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Electrodeposition of dendritic palladium nanostructures on carbon support for direct formic acid fuel cells
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Available online at www.sciencedirect.com
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Electrodeposition of dendritic palladium nanostructures on carbon support for direct formic acid fuel cells Kranthi Kumar Maniam, Volga Muthukumar, Raghuram Chetty* Department of Chemical Engineering, Indian Institute of Technology Madras, India
article info
abstract
Article history:
Palladium (Pd) dendrites were synthesized on electrochemically activated carbon (Vulcan
Received 1 October 2015
XC-72R) support by a simple template free potentiostatic (constant voltage) deposition.
Received in revised form
Scanning electron micrographs displayed a dendritic morphology when deposited at po-
25 November 2015
tentials <0.5 V (vs. RHE), whereas spherical aggregates were obtained at potentials >0.5 V.
Accepted 10 August 2016
Transmission electron micrographs and X-ray diffraction pattern confirmed the difference
Available online 31 August 2016
in morphological features and growth of Pd dendrites. Pd deposited at 0.4 V showed higher electrochemical surface area of 128.5 m2 g1 in comparison to 25.1 m2 g1 obtained for Pd
Keywords: Electrodeposition
deposited at 0.5 V and also displayed enhanced activity towards formic acid oxidation. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Formic acid oxidation Palladium Dendrites Electrochemical activation Fuel cells
Introduction Fuel cells which use small organic molecules such as methanol, ethanol, formic acid and formaldehyde are regarded as promising electrochemical systems for direct conversion of fuel into electrical energy for a wide range of portable and transportation applications [1]. Among the fuels employed, formic acid is considered to be a potential fuel because it has certain advantages such as (i) less crossover from anode to cathode, (ii) can be used in high concentrations, (iii) less toxic and (iv) has high open circuit potential (1.48 V) relative to methanol [2e4]. An improved understanding of the formic acid oxidation mechanism and development of an efficient
electrocatalysts underlie much of its research [5,6]. The oxidation mechanism of formic acid in direct formic acid fuel cell is described by two ways. One is “direct pathway” and other one is “indirect pathway”. In the “direct pathway”, formic acid is directly oxidized to CO2 without any intermediate step. In the “indirect pathway”, formic acid is first oxidized to CO and then to CO2 [7,8]. Though platinum is known to be the effective catalyst for oxidation of formic acid, strategies to avoid the indirect pathway requires further development. Since the oxidation of formic acid catalyzed by Pd electrocatalyst has been proven to be more active than Pt electrocatalyst [9,10], tailoring the morphology of Pd can be considered as one of the effective approaches to improve the oxidation reaction.
* Corresponding author. Fax: þ91 44 2257 4152. E-mail address: [email protected] (R. Chetty). http://dx.doi.org/10.1016/j.ijhydene.2016.08.064 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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A variety of micro/nano hierarchical architectures and structures of Pd such as cactoids, cubes, dendrites, flowers etc. have been prepared employing various synthesis routes. It has been shown that the as-synthesized Pd micro/nano structures exhibit a significant difference in catalytic activity toward electrooxidation of formic acid [11e13]. However, most of these synthesis routes employ organic surfactants and polymeric stabilizers at elevated temperature, which calls for a simple, economic, template and surfactant free method to develop the noble metal micro/nanostructures [14]. Considering the advantage of high degree of controllability towards the synthesis of desirable nanostructures, and electrode potential as a convenient parameter, electrodeposition is found to be one of the effective approaches [15,16]. Using electrons as the reducing agents, the metal precursor reacts on a conductive substrate which could primarily avoid the usage of stabilizers and surfactants. The supply of electrons can be easily controlled by applied potential and time [17]. Most of the recent literature reported on the synthesis of Pd dendrites and flowers use potentiostatic deposition on substrates such as glassy carbon, gold, indium tin oxide (ITO) etc. [18e21], which resulted in an increase in surface area and increased activity towards formic acid oxidation. Dendritic structures offer high porosity as well as better inter connectivity of the particles. These structures exhibited large electrochemical surface area and more active centers than other shapes. Moreover, dendritic structures have high surface roughness and surface steps which can contribute to the increased accessibility of reactant species and are believed to enhance the fuel cell reactions [22]. Since metal particles supported on carbon are employed as catalysts for a wide range of electrocatalytic applications, Pd with dendritic structures deposited on carbon-based substrates would be an ideal choice and advantageous for fuel cell applications. Though, there is a considerable ongoing research focused on the development of controlled Pt nanostructures on carbon based substrates [23,24], there is limited study which highlight the synthesis of electrodeposited Pd nanostructures on carbon based substrates. Despite the wide spread use of carbon as a catalyst support, the poor adsorption of metal precursors (due to hydrophobic nature of carbon) making the synthesis of hierarchical structures difficult. Considering the above, the present study aims at synthesizing Pd dendrites on a carbon support by potentiostatic (constant voltage) deposition which was previously subjected to an electrochemical activation process. Pd catalysts deposited at various deposition potentials were physically characterized and the activities were compared towards the electrooxidation of formic acid.
Experimental Electrochemically activated carbon black (Vulcan XC-72R) coated graphite electrodes (4 mm dia) were used for the deposition of Pd following the procedure reported in our previous publications [25]. In brief, graphite electrodes (4 mm) were coated with a thin layer of carbon black ink (Loading: 100 mg cm2), prepared by dispersing a known amount of carbon in a mixture of Nafion® (20 mL) and iso-propanol (1 mL)
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followed by ultrasonication for 30 min, and air dried. The resulting carbon coated substrate was subjected to electrochemical activation by potential cycling in N2 saturated 0.5 M H2SO4 for 100 cycles at a scan rate of 100 mV s1 [26]. Constant potential technique was employed for the deposition of Pd from 2 mM PdCl2 in 0.01 M HClO4 (PdCl2, Aldrich) solution on the electrochemically activated carbon based substrates. Potentials in the range of 0.4e0.7 V was chosen for the deposition. Pd loading was determined using ICP-OES measurements, from the concentrations of the precursor solution before and after electrodeposition. The deposited Pd catalysts were physically characterized by a field emission high resolution scanning electron microscopy (FE-SEM, Hitachi S4800) interfaced with an energy dispersive X-ray analysis, X-ray diffraction (XRD, Bruker D8 Discover X-ray diffractometer, Cu Ka source, l ¼ 1.54056 A) and transmission electron microscopy (TEM, Philips CM 12 model 120 kV). The catalysts were electrochemically characterized by cyclic voltammetry (CV) using a CHI 1100A electrochemical workstation (CH Instruments, Inc.) in a three electrodes cell housed in a BASi C3 cell stand. Pd electrodeposited on electrochemically activated carbon was used as the working electrode; Pt wire and Ag/AgCl(3 M NaCl) were used as counter and reference electrodes, respectively. Electrochemical tests were carried out at room temperature (23 ± 1 C) in 0.5 M H2SO4 solution and all potentials quoted in this work are referred to a reversible hydrogen electrode (RHE) scale. The current densities are referred to the geometrical area (0.125 cm2) of the electrode unless otherwise specified. CO stripping voltammograms were conducted in 0.5 M H2SO4 solution, in which electrodeposited Pd electrode is pre-adsorbed with carbon monoxide. Initially, CO was bubbled into 0.5 M H2SO4 solution for 20 min with the electrode potential fixed at 0.1 V. Then, the dissolved CO in solution was removed by bubbling N2 into the solution, and the stripping voltammograms were collected at a scan rate of 50 mV s1 [27]. The activity towards formic acid oxidation was studied in nitrogen saturated 0.5 M H2SO4 with 1 M HCOOH solution by scanning the potential from 0 V to 1.1 V at 20 mV s1.
Results and discussion It has been widely reported that the hydrophobic nature of untreated carbon materials affect the reaction characteristics of the active sites present on the catalyst surface due to the poor adsorption of catalyst precursors [28,29]. Therefore, carbon support are subjected to functionalization or surface treatment prior to metal particle deposition, which generally involve chemical oxidation of the support so as to make it hydrophilic [30e32]. The surface functional groups are responsible for anchoring the metal particles to the carbon support [27]. Electrochemical activation is one of the ways to introduce surface defects on carbon based support, and recently we have reported potential cycling of carbon in dilute acid (0.5 M H2SO4) facilitate the formation of surface functional groups [33].
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Effect of deposition potential Electrodeposition occurs by the process of nucleation and growth; nuclei appear at the active sites on a substrate according to nucleation rate, and then grow via the incorporation of further ions from the solution [34,35]. Deposition potential play a vital role during the electrochemical deposition of Pd as it is critical for controlling the nucleation and growth of structures [36,37]. In our study, Pd was deposited in the potential range of 0.4e0.7 V for 1800 s. Pd deposition potential range was chosen from the cyclic voltammogram of the electrochemically activated carbon support in acidic medium (0.01 M HClO4) containing chloride precursor (2 mM PdCl2) as reported in our previous publication [25]. In general, deposition of Pd is carried out at potentials near to the reductive peak potentials (i.e. 0.5 V in the present case) [38]. On decreasing the deposition potential below the reductive peak potential, the interference from hydrogen evolution with Pd deposition is expected to become more prominent [39]. Therefore Pd electrodeposition in acidic media is restricted to 0.4 V in this work. Fig. 1 shows the scanning electron micrographs of Pd electrodeposited at different potentials for 1800 s. Fig. 1a shows a representative SEM image of carbon without metal deposition. Pd when deposited at higher potentials i.e. at 0.7 and 0.6 V (Fig. 1b and c), spherical agglomerates are formed on the substrate. On decreasing the deposition potential, the
formation of quasi-spherical studded particles is observed at 0.5 V (Fig. 1d). When the deposition was carried out at 0.45 V (Fig. 1e), dendritic morphology is observed, while the deposition at 0.4 V (Fig. 1f), showed a denser coverage of dendrites. As evident from the SEM micrographs, a major transformation and difference in morphology was observed between the deposition potentials of 0.4 V and 0.5 V, hence in the rest of the work, Pd deposited at these two potentials are studied in detail for comparison. TEM imaging was used for the acquisition of morphological information of electrodeposited Pd as shown in Fig. 2. TEM micrographs were obtained by carefully scraping the electrodeposited catalyst from the graphite electrode and loaded on a carbon-coated copper grid. Fig. 2a shows the micrographs of Pd deposited at 0.4 V, where a single dendrite is seen distributed on the carbon support with porous structure. In contrast, Pd electrodeposited at 0.5 V (Fig. 2b) displayed an irregular spherical morphology. Microstructural investigations in Fig. 2c, d clearly revealed that Pd deposited at 0.4 V form a tapered dendritic structure with nanoscale corrugations on the surface, indicating that most of the dendrite branches consist of twins. This type of twinned growth was observed by Patra et al. [40] during the deposition of Pd dendrites on carbon paper by alternate current (AC) deposition. Pd dendrites seem to consist of many corrugations, which can provide high specific surface area.
Fig. 1 e (a) Scanning electron micrograph of Vulcan support without Pd deposition. Scanning electron micrographs of Pd catalyst electrodeposited on carbon support at different potentials for 1800 s: (b) 0.7 V, (c) 0.6 V, (d) 0.5 V, (e) 0.45 V, (f) 0.4 V. Insets for the (d), (e), (f) represent the higher magnification image. Electrodeposition bath consists of 2 mM PdCl2 in 0.01 M HClO4. Loading of carbon: 100 mg cm¡2. Number of electrochemical activation cycles before deposition: 100.
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Fig. 2 e Transmission electron micrographs of Pd catalyst electrodeposited on Vulcan support at different potentials for 1800 s: (a) 0.4 V, (b) 0.5 V and (c, d) higher magnification images of a single dendrite.
Fig. 3 compares the X-ray diffraction patterns of Pd deposited at 0.4 V and 0.5 V. The diffraction peak at 2q 40.1 , 47 , 68 corresponds to Pd (111), (200) and (220) planes of a typical face centered cubic (fcc) structure. Diffractions due to carbon appeared at 2q ~55 . As can be seen, there is an increase in intensity of (111) plane observed for the Pd deposited at 0.4 V relative to the Pd deposited at 0.5 V, suggesting and evidencing the dendritic feature of the metal deposited at lower reduction potentials. Similar XRD pattern for Pd dendrite was reported by several research groups [41e43].
in relation to hydrogen adsorption sites and metal oxide formation [44]. Fig. 4 compares the cyclic voltammograms of Pd electrodeposited at 0.4 V and 0.5 V in nitrogen saturated 0.5 M H2SO4 at a sweep rate of 50 mV s1 and room temperature. As can be seen, Pd surface electrochemistry representing the typical Hþ adsorption/desorption, oxide formation/stripping peaks are observed in both the cases. Pd deposited at 0.4 V displayed a predominant metal electrochemistry relative to
Pd oxide formation
3
Cyclic voltammetry (CV) provides a fundamental understanding of the metal surface electrochemistry, and the voltammetric peaks of Pd in acidic electrolyte are generally discussed
Current density, mA/cm
2
Electrocatalytic study
2
H desorption
1 0 -1 -2
Pd oxide reduction
-3 -4
Pd deposited at 0.4 V Pd deposited at 0.5 V
H adsorption
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential, V vs RHE
Fig. 3 e X-ray diffraction pattern of Pd catalyst electrodeposited on Vulcan support at 0.4 V and 0.5 V.
Fig. 4 e Comparison of the cyclic voltammograms of the Pd catalysts in nitrogen saturated 0.5 M H2SO4, deposited at 0.4 V (dashed lines) and 0.5 V (solid lines) on carbon support at a scan rate of 50 mV s¡1 and room temperature. Vulcan support was subjected to 100 cycles of electrochemical activation in nitrogen saturated 0.5 M H2SO4 prior to deposition. Electrodeposition bath consists of 2 mM PdCl2 in 0.01 M HClO4. Loading of support on graphite substrate: 100 mg cm¡2. Deposition time: 1800 s.
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the deposition at 0.5 V. First, it can be seen that the electrode exhibit the well-known hydrogen adsorption (Hads) and desorption (Hdes) in the potential region of 0e0.2 V, and Pd oxide formation and reduction within 1.3 to 0.6 V [45]. However, it is worth to note from the voltammograms (shown in Fig. 4) that there is a noticeable difference in the existence of peaks at 0 V and 0.2 V in nitrogen saturated 0.5 M H2SO4 solution for the Pd catalysts deposited at 0.4 V and 0.5 V. The sensitiveness of the peaks in the hydrogen region represents the differences in morphological features and Pd crystal faces. These further evidence the fact that deposition potential not only influences the morphological feature of Pd deposited but also effects the interaction between hydrogen and Pd. Electrochemical surface area (ESA) of Pd catalyst was evaluated using the areas of hydrogen adsorption/desorption after deduction of the double-layer region by adapting the assumption that 212 is the charge (mC cm2Pd) required to oxidize a monolayer of hydrogen on a smooth Pd surface [38]. The coulombic charge for hydrogen adsorption/desorption (Qa/Qd) was used to calculate the active palladium surface of the electrodes. The calculated ESA of Pd catalysts deposited at 0.4 V and 0.5 V are tabulated in Table 1. As can be seen, Pd deposited at 0.4 V display ESA of 122.4 m2 g1, which is higher than that of the Pd deposited at 0.5 V (32.5 m2 g1), indicative of the presence of more active centers/sites available for reaction. This could be due to the presence of many nanoscale corrugation and increased interconnectivity of Pd dendrites when deposited at 0.4 V [45]. CO stripping voltammetry was carried out for the Pd catalysts deposited at 0.4 V and 0.5 V to validate the electrochemical surface area. Fig. 5a shows the CO stripping voltammogram of Pd deposited at 0.4 V and the subsequent cyclic voltammogram in 0.5 M H2SO4 at a scan rate of 50 mV s1. Similar voltammogram was also observed for Pd deposited at 0.5 V as shown in Fig. 5b. The voltammogram showed a single oxidation peak during the first scan, whereas no CO oxidation was observed in the second scan confirming the complete removal of the adsorbed CO species [32]. The appearance of a shoulder peak in addition to the single oxidation peak for the Pd deposition at 0.5 V, can be attributed to the oxidation of weakly absorbed CO in accordance with the reported literature [4]. The ESA values was determined from CO stripping voltammetry, using the CO oxidation charge after subtracting the background current of the subsequent CV curves with the assumption of 430 mC cm2 as the oxidation charge for one monolayer of CO on a smooth Pd surface [32]. Pd deposited at 0.4 V displayed higher ESA of 128.5 m2 g1 compared to that of Pd deposition at 0.5 V (25.1 m2 g1), confirming the presence of more active surface sites on Pd deposited at 0.4 V. The ESA values obtained from CO stripping voltammetry are in relatively fair agreement with those obtained from hydrogen adsorption/desorption experiments.
Fig. 5 e Comparison of the CO stripping voltammograms at a scan rate of 50 mV s¡1 for Pd catalysts deposited at (a) 0.4 V and (b) 0.5 V on carbon support.
Formic acid oxidation Pd supported on carbon has its unique ability to catalyze the oxidation of formic acid in fuel cells [46e48]. In this context, morphology of Pd plays a vital role on the electrocatalytic activity. To understand the influence of Pd morphology, the performance of Pd catalysts deposited at 0.4 V and 0.5 V towards formic acid oxidation was compared by voltammetry and the results are shown in Fig. 6. As can be seen, a significant enhancement in the electrocatalytic activity towards formic acid oxidation was observed for the Pd deposited at 0.4 V relative to the deposition at 0.5 V. Fig. 7 compares the mass specific activity of both the catalysts, the peak current density for Pd deposited at 0.4 V was higher than that of Pd deposited at 0.5 V (65 mA mg1 vs 45 mA mg1). The mass specific current density for formic acid oxidation for the electrodeposited Pd at 0.4 V also increased compared to the deposition obtained at 0.5 V. In order to evaluate the intrinsic activity of electrocatalysts, the
Table 1 e Comparison of the morphology and electrochemical surface area (ESA) of Pd catalyst electrodeposited on Vulcan support at 0.4 and 0.5 V for 1800 s. Deposition potential (V) 0.4 0.5
Morphology of Pd
ESA from Hþ ads þ des (m2 g1)
ESA from CO stripping (m2 g1)
Dendritic Irregular spherical aggregates
122.4 32.5
128.5 25.1
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2.0
Current density, mA/cm
2
(a) 1.6
(b)
1.2 0.8 0.4 0.0 -0.4 0.0
0.2
0.4
0.6
0.8
Potential, V vs RHE
1.0
Fig. 6 e Comparison of the activities of the Pd catalysts deposited at (a) 0.4 V (dashed lines) and (b) 0.5 V (solid lines) on electrochemically activated carbon based support in N2 saturated 1 M HCOOH in 0.5 M H2SO4 at a scan rate of 20 mV s¡1 and room temperature.
that Pd deposited at 0.4 V has broader peak in the negative scan (at ~0.7 V) than that of 0.5 V, which evidence the better stripping of the absorbed intermediates for Pd deposition at 0.4 V [49,50]. The improved performance of Pd deposition at 0.4 V can be attributed to the combination of dendritic morphology with better interconnectivity of metal particles and to enhanced CO tolerance [51]. Thus it can be concluded that tailoring the morphology of Pd can improve the metal utilization and result in higher catalytic activity.
Mass specific activity, mA/mg
70
(a)
60
(b)
50
Fig. 8 e Comparison of the specific surface activities of the Pd catalysts deposited at (a) 0.4 V (dashed lines) and (b) 0.5 V (solid lines) on electrochemically activated carbon based support in N2 saturated 1 M HCOOH in 0.5 M H2SO4 at a scan rate of 20 mV s¡1 and room temperature.
40 30 20
Conclusion 10 0 0.0
0.2
0.4
0.6
0.8
1.0
Potential, V vs RHE Fig. 7 e Comparison of the mass specific activities of the Pd catalysts deposited at (a) 0.4 V (dashed lines) and (b) 0.5 V (solid lines) on electrochemically activated carbon based support in N2 saturated 1 M HCOOH in 0.5 M H2SO4 at a scan rate of 20 mV s¡1 and room temperature.
formic acid oxidation current is normalized to ESA values and the specific surface activity results are presented in Fig. 8. As can be seen, the current normalized to ESA plots showed a remarkable increase nearly 7e8 fold higher activity for Pd deposited at 0.4 V than that of 0.5 V. For instance, the current density values of formic acid oxidation for Pd deposited at 0.4 V is 2300 mA cm2 relative to 300 mA cm2 obtained for the Pd deposited at 0.5 V, as evidenced from Fig. 8. This confirms, Pd dendrites deposited at 0.4 V showed high activity (higher current density) towards formic acid oxidation. Besides, it is observed from the voltammograms
Pd was electrodeposited on electrochemically activated carbon coated substrate at different potentials using potentiostatic deposition. SEM images showed that the early deposition at lower potentials (<0.5 V) resulted in the formation of dendritic structures. On the other hand, deposition at higher potentials (>0.5 V) resulted in the formation of irregular aggregates. XRD pattern displayed the typical growth of Pd crystal faces confirming the differences in morphological features for the Pd deposited at 0.4 V and 0.5 V in accordance with SEM. Cyclic voltammograms showed a predominant metal electrochemistry and sensitiveness of the peaks in the hydrogen region for Pd deposited at 0.4 V relative to the Pd deposited at 0.5 V, indicative of morphological difference. Moreover, the dendritic structures showed higher ESA than that of irregular spherical aggregates (128.5 vs 25.1 m2 g1), indicative of increased metal utilization and interconnectivity of metal nanoparticles. Comparison of the response for the formic acid oxidation suggests that, Pd deposited at 0.4 V showed higher activity than that of 0.5 V. The combination of these is believed to improve the utilization of the catalyst resulting in higher activities towards formic acid oxidation. Thus, it can be concluded that the present method, is a unique way to synthesize dendritic Pd structures on carbon support with high catalytic activity.
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Acknowledgements We thank the Department of Science and Technology (DST), Government of India (Grant No. SR/FT/CS-037/2008) for the financial assistance.
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[36] Soreta TR, Strutwolf J, Sullivan CKO. Electrochemically deposited palladium as a substrate for self-assembled monolayers. Langmuir 2007;124:10823e30. [37] Yu J, Fujita T, Inoue A, Sakurai T, Chen M. Electrochemical synthesis of palladium nanostructures with controllable morphology. Nanotechnology 2010;21:85601e7. [38] Maniam KK, Muthukumar V, Chetty R. Approaches towards improving the dispersion of electrodeposited palladium on carbon supports. Energy Procedia 2014;54:281e91. [39] Soreta TR, Strutwolf J, Henry O, Sullivan CKO. Electrochemical surface structuring with palladium nanoparticles for signal enhancement. Langmuir 2010;26:12293e9. [40] Patra S, Viswanath B, Barai K, Ravishankar N, Munichandraiah N. High-surface step density on dendritic Pd leads to exceptional catalytic activity for formic acid oxidation. ACS Appl Mater Interfaces 2010;2:2965e9. [41] Tian N, Zhou Z, Yu N, Wang L, Sun S. Direct electrodeposition of tetrahexahedral Pd nanocrystals with high-index facets and high catalytic activity for ethanol electrooxidation. J Am Chem Soc 2010;132:7580e1. [42] Zhou R, Zhou W, Zhang H, Du Y, Yang P, Wang C, et al. Facile template-free synthesis of pine needle-like Pd micro/ nanoleaves and their associated electro-catalytic activities toward oxidation of formic acid. Nanoscale Res Lett 2011;6:381e6. [43] Niu W, Xu G. Crystallographic control of noble metal nanocrystals. Nano Today 2011;6:265e85. [44] Hoshi N, Kuroda M, Hori Y. Voltammograms of stepped and kinked stepped surfaces of palladium: Pd (S)-[(111) (100)]
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Electrodeposition of dendritic palladium nanostructures on carbon support for direct formic acid fuel cells Kranthi Kumar Maniam, Volga Muthukumar, Raghuram Chetty* Department of Chemical Engineering, Indian Institute of Technology Madras, India
article info
abstract
Article history:
Palladium (Pd) dendrites were synthesized on electrochemically activated carbon (Vulcan
Received 1 October 2015
XC-72R) support by a simple template free potentiostatic (constant voltage) deposition.
Received in revised form
Scanning electron micrographs displayed a dendritic morphology when deposited at po-
25 November 2015
tentials <0.5 V (vs. RHE), whereas spherical aggregates were obtained at potentials >0.5 V.
Accepted 10 August 2016
Transmission electron micrographs and X-ray diffraction pattern confirmed the difference
Available online 31 August 2016
in morphological features and growth of Pd dendrites. Pd deposited at 0.4 V showed higher electrochemical surface area of 128.5 m2 g1 in comparison to 25.1 m2 g1 obtained for Pd
Keywords: Electrodeposition
deposited at 0.5 V and also displayed enhanced activity towards formic acid oxidation. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Formic acid oxidation Palladium Dendrites Electrochemical activation Fuel cells
Introduction Fuel cells which use small organic molecules such as methanol, ethanol, formic acid and formaldehyde are regarded as promising electrochemical systems for direct conversion of fuel into electrical energy for a wide range of portable and transportation applications [1]. Among the fuels employed, formic acid is considered to be a potential fuel because it has certain advantages such as (i) less crossover from anode to cathode, (ii) can be used in high concentrations, (iii) less toxic and (iv) has high open circuit potential (1.48 V) relative to methanol [2e4]. An improved understanding of the formic acid oxidation mechanism and development of an efficient
electrocatalysts underlie much of its research [5,6]. The oxidation mechanism of formic acid in direct formic acid fuel cell is described by two ways. One is “direct pathway” and other one is “indirect pathway”. In the “direct pathway”, formic acid is directly oxidized to CO2 without any intermediate step. In the “indirect pathway”, formic acid is first oxidized to CO and then to CO2 [7,8]. Though platinum is known to be the effective catalyst for oxidation of formic acid, strategies to avoid the indirect pathway requires further development. Since the oxidation of formic acid catalyzed by Pd electrocatalyst has been proven to be more active than Pt electrocatalyst [9,10], tailoring the morphology of Pd can be considered as one of the effective approaches to improve the oxidation reaction.
* Corresponding author. Fax: þ91 44 2257 4152. E-mail address: [email protected] (R. Chetty). http://dx.doi.org/10.1016/j.ijhydene.2016.08.064 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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A variety of micro/nano hierarchical architectures and structures of Pd such as cactoids, cubes, dendrites, flowers etc. have been prepared employing various synthesis routes. It has been shown that the as-synthesized Pd micro/nano structures exhibit a significant difference in catalytic activity toward electrooxidation of formic acid [11e13]. However, most of these synthesis routes employ organic surfactants and polymeric stabilizers at elevated temperature, which calls for a simple, economic, template and surfactant free method to develop the noble metal micro/nanostructures [14]. Considering the advantage of high degree of controllability towards the synthesis of desirable nanostructures, and electrode potential as a convenient parameter, electrodeposition is found to be one of the effective approaches [15,16]. Using electrons as the reducing agents, the metal precursor reacts on a conductive substrate which could primarily avoid the usage of stabilizers and surfactants. The supply of electrons can be easily controlled by applied potential and time [17]. Most of the recent literature reported on the synthesis of Pd dendrites and flowers use potentiostatic deposition on substrates such as glassy carbon, gold, indium tin oxide (ITO) etc. [18e21], which resulted in an increase in surface area and increased activity towards formic acid oxidation. Dendritic structures offer high porosity as well as better inter connectivity of the particles. These structures exhibited large electrochemical surface area and more active centers than other shapes. Moreover, dendritic structures have high surface roughness and surface steps which can contribute to the increased accessibility of reactant species and are believed to enhance the fuel cell reactions [22]. Since metal particles supported on carbon are employed as catalysts for a wide range of electrocatalytic applications, Pd with dendritic structures deposited on carbon-based substrates would be an ideal choice and advantageous for fuel cell applications. Though, there is a considerable ongoing research focused on the development of controlled Pt nanostructures on carbon based substrates [23,24], there is limited study which highlight the synthesis of electrodeposited Pd nanostructures on carbon based substrates. Despite the wide spread use of carbon as a catalyst support, the poor adsorption of metal precursors (due to hydrophobic nature of carbon) making the synthesis of hierarchical structures difficult. Considering the above, the present study aims at synthesizing Pd dendrites on a carbon support by potentiostatic (constant voltage) deposition which was previously subjected to an electrochemical activation process. Pd catalysts deposited at various deposition potentials were physically characterized and the activities were compared towards the electrooxidation of formic acid.
Experimental Electrochemically activated carbon black (Vulcan XC-72R) coated graphite electrodes (4 mm dia) were used for the deposition of Pd following the procedure reported in our previous publications [25]. In brief, graphite electrodes (4 mm) were coated with a thin layer of carbon black ink (Loading: 100 mg cm2), prepared by dispersing a known amount of carbon in a mixture of Nafion® (20 mL) and iso-propanol (1 mL)
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followed by ultrasonication for 30 min, and air dried. The resulting carbon coated substrate was subjected to electrochemical activation by potential cycling in N2 saturated 0.5 M H2SO4 for 100 cycles at a scan rate of 100 mV s1 [26]. Constant potential technique was employed for the deposition of Pd from 2 mM PdCl2 in 0.01 M HClO4 (PdCl2, Aldrich) solution on the electrochemically activated carbon based substrates. Potentials in the range of 0.4e0.7 V was chosen for the deposition. Pd loading was determined using ICP-OES measurements, from the concentrations of the precursor solution before and after electrodeposition. The deposited Pd catalysts were physically characterized by a field emission high resolution scanning electron microscopy (FE-SEM, Hitachi S4800) interfaced with an energy dispersive X-ray analysis, X-ray diffraction (XRD, Bruker D8 Discover X-ray diffractometer, Cu Ka source, l ¼ 1.54056 A) and transmission electron microscopy (TEM, Philips CM 12 model 120 kV). The catalysts were electrochemically characterized by cyclic voltammetry (CV) using a CHI 1100A electrochemical workstation (CH Instruments, Inc.) in a three electrodes cell housed in a BASi C3 cell stand. Pd electrodeposited on electrochemically activated carbon was used as the working electrode; Pt wire and Ag/AgCl(3 M NaCl) were used as counter and reference electrodes, respectively. Electrochemical tests were carried out at room temperature (23 ± 1 C) in 0.5 M H2SO4 solution and all potentials quoted in this work are referred to a reversible hydrogen electrode (RHE) scale. The current densities are referred to the geometrical area (0.125 cm2) of the electrode unless otherwise specified. CO stripping voltammograms were conducted in 0.5 M H2SO4 solution, in which electrodeposited Pd electrode is pre-adsorbed with carbon monoxide. Initially, CO was bubbled into 0.5 M H2SO4 solution for 20 min with the electrode potential fixed at 0.1 V. Then, the dissolved CO in solution was removed by bubbling N2 into the solution, and the stripping voltammograms were collected at a scan rate of 50 mV s1 [27]. The activity towards formic acid oxidation was studied in nitrogen saturated 0.5 M H2SO4 with 1 M HCOOH solution by scanning the potential from 0 V to 1.1 V at 20 mV s1.
Results and discussion It has been widely reported that the hydrophobic nature of untreated carbon materials affect the reaction characteristics of the active sites present on the catalyst surface due to the poor adsorption of catalyst precursors [28,29]. Therefore, carbon support are subjected to functionalization or surface treatment prior to metal particle deposition, which generally involve chemical oxidation of the support so as to make it hydrophilic [30e32]. The surface functional groups are responsible for anchoring the metal particles to the carbon support [27]. Electrochemical activation is one of the ways to introduce surface defects on carbon based support, and recently we have reported potential cycling of carbon in dilute acid (0.5 M H2SO4) facilitate the formation of surface functional groups [33].
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Effect of deposition potential Electrodeposition occurs by the process of nucleation and growth; nuclei appear at the active sites on a substrate according to nucleation rate, and then grow via the incorporation of further ions from the solution [34,35]. Deposition potential play a vital role during the electrochemical deposition of Pd as it is critical for controlling the nucleation and growth of structures [36,37]. In our study, Pd was deposited in the potential range of 0.4e0.7 V for 1800 s. Pd deposition potential range was chosen from the cyclic voltammogram of the electrochemically activated carbon support in acidic medium (0.01 M HClO4) containing chloride precursor (2 mM PdCl2) as reported in our previous publication [25]. In general, deposition of Pd is carried out at potentials near to the reductive peak potentials (i.e. 0.5 V in the present case) [38]. On decreasing the deposition potential below the reductive peak potential, the interference from hydrogen evolution with Pd deposition is expected to become more prominent [39]. Therefore Pd electrodeposition in acidic media is restricted to 0.4 V in this work. Fig. 1 shows the scanning electron micrographs of Pd electrodeposited at different potentials for 1800 s. Fig. 1a shows a representative SEM image of carbon without metal deposition. Pd when deposited at higher potentials i.e. at 0.7 and 0.6 V (Fig. 1b and c), spherical agglomerates are formed on the substrate. On decreasing the deposition potential, the
formation of quasi-spherical studded particles is observed at 0.5 V (Fig. 1d). When the deposition was carried out at 0.45 V (Fig. 1e), dendritic morphology is observed, while the deposition at 0.4 V (Fig. 1f), showed a denser coverage of dendrites. As evident from the SEM micrographs, a major transformation and difference in morphology was observed between the deposition potentials of 0.4 V and 0.5 V, hence in the rest of the work, Pd deposited at these two potentials are studied in detail for comparison. TEM imaging was used for the acquisition of morphological information of electrodeposited Pd as shown in Fig. 2. TEM micrographs were obtained by carefully scraping the electrodeposited catalyst from the graphite electrode and loaded on a carbon-coated copper grid. Fig. 2a shows the micrographs of Pd deposited at 0.4 V, where a single dendrite is seen distributed on the carbon support with porous structure. In contrast, Pd electrodeposited at 0.5 V (Fig. 2b) displayed an irregular spherical morphology. Microstructural investigations in Fig. 2c, d clearly revealed that Pd deposited at 0.4 V form a tapered dendritic structure with nanoscale corrugations on the surface, indicating that most of the dendrite branches consist of twins. This type of twinned growth was observed by Patra et al. [40] during the deposition of Pd dendrites on carbon paper by alternate current (AC) deposition. Pd dendrites seem to consist of many corrugations, which can provide high specific surface area.
Fig. 1 e (a) Scanning electron micrograph of Vulcan support without Pd deposition. Scanning electron micrographs of Pd catalyst electrodeposited on carbon support at different potentials for 1800 s: (b) 0.7 V, (c) 0.6 V, (d) 0.5 V, (e) 0.45 V, (f) 0.4 V. Insets for the (d), (e), (f) represent the higher magnification image. Electrodeposition bath consists of 2 mM PdCl2 in 0.01 M HClO4. Loading of carbon: 100 mg cm¡2. Number of electrochemical activation cycles before deposition: 100.
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Fig. 2 e Transmission electron micrographs of Pd catalyst electrodeposited on Vulcan support at different potentials for 1800 s: (a) 0.4 V, (b) 0.5 V and (c, d) higher magnification images of a single dendrite.
Fig. 3 compares the X-ray diffraction patterns of Pd deposited at 0.4 V and 0.5 V. The diffraction peak at 2q 40.1 , 47 , 68 corresponds to Pd (111), (200) and (220) planes of a typical face centered cubic (fcc) structure. Diffractions due to carbon appeared at 2q ~55 . As can be seen, there is an increase in intensity of (111) plane observed for the Pd deposited at 0.4 V relative to the Pd deposited at 0.5 V, suggesting and evidencing the dendritic feature of the metal deposited at lower reduction potentials. Similar XRD pattern for Pd dendrite was reported by several research groups [41e43].
in relation to hydrogen adsorption sites and metal oxide formation [44]. Fig. 4 compares the cyclic voltammograms of Pd electrodeposited at 0.4 V and 0.5 V in nitrogen saturated 0.5 M H2SO4 at a sweep rate of 50 mV s1 and room temperature. As can be seen, Pd surface electrochemistry representing the typical Hþ adsorption/desorption, oxide formation/stripping peaks are observed in both the cases. Pd deposited at 0.4 V displayed a predominant metal electrochemistry relative to
Pd oxide formation
3
Cyclic voltammetry (CV) provides a fundamental understanding of the metal surface electrochemistry, and the voltammetric peaks of Pd in acidic electrolyte are generally discussed
Current density, mA/cm
2
Electrocatalytic study
2
H desorption
1 0 -1 -2
Pd oxide reduction
-3 -4
Pd deposited at 0.4 V Pd deposited at 0.5 V
H adsorption
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential, V vs RHE
Fig. 3 e X-ray diffraction pattern of Pd catalyst electrodeposited on Vulcan support at 0.4 V and 0.5 V.
Fig. 4 e Comparison of the cyclic voltammograms of the Pd catalysts in nitrogen saturated 0.5 M H2SO4, deposited at 0.4 V (dashed lines) and 0.5 V (solid lines) on carbon support at a scan rate of 50 mV s¡1 and room temperature. Vulcan support was subjected to 100 cycles of electrochemical activation in nitrogen saturated 0.5 M H2SO4 prior to deposition. Electrodeposition bath consists of 2 mM PdCl2 in 0.01 M HClO4. Loading of support on graphite substrate: 100 mg cm¡2. Deposition time: 1800 s.
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the deposition at 0.5 V. First, it can be seen that the electrode exhibit the well-known hydrogen adsorption (Hads) and desorption (Hdes) in the potential region of 0e0.2 V, and Pd oxide formation and reduction within 1.3 to 0.6 V [45]. However, it is worth to note from the voltammograms (shown in Fig. 4) that there is a noticeable difference in the existence of peaks at 0 V and 0.2 V in nitrogen saturated 0.5 M H2SO4 solution for the Pd catalysts deposited at 0.4 V and 0.5 V. The sensitiveness of the peaks in the hydrogen region represents the differences in morphological features and Pd crystal faces. These further evidence the fact that deposition potential not only influences the morphological feature of Pd deposited but also effects the interaction between hydrogen and Pd. Electrochemical surface area (ESA) of Pd catalyst was evaluated using the areas of hydrogen adsorption/desorption after deduction of the double-layer region by adapting the assumption that 212 is the charge (mC cm2Pd) required to oxidize a monolayer of hydrogen on a smooth Pd surface [38]. The coulombic charge for hydrogen adsorption/desorption (Qa/Qd) was used to calculate the active palladium surface of the electrodes. The calculated ESA of Pd catalysts deposited at 0.4 V and 0.5 V are tabulated in Table 1. As can be seen, Pd deposited at 0.4 V display ESA of 122.4 m2 g1, which is higher than that of the Pd deposited at 0.5 V (32.5 m2 g1), indicative of the presence of more active centers/sites available for reaction. This could be due to the presence of many nanoscale corrugation and increased interconnectivity of Pd dendrites when deposited at 0.4 V [45]. CO stripping voltammetry was carried out for the Pd catalysts deposited at 0.4 V and 0.5 V to validate the electrochemical surface area. Fig. 5a shows the CO stripping voltammogram of Pd deposited at 0.4 V and the subsequent cyclic voltammogram in 0.5 M H2SO4 at a scan rate of 50 mV s1. Similar voltammogram was also observed for Pd deposited at 0.5 V as shown in Fig. 5b. The voltammogram showed a single oxidation peak during the first scan, whereas no CO oxidation was observed in the second scan confirming the complete removal of the adsorbed CO species [32]. The appearance of a shoulder peak in addition to the single oxidation peak for the Pd deposition at 0.5 V, can be attributed to the oxidation of weakly absorbed CO in accordance with the reported literature [4]. The ESA values was determined from CO stripping voltammetry, using the CO oxidation charge after subtracting the background current of the subsequent CV curves with the assumption of 430 mC cm2 as the oxidation charge for one monolayer of CO on a smooth Pd surface [32]. Pd deposited at 0.4 V displayed higher ESA of 128.5 m2 g1 compared to that of Pd deposition at 0.5 V (25.1 m2 g1), confirming the presence of more active surface sites on Pd deposited at 0.4 V. The ESA values obtained from CO stripping voltammetry are in relatively fair agreement with those obtained from hydrogen adsorption/desorption experiments.
Fig. 5 e Comparison of the CO stripping voltammograms at a scan rate of 50 mV s¡1 for Pd catalysts deposited at (a) 0.4 V and (b) 0.5 V on carbon support.
Formic acid oxidation Pd supported on carbon has its unique ability to catalyze the oxidation of formic acid in fuel cells [46e48]. In this context, morphology of Pd plays a vital role on the electrocatalytic activity. To understand the influence of Pd morphology, the performance of Pd catalysts deposited at 0.4 V and 0.5 V towards formic acid oxidation was compared by voltammetry and the results are shown in Fig. 6. As can be seen, a significant enhancement in the electrocatalytic activity towards formic acid oxidation was observed for the Pd deposited at 0.4 V relative to the deposition at 0.5 V. Fig. 7 compares the mass specific activity of both the catalysts, the peak current density for Pd deposited at 0.4 V was higher than that of Pd deposited at 0.5 V (65 mA mg1 vs 45 mA mg1). The mass specific current density for formic acid oxidation for the electrodeposited Pd at 0.4 V also increased compared to the deposition obtained at 0.5 V. In order to evaluate the intrinsic activity of electrocatalysts, the
Table 1 e Comparison of the morphology and electrochemical surface area (ESA) of Pd catalyst electrodeposited on Vulcan support at 0.4 and 0.5 V for 1800 s. Deposition potential (V) 0.4 0.5
Morphology of Pd
ESA from Hþ ads þ des (m2 g1)
ESA from CO stripping (m2 g1)
Dendritic Irregular spherical aggregates
122.4 32.5
128.5 25.1
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2.0
Current density, mA/cm
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1.2 0.8 0.4 0.0 -0.4 0.0
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Potential, V vs RHE
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Fig. 6 e Comparison of the activities of the Pd catalysts deposited at (a) 0.4 V (dashed lines) and (b) 0.5 V (solid lines) on electrochemically activated carbon based support in N2 saturated 1 M HCOOH in 0.5 M H2SO4 at a scan rate of 20 mV s¡1 and room temperature.
that Pd deposited at 0.4 V has broader peak in the negative scan (at ~0.7 V) than that of 0.5 V, which evidence the better stripping of the absorbed intermediates for Pd deposition at 0.4 V [49,50]. The improved performance of Pd deposition at 0.4 V can be attributed to the combination of dendritic morphology with better interconnectivity of metal particles and to enhanced CO tolerance [51]. Thus it can be concluded that tailoring the morphology of Pd can improve the metal utilization and result in higher catalytic activity.
Mass specific activity, mA/mg
70
(a)
60
(b)
50
Fig. 8 e Comparison of the specific surface activities of the Pd catalysts deposited at (a) 0.4 V (dashed lines) and (b) 0.5 V (solid lines) on electrochemically activated carbon based support in N2 saturated 1 M HCOOH in 0.5 M H2SO4 at a scan rate of 20 mV s¡1 and room temperature.
40 30 20
Conclusion 10 0 0.0
0.2
0.4
0.6
0.8
1.0
Potential, V vs RHE Fig. 7 e Comparison of the mass specific activities of the Pd catalysts deposited at (a) 0.4 V (dashed lines) and (b) 0.5 V (solid lines) on electrochemically activated carbon based support in N2 saturated 1 M HCOOH in 0.5 M H2SO4 at a scan rate of 20 mV s¡1 and room temperature.
formic acid oxidation current is normalized to ESA values and the specific surface activity results are presented in Fig. 8. As can be seen, the current normalized to ESA plots showed a remarkable increase nearly 7e8 fold higher activity for Pd deposited at 0.4 V than that of 0.5 V. For instance, the current density values of formic acid oxidation for Pd deposited at 0.4 V is 2300 mA cm2 relative to 300 mA cm2 obtained for the Pd deposited at 0.5 V, as evidenced from Fig. 8. This confirms, Pd dendrites deposited at 0.4 V showed high activity (higher current density) towards formic acid oxidation. Besides, it is observed from the voltammograms
Pd was electrodeposited on electrochemically activated carbon coated substrate at different potentials using potentiostatic deposition. SEM images showed that the early deposition at lower potentials (<0.5 V) resulted in the formation of dendritic structures. On the other hand, deposition at higher potentials (>0.5 V) resulted in the formation of irregular aggregates. XRD pattern displayed the typical growth of Pd crystal faces confirming the differences in morphological features for the Pd deposited at 0.4 V and 0.5 V in accordance with SEM. Cyclic voltammograms showed a predominant metal electrochemistry and sensitiveness of the peaks in the hydrogen region for Pd deposited at 0.4 V relative to the Pd deposited at 0.5 V, indicative of morphological difference. Moreover, the dendritic structures showed higher ESA than that of irregular spherical aggregates (128.5 vs 25.1 m2 g1), indicative of increased metal utilization and interconnectivity of metal nanoparticles. Comparison of the response for the formic acid oxidation suggests that, Pd deposited at 0.4 V showed higher activity than that of 0.5 V. The combination of these is believed to improve the utilization of the catalyst resulting in higher activities towards formic acid oxidation. Thus, it can be concluded that the present method, is a unique way to synthesize dendritic Pd structures on carbon support with high catalytic activity.
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Acknowledgements We thank the Department of Science and Technology (DST), Government of India (Grant No. SR/FT/CS-037/2008) for the financial assistance.
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