Control of nanoparticle aggregation in PEMFCs using surfactants

Transcription

1 Control of nanoparticle aggregation in PEMFCs using surfactants... Jill E. Newton *, Jon A. Preece and Bruno G. Pollet Centre for Hydrogen and Fuel Cell Research, School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK... Abstract This research is aimed at investigating the aggregation behaviour of catalyst nanoparticles in proton exchange membrane fuel cells (PEMFCs). Electrocatalyst nanoparticles are prepared using various surfactants which are known to prevent aggregation. Well-dispersed nanoparticles are thought to have a higher available surface area, hence exhibiting higher catalytic activity than aggregated nanoparticles. Platinum nanoparticles have been successfully prepared in aqueous dispersion using tetradecyltrimethylammonium bromide (C 14 TAB), cetyltrimethylammonium bromide (C 16 TAB) and nonylphenolethoxylate (NP9). The aggregation behaviour of the particles was studied using transmission electron microscopy, Nanosight (Carr B, Hole P, Malloy A. Sizing of nanoparticles by visualizing and simultaneously tracking the Brownian motion of nanoparticles separately within a suspension. In: Eighth International Congress on Optical Particle Characterisation, Karl-Franzens University Graz, 2007) and dynamic light scattering. Electrochemistry results for the oxygen reduction reaction (ORR) on the platinum disc in the absence and in the presence of surfactants show that the surfactants have a marked effect on the reaction. The movement of the onset potential to lower voltages suggests the surfactant is poisoning the catalytic sites on the electrode. This shows that the surfactant molecule selection is vital to obtaining effective fuel cell catalyst. *Corresponding author: jen853@bham.ac.uk Keywords: PEM; fuel cell; platinum; nanoparticle; surfactant Received 8 June 2011; accepted 14 July INTRODUCTION Carbon-supported Pt nanoparticles are currently the most effective catalysts used in proton exchange membrane fuel cells (PEMFCs), because of their electrocatalytic activities for both the oxidation of hydrogen on the anode and the reduction of oxygen on the cathode. It is known that the catalytic activity depends on the particle size, shape, size distribution and dispersion over the catalyst support [1 5]. Pt is expensive and limited in supply. Therefore, minimization of Pt loading is necessary to achieve large-scale commercialization of PEMFCs. This could be achieved by good dispersion of the Pt nanoparticles because well-dispersed nanoparticles are thought to have a higher available surface area, hence exhibiting higher catalytic activity than aggregated nanoparticles. To tailor the size of Pt nanoparticles with uniform dispersion on the carbon support, some kind of stabilizing agents, such as surfactants, ligands or polymers, are usually employed during the preparative process (Figure 1) [6 15]. The stabilizer attaches to particle surfaces and prevents particle aggregation. Stabilizers generally operate either by steric or electrostatic repulsion, or in some cases a combination of the two. There is concern that the presence of stabilizers may reduce platinum catalytic activity by blocking active sites. For example, polyvinylpyrrolidone (PVP) is commonly used to stabilize Pt nanoparticles. However, the carbonyl group of PVP interacts strongly with the platinum surface and thus blocks a significant number of active sites [15]. Chemical reactions can only occur effectively on catalytically clean nanoparticles where the reactants adsorb more strongly to the particle surface than the stabilizing agents do. When the interaction between the stabilizing agent and the metal surface is too strong, the catalytic activity is greatly reduced [16]. Wang et al. [11] found that the presence of some surfactants did not affect electrochemical activity. They suggested that small molecules such as H 2 and O 2 can diffuse through the surfactant layer. Alkylammonium ions have been used in synthesizing Pt nanoparticles [16] and their interactions with Pt surfaces are considerably weaker than that of the carbonyl group. Therefore, these molecules could serve as ideal surface-stabilizing agents. International Journal of Low-Carbon Technologies 2012, 7, # The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@oup.com doi: /ijlct/ctr023 Advance Access Publication 21 December

2 Nanoparticle aggregation in PEMFCs using surfactants In this paper, platinum nanoparticles are prepared in aqueous dispersion using the cationic surfactants tetradecyltrimethylammonium bromide (C 14 TAB) and cetyltrimethylammonium bromide (C 16 TAB) and the non-ionic surfactant nonylphenolethoxylate (NP9). Electrochemical results for the oxygen reduction reaction (ORR) on the platinum disc in the absence and presence of surfactants are presented. 2 EXPERIMENTAL METHODS An aqueous colloidal route was chosen for the preparation of platinum nanoparticles, since water is cheaper, safer to use and more environmentally friendly than organic solvents. Sodium borohydride was chosen as the initial reducing agent. The method of Lee et al. [17] was used to prepare Pt nanoparticles. The method uses readily available, inexpensive reagents and the cationic surfactant, C 14 TAB. The Lee procedure was repeated using the cationic surfactant C 16 TAB and the nonionic surfactant (NP9). In a typical synthesis, aqueous solutions of K 2 PtCl 4 (Sigma Aldrich, 99.9%) and C 14 TAB (Aldrich, 99%) were mixed in a 20-ml round-bottomed flask at room temperature. The mixture was heated at 508C for about 5 min until the solution became clear. The vial was capped with a rubber septum immediately after adding ice-cold NaBH 4 (Sigma Aldrich) and the H 2 gas pressure inside the vial was released through a needle in the septum for 10 min. The needle was then removed Figure 1. Diagram to show possible arrangement of CTAB molecules on platinum nanoparticles. and the solution was kept at 508C for 6 h. The mixture was stirred using a magnetic stirrer. The nanoparticles were characterized using transmission electron microscopy (JEOL 1200ex TEM), dynamic light scattering (DLS) using a Beckman Coulter DelsaNano and nanoparticle tracking analysis (NTA) [18]. NTA is a newly developed method for the direct and real-time visualization and analysis of nanoparticles in liquids. Based on a laser illuminated microscopical technique, Brownian motion of nanoparticles is analysed in real time by a CCD camera, each particle is simultaneously but separately visualized and tracked by a dedicated particle tracking image analysis programme. Electrochemical measurements were carried out using an Autolab PG302N potentiostat and a Radiometer analytical rotating disc electrode. A platinum wire was used as the counter electrode. The reference electrode used was a reversible hydrogen electrode (RHE). An 0.1-M HClO4 solution ( prepared from 70% Sigma Aldrich, % and UHQ water, Millipore) was degassed by bubbling nitrogen for 20 min. Measurement of electrochemical surface area was then performed in the degassed electrolyte. Prior to measurement of ORR activity, the electrolyte was saturated with O 2 by bubbling for 20 min. Surfactants were added to the perchloric acid solution and allowed to equilibrate for 20 min before taking measurements. 3 RESULTS AND DISCUSSION Figure 2a shows a transmission electron micrograph (TEM) of Pt nanoparticles from which the size distribution of Figure 2b was derived. From Figure 2b it can be seen that the individual particle diameters from TEM lie between 2 and 15 nm. When the same particles were analysed by the Nanosight method (Figure 3a) and also by the DLS method (Figure 3b), somewhat different sizes were obtained (over 50 nm). The Nanosight and DLS techniques measure the size of particles dispersed in water. In all the results (Figures 2 7b), particle size distributions obtained using these techniques are much broader and the average particle size is much larger (over 50 nm) Figure 2. (a) TEM Pt nanoparticles and (b) particle size distribution from TEM Pt nanoparticles prepared with 150 mm C 16 TAB, 1.5 mm K 2 PtCl 4 and 30 mm NaBH 4. International Journal of Low-Carbon Technologies 2012, 7,

3 J.E. Newton et al. Figure 3. (a) Nanosight results and (b) DLS results for Pt nanoparticles prepared with 150 mm C 16 TAB, 1.5 mm K 2 PtCl 4 and 30 mm NaBH 4. Figure 4. (a) TEM and (b) particle size distribution from TEM Pt nanoparticles prepared with 150 mm C 14 TAB, 1.5 mm K 2 PtCl 4 and 30 mm NaBH 4. Figure 5. (a) Nanosight and (b) DLS Pt nanoparticles prepared with 150 mm C 14 TAB, 1.5 mm K 2 PtCl 4 and 30 mm NaBH 4. Figure 6. (a) TEM and (b) DLS Pt nanoparticles prepared with 77 mm NP9, 1.6 mm K 2 PtCl 4 and 22 mm ascorbic acid. than those obtained from TEM. The TEM images show particles dried onto a formvar-coated carbon-on-copper TEM specimen grid. The size of individual particles was measured from the images. The results suggest that the particles are forming aggregates when dispersed in water and the DLS and Nanosight techniques are measuring aggregate size. All the dispersions appeared stable to the eye, showing no particle sedimentation on standing for several days. 40 International Journal of Low-Carbon Technologies 2012, 7, 38 43

4 Nanoparticle aggregation in PEMFCs using surfactants Figure 7. (a) TEM and (b) DLS Pt nanoparticles prepared with 77 mm NP9, 1.3 mm H 2 PtCl 6 and 11 mm ascorbic acid. The procedure used to prepare CTAB stabilized nanoparticles gave totally unstable dispersions with the non-ionic surfactant NP9. Stable dispersions were prepared with NP9 when ascorbic acid was used as reducing agent. Figure 6a shows that these dispersions consist mainly of small particles in the size range of 2 5 nm. These may be the most effective fuel cell catalysts due to their small size. Figure 7a shows NP9-stabilized Pt particles prepared from H 2 PtCl 6 precursor. These particles are large (ca. 20 nm) and appear to have a porous nature. The study then turns to the effect of surfactant on the oxygen reduction reaction (ORR) on bare Pt electrode in the potential range [0; þ1.0 V vs. RHE] in the presence of 0.1 M HClO 4 electrolyte. Figure 8(a) shows RDE voltammograms of bare Pt in oxygenated perchloric acid at various rotation speeds, at 298 K and at 25 mv s 21. The figure clearly shows that as the rotation speed increases, the limiting currents also increases and follows a linear behaviour according to the Levich equation, i.e. I lim ¼ 0:620 nfad 2=3 0 y 1=6 C v 1=2 ð1þ where n is the number of electrons transferred, F is the Faraday number (96,500 C.mol 21 ), A is the geometric surface area of the electrode, D o is the diffusion coefficient of the electroactive species, y is the kinematic viscosity of the electrolyte solution, C* is the electroactive concentration, and v is the angular rotation rate. However, when 50 mm C 14 TAB surfactant is added to the perchloric acid solution, no limiting currents are obtained and instead cathodic peaks (with maximum peak currents at ca. þ0.1 V vs. RHE) are observed at various rotation speeds as shown in Figure 8(b). This finding suggests that either (i) C 14 TAB reacts with HClO 4 to give intermediate decomposition products which affect the reduction process in the potential range employed or (ii) C 14 TAB may be adsorbed onto the Pt surface blocking the available sites for the ORR. It is interesting to note that at low rotation speeds similar currents are obtained, þ0.2 V vs. RHE when comparing Figures 8(a) and 8(b), indicating that the surfactant may poison the catalytic sites of bare Pt. The addition of NP9 in the O 2 -saturated perchloric acid solutions affects the RDE voltammograms as shown in Figure 9(a). The figure also shows that no limiting currents are obtained and pseudo-cathodic peaks are observed around þ0.1 V vs. RHE at all rotation speeds employed. Furthermore, all voltammograms show an evident linear part in the range of potentials [þ0.2 to þ0.7 V vs. RHE], indicating a V ¼ RI Figure 8. ORR polarization curves for 5-mm Pt disk recorded during the anodic sweep ( V versus RHE) in (a) 0.1 M HClO 4 electrolyte and (b) 0.1 M HClO 4 electrolyte and 50 mm C 14 TAB. International Journal of Low-Carbon Technologies 2012, 7,

5 J.E. Newton et al. Figure 9. ORR polarization curves for 5-mm Pt disk recorded during the anodic sweep ( V versus RHE) in (a) 0.1 M HClO 4 electrolyte and 4 mm NP9 and (b) 0.1 M HClO 4 electrolyte and 50 mm NP9. relationship, i.e. the addition of NP9 increases the resistivity of the solution; in other words, NP9 is not electrochemically active. This is not surprising as poly(oxyethylene)9 nonylphenol is known to a non-ionic surfactant. Figure 9(b) shows that the increase in NP9 concentration by nearly 10-fold does not significantly affect the shapes of the RDE voltammogramms; however, the V ¼ RI region is further extended in the potential range indicating an increase in solution resistivity. Furthermore, pseudo-cathodic peaks are broader and less defined again, indicating that the overall reduction process is principally dominated by R. Cyclic voltammograms of 0.1 M perchloric acid solution (N2 saturated) on Pt disc electrode at 298 K and at 25 mv s 21 (not shown here) exhibited typical characteristics of a protonated acid (e.g. H 2 SO 4 ). For example, pronounced hydrogen adsorption and desorption peaks were observed in the potential region [0.0 þ 0.20 V vs. RHE] with OH adsorption peaks at more positive potentials. However, addition of NP9 at various concentrations in 0.1 M HClO 4 did affect the shape of the CV and no hydrogen adsorption, desorption and OH adsorption peaks were observed and instead a V ¼ RI response was obtained (not shown here). This clearly suggests that the non-ionic surfactant suppresses the oxido-reduction process occurring at the Pt surface. Interestingly, cyclic voltammograms of C 14 TAB (50 mm) in 0.1 M perchloric acid solutions (N 2 saturated) on Pt disc electrode at 298 K and at 25 mv s 21 (not shown here) clearly exhibited pronounced hydrogen adsorption and desorption peaks at low potentials, although no OH adsorption peaks were evident. This finding is very interesting as it implies that C 14 TAB (i) is electrochemically active and (ii) favours the ORR on the Pt surface. 4 CONCLUSIONS Platinum nanoparticles have been prepared in aqueous media using cationic surfactants (CTAB) and non-ionic surfactants (NP9) to prevent particle aggregation. Although the dispersions appear stable to the eye, DLS and Nanosight results suggest there is some particle aggregation. The electrochemical results for the ORR in the absence and presence of surfactants show that the surfactants have a marked effect on the reaction. The movement of the onset potential to lower voltages suggests the surfactant is poisoning the catalytic sites on the electrode. These results suggest that the surfactant will need to be removed from the catalyst surface to make it effective as a fuel cell catalyst. Other authors have successfully removed surfactant from catalyst nanoparticles by washing [11] or heating [8]. REFERENCES [1] Liu Z, Shamsuzzoha M, Ada E, et al. Synthesis and activation of Pt nanoparticles with controlled size for fuel cell electrocatalysts. J Power Sources 2007;164: [2] Park S, Xie Y, Weaver M. Electrocatalytic pathways on carbon-supported platinum nanoparticles: Comparison of particle-size-dependent rates of methanol, formic acid, and formaldehyde electrooxidation. Langmuir 2002;18: [3] Arenz M, Mayrhofer K, Stamenkovic V. The effect of the particle size on the kinetics of CO electrooxidation on high surface area Pt catalysts. JAm Chem Soc 2005;127: [4] Stamenkovic V, Fowler B, Mun B, et al. Improved oxygen reduction activity on Pt 3 Ni(111) via increased surface site availability. Science 2007:315: [5] Peng Z, Yang H. Designer platinum nanoparticles: Control of shape, composition in alloy, nanostructure and electrocatalytic property. Nano Today 2009;4: [6] Guo J, Zhao T, Prabhuram J, et al. Preparation and the physical/electrochemical properties of a Pt/C nanocatalyst stabilized by citric acid for polymer electrolyte fuel cells. Electrochim Acta 2005;50: [7] Lim D, Lee W, Choi D, et al. Preparation of platinum nanoparticles on carbon black with mixed binary surfactants: Characterization and evaluation as anode catalyst for low-temperature fuel cell. J Power Sources 2008;185: [8] Okaya K, Yano H, Uchida H, et al. Control of particle size of Pt and Pt alloy electrocatalysts supported on carbon black by the nanocapsule method. Appl Mat Interfaces 2010;2: International Journal of Low-Carbon Technologies 2012, 7, 38 43

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