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C electrocatalysts for ethanol electro-oxidation
Electrochimica Acta 52 (2007) 6622–6629
Comparison of different promotion effect of PtRu/C and PtSn/C electrocatalysts for ethanol electro-oxidation Huanqiao Li a,b , Gongquan Sun a,∗ , Lei Cao a,b , Luhua Jiang a , Qin Xin a,c a
Direct Alcohol Fuel Cell Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China b Graduate School of the Chinese Academy Sciences, Beijing 100039, China c State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Received 12 January 2007; received in revised form 9 April 2007; accepted 11 April 2007 Available online 25 April 2007
Abstract Well dispersed PtSn/C, PtRu/C and Pt/C electrocatalysts were synthesized by a modified polyol process and characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and inductively coupled plasma-atomic emission spectrometry techniques. XRD patterns show that Ru induces the contraction of Pt lattice parameter while Sn makes the Pt crystal lattice extended. Ethanol oxidation activities on the catalysts were studied via cyclic voltammetry (CV) and chronoamperometry (CA) methods at room temperature. It is found that the electrode potential plays an important role in the electrochemical behavior of ethanol oxidation on PtRu/C and PtSn/C catalysts. In the lower potential region, PtSn/C possesses higher performance for ethanol oxidation, while in the higher potential region PtRu/C is more active. The different promotion effects of PtSn/C and PtRu/C to ethanol oxidation can be explained by the structural effect and modified bi-functional mechanism in different potential region. Single cell test of a direct ethanol fuel cell (DEFC) was also carried out to elucidate the promotion effect of PtRu/C and PtSn/C catalysts on the ethanol oxidation at 90 ◦ C. © 2007 Elsevier Ltd. All rights reserved. Keywords: PtRu/C; PtSn/C; Promotion effect; Ethanol oxidation
1. Introduction Recently there has been an increasing interest in the development of direct alcohol fuel cells (DAFCs) because the storage and refilling of the liquid fuels are much easier than gas fuels such as H2 and no reformer subsystem is required [1]. Among different types of alcohol fuels, methanol is one of the most electro-active fuels and can be nearly completely electrooxidized to the final product of CO2 due to its simple molecular structure [2]. Another alternative fuel is ethanol, which is less toxic than methanol and can be mass-produced from fermentation process. In addition, ethanol has a higher theoretical mass energy density (8.01 kWh kg−1 ) in comparison with methanol (6.09 kWh kg−1 ) [3]. To achieve the maximum chemical energy from an alcohol molecule, the alcohol should be completely oxidized to the
∗
Corresponding author. Tel.: +86 411 84379063; fax: +86 411 84379063. E-mail address: [email protected] (G. Sun).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.04.056
final product of CO2 . Since the alcohol molecule (R CH2 OH) contains only one oxygen atom, the extra oxygen atoms must be provided by water, or water adsorbed residues (e.g. OHads species). Pt catalyst is well known as the best material for alcohol oxidation at low temperature; however, pure Pt can be easily poisoned by the strongly adsorbed species such as CO-containing materials, which are formed by the initial dehydrogenation of the alcohol molecules [4]. To enhance the activity of Pt catalyst towards alcohol oxidation, a secondary metal such as Ru, Sn, Mo, Rh or Pb is usually introduced as alloying metal [5–10]. As one of the simplest alcohols, the direct oxidation of methanol has been thoroughly studied and the reaction mechanism has been well established. PtRu nano-clusters have been considered to be the best catalysts in the literatures both experimentally and theoretically [11,12]. The superior performance of PtRu catalyst relative to pure Pt catalyst has been explained in terms of two models: the bi-functional mechanism and the ligand effect. In the bi-functional mechanism model, Ru atom was recognized as an oxygen-containing species provider for the CO oxidative removal. The ligand effect, on the other hand, known as
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the electronic model, was based on the modification of Pt electronic structure by the presence of Ru making Pt atoms less poisoned by CO or methanol dissociate adsorption. Compared with methanol, the complete oxidation of ethanol is difficult to be realized due to the presence of more stable C C bond, which is hard to be activated by the commonly employed catalysts at low temperature. Although different Pt-M catalysts (M = Ru, Sn, Mo, and Rh, . . .) have been explored as ethanol oxidation catalysts, only PtSn and PtRu with optimized compositions and structures are reported to exhibit the enhanced activity with respect to other catalysts [3,13–17]. However, the insight into the promotion function of PtRu and PtSn catalysts towards ethanol oxidation is under debate by far. Fujiwara had observed that high selectivity for CO2 formation during ethanol oxidation on PtRu catalyst at 5–40 ◦ C, while others suggested that PtSn was the most active catalyst to ethanol oxidation [18,19]. In our previous work, it was found that PtSn/C exhibited good performance in a direct ethanol fuel cell (DEFC) at 90 ◦ C. However, detailed work on the product analysis showed that CO2 was formed only in small amounts and the incomplete oxidation products such as acetaldehyde and acetic acid were the main reaction products on PtSn/C, similar to the circumstance on PtRu/C catalyst, which was also reported in others’ work [13–17,20]. This indicates that whether on PtRu or PtSn catalyst, the complete ethanol oxidation was restrained. To elucidate the promotion effect of PtRu/C and PtSn/C catalyst to ethanol oxidation, in this work, electrochemistry methods of cyclic voltammetery (CV) and chronoamperometry (CA) measurements in addition to single cell tests were employed. The experimental results show that the addition of the secondary metal Ru (or Sn) to Pt catalyst had a different promotion effect on the ethanol oxidation reaction.
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lyst with 45% metal loading was also obtained from this polyol process. 2.2. Catalyst characterizations Physical properties of the freshly prepared catalysts were characterized by inductively coupled plasma-atomic emission spectrometry (ICP-AES), X-ray diffraction (XRD) and transmission electron microscope (TEM) techniques. ICP-AES analyses of PtRu/C and PtSn/C catalysts were conducted to determine the metal content in the catalysts. For the ICP-AES analysis, the samples were firstly burned in an oven to remove the carbon support and then the samples were dissolved in the aqua regia solution to analyze on a PLASMA-SPEC-1 instrument. The crystalline structure of these supported catalysts was determined using the powder XRD technique. The data were obtained using a Rigaku Rotalflex (RU-2000B) X-ray power diffractometer with a Cu K␣ radiation source and a Ni filter. About 30 mg-powdered samples were kept in a quartz block. Then the powder was pressed onto the quartz block using a glass slide to obtain a uniform distribution. The 2θ Bragg angles were scanned over a range of 15–85◦ at a rate of 5◦ per minute with a 0.02◦ angular resolution. The tube current was 100 mA and the tube voltage was 40 kV. Bragg formula [23] was employed to calculate the lattice parameter of the catalysts. The morphology and the particle size distributions of PtRu/C and PtSn/C catalysts were analyzed by TEM observation. At first, the catalysts were ultrasonically dispersed in ethanol solution to obtain uniform catalyst ink and mounted onto a copper grids covered with holey carbon film. A JEOL JEL2000EX microscope, operating at 100 KV, was used for TEM observation.
2. Experimental details 2.3. Electrochemistry tests 2.1. Catalyst preparation In a typical process, PtSn/C catalyst with the total metal loading of 45% and a nominal Pt:Sn atomic ratio of 1:1 was obtained by a modified polyol process described in previous work [21,22]. Vulcan XC-72R carbon black (500 mg) was pretreated with 5 M HCl and 2 M HNO3 solution to remove some ash and then was suspended in 50 mL ethylene glycol under ultrasonic stirring. Appropriate amounts of H2 PtCl6 ·6H2 O and SnCl2 aqueous solution were added drop by drop to carbon suspension and 1 M NaOH aqueous solution was added to adjust the pH value of the mixture to 12–13 approximately. Reduction reaction was performed by heating the mixture solution at 130 ◦ C for 3 h, during which high purity argon gas was passed through the reaction system to remove the organic by-products. After cooling to the room temperature, the resulted catalyst was washed with hot distilled water until no chloride anion in the filtrate could be detected by 1 M AgNO3 reagent. The obtained catalyst powder was dried in a vacuum oven at 70 ◦ C overnight. Similarly, PtRu/C with the same total metal loading (45%) and a nominal Pt:Ru atomic ratio of 1:1 was also synthesized in the same procedure. As a comparison, Pt/C cata-
A CHI 760B potentistat/galvanostat was used for the electrochemistry measurements in a standard three-compartment electrochemical cell. All the electrochemistry experiments were carried out at 25 ± 0.5 ◦ C. The working electrode was a glass carbon disk with a diameter of 4 mm held in a Teflon cylinder. A Pt-foil counter-electrode and a saturated calomel reference electrode (SCE) (separated by a KNO3 salt bridge) were used. The potentials in this work are referred to normal hydrogen electrode (NHE). Thin porous coating disk electrode design had been described in previous paper [24]: About 5 mg the powdered catalyst was ultrasonically suspended in 4 ml ethanol and 50 L Nafion® solution (5 wt.%, Du Pont Corp., USA) for about 30 min to obtain the catalyst ink, then 20 L of the ink was spread on the surface of the micro-electrode using a micropipette and dried at room temperature to eliminate the solvent. The base cyclic voltammetry experiments were carried out in 0.5 M HClO4 solution, which was deaerated by ultra-high-purity nitrogen before each experiment. For the evaluation of the ethanol oxidation activity, ethanol was added to the base electrolyte to obtain the required concentration of 1 M and the corresponding CV curves were recorded at a scan rate of 25 mV s−1 . Chronoamperometry
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tests were carried out to evaluate the effect of the electrode potential on ethanol oxidation in the same electrolyte 0.5 M HClO4 containing 1 M ethanol. The potential was fixed at 0.45 V in the lower potential region and 0.8 V in the higher potential region. 2.4. Single cell tests The membrane electrode assembly (MEA) used for the single cell test was fabricated by hot-pressing the anode and cathode gas diffusion electrodes onto a pretreated Nafion 115 membrane at 130 ◦ C and with a pressure of 12 bar for 3 min. The resulted MEA was with 2 mg cm−2 total metal loading of PtRu/C or PtSn/C on the anode and 1 mg cm−2 metal loading of Pt/C40 wt% from Johnson Matthey Corporation on the cathode. The performance of the MEA was evaluated with a commercial fuel cell test instrument (Arbin Corporation, USA). Galvanostatic polarization curves were obtained at 90 ◦ C with 1 M liquid ethanol solution fed on the anode at a rate of 1 mL min−1 and the pressure of the oxygen gas reactants in the cathode was 2 bar. The anode polarization curve for ethanol oxidation in a single cell test with 1 M liquid ethanol solution was obtained with an EG&G 273 potentistat/galvanostat on the same rig by employing hydrogen saturated cathode as the counter and reference electrode. 3. Results and discussion 3.1. Catalyst characterizations XRD is a bulk method, and reveals the information on the bulk structure of the catalyst and its support. Fig. 1 presents the XRD patterns of the as prepared PtSn/C, PtRu/C and Pt/C catalysts. It could be clearly seen that PtRu/C and Pt/C catalysts display the typical Pt face centered cubic (f.c.c.) crystal structure, except that the first peak was associated with the XC-72 R carbon support. However, for PtSn/C sample, there are a few more tin oxide diffraction peaks (SnO2 , JCPDS #411445, denoted with * sig-
Table 1 Mean particle sizes and bulk compositions of the catalysts Sample
Lattice parameter (nm)
Particle size (nm)
Bulk composition
PtRu/C PtSn/C Pt/C
0.3853 0.3961 0.3913
1.6 ± 0.7 nm 2.7 ± 0.9 nm 3.4 ± 1.0 nm
1.09:1 3.1:1 N
nals) in addition to the main characteristic feature of Pt f.c.c. structure. The (1 1 0) diffraction peak of SnO2 at 26.6◦ overlaps with the (0 0 2) diffraction peak of carbon support, which results in the higher intensity of carbon support in the XRD patterns of PtSn/C catalyst than in the other two catalysts. It can be clearly observed that the diffraction peaks of PtSn/C and PtRu/C catalysts shift in opposite directions in comparison with Pt/C catalyst. Pt lattice parameters of these carbon-supported catalysts are calculated from the (2 2 0) diffraction peak positions in the XRD patterns and listed in Table 1. The lattice parameter of Pt/C catalyst (a = 0.3913 nm) is a little smaller than that of the bulky Pt metal (JCPDS # 040802, a = 0.3923 nm), which has been ascribed to the platinum–carbon interactions or size effect [25]. In consistent with the literature data, PtRu/C has a much smaller lattice parameter (a = 3.853 nm) and PtSn/C has the enlarged one (a = 0.3961 nm) [26,27]. This may indicate that the addition of the secondary metal Ru or Sn had different effect on the crystal structure of Pt/C catalyst. This can be explained by their ˚ RRu = 1.34 A ˚ different atomic sizes to Pt element (RPt = 1.39 A, ˚ and Rsn = 161 A). Because Sn has a comparable bigger atomic radius than Pt metal, the addition of some amount of Sn to Pt/C could induce the extension of Pt crystal structure. Oppositely, the addition of the smaller atomic sized Ru could result in the contraction of Pt lattice parameter. Based on Vegard’s law, the composition of the PtRu/C alloy could be determined from the shift of Pt diffraction peaks, or the variation of the lattice parameter from the XRD patterns. From literature data [28], a linear relation ship of the lattice parameter and the alloyed Ru atomic fraction (<0.7) has been proposed. a(Pt−M) = a(Pt) − kx(Ru)
Fig. 1. XRD patterns of (a) PtRu/C, (b) PtSn/C and (c) Pt/C electrocatalysts at a scan rate of 5◦ min−1 . The diffraction peaks of SnO2 (JCPDS # 411445) were denoted with * signal.
(1)
where a(Pt) = 0.3913 nm is the lattice parameter of Pt/C catalyst and k is a constant = 0.0124 nm. According to this equation, the atomic content of alloyed Ru (x(Ru) ) in our PtRu/C sample is 0.47. This indicates that most of Ru is alloyed with Pt in PtRu/C catalyst. For PtSn/C catalyst, if the Vegard’s law is still suitable to the occasion of PtSn/C catalyst, a similar linear relation between the lattice parameter and the alloyed Sn content will be proposed here, taking lattice parameter of the Pt3 Sn1 /C as 0.4000 nm for a reference. The calculated Sn content in our PtSn/C catalyst is about 0.14, indicating only 50% is alloyed with Pt metal, if the bulk composition of PtSn/C determined from ICP-AES test is considered. The alloyed Sn content in PtSn/C is much lower than its nominal values, which has been found in Pt-M (M is non noble metal like Fe or Sn element) in our work. The obtained bulk atomic ratios of the catalysts from ICP-AES are also listed in Table 1. TEM images of PtRu/C and PtSn/C catalysts are presented in Fig. 2. It could be seen that the approximately spherical metal
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Fig. 2. TEM images of (a) PtSn/C and (b) PtRu/C catalysts and the size distributions of (c) PtSn/C and (d) PtRu/C.
nanoparticles of the two catalysts are uniformly dispersed on the surface of Vulcan XC-72R. In comparison with PtSn/C catalyst, the mean particle size of PtRu/C is much smaller than that of PtSn/C and the distribution is more uniform than PtSn/C. A little agglomeration observed in PtSn/C catalyst could be ascribed to the presence of Sn hydroxide precipitation originated from the hydrolysis of Sn in alkaline solution. Fig. 2c and d show the histograms of particle size distributions for PtSn/C and PtRu/C catalysts. The particle size of PtSn/C catalyst ranges between 1 and 5 nm, with a mean diameter of 2.7 nm. For PtRu/C catalyst, the mean particle size is only 1.6 nm and the size distribution is in the range of 1–3 nm. The small particle sizes and homogeneous size distributions of both catalysts with high metal loading of 45 wt% should be attributed to the advantages of the modified polyol process in ethylene glycol solution. Because of its comparable big viscosity, ethylene glycol could prevent the agglomerations of the obtained nanoparticles and guarantee the uniform distribution of the obtained nanoparticles [22].
Fig. 3. Primary work showed that the upper potential limit had an important effect on the stability of Ru in PtRu/C catalyst. To avoid the leaching of Ru element, the upper limits are set to 1.0 V for PtRu/C, and 1.2 V for both Pt/C and PtSn/C catalysts [29]. As shown in Fig. 3, the base CV in 0.5 M HClO4 of Pt/C resembles the characteristic features similar to those of
3.2. Ethanol oxidation activity Before the evaluation of the ethanol oxidation activity, the base CV curves of the catalysts were obtained in the supporting electrolyte of 0.5 M HClO4 and the results are presented in
Fig. 3. Cyclic voltammetry of PtSn/C (–), PtRu/C (- - -) and Pt/C (· · ·) electrocatalysts in 0.5 M HClO4 with a sweep rate of 50 mV s−1 at 25 ± 0.5 ◦ C.
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polycrystalline Pt electrodes. The addition of secondary metal Sn in PtSn/C catalyst has a little effect on the CV response. The absence of characteristic features peaks of PtSn/C from Pt/C in hydrogen adsorption–desorption region possibly originates from blockage of Sn-containing species on Pt sites. This phenomenon has also been observed by other researchers for the Pt-M catalysts (M refers to the base metal like Ni or Cr) [30]. However, for PtRu/C catalyst, the base CV exhibits different characteristics from Pt/C and PtSn/C in the entire exploited potential region. Especially in the double layer region, PtRu/C represents much bigger double layer current than Pt/C or PtSn/C does. The big capacitive charging current has been usually ascribed to the activation of H2 O on Ru sites in PtRu/C catalyst. Most experimental and theoretical studies in the literatures has demonstrated that H2 O could be easily activated and dissociated on Ru surface with Ru (OH)ads species at the potential lower than 0.2 V versus NHE [30–32]. As mentioned in the preface of this work, the resulted Ru (OH)ads species could facilitate the removal of COlike poisoning species during the methanol oxidation at lower potential and thus enhance the methanol oxidation activity. Ethanol oxidation activities on PtRu/C and PtSn/C catalysts are evaluated in CV test and the results are displayed in Fig. 4. It could be found from Fig. 4 that the activity of ethanol oxidation on PtRu/C and PtSn/C catalysts varies with the electrode potentials. In the lower potential region, PtSn/C catalyst presents higher activity to ethanol oxidation. When the potential is increased to the higher value, the ethanol oxidation current on PtRu/C exceeds PtSn/C. To elucidate the potential effect to ethanol oxidation activity on PtRu/C and PtSn/C catalysts, CA tests for the ethanol oxidation at different potential were carried out and the results are shown in Fig. 5. It can be clearly seen that the currents for ethanol oxidation on all the catalysts dropped rapidly at first and then became relatively stable. The initial surge of the current is possible due to the charging current or the catalyst poisoning during the ethanol oxidation. Consistent with the CV results, it is found that at a lower potential of 0.45 V, the current for ethanol oxidation on PtSn/C catalyst is significantly
Fig. 5. Chronoamperometry test for ethanol oxidation in 0.5 M HClO4 containing 1 M ethanol on Pt/C (–), PtRu/C (- - -) and PtSn/C (· · ·) catalysts at 0.45 V vs. NHE (a) and 0.8 V vs. NHE (b) at 25 ± 0.5 ◦ C.
larger than on Pt/C or PtRu/C. While at higher potential of 0.8 V, the oxidation current on PtRu/C is the biggest one. This effect of electrode potential on the activity of these catalysts may indicate that different promotion effects PtRu/C and PtSn/C catalysts on the ethanol oxidation reaction. In the hypothesis that the ethanol oxidation on Pt based catalyst is similar to the methanol oxidation behavior. The rate-determining step (RDS) of ethanol oxidation in the lower potential is the dissociative adsorption of ethanol on Pt surface, as displayed as follows. CH3 CH2 OHbulk → CH3 CH2 OHads(Pt)
(2) +
CH3 CH2 OHads(Pt) → CH3 CHOHads(Pt) + H + e
−
(3)
(4)
Fig. 4. Ethanol oxidation on PtRu/C (a) and PtSn/C (b) electrocatalysts in 0.5 M HClO4 + 1 M CH3 CH2 OH with a sweep rate of 25 mV s−1 at 25 ± 0.5 ◦ C. The solid lines are the recorded 10th circles and the dotted lines are the 15th circles intercepted from the full CV curves.
Part of the dissociated formed intermediates like acetaldehyde species may diffuse into the bulk electrolyte, while most of the intermediates are strongly adsorbed on the Pt catalyst surface and contaminate the catalyst by inhibiting the following reaction. The contamination of the catalysts could decrease
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the ethanol oxidation current greatly in the following scans [34]. While in the higher potential region, H2 O could be activated and the resulted OH species could facilitate the incomplete ethanol oxidation to acetic acid. H2 O(M) → OHads(M) + H+ + e−
(5)
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catalysts at low temperature in acidic solution. Apparently, both the introduction of Ru and Sn to Pt/C catalyst could enhance the activity of ethanol oxidation greatly in comparison with pure Pt/C catalyst. However, the promotion effect of Ru and Sn were totally different in different potential region, which was due to the different RDS of ethanol oxidation in different poten-
(6) This indicates at higher potential region the activation of H2 O to result in active oxygen containing species is the RDS for the ethanol oxidation. However, many experimental tests on the electro-oxidation of acetic acid in acid solution reveals that acetic acid is fairly resistant to the electro-oxidation with all the three tested catalysts (Pt/C, PtRu/C and PtSn/C); with almost zero oxidation current in the exploited potential region. This indicates that the incomplete ethanol oxidation to C C compounds like the product of acetic acid, not CO2 , is main component for ethanol oxidation in the explored potential region. Based on the above discussion, different promotion effect of PtRu/C and PtSn/C can be clearly explained. As displayed by the XRD results, the addition of Sn could induce the extension of Pt–Pt distance. The extended Pt–Pt distance could facilitate the dissociation adsorption of much bigger ethanol molecule at lower potential region and thus enhance the ethanol oxidation. The well-defined peak near 0.5 V versus NHE on the first CV curve of PtSn/C is mainly originated from dehydrogenation reaction during the ethanol dissociation adsorption on the catalyst surface. Of course, besides structural effects, a strong ligand effect has been envisaged in the literatures for PtSn making this latter system is less prompt to poisoning by organic species than pure Pt. A number of studies of electronic and surface properties of PtSn/C have confirmed this aspect. However, in this work, we mainly keep more attention on the effect of structure of the ethanol oxidation catalyst. On PtRu/C catalyst, the less pronounced ethanol oxidation current in the lower potential region indicates that the dissociation adsorption of ethanol is unfavorable, which may ascribe to the contracted Pt–Pt distance originated from the addition of Ru. However, in the higher potential region, the electro-oxidation current of ethanol on PtRu/C catalyst is enhanced due to the strong capacity of H2 O activation on PtRu sites. It is interesting to note that PtSn/C catalyst in this potential region presents almost the same poor catalytic activity for ethanol oxidation as Pt/C catalyst, both from the CV and CA results. This is because the dissociation of H2 O on PtSn/C is difficult as on Pt/C catalyst and relatively high positive electrode potentials are needed in comparison with PtRu/C catalyst [33]. The product analysis of ethanol oxidation on PtSn/C and PtRu/C via DEMS technique indicated that the products were mainly consisted of incomplete oxidation products of C C compounds such as acetaldehyde or acetic acid [20], which was consistent with the literature works [34,35]. Based on the above results, we could infer the possible reaction mechanism of ethanol oxidation on PtRu/C and PtSn/C
tial regions. Similarly, the differences in product distribution of ethanol oxidation on PtRu/C and PtSn/C could be also explained by the different role of the foreign element Ru or Sn on Pt/C catalyst in promoting the ethanol oxidation in different potential regions. 3.3. DEFC single cell performance The DEFC single cell performances tested on the homemade equipment are compared in Fig. 6. When the currents are normalized to the geometric area of the single cell, it could be clearly seen that the cell performance of PtSn/C catalyst is better than PtRu/C catalyst in a DEFC in the low discharging current region. The cell voltage of PtSn/C is 170 mV higher than that of PtRu/C catalyst at a current density of 20 mA/cm2 . However, when the discharging current is increased, the cell performance of PtRu/C starts to exceed that of PtSn/C catalyst. To evaluate the ethanol oxidation activity in a gas diffusion electrode at higher temperature, the anode polarizations curves of PtRu/C and PtSn/C catalysts with 1 M ethanol at 75 ◦ C and 90 ◦ C were obtained and are displayed in Fig. 7. As consistent with the halfcell microelectrode CV results at lower temperature (25 ◦ C), the activity of ethanol oxidation of PtRu/C is far lower than
Fig. 6. Polarization curves and power density curves of a direct ethanol fuel cell employing PtSn/C () and PtRu/C () as anode catalyst, respectively. (Anode fuel feeding: 1 M ethanol at l mL min−1 , cell temperature 90 ◦ C, Pcathode : 2 bar, 2 mg cm−2 metal loading of PtRu/C or PtSn/C anode catalyst, 1 mg cm−2 Pt cathode catalyst, Pt/C-40% from JM.) The currents in this figure were normalized to the geometric area of the single cell.
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Fig. 7. The anode polarization curves of ethanol oxidation in a single cell employing PtSn/C (–) and PtRu/C (- - -) as anode catalyst at 75 ◦ C and 90 ◦ C, respectively. 1 M ethanol was fed at l mL min−1 . The cathode, fed with humidified H2 gas, was used as counter electrode and reference electrode.
that on PtSn/C catalyst in the lower discharging current density region at 75 ◦ C. When the potential is increased, the activity of PtRu/C is increased and begins to precede PtSn/C catalyst at 436 mV with a current density of 58 mAcm−2 . When the temperature is increased to 90 ◦ C, the potential at the crossing is decreased to 416 mV and the current is increased to 80 mAcm−2 . This indicates that the temperature has an important effect on the ethanol oxidation behavior on PtRu/C and PtSn/C catalysts. However, whether on the micro electrode in CV and CA test at room temperature or in a gas diffusion electrode at 90 ◦ C (or 75 ◦ C), almost no CO2 bubble could be observed on the surface of the electrode, indicating that the cleavage of C C bond during the oxidation of ethanol is negligible under our experimental conditions. Thus, we could conclude that only simple dehydrogenated reaction on Pt sites (in case of PtSn/C) or partial oxidation to the formation of acetic acid (on PtRu/C catalyst) had happened during the oxidation of ethanol between the room temperature and 90 ◦ C. Because Pt catalyst is well known as the best material for the dehydrogenation reaction of the organic molecule, the cell performance of a DEFC might have a relationship with the Pt metal amount in the MEA. To explore the effect of Pt amount on the cell performance of a DEFC, the current is normalized to the total Pt metal mass loading on the anode, which is determined by the ICP-AES, and it is found that the peak power density of the DEFC employing PtRu/C as anode catalyst is almost equivalent to that of a PtSn/C anode, as shown in Fig. 8. This indicates that the cell performance of a DEFC is determined by the total amount of platinum metal in a MEA to some extent, whether on PtSn/C or PtRu/C catalysts. This may be the reason that Pt3 Sn1 /C even Pt9 Sn1 /C and Pt85 Ru15 /C had been proposed as the most promising catalyst for low temperature ethanol oxidation reaction [1]. However, it should also be noted that the DEFC cell performance whether on PtSn/C or PtRu/C is far from its large-scale applications. To achieve higher ethanol oxidation performance, the C C bond in ethanol molecule must be broken for its further complete oxidation. Because the complete oxidation of ethanol to CO2
Fig. 8. Polarization curves and power density curves of a direct ethanol fuel cell employing PtSn/C () and PtRu/C () as anode catalyst, the currents in this figure were normalized to the total amount of Pt noble metal.
had been proved to be absolutely difficult at lower temperature, the operation temperature should be increased to enhance the ethanol oxidation activity of the catalysts. As displayed in this work, PtRu and PtSn seem to be indispensable components of a highly active ethanol oxidation catalyst. And it is no doubt that the C C bond cleavage process is promoted by high temperature operation, from the point of thermodynamic consideration. However, the biggest bottleneck of high temperature DEFC lies in the development of high temperature membrane. Due to the lack of appropriate proton exchange membrane for DEFC for high temperature operation, most of work is based on Nafion type membrane and the performance is still unsatisfactory. Further work on the key materials of membrane and catalysts at elevated temperature (up to 150–200 ◦ C) is under progress in our group. 4. Conclusions The modified polyol process in the ethylene glycol solution is proved to be suitable to prepare highly dispersed carbon supported Pt-based catalysts with uniform size distribution. The structural characterization of these Pt-based catalysts show that the addition of the secondary metal to Pt/C catalyst has different effect on the crystal lattice of Pt metal, which is determined by the relative atomic radii of M in comparison with that of Pt metal. The addition of bigger atomic sized Sn compared with Pt metal could induce the extension of Pt crystal structure, while the addition of smaller atomic sized Ru could result in the contraction of Pt lattice parameter in PtRu/C catalyst. It is found that the activity of ethanol oxidation on Pt-based catalysts is greatly affected by the secondary metal and the electrode potential. The enhancement of ethanol oxidation on PtSn/C is mainly from the structural effect due to the extend crystal structure, while the promotion of PtRu/C catalyst could be explained by the modified bi-functional mechanism, originated from the enhanced activity of H2 O activation on Ru sites. The maximum power density of PtSn/C is equivalent with that of PtRu/C catalyst when the currents are normalized to the
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total Pt mass loading, indicating that the amount of Pt plays an important role in a DEFC cell performance. The cell performance of a DEFC at low temperature is far from its large-scale application. To enhance the DEFC cell performance, high temperature (up to 150–200 ◦ C) operation should be selected to ensure the complete oxidation of ethanol. Acknowledgement This work was financially supported by National Natural Science Foundation of China (Grant No. 50575036). References [1] C. Lamy, E.M. Belgsir, J.M. Leger, J. Appl. Electrochem. 31 (2001) 799. [2] X.M. Ren, P. Zelenay, S. Thomas, J. Davey, S. Gottesfeld, J. Power Sources 86 (2000) 11. [3] F. Delime, J.M. Leger, C. Lamy, J. Appl. Electrochem. 28 (1998) 27. [4] S.C. Hall, V. Subramanian, G. Teeter, B. Rambabu, Solid State Ionics 175 (2004) 809. [5] O. Neto, M.J. Giz, J. Perez, E.A. Ticianelli, E.R. Gonzalez, J. Electrochem. Soc. 149 (2002) A272. [6] H. Wang, Z. Jusys, R.J. Behm, J. Power Sources 154 (2006) 351. [7] F. Delime, J.M. Leger, C. Lamy, J. Appl. Electrochem. 29 (1999) 1249. [8] B.N. Grgur, N.M. Markovic, P.N. Ross Jr., J. Phys. Chem. B 102 (1998) 2494. [9] J.P.I. de Souza, S.L. Queiroz, K. Bergamaski, E.R. Gonzalez, F.C. Nart, J. Phys. Chem. B 106 (2002) 9825. [10] G.C. Li, P.G. Pickup, Electrochim. Acta 52 (2006) 1033. [11] M. Watanabe, S. Motoo, J. Electroanal. Chem. 60 (1975) 275. [12] M.M.P. Janssen, J. Moolhuysen, J. Catal. 46 (1977) 289. [13] W.J. Zhou, Z.Z. Zhou, S.Q. Song, W.Z. Li, G.Q. Sun, P. Tsiakaras, Q. Xin, Appl. Catal. B 46 (2003) 273. [14] S.Q. Song, G.X. Wang, W.J. Zhou, X.S. Zhao, G.Q. Sun, Q. Xin, S. Kontou, P. Tsiakaras, J. Power Sources 140 (2005) 103.
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[15] L.H. Jiang, G.Q. Sun, S.G. Sun, J.G. Liu, S.H. Tang, H.Q. Li, B. Zhou, Q. Xin, Electrochim. Acta 50 (2005) 5384. [16] W.J. Zhou, S.Q. Song, W.Z. Li, Z.H. Zhou, G.Q. Sun, Q. Xin, S. Douvartzides, P. Tsiakaras, J. Power Sources 140 (2005) 50. [17] W.J. Zhou, S.Q. Song, W.Z. Li, G.Q. Sun, Q. Xin, S. Kontou, K. Pouhanitis, P. Tsiakaras, Solid State Ionics 175 (2004) 797. [18] H. Hitmi, E.M. Belgsir, J.M. Leger, C. Lamy, R.O. Lezna, Electrochim. Acta 39 (1994) 407. [19] N. Fujiwara, K.A. Friedrich, U. Stimming, J. Electroanal. Chem. 472 (1999) 120. [20] L. Colmenares, H. Wang, Z. Jusys, L.H. Jiang, S.Y. Yan, G.Q. Sun, R.J. Behm, Electrochim. Acta 52 (2006) 221. [21] H.Q. Li, Q. Xin, W.Z. Li, Z.Z. Zhou, L.H. Jiang, S.H. Yang, G.Q. Sun, Chem. Commun. (2004) 2776. [22] Y. Wang, J.W. Ren, K. Deng, L.L. Gui, Y.Q. Tang, Chem. Mater. 12 (2000) 1622. [23] V. Radmilovic, H.A. Gasteiger, P.N. Ross, J. Catal. 154 (1995) 98. [24] G. Tamizhmam, J.P. Dodelet, D. Guay, J. Electrochem. Soc. 143 (1996) 18. [25] A.K. Shukla, M.K. Ravikumar, A. Roy, S.R. Baraman, D.D. Sarma, A. Arico, V. Antonucci, L. Pino, N. Giordano, J. Electrochem. Soc. 141 (1994) 1517. [26] F. Colmati, E. Antolini, E.R. Gonalez, J. Electrochem. Soc 154 (2007) B39. [27] E. Antolini, F. Cardellini, L. Giorgi, J. Mater. Sci. Lett. 19 (2000) 2099. [28] D. Chu, S. Gilman, J. Electrochem. Soc. 143 (1996) 1686. [29] H. Yang, N. Alonso-Vante, J.M. Leger, C. Lamy, J. Phys. Chem. B 108 (2004) 1938. [30] C. Roth, N. Benker, T. Buhrmester, M. Mazurek, M. Loster, H. Fuess, D.C. Koningsberger, D.E. Ramaker, J. Am. Chem. Soc. 127 (2005) 14607. [31] T. Lei, M.S. Zei, G. Ertl, Surf. Sci. 581 (2005) 142. [32] P. Waszczuk, G.Q. Lu, A. Wieckowski, C. Lu, C. Rice, R.I. Masel, Electrochim. Acta 47 (2002) 3637. [33] A.B. Anderson, E. Grantscharova, S. Seong, J. Electrochem. Soc. 143 (1996) 2075. [34] T. Iwasita, J. Braz. Chem. Soc. 13 (2002) 401. [35] H. Wang, Z. Jusys, R.J. Behm, J. Phys. Chem. B 108 (2004) 19413.
Comparison of different promotion effect of PtRu/C and PtSn/C electrocatalysts for ethanol electro-oxidation Huanqiao Li a,b , Gongquan Sun a,∗ , Lei Cao a,b , Luhua Jiang a , Qin Xin a,c a
Direct Alcohol Fuel Cell Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China b Graduate School of the Chinese Academy Sciences, Beijing 100039, China c State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Received 12 January 2007; received in revised form 9 April 2007; accepted 11 April 2007 Available online 25 April 2007
Abstract Well dispersed PtSn/C, PtRu/C and Pt/C electrocatalysts were synthesized by a modified polyol process and characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and inductively coupled plasma-atomic emission spectrometry techniques. XRD patterns show that Ru induces the contraction of Pt lattice parameter while Sn makes the Pt crystal lattice extended. Ethanol oxidation activities on the catalysts were studied via cyclic voltammetry (CV) and chronoamperometry (CA) methods at room temperature. It is found that the electrode potential plays an important role in the electrochemical behavior of ethanol oxidation on PtRu/C and PtSn/C catalysts. In the lower potential region, PtSn/C possesses higher performance for ethanol oxidation, while in the higher potential region PtRu/C is more active. The different promotion effects of PtSn/C and PtRu/C to ethanol oxidation can be explained by the structural effect and modified bi-functional mechanism in different potential region. Single cell test of a direct ethanol fuel cell (DEFC) was also carried out to elucidate the promotion effect of PtRu/C and PtSn/C catalysts on the ethanol oxidation at 90 ◦ C. © 2007 Elsevier Ltd. All rights reserved. Keywords: PtRu/C; PtSn/C; Promotion effect; Ethanol oxidation
1. Introduction Recently there has been an increasing interest in the development of direct alcohol fuel cells (DAFCs) because the storage and refilling of the liquid fuels are much easier than gas fuels such as H2 and no reformer subsystem is required [1]. Among different types of alcohol fuels, methanol is one of the most electro-active fuels and can be nearly completely electrooxidized to the final product of CO2 due to its simple molecular structure [2]. Another alternative fuel is ethanol, which is less toxic than methanol and can be mass-produced from fermentation process. In addition, ethanol has a higher theoretical mass energy density (8.01 kWh kg−1 ) in comparison with methanol (6.09 kWh kg−1 ) [3]. To achieve the maximum chemical energy from an alcohol molecule, the alcohol should be completely oxidized to the
∗
Corresponding author. Tel.: +86 411 84379063; fax: +86 411 84379063. E-mail address: [email protected] (G. Sun).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.04.056
final product of CO2 . Since the alcohol molecule (R CH2 OH) contains only one oxygen atom, the extra oxygen atoms must be provided by water, or water adsorbed residues (e.g. OHads species). Pt catalyst is well known as the best material for alcohol oxidation at low temperature; however, pure Pt can be easily poisoned by the strongly adsorbed species such as CO-containing materials, which are formed by the initial dehydrogenation of the alcohol molecules [4]. To enhance the activity of Pt catalyst towards alcohol oxidation, a secondary metal such as Ru, Sn, Mo, Rh or Pb is usually introduced as alloying metal [5–10]. As one of the simplest alcohols, the direct oxidation of methanol has been thoroughly studied and the reaction mechanism has been well established. PtRu nano-clusters have been considered to be the best catalysts in the literatures both experimentally and theoretically [11,12]. The superior performance of PtRu catalyst relative to pure Pt catalyst has been explained in terms of two models: the bi-functional mechanism and the ligand effect. In the bi-functional mechanism model, Ru atom was recognized as an oxygen-containing species provider for the CO oxidative removal. The ligand effect, on the other hand, known as
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the electronic model, was based on the modification of Pt electronic structure by the presence of Ru making Pt atoms less poisoned by CO or methanol dissociate adsorption. Compared with methanol, the complete oxidation of ethanol is difficult to be realized due to the presence of more stable C C bond, which is hard to be activated by the commonly employed catalysts at low temperature. Although different Pt-M catalysts (M = Ru, Sn, Mo, and Rh, . . .) have been explored as ethanol oxidation catalysts, only PtSn and PtRu with optimized compositions and structures are reported to exhibit the enhanced activity with respect to other catalysts [3,13–17]. However, the insight into the promotion function of PtRu and PtSn catalysts towards ethanol oxidation is under debate by far. Fujiwara had observed that high selectivity for CO2 formation during ethanol oxidation on PtRu catalyst at 5–40 ◦ C, while others suggested that PtSn was the most active catalyst to ethanol oxidation [18,19]. In our previous work, it was found that PtSn/C exhibited good performance in a direct ethanol fuel cell (DEFC) at 90 ◦ C. However, detailed work on the product analysis showed that CO2 was formed only in small amounts and the incomplete oxidation products such as acetaldehyde and acetic acid were the main reaction products on PtSn/C, similar to the circumstance on PtRu/C catalyst, which was also reported in others’ work [13–17,20]. This indicates that whether on PtRu or PtSn catalyst, the complete ethanol oxidation was restrained. To elucidate the promotion effect of PtRu/C and PtSn/C catalyst to ethanol oxidation, in this work, electrochemistry methods of cyclic voltammetery (CV) and chronoamperometry (CA) measurements in addition to single cell tests were employed. The experimental results show that the addition of the secondary metal Ru (or Sn) to Pt catalyst had a different promotion effect on the ethanol oxidation reaction.
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lyst with 45% metal loading was also obtained from this polyol process. 2.2. Catalyst characterizations Physical properties of the freshly prepared catalysts were characterized by inductively coupled plasma-atomic emission spectrometry (ICP-AES), X-ray diffraction (XRD) and transmission electron microscope (TEM) techniques. ICP-AES analyses of PtRu/C and PtSn/C catalysts were conducted to determine the metal content in the catalysts. For the ICP-AES analysis, the samples were firstly burned in an oven to remove the carbon support and then the samples were dissolved in the aqua regia solution to analyze on a PLASMA-SPEC-1 instrument. The crystalline structure of these supported catalysts was determined using the powder XRD technique. The data were obtained using a Rigaku Rotalflex (RU-2000B) X-ray power diffractometer with a Cu K␣ radiation source and a Ni filter. About 30 mg-powdered samples were kept in a quartz block. Then the powder was pressed onto the quartz block using a glass slide to obtain a uniform distribution. The 2θ Bragg angles were scanned over a range of 15–85◦ at a rate of 5◦ per minute with a 0.02◦ angular resolution. The tube current was 100 mA and the tube voltage was 40 kV. Bragg formula [23] was employed to calculate the lattice parameter of the catalysts. The morphology and the particle size distributions of PtRu/C and PtSn/C catalysts were analyzed by TEM observation. At first, the catalysts were ultrasonically dispersed in ethanol solution to obtain uniform catalyst ink and mounted onto a copper grids covered with holey carbon film. A JEOL JEL2000EX microscope, operating at 100 KV, was used for TEM observation.
2. Experimental details 2.3. Electrochemistry tests 2.1. Catalyst preparation In a typical process, PtSn/C catalyst with the total metal loading of 45% and a nominal Pt:Sn atomic ratio of 1:1 was obtained by a modified polyol process described in previous work [21,22]. Vulcan XC-72R carbon black (500 mg) was pretreated with 5 M HCl and 2 M HNO3 solution to remove some ash and then was suspended in 50 mL ethylene glycol under ultrasonic stirring. Appropriate amounts of H2 PtCl6 ·6H2 O and SnCl2 aqueous solution were added drop by drop to carbon suspension and 1 M NaOH aqueous solution was added to adjust the pH value of the mixture to 12–13 approximately. Reduction reaction was performed by heating the mixture solution at 130 ◦ C for 3 h, during which high purity argon gas was passed through the reaction system to remove the organic by-products. After cooling to the room temperature, the resulted catalyst was washed with hot distilled water until no chloride anion in the filtrate could be detected by 1 M AgNO3 reagent. The obtained catalyst powder was dried in a vacuum oven at 70 ◦ C overnight. Similarly, PtRu/C with the same total metal loading (45%) and a nominal Pt:Ru atomic ratio of 1:1 was also synthesized in the same procedure. As a comparison, Pt/C cata-
A CHI 760B potentistat/galvanostat was used for the electrochemistry measurements in a standard three-compartment electrochemical cell. All the electrochemistry experiments were carried out at 25 ± 0.5 ◦ C. The working electrode was a glass carbon disk with a diameter of 4 mm held in a Teflon cylinder. A Pt-foil counter-electrode and a saturated calomel reference electrode (SCE) (separated by a KNO3 salt bridge) were used. The potentials in this work are referred to normal hydrogen electrode (NHE). Thin porous coating disk electrode design had been described in previous paper [24]: About 5 mg the powdered catalyst was ultrasonically suspended in 4 ml ethanol and 50 L Nafion® solution (5 wt.%, Du Pont Corp., USA) for about 30 min to obtain the catalyst ink, then 20 L of the ink was spread on the surface of the micro-electrode using a micropipette and dried at room temperature to eliminate the solvent. The base cyclic voltammetry experiments were carried out in 0.5 M HClO4 solution, which was deaerated by ultra-high-purity nitrogen before each experiment. For the evaluation of the ethanol oxidation activity, ethanol was added to the base electrolyte to obtain the required concentration of 1 M and the corresponding CV curves were recorded at a scan rate of 25 mV s−1 . Chronoamperometry
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tests were carried out to evaluate the effect of the electrode potential on ethanol oxidation in the same electrolyte 0.5 M HClO4 containing 1 M ethanol. The potential was fixed at 0.45 V in the lower potential region and 0.8 V in the higher potential region. 2.4. Single cell tests The membrane electrode assembly (MEA) used for the single cell test was fabricated by hot-pressing the anode and cathode gas diffusion electrodes onto a pretreated Nafion 115 membrane at 130 ◦ C and with a pressure of 12 bar for 3 min. The resulted MEA was with 2 mg cm−2 total metal loading of PtRu/C or PtSn/C on the anode and 1 mg cm−2 metal loading of Pt/C40 wt% from Johnson Matthey Corporation on the cathode. The performance of the MEA was evaluated with a commercial fuel cell test instrument (Arbin Corporation, USA). Galvanostatic polarization curves were obtained at 90 ◦ C with 1 M liquid ethanol solution fed on the anode at a rate of 1 mL min−1 and the pressure of the oxygen gas reactants in the cathode was 2 bar. The anode polarization curve for ethanol oxidation in a single cell test with 1 M liquid ethanol solution was obtained with an EG&G 273 potentistat/galvanostat on the same rig by employing hydrogen saturated cathode as the counter and reference electrode. 3. Results and discussion 3.1. Catalyst characterizations XRD is a bulk method, and reveals the information on the bulk structure of the catalyst and its support. Fig. 1 presents the XRD patterns of the as prepared PtSn/C, PtRu/C and Pt/C catalysts. It could be clearly seen that PtRu/C and Pt/C catalysts display the typical Pt face centered cubic (f.c.c.) crystal structure, except that the first peak was associated with the XC-72 R carbon support. However, for PtSn/C sample, there are a few more tin oxide diffraction peaks (SnO2 , JCPDS #411445, denoted with * sig-
Table 1 Mean particle sizes and bulk compositions of the catalysts Sample
Lattice parameter (nm)
Particle size (nm)
Bulk composition
PtRu/C PtSn/C Pt/C
0.3853 0.3961 0.3913
1.6 ± 0.7 nm 2.7 ± 0.9 nm 3.4 ± 1.0 nm
1.09:1 3.1:1 N
nals) in addition to the main characteristic feature of Pt f.c.c. structure. The (1 1 0) diffraction peak of SnO2 at 26.6◦ overlaps with the (0 0 2) diffraction peak of carbon support, which results in the higher intensity of carbon support in the XRD patterns of PtSn/C catalyst than in the other two catalysts. It can be clearly observed that the diffraction peaks of PtSn/C and PtRu/C catalysts shift in opposite directions in comparison with Pt/C catalyst. Pt lattice parameters of these carbon-supported catalysts are calculated from the (2 2 0) diffraction peak positions in the XRD patterns and listed in Table 1. The lattice parameter of Pt/C catalyst (a = 0.3913 nm) is a little smaller than that of the bulky Pt metal (JCPDS # 040802, a = 0.3923 nm), which has been ascribed to the platinum–carbon interactions or size effect [25]. In consistent with the literature data, PtRu/C has a much smaller lattice parameter (a = 3.853 nm) and PtSn/C has the enlarged one (a = 0.3961 nm) [26,27]. This may indicate that the addition of the secondary metal Ru or Sn had different effect on the crystal structure of Pt/C catalyst. This can be explained by their ˚ RRu = 1.34 A ˚ different atomic sizes to Pt element (RPt = 1.39 A, ˚ and Rsn = 161 A). Because Sn has a comparable bigger atomic radius than Pt metal, the addition of some amount of Sn to Pt/C could induce the extension of Pt crystal structure. Oppositely, the addition of the smaller atomic sized Ru could result in the contraction of Pt lattice parameter. Based on Vegard’s law, the composition of the PtRu/C alloy could be determined from the shift of Pt diffraction peaks, or the variation of the lattice parameter from the XRD patterns. From literature data [28], a linear relation ship of the lattice parameter and the alloyed Ru atomic fraction (<0.7) has been proposed. a(Pt−M) = a(Pt) − kx(Ru)
Fig. 1. XRD patterns of (a) PtRu/C, (b) PtSn/C and (c) Pt/C electrocatalysts at a scan rate of 5◦ min−1 . The diffraction peaks of SnO2 (JCPDS # 411445) were denoted with * signal.
(1)
where a(Pt) = 0.3913 nm is the lattice parameter of Pt/C catalyst and k is a constant = 0.0124 nm. According to this equation, the atomic content of alloyed Ru (x(Ru) ) in our PtRu/C sample is 0.47. This indicates that most of Ru is alloyed with Pt in PtRu/C catalyst. For PtSn/C catalyst, if the Vegard’s law is still suitable to the occasion of PtSn/C catalyst, a similar linear relation between the lattice parameter and the alloyed Sn content will be proposed here, taking lattice parameter of the Pt3 Sn1 /C as 0.4000 nm for a reference. The calculated Sn content in our PtSn/C catalyst is about 0.14, indicating only 50% is alloyed with Pt metal, if the bulk composition of PtSn/C determined from ICP-AES test is considered. The alloyed Sn content in PtSn/C is much lower than its nominal values, which has been found in Pt-M (M is non noble metal like Fe or Sn element) in our work. The obtained bulk atomic ratios of the catalysts from ICP-AES are also listed in Table 1. TEM images of PtRu/C and PtSn/C catalysts are presented in Fig. 2. It could be seen that the approximately spherical metal
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Fig. 2. TEM images of (a) PtSn/C and (b) PtRu/C catalysts and the size distributions of (c) PtSn/C and (d) PtRu/C.
nanoparticles of the two catalysts are uniformly dispersed on the surface of Vulcan XC-72R. In comparison with PtSn/C catalyst, the mean particle size of PtRu/C is much smaller than that of PtSn/C and the distribution is more uniform than PtSn/C. A little agglomeration observed in PtSn/C catalyst could be ascribed to the presence of Sn hydroxide precipitation originated from the hydrolysis of Sn in alkaline solution. Fig. 2c and d show the histograms of particle size distributions for PtSn/C and PtRu/C catalysts. The particle size of PtSn/C catalyst ranges between 1 and 5 nm, with a mean diameter of 2.7 nm. For PtRu/C catalyst, the mean particle size is only 1.6 nm and the size distribution is in the range of 1–3 nm. The small particle sizes and homogeneous size distributions of both catalysts with high metal loading of 45 wt% should be attributed to the advantages of the modified polyol process in ethylene glycol solution. Because of its comparable big viscosity, ethylene glycol could prevent the agglomerations of the obtained nanoparticles and guarantee the uniform distribution of the obtained nanoparticles [22].
Fig. 3. Primary work showed that the upper potential limit had an important effect on the stability of Ru in PtRu/C catalyst. To avoid the leaching of Ru element, the upper limits are set to 1.0 V for PtRu/C, and 1.2 V for both Pt/C and PtSn/C catalysts [29]. As shown in Fig. 3, the base CV in 0.5 M HClO4 of Pt/C resembles the characteristic features similar to those of
3.2. Ethanol oxidation activity Before the evaluation of the ethanol oxidation activity, the base CV curves of the catalysts were obtained in the supporting electrolyte of 0.5 M HClO4 and the results are presented in
Fig. 3. Cyclic voltammetry of PtSn/C (–), PtRu/C (- - -) and Pt/C (· · ·) electrocatalysts in 0.5 M HClO4 with a sweep rate of 50 mV s−1 at 25 ± 0.5 ◦ C.
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polycrystalline Pt electrodes. The addition of secondary metal Sn in PtSn/C catalyst has a little effect on the CV response. The absence of characteristic features peaks of PtSn/C from Pt/C in hydrogen adsorption–desorption region possibly originates from blockage of Sn-containing species on Pt sites. This phenomenon has also been observed by other researchers for the Pt-M catalysts (M refers to the base metal like Ni or Cr) [30]. However, for PtRu/C catalyst, the base CV exhibits different characteristics from Pt/C and PtSn/C in the entire exploited potential region. Especially in the double layer region, PtRu/C represents much bigger double layer current than Pt/C or PtSn/C does. The big capacitive charging current has been usually ascribed to the activation of H2 O on Ru sites in PtRu/C catalyst. Most experimental and theoretical studies in the literatures has demonstrated that H2 O could be easily activated and dissociated on Ru surface with Ru (OH)ads species at the potential lower than 0.2 V versus NHE [30–32]. As mentioned in the preface of this work, the resulted Ru (OH)ads species could facilitate the removal of COlike poisoning species during the methanol oxidation at lower potential and thus enhance the methanol oxidation activity. Ethanol oxidation activities on PtRu/C and PtSn/C catalysts are evaluated in CV test and the results are displayed in Fig. 4. It could be found from Fig. 4 that the activity of ethanol oxidation on PtRu/C and PtSn/C catalysts varies with the electrode potentials. In the lower potential region, PtSn/C catalyst presents higher activity to ethanol oxidation. When the potential is increased to the higher value, the ethanol oxidation current on PtRu/C exceeds PtSn/C. To elucidate the potential effect to ethanol oxidation activity on PtRu/C and PtSn/C catalysts, CA tests for the ethanol oxidation at different potential were carried out and the results are shown in Fig. 5. It can be clearly seen that the currents for ethanol oxidation on all the catalysts dropped rapidly at first and then became relatively stable. The initial surge of the current is possible due to the charging current or the catalyst poisoning during the ethanol oxidation. Consistent with the CV results, it is found that at a lower potential of 0.45 V, the current for ethanol oxidation on PtSn/C catalyst is significantly
Fig. 5. Chronoamperometry test for ethanol oxidation in 0.5 M HClO4 containing 1 M ethanol on Pt/C (–), PtRu/C (- - -) and PtSn/C (· · ·) catalysts at 0.45 V vs. NHE (a) and 0.8 V vs. NHE (b) at 25 ± 0.5 ◦ C.
larger than on Pt/C or PtRu/C. While at higher potential of 0.8 V, the oxidation current on PtRu/C is the biggest one. This effect of electrode potential on the activity of these catalysts may indicate that different promotion effects PtRu/C and PtSn/C catalysts on the ethanol oxidation reaction. In the hypothesis that the ethanol oxidation on Pt based catalyst is similar to the methanol oxidation behavior. The rate-determining step (RDS) of ethanol oxidation in the lower potential is the dissociative adsorption of ethanol on Pt surface, as displayed as follows. CH3 CH2 OHbulk → CH3 CH2 OHads(Pt)
(2) +
CH3 CH2 OHads(Pt) → CH3 CHOHads(Pt) + H + e
−
(3)
(4)
Fig. 4. Ethanol oxidation on PtRu/C (a) and PtSn/C (b) electrocatalysts in 0.5 M HClO4 + 1 M CH3 CH2 OH with a sweep rate of 25 mV s−1 at 25 ± 0.5 ◦ C. The solid lines are the recorded 10th circles and the dotted lines are the 15th circles intercepted from the full CV curves.
Part of the dissociated formed intermediates like acetaldehyde species may diffuse into the bulk electrolyte, while most of the intermediates are strongly adsorbed on the Pt catalyst surface and contaminate the catalyst by inhibiting the following reaction. The contamination of the catalysts could decrease
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the ethanol oxidation current greatly in the following scans [34]. While in the higher potential region, H2 O could be activated and the resulted OH species could facilitate the incomplete ethanol oxidation to acetic acid. H2 O(M) → OHads(M) + H+ + e−
(5)
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catalysts at low temperature in acidic solution. Apparently, both the introduction of Ru and Sn to Pt/C catalyst could enhance the activity of ethanol oxidation greatly in comparison with pure Pt/C catalyst. However, the promotion effect of Ru and Sn were totally different in different potential region, which was due to the different RDS of ethanol oxidation in different poten-
(6) This indicates at higher potential region the activation of H2 O to result in active oxygen containing species is the RDS for the ethanol oxidation. However, many experimental tests on the electro-oxidation of acetic acid in acid solution reveals that acetic acid is fairly resistant to the electro-oxidation with all the three tested catalysts (Pt/C, PtRu/C and PtSn/C); with almost zero oxidation current in the exploited potential region. This indicates that the incomplete ethanol oxidation to C C compounds like the product of acetic acid, not CO2 , is main component for ethanol oxidation in the explored potential region. Based on the above discussion, different promotion effect of PtRu/C and PtSn/C can be clearly explained. As displayed by the XRD results, the addition of Sn could induce the extension of Pt–Pt distance. The extended Pt–Pt distance could facilitate the dissociation adsorption of much bigger ethanol molecule at lower potential region and thus enhance the ethanol oxidation. The well-defined peak near 0.5 V versus NHE on the first CV curve of PtSn/C is mainly originated from dehydrogenation reaction during the ethanol dissociation adsorption on the catalyst surface. Of course, besides structural effects, a strong ligand effect has been envisaged in the literatures for PtSn making this latter system is less prompt to poisoning by organic species than pure Pt. A number of studies of electronic and surface properties of PtSn/C have confirmed this aspect. However, in this work, we mainly keep more attention on the effect of structure of the ethanol oxidation catalyst. On PtRu/C catalyst, the less pronounced ethanol oxidation current in the lower potential region indicates that the dissociation adsorption of ethanol is unfavorable, which may ascribe to the contracted Pt–Pt distance originated from the addition of Ru. However, in the higher potential region, the electro-oxidation current of ethanol on PtRu/C catalyst is enhanced due to the strong capacity of H2 O activation on PtRu sites. It is interesting to note that PtSn/C catalyst in this potential region presents almost the same poor catalytic activity for ethanol oxidation as Pt/C catalyst, both from the CV and CA results. This is because the dissociation of H2 O on PtSn/C is difficult as on Pt/C catalyst and relatively high positive electrode potentials are needed in comparison with PtRu/C catalyst [33]. The product analysis of ethanol oxidation on PtSn/C and PtRu/C via DEMS technique indicated that the products were mainly consisted of incomplete oxidation products of C C compounds such as acetaldehyde or acetic acid [20], which was consistent with the literature works [34,35]. Based on the above results, we could infer the possible reaction mechanism of ethanol oxidation on PtRu/C and PtSn/C
tial regions. Similarly, the differences in product distribution of ethanol oxidation on PtRu/C and PtSn/C could be also explained by the different role of the foreign element Ru or Sn on Pt/C catalyst in promoting the ethanol oxidation in different potential regions. 3.3. DEFC single cell performance The DEFC single cell performances tested on the homemade equipment are compared in Fig. 6. When the currents are normalized to the geometric area of the single cell, it could be clearly seen that the cell performance of PtSn/C catalyst is better than PtRu/C catalyst in a DEFC in the low discharging current region. The cell voltage of PtSn/C is 170 mV higher than that of PtRu/C catalyst at a current density of 20 mA/cm2 . However, when the discharging current is increased, the cell performance of PtRu/C starts to exceed that of PtSn/C catalyst. To evaluate the ethanol oxidation activity in a gas diffusion electrode at higher temperature, the anode polarizations curves of PtRu/C and PtSn/C catalysts with 1 M ethanol at 75 ◦ C and 90 ◦ C were obtained and are displayed in Fig. 7. As consistent with the halfcell microelectrode CV results at lower temperature (25 ◦ C), the activity of ethanol oxidation of PtRu/C is far lower than
Fig. 6. Polarization curves and power density curves of a direct ethanol fuel cell employing PtSn/C () and PtRu/C () as anode catalyst, respectively. (Anode fuel feeding: 1 M ethanol at l mL min−1 , cell temperature 90 ◦ C, Pcathode : 2 bar, 2 mg cm−2 metal loading of PtRu/C or PtSn/C anode catalyst, 1 mg cm−2 Pt cathode catalyst, Pt/C-40% from JM.) The currents in this figure were normalized to the geometric area of the single cell.
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Fig. 7. The anode polarization curves of ethanol oxidation in a single cell employing PtSn/C (–) and PtRu/C (- - -) as anode catalyst at 75 ◦ C and 90 ◦ C, respectively. 1 M ethanol was fed at l mL min−1 . The cathode, fed with humidified H2 gas, was used as counter electrode and reference electrode.
that on PtSn/C catalyst in the lower discharging current density region at 75 ◦ C. When the potential is increased, the activity of PtRu/C is increased and begins to precede PtSn/C catalyst at 436 mV with a current density of 58 mAcm−2 . When the temperature is increased to 90 ◦ C, the potential at the crossing is decreased to 416 mV and the current is increased to 80 mAcm−2 . This indicates that the temperature has an important effect on the ethanol oxidation behavior on PtRu/C and PtSn/C catalysts. However, whether on the micro electrode in CV and CA test at room temperature or in a gas diffusion electrode at 90 ◦ C (or 75 ◦ C), almost no CO2 bubble could be observed on the surface of the electrode, indicating that the cleavage of C C bond during the oxidation of ethanol is negligible under our experimental conditions. Thus, we could conclude that only simple dehydrogenated reaction on Pt sites (in case of PtSn/C) or partial oxidation to the formation of acetic acid (on PtRu/C catalyst) had happened during the oxidation of ethanol between the room temperature and 90 ◦ C. Because Pt catalyst is well known as the best material for the dehydrogenation reaction of the organic molecule, the cell performance of a DEFC might have a relationship with the Pt metal amount in the MEA. To explore the effect of Pt amount on the cell performance of a DEFC, the current is normalized to the total Pt metal mass loading on the anode, which is determined by the ICP-AES, and it is found that the peak power density of the DEFC employing PtRu/C as anode catalyst is almost equivalent to that of a PtSn/C anode, as shown in Fig. 8. This indicates that the cell performance of a DEFC is determined by the total amount of platinum metal in a MEA to some extent, whether on PtSn/C or PtRu/C catalysts. This may be the reason that Pt3 Sn1 /C even Pt9 Sn1 /C and Pt85 Ru15 /C had been proposed as the most promising catalyst for low temperature ethanol oxidation reaction [1]. However, it should also be noted that the DEFC cell performance whether on PtSn/C or PtRu/C is far from its large-scale applications. To achieve higher ethanol oxidation performance, the C C bond in ethanol molecule must be broken for its further complete oxidation. Because the complete oxidation of ethanol to CO2
Fig. 8. Polarization curves and power density curves of a direct ethanol fuel cell employing PtSn/C () and PtRu/C () as anode catalyst, the currents in this figure were normalized to the total amount of Pt noble metal.
had been proved to be absolutely difficult at lower temperature, the operation temperature should be increased to enhance the ethanol oxidation activity of the catalysts. As displayed in this work, PtRu and PtSn seem to be indispensable components of a highly active ethanol oxidation catalyst. And it is no doubt that the C C bond cleavage process is promoted by high temperature operation, from the point of thermodynamic consideration. However, the biggest bottleneck of high temperature DEFC lies in the development of high temperature membrane. Due to the lack of appropriate proton exchange membrane for DEFC for high temperature operation, most of work is based on Nafion type membrane and the performance is still unsatisfactory. Further work on the key materials of membrane and catalysts at elevated temperature (up to 150–200 ◦ C) is under progress in our group. 4. Conclusions The modified polyol process in the ethylene glycol solution is proved to be suitable to prepare highly dispersed carbon supported Pt-based catalysts with uniform size distribution. The structural characterization of these Pt-based catalysts show that the addition of the secondary metal to Pt/C catalyst has different effect on the crystal lattice of Pt metal, which is determined by the relative atomic radii of M in comparison with that of Pt metal. The addition of bigger atomic sized Sn compared with Pt metal could induce the extension of Pt crystal structure, while the addition of smaller atomic sized Ru could result in the contraction of Pt lattice parameter in PtRu/C catalyst. It is found that the activity of ethanol oxidation on Pt-based catalysts is greatly affected by the secondary metal and the electrode potential. The enhancement of ethanol oxidation on PtSn/C is mainly from the structural effect due to the extend crystal structure, while the promotion of PtRu/C catalyst could be explained by the modified bi-functional mechanism, originated from the enhanced activity of H2 O activation on Ru sites. The maximum power density of PtSn/C is equivalent with that of PtRu/C catalyst when the currents are normalized to the
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total Pt mass loading, indicating that the amount of Pt plays an important role in a DEFC cell performance. The cell performance of a DEFC at low temperature is far from its large-scale application. To enhance the DEFC cell performance, high temperature (up to 150–200 ◦ C) operation should be selected to ensure the complete oxidation of ethanol. Acknowledgement This work was financially supported by National Natural Science Foundation of China (Grant No. 50575036). References [1] C. Lamy, E.M. Belgsir, J.M. Leger, J. Appl. Electrochem. 31 (2001) 799. [2] X.M. Ren, P. Zelenay, S. Thomas, J. Davey, S. Gottesfeld, J. Power Sources 86 (2000) 11. [3] F. Delime, J.M. Leger, C. Lamy, J. Appl. Electrochem. 28 (1998) 27. [4] S.C. Hall, V. Subramanian, G. Teeter, B. Rambabu, Solid State Ionics 175 (2004) 809. [5] O. Neto, M.J. Giz, J. Perez, E.A. Ticianelli, E.R. Gonzalez, J. Electrochem. Soc. 149 (2002) A272. [6] H. Wang, Z. Jusys, R.J. Behm, J. Power Sources 154 (2006) 351. [7] F. Delime, J.M. Leger, C. Lamy, J. Appl. Electrochem. 29 (1999) 1249. [8] B.N. Grgur, N.M. Markovic, P.N. Ross Jr., J. Phys. Chem. B 102 (1998) 2494. [9] J.P.I. de Souza, S.L. Queiroz, K. Bergamaski, E.R. Gonzalez, F.C. Nart, J. Phys. Chem. B 106 (2002) 9825. [10] G.C. Li, P.G. Pickup, Electrochim. Acta 52 (2006) 1033. [11] M. Watanabe, S. Motoo, J. Electroanal. Chem. 60 (1975) 275. [12] M.M.P. Janssen, J. Moolhuysen, J. Catal. 46 (1977) 289. [13] W.J. Zhou, Z.Z. Zhou, S.Q. Song, W.Z. Li, G.Q. Sun, P. Tsiakaras, Q. Xin, Appl. Catal. B 46 (2003) 273. [14] S.Q. Song, G.X. Wang, W.J. Zhou, X.S. Zhao, G.Q. Sun, Q. Xin, S. Kontou, P. Tsiakaras, J. Power Sources 140 (2005) 103.
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