Electrocatalysis of oxygen reduction on electrodeposited Pd coatings on gold

Journal of Electroanalytical Chemistry 691 (2013) 35–41

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Electrocatalysis of oxygen reduction on electrodeposited Pd coatings on gold Heiki Erikson a, Madis Liik a, Ave Sarapuu a,⇑, Margus Marandi b, Väino Sammelselg a,b, Kaido Tammeveski a a b

Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia

a r t i c l e

i n f o

Article history: Received 18 November 2012 Received in revised form 22 December 2012 Accepted 24 December 2012 Available online 3 January 2013 Keywords: Electrochemical deposition Pd nanoparticles Oxygen reduction Electrocatalysis

a b s t r a c t The electrochemical reduction of oxygen on electrodeposited palladium coatings was studied in acid and alkaline media employing the rotating disc electrode (RDE) method. Gold was used as an electrode substrate and Pd was deposited at a constant potential varying the deposition time. Cyclic voltammetry and CO stripping experiments were used for surface characterisation of Pd coatings. The surface morphology of the as-prepared electrodes was examined by atomic force microscopy (AFM). The AFM images revealed the formation of Pd islands on the substrate surface. It was found that in alkaline solution the palladised electrodes show high specific activity for oxygen reduction and it does not depend on deposition time. In acid solution the dependence on the deposition time was in evidence and the specific activity surpassed that of bulk palladium. The oxygen reduction reaction (ORR) on electrodeposited Pd proceeds mainly via a four-electron pathway in both solutions and the reduction mechanism is similar to that on bulk Pd. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In recent years palladium has gained much attention in the search for an active electrocatalyst for oxygen reduction reaction (ORR) [1,2]. Pd has similar properties to platinum and the mechanism of the ORR is the same on these metals [3,4]. Pd is more abundant on the Earth and less expensive than Pt [1]. It has been established that Pd catalyses the four-electron reduction of oxygen [3,4]. Two-electron reduction of oxygen prevails on Au and on Au– Pd nanoalloys of low Pd content, where individual surface Pd atoms are surrounded by a Au matrix. In contrast, the presence of contiguous Pd atoms on the surface of Au–Pd nanoparticles of higher Pd content leads to H2O formation [5]. Jirkovsky et al. investigated O2 reduction on Au–Pd core–shell structures and found that the electrode preconditioning changes significantly the catalyst surface composition [6]. This leads to changes in H2O2 selectivity. Frequently Pd is alloyed with different transition metals to increase the ORR activity in acid media [1,2]. A low content of platinum has been used for this purpose, but if the goal is to achieve Pt-free catalysts then different non-precious metals have been employed [1,2]. Modification of Pt single crystal surfaces with Pd monolayer enhances the ORR kinetics in alkaline solution 2–4 times, but on Au single crystals the enhancement is more than an order of magnitude [7]. Dursun et al. studied O2 reduction on a Pd ad-layer modified Au(1 1 1) electrodes in alkaline media and concluded that Pd overlayer accelerated the electron transfer process [8]. Qian et al. prepared Au/Pd bimetallic nanostructures with ⇑ Corresponding author. Tel.: +372 7375277; fax: +372 7375181. E-mail address: [email protected] (A. Sarapuu). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2012.12.018

different atomic ratios and these bimetallic catalysts showed improved ORR activity in H2SO4 solution compared with Au dendritic structures [9]. Shao et al. studied ORR on Pd monolayers on various surfaces and found that Pd on Au(1 1 1) monocrystal is electrocatalytically active and the activity was higher only with Pd on Pt(1 1 1) [10]. The PdAu nanowires prepared by Koenigsmann et al. showed higher specific activity than that of commercial Pt/ C catalyst [11]. In an early work it has been shown using Pd–Au alloys that the O2 reduction activity decreases as Au content increases and thermal pre-treatment of these alloys had minimal effect on the electrocatalytic activity [12]. The electrochemical deposition process of Pd on different Au monocrystal facets has been thoroughly studied [13–17]. It has been suggested that the first step of Pd deposition is the adsorption of Pd chloro complex onto gold and then the reduction process takes place [13]. Pd/Au alloy formation is also possible in the deposition process [16,17]. Naohara et al. studied the electrochemical reduction of oxygen on the epitaxially grown ultra-thin Pd layers on Au(1 1 1) and Au(1 0 0) single crystal surfaces in HClO4 solution. It was observed that modifying Au electrode even by a submonolayer of Pd increases significantly its catalytic activity towards the ORR [18]. Xiao et al. prepared Pd nanorods on a gold substrate using electrodeposition and found that these structures have high electrocatalytic activity for ORR in perchloric acid solution [19]. They attributed the higher activity to Pd(1 1 0) facet which is in contradiction with the study of the ORR on Pd monocrystals where Pd(1 0 0) was found to be the most active plane [20]. More recently Hoshi and co-workers studied the reduction of oxygen on n(1 1 1)– (1 0 0) series of Pd in HClO4 [21]. They concluded that the oxide film is not relevant for ORR on these surfaces.

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Štrbac and co-workers investigated O2 reduction on Au electrodes modified with spontaneously deposited Pd. These submonolayer Pd films showed good electrocatalytic properties for ORR in acid media [22]. The dependence of specific activity of oxygen reduction on Pd film thickness has been studied on thin Pd films prepared by electron beam evaporation onto gold [23] and glassy carbon [24] substrates. In sulphuric acid solution, the specific activity increased with Pd film thickness [23,24]. Recently, several groups have shown that palladium nanocubes with (1 0 0) planes on the surface exhibit higher electrocatalytic activity towards ORR than spherical Pd nanoparticles [25,26] and Pd octahedra [27] in acid media. Cubic Pd catalysts have shown enhanced activity also in alkaline media [26,28]. Comparing Pd nanocubes of different size it was found that 48 nm particles had higher specific activity than 27 nm and 63 nm Pd nanocubes [29]. The ORR results obtained with cubic Pd nanoparticles [25–29] are in line with electrocatalytic activities reported for Pd(hkl) single crystal electrodes [20]. Pd(1 0 0) was found to be the most active low-index plane for oxygen reduction. There are numerous studies in which Pd nanoparticles supported on carbon nanomaterials have been used as catalysts for O2 reduction [30–38]. The pre-treatment of carbon support has shown to have significant effect on the activity of the Pd catalyst [34]. Palladium nanoparticles have been used to make composite materials with carbon nanotubes and these hybrid catalysts possessed promising electrocatalytic activity towards the ORR [36]. Recently, we explored the reduction of oxygen on electrodeposited Pd coatings on glassy carbon [39]. In this work the oxygen reduction reaction has been studied on electrochemically deposited Pd coatings on gold substrate in acid and alkaline media using the rotating disc electrode method. The deposition time was varied in order to compare the electrocatalytic properties of Pd/Au electrodes of different Pd loading.

During the electrochemical deposition the electrode continuously rotated at 960 rpm to ensure constant mass-transfer conditions. First, to prevent the spontaneous deposition of Pd, the working electrode was kept at 1.1 V, then the potential was swept at 50 mV s1 to 0.78 V where the electrode was kept for 300, 600 or 900 s. Immediately after deposition the electrode was rinsed with Milli-Q water and transferred to another cell containing either 0.05 M H2SO4 or 0.1 M KOH, where the electrode was kept at 0.1 V for 300 s so that chloride would desorb. Prior to oxygen reduction studies the electrodes were electrochemically pre-treated in Ar-saturated 0.05 M H2SO4 or 0.1 M KOH solution by scanning potential between 0.1 and 0.7 V at a sweep rate (v) of 50 mV s1. To further clean the surface of Pd/Au electrodes carbon monoxide (AGA) was adsorbed onto the Pd catalyst by bubbling CO through the electrolyte at 0.1 V until complete blockage of the surface, which was monitored by cycling the electrode between 0.1 and 0.35 V [40]. After that CO was removed from the solution by bubbling Ar for 45 min. Finally CO was oxidatively stripped off from the surface by scanning the potential up to 1.0 V and the voltammogram corresponding to the CO-free surface was again recorded. After these procedures the electrode was transferred immediately to O2-saturated solution in another cell in order to avoid surface contamination in air. For O2 reduction measurements the potential was scanned between 1.0 and 0.1 V at 20 mV s1 at different electrode rotation rates (x). The surface morphology of as-prepared electrodes was characterised by multimode atomic force microscope (AFM) Autoprobe CP II (Veeco). All AFM images were recorded in non-contact mode using UL20 (PSI) series cantilevers under ambient conditions. The Gwyddion™ free software (Czech Metrology Institute) vs. 2.25 was employed for image processing and surface roughness calculations. All images were processed by the first order flattening for background slope removal, and if necessary, the contrast and brightness were adjusted.

2. Experimental Bulk gold and palladium electrodes were prepared by mounting Au (99.99%, Alfa Aesar) and Pd (99.95%, Alfa Aesar) discs (diameter 5 mm) into Teflon holders. The surface of the electrodes was polished to a mirror finish using 1.0, 0.3 and 0.05 lm alumina slurries (Buehler). After alumina polishing, the electrodes were ultrasonically cleaned in Milli-Q (Millipore, Inc.) water for 5 min. Oxygen reduction was studied in 0.05 M H2SO4 and 0.1 M KOH solutions using the rotating disc electrode (RDE) method. The sulphuric acid solution was prepared from 96% H2SO4 (Suprapur, Merck) and potassium hydroxide solution was prepared from KOH pellets (p.a. quality, Merck). The solutions were made up using Milli-Q water and were saturated with pure O2 (99.999%, AGA) or deaerated with Ar gas (99.999%, AGA). A reversible hydrogen electrode (RHE) connected to the cell through a Luggin capillary was employed as a reference and all the potentials reported in this work are referred to this electrode. A Pt wire served as a counter electrode and the counter electrode compartment of the three-electrode glass cell was separated from the main cell compartment by a glass frit. The potential was applied with an Autolab potentiostat/galvanostat PGSTAT30 (Eco Chemie B.V., The Netherlands) and the experiments were controlled with General Purpose Electrochemical System (GPES) software. An EDI101 rotator and CTV101 speed control unit (Radiometer, Copenhagen) were used for the RDE experiments. All experiments were carried out at room temperature (23 ± 1 °C). The gold substrate was modified with palladium using the same procedure as was used with glassy carbon substrate [39]. The electrodeposition of palladium was carried out in deaerated 0.05 M H2SO4 solution containing 0.1 mM PdCl2 (99.9%, Sigma–Aldrich).

3. Results and discussion 3.1. Surface characterisation of Pd/Au electrodes Typical AFM images of polished Au substrate surface and Pd coatings deposited for different deposition times on Au substrates are presented in Fig. 1. In all cases of Pd deposition it can be seen that surfaces of Au electrodes are fully covered with island-like structures of Pd (Fig. 1b–d). It was found that the root mean square (RMS) roughness of surfaces of electrodes covered with Pd particles decreases compared to clean polished Au surface (Fig. 1a). The corresponding RMS roughness values were: 4.09 ± 0.9 nm, 3.64 ± 0.53 nm, 3.59 ± 0.64 nm and 2.21 ± 0.52 nm for polished Au, 300, 600 and 900 s deposited Pd, respectively. This decreasing trend of RMS roughness can be explained by preferred growing of Pd particles on Au surface defects caused by polishing and subsequent fulfilment of these scratches. It was also found that the average size of Pd particles was 19 ± 2 nm and it did not depend on deposition time. As compared to Pd coatings electrodeposited onto GC support at the same conditions [39], the size of the Pd particles was more uniform and formation of large agglomerates was not observed. 3.2. Cyclic voltammetry and CO stripping In order to clean Pd/Au electrode surface cyclic voltammetry as well as adsorption of CO was used [40]. The electrochemical stripping of adsorbed CO was performed in a single sweep up to 1 V. Typical stripping voltammograms are presented in Fig. 2. In

H. Erikson et al. / Journal of Electroanalytical Chemistry 691 (2013) 35–41

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Fig. 1. NC-AFM micrographs of (a) bulk Au and electrodeposited Pd coatings of various deposition times: (b) 300, (c) 600 and (d) 900 s.

acid medium well-defined CO stripping peaks in the range of 0.9– 0.95 V can be observed. In alkaline medium the CO stripping peak is located at 0.81 V. In alkaline solution the 300 s deposited Pd coating did not show a characteristic sharp CO stripping peak, the reason for that might be the formation of Pd–Au alloy. In acid the CO stripping peak shift is in evidence, which is most likely caused by alloying with Au as shown previously [41]. The increased current at more positive potentials than the CO stripping peak can be attributed to Pd surface oxidation. The broad cathodic peak centred at 0.77 V is due to the reduction of Pd surface oxides and cathodic current increase at E < 0.35 V is caused by hydrogen adsorption. The cyclic voltammetry curves (Fig. 3) registered after CO stripping experiments showed increased symmetry and definition of hydrogen adsorption and desorption peaks, thus indicating increased cleanliness of the surface. The CV curves of electrodeposited Pd electrodes showed the same characteristics as bulk Pd, the broad anodic peak from 0.75 V is a result of Pd surface oxidation and the well-defined cathodic peak at approximately 0.7 V is due to the reduction of these surface oxides. In alkaline media the CV of 300 s deposited Pd coating shows a small wave before PdO reduction peak which might be related to the formation of Pd–Au alloy. Note that a broad peak at approximately 1.15 V is due to the reduction of gold surface oxides which are formed at higher positive potentials on Au substrate surface uncovered by Pd layer. The current increase between 0.1 and 0.35 V is the result of hydrogen adsorption and desorption on electrodeposited Pd coatings. The charge corresponding to the Pd oxide reduction peak was used to determine the real electroactive area (Ar) of Pd. The value of 424 lC cm2 was employed as the charge density for the reduction of a monolayer of PdO [42]. As expected, the value of Ar increased with increasing the electrodeposition time at constant potential (Tables 1 and 2). For comparison, the Pd surface area

Fig. 2. CO stripping voltammograms of Pd/Au electrodes recorded in Ar-saturated (a) 0.05 M H2SO4 and (b) 0.1 M KOH solutions. v = 50 mV s1.

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where j is the measured current density, jk and jd are kinetic and diffusion-limited current densities, respectively, n is the number of electrons transferred per oxygen molecule, k is the rate constant for O2 reduction, F is the Faraday constant (96,485 C mol1), x is the electrode rotation rate, C bO2 is the concentration of oxygen in the bulk (1.22  106 mol cm3) [44], DO2 is the diffusion coefficient of oxygen (1.93  105 cm2 s1) [44] and m is the kinematic viscosity of the solution (0.01 cm2 s1) [45]. The K–L plots were constructed (inset to Fig. 4) and the number of electrons transferred per O2 molecule (n) was found. For all Pd/ Au electrodes the value of n was close to four in the whole range of potentials studied. The 4e pathway of oxygen reduction has been observed previously on Pd single crystals [20], Pd nanoparticles [25,35] and Pd monolayers deposited on various metals [10]. Comparison of the ORR polarisation curves (Fig. 5) shows that the onset potential of O2 reduction for all Pd/Au electrodes employed is similar, but half-wave potential (E1/2) differs more between 600 and 900 s (100 mV) deposited Pd than between 300 and 600 s (50 mV). The Pd coating deposited for 900 s exhibits a polarisation curve rather similar to that of bulk Pd showing a sharp current increase, but these for 300 and 600 s deposited Pd are gentle-sloped like those that have been obtained with nanoparticulate Pd catalysts [24,26,35]. This behaviour might be a consequence of uneven coating, possibly there are gaps between Pd particles where the gold substrate is exposed to the solution. Compared with bulk polycrystalline gold the Au electrode decorated with Pd shows much higher onset and half-wave potentials. In order to compare the intrinsic electrocatalytic activity of electrodeposited Pd the specific activities (SAs) of O2 reduction were calculated:

SA ¼ Ik =Ar Fig. 3. Cyclic voltammograms of Pd/Au electrodes recorded in Ar-saturated solutions: (a) 0.05 M H2SO4 and (b) 0.1 M KOH. v = 50 mV s1.

was also determined by charge integration under the HUPD peaks and CO stripping peaks and similar values were obtained. 3.3. Oxygen reduction on Pd/Au electrodes in acid media After applying the pre-treatment procedures the Pd/Au electrode was transferred to another cell where the solution was saturated with oxygen. A set of RDE polarisation curves recorded in O2saturated 0.05 M H2SO4 at various electrode rotation rates is presented in Fig. 4, the background current registered in O2-free solution has been subtracted from these data. Only cathodic sweeps are presented and further analysed. Similar single-wave polarisation curves were registered for all the electrodes studied and a comparison at a single electrode rotation rate is presented in Fig. 5. The RDE data were analysed using the Koutecky–Levich (K–L) equation [43]:

1 1 1 1 1  ¼ þ ¼ 2=3 b b j jk jd nFkC O2 0:62nFDO2 v 1=6 C O2 x1=2

ð1Þ

ð2Þ

where Ik is the kinetic current at a given potential and Ar is the real surface area of Pd. The Pd catalysts prepared with shorter deposition time had similar SA value, but it was higher for the coating with longest deposition time and surpassed that of bulk Pd. At this stage of work it is not clear why 900 s deposited Pd coating shows the highest SA value, but possibly it is due to morphological effects. It needs to be stressed that the values of SA determined for Pd-based catalysts in acid media containing strongly adsorbing (bi)sulphate anions is lower than that determined in the presence of weakly adsorbing anions (for instance in perchloric acid) [2]. This is due to site-blocking effect of (bi)sulphate anions that inhibit the ORR kinetics. On the basis of RDE data on O2 reduction collected at 1900 rpm the mass-transfer corrected Tafel plots were constructed (Fig. 6) and the values of Tafel slope were determined (Table 1). In the low current density region, the slope values for electrodeposited Pd catalysts were slightly lower than 60 mV dec1, which is the typical value for bulk polycrystalline Pd [3] and nanostructured Pd catalysts [24–26,33,34,38]. At high current densities the Tafel slope value was determined to be about 150 mV dec1, which is somewhat higher than 120 mV dec1 that has been confirmed for bulk palladium [3] as well as for nanoparticulate Pd catalysts [24,26,33,34,38]. There is general consensus that oxygen-contain-

Table 1 Kinetic parameters for oxygen reduction on electrodeposited Pd and bulk Pd in 0.05 M H2SO4. x = 1900 rpm.

a

Catalyst

Ar (cm2)

Tafel slope (mV dec1) (region I)a

Tafel slope (mV dec1) (region II)a

E1/2 vs. RHE (V)

SA at 0.8 V (mA cm2)

300 s deposited Pd 600 s deposited Pd 900 s deposited Pd Bulk Pd

0.18 ± 0.02 0.30 ± 0.01 0.40 ± 0.04 0.52 ± 0.03

52 ± 1 51 ± 1 47 ± 2 53 ± 3

146 ± 4 156 ± 10 152 ± 7 128 ± 5

0.54 ± 0.03 0.58 ± 0.02 0.71 ± 0.03 0.71 ± 0.01

0.18 ± 0.03 0.19 ± 0.03 0.39 ± 0.04 0.24 ± 0.02

Region I corresponds to low current densities and region II to high current densities.

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H. Erikson et al. / Journal of Electroanalytical Chemistry 691 (2013) 35–41 Table 2 Kinetic parameters for oxygen reduction on electrodeposited Pd and bulk Pd in 0.1 M KOH. x = 1900 rpm. Catalyst

Ar (cm2)

Tafel slope (mV dec1)

E1/2 vs. RHE (V)

SA at 0.85 V (mA cm2)

300 s deposited Pd 600 s deposited Pd 900 s deposited Pd Bulk Pd

0.21 ± 0.01 0.35 ± 0.01 0.46 ± 0.07 0.56 ± 0.04

58 ± 2 59 ± 1 60 ± 1 57 ± 2

0.77 ± 0.01 0.79 ± 0.01 0.81 ± 0.01 0.79 ± 0.02

0.60 ± 0.10 0.64 ± 0.02 0.60 ± 0.09 0.48 ± 0.02

ing species on Pd surface influence the Tafel behaviour of oxygen reduction at low overpotentials and as a result the value of 60 mV dec1 has been observed [3]. At higher overpotentials the surface oxide coverage of Pd catalysts is significantly lower and this yields a typical Tafel slope value (120 mV dec1) of charge transfer process rate limited by the transfer of the first electron to O2 molecule. 3.4. Oxygen reduction on Pd/Au electrodes in alkaline media

Fig. 5. Comparison of RDE voltammetry curves of oxygen reduction in O2-saturated 0.05 M H2SO4. x = 1900 rpm, v = 20 mV s1.

The oxygen reduction reaction on Pd/Au catalysts was also studied in 0.1 M KOH solution. A set of RDE polarisation curves is presented in Fig. 7. As expected, in alkaline medium the polarisation curves exhibit sharper current increase than in acid solution. It is clearly evident that Pd is more active catalyst for ORR in alkaline media than in acid and a well-defined diffusion-limited current plateau is formed. Based on the RDE data, the K–L plots were constructed and from Eq. (1) the n value was found to be 4 in the whole range of potentials, using the following values of diffusion coefficient and solubility of oxygen: DO2 ¼ 1:9 105 cm2 s1 and C bO2 ¼ 1:2  106 mol cm3 [46]. The fourelectron reduction of oxygen in alkaline media has been reported previously [7,30,32,47], but it has been suggested that on Pt group metals this reaction proceeds at least partly via peroxide intermediate [4]. Fig. 8 shows a comparison of the RDE polarisation curves of O2 reduction at a single rotation rate. The Pd coatings deposited for 300 s show slightly lower onset and half-wave potential than bulk Pd, 600 and 900 s deposited Pd coatings. The specific activities were determined at 0.85 V, at which bulk Au has still very low catalytic activity as can be observed in Fig. 8. The SA values are rather constant for various Pd coatings and this is in accordance with earlier studies, where it was found that the value of SA does not depend on the thickness of Pd film in alkaline media [24]. Recently, it has been found by Jiang et al. that SA for Pd/C catalysts increases continuously by a factor of 3 with increasing particle size from 3 to 16.7 nm [32]. The almost unchanged SA value in 0.1 M KOH of

Fig. 6. Mass-transfer corrected Tafel plots for oxygen reduction on Pd/Au and bulk Pd electrodes in 0.05 M H2SO4. x = 1900 rpm. Data derived from Fig. 5.

Fig. 7. RDE voltammetry curves for oxygen reduction in O2-saturated 0.1 M KOH solution on 900 s deposited Pd at various electrode rotation rates (v = 20 mV s1). Inset: Koutecky–Levich plot for O2 reduction at 0.3 V.

Fig. 4. RDE voltammetry curves for oxygen reduction in O2-saturated 0.05 M H2SO4 solution on 900 s deposited Pd at various electrode rotation rates (v = 20 mV s1). Inset: Koutecky–Levich plot for O2 reduction at 0.3 V.

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obtained in the present work are relevant for the development of cathode materials for alkaline membrane fuel cells. 4. Conclusions

Fig. 8. Comparison of RDE voltammetry curves of oxygen reduction in O2-saturated 0.1 M KOH. x = 1900 rpm, v = 20 mV s1.

electrodeposited palladium coatings observed in the present work can be rationalised on the basis of relatively large Pd particles as determined from the AFM data. Compared with bulk Pd, the slightly higher SA value might be caused by possible formation of Pd–Au surface alloy during the electrochemical deposition process [16,17]. It has been shown previously that by modifying Au(1 1 1) monocrystal with a monolayer of palladium the ORR kinetics in alkaline solution is enhanced by an order of magnitude [7]. For thicker films the surface alloying effect on the ORR is smaller and morphological effects have higher contribution. The Pd on Au has higher SA values than that of Pd on GC [39] which we suggest is most likely due to the substrate effects, for example alloy formation or possible change in the crystallographic structure. Additional experiments should be performed to determine the reason for this observation, as the Pd loading on the electrodes employed was rather high and therefore the substrate and alloying effects could not be studied in detail. Based on the RDE data on O2 reduction the mass-transfer corrected Tafel plots were constructed (Fig. 9) and the slope values were determined (Table 2). A single Tafel region was determined in which the slope value was 60 mV dec1, it is in accordance with earlier studies with Pd catalysts [30,32,48]. This suggests that the reaction mechanism on the electrodeposited Pd catalysts is similar to that on bulk Pd and indicates that the rate-determining step for ORR on these catalysts is the sluggish transfer of the first electron to O2 molecule. The Pd catalysts studied possess remarkable electrocatalytic activity for ORR in alkaline media and therefore the results

Fig. 9. Mass-transfer corrected Tafel plots for oxygen reduction on Pd/Au and bulk Pd electrodes in 0.1 M KOH. x = 1900 rpm. Data derived from Fig. 8.

The electroreduction of oxygen on electrodeposited palladium coatings on gold substrate has been studied in acid and alkaline media. The electrodeposited Pd coatings showed higher electrocatalytic activity towards the ORR in 0.1 M KOH solution. In 0.05 M H2SO4 the specific activity was found to depend somewhat on the deposition time, but in alkaline solution no such dependence was in evidence. The specific activity increased with increasing the deposition time in acid solution and for 900 s deposited Pd it surpassed that of bulk Pd. The oxygen reduction reaction was found to proceed via 4e pathway in both solutions. The Tafel analysis of O2 reduction indicated that the reaction mechanism on Pd/ Au electrodes studied is similar to that on bulk Pd. The results obtained in this work show that the electrodeposited Pd is a suitable catalyst for oxygen reduction. Acknowledgement This research was supported by the Estonian Science Foundation (Grant No. 8380). References [1] E. Antolini, Energy Environ. Sci. 2 (2009) 915–931. [2] M. Shao, J. Power Sources 196 (2011) 2433–2444. [3] L.M. Vracar, D.B. Sepa, A. Damjanovic, J. Electrochem. Soc. 133 (1986) 1835– 1839. [4] J.S. Spendelow, A. Wieckowski, Phys. Chem. Chem. Phys. 9 (2007) 2654–2675. [5] J.S. Jirkovsky´, I. Panas, E. Ahlberg, M. Halasa, S. Romani, D.J. Schiffrin, J. Am. Chem. Soc. 133 (2011) 19432–19441. [6] J.S. Jirkovsky´, I. Panas, S. Romani, E. Ahlberg, D.J. Schiffrin, J. Phys. Chem. Lett. 3 (2012) 315–321. [7] T.J. Schmidt, V. Stamenkovic, M. Arenz, N.M. Markovic, P.N. Ross Jr., Electrochim. Acta 47 (2002) 3765–3776. [8] Z. Dursun, S. Ulubay, B. Gelmez, F.N. Ertas, Catal. Lett. 132 (2009) 127–132. [9] L. Qian, X. Yang, Talanta 74 (2008) 1649–1653. [10] M.H. Shao, T. Huang, P. Liu, J. Zhang, K. Sasaki, M.B. Vukmirovic, R.R. Adzic, Langmuir 22 (2006) 10409–10415. [11] C. Koenigsmann, E. Sutter, R.R. Adzic, S.S. Wong, J. Phys. Chem. C 116 (2012) 15297–15306. [12] A. Damjanovic, V. Brusic´, Electrochim. Acta 12 (1967) 1171–1184. [13] H. Naohara, S. Ye, K. Uosaki, J. Phys. Chem. B 102 (1998) 4366–4373. [14] H. Naohara, S. Ye, K. Uosaki, J. Electroanal. Chem. 473 (1999) 2–9. [15] L.A. Kibler, M. Kleinert, R. Randler, D.M. Kolb, Surf. Sci. 443 (1999) 19–30. [16] L.A. Kibler, M. Kleinert, D.M. Kolb, Surf. Sci. 461 (2000) 155–167. [17] L.A. Kibler, M. Kleinert, V. Lazarescu, D.M. Kolb, Surf. Sci. 498 (2002) 175–185. [18] H. Naohara, S. Ye, K. Uosaki, Electrochim. Acta 45 (2000) 3305–3309. [19] L. Xiao, L. Zhuang, Y. Liu, J. Lu, H.D. Abruna, J. Am. Chem. Soc. 131 (2009) 602– 608. [20] S. Kondo, M. Nakamura, N. Maki, N. Hoshi, J. Phys. Chem. C 113 (2009) 12625– 12628. [21] A. Hitotsuyanagi, S. Kondo, M. Nakamura, N. Hoshi, J. Electroanal. Chem. 657 (2011) 123–127. [22] I. Srejic´, M. Smiljanic´, B. Grgur, Z. Rakocˇevic´, S. Štrbac, Electrochim. Acta 64 (2012) 140–146. [23] A. Sarapuu, A. Kasikov, N. Wong, C.A. Lucas, G. Sedghi, R.J. Nichols, K. Tammeveski, Electrochim. Acta 55 (2010) 6768–6774. [24] H. Erikson, A. Kasikov, C. Johans, K. Kontturi, K. Tammeveski, A. Sarapuu, J. Electroanal. Chem. 652 (2011) 1–7. [25] H. Erikson, A. Sarapuu, K. Tammeveski, J. Solla-Gullón, J.M. Feliu, Elecrochem. Commun. 13 (2011) 734–737. [26] H. Erikson, A. Sarapuu, N. Alexeyeva, K. Tammeveski, J. Solla-Gullón, J.M. Feliu, Electrochim. Acta 59 (2012) 329–335. [27] M. Shao, T. Yu, J.H. Odell, M. Jin, Y. Xia, Chem. Commun. 47 (2011) 6566–6568. [28] C.-L. Lee, H.-P. Chiou, C.-R. Liu, Int. J. Hydrogen Energy 37 (2012) 3993–3997. [29] C.-L. Lee, H.-P. Chiou, Appl. Catal. B 117–118 (2012) 204–211. [30] Y.-F. Yang, Y.-H. Zhou, C.-S. Cha, Electrochim. Acta 40 (1995) 2579–2586. [31] L. Jiang, A. Hsu, D. Chu, R. Chen, J. Electrochem. Soc. 156 (2009) B370–B376. [32] L. Jiang, A. Hsu, D. Chu, R. Chen, J. Electrochem. Soc. 156 (2009) B643–B649. [33] G.F. Alvarez, M. Mamlouk, S.M. Senthil Kumar, K. Scott, J. Appl. Electrochem. 41 (2011) 925–937. [34] S.M. Senthil Kumar, J. Soler Herrero, S. Irusta, K. Scott, J. Electroanal Chem. 647 (2010) 211–221.

H. Erikson et al. / Journal of Electroanalytical Chemistry 691 (2013) 35–41 [35] J.J. Salvador-Pascual, S. Citalan-Cigarroa, O. Solorza-Feria, J. Power Sources 172 (2007) 229–234. [36] K. Jukk, N. Alexeyeva, C. Johans, K. Kontturi, K. Tammeveski, J. Electroanal. Chem. 666 (2012) 67–75. [37] S. Chakraborty, C.R. Raj, Carbon 48 (2010) 3242–3249. [38] J.-S. Ye, Y.-C. Bai, W.-D. Zhang, Microchim. Acta 165 (2009) 361–366. [39] H. Erikson, M. Liik, A. Sarapuu, J. Kozlova, V. Sammelselg, K. Tammeveski, Electrochim. Acta 88 (2013) 513–518. [40] J. Solla-Gullon, V. Montiel, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 491 (2000) 69–77. [41] F. Maroun, F. Ozanam, O.M. Magnussen, R.J. Behm, Science 293 (2001) 1811– 1814.

41

[42] M. Grden, M. Lukaszewski, G. Jerkiewicz, A. Czerwinski, Electrochim. Acta 53 (2008) 7583–7598. [43] A.J. Bard, L.R. Faulkner, Electrochemical Methods, second ed., Wiley, New York, 2001. [44] R.R. Adzic, J. Wang, B.M. Ocko, Electrochim. Acta 40 (1995) 83–89. [45] D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, 2001. [46] R.E. Davis, G.L. Horvath, C.W. Tobias, Electrochim. Acta 12 (1967) 287–297. [47] M.H. Seo, S.M. Choi, H.J. Kim, W.B. Kim, Electrochem. Commun. 13 (2011) 182– 185. [48] M.H. Shao, K. Sasaki, P. Liu, R.R. Adzic, Z. Phys. Chem. 221 (2007) 1175–1190.