. 2005, 152, A1466. (21) Demarconnay, L.; Coutanceau, C.; Leger, J. M. Electrochim. Acta 2004, 49, 4513. (22) Coutanceau, C.; Demarconnay, L.; Lamy, C.; Leger, J. M. J. Power Sources 2006, 156, 14. (23) Zeng, J.; Yang, J.; Lee, J.; Zhou, W. J. Phys. Chem. B 2006, 110, 24606. (24) Hepel, M.; Tomkiewicz, M. J. Electrochem. Soc. 1984, 131, 1288. (25) Bard, A. J.; Faulken, L. R. Electrochemical Methods: Fundamental and Applications, 2nd ed.; Wiley: New York , 2001. (26) Perez, J.; Gonzalez, E. R.; Ticianelli, E. A. Electrochim. Acta 1998, 44, 1329. (27) Cifrain, M.; Kordesch, K. V. J. Power Sources 2004, 127, 234. (28) Lee, H.; Shim, J.; Shim, M.; Kim, S.; Lee, J. Mater. Chem. Phys. 1996, 45, 238.

JP910790U

Improving Oxygen Reduction Reaction Activities on Carbon

Recommend Documents