Acetic acid internal reforming in a solid oxide fuel cell-reactor using Cu–CeO2 anodic composites

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 6 7 2 2 e1 6 7 3 2

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Acetic acid internal reforming in a solid oxide fuel cell-reactor using CueCeO2 anodic composites N. Kaklidis a, V. Besikiotis b, G. Pekridis a, G.E. Marnellos a,c,* a

Department of Mechanical Engineering, University of Western Macedonia, Bakola & Sialvera, GR-50100 Kozani, Greece Department of Chemistry, University of Oslo, Center for Materials Science and Nanotechnology (SMN) FERMiO, Gaustadalle`en 21, NO-0349 Oslo, Norway c Chemical Process Engineering Research Institute, Centre for Research & Technology Hellas, 6th km, Charilaou-Thermi Rd., P.O. Box 361, GR-57001 Thermi, Thessaloniki, Greece b

article info

abstract

Article history:

This work targets to explore the performance of CueCeO2 anodes for the production of

Received 15 November 2011

hydrogen and power generation during the internal steam reforming of CH3COOH in SOFC

Accepted 7 February 2012

reactors. When the cell operated as an electrochemical membrane reactor, the effect of

Available online 22 March 2012

temperature, reactants’ partial pressures and imposed overpotentials on the catalytic

Keywords:

were investigated. The results show that at open circuit conditions, CH3COOH is efficiently

Acetic acid internal reforming

reformed by H2O to syngas, where the observed products’ distribution is influenced by both

activity and selectivity of Cu/CeO2 electrodes, at both open and closed circuit operations,

Electro-catalytic hydrogen produc-

the associated CH3COOH thermal/catalytic decomposition reactions and by the reverse

tion

water gas shift reaction. In all cases examined, neither acetone nor carbon is observed at

Direct hydrocarbon SOFC

the effluents, with the latter being attributed to the gasification of carbonaceous deposits

CueCeO2 anodes

by H2O to CO and H2. At anodic polarization conditions, CueCeO2 exhibits high catalytic activity toward the electro-oxidation of all combustible species. Kinetic experiments show that the reaction mechanism involves the dissociative adsorption and decomposition of both reactants on the catalyst surface, where the subsequent interaction of the resulted fragments leads to the observed final products. In the fuel cell mode, the electrochemical performance of CueCeO2 was investigated by voltageecurrent densityepower density and AC impedance measurements. Ohmic losses are the prevailing source of polarization, mainly attributed to the anodic interfacial resistance, which is significantly influenced by cell temperature and reactants composition. The electrode performance is mainly limited by the diffusion of the charged or neutral species to triple phase boundary while in the case of dry feeding mixtures, the charge transfer processes determine the overall efficiency. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Concerns related to fossil fuels’ depletion, as well as the environmental and public health problems that arise from the

emissions of stationary and mobile power generation processes make hydrogen, in combination with fuel cells, an attractive energy alternative [1]. There are various established processes to produce hydrogen from fossil fuels [2], resulting

* Corresponding author. Department of Mechanical Engineering, University of Western Macedonia, Bakola & Sialvera, GR-50100 Kozani, Greece. Tel.: þ30 2461 0 56690; fax: þ30 2461 0 56601. E-mail addresses: [email protected], [email protected] (G.E. Marnellos). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.02.030

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inevitably to greenhouse gas emissions. On the other hand, hydrogen production from renewable sources, e.g., biomass, is gaining attention for a CO2 neutral energy supply. Recent developments in flash pyrolysis make it possible to convert lignocellulosic biomass efficiently to a bio-oil, which is easier for handling and transport [3]. Bio-oils produced from biomass pyrolysis are separated into the oil and water-rich phases. The water-rich phase contains mostly carbohydrate-derived compounds that can be catalytically reformed with steam [1]. Since acetic acid is one of the major constituents of the water-rich phase of bio-oil (considered as its common surrogate), its steam reforming has been extensively investigated over various noble metals (Pt, Pd, Rh, Ru and Ni) supported on mixed oxides (CeO2eAl2O3, La2O3eAl2O3, MgO/Al2O3, CaOeAl2O3, CeO2eZrO2) catalyst formulations [3e10]. Fuel cells, due to their high efficiencies, are considered as a promising and environmental friendly technology to replace conventional thermal engines. However, their high cost has prevented wide-spread adoption and as a consequence their commercialization [2]. One of the key factors that contribute to this high cost is the luck of fuel flexibility. Fuel cells generally operate on hydrogen, which is neither readily available nor easily stored [2]. To utilize hydrocarbons, fuel cells usually employ steam reforming, which converts fuels into hydrogen. Reforming and exhaust-gas recirculation (providing the steam for reforming) lead to additional plant complexity and volume, increasing the overall cost [2]. On the other hand, Solid Oxide Fuel Cells (SOFCs) due to their high temperatures can be potentially operated directly (internal reforming or electro-oxidation) on hydrocarbon fuels [11]. Ni-cermets are the most commonly used materials for SOFC anodes [2,12e14]. Ni-based catalysts have been also used for steam reforming of hydrocarbons including oxygenate hydrocarbons [1]. Although, Ni-based catalysts/ electrodes are characterized by their high electronic conductivity and catalytic activity toward steam reforming reactions [2,11], it has been reported that their major problem is their fast deactivation due to carbon deposition at low H2O/C ratios and their inadequate redox stability owing to Ni/NiO volume variations [13e18]. To avoid coking, high H2O/C ratios [19e21] and/or lower operating temperatures are required, which inevitably is expected to suppress the overall efficiency due to further fuel dilution and high ohmic resistances, respectively. On the other hand, the research group of Gorte has recently proposed the substitution of Ni with equally good electronic conductors that do not favor carbon formation, such as Cu or conducting oxides [12,22e26]. It has been demonstrated that by using the proposed anodic formulations, CueCeO2/YSZ, electrical power generation can be achieved constantly through the direct electrochemical oxidation of a wide variety of hydrocarbon fuels. In the present communication, the endothermic steam reforming of acetic acid is studied in a CueCeO2/YSZ/Pt type SOFC reactor, at atmospheric pressure. Under open circuit and “pumping” mode of operation, the effect of cell temperature, reactants’ partial pressure and imposed overpotentials, on products’ distribution and overall catalytic and electrocatalytic activity of the CueCeO2 anodic composites, was examined. Furthermore the as prepared cell was electrochemically characterized using AC impedance

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measurements. Finally, the electrochemical performance and stability of CueCeO2 electrodes were investigated by measuring the voltageecurrent density and power densityecurrent density characteristics at various cell temperatures and CH3COOH/H2O ratios.

2.

Experimental

2.1.

Experimental apparatus

The apparatus used for the (electro)-catalytic and fuel cell measurements consisted of the feed unit, the cell-reactor and the gas analysis system [11]. Acetic acid and steam were fed into the reactor cell by bubbling pure Helium through two different vessels containing liquid CH3COOH of 99.99% purity (Riedel-de Haen) and distilled water, respectively, at various temperatures depending on their vapor pressures. Both the saturators and the tubing employed to introduce the feeding mixture to the fuel cell e membrane reactor were wrapped around with heating resistances and their temperatures were continuously controlled and monitored using four thermocontrollers of 1  C accuracy (JUMO) and the appropriate thermocouples. The electrochemical reactor consisted of a YSZ tube (15 cm long and 1 mm thickness), closed flat at one end and was supplied by CERECO. The open end of the YSZ tube was clamped to a stainless-steel cap, which had provisions for inlet and outlet gas lines. A Gold (Au) wire was used to establish electrical contact with the inner CueCeO2 catalysteelectrode via a spirally shaped end. The electrical circuit was closed by two other Au wires connected to counter and reference electrodes, respectively. In all cases, the temperature was monitored using a K-thermocouple. Gas analysis was performed using an online, SHIMADJU 14B, gas chromatograph, equipped with a thermal conductivity detector (TCD). A molecular sieve 13 column (10 ft  1/8 in.) was used to separate H2, CO and CH4, while a Porapack QS column (10 ft  1/8 in.) was employed for the CO2, H2O, and CH3COOH separation. Additionally a setup of IR gas analyzers in series e CH4 (ANARAD), CO2 (ANARAD) and CO (MIRAN 203) e was also used for the continuous monitoring of these chemical species. The carbon and water contents were always calculated using the carbon, hydrogen and oxygen elemental mass balances, respectively. In order to verify, the amount of deposited carbon, after each experiment a TPO (Temperature Programmed Oxidation) analysis was performed at standard conditions (T ¼ 800  C, Feed: 5% O2/He), where the COx formation rates were continuously monitored.

2.2.

Materials preparation and characterization

CueCeO2 (70 and 30% wt.) working electrode which also served as the catalyst for the CH3COOH reforming reaction, was prepared from Cerium(IV) oxide (99.9%, Alfa Aesar) and Copper powders (99%, Alfa Aesar), respectively. Stoichiometric quantities were mixed in 20 ml of ethyl glycol, fired at 200  C and stirred at 400 rpm until half of the volume was evaporated. The as formed viscous suspension was deposited on the inside bottom of the YSZ tube by painting, resulting in a catalyst weight equal to z1230 mg. The tube was then

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heated up to 900  C (heating rate: 3  C/min), and calcined for 4 h. The CueCeO2 electrode thus formed had a superficial surface area of 2 cm2. Finally, two Platinum (Metalor) films were deposited (calcination at 900  C for 4 h) on the outside wall of the YSZ tube, and they served as counter and reference electrodes. Their apparent surface areas were approximately 2 and 0.2 cm2, respectively. The analysis of the microstructure, morphology and chemical composition of the Cu/CeO2 electrode was carried out using various physicochemical characterization techniques [11,27]. The surface area was measured by N2 adsorption at 77 K, using the multipoint BET analysis method with an Autosorb-1 Quantachrome flow apparatus, and was found equal to 2 m2/g. For metal loading determination the inductively coupled plasma atomic emission spectroscopy (Optima 4300 DV, PerkineElmer) was used, showing a Cu loading of 73.4  2.5%. The morphology of the synthesized materials was examined by scanning electron microscopy (SEM) on a JEOL 6300 microscope, coupled with energy dispersive X-ray analysis (EDX; Oxford Link ISIS-2000) for local elemental composition determination. The elemental mapping revealed that the microstructure/microchemistry is non-uniform, with the CeO2 and Cu phases randomly distributed [11]. The grain size of the CeO2-phase varies from 1 to 10 mm while the Cu-phase has a liquid morphology without clear formed grains. Finally, crystallographic information was established with the aid of the powder X-ray diffraction technique [27]. The diffraction intensity e 2q spectra were measured in a Siemens D 500/501  A) at a scanning rate of 0.04 with Cu-Ka radiation (l ¼ 1.54178  over 2 s. In the X-ray spectra of the CueCeO2 catalyst pretreated at 850  C with pure H2 under open and closed circuit conditions (h ¼ 3000 mV), the Cu, CeO2 and Cu2O phases were observed, without any noteworthy differences between both samples [27]. Cu2O is mainly attributed to the catalyst preparation procedure, where the calcination was performed at stagnant air conditions.

inductance of the wires, Rbulk is the ohmic resistance of the electrolyte, Cdl is the double layer capacitance, RCT is the resistance of the charge transfer reaction between the electrodeeelectrolyte interface, and (RDiffQDiff) denotes a resistor and a constant phase element (CPE) in parallel attributed to the diffusion. In order to correlate the different contributions of the impedance spectra to physical processes, the pseudocapacitances, Ci, of the sub-circuits were calculated using equation (1): 1 n

1 n

Ci ¼ Yi i Ri i

1

(1)

Yi and ni define the admittance of the CPE given by pffiffiffiffiffiffiffi YQ ¼ Yi ðjuÞni , where u is the angular frequency and j ¼ 1 [30]. In all experiments, a total volumetric flow rate of 75 cm3/ min STP was employed. It should be noticed that prior to each experiment the anodic electrode was pretreated with pure H2 at 850  C for 3 h.

3.

Results and discussion

3.1. Catalytic and electro-catalytic steam reforming of acetic acid Initially, the reaction of acetic acid steam reforming was examined in the CueCeO2/YSZ/Pt oxygen-ion conducting solid electrolyte membrane reactor. The overall system of reactions which may be taking place is quite complex and may be represented by the following reactions [4,8]: CH3 COOH þ 2H2 O/2CO2 þ 4H2 CH3 COOH/2CO þ 2H2

Reforming

(2)

Thermal or catalytic decomposition (3)

CH3 COOH/CH4 þ CO2

Thermal or catalytic decomposition (4)

2.3. Potentiostatic, AC impedance and fuel cell measurements

CH3 COOH/C þ Cx Hy

Thermal or catalytic decomposition (5)

The open and closed circuit activity was examined in the reactor cell described before, using various reacting mixtures of CH3COOH (0.5e6 kPa) and H2O (1.5e12 kPa) diluted in He, at temperatures between 750 and 850  C. The potentiostatic mode of operation was implemented by applying anodic and cathodic overpotentials varying from 1000 to 3000 mV. An AMEL model 2053 galvanostatepotentiostat and two differential voltmeters (Digital Multimeter DT9205A) were used to impose overpotentials and to measure the developed currents. For the fuel cell tests, a Resistance box 1051 by Time Electronics was equipped, while the electrochemical impedance spectra were obtained under open circuit conditions in the frequency range from 0.01 Hz to 1 MHz, using the IVIUM technologies electrochemical workstation and the corresponding software (IVIUMSOFT) for data processing. The impedance data were validated with a KramerseKronig transform [28] and analyzed with the “Equivalent circuit for Windows” software [29]. Impedance spectra were fitted to the Randles circuit LRbulk(Cdl(RCT(RDiffQDiff))) [27], where L is the

CH3 COOH/ðCH3 Þ2 CO þ H2 O þ CO2

Ketonization

(6)

which are followed by a parallel network of reactions, where the extent of each reaction depends mainly on the operating conditions and the reacting mixture composition: CH4 þ H2 O/CO þ 3H2 CO þ H2 O4CO2 þ H2

Methane steam reforming Wateregas shift

CO2 þ 4H2 4CH4 þ 2H2 O C þ H2 O42CO þ 2H2 2CO4C þ CO2

(7) (8)

Methanation

(9)

Coal gasification

(10)

Boudouard reaction

(11) 

Fig. 1 presents the effect of temperature (750e850 C) on products’ formation rates distribution, acetic acid and steam conversions and on the developed electromotive force (EMF)

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Fig. 1 e Effect of temperature on CH3COOH and H2O conversions, products’ distribution and EMF, at open circuit conditions.

under open circuit conditions, for a stoichiometric mixture consisting of PCH3 COOH ¼ 3 kPa and PH2 O ¼ 6 kPa: It is clear that both CH3COOH and H2O conversions increase with temperature. In all temperatures examined the conversion of acetic acid was always higher than the corresponding conversion of H2O, denoting the significant role of the thermal and catalytic decomposition processes. However, it should be noticed that the conversion of steam is apparent, since additional amounts of H2O are formed through the existing reaction network. The final products during steam reforming of acetic acid consisted of CH4, H2, CO2 and CO, while their production rates were increased with temperature. The observed H2/CO2 ratios at temperatures above 800  C, were always higher than the corresponding expected stoichiometric ratio of reaction (2), verifying the importance of decomposition reactions at high operation temperatures. A similar behavior was obtained in the relevant works of Basagiannis and Verykios [4,5]. They observed that hydrogen production is increased sharply with temperature due to the thermal decomposition reaction of CH3COOH leading to CO and H2, as also observed here. The same group, over Rh-based catalysts, noticed that the selectivity toward CO is favored at high temperatures, due to the enhancement of the Reverse Water Gas Shift (RWGS) reaction [5]. In the present work, the H2/CO molar ratio is clearly decreased with cell temperature. At temperatures higher than 775  C, the ratio is instantaneously dropped from 1.27 to 1.08 at 800  C and then remains almost unaffected up to 850  C. This behavior is consistent with the CH3COOH decomposition and RWGS reactions, prevailing at high temperatures. In a relevant thermodynamic analysis performed by Vagia and Lemonidou [31], H2/CO molar ratios close to unity were calculated at temperatures higher than 750  C during CH3COOH decomposition. The corresponding results at CH3COOH steam reforming reacting mixtures showed that the H2/CO ratios were decreased with temperature and steam/fuel

ratios [31]. Furthermore, as in the work of Takanabe et al. [10], no carbon formation was observed due to the fact that the activated H2O species gasify CH3COOH-oriented carbon residues on the catalyst surface producing H2 and COx, making the CueCeO2 surface available for further reaction [10]. Therefore, the presence of H2O seems to protect the catalyst from carbon deposition, since at corresponding experiments performed without steam, carbon was inevitably formed [27]. This behavior is also verified by the thermodynamic analysis performed in Ref. [31]. Taking into account the reaction network and the relative formation rates, it is apparent that apart from the reforming reaction (2), the thermal and catalytic decomposition and RWGS reactions play a significant role in the achieved products’ distribution, especially at high operating temperatures. Thus, at the examined reaction conditions, CH3COOH seems to be reformed by H2O at low temperatures, while at higher temperatures is decomposed mainly through reactions (3) and to a lesser extent through the decarboxylation reaction (4). Finally, no acetone was identified at the reactor exit, since the ketonization reaction (6) dominates at significantly lower temperatures (450e650  C) and becomes essentially negligible at higher temperatures [4,8]. The apparent activation energy of the CH3COOH consumption rate, calculated at intrinsic reaction conditions, was equal to 11.5  0.8 kcal/mol, a value close to that observed over Ru/ La2O3/Al2O3 [4], however slightly lower compared to the acetic acid decomposition case, where it was found equal to 15.4  1.5 kcal/mol [27]. Fig. 2 depicts the dependence of products’ formation rates, CH3COOH and H2O conversions and EMF on both reactants’ partial pressures at a constant cell temperature equal to 850  C. In general, by increasing PCH3 COOH (Fig. 2a), all products’ formation rates increased. On the other hand, CH3COOH conversion decreases with acetic acid partial pressure (from

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a

b

Fig. 2 e Effect of a) CH3COOH and b) H2O partial pressures on CH3COOH and H2O conversion, products’ distribution and EMF, at open circuit conditions.

92.9 to 32.9%), while the apparent H2O conversion increases from 11.1 to 18.7%. The increase of PCH3 COOH is leading to a significant increment of H2 and CO selectivities implying the crucial role of reaction (3) in the observed products’ distribution. Their molar ratio, H2/CO, follows an inverted volcano behavior exhibited its minimum value (1.10) at stoichiometric conditions. At the lowest PCH3 COOH ¼ 0:5 kPa, the H2/CO ratio equals to 1.67, while at PCH3 COOH ¼ 6 kPa the ratio equals to 1.30. As can be seen, the EMF values are decreasing with PCH3 COOH , indicating that both acetic acid and its decomposition carbonaceous fragments are acting as electron donor species, i.e. their adsorption strength is expected to increase by supplying oxygen anions, O2, toward the anodic electrode/catalyst.

Fig. 2b shows the effect of steam partial pressure on products’ distribution, reactants’ conversions and the developed EMF values. Contrary to the results of Fig. 2, EMF values were increased with PH2 O , denoting that steam is acting as an electron acceptor species. CH3COOH conversion increases with increasing H2O partial pressure, while the corresponding H2O conversion decreases. Finally, the observed products and their relevant formation rates were qualitatively similar to Fig. 2a, where the decomposition products are favored at high CH3COOH/H2O ratios. Again the H2/CO ratio exhibited a similar to Fig. 2a inverted volcano behavior. In the case of the stoichiometric mixture, the H2/CO ratio was equal to 1.10, while at both low and high steam partial pressures the H2/CO ratios were increased. In general, at high PH2 O =PCH3 COOH , the

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H2/CO ratio is increased, according to Ref. [31]. The increased H2/CO ratios can be mainly attributed to the acetic acid reforming reaction (2) and the shift in the equilibrium of the RWGS reaction (8). On the other hand at CH3COOH excess conditions, the H2/CO molar ratio is also increased, however to a lower extent compared to the previous case. This behavior can be attributed to the methane reforming reaction which leads to H2/CO ratios equal to 3 according to reaction (7); methane is produced from reaction (4) which is favored at high acetic acid concentrations. Kinetic experiments showed that the partial reaction orders for acetic acid and steam were equal to 0.55  0.04 and 0.71  0.04, respectively, close to values reported in the related literature [4]. These values, possibly suggest a rate determining step that involves the dissociative adsorption and decomposition of both reactants on the catalyst surface and the subsequent interaction of the resulted fragments to final products. Fig. 3 shows the effect of overpotentials (1000e3000 mV) on products’ formation rates, CH3COOH and H2O conversions and on the developed current density, at a constant cell temperature equal to 850  C and at CH3COOH/H2O stoichiometric ratio. It can be seen that at anodic polarization conditions, i.e. when oxygen anions, O2, are electrochemically supplied toward the working electrode/catalyst, the same range of products were formed as in the case observed at open circuit conditions. As the applied overpotential increases, acetic acid conversion along with the formation rate of CO2 are slightly enhanced, while the corresponding CO, CH4 and H2 production rates along with H2O conversion are decreased. Therefore, it seems that all present combustible species are electro-oxidized at the triple phase boundary (TPB) by O2 to CO2 and H2O, indicating the high electro-catalytic activity of CueCeO2 electrodes. The additional amount of produced H2O, is essentially reflected in the decreased apparent H2O conversion. The corresponding H2/CO ratio increases linearly with the applied overpotential and reaches values above 1.3 at

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h ¼ 3000 mV. This behavior can be attributed to the preferred electro-oxidation of hydrogen compared to CO at TPB. At cathodic polarization conditions, where, by applying a constant cathodic overpotential, O2 are removed from the catalyst surface, both conversions are increased due to the abstraction of the oxygen species that are contained in both reactants molecules, indicating their electro-reduction at the TPB. Hydrogen is increased due to water electrolysis, while for the same reason CO2 formation rate is decreased leading to CO enhancement. Electro-kinetic measurements showed that all kinetic parameters (activation energy, reaction orders) are essentially similar to those obtained at open circuit operation, denoting that the same rate determining step is governing the overall reaction rate, under both open and closed circuit conditions.

3.2.

Fuel cell operation

Fig. 4a depicts the voltage and power density versus current density curves at various cell temperatures ranged between 750 and 850  C, at a stoichiometric acetic acid to steam ratio. The CueCeO2/YSZ/Pt cell exhibited an open circuit voltage (OCV) ranged from 0.95 V at 750  C to 0.90 V at 850  C. The results show a clear linear dependence of cell voltage on current density, indicating the significant contribution of the ohmic resistance on the overall cell polarization compared to activation and concentration polarizations. As expected, the slopes of the voltageecurrent density curves are decreasing with temperature, indicating that the overall ohmic resistance of the cell is reduced as cell temperature increases. The overall resistance contains the ohmic contribution from the electrolyte as well as the resistances attributed to cathode and anode micro-structures and interfaces. The open circuit AC impedance spectra obtained at conditions identical to those employed in Fig. 4a, showed that the ohmic portion of the area specific resistance (ASR) of the

Fig. 3 e Effect of applied overpotential on CH3COOH and H2O conversions, products’ distribution and developed current density.

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Fig. 4 e Effect of temperature a) on the current densityevoltage and current densityepower density characteristics of the cell and b) on the electrode polarization resistance.

cell, is decreased by increasing cell temperature from 3 U cm2 at 750  C to 1.2 U cm2 at 850  C (see Supp. 1). In all temperatures examined, two arcs were contained in each impedance spectrum, one centered at higher frequencies (6.3, 25.1 and 39.8 Hz, respectively), and the other at lower frequencies. The high frequency region arcs were attributed to the anodic electrochemical processes taking place during CH3COOH internal reforming, while the low frequency arcs corresponded to gas diffusion transport. Fig. 4b presents a typical Arrhenius plot of the electrode polarization resistance at

PCH3 COOH ¼ 3 kPa and PH2 O ¼ 6 kPa. It is clear that the charge transfer, RCT, and gas diffusion, RDiff, components of the overall electrode polarization resistance follow an Arrheniustype behavior with activation energies equal to 32.9 and 43.8 kcal/mol, respectively. It is evident that the diffusion of charged or neutral species at the CueCeO2 electrode seems to be the rate limiting step contributing with approximately 73% to the total electrode polarization resistance, while in the absence of water and at PCH3 COOH ¼ 1 kPa the charge transfer process taking place at the electrodeeelectrolyte interface

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was the controlling step, limiting the anode performance as it was shown in Ref. [27]. At the same time, substantial improvements in power output are obtained by increasing operation temperature. A 290% increase in power generation was achieved by increasing cell temperature from 750 to 850  C (Fig. 4a). The maximum power output was obtained at 850  C, and it was equal to 18.8 mW/cm2 at a cell voltage of 552 mV and a current density of 34 mA/cm2. Fig. 5a depicts the voltageecurrent density and power densityecurrent density characteristics of the cell at three different acetic acid/steam ratios and at a constant cell temperature equal to 850  C. The main source of polarization was again, as in Fig. 4a, attributed to ohmic resistance. Figs. 5b and c, depict the effect of acetic acid and H2O partial pressures, respectively, on the electrolyte ohmic and electrode polarization resistances. The corresponding open circuit ACimpedance spectra obtained at 850  C for the same acetic

16729

acid/steam ratios, showed that the electrolyte ohmic resistance, Rbulk, is almost similar and independent from the CH3COOH/H2O ratio (see Supp. 2), suggesting a good electrode adherence [32]. In addition as it can be also seen in Fig. 5b and c, there is a very good agreement on the obtained values for the pseudocapacitances of the gas diffusion and the double layer, building confidence for the fitting of the impedance spectra. Fig. 5b, shows that the electrode polarization resistance at T ¼ 850  C and PH2 O ¼ 6 kPa, is drastically decreasing with PCH3 COOH , where the gas diffusion component seems to mainly contribute to this decrease. The charge transfer resistance is essentially independent from the acetic acid partial pressure and it does not contribute to the observed decrease of the overall electrode polarization resistance. On the other hand, when the H2O partial pressure was varied (Fig. 5c, T ¼ 850  C, PCH3 COOH ¼ 3 kPa), a different behavior was observed.

Fig. 5 e Effect of CH3COOH/H2O ratio a) on the current densityevoltage and current densityepower density characteristics of the cell and b) on the electrode polarization resistance and electrolyte ohmic resistance. Cdl is the double layer capacitance of the electrode electrolyte interface, and CDiff is the pseudocapacitance of the gas diffusion.

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Fig. 6 e Power density at a cell potential of L0.55 V as a function of time on stream for different fuel mixtures (H2/He and CH3COOH/H2O/He).

Although, again the gas diffusion of charged and neutral species on the anode was the main contributor to the overall electrode polarization resistance, its variation on the PH2 O follows an inverted volcano behavior, exhibiting the minimum resistance at the stoichiometric ratio. At low H2O partial pressures, excess acetic acid may favor carbon deposition on the electrode’s surface limiting the diffusion of combustible chemical species to the active electrochemical zone. Therefore, diffusion is the process that determines the overall cell performance and this particular behavior can be explained, if one takes into consideration the high values of the gas diffusion pseudo-capacitances, in the order of 1 F cm2, which are characteristic for electrodes with poor porosity, as further evidenced by the rather limited BET surface area of the CueCeO2 electrodes. Concerning power generation, it is observed that power production is increased as PCH3 COOH =PH2 O is increasing. The maximum power output equaled to 21.7 mW/cm2 and was attained at 451 mV cell voltage and a current density of 48.1 mA/cm2, at PCH3 COOH =PH2 O ¼ 1: In order to examine the durability of the CueCeO2 anode long-term stability tests were performed, where the obtained power density was continuously recorded as a function of time by consecutively interchange the feed mixture. In Fig. 6 the variation of power density as a function of time, at a constant cell potential of 0.552 V and at T ¼ 850  C, is depicted. Initially, the fuel cell was fed with 15% H2 diluted in He gas mixture. The achieved power density was sharply decreased during the first half hour while for the rest three and a half hours remained almost constant and equal to 30.2 mW/cm2. At the end of the fourth hour, the hydrogen containing mixture was substituted by an acetic acid/steam balanced with He mixture ðPCH3 COOH ¼ 3 kPa; PH2 O ¼ 6 kPaÞ, and as can be seen the power output was instantaneously

dropped down to 14 mW/cm2 and remained unaffected for the rest of the cycle. In the following and in order to get an insight on possible degradation effects, the feed mixture was turned again to hydrogen using the same concentration as in the first cycle. The power output was gradually increased and the cell almost regained the initial efficiency after 1 h, achieving a final power density equal to 27 mW/cm2. Therefore, the power density was decreased at about 10% compared to the 1st step, where the cell was again fed with 15% H2/He. This degradation can be possibly attributed to permanent electrode morphological changes taking place after its exposure to the acetic acid/steam reacting mixture (2nd step). Ceria, CeO2, has a great ability to store O2 and as a consequence under oxidizing conditions (the acetic acid/steam mixture is a more oxidized feed mixture compared to 15% H2/He) the oxidation of Cu is favored due to the ability of CeO2 to hold high amounts of O2. It is possible that this transition of the anodic electrode may have caused harmful effects on its body [33].

4.

Conclusions

CH3COOH was efficiently reformed by H2O to syngas, where the overall operation was described by a complex network of physicochemical processes, including the reforming reaction, the CH3COOH thermal/catalytic decomposition and RWGS reactions as well as the electrochemical processes taking place at the TPB. In all cases examined, neither acetone nor carbon was observed at the effluents, with the latter being attributed to the gasification of carbonaceous deposits by H2O to CO and H2. CueCeO2 exhibited high catalytic activity toward the electro-oxidation of all combustible species that are present in the anode (H2, CO and CH4). Under both modes of operation, kinetic experiments showed that the reaction

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 6 7 2 2 e1 6 7 3 2

mechanism involved the dissociative adsorption and decomposition of both reactants on the catalyst surface, where the subsequent interaction of the resulted fragments led to the observed final products. Under the “fuel cell” mode, the achieved power densities were substantially increased with temperature and PCH3 COOH =PH2 O ratios. Fuel cell and AC impedance measurements exhibited that ohmic losses were the prevailed source of polarization, the main part being essentially attributed to the anodic interfacial resistance, which is significantly affected from cell temperature and reactants composition. The electrode performance was limited by the diffusion of the charged or neutral species to triple phase boundary in contrary to the case of dry feeding mixtures, where the charge transfer processes determined the overall efficiency. The increase of PCH3 COOH decreases the diffusion resistance resulting in the increase of the overall cell performance.

Ethical statement None.

Acknowledgments The present research was co-funded from the European Union (ESF) by 70% and the Hellenic State by 30% through the framework of scientific & technological cooperation between RTD organizations in Greece and in Cyprus.

Appendix. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ijhydene.2012.02.030.

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