Growth of Copper Nanocubes on Graphene Paper as Free-Standing

Mar 23, 2012 - Abstract. Abstract Image. We have developed a new type of flexible electrodes based on Cu nanocube-decorated free-standing graphene pap...
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Growth of Copper Nanocubes on Graphene Paper as Free-Standing Electrodes for Direct Hydrazine Fuel Cells Hongcai Gao,† Yuxi Wang,‡ Fei Xiao,† Chi Bun Ching,† and Hongwei Duan*,† †

School of Chemical and Biomedical Engineering, Nanyang Technological University, 70 Nanyang Drive, Singapore 637457 School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

‡

S Supporting Information *

ABSTRACT: We have developed a new type of flexible electrodes based on Cu nanocube-decorated free-standing graphene paper (GP) using a facile electrodeposition method. The Cu nanocubes− graphene paper (Cu−GP) hybrid electrode processes remarkable electrocatalytic activity with an onset potential of −0.10 V toward hydrazine oxidation in alkaline solutions and can serve as the catalyst layer for direct hydrazine fuel cells. One interesting finding is that a copper hydroxide/oxide layer in situ formed on Cu nanocube surfaces plays an important role in enhancing the electrocatalytic activity and durability of the electrocatalyst. A totally irreversible and diffusion-controlled oxidation of hydrazine occurs on the electrocatalyst, eventually leading to environmentally friendly products such as nitrogen and water. and catalysts16) and developing innovative approaches17−19 for better structural integration of the components. Graphene paper (GP), assembled from individual graphene nanosheets, is characterized by a unique set of electrical and structural properties, such as high electrical conductivity, outstanding mechanical strength, and excellent chemical and thermal stability. These properties make GP-based composites intriguing electrode materials in a wide range of applications from supercapacitors20−22 and lithium ion batteries23,24 to electrochemical biosensors.25 However, the potential of using GP to improve the performance of fuel cells has not been demonstrated, although GP appears to be a suitable carbon substrate for the catalyst layer given its mechanical stability and chemical inertness.26 On the other hand, there has been considerable interest in developing noble-metal-free catalysts, such as Ni-, Co-, and Cu-based catalysts for DHFCs to reduce the cost and minimize the poisoning effect associated with classical Pt-based catalysts.27 Previous reports have shown that Ni- and Co-based catalysts have to function at relatively higher temperature (50−80 °C) because of decreased activity induced by surface oxidation at low temperature.28 Herein, we present the development of Cu nanocube-loaded free-standing GP (Cu−GP) as a new type of flexible electrode material to catalyze hydrazine oxidation. Our results have demonstrated that Cu nanocubes directly grown on GP substrates through template-free electrodeposition exhibit high catalytic activity and stability against hydrazine oxidation in alkaline conditions at room temperature. One interesting finding is that a copper

1. INTRODUCTION Direct liquid fuel cells, a sort of power source especially suitable for hybrid vehicles and portable electric devices, are actively explored in recent years due to their high efficiency and low environmental impact. 1,2 Hydrazine has emerged as a promising fuel candidate for direct liquid fuel cells because of a number of attractive features, such as high theoretical cell voltage of 1.61 V with expected high power density, high hydrogen content equivalent to methanol, easier transportation and storage than hydrogen, and no greenhouse gas emissions.3 Since the early development of hydrazine-based alkaline fuel cells,4,5 research on direct hydrazine fuel cells (DHFCs) has been focused on developing catalysts with improved performance, particularly Pt-based systems and new device designs.6−8 Rapid progressions in the past decade have manifested that DHFCs hold great promise in the commercialization of highperformance and portable fuel cells.9−11 The catalyst layer, structurally sandwiched between a gas diffusion layer and a polymer electrolyte membrane in the fuel cell assembly,12 is a critical component of fuel cells to convert chemical energy into electricity. A conventional two-step method to prepare the catalyst layer involves dispersing catalyst in a binder (such as Teflon or Nafion) to form a slurry, followed by coating the slurry on carbon substrates.13 This process usually results in serious agglomeration of catalyst particles and poor adhesion of catalyst to the substrate, which typically leads to reduced active surface areas and stability of the catalysts. An addition of the binder can possibly block active sites of the catalyst. To overcome these problems, great efforts have recently been devoted to improving the existing systems from two aspects such as designing new building blocks (carbon substrates14,15 © 2012 American Chemical Society

Received: March 5, 2012 Revised: March 19, 2012 Published: March 23, 2012 7719

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Figure 1. AFM images of (A) as-synthesized GO and (B) hydrothermally reduced GO spin-coated on silicon wafer. Inset shows their corresponding aqueous dispersions. High-resolution XPS spectra of C 1s for (C) GO and (D) hydrothermally reduced GO.

hydrothermally reduced GO on a silicon wafer with a 300 nm SiO2 top layer and characterized with a silicon cantilever operating in tapping mode. X-ray diffraction (XRD) patterns were collected with a Bruker AXS D8 X-ray diffractometer equipped with monochromatized Cu Kα radiation (λ = 1.54056 Å, 40 kV, and 20 mA). Raman spectra were acquired with a micro-Raman spectrometer (Reinshaw Raman Scope RM3000) at room temperature in the backscattering configuration with wavelength of 633 nm (1.96 eV). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos-Axis spectrometer with monochromatic Al Kα (1486.71 eV) X-ray radiation (15 kV and 10 mA) and hemispherical electron energy analyzer. Curve fitting and background subtraction were accomplished using Casa XPS software. Electrochemical measurements of hydrazine oxidation were performed in KOH (0.1 M) aqueous solution containing hydrazine (10 mM) at room temperature (20 °C) on a CHI 660D electrochemical workstation with a conventional threeelectrode system. The working electrode was a modified GP electrode, and the counter and reference electrodes were Pt wire and a saturated calomel electrode (SCE), respectively. All potentials reported here are relative to SCE.

hydroxide/oxide layer in situ formed on Cu nanocube surfaces plays an important role in enhancing the electrocatalytic activity and stability of the electrocatalyst. We envision that Cu−GP nanocomposites can serve as a binder-free catalyst layer in DHFC, thus effectively minimizing the above-mentioned problems.

2. EXPERIMENTAL SECTION Graphene oxide (GO) was synthesized from graphite power (Sigma-Aldrich) according to the modified Hummers’ method and suspended in water to give a stable dispersion.29,30 The pH of the solution was adjusted with ammonia solution to 10, which ensures to yield a stable reduced GO (rGO) dispersion. Then a total volume of 30 mL of GO aqueous solution (0.1 mg mL−1) was transferred to a 45 mL capacity Teflon-lined autoclave and heated at 180 °C for 12 h. The autoclave was then cooled to room temperature naturally. GP was prepared by filtration of the dispersion of rGO through a cellulose acetate membrane filter (47 mm in diameter and 200 nm in pore size, Sartorius-Stedim), followed by washing, air drying, and peeling off from the filter. The thickness of each paper was controlled by adjusting the volume of the suspension. The electrodeposition of Cu nanocubes on GP was performed in KCl solution (0.1 M) containing CuCl2 (10 mM) by applying a negative potential of −0.4 V (vs SCE), which is negative enough for the electrochemical reduction of Cu2+ to Cu0.31 The size and morphology of the samples were investigated by atomic force microscope (AFM, Asylum Research) and fieldemission scanning electron microscopy (SEM, JSM-6700F). SEM is equipped to perform elemental chemical analysis by energy dispersive X-ray spectroscopy (EDS). The AFM samples were prepared by spin-coating the GO dispersions or

3. RESULTS AND DISCUSSION To fabricate the free-standing GP, the bright yellow dispersion of graphene oxides (GO) was first synthesized from graphite powder by the commonly used Hummers’ method (inset of Figure 1A). After hydrothermal treatment of GO dispersion at 180 °C for 12 h, a stable and black dispersion of graphene was obtained (inset of Figure 1B). The distinct color change of the dispersion before and after hydrothermal reaction indicates that reduced GO (rGO) is formed. AFM images reveal that 7720

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Figure 2. (A) Photographs of 2 cm × 2 cm GP. (B and C) SEM images of cross-section views of GP at different magnifications. (D) SEM image of a top view of GP. (E) SEM images of Cu nanocubes electrodeposited on GP. (F) EDS spectra of GP (a) and Cu−GP (b).

nanoparticles.34,35 After applying a negative potential of −0.4 V in 0.1 M KCl containing 0.01 M CuCl2 solution for 60 s, a dense layer of well-dispersed Cu nanocubes with a type size of around 50 nm was decorated on the surface of GP (Figure 2E). Energy-dispersive X-ray spectroscopy (EDS) spectra of GP and Cu−GP show the predominant peaks corresponding to C and Cu elements (Figure 2F), also confirming the successful electrodeposition of Cu nanocubes on GP. Typical XRD patterns of GO, GP, and Cu−GP are shown in Figure 3A. GO has a large interlayer distance (∼0.8 nm, 2θ = 10.3°) due to the presence of hydroxyl, epoxy, and carboxyl groups. The interlayer distance of GP decreases to 0.34 nm (2θ = 25.0°) after the oxygen-containing functional groups are

resultant GO and rGO deposited on a silicon wafer by spincoating have a typical lateral dimension of hundreds of nanometers (Figure 1A and B). The apparent thickness of GO is around 1.0 nm (Figure 1A), suggesting complete exfoliation of oxidized graphite into single-layered GO nanosheets. The thickness of hydrothermally reduced GO is about 0.8 nm (Figure 1B), which is smaller than that of GO owing to the removal of the surface oxygen functional groups on GO sheets. XPS was further employed to detect the constitutional change of GO before and after hydrothermal reduction. As shown in Figure 1C, deconvolution of the C 1s signal in the GO sample displays the presence of CC/C−C (∼284.6 eV), C−O (hydroxyl and epoxy, ∼286.5 eV), CO (carbonyl, ∼288.3 eV), and O−CO (carboxyl, ∼290.3 eV) functional groups, with a low atomic percentage of C/O (2/1). After the hydrothermal reduction, the intensities of the peaks assigned to the heavily oxygenated carbon species decrease significantly, and the peak associated with CC/C−C (∼284.6 eV) becomes predominant (Figure 1D). The resultant rGO sample shows a much higher atomic percentage of C/O (8/1). The effective deoxygenation of GO by the hydrothermal reduction process is expected to improve the electrical conductivity of graphene which is an essential criteria for the support material in catalyst layers.32 The flexible free-standing GP, prepared by the vacuum filtration method, exhibits metallic luster on its surface (Figure 2A). The thickness of the paper can be readily controlled by adjusting the volume and concentration of the graphene suspension. During the filtration process, graphene nanosheets tend to self-assemble in a sheet-by-sheet manner at the liquid− solid interface, driving the formation of a uniform and layered GP through the entire cross-section (Figure 2B and C).33 Figure 2D shows a typical top-view SEM image of pristine GP, in which the graphene nanosheets assembled to form a smooth surface with characteristic wrinkles arising from the flexibility of graphene nanosheets. GP with high electrical conductivity can be directly used as a working electrode for the electrodeposition of Cu nanocubes. The surface oxygen-functional groups on graphene nanosheets can serve as the nucleation sites for metal

Figure 3. (A) XRD patterns and (B) Raman spectra of GO (a), GP (b), and Cu−GP (c). 7721

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partially removed in the reduction process.36 The XRD pattern of Cu−GP clearly shows three major peaks at 43.5, 50.5, and 74.4° in the range of 10−80°, which can be assigned to the diffraction from (111), (200), and (220) planes of the facecentered cubic (fcc) lattice of Cu(0).37 Raman spectra were further used to determine the microstructures of GO, GP, and Cu−GP (Figure 3B). GO displays two prominent peaks at 1352 and 1594 cm−1, corresponding to the well-documented D and G bands. The intensity ratio (ID/IG) of D and G bands of GO is 0.8. In GP, these two peaks remain at the same position. However, ID/IG increases to 1.0, indicating a decrease in the size of the in-plane sp2 domains and a partially ordered crystal structure of graphene.38 Deposition of Cu nanocubes did not have any impact on the Raman spectral profile of the GP substrate. Electrochemical impedance spectroscopy (EIS) was carried out to probe the impedance changes of GP and Cu−GP. Figure 4 shows the Nyquist plots of GP and Cu−GP in 0.1 M KCl

Figure 5. Cyclic voltammograms of Cu−GP (a) and GP (b) in 0.1 M KOH containing 10 mM hydrazine at a scan rate of 100 mV s−1.

remarkable activity toward hydrazine oxidation in comparison with the rigid electrode materials previously reported, such as Ni−Co alloy (−0.39 V vs SCE),41 TiO2−Pt hybrid nanofibers (−0.14 V vs SCE),42 Nano-Au/Porous-TiO2 (0.00 V vs SCE),43 Pd/WO3 (0.10 V vs SCE),44 porous Mn2O3 nanofibers (0.06 V vs SCE),45 and Fe2O3 graphite composite (0.30 V vs SCE).46 In addition, the free-standing Cu−GP electrode offers great flexibility for potential applications in portable and microelectronic devices.47 Another interesting observation is that an anodic peak at −0.10 V appeared in the first CV cycle of Cu−GP and totally disappeared in the following cycles (Figure 6A). XPS spectra

Figure 4. Nyquist plots of GP (a) and Cu−GP (b) in 0.1 M KCl electrolyte solution containing 1.0 mM K3Fe(CN)6 + 1.0 mM K4Fe(CN)6 in the frequency range of 0.1−105 Hz.

solution containing 1.0 mM K 3Fe(CN)6 and 1.0 mM K4Fe(CN)6. The semicircular part of the plot at higher frequencies corresponds to the electron transfer limited process, and the diameter is equivalent to the electron transfer resistance (Ret). The linear part at lower frequencies is related to the diffusion process.39 Ret of GP was estimated to be 315 Ω, which dropped to be 141 Ω after the electrodeposition of Cu nanocubes, indicating that Cu nanocubes enhanced electron transfer on the Cu−GP electrode. To evaluate the electrocatalytic activity, the stable cyclic voltammograms (CVs) of GP and Cu−GP in 0.1 M KOH containing 10 mM hydrazine are compared in Figure 5. On the bare GP composed of graphene nanosheets with defects and oxygen functional groups as the catalytic sites, a peak current (ip) at 0.80 V was observed due to the hydrazine oxidation.40 In contrast, Cu−GP exhibits higher electrocatalytic activity toward hydrazine electrooxidation than GP, which is supported by a 10-fold increase of peak current and an overall negative shift of peak potential (Ep) to 0.30 V. The onset potential for hydrazine oxidation on Cu−GP is as low as −0.10 V, reflecting a fast electron-transfer reaction on the Cu−GP due to the high catalytic effect of Cu nanocubes. Furthermore, the reverse scan gives no corresponding cathodic peak, suggesting a totally irreversible oxidation of hydrazine in alkaline solutions on Cu− GP. Taking into account the onset potentials, Cu−GP exhibits

Figure 6. (A) Cyclic voltammograms of the first (a) and second (b) scans of Cu−GP in 0.1 M KOH containing 10 mM hydrazine at a scan rate of 100 mV s−1. (B) XPS spectra of Cu 2p3/2 and Cu 2p1/2 before (a) and after (b) the first cyclic voltammetry cycle in 0.1 M KOH containing 10 mM hydrazine at a scan rate of 100 mV s−1. 7722

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(Figure S1, Supporting Information). According to the equation Ep = 1/2b log(v) + constant, where b = 2.3RT/[(1 − α)nαF], if assuming the electron transfer coefficient (α) is approximately 0.5,53,54 the value of nα is estimated to be 1, indicating that one electron transfer is the rate-determining step for hydrazine oxidation on Cu−GP. Additionally, ip varies linearly with the square root of scan rate (v1/2), ip = 1.181 + 42.962v1/2 (r2 = 0.998; ip, mA cm−1; v, V s−1), suggesting a typical diffusion-controlled process according to the equation ip = 0.4958 × 10−3nF3/2(RT)−1/2(αn)1/2ACD1/2v1/2,55 where A stands for electrode surface area (Figure S2, Supporting Information). Chronocoulometry is used to measure the diffusion coefficient (D) of hydrazine according to the integrated Cottrell equation, Q = (2nFAD1/2Ct1/2)/π1/2 + Qdl + nFAΓ0, where C is the bulk concentration of hydrazine (Figure 7B).56 After subtracting the background charge, plotting Q against t1/2 yields a straight line: Q = −1.768 + 20.157t1/2 (r2 = 0.999; Q, mC cm−2; t, s) (Figure S3, Supporting Information). D of hydrazine is then calculated to be 2.1 × 10−5 cm2 s−1, which is in agreement with the value previously reported in the literature.40 On the basis of the value of D, from the slope of the ip versus v1/2 plot, the total number of electrons (n) involved in hydrazine oxidation is then calculated to be 4. The mechanism of hydrazine oxidation depends significantly on the electrolyte solution and the nature of the electrodes. According to the previous reports,57 the following mechanism is proposed for the oxidation of hydrazine at Cu−GP.

were used to identify the change of Cu surface species before and after the first CV cycle (Figure 6B). The presence of Cu(II) on Cu nanocubes after the first CV cycle is supported by the considerably broader Cu 2p3/2 and Cu 2p1/2 peaks than that of Cu2O and Cu metal, and the new shakeup satellite peaks at ca. 10 eV higher binding energy than the primary Cu 2p3/2 and Cu 2p1/2 peaks (933.7 and 953.6 eV). No such satellite peaks are present in the case of Cu metal and Cu2O.48 It is more difficult to distinguish between Cu metal and Cu2O since the shape and binding energy of their Cu 2p3/2 and Cu 2p1/2 peaks are quite similar.49 Nevertheless, the XPS results support the formation of a stable layer of Cu hydroxides/oxides on the Cu nanocube surfaces. The in situ formed Cu hydroxide/oxide layer can transfer between different oxidation states of Cu(II) and Cu(I) and consequently act as mediators to shuttle electron transfers between hydrazine and the catalysts.50,51 In the first cycle, hydrazine is mainly oxidized on the bare Cu surfaces that have higher overpotential for the oxidation of hydrazine.52 In the following cycles, hydrazine is oxidized on the Cu hydroxides/oxides layer that formed on Cu surfaces, leading to a negative shift of peak potential for hydrazine oxidation from 0.40 to 0.30 V. To get insight into the oxidation mechanism of hydrazine on Cu−GP, the effect of scan rate (v) is investigated in the range of 10−100 mV s−1 in 0.1 M KOH aqueous solution containing 10 mM hydrazine (Figure 7A). Ep shifted toward positive direction with increasing v, and a linear relationship is found between Ep and log(v), Ep = 0.397 + 0.104 log(v) (r2 = 0.998; r, correlation coefficient; Ep, V; v, V s−1), demonstrating that the oxidation of hydrazine on Cu−GP is an irreversible process

N2H 4 + OH− → N2H3 + H2O + e− (slow) N2H3 + 3OH− → N2 + 3H2O + 3e− (fast)

The rate-determining step is one electron transfer followed by a three-electron process to give nitrogen and water as the final products. The overall oxidation reaction of hydrazine in alkaline media can be expressed as N2H4 + 4OH− → N2 + 4H2O + 4e−. Thus, Cu−GP should not suffer from the poisoning effect of oxidation products. The high stability of the nitrogen molecules also helps to explain why the overall reaction is totally irreversible as observed earlier in the CVs, which show no cathodic peak for hydrazine oxidation on Cu−GP. Catalyst stability is crucial for the long-term operation of fuel cells. The stability of the modified electrodes is first investigated by chronoamperometry under the near-peak potentials (Figure S4, Supporting Information). The result shows that the Cu− GP possesses a higher current density than GP, and the current density remains steady for continuous reaction of 3000 s. The stability of the flexible Cu−GP electrode is also evaluated by monitoring the change of current density in successive detections of 10 mM hydrazine using one electrode. Agglomeration or migration of electrocatalysts on carbon support is a vital factor leading to their inferior stability and decreased activity.58 In our approach, Cu nanocubes were directly grown on GP by electrodeposition, and a robust and intimate contact is expected to form between Cu nanocubes and GP, efficiently preventing the agglomeration and enhancing the stability of the electrocatalysts. In addition, the in situ formed Cu hydroxides/oxides constitute a stable passivating layer and prevent Cu nanocubes from further corrosion.59 Cu− GP also showed excellent stability for hydrazine oxidation, with only 4.7% loss of the initial peak current density after 200 cycles (Figure S5, Supporting Information), and nearly no morphological (Figure S6) and composition (Figures S7 and S8,

Figure 7. (A) Cyclic voltammograms of Cu−GP in 0.1 M KOH containing 10 mM hydrazine at a scan rate of 10, 20, 40, 60, 80, and 100 mV s−1 (from bottom to top). (B) Chronocoulometric response of Cu−GP in 0.1 M KOH solutions containing 10.0 (a) and 0.0 mM (b) hydrazine. 7723

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Supporting Information) changes of Cu−GP were observed after 200 cycles.

4. CONCLUSIONS In conclusion, a free-standing electrode is obtained by electrodeposition of Cu nanocubes on GP. The resultant Cu−GP, taking advantage of the high electrical conductivity of GP, well-dispersed Cu nanocubes, and the strong interaction between Cu nanocubes and GP substrate endowed by the electrodeposition method, exhibits high catalytic activity and stability toward the oxidation of hydrazine. An in situ formed thin layer of Cu hydroxides/oxides on Cu nanocube surfaces can lower the overpotential and stabilize the catalyst in alkaline conditions. A totally irreversible and diffusion-controlled oxidation of hydrazine occurs on Cu−GP with nitrogen and water as the reaction products. The use of free-standing Cu− GP electrodes as the catalyst layer for DHFC applications would greatly simplify the manufacturing of electrode assemblies of fuel cells and offers new possibilities for the development of flexible power sources.

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ASSOCIATED CONTENT

S Supporting Information *

Supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.D. thanks the program of Nanyang Assistant Professorship for final support. H. G. is a recipient of a graduate research scholarship supported by Nanyang Technological University, Singapore.

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REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp3021276 | J. Phys. Chem. C 2012, 116, 7719−7725