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Feasibility of hydrogen production in a steam-carbon electrochemical cell
Solid State Ionics 192 (2011) 607–610
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
Feasibility of hydrogen production in a steam-carbon electrochemical cell Andrew C. Lee a,⁎, Reginald E. Mitchell a, Turgut M. Gür b,c a b c
Dept. of Mechanical Engineering, Stanford University, Stanford, CA 94305, United States Dept. of Materials Science and Engineering, Stanford University, Stanford, CA 94305, United States Direct Carbon Technologies, LLC, Palo Alto, CA 94301, United States
a r t i c l e
i n f o
Article history: Received 1 September 2009 Received in revised form 9 May 2010 Accepted 24 May 2010 Available online 26 June 2010 Keywords: Carbon Hydrogen production Steam electrolysis Yttria stabilized zirconia Solid oxide fuel cell
a b s t r a c t A high temperature electrochemical cell with a bed of solid carbon at the anode and steam at the cathode is proposed for carbon-assisted hydrogen production. This scheme eliminates the uphill potential barrier and provides a significant reduction in the required electrical work input to produce hydrogen from steam. The electrochemical cell is made of an yttria stabilized zirconia electrolyte with porous platinum electrodes. Current–voltage measurements and gas chromatographic analysis indicate steam utilization and production of carbon-free hydrogen. Measured open circuit potentials of 0.1–0.6 V agree with theoretical values. This downhill driving force allows for spontaneous hydrogen production and cogeneration of electricity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is an important and effective energy carrier. Once available, it is also an environmentally friendly fuel since it produces only water as the oxidation or combustion product. If a hydrogen economy is to be realized, distributed generation of hydrogen is desirable due to the cost and technical challenges posed by hydrogen transportation and storage. Unfortunately, no direct natural source exists for hydrogen; it must be generated from other chemicals. Currently, hydrogen is produced in large quantities mostly by steam reforming of methane, and to a lesser extent, of coal. However, steam reforming is cost effective only for central generation of large volumes. Unfortunately, this approach also produces a hydrogen stream that is diluted and/or contaminated by a number of other species, thus requiring expensive separation processes. Particularly critical impurities are the oxides of carbon, especially CO. Even at trace quantities, CO makes it undesirable to employ such contaminated hydrogen in many catalytic processes, and particularly in low temperature fuel cells. An alternative process to steam reforming is electrolysis of water (or steam) using an electrochemical cell, which is modular in nature and well suited for distributed generation. Generally, it is accomplished by using alkaline [1] or polymer exchange membrane (PEM) [2] electrolyzers, or solid oxide type electrolyzers as discussed below. Alkaline and PEM-type water electrolyzers operating slightly above room temperature are available commercially. The integration of
⁎ Corresponding author. E-mail address: [email protected] (A.C. Lee). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.05.034
biomass gasification with similar low temperature water electrolysis into a hydrogen production plant has also been investigated [3]. High temperature electrolyzers based on yttria stabilized zirconia (YSZ) electrolyte membranes have been developed by Dornier Systems GmbH in Germany under the name HotELLY in the 80's for commercial production of hydrogen by steam electrolysis [4,5]. The HotELLY process employed steam at the cathode and air at the anode side of the electrolyzer. However, this effort was later abandoned. Using the same concept, more recent work on steam electrolyzers and reversible solid oxide fuel cells have demonstrated high production rates for hydrogen [6–11]. Unlike steam reforming, where the product stream always contains small or trace amounts of CO, electrolysis of either water or steam produces a high purity hydrogen stream containing no CO. That is a major advantage especially for low temperature fuel cell power applications for transportation or stationary systems. Direct electrolysis of water or steam is energy intensive, as the open circuit voltage (OCV) of the cell, corresponding to the free energy of formation of water, needs to be overcome before electrolysis can be achieved. At room temperature, the open circuit voltage of the cell is 1.23 V. This quantity is dependent on temperature with a negative coefficient, and also on activity, and opposes the electrolysis process. Recently, there has been renewed interest in utilizing a suitable carbon-containing depolarizing agent at the anode for electrolytic production of hydrogen. Several studies have demonstrated the use of methane [12–14], and CO [14] at the anode of a solid oxide electrolyte based cell to facilitate removal of oxygen from steam. Carbon in a molten anode solid oxide cell was also reported [15]. A similar approach using high surface area carbon has recently reported for carbon assisted hydrogen production near room temperature [16,17].
608
A.C. Lee et al. / Solid State Ionics 192 (2011) 607–610
Much of the prior work and attempts to make electrolytic hydrogen production more efficient have been reviewed in a recent report [18]. This paper reports on the feasibility of spontaneous co-production of carbon-free hydrogen and electricity in a solid oxide fuel cell configuration containing steam at the cathode compartment and solid carbonaceous fuel at the anode. This work was motivated by earlier studies elsewhere [9–18] and inspired by recent reports from our laboratory on a carbon-air type fuel cell [19–22]. This electrochemical cell arrangement eliminates the need to overcome the OCV for steam electrolysis and significantly reduces the required electrical work input. Carbon, or carbonaceous solids, at the anode are able to maintain oxygen activities sufficiently low, and provide the electrochemical driving force for abstraction of oxygen from steam. Under specific conditions, this process can occur spontaneously, requiring no electrical work input in the form of an external bias. If desired, an external bias can be applied to supplement the downhill chemical potential gradient and thus increase the rate of hydrogen production. Consequently, this scheme renders pure, carbon-free hydrogen production energetically. 2. Description of the steam–carbon approach The arrangement investigated here is a solid oxide fuel cell configuration, with yttria stabilized zirconia (YSZ) solid oxide electrolyte and porous Pt electrodes. The anode compartment contains a bed of carbon particles in contact with the anode and a stream of steam is introduced to the cathode. The cell configuration can be described as shown in Eq. (1). H2 OðgÞ ; H2=Pt=YSZ=Pt=CðsÞ ; CO; CO2
ð1Þ
A schematic representation of the cell is provided in Fig. 1. The cathode reaction is the electrochemical reduction of steam by, −
H2 O + 2e →H2 + O
=
R:1
At the anode, the electrochemical oxidation reaction is given by R.2. =
−
CO + O →CO2 + 2e
R:2
Earlier work [19–21] has indicated that under these experimental conditions (C–O system at anode with excess carbon and temperatures above 1000 K), the Boudouard reaction R.3 governs the formation of CO. CðsÞ+ CO2 →2CO
R:3
Oxygen ions formed during the reduction of H2O at the cathode are transferred through the YSZ electrolyte and oxidize the CO at the anode by R.2. The net cell reaction resulting from the overall cathode and anode processes, and chemical interaction from the carbon bed, is given by, H2 Ocath +CðsÞan →H2;cath +COan
R:4
where the hydrogen containing species are located at the cathode (cath) and carbon-containing species are located at the anode (an). The theoretical open circuit voltage (OCV) from this system can be calculated for R.4 in terms of partial pressures using Eq. (2), with n = 2 for this case and for unit activity for solid carbon. 0
EOCV = E +
PH2O;cath RT ln nF PH2;cath PCO;an
! ð2Þ
The temperature-dependent standard potential, E0, for reaction R.4 at 1173 K is 169 mV, signifying that less cell bias is required for electrolysis over direct means. The formulation given by Eq. (2) is essentially the driving force for steam gasification (SG), which is endothermic, with heat of reaction 135 kJ/molC, at 1173 K. Since the carbon and hydrogen species are physically and chemically separated in the proposed scheme, the SG reaction can be forced to provide larger H2 concentrations than would be possible with a completely mixed system. Also evident from this formulation is that larger H2O/ H2 ratios are preferred for increased potentials. Depending upon the cathode material and cell operating conditions, there may be benefit to providing some H2 in the inlet steam stream, either directly added or from re-circulated steam-side products (i.e., cathode recycle), if oxidation of the electrode species such as Ni is a possible concern. The standard potential of Ni oxidation is 0.66 V at 1173 K, while the standard potential of pure steam at the anode is only 0.29 V, referenced to oxygen in air. Hence, a pure steam environment at the cathode does not maintain a sufficiently low oxygen activity to prevent NiO formation. 3. Experimental In order to verify the expected open circuit potential for the steam–carbon cell, several measurements were conducted using an activated carbon (available from Fisher Scientific) [21]. A closed-end partially stabilized zirconia (PSZ) electrolyte tube (0.5 in OD, 1.3 mm wall, 15 in long) was coated with Pt ink and fired at 1173 K to form porous Pt electrodes, similar to the arrangements described by Lee et al. [19]. The active area of the cell is roughly 20 cm2. The electrolyteelectrode tubular assembly is placed inside a quartz housing. Carbon was loaded into the PSZ tube interior (anode) at the closed-end and H2/H2O mixture was supplied to the PSZ tube exterior (cathode) within the quartz housing. The anode volume was purged with a small He flow away from the bed to create a positive pressure differential to reduce air transport into the carbon bed. A tube furnace was used to control the cell temperature via a thermocouple located in the carbon bed that was connected to a process controller. This type of cell was also used for gas chromatography measurements of the cathode exhaust to confirm hydrogen production. A steady, metered flow of steam was produced using an HPLC pump (Eldex) and boiler/ superheater section of the quartz housing. The cell is heated with ambient air at the cathode, and a small He purge above the carbon bed. Once the system has reached its
Fig. 1. Schematic concept of the steam-carbon electrochemical cell, utilizing a YSZ electrolyte with porous Pt electrodes.
A.C. Lee et al. / Solid State Ionics 192 (2011) 607–610
operating temperature, the OCV referenced to air is noted (nominally 1.2 V), giving an accurate measure of the carbon-side oxygen activity. The air is then displaced with a specified H2/H2O mixture. This method allows for the impact of the small He purge at the anode on the OCV to be included. An yttria stabilized zirconia (YSZ) electrolyte-supported button cell (2.5 cm diameter and 0.3 mm thick) was used for measurement of the current–voltage behavior. As with the tube cell, Pt ink was applied to both sides of the button cell, and fired at 1372 K to form porous Pt electrodes. The active area of the button cells was 2 cm2 on each side. Button cells were affixed in between two 2.5 cm diameter alumina tubes with refractory gaskets and a ceramic sealant. Platinum mesh current collectors were placed in contact with each electrode, and a thermocouple was affixed 1 cm from each face of the button cell to monitor temperature. One gram of activated carbon was loaded onto the top face of the button cell (anode) to form a free particle bed. Galvanostatic DC measurements were conducted using a Solartron 1286 electrochemical interface, and the impedance spectra of the cells were taken by coupling a Solartron 1250 frequency response analyzer. A root-mean-square amplitude of 20 mV was employed for AC measurements, over a frequency range of 0.1 to 60 k Hz. Gas compositions were measured with a Varian 3400 gas chromatograph (GC) with thermal conductivity detector and Haysep Q column for separation. 4. Results and discussion Theoretically expected OCV values are compared in Fig. 2 with those measured using the tubular cell geometry. The exceptional agreement between the two sets spanning 3 orders of magnitude in H2/H2O ratio at both 1073 and 1173 K demonstrates again that the steam-carbon cell is governed by the Boudouard reaction at the anode, and the H2/H2O ratio at the cathode. The oxygen potential at the cathode is larger than that at the anode, implying that hydrogen production can proceed spontaneously (without electrical work input). Recent thermodynamic analyses have shown that Boudouard equilibrium is reached and maintained at temperatures near 1173 K [22]. High temperature operation results in larger driving forces as the carbon bed causes the anode oxygen activity to decrease while the oxygen activity in a fixed H2/H2O mixture increases with temperature. During steam-carbon operation, larger H2O/H2 ratios allow for better performance by increasing the oxygen activity. The effect of the ratio on cell voltage was experimentally verified in Fig. 3, which shows the cell potential during galvanostatic operation (50 mA) at 1173 K with step changes in water flow. Steam flow was turned on and off during this experiment while a steady flow of 3% H2 (N2 balance) was introduced to set the hydrogen input into the reactor. The intervals without water flow reduced the H2O/H2 ratio and thus the operating voltage. The introduction of water flow increases this ratio and also the operating
Fig. 2. OCV measurements of the steam-carbon cell at 1173 and 1073 K.
609
Fig. 3. Measured voltage across a button cell at 1173 K, with 0 and 1 mL/min H2O flow under constant 25 mA/cm2 current.
voltage. The time intervals of the water flow and the cell geometry dictate the minimum and maximum potentials reached with this arrangement, but the trend is repeatable. This trend agrees with the functional dependencies included in Eq. (1). The cell potential given by Eq. (1) is measured as the voltage difference of the cathode minus the anode, and thus the open circuit potential is positive. A positive cell potential and current represent fuel cell behavior, and spontaneous hydrogen production. When the cell potential becomes negative, for a positive current, there is work input into the cell to force higher rates for oxygen transport. Conversely, if the cell is operated under negative currents for a positive potential, oxygen is forced from the carbon bed to the steam side. This scenario also requires work input, denoted by a negative product of I ⁎ V. A unique point exists at short circuiting, where the cell potential diminishes to zero, and a positive current is achieved. This represents the maximum hydrogen production without work input. The I–V data shown in Fig. 4a were taken with the same button cell and carbon bed, but differing only in the cathode environment. Quiescent air was used in the air–carbon experiment, and during steam–carbon operation inputs of steam and 3% H2 were maintained. The OCV of the steam–carbon system was much less than that of air– carbon system as expected. The shape of the I–V curve also suggests two approaches in order to achieve higher rates for hydrogen production; namely, increasing the OCV between the carbon bed and steam, and reducing the cell losses. In addition, AC impedance spectroscopy (EIS) measurements (Fig. 4b) during both steam and air operation suggest that oxygen removal from steam is more resistive than removal from air, in agreement with the trend of the I–V curves. This could be due to fast rate of the reverse of reaction R.1 compared to reaction R.2. The significant losses evident from the steam–carbon I–V data and the impedance results suggest that reducing the cell losses, which would involve minimizing ohmic resistances and enhancing electrode kinetics, can greatly improve cell performance and hydrogen production rate. Measurements of the exhaust stream compositions during DC operation were also performed on a tubular cell to verify the presence of H2. The cell was operated galvanostatically (100 mA), under steady flow conditions to compare steady state hydrogen production and carbon consumption. Argon was used as the carrier gas through the carbon bed and the GC, so that no extraneous peaks were present. Pure steam was introduced to the cathode side. This arrangement ensures that any oxygen contained in the anode exhaust along with any hydrogen produced at the cathode arise from a Faradaic process. In order to facilitate simultaneous and accurate measurement, the cathode and anode effluent streams were mixed, and the H2O was condensed out prior to GC analysis. Only H2 and CO were detected, and the molar H2/CO ratio ranged from 0.96 to 1.0, signifying that
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sufficient time in the carbon bed [22]. This signifies that Boudouard equilibrium was reached in the carbon bed. 5. Summary The feasibility of electrochemical hydrogen production from carbon and steam is demonstrated using a modified solid oxide fuel cell configuration. In this arrangement, carbon-containing species are kept physically and chemically separate from the cathode stream by a solid oxide electrolyte, producing pure hydrogen. The system was shown to operate spontaneously as a fuel cell under certain conditions, co-producing hydrogen and electrical work. Gas analysis validated hydrogen production at the cathode during galvanostatic operation. Acknowledgements The authors would like to thank Wiley Neel and Jonah Greenberger for their efforts and assistance. Partial support from Direct Carbon Technologies, LLC is greatly appreciated. References
Fig. 4. Measured (a) I–V and (b) EIS behavior of both air–carbon and steam–carbon operation at 900C, with 2 cm2 active area. The numbers in the figure legend indicate current density in mA/cm2.
practically all of oxygen removed from steam at the cathode ended up bound in CO at the anode. The carbon bed at the anode caused the oxygen to elude primarily as CO, with CO2 below detectable limits. The oxygen containing species in the carbon bed reach equilibrium given
[1] A.J. Appleby, G. Crepy, J. Jacquelin, Int. J. Hydrogen Energy 3 (1978) 21. [2] S.A. Grigoriev, V.I. Porembsky, V.N. Fateev, Int. J. Hydrogen Energy 31 (2) (2006) 171. [3] P.C. Hulteberg, H.T. Karlsson, Int. J. Hydrogen Energy 34 (2) (2009) 772. [4] W. Donitz, Int. J. Hydrogen Energy 9 (10) (1984) 817. [5] W. Donitz, G. Dietrich, E. Erdle, R. Streicher, Int. J. Hydrogen Energy 13 (5) (1988) 283. [6] J.S. Herring, J.E. O'Brien, C.M. Stoots, G.L. Hawkes, J.J. Hartvigsen, M. Shahnam, Int. J. Hydrogen Energy 32 (4) (2007) 440. [7] S.H. Jensen, P.H. Larsen, M. Mogensen, Int. J. Hydrogen Energy 32 (15) (2007) 3253. [8] A.O. Isenberg, Solid State Ionics 3–4 (1981) 431. [9] M.A. Laguna-Bercero, J.A. Kilner, S.J. Skinner, Solid State Ionics 192 (2011) 501–504 (this issue). [10] M.A. Laguna-Bercero, S.J. Skinner, J.A. Kilner, J. Power Sources 192 (2009) 126. [11] W. Wang, Y. Huang, S. Jung, J.M. Vohs, R.J. Gorte, J. Electrochem. Soc. 153 (11) (2006) A2066. [12] J. Martinez-Frias, A.Q. Pham, A.M. Aceves, Int. J. Hydrogen Energy 28 (5) (2003) 483. [13] W. Wang, R.J. Gorte, J.M. Vohs, Chem. Engr. Sci. 63 (2008) 767. [14] W. Wang, J.M. Vohs, R.J. Gorte, Top. Catal. 46 (2007) 380. [15] S. Gopalan, G. Ye, U.B. Pal, J. Power Sources 162 (1) (2006) 74. [16] M.S. Seehra, S. Ranganathan, A. Manivannan, Appl. Phys. Lett. 90 (2007) 044104. [17] M.S. Seehra, S. Bollineni, Intern. J. Hydrogen Energy 34 (15) (2009) 6078. [18] A. Hauch, S.D. Ebbesen, S.H. Jensen, M. Mogensen, J. Mater. Chem. 18 (2008) 2331. [19] S. Li, A.C. Lee, R.E. Mitchell, T.M. Gür, Solid State Ionics 179 (2008) 1549. [20] A.C. Lee, S. Li, R.E. Mitchell, T.M. Gür, Electrochem. Solid-State Lett. 11 (2) (2008) B20. [21] A.C. Lee, R.E. Mitchell, T.M. Gür, AIChE J. 55 (4) (2009) 983. [22] A.C. Lee, R.E. Mitchell, T.M. Gür, J. Power Sources 194 (2) (2009) 774.
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
Feasibility of hydrogen production in a steam-carbon electrochemical cell Andrew C. Lee a,⁎, Reginald E. Mitchell a, Turgut M. Gür b,c a b c
Dept. of Mechanical Engineering, Stanford University, Stanford, CA 94305, United States Dept. of Materials Science and Engineering, Stanford University, Stanford, CA 94305, United States Direct Carbon Technologies, LLC, Palo Alto, CA 94301, United States
a r t i c l e
i n f o
Article history: Received 1 September 2009 Received in revised form 9 May 2010 Accepted 24 May 2010 Available online 26 June 2010 Keywords: Carbon Hydrogen production Steam electrolysis Yttria stabilized zirconia Solid oxide fuel cell
a b s t r a c t A high temperature electrochemical cell with a bed of solid carbon at the anode and steam at the cathode is proposed for carbon-assisted hydrogen production. This scheme eliminates the uphill potential barrier and provides a significant reduction in the required electrical work input to produce hydrogen from steam. The electrochemical cell is made of an yttria stabilized zirconia electrolyte with porous platinum electrodes. Current–voltage measurements and gas chromatographic analysis indicate steam utilization and production of carbon-free hydrogen. Measured open circuit potentials of 0.1–0.6 V agree with theoretical values. This downhill driving force allows for spontaneous hydrogen production and cogeneration of electricity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is an important and effective energy carrier. Once available, it is also an environmentally friendly fuel since it produces only water as the oxidation or combustion product. If a hydrogen economy is to be realized, distributed generation of hydrogen is desirable due to the cost and technical challenges posed by hydrogen transportation and storage. Unfortunately, no direct natural source exists for hydrogen; it must be generated from other chemicals. Currently, hydrogen is produced in large quantities mostly by steam reforming of methane, and to a lesser extent, of coal. However, steam reforming is cost effective only for central generation of large volumes. Unfortunately, this approach also produces a hydrogen stream that is diluted and/or contaminated by a number of other species, thus requiring expensive separation processes. Particularly critical impurities are the oxides of carbon, especially CO. Even at trace quantities, CO makes it undesirable to employ such contaminated hydrogen in many catalytic processes, and particularly in low temperature fuel cells. An alternative process to steam reforming is electrolysis of water (or steam) using an electrochemical cell, which is modular in nature and well suited for distributed generation. Generally, it is accomplished by using alkaline [1] or polymer exchange membrane (PEM) [2] electrolyzers, or solid oxide type electrolyzers as discussed below. Alkaline and PEM-type water electrolyzers operating slightly above room temperature are available commercially. The integration of
⁎ Corresponding author. E-mail address: [email protected] (A.C. Lee). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.05.034
biomass gasification with similar low temperature water electrolysis into a hydrogen production plant has also been investigated [3]. High temperature electrolyzers based on yttria stabilized zirconia (YSZ) electrolyte membranes have been developed by Dornier Systems GmbH in Germany under the name HotELLY in the 80's for commercial production of hydrogen by steam electrolysis [4,5]. The HotELLY process employed steam at the cathode and air at the anode side of the electrolyzer. However, this effort was later abandoned. Using the same concept, more recent work on steam electrolyzers and reversible solid oxide fuel cells have demonstrated high production rates for hydrogen [6–11]. Unlike steam reforming, where the product stream always contains small or trace amounts of CO, electrolysis of either water or steam produces a high purity hydrogen stream containing no CO. That is a major advantage especially for low temperature fuel cell power applications for transportation or stationary systems. Direct electrolysis of water or steam is energy intensive, as the open circuit voltage (OCV) of the cell, corresponding to the free energy of formation of water, needs to be overcome before electrolysis can be achieved. At room temperature, the open circuit voltage of the cell is 1.23 V. This quantity is dependent on temperature with a negative coefficient, and also on activity, and opposes the electrolysis process. Recently, there has been renewed interest in utilizing a suitable carbon-containing depolarizing agent at the anode for electrolytic production of hydrogen. Several studies have demonstrated the use of methane [12–14], and CO [14] at the anode of a solid oxide electrolyte based cell to facilitate removal of oxygen from steam. Carbon in a molten anode solid oxide cell was also reported [15]. A similar approach using high surface area carbon has recently reported for carbon assisted hydrogen production near room temperature [16,17].
608
A.C. Lee et al. / Solid State Ionics 192 (2011) 607–610
Much of the prior work and attempts to make electrolytic hydrogen production more efficient have been reviewed in a recent report [18]. This paper reports on the feasibility of spontaneous co-production of carbon-free hydrogen and electricity in a solid oxide fuel cell configuration containing steam at the cathode compartment and solid carbonaceous fuel at the anode. This work was motivated by earlier studies elsewhere [9–18] and inspired by recent reports from our laboratory on a carbon-air type fuel cell [19–22]. This electrochemical cell arrangement eliminates the need to overcome the OCV for steam electrolysis and significantly reduces the required electrical work input. Carbon, or carbonaceous solids, at the anode are able to maintain oxygen activities sufficiently low, and provide the electrochemical driving force for abstraction of oxygen from steam. Under specific conditions, this process can occur spontaneously, requiring no electrical work input in the form of an external bias. If desired, an external bias can be applied to supplement the downhill chemical potential gradient and thus increase the rate of hydrogen production. Consequently, this scheme renders pure, carbon-free hydrogen production energetically. 2. Description of the steam–carbon approach The arrangement investigated here is a solid oxide fuel cell configuration, with yttria stabilized zirconia (YSZ) solid oxide electrolyte and porous Pt electrodes. The anode compartment contains a bed of carbon particles in contact with the anode and a stream of steam is introduced to the cathode. The cell configuration can be described as shown in Eq. (1). H2 OðgÞ ; H2=Pt=YSZ=Pt=CðsÞ ; CO; CO2
ð1Þ
A schematic representation of the cell is provided in Fig. 1. The cathode reaction is the electrochemical reduction of steam by, −
H2 O + 2e →H2 + O
=
R:1
At the anode, the electrochemical oxidation reaction is given by R.2. =
−
CO + O →CO2 + 2e
R:2
Earlier work [19–21] has indicated that under these experimental conditions (C–O system at anode with excess carbon and temperatures above 1000 K), the Boudouard reaction R.3 governs the formation of CO. CðsÞ+ CO2 →2CO
R:3
Oxygen ions formed during the reduction of H2O at the cathode are transferred through the YSZ electrolyte and oxidize the CO at the anode by R.2. The net cell reaction resulting from the overall cathode and anode processes, and chemical interaction from the carbon bed, is given by, H2 Ocath +CðsÞan →H2;cath +COan
R:4
where the hydrogen containing species are located at the cathode (cath) and carbon-containing species are located at the anode (an). The theoretical open circuit voltage (OCV) from this system can be calculated for R.4 in terms of partial pressures using Eq. (2), with n = 2 for this case and for unit activity for solid carbon. 0
EOCV = E +
PH2O;cath RT ln nF PH2;cath PCO;an
! ð2Þ
The temperature-dependent standard potential, E0, for reaction R.4 at 1173 K is 169 mV, signifying that less cell bias is required for electrolysis over direct means. The formulation given by Eq. (2) is essentially the driving force for steam gasification (SG), which is endothermic, with heat of reaction 135 kJ/molC, at 1173 K. Since the carbon and hydrogen species are physically and chemically separated in the proposed scheme, the SG reaction can be forced to provide larger H2 concentrations than would be possible with a completely mixed system. Also evident from this formulation is that larger H2O/ H2 ratios are preferred for increased potentials. Depending upon the cathode material and cell operating conditions, there may be benefit to providing some H2 in the inlet steam stream, either directly added or from re-circulated steam-side products (i.e., cathode recycle), if oxidation of the electrode species such as Ni is a possible concern. The standard potential of Ni oxidation is 0.66 V at 1173 K, while the standard potential of pure steam at the anode is only 0.29 V, referenced to oxygen in air. Hence, a pure steam environment at the cathode does not maintain a sufficiently low oxygen activity to prevent NiO formation. 3. Experimental In order to verify the expected open circuit potential for the steam–carbon cell, several measurements were conducted using an activated carbon (available from Fisher Scientific) [21]. A closed-end partially stabilized zirconia (PSZ) electrolyte tube (0.5 in OD, 1.3 mm wall, 15 in long) was coated with Pt ink and fired at 1173 K to form porous Pt electrodes, similar to the arrangements described by Lee et al. [19]. The active area of the cell is roughly 20 cm2. The electrolyteelectrode tubular assembly is placed inside a quartz housing. Carbon was loaded into the PSZ tube interior (anode) at the closed-end and H2/H2O mixture was supplied to the PSZ tube exterior (cathode) within the quartz housing. The anode volume was purged with a small He flow away from the bed to create a positive pressure differential to reduce air transport into the carbon bed. A tube furnace was used to control the cell temperature via a thermocouple located in the carbon bed that was connected to a process controller. This type of cell was also used for gas chromatography measurements of the cathode exhaust to confirm hydrogen production. A steady, metered flow of steam was produced using an HPLC pump (Eldex) and boiler/ superheater section of the quartz housing. The cell is heated with ambient air at the cathode, and a small He purge above the carbon bed. Once the system has reached its
Fig. 1. Schematic concept of the steam-carbon electrochemical cell, utilizing a YSZ electrolyte with porous Pt electrodes.
A.C. Lee et al. / Solid State Ionics 192 (2011) 607–610
operating temperature, the OCV referenced to air is noted (nominally 1.2 V), giving an accurate measure of the carbon-side oxygen activity. The air is then displaced with a specified H2/H2O mixture. This method allows for the impact of the small He purge at the anode on the OCV to be included. An yttria stabilized zirconia (YSZ) electrolyte-supported button cell (2.5 cm diameter and 0.3 mm thick) was used for measurement of the current–voltage behavior. As with the tube cell, Pt ink was applied to both sides of the button cell, and fired at 1372 K to form porous Pt electrodes. The active area of the button cells was 2 cm2 on each side. Button cells were affixed in between two 2.5 cm diameter alumina tubes with refractory gaskets and a ceramic sealant. Platinum mesh current collectors were placed in contact with each electrode, and a thermocouple was affixed 1 cm from each face of the button cell to monitor temperature. One gram of activated carbon was loaded onto the top face of the button cell (anode) to form a free particle bed. Galvanostatic DC measurements were conducted using a Solartron 1286 electrochemical interface, and the impedance spectra of the cells were taken by coupling a Solartron 1250 frequency response analyzer. A root-mean-square amplitude of 20 mV was employed for AC measurements, over a frequency range of 0.1 to 60 k Hz. Gas compositions were measured with a Varian 3400 gas chromatograph (GC) with thermal conductivity detector and Haysep Q column for separation. 4. Results and discussion Theoretically expected OCV values are compared in Fig. 2 with those measured using the tubular cell geometry. The exceptional agreement between the two sets spanning 3 orders of magnitude in H2/H2O ratio at both 1073 and 1173 K demonstrates again that the steam-carbon cell is governed by the Boudouard reaction at the anode, and the H2/H2O ratio at the cathode. The oxygen potential at the cathode is larger than that at the anode, implying that hydrogen production can proceed spontaneously (without electrical work input). Recent thermodynamic analyses have shown that Boudouard equilibrium is reached and maintained at temperatures near 1173 K [22]. High temperature operation results in larger driving forces as the carbon bed causes the anode oxygen activity to decrease while the oxygen activity in a fixed H2/H2O mixture increases with temperature. During steam-carbon operation, larger H2O/H2 ratios allow for better performance by increasing the oxygen activity. The effect of the ratio on cell voltage was experimentally verified in Fig. 3, which shows the cell potential during galvanostatic operation (50 mA) at 1173 K with step changes in water flow. Steam flow was turned on and off during this experiment while a steady flow of 3% H2 (N2 balance) was introduced to set the hydrogen input into the reactor. The intervals without water flow reduced the H2O/H2 ratio and thus the operating voltage. The introduction of water flow increases this ratio and also the operating
Fig. 2. OCV measurements of the steam-carbon cell at 1173 and 1073 K.
609
Fig. 3. Measured voltage across a button cell at 1173 K, with 0 and 1 mL/min H2O flow under constant 25 mA/cm2 current.
voltage. The time intervals of the water flow and the cell geometry dictate the minimum and maximum potentials reached with this arrangement, but the trend is repeatable. This trend agrees with the functional dependencies included in Eq. (1). The cell potential given by Eq. (1) is measured as the voltage difference of the cathode minus the anode, and thus the open circuit potential is positive. A positive cell potential and current represent fuel cell behavior, and spontaneous hydrogen production. When the cell potential becomes negative, for a positive current, there is work input into the cell to force higher rates for oxygen transport. Conversely, if the cell is operated under negative currents for a positive potential, oxygen is forced from the carbon bed to the steam side. This scenario also requires work input, denoted by a negative product of I ⁎ V. A unique point exists at short circuiting, where the cell potential diminishes to zero, and a positive current is achieved. This represents the maximum hydrogen production without work input. The I–V data shown in Fig. 4a were taken with the same button cell and carbon bed, but differing only in the cathode environment. Quiescent air was used in the air–carbon experiment, and during steam–carbon operation inputs of steam and 3% H2 were maintained. The OCV of the steam–carbon system was much less than that of air– carbon system as expected. The shape of the I–V curve also suggests two approaches in order to achieve higher rates for hydrogen production; namely, increasing the OCV between the carbon bed and steam, and reducing the cell losses. In addition, AC impedance spectroscopy (EIS) measurements (Fig. 4b) during both steam and air operation suggest that oxygen removal from steam is more resistive than removal from air, in agreement with the trend of the I–V curves. This could be due to fast rate of the reverse of reaction R.1 compared to reaction R.2. The significant losses evident from the steam–carbon I–V data and the impedance results suggest that reducing the cell losses, which would involve minimizing ohmic resistances and enhancing electrode kinetics, can greatly improve cell performance and hydrogen production rate. Measurements of the exhaust stream compositions during DC operation were also performed on a tubular cell to verify the presence of H2. The cell was operated galvanostatically (100 mA), under steady flow conditions to compare steady state hydrogen production and carbon consumption. Argon was used as the carrier gas through the carbon bed and the GC, so that no extraneous peaks were present. Pure steam was introduced to the cathode side. This arrangement ensures that any oxygen contained in the anode exhaust along with any hydrogen produced at the cathode arise from a Faradaic process. In order to facilitate simultaneous and accurate measurement, the cathode and anode effluent streams were mixed, and the H2O was condensed out prior to GC analysis. Only H2 and CO were detected, and the molar H2/CO ratio ranged from 0.96 to 1.0, signifying that
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sufficient time in the carbon bed [22]. This signifies that Boudouard equilibrium was reached in the carbon bed. 5. Summary The feasibility of electrochemical hydrogen production from carbon and steam is demonstrated using a modified solid oxide fuel cell configuration. In this arrangement, carbon-containing species are kept physically and chemically separate from the cathode stream by a solid oxide electrolyte, producing pure hydrogen. The system was shown to operate spontaneously as a fuel cell under certain conditions, co-producing hydrogen and electrical work. Gas analysis validated hydrogen production at the cathode during galvanostatic operation. Acknowledgements The authors would like to thank Wiley Neel and Jonah Greenberger for their efforts and assistance. Partial support from Direct Carbon Technologies, LLC is greatly appreciated. References
Fig. 4. Measured (a) I–V and (b) EIS behavior of both air–carbon and steam–carbon operation at 900C, with 2 cm2 active area. The numbers in the figure legend indicate current density in mA/cm2.
practically all of oxygen removed from steam at the cathode ended up bound in CO at the anode. The carbon bed at the anode caused the oxygen to elude primarily as CO, with CO2 below detectable limits. The oxygen containing species in the carbon bed reach equilibrium given
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