Electrocatalytic Properties of Multiwalled Carbon ... - Springer Link

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2012, Vol. 85, No. 10, pp. 1536−1540. © Pleiades Publishing, Ltd., 2012. Original Russian Text © M.O. Danilov, G.Ya. Kolbasov, I.A. Rusetskii, I.A. Slobodyanyuk, 2012, published in Zhurnal Prikladnoi Khimii, 2012, Vol. 85, No. 10, pp. 1601−1605.

TECHNOLOGY OF ELECTROCHEMICAL AND OTHER INDUSTRIES

Electrocatalytic Properties of Multiwalled Carbon Nanotubes-Based Nanocomposites for Oxygen Electrodes M. O. Danilov, G. Ya. Kolbasov, I. A. Rusetskii, and I. A. Slobodyanyuk Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, Kyiv, Ukraine Received April 27, 2012

Abstract—Metals (platinum, nickel, and lead) and oxides (manganese dioxide and molybdenum, chromium, cobalt, and niobium oxides) were deposited onto multiwalled carbon nanotubes by chemical and electrochemical methods. The resulting nanocomposites were tested as oxygen electrode materials for electrochemical power sources with alkaline electrolytes. A correlation was revealed between the catalytic activity of oxygen electrodes manufactured from composites based on carbon nanotubes with catalyst deposited, on the one hand, and the coefficient a in the Tafel equation for the oxygen evolution reaction on this catalyst, on the other. A possibility of predicting and evaluating the catalytic properties of oxygen electrode materials for power sources was suggested. DOI: 10.1134/S1070427212100084

Electrochemical power sources and fuel cells in which electrocatalytic processes occur belong to the scope of alternative or small-scale power generation. Today, the power generated by all existing power sources in the world is equal to that produced by nuclear, thermal, and hydro power plants combined. Therefore, studies into electric current generation are very topical nowadays. As demonstrated in review by Pletcher [1], the major mechanism used then for interpreting the features of electrocatalytic processes was that of activated chemisorption. In more recent publications [2–4], an alternative mechanism, referred to as the incipient hydrous oxide/adatom mediator (IHOAM) model [5], was described. The need for new approaches to catalysis has become particularly urgent after Haruta et al. [6] revealed anomalous catalytic activity of oxide-supported gold nanoparticles. Burke et al. [5] described the IHOAM model with emphasis on the significance of metastable state behavior of metals with active atoms on the surface being capable of undergoing fast redox electronic transitions at low potentials and of functioning as mediators in electrocatalytic processes. This model treats the surface active sites (atoms or oxide groups) not simply as “anchoring sites” for adsorption: At low surface coverages they undergo chemical or electrochemical transformations as part of electrocata-

lytic processes. For platinum, two mechanisms, activated chemisorption and IHOAM, were suggested for different reactions proceeding on this catalyst [7, 8]. Guerrini and Trasatti [9] demonstrated the importance of intensive parameters such as the Tafel slope and point of zero charge for oxide electrodes in differentiating the electronic from geometric factors in electrocatalyis. However, the abovementioned studies suggest that a unified theory describing the electrocatalytic processes is lacking. Therefore, based on the approaches described in the literature, we can presume the following. All electrocatalytic processes are based on the compromise potential of electrochemical reactions proceeding at an electrode consisting of a catalyst and a support. The value of the compromise potential depends on the potentials and exchange currents of the electrochemical reactions proceeding at this electrode simultaneously. Under presumption that the reactant adsorption and electron attachment are localized and occur simultaneously on the catalyst or on its support and that the reactants diffuse through the catalyst/support interface, it is possible, by selecting the appropriate catalystsupport pair, to influence the electrocatalytic properties of the electrode. Below, we will consider the possibility of this influence on the catalytic properties of electrodes employed in oxygen reduction reactions proceeding at

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ELECTROCATALYTIC PROPERTIES

oxygen electrodes of fuel cells. Here, we examined the electrocatalytic properties of the prepared nanostructured composites intended as oxygen electrode materials for electrochemical power sources and explored the possibility of evaluating their catalytic activity. EXPERIMENTAL

The electrochemical characteristics were recorded under galvanostatic conditions. The oxygen source was a U-shaped cell with alkaline electrolyte. Oxygen was supplied to the gas-diffusion electrodes under an excess pressure of 0.01 MPa. Prior to the experiments, the oxygen electrode was blown through with oxygen for 1 h. The polarization curves of oxygen evolution were recorded using 1-cm2 smooth nickel plates whose surface was modified by catalysts under conditions identical to those employed in the case of carbon nanotubes. The examinations were carried out using an IPS-Pro (Russia) potentiostat in the standard three-electrode mode. A 7-cm2 cylindrical platinum electrode served as auxiliary electrode. The potential was linearly scanned in 6 M KOH from the established value to +0.5 V at the scan rate of 1 mV s–1. All the potentials were measured against silver-chloride reference electrode connected through a salt bridge. The electron micrographs were recorded on a JEM100 CXII electron microscope. Also, the composites were examined on a DRON-4 X-ray diffractometer with CuKα radiation. In our experiments we used metals (platinum, nickel, and lead) and oxides (manganese dioxide and molybdenum, chromium, cobalt, and niobium oxides) as catalysts. For catalyst deposition we employed the previously developed electrochemical and chemical techniques [11–15]. Figures 2–4 show the electron micrographs of the nanocomposites based on molybdenum and chromium

Oxygen

As supports for catalysts we used multiwalled carbon nanotubes (MWNTs) prepared by catalytic pyrolysis of ethylene over a catalyst [10]. The resulting product had the bulk density of 25–30 g dm–3. The outer diameter of the nanotubes was 10–30 nm, specific surface area, 230 m2 g–1, and content of mineral impurities in unpurified product, 15–20 wt%. The catalyst impurities were removed from the MWNTs by treating with a hydrofluoric acid solution. The double-layer oxygen electrodes were prepared by pressing. The hydrophobic layer contained 0.07 g cm–2 acetylene black with 25% polytetrafluoroethylene, and the active layer, 0.02 g cm–2 MWNTs modified by different catalysts, with 5% polytetrafluoroethylene. The electrochemical examinations were carried out in 6 M potassium hydroxide electrolyte. A fuel cell mockup made of Plexiglas served as electrochemical cell; a zinc anode was used. The cell is shown schematically in Fig. 1.

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20 nm Fig. 1. Schematic of the cell: (1) cell body, (2) clamp sleeve, (3) metallic current lead and oxygen supply pipe, (4) metal grid of oxygen electrode, (5) hydrophobic layer of oxygen electrode, (6) active layer of oxygen electrode, (7) polytetrafluoroethylene gasket, (8) reference electrode, and (9) zinc anode.

Fig. 2. Electron micrograph of the nanocomposites based on multiwalled carbon nanotubes modified with molybdenum oxide containing 5 wt% elemental molybdenum; average particle size ranges from 3 to 10 nm.

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100 nm

Fig. 4. Electron micrograph of the nanocomposites based on multiwalled carbon nanotubes modified with manganese dioxide (20 wt%); average particle size ranges from 10 to 15 nm.

100 nm Fig. 3. Electron micrograph of the nanocomposites based on multiwalled carbon nanotubes modified with chromium oxide containing 5 wt% elemental chromium; average particle size ranges from 10 to 15 nm.

oxides and manganese dioxide. It is seen that the catalysts deposited are comprised of particles with nearly identical average size of 10–15 nm. Figure 5 shows the potential vs. current density plots for the oxygen electrodes whose active mass consists of composites based on carbon nanotubes modified with different catalysts: metallic platinum, Electrochemical characteristics of various catalytic materials

Catalyst

i, mA cm–2, for Coefficient a for oxygen electrodes oxygen evolution under 350 mV reaction polarization

Platinum

487

0.76

Lead

228

0.72

Manganese dioxide

211

0.7

Oxide of indicated element: niobium molybdenum

205 186

0.7 0.62

cobalt

148

0.61

chromium

120

0.61

40

0.52

nickel

lead, and nickel, as well as molybdenum, chromium, and niobium oxides and manganese dioxide. The polarization curves of oxygen evolution on these catalysts, deposited on the nickel electrodes, are presented in Fig. 6. The coefficient a in the Tafel equation for the molecular oxygen evolution reaction on these catalysts was calculated for low current densities and small polarization. The table compares the electrochemical characteristics of the oxygen electrodes manufactured from nanocomposites, based on carbon nanotubes modified by catalysts, with their corresponding values of the coefficient a for oxygen evolution reaction on these same catalysts. Figure 5 and the tabulated data are indicative of a correlation between the value of the coefficient a and the electrochemical characteristics examined: The larger the coefficient a, the higher the electrochemical characteristics.

Fig. 5. Variation of the potential E, V, with current density, i, mA cm–2, for oxygen electrodes with active layer containing 0.02 g cm–2 composites based on multiwalled carbon nanotubes modified with 10 wt% catalyst. Catalyst: (1) platinum, (2) molybdenum oxide, (3) niobium oxide, (4) cobalt oxide, (5) manganese dioxide, (6) lead, (7) chromium oxide, and (8) nickel.


ELECTROCATALYTIC PROPERTIES

Fig. 6. Polarization curves for oxygen evolution in 6 M KOH solution, recorded on the nickel electrode with different coatings. Coating: (1) metallic nickel, (2) chromium oxide, (3) cobalt oxide, (4) niobium oxide, (5) manganese dioxide, and (6) platinum black.

The experimental data obtained by us can be interpreted as follows. As the electrocatalytic oxygen reduction reaction proceeds at the electrode, the reactant supply stage, the electrochemical electron transfer event, and the reactant withdrawal occur. Under presumed different localizations of the electrochemical process stages on the surface of the heterogeneous catalyst-support system, the catalytic activity of this composition can be modified by selecting the appropriate catalyst-support pair with different energy barriers. Enhanced catalytic activity exhibited by composite electrodes compared to their unmodified counterparts corroborates this fact [16]. No bond rupture in the oxygen molecule occurs on active carbon, pyrographite, and some other catalysts because of a relatively low energy of adsorption of molecular oxygen [17]. In this case, the process involves electron attachment to the oxygen molecule adsorbed, which limits the entire oxygen reduction process. This is also true of another type of carbon materials, carbon nanotubes. As known, catalysts with higher energy of adsorption of molecular oxygen have higher oxygen evolution overpotentials [18] and, consequently, higher values of the coefficient a in the Tafel equation for molecular oxygen evolution:

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on which material the adsorption of oxygen molecule and bond stretching with possible electron attachment proceed more readily. As known, the value of the coefficient a depends on the nature of the material, specific surface area, orientation of single crystal faces, solution composition, and degree of surface oxidation [19]. The orientation of the crystalline faces of the catalyst influences the catalytic activity in the oxygen reduction reaction [20], thereby affecting the coefficient a. The revealed correlation between the catalytic activity of oxygen electrodes and overpotential of the molecular oxygen evolution on the catalyst (Fig. 5 and table) fits very well to the presumed localization of reactions. Adding iron to manganese dioxide is known to be responsible for deterioration of the electrochemical characteristics of manganese dioxide-based oxygen electrodes because of a low overpotential of oxygen evolution on iron, which fact also confirms the presumed localization of reactions. The above-described results provide explanation for why certain materials are catalysts for the reaction chosen and enable preliminary evaluation of their electrocatalytic properties. For example, when oxygen reduction proceeds in alkaline electrolyte and the catalyst support is represented by a carbon material with fairly low oxygen evolution overpotential in an alkaline medium, the catalyst to be chosen for this medium should be characterized by high oxygen evolution overpotential in an alkaline medium. Therefore, based on the electrochemical overpotential of oxygen evolution on the catalyst in this medium it is possible to evaluate the catalytic activity of the chosen composite in the oxygen reduction reaction. This enables directed synthesis or selection of catalytic materials for electrochemical power sources, as well as for other electrochemical systems and reactions. The limiting factor in selection of materials based on the principle of localization of electrochemical reactions is the corrosion resistance of materials in the electrolyte.

ŋ = а + blog І

CONCLUSIONS

Taking the IHOAM model [5] as the basis and considering the data reported in other relevant publications [7, 8], we can conclude the following. Based on the principle of localization of reactions, the catalyst to be used with carbon nanotubes having low energy of oxygen adsorption should be that with high energy of molecular oxygen adsorption (and, consequently, with large coefficient a),

(1) It was found that, for oxygen electrode with multiwalled nanotubes-based carbon support, materials characterized by high overpotentials of molecular oxygen evolution are good catalysts in alkaline electrolytes of fuel cells. (2) A presumption was made concerning localization of stages of electrochemical reactions on the catalyst and


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the support, which allows evaluating the catalytic activity of the resulting composite electrodes in the oxygen reduction reaction from the difference in the oxygen evolution overpotentials for the catalyst and the support. REFERENCES 1. Pletcher, D., J. Appl. Electrochem., 1984, vol. 14, no. 4, pp. 403–415. 2. Burke, L.D., Electrochim. Acta, 1994, vol. 39, nos. 11–12, pp. 1841–1848. 3. Burke, L.D. and Nugent, P.F., Gold Bull., 1998, vol. 31, no. 2, pp. 39–50. 4. Burke, L.D., Collins, J.A., and Murphy, M.A., J. Solid State Electrochem., 1999, vol. 4, no. 1, pp. 34–41. 5. Burke, L.D., Kinsella, L.M., and O’Connell, A.M., Electrokhimiya, 2004, vol. 40, no. 11, pp. 1289–1300. 6. Haruta, M., Yamada, M., Kobayashi, T., and Iijima, S., J. Catal. 1989, vol. 115, no. 2, pp. 301–309. 7. Parsons, R. and VanderNoot, T., J. Electroanal. Chem., 1988, vol. 257, nos. 1–2, pp. 9–45. 8. Burke, L.D. and Nugent, P.F., Electrochim. Acta, 1997, vol. 42, no. 3, pp. 399–411. 9. Guerrini, E. and Trasatti, S., Elektrokhimiya, 2006, vol. 42, no. 10, pp. 1131–1140. 10. Melezhik, A.V., Sementsov, Yu.I., and Yanchenko, V.V., Zh. Prikl. Khim., 2005, vol. 78, no. 6, pp. 938–944.

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