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Materials Chemistry and Physics 143 (2013) 109e115

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Development of silver-gas diffusion electrodes for the oxygenreduction reaction by electrodeposition

Sónia Salomé, Rosa Rego, M. Cristina Oliveira*

Chemistry Department, Chemistry Reseach Unit e Vila Real, University of Trás-os-Montes e Alto Douro, Apartado 1013, 5001-801 Vila Real, Portugal

h i g h l i g h t s

* Corresponding author. Tel.: þ 351 259 350 286; fE-mail address: mcris@utad.pt (M.C. Oliveira).

0254-0584/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2013.08.026

g r a p h i c a l a b s t r a c t

� A facile and simple way to success-fully prepare catalyzed gas diffusionelectrodes.

� Ultra-low loadings of Ag-GDEs can beachieved.

� Good tolerance to methanol and ahigh mass activity (3.14 mAAg mg�1).

� ORR occurs via a four-electronpathway.

a r t i c l e i n f o

Article history:Received 8 February 2013Received in revised form2 July 2013Accepted 16 August 2013

Keywords:MetalsNanostructuresElectrochemical techniquesElectrochemical properties

a b s t r a c t

Silver-gas diffusion electrodes (Ag-GDE) were prepared by direct deposition of the catalyst onto a carbonpaper support by electrodeposition. This deposition technique, under potentiostatic and galvanostaticmode, allows the production of well dispersed ultra-low Ag loading levels. The catalytic activity of theprepared materials towards the oxygen reduction reaction (ORR) was investigated in the alkaline solu-tion and its tolerance to methanol was evaluated. Based on an Ag-ink prepared from the electrodepositmaterial and RDE experiments, it was concluded that the ORR occurs via a four-electron pathway on theAg electrodeposit. The combination of reasonably high catalytic activity, efficiency, low price, facile andgreen synthesis makes the electrodeposited Ag-GDE attractive for the ORR in alkaline fuel cells.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

It is well known that silver has a relatively high catalytic activitytowards the oxygen reduction reaction (ORR) in alkaline fuel cells.Silver is a very attractive catalysts as it is much cheaper andabundant than platinum, the typical catalytic material for the ORR.Numerous examples of Ag based materials have been investigatedfor application as catalysts for ORRs in alkaline solutions, such asnanorods [1], nanowires [2], nanodentrites [3], nanoclusters [4],nanospheres [4e9] and nanoplates [10]. In all these studies the Agcatalyst is synthesized as a carbon supported catalyst powder,

ax: þ351 259 350 480.

All rights reserved.

which is thenmixedwithwater, or a Nafion solution, giving rise to acatalyst ink. Commonly, this ink is then dispersed onto an electrodesupport (a gas diffusion carbon paper) for membrane electrodeassembly preparation and fuel cell tests, or it is droped-casted ontoa glass carbon electrode for kinetic studies.

Recently, our group developed a novel procedure of preparingthe catalyst layer of the membrane electrode assembly by directlydepositing the electrocatalyst onto a gas diffusion material [11e14].In such approach, nanoparticles of palladium and palladium alloycatalysts have been deposited on a carbon paper substrate using theelectroless or electrodeposition processes. It has been demon-strated that such methods afford the catalyst to be well dispersedon the top of the gas diffusion support, providing very low catalystloadings and a high catalytic activity.

Solution A

Solution B

-3.5

-2.0

-0.5

1.0

2.5

4.0

-0.3 -0.2 -0.1 0 0.1 0.2 0.3

j / m

A cm

-2

E vs SCE / V

-4.0

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0.0

2.0

4.0

6.0

-0.1 0 0.1 0.2 0.3 0.4 0.5

j / m

A cm

- 2

E vs SCE / V

Fig. 1. Cyclic voltammograms of the carbon paper substrate in solution A (17.5 mMAgNO3, 0.11 M EDTA, 3.5 M NH4OH) and in solution B (0.10 M AgNO3, 0.50 M NH4OH,0.1 M NH4NO3). n ¼ 10 mV s�1.

S. Salomé et al. / Materials Chemistry and Physics 143 (2013) 109e115110

Despite the advantages of depositing the catalyst directly ontothe gas diffusion electrode, very scarce examples are found on theliterature for the Ag based electrodes [15]. This work aims toinvestigate the capability of using the electrodeposition process, asimple and green method, to prepare a silver-gas diffusion elec-trode (Ag-GDE) and to evaluate the ORR activity of the assembledelectrode.

2. Experimental

2.1. Preparation and physical characterization

The Ag catalyst was directly electrodeposited onto a porouscarbon paper with 5% PTFE (GDL 24 BC, Sigracet) to form thecatalyzed gas diffusion electrode. Before deposition, the carbonpaper was first wetted in a 0.1% (w/w) Triton X-100 (Plusone) so-lution and then washed with de-ionized water.

The electrodeposits of Ag catalyst were prepared from twocurrently used plating solutions described in the literature. Onecontaining 17.5 mMAgNO3, 0.11 M EDTA, 3.5 M NH4OH (solution A)[adapted from [16]] and another containing 0.10 M AgNO3, 0.50 MNH4OH, 0.1 M NH4NO3 (solution B) [17]. The electrodepositionswere performed at room temperature in a three-compartment cellusing a graphite rod as the counter electrode and a saturatedcalomel electrode as the reference electrode.

After the carbon paper-supported catalyst has been prepared, itwas sealed in a PTFE holder and then inserted in a three electrodecell for voltammetric experiments. The working electrode geo-metric surface area was 0.196 cm2.

The Ag loading (mg cm�2) of the prepared samples was obtainedfrom the Ag stripping charge (Qoxid/C) using the Faraday’s law (Eq.(1)).

m ¼ ðQox � 107:9� 1000Þ=�96485� Ageom�

(1)

The anodic stripping charge was acquired by scanning the po-tential at a scan rate of 5 mV s�1 from �0.05 V to 0.30 V in solutionA or from 0.04 V to 0.50 V in solution B. Further scans in the samepotential range allowed to confirm that Ag was quantitativelyremoved from the carbon paper.

The morphology and composition of the prepared samples wereanalyzed by a FEI Quanta 400 FEG ESEM/EDAX Pegasus X4M sys-tem. Structural analysis of the films was carried out by X-raydiffraction in a PAN’Analytical X’Pert Pro diffractometer, equippedwith a X’Celerator detector and secondary monochromator, usingCuKa radiation.

2.2. Electrochemical characterization

Once the Ag samples have been prepared, these were thereafterelectrochemically characterized in 0.1 M NaOH solution. Electro-chemical measurements were performed using an Autolab poten-tiostat/galvanostat (model PGSTAT100), a Pt foil as counterelectrode and a double junction AgjAgCl,KCl (0.1 M) as a referenceelectrode (0.29 V vs NHE and 1.06 V vs RHE), at ambient temper-ature. Deaeration was achieved by N2 bubblying for approximately1 h. To ensure O2 saturation, the solution was purged with highpurity oxygen gas for at least 1 h. The ORR polarization curves werecorrected for an uncompensated resistance of 50.0 U, which wasdetermined by electrochemical impedance spectroscopy, using apotentiostat PGSTAT 302N (Metrohm Autolab) equipped with aFRA2 module.

In order to determine the number of electrons involved on theORR, rotating disc electrode (RDE) essays were required. To addressit, an Ag-ink was prepared from the Ag catalyst deposited on the

carbon paper at�0.20 V (Q¼�0.26 C cm�2) from solution B, whichwas afterward carefully brushed off. The obtained powder wasdispersed ultrasonically in 300 ml of water giving rise to the Ag-ink.Then, an aliquot of 19 ml catalyst suspension was pipetted onto aRDE of glassy carbon and dried at room temperature. The Agloading (0.027mg cm�2) was determined from the anodic strippingcharge, alike the procedure on the carbon paper-supported cata-lysts. For the RDE experiments, a Radiometer speed control unitfrom Autolab was employed.

3. Results and discussion

3.1. Preparation and physical characterization

In order to deposit Ag on the carbonpaper substrate, twodistinctplating solutions commonly used on the Ag deposition on othersubstrates than carbon paper, were used. The typical voltammo-grams of these electrolyte solutions on the carbon paper substrateare shown in Fig. 1. The cathodic sweep in the voltammetric curvesindicates that the potential onset of silver deposition is �0.16 V insolution A and �0.05 V in solution B. In the reverse sweep, a cross-over loop is observed, which is indicative that silver nucleation oc-curs on the carbonpaper. At potentialsmorepositive than�0.04V insolution A or 0.06 V in solution B, an anodic peak assigned to thesilver dissolution is depicted. The potentiostatic conditions for theAg electrodeposition, i.e. �0.30 V and �0.20 V in solution A and B,respectively, were chosen in order to have approximately the sameoverpotential deposition (i.e. h ¼ Eapplied � Eonset) in both plating

Table 1Experimental conditions used on the preparation of Ag-GDE.

Sample Depositionmode

Depositionconditions

Platingsolution

Ag load(mg cm�2)

1 Galvanostatic i ¼ �7.6 mA cm�2 B 0.24Q ¼ �0.24 C cm�2

2 Potentiostatic E ¼ �0.20 V 0.24Q ¼ �0.22 C cm�2

3 Potentiostatic E ¼ �0.20 V 0.039Q ¼ �0.04 C cm�2

4 Galvanostatic i ¼ �7.6 mA cm�2 A 0.067Q ¼ �0.23 C cm�2

5 Potentiostatic E ¼ �0.30 V 0.038Q ¼ �0.04 C cm�2

S. Salomé et al. / Materials Chemistry and Physics 143 (2013) 109e115 111

solutions (hz0.15 V). Ag-GDE samples were also prepared by gal-vanostatic deposition from solution A and B, while maintaining thesame reduction charge. The metal loading of selected electro-deposited Ag-GDEs samples (Table 1) revealed that the electrode-position technique is able to provide a catalyst layer containing acatalyst loading that can range from ultra-low (an example of0.038 mg cm�2 is given) to high (e.g. 0.24 mg cm�2).

Fig. 2 shows SEM images of the obtained electrodeposits. On allsamples the obtained particles are spherical and well dispersedoverall the carbon paper support. It is found that particles obtainedfrom solution A are much smaller than those from solution B. Thesmaller dimensions of the electrodeposited particles from solutionA may be associated to its lower AgNO3 concentration but may alsobe related to the presence of EDTA and its high ability to adsorb onthe silver surface [18]. This hypothesis is also supported by thedeposition carried out in a solution containing the same composi-tion of solution A, but in the absence of EDTA in the same deposi-tion conditions, where much larger particles were obtained, with aparticle size of 1.33 mm contrasting to 0.28 mm in the presence ofEDTA (results not presented here).

Fig. 2. SEM images of Ag electrodeposited on the

Comparing the deposits obtained from solution A containingclose metal loadings (samples 4 and 5), it is concluded that thedeposition mode neither affect the particles morphology, nor theparticles size and distribution over the surface. In contrast, onelectrodepositing Ag from solution B, it is observed that thepotentiostatic mode yields a much more uniform particle size onthe entire carbon paper surface than the galvanostatic depositionmode (sample 1 and 2).

On analyzing the influence of the time deposition under thepotentiostatic deposition mode on the silver catalyst layer, signifi-cant differences are depicted between both solutions, Fig. 3. Whilethe increase of the time deposition in solution A (t ¼ 10 and 30 s)induces an increase of Ag particles size, no size effect is observed onthe electrodeposit obtained from solution B (t ¼ 2 and 15 s). Incontrast, an increase on the number of Ag particles overall thecarbon paper surface seems to occur, which may be indicative thatmore active sites on the carbon surface become available along thedeposition time. A possible explanation for this unexpectablephenomena relies on the adsorption of the surfactant that is usedon the pretreatment of the carbon paper which may remain on thecarbon paper surface, even after washing, being slowly displaced bythe water solvent during the deposition process. This phenomenondoes not seem to occur in solution A, probably due to the strongadsorbability of EDTA on the carbon surface, replacing at once thesurfactant molecules [19]. Differences on the catalyst nucleationand growthmechanism in both plating solutions are also evidencedby the different deviations found between the experimental cur-rent transients plotted in the non-dimensional form and thetheoretical curves corresponding to 3D nucleation and diffusioncontrolled growth (Supplementary data).

To examine the crystalline structure of the Ag electrodeposited,X-ray diffraction measurements were carried out. Fig. 4 shows therepresentative XRD pattern of an electrodeposited Ag on the carbonpaper. Diffraction peaks at 38.3�, 44.5� and 64.6� are ascribed,respectively, to the (111), (2 0 0) and (220) planes of metallic silver,

carbon paper from plating solution A and B.

Fig. 3. SEM images of Ag electrodeposited on the carbon paper from two different plating solutins at increament deposition charges: in solution A, at �0.30 V, for depositioncharges of 0.027 and 0.069 C cm�2 (a, b) in solution B, at �0.20 V, for deposition charges of 0.040 and 0.22 C cm�2 (c, d).

S. Salomé et al. / Materials Chemistry and Physics 143 (2013) 109e115112

which are in agreement with the face-centered cubic (fcc) structureof silver.

Once a set of Ag catalyzed gas diffusion electrodes displayingdifferent Ag loads, and different particles sizes, have been prepared(Table 1) and physically characterized, their catalytic activity to-wards the ORR was evaluated and compared.

Fig. 4. Representative XRD of Ag electrodeposited on the carbon paper substrate(sample 2).

3.2. Study of the ORR on carbon paper-supported Ag catalyst

Prior to the ORR measurements the Ag samples were char-acterized by cyclic voltammetry in N2 saturated 0.1 M NaOHsolution. Fig. 5 shows the typical cyclic voltammetric response.As it was expectable, a small peak at 0.10 V (peak A1) andanother at 0.19 V (peak A2) are recorded in the anodic sweep.These peaks are assigned to the formation of AgOH and Ag2O,respectively [20,21]. Likewise, a peak appears in the cathodicsweep which is assigned to the Ag2O reduction (peak C1). Unlessotherwise stated the ORR experiments were carried out within apotential window of 0.0 to �0.90 V in order to avoid Ag oxida-tion. The large peaks depicted in the �0.4 to �1.1 V potentialrange is characteristic of the carbon paper behavior in the alka-line medium, as evidenced by voltammogram obtained in an Ag-free sample.

Fig. 6 is representative of the polarization curves of the differentAg samples in an oxygen-saturated 0.1 M NaOH solution (e.g.sample 2). Inset, the magnification of the current response in apotential range dominated by the kinetic controlled region isshown. Note that all the polarization curves were recorded in thecathodic scan because a small hysteresis between the positive andnegative-going sweep directions was found. The voltammogram ofthe carbon-paper substrate was also included for comparison. The

Tafel slope, as well as the mass activity extracted from the kineticregion are shown Table 2.

The Tafel slope of about 80 mV dec�1 is in accordance withearlier results for the O2 reduction on silver low-index single crystalsurfaces in alkaline solution at low overpotentials [22]. Such valuehas been demonstrated for a first electron transfer as the rate-determining step, assuming that two sites are required for the O2

�

adsorption and that all adsorbates follow a Frumkin isotherm.

Fig. 5. Representative cyclic voltammograms of carbon paper (- - - -) and Ag elec-trodeposited on the carbon paper support (______) in a 0.1 M NaOH solution(sample 3).

Table 2Kinetics parameters for the ORR on Ag-GDE samples in O2 saturated 0.1 M NaOHsolution.

Samples Loading(mg cm�2)

Particlesize (nm)

Eonset (V) Tafel(mV dec�1)

Mass activityat E ¼ �0.20 V(mA mg�1)

1 0.24 550e2000 �0.14 86.8 0.8512 0.24 870 �0.14 84.0 0.9153 0.039 870 �0.17 86.0 1.894 0.067 260 �0.16 82.1 1.645 0.038 280 �0.16 82.6 3.79

S. Salomé et al. / Materials Chemistry and Physics 143 (2013) 109e115 113

In commercial applications, it is the mass activity that de-termines the viability of a catalyst. The mass activity was deter-mined by normalizing the current at 0.20 V to the catalyst loading.It is important to highlight that the mass activity was corrected forthe capacitive current by subtracting the corresponding currentrecorded in the deaerated solutions. Unfortunately, there is not awell established and routine procedure for the determination of theAg electrochemical surface area, and for this reason, the specificactivity, i.e. the current density normalized to the surface area, wasnot calculated.

The long straight segment in the polarization curve for poten-tials below �0.40 V, do not seem to reach a diffusion controlledlimiting current, resembling the behavior observed on a mini-gasdiffusion electrode [23] and other catalysts earlier prepared byour group which were also anchored on a carbon paper support[11e14]. Apparently it seems that no concentration polarizationoccurs inside the catalyst layer since the interfacial oxygen masstransport may be very fast. However, electrochemical impedancespectroscopy analysis of these type of electrodes is understudy inorder to better understand this behavior.

Fig. 6. Representative linear voltammograms in O2-saturated 0.1 M NaOH solution ofcarbon paper and Ag-GDEs (sample 2). n ¼ 5 mV s�1.

The effect of the Ag particles size on the ORR polarization isrepresented by the kinetic data of samples 3 and 5 (same Ag load).It is shown that the onset for the ORR is approximately the same onboth samples, but the mass activity is two-fold higher on thesample with a smaller particle size. On comparing samples havingthe same particles size, but distinct Ag loading (samples 2 and 3 orsamples 4 and 5), a two-fold increment on the mass activity isobtained on the lowest Ag loading samples. The highest activitywas found on sample 5, which has both the lowest particle size andlowest catalyst loading. Comparing the mass activity of the Ag-GDEwith the literature data, we conclude that our results are compa-rable those obtained with silver nanoclusters of 3.3 nm size pre-pared by chemical reduction and coated with a capping ligand(1.36 mA mg�1 at 0.76 V vs RHE) [4] or silver nanoparticles of 100 nmsize (2.0 mA mg�1 at 0.90 V vs RHE) [24]. However, a better catalyticactivity has been found for Ag nanoclusters of 0.9 nm diameter(12.48 mA mg�1 at 0.76 V vs RHE) [4].

While this study reveals the promising potential of the elec-trodeposition as an effective electrode making process to prepareAg-GDE showing a reasonably catalyst utilization and an ultra-lowcatalyst loading, the preparation of particles in the range of fewnanometers are not straightforwardly obtained, remaining a sub-ject of research in the future.

3.3. The effect of methanol on the ORR

In order to investigate the activity of the Ag-GDEs for the ORRundermethanol crossover conditions, the polarization curves in theO2 - saturated solution were recorded in the presence of 0.1 MMeOH on two samples containing different Ag loadings (samples 2and 5), Fig. 7. Albeit in general a good tolerance to methanol isshown, some particularities are observed on these two samples.Both reveal in the mixed controlled potential range an increase ofthe reduction current density in the presence of methanol,reminding other GDE onwhich the catalyst is directly deposited ona carbon paper support [7]. Such behavior is attributed to an in-crease of the carbon paper hydrophilicity in the presence ofmethanol, contrasting to conventional electrodes, prepared fromcarbon powder support catalysts, where the oxygen reductioncurrent is known to decrease with the addition of methanol[7,10,25,26].

In the kinetic controlled potential region the two samples showa slightly different response. On the electrode containing the lowestload (sample 5), there is a 50 mV shift of the potential onset to-wards more negative potentials with a concomitant decrease of thecurrent density, while on the electrode having the highest load(sample 2) the potential onset and current density remain practi-cally unchanged in the presence of the alcohol. These resultsreinforce the conclusion that electrodeposited Ag catalyst shows avery good tolerance to methanol.

Hence, it is concluded that the direct deposition of Ag on a gasdiffusion substrate by the electrodeposition technique is an envi-ronmental attractive alternative on the preparation of the catalyst

-0.8 -0.6 -0.4 -0.2 0.0-50

-40

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0

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-4

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0

NaOH NaOH+MeOH

Sample 2

j / m

A cm

-2

Evs (Ag/AgCl) / V

Sample 5

Fig. 7. Comparison of polarization curves for ORR on Ag-GDEs in 0.1 M NaOH with(- - - - -) and without (_______) 0.1 M MeOH (sample 2 and 5).

-3

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E vs (Ag/AgCl)/V

Ag-ink

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Ag-ink N2

0 rpm

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w-1/2/rpm-1/2

-0.60 V-0.70 V-0.90 Vn=4

a)

b)

c)

j/mA

cmj/m

A cm

Fig. 8. a) Cyclic voltammograms in a 0.1 M NaOH solution of Ag-ink catalyst preparedfrom the Ag electrodeposited; b) RDE measurements in O2 saturated solution atdifferent rotation rates; c) KouteckyeLevich plot.

S. Salomé et al. / Materials Chemistry and Physics 143 (2013) 109e115114

layer of membrane electrode assembly for alkaline fuel cells of H2or methanol.

3.4. RDE experiments

Typically, investigation on the mechanism of the oxygenreduction reaction requires rotating disk electrode (RDE) essays inorder to determine the number of electrons transferred per O2molecule. The oxygen reduction reaction can take place via either adirect four-electron or by an indirect two-electron transfer. Whilethe four-electron reaction produces hydroxide ions, the two-electron transfer pathway produces hydroxide and peroxide ions,contributing to a decrease of the overall efficiency of the cell, aswell as to a decrease on the catalyst and membrane durability[26,27]. Previous work have shown that the number of electronsinvolved in the ORR on Ag bulk is four, but lower values have beenfound on Ag/C catalyst due to the two-electron process that occurson carbon in the alkaline medium [28,29].

To perform the RDE experiments, a Ag-ink was prepared fromAg electrodeposited on the carbon paper substrate (see descriptionabove). Fig. 8a shows the cyclic voltammogram of the Ag-inkcatalyst deposited onto the glass carbon electrode in an O2-free0.1 M NaOH solution. The cyclic voltammogram presents thecharacteristic peaks of Ag catalyst in the alkaline medium. Thepeaks characteristic of the carbon paper are not visible, which isindicative that not much carbon paper has been dragged whilebrushing off the Ag catalyst from its surface.

Contrasting to the polarization curves obtained on the Ag-GDEsin the O2-saturated solution, a well defined diffusion limiting cur-rent is reached on the Ag ink electrocatalyst at different rotationrates, at potentials below �0.60 V, Fig. 8b. The diffusion limitedcurrent densities are close to the theoretical values for a fourelectron process (for example 4.94 mA cm�2 at 1200 rpm).

Fig. 8c shows a plot of the inverse of the current density (j�1) as afunction of the inverse square root of the rotating rate (u�1/2),known as the KouteckyeLevich (KeL) plot at different potentials.

The linear relationship between u�1/2 and j�1 and the paral-lelism of the obtained plots indicate, on one hand, a first order ki-netics of the Ag sample with respect to the oxygen reductionreaction, and on the other hand, that the number of electronstransferred does not change within the potential range studied.Note that the plot for �0.60, �0.70 and �0.90 V potentials do notintercept the j�1 axis at the origin, which means that at those po-tentials the reaction is not totally diffusion-limited.

From the slopes of the KeL plots, the number of electronsexchanged in the reduction of oxygenwere obtained: n ¼ 4.0 for allthe studied potentials. The dashed line represents the theoreticalslope for a four-electron reduction under diffusion control. Thecalculation was performed using the published data for oxygensolubility (1.2 � 10�6 mol cm�3), the solution’s kinematic viscosity(1.0 � 10�2 cm2 s�1) and oxygen diffusivity (1.9 � 10�5 cm2 s�1)[30]. A four electron pathway mechanism is consistent with amechanism on which O2 is reduced directly to H2O or OH- withoutintermediate formation of H2O2 [31]. This result shows that the

S. Salomé et al. / Materials Chemistry and Physics 143 (2013) 109e115 115

electrodeposited Ag on the carbon paper substrate is an efficientcatalyst for the ORR.

According to a literature survey, there are very scarce worksreporting the direct deposition of Ag on a GDE. According to C.Hsieh et al. [15], microwave-assisted and chemical impregnationwere applied to deposit directly Ag particles on oxidized carbonpaper. Compared to the electrodeposited Ag-GDE, these differentprepared Ag-GDEs showedmuch smaller particles size (20 nm), butthe particles were not uniformly distributed over the carbon papersurface and the obtained loadings were rather high (0.14e0.32 mg cm�2). Their catalytic activity towards the ORR was notevaluated.

The direct deposition of the catalyst onto the carbon paper byelectrodeposition was recently applied by our group to Pd [14].However, very distinct morphologies were obtained, none of themexhibiting discrete spherical particles.

4. Conclusions

Electrodeposition reveals to be a facile, low cost and versatiletechnique to prepare gas diffusion electrodes displaying variableAg loadings, even with ultra-low loading levels. The size of thedeposited particles was found to depend significantly on theplating composition solution. Among two currently Ag platingsolutions, the smallest particles size (within micrometer range)were obtained in a EDTA containing plating solution. Among thedifferent prepared Ag-GDEs, the highest catalytic activity, interms of the mass activity, was found on the samples containingthe smallest particle size and lowest catalyst loading(3.8 mAAg mg�1). Based on RDE experiments it was concludedthat the ORR occurs via a four-electron pathway on the Ag-GDEs.It was also demonstrated that the electrodeposited Ag samplesshow a good tolerance to methanol. The combination of reason-ably high catalytic activity, low price, tolerance to methanol,simple, fast and green synthesis, makes the electrodeposited Ag-GDE attractive for the ORR in alkaline fuel cells, particularly indirect methanol fuel cells.

Acknowledgments

This work was supported by Fundação para a Ciência e a Tec-nologia (FCT) and COMPETE (projects PTDC/QUIeQUI/110855/2009and UI 686 - 2011e2012, PEst-C/QUI/UI0686/2011).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.matchemphys.2013.08.026.

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