Electrochemical oxidation of ethanol in acid media on titanium nitride supported fuel cell catalysts

Electrochimica Acta 56 (2011) 3549–3554

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Electrochemical oxidation of ethanol in acid media on titanium nitride supported fuel cell catalysts M.M. Ottakam Thotiyl, S. Sampath ∗ Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, Karnataka 560012, India

a r t i c l e

i n f o

Article history: Received 30 June 2010 Received in revised form 22 December 2010 Accepted 28 December 2010 Available online 8 January 2011 Keywords: Platinized titanium nitride Electrocatalyst Ethanol oxidation Metal–support interaction

a b s t r a c t In the present study, titanium nitride, TiN that possesses good electronic conductivity, high corrosion resistance combined with the ability to support metallic particles, has been used to anchor Pt catalysts and subsequently used for ethanol oxidation. Platinum deposited on TiN (Pt–TiN) surface is contrasted with the conventional support material, Vulcan carbon for the electrochemical oxidation of ethanol in acidic medium. Though the comparison is not straight forward due to different morphology/particle size of the Pt catalyst on the two supports, the present investigations reveal that the TiN support lead to surface Ti–OH type functional groups that help in reducing the accumulation of carbon monoxide on the catalyst surface. The Tafel slopes are similar but the exchange current density on Pt–TiN is approximately twice that of the value observed on Pt–C. X-ray photoelectron spectroscopy data support the long term stability and electrocatalytic activity of Pt–TiN electrocatalyst. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Low molecular weight alcohols like methanol, ethanol, etc. that are amenable for complete oxidation have been extensively used in direct alcohol fuel cells [1–5]. However, due to the toxicity of methanol [6], its next higher analogue, ethanol has received considerable attention as a promising fuel for low temperature applications. Ethanol is considered a renewable fuel because it can be easily obtained in large quantities from renewable sources, such as sugar cane [7,8]. Ethanol is also very promising in terms of high theoretical energy density values (8 kWh kg−1 against 6.1 kWh kg−1 for methanol) [9,10]. Reduced cross-over to the cathode compartment is an added advantage of ethanol as the fuel of choice [11,12]. Even though ethanol is projected as a promising fuel, its complete electrochemical oxidation involves 12 electrons and the pathways seem quite complicated. This is mainly because of the required C–C bond cleavage (which is absent in methanol) and hence complete oxidation requires very high over potentials which is practically difficult [11,13–16]. The incomplete oxidation results in intermediates like CO and –CHO that block the catalytic sites severely [11]. These issues are mainly addressed by alloying the main electrocatalyst, Pt with a second metal such as Sn and Ir to reduce poisoning effect [11–16]. However, this makes the Pt-based catalyst very expensive.

∗ Corresponding author. Tel.: +91 80 22933315. E-mail address: [email protected] (S. Sampath). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.12.091

Another aspect that greatly influences the performance of an electrocatalyst is the nature of the support material. For example, carbon support undergoes severe corrosion during fuel cell operation resulting in the aggregation or coagulation of Pt nanoparticles apart from its loss due to poor adhesion on the corroded support [17–21]. The support material should provide a good platform for Pt particles in corrosive media which is often encountered in fuel cells. In this study, we follow the characteristics of stable, extremely corrosion resistant titanium nitride (TiN) as an active support material for catalysts such as Pt and PtRu for ethanol oxidation in acidic medium. Titanium nitride (TiN) belongs to the category of conducting ceramic materials which is widely used in abrasive coatings [22,23]. Its very good diffusion barrier characteristics make it an interesting component in electronic industry [24,25]. Even though this conducting and biocompatible [26] material has a highly reproducible surface for electron transfer, the reports on its use in electrochemical studies are only few. It has been used in supercapacitors [27]; as a conducting support for electrodeposition of metals such as Pt, Zn, Cu and Ag [28–30]; as a pH sensor [31]; as a remedy for biofouling [32] and in electroanalysis [33]. We have recently reported that Pt particles anchored on TiN thin films are stable and very good catalysts for methanol oxidation in acidic media [34]. In another recent study, Pt–TiN is projected as a promising material for electrochemical reduction of oxygen in sulphuric acid medium [35]. In the present study, TiN is used for anchoring fuel cell catalysts like Pt and PtRu and subsequently used for ethanol oxidation. Various physicochemical and electrochemical techniques such as

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XPS, reflectance absorption IR (RAIR) spectroscopy, atomic force microscopy, Kelvin probe microscopy and electrochemistry have been used in the present study. The performance of TiN supported catalyst is compared with the corresponding Vulcan carbon supported catalyst. It is found that TiN helps in scavenging CO formed on Pt sites and promotes ethanol oxidation. 2. Experimental 2.1. Materials All the reagents and chemicals used were of analytical grade. H2 PtCl6 (Ranbaxy, India), ethanol (Qualigens, India), Pt black (particle size ∼5 nm, Aldrich, USA), Pt–Ru (50:50, Johnson Matthey, UK, particle size ∼4 nm), TiN powder (average size 3 ␮m, Aldrich, USA) and carbon powder (Vulcan XC 72, average size 40–50 nm) were used as received. De-ionized water was used for all experiments. All glasswares were cleaned with chromic acid, washed with de-ionized water, rinsed with acetone and dried prior to the experiments. 2.2. Preparation of TiN coating TiN was coated on thin flexible stainless steel sheets (SS-304) by cathodic arc deposition technique [31,32]. In the Multi-Arc chamber used for the deposition a base vacuum of 10−6 Torr was maintained before deposition. While deposition, reactive nitrogen gas pressure was maintained at 10 mTorr and the substrate voltage was −200 V. The deposition time was kept at 30 minutes and the thickness of the TiN coating on stainless steel was found to be 2–3 ␮m. 2.3. Preparation of TiN working electrode The working electrodes were made as follows. TiN coated SS sheets were cut into small pieces by high quality metal cutter. A Cu wire is attached to TiN pieces by spot welding technique and the exposed Cu wire and stainless steel areas were carefully glued by epoxy adhesive. Before the electrochemical measurements, TiN electrodes were degreased by cleaning with soft tissue soaked in absolute ethanol and washed with copious amount of de-ionized water. 2.4. Platinum deposition on TiN Chronopotentiometry technique was used for the electrodeposition of Pt from a solution of 100 mM H2 PtCl6 using TiN as the working electrode, a large area Pt foil as the auxiliary electrode and saturated calomel electrode (SCE) as the reference electrode. The amount of electrodeposited Pt was tuned by the charge passed through the cell, at a fixed current density of 5 mA/cm2 . The mass of deposited Pt is calculated using the charge passed during the deposition [36]. H2 reduction method was used for the preparation of platinised titanium nitride powder. About 200 mg of TiN powder was thoroughly mixed with aqueous solution of H2 PtCl6 to yield the required weight % of Pt on TiN. The solvent was allowed to evaporate overnight under constant stirring. The so obtained dry powder is ground well to make it fine powder and then treated at 350 ◦ C in presence of flowing H2 for 2 h in a tubular furnace to reduce platinum ions to Pt on TiN. The powder so obtained was sonicated in minimum amount of water with 5 wt% Nafion binder and the slurry was then used to coat the electrode surface [37]. Similar procedures were used for preparing Vulcan carbon supported Pt for comparative studies. A physical mixture of TiN powder with PtRu black was also used whenever PtRu was used for comparative studies. The TiN powder was mixed with PtRu black as a slurry in a

solvent. The powder was dried and sonicated in minimum amount of water. The slurry was then coated on the electrode surface using Nafion binder as mentioned earlier. A physical mixture of Pt powder and transition metal oxide was earlier reported to show good electroactivity for methanol oxidation [38]. Electrochemical measurements were carried out using a conventional three electrode system employing the Pt or PtRu coated electrode as the working electrode, a larger Pt foil as the auxiliary electrode and saturated calomel electrode (SCE), i.e. Hg/Hg2 Cl2 /KCl (saturated solution) as the reference electrode. The potential of the SCE electrode is 0.2412 vs. NHE [39]. 2.5. Characterization Pt–TiN, PtRu–TiN and TiN were characterized by X-ray diffraction (JEOL JDX 8030), X-ray photoelectron spectroscopy (XPS, Thermoscientific Multilab 2000), atomic force microscopy (Digital Nanoscope IVA, USA), scanning electron microscopy (FEI 200 kV, The Netherlands), electrochemical (EG&G PARC, 263A or CHI 660A) and spectroscopic techniques (Thermonicolet 6700 FTIR with liquid N2 cooled HgCdTe detector). Scanning Kelvin probe (SKPM, Digital Nanoscope IVA, USA) and conductive AFM (C-AFM, Digital Nanoscope IVA, USA) images were acquired using conducting Co–Cr probe. An appropriate sample bias with respect to the tip was applied for current measurements. 2.6. Electrochemical oxidation of ethanol using Pt–TiN Electrochemical measurements were carried out in a three electrode cell as mentioned earlier. The electrolyte consisted of 0.5 M ethanol + 0.5 M H2 SO4 . Cyclic voltammograms were recorded in the potential range, 0–0.85 V using TiN or carbon supported electrodes as the working electrode. The electrodes were kept in the electrolyte for several minutes for equilibration before the measurements. All the potentials in the present paper are referred to SCE electrode unless otherwise stated. All the experiments were carried out at 25 ± 0.2 ◦ C. 3. Results and discussion 3.1. Characterization of TiN and Pt–TiN The SEM of TiN powder along with the EDS pattern is shown in Fig. 1. The SEM pictures clearly reveal large particles of TiN and EDS confirms the presence of Ti and N. The X-ray diffraction (XRD) patterns of TiN powder as well as the film reveal highly oriented TiN (2 0 0) surface (2Â = 43.3◦ ) corresponding to cubic NaCl-type crystal lattice [40]. The diffraction pattern also shows additional reflections corresponding to (1 1 1), (2 0 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1) and (4 2 0) which are characteristics of TiN crystallizing in the face centered cubic pattern. The lattice constant calculated using Bragg’s equation is 0.4244 nm, which is in good agreement with the reported value (0.4241 nm, JCPDF card number, 38-1420). The Ti(2p3/2 ) region of TiN in the XPS spectrum is observed at binding energies in the range 454–460 eV [40]. Components corresponding to TiN, titanium oxynitride and TiO2 are clearly observed [41]. The Ti(2p1/2 ) signal observed at binding energies in the range 460–464 eV, possesses three components and are assigned to TiN, titanium oxynitride and TiO2 respectively, from low to high binding energies. Similar pattern comprising TiN, oxynitride and oxide is observed on TiN films prepared by cathodic arc deposition technique [31,32] which in turn suggest that there is an oxygen enriched surface that coexists with pure TiN phase. The N(1S) spectrum [40] shows components corresponding to nitridic nitrogen and oxynitride phases [41,42]. The low binding energy component stems from atomic nitrogen species [43].

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Fig. 1. (A) Scanning electron micrograph of titanium nitride powder. (B) Corresponding EDS pattern.

Fig. 2. (A) Scanning electron micrograph of platinized titanium nitride powder (Pt–TiN). (B) Corresponding EDS pattern.

Pt is deposited onto TiN powder by chemical reduction method as described in the experimental section. The SEM with EDS of platinum coated TiN (Pt–TiN) is shown in Fig. 2. Spherical Pt particles supported on large TiN particles can be clearly visualized in the SEM image. The average Pt particle size is found to be close to 200 nm. The presence of Pt is further confirmed by the EDS pattern. The physicochemical characterization reveals that TiN both in the powder form as well as in thin film form on SS substrate behave very similar. The Pt domains on TiN are further investigated by AFM coupled with scanning Kelvin probe (SKPM) microscopy and conductive AFM. Pt electrodeposited onto TiN thin film is used for the measurements. Since Pt is more conducting than TiN, the work function is expected to be relatively lower on Pt domains as compared to TiN domains and is indeed observed. Comparative analysis of topographical and work function images shows the Pt domains as dip in the work function image as reported earlier [44]. The dip domains are the zones where metallic Pt is present and is clearly observed in the topographic image. An interesting observation in SKPM image is the relative work function of Pt–TiN interface. The interface of support–catalyst clearly reveals different relative work function values and is ascribed to possible metal–support interaction. The metal–support interaction may lead to a change in the electronic environment at the interface which in turn may play a role in alleviating CO and –CHO poisoning of the catalyst surface.

dation. In the reverse scan, another anodic wave is observed. This is characteristic of ethanol oxidation in acidic media on Pt-based electrodes [11–16] and the anodic current observed in the reverse scan is generally attributed to the oxidation of intermediates formed in the forward scan that get adsorbed on the electrode surface and/or oxidation of ethanol on oxidized Pt surface. The mass activity of the catalysts Pt–TiN and Pt–C are shown in the cyclic voltammograms (Fig. 3). Mass activity for Pt loaded onto TiN is found to be higher than that observed for Pt on carbon support. The onset of ethanol oxidation is negatively shifted by at least 100 mV on Pt–TiN as compared to Pt–C (Table 1). The peak currents are more than doubled when Pt is loaded onto TiN showing the positive effect of TiN support for Pt towards ethanol oxidation reaction. A good support should help in better dispersion of electrocatalysts apart

3.2. Electrochemical oxidation of ethanol It is found that bare TiN is inactive towards ethanol electrooxidation in acid medium (results not shown). A comparison of TiN support with a widely used Vulcan carbon support for Pt particles for the same amount of Pt loading is provided in Fig. 3. On both the electrodes (Pt–TiN and Pt–C), well-defined voltammetric peak is observed at 0.75 V in the forward scan due to ethanol electrooxi-

Fig. 3. Cyclic voltammograms of Pt–TiN and Pt–C electrodes in 0.5 M sulfuric acid containing 0.5 M ethanol at a scan rate of 10 mV/s. Loading of the catalyst is 1 mg of Pt/cm2 .

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Table 1 Voltametric parameters on Pt–TiN and Pt–C for ethanol oxidation based on Fig. 3. Electrode material

Onset potentiala (mV)

Peak potentiala (mV)

i (mA/mg)

EAA (cm2 /mg)b

j (mA/cm2 )c

Ipf /Ipb

Pt–TiN Pt–C

75 180

742 740

44.5 17.5

77.7 66.6

0.566 0.262

1.44 1.12

a b c

Potential vs. SCE. Active area. Obtained by dividing by EAA.

Fig. 4. Cyclic voltammograms of Pt–TiN and Pt–C powder in 0.5 M sulfuric acid containing at a scan rate of 100 mV/s. The loading of the catalyst is 180 ␮g of Pt/cm2 .

from providing favorable metal–support interaction [45,46]. The Pt deposition has been carried out by the same procedure on TiN as well as Vulcan carbon and the particle size range of Pt as observed by scanning electron microscopy remains very similar on the two supports (∼200 nm). The electrochemically active area of Pt on the two supports (TiN and Vulcan carbon) as observed in hydrogen desorption region [47] by voltammetry (Fig. 4, Table 1) is almost the same. Hence, any difference observed in the activity for ethanol oxidation stems from the support material. The normalized current density with respect to the electrochemically active area is found to be twice higher on Pt–TiN indicating the promoting effect of TiN on Pt towards ethanol oxidation. Since the forward peak in the voltammogram is associated with the oxidation of ethanol and the reverse peak stems mostly from the removal of accumulated intermediates during the forward scan, the ratio of forward oxidation current (Ipf ) to reverse oxidation current (Ipb ) is sometimes used to understand the anti-poisoning effect of the catalyst during alcohol oxidation. However, it should be noted that, the reversal voltage plays an important role in the ratio. Higher the ratio, better is the catalyst efficiency. The Ipf /Ipb is found to be 1.44 for Pt–TiN while it is 1.12 for Pt–C (Table 1). This indirectly points out that the CO tolerance of the Pt–TiN surface is better than that of Pt–C. Possible reasons are discussed later.

From a fuel cell point of view, a good fuel cell catalyst should have high current densities at low overpotentials. This is the main reason why PtRu–C and PtSn–C are considered very good catalysts for ethanol oxidation reaction as compared to Pt–C [48,49] though peak current densities are higher on the latter electrode. The voltammetric profile (Fig. 3) clearly reveals that Pt–TiN exhibits higher anodic currents at low over potentials. The long term stability can be understood by following the current–time transients based on chronoamperometry (I vs. t) at different overpotentials (Fig. 5). I vs. t transients (mass activity) recorded at low overpotentials of 0.35 V (rising part of the i–v curve) show that the stabilized currents on Pt–TiN are almost 3 times higher than that observed on Pt–C. The transients at still lower over voltages, 0.2 V where the extent of ethanol oxidation is low and CO poisoning of Pt can be severe, reveals better performance for Pt–TiN than that of Pt–C. The stabilized value of current is found to be almost 2.3 times higher on Pt–TiN electrode as compared to Pt–C. It should be noted that at low overpotentials, the reaction kinetics is likely to be limited by electron transfer and therefore I vs. t transients display the true catalytic activity of the electrodes. Therefore, high current values observed on TiN supported catalysts at low overpotentials clearly confirm the reduced poisoning effect. Polarization measurements are carried out on Pt–TiN and Pt–C electrodes to understand the kinetics of the reaction (Fig. 6). The experiments are carried out by very slow scanning of the potential (0.5 mV/s) of the working electrode from the open circuit value (OCV) to 100 mV positive value. The slopes are found to be 120 mV on Pt–TiN and 105 mV on Pt–C which are similar to the ones reported on other Pt-based electrodes [50,51]. From the observed Tafel slope values, it can be presumed that the first electron transfer is the rate limiting step [50]. Similar Tafel slopes on the two supports indicate that mechanism of electrooxidation is probably the same on both. Very importantly, the exchange current density is found to be 30 ␮A/cm2 on the Pt–TiN and 12 ␮A/cm2 on Pt–C electrodes respectively. A 2 times increase in the exchange current density suggests that rate of ethanol electrooxidation is higher on Pt–TiN than that on Pt–C. To further understand the nature of TiN support, commercially available PtRu (50:50) catalyst is loaded on to TiN support and the performance compared with the corresponding Vulcan carbon supported catalysts (Fig. 7). As observed earlier for Pt–TiN and Pt–C,

Fig. 5. Chronoamperometry traces on Pt–TiN and Pt–C electrodes at 0.35 V (A) and 0.2 V (B) in 0.5 M sulfuric acid containing 0.5 M ethanol. Loading of the catalyst is 1 mg of Pt/cm2 .

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Fig. 6. Tafel plots on Pt–C (open circles) and Pt–TiN (closed stars) in 0.5 M sulfuric acid containing 0.5 M ethanol at a scan rate of 0.5 mV/s. Loading of the catalyst is 1 mg of Pt/cm2 .

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Fig. 9. Tafel plots on PtRu–C (open circles) and PtRu–TiN (closed stars) in 0.5 M sulfuric acid containing 0.5 M ethanol at a scan rate of 0.5 mV/s. Loading of the catalyst is 1 mg of Pt/cm2 .

3.3. XPS and RAIR analysis

Fig. 7. Cyclic voltammograms of PtRu–TiN and PtRu–C electrodes in 0.5 M sulfuric acid containing 0.5 M ethanol at a scan rate of 10 mV/s. Loading of the catalyst is 1 mg of Pt/cm2 .

the bimetallic PtRu–TiN shows better characteristics than that of PtRu–C. Interestingly, the capacitive regions on both the electrodes are found to be very similar and hence the electrochemically active areas of Pt particles are the same on both supports. It is found that peak currents are higher on PtRu loaded on to TiN than when it is loaded on to carbon. The I vs. t transients recorded at different over potentials reveal similar characteristics as observed earlier (Fig. 8). The Tafel slopes deduced from polarization studies are 131 mV and 145 mV for PtRu–TiN and PtRu–C electrodes respectively (Fig. 9). The exchange current densities are found to be 46 ␮A/cm2 and 31 ␮A/cm2 on PtRu–TiN and PtRu–C respectively.

The XPS data for Pt–TiN surface before and after 200 cycles of electrochemical oxidation of ethanol (not shown) show similar behaviour to that observed for the oxidation of methanol in alkaline media [44]. The observations point to Pt in metallic state [52,53]. No change in the Pt oxidation state is observed even after 200 cycles of ethanol oxidation and the spectrum after oxidation exactly can be overlaid with the one before oxidation. This clearly confirms that the Pt particles are intact on TiN surface and TiN provides a very good platform for the catalytic particles. Additionally, voltammetric studies using standard ferrocyanide/ferricyanide redox couple show very little changes on Pt–TiN before and after ethanol oxidation thus confirming the observations of XPS studies. RAIR analysis of TiN before and after cycling the potential in ethanolic sulphuric acid shows a large increase in the intensity of the band at 3250 cm−1 (–OH stretching intensity) after potential cycling as reported earlier [34]. This ability of TiN to accumulate –OH groups on its interface makes it an attractive candidate to replace Ru in conventional PtRu electrocatalyst. Additionally, the ability of conducting TiN support to hold Pt particles for a number of cycles point to the fact that it can replace carbon support as well. The increased ethanol oxidation activity of TiN supported electrocatalysts could be due to a number of reasons. As described earlier, the metal–support interaction leads to flow of electrons from Pt to TiN making Pt slightly positive [44]. This electron deficient Pt may have d-orbital vacancies which will decrease the back donation to CO bonding orbital of the adsorbed CO formed during ethanol oxidation. The difference in work function observed at the interface of Pt–TiN supports the existence of significant metal–support interaction. Similar interactions have been reported when Ru, Fe, Co and Ni [54,55] are used along with Pt as bimetallic

Fig. 8. Current–time (I–t) transients recorded on PtRu–TiN and PtRu–C in 0.5 M sulfuric acid containing 0.5 M ethanol for different over potentials. Loading of the catalyst is 1 mg of Pt/cm2 .

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Scheme 1. Scheme of ethanol oxidation on Pt–TiN electrocatalysts.

catalysts. Electron deficiency on Pt may also enhance accumulation of hydroxyl groups on its surface which would help in the removal of adsorbed CO on Pt. It should be pointed out that Pt loaded on W, Zn, Mo, WC, etc. shows a decrease in CO desorption temperature [56–59]. The charge separation between the Pt and TiN is similar to that reported for sulphated Pt/Al2 O3 . This has been reported to result in C–H bond activation in the combustion of propane [60] on Pt/Al2 O3 catalyst. Similar mechanism may operate here as well. The ability of TiN to decompose water to form Ti–OH type groups at low over potentials, will help oxidize the adsorbed poisonous intermediates on Pt [61,62]. Based on the above arguments, Scheme 1 is suggested for ethanol oxidation on Pt–TiN interface. 4. Conclusions Electrochemical oxidation of ethanol on Pt and PtRu supported on TiN in sulfuric acid medium shows higher efficiency than the corresponding conventional carbon supported catalysts. This may have very positive implications in direct ethanol fuel cells in acid medium. References [1] C. Lamy, S. Rousseau, E.M. Belgsir, C. Coutanceau, J.-M. Léger, Electrochim. Acta 49 (2004) 3901. [2] E. Peled, T. Duvdevani, A. Aharon, A. Melman, Electrochem. Solid-State Lett. 4 (4) (2001) A38. [3] C. Lamy, A. Lima, V. Le Rhun, F. Delime, C. Coutanceau, J.-M. Léger, J. Power Sources 105 (2002) 283. [4] M. Huang, L. Li, Y. Guo, J. Solid State Electrochem. 13 (2009) 1403. [5] E.V. Spinacé, A.O. Neto, M. Linardi, J. Power Sources 129 (2004) 121. [6] E.V. Spinace, A.O. Neto, M. Linardi, J. Power Sources 124 (2003) 426. [7] C. Cosmi, M. Macchiato, L. Mangiamele, G. Marmo, F. Pietrapertos, M. Salvia, Energy Policy 31 (2003) 443. [8] U. Wachsmann, M.T. Tolmasquim, Renew. Energy 28 (2003) 1029. [9] W.J. Zhou, S.Q. Song, W.Z. Li, Z.H. Zhou, G.Q. Sun, Q. Xin, S. Douvartzides, P. Tsiakaras, J. Power Sources 140 (2005) 50. [10] A. Caillard, C. Coutanceau, P. Brault, J. Mathias, J.-M. Léger, J. Power Sources 162 (2006) 66. [11] S.Q. Song, W.J. Zhou, Z.X. Liang, R. Cai, G.Q. Sun, Q. Xin, V. Stergiopoulos, P. Tsiakaras, Appl. Catal. B 55 (2005) 65. [12] C.Y. Wang, Chem. Rev. 104 (2004) 4727. [13] Z.B. Wang, G.P. Yin, Y.G. Lin, J. Power Sources 170 (2007) 242. [14] F. Vigier, C. Coutanceau, F. Hahn, E.M. Belgsir, C. Lamy, J. Electroanal. Chem. 563 (2004) 81.

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