Abstract
The need to lower the construction cost of fuel cells calls for the development of non-Pt based electrocatalysts. Among others, Pd has emerged as a promising alternative to Pt for fuel cell catalysis. This research aims to investigate the synthesis and characterization of nanostructured Pd-based catalysts dispersed on carbon support as anode materials in direct ethanol fuel cells. For the preparation of the first Pd-based electrocatalyst, palladium nanoparticles (NPs) were synthesized via oleylamine (OAm)-mediated synthesis and precursor method with a mean particle size of 3.63 ± 0.59 nm as revealed by transmission electron microscopy (TEM). Carbon black was used as a supporting matrix for the OAm-capped Pd NPs. Thermal annealing and acetic acid washing were used to remove the OAm capping agent. To evaluate the electrocatalytic activity of the prepared electrocatalyst towards ethanol oxidation, cyclic voltammetry (CV) studies were performed using 1.0 M ethanol in basic medium. The CV data revealed the highest peak current density of 11.05 mA cm−2 for the acetic acid-washed Pd/C electrocatalyst. Meanwhile, the fabrication of the second Pd-based electrocatalyst was done by functionalization of the carbon black support using 3:1 (v/v) H2SO4:HNO3. The metal oxide, NiO, was deposited using precipitation method while polyol method was used for the deposition of Pd NPs. X-ray diffraction (XRD) analysis revealed that the estimated particle size of the synthesized catalysts was at around 9.0–15.0 nm. CV results demonstrated a 36.7% increase in the catalytic activity of Pd–NiO/C (functionalized) catalyst towards ethanol oxidation compared to the non-functionalized catalyst.
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1. Introduction
Fuel cells are becoming the subject of intense applied research compared to conventional power sources, such as internal combustion engines or batteries because of their high energy density, low-temperature operation and low pollution. Alcohol, specifically methanol, is often used as possible fuel for mobile applications because of its availability, simplicity, and its ease of storage as a liquid [1]. However, its toxicity and the possible environmental problems in relation to its miscibility with water have always been a problem. For these reasons, ethanol as an alternative fuel has been studied. Ethanol has a higher theoretical energy density than methanol and it can be produced in large quantities from agricultural products. Despite the attraction that direct ethanol fuel cells (DEFCs) have been receiving, there is still a need for their development by exploring for a highly active catalyst for the ethanol oxidation reaction that takes place at the anode. Studies showed that the kinetics of both alcohol anodic oxidation and the oxygen cathodic reduction are significantly improved in basic electrolyte, thus making it possible to use Pt-free electrocatalysts and to reduce the catalyst loading [2]. At present, palladium emerges as the most attractive replacement for platinum in direct alcohol fuel cells because it is fifty times more abundant in nature and less expensive [3]. The activity of an electrocatalyst is dependent on the size and the dispersion of metal particles. Preparation methods have been investigated to synthesize catalyst with well-dispersed and nano-sized metal particles [4, 5].
There is also a need for the addition of a secondary metal to improve the electrocatalysis of ethanol and to completely oxidize the remaining adsorbed intermediates; thus, a complete oxidation of ethanol to CO2 can be achieved. Shen and Xu [6] reported that the performance of Pd/C electrocatalysts promoted with oxides (CeO2, Co3O4, Mn3O4 and NiO) is superior to Pt-based electrocatalysts in terms of activity and poisoning tolerance in alkaline oxidation of methanol, ethanol, glycerol and ethylene glycol.
Carbon black is frequently used as the catalyst support because of its relatively high stability in both acid and basic media, good electronic conductivity and high specific area [5]. Since the support material has a strong influence on the properties of the catalysts, carbon black should be modified at the micro(nano)scopic level. To accomplish this, chemical modification of the carbon black surface should be done to anchor the nanoparticles/electrocatalysts; thus, modifying their reactivity and catalytic properties [7].
This study describes the synthesis and characterization of nanostructured Pd-based catalysts dispersed on carbon support as potential anode materials relevant to DEFC applications.
2. Experimental
2.1. Synthesis of Pd-based electrocatalysts
2.1.1. Pd nanoparticles.
The synthesis of Pd nanoparticles (NPs) was done according to a method reported in the literature with minor modifications [4]. The Pd NPs were synthesized by the reduction of the palladium (II) acetylacetonate precursor in oleylamine (OAm) and borane-tert-butylamine (Sigma-Aldrich) under N2 atmosphere. Transmission electron microscopy (TEM) images of the NPs were obtained using a transmission electron microscope (Carl Zeiss Libra 120). The prepared Pd NPs were supported on carbon (Vulcan®XC-72) by sonication of an equal mixture of the prepared colloidal metal NPs and the supporting matrix. This was dispersed in hexane for two hours, which then becomes the metal/support suspension.
Two different methods to remove the capping agent, OAm (Sigma-Aldrich), were studied: acetic acid washing and thermal annealing. For the acetic acid washing method, acetic acid (JT Baker) was added to the metal/support dispersion and heated for 10 h at 70 °C. Energy dispersive x-ray (EDX) analysis was done to verify the removal of the capping agent. Based on the thermal properties of OAm, the thermal annealing method proposed elsewhere [8] was also employed to activate the metal NPs. The supported metal NPs were heated in an oven at 185 °C for 5 h.
2.1.2. PdNiO catalyst.
The synthesis of PdNiO catalyst was done as follows. A mass of 0.5 g of carbon black (Vulcan®XC-72) was dissolved in 25 ml of 3:1 (v/v) H2SO4: HNO3. The solution was ultrasonicated for 1 h and was refluxed for 3 h in an oil bath at 140 °C. The resulting solution was filtered through vacuum filtration. The residue was washed with ultrapure water to remove the acid traces then it was dried at 70 °C for 12 h. NiO was supported on functionalized Vulcan®XC-72 by precipitation method. A computed volume of 0.2 M Ni(NO3)2.6H2O (0.69 ml) was introduced to a suspension of functionalized Vulcan®XC-72 (195 mg) in ultrapure water. The NiO loading amount was fixed at 5 wt% based on a previous study [9]. A pH of 10 was established using ammonia solution. The mixture was stirred vigorously for 3 h. After filtering and washing the precipitation mixture with ultrapure water, the solid residue was dried in vacuum for 12 h. The dried residue was then calcined at 300 °C for 3 h in the oven to form NiO–C. Pd was added onto the prepared NiO–C using the polyol method. The Pd loading amount was fixed at 20 wt% [9]. To 20 mg C–NiO, 1.76 ml of 0.02 M of PdCl2 in ethylene glycol was added. The pH was increased to 13.0 ± 0.1 using 2.5 M NaOH. The mixture was then refluxed at 160 °C for 3 h in an oil bath. After cooling to room temperature, 3 M of HCl solution was added to the mixture in order to lower the pH to 2.0 ± 0.1. The fabricated Pd/NiO–C was filtered, washed with ultrapure water, and dried in vacuum oven for 12 h at 65 °C.
2.2. Electrochemical characterization of the synthesized electrocatalysts
Metal/support solutions in ultrapure water were prepared and a 20 μl of this solution was dropcast on a clean glassy carbon electrode and dried in an oven at 60 °C. An additional layer of nafion (0.1 wt%) was drop-cast on the electrode to seal the NPs in place and dried at 60 °C in an oven. The prepared working electrode was then connected to a potentiostat (EDAQ, Australia) for cyclic voltammetry (CV) studies. The electrocatalytic activity of the prepared electrodes towards ethanol oxidation in alkaline medium was characterized by CV in 1.0 M ethanol + 1.0 M NaOH at a potential range of −900–300 mV (versus Ag/AgCl) for 50 cycles at a scan rate of 50 mV s−1. Electrochemical measurements were performed in a conventional three-electrode single compartment electrochemical cell at room temperature, under nitrogen (N2) atmosphere after purging the solution for at least 30 min.
3. Results and discussion
3.1. Pd NPs/carbon composite
The synthesized Pd NPs are monodispersed with size of 4 nm as shown in figure 1. OAm played an essential role during the synthesis of the metal NPs as it controlled the growth of the NPs, acted as a solvent and reductant, and prevented the metals from agglomeration as shown in the TEM image. However, the capping agent, OAm, also blocks the catalytic sites of the NPs [4] and is thus removed by two methods: acetic acid (AA) washing and thermal annealing (TA). The removal of OAm was achieved by acid–base reaction in the AA washing method and by evaporating OAm (flash point around 160 °C) in the TA method [8]. EDX analysis revealed that OAm was indeed removed by the AA washing method.
Figure 1. TEM image of Pd NPs at 63 000 × magnification with average size of 3.63 ± 0.59 nm.
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Standard imageCV studies were performed to evaluate the electrocatalytic activity of the prepared electrocatalysts towards ethanol oxidation. The cyclic voltammograms of all prepared electrodes are shown in figure 2. The AA-washed and TA Pd/C and Pt/C modified-electrodes are both electro-catalytically active towards ethanol oxidation. However, AA-washed Pd/C composite gave the highest electrocatalytic activity towards ethanol oxidation in basic medium with a peak current density of 11.05 mA cm−2.
Figure 2. Cyclic voltammograms of carbon-supported metallic NPs subjected to two different methods of OAm removal AA washing and TA.
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Standard image3.2. PdNiO/carbon composite
The inert surface of carbon requires suitable chemical modification in order to interact favorably with the catalyst particles. One method of chemical modification is functionalization of the carbon surface with concentrated mineral acids (e.g. 3:1 (v/v) H2SO4:HNO3). With the functionalization process, the carbon black became much more hydrophilic and the dispersion of the carbon black in water was easily achieved. Furthermore, the functionalities introduced onto the carbon support act as anchoring sites for the metal catalysts and aid in their adsorption on the support matrix by numerous mechanisms which include adsorption, ion-exchange or coordination reactions. The presence of oxygen-carrying polar functional groups enhances the surface hydrophobicity which aids carbon dispersion in various solvents.
Figure 3 shows the CV profile for ethanol electro-oxidation of 1:1 ratio of PdNiO/C(non-functionalized) and PdNiO/C(functionalized). Based on the comparison of the current density of the ethanol oxidation, PdNiO/C (functionalized) exhibited higher current compared to PdNiO/C(non-functionalized) by 36.7%. This enhanced performance can be inferred from the functional mechanism since there is a greater interaction (adsorption, ion-exchange and coordination reactions) of the metal particles and the support which resulted in the increased metal dispersion. Also, this behavior is probably related to a breaking of hydrophobicity and to an increase in the amount of surface electrochemically active groups. Due to the oxygenated groups on the carbon surface, the NPs are located outside the pore structure and are now available for the oxidation reaction and diminishing the waste of noble catalyst.
Figure 3. CV profile of 1:1 ratio Pd:NiO/C(functionalized) and Pd:NiO/C(non-functionalized) electrocatalysts.
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Standard imagePresented in figure 4 are the SEM images together with the EDX profiles of the 1:1 ratio of both PdNiO/C(non-functionalized) and PdNiO/C(functionalized) electrocatalysts. The latter was functionalized by acid treatment (e.g. 3:1 (v/v) H2SO4:HNO3). Based on figure 4(a), the carbon black appears clean and devoid of particulate impurities. No evidence of any damage to the structure of the carbon black as a result of the acid treatment was observed. It could be seen that the surface of the PdNiO/C(functionalized) electrocatalyst has a more homogeneous surface, which means that the dispersion of the metal particles was successfully done compared to PdNiO/C(non-functionalized) electrocatalyst.
Figure 4. SEM and EDX profiles of (a) 1:1 PdNiO/C(functionalized) and (b) PdNiO/C(non-functionalized) electrocatalysts.
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Standard imageShown also in figure 4 are the EDX profiles of both the PdNiO/C(non-functionalized) and PdNiO/C (functionalized). EDX analysis was performed to verify the stoichiometric ratio of the elements present on the surface of the carbon black electrode. EDX analysis showed that more metallic particles (i.e. Pd and Ni combined) are dispersed on the functionalized carbon black compared to the non-functionalized carbon black. The presence of carbon (C) could be due to exposed C surface, which could mean that C electrode has not been fully covered by the metal particles.
The XRD diffractograms of the composite materials (figure 5) displayed strong diffraction peaks at the Bragg angles of 40.10°, 46.49° and 68.08° which correspond to the (111), (200) and (220) facets of Pd crystals. All diffraction peaks of Pd and NiO can be observed indicating their coexistence in the composite. The XRD pattern exhibited a typical face-centered-cubic (fcc) lattice structure. The average crystal size, L, could be estimated according to the Scherer equation
using the Pd (220) peak where λkα is 1.540 56 Å and B2θ is in radians.
Figure 5. X-ray diffractogram of 1:1 PdNiO/C(non-functionalized) composite.
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Standard imageThe XRD average crystallite sizes of the 1:1 PdNiO/C (non-functionalized) and 1:1 PdNiO/C(functionalized) are around 9.0–15.0 nm. It is worthy of note that the oxidation of the support does not lead to the changes in the particle size of PdNiO [5].
4. Conclusion
This study demonstrated two methods of synthesizing Pd-based catalysts supported on carbon black. The first method was successful in synthesizing Pd nanoparticles with a mean particle size of 3.63 ± 0.59 nm as revealed by TEM. The OAm-capped Pd NPs were activated by AA washing giving the highest electrocatalytic activity with a current density of 11.05 mA cm−2 towards ethanol oxidation in basic medium. On the other hand, the second method was successful in functionalizing the carbon support for the deposition of PdNiO particles. CV results demonstrated a 36.7% increase in the catalytic activity of Pd–NiO/C(functionalized) catalyst towards ethanol oxidation compared to the non-functionalized catalyst.
Acknowledgments
This study was partly supported by the Research Center for the Natural and Applied Sciences of the University of Santo Tomas and partly by the Philippine Council for Industry, Energy and Emerging Technology Research and Development (PCIEERD) of the Department of Science and Technology (DOST), Philippines.