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electrodeposited on different carbon supports such as Vulcan XC-. 72R (VCX), carbon nanotubes (CNTs) and wood apple shell carbon (WASC), and presents a ...
ECS Transactions, 61 (12) 11-20 (2014) 10.1149/06112.0011ecst ©The Electrochemical Society

Palladium Nanodendrites Deposited on Electrochemically Activated Carbon Based Support for Electrocatalytic Applications K. K. Maniama and R. Chettya a

Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai, 600036, India

This paper focuses on the morphological nature of Pd electrodeposited on different carbon supports such as Vulcan XC72R (VCX), carbon nanotubes (CNTs) and wood apple shell carbon (WASC), and presents a comparative assessment of the catalytic activity on different supports towards formic acid (HCOOH) oxidation and oxygen reduction reaction (ORR) in lowtemperature fuel cells. The carbon supports subjected to an electrochemical activation by potential cycling in an acidic media followed by deposition of Pd showed dendrite formation as confirmed by scanning electron micrographs irrespective of the morphology of carbon material. Pd dendrites on WASC showed the highest electrochemical surface area of 86.1 m2g-1 among the electrocatalysts studied. The carbon supported Pd dendrite showed the following order towards the activity of formic acid oxidation: WASC > VXC > CNTs, whereas for oxygen reduction reaction the activity was in the order CNTs > WASC > VXC. The results suggest the need for different carbon support for anode and cathode reaction of direct liquid fuel cells.

Introduction

Development of noble metal nanostructures on carbon support to improve the catalytic activity plays a key role in fuel cell research, and electrochemical deposition offers a simple and feasible way of synthesizing nanostructures (1, 2). On the other hand, considerable research is being focused on the development of new carbon materials as catalyst supports with improved stability and durability for low temperature fuel cells. In the recent years, carbon materials such as carbon nanotubes (CNTs), carbon nanofibers (CNFs) and graphene etc. showed enhanced catalytic activities when used as electrocatalyst support towards fuel cell reactions (3-5). The significant advantages over traditional supports such as high surface to volume ratio, excellent electrical conductivity, impurity free surface and resistance to corrosion motivated the research community to investigate noble metals (such as platinum and palladium) deposition on these newly developed carbon materials for low temperature fuel cells (6,7).

It is expected that nanostructured catalyst will lead to better utilization of noble metals and hence low electrode costs (8, 9). The development of noble metal structures on different carbon supports remains to be of particular interest and is likely to play a

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significant role in fuel cell catalysis. Moreover, palladium is seen as ideal alternative to Pt and has been widely used in various electrochemical reactions due to the relatively abundant and less expensive resource (10). Besides, studies on the morphological nature of Pd catalyst structures on different carbon supports remained unexplored. The development of dendritic Pd structures on different carbon supports may open up new opportunities for catalysing the electrochemical reactions in fuel cells and hydrogen storage. The geometrical structure of carbon support is expected to play an important role on the morphology of Pd and in turn on the performance of low temperature polymer electrolyte membrane (PEM) fuel cells (11-13). Because of the hydrophobic nature of untreated carbons, they are subjected to surface modification/functionalization, which involve oxidation of carbon surface so as to make it hydrophilic (14). Surface modification by electrochemical activation improves the hydrophilicity of carbon black surface and favors a good deposition (15). However, most of the reported work showed, deposition of metals on carbon black favored a good deposition with well dispersed “spherical” morphology (16, 17). In this study, we report on the synthesis of Pd with “dendrite” morphology by electrochemically activating the carbon supports prior to metal deposition. Carbon supports used in this work are: Vulcan XC-72R (VXC), carbon nanotubes (CNTs) and wood apple shell carbon (WASC, Limonia Acidissima; a waste biomass as a raw material to produce activated carbon). The main focus is to assess the morphological nature of Pd deposition on different carbon supports (which has varying morphologies), and compare their catalytic activity towards formic acid (HCOOH) oxidation and oxygen reduction reaction (ORR) in fuel cells. Experimental Graphite electrodes (4 mm) were coated with a thin layer of carbon (VXC, CNTs, or WASC). 5 mg of the carbon support was dispersed in a mixture of Nafion® (20 µL), isopropanol (1mL) and ultrasonically blended for 30 min, and a known amount of the ink was dropped on the surface of a polished graphite electrode and air dried. The carbon coated substrate was subjected to an electrochemical activation process in nitrogen saturated 0.5 M H2SO4 solution at a sweep rate of 100 mV s-1 for 100 cycles as reported in our previous publication (17). The resulting carbon coated and electrochemically activated substrate was used for the deposition of Pd. Electrodeposition of Pd on the carbon coated graphite electrode was carried out in an inert atmosphere at room temperature using 2 mM PdCl2 in 0.01 M HClO4 (PdCl2, Aldrich) solution by potential cycling between 0 to 1.3 V at a scan rate of 20 mV s-1 for 10 to 25 cycles. Pt mesh and Ag/AgCl (3M NaCl) were used as counter and reference electrodes, respectively. All potentials quoted in this work are referred to a reversible hydrogen electrode (RHE) scale. Preparation of activated carbon from wood apple shell is reported elsewhere (6). In brief, a known amount of dried shells of wood apple fruit is soaked in 50 wt.% KOH solution for 2 h followed by decantation, drying and subjecting to a thermal treatment in an inert (nitrogen) atmosphere at 800 °C for 2 h. The char material thus obtained was subsequently treated with conc. HNO3 and filtered. Pd loading was determined using ICP-OES measurements, from the concentration of 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.

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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(3M NaCl) were used as counter and reference electrodes, respectively. Electrochemical tests were carried out at room temperature (23 ± 1oC) 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 of the electrode (0.125 cm2) unless otherwise specified. The performance of electrodeposited Pd catalyst towards towards formic acid oxidation was studied in nitrogen saturated 0.5 M H2SO4 with 1 M HCOOH solution by scanning the potential from 0.0 V to 1.1 V. The activity towards oxygen reduction reaction (ORR) was evaluated by linear sweep voltammetry (LSV) in an oxygen saturated 0.5 M H2SO4 solution by scanning the potential from 1.3 V to 0.0 V.

Results and Discussion Scanning Electron Microscopy (SEM) The implementation of the present approach of synthesizing Pd dendrites by electrochemical deposition on the carbon supports mentioned above is shown in Figure 1. The figure shows the SEM images of Pd electrodeposited on different carbon materials, which was subjected to electrochemical activation in nitrogen saturated 0.5 M H2SO4 solution for 100 cycles at a scan rate of 100 mV s-1. The micrographs confirm the formation of Pd dendrites on different carbon materials namely: VXC (Figure 1a), CNTs (Figure 1b) and WASC (Figure 1c). A representative EDX image shown in Figure 1d confirms the presence of Pd. As can be seen from the Figure 1, Pd with dendritic morphology is formed on all the electrochemically activated carbon supports. In accordance with the SEM images, it can thus be considered that the carbon substrates subjected to electrochemical activation process by potential cycling in an acidic electrolyte followed by the cyclic deposition of Pd from chloride precursor can be considered as a universal approach for the synthesis of dendritic nanostructures on carbon substrates irrespective of nature (morphology) of the carbon material. On comparing the morphologies of Pd on different carbon supports, it can be observed that, the morphology of dendrites formed on WASC were slightly different than that of VXC or CNT. Pd dendrites on VXC and CNT showed continuous long dendrites, whereas Pd on WASC showed a dispersed dendritic morphology. SEM image from Figure 2 evidence the porous nature of wood apple carbon. It is generally believed that the porous nature of the amorphous materials like WASC leads to large internal surface area per unit weight and facilitate the dispersion of noble metal particles during the electrochemical deposition (6, 18). Deposition of Pd on WASC support might have resulted in discrete nuclei, which could have resulted in Pd dendrites with good dispersion. Slow scan cyclic deposition, uniform electric field between the working and counter electrode, and the perchlorate ions present in the supporting electrolyte could have assisted the growth along perpendicular direction, accelerating the noble metal nanoparticle coalescence and

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3D growth thereby resulting in the formation of dendrites with dispersion. These evidence that the morphology of carbon support plays a crucial role on the formation of Pd dendritic structures either with or without dispersion.

(a)

(b)

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Figure 1. Scanning electron micrograph of Pd deposited on an electrochemically activated carbon based substrates: (a) Vulcan XC-72R (b) Carbon nanotubes and (c) Wood apple shell carbon. (d) Representative EDX image showing the presence of Pd. Carbon based substrates were 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 carbon on graphite substrate: 400 µg cm-2. Electrochemical Surface Area (ESA) Electrochemical surface area (ESA) provides information on the available active sites of catalysts, which account for the access of a conductive path available to transfer electrons to and from the electrode surface (19). The ESA of Pd catalyst was evaluated using the hydrogen underpotential deposition method. The electrochemical surface area can be calculated from the areas of hydrogen adsorption/desorption after deduction of the double-layer region (17). The calculated ESA of Pd dendritic structures on different supports are tabulated in Table 1. As can be seen from the table, Pd deposited on WASC (86.1 m2g-1) and CNTs (47.1 m2g-1) showed higher ESA values than VXC (41.6 m2g-1).

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The high ESA of the Pd dendrites on WASC can be attributed to the porous nature of the support which facilitate the dispersion of Pd dendrites

TABLE 1. Comparison of the Electrochemical Surface Area (ESA) Values of the Pd Dendrites on Different Carbon Supports. Type of Carbon ESA (m2g-1) Vulcan XC-72R 41.6 Carbon nanotubes 47.1 Wood Apple Shell Carbon 86.1

Figure 2. Scanning electron micrograph of the wood apple shell carbon.

Formic Acid Oxidation Pd supported on carbon has its unique ability to catalyze the oxidation of formic acid in fuel cells (20). To evaluate the performance of the electrodeposited Pd catalysts, cyclic voltammograms are recorded in nitrogen saturated 1 M HCOOH in 0.5 M H2SO4 in the potential range from 0 V to 1.1 V at a scan rate of 100 mV s-1 and room temperature. Figure 3 shows the comparison of the cyclic voltammograms of electrodeposited Pd on different carbon support. Shape of the voltammogram of Pd deposited on different carbon supports is as expected for Pd with the existence of one forward oxidation peaks and one backward oxidation peak. The first peak at ~ 0.3 V corresponds to the direct oxidation of formic acid leading to the formation of CO2 as the end product. A minor hump observed at about 0.65 V is due to Pd oxidation. As a result, there is a decrease in activity and drop in the oxidation current. On reversing the scan, a peak is observed at ~ 0.7 V. wherein the anodic current densities arise due to reduction of palladium oxides, exposing fresh Pd atoms sites for the electro-oxidation of formic acid (21). The peak appeared at ~ 0.7 V evidence the stripping of the absorbed intermediates and regeneration of the Pd surface on carbon-based supports (22). The onset potential for formic acid oxidation is nearly 50 mV higher for the CNTs and WASC in comparison to the conventional VXC support. This suggests that changing the catalyst support also has a positive effect on the onset potential. Pd dendrites deposited on WASC showed the highest activity towards HCOOH oxidation.

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Figure 3. Cyclic voltammograms of Pd catalysts deposited on different carbon based support in nitrogen saturated 0.5 M H2SO4 + 1 M HCOOH, at a scan rate of 100 mV s-1: (a) Vulcan XC -72 R, (b) Carbon nanotubes and (c) Wood apple shell carbon. In order to evaluate the intrinsic activity of electrocatalysts, the HCOOH oxidation current is normalized to ESA values and the results are presented in Figure 4. As can be seen, the current normalized to ESA of these catalysts showed the same order as of the currents normalized to geometrical surface area. From the above results, Pd deposited on wood apple carbon coated support showed the highest activity (current density) towards HCOOH oxidation. The electrocatalytic activities followed the order in terms of peak current densities: WASC (5 mA cm-2) > VXC (1.4 mA cm-2) > CNTs (1.05 mA cm-2). Pd deposited on wood apple carbon has a beneficial effect on the catalytic activity toward formic acid oxidation over that of the Pd deposition on VXC support. Besides, it is observed from the voltammograms that Pd dendritic structures deposited on WASC has broader peak in the negative scan (at ~ 0.7 V) than that of the Pd deposition on other carbon supports. These clearly evidence the better stripping of the absorbed intermediates and regeneration of the Pd surface (22, 23) on WASC and activity towards formic acid oxidation. The improved performance of Pd dendrites on WASC towards the formic acid oxidation can be attributed to two factors. One is the better CO tolerance and the other is the porous nature of WASC support. Improved CO tolerance can be evidenced from the CVs (observed in terms of stripping of the adsorbed intermediates and regeneration of the Pd surface) shown in Figures 3 and 4. Porous nature of the carbon material is expected to

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offer an effective metal utilization which can be evidenced from the higher ESA and peak current density values. Thus it can be concluded that the use of WASC as a support for Pd improves the metal utilization resulted in higher activities towards the electrooxidation of formic acid with better CO tolerance than the other carbon supports.

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Figure 4. Normalized cyclic voltammograms of the Pd catalysts deposited on different carbon based support in nitrogen saturated 0.5 M H2SO4 + 1 M HCOOH, at a scan rate of 100 mVs-1: (a) Vulcan XC-72R, (b) CNT, (c) Wood apple shell carbon.

Oxygen Reduction Reaction The activity of Pd catalyst deposited on different carbon support was also evaluated towards cathodic oxygen reduction reaction (ORR) of fuel cells. Figure 5 compares the linear sweep voltammetry (LSV) recorded for electrodeposited Pd on different carbon supports in 0.5 M H2SO4 solution saturated and blanketed with oxygen at a scan rate of 5 mV s-1 and room temperature. Comparison of cathodic current density values in the presence of oxygen showed highest reduction current for Pd deposited on CNTs. Inset in Figure 5 compares the current density values taken at 0.5 V. As can be seen, Pd deposited on CNTs and WASC displayed higher catalytic activity towards the reduction of oxygen than Pd/VXC. The order of catalytic activities is as follows: CNTs (1.45 mA cm-2) > WASC (0.9 mA cm-2) > VXC (0.24 mA cm-2). The ESA of Pd estimated from underpotential deposition method was also used to normalize the ORR current, and the results are shown in Figure 6.

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It can be seen that the surface area-specific activities of these catalyst are of the same order as their specific activities (currents normalized to geometrical surface area). Inset in Figure 6 which compares the normalized current density values, taken at 0.5 V showed higher reduction current for Pd dendrites on CNTs. On the other hand, it can be observed that the Pd dendrites on WASC show better activity than that of Pd dendrites on VXC. This evidence the fact that alternative to VXC like CNT or carbon microspheres like WASC can induce significant changes in improving the catalytic activity towards ORR.

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Potential, V vs RHE Figure 5. Linear sweep voltammograms of Pd catalysts in oxygen saturated 0.5 M H2SO4 deposited on different carbon based substrates, at a scan rate of 5 mVs-1: (a) Vulcan XC72R, (b) CNTs, (c) Wood apple shell carbon. Inset shows the current density values taken at 0.5 V. Pd dendrites on CNTs showed higher activities towards the reduction of oxygen than the Pd dendrites on other carbon supports. The tubular morphology of CNTs could allow better access to the gases (oxygen) than the VXC which has randomly distributed pores of different sizes (24, 25). On the other hand, highly porous WASC with microsphere nature might result in excessive flooding of the electrode caused by the water formation (product) during oxygen reduction (26). These results suggest the need for different carbon support for the anode and the cathode reaction of the fuel cells in low-temperature direct liquid fuel cells. Pd dendrites deposited on CNTs has a beneficial effect on the catalytic activity toward oxygen reduction reaction, while Pd dendrites on WASC showed higher activity towards formic acid oxidation. Further work will be carried out to test these electrocatalysts in single fuel cell set-up.

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Potential, V vs RHE Figure 6. Normalized linear sweep voltammograms of Pd catalysts in oxygen saturated 0.5 M H2SO4 deposited on different carbon based support, at a scan rate of 5 mV s-1: (a) Vulcan XC-72R, (b) Carbon nanotubes, (c) Wood apple shell carbon. Inset shows the current density values taken at 0.5 V.

Conclusions

Pd dendrites were synthesized by subjecting carbon-based supports to an electrochemical activated process in acidic media followed by electrodepostion in PdCl2 solution. SEM images confirmed the formation of dendritic Pd on the supports irrespective of the nature (morphology) of the carbon material and evidenced that the adopted methodolgy can be considered as a universal approach for the synthesis of dendritic nanostructures. Pd dendrites on wood apple shell carbon showed increased activity towards formic acid oxidation, while Pd dendrites deposited on carbon nanotubes showed beneficial effect toward oxygen reduction reaction, suggesting the need for different carbon support for anode and cathode reaction of the fuel cells.

Acknowledgments The authors would like to thank Indian Institute of Technology (IIT) Madras for the financial support.

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