Anchoring ternary CuFePd nanocatalysts on reduced graphene oxide to improve the electrocatalytic activity for the methanol oxidation reaction

Abstract

Ternary CuFePd nanocatalysts were anchored on reduced graphene oxide (rGO) by a simple one-pot chemical reduction with NaBH₄ at room temperature, and used as a novel CuFePd/rGO electrocatalyst toward the methanol oxidation reaction (MOR). The CuFePd/rGO nanocatalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS), and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The electrocatalytic performance for MOR was evaluated by cyclic voltammetry. The as-prepared ternary CuFePd/rGO exhibited improved activity and stability in comparison with Pd/rGO, binary CuPd/rGO, and FePd/rGO. Furthermore, the effects of the Cu to Fe composition ratio on the electrocatalytic performance are also discussed.

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1. Introduction

The extensive utilization of fossil fuels as energy sources has caused worldwide environmental contamination and global warming. Thus, the exploration of renewable and clean energy source has become an urgent task. Recently, direct methanol fuel cells (DMFCs), as a typical promising sustainable power source, have attracted increasing attention due to their unique advantages, such as high power density, low pollutant emission, cheap price of fuel, portability and low operating temperature.^1–4 However, the relatively poor methanol oxidation kinetics prevents the practical applications of DMFCs. To a large extent, the electrochemical performance in fuel cells depends primarily on anode electrocatalysts.^5–7 It has been well-known that Pt-based catalyst is one of the most efficient and extensively used catalysts in DMFCs due to its extraordinary electric activity and chemical stability.^7,8 However, Pt-based catalysts still have some disadvantages such as their high cost due to the limited supply of Pt and serious kinetic constraints from CO poisoning.^9–11 Alternatively, Pd, whose abundance is at least fifty times higher than that of Pt on Earth, could serve as a good candidate for the methanol oxidation reaction (MOR) under alkaline conditions.^12–15 Nevertheless, the electrocatalytic activity and stability of pure Pd nanoparticles are still not satisfactory for practical applications in fuel cells technology, due to the low utilization efficiency of Pd, wherein only the outer-most Pd atom can come in direct contact with methanol and exhibit its electrocatalytic activity. Therefore, the development of highly efficient Pd-based anode catalysts is highly desired.

One approach to improve the performance is tuning of the electronic structures of Pd by forming bimetallic structures with a second noble metal such as Ru, Au and Rh.^16–20 Most promisingly, combining Pd with non-noble metals could not only improve the catalytic activity but also reduce the cost of the catalysts. For example, Ni@Pd core–shell nanoparticles supported on multi-walled carbon nanotubes exhibit high electrocatalytic activity and stability compared to its pure Pd counterpart.²¹ Furthermore, ternary metallic catalysts are considered more efficient than the binary metallic catalysts in DMFCs.^22,23

In addition, it has been recognized that support materials could also play a crucial role in the performance of electrocatalysts via a support–catalyst interaction.^24,25 An ideal catalyst support should have (i) high specific surface area to achieve high metal dispersion; (ii) good electric conductivity to promote fast electron transfer in redox reactions; (iii) high stability to maintain a stable catalyst structure; and (iv) strong affinity to immobilize catalyst nanoparticles.^26,27 Carbon materials, such as carbon black,²⁸ carbon nanospheres,²⁹ carbon nanotubes,^30–32 fullerene,^33–35 and graphene,^36–38 have been considered as potential support materials. Graphene, a unique two-dimensional carbon material, has received much attention for utilization as a novel support material owing to its large surface area and high electrical conductivity.^39–41 In particular, reduced graphene oxide (rGO), holding many residual epoxides, hydroxides and carboxylic acid groups that endow many advantages such as providing anchor sites for metal nanoparticles, became more popular in support materials.^42–45

In this study, novel CuFePd/rGO electrocatalyst, ternary CuFePd nanocatalysts anchored on reduced graphene oxide (rGO), were developed by a simple one-pot chemical reduction with NaBH₄ at room temperature. The as-prepared CuFePd/rGO exhibited improved activity and stability compared to Pd/rGO, binary CuPd/rGO, and FePd/rGO. Furthermore, the effects of the Cu to Fe composition ratio on the electrocatalytic performance were also examined.

2. Experimental section

Graphite powder (99.85%) and Nafion (5%) were purchased from XFNANO (Nanjing, China) and Alfa, respectively. Palladium chloride (PdCl₂), copper sulfate pentahydrate (CuSO₄·5H₂O), iron sulfate heptahydrate (FeSO₄·7H₂O), potassium permanganate (KMnO₄), hydrogen peroxide (H₂O₂, 30%), concentrated sulfuric acid (98%, H₂SO₄), and concentrated nitric acid (HNO₃, 65%) were obtained from Sinopharm Chemical Reagent Corp. (Shanghai, China). All chemicals were of analytical grade and used as received.

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were carried out on a Rigaku D/max 2550 with Cu-Kα radiation and PHI 5400, respectively. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) were carried out on a JEM-2100F. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was measured on Leeman ICP-AES Prodigy.

Graphene oxide (GO) was prepared using a modified Hummers method.^46,47 In brief, graphite powder (0.5 g) was treated with concentrated acid (1.5 mL HNO₃ and 15 mL H₂SO₄) in an ice-water bath and KMnO₄ (2 g) was slowly added to the mixture solution within 20 min and warmed to 45 °C for another 1 h. Water (25 mL) was then added and the temperature was raised to 90 °C for 30 min. Finally, the solution was cooled down to room temperature and diluted with water (60 mL), followed by the addition of H₂O₂ (2 mL, 30%) within 30 min. GO was obtained by centrifugation, washing with water and drying in a vacuum at 40 °C.

Ternary CuFePd/rGO catalysts were synthesized using a one-pot procedure at ambient temperature. Briefly, GO (20 mg) was dissolved in deionized water (30 mL) under ultrasonication for 0.5 h to obtain a homogeneous solution. FeSO₄ (160 μL, 0.4 M) and freshly prepared NaBH₄ (5 mL, 0.1 M) were added sequentially into the abovementioned GO solution. CuSO₄ (160 μL, 0.4 M) and NaBH₄ (5 mL, 0.1 M) were then added sequentially into the abovementioned mixture, followed by the addition of PdCl₂ (6.4 mL, 0.01 M) and NaBH₄ (5 mL, 0.1 M). Finally, more NaBH₄ (5 mL, 0.1 M) was added into the abovementioned mixture and stirred overnight. The ternary CuFePd/rGO catalysts were obtained by centrifuging, washing 3 times with deionized water and ethanol, and finally drying at 50 °C overnight. For comparison, ternary CuFePd/rGO with different Cu to Fe ratio, binary FePd/rGO and CuPd/rGO as well as pure Pd/rGO catalysts were also prepared using the same procedure with a different metal precursor.

Electrochemical measurements, including cyclic voltammetry (CV) and chronoamperometry (CA), were carried out on a CHI 660D electrochemical workstation (CH Instruments, Inc., Shanghai) at ambient temperature. The counter and reference electrodes were a platinum wire and a saturated calomel electrode (SCE), respectively. The working electrode was prepared as described below. The as-prepared catalyst (2 mg) was dispersed ultrasonically in 1.0 mL of an ethanol solution for 0.5 h, and the ink (5 μL) was transferred onto a glassy carbon electrode that was pre-polished sequentially with 0.3 and 0.05 mm alumina oxide powder, followed by coating a Nafion solution (5 μL, 0.5%) and drying at ambient temperature. The as-prepared working electrode was then activated by CV in 1.0 M H₂SO₄ under a potential window of −0.2 to 1.0 V (vs. SCE) with a scan rate of 50 mV s⁻¹. The electrocatalytic activities of the catalysts for methanol oxidation reaction were performed by CV in 1.0 M NaOH solution containing 1.0 M MeOH under a potential window of −1.0 to 0.2 V (vs. SCE) with a scan rate of 50 mV s⁻¹. The stability was examined by CA at a potential of −0.2 V for 6000 s.

3. Result and discussion

3.1. Characterization of the electrocatalyst

The XPS spectra of GO and CuFePd/rGO are shown in Fig. 1. The C1s band could be deconvoluted into three main peaks centered at 284.08, 286.68 and 288.58 eV, corresponding to the alkyl C and sp²-bonded carbon network (C–C/C=C), the hydroxyl and epoxy groups (C–O), and the carbonyl C (C=O), respectively. The peak intensity ratio of C–C to C–O in the rGO spectra increased compared to that of GO (Fig. 1A and C), indicating that the GO has been largely changed to rGO with NaBH₄ as a reductant. In addition, the peaks of Pd, Cu and Fe were also observed in the XPS spectra and could be deconvoluted into Pd 3d_5/2 and 3d_3/2, Cu 2p_1/2 and 2p_3/2, Fe 2p_1/2 and 2p_3/2 peaks, respectively (Fig. 1B–F).^4,48–50 This indicates that CuFePd/rGO were fabricated successfully.

Fig. 1 XPS spectra of GO C1s (A), CuFePd/rGO (B), rGO C1s (C), Pd 3d (D), Cu 2p (E) and Fe 2p (F).

Fig. 2 shows XRD patterns of the as-prepared electrocatalysts. The four diffraction peaks could be assigned to (111), (200), (220) and (311) planes of the crystalline face centered cubic (fcc) structure of Pd. The average crystallite size (D) was then estimated to be 3–5 nm from the (111) diffraction peak by Scherrer's equation:

where D is the mean particle size in nm, λ is 0.1542 nm for Cu-Kα, B is the full width at half maximum (FWHM) of the diffraction peak in radians, and θ is the Bragg angle. In particular, the diffraction peaks of CuFePd/rGO, CuPd/rGO and FePd/rGO slightly shifted to higher angles compared to Pd/rGO, likely due to the incorporation of Cu and Fe into the Pd lattice.

Fig. 2 XRD patterns of Pd/rGO, FePd/rGO, CuPd/rGO and CuFePd/rGO catalysts.

The as-prepared CuFePd/rGO catalysts were then characterized by TEM and EDS. The representative TEM images are shown in Fig. 3A–C. The catalyst nanoparticles are dispersed uniformly on the rGO surface with a mean size of 3–5 nm, which is in a good agreement with the value estimated from XRD. The high-resolution TEM (HR-TEM) image of the CuFePd/rGO catalysts revealed a fringe spacing of 0.224 nm (inset in Fig. 3B), which is close to the lattice spacing of the (111) plane fcc structure of Pd. The observation of a clear SAED pattern indicates the crystalline nature of Pd (inset in Fig. 3A). As shown in Fig. 3D, EDS analysis clearly showed the signals of Cu, Fe and Pd in the as-prepared CuFePd/rGO catalysts, where C and O could originate from the graphene support. Pd loading was determined further by ICP-AES to be 24–26% for the ternary CuFePd/rGO, binary FePd/rGO and CuPd/rGO as well as the mono-metallic Pd/rGO. The slightly higher Pd loading on these catalysts than the expected theoretical value of 20% could be attributed to the loss of graphene in the preparation process. On the other hand, the mass factions of Cu and Fe in CuFePd/rGO were 11% and 9.3%, respectively, which is in good agreement with the values expected from the precursors concentration used. These results show that the ternary CuFePd nanocatalysts were anchored successfully to the graphene sheets.

Fig. 3 TEM images (A–C) and EDS analysis (D) of CuFePd/rGO. Insets are SAED pattern (A) and magnified HR-TEM image (B).

3.2. Electrocatalytic performance

To demonstrate the electrocatalytic performance of the as-prepared CuFePd/rGO catalyst towards the MOR, CV measurements were conducted in 1 M H₂SO₄ and 1 M NaOH + 1.0 M MeOH solutions. Fig. 4 presents the cyclic voltammetry profiles of CuFePd/rGO, FePd/rGO, CuPd/rGO and Pd/rGO in 1 M H₂SO₄. The electrochemical surface area (ECSA) was estimated by calculating the charges accumulated during hydrogen desorption according to the following equation:
where Q (μC) is the integrated charge from the hydrogen desorption region by excluding the double electric layer; k is 210 μC cm⁻², corresponding to the charge density for a monolayer of Pd from the adsorption/desorption of hydrogen; and m is the catalyst amount.⁵¹ The ECSA was estimated to be 43.5, 32.6, 28.3 and 9.70 m² g⁻¹ for CuFePd/rGO, FePd/rGO, CuPd/rGO and Pd/rGO, respectively. The ECSA was also measured from the electric charge of the reduction of the monolayer Pd oxide with the assumption of 424 μC cm⁻²,⁵² and the corresponding value was estimated to be 46.4, 38.2, 36.5 and 13.7 m² g⁻¹. The larger ECSA value of CuFePd/rGO suggested that the ternary metallic catalyst could have more active sites compared to the binary and mono-metallic counterparts.

Fig. 4 CV curves of Pd/rGO, CuPd/rGO, FePd/rGO and CuFePd/rGO catalysts in 1 M H₂SO₄ solution with a scan rate of 50 mV s⁻¹.

The cyclic voltammograms of CuFePd/rGO, FePd/rGO, CuPd/rGO and Pd/rGO for the MOR in 1 M NaOH + 1.0 M MeOH solutions are shown in Fig. 5. The peaks in the forward scans correspond to methanol oxidation, whereas the peaks in the backward scans are attributed to the removal of incompletely oxidized CO-like carbonaceous species.¹² The mass activity of CuFePd/rGO for MOR was 835 mA mg⁻¹, which is higher than those of FePd/rGO (697 mA mg⁻¹), CuPd/rGO (697 mA mg⁻¹) and Pd/rGO (406 mA mg⁻¹), respectively. This clearly demonstrates enhanced catalytic activity on the CuFePd/rGO, because the presence of dopant Cu and Fe not only increased the utilization efficiency, but also decreased the d-band center of Pd and thereby altered the electronic properties of the overall catalyst.^16–21,53 As a result, the coverage of CO-like carbonaceous species on the Pd surface was reduced, making more active sites available for MOR; this resulted from the diminished affinity toward CO due to the presence of defect sites at the interconnects between Pd and the dopant Cu/Fe segments.⁵³ The onset potential of CuFePd/rGO was more negative than those of the others, showing that methanol is easier to oxidize on the ternary alloyed catalyst.

Fig. 5 CV of Pd/rGO, FePd/rGO, CuPd/rGO, and CuFePd/rGO catalysts in 1 M NaOH + 1 M MeOH solution with a scan rate of 50 mV s⁻¹.

The stability of these catalysts was evaluated by CA at −0.2 V in 1 M NaOH + 1 M MeOH solution for 6000 s (Fig. 6). The currents of all the catalysts initially decayed and then reached a steady state. Overall, the ternary CuFePd/rGO catalyst maintained the highest steady state current density over the whole time region of 6000 s than the corresponding binary CuPd/rGO and FePd/rGO as well as the mono-metallic Pd/rGO. This suggests that the as-prepared ternary CuFePd/rGO catalyst has enhanced stability for MOR.

Fig. 6 CA curves of Pd/rGO, FePd/rGO, CuPd/rGO and CuFePd/rGO catalysts in 1 M NaOH + 1 M MeOH solution at −0.2 V.

To gain further insights into the influence of the chemical composition on electrocatalytic performance, ternary catalysts, Cu_0.5Fe_1.5Pd₁/rGO and Cu_1.5Fe_0.5Pd₁/rGO, with various Cu to Fe precursor ratios, were prepared and their electrocatalytic performance for MOR were compared with that of CuFePd/rGO. The change in metal composition ratio could tune the electrocatalytic performance toward MOR. As shown in Fig. 7, the ECSA was estimated to be 58.6, 43.5, and 34.6 m² g⁻¹ for Cu_0.5Fe_1.5Pd₁/rGO, Cu₁Fe₁Pd₁/rGO, and Cu_1.5Fe_0.5Pd₁/rGO catalysts, respectively. The CV obtained in 1 M NaOH solution containing 1 M MeOH showed the highest forward peak current density on CuFePd/rGO among the ternary catalysts examined (Fig. 8). In addition, the CA curves obtained at −0.2 V showed that CuFePd/rGO also holds the highest steady state current density for MOR (Fig. 9). These results suggest that a fine tailoring of the chemical composition in ternary metallic catalyst could efficiently tune the electrocatalytic performance.

Fig. 7 CV of CuFePd/rGO catalysts with various ratio of Cu to Fe in 1 M H₂SO₄ solution with a scan rate of 50 mV s⁻¹.

Fig. 8 CV curves of CuFePd/rGO catalysts with various ratio of Cu to Fe in 1 M NaOH + 1 M MeOH solution with a scan rate of 50 mV s⁻¹.

Fig. 9 CA curves of CuFePd/rGO catalysts with various Cu to Fe ratios in 1 M NaOH + 1 M MeOH solution at −0.2 V.

4. Conclusion

Ternary CuFePd nanocatalysts were anchored onto graphene by a simple one-pot chemical reduction with NaBH₄ at room temperature, and used as a novel CuFePd/rGO electrocatalyst toward MOR. Compared to the binary CuPd/rGO and FePd/rGO, as well as mono-metallic Pd/rGO, the as-prepared ternary electrocatalyst CuFePd/rGO displayed substantially enhanced activity and improved stability for MOR. This was attributed to the synergistic interactions between Pd and dopant metals, Cu and Fe, as well as the catalyst and graphene support. The electrocatalytic performance was dependent on the chemical composition ratios of dopant Cu and Fe in CuFePd/rGO, suggesting that fine tailoring of the chemical composition in the ternary metallic catalyst could efficiently tune the electrocatalytic performance. These findings provide a straightforward one-pot synthesis strategy and insights into the design of novel high-performance and cheap Pd-based catalysts for MOR in fuel cells.

Acknowledgements

This study was financially supported by Fundamental Research Funds for the Central Universities (No. EG2015020, 2232014D3-11, 2232015G1-61).

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