Facile synthesis of Pd nanostructures with enhanced electrocatalytic performance for ethanol oxidation by a bio-based method

Abstract

A facile method was developed for the controlled synthesis of well-defined Pd nanoparticles and nanowires, with the assistance of polyhedrin as a growth-directing agent. The concentration of polyhedrin plays a key role in the final morphology. The cyclic voltammetry and current–time tests demonstrate that the Pd samples show vastly superior electrocatalytic properties compared to the commercial Pd/C catalysts. Moreover, the Pd supported by carbon black or multi-wall carbon nanotube exhibits significantly enhanced electrocatalytic activity toward ethanol oxidation, indicating a great potential for application in fuel cells.

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Introduction

Direct methanol fuel cells (DMFCs) are considered to be promising power sources because of the advantages of high energy density and low environmental pollution.^1,2 Recently, direct ethanol fuel cells (DEFCs) also attracted considerable interest because their energy supplier has abundant availability and lower toxicity.³ Platinum is an excellent electrocatalyst for fuel cells. Pd-based catalysts have been reported that provide a higher electrocatalytic activity toward ethanol oxidation in alkaline solutions than the Pt-based catalysts.^4,5 Because electrocatalysis for the oxidation reaction is well known to be structure sensitive,⁶ tremendous efforts have been devoted to the effective control of the morphology, distribution, and size of the Pd-based nanocatalysts.^7–9 In the last decade, a variety of Pd nanocrystals with different shapes have been synthesized.^10–22 Considerable efforts have been made to find controllable synthetic routes for Pd/C catalysts with designed microstructure. Wang's group have reported a facile one-pot and stabilizer-free preparation of monodispersed ultrafine palladium nanoparticles supported on Vulcan XC-72 and their excellent electrocatalytic activity toward ethanol.²³ Xing and co-workers devised a surface replacement reaction method for the synthesis of carbon-supported Pd hollow nanospheres and nanoparticles.²⁴ However, the operation process is complex and difficult to control. Ding's group described a dry-grinding approach for preparing PdO nanoparticles supported on multi-walled carbon nanotubes (MWCNT);²⁵ the prepared catalysts showed the best electrocatalytic activity performance for the ethanol electro-oxidation reaction (EOR). Controllable synthesis of carbon-supported Pd catalysts with designed size and nanostructure is therefore significantly desirable.

In our study, we developed a new type of carbon-supported Pd catalyst consisting of Pd nanoparticles and Pd nanowire networks using polyhedrin as the shape-directing agent, multi-walled carbon nanotubes (MWCNT) or carbon black (CB) as the support, and NaBH₄ as the reducing agent. Highly dispersed Pd nanoparticles and uniform Pd nanowire networks were successfully prepared. Furthermore, the use of polyhedrin could enhance the bonding force between the catalysts and carbon supports, further improving the catalytic activity and stability of carbon-supported Pd nanowires and Pd nanoparticles.

Experimental

Purification of polyhedrin

Polyhedrin was purified using a modification of the procedure reported by Summers and Smith.²⁶ In brief, highly purified polyhedral inclusion bodies were suspended in deionized water and solubilized in an alkaline solution (pH = 10.4) for 24 h. The solution was centrifuged at 6000 rpm to remove all the insoluble material, and then the supernatant containing polyhedrin was ultracentrifuged at 28 000 rpm for 1 h to remove the virions. The polyhedrin in the supernatant was recovered as a pellet by addition of 0.1 M HCl; the pellet was extensively washed with water and then lyophilized.

Synthesis of Pd nanomaterials

Prior to the preparation of palladium catalysts, the polyhedrin lyophilized powder (0.01 mg mL⁻¹ or 0.1 mg mL⁻¹) was dissolved in water and then stored at 4 °C for 24 h. During this process, the polyhedrin self-assembled and formed a polyhedrin framework. To synthesize Pd nanostructures, 200 μL Na₂PdCl₄ (10 mM) was mixed with 600 μL polyhedrin aqueous solution. The mixture was adjusted to pH of 8 and stirred for 24 h at room temperature, followed by the addition of NaBH₄ (10 mM). The mixture was stirred for another 0.5 h. Fabrication of carbon-supported catalysts (10 wt% metal loading) was carried out as follows: CB or MWCNT was dispersed in the solution containing Pd nanoparticles (PdNPs) or Pd nanowires (PdNWs) and then subjected to ultrasound treatment for 30 min. The final catalysts were washed with redistilled water and the combinations were denoted as PdNP/CB, PdNW/CB, PdNP/MWCNT and PdNW/MWCNT.

Electrochemistry

Electrochemical experiments were carried out with a CHI 832C electrochemical workstation. Before each experiment, all the electrolyte solutions were deoxygenated using high purity nitrogen. The glassy carbon (GC) was polished with alumina slurries, and then rinsed with ethanol and deionized water. Finally, the electrode was dried by nitrogen. The as-synthesized catalysts or commercial Pd/C catalyst (8 μL, 0.16 mg mL⁻¹) was drop-coated on the GC electrode and dried with an infrared lamp. The catalyst-coated glassy carbon electrode was used as the working electrode. A saturated calomel electrode and a Pt foil were used as the reference and counter electrodes, respectively.

Material characterization

The morphology of palladium nanostructures was characterized with transmission electron microscopy (TEM) on a JEOL JEM-200EX at 120 kV. The high resolution transmission electron microscopy (HRTEM) was carried out on a JEOL JEM-2010 at 200 kV. Powder X-ray diffraction (XRD) was carried out on a D/max-2500/PC X-ray diffractometer with Cu-K_α radiation. XPS measurements were performed on a PHI QUANTERA-II SXM X-ray photon-electron spectrometer.

Results and discussion

The Pd materials were prepared using a bio-based method. By changing the concentration of the polyhedrin, Pd catalysts with different morphologies were obtained. Fig. 1 shows the representative TEM images of the as-prepared PdNWs and PdNPs under different magnification. It can be seen that the products consist of uniform nanowire networks with high yield of PdNWs at a low concentration (0.01 mg mL⁻¹) of polyhedrin (Fig. 1a and b). The average diameter of the PdNWs is about 4 nm. For the high concentration of polyhedrin (0.1 mg mL⁻¹), only spherical Pd nanoparticles with a relatively narrow particle size distribution are observed, as show in Fig. 1d and e. The corresponding high resolution TEM (HRTEM) images (Fig. 1c and f) show that the PdNWs and PdNPs are in the polycrystalline state, and the d spacings of PdNWs are observed to be 0.227 and 0.196 nm, which can be assigned to the (111) and (200) planes of Pd, respectively. The d spacing of PdNPs is 0.229 nm, which corresponds to the (111) plane of face-centered cubic Pd.

Fig. 1 TEM images of PdNWs obtained at a low concentration (0.01 mg mL⁻¹) of polyhedrin (a, b) and corresponding HRTEM image (c); TEM images of PdNPs obtained at a high concentration (0.1 mg mL⁻¹) of polyhedrin (d, e) and corresponding HRTEM image (f).

The XRD patterns of the two catalysts are show in Fig. 2. The result confirms the face-centered cubic (fcc) crystal structure in both PdNPs and PdNWs. The characteristic peaks at 2θ = 39.8°, 46.3°, 67.5° and 81.9° correspond to (111), (200), (220), and (311) planes of Pd, respectively. It is worth noting that the 2θ values of PdNPs exhibit a slight negative shift relative to that of the PdNWs, indicating that the Pd–Pd interatomic distance increases in the Pd nanoparticles.²⁷

Fig. 2 XRD patterns of PdNWs and PdNPs.

To study the effects of the polyhedrin framework for the production of Pd nanomaterials, Na₂PdCl₄ solution in various concentrations was employed, while the concentration of the polyhedrin remained constant. The as-obtained Pd nanostructures were examined using TEM, as shown in Fig. 3 and 4. The low concentration of polyhedrin, using 2 mM and 5 mM Na₂PdCl₄ as precursor, produced short nanowires with diameters of about 3–4 nm (Fig. 3a and b). Uniform nanowire networks with smooth surface were observed (Fig. 3c). It was found that by increasing the concentration of Na₂PdCl₄ from 10 mM to 20 mM, the diameter increased from about 4 nm to 5 nm, and the surfaces of the nanowire networks became rough (Fig. 3d). Furthermore, when the concentration of polyhedrin increased to 0.1 mg mL⁻¹, only nanoparticles were observed. Comparing the results in Fig. 4, it appeared that by increasing the concentration of Na₂PdCl₄ from 2 to 20 mM, the diameter increased slightly. In addition, it can be seen from the insets that an average size of 3.0 ± 0.5 nm was observed for all the samples. Interestingly, when polyhedrin solutions with various concentrations were employed while the metal/polyhedrin ratio remained constant, short nanowires and nanoparticles were observed, as shown in Fig. 3a and 4d, respectively. Previous studies have proposed possible mechanisms for the generation of Pd nanostructures employing the R5-based template,^28,29 indicating that the metal/peptide ratio is the key point to control the morphology of the inorganic materials. However, in our study, we found that the concentration of polyhedrin plays a key role in the final morphology.

Fig. 3 TEM images of PdNWs obtained at a low concentration of polyhedrin with various concentrations of Na₂PdCl₄: (a) 2 mM, (b) 5 mM, (c) 10 mM, and (d) 20 mM.

Fig. 4 TEM images of PdNPs obtained at a high concentration of polyhedrin with various concentrations of Na₂PdCl₄: (a) 2 mM, (b) 5 mM, (c) 10 mM, and (d) 20 mM. The corresponding particle size distributions are shown as insets.

The aforementioned investigations led us to propose a possible mechanism for the growth of PdNWs and PdNPs, as shown in Scheme 1. First, the polyhedrin, dissolved in an aqueous solution with different concentrations, was able to self-assemble and form polyhedrin frameworks; low concentration produces a sparse framework, while high concentration results in a dense framework. When the Na₂PdCl₄ stock solution was added, Pd²⁺ ions were introduced to the polyhedrin scaffold, and possibly sequestered within localized areas of amines. Once the NaBH₄ solution was added, in situ nucleation occurs while the growth process significantly depended on steric confinement effects of the assembled polyhedrin framework. As anticipated, at a low concentration of polyhedrin, the sparse framework has sufficient space to allow particles to aggregate to form linear structures based on the steric confinement effects of peptide branches. As the concentration increases, the decrease in the interparticle spacing decreases interparticle interactions, resulting in spherical materials. To prove the proposed mechanism, time-dependent TEM experiments were used to track the experimental process, as shown in Fig. S1 and S2 (in the ESI†). As time goes on, the nanoparticles aggregate to form short nanowires. Finally, nanowire networks were formed at a low concentration of polyhedrin, while only nanoparticles were formed during the process at the high concentration of polyhedrin.

Scheme 1 Schematic illustration of the formation of Pd nanowires (upper) or Pd nanoparticles (lower) employing the polyhedrin framework.

Fig. 5 shows the Pd structures supported on the two supports. The supported palladium particles and nanowires still maintain a well-dispersed configuration and small particle sizes. Compared with those supported on MWCNT (Fig. 5c and d), PdNWs and PdNPs supported on CB are more uniform and complete (Fig. 5a and b). Mironenko pointed out that the surfaces of MWCNT are extremely poor in functional groups as compared to CB.³⁰ The surface of carbon black is characterized by the presence of phenol, ether, lactone, quinone, carboxyl and anhydride. Moreover, the intrinsic property of the polyhedrin is that it is rich in functional groups such as amino, carboxyl, and peptide.³¹ These groups decrease the hydrophobicity of the carbon, making the surface more accessible to the aqueous solution of the metal precursor. It is therefore speculated that the presence of the polyhedrin framework can also potentially increase the binding force between the supports and the Pd catalysts leading to enhancement of Pd adsorption.

Fig. 5 TEM images of PdNW/CB (a), PdNP/CB (b), PdNW/MWCNT (c) and PdNP/MWCNT (d).

XPS analysis was used to determine the surface composition of the as-prepared catalysts. Fig. 6a shows the XPS wide spectra of PdNWs, PdNW/MWCNT, PdNW/CB, PdNPs, PdNP/MWCNT, and PdNP/CB. All spectra contain a C 1s peak at about 284 eV, an O 1s peak at about 531 eV and a Pd 3d peak at about 340 eV, indicating that all catalysts contain C, O and Pd. The carbon in the supported catalysts is mainly from the carbon material, whereas the C in PdNWs and PdNPs is from the polyhedrin. The high-resolution C 1s XPS spectra of PdNPs, PdNP/MWCNT and PdNP/CB (Fig. 6b–d) show that the samples contain the following carbon functional groups: C=C, C–C and C–H (284.6 eV), C–N (285.37 eV), C–O (286.4 eV) and O–C–O (288 eV).^32,33 The high-resolution C 1s XPS spectra of PdWPs, PdWP/MWCNT and PdWP/CB are shown in Fig. S3.† The C–C peak intensity in the Pd-supported spectra obviously increases compared to that of the unsupported catalysts due to the introduction of the carbon support material.

Fig. 6 XPS spectra of the as-prepared catalysts (a). C 1s spectra of PdNPs (b), PdNP/MWCNT (c) and PdNP/CB (d).

The catalytic activity of the prepared catalysts towards ethanol oxidation was investigated in an alkaline medium through electrochemical measurements and further compared with a commercial Pd/C catalyst. Fig. 7a shows the cyclic voltammetric curves (CVs) of Pd catalysts obtained in a 1 M KOH + 0.5 M CH₃CH₂OH solution. It is found that the forward anodic peak value of mass activity for PdNWs is 2430 A g⁻¹, which is about 8.1 times higher than the 300 A g⁻¹ for the commercial Pd/C, and the value for the prepared PdNPs (550 A g⁻¹) is also much higher than that for the commercial Pd/C. It can also be noted that the onset potential toward ethanol oxidation of PdNWs is lower than that of the Pd/C, indicating enhancement in the kinetics of the ethanol electro-oxidation reaction. As shown in Fig. 7c, all the CVs were also normalized by the electrochemical surface area (ECSA), which was obtained by calculating the charge of metal oxide reduction area according to the CVs obtained in N₂-purged 0.5 M H₂SO₄ (Fig. 7b).

Fig. 7 Comparison of the electrochemical performance of PdNWs, PdNPs and commercial Pd/C catalysts: mass activity evaluated by CV, (a) CVs obtained in N₂-purged 0.5 M H₂SO₄ (b), specific activity (c), and stability evaluated by current–time curves recorded at −0.3 V in 1 M KOH and 0.5 M ethanol (d).

The stability of the as-prepared catalysts toward ethanol electro-oxidation was investigated by the current–time method (Fig. 7d). Compared with PdNPs and Pd/C, the PdNWs show a relatively slower decay of current density, indicating their better tolerance toward ethanol electro-oxidation. After 8000 s, the residue current of PdNWs is much higher than that of the PdNPs and Pd/C. The results indicate that the PdNWs have an outstanding catalytic performance for ethanol electro-oxidation in alkaline media.

The effect of carbon-supported materials on the electrocatalytic activity of supported PdNPs and PdNWs for the ethanol oxidation reaction was evaluated and the results are shown in Fig. 8. Corresponding specific activities are shown in Fig. S4 (in the ESI†). The electrocatalytic activity of supported Pd catalysts strongly depends on the type of support. The best result is observed for PdNW/CB, achieving a peak current density of 3225 A g⁻¹. The value of PdNW/MWCNT is 2900 A g⁻¹ and they are all higher than that of the unsupported PdNWs, as shown in Fig. 8a. Moreover, the onset potentials for PdNW/MWCNT and PdNW/CB are 0.70 V and 0.68 V, respectively, which are both more negative than 0.64 V observed for the unsupported PdNWs. Fig. 8c shows a significant promotion of the forward current density of PdNP/MWCNT (2400 A g⁻¹) and PdNP/CB (2000 A g⁻¹), compared with the unsupported PdNPs catalyst. Because the synthesized Pd nanowires have better electron transport properties than the nanoparticles, and the addition of carbon materials can further improve the electron transfer between catalyst and electrode, this enhanced effect is therefore relatively small for nanowires compared with nanoparticles. The electrochemical stability of supported Pd catalysts for the EOR was also investigated. The results indicate that MWCNT and CB supports significantly promote the tolerance and resistance of PdNPs and PdNWs toward the poisoning effect of intermediate species in the EOR (Fig. 8b and d).

Fig. 8 Comparison of the electrochemical performance of carbon-supported materials: mass activity of PdNW, PdNW/MWCNT and PdNW/CB toward ethanol electro-oxidation (a). Current–time curves recorded on PdNW, PdNW/MWCNT and PdNW/CB (b). Mass activity of PdNP, PdNP/MWCNT and PdNP/CB toward ethanol electro-oxidation (c). Current–time curves recorded on PdNP, PdNP/MWCNT and PdNP/CB (d).

Many literature reports have focused on synthetic strategies to produce PdNPs and PdNWs; however, there are few examples of electronic devices being directly fabricated from biomaterials owing to a lack of charge transport through them. However, our prepared carbon-supported PdNPs and PdNWs exhibit superior electrocatalytic performance towards ethanol electro-oxidation. This is due to the following reasons: (a) the reagents could penetrate through the polyhedrin scaffold, react at the metallic component, and then traverse back through the template for release to the solution. (b) The nanowire networks possess the advantages of a one-dimensional structure such as improved electron transport characteristics during the catalysis as a result of the path-directing effect of the structural anisotropy.^18,34,35 Moreover, the small diameter of the prepared nanoparticles offers a great opportunity to provide more active sites. (c) The ideal support with large surface area, good conductivity and strong adsorption of metal shows the ability to improve the dispersion of metal nanoparticles, and thereby enhance the utilization and efficiency of the noble metal electrocatalysts.^36,37

Conclusion

In this study, we have successfully synthesized Pd nanoparticles and Pd nanowires through a bio-inspired method, which is a very economical, environmentally friendly and highly efficient route. The influence of different carbon supports and the shapes of Pd catalysts on the ethanol oxidation reaction were studied. The PdNWs supported on CB show much higher activity and stability than PdNWs supported on MWCNT. The combination of support and Pd nanostructure can also enhance the kinetics of the ethanol electro-oxidation reaction.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21371149 and 21101134), Natural Science Foundation of Hebei (Grant No. B2012203005, 11965152D and 20131333110010) and China Postdoctoral Science Foundation funded project (Grant No. 2015M581315).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00560h

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