Synthesis and Characterization of PdSn Oxygen Reduction Electrocatalyst

Transcription

1 / The Electrochemical Society Synthesis and Characterization of PdSn Oxygen Reduction Electrocatalyst J. Salvador Pascual a, J.A. Chávez Carvayar b, O. Solorza Feria *a a Depto. Química. Centro de Investigación y de Estudios Avanzados del IPN. A. Postal C.P México, D.F. México. b Instituto de Investigaciones en Materiales, UNAM. Av. Universidad Del. Coyoacan. C.P México, D.F. México. ( corresponding author: osolorza@cinvestav.mx) PdSn synthesized by a NaBH 4 reduction of PdCl 2 and SnCl 2 in a THF solution, was examined as electrocatalyst for the oxygen reduction reaction (ORR) in acid media by rotating disk electrode (RDE). The structure, morphology and composition of the catalyst were characterized by using XRD, SEM, TEM and AFM techniques. Nanocrystalline particles with less than 10 nm in size were observed. The PdSn catalysts exhibited high current densities for the ORR compared to that of pure Pd catalysts. The effect of temperature on the kinetics was analyzed with an apparent activation energy of ± 0.08 kj mol 1. Introduction Interest in clean energy generation such as fuel cells technology has motivated research on the synthesis and characterization of novel and stable electrocatalysts for the direct four electron transfer process, of the oxygen reduction reaction, ORR, to water formation. These novel electrocatalysts are specially aimed to promote the dissociation of the O O bond during the ORR mechanism, and to maintain better stability (1, 2). Bimetallic and alloy catalysts have been particularly active area of research because it has been established that for the cathode reaction, multi metallic surfaces have superior activity than that of pure metal (3, 4). The enhanced reactivity of the alloy surfaces is attributed to bi functional effects, in which the unique catalytic properties of each of the elements in the alloy combine in a synergetic manner to yield a surface which results more active than each of the elements alone. Also an electronic effect could modify the electronic properties of the alloy to enhance the active catalytic surface. Among the electrocatalysts, which have shown interesting electrochemical properties both in terms of tolerance to alcohol and high activity toward the ORR, are Pd and Pd alloys (5, 6). Recently, it has been shown that Pd of nanometric size exhibited a reduced catalytic activity for the ORR (7). The interest of this work in palladium bearing tin atoms is to improve the stability of the bimetallic compound as well as to enhance the electrocatalytic activity through the synergistic effects of the two metals. The effect of temperature on the kinetics of the ORR study of the PdSn catalyst also constitutes part of this work. Experimental PdSn electrocatalyst was synthesized through NaBH 4 reduction of PdCl 2 (0.86 mm) and SnCl 2 (0.086 mm) in a 100 ml tetrahydrofuran, THF solution, as previously 3

2 reported for the preparation of nanometric Pd (7). Briefly, a chemical reactor was charged with the reagents and vigorously stirred for the chemicals salvation. Afterwards, NaBH 4 (0.06 mm) solution was slowly added maintaining the stirred solution at 0 C under N 2 atmosphere for 16 h. The reaction product was washed with distilled water to eliminate the sodium chloride formed during the reaction. After filtered, the powder was dried and weighted obtaining a 99% of yield. The powder was subject to thermal treatment under H 2 atmosphere at 200 C for 3 h. Phase identification of the powdered catalyst, after thermal treatment, was carried out with a Bruker D8 Advance Plus X ray diffractometer, with CuK radiation ( = nm). Scanning steps of were used in the range Particle size was determined from transmission electron microscopy (TEM) analyses, with a Jeol JEM 1200 EX microscope, at 100 kev. For superficial studies, a Leica-Cambridge Stereoscan 440 SEM, at 20 kv, was used to analyze the morphology and distribution of the materials. An Oxford ISIS energy dispersive spectroscopy (EDS) device linked to the SEM was used to obtain the elemental composition of the samples. To analyze the topography, profiles, and particle size, a JEOL JSPM-4212 Scanning Probe Microscope, with the tapping mode under normal pressure conditions, was used. For electrochemical studies, experiments were carried out in a temperature controlled double compartment three electrode test cell. A platinum mesh was used as the counter electrode and Hg/Hg 2 SO 4 /0.5M H 2 SO 4 (MSE = V/NHE) as reference electrode. This electrode was outside the cell, kept at room temperature and connected by a porcelain Luggin capillary. The potentials were referred to NHE. The electrolyte was a 0.5M H 2 SO 4 solution prepared from double distilled water which was degassed with N 2 for the working electrode activation and saturated with O 2 for 20 min before each ORR electrochemical measurements. Electrochemical studies were performed in an EG&G potentiostat/galvanostat model 273A. Cell temperature was controlled by a Haake thermostat (model F3) from 308 to 328 K. RDE measurements were conducted on a thin film catalyst deposited on a glassy carbon disk electrode (0.196 cm 2 ) mounted in a interchangeable RDE holder (Pine Instruments). The thin film was deposited from a solution which was prepared by adding 3 L of a suspension resulting by mixing 75L 2 propanol, 7 L Nafion (5wt%, Du Pont 1000EW) and 3 mg PdSn (50 wt%/c). The estimated amount of catalyst in the thin film was about 0.6 mg cm 2. The rotation rate of the working electrode was in the range of 200 and 2500 rpm at 5 mv s 1. Results and discussion X ray diffraction results of thermally treated PdSn showed an orthorhombic structure Pnma (62), which experimental XRD pattern matches well the standard JCPDS card Broad peaks in the PdSn XRD pattern indicate the presence of nanocrystallites which percentage, about 90%, was determined by MDI jade 5.0 software. The average size of the PdSn catalyst, about 8.6 nm (Rpw 10.3), was deduced by fitting the diffraction patterns using the academic Topas software. Studies by SEM, Fig. 1, showed an homogenous distribution of particles which form clusters with a variety of sizes. These clusters were analyzed by EDS and results indicated that metal composition in atomic percentage was: Pd at% and Sn at%. The morphology of PdSn was analyzed by TEM and results exhibited semispherical and spherical agglomerates which are built of fine particles. Figure 2, a TEM micrograph of PdSn catalyst, shows clusters of 4

3 nm which are formed by semispherical and spherical nanoparticles in a range 5 20 nm. Figure 1. SEM micrograph of PdSn electrocatalyst. Figure 2. TEM micrgraph of PdSn electrocatalyst. 5

4 In a previous work (7) it has been shown that ink type Pd film deposited on a glassy carbon electrode catalyzed the reduction of oxygen in acid medium. Fig. 3 shows a set of RDE current density potential curves obtained on PdSn catalyst in O 2 saturated 0.5M H 2 SO 4, at 35 C. The electrochemical behavior at elevated temperatures exhibited similar polarization curves with a variation of the open circuit potential with temperature rpm E (NHE) / V Figure 3. Oxygen reduction reaction on PdSn at different rotation rates. The main characteristic in the polarization curves was the defined charge transfer control ( V/NHE), mixed ( V/NHE) and a reduced mass transfer region. The disk currents did not reach a plateau of the diffusion limited current and this is due to the small amount of catalysts (0.6 mg cm 2 ) on the thin film formed on the glassy carbon electrode, but it is clearly defined its dependency with the rotation rate. The overall measured current density, i, of the ORR can be expressed as being dependent of the kinetic current density, i k, the boundary layer diffusion limited current density, i d, and films diffusion limited current density. However, in thin film ink type electrode, the resistance of the Nafion film which covers the dispersed catalyst is sufficiently small that can be neglected (1). Figure 4 shows the mass transfer corrected Tafel plots for the molecular oxygen reduction at 308 K and at different temperatures, which was obtained from the equation i k i id. [1] i i The Tafel plots, at all temperatures, showed a linear region at low current density and a deviation of the kinetic current occurs with higher slope at high current density. The increase of the current densities reflects the temperature dependence of the chemical E / RT rate constant, which is approximately proportional to e, where E is the apparent activation energy at the reversible potential. Electrode kinetic parameters were d 6

5 determined at these temperatures. The exchange current density, i 0, was evaluated as a function of temperature, taking into consideration the reversible oxygen electrode potential, E r, at each temperature. The dependence of Er on temperature was evaluated using equations [2] and [3] 0 G 296, T ln T 389. T, (J mol 1 ) [2] ( H / O ) E 0 G / F. [3] r ( H / O ) K E (NHE) / V Figure 4. Mass transfer corrected Tafel plot for ORR at different temperatures. The basis of the potential dependence of the electrochemical reaction rates, expressed as current density is the Tafel equation, a b log i, where b is the so called Tafel slope, conventionally written as b 2.303RT / n F, [4] where n and are the number of electrons transferred and the transfer coefficient, respectively. Temperature dependence of the Tafel slope and transfer coefficient showed a linear variation of Tafel slopes with temperature, rarely observed in ORR electrocatalysis. Here the experimental average transfer coefficient of 0.45 was practically invariant with temperature for all the samples. The apparent activation energy, E, for the ORR on PdSn electrocatalyst were evaluated in the range of temperature K, from the slope of the Arrhenius equation represented by the relationship 7

6 d log i 0 E R. [5] d (1/ T ) Figure 5 shows the Arrhenius plot from the exchange current density deduced from Fig.4. An average value of E = ± 0.08 kj mol 1 was calculated. This value is lower in comparison to that of nanometric Pd electrocatalyst ( E = 89.5 ±0.5 kj mol 1 ) (7). This result indicates that the incorporation of tin to palladium improves the catalytic activity and selectivity towards the reduction of molecular oxygen to water formation E # = ± 0.08 kj mol T -1 / K -1 Figure 5. Electrochemical Arrhenius plot for the activation energy determination. Conclusions The borohydride reduction process has been proven to be a significant synthetic tool for the preparation of PdSn catalyst, with particles less than 10 nm in size. The electrocatalytic activity of this compound, for the oxygen reduction reaction in acid media, showed that the electrocatalyst allowed multielectron charge transfer to water formation, with an apparent activation energy of ± 0.08 kj mol 1. Acknowledgements We gratefully acknowledge to Carlos Flores M. (IIM UNAM) for technical assistance. This work was supported by Mexican Council of Science and Technology, CONACYT (Grant No ) and Science and Technology Institute of Mexico City, ICYTDF (Grant No. OCF OSF). JJSP thanks CONACYT for doctoral fellowship. 8

7 References [1] K. Suárez Alcántara and O. Solorza Feria, Electrochim. Acta, 53, 4981 (2008). [2] K. Suárez Alcántara, A. Rodríguez Castellanos, S. Durón Torres and O. Solorza Feria, J. Power Sources, 171, 381 (2007). [3] A. Gebert, N. Mattern, U. Kuehn, J. Eckert and L. Schultz, Intermetallics, 15, 1183 (2007). [4] M.T.M. Koper, Surface Science, 548, 1 (2004). [5] F.P. Hu, Z. Wang, Y. Li, Ch. Li, X. Zang and P.K. Shen, J. Power Sources, 177, 61 (2008). [6] F.J. Rodríguez Varela and O. Savadogo, ECS Transactions, 3, 181 (2006). [7] J.J. Salvador Pascual, S. Citalán Cigarroa and O. Solorza Feria, J. Power Sources, 172, 229 (2007). [8] L. Jiang, Z. Zhou, W. Li, W. Zhou, S. Song, H. Li, G. Sun and Q. Xin, Energy & Fuels, 18, 866 (2004). 9