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Highly dispersed molybdenum carbide as non-noble electrocatalyst for PEM fuel cells: Performance for CO electrooxidation
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 5 ( 2 0 1 0 ) 7 8 8 1 e7 8 8 8
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Highly dispersed molybdenum carbide as non-noble electrocatalyst for PEM fuel cells: Performance for CO electrooxidation R. Guil-Lo´pez a,*, M.V. Martı´nez-Huerta a, O. Guille´n-Villafuerte b, M.A. Pen˜a a, J.L.G. Fierro a, E. Pastor b a b
Instituto de Cata´lisis y Petroleoquı´mica (CSIC), Marie Curie 2, Cantoblanco, E-28049 Madrid, Spain Departamento de Quı´mica Fı´sica, Universidad de La Laguna, Astrofı´sico Francisco Sa´nchez s/n, E-38071 La Laguna, Tenerife, Spain
article info
abstract
Article history:
CO electrooxidation on nanocrystalline molybdenum carbide has been studied through CO
Received 29 January 2010
stripping measurements using cyclic voltammetry. The active molybdenum carbide was
Received in revised form
obtained from the carbothermic reduction of really very small molybdenum oxide particles
29 March 2010
supported on Vulcan XC-72 carbon black (CB). In order to obtain highly dispersed molyb-
Accepted 8 May 2010
denum carbide particles, low molybdenum loading and control of the carbothermic
Available online 16 June 2010
reduction conditions of CB-supported molybdenum oxide were employed to avoid Mo sintering during the carburization process. This work provides experimental evidence on
Keywords:
the CO electrooxidation capability of the Mo carbide phase, which to the best of our
Non-noble metal electrocatalyst
knowledge is reported for the first time. The small particle size of carbide electrocatalyst
Molybdenum carbide
exhibited better performance for CO electrooxidation than the commercial bulk molyb-
PEMFC
denum carbide sample.
CO electrooxidation
1.
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Introduction
The parallel growth of energy demand and environmental awareness has driven research into new energy sources, as well as into new technologies for energy interconversions. Several options are being studied to solve this latter problem, and fuel cells (FCs) seem to stand out from all the other options. The proton exchange membrane fuel cell (PEMFC) is perhaps the most promising power source to optimize energy use for mobile applications and standalone utilities. Electrocatalysts for hydrogen in PEMFC or methanol electrooxidation in direct methanol fuel cell (DMFC) anode comprise mainly Pt or its alloys, thereby providing the best electroactivity [1]. However, both fuels, H2 streams from
hydrocarbons reforming and methanol, content carbon monoxide (CO) or it is a reaction intermediate that poisons the Pt centres by CO chemisorption and reduces fuel cell performance by decreasing the number of Pt active centres, even at very low CO concentration (10 ppm) [2]. To increase CO tolerance, Ru is added to the electrocatalyst, which allows reducing the amount of Pt [3,4]. Nevertheless, the addition of Ru involves another noble metal and so adds cost, hence rendering the PEMFC and DMFC electrocatalyst less attractive. Accordingly, the complete and immediate industrial development of these fuel cells is economically less viable. Moreover, it still requires increasing noble metal loadings, and the tolerance of PteRu catalysts is not high enough when used at economic electrode Pt loadings (below 0.25 mgPt$cm2) with higher levels of CO (above 10 ppm) [5].
* Corresponding author. Inst. Cata´lisis & Petroleoquimica-CSIC, C/Marie Curie 2, Cantoblanco, E‑28049 Madrid, Spain. Tel.: þ34 915854947; fax: þ34 915854760. E-mail address: [email protected] (R. Guil-Lo´pez). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.05.044
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It has been shown that systems based on the addition of Mo [5e7] or W [8] to Pt or to PteRu electrocatalysts have a greater CO tolerance than PteRu catalysts, thus a new nonnoble metal electrocatalyst appears to be an interesting future alternative. Likewise, it is known that metallic carbides of molybdenum, vanadium or tungsten mimic the catalytic behaviour of certain noble metals [9,10]. Thereby, some preliminary studies on the electrocatalytic behaviour of molybdenum carbides [11,12] and tungsten carbides [13] showed activity in methanol or hydrogen electrooxidation using bimodal Pt and carbide electrocatalysts [14]. These results make the electrocatalytic capacity of molybdenum carbides (MoxC) systems for CO electrooxidation highly attractive because MoxC loaded electrocatalysts perform well for both methanol or hydrogen and CO electrooxidations. In this case, the increase of Pttolerance to CO could decrease the amount of Pt required in the electrocatalyst even in Ru-absence. This work focuses on the preparation of MoxC electrocatalysts in the absence of noble metals and on the use of the cyclic voltammetry technique of CO electrooxidation (CO stripping) to evaluate the CO electrooxidation capacity of MoxC electrocatalyst, which is tested for the first time. The importance of a small particle size of metal carbide phase in the methanol electrooxidation capacity has already been reported [11e13]. Therefore, to guarantee high dispersion of MoxC, electrocatalysts containing low molybdenum loadings were prepared via the carbothermic reduction method [15e19].
2.
Experimental
2.1.
Electrocatalyst preparation
The preparation of an electrocatalyst supported on carbon black (CB) involves two steps: (i), deposition of molybdenum precursor on the CB substrate; and (ii), carburization of precursor to form MoxC electrocatalyst followed by surface passivation to stabilize the molybdenum carbide formed. The electrocatalyst precursor (EC-precursor) was prepared by a modified impregnation method with molybdenum pentachloride, with precipitation of hydrous molybdenum oxide (MoOxHy) suspension on the CB surface using tetramethylammonium hydroxide solution as precipitating agent [20]. The EC-precursor was subsequently carburized using temperature-programmed reduction (TPR) with hydrogen. Carburization processes were carried out using a U-shaped quartz fixed-bed reactor (4 mm ID) with 100 mL min1 of 10 vol % H2/N2 mixture and atmospheric pressure, with 60 mg of EC-precursor. The reactor was on-line with a Baltzer Prisma QMS 200 TM quadrupole mass spectrometer to monitor all processes. Two different carbothermic reduction methods were carried out. In carbothermic reduction-1, the temperature was increased from room temperature (r.t.) to 840 C at 10 C$min1 (coded EC-1). In carbothermic reduction-2, the temperature was increased from r.t. to 600 C at 10 C$min1, and then, maintained at 600 C for 2 h (coded EC-2). In addition, EC-3 molybdenum carbide electrocatalyst was prepared as the EC-1 (carbothermic reduction-1) except that the
temperature of carburization was 975 C. The EC-3 sample was prepared in order to sinterize MoxC particles, thus facilitating their detection by XRD technique. All electrocatalysts were obtained from the EC-precursor. Since the carbide catalysts are pyrophoric, after cooling the samples to room temperature, the electrocatalysts were passivated by switching to a flow of O2/Ar (1% O2/Ar at a total flow of 150 mL min1) for 3 h. During the passivation step, a protective oxide layer was developed on the surface of the carbide phase. All steps were followed by monitoring the main [m/z] signals relative to H2, H2O, CO, CO2 and Ar among the products. No other products were detected. To remove any error due to changes in pressure, all signals were referred to the Ar signal. CB support (Vulcan XC-72) was provided by Cabot Co. Commercial bulk of molybdenum carbide was used as reference (coded MoCcommercial). Chemicals, reagents and reference material were purchased from Aldrich.
2.2.
Catalyst characterization
Mo-loading was determined by chemical analysis using an ICP-AES PerkineElmer Optima 3300 DV spectrometer. Additional data on chemical analysis by thermogravimetric (TG) measurements were obtained through the loss of weight during carbon support combustion with a flow of air (100 mL min1) using a Mettler Toledo TGA/SDTA 851 thermal analyser. Powder X-ray diffraction (XRD) patterns of materials were recorded in a Seifert 3000 P diffractometer, using nickelfiltered Cu Ka1 radiation, and a scanning rate of 0.04 2q s1, in the 2q range of 10 e65 . TPR analyses were performed on a Micromeritics 2900 TPD/TPR instrument. The reducing agent was 10% vol. H2/Ar, with a gas flow of 50 mL min1. The samples (20 mg) were heated from r.t. to 1000 C at a rate of 10 C$min1. Transmission electron microscopy (TEM) was recorded using a Philips Technai 20 microscope operating at 200 kV. The samples were crushed in an agate mortar, dispersed in acetone and dropped onto a holey copper microgrid.
2.3. Electrochemical methods: CO-stripping voltammetry Electrochemical measurements were carried out in a standard three-electrode electrochemical cell at room temperature, with a Radiometer Analytical Model PGZ 301 potentiostat. The working electrode was a glassy carbon electrode of 3 mm in diameter. For the preparation of the ink, electrocatalysts (10 mg) were dispersed in water (1 mL) and Nafion (30 mL) in an ultrasonic bath for 30 min. The ink (5 mL) was deposited onto the electrode and dried under Ar flow for 30 min. Potentials in this work are reported versus a reversible hydrogen electrode (RHE). The CO electrooxidation capacity of the molybdenum carbide materials was characterized by COads stripping in 0.5 M H2SO4 (98% Merck p.a.) solution applying the cyclic voltammetry technique, using the adsorption and subsequent CO electrooxidation as reaction test. This procedure was used to prove: (i) the CO electrooxidation capacity of MoxC; and (ii) the presence of the carbide on the electrocatalysts. For this purpose, gaseous CO was bubbled into the cell for 30 min at a constant voltage of 0.02 V vs. RHE. After CO removal
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3.
Results and discussion
enlargament: x 25 Temperature
600
500
Mass signal / a.u.
400
300
H2O
200
T / ºC
(Ar purging for 30 min), the potential was scanned from the adsorption potential up to 1.10 V and three consecutive cycles were recorded at a 10 mV/s scan rate. The stability of MoxC electrocatalyst has been tested by doing multiple cycles. Activation of the electrode has been performed by recording 25 potential cycles between 0 and 1.10 V at 0.10 V/s in 0.5 M H2SO4. All reagents used were of analytical-grade.
CO2 100
CO
3.1.
Electrocatalyst preparation
0
Fig. 1 shows the carburization results of EC-1, which are expressed in terms of the corresponding [m/z] mass spectra as a function of time and temperature (thermo-programmed carbothermic reduction experiments). The only products detected were water and carbon oxides, CO and CO2, among H2 and Ar. Fig. 1b enlarges the signals to detect the CO2 and H2O peaks (note the 25-fold smaller scale). Two processes can be observed: (i), a first one at lower temperature is mainly associated to H2O production, which maintains a maximum value from 510 to 675 C; and (ii), a second one at 780 C is associated to a massive CO and CO2 production. Therefore, COx evolution occurs at higher temperature up to 700 C (Fig. 1). As no oxygen is present in the environment, the molybdenum reduced species (MoO3x) in the first process react at higher temperatures with the carbon substrate to form COx in the gas, while simultaneously forming the carbide phase (MoxC). The first assignment of both processes relates the low-temperature process to partial Mo-reduction and the high temperature process to Mo-carburization. In order to isolate the first process, carbothermic reduction-2 was carried out at 600 C (see Experimental Section 2.1). These conditions ensure the development of the lowtemperature process (EC-2). Fig. 2 shows the results obtained
900
(a)
(b)
CO
800
CO
Mass signal / a.u.
700
tem pe rat ur e
enlargament: x 25
500
H2O
400
T / ºC
600
300
CO2
0
200
CO2
H2O
100 20
40
60
80 0
20
40
60
80
t / min Fig. 1 e Results from the carburization stage of EC-1 preparation: (a) The results for CO (solid line), CO2 (dash line) and H2O (dot line) production are represented by their corresponding [m/z] mass profiles as a function of temperature; (b) The [m/z] mass profiles have been enlarged in order to facilitate the identification of the peaks from CO2 and H2O profiles.
0
30
60
90
120
150
-100 180
t / min Fig. 2 e Results from the carburization stage of EC-2 preparation: The results for CO (solid line), CO2 (dash line) and H2O (dot line) production are represented by their corresponding [m/z] mass profiles as a function of temperature. The [m/z] mass profiles have been enlarged in order to facilitate the identification of the peaks.
from EC-2 carburization. In this case, no massive CO production is observed (Fig. 2), which indicates the involvement of the low-temperature process. As a result, EC-2 develops partially reduced (MoO3x) species but no carburization takes place, as there are no COx species in the gas phase (Fig. 2). Fig. 3 compares the H2O (Fig. 3a) and CO2 (Fig. 3b) evolution of both carbothermic reductions-1 and -2 as a function of time (note that process-2 presents an isothermal stage at 600 C). The same H2O quantity is produced in both cases (0.01 a.u. below each curve), indicating that the same quantity of molybdenum is reduced in both cases. Taking into account the high temperature of carbothermic reduction-1 conditions, it can be concluded that the reduction of all the molybdenum oxide supported phase is completed using both carbothermic conditions. Therefore, CO and CO2 formation seems to be linked to Mo carburization. Figure 3b shows the low-intensity signal of CO2 production with both carbothermic reduction conditions. Two peaks are detected with carbothermic reduction-1 conditions (solid line in Fig. 3b). The CO2 peak at low temperature (at 350 C) appears when both carbothermic conditions were used. This peak is connected to the loss of oxygenated groups on the carbon support surface. The CO2 peak at high temperature (ca. 780 C) appears when carbothermic reduction-1 conditions were used. This CO2 production at 780 C is therefore linked to the Mo-carburization process. In short, it can be proposed that: (i), water production (from 510 to 675 C) is associated to the formation of the partiallyreduced Mo oxide phases; (ii), the low-temperature CO2 peak (at 350 C) stems from the loss of oxygenated groups on the carbon surface; and (iii), the high temperature CO2 peak arises from the carburization of partially-reduced Mo oxide phases. Thus, molybdenum carbide phases are only expected in EC-1 electrocatalyst. The electrochemical results reveal the success of the carburization process.
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Fig. 3 e Comparative study of the results from the carburization stage of EC-1 (solid line) and EC-2 (dot line) preparations. The results for H2O (a) and CO2 (b) production are represented by their corresponding [m/z] mass profiles as a function of time.
3.2.
Catalyst characterization
ICP analysis of EC-precursor and EC electrocatalysts showed low metal loading: 4% wt Mo/CB (0.5% atomic Mo/CB), which is very close to the nominal value. Similar values of Mo-loading were obtained through the weight lost of the samples during TG measurements. Mo-loadings between 3.5 and 4.0% wt were recorded by TG analysis for all samples. For these samples, the crystallite size of molybdenum phase was found to be very low, which meant that no diffraction lines were observed in EC-1 sample, suggesting that crystallite size falls below the XRD technique’s detection limit (below ca. 3 nm). Fig. 4 compares the XRD patterns of the Mo/CB-precursor, CB substrate and commercial reference compound with the molybdenum carbide electrocatalysts (EC-1 and EC-3). Crystalline phases were identified with the Joint Committee on Powder Diffraction Standards (JCPDS) files [21]. Two weak and broad peaks (note the enlargement: x100, regarding MoCcommercial in Fig. 4e) are presented in the diffraction patterns of both carburized electrocatalysts (EC-1 and EC-3, Fig. 4a and b, respectively) and the precursor (Fig. 4c). These peaks correspond to the basal planes of graphene in CB-support (Fig. 4d). No signal from the molybdenum phases were detected in the XRD patterns of the EC-precursor, EC-1 and EC-2 samples (diffraction pattern not shown) samples. Two hypotheses can we proposed to explain this fact. In the first one it is assumed that the absence of Mo signals might well account for the presence of molybdenum crystallites of very small size, even after the carbothermic reduction process at 840 C (see EC-1, Fig. 4a). In the second hypothesis, the formation of an
Fig. 4 e XRD patterns of the materials studied: (a) electrocatalyst-1 carburized with carbothermic reduction-1 conditions at 840 C, EC-1; (b) electrocatalyst-3 carburized at 975 C, EC-3; (c) precursor of electrocatalyst (ECprecursor); (d) CB-support vulcan; and (e) commercial bulk MoC (MoC-commercial). The XRD patterns ofEC-1, EC-3, ECprecursor and CB-support (aed) have been enlarged (x100) in order to facilitate the identification of the peaks. Phase identification: B graphite (JPDFN: 75-1621); - Mo2C (JPDFN: 79-744); and , MoC (JPDFN: 45-1015).
amorphous Mo phases is considered. Such amorphous phases would not show XRD signals. Additional experiments were carried out to distinguish between both tentative explanations. Accordingly, the EC-3 sample was treated at 975 C with the aim to sinterize as much as possible crystalline Mo carbide phases, which could be present in EC-1 (carburized at 840 C). XRD patterns of electrocatalyst EC-3 carburized at 975 C shows small crystallite domains (Fig. 4b) whose diffraction lines belong to a less common phase of molybdenum carbide: MoC‑phase (JPDFN: 45-1015). It could be possible that small crystallites of MoC-phase were present on EC-1; they are not be detectable by XRD, but became observable to this technique upon sintering at 975 C (EC-3). It is likely that amorphous molybdenum phases in EC-1sample are transformed into MoCphase at 975 C. Fig. 5 shows the TEM-micrograph of a representative area of EC-1 electrocatalyst at two different magnifications. Well defined characteristic concentric graphene layers in the carbon black particles support are distinguished. No-molybdenum particles are detected in the TEM-micrograph. However, EDX microanalysis of three different areas of Fig. 5 revealed nominal Mo contents of 4.7, 4.7 and 4.9% wt of Mo, which agree with the Mo content determined by ICP and TG (4.0% wt by ICP analysis or 3.6% wt by TG). This nominal Mo
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10
a) MoC-commercial: 94 % wt Mo
0
Current / A
10
b) EC-1: 4 % wt Mo
0
10
c) EC-precursor: 4 % wt Mo
Fig. 5 e TEM images of EC-1 electrocatalyst (x 29,000). Enlarged area recorded at higher magnification (x 200,000).
0
0.2
content proves the presence of Mo on the carbon black particles, even though the Mo-particles over concentric carbon black particles are not visible. Taking into account that Mo-particles are not detected but the EDX‑microanalysis ascertains the presence of Mo, the TEM results agree with a small Mo‑particle size, which is one of the possibilities inferred from XRD results. The extremely small particle size of carbide crystallites (not detected by TEM) could be due to a well known low contrast between carbon and Mo phases by TEM, which would make it difficult to distinguish between small particles of MoxC and carbon support. On the other hand, the MoxC formation involves the metallic Mo diffusion into the carbon support [15,22], which favors Mo dispersion in the region of low Mo-loadings. Regarding the reference material (MoC-commercial), the XRD patterns revealed only the development of the common Mo2C‑phase (JPDFN: 79-744). As additional information, reduction of EC-1 carburized material was carried out. TPR profiles for EC-1 electrocatalyst showed negligible H2-consumption indicating that the Mo carbide phases are completely reduced. However, the low Mo-loading used to avoid the MoxC sinterization makes it difficult to conduct an in-depth analysis of the molybdenum structures in these materials by standard characterization techniques. Apart from this, CO-stripping voltammetry measurements were taken to show the formation of a molybdenum carbide phases.
3.3.
CO-Stripping voltammetry
Fig. 6 presents the results obtained in CO electrooxidation by cyclic voltamperometry of CO stripping with the commercial bulk MoxC (Fig. 6a, MoC-commercial), Mo carbide electrocatalyst (Fig. 6b, EC-1), and its precursor (Fig. 6c, EC-precursor). The resulting curves were obtained by subtracting the second voltamperometric curve from the first one. The second and the third cycles revealed that CO was completely oxidized in the first cycle. The second cycle was used as a background and subtracted from the first cycle in order to show only the current related to CO oxidation that occurs during the
0.4
0.6
0.8
1.0
Potential v.s. RHE / V
Fig. 6 e Results from CO electrooxidation cyclic voltammograms using as electrocatalysts: (a) a commercial bulk MoC (MoC-commercial), (b) EC-1; and (c) EC-precursor. The resulting curves were obtained by subtraction of the second voltamperometric cycle from the first one. Constant voltage CO-adsorption: 0.02 V. Scan rate: 0.01 V sL1.
positive-going scan at E > 0.60 V vs. RHE. It can be observed that the Mo carbide-containing electrocatalysts, mainly the commercial MoC-reference (Fig. 6a) show CO electrooxidation currents in the above mentioned potential range. It is therefore proven that the molybdenum carbide phase is able to perform the CO electrooxidation reaction. If a comparison is made between the performances of the carburized material (EC-1) (Fig. 6b) with the non-carburized EC-precursor (Fig. 6c), it can be concluded that the carburized material (EC-1) presents electroactivity. This result is direct proof that the carburization process has taken place during carbothermic reduction-1 conditions. On the other hand, no CO electrooxidation current was recorded for EC-2 (not shown in Fig. 6), which means that MoxC is not developed in EC-2 sample. This result agrees with the previous assignment of thermal process occurring during the different carburization treatments: the first process at low temperature, found in EC-1 and EC-2 samples, is due to partial reduction of molybdenum oxide, whereas the second process at high temperature, observed only for EC-1, that is, the catalysts with the highest electrooxidation activity towards CO, stems from the carburization of the partially-reduced MoO3x phase produced in the first step. It is therefore proven that carburization is the process at high temperature (780 C) that generates CO and CO2 in the gas phase during its formation and is then active in the CO electrooxidation reaction. These assignments are consistent with the mechanism of molybdenum carbide formation found in the literature, which assumes carbide formation through molybdenum reduced intermediates [15,22]. However, the oxygenated nature of the partially-reduced molybdenum
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intermediate, justified by COx production during the last step of the carburization process, has not yet been proven. The shape and position of the CO electrooxidation peak in the cyclic voltammograms depend on the nature of the molybdenum samples. Four magnitudes were defined: (i) the threshold potential (ET), as the on-set potential where the increase in the anodic current is observed during the positive-going potential scan; (ii) the peak potential (Emax), as the potential at which the maximum CO oxidation current density is achieved; (iii) the maximum current (Imax), which is the current at Emax; and (iv) the electrooxidation charge (QCO), as the result of the integration of the current at E > 0.60 V from the curves given in Fig. 6. Two additional magnitudes were calculated from the maximum current (Imax), and the electrooxidation charge (QCO), which are normalized to Mo-loadings. The normalized magnitudes were the Mo-mass Momass ) and the Mo-mass normalized maximum current (Imax MOmass ). Table 1 normalized electrooxidation charge (QCO contains the main CO electrooxidation results obtained with EC-precursor, EC-1, EC-2 and the reference commercial bulk MoxC material (MoC-commercial). The active electrocatalysts (EC-1 and both reference materials) carry out CO electrooxidation at E > 0.6 V. It is remarkable that similar threshold potentials are obtained with both molybdenum carbide electrocatalysts (see ET in Table 1), suggesting a similar nature of the phases present in both electrocatalysts. Nevertheless, the molybdenum carbide electrocatalysts have different electrocatalytic behaviour regarding the current density produced by each electrocatalyst (see Imax and QCO Table 1). The most active electrocatalyst is EC-1. It should be noted that the catalyst with the lowest metal carbide loading and lowest particle size (EC-1, see XRD pattern in Fig. 4a or TEM-micrograph in Fig. 5) is the catalyst with the highest CO electrooxidation activity (Table 1). Moreover, the EC-1 sample with 4% wt of Mo produces 60% more current density than the reference MoC-commercial sample, whose Mo-loading is 94% wt of Mo. The small particle size of molybdenum carbide on EC-1 surface, which cannot be revealed by TEM and XRD techniques, gives more reactivity to exposed molybdenum carbide than in the reference samples with larger particles size. Accordingly, the highly dispersed molybdenum carbide obtained in EC-1 surface increases the electroactivity of this catalyst. Therefore, the use of the carbothermic reduction process seems to
be the key factor for yielding the highest current density produced by these kinds of electrocatalysts. Another factor to analyze the electrochemical results is the nature of molybdenum carbide phases presented on each material. In this sense, MoC-commercial sample shows an only crystalline phase, Mo2C-phase (see XRD Section and Fig. 4e), while, we proposed in the XRD discussion, that really small crystal of MoC-phase could be present on EC-1, which became observable by XRD upon sintering at 975 C (EC-3). The different nature of the molybdenum carbide phases, on EC-1 and MoC-commercial, could be involved in this different CO electrooxidation capacity, suggesting that MoC-phase shows more CO electrooxidation capacity than the more stable Mo2C-phase. In summary, it is highly probable that small molybdenum carbide particle size and different nature of the molybdenum carbide phases, were the causes of the higher electroactivity of the lower Mo-containing electrocatalysts, EC-1. The electrochemical results (normalized to Mo-loading) (Table 1) enlarge the difference between EC-1 results and the ones belonging to commercial sample, emphasizing the importance of molybdenum carbide particle size and the nature of the molybdenum carbide phases. However, MoC electrocatalysts begin to catalyze this reaction at potentials close to 0.60 V (see ET in Table 1). This value is fairly positive when compared with other catalysts, as PteRu supported systems [23]. In order to decrease this on-set potential, binary PteMoC electrocatalysts can be prepared. In binary electrocatalysts, molybdenum carbides work as co-electrocatalyst assisting the Pt in the electrooxidation of CO, H2 or methanol, which decreases the binary electrocatalysts on-set potential compared to MoC materials (without Pt). Moreover, as MoC works as co-catalyst, the required Pt quantity would be lower than usual commercial electrocatalysts, thereby decreasing the electrocatalyst’s production cost. As the stability is a key factor in any fuel cell catalyst, the stability of MoxC electrocatalyst has been tested by doing multiple cycles. This cyclic voltammograms are shown in Fig. 7. It is observed that a drastic decrease in the current occurs from the first to the second cycle, then a continuous decrease is observed but after 15 cycles the cyclic voltammogram remains constant. Then, potential cycling in the same range mentioned before after CO adsorption or in a methanol solution displays a stable catalyst.
Table 1 e Threshold potential (ET), peak potential (Emax), maximum current (Imax), and electrooxidation charge (QCO) of samples. EC-precursor ET/V Emax/V Imax/mA Momass =mA,mg1 Imax Mo QCO/mC Momass =mC,mg1 QCo Mo a EC-2 results are not shown in Fig. 6. b N/React: no-reaction was detected.
b
N/React N/Reactb N/Reactb N/Reactb N/Reactb N/Reactb
EC-1 0.63 0.85 8.44 42.20 0.875 4.375
EC-2a
MoCcommercial b
N/React N/Reactb N/Reactb N/Reactb N/Reactb N/Reactb
0.60 0.90 3.47 0.07 0.355 0.008
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0.006
1
Current / A
0.004
0.002
0.000
-0.002
-0.004
1 -0.006 0.0
0.1
0.2
0.3
0.4
0.5
0.6
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0.8
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1.0
1.1
Potential vs RHE / V Fig. 7 e Cyclic voltammograms recorded during activation of EC-1. v [ 0.10 V/s; base electrolyte: 0.5 M H2SO4.
4.
Conclusions
The results presented in this report on molybdenum carbide as electrocatalyst for the PEMFC anode allow drawing the following conclusions: (i) The CO electrooxidation capacity of MoxC electrocatalyst systems has been proven. Therefore, the CO electrooxidation test is a reliable method for detecting the presence of molybdenum carbide phases, even though traditional characterization techniques are unable to detect them. (ii) To obtain scattered particles of MoxC on the CB substrate, the carbothermic reduction process, using H2 as reduction agent and carbon support as C source for the carburization, yield small MoxC particles. (iii) The small size of MoxC enhances electrocatalyst activity in CO electrooxidation. Therefore, highly dispersed MoxC on carbon support produces 60% more current density than the commercial MoxC sample, although the Mo content of this latter sample is more than 20 times higher than in the former. (iv) Since MoC shows activity in the electrooxidation of both CO and methanol and hydrogen [11e13], it is a promising candidate as co-catalysts with Pt for the electrocatalytic materials for PEMFC.
Acknowledgements The authors wish to thank for financial support for the Project MAT2008-06631/C03-02/MAT. RGL and MVMH acknowledge the Spanish Ministry of Science and Innovation and the European Social Fund for “Ramon-y-Cajal” contracts, OVG acknowledges them for technical contracts.
references
[1] Jha N, Mohana-Reddy AL, Shaijumon MM, Rajalakshmi N, Ramaprabhu S. Pt-Ru/Multi-walled carbon nanotubes as electrocatalysts for direct methanol fuel cell. Int J Hydrogen Energ 2008;33(1):427e33.
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[2] Steele BCH, Heinzel A. Materials for fuel-cell technologies. Nature 2001;414:345e52. [3] MacDonald JP, Gualtieri B, Runga N, Teliz E, Zinola CF. Modification of platinum surfaces by spontaneous deposition: methanol oxidation electrocatalysis. Int J Hydrogen Energ 2008;33(23):7048e61. [4] Jow J-J, Yang S-W, Chen H-R, Wu M-S, Ling T-R, Wei T-Y. Co-electrodeposition of PteRu electrocatalysts in electrolytes with varying compositions by a double-potential pulse method for the oxidation of MeOH and CO. Int J Hydrogen Energ 2009;34(2):665e71. [5] Ball S, Hodgkinson A, Hoogers G, Maniguet S, Thompsett D, Wong B. The proton exchange membrane fuel cell performance of a carbon supported ptmo catalyst operating on reformate. Electrochem Solid-State Lett 2002; 5:31e4. [6] Han DM, Guo ZP, Zeng R, Kim CJ, Meng YZ, Liu HK. Multiwalled carbon nanotube-supported Pt/Sn and Pt/Sn/ PMo12 electrocatalysts for methanol electro-oxidation. Int J Hydrogen Energ 2009;34(5):2426e34. [7] Ordo´n˜ez LC, Roquero P, Sebastian PJ, Ramı´rez J. CO oxidation on carbon-supported PtMo electrocatalysts: effect of the platinum article size. Int J Hydrogen Energ 2007;32(15): 3147e53. [8] Zeng J, Lee J-Y. Ruthenium-free, carbon-supported cobalt and tungsten containing binary & ternary pt catalysts for the anodes of direct methanol fuel cells. Int J Hydrogen Energ 2007;32(17):4389e96. [9] Levy RB, Boudart M. Platinum-like behavior of tungsten carbide in surface catalysis. Science 1973;181:547e9. [10] Bennett LH, Cuthill JR, McAlister AJ, Erickson NE. Electronic structure and catalytic behavior of tungsten carbide. Science 1974;184:563e5. [11] Jin-Ham D, Ganesan R, Sung-Lee J. Tungsten carbide microsphere as an electrode for cathodic hydrogen evolution from water. Int J Hydrogen Energ 2008;33(23): 6865e72. [12] Weigert EC, South J, Rykov SA, Chen JGG. Multifunctional composites containing mlybdenum carbides as potential electrocatalysts. Catal Today 2005;99:285e90. [13] Ma C, Sheng J, Brandon N, . Zhang Ch, Li G. Preparation of tungsten carbide-supported nano platinum catalyst and its electrocatalytic activity for hydrogen evolution. Int J Hydrogen Energ 2007;32(14):2824e9. [14] K. Kourtakis, US 2003059666 A1 (2003). [15] Chaudhury S, Mukerjee SK, Vaidya VN, Venugopal V. Kinetics and mechanism of carbothermic reduction of MoO3 to Mo2C. J Alloy Compd 1997;261:105e13. [16] Szymanska-Kolasa A, Lewandowski M, Sayag C, Brodzki D, Djega-Mariadassou G. Comparison between tungsten carbide and molybdenum carbide for the hydrodenitrogenation of carbazole. Catal. Today 2007;119:35e8. [17] Struck BD, Neumeister H, Naoumidis A. Tungsten carbide cathodes for hydrogen production in acidic electrolytes. Int J Hydrogen Energ 1986;11-8:541e8. [18] Moreno-Castilla C, Alvarez-Merino MA, Carrasco-Marin F, Fierro JLG. Tungsten and tungsten carbide supported on activated carbon: surface structures and performance for ethylene hydrogenation. Langmuir 2001;17:1752e6. [19] Hatano Y, Ishiyama K, Homma H, Watanabe K. Improvement in high temperature stability of Pd coating on Nb by Nb2C intermediate layer. Int J Hydrogen Energ 2007;32(5):615e9. [20] Martı´nez-Huerta MV, Rodrı´guez JL, Tsiouvaras N, Pen˜a MA, Fierro JLG, Pastor E. Novel synthesis method of CO-Tolerant PtRuMoO x nanoparticles: structural characteristics and performance for methanol electrooxidation. Chem Mater 2008;20:4249e59.
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[21] Joint Committee on Powder Diffraction Standard. International centre for diffraction data; 1997. Pennsylvania. [22] Yang ZH, Cai PJ, Shi L, Gu YL, Chen LY, Qian YT. A facile preparation of nanocrystalline Mo2C from graphite or carbon nanotubes. J Solid State Chem 2006;179:29e32.
[23] Stevens DA, Rouleau JM, Mar RE, Bonakdarpour A, Atanasoski RT, Schmoeckel AK, et al. Characterization and PEMFC testing of Pt1 xM x (M¼Ru, Mo, Co, Ta, Au, Sn) anode electrocatalyst composition spreads. J Electrochem. Soc 2007; 154:566e76.
Available at www.sciencedirect.com
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Highly dispersed molybdenum carbide as non-noble electrocatalyst for PEM fuel cells: Performance for CO electrooxidation R. Guil-Lo´pez a,*, M.V. Martı´nez-Huerta a, O. Guille´n-Villafuerte b, M.A. Pen˜a a, J.L.G. Fierro a, E. Pastor b a b
Instituto de Cata´lisis y Petroleoquı´mica (CSIC), Marie Curie 2, Cantoblanco, E-28049 Madrid, Spain Departamento de Quı´mica Fı´sica, Universidad de La Laguna, Astrofı´sico Francisco Sa´nchez s/n, E-38071 La Laguna, Tenerife, Spain
article info
abstract
Article history:
CO electrooxidation on nanocrystalline molybdenum carbide has been studied through CO
Received 29 January 2010
stripping measurements using cyclic voltammetry. The active molybdenum carbide was
Received in revised form
obtained from the carbothermic reduction of really very small molybdenum oxide particles
29 March 2010
supported on Vulcan XC-72 carbon black (CB). In order to obtain highly dispersed molyb-
Accepted 8 May 2010
denum carbide particles, low molybdenum loading and control of the carbothermic
Available online 16 June 2010
reduction conditions of CB-supported molybdenum oxide were employed to avoid Mo sintering during the carburization process. This work provides experimental evidence on
Keywords:
the CO electrooxidation capability of the Mo carbide phase, which to the best of our
Non-noble metal electrocatalyst
knowledge is reported for the first time. The small particle size of carbide electrocatalyst
Molybdenum carbide
exhibited better performance for CO electrooxidation than the commercial bulk molyb-
PEMFC
denum carbide sample.
CO electrooxidation
1.
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Introduction
The parallel growth of energy demand and environmental awareness has driven research into new energy sources, as well as into new technologies for energy interconversions. Several options are being studied to solve this latter problem, and fuel cells (FCs) seem to stand out from all the other options. The proton exchange membrane fuel cell (PEMFC) is perhaps the most promising power source to optimize energy use for mobile applications and standalone utilities. Electrocatalysts for hydrogen in PEMFC or methanol electrooxidation in direct methanol fuel cell (DMFC) anode comprise mainly Pt or its alloys, thereby providing the best electroactivity [1]. However, both fuels, H2 streams from
hydrocarbons reforming and methanol, content carbon monoxide (CO) or it is a reaction intermediate that poisons the Pt centres by CO chemisorption and reduces fuel cell performance by decreasing the number of Pt active centres, even at very low CO concentration (10 ppm) [2]. To increase CO tolerance, Ru is added to the electrocatalyst, which allows reducing the amount of Pt [3,4]. Nevertheless, the addition of Ru involves another noble metal and so adds cost, hence rendering the PEMFC and DMFC electrocatalyst less attractive. Accordingly, the complete and immediate industrial development of these fuel cells is economically less viable. Moreover, it still requires increasing noble metal loadings, and the tolerance of PteRu catalysts is not high enough when used at economic electrode Pt loadings (below 0.25 mgPt$cm2) with higher levels of CO (above 10 ppm) [5].
* Corresponding author. Inst. Cata´lisis & Petroleoquimica-CSIC, C/Marie Curie 2, Cantoblanco, E‑28049 Madrid, Spain. Tel.: þ34 915854947; fax: þ34 915854760. E-mail address: [email protected] (R. Guil-Lo´pez). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.05.044
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It has been shown that systems based on the addition of Mo [5e7] or W [8] to Pt or to PteRu electrocatalysts have a greater CO tolerance than PteRu catalysts, thus a new nonnoble metal electrocatalyst appears to be an interesting future alternative. Likewise, it is known that metallic carbides of molybdenum, vanadium or tungsten mimic the catalytic behaviour of certain noble metals [9,10]. Thereby, some preliminary studies on the electrocatalytic behaviour of molybdenum carbides [11,12] and tungsten carbides [13] showed activity in methanol or hydrogen electrooxidation using bimodal Pt and carbide electrocatalysts [14]. These results make the electrocatalytic capacity of molybdenum carbides (MoxC) systems for CO electrooxidation highly attractive because MoxC loaded electrocatalysts perform well for both methanol or hydrogen and CO electrooxidations. In this case, the increase of Pttolerance to CO could decrease the amount of Pt required in the electrocatalyst even in Ru-absence. This work focuses on the preparation of MoxC electrocatalysts in the absence of noble metals and on the use of the cyclic voltammetry technique of CO electrooxidation (CO stripping) to evaluate the CO electrooxidation capacity of MoxC electrocatalyst, which is tested for the first time. The importance of a small particle size of metal carbide phase in the methanol electrooxidation capacity has already been reported [11e13]. Therefore, to guarantee high dispersion of MoxC, electrocatalysts containing low molybdenum loadings were prepared via the carbothermic reduction method [15e19].
2.
Experimental
2.1.
Electrocatalyst preparation
The preparation of an electrocatalyst supported on carbon black (CB) involves two steps: (i), deposition of molybdenum precursor on the CB substrate; and (ii), carburization of precursor to form MoxC electrocatalyst followed by surface passivation to stabilize the molybdenum carbide formed. The electrocatalyst precursor (EC-precursor) was prepared by a modified impregnation method with molybdenum pentachloride, with precipitation of hydrous molybdenum oxide (MoOxHy) suspension on the CB surface using tetramethylammonium hydroxide solution as precipitating agent [20]. The EC-precursor was subsequently carburized using temperature-programmed reduction (TPR) with hydrogen. Carburization processes were carried out using a U-shaped quartz fixed-bed reactor (4 mm ID) with 100 mL min1 of 10 vol % H2/N2 mixture and atmospheric pressure, with 60 mg of EC-precursor. The reactor was on-line with a Baltzer Prisma QMS 200 TM quadrupole mass spectrometer to monitor all processes. Two different carbothermic reduction methods were carried out. In carbothermic reduction-1, the temperature was increased from room temperature (r.t.) to 840 C at 10 C$min1 (coded EC-1). In carbothermic reduction-2, the temperature was increased from r.t. to 600 C at 10 C$min1, and then, maintained at 600 C for 2 h (coded EC-2). In addition, EC-3 molybdenum carbide electrocatalyst was prepared as the EC-1 (carbothermic reduction-1) except that the
temperature of carburization was 975 C. The EC-3 sample was prepared in order to sinterize MoxC particles, thus facilitating their detection by XRD technique. All electrocatalysts were obtained from the EC-precursor. Since the carbide catalysts are pyrophoric, after cooling the samples to room temperature, the electrocatalysts were passivated by switching to a flow of O2/Ar (1% O2/Ar at a total flow of 150 mL min1) for 3 h. During the passivation step, a protective oxide layer was developed on the surface of the carbide phase. All steps were followed by monitoring the main [m/z] signals relative to H2, H2O, CO, CO2 and Ar among the products. No other products were detected. To remove any error due to changes in pressure, all signals were referred to the Ar signal. CB support (Vulcan XC-72) was provided by Cabot Co. Commercial bulk of molybdenum carbide was used as reference (coded MoCcommercial). Chemicals, reagents and reference material were purchased from Aldrich.
2.2.
Catalyst characterization
Mo-loading was determined by chemical analysis using an ICP-AES PerkineElmer Optima 3300 DV spectrometer. Additional data on chemical analysis by thermogravimetric (TG) measurements were obtained through the loss of weight during carbon support combustion with a flow of air (100 mL min1) using a Mettler Toledo TGA/SDTA 851 thermal analyser. Powder X-ray diffraction (XRD) patterns of materials were recorded in a Seifert 3000 P diffractometer, using nickelfiltered Cu Ka1 radiation, and a scanning rate of 0.04 2q s1, in the 2q range of 10 e65 . TPR analyses were performed on a Micromeritics 2900 TPD/TPR instrument. The reducing agent was 10% vol. H2/Ar, with a gas flow of 50 mL min1. The samples (20 mg) were heated from r.t. to 1000 C at a rate of 10 C$min1. Transmission electron microscopy (TEM) was recorded using a Philips Technai 20 microscope operating at 200 kV. The samples were crushed in an agate mortar, dispersed in acetone and dropped onto a holey copper microgrid.
2.3. Electrochemical methods: CO-stripping voltammetry Electrochemical measurements were carried out in a standard three-electrode electrochemical cell at room temperature, with a Radiometer Analytical Model PGZ 301 potentiostat. The working electrode was a glassy carbon electrode of 3 mm in diameter. For the preparation of the ink, electrocatalysts (10 mg) were dispersed in water (1 mL) and Nafion (30 mL) in an ultrasonic bath for 30 min. The ink (5 mL) was deposited onto the electrode and dried under Ar flow for 30 min. Potentials in this work are reported versus a reversible hydrogen electrode (RHE). The CO electrooxidation capacity of the molybdenum carbide materials was characterized by COads stripping in 0.5 M H2SO4 (98% Merck p.a.) solution applying the cyclic voltammetry technique, using the adsorption and subsequent CO electrooxidation as reaction test. This procedure was used to prove: (i) the CO electrooxidation capacity of MoxC; and (ii) the presence of the carbide on the electrocatalysts. For this purpose, gaseous CO was bubbled into the cell for 30 min at a constant voltage of 0.02 V vs. RHE. After CO removal
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3.
Results and discussion
enlargament: x 25 Temperature
600
500
Mass signal / a.u.
400
300
H2O
200
T / ºC
(Ar purging for 30 min), the potential was scanned from the adsorption potential up to 1.10 V and three consecutive cycles were recorded at a 10 mV/s scan rate. The stability of MoxC electrocatalyst has been tested by doing multiple cycles. Activation of the electrode has been performed by recording 25 potential cycles between 0 and 1.10 V at 0.10 V/s in 0.5 M H2SO4. All reagents used were of analytical-grade.
CO2 100
CO
3.1.
Electrocatalyst preparation
0
Fig. 1 shows the carburization results of EC-1, which are expressed in terms of the corresponding [m/z] mass spectra as a function of time and temperature (thermo-programmed carbothermic reduction experiments). The only products detected were water and carbon oxides, CO and CO2, among H2 and Ar. Fig. 1b enlarges the signals to detect the CO2 and H2O peaks (note the 25-fold smaller scale). Two processes can be observed: (i), a first one at lower temperature is mainly associated to H2O production, which maintains a maximum value from 510 to 675 C; and (ii), a second one at 780 C is associated to a massive CO and CO2 production. Therefore, COx evolution occurs at higher temperature up to 700 C (Fig. 1). As no oxygen is present in the environment, the molybdenum reduced species (MoO3x) in the first process react at higher temperatures with the carbon substrate to form COx in the gas, while simultaneously forming the carbide phase (MoxC). The first assignment of both processes relates the low-temperature process to partial Mo-reduction and the high temperature process to Mo-carburization. In order to isolate the first process, carbothermic reduction-2 was carried out at 600 C (see Experimental Section 2.1). These conditions ensure the development of the lowtemperature process (EC-2). Fig. 2 shows the results obtained
900
(a)
(b)
CO
800
CO
Mass signal / a.u.
700
tem pe rat ur e
enlargament: x 25
500
H2O
400
T / ºC
600
300
CO2
0
200
CO2
H2O
100 20
40
60
80 0
20
40
60
80
t / min Fig. 1 e Results from the carburization stage of EC-1 preparation: (a) The results for CO (solid line), CO2 (dash line) and H2O (dot line) production are represented by their corresponding [m/z] mass profiles as a function of temperature; (b) The [m/z] mass profiles have been enlarged in order to facilitate the identification of the peaks from CO2 and H2O profiles.
0
30
60
90
120
150
-100 180
t / min Fig. 2 e Results from the carburization stage of EC-2 preparation: The results for CO (solid line), CO2 (dash line) and H2O (dot line) production are represented by their corresponding [m/z] mass profiles as a function of temperature. The [m/z] mass profiles have been enlarged in order to facilitate the identification of the peaks.
from EC-2 carburization. In this case, no massive CO production is observed (Fig. 2), which indicates the involvement of the low-temperature process. As a result, EC-2 develops partially reduced (MoO3x) species but no carburization takes place, as there are no COx species in the gas phase (Fig. 2). Fig. 3 compares the H2O (Fig. 3a) and CO2 (Fig. 3b) evolution of both carbothermic reductions-1 and -2 as a function of time (note that process-2 presents an isothermal stage at 600 C). The same H2O quantity is produced in both cases (0.01 a.u. below each curve), indicating that the same quantity of molybdenum is reduced in both cases. Taking into account the high temperature of carbothermic reduction-1 conditions, it can be concluded that the reduction of all the molybdenum oxide supported phase is completed using both carbothermic conditions. Therefore, CO and CO2 formation seems to be linked to Mo carburization. Figure 3b shows the low-intensity signal of CO2 production with both carbothermic reduction conditions. Two peaks are detected with carbothermic reduction-1 conditions (solid line in Fig. 3b). The CO2 peak at low temperature (at 350 C) appears when both carbothermic conditions were used. This peak is connected to the loss of oxygenated groups on the carbon support surface. The CO2 peak at high temperature (ca. 780 C) appears when carbothermic reduction-1 conditions were used. This CO2 production at 780 C is therefore linked to the Mo-carburization process. In short, it can be proposed that: (i), water production (from 510 to 675 C) is associated to the formation of the partiallyreduced Mo oxide phases; (ii), the low-temperature CO2 peak (at 350 C) stems from the loss of oxygenated groups on the carbon surface; and (iii), the high temperature CO2 peak arises from the carburization of partially-reduced Mo oxide phases. Thus, molybdenum carbide phases are only expected in EC-1 electrocatalyst. The electrochemical results reveal the success of the carburization process.
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Fig. 3 e Comparative study of the results from the carburization stage of EC-1 (solid line) and EC-2 (dot line) preparations. The results for H2O (a) and CO2 (b) production are represented by their corresponding [m/z] mass profiles as a function of time.
3.2.
Catalyst characterization
ICP analysis of EC-precursor and EC electrocatalysts showed low metal loading: 4% wt Mo/CB (0.5% atomic Mo/CB), which is very close to the nominal value. Similar values of Mo-loading were obtained through the weight lost of the samples during TG measurements. Mo-loadings between 3.5 and 4.0% wt were recorded by TG analysis for all samples. For these samples, the crystallite size of molybdenum phase was found to be very low, which meant that no diffraction lines were observed in EC-1 sample, suggesting that crystallite size falls below the XRD technique’s detection limit (below ca. 3 nm). Fig. 4 compares the XRD patterns of the Mo/CB-precursor, CB substrate and commercial reference compound with the molybdenum carbide electrocatalysts (EC-1 and EC-3). Crystalline phases were identified with the Joint Committee on Powder Diffraction Standards (JCPDS) files [21]. Two weak and broad peaks (note the enlargement: x100, regarding MoCcommercial in Fig. 4e) are presented in the diffraction patterns of both carburized electrocatalysts (EC-1 and EC-3, Fig. 4a and b, respectively) and the precursor (Fig. 4c). These peaks correspond to the basal planes of graphene in CB-support (Fig. 4d). No signal from the molybdenum phases were detected in the XRD patterns of the EC-precursor, EC-1 and EC-2 samples (diffraction pattern not shown) samples. Two hypotheses can we proposed to explain this fact. In the first one it is assumed that the absence of Mo signals might well account for the presence of molybdenum crystallites of very small size, even after the carbothermic reduction process at 840 C (see EC-1, Fig. 4a). In the second hypothesis, the formation of an
Fig. 4 e XRD patterns of the materials studied: (a) electrocatalyst-1 carburized with carbothermic reduction-1 conditions at 840 C, EC-1; (b) electrocatalyst-3 carburized at 975 C, EC-3; (c) precursor of electrocatalyst (ECprecursor); (d) CB-support vulcan; and (e) commercial bulk MoC (MoC-commercial). The XRD patterns ofEC-1, EC-3, ECprecursor and CB-support (aed) have been enlarged (x100) in order to facilitate the identification of the peaks. Phase identification: B graphite (JPDFN: 75-1621); - Mo2C (JPDFN: 79-744); and , MoC (JPDFN: 45-1015).
amorphous Mo phases is considered. Such amorphous phases would not show XRD signals. Additional experiments were carried out to distinguish between both tentative explanations. Accordingly, the EC-3 sample was treated at 975 C with the aim to sinterize as much as possible crystalline Mo carbide phases, which could be present in EC-1 (carburized at 840 C). XRD patterns of electrocatalyst EC-3 carburized at 975 C shows small crystallite domains (Fig. 4b) whose diffraction lines belong to a less common phase of molybdenum carbide: MoC‑phase (JPDFN: 45-1015). It could be possible that small crystallites of MoC-phase were present on EC-1; they are not be detectable by XRD, but became observable to this technique upon sintering at 975 C (EC-3). It is likely that amorphous molybdenum phases in EC-1sample are transformed into MoCphase at 975 C. Fig. 5 shows the TEM-micrograph of a representative area of EC-1 electrocatalyst at two different magnifications. Well defined characteristic concentric graphene layers in the carbon black particles support are distinguished. No-molybdenum particles are detected in the TEM-micrograph. However, EDX microanalysis of three different areas of Fig. 5 revealed nominal Mo contents of 4.7, 4.7 and 4.9% wt of Mo, which agree with the Mo content determined by ICP and TG (4.0% wt by ICP analysis or 3.6% wt by TG). This nominal Mo
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10
a) MoC-commercial: 94 % wt Mo
0
Current / A
10
b) EC-1: 4 % wt Mo
0
10
c) EC-precursor: 4 % wt Mo
Fig. 5 e TEM images of EC-1 electrocatalyst (x 29,000). Enlarged area recorded at higher magnification (x 200,000).
0
0.2
content proves the presence of Mo on the carbon black particles, even though the Mo-particles over concentric carbon black particles are not visible. Taking into account that Mo-particles are not detected but the EDX‑microanalysis ascertains the presence of Mo, the TEM results agree with a small Mo‑particle size, which is one of the possibilities inferred from XRD results. The extremely small particle size of carbide crystallites (not detected by TEM) could be due to a well known low contrast between carbon and Mo phases by TEM, which would make it difficult to distinguish between small particles of MoxC and carbon support. On the other hand, the MoxC formation involves the metallic Mo diffusion into the carbon support [15,22], which favors Mo dispersion in the region of low Mo-loadings. Regarding the reference material (MoC-commercial), the XRD patterns revealed only the development of the common Mo2C‑phase (JPDFN: 79-744). As additional information, reduction of EC-1 carburized material was carried out. TPR profiles for EC-1 electrocatalyst showed negligible H2-consumption indicating that the Mo carbide phases are completely reduced. However, the low Mo-loading used to avoid the MoxC sinterization makes it difficult to conduct an in-depth analysis of the molybdenum structures in these materials by standard characterization techniques. Apart from this, CO-stripping voltammetry measurements were taken to show the formation of a molybdenum carbide phases.
3.3.
CO-Stripping voltammetry
Fig. 6 presents the results obtained in CO electrooxidation by cyclic voltamperometry of CO stripping with the commercial bulk MoxC (Fig. 6a, MoC-commercial), Mo carbide electrocatalyst (Fig. 6b, EC-1), and its precursor (Fig. 6c, EC-precursor). The resulting curves were obtained by subtracting the second voltamperometric curve from the first one. The second and the third cycles revealed that CO was completely oxidized in the first cycle. The second cycle was used as a background and subtracted from the first cycle in order to show only the current related to CO oxidation that occurs during the
0.4
0.6
0.8
1.0
Potential v.s. RHE / V
Fig. 6 e Results from CO electrooxidation cyclic voltammograms using as electrocatalysts: (a) a commercial bulk MoC (MoC-commercial), (b) EC-1; and (c) EC-precursor. The resulting curves were obtained by subtraction of the second voltamperometric cycle from the first one. Constant voltage CO-adsorption: 0.02 V. Scan rate: 0.01 V sL1.
positive-going scan at E > 0.60 V vs. RHE. It can be observed that the Mo carbide-containing electrocatalysts, mainly the commercial MoC-reference (Fig. 6a) show CO electrooxidation currents in the above mentioned potential range. It is therefore proven that the molybdenum carbide phase is able to perform the CO electrooxidation reaction. If a comparison is made between the performances of the carburized material (EC-1) (Fig. 6b) with the non-carburized EC-precursor (Fig. 6c), it can be concluded that the carburized material (EC-1) presents electroactivity. This result is direct proof that the carburization process has taken place during carbothermic reduction-1 conditions. On the other hand, no CO electrooxidation current was recorded for EC-2 (not shown in Fig. 6), which means that MoxC is not developed in EC-2 sample. This result agrees with the previous assignment of thermal process occurring during the different carburization treatments: the first process at low temperature, found in EC-1 and EC-2 samples, is due to partial reduction of molybdenum oxide, whereas the second process at high temperature, observed only for EC-1, that is, the catalysts with the highest electrooxidation activity towards CO, stems from the carburization of the partially-reduced MoO3x phase produced in the first step. It is therefore proven that carburization is the process at high temperature (780 C) that generates CO and CO2 in the gas phase during its formation and is then active in the CO electrooxidation reaction. These assignments are consistent with the mechanism of molybdenum carbide formation found in the literature, which assumes carbide formation through molybdenum reduced intermediates [15,22]. However, the oxygenated nature of the partially-reduced molybdenum
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intermediate, justified by COx production during the last step of the carburization process, has not yet been proven. The shape and position of the CO electrooxidation peak in the cyclic voltammograms depend on the nature of the molybdenum samples. Four magnitudes were defined: (i) the threshold potential (ET), as the on-set potential where the increase in the anodic current is observed during the positive-going potential scan; (ii) the peak potential (Emax), as the potential at which the maximum CO oxidation current density is achieved; (iii) the maximum current (Imax), which is the current at Emax; and (iv) the electrooxidation charge (QCO), as the result of the integration of the current at E > 0.60 V from the curves given in Fig. 6. Two additional magnitudes were calculated from the maximum current (Imax), and the electrooxidation charge (QCO), which are normalized to Mo-loadings. The normalized magnitudes were the Mo-mass Momass ) and the Mo-mass normalized maximum current (Imax MOmass ). Table 1 normalized electrooxidation charge (QCO contains the main CO electrooxidation results obtained with EC-precursor, EC-1, EC-2 and the reference commercial bulk MoxC material (MoC-commercial). The active electrocatalysts (EC-1 and both reference materials) carry out CO electrooxidation at E > 0.6 V. It is remarkable that similar threshold potentials are obtained with both molybdenum carbide electrocatalysts (see ET in Table 1), suggesting a similar nature of the phases present in both electrocatalysts. Nevertheless, the molybdenum carbide electrocatalysts have different electrocatalytic behaviour regarding the current density produced by each electrocatalyst (see Imax and QCO Table 1). The most active electrocatalyst is EC-1. It should be noted that the catalyst with the lowest metal carbide loading and lowest particle size (EC-1, see XRD pattern in Fig. 4a or TEM-micrograph in Fig. 5) is the catalyst with the highest CO electrooxidation activity (Table 1). Moreover, the EC-1 sample with 4% wt of Mo produces 60% more current density than the reference MoC-commercial sample, whose Mo-loading is 94% wt of Mo. The small particle size of molybdenum carbide on EC-1 surface, which cannot be revealed by TEM and XRD techniques, gives more reactivity to exposed molybdenum carbide than in the reference samples with larger particles size. Accordingly, the highly dispersed molybdenum carbide obtained in EC-1 surface increases the electroactivity of this catalyst. Therefore, the use of the carbothermic reduction process seems to
be the key factor for yielding the highest current density produced by these kinds of electrocatalysts. Another factor to analyze the electrochemical results is the nature of molybdenum carbide phases presented on each material. In this sense, MoC-commercial sample shows an only crystalline phase, Mo2C-phase (see XRD Section and Fig. 4e), while, we proposed in the XRD discussion, that really small crystal of MoC-phase could be present on EC-1, which became observable by XRD upon sintering at 975 C (EC-3). The different nature of the molybdenum carbide phases, on EC-1 and MoC-commercial, could be involved in this different CO electrooxidation capacity, suggesting that MoC-phase shows more CO electrooxidation capacity than the more stable Mo2C-phase. In summary, it is highly probable that small molybdenum carbide particle size and different nature of the molybdenum carbide phases, were the causes of the higher electroactivity of the lower Mo-containing electrocatalysts, EC-1. The electrochemical results (normalized to Mo-loading) (Table 1) enlarge the difference between EC-1 results and the ones belonging to commercial sample, emphasizing the importance of molybdenum carbide particle size and the nature of the molybdenum carbide phases. However, MoC electrocatalysts begin to catalyze this reaction at potentials close to 0.60 V (see ET in Table 1). This value is fairly positive when compared with other catalysts, as PteRu supported systems [23]. In order to decrease this on-set potential, binary PteMoC electrocatalysts can be prepared. In binary electrocatalysts, molybdenum carbides work as co-electrocatalyst assisting the Pt in the electrooxidation of CO, H2 or methanol, which decreases the binary electrocatalysts on-set potential compared to MoC materials (without Pt). Moreover, as MoC works as co-catalyst, the required Pt quantity would be lower than usual commercial electrocatalysts, thereby decreasing the electrocatalyst’s production cost. As the stability is a key factor in any fuel cell catalyst, the stability of MoxC electrocatalyst has been tested by doing multiple cycles. This cyclic voltammograms are shown in Fig. 7. It is observed that a drastic decrease in the current occurs from the first to the second cycle, then a continuous decrease is observed but after 15 cycles the cyclic voltammogram remains constant. Then, potential cycling in the same range mentioned before after CO adsorption or in a methanol solution displays a stable catalyst.
Table 1 e Threshold potential (ET), peak potential (Emax), maximum current (Imax), and electrooxidation charge (QCO) of samples. EC-precursor ET/V Emax/V Imax/mA Momass =mA,mg1 Imax Mo QCO/mC Momass =mC,mg1 QCo Mo a EC-2 results are not shown in Fig. 6. b N/React: no-reaction was detected.
b
N/React N/Reactb N/Reactb N/Reactb N/Reactb N/Reactb
EC-1 0.63 0.85 8.44 42.20 0.875 4.375
EC-2a
MoCcommercial b
N/React N/Reactb N/Reactb N/Reactb N/Reactb N/Reactb
0.60 0.90 3.47 0.07 0.355 0.008
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0.006
1
Current / A
0.004
0.002
0.000
-0.002
-0.004
1 -0.006 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Potential vs RHE / V Fig. 7 e Cyclic voltammograms recorded during activation of EC-1. v [ 0.10 V/s; base electrolyte: 0.5 M H2SO4.
4.
Conclusions
The results presented in this report on molybdenum carbide as electrocatalyst for the PEMFC anode allow drawing the following conclusions: (i) The CO electrooxidation capacity of MoxC electrocatalyst systems has been proven. Therefore, the CO electrooxidation test is a reliable method for detecting the presence of molybdenum carbide phases, even though traditional characterization techniques are unable to detect them. (ii) To obtain scattered particles of MoxC on the CB substrate, the carbothermic reduction process, using H2 as reduction agent and carbon support as C source for the carburization, yield small MoxC particles. (iii) The small size of MoxC enhances electrocatalyst activity in CO electrooxidation. Therefore, highly dispersed MoxC on carbon support produces 60% more current density than the commercial MoxC sample, although the Mo content of this latter sample is more than 20 times higher than in the former. (iv) Since MoC shows activity in the electrooxidation of both CO and methanol and hydrogen [11e13], it is a promising candidate as co-catalysts with Pt for the electrocatalytic materials for PEMFC.
Acknowledgements The authors wish to thank for financial support for the Project MAT2008-06631/C03-02/MAT. RGL and MVMH acknowledge the Spanish Ministry of Science and Innovation and the European Social Fund for “Ramon-y-Cajal” contracts, OVG acknowledges them for technical contracts.
references
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