Iso-octane internal reforming in a solid oxide cell reactor

Solid State Ionics 288 (2016) 135–139

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Iso-octane internal reforming in a solid oxide cell reactor A. Al-Musa a, N. Kaklidis b, M. Al-Saleh a, A. Al-Zahrani a, V. Kyriakou c,d,⁎, G.E. Marnellos b,d,e a

National Center for Combustion and Plasma Technologies, Water & Energy Research Institute, King Abdulaziz City for Science & Technology, 11442, Riyadh, Saudi Arabia Department of Mechanical Engineering, University of Western Macedonia, 50100 Kozani, Greece c Department of Chemical Engineering, Aristotle University of Thessaloniki, Building E13, 54124, Greece d Chemical Process & Energy Resources Institute, Centre for Research & Technology Hellas, 57001 Thermi, Thessaloniki, Greece e Department of Environmental Engineering, University of Western Macedonia, 50100 Kozani, Greece b

a r t i c l e

i n f o

Article history: Received 17 July 2015 Received in revised form 27 November 2015 Accepted 6 December 2015 Available online 30 December 2015 Keywords: Iso-octane internal reforming SOFC Cu/CeO2 anodic composite YSZ

a b s t r a c t This study reports on the feasibility of internal iso-octane steam reforming process in an YSZ solid oxide fuel cell reactor by employing Cu/CeO2 as catalyst/anodic electrode. The Cu/CeO2 anode is evaluated for its both catalytic and electro-catalytic performance. In all cases, i-C8H18 was successfully reformed by H2O to syngas. In addition, appreciable amounts of CO2 and CH4 were also produced. The distribution of products was also influenced by i-C8H18 thermal pyrolysis and catalytic decomposition processes leading mainly to olefins formation. At closedcircuit operation, and by applying anodic overpotentials, mainly H2 and CO were electro-oxidized to H2O and CO2, while at cathodic polarization conditions the co-electrolysis of H2O and CO2 to H2 and CO was taking place, affecting the equilibrium reactions at the anodic chamber. During fuel cell operation, the electrochemical performance increased with cell temperature and i-C8H18/H2O feed ratio. The AC impedance spectroscopy analysis showed contributions both from charge and mass transfer processes, with the latter to dominate the overall cell performance. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The increasing energy demands are still mainly compensated by fossil fuels employing thermodynamic limited conventional heat cycle configurations [1]. In recent years great advances have been accomplished in Renewable Energy Sources (RES) technologies. However, the high cost and complexity for RES adaptation and exploitation have made them prohibitive to be implemented currently in stationary power plants. This “business as usual” scenario is leading to natural sources depletion and significant environmental implications, which is in contrast with the strategies for a sustainable future imposed by international organizations. Fuel cells are considered as a promising energy conversion technology, capable of satisfying the needs of high efficiency and low environmental footprint [1]. Although, H2 is the ideal fuel for fuel cells, several obstacles related to its limited availability, the absence of relevant infrastructure and the difficulties associated with its storage and transport retard the market roll out of fuel cells [2]. In the meantime, the use of conventional fuels with established infrastructure, like gasoline, could be an alternative way to accelerate the commercialization of fuel cells. SOFCs, due to their high operating temperature can be potentially operated directly on conventional hydrocarbon fuels [3–9]. ⁎ Corresponding author at: Department of Chemical Engineering, Aristotle University of Thessaloniki, Building E13, 54124, Greece. Tel.: +30 2310 996145. E-mail address: [email protected] (V. Kyriakou).

http://dx.doi.org/10.1016/j.ssi.2015.12.013 0167-2738/© 2015 Elsevier B.V. All rights reserved.

The key-issue of liquid fuel-fed SOFCs development is the selection of the anodic composite materials. An electro-catalyst for this application should mainly exhibit a) high catalytic activity for hydrocarbon oxidation and reforming, b) adequate electron conductivity and c) high tolerance to carbon deposition. The Ni-based anodes are currently the state of the art materials for SOFCs [2–6,8–13]. Nickel electrodes are fulfilling the first condition, but it has been proved that they are carbon sensitive at low C/H2O ratios [14]. Furthermore, Ni in these cermets is oxidized easily to NiO, due to its poor redox properties suffering an important decrease in electronic conductivity [15–18]. Hence, the development of electro-catalytic active and conductive anodic materials, tolerant toward carbon poisoning, is of crucial importance [3,4]. Among others, Cu-based cermets have been proved as the most promising candidates for direct hydrocarbon SOFCs, because they perfectly fulfill all the aforementioned prerequisites. In a recent study it was demonstrated that their performance is improved when a thin layer of carbonaceous compounds is deposited on their surface after paraffin fuel treatment [3]. In addition, Gorte et al. also demonstrated that the Cu-CeO2 cermets show excellent stability in their relevant direct hydrocarbon SOFC works [6,7,15,19]. In the present study, typical fuel cell measurements and AC impedance spectroscopy studies are combined to investigate the performance of an iso-octane internal steam reforming solid oxide reactor cell of the type Cu-CeO2/YSZ/Pt. Iso-octane is used as fuel, because it is a common surrogate for gasoline and has lower tendency for cracking than other heavier hydrocarbons [20]. Furthermore, the catalytic and electro-

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catalytic behavior of Cu-CeO2 anodic composite is also evaluated under open- and closed-circuit modes of operation.

3. Results and discussion 3.1. Open-circuit studies

CO2 þ H2 ←→CO þ H2 O

ð1Þ

In addition, the catalytic decomposition and thermal pyrolysis of isooctane could also explain the observed differences in reactants conversions. The latter is confirmed by the formation of C2–C4 olefins which are derived as carbonaceous fragments via the scission reactions of iso-octane [21]. Saunders and Kendall observed a similar product distribution over Ni/YSZ electrodes which was assigned to the simultaneous polymerization and decomposition processes of produced olefins [17]. The observed H2/CO ratio is equal to 2.8 at 750 °C, a value higher than the stoichiometric ratio (2.1) expected for ISR, and decreases with temperature becoming equal to 2 at 850 °C. At the same time, the CO/CO2 ratio and methane production rate are also increasing with temperature. The above results clearly reveal the importance of reverse water gas shift (RWGS) reaction at these operating conditions. A similar H2/CO ratio equaled to 2.57 was observed in the work of Flores-Marin and Su [22], who proposed that the produced CO was consumed by one or more parallel reaction pathways carried out in the reactor. Fig. 2 depicts the dependence of the formation rate of various products, reactants' conversion and OCV on the iso-octane (Fig. 2a) and H2O (Fig. 2b) inlet concentration. The partial pressures were ranged from 0.5–3 kPa for iso-octane under a constant PH2O = 12 kPa (Fig. 2a) and from 3 to 24 kPa for H2O at Pi-C8H18 = 1.5 kPa (Fig. 2b), respectively. The operation temperature was kept constant at 850 °C and the total flowrate was remained always equal to 75 cm3/min. The observed

Electromotive Force, mV

14 - 908

50

- 898

- 900

- 905

- 897

12 40 10 H2

30

8 CO

i-C8H18

6

20 H2O

4

CH4

10

CO2

2

C3+ C2s

0

Conversion of H 2O - i-C8H18, %

The apparatus employed for the electrochemical measurements during iso-octane internal reforming, has been described in detail in previous communications [5,12]. The employed cell is consisted of an 8 mol% Y2O3-stabilized ZrO2 (YSZ) solid electrolyte tube (15 cm long, 16 mm ID, 18 mm OD, 1.2 mm thickness, supplied by CERECO) closed at its one end and three electrodes applied on its both sides. On the outer surface of the closed-end, two porous Pt films were deposited and served as counter and reference electrodes similarly to our previous works [5,11,12], while on the inside bottom surface a 70 wt.%Cu–30 wt.%CeO 2 mixed oxide composite was employed as anode. The Cu/CeO2 working electrode was prepared from cerium (IV) oxide (99.9%, Alfa Aesar) and copper powders (99%, Alfa Aesar), respectively [5,12]. Appropriate quantities of both chemicals were diluted in 20 ml of ethyl glycol, fired at 200 °C and stirred at 400 rpm until half of the volume was evaporated. The resulted viscous suspension was deposited on the inside bottom of the YSZ tube by painting. The tube was then heated up to 900 °C, and calcined for 4 h. The Cu/CeO2 electrode thus formed had a superficial surface area of 1.7 cm2 and its morphology, structure and composition was analyzed employing various characterization techniques, using a previously removed from the YSZ surface Cu/CeO2 electrode film [5,12]. The surface area was measured using the BET method, and was found equal to 2 m2/g. The elemental mapping revealed that the microstructure is non-uniform, with the CeO2 and Cu phases randomly distributed. For XRD analysis, the Cu/CeO2 catalyst was first deposited as anodic electrode and it was treated with pure H2 under open circuit and anodic polarization conditions (η = 3000 mV) at 850 °C. Then, the electrode was removed from the solid electrolyte surface and subjected to XRD analysis, where Cu, CeO2 and Cu2O phases were observed in the corresponding diffraction spectra [5]. The metallic Pt counter and reference electrodes were prepared from an organometallic paste (Metalor) after calcination in static air at 900 °C for 4 h. The heating and cooling rates in all steps were kept at 4 °C/min. The experimental apparatus [5,12] is comprised of a liquid reactants feed unit equipped with the necessary heated saturators, the solid electrolyte cell reactor and the gas analysis system. The measurements were carried out in the temperature range of 750–850 °C under atmospheric pressure and a total flowrate of 75 cm3/min. Various mixtures of isooctane and water vapors were fed into the cell reactor by bubbling pure He (Air Liquide) through two different vessels containing i-C8H18 (99.5% purity, Riedel-de Haen) and twice-distilled water. Both vessels were insulated and heated at specific temperatures in order to obtain the desired amounts of iso-octane and water vapors, while the remaining flow lines up to cell reactor inlet provisions and the GC were heated at 100 °C. The reactant and effluent composition was monitored with a SHIMADJU 14B gas chromatograph (GC) equipped with thermal conductivity (TCD) and flame ionization (FID) detectors as well as with a Molecular Sieve 13X and a Porapack QS column for gas separation. The carbon and water contents were always calculated using the carbon, hydrogen and oxygen mass balances. The cell voltage and developed electrical current were controlled and monitored by means of an AMEL model 2053 PotentiostatGalvanostat and two differential voltmeters (Digital Multimeter DT9205A). For the fuel cell measurements, the cell characteristics were acquired employing a resistance box 1051 by Time Electronics. AC electrochemical impedance spectra were acquired under open circuit conditions in the frequency range from 0.01 Hz to 1 MHz, with an amplitude of 30 mV RMS, using the IVIUM technologies electrochemical workstation and the corresponding software (IVIUMSOFT) for data processing. Prior to each experiment the anodic electrode was pretreated with pure H2 at 850 °C for 3 h.

Open-circuit experiments were firstly carried out to demonstrate the catalytic activity of Cu-CeO2 anodic electrode for iso-octane steam reforming (ISR). Fig. 1 shows the effect of temperature on the products' formation rate, open-circuit voltage (OCV) and reactants conversions. The inlet partial pressures of iso-octane and steam was 1.5 and 12 kPa, respectively, corresponding to a H2O/C feed ratio equal to unity. The reactor cell effluent stream consisted mainly from H2, CO, CH4 and CO2, while minor quantities of C2s (C2H4, C2H6), and C3 + olefins (C3H6, C4H8) were also detected. Concerning the conversions of reactants, it seems that they both increase with cell temperature. However, isooctane conversion is higher and its increase is more intense compared to H2O consumption rate. The main reason is that H2O is also a product of the parallel running reverse water gas shift (1) reaction, which is favored at elevated temperatures:

Production Rate, 107 mol/sec

2. Experimental

0 750

775

800

Temperature,

825

850

oC

Fig. 1. Dependence of products' formation rates, i-C8H18 and H2O conversions and developed OCV on cell temperature at open circuit conditions. Pi-C8H18 = 1.5 kPa, PH2O = 12 kPa, FT = 75 cm3/min.

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of Wang and Gorte [24]. On the other hand, the apparent reaction orders for i-C8H18 and H2O were found equal to 0.36 and 0.44, respectively. 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. The open literature contains only few works dealing with the ISR process. Most of the studies agree that the ISR mechanism involves several reactions carried out both on the catalytic surface and in the gas-phase [21,25]. However, two are the most common reaction mechanisms. The first proposed by Savage, describes that hydrogen production occurs through multiple pyrolysis reactions, including homolytic dissociation of C–C bond, radicals recombination, beta scission, isomerization and hydrogen removal with the distribution of products to be determined by the strength of C–H and C–C bonds of the reactants [21]. In the second, suggested by Kopasz et al., hydrogen production occurs after iso-octane pyrolysis into lighter hydrocarbons and their consequent catalytic reforming by Η2Ο [25].

3.2. Closed circuit operation

Current Density, mA/cm2

16 - 29.3 - 19.6

0

9.6

5.7

16.2

13.7

50

19.6 25.3

14 H2

40

12 i-C8H18

10

30

8 CO

20

6 H 2O

CH4

4

10

CO2 C3+

2

C2s

0 -1000

-500

0

500

1000

1500

2000

2500

Conversion of H 2O - i-C8 H 18 , %

products and their distribution were similar as in Fig. 1, i.e., H2, CO, CO2, CH4 and small amounts of heavier hydrocarbons. As the feed concentration of iso-octane increases, an important improvement in H2 production is observed (Fig. 2a). However, the CO formation rate is only slightly affected, which leads to an increase of the H2/CO ratio from 2 to 2.35 at Pi-C8H18 = 0.5 kPa and Pi-C8H18 = 3 kPa, respectively. In addition, the formation rates of CH4 and C2–C4 olefins drastically increase at H2O/C ratios below unity indicating that at steam deficient conditions iso-octane cracking is prevailing the reaction system. The enriched atmosphere in H2, CO, CH4 and C2–C4 is the reason for OCV increase from −850 to −925 mV at Pi-C8H18 = 0.5 kPa and Pi-C8H18 = 3 kPa, respectively. The effect of H2O partial pressure on the products' formation rates (Fig. 2b) is rather marginal for inlet steams with steam to carbon ratio above the reaction stoichiometry (H2O/C = 1). Hydrogen formation tends to stabilize over this value and at the same time H2/CO ratio slightly decreases due to the RWGS reaction. Of crucial importance is the coke formation on the catalyst which was calculated from the carbon balance. In all cases, this amount was found to be negligible because of the H2O presence, which via gasification prevents the accumulation of carbon deposits on the electrode, in contrast to what has been observed during the corresponding H2Ofree experiments [5]. In addition, it has been demonstrated that Cu catalysts on redox supports, such as CeO2, exhibit enhanced tolerance to coke formation due to the high oxygen mobility of the support [23]. Finally, by performing kinetic studies at differential conditions, the apparent activation energy of the iso-octane steam reforming reaction was calculated and was found equal to 24.1 ± 1.5 kcal/mol, a value slightly higher compared to 17.7 ± 2 kcal/mol estimated in the work

Production Rate, 10 7 mol/sec

Fig. 2. Dependence of products' formation rates, i-C8H18 and H2O conversions and developed OCV on a) Pi-C8H18 and b) PH2O feed partial pressures at open circuit conditions. T = 850 °C, FT = 75 cm3/min.

Fig. 3 contains results obtained under closed-circuit operation, i.e. when an overpotential is applied to the cell in order to pump O2 − through the electrolyte. A positive applied overpotential corresponds to the electrochemical supply of oxygen anions (O2 −), according to Faraday's Law, from the air exposed cathode (Pt) through the solid electrolyte (YSZ) to the anode (Cu-CeO2). On the contrary, at cathodic polarization conditions the opposite electrochemical flux is obtained; O2− are removed from the anode chamber to cathode. The inlet partial pressure of iso-octane and steam was 1.5 and 12 kPa, respectively, and the operation temperature was kept at 850 °C. The applied overpotentials ranged from −1 to +3 V with the current densities achieved to be relatively low and took values from −29.3 to 25.3 mA/cm2, respectively. During anodic polarization, the formation rates of H2, CO and CH4 are decreasing as the applied overpotential and developed current increase, due to their electro-oxidation at three phase boundary. As expected, most of the applied current is consumed by H2 and CO to produce H2O and CO2, respectively, because they are both presenting higher electro-oxidation kinetics and better diffusion characteristics compared to methane. On the other hand, the effect of anodic overpotentials on higher hydrocarbons (C2s and C3 +) is less pronounced. At negative overpotentials, the hydrogen formation rate is increased due to water electrolysis, while for the same reason the CO2 formation rate is decreasing leading to enhanced CO formation. As far as other products

0 3000

Overpotential, mV Fig. 3. Dependence of the products' formation rate, i-C8H18 and H2O conversions and developed current densities on the applied cell overpotential. Pi-C8H18 = 1.5 kPa, PH2O. = 12 kPa, T = 850 °C, FT = 75 cm3/min.

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are concerned, the formation rates of CH4 is increasing at negative currents, while the formation rates of higher hydrocarbons are not notably affected under both anodic and cathodic polarization conditions. The corresponding H2/CO ratio is close to stoichiometry and is not essentially affected by the applied overpotentials. Thus, the supply or removal of oxygen anions to or from the Cu/CeO2 electrode, respectively, does not influence the main reaction pathways occurring at open circuit catalytic operation but it does essentially affect the amounts of H2, CO, CO2 and H2O species, and thus the equilibrium of the RWGS reaction. At anodic polarization, H2 and CO are electro-oxidized to H2O and CO2, while at cathodic overpotentials the co-electrolysis of H2O and CO2 to H2 and CO is taking place.

-25 -20 -15 0.03Hz

Z - Im (Ωcm2 )

138

6.3 Hz

-10

750 oC

0.63 Hz

2.5 Hz 15.9 Hz 6.3 Hz

-5

800 oC

39.8 Hz

850oC

0 9.1 3.6

5 1.9

12.7

10 0

3.3. Fuel cell & AC impedance spectroscopy studies

PH2O = 12 kPa

6

500 800 oC

4

300 T = 750 o C

2

100 0

0 15

20

25

Curremt density,

40

In the present work, the performance of an iso-octane internal steam reforming SOFC of the type Cu-CeO2/YSZ/Pt was examined. At opencircuit studies, the results showed that i-C8H18 was successfully 16

1000

14

800

Pi-C8H18 = 3 kPa

30

35

40

45

mA/cm2

Fig. 4. Effect of operation temperature on fuel cell characteristics. Pi-C8H18 = 1.5 kPa, PH2O = 12 kPa, FT = 75 cm3/min.

12

700 10

600 FT = 75 ml/min

500

8

T = 850oC

400

PH2O = 12 kPa

6

300 4 200 Pi-C 8H18 = 0.5 kPa

100 10

35

900

600

5

30

4. Conclusions

10

8

T = 850oC

0

25

Pi-C8H18 = 1.5 kPa

2

0

Power density, mW/cm2

Pi-C8H18 = 1.5 kPa

800 700

Voltage, mV

20

total electrode polarization resistance, a behavior which was also noticed in similar experiments when acetic acid and H2O were co-fed to the cell [12]. On the other hand, the contribution of charge transfer processes increases with temperature being ca. 20% at 850 °C. Fig. 6 presents the voltage–current density and power density–current density characteristics of the cell at three different iso-octane feed partial pressures (0.5, 1.5 and 3 kPa), at a constant PH2O = 12 kPa and cell temperature equal to 850 °C. It is obvious that as the iso-octane/ H2O ratio increases the achieved electrochemical performance is enhanced reaching at Pi-C8H18 = 3 kPa a maximum power density value equal to 15.2 mW/cm2 at a cell voltage of 460 mV and a current density of 33.0 mA/cm2. Concerning the developed OCV, a clear increase in the absolute OCV values is observed with increasing iso-octane feed partial pressure. Open-circuit voltage values of 834, 864 and 914 mV are recorded at 850 °C for Pi-C8H18 equal to 0.5, 1.5 and 3 kPa, respectively. The corresponding open circuit AC impedance spectra (data not shown) showed that as expected the ohmic resistance is almost similar (ca. 2 Ωcm2) and independent from the reactants ratio. On the other hand, the electrode resistance and specifically the mass transport part is slightly improved upon increasing the iso-octane/H2O feed ratio.

Voltage, mV

900

200

15

Fig. 5. AC impedance spectra at different cell temperatures. Pi-C8H18 = 1.5 kPa, PH2O = 12 kPa, FT = 75 cm3/min.

12

FT = 75 ml/min

Power density, mW/cm2

1000

T=

10

Z Re (Ωcm2)

Fig. 4 depicts the fuel cell characteristics at temperatures ranging from 750 to 850 °C, at a stoichiometric iso-octane to steam ratio (H2O/C = 1). The absolute open circuit voltage values are decreasing with temperature increase varying from 925 to 907 and 869 mV at 750, 800 and 850 °C, respectively. The slopes of the voltage–current density curves are decreasing with temperature, implying that the overall cell resistance is reduced as cell temperature increases resulting to higher achieved power output. The maximum power density is almost doubled by increasing the cell temperature from 750 to 850 °C and it was equal to 12.7 mW/cm2 at a cell voltage of 460 mV and a current density of 23.4 mA/cm2. The corresponding open circuit AC impedance spectra are shown in Fig. 5. The ohmic resistance is decreased by increasing cell temperature from 4.1 Ωcm2 at 750 οC to 1.9 Ωcm2 at 850 οC. Furthermore, the electrode resistance is substantially reduced from 55.1 Ωcm2 at 750 °C to 17.3 Ωcm2 at 850 °C with an apparent activation energy equal to 26.5 kcal/mol. The feature of the AC impedance spectra is comprised of two distinct arcs, one centered at high frequencies (HF) and the other one at lower frequencies (LF), with the latter dominating the polarization resistance. Based on the pseudo-capacitance values, which are in the order of 10−3 and 10−1 F.cm−2 for the HF and LF arcs, respectively, it can be assumed that the HF arc is attributed to charge transfer and surface reaction processes, while the LF arc is reflecting mass transfer processes of charge and/or neutral chemical species. By further analyzing the AC impedance data it was revealed that the charge transfer and gas diffusion components of the overall electrode resistance follow an Arrhenius-type behavior with activation energies equal to 36.7 and 24.7 kcal/mol, respectively. In addition, it is evident that the diffusion of charge and/or neutral species seems to prevail the overall process contributing with approximately 80–90% to the

400

5

0

0

10

20

30

40

50

60

Current density, mA/cm2 Fig. 6. Effect of iso-octane feed partial pressure on fuel cell characteristics. T = 850 °C, PH2O = 12 kPa, FT = 75 cm3/min.

A. Al-Musa et al. / Solid State Ionics 288 (2016) 135–139

reformed by steam and effluent mixtures rich in Η2, CO, CO2 and CH4 were produced. The effluent distribution was dominated by i-C8H18 reforming reactions, but at the same time it was slightly affected by the reactions of thermal pyrolysis and catalytic decomposition of i-C8H18, as well as, by the RWGS reaction. At electrocatalytic studies (overpotential effect) and when O2 − were pumped to Cu-CeO2 electrode (anodic polarization), the electro-oxidation of Η2 and CO were the prevailing processes, while during O2 − removal from Cu-CeO2 (cathodic polarization) the co-electrolysis of H2O and CO2 dominated the reaction scheme, influencing the equilibrium reactions carried out at anode chamber. Under both open- and closed-circuit conditions at the cell reactor, insignificant carbon formation and consequently stable operation of the catalyst was observed. During fuel cell operation, the power output was increased with the cell temperature and the i-C8H18/H2O feed ratio. The AC impedance analysis showed contributions, both from charge and mass transfer processes. The increase in temperature reduced the ohmic and electrode resistances with the latter having contributions both from charge transfer (Ea = 36.7 kcal/mol) and mass transport (Ea = 24.7 kcal/mol) processes. On the other hand, the i-C8H18/H2O feed ratio affected only the electrode resistance and more notably the mass transfer component. Overall, the diffusion of charged and/or neutral species at the electrode surface was possibly the rate limiting step during fuel cell operation at all temperatures examined, with the charge transfer processes being significant at higher operation temperatures. Acknowledgments The authors would like to acknowledge the financial support provided by King Abdulaziz City for Science and Technology (KACST Grant number 671-32: Novel anodes for solid oxide fuel cells)

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