Electrooxidation of Sodium Borohydride at Pd, Au ... - ACS Publications

J. Phys. Chem. C 2009, 113, 13369–13376

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Electrooxidation of Sodium Borohydride at Pd, Au, and PdxAu1-x Carbon-Supported Nanocatalysts Ma´rio Simo˜es, Ste`ve Baranton, and Christophe Coutanceau* Laboratoire de Catalyse en Chimie Organique, Equipe Electrocatalyse, UMR 6503 CNRS, UniVersité de Poitiers, 40 aVenue du Recteur Pineau, F-86022 Poitiers Cedex, France ReceiVed: March 26, 2009

Carbon-supported palladium, gold, and bimetallic Pd-Au nanocatalysts with different compositions were synthesized by a “water-in-oil” microemulsion method. Their catalytic activity toward borohydride electrooxidation was evaluated in alkaline medium. Physical and electrochemical methods where applied to characterize the structure and surface of the synthesized catalysts. It was shown that PdxAu1-x/C catalysts were alloys, which present an increase of crystallite (X-ray diffraction) and particle (transmission electron microscopy) sizes with increasing Au atomic fraction. Their surfaces were palladium-rich whatever the Pd atomic ratios. The onset potential of NaBH4 oxidation was close to -0.2 V versus reversible hydrogen electrode (RHE) on Pd/C. PdxAu1-x/C catalysts presented lower onset potential for BH4- oxidation than Au and Pt (in the range from -0.2 to -0.1 V vs RHE against 0 and 0.3 V vs RHE for Au/C and Pt/C, respectively). The NaBH4 oxidation on Pd/C catalyst was found to be a first-order reaction with respect to borohydride concentration. From voltammetric measurements, rotating disk electrode experiments, and hydrogen production estimations it was proposed that NaBH4 oxidation on palladium-based nanocatalysts followed two pathways. The first one, at negative potentials, involved the formation of BH3OH- intermediate with H2 generation. The second one, at higher overpotentials, occurred mainly via the direct BH4- oxidation reaction, involving 6 mol of exchanged electrons per mole of borohydride. However, it was shown that addition of gold to palladium leads to increase significantly the hydrogen evolution rate. At last, comparison of the activity of the different catalysts toward the borohydride oxidation reaction showed that up to 50% of the palladium atoms can be replaced by a noncatalytic foreign metal like gold, while keeping identical catalytic activity than that of the monometallic Pd/C catalyst. 1. Introduction The interest for SAMFCs (solid alkaline membrane fuel cells) has recently increased for different reasons: first, the activation of the oxidation and reduction reactions occurring in fuel cells is easier in alkaline medium1,2 than in acidic medium. Therefore, less platinum or even non-noble platinum based catalysts can be used due to higher electrode kinetics. Second, the recent development of hydroxyl conductive membrane makes this technology available.3-7 However, the question of the fuel to be used in this system is still a key parameter. Pure hydrogen or hydrogen-rich gases allow high electric efficiency, but their production, storage, and distribution are still constraints for a large-scale development.8,9 The electrooxidation of different alcohols and polyols in a direct membrane alkaline fuel cell led one to achieve from a few to a few tens of milliwatts per square centimeter as best cell performance.10-12 Such systems may be improved, and the cell performance enhanced, but until now, electric performance remains relatively low: the oxidation reaction of alcohols is difficult to activate even in alkaline medium. Sodium borohydride (NaBH4) has then arisen as an interesting alternative because of its reactivity.13-15 It has a specific energy of around 9.3 kW · h kg-1,16,17 i.e., higher than that of ethanol with 8.0 kW · h kg-1,18 and it is stable in alkaline medium at pH > 12.19 Moreover, electrical performance as high * To whom correspondence should be addressed. E-mail: christophe. [email protected].

as 290 mW cm-2 was achieved in DBFCs (direct borohydride fuel cells).20 However, NaBH4 can undergo several reaction pathways: (i) direct reaction pathway

BH4- + 8OH- f BO2- + 6H2O + 8e◦ EBO - ) -1.24 V vs SHE 2 /BH4

at

(1)

pH ) 14

(ii) indirect reaction pathway via hydrogen evolution

BH4- + 2H2O f BO2- + 4H2

(2)

H2 + 2OH- f 2H2O + 2e-

(3)

EH◦ 2O/H2 ) -0.83 V vs SHE

at

pH ) 14

The direct reaction pathway has to be favored in order to increase the efficiency of a DBFC. According to Elder and Hickling,21 platinum, palladium, and nickel are virtually able to oxidize NaBH4 at low potentials. Gold is also known to be active for the NaBH4 electrooxidation. According to Chatenet et al.,22 gold particles lead to the exchange of 7.5 mol of electrons per mole of borohydride against 2 to 4 with platinum.21

10.1021/jp902741z CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

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TABLE 1: DTA-TGA, ICP-OES, TEM, XRD, and Electrochemical Catalyst Characterization Data DTA-TGA [wt %] lattice parameter [Å] mean particle diameter (TEM) [nm] Ealloy [V vs RHE] p atomic Pd on surface [%] atomic composition [atom %]

Pd Au

Pd

Pd0.9Au0.1

Pd0.7Au0.3

Pd0.5Au0.5

Pd0.3Au0.7

Pd0.1Au0.9

Au

41 3.923 4.0 0.626 100

35 3.950 4.7 0.640 97

37 3.986 4.9 0.680 88

37 4.001 5.0 0.730 76

40 4.018 5.1 0.795 61

38 4.048 5.3 0.726 77

39 4.067 7.4 1.065 0

90 10

ICP-OES 69 31

50 50

31 69

However, the onset of the oxidation wave is shifted 0.3 V toward higher potentials compared to platinum. In the case of palladium, Li et al.14 proposed that six electrons were exchanged for the oxidation of BH4- associated with the determination of the amount of hydrogen formed in the high-current region (higher than 1 or 2 A). But, to our knowledge, few information has been reported about the kinetics and mechanism of the electrooxidation of NaBH4 at nanodispersed Pd-based catalysts. The aim of this paper is to evaluate the electroactivity of such catalysts toward the oxidation of NaBH4 in terms of onset potential and number of exchanged electrons and also to enlarge the study to PdxAu1-x/C palladium-gold alloys as NaBH4 oxidation on gold electrodes has attracted more and more attention. 2. Experimental Section 2.1. Synthesis of Catalysts by the “Water-in-Oil” Microemulsion Method. Catalysts were prepared by mixing NaBH4 (99% from Acros Organics) as reducing agent, with a microemulsion carrying the specific reactants dissolved in an aqueous phase (Milli-Q Millipore, 18.2 MΩ cm). In particular, K2PdCl4 and HAuCl4 · 3H2O (from Alfa Aesar, 99.9%) were used. Poly(ethylene glycol)-dodecyl ether (BRIJ 30 from Fluka) was chosen as surfactant, and the organic phase was n-heptane (99% from Acros Organics). The desired amount of metal salts was dissolved in ultrapure water in order to obtain metallic nanoparticles with controlled compositions after the reduction process with NaBH4. Carbon (Vulcan XC72), previously treated under N2 at 400 °C for 4 h, was added directly in the colloidal solution to obtain the desired metal loading, and the mixture was kept under stirring for 2 h. In the present work all the catalysts were synthesized in order to obtain 40 wt % metal loading. The mixture was filtered on a 0.22 µm Durapore membrane filter (Millipore). The resulting powder was abundantly rinsed with ethanol, acetone, and ultrapure water. The carbon-supported catalysts were dried overnight in an oven at 75 °C. 2.2. Electrochemical Measurements. Catalytic powders were deposited on a glassy carbon substrate according to a method proposed by Gloaguen et al.23 The catalytic powder (25 mg) is added to a mixture of 0.5 mL of Nafion solution (5 wt % from Aldrich) in ultrapure water. After ultrasonic homogenization of the catalyst/XC72-Nafion ink, a given volume is deposited from a syringe onto a fresh polished glassy carbon substrate yielding a catalytic powder loading of 354 µg cm-2. The solvent is then evaporated in a stream of ultrapure nitrogen at room temperature. By this way, a catalytic layer is obtained with a thickness lower than 1 µm. The electrochemical setup consists of a Voltalab PGZ 402 computer-controlled potentiostat, a Radiometer speed control unit CTV 101, and a rotating disk electrode (RDE) Radiometer BM-EDI 101. The solutions were prepared from NaOH (semiconductor grade 99.99%, SigmaAldrich), NaBH4 (99% from Acros), and ultrapure water. The electrochemical experiments were carried out at 20 °C in N2-

0.649 95

0.834 53

11 89

purged supporting electrolyte, using a conventional thermostatted standard three-electrode electrochemical cell. The working electrode was a glassy carbon disk (0.071 cm2 geometric surface area), the counter electrode was a glassy carbon plate (8 cm2 geometric surface area), and the reference electrode was a reversible hydrogen electrode (RHE). 2.3. Hydrogen Measurement. A conventional three-electrode electrochemical cell with gas recuperation on the top was used. The volume of generated H2 was measured by water displacement in a cylindrical compartment connected to the cell. The working electrodes were prepared as follows: 5 mg of catalyst powder was mixed with 1.5 mL of a solution composed by 20 vol % of isopropyl alcohol and 80 vol % of water. A volume of water/PTFE emulsion was added to the solution in order to reach 30 wt % of the catalyst powder in PTFE on the prepared ink. The ink was then deposited on a 2.5 cm2 diffusion layer electrode (from E-TEK, 40 wt % PTFE) to reach 0.4 mg cm-2 deposited metal and was then dried overnight at 75 °C. 2.4. Transmission Electron Microscopy and X-ray Diffraction Characterization. Catalysts were characterized by transmission electron microscopy (TEM) using a Philips CM 120 microscope (120 kV) equipped with a LaB6 filament. The mean particle size and size distribution were determined by measuring the diameter of isolated particles using ImageJ free software,24 although particle agglomeration is present in all catalysts. Between 200 and 300 particles were considered for each catalyst in order to have an acceptable statistical sample. The microstructure of the catalytic powders was evaluated by X-ray diffraction (XRD). The powder diffraction patterns were recorded on a Bruker D5005 Bragg-Brentano (θ - θ) diffractometer operated with a copper tube powered at 40 kV and 40 mA (CuKa1 ) 1.54060 Å and CuKa2 ) 1.54443 Å). Measurements were effectuated from 2θ ) 15° to 2θ ) 90° in step mode, with steps of 0.06° and a fixed acquisition time of 10 s/step. 3. Results and Discussion 3.1. Characterization of the Catalysts. The synthesized catalysts were characterized by differential thermal analysisthermogravimetric analysis (DTA-TGA), TEM, XRD, inductively coupled plasma optical emission spectroscopy (ICP-OES), and electrochemical methods. 3.1.1. DTA-TGA and ICP-OES. The metallic charge of the carbon-supported catalysts was confirmed by DTA-TGA, and results lie between 35 and 40 wt %, which is in accordance with the theoretical 40 wt % metal loading. Results are shown in Table 1. The composition of the PdxAu1-x/C alloys was confirmed by ICP-OES, and results are given in Table 1. All Pd-Au catalysts present the expected atomic ratios, which confirms that metals were completely reduced during the synthesis process, but this method cannot provide any information about the microstructure of the formed alloys. Therefore, to confirm the structural

Catalytic Electrooxidation of Sodium Borohydride

J. Phys. Chem. C, Vol. 113, No. 30, 2009 13371

Figure 1. TEM pictures and related particle size distribution of the Pd and Au based catalysts (40 wt % metal loading on carbon): (a) Pd/C; (b) Pd0.9Au0.1/C; (c) Pd0.7Au0.3/C; (d) Pd0.5Au0.5/C; (e) Pd0.3Au0.7/C; (f) Pd0.1Au0.9/C; (g) Au/C.

composition of the nanoparticles TEM, XRD, and electrochemical methods were employed. 3.1.2. TEM and XRD Catalyst Characterization. The mean size of nanoparticles and their dispersion on the support were evaluated from TEM pictures (Figure 1). Monometallic palladium and gold nanoparticles supported on carbon present the

smaller and higher mean particle size with 4.0 and 7.4 nm, respectively. However, Au/C catalyst presents nonagglomerated gold nanoparticles with size greater than 20 nm, which were not taken into account for the statistical size distribution. The synthesized PdxAu1-x/C alloys exhibit an increasing mean particle size with the Au atomic ratio (Table 1).

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Figure 2. XRD diffraction patterns of the Pd and Au based catalysts (40 wt % metal loading on carbon).

Figure 3. Cyclic voltammograms of Pd/C and Au/C catalysts (V ) 20 mV s-1, N2-saturated 0.1 M NaOH electrolyte, T ) 293 K).

XRD measurements were performed in order to evaluate the microstructure of the catalysts (Figure 2). Considering the position of the diffraction peaks of the different crystallographic planes, it can be seen that all catalysts present a face-centered cubic (fcc) crystalline structure. For monometallic Pd/C catalyst, a few PdO domains were present as evidenced by low intensity and large peak present at 2θ close to 34°. The monometallic nanoparticles exhibited mainly the crystalline structure of the pure metals, confirmed by the calculated lattice parameters, presented in Table 1. The lattice parameter of each catalyst was calculated considering the mean value of the four lattice parameters obtained for (111), (200), (220), and (311) planes using the Bragg equation:25

for the complete formation of the first oxide monolayer28 on the Pd catalyst surface. Under these conditions, a charge density of 424 µC cm-2 is associated to the reduction of the formed PdO monolayer. The value of the active surface area, obtained by integration of the oxide reduction zone on the voltammogram presented in Figure 3, is close to 28 m2 g-1. The active surface area of the Au/C catalyst was also estimated by the charge involved for the reduction of the first monolayer of AuO.29,30 Considering an upper anodic potential of 1.486 V versus RHE, the charge density associated to the reduction of the oxide formed on the gold surface is 493 µC cm-2. Integration of the oxide reduction region on the gold voltammogram presented in Figure 3 led to an estimated active surface value of ca. 8 m2 g-1. These results are consistent with TEM and XRD measurements since the catalyst active surface area is related to the mean nanoparticle size. Assuming that the synthesized particles are spherical, a relation between the active surface S [m2 g-1] and the mean particle size d [nm] is found:31

4 sin2 θ 1 h2 + k2 + l2 λ√(h2 + k2 + l2) ) ) S a ) 2 sin θ λ2 d2 a2 (4) The value of the lattice parameter of a Pd/C nanoparticle (a ) 3.923 Å) is higher than that for bulk Pd (a ) 3.8908 Å). Such a high value of lattice parameter corresponds rather to the Pd-H system, which leads to enlarge the palladium lattice parameter.26 The formation of palladium hydride can occur during the Pd(II) reduction step of the colloid synthesis, where hydrogen evolution coming from the hydrolysis reaction of BH4- with water is involved,27 part of which can be absorbed by palladium metal. However, the lattice parameter calculated for PdxAu1-x/C catalysts increases linearly with the Au atomic ratio, which is convenient with the formation of PdAu alloy structures. XRD results are also in agreement with the catalyst composition determined by ICP-OES. In the same time, a decreasing behavior is observed for the widening of PdxAu1-x/C diffraction peaks from monometallic Pd to Au (Figure 2), indicating an increase of the crystallite size, confirming the TEM characterization of particle size. 3.1.3. Electrochemical Catalyst Surface Characterization. The catalytic surface was characterized by cyclic voltammetry by considering the zone of surface oxide reduction. The voltammograms were recorded at 20 mV s-1 in a N2-saturated 0.1 M NaOH electrolyte without electrode rotation. Several techniques where employed depending on the catalyst composition. The active surface area of monometallic palladium catalyst was characterized by the quantification of the electric charge involved in the reduction of the first PdO monolayer formed on the metal surface, as described by Grde´n et al.28 A voltammetric cycle was performed between 0.35 and 1.45 V versus RHE. This latter potential corresponds to the potential

S)K

1 dF

(5)

where K is a constant independent of the nature of the metal composing the nanoparticles and F is the metal density [g cm-3]. Considering the TEM results obtained for the monometallic catalysts and using eq 5, a relation between Pd and Au mean particle sizes and the active surface areas is obtained:

SPd ) SAu

1 dAuFAu dPdFPd ) dPdFPd 1 K dAuFAu K

(6)

When comparing the ratio between the active surface areas of Pd/C and Au/C catalysts, as determined by electrochemical methods (SPd/SAu ) 3.5), with that involving the mean particle size of Pd/C and Au/C as determined by TEM measurements ((dAuFAu)/(dPdFPd) ≈ 3.0), the relative error between both values has been found to be inferior to 20%. This relative error can be explained by the fact that Au/C catalyst exhibits nonagglomerated particles with size greater than 20 nm (not shown), which were not taken into account for the determination of the mean particle size. This leads to a higher value of the active surface area calculated by TEM measurements for the Au/C catalyst, when compared to electrochemical methods.



Figure 4. Cyclic voltammograms of PdxAu1-x/C catalysts (V ) 20 mV s-1, N2-saturated 0.1 M NaOH electrolyte, T ) 293 K, Emax ) 1.45 V vs RHE).

As catalytic reaction takes place at the electrode surface, it is important to evaluate the surface composition of PdxAu1-x/C alloys. An electrochemical method presented by Rand and Woods32 was used to estimate the atomic composition of the Pd-Au electrode surfaces. Bimetallic catalysts can present common or separated potential regions for the reduction of surface oxides. The Pd-Au alloy system exhibits a common zone where those oxides are reduced. As shown by Rand and Woods, the potential where the reduction peak of the alloy surface oxides (Epalloy) is observed depends on the surface composition and can, therefore, be used to estimate its composition using the following equation:32

Au Ealloy ) XPdEPd p p + XAuEp S XPd )

Ealloy - EAu p p Au EPd p - Ep

(7) where XPd and XAu are the Pd and Au surface atomic fractions and EpPd and EpAu are the oxide reduction peak potentials for the monometallic Pd and Au nanoparticles, respectively. The electrochemical measurements were carried out at 20 mV s-1 in 0.1 M NaOH between 0.35 and 1.45 V versus RHE, for all catalysts. Results are given in Table 1. Voltammograms are presented in Figure 4. The surface is palladium-rich for all PdxAu1-x/C alloys. This segregation of Pd atoms may only affect the few first atomic layers since XRD measurements indicate a linear variation of the lattice parameter with the Au atomic fraction. For Pd atomic content lower than 30%, a gold oxide reduction peak appears on the voltammograms, which means that nonalloyed Au catalytic sites are present on the PdAu surface. The Pd0.1Au0.9/C surface is mainly composed by gold atoms as can be seen from the voltammogram, which is similar to that of Au/C with clearly identified peak current coming from gold oxide formation and reduction. The Pd0.1Au0.9/C voltammogram also presents several palladium oxide reduction peaks at different potentials in the cathodic scan, indicating different alloyed forms of the Pd-Au atoms on the surface. This catalyst clearly presents a heterogeneous distribution of palladium on the nanoparticle surface. It is interesting to remark that no noticeable gold oxide formation or reduction zone in the voltammograms is observed for alloys with palladium atomic content over 50%, which indicates that the catalyst surface is mainly composed by Pd-Au alloy catalytic sites. 3.2. Evaluation of the Catalytic Activity. The polarization curves of the oxidation of 10-2 M NaBH4 in 1.0 M NaOH were

Figure 5. Polarization curves of the NaBH4 oxidation recorded on Pd/C, Au/C, and PdxAu1-x/C catalysts without electrode rotation (V ) 5 mV s-1, N2-saturated, 10-2 M NaBH4 + 1 M NaOH electrolyte, T ) 293 K, Ω ) 0 rpm).

recorded at 5 mV s-1 with different rotation rates of the electrode in order to evaluate the kinetics of BH4- oxidation on the different synthesized catalysts. Polarization curves recorded for all catalysts without electrode rotation are presented in Figure 5. Monometallic palladium catalyst presents a negative onset potential for the NaBH4 oxidation close to -0.2 V versus RHE, which is very interesting, considering a fuel cell application, especially when compared to the gold catalyst which presents an onset potential for borohydride oxidation reaction close to 0.25 V versus RHE, in agreement with the literature.22,33 Bimetallic Pd-Au catalysts having a Pd atomic fraction between 90% and 50% present identical onset potential than that with Pd/C catalyst. When the Pd atomic ratio is lower than 50%, the oxidation onset potential is shifted toward higher potentials, as Au atoms are more present on the surface. However, the onset potential of borohydride oxidation reaction on Pd-Au alloys remains lower than that measured on Au/C catalyst, even with only 10 atom % of Pd. All palladium-containing catalysts exhibit two oxidation waves occurring at separated potentials. The first one is present at negative potentials with an oxidation plateau that ends when the second one starts at around 0.1 V versus RHE. The first oxidation wave is accompanied by the formation of small hydrogen bubbles on the electrode surface and can be related to a first oxidation reaction of BH4-, involving the BH3OHintermediate:22,33-35

BH4- + H2O f BH3OH- + H2

(8)

3 BH3OH- + 3OH- f BO2- + H2 + 2H2O + 3e2

(9) The reaction pathway described by eqs 8 and 9 is catalyst promoted, by platinum-based materials, for example,33,35 or associated to low [OH-]/[BH4-] ratio (typically inferior than 4.433-35). It takes place at potentials more negatives than BH4direct oxidation,22,33,35 which can explain the potential difference between the first and the second oxidation waves on the palladium-based catalysts. The obtained results show that reactions 8 and 9 seem to be promoted by the presence of palladium atoms on the catalyst surface. Santos and Sequeira36 have recently studied the open circuit potential (OCP) dependence on BH4- concentration with different electrode materials; it was shown that even in 4 M NaOH electrolyte, with [BH4-] ) 10-2 M, the palladium electrode presented a negative OCP

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Figure 6. Polarization curves of the NaBH4 oxidation recorded on Pd/C and Au/C catalysts with electrode rotation (V ) 5 mV s-1, N2saturated, 10-2 M NaBH4 + 1 M NaOH electrolyte, T ) 293 K, Ω ) 400, 900, 1600, and 2500 rpm).

versus RHE. Our results are in agreement with this observation. The gold electrode displays a similar OCP than that of palladium, but for a much lower [OH-]/[BH4-] ratio and with [BH4-] > 1 M.36 The second oxidation wave starts at about 0.1 V versus RHE for the catalysts containing more than 10 atom % of Pd and has different current profiles, depending on the Au loading. These different current profiles can be explained by the BH4oxidation occurring between -0.2 and 0.1 V versus RHE on PdxAu1-x/C catalysts with x > 0.1, which is accompanied by H2 production (eqs 8 and 9). Part of the generated hydrogen remains on the electrode surface and is then oxidized in the potential range from 0.1 to 0.4 V versus RHE. It can be seen that the presence of Au atoms on the catalyst surface (inactive for H2 oxidation reaction22,33) leads to increase the peak potential of the oxidation peak observed between 0.2 and 0.5 V versus RHE that is related to the oxidation of the accumulated hydrogen on the electrode surface. This phenomenon is almost inexistent on Pd/C catalyst due to a lower H2 production from the reaction pathway described by eqs 8 and 9, in comparison to Au and Pd-Au catalysts. This aspect will be discussed in the latter part of the Results and Discussion section. Pd0.1Au0.9/C presents the same catalytic behavior for the BH4- oxidation reaction than that of other PdAu alloys (with higher Pd atomic fractions), but it exhibits a shift of the oxidation current curve toward more positive potentials due to the lack of Pd atoms on the surface. Current densities of the BH4- oxidation reaction are always higher for Pd-containing catalysts, compared to monometallic Au. This difference is possibly due to the higher active surface area of Pd-based catalysts or to the intrinsically better activity of palladium catalytic sites with respect to the BH4- oxidation reaction. The polarization curves of the BH4- oxidation on the synthesized catalysts were recorded at different electrode rotation rates. The curves obtained for Pd/C and Au/C catalysts are presented in Figure 6. The current densities recorded for Pd/C catalyst in the 0.5-0.8 V versus RHE potential range at Ω ) 1600 rpm are twice higher than those recorded at Ω ) 400 rpm. A similar result is obtained for the Au/C catalyst in the 0.8-1.1 V versus RHE potential range. This indicates that the kinetics is controlled by the BH4- ion diffusion/convection in the referred potential ranges. With the gold catalyst some hydrogen evolution punctually perturbs the diffusion mechanism. It seems that gold nanoparticles promote BH4- hydrolysis during the oxidation reaction, leading to lower faradic efficiency compared to the gold bulk electrode where none or negligible hydrogen evolution has been reported.33

Simo˜es et al. BH4- oxidation curves recorded at Ω ) 400 rpm and Ω ) 1600 rpm on PdxAu1-x/C are presented in Figure 7. As for monometallic catalysts, Pd-Au bimetallic catalysts present current densities that are twice higher for Ω ) 1600 rpm than for Ω ) 400 rpm, for potentials near the diffusion plateau, indicating that the kinetics is controlled by the BH4- ion diffusion/convection. When the electrode is rotated, the hydrogen formed on its surface at negative potentials during the first oxidation wave is partially evacuated from the electrode surface. Hydrogen is removed more efficiently from the surface for higher rotation rate of the electrode, but even at Ω ) 2500 rpm, H2 is not totally removed from the catalyst surface. Analyzing the polarization curves of Figure 7, the catalytic activity toward borohydride oxidation of PdxAu1-x/C alloys with x g 0.5 is similar to that of monometallic Pd catalyst. This observation implies that replacing 50% of the palladium atoms by another metal which presents a catalytic activity for the direct BH4oxidation reaction, but at higher overpotentials, leads to identical electrooxidation profiles than that obtained on the Pd/C catalyst. The reaction order of the NaBH4 oxidation on Pd nanocatalyst was evaluated. Considering that the increase of the borohydride concentration in solution leads to higher disk currents, the rate of the oxidation reaction can be expressed as a kinetic current density with the following equation:37

jk ) A[BH4-]R

(10)

where jk is the kinetic current density, A is a constant, and R is the reaction order with respect to borohydride ions concentration. Polarization curves of the BH4- oxidation reaction on Pd/C electrode with different borohydride concentrations, ranging from 10-4 to 10-2 M, were recorded. From the plot of ln(jk) ) f(ln([BH4-])), the reaction order R was determined. In the potential range from 0.45 to 1.20 V versus RHE, the slope value laid between 0.8 and 1.0, meaning that the BH4- oxidation on palladium is a first-order reaction. Cheng and Scott37 have also confirmed that on a gold electrode the BH4- oxidation is a firstorder reaction with respect to the borohydride concentration. This result allows the use of the Koutechy-Levich eq 11 to determine the total number of exchanged electrons per BH4ion (nt):38 1 1 1 1 1 + ) + ) j jlim jk jk 0.2ntFD2/3CBH4-ν-1/6Ω1/2

(11)

where j is the current density, jlim is the diffusion limiting current density, and jk is the kinetic current density. D ) 1.6 × 10-5 cm2 s-1 is the BH4- diffusion coefficient, CBH4- (mol L-1) is the BH4- concentration in solution, ν ) 1.19 × 10-2 cm2 s-1 is the NaOH solution kinematic viscosity, F ) 96 485 C mol-1, and Ω (rpm) is the electrode rotation rate. From the data used by Chatenet et al.22 (references therein) the total number of electrons involved in the reaction, nt in eq 11, can be calculated for each catalyst. Considering the constant B ) 0.2FD2/3 CBH4-ν-1/6 and by plotting j-1 ) f(Ω-1/2) for different rotation ratios, the slope sKL of resulting curve can be used to calculate nt by eq 12:

nt )

1 BsKL

(12)



Figure 7. Polarization curves of the NaBH4 oxidation recorded on Pd/C, Au/C, and PdxAu1-x/C catalysts with electrode rotation: (a) Ω ) 400 rpm; (b) Ω ) 1600 rpm (V ) 5 mV s-1, N2-saturated, 10-2 M NaBH4 + 1 M NaOH electrolyte, T ) 293 K).

Figure 8. Calculated number of exchanged electrons on Pd/C, Au/C, and PdxAu1-x/C catalysts.

Figure 8 presents the calculated total number of exchanged electrons per BH4- ion on the whole potential range. For Pdbased catalysts (x > 0.1), six electrons per BH4- oxidized molecule are exchanged for potentials higher than 0.6 V versus RHE, when hydrogen is no longer oxidized on the surface. The Pd0.1Au0.9/C catalyst exhibits a shift of the oxidation curve toward higher potentials, as mentioned earlier. Between six and seven electrons are exchanged per BH4- ion on the Au/C catalyst, but this result is obtained for higher overpotentials than with Pd-based catalysts. The curves presented in Figure 8 show a peak for all PdAu catalysts, which leads to a total number of exchanged electrons superior to 6e-. This peak is related to the oxidation of the hydrogen generated in the potential range of the first oxidation wave (eqs 8 and 9) that is still present on the catalyst surface. It was found that, for some compositions of the Pd-Au alloys, the number of exchanged electrons is superior to the stoichiometric value of the direct borohydride oxidation reaction pathway (eq 1) which can only be explained by a mixed oxidation mechanism involving both borohydride and gaseous hydrogen oxidation reactions on the Pd catalyst surface in the potential domain below 0.6 V versus RHE. 3.3. H2 Generation Measurement. The hydrogen produced during the borohydride oxidation on Pd/C, Au/C, and Pd0.7Au0.3/C catalysts was measured in 0.134 M NaBH4 + 0.126 M NaOH electrolyte at different potentials, and the electric charge involved was recorded. Chronoamperometries were carried out for each catalyst at OCP and 0, 0.2, 0.4, and 0.6 V versus RHE for 1 h. A [OH-]/[BH4-] ratio close to 1 was chosen to promote the BH4- ion instability while maintaining a strongly alkaline electrolyte. The generated H2 measurements are presented in Figure 9 as a function of the applied potential.

Figure 9. Generated H2 on Pd/C, Au/C, and Pd0.7Au0.3/C catalysts at different potentials: ([) Pd/C, (b) Au/C, (9) Pd0.7Au0.3/C (0.124 M NaOH + 0.136 M NaBH4, 50 mL solution, T ) 293 K, electrode metal density ) 0.4 mg cm-2, electrode surface ) 2.42 cm2).

As expected, the higher borohydride concentration leads to lower measured OCP for all catalysts.36 No borohydride hydrolysis is observed at OCP on Au/C catalyst, but a small volume of H2 was measured on Pd-based catalysts. As the potential is increased, the hydrogen generation increases too. With the Au/C catalyst, an abrupt increase in H2 production is observed for potentials higher than 0.2 V versus RHE, which corresponds to the potential region where gold starts to be active for the BH4- oxidation. With the Pd/C catalyst, the increase of the H2 evolution flow with potential is lower than that recorded at the gold nanocatalyst. The alloyed Pd0.7Au0.3/C catalyst presents higher H2 evolution flow than the Pd/C catalyst. This can be related to the presence of gold atoms on the surface. These behaviors confirm the voltammetric results showing that the kinetics of hydrogen production was higher and that the kinetics of direct oxidation of borohydride was lower at Pd-Au than at Pd/C catalysts. From the integration of the Coulombic charge, H2 production flow versus the oxidation current I can be plotted (Figure 10). The number of exchanged electrons per borohydride molecule in the oxidation process can then be estimated using eq 13:

BH4- + xOH- f BO2- + (x - 2)H2O + 1 (4 - x)H2 + xe2

(13)

It is found that under the present conditions, with [OH-]/ [BH4-] close to 1, the total number of exchanged electrons is

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Simo˜es et al. Acknowledgment. This work was carried out under the framework of a contract (BODIPAC) from the French National Research Agency (ANR-PAN-H), funded by the French ministry of research. References and Notes

Figure 10. Chronoamperometric relationship with H2 volume generation for different potentials: ([) Pd/C, (b) Au/C, (9) Pd0.7Au0.3/C (0.124 M NaOH + 0.136 M NaBH4, 50 mL solution, T ) 293 K, electrode metal density ) 0.4 mg cm-2, electrode surface ) 2.5 cm2).

inferior to six. The presence of gold in the catalysts decreases the faradic efficiency since gold is not an active catalyst toward the hydrogen oxidation reaction. Therefore, generated H2 is not oxidized, and if it is not removed from the electrode, the surface can be partially covered by gaseous H2 reducing the available catalytic sites for the BH4- oxidation. 4. Conclusion In this work the activity of Pd/C, Au/C, and PdxAu1-x/C catalysts toward the electrooxidation of NaBH4 was compared. The catalyst surfaces were characterized by physical and electrochemical methods. It was shown that the synthesized bimetallic catalysts have a palladium-rich surface, with respect to the atomic ratio of the alloys that was verified by ICP-OES. The OCP of the BH4- oxidation is lowered by the presence of palladium atoms in the alloy. The measured OCP on Pd/C catalyst is 0.45 V inferior to that of Au/C and is also inferior to the reported one for platinum.33,36 Several insights were highlighted in this work. On monometallic gold catalyst, NaBH4 oxidation occurs mainly via the direct pathway; however, gold is active at higher potentials than Pd-based catalysts. Hydrogen generation on the Au nanocatalyst was verified in the potential range where BH4oxidation occurs, justifying that only six to seven electrons are exchanged per BH4- ion. On palladium-based electrodes, borohydride electrooxidation appears to follow several different mechanisms according to the electrode potential. In the potential range from -0.2 to 0.1 V versus RHE, the mechanism involves the hydrolysis reaction of BH4- to BH3OH- followed by the oxidation of this latter species. In the high-overpotential range (potential higher than 0.4 V vs RHE) the direct borohydride oxidation is favored, and six electrons are exchanged per BH4- ion. In the potential range from 0.1 to 0.4 V versus RHE, both oxidation of hydrogen formed on the surface and direct oxidation of NaBH4 reactions occurs. At last, it was shown that the catalytic activity toward the BH4- oxidation at low potentials of alloyed palladium-based catalyst can be maintained even after exchanging up to 50% of the palladium atoms with noncatalytic metal. This fact is remarkable considering the possibility to use non-noble metal in the alloy composition.

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