Muhammad Ali Ehsana,
Abbas Saeed Hakeema,
Hamid Khaledib,
Muhammad Mazhar*b,
Muhammad Mehmood Shahidc,
Alagarsamy Pandikumarc and
Nay Ming Huangc
aCenter of Research Excellence in Nanotechnology (CENT), King Fahd University of Petrolium & Minerals, Saudia Arabia
bDepartment of Chemistry, Faculty of Science, University of Malaya, Lembah Pantai, 50603-Kuala Lumpur, Malaysia. E-mail: mazhar42pk@yahoo.com; Fax: +60 379674193; Tel: +60 379674269
cDepartment of Physics, Faculty of Science, University of Malaya, Lembah Pantai, 50603-Kuala Lumpur, Malaysia
First published on 30th November 2015
A heteronuclear coordination complex [Cu4Zr6(μ-O)8(dmap)4(OAc)12]·H2O (1), where dmap = N,N-dimethylaminopropanolato and −OAc = acetato, has been isolated in pure form by the chemical interaction of Zr(dmap)4 with Cu(OAc)2·H2O in THF. Complex (1) has been examined by melting point, elemental analysis, FT-IR spectroscopy and single crystal X-ray diffraction. The thermal decomposition behavior of the complex has been explored by thermogravimetric, derivative thermogravimetric and differential scanning calorimetric analyses which reveal that complete conversion of (1) into 1:
1.5 composite oxides, CuO
:
ZrO2, treated at 500 °C. The ability of complex (1) to act as a single-source precursor for the formation of advanced composite oxides thin film has been investigated by aerosol assisted chemical vapor deposition at 550 °C in air ambient. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX), X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopic (XPS) analyses of the developed thin films suggest the formation of good quality crystalline microspherical-shaped CuO–1.5ZrO2 composite oxide with high purity. The electrocatalytic activity of CuO–1.5ZrO2 composite oxide film was studied toward methanol oxidation in an alkaline medium and it showed high oxidation peak current of 14 μA during a forward scan which is ∼3.5-fold higher than the bare Pt electrode. The ease and low cost fabrication and high electrocatalytic activity of composite oxide film could make it potential candidate for direct methanol fuel cells application.
A survey of known heterobimetallic Cu–Zr complexes includes a series of compounds, [(RCOCHCOR′)Cu{Zr2(OPr-i)}] (R = R′ = Me (1); R = R′ = CF3 (2) and R = Me, R′ = CF3 (3)), which have been spectroscopically probed.16 Some interesting zirconium copper(I) [Zr4Cu4(μ4-O)(μ-OPri)10(OPri)8]17 and zirconium copper(II)oxo-isopropoxo [Zr4Cu4(μ4-O)3(μ-OPri)10(OPri)8]18 complexes have also been structurally characterized. However, the thermal degradation pathways of these Cu–Zr complexes and their potential as CVD precursor for the synthesis of composite oxides nanopowders or thin films have not been investigated so far.
Previously we designed and synthesized a well-defined heterobimetallic complex [Cu4Zr2(μ4-O)2(dmae)4(OAc)8]·2H2O using N,N-dimethylaminoethanolato (dmae) as bridging moiety between two metal centers and the thermal decomposition of this complex resulted in formation of CuZrO3–CuO thin films.19 The continuation of similar synthetic strategy enabled us to construct a new Cu–Zr heterometallic derivative complex [Cu4Zr6(μ-O)8(dmap)4(OAc)12]·H2O through a different aminoalcoholic bridging moiety, 3-dimethylamino-1-propanolato (CH3)2N(CH2)3O−; (dmap), which exhibits different stoichiometry as compare to the earlier reported complex.19 A new bridging connectivity pattern has been achieved within the heterometallic assembly to produce a novel and modified heterometallic complex with improved physicochemical properties. The complex (1) decomposes under ordinary aerosol assisted chemical vapor deposition (AACVD) conditions to yield impurity free target material (CuO–1.5ZrO2) inspite of increasing number of carbons in damp ligand. Further, the CuO–1.5ZrO2 thin film electrodes of high temperature material have been fabricated at relatively low temperature which is another unique prospective achieved through complex (1).
Considering future commercial-scale availability from coal gasification and natural gas modification, high sensitivity to catalysts and easy conversion to chemicals, methanol has gained importance as an important feedstock for chemical industry, especially in fuel cells.20 In an electrochemical oxidation of methanol, the electrode material is an important parameter where high efficiency electrocatalyst is needed. Zirconia based copper oxide nanocomposites have been shown to perform numerous catalytic operations such as methanol synthesis,21,22 water gas shift reactions,23,24 NO and N2O decompositions25 and various hydrogenation reactions.26,27 The high catalyst activity is mainly accounted by the presence of zirconia which presents special characteristics such as high fracture toughness, ionic conductivity and stability even under reducing conditions.28 Moreover, the possession of both amphoteric and redox functions, rich surface oxygen vacancies29 and surface hydroxyl groups30,31 make it superior when compared with conventional copper oxide–zinc oxide catalyst for numerous catalytic applications. Recent literature reveals many ways of producing binary CuO–ZrO2 nanopowders including deposition–precipitation,23 precipitation,32 surfactant assisted method26 and sol–gel method.33 However, the fabrication of copper oxide–zirconia composite thin film electrodes is not frequently addressed in the literature.
In the present work, we aim to report the synthesis, structural characterization and thermal characteristics of a new heterobimetallic complex [Cu4Zr6(μ-O)8(dmap)4(OAc)12]·H2O (1), (dmap = N,N-dimethylaminopropanolate; −OAc = acetato) for the single step growth of CuO–1.5ZrO2 composite oxide thin film electrodes on fluorine doped tin oxide coated (FTO) conducting glass substrate at 550 °C via AACVD technique. The deposited electrodes were characterized by XRD, SEM, EDX and XPS for their phase, structural, textural and compositional recognition. The electrocatalytic activity of CuO–1.5ZrO2 composite oxide electrodes was studied toward methanol oxidation in an alkaline medium and it showed high oxidation peak current of 14 μA during a forward scan which is ∼3.5-fold higher than the bare Pt electrode. Considering the high cost of Pt, the present metal oxide composite catalyst is potential alternative for direct methanol fuel cells towards methanol oxidation.
The surface morphology and chemical composition of thin films were analyzed by field-emission gun scanning electron microscope (Hitachi FESEM SU 8000) equipped with an energy dispersive X-ray spectrometer EDX (INCA Energy 200, Oxford Inst.) operated at an accelerating voltage of 20 kV and a working distance of 9.2 mm. X-ray photoelectron spectroscopy of thin films were studied using an ULVAC-PHI Quantera II with a 32-channel Spherical Capacitor Energy Analyzer under vacuum (1 × 10−6 Pa) using Monochromated Al Kα radiation (1486.8 eV) and natural energy width of 680 meV. The carbonaceous C 1s line (284.6 eV) was used as a reference to calibrate the binding energies.
As reported earlier10–15 the reaction of metal alkoxides or aminoalkoxides with metal acetate is a promising method to design heterobimetallic complex for the deposition of thin films of a desired oxide material. This type of synthetic strategy aims to coordinatively saturate each metal center by the use of chelating ligands such as carboxylate and a functionalized alcohol which strictly restrict the resulting complexes into a molecular regime and enhance their stability, solubility, volatility and shelf life. Metal alkoxides and carboxylates generally react with one another by the elimination of an ester as a volatile by-product, but in the reaction observed here, it is more likely that the oligomeric oxo complex was formed through hydrolysis from the hydrated starting materials in combination with the loss of acetate and dmapH ligands. However, such reactions can occur under very mild conditions (room temperature and nonpolar solvents), and thus the complexes can be synthesized by simple mixing of the starting materials in an appropriate solvent. The reactions between Cu(OAc)2·H2O with Zr(dmap)4 demonstrate these features. These reactions proceed smoothly and quantitatively in hydrocarbon solvents over a period of 1 or 2 h with progressive dissolution of the acetate. The unreacted excess of metal acetate is easily removed by filtration, and the complexes can be crystallized out almost quantitatively from the filtrate. Thus the complex [Cu4Zr6(μ-O)8(dmap)4(OAc)12]·H2O (1), was prepared in yield of 70% by reacting appropriate amounts of tetrameric N,N-dimethylaminopropanolato Zr(IV) with Cu(II)acetate in THF at room temperature. The compound was found to be soluble in polar solvents and solubility was found to be poorer in solvents like benzene and toluene. The compound was stable under ambient conditions. The microanalysis of complex (1) was found to be consistent with the empirical formula C44H86Cu4Zr6N4O37. The appearance of a broad band at 3436 cm−1 is assignable to the ν(O–H) vibration of the solvated water molecules present in (1). The IR spectrum of complex (1) shows the presence of characteristic vibrations of both carboxylato and aminoalcoholato functional groups attached to the copper and zirconium atoms. In complex (1), the different modes of the carboxylate binding, namely bidentate bridging, bridging-monodentate and monodendate terminal coordination of the acetate ligand have also been established from the IR spectrum. The two strong bands at 1563 and 1430 cm−1 are due to the asymmetric and symmetric stretching vibrations of a bridging acetate functionality of the ligand, respectively. The difference of Δ = 133 cm−1 between the asymmetric and symmetric stretching vibrations is attributed to the bidentate bridging of the acetate group.40 In addition, the asymmetric and symmetric carboxylate stretches are observed at 1608 and 1330 cm−1 respectively, (Δ = 278 cm−1), suggesting the presence of bridging monodentate coordination mode of the acetate ligand. Generally the order of Δ, (Δ = νas(COO−) − νs(COO−)), for metal carboxylates is of the order Δ(bridging) < Δ(monodentate). The lower Δ value (133 cm−1) compared to that of higher value (278 cm−1) strongly suggest the presence of bidentate bridging along with the bridging monodentate coordination.41 The absorptions at the low frequencies of 531 and 463 cm−1 are probably due to M–O and M–N stretching vibrations respectively.42
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Fig. 1 Crystal structure of complex [Cu4Zr6(μ-O)8(dmap)4(OAc)12] (1). H-atoms and the hydration water molecules are omitted for clarity. Zr: golden, Cu: turquoise, O: red, N: blue, C: dark grey. |
Empirical formula | C44H86Cu4N4O37Zr6 |
Formula weight | 2064.64 |
Crystal system, space group | Triclinic, P![]() |
a | 12.0851(2) Å |
b | 13.2867(3) Å |
c | 23.8841(4) Å |
α | 77.8260(10)° |
β | 79.3910(10)° |
γ | 70.9570(10)° |
Volume | 3516.00(12) Å3 |
Z | 2 |
Calculated density | 1.950 mg m−3 |
Absorption coefficient | 2.126 mm−1 |
F(000) | 2060 |
Crystal size | 0.360 × 0.210 × 0.150 mm3 |
Crystal color and habit | Green block |
θ range for data collection | 1.643 to 29.130° |
Reflections collected | 37![]() |
Independent reflections | 18![]() |
Observed reflections [I > 2σ(I)] | 13![]() |
Completeness to θ = 25.242° | 99.5% |
Data/restraints/parameters | 18![]() |
Goodness-of-fit on F2 | 1.032 |
Final R indices [I > 2σ(I)] | R1 = 0.0390, wR2 = 0.0820 |
R indices (all data) | R1 = 0.0613, wR2 = 0.0913 |
Largest diff. peak and hole | 0.962 and −0.879 (e Å−3) |
Zr(1)–O(4) | 2.073(2) | Zr(5)–O(27) | 2.133(2) |
Zr(1)–O(2) | 2.077(2) | Zr(5)–O(28) | 2.153(2) |
Zr(1)–O(3) | 2.191(2) | Zr(5)–O(30) | 2.175(3) |
Zr(1)–O(1) | 2.203(2) | Zr(5)–O(5) | 2.190(2) |
Zr(1)–O(9) | 2.259(2) | Zr(5)–O(3) | 2.201(2) |
Zr(1)–O(12) | 2.265(2) | Zr(6)–O(4) | 2.053(2) |
Zr(1)–O(10) | 2.302(2) | Zr(6)–O(8) | 2.079(2) |
Zr(1)–O(11) | 2.336(2) | Zr(6)–O(32) | 2.098(2) |
Zr(2)–O(6) | 2.055(2) | Zr(6)–O(7) | 2.159(2) |
Zr(2)–O(4) | 2.083(2) | Zr(6)–O(33) | 2.191(2) |
Zr(2)–O(13) | 2.107(2) | Zr(6)–O(3) | 2.193(2) |
Zr(2)–O(16) | 2.144(3) | Zr(6)–O(35) | 2.197(3) |
Zr(2)–O(1) | 2.182(2) | Cu(1)–O(13) | 1.949(2) |
Zr(2)–O(14) | 2.191(2) | Cu(1)–O(7) | 1.949(2) |
Zr(2)–O(7) | 2.208(2) | Cu(1)–O(34) | 1.954(3) |
Zr(3)–O(2) | 2.056(2) | Cu(1)–N(1) | 2.036(3) |
Zr(3)–O(6) | 2.079(2) | Cu(1)–O(35) | 2.465(3) |
Zr(3)–O(18) | 2.109(2) | Cu(2)–O(15) | 1.954(2) |
Zr(3)–O(5) | 2.189(2) | Cu(2)–O(1) | 1.956(2) |
Zr(3)–O(1) | 2.191(2) | Cu(2)–O(18) | 1.970(2) |
Zr(3)–O(21) | 2.191(2) | Cu(2)–N(2) | 2.052(3) |
Zr(3)–O(19) | 2.199(2) | Cu(2)–O(17) | 2.349(3) |
Zr(4)–O(8) | 2.074(2) | Cu(3)–O(27) | 1.947(2) |
Zr(4)–O(6) | 2.080(2) | Cu(3)–O(20) | 1.949(3) |
Zr(4)–O(7) | 2.166(2) | Cu(3)–O(5) | 1.952(2) |
Zr(4)–O(5) | 2.182(2) | Cu(3)–N(3) | 2.022(3) |
Zr(4)–O(23) | 2.256(2) | Cu(3)–O(21) | 2.448(2) |
Zr(4)–O(26) | 2.258(2) | Cu(4)–O(31) | 1.947(3) |
Zr(4)–O(25) | 2.325(3) | Cu(4)–O(3) | 1.951(2) |
Zr(4)–O(24) | 2.340(3) | Cu(4)–O(32) | 1.975(3) |
Zr(5)–O(8) | 2.065(2) | Cu(4)–N(4) | 2.055(3) |
Zr(5)–O(2) | 2.088(2) | Cu(4)–O(29) | 2.269(3) |
In the molecule, six octahedrally arranged ZrIV atoms are bridged by eight oxides forming a rhombic dodecahedral Zr6O8 cage. The cage is surrounded by four tetrahedrally located CuII atoms. Four of the cage O-atoms are of μ4-type, each bridges one CuII and three ZrIV centers. The other four oxides are μ3 and link only three ZrIV centers. The Zr-(μ4-O) distances range between 2.159 and 2.208 Å, which are slightly longer than Zr-(μ3-O) bond lengths (2.055–2.088 Å). Two of the Zr centers (Zr1 and Zr4) are eight coordinated by four cage O-atoms and two chelating acetato ligands. The other four Zr centers are seven coordinated by four cage O-atoms, and three O atoms from one dmap and two acetato ligands. Each Cu atom is coordinated by one cage O-atom, one N,O-chelating dmap and two bridging acetato ligands in a distorted square-pyramidal geometry. In the crystal, hydration water molecules link the neighbouring complex molecules through O–H⋯O interactions.
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Fig. 3 TG (black), DTG (red) and DSC (blue) profiles showing thermal decomposition of complex (1) as a function of temperature. |
DSC (blue curve in Fig. 3) recognizes these weight loss stages in form of endothermic peaks at 90, 256, 290 and 334 °C and designates as the desolvation of water molecules, melting and decomposition phases, respectively occurring in complex (1) during its heat treatment process. The weight loss featured in Fig. 3 terminates at 500 °C providing stable residues of 50.95% of its original mass which is in reasonable agreement with the formation of the expected 1:
1.5 of CuO
:
ZrO2 (51.15%) composite oxide material from (1). The exothermic peak appearing at 495 °C in DSC is attributed with the crystallization process occurring in the residual CuO–1.5ZrO2 material.
The XRD qualitative phase analysis reveals that all the copper oxide–zirconia composite films deposited from two different solvents have similar crystalline phases of tenorite CuO and a zirconium oxide-Ht. Subsequently, the XRD semi-quantification analysis was applied on each X-ray diffractogram to measure the proportion of crystallinity of each phase in the crystalline composite product. The crystalline composition of CuO–1.5ZrO2 deposit obtained from methanol is poised at 30% CuO and 70% ZrO2 respectively (inset Fig. 4(a)). However the film deposited from ethanol contains the crystalline contents of 53% CuO and 47% ZrO2 respectively. It is worth emphasizing that the films obtained from two different solvents differ in terms of their percentage crystalline composition of the phases, however the overall phase composition of 1:
1.5 for CuO
:
ZrO2 can be further confirmed from energy dispersive X-ray and XPS analyses.
The disparities in crystalline composition of the films prepared from different solvents propose that solvents play a key role in dictating the crystalline phases of the films grown on the substrate surface by AACVD, and not just as a transport medium. In aerosol deposition, solvents play an important role in the determination of the extent of a reaction. The precursor can react differently in various solvents in the gas phase which may lead to the formation of different intermediates and thus to different phases of the deposit. There have been similar reports whereby a variety of solvents have been used to alter the phase composition of materials using AACVD technique.45,46
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Fig. 5 SEM images of CuO–1.5ZrO2 composite oxide films deposited on FTO glass substrate at 550 °C from solution of precursor (1) in (a) methanol (b) ethanol. |
Fig. 5(a) shows that the surface architecture of the film obtained from methanol solution is composed of well-defined and orderly packed microspheres having a size range of 0.2–0.7 μm. Fig. 5(a) also demonstrates that textures of some microspheres belong to highly wrinkled morphology while the other type displays a smooth and simple design.
Fig. 5(b) shows the surface morphology of CuO–1.5ZrO2 composite thin films grown from the ethanol solution of (1). The film consists of interconnected spherical objects of heterogeneous design, shape and size which are grown in the vertical direction of the substrate plane. One type of microspherical object attain donuts shape structure while the others are round ball shaped entities of size range 0.5–1.3 μm.
The mechanism of formation of different design, shape and size microspheres thin films by AACVD is mainly controlled by homogenous and heterogeneous type of reactions occurring between precursor gaseous intermediate and substrate surface. The gas phase reaction pathways are significantly inveigled by the type of solvent used and are explicated well by us and others previously.47,48 In the present scenario we used two different types of solvents such as methanol and ethanol which because of their different physical properties (densities, boiling points and enthalpies) affect the course of gas phase reactions and help in building microsphere structures of different shape and architectures.
The elemental composition of CuO–1.5ZrO2 composite oxide thin films were determined by energy dispersive X-ray (EDX) analysis and are presented in Fig. S1 and S2.† The EDX analysis executed on several randomly selected large regions reveal that the metallic ratio of Cu:
Zr in the films is close to 1
:
1.5 confirming the retention of the same metallic ratio in the films as found in complex (1).
Further the composite nature of the CuO–1.5ZrO2 films invented from methanol and ethanol solutions of precursor (1) was established from EDX mapping (Fig. 6(a) and (b)) which reveals that Cu, Zr and O atoms are evenly distributed throughout the films matrix.
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Fig. 7 (a) Wide scan XPS spectra of CuO–1.5ZrO2 composite thin films prepared from methanol (blue line) and ethanol (black line); high resolution spectra CuO–1.5ZrO2 for (b) Zr 3d (c) O 1s (d) Cu 2p. |
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Fig. 8 Cyclic voltammograms observed for the (a) bare Pt and (b) CuO–1.5 ZrO2 composite oxide film modified Pt electrode in the presence of 0.5 M CH3OH and 0.1 M KOH at a scan rate of 50 mV s−1. |
It is observed that the CuO–1.5ZrO2 composite electrode showed a higher electrocatalytic oxidation current during forward (14 μA) and backward (4 μA) scans compared to the bare Pt electrodes. The peak currents observed during the forward and reverse scans were due to the methanol oxidation and removal of the residual carbonaceous species formed in the forward scan, respectively.52,53 The higher electrocatalytic activity of the CuO–1.5ZrO2 modified electrode compared with bare Pt electrode was attributed to the catalytic properties of copper oxide–zirconia composite material and tightly packed spherical architecture of the film.
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Fig. 9 CV recorded for the CuO–1.5ZrO2 composite oxide film modified Pt electrode in presence of 0.5 M CH3OH and 0.1 M KOH at different scan rates in range of 10–350 mV s−1. |
The CuO–1.5ZrO2 composite oxide film modified Pt electrode showed a linearly increase anodic peak current towards forward and backward oxidation peak. This suggests that the electrocatalytic oxidation of methanol at the CuO–1.5ZrO2 composite oxide film modified Pt electrode is a diffusion-controlled process.
In practical application of direct methanol fuel cells (DMFC), methanol concentration plays a vital role. In this regard effect of methanol concentration was studied by varying the concentration of methanol in the presence of CuO–1.5ZrO2 composite oxide film modified Pt electrode at a scan rate of 50 mV s−1 in the presence of 0.1 M KOH and the results are shown in Fig. 10. It can be noticed that anodic peak current increases and the anodic peak potential have a slightly shifted to positive potential while increase in the methanol content due to the saturation of active catalytic sites at the CuO–1.5ZrO2 composite oxide film modified Pt electrode surface.
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Fig. 10 CV recorded for the CuO–1.5ZrO2 composite oxide film modified Pt electrode at scan rate 50 mV s−1 in presence of 0.1 M KOH and various concentrations of CH3OH. |
Table 3 shows the comparison of electrocatalytic performances of some metal oxides catalysts towards methanol oxidation.
Catalysts | Preparation method | Electrolyte | Medium | Scan rate (mV s−1) | Peak potential (mV) | Current | Ref. | |
---|---|---|---|---|---|---|---|---|
Forward scan | Reverse scan | |||||||
Pt/CeO2/CNTs | Two steps procedure sol–gel and adsorption | 0.5 M H2SO4 | Acidic | 50 | 660 | 370 | 0.87 mA cm−2 | 54 |
Pt/TiO2/CNTs | Two steps synthesis sol–gel and adsorption | 0.5 M H2SO4 | Acidic | 50 | 680 | 390 | 0.55 mA cm−2 | |
Pt/SnO2/CNTs | Two steps synthesis sol–gel and adsorption | 0.5 M H2SO4 | Acidic | 50 | 670 | 420 | 0.43 mA cm−2 | |
Pt–ZrO2/C | Two-step chemical reduction | 0.1 M KOH | Alkaline | 50 | −270 | 400 | 25 mA cm−2 | 55 |
Pt/MnO2/rGO | Spontaneous and electroless method | 0.5 M H2SO4 | Acidic | 50 | 700 | 500 | 425 mA mg−1 | 56 |
Pt/h-rGO@TiO2–rGO | Deposition and hydrothermal method | 0.5 M H2SO4 | Acidic | 50 | 700 | 500 | 324 mA mg−1 | 57 |
RGO–Co3O4 | Hydrothermal method | 0.1 M KOH | Alkaline | 50 | −150 | −225 | 362 μA cm−2 | 58 |
Pd/TiO2–C | Hydrothermal growth method | 0.1 M KOH | Alkaline | 50 | −200 | −400 | 4 mA cm−2 | 59 |
CuO–1.5ZrO2 | AACVD | 0.1 M KOH | Alkaline | 50 | −250 | −350 | 14 μA | Present work |
It is widely accepted that CO species produced in the process of methanol electrooxidation are the main poising intermediate that slow down the oxidation kinetics.54 This problem can be solved by using metal oxides (TiO2, ZrO2 and MoO2) which can effectively promote the electrocatalytic oxidation of methanol and show perfect tolerance to CO poisoning.54 At the same time the low price and abundance of metal oxide can help to reduce the cost of DMFC. Further, to improve the performance of electrocatalytic oxidation of methanol, carbon nanotubes (CNTs) and reduced graphene oxide (rGO) have been frequently used as the support which provides large network for collecting electrons from oxidation process thereby assisting efficient current generation. It can be seen from Table 3 that the metal oxide composite with CNTs and rGO showed higher methanol oxidation current than the present catalyst, CuO–1.5ZrO2. However, considering the high cost of Pt metals, present catalyst prepared from AACVD technique is suitable alternative to Pt free electrocatalyst for the methanol oxidation due to the low cost and ease of fabrication.
Footnote |
† Electronic supplementary information (ESI) available: Microanalysis result, energy dispersive X-ray spectra. CCDC 1401618. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra18053h |
This journal is © The Royal Society of Chemistry 2015 |