Enhanced hydrogen/oxygen evolution and stability of nanocrystalline (4–6 nm) copper particles

Journal ofMaterials Chemistry A

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Department of Chemistry, Indian Institute

110016, India. E-mail: [email protected]

1126591511

Cite this: DOI: 10.1039/c3ta01194a

Received 22nd November 2012Accepted 4th February 2013

DOI: 10.1039/c3ta01194a

www.rsc.org/MaterialsA

This journal is ª The Royal Society of

Enhanced hydrogen/oxygen evolution and stability ofnanocrystalline (4–6 nm) copper particles

Bharat Kumar, Soumen Saha, Mrinmoyee Basu and Ashok K. Ganguli*

The exploration of nanomaterials as catalysts for hydrogen and oxygen evolution is highly desirable for

renewable and clean energy applications. We have obtained highly efficient and stable copper

nanoparticles (4–6 nm) by thermal decomposition of aligned copper oxalate nanorods (obtained by a

microemulsion method) in an argon atmosphere. Hydrogen and oxygen evolution reactions (HER and

OER) were carried out on glassy carbon as well as platinum as working electrodes in KOH solution. In

the case of HER the current densities were found to be 12 mA cm�2 (glassy carbon electrode) and 46

mA cm�2 (Pt electrode) which are significantly higher than reported values (maximum 1 mA cm�2). In

the case of OER the current density was found to be 1.6 mA cm�2 (which is slightly higher) for the

glassy carbon electrode and 15 mA cm�2 for Pt as the working electrode which is 4–30 times higher

than earlier reports. The high efficiency can be related to the high surface area (34 m2 g�1) of these tiny

well-crystalline copper nanoparticles obtained by the microemulsion mediated synthesis. The copper

nanoparticles obtained by us show excellent stability as electrocatalysts and retain their activity even

after 50 cycles.

Introduction

Nano-sized materials1,2 have been of interest for their applica-tions in magnetic,3 electrical,4 optical5 and catalytic6,7 propertiesdue to the higher surface area to volume ratio. Among metalnanoparticles, copper is of special interest compared to silver,gold and platinum nanoparticles due to its lower cost and wideavailability. It has importance in catalytic,8,9 electrical,10 anti-fouling11 and optical12 applications and also as conductive ink,13

antibacterials,14 and in electronic industry.15

Renewable and clean energy is a major challenge for scien-tists due to the dwindling natural resources like coal and oil.Hydrogen is an important form of energy because it is a cleanfuel (byproduct is H2O) and has a higher energy content.Splitting of water to obtain hydrogen and oxygen has a greatpotential to provide sustainable energy through fuel cells andbattery devices. In the fuel cells, hydrogen is used as a fuel byconverting the chemical energy contained in the hydrogenmolecule into electrical energy. Hydrogen also reacts withoxygen to produce electricity in a fuel cell. Hydrogen is used as afuel in automobile and aerospace industries. Oxygen is alsoused in fuel cells which helps in power generation by enhancingthe temperature of the reactor due to complete combustion ofthe fuels.

of Technology, Hauz Khas, New Delhi

td.ac.in; Fax: +91 1126854715; Tel: +91

Chemistry 2013

One of the major drawbacks to produce energy through theelectrochemical route on the industrial scale is the unavail-ability of cheap and highly efficient electrocatalysts for thehydrogen evolution reaction (HER) and oxygen evolution reac-tion (OER). Copper metal is a very good electrocatalyst16 forproducing hydrogen for fuel cells. In addition to its role inproducing H2/O2, it is also used as an electrocatalyst for otherreactions. Reduction of nitrite and nitric oxide is possible byusing copper nanoparticles as electrocatalysts and they showefficient electron transfer characteristics.17 Electrocatalyticoxidation of amino acids has been reported using coppernanoparticles.18 There have been reports on hydrogen evolutionreaction and oxygen evolution reaction (HER and OER) usingnano-sized metals and alloys like Co, Co–Ni, Cu and Fe–Co.19–21

There are various methods reported for the synthesis ofcopper nanoparticles including a electrochemical depositionroute,22 hydrothermal method,23 electrolysis,24 microwave-assisted polyol method,25 reverse micelle synthesis,20,26,27 metalvapor synthesis,28 sonochemical,29 thermal reduction,29,30 pho-tocatalytic decomposition,31 gas phase evaporation,32 and usinghydrophobically immobilized surfactant template.33 EarlierAhmed et al.20 synthesized Cu nanoparticles of differentmorphology by changing the aqueous phase concentration inthe reverse micelle method. However not much was discussedregarding the role of the precursor in the controlled synthesis ofCu nanoparticles. A highmetal concentration (1 M) was used forthe synthesis of spherical nanoparticles in the above studies.The nanoparticles were obtained at 400 �C which variedbetween 5 and 50 nm in size.

J. Mater. Chem. A

http://dx.doi.org/10.1039/c3ta01194a

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Fig. 2 Powder X-ray diffraction pattern of copper nanoparticles obtained fromthermal decomposition of copper oxalate {inset shows X-ray line broadening ofthe (111) reflection}.

Fig. 3 Thermogravimetric analysis of copper oxalate nanorods (inset DTG).

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4AView Article Online

Here we report the synthesis of very ne and uniform nano-particles (�4 to 6 nm) by the thermal decomposition of copperoxalate nanorods in an argon atmosphere. These nanorods wereobtained using a microemulsion method. We have extensivelycharacterized the nanoparticles using X-ray diffraction, eldemission scanning electron microscopy, transmission electronmicroscopy (TEM) and X-ray photoelectron spectroscopy (XPS).The electrocatalytic properties for hydrogen and oxygen evolutionreaction (HER and OER) were investigated which show signi-cantly higher catalytic activity than reported earlier.

Results and discussion

Fig. 1 shows X-ray diffraction patterns of the initial precursor ofcopper oxalate crystallized in the orthorhombic system (JCPDS-210297) with the average crystallite size of 39 nm (inset of Fig. 1).Monophasic copper nanoparticles (Fig. 2) are obtained aerheating the copper oxalate at 325 �C in argon. All the observedreections were indexed on the basis of a face centered cubic(Fm3m) crystal systemwith rened cell parameters of a¼ 3.6142(1)A. The crystallite size of the particles was observed to be 7 nm fromX-ray line broadening (Fig. 2, inset) using Scherrer's formula.

In the TGA studies copper oxalate monohydrate shows aweight loss (Fig. 3) in nitrogen atmosphere in the range of 200–305 �C due to the loss of one water and two carbon dioxidemolecules. In the derivative plot the sharp loss observed at 240 �Cand 300 �C is due to loss of H2O and CO2molecules (Fig. 3, inset).The total weight loss was nearly 62% which conforms to theexpected loss. Earlier Mohamed et al.30 reported thermaldecomposition of anhydrous copper oxalate in different gasatmospheres (N2, CO2, H2, air) which shows the decomposition at299.8 �C, 309.6 �C, 283.5 �C and 307.1 �C respectively. Differentialscanning calorimetry (Fig. 4) was carried out for the copperoxalate nanorods which show two transitions. The transition at180 �C is due to the removal of water molecules which is endo-thermic in nature and the transition at 303 �C is due to the loss ofcarbon dioxide molecule which is exothermic. The changes inenthalpy were 3.3 J g�1 and 113 J g�1 respectively. Mohamed

Fig. 1 Powder X-ray diffraction pattern of copper oxalate nanorods(CuC2O4$2H2O) obtained from reverse micelle {inset shows X-ray line broadeningof the (110) reflection}. Fig. 4 Differential scanning calorimetry of copper oxalate nanorods.

J. Mater. Chem. A This journal is ª The Royal Society of Chemistry 2013


Fig. 7 HRTEM micrograph of copper nanoparticles.

Fig. 8 FESEM micrograph of copper nanoparticles.

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et al.30 also reported DSC for anhydrous copper oxalate indifferent gas atmospheres (N2, CO2, H2, air). They observedexothermic peaks at 296 �C, 299 �C, 304 �C, and 318 �C respec-tively. The change in enthalpies are 78.5 J g�1, 118.7 J g�1, 139J g�1 and 287 J g�1 for N2, CO2, H2 and air respectively. The highervalue (113 J g�1) obtained in our study compared to the results ofMohamed et al. obtained in a N2 atmosphere is due to the pres-ence of water molecules in aligned nanorods of copper oxalatemonohydrate.

Transmission electron microscopy of copper oxalate mono-hydrate shows the formation of aligned nanorods having thediameter of 215–240 (�7) nm and length of 750–1000 (�20) nm(Fig. 5a). Earlier studies34,40 reported the synthesis of copperoxalate nanorods having the diameter of 130 nm and length of480 nm.We believe that the increase in length, diameter and theiralignment is due to the increase in stirring time during synthesis.At higher magnication, these aligned nanorods of copper oxalateare seen to be composed of spherical nanoparticles having a sizeof 4–5 (�0.2) nm (Fig. 5b). Fig. 6 shows the TEM micrograph ofcopper nanoparticles obtained from the thermal decomposition ofcopper oxalate. These particles were monodisperse and sphericalin nature. The average diameter of the spherical nanoparticles was4–6 (�0.2) nm. The high resolution TEM micrograph (Fig. 7b) ofthe copper nanoparticles shows the presence of (111) latticefringes which conrms the single crystalline nature of the nano-particles. Field emission scanning electron microscopy (FESEM)studies were carried out on the copper nanoparticles which show(Fig. 8) the formation of dense agglomerated nanoparticles ofspherical morphology with individual sizes of 10–15 nm. XPS

Fig. 5 TEM micrograph of copper oxalate nanorods.

Fig. 6 TEM micrograph of copper nanoparticles.

This journal is ª The Royal Society of Chemistry 2013

studies were carried out to conrm the nature of the Cu nano-particles. The XPS spectra of the synthesized copper nanoparticlesshow the presence of two peaks at 932.4 and 952.3 eV (ref. 20 and29) due to the Cu (2p3/2) and Cu (2p1/2) respectively (Fig. 9), whichsuggest that the copper nanoparticles are in zero oxidation state.Ahmed et al.20 using the reverse micellar method showed theformation of copper nanoparticles of various morphologies(spheres, cubes and nanorods) and size (5–50 nm). Brege et al.33

have synthesized monodispersed copper nanoparticles having asize of 2 nm using N,N,N0,N0-tetramethyl-p-phenylenediamine(TMPDA) and sodium dodecylbenzenesulfonate (SDBS) surfactant

Fig. 9 XPS spectra of Cu nanoparticles.

J. Mater. Chem. A


Fig. 11 HER stability of copper nanoparticles on (a) glassy carbon and (b)platinum electrodes.


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template in the presence of sodium citrate at room temperature.Wen et al.35 reported the synthesis of cube shaped copper nano-particles of 15 nm size. However most of these studies (exceptref. 20) did not investigate the electrochemical properties (HERand OER). The surface area of the synthesized Cu nanoparticleswas found to be 34 m2 g�1 which is much higher than reportedearlier (6.1 m2 g�1, Patra et al.36).

We have investigated the electrochemical properties of theCu nanoparticles using cyclic voltammetry. Electroactivesurface area (ESA)37–39 is a very important parameter in electro-chemistry for calculating the current density. ESA was deter-mined electrochemically by applying the following formula:40

ESA ¼ QH/{Qref (Cu loading)}

QH is calculated by extrapolating the curve obtained fromcyclic voltammetry. The Qref for copper is 420 mC cm�2.41,42 Theelectroactive surface areas of Cu nanoparticles on the glassycarbon electrode and on the Pt electrode are found to be 0.052cm2 mg�1 and 0.119 cm2 mg�1 respectively. These nano-particles were used as electrocatalysts for hydrogen evolutionreaction by applying a negative potential from �1.5 to 0 V forboth the working electrodes (glassy carbon as well as platinum)in 0.5 M KOH solution at the scan rate of 0.025 V s�1. Thehydrogen generation is according to the equations given below:

M + H2O + e� / MHads + OH�

MHads + H2O + e� / H2 + OH� + M

where M ¼ electrocatalyst.The net reaction will be the following,

2H2O + 2e� / H2 + 2OH�

Fig. 10 shows the cyclic voltammograms of copper nano-particles for HER. In the case of glassy carbon electrode

Fig. 10 HER study of copper nanoparticles on (a) glassy carbon and (b) platinum

J. Mater. Chem. A

(Fig. 10a), two redox peaks were observed at �0.61 V and �0.36V. These peaks are observed due to the oxidation of Cu to Cu+

and to Cu++. The maximum current density was found to be 12(�0.2) mA cm�2 at an applied potential of �1.50 V which is thecurrent density for HER. Also in the case of platinum as theworking electrode (Fig. 10b), two redox peaks were observed atpotentials �0.62 V and �0.37 V – due to the oxidation of Cunanoparticles. The maximum current density was 46 (�0.6) mAcm�2 at�1.50 V. The current densities in this study using glassycarbon and platinum electrodes are signicantly higher thanthose reported earlier20 due to themonodisperse, small size (4–6nm) as well as high surface area of the nanoparticles. Thecurrent density depends critically on the surface area andmorphology. The copper nanoparticles obtained by us showexcellent stability for 50 cycles (recorded continuously) (Fig. 11).Stability is also conrmed by the monophasic nature of coppernanoparticles shown by the X-ray diffraction pattern (Fig. 14)aer the hydrogen evolution reaction. The broad hump in PXRDis due to the glass sample holder. The particles remainedspherical in nature as they were before electrocatalysis, havingaverage diameters of 15–20 (�1) nm and 4–6 (�0.2) nm (Fig. 16).The increase in size is due to the agglomeration of the particlein the presence of Naon and KOH solution. The high

electrodes.




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resolution TEM micrograph (Fig. 17) of these copper nano-particles also shows the presence of (111) lattice fringes whichconrms that the particles are still single crystalline.

Oxygen evolution reaction was also studied using cyclic vol-tammetry on glassy carbon as well as platinum as workingelectrodes. These measurements were carried out at the scanrate of 0.025 V s�1 in 0.5 M KOH solution at room temperaturein the potential range of 0.0 to 0.8 V. Fig. 12 shows the cyclicvoltammograms of the OER study using copper nanoparticles.The electrode containing copper nanoparticles as electro-catalysts showed an increase in oxidation current at 0.52 Vpotential for both the working electrodes (glassy carbon and Pt).The following reaction occurs at the electrode surface.

4OH� / 2H2O + O2 + 4e�

The peak current is proportional to the amount of oxygengenerated during the electrochemical reaction. The amount ofcurrent generation is a function of surface area and size ofnanoparticles. The maximum current density at 0.8 V potential is1.6 (�0.01) mA cm�2 and 15 (�0.3) mA cm�2 using the glassycarbon and platinum electrode respectively. In an earlier report20

copper nanoparticles of size of 5–50 nm showed a current densityof 1–4 mA cm�2 on the glassy carbon electrode. Oxygen reduction

Fig. 12 OER study of copper nanoparticles on (a) glassy carbon and (b) platinumelectrodes.

Fig. 13 OER stability of copper nanoparticles on (a) glassy carbon and (b) platinu


reaction (ORR) studies have also been reported using Pt and PtNi43

nanoparticles in the potential range of 0.05 to 1.2 V at a scan rate100 mV s�1 in 0.1 M H2SO4 at 30 �C. Pt–Ni nanoparticles generatehigher current density than Pt nanoparticles. Reier et al.44 recentlyreported OERs for Ru, Ir, Pt nanoparticles and compared withtheir bulk counterparts in 0.1 M HClO4 and 0.05 M H2SO4 whichshow that the nanoparticles generate higher current density. Theyobtained a maximum current density of 4 mA cm�2 fromgeometric surface area for Ir nanoparticles using the glassy carbonelectrode. Kong et al.45 studied the oxygen reduction reaction bycauliower structured Cu nanoparticles in KOH solution andproduced a maximum current of 0.1 mA using the glassy carbonelectrode. Compared to the above reported values the Cu nano-particles obtained by us show a much higher current density (1.6to 4 times) in the case of glassy carbon electrode. The currentdensity is even much higher in the case of Pt as the workingelectrode. The copper nanoparticles show excellent stability for 50cycles (Fig. 13). Powder X-ray diffraction pattern of the electro-catalyst (Cu) aer OER (Fig. 15) shows the presence of a mixedphase of Cu, Cu2O and a small amount of CuO (2%). Trans-mission electron microscopy (Fig. 18) shows agglomeration of theparticles. These agglomerated structures are due to assembly(Fig. 19) of spherical particles having a size of 4–5 (�0.2) nm.

Experimental

Copper oxalate nanorods were synthesized by the reverse micellarroute.46 Two microemulsions containing cetyltrimethy-lammonium bromide (Spectrochem, 98%) as the surfactant,n-butanol (Qualigens, 99.5%) as the co-surfactant and isooctane(Spectrochem, 98%) as the oil phase were taken. 0.1 M coppernitrate (CDH, 98%) and ammonium oxalate (CDH, 99%) wereadded as aqueous phases to the two solutions respectively. Theweight fractions of these constituents were 16.76% CTAB, 13.90%n-butanol, 59.29% isooctane and 10.05% aqueous phases. Theabove two microemulsions were mixed and stirred for two daysand the precipitate was centrifuged and washed using chloroformand methanol (1 : 1 ratio) and dried in air. These gave copperoxalate nanorods.34 The copper nanoparticles were obtained bythermal decomposition of the above copper oxalate nanorods by

m electrodes.

J. Mater. Chem. A


Fig. 15 Powder X-ray diffraction pattern of copper nanoparticles (after OER overthe glassy carbon electrode).

Fig. 18 TEM micrograph of copper nanoparticles (after OER over the glassycarbon electrode).

Fig. 14 Powder X-ray diffraction pattern of copper nanoparticles (after HER overthe glassy carbon electrode).

Fig. 16 TEM micrograph of copper nanoparticles (after HER over the glassycarbon electrode).

Fig. 17 HRTEM micrograph of copper nanoparticles (after HER over the glassycarbon electrode).

Fig. 19 HRTEM micrograph of copper nanoparticles (after OER over the glassycarbon electrode).


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heating at 325 �C for 6 h in an argon atmosphere (the heating andcooling rate was kept at 5 �C per hour).

Powder X-ray diffraction studies (PXRD) were carried outusing Ni ltered Cu-Ka radiation. Normal scans were recordedwith a step size of 0.02� and a step time of 1 s. The Ka2 reec-tions were removed to obtain accurate lattice constants. Thecrystallite sizes of the particles were determined from X-ray linebroadening studies using Scherrer's formula (t ¼ 0.9l/Bcos q)where t is the crystallite size, l is the wavelength (for CuKa, l ¼1.5418 A) and B ¼ O(BM

2 � BS2) (BM is the full width at half

maximum for a particular reection of the sample and BS is thatof standard (with a crystallite size of around 2 mm)). A quartz

J. Mater. Chem. A

standard was chosen such that some reections of the sampleand the standard have similar 2q values (quartz gives a reec-tion at 2q¼ 45.8�) which is near the (111) and (200) reection ofthe copper nanoparticles. The crystallite size analysis based onthe above two reections was identical. The lattice constant wasdetermined using a least-squares tting procedure on all theobserved reections.

Thermogravimetric analysis (TGA), differential thermalanalysis (DTA) and differential scanning calorimetry (DSC) werecarried out using a Perkin-Elmer TGA–DTA–DSC system on wellground samples in a owing nitrogen atmosphere with a heat-ing rate of 5 �C per min.

Transmission electron microscopy (TEM) studies werecarried out using a Tecnai G2 20 electron microscope operated




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at 200 kV. TEM specimens were prepared by dispersing thecopper nanoparticles in ethanol by ultrasonic treatment, drop-ping onto a porous carbon lm supported on a copper grid, andthen drying in air. Field emission scanning electron microscopy(FESEM) of the copper nanoparticles was carried out on a FEIquanta 3D FEG – FESEM by coating the powder samples withsilver. XPS studies were performed using an X-ray PhotoelectronSpectroscopy system (Omicron Inc.), consisting of a Mg Ka(1253.6 eV) source and a high resolution hemi-sphericalanalyzer with a 7 channel detector. The survey scan and corelevel scan were acquired with 100 eV and 25 eV pass energy,respectively. Nitrogen adsorption–desorption isotherms wererecorded at liquid nitrogen temperature (77 K) using Nova2000e (Quantachrome Corp.) equipment and the specic areawas determined by the Brunauer–Emmett–Teller (BET) method.The Cu nanoparticles sample was degassed at 150 �C for 4 hprior to the surface area measurements.

Cyclic voltammetry (CV) was carried out with a computercontrolled electrochemical workstation (Autolab PGSTAT302N). Hydrogen evolution and oxygen evolution reactions werestudied by using Ag/AgCl as the reference electrode while Pt wasused as the counter electrode. Glassy carbon (0.02 cm2) andplatinum (0.03 cm2) were used as working electrodes. Both theworking electrodes were polished using (0.05 mm) aluminapaste, ultrasonicated in distilled water and then in ethanol forthe electrocatalytic (HER and OER) studies. 2 mg of coppernanoparticles were sonicated in 20 mL of isopropanol and then10 mL of Naon was added. This paste was placed on theworking electrode and dried for half an hour. Naon47 acts as aproton conducting binder for nanoparticles which forms amembrane over the surface of electrodes (membrane electrodeassembly, MEA). All the three electrodes were placed in a freshlyprepared 0.5 M KOH (potassium hydroxide solution). For eachexperiment, freshly prepared KOH solution was used. Cyclicvoltammetry was carried out at a scan rate of 0.025 V s�1 in thepotential range of �1.5 to 0 V for both electrodes (glassy carbonand platinum) in HER studies whereas 0 to 0.8 V potential wasapplied to both the electrodes for the OER study. The electro-active surface areas of the catalyst on the working electrodeswere measured by CV in 0.5 M KOH.

Conclusions

Spherical shaped copper nanoparticles (4–6 nm) have beenobtained from copper oxalate nanorods. Hydrogen evolutionreaction (HER) efficiencies are found to be signicantlyenhanced by 12-fold compared to earlier reports and oxygenevolution reaction (OER) show 1.5 times enhancement usingthe glassy carbon electrode. The current densities are very high46 (�0.6) mA cm�2 (46 times) in the case of HER and 15 (�0.3)mA cm�2 (4–30 times) for OER when the catalyst is used with thePt electrode. The electrocatalyst (Cu nanoparticles) is quitestable over 50 cycles in the case of HER whereas in the case ofOER there is some oxidation of Cu nanoparticles to Cu2O. Ourstudies show that improvement in activity and stability ofelectrocatalysts can be achieved by using the microemulsionmethodology for obtaining ne nanoparticles. This


methodology may be expanded to obtaining other electro-catalysts with increased activity.

Acknowledgements

Financial assistance from IIT Delhi, CSIR and DST, Govt. ofIndia is gratefully acknowledged.

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