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Electrochimica Acta 76 (2012) 292– 299

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

j ourna l ho me pag e: www.elsev ier .com/ locate /e lec tac ta

lectrodeposited Pd–MoOx catalysts with enhanced catalytic activityor formic acid electrooxidation

uoshi Li, Hao Hao, Tao Huang ∗, Aishui Yu ∗∗

epartment of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200438, China

r t i c l e i n f o

rticle history:eceived 11 April 2012ccepted 7 May 2012vailable online 15 May 2012

eywords:

a b s t r a c t

Pd–MoOx catalysts supported on glassy carbon electrode were co-deposited using cyclic voltammetry.The influence of two key deposition parameters (i.e. scanning potential range and concentration of sodiummolybdate in the electrolyte) on the catalysts was investigated by X-ray diffraction, energy dispersiveX-ray spectroscopy, element analysis mapping, scanning electron microscopy, X-ray photoelectron spec-troscopy and typical electrochemical measurements, respectively. It was found both of the parameters

d–MoOx catalystslectrodepositionyclic voltammetryormic acid electrooxdiation

had great effect on the morphology, chemical states, composition and electrochemical performance ofthe catalysts. By tuning the electrodeposition parameters, we found the optimal condition to prepare thecatalysts. The as-prepared catalysts showed much improved catalytic activity and stability for formic acidelectrooxidation. The enhanced performance can be attributed to the fine porous structure composed ofsmall particles, hydrogen spillover effect and unique element distribution (different composition betweensurface and bulk).

. Introduction

Direct fuel cells using small organic compounds as liquiduels are promising power sources for portable electronic devicesecause these fuels are convenient to store, transport and refill,uitable for simple system design and cell operation [1–3]. Amongifferent candidates, formic acid has the advantage of nontoxicity,ninflammability at room temperature, higher theoretical open cir-uit potential and lower crossover through the membrane [4–6].owever, for practical applications of direct formic acid fuel cells

DFAFCs) one of the key issues is the design of effective and low-riced anode catalysts.

Pd-based catalysts have been widely investigated recentlyecause of its lower cost and superior performance compared to Pt-ased catalysts, however, the stability of pure Pd is not satisfactorynd its activity needs further improvement for commercializa-ion [7–11]. To achieve the above goal an interesting method fornhancing catalytic activity is a combination of Pd-based sys-ems with large-surface-area inorganic oxide matrices that ensure

utual metal–support interactions, high dispersion of catalyticenters, unimpeded charge distribution, and good overall stabil-ty [12–15]. Among applicable inorganic compounds, molybdenum

∗ Corresponding author. Tel.: +86 21 51630321; fax: +86 21 51630320.∗∗ Corresponding author. Tel.: +86 21 51630320, fax: +86 21 51630320.

E-mail addresses: huangt@fudan.edu.cn (T. Huang), asyu@fudan.edu.cn (A. Yu).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.05.041

© 2012 Elsevier Ltd. All rights reserved.

oxide species (MoOx) seem to act as promising matrix for the fol-lowing reasons: firstly, they are demonstrated to have relativelyhigh electronic conductivity owing to their rutile-type structurewith the short metal–metal distance along the shared edges; sec-ondly, the presence of OHads groups on the oxide surface wouldfacilitate oxidation of poisoning CO intermediates and improve theCO-tolerance; thirdly, the interacting with Pt or Pd via hydrogenspillover effect can promote the electrooxidation of small organicmolecules; fourthly, they can modify the electronic structure ofnoble metals through synergistic effect and prevent the agglom-eration of catalysts via physical separation. In addition, MoOx arerelatively stable in acidic solution [16–21]. To our knowledge, theuse of MoOx as co-catalysts for formic acid electrooxidation hasseldom been reported. Pickup and co-workers reported Pt blackanodes modified with Mo oxide which showed improved activ-ities in DFAFCs [22]. Feng et al. added MoOx to Pd/C catalystsusing a thermal decomposition method and the catalysts obtainedrevealed enhanced performance for formic acid electrooxidation[13].

In this work, Pd–MoOx catalysts were successfully electrode-posited on glassy carbon electrode by cyclic voltammetry in asolution containing palladium chloride and sodium molybdate.It is found that the scanning potential and the concentration ofsodium molybdate in the plating electrolyte had a great influence

on the morphology, composition, chemical states and electro-catalytic activity of the catalysts. By tuning them, we wereable to determine the optimal condition to prepare the cata-lysts.

ica Acta 76 (2012) 292– 299 293

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Fig. 1. Cyclic voltammograms of Pd–MoO electrodeposited in different scanning

R. Li et al. / Electrochim

. Experimental

.1. Pd–MoOx co-deposition

Electrochemical co-deposition of Pd–MoOx was carried out onn electrochemical workstation (CHI660A, CHI company) using aonventional three-electrode electrochemical cell with a platinumheet and a Hg/Hg2SO4, K2SO4 electrode (MSE, 0.615 V vs. NHE) ashe counter and reference electrodes at room temperature. Poten-ials in the text are referred to MSE. A glassy carbon electrode (GC, �

mm) sealed by PTFE was used as working electrode for Pd–MoOx

o-deposition. It was polished with 0.05 �m alumina powder until ahiny, mirror-like surface was obtained and then ultrasonicated foreveral minutes in deionized water. The GC electrode was furtherleaned in 0.5 M H2SO4 solution by electrochemical cyclic voltam-etry within the potential range from −0.75 to 0.85 V at a scan rate

f 50 mV s−1. Then Pd–MoOx catalysts supported on GC electrodesere prepared by cyclic voltammetry at a scan rate of 50 mV s−1

or 10 cycles in a solution containing palladium chloride, sodiumolybdate and hydrochloric acid. All chemicals were of analytical

rade and used without further purification. Deionized water wassed to prepare all solutions.

To investigate the effect of scanning potential range for elec-rodeposition on the catalyst, the plating electrolyte was fixed as

solution of 2 mM palladium chloride, 15 mM sodium molybdatend 0.2 M hydrochloric acid. The as-deposited catalysts with theotential ranges of −0.73 to −0.2, −0.62 to −0.2 and −0.73 to 0.2 Vere sequentially labeled as S15, S15-1 and S15-2, respectively.

To examine the influence of the concentration of sodium molyb-ate in the plating electrolyte, the scanning potential range wasxed as −0.73 to −0.2 V. The as-prepared catalysts with the sodiumolybdate concentration of 0, 5, 15 and 30 mM were labeled as S0,

5, S15 and S30, respectively.

.2. Materials characterizations

The as-prepared catalysts were characterized by X-ray diffrac-ion (XRD), which was performed on a Bruker D8 Advance X-rayiffractometer using Cu K� radiation with a k of 1.5406 A. The sig-als of the as-prepared catalysts were too weak to detect, so theeposition time was increased to 60 cycles for the measurement.he Pd loadings of the catalysts were analyzed by means of induc-ively coupled plasma-atomic emission spectroscopy (ICP-AES) on

Hitachi P-4010. The morphology was observed by scanning elec-ron microscope (SEM, Hitachi, FE-SEM S-4800A). The compositionas analyzed by an energy dispersive X-ray spectrometer (EDX)

perated at 20 keV (Bruker, QUANTAX 400, attached to SEM), andhe element analysis mapping (EM) was carried out on the samequipment. X-ray photoelectron spectroscopy (XPS) experimentsere carried out on a RBD upgraded PHI-5000C ESCA system

Perkin Elmer) with Al K� radiation (h� = 1486.6 eV). Binding ener-ies were calibrated using the containment carbon (C1s = 284.6 eV).

.3. Electrochemical measurement

All electrochemical measurements were carried out on theame workstation using the same three-electrode electrochemi-al cell as electrodeposition. The as-prepared Pd–MoOx catalystsere firstly characterized in 0.5 M H2SO4 solution at a scan rate

f 50 mV s−1 by cyclic voltammetry and their electrochemical per-ormance for formic acid oxidation was studied by the means ofyclic voltammetry and chronoamperometry. The cyclic voltam-

etry measurements were carried out in 0.5 M H2SO4 and 0.5 MCOOH at a scan rate of 50 mV s−1, and for chronoamperometry

est, the working electrodes were polarized at −0.32 V in the sameolution for 1000 s.

x

potential range in 2 mM PdCl2 + 15 mM Na2MoO4 + 0.2 M HCl: (A) S15-1, (B) S15 and(C) S15-2.

3. Results and discussion

3.1. The influence of the scanning potential range forelectrodeposition

Fig. 1 presents cyclic voltammograms (CV) of Pd–MoOx elec-trodeposited in different scanning potential range in a solution

containing 2 mM palladium chloride, 15 mM sodium molybdateand 0.2 M hydrochloric acid. The peak current densities increasewith increasing cycle number for all of the three catalysts, whichimplies growth of Pd–MoOx catalysts on the GC surface. As

294 R. Li et al. / Electrochimica Acta 76 (2012) 292– 299

15, (C

oW−p(sbi[fttc

b1doataPpEoatcudt

ssaa

Fig. 2. (A) SEM image of S15-1, (B) SEM image of S

bserved, there is much difference among the three CV profiles.hen the lower limit of scanning potential was extended to

0.73 V, characteristic peaks of hydrogen adsorption/desorptionrocess could be clearly observed between −0.73 and −0.6 VFig. 1B). It means that the evolution of hydrogen and electrodepo-ition of the catalysts happen concurrently. The adsorbed hydrogenubbles would play the role of dynamic template, prevent the

ncrease of particle size and assist the formation of porous structure23]. In other words, they would help increase the electroactive sur-ace areas (EASA), improve the noble metal utilization and benefithe mass activity. While when the upper limit of scanning poten-ial reached 0.2 V, a peak after 0 V was discovered, which may beaused by the formation of palladium oxide (Fig. 1C).

The morphology of the as-prepared catalysts is characterizedy SEM and shown in Fig. 2. It is evident that both of S15 and S15-

display porous structure composed of nanosized and uniformlyispersed particles on GC surface, while S15-2 is agglomerated seri-usly. Compared to S15-1, the nanoparticles of S15 are smallernd the structure is more porous. The difference is aroused byhe hydrogen evolution and would increase the EASA of S15. Thegglomeration of S15-2 is probably caused by the oxidation ofd [24]. In order to confirm the elemental composition of the as-repared catalysts, the EDX investigation was carried out and theDX pattern of S15 is displayed in Fig. 2D. It manifests the existencef Pd and Mo. The atomic ratios of Pd:Mo for S15, S15-1 and S15-2re 80:20, 80:20 and 60:40, respectively. Compared to S15-1, whenhe lower limit of scanning potential is extended to −0.73 V, theomposition of the catalysts would not change. However, when thepper limit of scanning potential reaches 0.2 V, the Pd content willecrease. In other words, the former only affect the morphology ofhe catalysts, while the latter would influence the composition.

XPS measurements were performed to determine the chemical

tates of the catalyst surfaces. Fig. 3 displays the representative XPSpectra for the as-prepared catalysts. In Fig. 3A, two peaks at 335.1nd 340.4 eV are attributed to Pd 3d5/2 and Pd 3d3/2, which are char-cteristic peaks for Pd metal. In Fig. 3B, the two main peaks for Pd (0)

) SEM image of S15-2 and (D) EDX pattern of S15.

is also observed, however, it shifts negatively for about 1.2 eV. Theobvious shift can be attributed to the serious increase of particlesize caused by agglomeration [25,26]. In addition, two shouldersare found at about 336.6 and 341.9 eV, which can be ascribed tothe contribution of PdO2. Comparison of the two spectra indicatesthat most of the palladium in the catalysts exists as metallic Pd inboth catalysts and S15-2 contains a small amount of PdO2. A pos-sible reason for this is that the deposited Pd particles formed atthe beginning deposition were oxidized to PdO2 due to the higherupper potential of S15-2 [24]. The results are in accordance to theelectrodepositing voltammograms in Fig. 1. The XPS spectra of theMo 3d doublet are displayed in Fig. 3C. Comparison with the ver-tical lines indicates that Mo (VI) may be the dominant species. Itis noteworthy that no signal was observed at 227.7 eV, which wasattributed to metallic Mo. Therefore, no Pd–Mo alloy was formedin the prepared Pd–MoOx catalysts [27].

The electrochemical characterization of the catalysts was car-ried out by CV experiments in 0.5 M H2SO4 (Fig. 4). The shape of theprofile is similar to the literature [28]. The multiple peaks between−0.7 V to −0.4 V are attributed to the so-called hydrogen absorp-tion/desorption (Hab) process and hydrogen adsorption/desorption(Had) process [29]. The smaller pair of peak at the more positivepotentials is thought to be an indication of real surface area of cata-lysts and is missing or merged into the broad Hab peaks sometimes[10,30]. The significant augmentation of it indicates that the realsurface area of S15 is the largest while that of S15-2 is the small-est. This result corroborates the conclusion from electrodepositingvoltammograms and SEM described above. In addition, the surfacePd oxide reduction peak around 0 V shifts negatively with the con-comitant decrease in particle size, which may be due to an increasein the OH adsorption strength [31]. The accurate EASA of each cat-alyst was calculated from the amount of charge corresponding to

the reduction of a full monolayer of Pd oxides [28] and the valuesare 59.25, 46.33 and 9.25 m2 g −1 for S15, S15-1 and S15-2, respec-tively. Therefore the EASA of the catalysts can be tuned simply bychanging the scanning potential range.

R. Li et al. / Electrochimica Acta 76 (2012) 292– 299 295

Fo

t0csIaao

catalysts, the current is normalized to EASA. It could be seen thatafter normalization, the SSC of S15 and S15-1 is similar, while thatof S15-2 is still much lower. It indicates that the superior activity

ig. 3. Representative XPS spectra for the as-prepared catalysts: (A) Pd (3d) regionsf S15, (B) Pd (3d) regions of S15-2 and (C) Mo (3d) regions of S15.

Fig. 5 shows cyclic voltammograms of formic acid electrooxida-ion on the as-prepared catalysts in a solution of 0.5 M H2SO4 and.5 M HCOOH. The catalytic activity was evaluated as mass specificurrent (MSC, normalized to the mass of Pd, shown in Fig. 5A) andurface specific current (SSC, normalized to EASA, shown in Fig. 5B).

n the forward scan, formic acid oxidation produced an anodic peaknd in the reverse scan, there was also an oxidation peak, which isttributed to formic acid oxidation after the reduction of the Pdxide and the removal of the incompletely oxidized carbonaceous

Fig. 4. Cyclic voltammograms of the catalysts in 0.5 M H2SO4 solution at a scan rateof 50 mV s−1.

species formed in the forward scan. The oxidation peak in the for-ward scan is usually used to evaluate the electrocatalytic activityof the catalysts. As observed, the rank of MSC of the catalysts forformic acid electroxidation is in the order of S15 > S15-1 > S15-2.The discrepancy of the activity can be ascribed to the different EASAof the catalysts. In order to evaluate the intrinsic activity of the

Fig. 5. Cyclic voltammograms of formic acid electrooxidation on different catalystsin 0.5 M H2SO4 + 0.5 M HCOOH at a scan rate of 50 mV s−1: (A) mass specific currentand (B) surface specific current.

296 R. Li et al. / Electrochimica Acta 76 (2012) 292– 299

Fa

otEe

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nwtv

Fig. 7). Other peaks of Pd were too weak to see because of the low

ig. 6. Chronoamperometric curves on the catalysts in 0.5 M H2SO4 + 0.5 M HCOOHt −0.32 V.

f S15 over S15-1 is mainly due to the much larger EASA, whilehe inferior activity of S15-2 is not only attributed to the smallerASA, it may also be caused by the oxidation of the Pd during thelectrodeposition.

The stability of the electrocatalysts is extremely important forheir real applications in DFAFCs. The long-term activity and dura-ility of the catalysts were further assessed by chronoamperometryest, conducted on catalyst-coated GC with the potential fixed at0.32 V, a typical working potential for formic acid oxidation inFAFC (Fig. 6). As observed, the formic acid electrooxidation on15 possesses highest initial and limiting current densities, whichndicates the best catalytic activity and stability for formic acidlectrooxidation, whereas S15-2 has the lowest.

It also should be mentioned more negative lower limits of scan-ing potential range were also tried, however, their performance

ere not as good as S15. We considered the possible reason: when

he lower limit is too low that hydrogen bubbles generated reallyiolently, it may influence the deposition of catalysts and caused the

Fig. 8. SEM images of the as-prepared cataly

Fig. 7. XRD patterns of the as-prepared catalysts.

instability of the catalyst’s structure, for big and excessive hydrogenbubbles may induce the collapse of structure.

3.2. The influence of the concentration of sodium molybdate inthe electrodepositing electrolyte

From the results above, we fixed the scanning potential range as−0.73 to −0.2 V and examined the influence of the concentration ofsodium molybdate in the plating electrolyte. Fig. 7 shows the XRDpatterns of the as-prepared catalysts. The peaks located at 26◦ and43◦ correspond to the hexagonal graphite (0 0 2) and (1 0 0) planeof GC substrate (JCPDS, Card No. 75-1621). The small peak around40.1◦ can be attributed to (1 1 1) facet of Pd face-centered cubicphase (JCPDS, Card No. 46-1043, indicated by the vertical line in

sts: (A) S0, (B) S5, (C) S15 and (D) S30.

loading of catalysts. There are also no peaks for MoOx, which meansthey exist as amorphous structure or their signals are too weak. Noshift of peak position is observed for Pd–MoOx catalysts compared

R. Li et al. / Electrochimica Acta 76 (2012) 292– 299 297

and t

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sFftwHadeEotocrdttib

EASA of catalysts greatly. In order to evaluate the intrinsic activityof the catalysts, the current is normalized to EASA. For clarity onlythe positive scans are shown in Fig. 10C. The intrinsic activity of

Table 1Physicochemical and electrochemical characteristics of the catalysts.

Catalysts S0 S5 S15 S30

Pd:Mo (at%, EDX) 100:0 85:15 80:20 70:30

Fig. 9. SEM image of S15 at low magnification

ith pure Pd catalyst. It indicates that no Pd–Mo alloy forms in theatalysts, in accordance with the above XPS results. According tohe Debye–Scherrer formula, particle size is inversely proportionalo the FWHM of the diffraction peak. Apparently, the Pd (1 1 1) peakf S15 is broader than others, which implies smaller particle size.

Fig. 8 displays the SEM images of the as-prepared catalysts. Itould be seen clearly almost all of the catalysts dispersed uniformlyn the GC without agglomeration. The particle size of S0, the pured catalyst, is about 20–50 nm. After the introduction of MoOx, thearticle size decreased greatly. As observed, S5 and S15 have a par-icle size of 10–20 nm. However, as the concentration of sodium

olybdate increased to 30 mM, the small particles agglomeratedogether to form big particles about 100 nm (Fig. 8D). It also shoulde mentioned the nanoparticles of S0 and S5 are closely packed,hile that of S15 and S30 possess porous structure composed of

oosely packed nanoparticles. Obviously, the latter porous structures superior over the former compact one, because it would facilitatehe mass transfer and improve the electrochemical performance.

The distribution of Pd and MoOx in the whole catalysts wastudied with element analysis mapping (EM) at low magnification.rom the Pd, Mo element maps shown in Fig. 9, the two are uni-ormly distributed in the catalysts without phase separation. Fromhe image at so low magnification, we could conclude the catalystsere dispersed uniformly in the entire range without aggregation.owever, EM only can show the element distribution horizontallylong the electrodeposited film, the vertical distribution in differentepth of the film is still unknown. By taking advantage of the differ-nt detecting depth of EDX and XPS, we could study this problem.DX have a detecting depth of a few micrometers and the thicknessf the electrodeposited film is less than 1 �m. So we could considerhe EDX results as the bulk composition. While the detecting depthf XPS is only a few nanometers, so it could indicate the surfaceomposition. The corresponding results are listed in Table 1. It waseasonable to find out that with the increase of the sodium molyb-ate concentration, the Mo content in the catalysts would rise at

he same time. By comparing the results in Table 1 we could findhe surface Mo content is higher than that of the bulk. Interest-ngly, a recent XANES investigation of Pd–Mo catalysts reportedy Consul et al. demonstrated the Mo in the catalysts would stay

he corresponding element mapping of Pd/Mo.

at the surface. In addition, Pd had stronger chemical stability inthe presence of Mo and was always in metallic form because of thisunique structure [32]. It is probably the same case for our Pd–MoOx

catalysts. In addition, according to previous reports, avoiding theformation of Pd oxides on the surface contributes to improvementof the catalytic activity and stability for Pd-based catalysts [33].

Fig. 10 displays the electrochemical measurements results ofthe as-prepared catalysts. The profiles and characteristic peaks aresimilar to the above results (Figs. 4–6). By electrochemical char-acterization shown in Fig. 10A we could calculate the EASA of thecatalysts and the results are listed in Table 1. Clearly, the EASA ofS0 is the smallest. That of S5 and S30 is bigger because of the smallparticle size or porous structure proved by SEM images in Fig. 8. S15has both the advantage of porous structure and the small particlesize thus it has much larger EASA than other catalysts. In Fig. 10Ait was also noticeable the hydrogen desorption peaks for S15 andS30 shifted to lower potentials, indicating a weakened adsorptionstrength of hydrogen on the Pd surface, which may be attributed tothe hydrogen spillover effect of MoOx. The hydrogen adsorbed onPd can diffuse onto the surface of MoOx to form a so-called hydro-gen bronze and release the Pd active sites for further adsorptionof hydrogen [13,34]. Fig. 10B and C shows the catalytic activity ofthe catalysts and the peak current density are listed in Table 1. Asobserved, all of the Pd–MoOx catalysts show better catalytic abil-ity than pure Pd catalysts. Especially, S15 has a jp (MSC) 3.5 timesof S0. It is reasonable because the addition of MoOx increased the

Pd:Mo (at%, XPS) 100:0 84:16 64:36 46:54EASA (m2 g−1) 20.25 27.5 59.25 30.5jp (MSC) (A mg−1 Pd) 0.66 0.98 2.31 1.18jp (SSC) (mA cm−2) 3.25 3.57 3.90 3.88

298 R. Li et al. / Electrochimica Acta 76 (2012) 292– 299

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ig. 10. Electrochemical measurements of the as-prepared catalysts: (A) cyclic voyclic voltammograms of formic acid electrooxidation on different catalysts in 0.5 Mn the catalysts in 0.5 M H2SO4 + 0.5 M HCOOH at −0.32 V.

d–MoOx catalysts without the effect of EASA is still a little betterhan pure Pd catalysts. The minor improvement can be ascribed tohe hydrogen spillover effect: the adsorbed hydrogen from formiccid dehydrogenation on the Pd surface transferred onto the MoOx

urface and more active sites of Pd were released for formic aciddsorption and oxidation. Accordingly, the dominant reason forhe much enhanced activity is the greatly increased EASA obtainedrom the excellent structure. The catalytic activity improvementaused by hydrogen spillover effect is not large, similar to a previouseport [13]. As displayed in Fig. 10D, all of the Pd–MoOx catalystshowed superior stability over pure Pd catalysts, among them S15s the best. The enhancement can be attributed to the unique ele-

ent distribution substantiated by EDX and XPS results mentionedbove.

. Conclusions

In summary, a series of Pd–MoOx catalysts supported on GCynthesized via a simple electrochemical method. The morphol-gy, composition and electrocatalytic properties of the as-preparedatalysts can be simply adjusted by two deposition parameters (i.e.canning potential range and content of the electrolyte). By tuninghem, the optimal conditions were found. The as-prepared cata-yst showed excellent porous structure, good stability and muchnhanced catalytic activities (3.5 times of pure Pd catalysts). More-ver, it is more economic because with the addition of MoOx, thesing of noble metal Pd is less. The Pd–MoOx catalyst shows greatotential as less expensive electrocatalyst for formic acid electroox-

dation in DFAFCs. And further work will aim to study the influencef other inorganic oxides over Pd or Pt-based catalysts for formiccid electrooxidation.

cknowledgments

This work is financially supported by a grant from Nationalatural Science Foundation of China (No. 21003025) and Science &

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ograms of the catalysts in 0.5 M H2SO4 solution at a scan rate of 50 mV s−1, (B, C)O4 + 0.5 M HCOOH at a scan rate of 50 mV s−1 and (D) chronoamperometric curves

Technology Commission of Shanghai Municipality (No.08DZ2270500), China.

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