Innovative electrodes for direct methanol fuel cells based on carbon nanofibers and bimetallic PtAu nanocatalysts

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 1 6 0 1 e2 1 6 1 2

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Innovative electrodes for direct methanol fuel cells based on carbon nanofibers and bimetallic PtAu nanocatalysts L. Giorgi a,*, E. Salernitano b, Th. Dikonimos Makris c, S. Gagliardi c, V. Contini c, M. De Francesco d a

Materials Science & Electrochemistry, Via Mantova 11, 00042 Anzio, Rome, Italy ENEA, Technical Unit on Material Technologies, Faenza Laboratories, Via Ravegnana 186, 48018 Faenza, RA, Italy c ENEA, Technical Unit on Material Technologies, Casaccia Research Centre, Via Anguillarese 301, 00123 S. Maria di Galeria, Rome, Italy d ENEA, Technical Unit for Renewable Energy Sources, Casaccia Research Centre, Via Anguillarese 301, 00123 S. Maria di Galeria, Rome, Italy b

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abstract

Article history:

Direct methanol fuel cells are very promising power sources, but the easy poisoning of the

Available online 5 July 2014

platinum anode electrocatalyst by CO-like reaction intermediates, restricts their industrial application and commercialization. The development of Pt-based alloys or bimetallic catalysts,

Keywords:

in which the second metal acts as Pt poisoning inhibitor, is one of the main promising solutions

Direct methanol fuel cell

to this problem. In this work we have combined the use of unconventional methods to deposit

Carbon nanofibers

the catalyst nanoparticles with unconventional carbon supports. Innovative electrodes made

Electrodeposition

of platelet carbon nanofibers, directly grown on graphite paper, as substrate for electro-

Bimetallic catalyst

deposited platinum and gold bimetallic nanoparticles, have been developed. These electrodes

Electrocatalytic activity

allow having a single layer with both the diffusive and catalytic function, and a considerable decrease of noble metals amount (about five times), with consequent large cost reduction. Moreover, the replacement of the conventional ink deposition methods with electrodeposition for platinum and gold dispersion, considerably increases the catalytic activity. The electrocatalytic performance results were encouraging. Gold allows increasing the catalyst poisoning tolerance and then the electrode long term stability. The innovative electrodes show a performance improvement up to three times compared to a commercial carbon substrate electrode (Vulcan XC-72R) with ink-spray deposited PtRu nanoparticles as catalyst. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Direct methanol fuel cells (DMFC) are considered as one of the most promising power sources because they offer a highly

efficient and environmental-friendly technology for energy conversion, especially attractive for mobile and portable applications [1]. With respect to other fuel cells systems, DMFC offer improved logistics because methanol is a liquid fuel at

* Corresponding author. Tel.: þ39 3284360938; fax: þ39 06 9844047. E-mail address: [email protected] (L. Giorgi). http://dx.doi.org/10.1016/j.ijhydene.2014.06.053 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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room temperature and atmospheric pressure and therefore easy to store and transport. The use of DMFC does not require preprocessing modules such as external reformers. Methanol has high energy density (theoretically about 6 kWh kg1) [2] and DMFC are characterized by low operative temperature, quick recharge of fuel, light weight and simple design. However, competitive industrial application and wide commercialization of DMFC is still restricted by the easy poisoning of the platinum anode electrocatalyst by CO-like reaction intermediates [3], which implies the need of high Pt load (with consequent increase of cost) in order to obtain reasonable performances. Two approaches can be followed to overcome these obstacles: on one side increasing the Pt utilization and electrode performance with the aim to reduce the catalyst load, on the other enhancing the catalyst poisoning tolerance in order to improve the electrode long term stability. The combination of unconventional methods to deposit the catalyst nanoparticles only on the uppermost surface of the electrode with unconventional carbon supports seems to be promising for both the approaches [4,5]. Carbon nanofibers (CNF) have been proved to be a promising substrate for electrocatalyst [6e8], especially the platelet morphology (pCNF). This CNF type has the graphene layers oriented perpendicularly to the fiber axis [9,10] and, as a consequence, is characterized by high surface area and a great number of plane edges with high energy. Such a structure allows a good dispersion of catalyst nanoparticles, which are strongly anchored to the substrate with negligible coalescence, with a gain in the catalyst utilization and electrocatalytic activity with respect to the commercial electrodes [11e13]. The platelet CNF surface has some containing oxygen functional groups contributing to the oxidation of some poisoning products of methanol oxidation, thus improving the electrode stability. Our previous paper [13] shows that electrodes made of platelet carbon nanofibers, directly grown on graphite paper by plasma enhanced chemical vapor deposition, as substrate for electrodeposited platinum nanoparticles exhibited good and encouraging results in terms of electrochemical performance and stability, both higher then a commercial carbon substrate electrode (Vulcan XC-72R). These electrodes allow having a single layer with both the diffusive and catalytic function and a considerable decrease of noble metal amount with consequent large cost reduction. As a matter of fact, the electrodeposition places the metal nanoparticles only on the exposed surface of the electrode providing a full exploitation of the catalytic properties and then a remarkable reduction of the Pt load, but a significant increase of its utilization. The fuel cell cost, mainly resulting from the use of platinum as catalyst, still remains the main obstacle to the DMFC industrial production and penetration into the market. Moreover, the not yet adequate long term stability, due to platinum CO induced poisoning, implies the need of high catalyst load. So, in addition to the use of innovative carbon based electrocatalyst substrates and unconventional methods to deposit the catalyst nanoparticles, a further decrease in the noble metal amount, still ensuring high electrocatalytic performance over time, can be achieved by increasing the poisoning tolerance of electrocatalyst. Great

efforts have been made in recent years to develop Pt-based bimetallic catalysts or alloys, whose properties often significantly differ from those of the constituting metals, offering better long term stability [14e16]. The metals most widely matched to platinum for this purpose are transition metals (Ru, Pd, Ag, Au, etc.). The second metal in the reaction mechanism called “bi-functional” preserves the platinum from CO poisoning since it forms chemical species containing oxygenated groups (metal-OH) to lower potential than platinum [17]. These chemical species promote the oxidation of the CO adsorbed on platinum to CO2, reactivating the Pt active sites and allowing adsorption/oxidation of fuel (hydrogen or methanol) [18e21]. The catalytic properties of Pt-containing nanostructures are strongly dependent on their morphology (size and/or shape) and composition. The morphology effect, arising from differences in the surface electronic structure of metallic nanoparticles, can lead to considerable increase in Pt activity for a specific reaction by selective exposure of certain Pt surface structures [22,23]. Combining Pt with other metal elements to form bi- or multi-metallic nanostructures could be efficient to tune the structure and catalytic activity of Pt [24,25]. Surface science studies have shown that the creation of a hetero-(metalemetal) bond in well-defined Pt surfaces may induce significant changes in the electronic structure and catalytic property of Pt [26,27]. Recently, nanostructures based on Pt deposited onto Au nanoparticles were shown to be viable for making good use of Pt for anodic electrocatalysis [28e31]. Pt dispersion or utilization efficiency (exposed percentage of Pt atoms or UPt) in these nanostructures may be enhanced with proper control of Pt loading and Au particle size. In this work, innovative electrodes based on platelet carbon nanofibers and bimetallic platinumegold nanoparticles are developed. pCNF are directly grown on graphite paper by plasma enhanced chemical vapor deposition (PECVD), using methane and hydrogen as precursors. Bimetallic PtAu nanoparticles were deposited on pCNF by electrodeposition. The choice of gold as the second metal was motivated by its abundance in nature, three times higher than that of Ru, normally used in Pt-based alloy electrocatalysts, and by its high catalytic activity at the nanoscale, dependent on the particle size and the interaction with the substrate [32]. The use of gold in combination with platinum gave interesting results with respect to alcohols oxidation and oxygen reduction [28,33e36]. Many past studies have therefore been directed to the development of Au-based catalysts to be used in low temperature combustion processes (partial hydrocarbons oxidation, hydrogenation of unsaturated hydrocarbons, etc.). It has also been shown that nanostructured gold exhibits high activity towards carbon monoxide oxidation [37]. These aspects make interesting the possibility of using platinum in combination with gold in low temperature fuel cells. Moreover, gold has an electronic structure very similar to that of platinum and it is inert in acidic electrolytes [38]. Finally, the performances of the PtAu/pCNF electrodes are compared to those of a commercial carbon substrate electrode (Vulcan XC-72R) with ink-sprayed PtRu nanoparticles as catalyst, showing better performance.

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Experimental

catalyst deposition. A N2 atmosphere was maintained over the electrolyte during the electrodeposition.

pCNF growth

Characterization

The pCNF growth substrates consisted of graphite paper disks Papyex (Groupe Carbone Lorraine) with 10 mm diameter and 0.4 mm thickness, catalyzed by electrodeposited nickel clusters using a 0.5 M NiCl2∙6H2O solution with de-ionized water (pH ¼ 3 adjusted by suitable HCl addition) [39]. A disk of graphite paper was used as working electrode, the counterelectrode was a Pt wire and a Saturated Calomel Electrode (SCE) was used as reference. The Ni electrodeposition was potentiostatically carried out in a three electrodes Pyrex cell purged with N2 at 30  C. A N2 atmosphere was maintained over the electrolyte during the electrodeposition. The potentiostatic experiments were performed by a PAR potentiostat model 273 A, in remote control. Different deposition conditions were tested in order to optimize the dimension, morphology and density of Ni clusters. The optimal process parameters were 0.75 V vs. SCE potential and a duration that deposits 100 mC. The pCNF growth was performed into a “home built” original PECVD reactor [40], consisting of a vacuum sealed quartz chamber, coaxial to a tube furnace, containing the substrate support and a twin electrode system. The reactor design allows activating a DC glow discharge plasma inside the hot zone of the quartz tube chamber and uses the reactor chamber itself as an electric break. The chamber was first pumped down to rotary pump vacuum, and then the furnace was heated up to the process temperature in H2 stream. A high voltage was then applied between the electrodes, and when stable in hydrogen, the gas flow was switched to the process gas mixture. A needle valve was used to adjust the process pressure to the desired value. The gaseous precursors were CH4 and H2 in the fixed flow ratio 7:1 and the process parameters were the following: total flow rate 480 sccm, process pressure 50 mbar, process temperature 780  C and process duration 30 min [13,41].

The electrodes morphological analysis was performed by Field Emission Gun Scanning Electron Microscopy (FEG-SEM LEO mod.1530) equipped with a high-resolution secondary electrons detector (in-lens detector). The deposited catalyst structure was analyzed by X-Ray Diffraction (XRD). XRD patterns were obtained using a Cu Ka radiation in a BraggeBrentano powder diffractometer equipped with a graphite monochromator positioned in the diffracted beam. The catalyst nanoparticles stoichiometry was calculated by X-Ray Photoelectron Spectroscopy (XPS) and Energy Dispersive XRay analysis (EDX). XPS spectra were acquired by a V.G. ESCALAB MKII using a Mg anode (Ka radiation line at 1253.6 eV unmonochromatised) as X-ray source, operating at a voltage of 10 kV and a power of 240 W, with a scanning area of about 5 mm2. During the measurements the pressure in the ultra high vacuum chamber was lower than 5  107 mbar. Survey spectra were acquired at pass energy of 50 eV and step channel of 0.5 eV, while detailed spectra at pass energy of 20 eV and step channel of 0.1 eV. The spectrometer was operated in constant analyzer energy (CAE) mode during all acquisitions. EDX spectra were collected by a Shimadzu spectrometer mod. EDX-720 RayeNy. The EDX analysis was performed on different positions of sample surface and on areas of different size. The electrocatalytic performance and the long term stability of the electrodes were investigated using cyclic voltammetry (CV). CV measurements were carried out in the same electrochemical setup used for PtAu electrodeposition. The electrochemical behavior of the PtAu/pCNF electrodes was studied by CV in H2SO4 1 M, purged with N2, at a potential scan rate of 100 mV s1. The electrocatalytic activity of PtAu/ pCNF electrodes was tested by measuring the oxidation of methanol in CH3OH 0.5 M þ H2SO4 1 M, purged with N2, at 100 mV s1. The potential window between 0.3 V and 1.3 V vs. SCE was used in every acquisition. Pt/CNF and Au/CNF electrodes were considered as reference.

Pt and Au electrodeposition A galvanostatic electrodeposition (GED), at applied charge range between 300 and 750 mC for an electrolysis time of 150 s, was carried out in a three electrodes Pyrex cell purged with N2 at 30  C using solutions of Pt and Au precursors in H2SO4 1 M. The tested solutions were H2PtCl6 2.5 mM þ AuCl3 2.5 mM (denoted as 2.5Pt2.5Au) and H2PtCl6 4 mM þ AuCl3 1 mM (denoted as 4Pt1Au); solutions of only H2PtCl6 5 mM (denoted as 5Pt) and AuCl3 5 mM in H2SO4 1 M (denoted as 5Au) were considered as reference. The working electrode was a graphite paper disk completely and uniformly covered by dense pCNF, inserted in a Teflon holder with an exposing area of 0.79 cm2, the counter electrode was a high purity (99.9999%) graphite rod placed in front of the working electrode, and the reference electrode was a saturated calomel Hg/Hg2Cl2 electrode (SCE). The electrodeposition was carried out by using a potentiostategalvanostat EG&G PAR mod.273 A controlled with dedicated software (CorrWare, Scribner Inc.). The electrolyte solution was bubbled with N2 gas for 30 min before every

The platinum and gold loads were determined by a spectrophotometric method, after metal dissolution in boiling HNO3/HCl (1:3 v/v). The sample absorption spectra were recorded between l ¼ 380 and 700 nm using a Perkin Elmer UV-VIS-NIR mod. 330 and a mathematical method (using Matlab© software) was implemented to determine the metals amount.

Results and discussion Electrodeposition of Pt and Au Platelet carbon nanofibers, with graphene layers oriented perpendicular to the fiber axis, were grown on graphite paper substrates catalyzed by electrodeposited nickel clusters. Plasma enhanced chemical vapor deposition, using methane and hydrogen as precursors, allowed to obtain pCNF with high

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Fig. 1 e TEM (a) and FEG-SEM ((b): high magnification; (c): low magnification) micrographs of platelet carbon nanofibers (pCNF) grown by plasma enhanced chemical vapor deposition (PECVD). Process parameters: CH4/H2 in the fixed flow ratio 7:1, total flow rate 480 sccm, process pressure 50 mbar, process temperature 780  C, process duration 30 min.

density, narrow diameter distribution (average diameter of about 100 nm) and good quality, i.e. high purity, and controlled morphology (Fig. 1). pCNF were use as electrocatalyst substrate after a cleaning/activation treatment by CV in H2SO4 1 M. The cleaning/activation treatment consists of a cyclic voltammetry (CV) in the potential range from 0.3 to þ1.3 V vs. SCE at a scan rate of 100 mV s1. The CV was repeated up to 50 times to clean the surface of the electrode and activate the nanofibers by producing oxygenated species on their surface [13]. In the literature, only very few papers report on the coelectrodeposition of Pt and Au and generally it is not provided evidence of the formation of an alloy or the presence of two separate components, but only the deposit catalytic behavior is analyzed [42e44]. Our PtAu/pCNF electrodes were prepared by single-pulse galvanostatic electrodeposition according to the same procedure described in details in a previous paper for Pt catalyst [13]. When two different metals are present in the electrolyte solution, the reduction of the species does not occur simultaneously. The standard redox potentials E for the platinum and gold couples are [45,46]: 

PtCl6 þ 4e /Pt þ 6Cl 2





AuCl4 þ 3e /Au þ 4Cl

E ¼ þ0:74 V vs: NHE E ¼ þ0:99 V vs: NHE

(1) (2)

Thus, gold ions should be reduced before platinum ions. Once the initial nucleation has occurred, the growth of the

existing particles is favored over the formation of new particle nuclei [47]. For sufficiently negative potentials, Au and Pt are deposited simultaneously. Probably, each AuePt bimetallic nanoparticle consists of several microclusters that have a modified Au core. Au and Pt atoms are deposited on the surface of this Au core to form AuePt bimetallic clusters [48]. The electrodeposition of both metals is a multistep process. Gold requires more induction time but then its kinetics significantly increases with respect to platinum because of the higher reduction potential and the need of only three electrons to be reduced in metallic form. 

PtCl6 þ 2e /PtCl4 þ 2Cl 2

2



PtCl4 þ 2e /PtY þ 4Cl 2





(4) 

AuCl4 þ 2e /AuCl2 þ 2Cl 



AuCl2 þ e /AuY þ 2Cl

(3)

(5) (6)

Both the solutions of Pt and Au precursors give rise to similar trend of chronopotentiometric plots: the increase of the deposition charge results in a faster kinetics reducing the time needed to reach the final potential. The comparison of the electrodeposition curves from solutions with different metals content, at constant deposition charge, highlights that the addition of gold in the electrolyte results in longer initial induction time (nucleation of a new phase) followed by faster kinetics and lower final potential, in accordance with Au chronopotentiometric plot (Fig. 2). At the

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potential plateau the hydrogen evolution takes place on the deposited catalyst. Pt and Au codeposit on the nanofibers with different rate due the different kinetics.

SEM analysis The different electrodeposition kinetics results in a different deposit morphology. The comparison was carried out on glassy carbon (GC) substrate, with smooth and mirror surface, not affecting in any way the electrodeposition process. SEM analysis shows that Pt deposit is characterized by nanoparticles with dendritic morphology and highly nanostructured surface, while Au is deposited in the form of globular nanoparticles with negligible surface nanostructuring (Fig. 3). The same morphology difference is observed when pCNF are used as substrate for Pt and Au electrodeposition. The coelectrodeposition of platinum and gold, depending on the electrolyte solution composition and electrodeposition charge, gives rise to different deposit morphologies from very faceted to more globular nanoparticles, but always with a great nanostructuring surface, that increases the metals exposed surface (Fig. 4). The deposit is uniformly distributed on the electrode surface, but the nanoparticles have not been deposited even in the nanofibers mat porosity under all tested conditions.

were considered as reference because of the difficulty to have a perfect sample alignment during the spectrum acquisition due to the nanofibers entanglement. The cell parameter was found to be 0.39004 nm for Pt nanoparticles and 0.40743 nm for Au deposition. The slight difference with respect to the theoretical values (0.39231 nm for Pt and 0.40786 nm for Au) may be structural, due to small deformations of the crystal lattice, or instrumental, attributable to small quota variations because the sample does not have a flat surface. The specific peaks of the two metals are clearly distinguishable in the spectra acquired for the PtAu/pCNF electrodes, demonstrating that the obtained deposit is not an alloy but is constituted by bimetallic nanoparticles (Fig. 5). It is predominant the peak of the metal whose content in the deposit is higher, as confirmed by stoichiometry derived by XPS and EDX analysis (Table 1). XPS and EDX quantitative analysis reveal always a Pt/Au atomic ratio lower than that of the starting electrolyte solutions, confirming a faster Au electrodeposition kinetics compared to that of platinum. XPS is a surface analysis instead EDX is a bulk one and the comparison between the deposit composition obtained by the two techniques reveals a discrepancy indicative of a metal nanoparticles surface

Structural and surface analysis XRD analysis was used to verify if the electrodeposited nanoparticles were made of a single lattice structure, as a result of the formation of an alloy, or showed both Pt and Au crystalline structures, if the two deposition processes were independent and the deposit was then constituted by bimetallic nanoparticles. pCNF, Pt/pCNF and Au/pCNF electrodes

Fig. 2 e Typical chronopotentiometric plots for the galvanostatic electrodeposition of Pt, Au and PtAu on platelet carbon nanofibers (pCNF). (Overvoltage: difference between the actual electrode potential and that at zero current).

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Fig. 3 e FEG-SEM micrographs of Pt (a) and Au (b) galvanostatically electrodeposited on glassy carbon electrodes.

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Fig. 4 e Typical FEG-SEM micrographs of PtAu catalyst electrodeposited on platelet carbon nanofibers (pCNF). Fig. 5 e XRD spectra of platelet carbon nanofibers (pCNF), Pt, Au and PtAu catalyst electrodeposited on pCNF. composition different from that of the bulk. The electrocatalytic processes are surface phenomena; therefore the XPS data are more correlated to the catalytic performance. The applied charge (Qdep) has a noticeable effect on the surface Pt/Au ratio. Particularly, an increase of Qdep leads to a higher Pt content irrespective of Pt/Au ratio in the electrolyte. The Pt/Au ratio mostly increases with the charge deposition, consistently with an increase in the deposition rate of both metals, attenuating the kinetics differences. It is interesting to note a surface enrichment in gold in the electrode obtained from the 4Pt1Au solution, which can contribute to an increase in the platinum poisoning tolerance. The EDX analysis was used with the dual purpose to verify the bimetallic deposit uniformity on the electrodes exposed surface and to measure the catalyst stoichiometry. For this reason, the measurement of each electrode was carried out in three different areas: a central area with a diameter of 1 mm, a lateral area of a diameter of 1 mm and a central area with a diameter of 5 mm. The obtained results show that the best nanoparticles uniformity and homogeneity is observed in the case of the solution 4Pt1Au.

Chemical analysis The electrocatalytic performances of the PtAu/pCNF electrodes are expressed as the electrochemical active surface (EAS) and mass specific activity (MSA). For both values determination it is necessary to know the platinum loading (LPt), since gold is not directly involved in the hydrogen and methanol reactions. LPt is generally determined by spectrophotometric investigation of platinumetin complexes characterized by a great stability and color intensity [49]. The presence of Au makes the chemical analysis more difficult, as result of the formation of colloids after reduction of gold with tin. The AueSn colloids are thermodynamically unstable and subjected to spontaneous nanoparticles coalescence, responsible for great errors in the quantitative Au determination based on its absorbance [50]. Therefore a spectrophotometric analysis without Sn was

developed and a mathematical method was implemented to determine the metals amount. The method is based on the assumption that the Beer's law, stating that absorbance of electromagnetic radiation is directly proportional to analyte concentration, also applies to solutions of several absorbing species. The total absorbance is the sum of the individual absorbance, provided there is no interaction among the various species (dilute solutions). First of all the analytical curves (20 polynomial), approximating average experimental absorption spectra of Pt and Au 1 ppm solutions, were determined by least squares method. Then these curves were used in a least squares fitting for estimating the unknown platinum and gold concentrations, which minimize the deviation between the measured and calculated absorbance, according to the following equation: A ¼ APt þ AAu ¼ ½Pt$PPt ðxÞ20 þ ½Au$PAu ðxÞ20

(7)

where A is the absorbance of the solution obtained after the acid dissolution of the PtAu bimetallic nanoparticles electrodeposited on pCNF, PPt(x)20 and PAu(x)20 are the 20 polynomial of 1 ppm Pt and Au solution respectively. The cost function of the system, defined as the sum of the standard deviations between the analytical and the corresponding experimental absorbance values, showed that the Au local minimum is well defined, while the Pt partial

Table 1 e PtAu deposit stoichiometry. Electrolyte solution 2.5Pt2.5Au

4Pt1Au

Qdep (mC)

(Pt/Au)XPS

(Pt/Au)EDX

300 525 750 300 525 750

0.73 0.83 0.87 1.38 1.62 2.04

0.51 0.64 1.11 2.33 2.09 2.23

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derivative is almost constant. The mathematical method therefore has a strong sensitivity to Au concentration but any Pt concentration, in a wide range of possible solutions, is numerically compatible with the cost function minimum. For this reason it was imposed as a constraint to the problem that the Pt/Au ratio was that measured by EDX analysis. Fig. 6 shows the result obtained for the electrode deposited at 525 mC by 2.5Pt2.5Au solution.

CV characteristics The electrodes were characterized by cyclic voltammetry in H2SO4 to determine the electrochemical real surface (ERS). CV of Pt/pCNF and Au/pCNF electrodes were considered as reference (Fig. 7) showing respectively: a) hydrogen adsorption/desorption peaks (cathodic/anodic current between 0.1 and 0.3 V) and PtOx reduction peak (cathodic current at about 0.5 V); b) AuOx formation/reduction peaks (anodic current at about 1.1 V and cathodic current at about 0.65 V). PtAu/pCNF electrodes exhibit the specific peaks of both metals, more or less evident depending on the deposit stoichiometry, confirming the bimetallic nature of catalytic nanoparticles (Fig. 8). ERS is calculated by the integration of the charge associated to the hydrogen desorption in the voltammetry positive scan (QH [mC]), after correction of the double layer charging: ERS ¼

QH QHo

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electrodes performance even better. Matching up the ERS data with those of catalyst loads, the most promising electrodes were found to be the two obtained at Qdep ¼ 525 mC from 2.5Pt2.5Au solution (hereafter denoted as sample A) and Qdep ¼ 300 mC from 4Pt1Au solution (hereafter denoted as sample B), showing high ERS (13.8 and 5.4 respectively) with low Pt/Au ratio (LPt ¼ 54 mg/cm2 and LAu ¼ 117 mg/cm2 for sample A; LPt ¼ 11 mg/cm2 and LAu ¼ 6 mg/cm2 for sample B). The platinum amount for both the electrodes is very similar to that obtained in case of Pt/pCNF electrodes [13] confirming that the two metals are electrodeposited independently. The lower loading of sample B is due to the increase in the hydrogen discharge when the applied deposition charge is increased, with a consequent decrease of the deposition efficiency. The value of ERS is concerning only to the platinum surface. The low value of ERS is due to the fact that we obtained deposit made up clusters of nanoparticles and not by separated single nanoparticles. In any case the ERS values are

(8)

where QHo (0.21 mC) is the same value for a hydrogen monolayer on smooth platinum (1 cm2) considering the (100) crystallographic orientation of fcc Pt [51]. The ERS values are in the range between 4 and 16 cm2 cm2 and increase with the Pt/Au ratio because gold is not catalytically active towards the hydrogen adsorption/desorption reaction. The ERS values concern only the platinum surface, so they are quite small because the obtained deposit is made of nanoparticles clusters and not of separated single nanoparticles. Therefore, they could be underestimated and the

Fig. 6 e Calculated and measured absorption spectrum of Pt þ Au solution.

Fig. 7 e Typical cyclic voltammetry in 1 M H2SO4 of Pt/pCNF (a) and Au/pCNF (b) electrodes obtained by galvanostatic electrodeposition (potential window between ¡0.3 V and 1.3 V vs. SCE, potential scan rate of 100 mV s¡1).

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Methanol electro-oxidation

Fig. 8 e Typical cyclic voltammetry in 1 M H2SO4 of PtAu/ pCNF electrode obtained by galvanostatic electrodeposition (potential window between ¡0.3 V and 1.3 V vs. SCE, potential scan rate of 100 mV s¡1).

conservative and therefore the electrode performances may be better. Given the platinum load (LPt [mg cm2]) the electrochemical active surface (EAS [m2 g1]) was obtained by measuring the charges associated with the hydrogen desorption signals (QH) in the CV curves using the following equation: EAS ¼

ERS 10LPt

(9)

where LPt is the load of Pt [mg cm2] in the working electrode. The achieved values are the following: 26 m2 g1 for sample A and 49 m2 g1 for sample B. The electrode obtained by the solution with higher Pt content exhibited the best performance showing a lower platinum load but a higher EAS, that is the catalyst amount has been minimized and its activity simultaneously maximized. Given the well-established hydrogen-adsorption stoichiometry at a Pt surface (H:Pt ¼ 1:1) [52,53], the number of the exposed Pt atoms (Ns) can be counted as the number of adsorbed hydrogen atoms (NH) which can be easily derived from QH. The data of the CV measurements in H2SO4 allow to calculate the number of exposed atoms as the number of adsorbed H atoms on the electrode catalyst, that is: NS ¼ NH ¼

QH Qe

The PtAu/pCNF electrodes electrocatalytic performances were evaluated by the methanol oxidation reaction (MOR), using the cyclic voltammetry in 1 M H2SO4 þ 0.5 M CH3OH (Fig. 9). PtAu/pCNT electrodes presented cyclic voltammetries with hydrogen adsorption/desorption peaks (between 0.2 and 0.1 V), with the electrical charge QH lower than in the case of pure H2SO4, due to the poisoning of the catalyst by methanol and its oxidation products (e.g.,: CO, HCOOH, HCHO, etc.). The methanol oxidation peaks were detected at electrode potential higher than 0.5 V. The remarkable increase in the anodic current was due to the dehydrogenation of methanol followed by oxidation of adsorbed intermediates and poisoning of Pt by CO. In the back-scan in cathodic direction, an anodic reaction started, with a peak around þ0.40 V. This peak was due to the oxidation of residual adsorbed methanol and desorption of chemical products generated during the methanol oxidation in the direct scan. The slow current increase in anodic direction was ascribed to the formation of intermediate products and the quick increase to the formation of platinum oxide particles which promote the conversion of intermediates to carbon dioxide. The current increased with electrode potential, so the reaction was controlled by charge transfer step. The current reached a maximum and then decreased, indicating the MOR became difficult on the Pt surface covered with PtOx. On the backward scan (cathodic direction) the current peak of MOR occurred at lower potential than on the forward potential scan. This could be ascribed to the naked Pt available due to the reduction of PtOx.Variations in the catalyst activity could be connected with a change in the bond strength of the methanol adsorbate with platinum particles, which was determined primarily by the structural properties of the deposits. The long term stability of the electrocatalyst is a strong requirement for the practical applications. For this reason, the CV scans were repeated up to 200 times to investigate the aging of the electrocatalysts. The electrodes are characterized

(10)

where Qe is the charge of one electron (1.602∙1019 C). The ratio of NS to the total number of Pt atoms in the electrocatalyst, obtained considering the Pt loading, gives the utilization percentage of Pt (UPt): UPt ¼ 100$

NS Nt

(11)

The achieved values are the following: 11% for sample A and 21% for sample B.

Fig. 9 e Typical cyclic voltammetry in 1 M H2SO4 þ CH3OH 0.5 M of PtAu/pCNF electrode obtained by galvanostatic electrodeposition (potential window between ¡0.3 V and 1.3 V vs. SCE, potential scan rate of 100 mV s¡1).

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by a limited QH decrease during the cycling proving that the presence of oxygen containing functional groups on pCNF surface and the combination of Pt with Au really contribute to the oxidation and removal of CO, increasing the electrode poisoning tolerance. The catalysts showed an initial increase of MSA with the cycle number probably due to the electrode surface cleaning. After this step the MSA reached a maximum value and then the plateau or a decrease. The electrocatalytic performances in terms of mass specific activity (MSA) were calculated as follows:  QMOR  MSA mC mg1 ¼ LPt

(12)

where LPt is the platinum load and QMOR [mC cm2] is the charge associated to the methanol oxidation in the voltammetry positive scan in H2SO4 þ CH3OH, after correction of the double layer charging. The gold load is not considered in the equations because Au does not contribute to the hydrogen desorption and methanol oxidation [13]. The MSA changes over time are an indication of the electrodes long term stability, which is a strong requirement for fuel cell applications. The MSA time evolution revealed again better performance for the sample B (Fig. 10). MSA is six times higher and stable also after 200 MOR cycles not showing any decrease. This behavior is ascribable mainly to the different Pt/Au ratio in the two electrodes: the sample A has approximately a double gold content compared to platinum, while the sample B has a ratio Pt/Au of about 1.38 (Table 2). The gold is not catalytically active towards the oxidation of methanol, so when its content is high, the electrode shows lower performance in terms of MSA, while when the deposit has a low content of gold, covering the platinum surface as shown by XPS analysis, it increases the Pt poisoning tolerance promoting the oxidation of the intermediates of the methanol oxidation reaction. Therefore bimetallic composition can significantly modify the electrocatalytic properties of both Pt and Au and the electrocatalytic activity of pCNF supported PtAu nanoparticles. The gold present in the PtAu nanoparticles may play an important role in removing the intermediate CO-like species or providing oxygenated species in the methanol oxidation process. This affirmation is consistent with the fact that gold at nanoscale is catalytically active for CO oxidation [32,37]. Bi-functional electrocatalytic properties may be operative for MOR on the AuPt nanoparticle catalysts [54]. Mechanistically, the bi-functional properties could then involve a possible combination of the following reactions: CH3 OH þ Pt////Pt  COads þ 4Hþ þ 4e

(13a)

Au þ H2 O/Au  OHads þ Pt

(13b)

Pt  COads þ Au  OHads /Pt þ CO2 þ Au þ Hþ þ e

(13c)

In Fig. 10 it is also shown as comparison the MSA variation as function of the MOR cycles of a commercial catalyst Pt/ Ru ¼ 1 on Vulcan XC-72R carbon (BASF Fuel Cell Inc., NJ, USA).

Fig. 10 e Mass specific activity (MSA) time evolution of PtAu/pCNF electrodes and a PtRu/Vulcan reference electrode.

The PtRu/C electrode was prepared by depositing the PtRu/C catalyst with a spray technique [21] on a diffusive layer (composed of Vulcan XC-72R carbon powder, 2 mg cm2, and PTFE, 20% w). This electrode shows an MSA about two times lower than sample B, despite the higher platinum load. The results highlights a large increase of MSA, up to three times for the innovative electrode obtained from the 4Pt1Au solution respect to 2.5Pt2.5Au one, together with a considerable decrease in the catalyst load (about five times for the same electrode). Despite the low Pt load, the sample B catalyst shows the higher utilization and the higher MSA, with a great advantage in terms of cost and performance (Table 2). The bimetallic nanoparticles composition was measured again by XPS and EDX after 200 MOR cycles (Table 3). The Pt/Au ratio changes with respect to that measured after the electrodeposition (Table 1). Both the electrodes loss some platinum during the MOR cycling, as confirmed by SEM observation showing a reduction in the nanoparticles density and in particular the disappearance of the nanoparticles of smaller dimensions. It is interesting to note that the bimetallic nanoparticles deposited by the solution with higher Pt content show a considerable Au surface enrichment, that could enhance the catalyst poisoning long term stability. SEM analysis at high magnification highlights different deposit morphology for the two electrodes (Fig. 11). It is observed that the nanoparticles deposited by the 2.5Pt2.5Au solution partially coalesce, with a loss of active surface, while those deposited by the 4Pt1Au solution lose the surface nanostructuring assuming the more globular morphology, typical of gold (Fig. 3), confirming the surface Au enrichment of the deposited catalyst.

Conclusions PtAu bimetallic nanoparticles were deposited on platelet carbon nanofibers. The electrocatalytic properties for the

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Table 2 e Electrochemical active surface (EAS), platinum utilization (UPt) and mass specific activity (MSA), after 200 MOR cycles for different PtAu/pCNF (Sample A and B) and commercial PtRu/C electrocatalysts. Electrode

Qdep/mC

Pt/Me

Pt/mg cm2

EAS/m2 g1

UPt/%

MSA/mC mg2

525 300 e

0.83 1.38 1.10

0.054 0.011 0.150

26 49 54

11 21 8

56 302 166

Sample A Sample B PtRu/Vulcan

Table 3 e PtAu deposit stoichiometry after galvanostatic electrodeposition (GED) and after 200 methanol oxidation reaction (MOR) cycles. Electrode

(Pt/Au)XPS After GED

(Pt/Au)EDX After GED

(Pt/Au)XPS After MOR

(Pt/Au)EDX After MOR

Sample A Sample B

0.83 1.38

0.64 2.33

0.53 0.75

0.46 1.79

methanol oxidation of PtAu/pCNF with different Pt/Au atomic ratio have been investigated and the results showed better performance with respect to an electrode made of a commercial nanometer carbon based substrate (PtRu on Vulcan XC-72R). The particular morphology of platelet CNF determines a strong anchorage of catalyst nanoparticles to the substrate with reduced coalescence phenomena. Moreover, the oxygen containing functional groups on nanofibers surface contributes to the oxidation and removal of carbonaceous species accumulated on the electrode during cell operation, such as CO, thus reducing platinum poisoning and increasing the electrode long-term stability. The increased catalytic activity of electrodeposited PtAu respect to PtRu might be attributed to the higher utilization of Pt atoms for the electrocatalytic reaction of methanol oxidation.

The direct growth of CNF on carbon based substrates allows a cost effective production process of the electrodes, reducing the number of needed steps through the elimination of the diffusive and catalytic ink conventional spraying process. Furthermore the electrodeposition localizes the catalyst nanoparticles only on the exposed surface of the electrode with great catalyst load reduction. Finally, bimetallic catalysts led to an increase in Pt tolerance to poisoning with a gain in terms of long-term stability and decrease of catalyst load with cost reduction. In addition to the electrocatalytic performance, another aspect to be considered in the comparison between the innovative and the traditional electrodes is the cost effectiveness of the production process. The economic evaluation was performed based on the raw materials and energy cost for the electrodes production [55]. The combination of chemical vapor deposition for CNF growth and electrodeposition for catalyst deposition leads to a four times cost reduction. This is an important result since nearly a quarter of the whole fuel cell cost is related to the catalytic layer, of which the catalyst counts for more than half. Besides cost savings, the production of the innovative PtAu/pCNF electrodes offers a number of other advantages, such as the no need to perform hydrogen treatments, the absence of precursors reduction residues, a much greater process simplicity due to

Fig. 11 e High and low magnification FEG-SEM micrographs after methanol oxidation reaction (MOR) cycling of PtAu nanoparticles [sample A (a,c) and sample B (b,d)].

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 2 1 6 0 1 e2 1 6 1 2

much lower number of steps and the presence of a single layer with both diffusive and catalytic function.

Acknowledgments The experimental assistance of Dr. Francesco Mura from CNIS at Rome University, for SEM analysis, and Dr. E. Piscopiello from ENEA Brindisi Research Center, for TEM analysis, is acknowledged.

Nomenclature CNF carbon nanofibers CV cyclic voltammetry DMFC direct methanol fuel cells EAS electrochemical active surface EDX energy dispersive X-ray analysis ERS electrochemical real surface FEG-SEM field emission gun scanning electron microscopy GED galvanostatic electrodeposition MSA mass specific activity MOR methanol oxidation reaction pCNF platelet carbon nanofibers PECVD plasma enhanced chemical vapor deposition PEFC proton exchange fuel cells SCE saturated calomel electrode XPS X-ray photoelectron spectroscopy XRD X-ray diffraction

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