- Email: [email protected]
Carbon supported Ag and Ag–Co catalysts tolerant to methanol and ethanol for the oxygen reduction reaction in alkaline media
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Carbon supported Ag and AgeCo catalysts tolerant to methanol and ethanol for the oxygen reduction reaction in alkaline media ndez-Rodrı´guez, M.C. Goya, M.C. Arevalo, J.L. Rodrı´guez, M.A. Herna * E. Pastor Departamento de Quı´mica, Instituto de Materiales y Nanotecnologı´a, Universidad de La Laguna, Apdo. 456, 38206, La Laguna, Tenerife, Spain
article info
abstract
Article history:
Ag/C and AgeCo/C catalysts with high activity towards the oxygen reduction reaction in
Received 29 January 2016
alkaline media, different metal loadings and average sizes below 20 nm were synthesized
Received in revised form
by glycerol and borohydride reduction methods without stabilizers. Physicochemical
23 July 2016
characterization of the materials was performed by X-ray techniques (diffraction, energy
Accepted 24 July 2016
dispersive and photoelectron spectroscopy) and transmission electron microscopy. Well
Available online xxx
dispersed small nanoparticles were obtained in all cases, mainly formed by Ag(0). For Ag eCo/C catalyst, it was observed that Co was not alloyed but presented as cobalt oxides. The
Keywords:
electrocatalytic activity towards oxygen reduction reaction (ORR) in alkaline solution was
Silver nanoparticles
evaluated by cyclic voltammetry and rotating disk techniques. A four electron transfer
Oxygen reduction reaction
mechanism was established although increasing Ag loading produces a decrease of this
Alcohol tolerance
number, indicating that hydrogen peroxide produced as intermediate in a first two electron
Alkaline fuel cell
step was not completely reduced. Alcohol tolerance of the catalysts was also established in methanol and ethanol solutions. Materials were not active for the electro-oxidation of alcohols, although it was observed that both methanol and ethanol were adsorbed on the catalyst. Highest activity and alcohol tolerance was observed for the 60 wt.% Ag loading material. Also the introduction of Co produces an increase in the activity (higher ORR limiting currents) and in the alcohol tolerance. In comparison to Pt and Pd, Ag and AgeCo present more appropriate activities for ORR when alcohol tolerance is considered, being good candidates for the use as catalysts in alkaline direct alcohol fuel cells. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Alkaline direct alcohol fuel cells (ADAFCs) are considered good candidates as power sources for future portable and vehicle devices owing to their high theoretical specific energy, low
cost and environmental friendliness [1e3]. These systems have some advantages compared to the common polymer electrolyte membrane fuel cells (PEMFCs), such as the usage of non-precious metal catalyst, the decrease of cathode overpotential and the favorable water management. Thus, it is the
* Corresponding author. E-mail address: [email protected] (E. Pastor). http://dx.doi.org/10.1016/j.ijhydene.2016.07.188 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
possibility to replace Pt-based alloys by non-noble (and cheaper) metals as electrocatalysts for both electrodes the most relevant characteristic of the ADAFCs [4,5]. Previous works have shown that several metals could be used as anode catalyst [6] and silver appears as a good candidate for the cathode [7e9]. However, there are still notable challenges for ADAFCs concerning the oxygen reduction reaction (ORR), including the improvement of the low reaction rates, high overpotentials and low stabilities. Silver is a promising candidate for replacing Pt for ORRs in alkaline solution due to its high activity and comparatively low cost [10,11]. Moreover, Ag cathodes have been reported to be more stable than Pt ones during long-term operation [12]. On the other hand, Ag and its alloys show better tolerance toward methanol [7e11,13] and ethanol [14] poisoning than Pt. It has been reported that Ag/C promotes the 4-electron pathway for ORR in alkaline solutions and increasing the content of Ag in the carbon supported catalyst produces a positive shift of the onset potential for this reaction [15]. Ying et al. [16] have reported that Ag nanoparticles supported on Co3O4 show large electrochemical active surface area (ESA), high catalytic activity towards ORR and also exhibit good methanol tolerance and stability in alkaline media. In a recent work Eun et al. [17] described that highly dispersed Ag nanoparticles on reduced graphene oxide (RGO) displayed higher catalytic activity than Ag/C. Tammeveskia et al. reported that Ag nanoparticle/multi-walled carbon nanotube (AgNP/ MWCNT) presented a high electrocatalytic activity towards ORR, with similar specific activity to bulk Ag [18], whereas Sekol and colab. have shown that Ag@Pd/MWCNTs catalysts also presented high tolerance to methanol and ethanol [19]. On the other hand, it was evidenced that the presence of a second metal also improved the Ag activity. Lima et al. have studied AgeCo/C as cathode catalyst for ORR in AFCs and found that Co, in the form of Co3O4, contributed to the 4electron pathway with respect to Ag/C [20]. Bard et al. have proposed a simple mechanism to illustrate the electroactivity enhancement of Ag-based binary catalysts for ORR: the OeO bond first breaks down on one metal-atom of the binary Agbased catalyst to form an adsorbed O atom (Oad), and then the Oad transfers into another metal-atom (usually Ag) to be reduced. AgePt, AgeAu and AgePd have been extensively studied as effective Ag-based binary electrocatalysts for ORR [21,22]. In addition, Lima et al. have prepared a low cost carbon supported binary AgeCo catalyst, with high electroactivity for this reaction, by a chemical reduction procedure [20]. Finally, other no-noble metals have been used for the preparation of bimetallic Ag electrocatalysts. Thus, it was observed that the increment of the amount of Ni in the AgeNi alloy and the heat treatment at 500 C decreases the ORR overpotential and increases the limiting current density for this reaction [23]. Alternatively, it has been reported that AgeNi catalyst can be used in borohydride fuel cells exhibiting higher discharge voltage and capacity [24]. In this work, Ag/C materials with low nanoparticle sizes were synthesized by the glycerol reduction method to be used as catalyst at the cathode of alkaline fuel cells, varying the Ag loading between 10 and 60 wt.%. On the other hand, carbon supported AgeCo catalysts have been prepared using NaBH4 as reduction agent with 20 wt.% metal content. The
composition, morphology, particle size and metal loading of the catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive Xray (EDX) and photoelectron spectroscopy (XPS) analysis. The electrocatalytic activity towards the ORR was studied applying two electrochemical techniques: cyclic voltammetry and rotating disk electrode, in the presence of different methanol or ethanol concentrations. Finally, the catalytic activity of 20 wt.% Ag/C and AgeCo catalysts was compared to the performances for 20 wt.% Pd/C and Pt/C materials.
Material and methods Preparation of Ag/C catalysts Ag/C catalysts with different Ag content were synthesized following the glycerol reduction method reported in the literature [25]. Briefly, an appropriate amount of Vulcan XC-72R (Cabot Corp.) in a known volume of Milli-Q water was sonicated and a certain quantity of AgNO3 (Aldrich, 99.9999%) was added under stirring to promote the homogenization of the mix. Afterward, another solution containing glycerol (SigmaeAldrich, 99%) and NaOH (SigmaeAldrich, 99.99%) was added to give the following concentrations: 0.82 mM AgNO3, 1 M glycerol and 0.1 M NaOH. The mixture was kept during 24 h under stirring at room temperature and then washed, filtered and dried at 80 C during 12 h. Three Ag/C catalysts with silver loadings of 10, 20 and 60 wt.% were prepared with this method.
Preparation of AgeCo/C, Co/C and Pd/C catalysts Pd, Co, Ag and AgeCo catalysts dispersed on carbon Vulcan XC-72R with 20 wt.% metal content were prepared using NaBH4 as reduction agent. A stronger reduction agent was used to guarantee metal reduction in all cases. The bimetallic particles were obtained with different nominal atomic ratios, ranging from pure Ag and Co (for a comparative purpose) to Ag:Co (3:1). Materials were prepared by simultaneous reduction of the precursor metal salts Ag (NO3)2 and Co(NO3)2$6H2O (SigmaeAldrich, 99%). The reduction process was carried out at room temperature by drop wise addition of the precursor solution onto a carbon black dispersion, which was prepared by suspending carbon powder in ultrapure water ultrasonically blending for 10 min and then 12 h under magnetic agitation. This is followed by drop wise addition of an excess of NaBH4 solution for reduction and precipitation of the precursor metal particles. The mixture was kept during 12 h under stirring at room temperature and then washed, filtered and dried at 60 C during 12 h. Similar procedure was applied for preparation of Pd/C material using PdCl2 (SigmaeAldrich, p.a.) as precursor. Commercial E-TEK 20 wt.% Pt/C catalyst was also employed for comparison.
Physicochemical characterization The real Ag content and the atomic composition of the electrocatalysts were determined by energy dispersive X-ray analysis (EDX) coupled to a scanning electron microscopy (SEM) Jeol JEM Mod. 6300 with a silicon doped with lithium
ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
Oxford 6699 ATW detector applying 20 keV. Particle dispersion on the support was established from transmission electron microscopy (TEM) images obtained with a Jeol JEM Mod. 2100 equipment. X-ray diffraction (XRD) patterns were obtained using a universal diffractometer Carl Zeiss-Jena, URD-6, equipped with a PANalytical X'Pert Pro X-ray diffractometer operating with Cu Ka radiation (l ¼ 0.15406 nm) generated at 40 kV and 20 mA. The average crystallite size was calculated using Scherrer's equation: D¼
kl BCosq
where D is the average crystallite size in angstrom, k is a coefficient taken here as 0.9, l the wavelength of the X-rays used (1.5406 Å), B the width of the diffraction peak at half height in radians and q the angle at the position of the peak maximum. High-resolution XPS spectra were collected with an ESCALAB 250 spectrometer equipped with dual aluminummagnesium anodes, using a monochromatized Al Ka X-ray radiation (1486.6 eV) with a spot size of 650 mm. The spectrometer energy calibration was performed using the Au 4f7/2 and Cu 2p3/2 photoelectron lines. The spectra were collected in constant analyzer energy (CAE) mode, with pass energy of 20 eV and with an energy resolution of about 0.1 eV. For all the measurements, pressure in the ultrahigh vacuum analysis chamber was less than 9 109 mbar, avoiding that the ejected photoelectron interact with gas molecules.
Electrochemical characterization A three-electrode configuration cell was used for these experiments. A pre-calibrated reference hydrogen electrode (RHE) and a glassy carbon bar were used as reference and counter electrodes, respectively. A glassy carbon (GC) disk supporting the catalyst, with a geometric area of 0.071 cm2, was used as working electrode. Before each test, the electrode was polished with 1 mm alumina on a polishing cloth, followed by washing the electrode with abundant Milli-Q water. For the preparation of the working electrode, a catalyst suspension was prepared with 3 mg of the catalyst in 1 mL of Milli-Q water. Then 20 mL of catalyst suspension was deposited on the surface of the polished GC disk. The electrode was dried at room temperature assisted by a nitrogen flow.
3
The electrochemical characterization was carried out using cyclic voltammetry (CV) and the ORR was evaluated by linear sweep voltammetry (LSV), in both cases using a computer-controlled electrochemical analyzer mAutolab III, Eco Chemie BV. Once deposited and dried the ink with the catalyst on the GC surface, the electrode was cycled for activation between 0.05 and 0.80 V at 100 mV s1 in Ar purged electrolyte solution (0.1 M NaOH) during 50 cycles. Then the catalyst was cycled in the working range (0.05e1.44 V) at 10 mV s1 until stable and reproducible voltammograms were obtained. LSV experiments were performed using a rotating disk electrode (RDE) model Eco Chemie BV Autolab rotator, in oxygen saturated 0.1 M NaOH solution. LSV data were recorded in the negative going sweep direction running from 1.00 to 0.05 V vs. RHE over a range of rotation speeds (from 400 to 2500 rpm) at a scan rate of 2 mV s1.
Results and discussion Physicochemical characterization The crystalline structure of the Ag/C catalysts was characterized by XRD, as can be seen in Fig. 1A. The diffraction patterns for the three materials are similar, with peaks at 2q values of 38.0 , 44.4 , 64.5 and 77.4 that can be assigned to the (111), (200), (220), and (311) crystalline planes of Ag with a face-centered cubic structure, respectively. Characteristic diffraction peaks for crystalline Ag2O or silver hydroxide appear only in the spectra for 10 wt.% Ag loading as two small peaks at 30.1 and 40.2 . The mean sizes of Ag crystallites calculated from the (220) diffraction peak applying Scherrer's equation are around 16e21 nm (Table 1). It is remarkable that similar values were obtained for 20% wt. Ag catalysts prepared with the glycerol (21.0) and borohydride (19.4) reduction methods (Table 2). On the other hand, the crystalline structure of the AgeCo nanoparticles resembles the face centered cubic (fcc) structure of silver (Fig. 1B). There is no shift in the peak positions with respect to Ag/C, indicating a negligible Co insertion in the Ag lattice. However, no peaks due to the presence of Co oxides are apparent in the spectra (Coþ2 and Coþ3 are present in the XPS spectra), and therefore, have to be amorphous. The mean crystallite size estimated for AgCo/C (3:1) catalyst is 16.9 nm,
Fig. 1 e XRD patterns A) for Ag/C catalysts and B) for 20 wt.% Ag/C and AgCo/C. ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
Table 1 e Metal loadings (EDX), crystallites sizes and lattice parameters (XRD), and XPS data for the different Ag-based supported catalysts. Metal content (nominal) (wt.%)
Metal loading (wt.%) EDX
Crystallite (nm) XRD
Lattice parameter (Å) XRD
Binding energy (eV)
Binding energy (eV)
Ag0 (%)
Agþ (%)
9 22 59 18 13 (Ag), 3 (Co)
18.1 21.0 17.7 19.4 17.0
4.097 4.094 4.097 4.098 4.094
368.45 (93.67) 368.43 (93.64) 368.43 (100) 368.43 (100) 368.43 (100)
367.81 (6.63) 367.82 (6.34) e e e
10 20 (glycerol) 60 20 (borohydride) 20
Table 2 e Kinetic parameters for the ORR derived from hydrodynamic polarization curves. Ag/C catalyst Nominal content (wt.%) 10 20 60
Onset n (mV)
826 829 890
3.9 3.9 3.7
Kinetic current density (mA cm2) 36 209 1639
Tafel slope Tafel slope small h large h (mV/ (mV/ decade) decade) 85 89 75
104 112 117
indicating that the presence of Co induces a decrease in the mean size of Ag nanoparticles. The same result was reported by Lima et al. [20]. XRD patterns indicate the predominance of the (111) plane in both catalysts, being more relevant in the case of the bimetallic one. TEM images for 10, 20 and 60 wt.% Ag/C catalysts are shown in Fig. 2. The image for the 60 wt.% Ag/C (Fig. 2B) shows well dispersed Ag nanoparticles with sizes in the same range obtained from X-ray diffractograms, but at 60 wt.% the particles appear agglomerated. Surprisingly, in the image for the 10 wt.% Ag/C (Fig. 2A), it can be observed a big number of metal nanoparticles lower than 2e3 nm and some in the range 10e15 nm. The crystallite size obtained from XRD in Fig. 1 is in agreement with the last values, and it seems that the small nanoparticles are below the detection limit of the technique, only recording the contributions of the bigger ones. XPS spectra of Ag/C catalysts prepared by the glycerol route are shown in Fig. 3. In the corel level spectrum of Ag, peaks at 368 and 374 eV are assigned to Ag3d5/2 and Ag3d3/2 transitions, respectively. These energies and the 6 eV spineorbit splitting of the 3d doublet are characteristic of silver in the metallic form (Ag0) [26]. Only a small contribution related to silver oxide (Ag2O) can be found in the 10 and 20 wt.% Ag/C catalysts (Fig. 3 and Table 1). Accordingly, peaks corresponding to crystalline oxides were observed for 10 wt.% Ag/C catalyst in Fig. 1A but not for 20% catalysts, suggesting that in the latter case the oxides are amorphous. It is remarkable that for the 20 wt.% material synthesized with the borohydride reduction method, all Ag is present in the metallic form (Table 1). On the other hand, no XPS peak features of Ag2O or other silver oxides are observed for the 60 wt.% Ag/C catalyst, indicating that only Ag0 is present in this material, in concordance with the XRD results. Finally, the XPS spectrum of the AgeCo/C catalyst (Fig. 4) indicates that Ag is also present as Ag0 in this material (Fig. 4A). Concerning Co (Fig. 4B and C), peaks at 782 and 798 eV are related to Co2p1/2 and Co2p3/2 transitions, respectively, and are
Fig. 2 e TEM images for A) 10, B) 20 and C) 60 wt.% Ag/C catalysts. associated to Coþ2 and Coþ3 (it is difficult to separate both signals). The shoulders at 787 and 803 eV in these figures are satellite signals associated only to Coþ2. Comparing the 3:1 AgeCo/C catalyst with the Co/C (19 wt.% real metal loading),
ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
Fig. 3 e XPS spectra for the Ag 3d region of Ag/C.
the spectra are very similar, although a slight shift is observed from 782.4 to 798.1 eV signals in pure Co to 781.9 and 797.6 eV in the presence of Ag, which indicates that electrons goes from Ag to Co in the bimetallic materials increasing the amount of Coþ2. Accordingly, the intensity of the satellite signals slightly increases with the addition of Ag (see Fig. 4B and C).
Electrochemical characterization Oxygen electroreduction in absence of alcohol Fig. 5A displays the cyclic voltammograms of Ag/C catalysts with 10, 20 and 60 wt.% metal loading in freshly prepared 0.1 M NaOH solution purged with nitrogen. The curves exhibit two main anodic contributions (C1 and C2) and an unique cathodic peak for all catalysts, which are in good agreement with the CVs reported in the literature [15].The anodic features around 1.21 and 1.31 V vs. RHE are due to the formation of the Ag2O layer and the cathodic peak centered at 1.05 V vs. RHE is
5
associated to the oxide reduction to metallic silver [27]. Current densities in CV experiments were calculated using the electrochemical surface area obtained from the charge involved in the Ag oxide reduction peak assuming 420 mC/cm2 [28]. The curves for AgeCo/C catalyst in the base electrolyte present a broad current contribution previous to the formation of the silver oxide (Fig. 5B), describing two peaks at 1.03 and 1.17 V during the positive-going scan. Moreover, comparison of the CVs for Ag and AgeCo catalysts shows a shift to more negative potentials in the reduction peak recorded during the negative sweep for the bimetallic material, indicating a stronger Oad interaction with Ag surface sites in the presence of Co which could facilitate the splitting of the OeO bond [16]. The ORR was studied at these materials in an oxygensaturated 0.1 M NaOH solution by the RDE technique. Steady-state polarization curves for 10, 20 and 60 wt.% Ag/C catalysts were recorded at a scan rate of 2 mV s1 and various rotation speeds, namely, 400, 600, 900, 1600 and 2500 rpm. It was observed that the current densities (referenced to the geometric area of the working electrode) increase, as expected, with the increment of the rotation speed (not shown). In Fig. 6A the polarization curves of these Ag/C catalysts at 1600 rpm are given in order to compare the catalytic activity of the materials. It is observed that final limiting diffusional currents are similar for 20 and 60 wt.% Ag loading, but an important shift in the onset potential (see the values in Table 2) of almost 70 mV to positive potentials is displayed by the latter material. Similar behavior was described previously [28]. Then, this catalyst appears as the most appropriate for the ORR reaction. It is remarkable that 60 wt.% Ag/C shows the worst dispersion in Fig. 2C, suggesting that the ORR could be facilitated at the grain boundary of the agglomerated nanoparticles. Other factor that could contribute is the fact that only metallic silver is present in this material, whereas oxides were detected for 10 and 20 wt.% Ag/C catalysts prepared by the glycerol reduction method. Information about the mechanism of the ORR at these catalysts can be obtained from the polarization curves. The oxygen reduction reaction is a multi-electron reaction which has two possible pathways in basic media. One is an indirect four-electron transfer process, which produces hydrogen peroxide as intermediate while the second is a direct fourelectron transfer process producing hydroxyl ion as the final
Fig. 4 e XPS spectra for 20 wt.% catalysts. A) Ag 3d region of Ag/C and AgCo/C; B) and C) Co2p region of AgCo/C and Co/C, respectively. ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
Fig. 5 e Cyclic voltammograms in Ar purged 0.1 M NaOH solution at a sweep rate of 10 mV s¡1. A) 10, 20 and 60 wt.% Ag/C catalysts and B) 20 wt.% Ag/C and AgCo/C. solubility in the solution, n ¼ 0.0109 cm2 s1 is the NaOH solution kinematic viscosity, all at 25 C, and u (radian per second) is the RDE revolution speed. B is the Levich coefficient given by: B ¼ 0:62nFDO2=3 n1=6 CO
Fig. 6 e A) RDE polarization curves for 10, 20 and 60 wt.% Ag/C catalysts in oxygen-saturated 0.1 M NaOH solution at 1600 rpm and 2 mV s¡1. B) KeL plots for the Ag/C catalysts.
product. The overall number of electrons exchanged per oxygen molecule can be calculated from the limiting diffusion current density plateaus using the classical KoucteckyeLevich (KeL) equation [28]: 1 1 1 1 1 ¼ þ þ ¼ j jk jD nFkCo Bu1=2 where jD is the limiting current density, jk the kinetic current density, n is the number of electrons involved in the reaction, k is the rate constant for O2 reduction, DO ¼ 1.93 105 cm2 s1 is the oxygen diffusion coefficient, F ¼ 96485.34 C mol1 the Faraday's constant, CO ¼ 1.26 107 mol cm3 is the oxygen
From the slope of KeL plots (Fig. 6B), the calculated value of n (Table 2) resulted to be close to 4 for 10 and 20 wt.% Ag/C catalysts indicating that the ORR proceeds through a direct four-electron pathway. For catalysts with 60 wt.% loading, n is 3.7, respectively, demonstrating the non-completely elimination of the hydrogen peroxide produced as an intermediate in a first two-electron process [3] even this is the catalyst with the highest activity. Extrapolation of KeL curves to 1/u1/2 ¼ 0 allows to obtain 1/jk and calculate the kinetic current density for these materials [28]. Values for jk in Table 2 show that the 60 wt.% Ag/C catalyst presents the highest jk in agreement with the shift in the curve observed in Fig. 6A. The Tafel plots for normalized kinetic currents for Ag/C catalysts can be seen in Fig. 7. Two different linear regions can be distinguished: one at low overpotentials (l.o.r., 0.95 > E > 0.75) and the second at high overpotentials (h.o.r., 0.75 > E > 0.60). The Tafel slopes obtained from the above figure are given in Table 2 and the values are in accurate agreement with those reported in the literature [29]. In the l.o.r. slopes closed to 90 mV dec1 were determined, whereas values near 120 mV dec1 were calculated in the h.o.r. A Tafel slope of 120 mV dec1 can be interpreted through a mechanism involving the first single electron transfer as rate determining step, r.d.s., and Langmuirian conditions for the reaction intermediates, whereas that of 90 mV dec1 would probably be the result of a competition of, at least, two electrochemical processes: a single electron transfer as r.d.s. and a chemical r.d.s. following the first electron transfer [30,31]. Such Tafel slope could also be explained by the presence of surface oxides with adsorption isotherms different from Langmuir. The dependence of surface coverage of those species with potential modifies the classical Tafel slope of 120 mV dec1e90 mV dec1.
Oxygen electroreduction in presence of alcohols For ADAFCs, the crossover of the alcohols used as fuels at the anode to the cathode unavoidably occurs and these molecules can competitively adsorb on active sites with oxygen, leading to a mixed potential at the cathode and a notably decrease of fuel cells performances. Thus, it is important for the cathodic
ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
Fig. 7 e Tafel plots for ORR at Ag/C catalysts. a Low overpotential region (l.o.r.) and b high overpotential region (h.o.r.).
materials to be tolerant to alcohols. Hence, the ORR was studied for Ag/C and AgeCo/C catalysts in the presence of methanol (MeOH) or ethanol (EtOH) with the purpose to establish their tolerance. Firstly, to investigate the activity of Ag/C and AgeCo/C towards alcohol oxidation in alkaline medium, cyclic voltammetry were performed in 0.1 M NaOH þ xM MeOH or EtOH (x ¼ 0.5, 1, 2, 3 M) solutions. Fig. 8A and B shows the CVs for the 20 wt.% Ag/C catalyst in presence of EtOH and MeOH, respectively. In both cases, the curves do not exhibit alcohol oxidation features implying that catalysts are not active for their oxidation in alkaline media. Nevertheless, an inhibition of the peaks related to oxide formation and reduction is observed, especially remarkable with ethanol, which indicates that the alcohol is adsorbed without oxidation. For the other Ag/C catalysts with different metal loadings, the behavior in the presence of alcohols is similar. Moreover, no alcohol oxidation peak was also observed for AgeCo/C in the presence of several concentrations of alcohol (not shown), confirming that the prepared Ag-based catalysts are electrochemically inactive for this electrooxidation reaction.
7
RDE measurements for the ORR in the presence of different methanol or ethanol concentrations were accomplished in the same conditions previously described in Section Oxygen electroreduction in absence of alcohol. Fig. 9 shows the ORR polarization curves of the three Ag/C catalysts for an oxygensaturated 0.1 M NaOH solution in the presence of 0.5 M methanol and ethanol at 1600 rpm. A decrease in the cathodic diffusional limiting current was recorded in all cases, with a maximum diminution around 10% in the presence of ethanol and 5% in the presence of methanol. Increasing the alcohol concentration up to 3 M (not shown), a further decrease in the limiting current was observed achieving a 40 and 20% for ethanol and methanol, respectively. But it has to be considered that this is a very high concentration of the alcohol, and accordingly, it can be concluded that silver materials present a high alcohol tolerance (significant when compared with most active metals for the ORR, i.e. Pt and Pd, see Section Comparison of ORR catalytic activities for 20 wt.% Ag, Pd and Pt supported catalysts). It is remarkable that the diffusionlimited current density of the ORR polarization curve gradually decreases with increasing the alcohol concentration although the ORR currents at the kinetics-controlled region keep almost unvaried. Two possible reasons might account for this phenomenon: i) the adsorbed methanol or ethanol on the surface of thin film electrode covers partially active sites; and ii) the solubility of O2 in electrolyte decreases with the increase in methanol or ethanol concentration. Polarization curves for the ORR at AgeCo/C in the presence of methanol display higher mass cathodic current densities (per silver content) for the bimetallic catalyst compared to Ag/ C (Fig. 10 shows the curves recorded at 1600 rpm for 20 wt.% metal content materials), that is, inhibition is also observed with the AgeCo/C catalyst but the latter is an improved alcohol tolerant material.
Comparison of ORR catalytic activities for 20 wt.% Ag, Pd and Pt supported catalysts The polarization curves for ORR at 1600 rpm for 20 wt.% Ag/C, Pd/C and Pt/C catalysts in absence of alcohols are compared in Fig. 11. It can be seen that both 20% Pd/C and 20 wt.% Pt/C
Fig. 8 e Cyclic voltammograms for the 20 wt.% Ag/C catalyst in Ar purged 0.1 M NaOH solution at a sweep rate of 10 mV s¡1 in presence of different ethanol and methanol concentrations. ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
Fig. 11 e RDE polarization curves at 1600 rpm rotation rate and 2 mV s¡1 for 20 wt.% Ag/C, AgCo/C, Pd/C and Pt/C in oxygen-saturated 0.1 M NaOH solution.
Fig. 9 e RDE polarization curves at 1600 rpm rotation rate and 2 mV s¡1 for 10, 20 and 60 wt.% Ag/C catalysts in oxygen-saturated 0.1 M NaOH and in presence of 0.5 M methanol (red curves) and ethanol (blue curves). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
present slightly higher limiting diffusional reduction currents respect to 20 wt.% Ag/C. However, the main difference is observed in the onset potential for the ORR, which is shift towards positive potentials both for Pt and Pd (994 and 948 mV, respectively). So, from the onset potential point of view, the later metals are more active than Ag. In presence of 0.5 M of methanol or ethanol (Fig. 12), Pt and Pd present large anodic currents, which increases notably with the increment of methanol or ethanol concentrations (not shown), superimposed to the oxygen reduction one. These anodic signals are related to methanol oxidation reaction (MOR) [32,33] and ethanol oxidation reaction (EOR), processes that take place when the crossover occurs in a fuel cell producing a mixed potential at the cathode. It is known that such mixed potentials could negatively affect the cathode performance of the ADAFCs and, according to Fig. 12, it is clearly shown that the effect is important for Pt and Pd. However, Ag and AgeCo materials present low sensitivity to the presence of alcohols, as can be seen also in Fig. 12, and therefore mixed potentials are not expected in this case. These results justify the usage of Ag/C and AgeCo/C catalysts instead of Pd or Pt based ones for the cathode of ADAFCs.
Fig. 10 e Linear sweep voltammograms at 1600 rpm for A) Ag/C and B) AgCo/C catalysts recorded in an oxygen-saturated 0.1 M NaOH solution in the presence of different concentrations methanol. ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
9
Fig. 12 e RDE polarization curves at 1600 rpm rotation rate and 2 mV s¡1 for 20 wt.% Ag/C, AgCo/C, Pd/C and Pt/C catalysts in oxygen-saturated 0.1 M NaOH and in presence of 0.5 M methanol and ethanol.
Conclusions Carbon-supported Ag and AgeCo nanoparticles with sizes below 20 nm were prepared without stabilizers to be used as catalysts for the oxygen reduction reaction (ORR) in alkaline media. It was observed that the ORR took place exchanging 4 electrons per O2 molecule although complete reduction was disfavored with increasing Ag loading. Better activities were achieved with the increment of Ag content and with the introduction of Co. In these materials, Ag was in the metallic form and Co as oxides. Cyclic voltammograms displayed a shift to more negative potentials of the silver oxide reduction peak for the AgeCo/C catalyst compared to Ag/C. This result suggests that the increased activity of AgeCo/C materials towards the ORR is due to the stronger interaction of adsorbed oxygen with Ag atoms. This facilitates the OeO bond splitting, increasing the ORR kinetics compared to pure Ag. The analysis of the polarization curves for the ORR recorded with the rotating disk electrode in the presence of alcohols shows great tolerance for silver-containing materials and describes higher activities in the presence of methanol for AgeCo/C with respect to Ag/C. Thus, the addition of Co seems to enhance the alcohol tolerance of Ag-based catalysts. Finally, comparing the activity with Pd/C and Pt/C, it can be established that Ag/C develops the best tolerance of the three metals to alcohols in ORR experiments. Accordingly, it can be suggested the use of Ag based catalysts instead of Pd- or Ptbased ones as cathode in ADAFCs.
Acknowledgments This work was financially supported by MINECO and FEDER (Spain, projects CTQ2011-28913-C02-02 and ENE2014-52158-
C2-2-R). M.A.H-R thanks CajaCanarias for the pre-doctoral fellowship. Authors thanks G. Garcı´a and L.M. Rivera for helpful discussions.
references
[1] Bidault F, Brett DJL, Middleton PH, Brandon NP. Review of gas diffusion cathodes for alkaline fuel cells. J Power Sources 2009;187:39e48. [2] McLean GF, Niet T, Prince-Richard S, Djilali N. An assessment of alkaline fuel cell technology. Int J Hydrogen Energy 2002;27:507e26. [3] Spendelow JS, Wieckowski A. Electrocatalysis of oxygen reduction and small alcohol oxidation in alkaline media. Phys Chem Chem Phys 2007;9:2654e75. [4] Cifrain M, Kordesch KV. Advances, aging mechanism and lifetime in AFCs with circulating electrolytes. J Power Sources 2004;127:234e42. [5] Varcoe JR, Slade RCT. Prospects for alkaline anion-exchange membranes in low temperature fuel cells. Fuel Cells 2005;5:187e200. [6] Antolini E. Catalysts for direct ethanol fuel cells. J Power Sources 2007;170:1e12. ger JM. Electroreduction of [7] Demarconnay L, Coutanceau C, Le dioxygen (ORR) in alkaline medium on Ag/C and Pt/C nanostructured catalystsdeffect of the presence of methanol. Electrochim Acta 2004;49:4513e21. [8] Meng H, Kang Shen P. Novel Pt-free catalyst for oxygen electroreduction. Electrochem Commun 2006;8:588e94. [9] Antolini E, Gonzalez ER. Alkaline direct alcohol fuel cells. J Power Sources 2010;195:3431e50. [10] Kostowskyj MA, Gilliam RJ, Kirk DW, Thorpe SJ. Silver nanowire catalysts for alkaline fuel cells. Int J Hydrogen Energy 2008;33:5773e8. [11] Wagner N, Schulze M, Gu¨lzow E. Long term investigations of silver cathodes for alkaline fuel cells. J Power Sources 2004;127:264e72.
ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
10
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
[12] Furuya N, Aikawa H. Comparative study of oxygen cathodes loaded with Ag and Pt catalysts in chlor-alkali membrane cells. Electrochim Acta 2000;45:4251e6. [13] Ju JF, Chen X, Shi YJ, Wu DH, Hua P. A novel TiO2 nanofiber supported PdAg catalyst for methanol electro-oxidation. Energy 2013;59:478e83. [14] Safavi A, Kazemi H, Momeni S, Tohidi M, Khanipour Mehrin P. Facile electrocatalytic oxidation of ethanol using Ag/Pd nanoalloys modified carbon ionic liquid Electrode. Int J Hydrogen Energy 2013;38:3380e6. [15] Junsong G, Andrew H, Deryn C, Rongrong C. Improving oxygen reduction reaction activities on carbon-supported Ag nanoparticles in alkaline solutions. J Phys Chem C 2010;114:4324e30. [16] Ying W, Xiaojian L, Yu L, Youquan D. Silver supported on Co3O4 modified carbon as electrocatalyst for oxygen reduction reaction in alkaline media. Electrochem Commun 2013;31:108e11. [17] Eun JL, Mook CS, Min HS, Youngmin K, Seonhwa L, Won BK. Highly dispersed Ag nanoparticles on nanosheets of reduced graphene oxide for oxygen reduction reaction in alkaline media. Electrochem Commun 2013;28:100e3. [18] Tammeveski L, Erikson H, Sarapuu A, Kozlova J, Ritslaid P, Sammelselg V, et al. Electrocatalytic oxygen reduction on silver nanoparticle/multi-walled carbon nanotube modified glassy carbon electrodes in alkaline solution. Electrochem Commun 2012;20:15e8. [19] Sekol RC, Li XK, Cohen P, Doubek G, Carmo M, Taylor AD. Silver palladium core-shell electrocatalyst supported on MWNTs for ORR in alkaline media. Appl Catal B Environ 2013;138:285e93. [20] Lima FHB, Castro JFR, Edson A. Silver-cobalt bimetallic particles for oxygen reduction in alkaline media. J Power Sources 2006;161:806e12. [21] Feng Y-Y, Ma J-H, Zhang G-R, Liu G, Xu B-Q. Dealloyed carbon-supported PtAg nanostructures: enhance electrocatalytic activity for oxygen reduction reaction. Electrochem Commun 2010;12:1191e4. [22] Lee C-L, Chiou H-P, Syu C-M, Liu C-R, Yang C-C, Syu C-C. Displacement triangular Ag/Pd nanoplate as methanol tolerant electrocatalyst in oxygen reduction reaction. Int J Hydrogen Energy 2011;36:12706e14.
[23] Xingjuan S, Dongming Z. Bimetallic AgeNi/C particles as cathode catalyst in AFCs (alkaline fuel cells). Energy 2014;70:223e30. [24] Feng RX, Dong H, Cao YL, Ai XP, Yang HX. AgNi-catalyzed anode for direct borohydride fuel cells. Int J Hydrogen Energy 2007;32:4544e9. [25] Garcia AC, Gasparotto LHS, Gomes JF, Tremiliosi-Filho G. Straight forward synthesis of carbon-supported Ag nanoparticles and their application for the oxygen reduction reaction. Electrocatalysis 2012;3:147e52. [26] Cheng Y, Li W, Fan X, Liu J, Xu W, Yan C. Modified multiwalled carbon nanotube/Ag nanoparticle composite catalyst for oxygen reduction reaction in alkaline solution. Electrochim Acta 2013;111:635e41. [27] Sheng-Li C, Bing-Liang W, Chuan-Sin C. Application of timeresolved EQCM to the study of the mechanism of silver (I) oxide formation on a polycrystalline silver electrode in alkaline solution. J Electroanal Chem 1996;416:53e9. [28] (a)Maheswari S, Sridhar P, Pitchumani S. Carbon supported silver as cathode electrocatalysts for alkaline polymer electrolyte Membrane Fuel Cells. Electrocatalysis 2012;3:13e21. (b)Bard AJ, Faulkner LR. Electrochemical methods. New Delhi: Wiley; 2006. p. 341. [29] Blizanac BB, Ross PN, Markovic NM. Oxygen reduction on silver low-index single-crystal surfaces in alkaline solution: rotating ring disk Ag(hkl) studies. J Phys Chem B 2006;110:4735e41. [30] Damjanovic A, Brusic V. Electrode kinetics of oxygen reduction on oxide-free platinum electrodes. Electrochim Acta 1967;12:615e28. [31] Wroblowa H, Rao M, Damjanovic A, Bockris JO’M. Adsorption and kinetics at platinum electrodes in the presence of oxygen at zero net current. Electrochim Acta 1967;12:139e50. [32] Maheswari S, Sridhar P, Pitchumani S. PdeTiO2/C as a methanol tolerant catalyst for oxygen reduction reaction in alkaline medium. Electrochem Commun 2013;26:97e100. [33] Maheswari S, Sridhar P, Pitchumani S. Pd-RuSe/C as ORR specific catalyst in alkaline solution containing methanol. Fuel Cells 2012;12:963e70.
ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Carbon supported Ag and AgeCo catalysts tolerant to methanol and ethanol for the oxygen reduction reaction in alkaline media ndez-Rodrı´guez, M.C. Goya, M.C. Arevalo, J.L. Rodrı´guez, M.A. Herna * E. Pastor Departamento de Quı´mica, Instituto de Materiales y Nanotecnologı´a, Universidad de La Laguna, Apdo. 456, 38206, La Laguna, Tenerife, Spain
article info
abstract
Article history:
Ag/C and AgeCo/C catalysts with high activity towards the oxygen reduction reaction in
Received 29 January 2016
alkaline media, different metal loadings and average sizes below 20 nm were synthesized
Received in revised form
by glycerol and borohydride reduction methods without stabilizers. Physicochemical
23 July 2016
characterization of the materials was performed by X-ray techniques (diffraction, energy
Accepted 24 July 2016
dispersive and photoelectron spectroscopy) and transmission electron microscopy. Well
Available online xxx
dispersed small nanoparticles were obtained in all cases, mainly formed by Ag(0). For Ag eCo/C catalyst, it was observed that Co was not alloyed but presented as cobalt oxides. The
Keywords:
electrocatalytic activity towards oxygen reduction reaction (ORR) in alkaline solution was
Silver nanoparticles
evaluated by cyclic voltammetry and rotating disk techniques. A four electron transfer
Oxygen reduction reaction
mechanism was established although increasing Ag loading produces a decrease of this
Alcohol tolerance
number, indicating that hydrogen peroxide produced as intermediate in a first two electron
Alkaline fuel cell
step was not completely reduced. Alcohol tolerance of the catalysts was also established in methanol and ethanol solutions. Materials were not active for the electro-oxidation of alcohols, although it was observed that both methanol and ethanol were adsorbed on the catalyst. Highest activity and alcohol tolerance was observed for the 60 wt.% Ag loading material. Also the introduction of Co produces an increase in the activity (higher ORR limiting currents) and in the alcohol tolerance. In comparison to Pt and Pd, Ag and AgeCo present more appropriate activities for ORR when alcohol tolerance is considered, being good candidates for the use as catalysts in alkaline direct alcohol fuel cells. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Alkaline direct alcohol fuel cells (ADAFCs) are considered good candidates as power sources for future portable and vehicle devices owing to their high theoretical specific energy, low
cost and environmental friendliness [1e3]. These systems have some advantages compared to the common polymer electrolyte membrane fuel cells (PEMFCs), such as the usage of non-precious metal catalyst, the decrease of cathode overpotential and the favorable water management. Thus, it is the
* Corresponding author. E-mail address: [email protected] (E. Pastor). http://dx.doi.org/10.1016/j.ijhydene.2016.07.188 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
possibility to replace Pt-based alloys by non-noble (and cheaper) metals as electrocatalysts for both electrodes the most relevant characteristic of the ADAFCs [4,5]. Previous works have shown that several metals could be used as anode catalyst [6] and silver appears as a good candidate for the cathode [7e9]. However, there are still notable challenges for ADAFCs concerning the oxygen reduction reaction (ORR), including the improvement of the low reaction rates, high overpotentials and low stabilities. Silver is a promising candidate for replacing Pt for ORRs in alkaline solution due to its high activity and comparatively low cost [10,11]. Moreover, Ag cathodes have been reported to be more stable than Pt ones during long-term operation [12]. On the other hand, Ag and its alloys show better tolerance toward methanol [7e11,13] and ethanol [14] poisoning than Pt. It has been reported that Ag/C promotes the 4-electron pathway for ORR in alkaline solutions and increasing the content of Ag in the carbon supported catalyst produces a positive shift of the onset potential for this reaction [15]. Ying et al. [16] have reported that Ag nanoparticles supported on Co3O4 show large electrochemical active surface area (ESA), high catalytic activity towards ORR and also exhibit good methanol tolerance and stability in alkaline media. In a recent work Eun et al. [17] described that highly dispersed Ag nanoparticles on reduced graphene oxide (RGO) displayed higher catalytic activity than Ag/C. Tammeveskia et al. reported that Ag nanoparticle/multi-walled carbon nanotube (AgNP/ MWCNT) presented a high electrocatalytic activity towards ORR, with similar specific activity to bulk Ag [18], whereas Sekol and colab. have shown that Ag@Pd/MWCNTs catalysts also presented high tolerance to methanol and ethanol [19]. On the other hand, it was evidenced that the presence of a second metal also improved the Ag activity. Lima et al. have studied AgeCo/C as cathode catalyst for ORR in AFCs and found that Co, in the form of Co3O4, contributed to the 4electron pathway with respect to Ag/C [20]. Bard et al. have proposed a simple mechanism to illustrate the electroactivity enhancement of Ag-based binary catalysts for ORR: the OeO bond first breaks down on one metal-atom of the binary Agbased catalyst to form an adsorbed O atom (Oad), and then the Oad transfers into another metal-atom (usually Ag) to be reduced. AgePt, AgeAu and AgePd have been extensively studied as effective Ag-based binary electrocatalysts for ORR [21,22]. In addition, Lima et al. have prepared a low cost carbon supported binary AgeCo catalyst, with high electroactivity for this reaction, by a chemical reduction procedure [20]. Finally, other no-noble metals have been used for the preparation of bimetallic Ag electrocatalysts. Thus, it was observed that the increment of the amount of Ni in the AgeNi alloy and the heat treatment at 500 C decreases the ORR overpotential and increases the limiting current density for this reaction [23]. Alternatively, it has been reported that AgeNi catalyst can be used in borohydride fuel cells exhibiting higher discharge voltage and capacity [24]. In this work, Ag/C materials with low nanoparticle sizes were synthesized by the glycerol reduction method to be used as catalyst at the cathode of alkaline fuel cells, varying the Ag loading between 10 and 60 wt.%. On the other hand, carbon supported AgeCo catalysts have been prepared using NaBH4 as reduction agent with 20 wt.% metal content. The
composition, morphology, particle size and metal loading of the catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive Xray (EDX) and photoelectron spectroscopy (XPS) analysis. The electrocatalytic activity towards the ORR was studied applying two electrochemical techniques: cyclic voltammetry and rotating disk electrode, in the presence of different methanol or ethanol concentrations. Finally, the catalytic activity of 20 wt.% Ag/C and AgeCo catalysts was compared to the performances for 20 wt.% Pd/C and Pt/C materials.
Material and methods Preparation of Ag/C catalysts Ag/C catalysts with different Ag content were synthesized following the glycerol reduction method reported in the literature [25]. Briefly, an appropriate amount of Vulcan XC-72R (Cabot Corp.) in a known volume of Milli-Q water was sonicated and a certain quantity of AgNO3 (Aldrich, 99.9999%) was added under stirring to promote the homogenization of the mix. Afterward, another solution containing glycerol (SigmaeAldrich, 99%) and NaOH (SigmaeAldrich, 99.99%) was added to give the following concentrations: 0.82 mM AgNO3, 1 M glycerol and 0.1 M NaOH. The mixture was kept during 24 h under stirring at room temperature and then washed, filtered and dried at 80 C during 12 h. Three Ag/C catalysts with silver loadings of 10, 20 and 60 wt.% were prepared with this method.
Preparation of AgeCo/C, Co/C and Pd/C catalysts Pd, Co, Ag and AgeCo catalysts dispersed on carbon Vulcan XC-72R with 20 wt.% metal content were prepared using NaBH4 as reduction agent. A stronger reduction agent was used to guarantee metal reduction in all cases. The bimetallic particles were obtained with different nominal atomic ratios, ranging from pure Ag and Co (for a comparative purpose) to Ag:Co (3:1). Materials were prepared by simultaneous reduction of the precursor metal salts Ag (NO3)2 and Co(NO3)2$6H2O (SigmaeAldrich, 99%). The reduction process was carried out at room temperature by drop wise addition of the precursor solution onto a carbon black dispersion, which was prepared by suspending carbon powder in ultrapure water ultrasonically blending for 10 min and then 12 h under magnetic agitation. This is followed by drop wise addition of an excess of NaBH4 solution for reduction and precipitation of the precursor metal particles. The mixture was kept during 12 h under stirring at room temperature and then washed, filtered and dried at 60 C during 12 h. Similar procedure was applied for preparation of Pd/C material using PdCl2 (SigmaeAldrich, p.a.) as precursor. Commercial E-TEK 20 wt.% Pt/C catalyst was also employed for comparison.
Physicochemical characterization The real Ag content and the atomic composition of the electrocatalysts were determined by energy dispersive X-ray analysis (EDX) coupled to a scanning electron microscopy (SEM) Jeol JEM Mod. 6300 with a silicon doped with lithium
ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
Oxford 6699 ATW detector applying 20 keV. Particle dispersion on the support was established from transmission electron microscopy (TEM) images obtained with a Jeol JEM Mod. 2100 equipment. X-ray diffraction (XRD) patterns were obtained using a universal diffractometer Carl Zeiss-Jena, URD-6, equipped with a PANalytical X'Pert Pro X-ray diffractometer operating with Cu Ka radiation (l ¼ 0.15406 nm) generated at 40 kV and 20 mA. The average crystallite size was calculated using Scherrer's equation: D¼
kl BCosq
where D is the average crystallite size in angstrom, k is a coefficient taken here as 0.9, l the wavelength of the X-rays used (1.5406 Å), B the width of the diffraction peak at half height in radians and q the angle at the position of the peak maximum. High-resolution XPS spectra were collected with an ESCALAB 250 spectrometer equipped with dual aluminummagnesium anodes, using a monochromatized Al Ka X-ray radiation (1486.6 eV) with a spot size of 650 mm. The spectrometer energy calibration was performed using the Au 4f7/2 and Cu 2p3/2 photoelectron lines. The spectra were collected in constant analyzer energy (CAE) mode, with pass energy of 20 eV and with an energy resolution of about 0.1 eV. For all the measurements, pressure in the ultrahigh vacuum analysis chamber was less than 9 109 mbar, avoiding that the ejected photoelectron interact with gas molecules.
Electrochemical characterization A three-electrode configuration cell was used for these experiments. A pre-calibrated reference hydrogen electrode (RHE) and a glassy carbon bar were used as reference and counter electrodes, respectively. A glassy carbon (GC) disk supporting the catalyst, with a geometric area of 0.071 cm2, was used as working electrode. Before each test, the electrode was polished with 1 mm alumina on a polishing cloth, followed by washing the electrode with abundant Milli-Q water. For the preparation of the working electrode, a catalyst suspension was prepared with 3 mg of the catalyst in 1 mL of Milli-Q water. Then 20 mL of catalyst suspension was deposited on the surface of the polished GC disk. The electrode was dried at room temperature assisted by a nitrogen flow.
3
The electrochemical characterization was carried out using cyclic voltammetry (CV) and the ORR was evaluated by linear sweep voltammetry (LSV), in both cases using a computer-controlled electrochemical analyzer mAutolab III, Eco Chemie BV. Once deposited and dried the ink with the catalyst on the GC surface, the electrode was cycled for activation between 0.05 and 0.80 V at 100 mV s1 in Ar purged electrolyte solution (0.1 M NaOH) during 50 cycles. Then the catalyst was cycled in the working range (0.05e1.44 V) at 10 mV s1 until stable and reproducible voltammograms were obtained. LSV experiments were performed using a rotating disk electrode (RDE) model Eco Chemie BV Autolab rotator, in oxygen saturated 0.1 M NaOH solution. LSV data were recorded in the negative going sweep direction running from 1.00 to 0.05 V vs. RHE over a range of rotation speeds (from 400 to 2500 rpm) at a scan rate of 2 mV s1.
Results and discussion Physicochemical characterization The crystalline structure of the Ag/C catalysts was characterized by XRD, as can be seen in Fig. 1A. The diffraction patterns for the three materials are similar, with peaks at 2q values of 38.0 , 44.4 , 64.5 and 77.4 that can be assigned to the (111), (200), (220), and (311) crystalline planes of Ag with a face-centered cubic structure, respectively. Characteristic diffraction peaks for crystalline Ag2O or silver hydroxide appear only in the spectra for 10 wt.% Ag loading as two small peaks at 30.1 and 40.2 . The mean sizes of Ag crystallites calculated from the (220) diffraction peak applying Scherrer's equation are around 16e21 nm (Table 1). It is remarkable that similar values were obtained for 20% wt. Ag catalysts prepared with the glycerol (21.0) and borohydride (19.4) reduction methods (Table 2). On the other hand, the crystalline structure of the AgeCo nanoparticles resembles the face centered cubic (fcc) structure of silver (Fig. 1B). There is no shift in the peak positions with respect to Ag/C, indicating a negligible Co insertion in the Ag lattice. However, no peaks due to the presence of Co oxides are apparent in the spectra (Coþ2 and Coþ3 are present in the XPS spectra), and therefore, have to be amorphous. The mean crystallite size estimated for AgCo/C (3:1) catalyst is 16.9 nm,
Fig. 1 e XRD patterns A) for Ag/C catalysts and B) for 20 wt.% Ag/C and AgCo/C. ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
Table 1 e Metal loadings (EDX), crystallites sizes and lattice parameters (XRD), and XPS data for the different Ag-based supported catalysts. Metal content (nominal) (wt.%)
Metal loading (wt.%) EDX
Crystallite (nm) XRD
Lattice parameter (Å) XRD
Binding energy (eV)
Binding energy (eV)
Ag0 (%)
Agþ (%)
9 22 59 18 13 (Ag), 3 (Co)
18.1 21.0 17.7 19.4 17.0
4.097 4.094 4.097 4.098 4.094
368.45 (93.67) 368.43 (93.64) 368.43 (100) 368.43 (100) 368.43 (100)
367.81 (6.63) 367.82 (6.34) e e e
10 20 (glycerol) 60 20 (borohydride) 20
Table 2 e Kinetic parameters for the ORR derived from hydrodynamic polarization curves. Ag/C catalyst Nominal content (wt.%) 10 20 60
Onset n (mV)
826 829 890
3.9 3.9 3.7
Kinetic current density (mA cm2) 36 209 1639
Tafel slope Tafel slope small h large h (mV/ (mV/ decade) decade) 85 89 75
104 112 117
indicating that the presence of Co induces a decrease in the mean size of Ag nanoparticles. The same result was reported by Lima et al. [20]. XRD patterns indicate the predominance of the (111) plane in both catalysts, being more relevant in the case of the bimetallic one. TEM images for 10, 20 and 60 wt.% Ag/C catalysts are shown in Fig. 2. The image for the 60 wt.% Ag/C (Fig. 2B) shows well dispersed Ag nanoparticles with sizes in the same range obtained from X-ray diffractograms, but at 60 wt.% the particles appear agglomerated. Surprisingly, in the image for the 10 wt.% Ag/C (Fig. 2A), it can be observed a big number of metal nanoparticles lower than 2e3 nm and some in the range 10e15 nm. The crystallite size obtained from XRD in Fig. 1 is in agreement with the last values, and it seems that the small nanoparticles are below the detection limit of the technique, only recording the contributions of the bigger ones. XPS spectra of Ag/C catalysts prepared by the glycerol route are shown in Fig. 3. In the corel level spectrum of Ag, peaks at 368 and 374 eV are assigned to Ag3d5/2 and Ag3d3/2 transitions, respectively. These energies and the 6 eV spineorbit splitting of the 3d doublet are characteristic of silver in the metallic form (Ag0) [26]. Only a small contribution related to silver oxide (Ag2O) can be found in the 10 and 20 wt.% Ag/C catalysts (Fig. 3 and Table 1). Accordingly, peaks corresponding to crystalline oxides were observed for 10 wt.% Ag/C catalyst in Fig. 1A but not for 20% catalysts, suggesting that in the latter case the oxides are amorphous. It is remarkable that for the 20 wt.% material synthesized with the borohydride reduction method, all Ag is present in the metallic form (Table 1). On the other hand, no XPS peak features of Ag2O or other silver oxides are observed for the 60 wt.% Ag/C catalyst, indicating that only Ag0 is present in this material, in concordance with the XRD results. Finally, the XPS spectrum of the AgeCo/C catalyst (Fig. 4) indicates that Ag is also present as Ag0 in this material (Fig. 4A). Concerning Co (Fig. 4B and C), peaks at 782 and 798 eV are related to Co2p1/2 and Co2p3/2 transitions, respectively, and are
Fig. 2 e TEM images for A) 10, B) 20 and C) 60 wt.% Ag/C catalysts. associated to Coþ2 and Coþ3 (it is difficult to separate both signals). The shoulders at 787 and 803 eV in these figures are satellite signals associated only to Coþ2. Comparing the 3:1 AgeCo/C catalyst with the Co/C (19 wt.% real metal loading),
ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
Fig. 3 e XPS spectra for the Ag 3d region of Ag/C.
the spectra are very similar, although a slight shift is observed from 782.4 to 798.1 eV signals in pure Co to 781.9 and 797.6 eV in the presence of Ag, which indicates that electrons goes from Ag to Co in the bimetallic materials increasing the amount of Coþ2. Accordingly, the intensity of the satellite signals slightly increases with the addition of Ag (see Fig. 4B and C).
Electrochemical characterization Oxygen electroreduction in absence of alcohol Fig. 5A displays the cyclic voltammograms of Ag/C catalysts with 10, 20 and 60 wt.% metal loading in freshly prepared 0.1 M NaOH solution purged with nitrogen. The curves exhibit two main anodic contributions (C1 and C2) and an unique cathodic peak for all catalysts, which are in good agreement with the CVs reported in the literature [15].The anodic features around 1.21 and 1.31 V vs. RHE are due to the formation of the Ag2O layer and the cathodic peak centered at 1.05 V vs. RHE is
5
associated to the oxide reduction to metallic silver [27]. Current densities in CV experiments were calculated using the electrochemical surface area obtained from the charge involved in the Ag oxide reduction peak assuming 420 mC/cm2 [28]. The curves for AgeCo/C catalyst in the base electrolyte present a broad current contribution previous to the formation of the silver oxide (Fig. 5B), describing two peaks at 1.03 and 1.17 V during the positive-going scan. Moreover, comparison of the CVs for Ag and AgeCo catalysts shows a shift to more negative potentials in the reduction peak recorded during the negative sweep for the bimetallic material, indicating a stronger Oad interaction with Ag surface sites in the presence of Co which could facilitate the splitting of the OeO bond [16]. The ORR was studied at these materials in an oxygensaturated 0.1 M NaOH solution by the RDE technique. Steady-state polarization curves for 10, 20 and 60 wt.% Ag/C catalysts were recorded at a scan rate of 2 mV s1 and various rotation speeds, namely, 400, 600, 900, 1600 and 2500 rpm. It was observed that the current densities (referenced to the geometric area of the working electrode) increase, as expected, with the increment of the rotation speed (not shown). In Fig. 6A the polarization curves of these Ag/C catalysts at 1600 rpm are given in order to compare the catalytic activity of the materials. It is observed that final limiting diffusional currents are similar for 20 and 60 wt.% Ag loading, but an important shift in the onset potential (see the values in Table 2) of almost 70 mV to positive potentials is displayed by the latter material. Similar behavior was described previously [28]. Then, this catalyst appears as the most appropriate for the ORR reaction. It is remarkable that 60 wt.% Ag/C shows the worst dispersion in Fig. 2C, suggesting that the ORR could be facilitated at the grain boundary of the agglomerated nanoparticles. Other factor that could contribute is the fact that only metallic silver is present in this material, whereas oxides were detected for 10 and 20 wt.% Ag/C catalysts prepared by the glycerol reduction method. Information about the mechanism of the ORR at these catalysts can be obtained from the polarization curves. The oxygen reduction reaction is a multi-electron reaction which has two possible pathways in basic media. One is an indirect four-electron transfer process, which produces hydrogen peroxide as intermediate while the second is a direct fourelectron transfer process producing hydroxyl ion as the final
Fig. 4 e XPS spectra for 20 wt.% catalysts. A) Ag 3d region of Ag/C and AgCo/C; B) and C) Co2p region of AgCo/C and Co/C, respectively. ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
Fig. 5 e Cyclic voltammograms in Ar purged 0.1 M NaOH solution at a sweep rate of 10 mV s¡1. A) 10, 20 and 60 wt.% Ag/C catalysts and B) 20 wt.% Ag/C and AgCo/C. solubility in the solution, n ¼ 0.0109 cm2 s1 is the NaOH solution kinematic viscosity, all at 25 C, and u (radian per second) is the RDE revolution speed. B is the Levich coefficient given by: B ¼ 0:62nFDO2=3 n1=6 CO
Fig. 6 e A) RDE polarization curves for 10, 20 and 60 wt.% Ag/C catalysts in oxygen-saturated 0.1 M NaOH solution at 1600 rpm and 2 mV s¡1. B) KeL plots for the Ag/C catalysts.
product. The overall number of electrons exchanged per oxygen molecule can be calculated from the limiting diffusion current density plateaus using the classical KoucteckyeLevich (KeL) equation [28]: 1 1 1 1 1 ¼ þ þ ¼ j jk jD nFkCo Bu1=2 where jD is the limiting current density, jk the kinetic current density, n is the number of electrons involved in the reaction, k is the rate constant for O2 reduction, DO ¼ 1.93 105 cm2 s1 is the oxygen diffusion coefficient, F ¼ 96485.34 C mol1 the Faraday's constant, CO ¼ 1.26 107 mol cm3 is the oxygen
From the slope of KeL plots (Fig. 6B), the calculated value of n (Table 2) resulted to be close to 4 for 10 and 20 wt.% Ag/C catalysts indicating that the ORR proceeds through a direct four-electron pathway. For catalysts with 60 wt.% loading, n is 3.7, respectively, demonstrating the non-completely elimination of the hydrogen peroxide produced as an intermediate in a first two-electron process [3] even this is the catalyst with the highest activity. Extrapolation of KeL curves to 1/u1/2 ¼ 0 allows to obtain 1/jk and calculate the kinetic current density for these materials [28]. Values for jk in Table 2 show that the 60 wt.% Ag/C catalyst presents the highest jk in agreement with the shift in the curve observed in Fig. 6A. The Tafel plots for normalized kinetic currents for Ag/C catalysts can be seen in Fig. 7. Two different linear regions can be distinguished: one at low overpotentials (l.o.r., 0.95 > E > 0.75) and the second at high overpotentials (h.o.r., 0.75 > E > 0.60). The Tafel slopes obtained from the above figure are given in Table 2 and the values are in accurate agreement with those reported in the literature [29]. In the l.o.r. slopes closed to 90 mV dec1 were determined, whereas values near 120 mV dec1 were calculated in the h.o.r. A Tafel slope of 120 mV dec1 can be interpreted through a mechanism involving the first single electron transfer as rate determining step, r.d.s., and Langmuirian conditions for the reaction intermediates, whereas that of 90 mV dec1 would probably be the result of a competition of, at least, two electrochemical processes: a single electron transfer as r.d.s. and a chemical r.d.s. following the first electron transfer [30,31]. Such Tafel slope could also be explained by the presence of surface oxides with adsorption isotherms different from Langmuir. The dependence of surface coverage of those species with potential modifies the classical Tafel slope of 120 mV dec1e90 mV dec1.
Oxygen electroreduction in presence of alcohols For ADAFCs, the crossover of the alcohols used as fuels at the anode to the cathode unavoidably occurs and these molecules can competitively adsorb on active sites with oxygen, leading to a mixed potential at the cathode and a notably decrease of fuel cells performances. Thus, it is important for the cathodic
ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
Fig. 7 e Tafel plots for ORR at Ag/C catalysts. a Low overpotential region (l.o.r.) and b high overpotential region (h.o.r.).
materials to be tolerant to alcohols. Hence, the ORR was studied for Ag/C and AgeCo/C catalysts in the presence of methanol (MeOH) or ethanol (EtOH) with the purpose to establish their tolerance. Firstly, to investigate the activity of Ag/C and AgeCo/C towards alcohol oxidation in alkaline medium, cyclic voltammetry were performed in 0.1 M NaOH þ xM MeOH or EtOH (x ¼ 0.5, 1, 2, 3 M) solutions. Fig. 8A and B shows the CVs for the 20 wt.% Ag/C catalyst in presence of EtOH and MeOH, respectively. In both cases, the curves do not exhibit alcohol oxidation features implying that catalysts are not active for their oxidation in alkaline media. Nevertheless, an inhibition of the peaks related to oxide formation and reduction is observed, especially remarkable with ethanol, which indicates that the alcohol is adsorbed without oxidation. For the other Ag/C catalysts with different metal loadings, the behavior in the presence of alcohols is similar. Moreover, no alcohol oxidation peak was also observed for AgeCo/C in the presence of several concentrations of alcohol (not shown), confirming that the prepared Ag-based catalysts are electrochemically inactive for this electrooxidation reaction.
7
RDE measurements for the ORR in the presence of different methanol or ethanol concentrations were accomplished in the same conditions previously described in Section Oxygen electroreduction in absence of alcohol. Fig. 9 shows the ORR polarization curves of the three Ag/C catalysts for an oxygensaturated 0.1 M NaOH solution in the presence of 0.5 M methanol and ethanol at 1600 rpm. A decrease in the cathodic diffusional limiting current was recorded in all cases, with a maximum diminution around 10% in the presence of ethanol and 5% in the presence of methanol. Increasing the alcohol concentration up to 3 M (not shown), a further decrease in the limiting current was observed achieving a 40 and 20% for ethanol and methanol, respectively. But it has to be considered that this is a very high concentration of the alcohol, and accordingly, it can be concluded that silver materials present a high alcohol tolerance (significant when compared with most active metals for the ORR, i.e. Pt and Pd, see Section Comparison of ORR catalytic activities for 20 wt.% Ag, Pd and Pt supported catalysts). It is remarkable that the diffusionlimited current density of the ORR polarization curve gradually decreases with increasing the alcohol concentration although the ORR currents at the kinetics-controlled region keep almost unvaried. Two possible reasons might account for this phenomenon: i) the adsorbed methanol or ethanol on the surface of thin film electrode covers partially active sites; and ii) the solubility of O2 in electrolyte decreases with the increase in methanol or ethanol concentration. Polarization curves for the ORR at AgeCo/C in the presence of methanol display higher mass cathodic current densities (per silver content) for the bimetallic catalyst compared to Ag/ C (Fig. 10 shows the curves recorded at 1600 rpm for 20 wt.% metal content materials), that is, inhibition is also observed with the AgeCo/C catalyst but the latter is an improved alcohol tolerant material.
Comparison of ORR catalytic activities for 20 wt.% Ag, Pd and Pt supported catalysts The polarization curves for ORR at 1600 rpm for 20 wt.% Ag/C, Pd/C and Pt/C catalysts in absence of alcohols are compared in Fig. 11. It can be seen that both 20% Pd/C and 20 wt.% Pt/C
Fig. 8 e Cyclic voltammograms for the 20 wt.% Ag/C catalyst in Ar purged 0.1 M NaOH solution at a sweep rate of 10 mV s¡1 in presence of different ethanol and methanol concentrations. ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
Fig. 11 e RDE polarization curves at 1600 rpm rotation rate and 2 mV s¡1 for 20 wt.% Ag/C, AgCo/C, Pd/C and Pt/C in oxygen-saturated 0.1 M NaOH solution.
Fig. 9 e RDE polarization curves at 1600 rpm rotation rate and 2 mV s¡1 for 10, 20 and 60 wt.% Ag/C catalysts in oxygen-saturated 0.1 M NaOH and in presence of 0.5 M methanol (red curves) and ethanol (blue curves). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
present slightly higher limiting diffusional reduction currents respect to 20 wt.% Ag/C. However, the main difference is observed in the onset potential for the ORR, which is shift towards positive potentials both for Pt and Pd (994 and 948 mV, respectively). So, from the onset potential point of view, the later metals are more active than Ag. In presence of 0.5 M of methanol or ethanol (Fig. 12), Pt and Pd present large anodic currents, which increases notably with the increment of methanol or ethanol concentrations (not shown), superimposed to the oxygen reduction one. These anodic signals are related to methanol oxidation reaction (MOR) [32,33] and ethanol oxidation reaction (EOR), processes that take place when the crossover occurs in a fuel cell producing a mixed potential at the cathode. It is known that such mixed potentials could negatively affect the cathode performance of the ADAFCs and, according to Fig. 12, it is clearly shown that the effect is important for Pt and Pd. However, Ag and AgeCo materials present low sensitivity to the presence of alcohols, as can be seen also in Fig. 12, and therefore mixed potentials are not expected in this case. These results justify the usage of Ag/C and AgeCo/C catalysts instead of Pd or Pt based ones for the cathode of ADAFCs.
Fig. 10 e Linear sweep voltammograms at 1600 rpm for A) Ag/C and B) AgCo/C catalysts recorded in an oxygen-saturated 0.1 M NaOH solution in the presence of different concentrations methanol. ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
9
Fig. 12 e RDE polarization curves at 1600 rpm rotation rate and 2 mV s¡1 for 20 wt.% Ag/C, AgCo/C, Pd/C and Pt/C catalysts in oxygen-saturated 0.1 M NaOH and in presence of 0.5 M methanol and ethanol.
Conclusions Carbon-supported Ag and AgeCo nanoparticles with sizes below 20 nm were prepared without stabilizers to be used as catalysts for the oxygen reduction reaction (ORR) in alkaline media. It was observed that the ORR took place exchanging 4 electrons per O2 molecule although complete reduction was disfavored with increasing Ag loading. Better activities were achieved with the increment of Ag content and with the introduction of Co. In these materials, Ag was in the metallic form and Co as oxides. Cyclic voltammograms displayed a shift to more negative potentials of the silver oxide reduction peak for the AgeCo/C catalyst compared to Ag/C. This result suggests that the increased activity of AgeCo/C materials towards the ORR is due to the stronger interaction of adsorbed oxygen with Ag atoms. This facilitates the OeO bond splitting, increasing the ORR kinetics compared to pure Ag. The analysis of the polarization curves for the ORR recorded with the rotating disk electrode in the presence of alcohols shows great tolerance for silver-containing materials and describes higher activities in the presence of methanol for AgeCo/C with respect to Ag/C. Thus, the addition of Co seems to enhance the alcohol tolerance of Ag-based catalysts. Finally, comparing the activity with Pd/C and Pt/C, it can be established that Ag/C develops the best tolerance of the three metals to alcohols in ORR experiments. Accordingly, it can be suggested the use of Ag based catalysts instead of Pd- or Ptbased ones as cathode in ADAFCs.
Acknowledgments This work was financially supported by MINECO and FEDER (Spain, projects CTQ2011-28913-C02-02 and ENE2014-52158-
C2-2-R). M.A.H-R thanks CajaCanarias for the pre-doctoral fellowship. Authors thanks G. Garcı´a and L.M. Rivera for helpful discussions.
references
[1] Bidault F, Brett DJL, Middleton PH, Brandon NP. Review of gas diffusion cathodes for alkaline fuel cells. J Power Sources 2009;187:39e48. [2] McLean GF, Niet T, Prince-Richard S, Djilali N. An assessment of alkaline fuel cell technology. Int J Hydrogen Energy 2002;27:507e26. [3] Spendelow JS, Wieckowski A. Electrocatalysis of oxygen reduction and small alcohol oxidation in alkaline media. Phys Chem Chem Phys 2007;9:2654e75. [4] Cifrain M, Kordesch KV. Advances, aging mechanism and lifetime in AFCs with circulating electrolytes. J Power Sources 2004;127:234e42. [5] Varcoe JR, Slade RCT. Prospects for alkaline anion-exchange membranes in low temperature fuel cells. Fuel Cells 2005;5:187e200. [6] Antolini E. Catalysts for direct ethanol fuel cells. J Power Sources 2007;170:1e12. ger JM. Electroreduction of [7] Demarconnay L, Coutanceau C, Le dioxygen (ORR) in alkaline medium on Ag/C and Pt/C nanostructured catalystsdeffect of the presence of methanol. Electrochim Acta 2004;49:4513e21. [8] Meng H, Kang Shen P. Novel Pt-free catalyst for oxygen electroreduction. Electrochem Commun 2006;8:588e94. [9] Antolini E, Gonzalez ER. Alkaline direct alcohol fuel cells. J Power Sources 2010;195:3431e50. [10] Kostowskyj MA, Gilliam RJ, Kirk DW, Thorpe SJ. Silver nanowire catalysts for alkaline fuel cells. Int J Hydrogen Energy 2008;33:5773e8. [11] Wagner N, Schulze M, Gu¨lzow E. Long term investigations of silver cathodes for alkaline fuel cells. J Power Sources 2004;127:264e72.
ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188
10
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 0
[12] Furuya N, Aikawa H. Comparative study of oxygen cathodes loaded with Ag and Pt catalysts in chlor-alkali membrane cells. Electrochim Acta 2000;45:4251e6. [13] Ju JF, Chen X, Shi YJ, Wu DH, Hua P. A novel TiO2 nanofiber supported PdAg catalyst for methanol electro-oxidation. Energy 2013;59:478e83. [14] Safavi A, Kazemi H, Momeni S, Tohidi M, Khanipour Mehrin P. Facile electrocatalytic oxidation of ethanol using Ag/Pd nanoalloys modified carbon ionic liquid Electrode. Int J Hydrogen Energy 2013;38:3380e6. [15] Junsong G, Andrew H, Deryn C, Rongrong C. Improving oxygen reduction reaction activities on carbon-supported Ag nanoparticles in alkaline solutions. J Phys Chem C 2010;114:4324e30. [16] Ying W, Xiaojian L, Yu L, Youquan D. Silver supported on Co3O4 modified carbon as electrocatalyst for oxygen reduction reaction in alkaline media. Electrochem Commun 2013;31:108e11. [17] Eun JL, Mook CS, Min HS, Youngmin K, Seonhwa L, Won BK. Highly dispersed Ag nanoparticles on nanosheets of reduced graphene oxide for oxygen reduction reaction in alkaline media. Electrochem Commun 2013;28:100e3. [18] Tammeveski L, Erikson H, Sarapuu A, Kozlova J, Ritslaid P, Sammelselg V, et al. Electrocatalytic oxygen reduction on silver nanoparticle/multi-walled carbon nanotube modified glassy carbon electrodes in alkaline solution. Electrochem Commun 2012;20:15e8. [19] Sekol RC, Li XK, Cohen P, Doubek G, Carmo M, Taylor AD. Silver palladium core-shell electrocatalyst supported on MWNTs for ORR in alkaline media. Appl Catal B Environ 2013;138:285e93. [20] Lima FHB, Castro JFR, Edson A. Silver-cobalt bimetallic particles for oxygen reduction in alkaline media. J Power Sources 2006;161:806e12. [21] Feng Y-Y, Ma J-H, Zhang G-R, Liu G, Xu B-Q. Dealloyed carbon-supported PtAg nanostructures: enhance electrocatalytic activity for oxygen reduction reaction. Electrochem Commun 2010;12:1191e4. [22] Lee C-L, Chiou H-P, Syu C-M, Liu C-R, Yang C-C, Syu C-C. Displacement triangular Ag/Pd nanoplate as methanol tolerant electrocatalyst in oxygen reduction reaction. Int J Hydrogen Energy 2011;36:12706e14.
[23] Xingjuan S, Dongming Z. Bimetallic AgeNi/C particles as cathode catalyst in AFCs (alkaline fuel cells). Energy 2014;70:223e30. [24] Feng RX, Dong H, Cao YL, Ai XP, Yang HX. AgNi-catalyzed anode for direct borohydride fuel cells. Int J Hydrogen Energy 2007;32:4544e9. [25] Garcia AC, Gasparotto LHS, Gomes JF, Tremiliosi-Filho G. Straight forward synthesis of carbon-supported Ag nanoparticles and their application for the oxygen reduction reaction. Electrocatalysis 2012;3:147e52. [26] Cheng Y, Li W, Fan X, Liu J, Xu W, Yan C. Modified multiwalled carbon nanotube/Ag nanoparticle composite catalyst for oxygen reduction reaction in alkaline solution. Electrochim Acta 2013;111:635e41. [27] Sheng-Li C, Bing-Liang W, Chuan-Sin C. Application of timeresolved EQCM to the study of the mechanism of silver (I) oxide formation on a polycrystalline silver electrode in alkaline solution. J Electroanal Chem 1996;416:53e9. [28] (a)Maheswari S, Sridhar P, Pitchumani S. Carbon supported silver as cathode electrocatalysts for alkaline polymer electrolyte Membrane Fuel Cells. Electrocatalysis 2012;3:13e21. (b)Bard AJ, Faulkner LR. Electrochemical methods. New Delhi: Wiley; 2006. p. 341. [29] Blizanac BB, Ross PN, Markovic NM. Oxygen reduction on silver low-index single-crystal surfaces in alkaline solution: rotating ring disk Ag(hkl) studies. J Phys Chem B 2006;110:4735e41. [30] Damjanovic A, Brusic V. Electrode kinetics of oxygen reduction on oxide-free platinum electrodes. Electrochim Acta 1967;12:615e28. [31] Wroblowa H, Rao M, Damjanovic A, Bockris JO’M. Adsorption and kinetics at platinum electrodes in the presence of oxygen at zero net current. Electrochim Acta 1967;12:139e50. [32] Maheswari S, Sridhar P, Pitchumani S. PdeTiO2/C as a methanol tolerant catalyst for oxygen reduction reaction in alkaline medium. Electrochem Commun 2013;26:97e100. [33] Maheswari S, Sridhar P, Pitchumani S. Pd-RuSe/C as ORR specific catalyst in alkaline solution containing methanol. Fuel Cells 2012;12:963e70.
ndez-Rodrı´guez MA, et al., Carbon supported Ag and AgeCo catalysts tolerant to methanol and Please cite this article in press as: Herna ethanol for the oxygen reduction reaction in alkaline media, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.07.188