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Direct electro-oxidation of iso-octane in a solid electrolyte fuel cell
Solid State Ionics 192 (2011) 435–443
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Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Direct electro-oxidation of iso-octane in a solid electrolyte fuel cell N. Kaklidis a, G. Pekridis a, C. Athanasiou b,c, G.E. Marnellos a,c,⁎ a b c
Department of Mechanical Engineering, University of Western Macedonia, Bakola and Sialvera, GR-50100 Kozani, Greece Department of Environmental Engineering, Democritus University of Thrace, University Campus, GR-67100 Xanthi, Greece Chemical Process Engineering Research Institute, Centre for Research and Technology Hellas, 6th km. Charilaou-Thermi Rd., P.O. Box 361, GR-57001 Thermi, Thessaloniki, Greece
a r t i c l e
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Article history: Received 1 September 2009 Received in revised form 6 June 2010 Accepted 11 June 2010 Available online 10 July 2010 Keywords: Iso-octane Direct hydrocarbon SOFC Cu–CeO2 electrodes
a b s t r a c t The present work aims to explore the activity of Cu/CeO2 composites as anodic electrodes in direct iso-octane SOFCs. When the cell was operated as a membrane reactor, the effect of temperature, Pi-C8H18 and applied anodic overpotentials on the electrocatalytic activity and products' distribution, at both open and closed circuit conditions, was examined. Additionally, in situ DRIFT spectroscopy was carried out in order to correlate the performance of Cu/CeO2 with its surface chemistry during iso-octane decomposition. Under the “fuel cell” mode of operation, the electrochemical performance and stability of Cu/CeO2 were investigated by voltage–current density–power density and AC impedance measurements. The results reveal that at high anodic polarization conditions, carbon formation can be noticeably restricted (verified also by EDAX analysis), while H2 production was enhanced due to partial oxidation, steam reforming, dehydrogenation and water gas shift reactions. Achieved power densities were found to substantially increase both with temperature and Pi-C8H18, while minor performance degradation was indicated in the step-change tests, where the overall activity of Cu–CeO2 electrodes remained essentially unaffected. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Fuel cells are considered as a promising and environmental friendly candidate technology to replace conventional thermal engines for power generation at high efficiencies (40–50%) [1,2]. However, the high cost of these systems has prevented their widespread adoption and as a consequence their commercialization [3]. One of the key factors that contribute to this high cost is the luck of fuel flexibility [3,4]. Fuel cells generally operate on hydrogen, which is neither readily available nor easily stored [3,5,6]. To utilize hydrocarbons, fuel cell power plants usually employ steam reforming, which converts fuels into hydrogen [3]. Reforming and exhaust-gas recirculation (providing the steam for reforming) lead to additional plant complexity and volume, increasing the overall cost [7]. 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 conventional hydrocarbon fuels ranging from natural gas to gasoline [6–12], for which their refining and distribution infrastructure is well established [5]. Therefore in SOFCs, any combustible material can, in principle, be used to generate electrons from the electro-oxidation reactions with O2− species. However, the selection of the fuel is limited by its
⁎ Corresponding author. Department of Mechanical Engineering, University of Western Macedonia, Bakola and Sialvera, GR-50100 Kozani, Greece. Tel.: +30 2461 0 56690; fax: +30 2461 0 56601. E-mail address: [email protected] (G.E. Marnellos). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.06.022
tendency to foul the anodes [8]. Ni-cermets are the most commonly used materials for SOFC anodes [3,6,13,14]. These anodic composites are characterized by their high electronic conductivity, adequate thermal stability and catalytic activity towards steam reforming reactions [13]. However, their tendency to catalyze the formation of carbon fibers at low H2O/C ratios and consequently their rapid degradation [7,13–16], renders their use in direct hydrocarbon SOFC applications improper. To avoid coking, high H2O/C ratios [17–19] 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 [6,13] has recently proposed the substitution of Ni with equally good electronic conductors that do not favor carbon formation, such as Cu [8,20,21] or conducting oxides [22–24]. It has been demonstrated that by using the proposed anodic formulations, Cu–CeO2/YSZ, electrical power generation can be achieved constantly through the direct electrochemical oxidation of a wide variety of hydrocarbon fuels, including also liquid fuels. To this end, the expansion of potential fuel range is enabled, co-currently eliminating the fuel pre-processing costs and increasing overall efficiency [5,6,8,9,20–22]. In the present study, iso-octane, a common surrogate for gasoline, is directly fed in a SOFC reactor of the Cu–CeO2/YSZ/Pt type. The selection of iso-octane as fuel feedstock instead of using other commercial fuels such as diesel or kerosene was based to its broaden applicability in both mobile and stationary applications and to its lower tendency for coke formation and deposition due to thermal cracking compared to heavier hydrocarbons [25]. Initially, the effect of
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cell temperature, i-C8H18 partial pressure and imposed anodic overpotentials on product's distribution was thoroughly examined by performing typical potentiostatic experiments (at both open and closed circuit conditions). In situ DRIFT spectroscopy studies were carried out in order to get an insight on the Cu–CeO2 surface chemistry during iso-octane decomposition. In the following and prior to fuel cell tests, the as prepared cell was electrochemically characterized using AC impedance measurements. Finally, the electrochemical performance and stability of Cu–CeO2 electrodes were investigated by measuring the voltage–current density and power density–current density characteristics of the direct iso-octane SOFC at various cell temperatures and i-C8H18 partial pressures. 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. Reactant gases included H2 and He of 99.999% purity, and were supplied by Air Liquide. Iso-octane was fed in the reactor cell by bubbling pure helium through a vessel containing liquid i-C8H18 of 99.5% purity (Riedel-de Haen), at ambient temperature. The electrochemical reactor used in our experiments 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 wire was used to establish electrical contact with the inner Cu-CeO2 catalyst–electrode 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 on-line, 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, C2's, C3's, H2O and i-C8H18 separation. Additionally a setup of IR gas analyzers in series – CH4 (ANARAD), CO2 (ANARAD) and CO (MIRAN 203) – was also used for the continuous monitoring of these chemical species. The carbon and water contents were always calculated using the C, hydrogen and oxygen mass balances, respectively. Specifically, in the case of carbon, the deposited amount was also verified by performing at standard conditions (T = 800 °C, feed: 5% O2/He) a temperature programmed oxidation (TPO) experiment. In all cases examined, the difference between the coke that was calculated using carbon mass balance and the coke that was estimated through the TPO experiment always ranged within 5%.
whiter CeO2-phase (1-spot) and the gray Cu-phase (2-spot) are randomly distributed. The grain size of the CeO2-phase varies from 1 to 10 μm while the Cu-phase has liquid morphology without clear formed grains. 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. 2.3. Potentiostatic, AC impedance and fuel cell measurements The electro-oxidation of i-C8H18 was examined in the reactor cell described in Section 2.1, using various reacting mixtures of i-C8H18 (0.1–6.0%) diluted in He, at temperatures between 750 and 850 °C. The potentiostatic mode of operation was implemented by applying anodic overpotentials varying from 0 to 3000 mV. An AMEL model 2053 galvanostat–potentiostat 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 0.1 Hz to 1 MHz, using the IVIUM technologies electrochemical workstation and the corresponding software (IVIUMSOFT) for data processing. 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. 2.4. In situ DRIFTS studies Diffuse reflectance IR spectra were obtained using a NICOLET 6700 spectrometer, equipped with a high temperature and high pressure cell, cooled by liquid nitrogen. Spectra were obtained using a resolution of 4 cm− 1 with accumulation of 64 scans. Catalyst samples (≈50 mg) in the form of powder were flattened in order to maximize the intensity of the reflected beam. The total flow rate through the IR cell was retained equal to 40 cm3/min. Before each experiment the catalyst was pretreated as follows: i) purging with Ar flow at 850 °C for 30 min, ii) reduction at the same temperature using pure H2 flow for 3 h, iii) purging with Ar for 30 min and iv) background spectra acquisition under Ar flow at the desired temperatures. The steadystate experiments were performed at constant feed composition (Pi-C8H18 = 0.4 kPa balanced with Ar) in the temperature range between 25 and 850 °C, and the system was allowed to stabilize for 30 min in order to achieve steady-state band intensities. 3. Results and discussion 3.1. Iso-octane decomposition at open circuit conditions
2.2. Material preparation Cu–CeO2 (70 and 30 wt.%, respectively) working electrode which also served as the catalyst for the i-C8H18 electro-oxidation 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 (application of four consecutive thin coatings) resulting in a catalyst weight equal to ≈1230 mg. The tube was then heated up to 900 °C (heating rate: 3 °C/min), and calcined for 4 h. The Cu/CeO2 electrode thus formed had a superficial surface area of 2 cm2. Fig. 1 represents two Scanning Electron Micrographs (SEM) of the Cu–CeO2 microstructure (JSM 6300, JEOL) at different magnification factors (pretreatment with pure H2 at 850 °C for 12 h). The SEM elemental mapping on the microstructure of the sample reveals that the microstructure/microchemistry are non-uniform, while the
Initially, the reaction of iso-octane decomposition was examined in the Cu–CeO2/YSZ/Pt oxygen-ion conducting solid electrolyte membrane reactor. Fig. 2 presents the effect of temperature (750– 850 °C) on products' formation rate distribution, iso-octane conversion and on the developed electromotive force (EMF) under open circuit conditions, for Pi-C8H18 = 0.4 kPa. It is clear that i-C8H18 conversion increases almost linearly with temperature. The products that were formed during iso-octane decomposition were C, CH4, H2, C2H4, C2H6, C3H6 and C3H8, while their production rates were increased with temperature. Specifically, up to 825 °C the product's distribution remains almost unaffected, however at higher temperatures the selectivities of both C and CH4 increase rapidly, because both decomposition and dehydrogenation reactions become thermally and kinetically more favorable [7]. As far as the formation of higher hydrocarbons concerns, the Cu–CeO2 catalyst seems to favor olefins (C2H4 and C3H6) production in contrast to saturated hydrocarbons (C2H6, C3H8). However it should be emphasized that olefins were also
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Fig. 1. SEM pictures of the Cu–CeO2 electrode (fresh sample) at different magnification factors and the corresponding elemental mapping. 1-spot: CeO2-phase and 2-spot: Cu-phase. Pretreatment: pure H2 at 850 °C for 12 h.
the main products observed in the absence of catalyst (blank experiments not shown for brevity's sakes) during i-C8H18 thermal decomposition. Qualitatively similar results were obtained in the work of Saunders and Kendall over Ni/YSZ electrodes using n-octane as a fuel [7]. However instead of C3's, C4's alkanes/alkenes were formed suggesting concurrent polymerization and degradation of alkene intermediates [7], a fact that was not observed in the present study. Fig. 3 depicts the dependence of products' formation rates, i-C8H18 conversion and EMF on i-C8H18 partial pressure at a constant cell temperature equal to 850 °C. In general by increasing Pi-C8H18 all
products' formation rates increased almost linearly. On the other hand, i-C8H18 conversion decreases smoothly at lower iso-octane partial pressures, while at Pi-C8H18 higher than 3 kPa remains essentially constant. As can be seen, the EMF values are decreasing with Pi-C8H18, indicating that both iso-octane and its decomposition products are acting as electron donor species, i.e. their adsorption strength increase by supplying oxygen anions, O2−, towards the anodic electrode/catalyst. Fig. 4 shows the IR spectra of adsorbed species present on the surface of the Cu–CeO2 catalyst at temperatures in the interval of
Fig. 2. Effect of temperature on i-C8H18 conversion, products' distribution and EMF, at open circuit conditions.
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Fig. 3. Effect of i-C8H18 partial pressure on i-C8H18 conversion, products' distribution and EMF, at open circuit conditions.
27–850 °C, using a feed mixture of iso-octane (Pi-C8H18 = 0.4 kPa) diluted in Ar. At 27 °C five well-resolved peaks at 1371, 1478, 2879, 2908 and 2962 cm− 1 are observed, which can be attributed to gas phase i-C8H18. Also, two bands at 1047 and 1646 cm− 1, which can be assigned to in-of-plane bending vibration of C–H and the stretching vibration of C C bonds, respectively [26], are starting to develop at 100 °C, indicating the existence of surface iso-octane fragments through its interaction with the Cu/CeO2 electrode. All the above peaks remained almost unaffected up to 600 °C, while above this temperature their intensities are starting to decrease, indicating that i-C8H18 and consequently its surface fragments are beginning to decompose. This observation is in accordance with the rather low conversion of iso-octane (15.6%) observed at 750 °C (Fig. 2). At the same time new bands at 729, 950, 1300, 3014 and 3088 cm− 1 start to develop and their intensities are increased with temperature. Absorption peaks at 729 and 1300 cm− 1 represent the out-plane and in-of-plane bending vibration of C–H bonds, while the bands at 950 and 3088 cm− 1 are characterizing the
bending and stretching vibration of C–H bond, verifying the formation of the saturated (C2H6, C3H8) and unsaturated (C2H4, C3H6) hydrocarbons observed in the catalytic experiments (Figs. 2 and 3) [26]. In addition, the bands between 1371 and 1478 cm− 1 correspond to symmetric and asymmetric bending modes of CH2 and CH3 groups [27], which are considered as active hydrocarbon components [28]. Finally, the peak at 3014 cm− 1, which its intensity is maximized at 850 °C, is characteristic for gas phase methane, justifying its high formation rate observed in Figs. 2 and 3. Based on the results obtained at open circuit conditions and taking into account the IR spectra presented in Fig. 4, it can be concluded that at these high temperatures (750–850 °C), i-C8H18 decomposes both thermally and catalytically towards H2, C, CH4, C2's and C3's hydrocarbons. Methane, C2's and C3's can be considered as products that are generated directly from iso-octane pyrolysis. However, at temperatures higher than 800 °C, methane formation can be probably also attributed to the decomposition of hydrocarbons that are formed during reaction, justifying the observed increase in CH4 selectivity at
Fig. 4. DRIFT spectra of Cu–CeO2 catalyst at various temperatures.
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Fig. 5. Effect of applied anodic overpotential on i-C8H18 conversion, products' distribution and developed current density.
those high temperatures. Finally, the rather high formation rates achieved for carbon and H2 can be assigned to the dehydrogenation of the hydrocarbons, CXHY, that are present in the reacting mixture during reaction, including iso-octane.
3.2. Iso-octane direct (electro-)oxidation Fig. 5 shows the effect of anodic overpotentials (0–3000 mV) on products' formation rates, i-C8H18 conversion and developed current density, at a constant cell temperature and i-C8H18 partial pressure equal to 850 °C and 0.4 kPa, respectively. It can be seen that at anodic polarization conditions, i.e. when oxygen anions, O2−, are electrochemically supplied towards the working electrode/catalyst, the same range of products were formed as in the case of open circuit conditions. In addition, through the (electro-)oxidation of mainly C but also due to the partial (electro-)oxidation of CXHY, the formation of COX was observed at a CO2/CO ratio equal to 2, indicating that at anodic polarization conditions the water gas shift reaction is enhanced.
As the applied overpotential increases, iso-octane conversion along with the formation rates of H2, C2's, C3's, CO and CO2 is enhanced, while the corresponding CH4 production rate is decreased. As far as the carbon formation concerns, its production rate is depending on the relative rates of carbon deposition and (electro-)oxidation. Therefore, it is obvious that at high electrochemical oxygen anion pumping rates (η N 2000 mV), where the electro-oxidation rate is high, the net formation rate of C is gradually decreased. This observation is also verified in the characteristic microphotographs and the corresponding X-ray energy dispersive analysis (EDAX) of two different pretreated samples, presented in Fig. 6. It should be pointed out here that initially, as in the case of Fig. 1, the samples were exposed to pure hydrogen for 12 h. In the following, the sample located at the left of Fig. 6, was pretreated with an iso-octane (Pi-C8H18 = 0.4 kPa) diluted in He gas mixture at open circuit conditions for 9 h, while the right hand side SEM picture corresponds to a different Cu/CeO2 sample, which was also pretreated using the same reacting mixture for 9 h, however at closed circuit conditions and specifically by applying a constant anodic overpotential equal to η = 3000 mV. In accordance to the work of Kim
Fig. 6. Scanning Electron Micrographs of the Cu–CeO2 electrode and the corresponding EDAX analyses. On the left picture the sample was treated with Pi-C8H18 = 0.4 kPa balanced with He mixture for 9 h at 850 °C. In the right picture the sample was treated with Pi-C8H18 = 0.4 kPa balanced with He mixture for 9 h at 850 °C at an applied overpotential, η = 3000 mV. 1-spot: CeO2-phase and 2-spot: Cu-phase. Sample pretreatment: pure H2 at 850 °C for 12 h.
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et al. the expected carbon deposition didn't cause any physical damage or changes in the morphology and composition of Cu/CeO2 catalyst, while on the contrary, in the same study, the formation of carbon fibers on the Ni-based electrodes resulted even in the removal of Ni particles (dusting) [29]. In both pretreated samples a SEM picture was selected to be obtained, where the different CeO2 (1-spot) and Cu (2-spot) phases are clearly depicted. In the former sample, EDAX analysis of both spots reveals that carbon is present only in the CeO2-phase suggesting that copper is not active to catalyze carbon formation. On the other hand by applying a certain anodic overpotential, where O2− were supplied electrochemically to the electrode at a rate equal to Faradaic's law (I/4F), the corresponding density of carbon amounts on the back scattered micrograph is dramatically reduced. Summarizing, at anodic polarization conditions it was observed that iso-octane conversion and in general the formation rates of pyrolysis products were increased. This was attributed to the enhancement of i-C8H18 and its derived decomposition products' adsorption strength (electron donor). At the same time, the reduction achieved in the formation rate of C at rather high anodic overpotentials (Fig. 5), indicates that this active carbon can be successfully (electro-)oxidized to COX. This is a very significant observation since it has been noticed repeatedly in the related literature that fuel cells operating directly on liquid hydrocarbons are suffering (gradual degradation in overall efficiency) due to carbon formation [7,13–16]. Methane is also decreased probably due to its partial oxidation and
oxidative coupling reactions with O2− producing H2, COX, C2's and C3's hydrocarbons. Moreover, H2O produced via the (electro-)oxidation reactions of CXHY and H2, is totally consumed since no water was observed at all reaction conditions examined, indicating its participation in steam reforming reactions towards COX and H2 and in the water gas shift reaction, which designates the observed CO2/CO ratio. The above observations are verified in the work of Zhan and Barnett where they used a standard free energy minimization calculation to predict the equilibrium reaction product composition versus the oxygen-to-iso-octane ratio at 750 °C, where only trace amounts of H2O were produced and specifically its molar fraction was diminished by decreasing the corresponding feed mixture ratio and increasing cell temperature [9]. Finally, the interaction of alkanes with O2− leads in dehydrogenation reactions giving rise in the corresponding formation rates of olefins and contributing co-currently to the observed increase in hydrogen production rate. As far as the interpretation of the observed kinetics is concerned, the results obtained in the electro-activity tests (Sections 3.1 and 3.2), revealed that at open circuit operation, the apparent energy activation was found equal to 26 ± 1.5 kcal/mol, while the iso-octane partial reaction order was equaled to 0.9 ± 0.1, implying that the rate determining step probably involves the adsorption and consequent decomposition of iso-octane on the electrode's surface. On the other hand, at anodic polarization conditions; when O2− species are supplied electrochemically towards the Cu/CeO2 electrode, the partial
Fig. 7. Effect of temperature a) on the current density–voltage and current density–power density characteristics of the cell and b) on the corresponding impedance spectra at open circuit conditions.
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reaction order of iso-octane remained almost unaffected while the apparent energy activation decreased to 18.6 ± 2.0 kcal/mol, denoting the promoting role of anodic overpotentials, facilitating iso-octane (electron donor molecule) adsorption and consequent decomposition/electro-oxidation. This fact is also supported by the similar apparent energy activation (19.1 ± 2.5 kcal/mol) of the charge transfer process, calculated by relevant electrode polarization studies (data not shown for brevity's sake). 3.3. Fuel cell operation In Sections 3.1 and 3.2, a clear picture was obtained concerning the iso-octane conversion, carbon formation and the corresponding products' distribution, when various i-C8H18/He gas mixtures were directly fed to the YSZ membrane reactor, at both open (i-C8H18 decomposition) and closed (i-C8H18 electro-oxidation) circuit conditions. In Section 3.3 the capability of the as prepared cell to generate power operating on direct iso-octane feed mixtures without using air or/and steam, will be explored. Fig. 7a depicts the voltage and power density versus current density curves at various cell temperatures ranged between 750 and 850 °C, at a constant iso-octane partial pressure equal to Pi-C8H18 = 1 kPa. The Cu–CeO2/YSZ/Pt cell exhibited an open circuit voltage (OCV) of approximately 1.00 V depending on cell temperature. 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 voltage–current 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. Fig. 7b presents the corresponding open circuit AC impedance spectra measured at conditions identical to those employed in Fig. 7a. It is clearly shown that the ohmic portion of the area specific resistance (ASR) of the cell, which corresponds to the intercept of the arc in the impedance spectrum with the real axis is decreased by increasing cell temperature from 5.2 Ω cm2 at 750 °C to 2.3 Ω cm2 at 850 °C. As can be seen for all temperatures examined the impedance spectra contain 1 arc, centered at different frequencies, i.e. 3.2, 12.6 and 39.8 Hz for 750, 800 and 850 °C, respectively, in the low frequency region. Since the cathode and electrolyte were identical in all cases examined, changes in the impedance spectra can be attributed only to anodic electrochemical processes, and more specifically to the overall steps involved in the electro-oxidation of i-C8H18 and its derived species. As the cell temperature is decreased the arcs become larger, denoting that the interfacial polarization resistance is increasing by lowering cell temperature, i.e. from N15.0 Ω cm2 at 750 °C to 1.7 Ω cm2 at 850 °C. Therefore, at low temperatures the interfacial polarization determines the whole process. However at 850 °C its contribution is decreased comprising essentially the 42% of the total cell resistance. Furthermore, it can be seen that all impedance spectra end up with the typical Warburg “look”, indicating that diffusion limitation problems are existing, being more pronounced at higher temperatures. At the same time, substantial improvements in power output are obtained by increasing operation temperature. An approximately three fold increase in power generation was achieved by increasing cell temperature from 750 to 850 °C (Fig. 7a). The maximum power output was obtained at 850 °C, and it was equal to 17.6 mW/cm2 at a cell voltage of 459 mV and a current density of 38.4 mA/cm2. Fig. 8a depicts the voltage–current density and power density– current density characteristics of the cell at three different iso-octane/ He feed mixtures (1, 2.5 and 5.6 kPa) and at a constant cell temperature equal to T = 850 °C. For comparison, the corresponding closed circuit data of a gas mixture containing 50% H2/He are also
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presented. The OCV of the cell was equal to 1.12 V in the case of the hydrogen containing mixture, while it decreased in the i-C8H18 gas mixtures (0.92 V at Pi-C8H18 = 5.6 kPa). The main source of polarization was again, as in Fig. 7a, attributed to ohmic resistance, however at higher iso-octane partial pressures (2.5 and 5.6 kPa), activation polarization was also observed. The corresponding open circuit AC impedance spectra obtained at 850 °C for the Pi-C8H18 = 5.6 kPa and PH2 = 50 kPa feed mixtures (Fig. 8b), show that as expected the electrolyte ohmic resistance is almost similar in both cases. Also here 1 arc was observed located in the low frequency region (31.6 and 15.9 Hz for the iso-octane and hydrogen containing mixtures, respectively). The interfacial polarization resistance decreased from 2.1 to 1.6 Ω cm2 when the fuel was changed from Η2 to iso-octane. The decrease could be primarily attributed to anode due to changes in the type of fuel and in the different fuel concentrations employed, since the cathode was exactly the same, exposed to stagnant atmospheric air. However contrary to previous results, at these high fuel concentrations the interfacial polarization is higher compared to the electrolyte resistance, which comprises the 59 and 55% of the overall cell resistance. Thus, at the present reaction conditions the physicochemical and electrochemical processes taking place over the Cu–CeO2 anode, determine the overall cell efficiency. Also, as in the previous case (Fig. 7b), mass transfer limitations were observed, which were more evident in the case of iso-octane containing mixture. Concerning power generation, it is observed that power production is increased as Pi-C8H18 is increasing. The maximum power output equaled to 36.6 mW/cm2 was achieved at 458 mV cell voltage and a current density of 79.9 mA/cm2, at Pi-C8H18 = 5.6 kPa. The corresponding maximum power output in the case of hydrogen was obtained at Vcell = 0.54 and I = 102.3 mA, and it was equal to 55.3 mW/cm2, a promising result which highlight the potentiality of iso-octane fuel to be directly used in Cu/CeO2 based anodes fuel cells for power generation. In order to examine the durability of the Cu/CeO2 anode stepchange tests were performed, where the obtained power density was continuously recorded as a function of time by consecutively interchanging the feed mixture. In Fig. 9 the variation of power density of time on stream, at a constant cell potential of 0.45 V and at T = 850 °C, is depicted. Initially, the fuel cell was fed with 50% H2 diluted in He gas mixture. The achieved power density was gradually increased during the first two hours while for the rest hour remained almost constant and equal to 52.8 mW/cm2. At the end of the third hour, the hydrogen containing mixture was substituted by an iso-octane balanced with He mixture (Pi-C8H18 = 1 kPa), and as can be seen the power output was instantaneously dropped down to 17.4 mW/cm2 and remained unaffected for the rest of the cycle. At the end of this step, the i-C8H18 concentration was increased to 5.6 kPa and the cell was immediately responded, increasing power density to 35.9 mW/cm2 (an approximately 100% increase), which essentially remained the same during the whole period. In the following and in order to get an insight on possible degradation effects due to coking, 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 2 h, achieving a final power density equal to 48.2 mW/cm2, although decreased by 8.7% compared to the first “hydrogen” step. Therefore, in contrast to the corresponding works of Saunders et al. using Ni/YSZ based anodes, where a strong degradation effect was observed within 30 min of operation on pure iso-octane [7,15], the power density in the present work not only remained almost constant at all i-C8H18 containing mixtures examined, but also maintained its initial activity when the H2/He gas mixture was re-introduced to fuel cell reactor (4th step). However, the power output achieved in the present study was rather low compared to Saunders et al. [7,15] as well as to the relevant works of Zhan et al. [9,10] and Ding et al. [12], where they have used iso-octane/air mixtures, where maximum power densities ranged
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Fig. 8. Effect of Pi-C8H18 and hydrogen a) on the current density–voltage and current density–power density characteristics of the cell and b) on the impedance spectra of the cell at open circuit conditions.
between 0.5 and 1 W/cm2, were achieved at much lower temperatures (550–650 °C). This difference can be attributed to both the lower concentrations of the employed fuels fed or produced internally in the
SOFC and to the rather large thickness of the YSZ electrolyte (≈1 mm). Moreover, it is worth to notice that in lab experiments it is difficult to set up an effective current collector configuration and
Fig. 9. Step-change stability tests of power density as a function of time on stream at a cell potential of − 0.45 V for different fuel (H2 and i-C8H18) and iso-octane partial pressures.
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usually significant power losses are revealed in such systems. Therefore, substantial improvements in overall cell efficiency are expected to be gained by using thinner YSZ components, state of the art interconnectors or operating at higher fuel concentrations.
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scientific and technological cooperation between RTD organizations in Greece and RTD organizations in USA, Canada, Australia, New Zealand, Japan, South Korea, Taiwan, Malaysia and Singapore. References
4. Conclusions Iso-octane was directly fed in a SOFC reactor with Cu/CeO2 anodic composites. The performance of Cu/CeO2 electrodes was investigated in terms of H2 production, carbon deposition, power generation and stability at various reaction conditions. In the “reactor” mode and open circuit conditions, catalytic and in situ DRIFT studies showed that i-C8H18 was both thermally and catalytically decomposed to solid carbon, rich CH4 and H2 mixtures, and small quantities of C2/C3 hydrocarbons (mainly olefins). EDAX analysis revealed that carbon is formed on the CeO2-phase, suggesting that Cu does not catalyze pyrolysis. At anodic polarization, COX were also formed (CO2/CO ∼ 2), indicating that the water gas shift reaction is enhanced. Carbon deposition was noticeably reduced at high anodic overpotentials due to its electro-oxidation, an observation that was also verified by the corresponding micrographs. On the other hand, H2 production was increased due to the partial oxidation, steam reforming, dehydrogenation and water gas shift reactions. However, under both open and closed circuit operation the rate determining step of the whole process seems to involve the adsorption and consequent decomposition/electro-oxidation of isooctane, which is facilitated at anodic polarization conditions. Under the “fuel cell” mode, the achieved power densities were substantially increased with temperature and Pi-C8H18 (maximum power output of 35.9 mW/cm2 at 850 °C and Pi-C8H18 = 5.6 kPa). 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 fuel type. Finally, no significant performance degradation was observed during the step-change tests. Acknowledgments The present research was co-funded from the European Union (ESF) by 75% and the Hellenic State by 25% through the framework of
[1] G. Hoogers, Fuel Cell Technology Handbook, CRC Press, 2003. [2] S.C. Singhal, K. Kendall, High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications, Elsevier Advanced Technology, 2003. [3] G.E. Marnellos, C. Athanasiou, S.S. Makridis, E.S. Kikkinides, “Integration of hydrogen energy technologies in autonomous power systems”, Chapter 3 in “Hydrogen based Autonomous Power Systems” edited by E.I. Zoulias and N. Lymberopoulos, Springer Eds. (2008). [4] J.R. Kaltzer, M.P. Ramage, A.V. Sapre, Chem. Eng. Prog. 7 (2000) 41. [5] R.J. Gorte, H. Kim, J.M. Vohs, J. Power Sources 106 (2002) 10. [6] R.J. Gorte, J.M. Vohs, S. McIntosh, Sol. St. Ionics 175 (2004) 1. [7] G.J. Saunders, K. Kendall, J. Power Sources 106 (2002) 258. [8] R.J. Gorte, J.M. Vohs, J. Catal. 216 (2003) 477. [9] Z. Zhan, S.A. Barnett, J. Power Sources 155 (2006) 353. [10] Z. Zhan, S.A. Barnett, J. Power Sources 157 (2006) 422. [11] A. Dhir, K. Kendall, J. Power Sources 181 (2008) 297. [12] D. Ding, Z. Liu, L. Li, C. Xia, Electrochem. Commun. 10 (2008) 1295. [13] H. He, J.M. Vohs, R.J. Gorte, J. Power Sources 144 (2005) 135. [14] R.M. Ormerod, Chem. Soc. Rev. 32 (2003) 17. [15] G.J. Saunders, J. Preece, K. Kendall, J. Power Sources 131 (2004) 23. [16] O. Costa-Nunez, R.J. Gorte, J.M. Vohs, J. Power Sources 141 (2005) 241. [17] C.V. Bartholomew, Catal, Rev.-Sci. Eng. 24 (1982) 67. [18] T. Takeguchi, Y. Kani, T. Yano, R. Kikuchi, K. Eguchi, K. Tsujimoto, Y. Uchida, A. Ueno, K. Omoshiki, M. Aizawa, J. Power Sources 112 (2002) 588. [19] K. Sasaki, Y. Teraoka, J. Electrochem. Soc. 150 (2003) A878. [20] S. Park, J.M. Vohs, R.J. Gorte, Nature 404 (2000) 265. [21] R.J. Gorte, S. Park, J.M. Vohs, C. Wang, Adv. Mater. 12 (2000) 1465. [22] H.P. He, Y.Y. Huang, J.M. Vohs, R.J. Gorte, Sol. St. Ionics 175 (2004) 171. [23] O.A. Marina, C. Bagger, S. Primdhal, M. Mogensen, Sol. St. Ionics 123 (1999) 199. [24] A. Sauvet, J.T.S. Irvine, Fuel Cells 1 (2001) 205. [25] I. Kang, J. Bae, G. Bae, J. Power Sources 163 (2006) 538. [26] J. Zhang, W. Yin, H. Shang, C. Liu, J. Nat. Gas Chem. 17 (2008) 165. [27] I.I. Ivanova, Y.G. Kolyagin, V.V. Ordomsky, E.V. Asachenko, E.M. Pasynkova, Y.A. Pirogov, J. Mol. Catal. A 305 (2009) 47. [28] H. Kishimoto, K. Yamaji, T. Horita, Y. Xiong, N. Sakai, M.E. Brito, H. Yokokawa, J. Power Sources 172 (2007) 67. [29] T. Kim, G. Liu, M. Boaro, S.-I. Lee, J.M. Vohs, R.J. Gorte, O.H. Al-Madhi, B.O. Dabbousi, J. Power Sources 155 (2006) 231.
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Direct electro-oxidation of iso-octane in a solid electrolyte fuel cell N. Kaklidis a, G. Pekridis a, C. Athanasiou b,c, G.E. Marnellos a,c,⁎ a b c
Department of Mechanical Engineering, University of Western Macedonia, Bakola and Sialvera, GR-50100 Kozani, Greece Department of Environmental Engineering, Democritus University of Thrace, University Campus, GR-67100 Xanthi, Greece Chemical Process Engineering Research Institute, Centre for Research and Technology Hellas, 6th km. Charilaou-Thermi Rd., P.O. Box 361, GR-57001 Thermi, Thessaloniki, Greece
a r t i c l e
i n f o
Article history: Received 1 September 2009 Received in revised form 6 June 2010 Accepted 11 June 2010 Available online 10 July 2010 Keywords: Iso-octane Direct hydrocarbon SOFC Cu–CeO2 electrodes
a b s t r a c t The present work aims to explore the activity of Cu/CeO2 composites as anodic electrodes in direct iso-octane SOFCs. When the cell was operated as a membrane reactor, the effect of temperature, Pi-C8H18 and applied anodic overpotentials on the electrocatalytic activity and products' distribution, at both open and closed circuit conditions, was examined. Additionally, in situ DRIFT spectroscopy was carried out in order to correlate the performance of Cu/CeO2 with its surface chemistry during iso-octane decomposition. Under the “fuel cell” mode of operation, the electrochemical performance and stability of Cu/CeO2 were investigated by voltage–current density–power density and AC impedance measurements. The results reveal that at high anodic polarization conditions, carbon formation can be noticeably restricted (verified also by EDAX analysis), while H2 production was enhanced due to partial oxidation, steam reforming, dehydrogenation and water gas shift reactions. Achieved power densities were found to substantially increase both with temperature and Pi-C8H18, while minor performance degradation was indicated in the step-change tests, where the overall activity of Cu–CeO2 electrodes remained essentially unaffected. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Fuel cells are considered as a promising and environmental friendly candidate technology to replace conventional thermal engines for power generation at high efficiencies (40–50%) [1,2]. However, the high cost of these systems has prevented their widespread adoption and as a consequence their commercialization [3]. One of the key factors that contribute to this high cost is the luck of fuel flexibility [3,4]. Fuel cells generally operate on hydrogen, which is neither readily available nor easily stored [3,5,6]. To utilize hydrocarbons, fuel cell power plants usually employ steam reforming, which converts fuels into hydrogen [3]. Reforming and exhaust-gas recirculation (providing the steam for reforming) lead to additional plant complexity and volume, increasing the overall cost [7]. 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 conventional hydrocarbon fuels ranging from natural gas to gasoline [6–12], for which their refining and distribution infrastructure is well established [5]. Therefore in SOFCs, any combustible material can, in principle, be used to generate electrons from the electro-oxidation reactions with O2− species. However, the selection of the fuel is limited by its
⁎ Corresponding author. Department of Mechanical Engineering, University of Western Macedonia, Bakola and Sialvera, GR-50100 Kozani, Greece. Tel.: +30 2461 0 56690; fax: +30 2461 0 56601. E-mail address: [email protected] (G.E. Marnellos). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.06.022
tendency to foul the anodes [8]. Ni-cermets are the most commonly used materials for SOFC anodes [3,6,13,14]. These anodic composites are characterized by their high electronic conductivity, adequate thermal stability and catalytic activity towards steam reforming reactions [13]. However, their tendency to catalyze the formation of carbon fibers at low H2O/C ratios and consequently their rapid degradation [7,13–16], renders their use in direct hydrocarbon SOFC applications improper. To avoid coking, high H2O/C ratios [17–19] 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 [6,13] has recently proposed the substitution of Ni with equally good electronic conductors that do not favor carbon formation, such as Cu [8,20,21] or conducting oxides [22–24]. It has been demonstrated that by using the proposed anodic formulations, Cu–CeO2/YSZ, electrical power generation can be achieved constantly through the direct electrochemical oxidation of a wide variety of hydrocarbon fuels, including also liquid fuels. To this end, the expansion of potential fuel range is enabled, co-currently eliminating the fuel pre-processing costs and increasing overall efficiency [5,6,8,9,20–22]. In the present study, iso-octane, a common surrogate for gasoline, is directly fed in a SOFC reactor of the Cu–CeO2/YSZ/Pt type. The selection of iso-octane as fuel feedstock instead of using other commercial fuels such as diesel or kerosene was based to its broaden applicability in both mobile and stationary applications and to its lower tendency for coke formation and deposition due to thermal cracking compared to heavier hydrocarbons [25]. Initially, the effect of
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cell temperature, i-C8H18 partial pressure and imposed anodic overpotentials on product's distribution was thoroughly examined by performing typical potentiostatic experiments (at both open and closed circuit conditions). In situ DRIFT spectroscopy studies were carried out in order to get an insight on the Cu–CeO2 surface chemistry during iso-octane decomposition. In the following and prior to fuel cell tests, the as prepared cell was electrochemically characterized using AC impedance measurements. Finally, the electrochemical performance and stability of Cu–CeO2 electrodes were investigated by measuring the voltage–current density and power density–current density characteristics of the direct iso-octane SOFC at various cell temperatures and i-C8H18 partial pressures. 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. Reactant gases included H2 and He of 99.999% purity, and were supplied by Air Liquide. Iso-octane was fed in the reactor cell by bubbling pure helium through a vessel containing liquid i-C8H18 of 99.5% purity (Riedel-de Haen), at ambient temperature. The electrochemical reactor used in our experiments 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 wire was used to establish electrical contact with the inner Cu-CeO2 catalyst–electrode 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 on-line, 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, C2's, C3's, H2O and i-C8H18 separation. Additionally a setup of IR gas analyzers in series – CH4 (ANARAD), CO2 (ANARAD) and CO (MIRAN 203) – was also used for the continuous monitoring of these chemical species. The carbon and water contents were always calculated using the C, hydrogen and oxygen mass balances, respectively. Specifically, in the case of carbon, the deposited amount was also verified by performing at standard conditions (T = 800 °C, feed: 5% O2/He) a temperature programmed oxidation (TPO) experiment. In all cases examined, the difference between the coke that was calculated using carbon mass balance and the coke that was estimated through the TPO experiment always ranged within 5%.
whiter CeO2-phase (1-spot) and the gray Cu-phase (2-spot) are randomly distributed. The grain size of the CeO2-phase varies from 1 to 10 μm while the Cu-phase has liquid morphology without clear formed grains. 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. 2.3. Potentiostatic, AC impedance and fuel cell measurements The electro-oxidation of i-C8H18 was examined in the reactor cell described in Section 2.1, using various reacting mixtures of i-C8H18 (0.1–6.0%) diluted in He, at temperatures between 750 and 850 °C. The potentiostatic mode of operation was implemented by applying anodic overpotentials varying from 0 to 3000 mV. An AMEL model 2053 galvanostat–potentiostat 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 0.1 Hz to 1 MHz, using the IVIUM technologies electrochemical workstation and the corresponding software (IVIUMSOFT) for data processing. 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. 2.4. In situ DRIFTS studies Diffuse reflectance IR spectra were obtained using a NICOLET 6700 spectrometer, equipped with a high temperature and high pressure cell, cooled by liquid nitrogen. Spectra were obtained using a resolution of 4 cm− 1 with accumulation of 64 scans. Catalyst samples (≈50 mg) in the form of powder were flattened in order to maximize the intensity of the reflected beam. The total flow rate through the IR cell was retained equal to 40 cm3/min. Before each experiment the catalyst was pretreated as follows: i) purging with Ar flow at 850 °C for 30 min, ii) reduction at the same temperature using pure H2 flow for 3 h, iii) purging with Ar for 30 min and iv) background spectra acquisition under Ar flow at the desired temperatures. The steadystate experiments were performed at constant feed composition (Pi-C8H18 = 0.4 kPa balanced with Ar) in the temperature range between 25 and 850 °C, and the system was allowed to stabilize for 30 min in order to achieve steady-state band intensities. 3. Results and discussion 3.1. Iso-octane decomposition at open circuit conditions
2.2. Material preparation Cu–CeO2 (70 and 30 wt.%, respectively) working electrode which also served as the catalyst for the i-C8H18 electro-oxidation 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 (application of four consecutive thin coatings) resulting in a catalyst weight equal to ≈1230 mg. The tube was then heated up to 900 °C (heating rate: 3 °C/min), and calcined for 4 h. The Cu/CeO2 electrode thus formed had a superficial surface area of 2 cm2. Fig. 1 represents two Scanning Electron Micrographs (SEM) of the Cu–CeO2 microstructure (JSM 6300, JEOL) at different magnification factors (pretreatment with pure H2 at 850 °C for 12 h). The SEM elemental mapping on the microstructure of the sample reveals that the microstructure/microchemistry are non-uniform, while the
Initially, the reaction of iso-octane decomposition was examined in the Cu–CeO2/YSZ/Pt oxygen-ion conducting solid electrolyte membrane reactor. Fig. 2 presents the effect of temperature (750– 850 °C) on products' formation rate distribution, iso-octane conversion and on the developed electromotive force (EMF) under open circuit conditions, for Pi-C8H18 = 0.4 kPa. It is clear that i-C8H18 conversion increases almost linearly with temperature. The products that were formed during iso-octane decomposition were C, CH4, H2, C2H4, C2H6, C3H6 and C3H8, while their production rates were increased with temperature. Specifically, up to 825 °C the product's distribution remains almost unaffected, however at higher temperatures the selectivities of both C and CH4 increase rapidly, because both decomposition and dehydrogenation reactions become thermally and kinetically more favorable [7]. As far as the formation of higher hydrocarbons concerns, the Cu–CeO2 catalyst seems to favor olefins (C2H4 and C3H6) production in contrast to saturated hydrocarbons (C2H6, C3H8). However it should be emphasized that olefins were also
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Fig. 1. SEM pictures of the Cu–CeO2 electrode (fresh sample) at different magnification factors and the corresponding elemental mapping. 1-spot: CeO2-phase and 2-spot: Cu-phase. Pretreatment: pure H2 at 850 °C for 12 h.
the main products observed in the absence of catalyst (blank experiments not shown for brevity's sakes) during i-C8H18 thermal decomposition. Qualitatively similar results were obtained in the work of Saunders and Kendall over Ni/YSZ electrodes using n-octane as a fuel [7]. However instead of C3's, C4's alkanes/alkenes were formed suggesting concurrent polymerization and degradation of alkene intermediates [7], a fact that was not observed in the present study. Fig. 3 depicts the dependence of products' formation rates, i-C8H18 conversion and EMF on i-C8H18 partial pressure at a constant cell temperature equal to 850 °C. In general by increasing Pi-C8H18 all
products' formation rates increased almost linearly. On the other hand, i-C8H18 conversion decreases smoothly at lower iso-octane partial pressures, while at Pi-C8H18 higher than 3 kPa remains essentially constant. As can be seen, the EMF values are decreasing with Pi-C8H18, indicating that both iso-octane and its decomposition products are acting as electron donor species, i.e. their adsorption strength increase by supplying oxygen anions, O2−, towards the anodic electrode/catalyst. Fig. 4 shows the IR spectra of adsorbed species present on the surface of the Cu–CeO2 catalyst at temperatures in the interval of
Fig. 2. Effect of temperature on i-C8H18 conversion, products' distribution and EMF, at open circuit conditions.
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Fig. 3. Effect of i-C8H18 partial pressure on i-C8H18 conversion, products' distribution and EMF, at open circuit conditions.
27–850 °C, using a feed mixture of iso-octane (Pi-C8H18 = 0.4 kPa) diluted in Ar. At 27 °C five well-resolved peaks at 1371, 1478, 2879, 2908 and 2962 cm− 1 are observed, which can be attributed to gas phase i-C8H18. Also, two bands at 1047 and 1646 cm− 1, which can be assigned to in-of-plane bending vibration of C–H and the stretching vibration of C C bonds, respectively [26], are starting to develop at 100 °C, indicating the existence of surface iso-octane fragments through its interaction with the Cu/CeO2 electrode. All the above peaks remained almost unaffected up to 600 °C, while above this temperature their intensities are starting to decrease, indicating that i-C8H18 and consequently its surface fragments are beginning to decompose. This observation is in accordance with the rather low conversion of iso-octane (15.6%) observed at 750 °C (Fig. 2). At the same time new bands at 729, 950, 1300, 3014 and 3088 cm− 1 start to develop and their intensities are increased with temperature. Absorption peaks at 729 and 1300 cm− 1 represent the out-plane and in-of-plane bending vibration of C–H bonds, while the bands at 950 and 3088 cm− 1 are characterizing the
bending and stretching vibration of C–H bond, verifying the formation of the saturated (C2H6, C3H8) and unsaturated (C2H4, C3H6) hydrocarbons observed in the catalytic experiments (Figs. 2 and 3) [26]. In addition, the bands between 1371 and 1478 cm− 1 correspond to symmetric and asymmetric bending modes of CH2 and CH3 groups [27], which are considered as active hydrocarbon components [28]. Finally, the peak at 3014 cm− 1, which its intensity is maximized at 850 °C, is characteristic for gas phase methane, justifying its high formation rate observed in Figs. 2 and 3. Based on the results obtained at open circuit conditions and taking into account the IR spectra presented in Fig. 4, it can be concluded that at these high temperatures (750–850 °C), i-C8H18 decomposes both thermally and catalytically towards H2, C, CH4, C2's and C3's hydrocarbons. Methane, C2's and C3's can be considered as products that are generated directly from iso-octane pyrolysis. However, at temperatures higher than 800 °C, methane formation can be probably also attributed to the decomposition of hydrocarbons that are formed during reaction, justifying the observed increase in CH4 selectivity at
Fig. 4. DRIFT spectra of Cu–CeO2 catalyst at various temperatures.
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Fig. 5. Effect of applied anodic overpotential on i-C8H18 conversion, products' distribution and developed current density.
those high temperatures. Finally, the rather high formation rates achieved for carbon and H2 can be assigned to the dehydrogenation of the hydrocarbons, CXHY, that are present in the reacting mixture during reaction, including iso-octane.
3.2. Iso-octane direct (electro-)oxidation Fig. 5 shows the effect of anodic overpotentials (0–3000 mV) on products' formation rates, i-C8H18 conversion and developed current density, at a constant cell temperature and i-C8H18 partial pressure equal to 850 °C and 0.4 kPa, respectively. It can be seen that at anodic polarization conditions, i.e. when oxygen anions, O2−, are electrochemically supplied towards the working electrode/catalyst, the same range of products were formed as in the case of open circuit conditions. In addition, through the (electro-)oxidation of mainly C but also due to the partial (electro-)oxidation of CXHY, the formation of COX was observed at a CO2/CO ratio equal to 2, indicating that at anodic polarization conditions the water gas shift reaction is enhanced.
As the applied overpotential increases, iso-octane conversion along with the formation rates of H2, C2's, C3's, CO and CO2 is enhanced, while the corresponding CH4 production rate is decreased. As far as the carbon formation concerns, its production rate is depending on the relative rates of carbon deposition and (electro-)oxidation. Therefore, it is obvious that at high electrochemical oxygen anion pumping rates (η N 2000 mV), where the electro-oxidation rate is high, the net formation rate of C is gradually decreased. This observation is also verified in the characteristic microphotographs and the corresponding X-ray energy dispersive analysis (EDAX) of two different pretreated samples, presented in Fig. 6. It should be pointed out here that initially, as in the case of Fig. 1, the samples were exposed to pure hydrogen for 12 h. In the following, the sample located at the left of Fig. 6, was pretreated with an iso-octane (Pi-C8H18 = 0.4 kPa) diluted in He gas mixture at open circuit conditions for 9 h, while the right hand side SEM picture corresponds to a different Cu/CeO2 sample, which was also pretreated using the same reacting mixture for 9 h, however at closed circuit conditions and specifically by applying a constant anodic overpotential equal to η = 3000 mV. In accordance to the work of Kim
Fig. 6. Scanning Electron Micrographs of the Cu–CeO2 electrode and the corresponding EDAX analyses. On the left picture the sample was treated with Pi-C8H18 = 0.4 kPa balanced with He mixture for 9 h at 850 °C. In the right picture the sample was treated with Pi-C8H18 = 0.4 kPa balanced with He mixture for 9 h at 850 °C at an applied overpotential, η = 3000 mV. 1-spot: CeO2-phase and 2-spot: Cu-phase. Sample pretreatment: pure H2 at 850 °C for 12 h.
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et al. the expected carbon deposition didn't cause any physical damage or changes in the morphology and composition of Cu/CeO2 catalyst, while on the contrary, in the same study, the formation of carbon fibers on the Ni-based electrodes resulted even in the removal of Ni particles (dusting) [29]. In both pretreated samples a SEM picture was selected to be obtained, where the different CeO2 (1-spot) and Cu (2-spot) phases are clearly depicted. In the former sample, EDAX analysis of both spots reveals that carbon is present only in the CeO2-phase suggesting that copper is not active to catalyze carbon formation. On the other hand by applying a certain anodic overpotential, where O2− were supplied electrochemically to the electrode at a rate equal to Faradaic's law (I/4F), the corresponding density of carbon amounts on the back scattered micrograph is dramatically reduced. Summarizing, at anodic polarization conditions it was observed that iso-octane conversion and in general the formation rates of pyrolysis products were increased. This was attributed to the enhancement of i-C8H18 and its derived decomposition products' adsorption strength (electron donor). At the same time, the reduction achieved in the formation rate of C at rather high anodic overpotentials (Fig. 5), indicates that this active carbon can be successfully (electro-)oxidized to COX. This is a very significant observation since it has been noticed repeatedly in the related literature that fuel cells operating directly on liquid hydrocarbons are suffering (gradual degradation in overall efficiency) due to carbon formation [7,13–16]. Methane is also decreased probably due to its partial oxidation and
oxidative coupling reactions with O2− producing H2, COX, C2's and C3's hydrocarbons. Moreover, H2O produced via the (electro-)oxidation reactions of CXHY and H2, is totally consumed since no water was observed at all reaction conditions examined, indicating its participation in steam reforming reactions towards COX and H2 and in the water gas shift reaction, which designates the observed CO2/CO ratio. The above observations are verified in the work of Zhan and Barnett where they used a standard free energy minimization calculation to predict the equilibrium reaction product composition versus the oxygen-to-iso-octane ratio at 750 °C, where only trace amounts of H2O were produced and specifically its molar fraction was diminished by decreasing the corresponding feed mixture ratio and increasing cell temperature [9]. Finally, the interaction of alkanes with O2− leads in dehydrogenation reactions giving rise in the corresponding formation rates of olefins and contributing co-currently to the observed increase in hydrogen production rate. As far as the interpretation of the observed kinetics is concerned, the results obtained in the electro-activity tests (Sections 3.1 and 3.2), revealed that at open circuit operation, the apparent energy activation was found equal to 26 ± 1.5 kcal/mol, while the iso-octane partial reaction order was equaled to 0.9 ± 0.1, implying that the rate determining step probably involves the adsorption and consequent decomposition of iso-octane on the electrode's surface. On the other hand, at anodic polarization conditions; when O2− species are supplied electrochemically towards the Cu/CeO2 electrode, the partial
Fig. 7. Effect of temperature a) on the current density–voltage and current density–power density characteristics of the cell and b) on the corresponding impedance spectra at open circuit conditions.
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reaction order of iso-octane remained almost unaffected while the apparent energy activation decreased to 18.6 ± 2.0 kcal/mol, denoting the promoting role of anodic overpotentials, facilitating iso-octane (electron donor molecule) adsorption and consequent decomposition/electro-oxidation. This fact is also supported by the similar apparent energy activation (19.1 ± 2.5 kcal/mol) of the charge transfer process, calculated by relevant electrode polarization studies (data not shown for brevity's sake). 3.3. Fuel cell operation In Sections 3.1 and 3.2, a clear picture was obtained concerning the iso-octane conversion, carbon formation and the corresponding products' distribution, when various i-C8H18/He gas mixtures were directly fed to the YSZ membrane reactor, at both open (i-C8H18 decomposition) and closed (i-C8H18 electro-oxidation) circuit conditions. In Section 3.3 the capability of the as prepared cell to generate power operating on direct iso-octane feed mixtures without using air or/and steam, will be explored. Fig. 7a depicts the voltage and power density versus current density curves at various cell temperatures ranged between 750 and 850 °C, at a constant iso-octane partial pressure equal to Pi-C8H18 = 1 kPa. The Cu–CeO2/YSZ/Pt cell exhibited an open circuit voltage (OCV) of approximately 1.00 V depending on cell temperature. 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 voltage–current 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. Fig. 7b presents the corresponding open circuit AC impedance spectra measured at conditions identical to those employed in Fig. 7a. It is clearly shown that the ohmic portion of the area specific resistance (ASR) of the cell, which corresponds to the intercept of the arc in the impedance spectrum with the real axis is decreased by increasing cell temperature from 5.2 Ω cm2 at 750 °C to 2.3 Ω cm2 at 850 °C. As can be seen for all temperatures examined the impedance spectra contain 1 arc, centered at different frequencies, i.e. 3.2, 12.6 and 39.8 Hz for 750, 800 and 850 °C, respectively, in the low frequency region. Since the cathode and electrolyte were identical in all cases examined, changes in the impedance spectra can be attributed only to anodic electrochemical processes, and more specifically to the overall steps involved in the electro-oxidation of i-C8H18 and its derived species. As the cell temperature is decreased the arcs become larger, denoting that the interfacial polarization resistance is increasing by lowering cell temperature, i.e. from N15.0 Ω cm2 at 750 °C to 1.7 Ω cm2 at 850 °C. Therefore, at low temperatures the interfacial polarization determines the whole process. However at 850 °C its contribution is decreased comprising essentially the 42% of the total cell resistance. Furthermore, it can be seen that all impedance spectra end up with the typical Warburg “look”, indicating that diffusion limitation problems are existing, being more pronounced at higher temperatures. At the same time, substantial improvements in power output are obtained by increasing operation temperature. An approximately three fold increase in power generation was achieved by increasing cell temperature from 750 to 850 °C (Fig. 7a). The maximum power output was obtained at 850 °C, and it was equal to 17.6 mW/cm2 at a cell voltage of 459 mV and a current density of 38.4 mA/cm2. Fig. 8a depicts the voltage–current density and power density– current density characteristics of the cell at three different iso-octane/ He feed mixtures (1, 2.5 and 5.6 kPa) and at a constant cell temperature equal to T = 850 °C. For comparison, the corresponding closed circuit data of a gas mixture containing 50% H2/He are also
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presented. The OCV of the cell was equal to 1.12 V in the case of the hydrogen containing mixture, while it decreased in the i-C8H18 gas mixtures (0.92 V at Pi-C8H18 = 5.6 kPa). The main source of polarization was again, as in Fig. 7a, attributed to ohmic resistance, however at higher iso-octane partial pressures (2.5 and 5.6 kPa), activation polarization was also observed. The corresponding open circuit AC impedance spectra obtained at 850 °C for the Pi-C8H18 = 5.6 kPa and PH2 = 50 kPa feed mixtures (Fig. 8b), show that as expected the electrolyte ohmic resistance is almost similar in both cases. Also here 1 arc was observed located in the low frequency region (31.6 and 15.9 Hz for the iso-octane and hydrogen containing mixtures, respectively). The interfacial polarization resistance decreased from 2.1 to 1.6 Ω cm2 when the fuel was changed from Η2 to iso-octane. The decrease could be primarily attributed to anode due to changes in the type of fuel and in the different fuel concentrations employed, since the cathode was exactly the same, exposed to stagnant atmospheric air. However contrary to previous results, at these high fuel concentrations the interfacial polarization is higher compared to the electrolyte resistance, which comprises the 59 and 55% of the overall cell resistance. Thus, at the present reaction conditions the physicochemical and electrochemical processes taking place over the Cu–CeO2 anode, determine the overall cell efficiency. Also, as in the previous case (Fig. 7b), mass transfer limitations were observed, which were more evident in the case of iso-octane containing mixture. Concerning power generation, it is observed that power production is increased as Pi-C8H18 is increasing. The maximum power output equaled to 36.6 mW/cm2 was achieved at 458 mV cell voltage and a current density of 79.9 mA/cm2, at Pi-C8H18 = 5.6 kPa. The corresponding maximum power output in the case of hydrogen was obtained at Vcell = 0.54 and I = 102.3 mA, and it was equal to 55.3 mW/cm2, a promising result which highlight the potentiality of iso-octane fuel to be directly used in Cu/CeO2 based anodes fuel cells for power generation. In order to examine the durability of the Cu/CeO2 anode stepchange tests were performed, where the obtained power density was continuously recorded as a function of time by consecutively interchanging the feed mixture. In Fig. 9 the variation of power density of time on stream, at a constant cell potential of 0.45 V and at T = 850 °C, is depicted. Initially, the fuel cell was fed with 50% H2 diluted in He gas mixture. The achieved power density was gradually increased during the first two hours while for the rest hour remained almost constant and equal to 52.8 mW/cm2. At the end of the third hour, the hydrogen containing mixture was substituted by an iso-octane balanced with He mixture (Pi-C8H18 = 1 kPa), and as can be seen the power output was instantaneously dropped down to 17.4 mW/cm2 and remained unaffected for the rest of the cycle. At the end of this step, the i-C8H18 concentration was increased to 5.6 kPa and the cell was immediately responded, increasing power density to 35.9 mW/cm2 (an approximately 100% increase), which essentially remained the same during the whole period. In the following and in order to get an insight on possible degradation effects due to coking, 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 2 h, achieving a final power density equal to 48.2 mW/cm2, although decreased by 8.7% compared to the first “hydrogen” step. Therefore, in contrast to the corresponding works of Saunders et al. using Ni/YSZ based anodes, where a strong degradation effect was observed within 30 min of operation on pure iso-octane [7,15], the power density in the present work not only remained almost constant at all i-C8H18 containing mixtures examined, but also maintained its initial activity when the H2/He gas mixture was re-introduced to fuel cell reactor (4th step). However, the power output achieved in the present study was rather low compared to Saunders et al. [7,15] as well as to the relevant works of Zhan et al. [9,10] and Ding et al. [12], where they have used iso-octane/air mixtures, where maximum power densities ranged
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Fig. 8. Effect of Pi-C8H18 and hydrogen a) on the current density–voltage and current density–power density characteristics of the cell and b) on the impedance spectra of the cell at open circuit conditions.
between 0.5 and 1 W/cm2, were achieved at much lower temperatures (550–650 °C). This difference can be attributed to both the lower concentrations of the employed fuels fed or produced internally in the
SOFC and to the rather large thickness of the YSZ electrolyte (≈1 mm). Moreover, it is worth to notice that in lab experiments it is difficult to set up an effective current collector configuration and
Fig. 9. Step-change stability tests of power density as a function of time on stream at a cell potential of − 0.45 V for different fuel (H2 and i-C8H18) and iso-octane partial pressures.
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usually significant power losses are revealed in such systems. Therefore, substantial improvements in overall cell efficiency are expected to be gained by using thinner YSZ components, state of the art interconnectors or operating at higher fuel concentrations.
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scientific and technological cooperation between RTD organizations in Greece and RTD organizations in USA, Canada, Australia, New Zealand, Japan, South Korea, Taiwan, Malaysia and Singapore. References
4. Conclusions Iso-octane was directly fed in a SOFC reactor with Cu/CeO2 anodic composites. The performance of Cu/CeO2 electrodes was investigated in terms of H2 production, carbon deposition, power generation and stability at various reaction conditions. In the “reactor” mode and open circuit conditions, catalytic and in situ DRIFT studies showed that i-C8H18 was both thermally and catalytically decomposed to solid carbon, rich CH4 and H2 mixtures, and small quantities of C2/C3 hydrocarbons (mainly olefins). EDAX analysis revealed that carbon is formed on the CeO2-phase, suggesting that Cu does not catalyze pyrolysis. At anodic polarization, COX were also formed (CO2/CO ∼ 2), indicating that the water gas shift reaction is enhanced. Carbon deposition was noticeably reduced at high anodic overpotentials due to its electro-oxidation, an observation that was also verified by the corresponding micrographs. On the other hand, H2 production was increased due to the partial oxidation, steam reforming, dehydrogenation and water gas shift reactions. However, under both open and closed circuit operation the rate determining step of the whole process seems to involve the adsorption and consequent decomposition/electro-oxidation of isooctane, which is facilitated at anodic polarization conditions. Under the “fuel cell” mode, the achieved power densities were substantially increased with temperature and Pi-C8H18 (maximum power output of 35.9 mW/cm2 at 850 °C and Pi-C8H18 = 5.6 kPa). 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 fuel type. Finally, no significant performance degradation was observed during the step-change tests. Acknowledgments The present research was co-funded from the European Union (ESF) by 75% and the Hellenic State by 25% through the framework of
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