Energy Efficiency Enhancement of Ethanol ...

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Apr 19, 2012 - troless procedure on a mixed ceria (CeO2)/carbon black (Vulcan. XC-72) support. The resulting material, Pd–CeO2/C, has been characterized ...
DOI: 10.1002/cssc.201100738

Energy Efficiency Enhancement of Ethanol Electrooxidation on Pd–CeO2/C in Passive and Active Polymer Electrolyte-Membrane Fuel Cells Valentina Bambagioni,[a] Claudio Bianchini,*[a] Yanxin Chen,[a] Jonathan Filippi,[a] Paolo Fornasiero,[a, b] Massimo Innocenti,[a, c] Alessandro Lavacchi,*[a] Andrea Marchionni,[a] Werner Oberhauser,[a] and Francesco Vizza*[a] Pd nanoparticles have been generated by performing an electroless procedure on a mixed ceria (CeO2)/carbon black (Vulcan XC-72) support. The resulting material, Pd–CeO2/C, has been characterized by means of transmission electron microscopy (TEM), inductively coupled plasma atomic emission spectroscopy (ICP–AES), and X-ray diffraction (XRD) techniques. Electrodes coated with Pd–CeO2/C have been scrutinized for the oxidation of ethanol in alkaline media in half cells as well as in passive and active direct ethanol fuel cells (DEFCs). Membrane electrode assemblies have been fabricated using Pd–CeO2/C anodes, proprietary FeCo cathodes, and Tokuyama anion-exchange membranes. The monoplanar passive and active DEFCs have been fed with aqueous solutions of 10 wt % ethanol and

2 m KOH, supplying power densities as high as 66 mW cm2 at 25 8C and 140 mW cm2 at 80 8C. A comparison with a standard anode electrocatalyst containing Pd nanoparticles (Pd/C) has shown that, at even metal loading and experimental conditions, the energy released by the cells with the Pd–CeO2/C electrocatalyst is twice as much as that supplied by the cells with the Pd/C electrocatalyst. A cyclic voltammetry study has shown that the co-support ceria contributes to the remarkable decrease of the onset oxidation potential of ethanol. It is proposed that ceria promotes the formation at low potentials of species adsorbed on Pd, PdI-OHads, that are responsible for ethanol oxidation.

Introduction The present production of ethanol from biomasses (first- and second-generation bioethanol) exceeds 20 billion gallons per year and is expected to grow at a rate of 5 % per year in the near future[1] pushed up both by environmental concerns and the instability in fossil fuel supply. Among the several applications of ethanol for a more sustainable development, its direct use to generate electrical energy in fuel cells is attracting considerable interest. Such devices are known under the name of direct ethanol fuel cells (DEFCs). Ethanol as anolyte in a fuel cell exhibits some advantages over hydrogen that is the fuel in proton exchange membrane fuel cells (PEMFCs).[2] Ethanol exhibits a higher volumetric energy density and its storage and transport are much easier compared to hydrogen. Conversely, the oxidation kinetics of ethanol on known electrocatalysts are considerably slower than those of hydrogen, and PEMFCs exhibit superior electrical performance as compared to DEFCs. Increasing research efforts are, therefore, required to design and develop anode electrocatalysts for DEFCs with improved activity, stability, and selectivity. The large majority of DEFCs described in the literature operate in acidic environment with cation-exchange membranes, typically Nafion, and Pt-based electrocatalysts at both electrodes.[3–5] In no case, however, have these membrane-electrode assemblies (MEAs) demonstrated the capacity to supply acceptable power densities as well as high fuel efficiency. The overall efficiency of acidic DEFCs is limited by slow oxidation

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kinetics; oxidation of ethanol to mixtures of acetaldehyde, acid acetic, CO, and CO2 ; and, last but not least, the propensity of Pt-based anode catalysts to be poisoned by CO.[3, 5] The performance of Pt-based electrocatalysts in alkaline media is even worse than in acidic media due to sluggish kinetics and fast catalyst deactivation.[6] Nanostructured Pd supported on carbon blacks, typically Vulcan XC-72, is emerging as an effective material for manufacturing anodes for DEFCs operating in alkaline media with anion-exchange membranes.[2] Both pas[a] Dr. V. Bambagioni, Dr. C. Bianchini, Y. Chen, Dr. J. Filippi, Prof. P. Fornasiero, Dr. M. Innocenti, A. Lavacchi, A. Marchionni, Dr. W. Oberhauser, Dr. F. Vizza Istituto di Chimica dei Composti Organometallici (ICCOM-CNR) Via Madonna del Piano 10, 50019 Sesto Fiorentino (Italy) Fax: (+ 39) 0555225203 E-mail: [email protected] [email protected] [email protected] [b] Prof. P. Fornasiero Department of Chemistry ICCOM-CNR-Trieste CENEMAT University of Trieste Via L. Giorgieri, 1 34127 Trieste (Italy) [c] Dr. M. Innocenti Dipartimento di Chimica Universit degli Studi di Firenze Via della Lastruccia 3, 50019 Sesto Fiorentino (Italy) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201100738.

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Energy Efficiency Enhancement of Ethanol Electrooxidation in Fuel Cells sive (air-breathing) and active DEFCs with Pd-catalyzed anodes have been recently reported to provide quite satisfactory power densities (up to 170 mW cm2) at relatively high voltages, in conjunction with non-noble metal (Fe–Co) electrocatalysts at the cathode side.[7–12] A major drawback of Pd-based electrodes is their capacity to oxidize ethanol selectively to acetate ions in strongly alkaline media [Eq. (1)].[2, 10, 11] Accordingly, each ethanol molecule can release a maximum of four electrons, instead of the twelve electrons theoretically obtainable from the total oxidation to carbonate [Eq. (2)]. C2 H5 OH þ 5 OH ! CH3 COO þ 4 H2 O þ 4 e

ð1Þ

C2 H5 OH þ 16 OH ! 2 CO3 2 þ 11 H2 O þ 12 e

ð2Þ

Conversely, it is worth stressing that the drawback represented by the low faradaic efficiency is largely compensated by the generally fast oxidation kinetics, leading to high current densities[2, 10, 11] as well as the selectivity to acetate [Eq. (1)] whose added value is higher than that of ethanol. The recent discovery that the acetate ion can be converted to ethanol in a renewable way by electro-bioreduction,[13] thus closing the fuel production/transformation cycle, offers a further strong argument in favor of the use of Pd-based electrocatalysts for DEFC anodes. DEFCs with Pd-based anodes are presently developed as reactors for the simultaneous production of energy and chemicals.[7, 10] In addition to the low faradaic efficiency, Pd-based electrocatalysts exhibit a second drawback: at anode potential values higher than 0.6 V vs. reversible hydrogen electrode (RHE) in half cells and 0.15 vs. RHE in monoplanar DEFCs, the surface adsorbed Pd-OHads species [Eq. (3)], active for the oxidation of ethanol, start converting into inactive Pd-O[14, 15] according to Equations (4) and (5). As a result, the number of active sites on the electrode surface decreases, and the fuel conversion is slowed down and ultimately inhibited. Pd þ OH ! Pd-OHads þ e

ð3Þ

Pd-OHads þ OH ! Pd-O þ H2 O þ e

ð4Þ

Pd-OHads þ Pd-OHads ! Pd-O þ H2 O

ð5Þ

The extent of Pd oxidation to Pd-O on DEFC anodes has been recently minimized by adding a small amount of NaBH4 into the anolyte compartment: such a reagent is able to reduce back Pd-O to Pd.[15] In this paper we propose an alternative way to magnify the capacity of Pd nanoparticles to oxidize ethanol in alkaline media. Our approach focuses on the process described in Equation (3) and, in particular, is aimed at anticipating the oxidation of Pd0 to PdI-OHads. We have found that an effective material to accomplish this goal is ceria (CeO2). CeO2 is a mixed conductor, showing both electronic and ionic conduction, with many applications in catalysis in conjunction with transition metals.[16] In particular, Pd/CeO2 catalysts are largely used in a variety of dehydrogenation and oxidation reactions.[17–26] One ChemSusChem 2012, 5, 1266 – 1273

of the key functions of this material is the ability of Ce to switch between the Ce4 + and Ce3 + oxidation states, which allows the reversible addition and removal of oxygen, the socalled oxygen storage capacity (OSC). Previous studies have shown that the addition of CeO2 to Pd/Al2O3 promotes the oxidation of Pd.[27] In the field of fuel cell technology, Shen and co-workers have reported that the addition of CeO2 to Pd/C (C = Vulcan XC-72) enhances the activity of ethanol electrooxidation in half cells.[28, 29] It has been suggested that this oxide promotes “the formation of OHads species at lower potential […] so as to transform CO-like poisoning species on Pd to CO2 or other products, which could be dissolved in water, releasing the active sites on Pd for further electrochemical reaction”.[28] Pd–CeO2 nanobundles have also been synthesized by electrodeposition and successfully tested as electrocatalysts for ethanol oxidation, yet no mechanistic interpretation has been provided.[30] An in depth-study of ethanol oxidation on electrocatalysts obtained by combining CeO2 with Vulcan XC-72-supported Pd nanoparticles is reported in this paper. The novel Pd–CeO2/C material has been used to manufacture MEAs for both passive and active DEFCs equipped with an anion-exchange membrane. In addition to a comparison with a state-of-the-art Pd/C electrocatalyst, an estimation of the energy efficiency of alkaline DEFCs, where ethanol is converted either to acetate [Eq. (1)] or to carbonate [Eq. (2)], is also reported. To the best of our knowledge, neither DEFCs with Pd–CeO2/C anodes nor an estimation of the energy efficiency of alkaline DEFCs has been reported in the relevant literature so far.

Results and Discussion Structural and morphological investigation X-ray diffraction (XRD) experiments were carried out on the three materials Pd/C, CeO2/C, and Pd–CeO2/C (Figure 1). The XRD patterns of Pd/C and Pd–CeO2/C revealed the presence of a metallic Pd phase that, in the case of the latter material, was accompanied by the presence of CeIV oxides.[31] The line broadening of these signals is consistent with the formation of nanostructured Pd and Ce oxides.

Figure 1. X-ray powder diffraction patterns of a) Pd/C, b) CeO2/C and c) Pd– CeO2/C.*: Pd, &: CeO2, ^: C.

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C. Bianchini, A. Lavacchi, F. Vizza et al. Transmission electron microscopy (TEM) showed the Pd/C system to contain Pd nanoparticles with dimensions ranging from 2–5 nm. No aggregate was observed for this sample, indicating a high metal dispersion (Figures 2 a and b), whereas both isolated nanoparticles and aggregates were observed for

Figure 3. Potentiodynamic (empty symbols) and power density curves (filled symbols) at 25 8C for passive DEFCs containing the Pd/C (*) and Pd–CeO2/C (&) anode electrocatalysts and fuelled with 2 m KOH and 10 wt % EtOH solutions. The cathode was exposed to an oxygen atmosphere.

with Pd–CeO2/C is higher than that with the Pd/C electrocatalyst in the whole potential range. This is a crucial point as it indicates that the CeO2-containing electrocatalyst leads to an important increase in the fuel energy efficiency. This point is even more evident when one examines the galvanostatic curves reported in Figure 4. For a galvanostatic experiment

Figure 2. Low and high magnification TEM micrographs of a, b) Pd/C, c, d) CeO2/C, and e, f) Pd–CeO2/C.

CeO2/C (Figures 2 c and d). In the case of Pd–CeO2/C (Figures 2 e and f), the TEM images showed the presence both of nanoparticles, in the range between 2 and 10 nm, and of large aggregates of nanoparticles. The low image contrast between CeO2 and Pd did not allow us to identify the chemical nature of the single nanoparticles. Energy dispersive microanalysis (EDS) was performed on Pd–CeO2/C, showing the presence of Pd and Ce signals in all analyzed sample areas, which does not allow the discrimination of single particles of Pd from particles of CeO2.

Figure 4. Galvanostatic curves for the passive DEFC containing the Pd/C (c) and Pd–CeO2/C (a) anode electrocatalysts and fuelled with 2 m KOH and 10 wt % EtOH solutions.

Direct ethanol fuel cells performance The potentiodynamic and power density curves shown in Figure 3 were recorded for air-breathing monoplanar passive fuel cells with MEAs containing either Pd/C or Pd–CeO2/C as anode electrocatalyst, a Tokuyama A-006 anion-exchange membrane, and Fe–Co/C as cathode electrocatalyst.[10–12] The fuel was an aqueous 2 m KOH solution containing 10 wt % EtOH. A perusal of such curves clearly shows that the cell with the Pd–CeO2/C electrocatalyst exhibits a considerably higher peak power density (66 mW cm2) compared to the cell with the Pd/ C electrocatalyst (18 mW cm2). The power output for the cell

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performed at 20 mA cm2, the cell potential for the DEFC containing Pd–CeO2/C is higher, except for the last hour. The energy released during the galvanostatic experiments is 795 J for the cell with Pd/C and 1500 J for the cell with Pd–CeO2/C. In the former case, 9.1 mmol of ethanol, out of the initial 22.1 mmol, have been selectively converted into acetate; in the other case, 8 mmol of ethanol, out of the initial 22.8 mmol, have been selectively converted into acetate. These experimental data have allowed us to estimate the energy efficiency in either case, which, to our knowledge, has never been reported for alkaline DEFCs.

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Energy Efficiency Enhancement of Ethanol Electrooxidation in Fuel Cells According to Equation (6) (see the Supporting Information), the energy efficiency for the ethanol oxidation to CO32 [Eq. (7)], as a reference, can be calculated by using the reaction enthalpy: Rtf I Vdt

eCO2 ¼ 3

ð6Þ

1 t0 0 F DHCO 2 MTOT 3

where eCO2 = efficiency referred to the ethanol oxidation to 3 CO32, I = current of the galvanostatic experiment (0.102 A), t0 = start time, tf = end time, V = cell potential (in the galvano0 static experiments dependent on time), DHCO 2 = enthalpy for 3 ethanol oxidation to carbonate (1342 kJ mol1), and MFTOT = total number moles of fuel in the cell. C2 H5 OH þ 3 O2 þ 4 OH ! 2 CO3 2 þ 5 H2 O

ð7Þ

Figure 5. Potentiodynamic (empty symbols) and power density (filled symbols) curves for active DEFCs containing the Pd/C anode electrocatalyst and fuelled with a 2 m KOH and 10 wt % EtOH solution at different temperatures. Temperature of fuel (left), cell (central), and oxygen gas (right) were *: 25/25/25 8C, ^: 40/40/30 8C, ~: 60/60/40 8C, !: 80/80/60 8C.

Once all parameters in Equation (6) were introduced, a net energy efficiency of 2.6 % and 5.3 % was calculated for the Pd/C and Pd–CeO2/C electrocatalysts, respectively. A twofold energy efficiency enhancement for the latter electrocatalyst was also calculated by using Equation (8) for the selective conversion of ethanol into acetate [Eq. (9)] that occurred in the present DEFCs: Rtf I Vdt eCH3 COO ¼

t0 0 CH3 COO

DH

ð8Þ

1 MFTOT

where eCH3 COO = the efficiency referred to ethanol oxidation re0 action to acetate standard enthalpy at pH 14, andDHCH  = 3 COO ethanol oxidation reaction standard enthalpy at pH 14 (590 kJ mol1). C2 H5 OH þ O2 þ 5 OH ! CH3 COO þ 2 H2 O

ð9Þ

Equation (8) gave an efficiency of 6 % for the DEFC equipped with the Pd/C electrocatalyst and an efficiency of 12.2 % for the DEFC containing Pd–CeO2/C as anode electrocatalyst. A tentative explanation for such a dramatic enhancement of the performance will be proposed in the next section on the basis of the results of the half-cell characterization of the present electrocatalysts. The performance of the active DEFCs with either Pd/C or Pd–CeO2/C as anode electrocatalyst was investigated by varying the temperature at fixed oxygen and fuel flows. Figure 5 reports a series of potentiodynamic and power output scans for the cell containing Pd/C. The room temperature performance was in good agreement with that observed for the airbreathing passive cell. Increasing the temperature to 80 8C provided a fivefold increase in the energy output (from 25 mW cm2@20 8C to 120 mW cm2@80 8C). The potentiodynamic and power density curves for the cell with Pd–CeO2/C as anode electrocatalyst are shown in Figure 6 in the temperature range from 20 to 80 8C: in this interval of temperatures, the ChemSusChem 2012, 5, 1266 – 1273

Figure 6. Potentiodynamic and power density curves for active DEFCs containing the Pd–CeO2/C anode electrocatalyst and fuelled with a 2 m KOH and 10 wt % EtOH solution at different temperatures. Temperature of fuel (left), cell (central) and oxygen gas (right) were *: 25/25/25 8C, &: 40/40/30 8C, ^: 60/60/40 8C, ~: 80/80/60 8C.

power output increased nearly 2.5 times, 60 mW cm2@20 8C to 140 mW cm2@80 8C.

from

Half-cell characterization In an attempt to rationalize the promoting effect of ceria on the electrochemical performance of Pd nanoparticles, a cyclic voltammetry (CV) study was performed on the three anode materials Pd/C, CeO2/C, and Pd–CeO2/C in half cells both in 2 m KOH and 2 m KOH + 10 wt % EtOH solutions. Figure 7 shows the CVs of all materials in the 2 m KOH electrolyte. The interpretation of the behavior of Pd/C (Figure 7 a) is straightforward as this electrocatalyst has been widely investigated in the past.[11, 14, 24, 32] Starting from the low potential region, one may notice the appearance of a very broad anodic peak (A’) due to oxidation of the adsorbed and absorbed hydrogen with the relative current density apparently limited by the amount of

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Figure 8. Cyclic voltammograms of a) Pd/C, b) CeO2/C, and c) Pd–CeO2/C electrodes in 2 m KOH+10 wt % EtOH solutions. Figure 7. Cyclic voltammograms of a) Pd/C, b) CeO2/C, and c) Pd–CeO2/C electrodes in 2 m KOH solutions.

hydrogen incorporated. The formation of a Pd-O coverage on the catalyst surface was accompanied by a gentle increase of the current density. The well-defined cathodic peak C’ at 0.69 V in the reverse scan can be straightforwardly assigned to the reduction of Pd-O,[27–30] whereas the cathodic peak C’’ at 0.20 V is assigned to hydrogen uptake.[28, 30] In the case of the CeO2/C electrode (Figure 7 b), one may observe the occurrence of a broad anodic peak (A’) culminating at 0.28 V and corresponding to hydrogen desorption. No other visible process was observed in the forward scan. The backward scan showed a broad cathodic peak C’ (0.20 V) due to the hydrogen uptake. The CV of the Pd–CeO2/C electrode in 2 m KOH can be interpreted as a combination of the CVs of Pd/C and CeO2/C (Figure 7 c). A very pronounced peak, due to hydrogen desorption (A’), was observed at 0.37 V (A’’). Moving toward more positive potentials, one may notice a slow current-density increase attributable to the oxidation of Pd, yet no clear evidence for the peak corresponding to the formation of surface-adsorbed PdI hydroxide species (PdI-OHads) was obtained. This can be ascribed to the superimposition of the PdI-OHads signal with the larger signal of the hydrogen desorption. In the backward scan, the cathodic peak at 0.78 V (C’) can be safely assigned to the reduction of Pd-O, whereas the broad cathodic region at potentials lower than 0.55 V (C’’) is attributed to the hydrogen uptake by both Pd and ceria. The CV experiments performed on the Pd/C electrode in 2 m KOH+10 wt % EtOH solutions (Figure 8 a) showed the ethanol oxidation peak (A’) at 0.78 V with an intensity controlled by the progressive coverage of the electrode surface by Pd-O.[14, 15, 32] At potentials higher than 1 V, the anodic current was comparable to that observed for the 2 m KOH solution

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alone (Figure 7 a), indicating that only a negligible fraction of the catalyst sites remains active for ethanol oxidation. The reverse scan showed a further anodic peak A’’ occurring at 0.65 V due to the oxidation of fresh ethanol on the catalyst surface freed from Pd oxide.[14, 29] The CeO2/C material (Figure 8 b) was inactive for ethanol oxidation in the potential range investigated. The anodic (A’, 0.38 V) and cathodic peaks (C’, 0.22 V) are attributed to the hydrogen adsorption and absorption processes, respectively. The CV of the Pd–CeO2/C electrode (Figure 8 c) showed an ethanol oxidation peak culminating at 0.82 V with a current density of 106 mA cm2 (Jp) comparable to that measured for the Pd/C electrode (107 mA cm2), yet with an onset potential (Eonset) for ethanol oxidation considerably lower than for Pd/C (0.24 V, Table 1). This finding is in agreement with the data reported in Refs. [28–30] for analogous systems. At a fixed potential of 0.5 V, current densities of 21 and 9 mA cm2 for Pd–CeO2/C and Pd/C were measured, respectively. Such a difference in power density provides an explanation for the power outputs supplied by the two electrocatalysts in the monoplanar DEFCs investigated (vide infra). The effect of the KOH concentration on the half-cell performance was investigated for either Pd/C or Pd–CeO2/C. The data reported in Table 1 show that Eonset of ethanol oxidation

Table 1. CV data for the electrooxidation of 2 m ethanol in different aqueous KOH solutions. cKOH [mol L1]

Jp [mA cm2]

0.5 1.0 2.0 4.0

59.0 90.9 107.0 94.5

Pd/C Eonset [V vs. RHE] 0.34 0.30 0.24 0.20

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Pd–CeO2/C Jp [mA cm2] Eonset [V vs. RHE] 65.5 99.4 106.0 95.6

0.26 0.22 0.18 0.14

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Energy Efficiency Enhancement of Ethanol Electrooxidation in Fuel Cells on either electrocatalyst decreases steadily with KOH concentration in the range between 0.5 and 4 m, down to a remarkable value of 0.14 V for the Pd–CeO2/C electrode in a 4 m KOH solution, which is the lowest potential ever observed for such a reaction on Pd-based electrocatalysts.[2] The Pd–CeO2/C catalyst exhibits a lower Eonset compared to Pd/C in the whole range of KOH concentrations investigated, whereas Jp did not appreciably vary for the two catalysts. In view of this finding, one may be tempted to use KOH concentrations higher than 2 m to increase the DEFC energy output, yet we have found that a 2 m KOH concentration is the best compromise between a low Eonset and good catalyst stability: the rate of Pd oxidation to inactive Pd-O increases with the KOH concentration. A generally accepted mechanism for the oxidation of ethanol on Pd in alkaline media is described in Equations (3) and (10)–(13):

mote the formation of Pd-OHads species. This evidence represents a breakthrough in the design of electrocatalysts for alcohol oxidation.

Pd0 þ CH3 CH2 OH ! Pd-ðCH3 CH2 OHÞads

The quantitative analysis of the fuel cell exhausts was obtained by performing 13C{1H} NMR spectroscopy using a Bruker Avance DRX400 instrument with the chemical shifts relative to external tetramethylsilane (TMS). The calibration curves for the quantitative analysis were obtained by using authentic samples of the various products in the appropriate range of concentrations and 1,4-dioxane as internal standard. Ionic chromatography (IC) was also used to identify the oxidation products (Metrohm 761 Compact instrument equipped with a Metrosep Organic Acids column). The sonication process was performed with a Bandelin Sonopuls probe instrument.

Pd-ðCH3 CH2 OHÞads þ 3 OH

ð10Þ



! Pd-ðCOCH3 Þads þ 3 H2 O þ 3 e

ð11Þ

Pd-ðCOCH3 Þads þ Pd-OHads ! Pd-ðCH3 COOHÞads þ Pd0

ð12Þ

Pd-ðCH3 COOHÞads þ OH ! Pd0 þ CH3 COO þ H2 O

ð13Þ

Liang et al. have proposed that the rate of ethanol oxidation on Pd in an alkaline environment is controlled at low overpotentials by hydroxide adsorption on Pd to give PdI-OHads species [Eq. (3)],[14] whereas at higher potentials, the rate-determining-step would be the insertion of hydroxides on the adsorbed acyl [Eq. (12)]. Irrespective of the kinetic regime, all experimental[14, 32, 33] and theoretical[34] studies agree to consider the OHads concentration on the Pd surface as a crucial parameter determining the oxidation of ethanol, which does not occur on PdII-O.[14, 15] Crucial for the oxidation is, therefore, the formation of PdI-OHads species at low potentials, a process that is apparently favored by ceria. Ceria is an active support in catalysis[16] also for the primary oxide spill-over,[33, 35] which enhances the ability of the supported metal to adsorb hydroxyl ions. This effect, rather than the ability to oxidize adsorbed CO,[28] may be invoked to account for the enhanced activity of ceria-supported Pd electrocatalysts for the ethanol oxidation reaction.

Conclusions In this paper are reported the first examples of direct ethanol fuel cells containing anode electrocatalysts made of Pd nanoparticles supported on ceria (Pd–CeO2/C). A comparison with a standard anode electrocatalyst containing Pd nanoparticles (Pd/C) has shown that, at even metal loading and experimental conditions, the energy efficiency of a DEFC assembled with the Pd–CeO2/C anode electrocatalyst is twice as much that supplied by the cells with the Pd/C electrocatalyst. A CV study has shown that the co-support ceria contributes to decreasing remarkably the Eonset of ethanol, by virtue of its ability to proChemSusChem 2012, 5, 1266 – 1273

Experimental Section Materials and general instrumentation All manipulations, except as stated otherwise, were routinely performed under a nitrogen atmosphere using standard airless techniques. Carbon black (Vulcan XC-72 pellets) was purchased from Cabot Corp., USA. The Fe-Co/C cathode electrocatalyst (C = Ketjen Black, 1.13 wt % Fe and 1.71 wt % Co) prepared by using a method reported elsewhere.[12] The alkaline solid electrolyte used in both passive and active DAFCs was a Tokuyama anion-exchange membrane A-006 (OH-type) obtained from Tokuyama Corporation. All metal salts and reagents were purchased from Aldrich and used as received. All the solutions were freshly prepared with doubly distilled-deionized water.

Transmission electron microscopy (TEM) was performed on a Philips CM12 microscope at an accelerating voltage of 100 kV. The microscope was equipped with an EDAX energy dispersive microanalysis system. X-ray powder diffraction (XRD) spectra were acquired at room temperature with a PANalytical X’PERT PRO diffractometer, employing Cu Ka radiation (l = 1.54187 ) and a PW3088/60-graded multilayer parabolic X-ray mirror for Cu radiation. The spectra were acquired in the 2 q range from 5.0 to 120.08, using a continuous scan mode with an acquisition step size of 2 q = 0.02638 and a counting time of 49.5 s.

Preparation of Pd–CeO2/C Vulcan XC-72 (4 g) was added to a solution of Ce(NO3)3·6 H2O (5.31 g, 12.23 mmol) in H2O (250 mL). The resulting mixture was stirred for 30 min at room temperature and then sonicated for 30 min. After adjusting the pH to 12 using an aqueous solution of 1.5 m KOH (100 mL), the resulting suspension was stirred for 1 h. The solid product was separated by filtration, then it was washed with distilled H2O to neutral pH and dried at 60 8C under vacuum to constant weight. After physical milling, the product was calcined in air at 250 8C for 2 h to give the material CeO2/C with a ceria content of 35 wt % (yield: 6.02 g). CeO2/C was suspended in water (500 mL), and the resulting slurry was sonicated for 10 min. To this mixture, a solution of K2PdCl4 (0.93 g, 2.84 mmol) in water (250 mL) was slowly added (ca. 3 h) under vigorous stirring, followed by an aqueous solution of 1 m KOH (14.5 mL). Next, ethanol (100 mL) was added to the resulting mixture, which was then refluxed for 30 min. The desired product Pd–CeO2/C was filtered off, washed several times with distilled water to neutrality and finally dried

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C. Bianchini, A. Lavacchi, F. Vizza et al. under vacuum at 40 8C until constant weight (yield: 6.25 g). Inductively coupled plasma atomic emission spectrometry (ICP–AES) analysis: 4.55 wt % Pd.

Preparation of Pd/C This electrocatalyst was prepared by using an electroless method reported elsewhere.[32] The sample used in this work contained 5 wt % Pd (ICP–AES analysis).

mined by weighing the amount of the deposited ink onto the glassy carbon electrode. Cyclic voltammetry (CV) experiments were conducted on a Parstat 2273A potentiostat/galvanostat (Princeton Applied Research) using a three-electrode arrangement with a KClsaturated Ag/AgCl reference electrode and a Pt wire as counter electrode. The potential scale of the CV curves was expressed according to the reverse hydrogen electrode (RHE).

Acknowledgements Fuel-cell testing The membrane-electrode assemblies (MEAs) for the passive monoplanar cells were realized by using a commercial Tokuyama A-006 anion-exchange membrane and cathodes containing a proprietary Fe–Co/C electrocatalyst.[12] The cell hardware was described previously.[11] The membrane was conditioned in a saturated KOH solution for 1 min before assembling the MEA. The volume of the anode compartment was approximately 25 mL for actual 13–15 mL of fuel solution. The anode was realized with a 5.13 cm2 nickel foam plate, onto which the appropriate amount of dense catalytic ink was deposited. This was prepared by introducing the catalyst (100 mg) into a 5 mL polyethylene container together with water (100 mg). The metal loading on each electrode was determined by weighing the amount of ink deposited. The MEA was obtained by mechanically pressing anode, cathode, and membrane. For a reliable evaluation of the oxidation products, the anode compartment was sealed under a nitrogen atmosphere, whereas the cathode was exposed to either air or oxygen with no significant variation of the cell performance. This was evaluated by means of an ARBIN BT-2000 5A 4 channels instrument. The fuel oxidation products were qualitatively and quantitatively determined by means of 13 1 C{ H} NMR spectroscopy using calibration curves. The active fuelcell system was purchased from Scribner-Associates (USA) (25 cm2 fuel cell fixture) and was modified in our laboratory with titanium end plates to tolerate the alkaline conditions. The MEA was fabricated by mechanically pressing anode (nickel foam), cathode (carbon cloth), and a Tokuyama A006 anion exchange membrane. A dense anode ink was prepared by mixing the powdered catalyst with a 5–10 wt % aqueous dispersion of PTFE. As a general procedure, an identical amount of the resulting paste was spread onto two identical nickel-foam plates. One of these was used almost immediately to fabricate the MEA, the other was dried until constant weight for the quantitative determination of the Pd loading, which was approximately 1 mg cm2 in all cases. The effective electrode area was 5 cm2. The fuel, a water solution containing 10 wt % ethanol in 2 m KOH, was delivered to the anode at 4 mL min1 rate, whereas the oxygen flow was regulated at 200 mL min1. The entry temperatures of the fuel and of the oxygen gas were regulated at the desired temperature, and the effective cell temperature under working conditions was determined by means of an appropriate sensor positioned inside the end plate at the cathode side.

Half-cell testing A portion (45 mg) of each electrocatalyst was introduced into a 5 mL high density polyethylene container together with water (1.2 g), ethanol (99.8 %, 0.70 g, Fluka), and 0.19 g. of Tokuyama OHtype anion-exchange ionomer in alcohols solution. The resulting suspension was sonicated for 90 min using a FALC bath. Each suspension was freshly prepared just before carrying out the experiment scheduled. The metal loading on each electrode was deter-

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Financial support from Ing. Guido Gay (Switzerland) project: “Conversion of CO2 in hydrocarbons and oxygenated compounds”, the MIUR (Italy) for the PRIN 2008 project prot. 2008N7CYL5, the MATTM (Italy) for the PIRODE project n.94, the MSE for the PRIT project Industria 2015, and the ICTP–TRIL Program is acknowledged. Keywords: electrooxidation · cerium · ethanol · fuel cells · palladium

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Received: November 17, 2011 Published online on April 19, 2012

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