ORIGINAL PAPER

Electrodeposited nanostructured Pt–Ru co-catalyst on graphenefor the electrocatalytic oxidation of formaldehyde

Hagar K. Hassan & Nada F. Atta & Ahmed Galal

Received: 9 September 2012 /Revised: 14 January 2013 /Accepted: 14 January 2013 /Published online: 20 February 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract The electrochemical oxidation of formalde-hyde over graphene surfaces modified with Pt–Ru co-catalyst is presented. Graphene was chemically con-verted from graphite and Pt–Ru co-catalyst was electro-chemically deposited using cyclic voltammetry. Thehybrid surface is prepared using “green approaches”and displayed electrocatalytic activity towards formalde-hyde in the form of current oscillations. The currentoscillations that were mainly due to adsorption/desorp-tion of carbonaceous oxidative products are a factor ofseveral parameters such as the concentrations of bothformaldehyde and supporting electrolyte in solution, theamount of catalyst loading, scan rate of potential, upperpotential limit, and the temperature change. CCG/Pt–Ruexhibited higher electrocatalytic activity toward formal-dehyde electro-oxidation, and intense electrochemicalcurrent oscillations were obtained at relatively lowHCHO concentrations compared to other work men-tioned in literature for CCG/Pt–Pd.

Keywords Chemically converted graphene . Pt–Ruco-catalyst . Formaldehyde electro-oxidation .

Electrochemical current oscillations . Fuel cells

Introduction

Although formaldehyde is toxic and not very suitablefor fuel cells, the study of its electrochemical oxidationis important to understand the methanol oxidation pro-cess because formaldehyde is one of the products pro-duced by partial oxidation of methanol [1–4]. It istherefore necessary to study formaldehyde electro-oxidation as a further step in methanol oxidation thattakes place in methanol fuel cells [5]. It will be alsocrucial to find a suitable catalyst and support for thisprocess. On the other hand, formaldehyde is becoming amajor indoor pollutant in airtight buildings, which ismainly emitted from the widely used constructive anddecorative materials [6, 7]. It is recognized that long-term exposure to indoor air even containing a few ppmof formaldehyde may cause adverse effects on humanhealth, so heterogeneous catalytic oxidation of HCHOinto harmless CO2 and H2O is very important and hasattracted a wide attention in recent years as a way forindoor air purification [7].

Another application of formaldehyde is its use intechnologically important processes such as electrolesscopper plating and in the textile industry. Thus, itsoxidation is also relevant to wastewater treatment [8].The study of formaldehyde oxidation reaction is there-fore one of the most important topics and finding themost suitable catalyst and catalyst support is very cru-cial to its application both in fuel cells and pollutionremediation.

Graphene is a one-atom-thick structure that consistsof a hexagonal array of Sp2-bonded carbon atoms[9–13] and looks like a honey comb [9, 14, 15]. Itwas discovered by Andre Geim’s research group atUniversity of Manchester in 2004, and was preparedby the so-called scotch-tape technique [16]. It can be

Electronic supplementary material The online version of this article(doi:10.1007/s10008-013-2008-4) contains supplementary material,which is available to authorized users.

This paper is a contribution to recognize Prof. Alexander Milchev onthe occasion of his 70th birthday, for his influential contribution to thefield of electrochemistry and for his dedication to his colleagues andstudents.

H. K. Hassan :N. F. Atta :A. Galal (*)Department of Chemistry, Faculty of Science, CairoUniversity, Giza 12613, Egypte-mail: galal@sci.cu.edu.eg

J Solid State Electrochem (2013) 17:1717–1727DOI 10.1007/s10008-013-2008-4

considered as the mother of all carbon materials [17].Graphene has some unique properties which makes itone of the most interesting materials nowadays andtakes a similar level in new applications as carbonnanotubes (CNTs). Some unique properties of grapheneare as follows: it has a large theoretical surface area ofabout 2,620 m2/g [14, 18, 19], chemically stable andalmost impermeable to gases, can withstand large cur-rent densities, has high thermal [10] and chemical con-ductivity [20, 21], possesses outstanding mechanicalproperties [20, 21], has a large amount of edge planes/-defects [20], and in addition to its relatively low costduring its production compared to CNTs [21]. Theseproperties make graphene promising for potential appli-cation in electrochemical field [20]; it particularlyrepresents a promising catalyst carrier in the next gen-eration of carbon-based supports for electrocatalysis[21].

Periodical changes in the oxidation rate, as mani-fested by current oscillations under potentiostatic con-ditions or potential oscillations under galvanostatic oropen-circuit conditions, have been observed for smallorganic molecules such as methanol, formic acid, form-aldehyde, ethanol, propanol, and many other species instrongly acidic to strongly alkaline electrolytes [22–29].The electrochemical oscillations are important in variousapplications such as the depoisoning of electrocatalysts.Moreover, electrochemical oscillations constitute a par-ticular example of periodic behavior in chemical sys-tems far from thermodynamic equilibrium. Therefore, itis of both practical and fundamental interest to gain abetter understanding of the mechanism of oscillations inelectrocatalytic reactions [26].

Schell and co-workers [26, 28] used a rotating poly-crystalline Pt disc electrode to study the potential-dependent oscillation of formaldehyde, while GuohuaZhao et al. [27] studied both the current and the poten-tial oscillation during formaldehyde oxidation on plati-num particles dispersed in three-dimensional porenetwork of Pt–TiOx supported by Ti. They found thatthe electrochemical oscillations are related to the micro-structure and morphology of Pt–TiOx/Ti electrode sur-face. In a previous work (Atta NF, Hassan HK, Galal A,submitted), we used graphene modified with a co-catalyst of Pt and Pd to study the current oscillationsof formaldehyde in acid medium. The results were verypromising and revealed that the presence of grapheneincreases the current oscillations and the presence of Pdas a co-catalyst with Pt enhances synergistically theseoscillations. Because the electrochemical oscillationsgenerated on the electrode have also a close relationto the morphology of electrode surface and the structureof the catalyst, the conditions and even mechanism for

the occurrence of the oscillations are likely to changewith different microstructures of electrode surface [29].

It will be important to study the effect of changingthe second metal catalyst, beside Pt, on the electrochem-ical oscillations. In the present study, we replaced Pdwith Ru in the composition of the co-catalyst. The aimof this work is to study the electrochemical oxidation offormaldehyde based on the number/amplitude of currentoscillations obtained and to investigate the effect ofvarious external factors that can affect the electrochem-ical response.

Experimental

Chemicals and materials

All chemicals were used as received without further purifi-cation. Graphite powder, perchloric acid, sulfuric acid, nitricacid, and hydrochloric acid were purchased from SigmaAldrich. Hydrazine hydrate (HH), dimethylformamide(DMF), commercial formaldehyde 37 %, and hexachloro-platinic acid were purchased from Aldrich while ruthenium(III) chloride was obtained from Fluka.

Electrochemical cells and equipment

Electrochemical experiments were carried out in a three-electrode/one-compartment glass cell. The working electrodewas GC electrode (ϕ=3 mm), reference electrode wasAg/AgCl (4 M KCl), and auxiliary electrode was a platinumwire. GC electrode was polished using an alumina(<2 μm)/water slurry until no scratches can be detected asobserved by the naked eye. The electrochemical characteriza-tion was performed using a BAS-100B electrochemical ana-lyzer (Bioanalytical Systems, BAS,West Lafayette, USA) thatwas connected to a personal computer. All potentials indicatedin the data section are referred to the Ag/AgCl (4 M KCl).

Preparation of graphene by chemical reduction of GO

Graphene oxide (GO) was prepared by oxidation ofspectroscopic grade graphite powder according to themodified method of Hummer and Offeman as previous-ly described [30]. The chemically converted graphenewas prepared by chemical reduction of GO with assis-tance of microwave irradiation in presence of HH as areducing agent [31]. The steps for the preparation are asfollows: 0.1 g of GO was sonicated in 20 ml deionizedwater until a homogeneous yellow dispersion isobtained. Then 120 μl of HH is added to the solutionand is placed in a conventional microwave oven (MC-9283 JLR, 900 W) operated at a full power 900 W in

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30-scycles (on for 10 s; off and stirring for 20 s) for atotal reaction time of 120 s.

After microwave irradiation, the yellow color of solutionwas converted into black that was a good indicator for thereduction of GO into CCG. The chemically converted gra-phene flakes were separated by Mark IV auto bench centri-fuge operated at 5,000 rpm for 15 min and dried overnight.

Preparation of electrode for electrochemical measurements

DMF (1 ml) is added to 1.5 mg of CCG and is sonicateduntil a homogeneous suspension is obtained. GC electrodewas polished thoroughly using 0.2 μm alumina, rinsed withdistilled water, sonicated, and then casted with suitable vol-ume of CCG suspension (10 μl). The electrode with CCGwasleft to dry at 70 °C. The surface was ready for electrodeposi-tion of catalyst and co-catalysts that was achieved by cyclicvoltammetry (CV) in a three-electrode, one-compartment cellusing the catalyst or co-catalyst solutions as electrolyte. Plat-inum or platinum–ruthenium (Pt–Ru) solution is prepared bydissolving H2PtCl6 or H2PtCl6 and RuCl3 in 0.5 M H2SO4,respectively. Pt–Ru co-catalyst solution is prepared to form aratio of 3(Pt):1(Ru) (7.5 mM H2PtCl6 and 2.5 mM RuCl3).The electrodeposition of Pt is performed by applying potentialfrom −250 mV to +650 mV vs. Ag/AgCl for 30 cycles at ascan rate of 50 mVs−1. For the electrodeposition of Pt–Ru, anapplied potential from −1,000 mV to 0.0 V vs. Ag/AgCl at ascan rate of 50 mVs−1 for 10–30 cycles was used.

Structural and surface characterization

Thin film X-ray diffraction (XRD) was recorded on Pana-lytical X’Pert using Cu-Kα radiation (λ=1.540Ǻ), whilefield emission scanning electron microscope (FE-SEM)images was measured by JEOL JSM-6360LA and PhilipsXL30 instruments. Atomic force microscope (AFM) wasmeasured using a Shimadzu Wet-SPM (scanning probe mi-croscope). AFM roughness data were used to calculate thereal surface area according to the following relation:

Surface roughness ¼ Real surface area

Geometric surface areað1Þ

The “electrochemically active” surface area was determinedusing a redox probe system of potassium ferricyanide, and theresults were compared to those obtained from AFM. Thus, weused the geometrical area during the redox process of ferro/-ferricyanide couple and compared it to the value estimatedfrom the surface roughness measured by AFM. The surfacearea for GC/CCG/Pt–Ru is based on that calculated from theexperiment of ferricyanide that was equal to 0.2107 cm2. It isworth to mention that all the samples are prepared as previ-ously described in the preceding sections to maintain consis-tency with sample surface throughout this study.

Results and discussion

Structural and surface characterization

Figure 1a shows the XRD of graphite, GO, and CCG. TheXRD results of CCG prove the disappearance of both thepeak related to graphite (26 Å) and that is related to GO(around 10 Å). This indicates the complete reduction of GOto CCG. On the other hand, the XRD patterns ofGC/CCG/Pt–Ru are displayed in Fig. 1b. The results showthat both Pt and Ru are deposited as polycrystalline par-ticles. Moreover, the results also reveal that Pt–Ru are notpresent as alloy but as a physical mixture of both.

Field-emission scanning electron microscope images ofCCG and GC/CCG/Pt–Ru as well as its corresponding EDXA(Energy-Dispersive X-ray Analysis) analyses are displayed inFig. 2a–c, respectively. It is shown that CCG is rippled withwell-identified edges while Pt–Ru is deposited on its surfacewith various sizes. Pt–Ru particles are accumulated in someareas and deposited smoothly on another that may be due to thehigh surface roughness of CCG as measured from AFM.Three-dimensional AFM images of both CCG andGC/CCG/Pt–Ru are shown in Fig. 2d and e, respectively. FromAFM analysis of CCG and GC/CCG/Pt–Ru, the surface rough-ness and real surface area were calculated and found equal to1.67 and 1.43 and 0.118 cm2 and 0.101 cm2, respectively. Thecalculated average particle size of Pt–Pdwas found to be 152 nm.

Electrocatalytic activity toward formaldehydeelectro-oxidation

The electrochemical oxidation of formaldehyde proceedsthrough two competitive processes [8, 21, 29]. The firstis the direct oxidation of HCHO [which exists in aque-ous solutions mainly as methylene glycol (Eq. (2)],where CO2 molecules are formed and leave the surface(Eq. 3). The second is the “indirect” oxidation occurringthrough the adsorbed CO (Eq. 4) [32].

HCHOþ H2O ! CH2 OHð Þ2 ð2Þ

CH2 OHð Þ2 ! CO2 þ 4Hþ þ 4e� ð3Þ

CH2 OHð Þ2 þ � ! COad þ H2Oþ 2Hþ þ 2e� ð4Þ

In the above simplified equation, the asterisk (*) designates afree catalyst site. The final product of the oxidation takes placethrough the reaction of adsorbed CO and neighboring adsorbedOH (or H2O). At low potentials, CO molecules are producedfrom formaldehyde in a dissociative step then adsorb on the Ptsurface according to Eq. 3, thus “poisoning” the electrode andprohibiting the direct oxidation of formaldehyde. At higher

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Fig. 1 XRD of a graphite, GO,and CCG, and b GC/CCG/Pt–Ru

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potentials, OH is adsorbed in the neighborhood of CO andreacts with the oxide, and both leave the surface through aCO2 molecule. Thus, the indirect path results in the formationof free sites, giving way for the direct path. Decreasing theelectrode potential makes the direct path preferable until thecomplete poisoning of the electrode surface takes place.

Figure 3 shows the cyclic voltammetry of 0.1 M HCHO in0.5 M H2SO4 from −500 to 1,800 mV with a scan rate of100 mVs−1 at CCG/Pt–Ru. Current oscillations are not usu-ally to be observed at low formaldehyde concentration duringits electrochemical oxidation. It is shown that a clear oxidationpeak is observed at GC/CCG/Pt–Ru surface during the anodicscan at 770 mVand another oxidation peak in the reverse scanappears at 652 mV. The oxidation peak in the anodic directionis assigned to the oxidation of CO into CO2 that is producedduring the indirect path of HCHO oxidation. The other oxi-dation peak that forms during the reverse scan is due to thedirect oxidation of HCHO into CO2. The onset potential foroxidation is about 230 mV and forward current (If) to

backward current (Ib) ratio is an indicator for CO toleranceof the catalyst. Thus, If/Ib for HCHO electro-oxidation at

Fig. 3 CVof GC/CCG/Pt–Ru in 0.1 M HCHO in 0.5 M H2SO4 from−500 to 1,800 mV at a scan rate 100 mVs−1

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Fig. 2 a FE-SEM of GC/CCG, b FE-SEM of GC/CCG/Pt–Ru, c the corresponding EDXA analysis of GC/CCG/Pt–Ru, d three-dimensional AFMimage of GC/CCG, and e three-dimensional AFM image of GC/CCG/Pt–Ru

GC/CCG/Pt–Ru was found to be 1.44, which indicates thatmore carbonaceous species are oxidized into CO2 during theanodic scan. Comparing these results with our previous resultsusing Pt–Pd co-catalyst, we found that Pt–Ru provided higher“overall” current density compared to Pt–Pd in relatively lowformaldehyde concentrations. This finding indicates that atrelatively lower formaldehyde concentrations, Pt–Ru co-catalyst provides a better surface that inhibits surface poison-ing compared to a rather slower charge transfer at the Pt–Pdmodifier. Adsorption affinity of the surface towards CO and“other” carbonaceous matters when substituting Pd by Ru inthe co-catalyst should be another factor that contributes toelectrocatalytic superiority of Pt–Ru over that of Pt–Pd.

On the other hand, CO is removed from Pt surface atpotentials higher than 0.8 V as indicated in a previous study[33]. Moreover, the current oscillations occur in the O2

evolution region over Ru surfaces. According to other stud-ies [27], OH is produced by the oxidation of water present inthe double layer and adsorbed in the neighborhood ofadsorbed CO that is removed from the electrode surfacethrough its reaction with OH (or H2O) to CO2. Therefore,the current oscillations are a combination of water oxidationthat results in O2 evolution and cyclic formation of CO andits reaction with co-adsorbed species.

Current oscillation under cyclic voltammetric conditions

The current oscillatory response during the electrochemicaloxidation of formaldehyde can be explained as follows: In theindirect path, formaldehyde is oxidized to CO which isadsorbed on electrode surface. The electrode is thus poisonedmomentarily allowing no further oxidation of formaldehyde.However, when OH is produced by the oxidation of wateradsorbed in the neighborhood of CO and in the double layer,the adsorbed CO is removed from the electrode surface throughits reaction with OH (or H2O) to produce CO2 [34]. Thissituation favors further oxidation of formaldehyde and thereproduction of CO. Thus, the cyclic formation of CO and itsreaction with co-adsorbed species is responsible for the elec-trochemical oscillatory response [35–37]. Furthermore, it isknown that the current oscillations are observed during theelectro-oxidation of high concentration of formaldehyde atdifferent surfaces [35–37]. Figure 4 shows the CVs of 1.0 MHCHO dissolved in 0.5 M H2SO4 at GC/CCG/Pt–Ru (a),GC/CCG/Pt (b), and GC/Pt–Ru (c). The synergism betweenthe presence of graphene as a catalyst support and presence ofRu as a co-catalyst with Pt clearly appears from the value ofonset potential as well as the number of current oscillationobtained.

Comparing between the results obtained at GC/CCG/Pt–Ruand GC/Pt–Ru surfaces (Fig. 4a and c, respectively), it is clearthat the presence of graphene lowers the onset potential from314 mV to 238 mV. Moreover, current oscillations appear

clearly more vigorous in the presence of graphene at the sur-face, as a catalyst support. The electrocatalytic activity that ismanifested in the lowering of the onset potential of currentoscillations and their frequency is related to the nature ofsurface of the catalyst. Several factors contribute to this finding;the intrinsic catalytic nature of the surface, the adsorptionaffinity of oxidative products, the electronic structure of thecatalyst atoms, and themorphology of the nanostructures. Fromthese results, we can also infer that the presence of graphenedecreases the overpotential during the electro-oxidation offormaldehyde as indicated from the lower value of the onsetpotential of oscillations and improves greatly the electrocata-lytic activity toward formaldehyde electro-oxidation.

Moreover, the synergistic role played by adding Ru to Pt inimproving the electrocatalytic activity can be verified by com-paring between the voltammograms of GC/CCG/Pt–Ru(Fig. 4a) and GC/CCG/Pt (Fig. 4b) electrodes. It is shown thatthe presence of Ru with Pt improves greatly the electrocatalyticactivity of the surface towards the HCHO oxidation. The pres-ence of Ru lowers the onset potential of oxidation of formalde-hyde (469 mV for GC/CCG/Pt and 238 mV for GC/CCG/Pt–Ru) and also improves the current oscillations. It is known thatRu provides additional sites for adsorption of oxygenated spe-cies that help in oxidation of adsorbed carbon monoxide(COads) into CO2 [38, 39]. This results in lowering the poison-ing of the catalyst surface that leads in turn to an increase in itscatalytic activity. However, at relatively higher potentials Ru isoxidized into RuO2 and/or RuO3 and their correspondingamounts increase with the increase in potential [40]. This resultsin inhibiting the oxidation of COads into CO2 that explains whythe current oscillations are not intense enough.

Oscillatory behaviors are known to depend on the exper-imental conditions, so the effect of changing of some exper-imental conditions on the intensity of current oscillationswas investigated on the following sections.

The catalyst was stable up to 1.7 V as indicated from theEDXA measurements that indicated that the catalystretained the same percent composition up to this potential.The highest current oscillations were obtained up 1.9 V.Beyond 1.9 V, the catalyst is subject to changes due to thepossible formation of volatile RuO2.

Effect of Pt–Ru loading

One of the important parametric factors that influence theelectrocatalytic activity is the “amount” of catalyst on thesurface that is manifested in its thickness, morphology, poros-ity, and distribution. Pt–Ru co-catalyst was deposited on thesurface of CCG by cyclic voltammetry from −1,000 mV to0.0 V at a scan rate 100 mVs−1 as reported before in the“Experimental” section. The thickness of Pt–Ru deposited overthe graphene substrate is controlled by the number of deposi-tion cycles that are related to the amount of charge consumed

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(assuming 100 % efficiency, CE). The deposition cycles variedfrom 10 to 30 cycles and the resulted GC/CCG/Pt–Ru wastested in 1.0 M HCHO in 0.5 M H2SO4. The relation betweenthe resulting number of current oscillation and the number ofdeposition cycles is shown in Fig. 5a. It could be noticed thatthe current oscillations varied erratically with the number ofcycles. Thus, current oscillations start to be observed when Pt–Ru is deposited by 15 cycles and disappear by increasing thenumber of deposition cycles till 30 cycles of deposition thenreappeared again as the number of cycles increase. However,the optimum number of deposition cycles that provide highercumulative current of oscillations is 15 cycles. Figure 5b showsa relation between the maximum peak current obtained duringthe oxidation of HCHO versus the number of cycles used forPt–Ru deposition. It is shown that by increasing the number ofdeposition cycles from 10 to 15 cycles, the peak current reachesa maximum and stabilizes with the number of cycles thensharply increases when using 30 cycles for deposition.

The results obtained are explained in terms of the differ-ent morphologies, particle size, amount (loading), and sur-face coverage of the co-catalyst. SEM and AFM microscopymeasurements confirm this justification, as the layer depos-ited starts to agglomerate that result in decreasing the sur-face area and the surface coverage as well. This should alsoaffect the “active” site of the catalyst that in turn affects theadsorption/desorption kinetics. The other factors influencingthe electrocatalytic behavior are examined on the surfacecontaining the co-catalyst deposited using 15 cycles assum-ing 100 % current efficiency. The amount of co-catalystdeposited on CCG that is estimated from the amount ofcharge passed during 15 cycles of deposition is 4.3 × 10−8

g Pt–Ru/electrode area. The corresponding catalyst loadingin weight percent with respect to CCG is 0.28 wt.%.

It is important to mention that the amount of Pt/Ruwith an almost 100 % CE was estimated as indicated bythe method reported by Tusseeva et al. [41].

Fig. 4 CVs of a GC/CCG/Pt–Ru, b GC/CCG/Pt, and cGC/Pt–Ru in 1.0 M HCHOdissolved in 0.5 M H2SO4

Fig. 5 a Relation between the number of deposition cycles of Pt–Ruand the obtained number of current oscillations during the anodic scan(solid line) and the cathodic scan (dotted line). b Relation between the

number of deposition cycles of Pt–Ru and the maximum peak currentobtained by testing GC/CCG/Pt–Ru in 1.0 M HCHO dissolved in0.5 M H2SO4

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Effect of formaldehyde concentration

The effect of HCHO concentration on GC/CCG/Pt–Ruelectrochemical performance was also studied in 0.5 MH2SO4 and 0.1–2.0 M HCHO. The corresponding rela-tion between the number of current oscillations in bothanodic and cathodic scan as well as the maximum peakcurrent versus HCHO concentration are shown inFig. 6a and b, respectively. The results indicate thatby increasing the concentration of formaldehyde, thenumbers of current oscillations in both anodic and ca-thodic scan increase up to a concentration of 1.5 M

after which they tend to decrease. On the other hand,the maximum peak current nearly does not change whenthe concentration of HCHO exceeds 0.5 M.

Effect of supporting electrolyte concentration

The concentration of supporting electrolyte solution largelyaffects the occurrence and frequency of current oscillations.Thus, Fig. 7a–d shows the CVs of electro-oxidation of1.0 M HCHO in 0.01 M (a), 0.1 M (b), 0.5 M (c), and1.0 M (d) H2SO4 at GC/CCG/Pt–Ru electrode surface usingscan rate 100 mVs−1. As depicted from the results, for

Fig. 6 a Relation between HCHO concentration and the obtainednumber of current oscillations during the anodic scan (solid line) andthe cathodic scan (dotted line). b Relation between HCHO

concentration and the maximum peak current obtained by testingGC/CCG/Pt–Ru in 0.1 to 2.0 M HCHO dissolved in 0.5 M H2SO4

Fig. 7 CVs of electro-oxidation of 1.0 M HCHO in a 0.01 M, b 0.1 M c 0.5 M, and d 1.0 M H2SO4 at GC/CCG/Pt–Ru electrode surface with ascan rate 100 mVs−1. e Histogram between log CH2SO4 and number of peaks observed during the positive-going direction

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relatively low concentrations of H2SO4 (ca. 0.01 M) theelectro-oxidation of HCHO is not accompanied with currentoscillations. By increasing the concentration of supportingelectrolyte, the number of current oscillations increases andreaches its maximum current with 0.5 M H2SO4. At aconcentration of 1.0 M H2SO4, the oscillations start todecrease again. The increase of the concentration ofH2SO4 results in an increase in the ionic strength in solutionthat causes the increase in the maximum currents of electro-chemical response as well as the frequency and number ofcurrent oscillation.

On the other hand, as the concentration of H2SO4

increases, the sulfate ions are adsorbed on the surface ofelectrode and block some active sites that inhibit theelectrochemical activity of the catalyst. As a result, thenumber of current oscillations in the anodic directiondecreases when using 1.0 M H2SO4 as shown in thehistogram between log CH2SO4 and number of peaksobserved during the positive-going direction (Fig. 7e).Therefore, H2SO4 with a concentration of 0.5 M is aconvenient concentration that facilitates the occurrenceof electrochemical oscillation during the electrochemicaloxidation of formaldehyde.

Effect of upper potential limit (UPL) and scan rate

The potential window and the scan rate affect the shape of CV,namely the current oscillations accompanying the electro-oxidation of small molecules such as HCHO. In this respect,the initial potential is fixed at −500 mV and the UPL ischanged from 1,000 to 3,000 mV. Figure 8a–h shows that atUPL below 1,200 mV, the electro-oxidation of HCHO did notshow current oscillations which indicate that the higher po-tential is favored during the electro-oxidation of formalde-hyde. The surface of the electrode is activated at thesepotentials and the reaction is thermodynamically more feasi-ble [29]. Moreover, by increasing the UPL the number ofcurrent oscillations as well as the frequency of oscillationsincreases sharply up to 1,900 mV after which both numbersstart to decrease as depicted in the relation in Fig. 8i. Thiscould be attributed to the oxidation of Ru into RuO2 or RuO3

that decreases the catalyst’s ability to oxidize CO into CO2.On the other hand, the effects of scan rate on the number

of current oscillation peaks and the maximum peak currentare shown in Fig. 4a and b, respectively. The results illus-trate that as the scan rate increases, the number of currentoscillations decreases. Thus, at low sweep rate the potential

Fig. 8 CVs of GC/CCG/Pt–Ru in 1.0 M HCHO in 0.5 M H2SO4, from−0.5 to a +1.0, b +1.2, c +1.5, d +1.8, e +1.9, f +2.0, g +2.5, andh +3.0 V at SR 100 mVs−1. The solid line represents the anodicdirection while the dotted line represents the cathodic direction. i

Relation between UPL and the number of current oscillation peaksobserved during anodic (solid line) and cathodic (dotted line) scan atGC/CCG/Pt–Ru in 1.0 M HCHO in 0.5 M H2SO4, starting the scanfrom −0.5 V at SR 100 mVs−1

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change allows enough time for the current oscillation that ismainly due to the adsorption/desorption events to take placeand consequently the frequency of oscillation increases bydecreasing the sweep rate. When plotting the maximumpeak current and the square root of the scan rate, a straightline is obtained that indicates that the oxidation process isunder diffusion control (Fig. 9b).

Effect of temperature

The effect of temperature on the electrocatalytic activity ofGC/CCG/Pt–Ru was also investigated. As shown inFig. 10a, the optimum temperature at which higher currentoscillations are observed during the electro-oxidation ofHCHO is 313 K (40 °C). By increasing the temperaturefrom 25 to 40 °C, the number of current oscillationsincreases. This result is in good agreement with those men-tioned in the literature related to the current oscillations offormaldehyde electro-oxidation in acid medium. This can beattributed to desorption of CO from the surface before itsfurther oxidation into CO2. It is worth to mention that themaximum peak currents observed at GC/CCG/Pt–Ru werenot nearly affected by the temperature as shown in Fig. 10b.Arrhenius plot will not be possible to obtain in the case ofusing GC/CCG/Pt–Ru under these experimental conditions.

Conclusion

The electrochemical oxidation of formaldehyde is catalyti-cally driven over the surface of a co-catalyst of Pt–Ru that issupported on graphene sheets prepared chemically via mi-crowave method. The synergetic role played by CCG wasobvious since Pt–Pd co-catalyst exhibited slow charge trans-fer kinetics when directly deposited on conventional surfa-ces. When comparing the current densities obtained, it isconcluded that the presence of graphene provides not onlyan “electronic precursor mat” from the sp2 hybrid intrinsicstructure but also an optimized distribution ratio of the

catalyst over the graphene surface. Moreover, CCG/Pt–Rusurfaces have higher electroactivity for the oxidation offormaldehyde and display higher current oscillation com-pared to CCG/Pt. This was explained in terms of the factthat Ru provides sites for adsorption of oxygenated species

Fig. 9 a Relation between the scan rate and the number of currentoscillation peaks observed during anodic (solid line) and cathodic(dotted line) scan. b Relation between the scan rate and the maximum

current observed during the anodic scan at GC/CCG/Pt–Ru in 1.0 MHCHO in 0.5 M H2SO4, from −500 to +1,800 mV

Fig. 10 a Relation between temperature and the number of currentoscillation peaks observed during anodic (solid line) and cathodic(dotted line) scan. b Relation between the temperature in Kelvin andthe maximum anodic peak current at GC/CCG/Pt–Ru in 1.0 M HCHOin 0.5 M H2SO4, starting the scan from −0.5 V at SR 100 mVs−1

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as OH which contributes in oxidation of COads into CO2. Astrong adsorption at Ru sites is thought when compared toPd that was discussed in another work (Atta NF, HassanHK, Galal A, submitted). Several factors studied showedthat (1) the increase in formaldehyde concentration affectedthe pattern of adsorption/desorption mechanism at co-catalyst interface, (2) a critical concentration of the support-ing electrolyte was necessary to initiate the oxidation pro-cess through the indirect route, (3) the increase in UPLaccelerates the adsorption/desorption step, and (4) the tem-perature did not display a linear relationship that followsArrhenius relation in the temperature interval studied. Inconclusion, the surface characterization proved the presenceof the co-catalyst in the electrodeposit with sub-microdimensions and the electrocatalytic activity that is mani-fested in the number of current oscillations and frequencywas a function in structural aspects, amount, and nature ofco-catalyst used.

Acknowledgment The authors would like to acknowledge the finan-cial support from Cairo University through the President Office forResearch Funds.

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