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Analysis of the carbon anode in direct carbon conversion fuel cells
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Analysis of the carbon anode in direct carbon conversion fuel cells John F. Cooper a,*, J. Robert Selman b a b
John F. Cooper Consulting, LLC, 1971 Arrowhead Drive, Oakland, CA 94611, USA Dept of Chem and Biol Eng, Illinois Institute of Technology, 10 W. 33rd St., Chicago, Il 60616, USA
article info
abstract
Article history:
The total electrochemical efficiency of a direct carbon fuel cell with molten carbonate
Received 1 July 2011
electrolyte is dominated by the product of coulombic efficiency (electron yield (n) per carbon
Received in revised form
atom, divided by 4) and voltaic efficiency (ratio of cell voltage to theoretical voltage). The
5 March 2012
voltaic efficiency is acceptably high (70e80%) for many atomically-disordered carbon
Accepted 17 March 2012
materials. High coulombic efficiency is more difficult to achieve but ranges from below 50%
Available online 24 April 2012
at low current densities in porous material to 100% in certain monolithic and particulate carbon anodes at high current densities where substantially pure CO2 is the product gas.
Keywords:
We find evidence for two competing anode reactions associated with distinct low- and high
Direct carbon fuel cell
polarization segments, respectively: (1) a charge-transfer controlled, linearepolarization
Carbon anode efficiency
reaction occurring predominately within pores, proportional to specific area, and tending
Carbon anode in molten carbonate
toward low efficiency by co-production of CO and CO2; and (2) a flow-dependent reaction
Reaction distribution
occurring on the exterior surface of the anode, requiring > 100 mV polarization and tending
in carbon anodes
to produce CO2. Based on this interpretation, high electrochemical efficiency of a carbon fuel cell is expected with anodes made of atomically disordered ("turbostratic") carbon that have negligible porosity, or with anodes of disordered carbon for which interior pores are intentionally blocked with an impervious solid material, such as an inert salt or readily carbonized pitch. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
A century-old goal of electrochemistry has been a fuel cell that could generate electricity from elemental carbon derived from fossil or biological resources. This would avoid the thermal inefficiencies of heat engines and the pollution associated with combustion. Early research on this type of fuel cell (which we call "direct carbon fuel cell", or DCFC) has been reviewed by Liebhafsky and Cairns [1]. Later work is
summarized by Cao [2] and by Cooper [3]. In the last five years, there has been a resurgence of interest in the DCFC that is driven by increased emphasis on the efficiency of use of carbon fuels [4]. Electrochemical conversion of carbon is found in a variety of concepts. For example, these include: (1) designs resembling the gas-fueled molten carbonate fuel cell (MCFC) but with a solid carbon anode; (2) solid oxide fuel cells in which carbon particles are directly fed into the anode channel, (3) cells with carbon anodes in combination with
* Corresponding author. Tel./fax: þ1 510 339 9401. E-mail address: [email protected] (J.F. Cooper). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.03.095
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parallel solid oxide and molten salt electrolyte, and (4) systems in which gasification of carbon to CO is integrated with electrochemical oxidation or combustion of CO. In general, direct carbon fuel cell (DCFC) refers to the galvanic oxidation of solid carbon with direct transfer of electrons from the solid carbon to a current collector, as depicted in Fig. 1. Such fuel cells operate at elevated temperatures (650e800 C) to overcome sluggish kinetics of reactions leading to the CO2 product and make use of molten carbonate and/or solid oxide electrolytes. In this communication, discussion of DCFC is limited to carbonate-based systems. Efficient operation requires balanced cathodic and anodic half reactions to produce a CO2 reaction product by means of the transfer of four electrons per unit of reaction: O2 þ 2CO2 þ 4 e ¼ 2CO2 3
Cathode reaction
(1)
¼ 3CO2 þ 4 e C þ 2CO2 3
Anode reaction
(2)
C þ O2 ¼ CO2
Net cell reaction; n ¼ 4
(3)
One of many possible alternative reactions yielding 2 electrons per mole of carbon may be represented: C þ CO2 3 ¼ CO2 þ CO þ 2e
Anode reaction; n ¼ 2
(4)
Following Coleman and White [5], Hemmes and Cassir [6] made use of matrix algebra techniques to determine the minimum number of chemical equations (three) necessary to describe the complex DCFC system under constraints of equilibrium. The carbonate will dissociate at equilibrium according to: 2 CO2 3 ¼ CO2 þ O
(5)
Assuming that CO and CO2 concentration in the off gas are independent variables and that the anode system is at equilibrium, then reactions (2) and (4) along with (5) fully describe the anode system, as will any linear combinations of these three. For example, subtracting reaction (2) from twice reaction (4) yields the familiar Boudouard reaction that may occur in the gas phase contacting the carbon: C þ CO2 ¼ 2CO
(6)
This equilibrium together with (2) also fully describes the anode half cell. However, in anticipation of the reaction pathway described in Fig. 2, we regard the net anode reaction as a combination of the dissociation reaction (5) and anode
Fig. 1 e Schematic of a direct carbon fuel cell. Electrons from anodic oxidation of carbon pass directly from elemental carbon to the current collector.
Fig. 2 e Reaction pathway for our model is related to that accepted for the Hall cell anode in cryolite-alumina melts, except the formation of the free oxide ion results from carbonate decomposition rather than by aluminooxyfluoride decomposition. In both models, step [6] is assumed to be rate-determining.
reactions (2) and (4). To add the Boudouard equilibrium (6) to a description already defined by (2), (4) and (5) over-constrains the system of equations. Still, Boudouard corrosion of carbon (forward direction of (6)) may occur at other locations such as parts of the fuel mix that are physically isolated from the electrochemical reaction interface or out of electrical contact with the current collector. Such parasitic losses increase strongly with temperature. Moreover, chemical equations are not the same as chemical reactions even at equilibrium [5]. Equations (2) and (4) must yet be shown to describe the anode reactions. It is the purpose of this communication to re-examine published data on the carbon/carbonate anode in order to explain why some carbon materials and configurations are reported to have extraordinarily high electrochemical efficiency, while other studies seem to find unacceptable levels of polarization or CO production for efficient fuel cell operation. Our approach begins by noting that most polarization curves show a distinct inflection that separate low- and high potential regions having different slopes and curvatures. We will use reaction pathway models from the literature to interpret the separate dependences of low- and high potential regions on melt convection and specific area (total pore area/electrode volume) that we report here. Finally, we will suggest that the inflection signifies a shift in the predominant location of the anodic reaction from the interior of the porous anode to the exterior. Then we will outline a possible route to improve fuel efficiency.
1.1.
Thermodynamic basis and definition of efficiency
An enduring interest in carbon fuel cells derives from uniquely favorable thermodynamics associated with the cell reaction (3). Not only is the entropy change of reaction nearly zero, but pure carbon reactant and the CO2 product occupy separate phases at unit activity. The carbon activity is not degraded by the CO2 reaction product (as hydrogen fuel activity may be decreased by dilution with product steam in SOFC). This indicates the possibility of full utilization of the carbon fuel in a single pass through the cell. The total efficiency h total of a carbon fuel cell is defined as the ratio of electrical energy output to the heat of reaction, typically referenced to the standard enthalpy of the combustion reaction, C þ O2 ¼ CO2, DH ¼ 393.5 kJ/mol-C
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(94.05 kcal/mol). Total efficiency may be formally resolved into a product of partial efficiency factors: htotal ¼ hutil hcoul hvðiÞ hnernst htheor htheor ¼
DG+ TDS y1:007 T ¼ 650 750C DHT
(7)
(8)
where hvðiÞ hNernst ¼
VðiÞ EN VðiÞ ¼ + DG+o =nF EN V
(9)
and hutil ¼ 1
(10)
Theoretical efficiency, htheor, is the ratio of Gibbs free energy to the enthalpy of reaction under standard conditions (3). The coulombic efficiency, hcoul, is defined as the ratio of integrated cell current to that calculated for 4-electron transfer per atom of carbon. The Nernst efficiency hNernst is that fraction of the theoretical potential of the cell available when the gas composition corresponds to operational partial pressures of oxygen and carbon dioxide at operating current. Typically, hNernst w0.91 for the average gas compositions in the DCFC at ambient total pressure, where cathode input gas molar composition is 1/7 O2, 2/7 CO2 and 4/7 N2. The voltage efficiency hV(i) is defined as the ratio of the current-dependent cell voltage V(i) to the Nernst potential, EN ¼ 0.93 V. However, we combined the Nernst and voltage efficiencies into a single factor V(i)/V where V ¼ DGo /nF ¼ 1.022 V and n ¼ 4. F is the Faraday constant, 96500 C/equiv. Utilization efficiency hutil represents the fraction of carbon that may be consumed in a single refueling operation and is taken as 1.0. With these definitions, total efficiency (7) reduces to a simple product of a coulombic efficiency and voltaic efficiency: htotal zhcoul
VðiÞ V+
(11)
2. Mechanisms for the carbon electrode in the hall process: Background and possible relevance The production of substantially pure CO2 from carbon/ carbonate electrodes is often reported (see section 4) and may involve kinetic inhibitions of reactions that otherwise would lead to CO as favored by thermodynamics. While cryolite/ alumina electrolytes used in Hall smelters have different acid/ base properties than carbonates, Hall cell reactions may still be relevant to C/CO2 3 anodes. The accepted reaction mechanism for the carbon anode in Hall-Heroult aluminum smelters consists of a series of 1electron oxidation steps preceded by adsorption of oxide ions. We refer to this mechanism as the "Alcoa model" [7]. This model is used to explain the formation of pure CO2 even at temperatures of over 1100 C in carbon/cryolite-alumina half cells [31]. The anode reaction sequence is initiated by the formation of oxide ions from the decomposition aluminum ¼ O2 þ Al2OF4). (cf. oxyfluoride decomposition (Al2O2F2 4 Fig. 2). Adsorption of oxide occurs twice in the sequence. The
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second adsorption constitutes the rate-determining step for the 4-electron net process and forms a eCOO2- group that is then rapidly oxidized to CO2. The Alcoa model is related to the mechanism that we propose (Fig. 2) in important respects. The decomposition of alumino-oxyfluoride to form O2- is replaced, in our model, by 2 dissociation of carbonate, CO2 3 ¼ CO2 þ O , as suggested by Cherepy et al. [23]. Vutetakis cites the dissociation constant (K973 K ¼ PCO2 xO2/xCO32) to be 6.99 108 atm [8]. The Alcoa model assumes step 6 to be rate-determining, and it is logical but un-proven that step 6 should be rate-determining in the carbonate melt. The second adsorption step of charged O2- ion adjacent to an electronegative -CsCO site is hindered by charge repulsion and may require considerable overpotential. The first five steps in our mechanism have the net overall reaction as reaction (4), leading to a mole fraction of 0.5 for the CO2 in the off-gas and hcoul ¼ 0.5. If the sequence continues after a second O2- adsorption at an active site -CsCO to form eCOO2- then a net four electron transfer occurs according to (2) and the coulombic efficiency should approach 100%.
3. Literature relevant to determination of efficiency Coulombic efficiency may be determined from weight loss of the carbon anode or by analysis of the off-gas composition for relative amounts of CO and CO2. We prefer to use gas analysis from which the apparent valence for carbon oxidation may be inferred from a mass balance on the cell, as described elsewhere [9,13]. Gas analysis avoids problems of accurate weight determinations of porous electrodes; moreover, electrodes may disintegrate in the cell and are difficult to clean. Coulombic efficiency could also be inferred from assumed reactions by comparing the moles of gas evolved per unit charge passed (e.g., for reaction (2) this ratio is 3/4 ), but this approach requires adoption of a certain reaction stoichiometry without proof of its validity.
3.1.
Rigid anodes
Early work by Baur [10] concluded that a practical cell voltage would require very high operating temperature (w900 C), where the predominant product would be CO and not CO2. Tamaru and Kamada used both weight loss and gas analysis to conclude that CO2 was the predominant product of the electrolysis of charcoal anodes contained in a porous magnesia crucible separator [11]. The electrolyte consisted of mixed alkali carbonates in some cases with addition of halides (Cl or F). Apparent coulombic efficiencies of 1.1e1.2 were found at 600e650 C from gas composition and the open circuit potential approached 1.0 V above 750 C, but experimental conditions were not fully reported. Hauser studied the anodic oxidation of very dense graphite rods from 650 to 880 C [12]. He found predominantly CO2 at 50e100 mA/cm2 and coulombic efficiencies of >95% in the range 650e800 C. Salient results appear in Fig. 3A, where we have converted reported mole ratios (x ¼ [CO]/[CO2]) to coulombic efficiency using the equation, hcoul ¼ (1/2)(xþ2)/ (2xþ1) (cf. equation (10) in Cooper and Selman [13].) Plots of
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Fig. 3 e A. Ratios [CO]/[CO2] reported by Hauser are reduced to coulombic efficiency by this author. Data for 870 C on right axis. B Anode polarization, graphed against log(current density), exhibits Tafel behavior. After Hauser [12].
overpotential (ref. Au/(0.5 O2, 0.5 CO2) against log (i) were linear, suggesting Butler-Volmer kinetics (Fig. 3B). Hauser noted a slight shift of the Tafel slope at high temperatures and current densities < 5 mA/cm2. Sluggish kinetics of the dense, highly ordered graphite severely limited power and therefore practical applications. The coulombic efficiency at 870 C is low at current densities below 50 mA/cm2, possibly indicating a larger contribution from Boudouard corrosion, C þ CO2 ¼ 2CO at low overall currents. Weaver and colleagues measured anode polarizations on a number of rigid or compacted carbon anodes. Rates ranged from 0.1 mA/cm2 for dense, monolithic graphite to 100 mA/cm2 for bituminous coal chars at a fixed anode potential of 0.8 V vs. Au/(2/3 CO2, 1/3 O2) [14]. Weaver and Nanis showed that the proportion of CO2 in the off-gas increases linearly with current density for anodes of pyrolyzed Pocahontas and Peabody coal at 709e712 C [15,16]. (Fig. 4A) Contamination of the electrolyte with up to 20% coal ash had no observable effect on anode polarization. Rods and plates of pyrolyzed Kentucky No. 9 bituminous coal were fabricated by extruding particles together with coal tar or pitch followed by pyrolysis. Electrolysis yielded CO2 with coulombic efficiencies reported to be above 98% in half cell experiments in the range 650e725 C, at an overpotential of 0.2 V [17] (Fig. 4B). In the latter experiments using anodes fabricated from pyrolyzed Kentucky No. 9 coal, anode potential and coulombic efficiency data were measured for the same anode. Cooper
Fig. 4 e A. The coulombic efficiency is given for rigid electrodes made of bituminous coal chars and for FC-12, a graphitized carbon (Pure Carbon Company, St. Marys, PA) at temperatures of 709e712 C. ([15,16]). B Current efficiency and mole percentage CO2 are given at 100 mA/ cm2 as function of temperature for Kentucky No. 9 char [17]. Curve-fitting is for 3-rd order polynomial linear regression.
and Selman [13] combined polarization and coulombic data for this electrode with air electrode polarization data for a NiO(Li) cathode [18] to estimate the total efficiency from cell performance as a function of superficial power density, showing 80% maximum efficiency and peak power density in the range of 0.9e1.1 kW/m2. Power curves are reproduced from [13] in Fig. 5.
3.2.
Slurry anodes
Vutetakis [20] and more recently Zhu et al. [21] measured anode potentials on particles of diverse morphology in stirred electrolyte (Li,K,Na)2CO3, using gold or graphite current collector rods. Zhu confirmed Vutetakis’ key result, namely that CO2 product predominates at 700 C at 0.1 A/cm2. Both measured the differential increase of CO2 and CO in the offgas upon stepping current from zero, then subtracting the background "current-off" gas evolution of CO2 and CO. From research using slurry anodes, Zhu found a dependence of rate on various anode pretreatments that promote wetting, and
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4. Analysis: segmentation of anode polarization curves and dependence on experimental conditions
Fig. 5 e Depicted is the dependence of total efficiency on power density for Kentucky No. 9 char [13], as derived from electrode polarization, inter-electrode IR drop, and the dependence of coulombic efficiency on temperature [17e19]. Cathode polarization based on data from Morita [19].
We find that the polarization curve of anodes of diverse substances and configurations often breaks into two distinct segments. A linear, low-current segment (which we call the "LCS") is followed by a high current segment ("HCS") which is curved and has a lower differential resistance. Examples shown in Fig. 6A and B cover a wide range of diverse transport conditions, anode configurations (rigid, particle bed, slurry), and anode morphologies (dense to porous; ordered graphitic to highly disordered). Therefore the segmentation cannot be an artifact of the experimental technique. Segmentation is
investigated of raw coal as well as nanofibers produced by catalytic decomposition of methane [21,22,27,29]. Net efficiency was degraded by Boudouard corrosion occurring in the bulk of the stirred slurry out of electrical contact with the current collector.
3.3.
Dependence of rate on atomic-level disorder
Weaver [14] noted the 3-orders of magnitude difference between rates at fixed polarization of graphite and disordered bituminous chars. Cherepy et al. [23] conducted tests in cells combining a packed bed anode, porous ceramic separator, and gas diffusion cathode (NiO[Li]) to produce cells with rates to 150 mW/cm2 using reactive carbons. This work confirmed a dependence of rate on crystallographic factor (a measure of atomic disorder combining graphene plane spacing and the scale of basal plane micro-crystallinity, La). Electronic conductivity was found to be important, but total area and purity were deemed to be of secondary importance. Cao also found strong effects of surface functional groups and fluoride treatments of charcoal. HF treatment of the carbon reportedly removed metal oxide impurities and increased the degree of disorder at the thus uncovered reacting interface. The increased disorder led to 100% increase in rate while the mesopore area increased by only 5e7% [24]. A strong dependence of rate on atomic-level disorder was derived from physical and electrochemical factors by Yongdan Li and coworkers [25]. This model defines a reduced electrochemical activity associated with highly ordered structures. They developed a physical model based on the known microstructure of graphitic carbon and scaled the model using polarization and complex impedance measurements on dense graphite anodes reported by Peelen [26]. This model gives a quantitative explanation for the dominant role of atomic disorder in reaction rates as observed by Weaver. The model is based on Butler-Volmer kinetics and the pathway of Fig. 2, but does not assign a rate-determining step.
Fig. 6 e A. Polarization curves showing an inflection point (arrow) separating low- and high potential regions in electrolysis of carbon slurries at 700 C. 6A-a is an disordered carbon black (Koppers Continex N220, o.1 mm particles) of low La [29]. 6A-b, activated carbon (Calgon BPC 0.25e0.45 mm) [21] 6A-c and -d are low and high rank raw coal, respectively [22]. B Traces a0, a1, a2 and a3 show polarization of Lignite-derived Darco activated carbon without stirring and with stirring at 600-, 800- and 1000 rpm, respectively [8,20]. 6B-b1 and 6B-b2 are cell voltages taken from a loosely packed bed of carbon particles, at 600 and 650 C respectively [28]. Curve 6B-c represents polarization of a carbon nanofiber produced by catalytic decomposition of methane at 600 C [27].
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most pronounced in the case of stirred-slurry anodes, particle beds at low temperatures, and rigid anodes at high temperatures (above 800 C). Many examples can be found in the publications of Vutetakis [8,20] and Zhu, Dicks et al. [20,21,27,29]. An example of an inflection in a rigid high density rod of graphite is found at 750 C by Peelen [26]. The sharpness of transition between segments rules out that it is merely the transition from low-current linear behavior to Tafel behavior in Butler-Volmer kinetics. Fig. 6A and B compare polarization in stirred slurries (600 rpm) at 700 C. Fig. 6A shows behavior of various carbons: 6A-a: carbon black (Koppers Continex N220, o.1 mm particles) of low La ¼ 2.9 nm and low conductivity, 1.5 S/cm [29]. 6A-b: Activated carbon (Calgon BPC 0.25e0.45 mm) [21]. 6 A -c and 6A-d are raw coals: Newland (low rank, 52% C) and Germancreek (high rank, 73% C), respectively [22]. Fig. 6B-a shows behavior of an activated carbon without stirring (a0) and at 600-(a1), 800- (a2) and 1000 rpm (a3), showing dependence on flow rate of the high potential branch, but not of the low potential branch; the carbon is a 5% slurry of lignite-derived Darco activated carbon (Vutetakis [8,20],). 6B-b shows cell voltage (not anode potential) of an unstirred carbon/air cell reacting fine activated carbon particles at 600 C (b1) and 650 C (b2) in a cell with a composite electrolyte (Li et al. [28]). 6B-c is for a carbon nanofiber produced by decomposition of methane at 600 C on NieCueAl2O3 catalyst [27]. In most cases, the inflections occur at a polarization of at least 150 mV relative to the standard potential of C/CO23 but a polarization as high as 600e700 mV are found at low temperatures in packed particle beds. In some cases the dependence of the polarization curves on convective transport or specific area can be inferred from published data. For Calgon activated carbon, the slope of the low-current segment (LCS) increases with specific area (pore area to volume ratio), such that current at fixed potential is directly proportional to this parameter. The current shows little dependence on specific area within the high current segment. (Fig. 7; after Zhu et al. [29]). Samples were prepared by progressively grinding an granular starting material (AC-L) with an initial specific area of 70 cm1. The rapid falloff of potential at high current density for the stock material alone may be an artifact of the preparation and the tendency of the material to fracture along large pores [Zhu, private discussions]. On another carbon black material (Koppers, Continex, 0.1e0.2 mm size) tested by Zhu [29], the current at a fixed overpotential (0.5 V) on the low current segment (LCS) shows weak dependence on external flow rate while the high current segment (HCS) shows a strong dependence on rotation rate. Moreover, anode potential falls off sharply at a critical current density on the HCS that increases with the stirring rate. There is insufficient data to determine a transport law, and a particle impact or diffusion model cannot be ruled out. (Fig. 8). A linear dependence of current density on impellor rotational rate may be found by plotting the data for Darco activated carbon (Fig. 6B, curves a0ea3.)
4.1.
Fig. 7 e A. Dependence of low-polarization segment on current density and on specific area, for samples of activated carbon (Calgon BPL, 4DA6 mm). Specific area (cmL1): AC-L, 84; AC, 131; AC-M, 181; and AC-S, 214. Reproduced with permission, from reference [29]. B Dependence of low-polarization segment on current density and on specific area, for indicated anode potential, for granulated activated carbon (Calgon BPL, 4DA6 mm). After Zhu [29].
often quasi-linear polarization segment at low current density and a segment of lower differential resistance at high current density. The break is most pronounced with slurry anodes of fine particles, and rigid porous anodes at high temperatures (see Peelen [26]). In packed bed anodes, the phenomenon is restricted to low temperatures. Vutetakis [20] and Zhu [29] noted that the low current segment is dominated by activation polarization; Zhu suggests that ohmic polarization and mass transport (concentration polarization) control rate at high current densities.
5.
Interpretation
5.1.
Low and high current segments
Summary
Our examination of scores of polarization curves brought us to the conclusion that nearly all show clear breaks between an
We interpret the typical segmentation of the carbon-anode polarization curve as a reflection of the changing relative contributions of interior reactions (at the carbon/electrolyte
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equilibrium expression ([CO2]/[CO]2 ¼ KB) and fixes anode potential by substitution of this expression into the Nernst equation for either reaction (2) or (4). As the current increases, the concentration of dissolved CO2 increases within the electrolyte-filled voids and its diffusion to the exterior surface of the anode is impeded by the tortuous path. The concentration can increase to supersaturated levels because of the excess pressure needed to induce bubble formation in pores of small dimensions. The increase of [CO2] depresses the oxide concentration (from dissociation equilibrium, 2 CO2 3 ¼ CO2 þ O ). This impedes the adsorption reaction (step [2], Fig. 2). Ionic resistance within the porous electrode may also increase if bubbles are formed in macro voids and block access to the mesopore surface. In summary, the potential rises with current density in the low current segment because of the progressive depletion of available sites for initial oxide adsorption, because of depletion of oxide by reaction, and depletion of oxide because of the shift in equilibrium (reaction [1], Fig. 2). Pore blockage by gas bubbles may also play a role, but there is no evidence of this.
5.1.2.
Fig. 8 e A. Dependence of anode potential on impellor rotation rate. (Koppers Continex 0.1e0.2 mm size). Reproduced with permission, from reference [29]. B Dependence of high polarization segment on current density and on stirring impeller rate for a sample of granular carbon black (Koppers Continex 0.1e0.2 mm size); after Zhu [29].
interface within in the pores) and exterior reactions (taking place on the outer surface of the electrode). This shift in predominate reaction site from the porous interior to the exterior surface follows the overall increase of anode potential. This increase in potential may be caused by the saturation of available sites for initial oxide adsorption (Rxn [2], Fig. 2). It is possibly enhanced by the depletion of free oxide within the pores (Rxn [1], Fig. 2) or by blockage of the larger pores with gas bubbles. At some critical overpotential, an exponential rise in the adsorption of an oxide ion on the carbon substrate near to an existing CseCO site shifts the predominate reaction from the 2-electron reaction (steps [2e5] in Fig. 2) to the 4-electron net transfer, i.e., reactions (steps [1]e [9]). The high current segment follows the Butler-Volmer model as derived by Y. Li and coworkers [25].
5.1.1.
Low currents
In the low current segment (LCS), current penetrates effectively into the porous structure and the electrode reacts with production of both CO and CO2, as described by the net stoichiometry (4). As both carbon oxides are present, coulombic efficiency is less than unity. Under open circuit conditions, the Boudouard equilibrium determines the ratio through the
High currents
As the potential increases and approaches that of the intersection of the LCS and HCS, progressively more of the CO that is produced by reaction (4) (steps [1]- [5], Fig. 2) remains electrosorbed on the surface and is converted to CO2 according to reaction (2). On the HCS, the rate-determining step [6] controls the overall reaction rate (steps [1] through [9]). Since the differential resistance of the high current segment is roughly an order of magnitude lower than that of the low current segment, reaction (2) quickly dominates the stoichiometry of the carbon oxidation as the total current is increased. Therefore the coulombic efficiency rapidly approaches unity as the potential is raised beyond that of the intersection of the two curves, and the onset of 4-electron transfer appears as a quasi-threshold effect.
5.1.3.
Support for the model
Where the low current density net reaction (2) predominates, the rate is proportional to the aggregate area of the pores and to specific area (units cm2/cm3). This is observed. To the extent that exterior surface reactions predominate, the reaction tends towards high overpotential and a 4-electron net transfer per mole (hcoul / 100%). Coulombic efficiency generally increases with current density (Figs. 3, 4). Because a soluble liquid product is formed (supersaturated CO2) or consumed (oxide concentration at the interface) in a way which directly controls rate, one expects that the HCS reaction(s) should be ultimately limited by solution-side mass-transport. This also is observed. Support for this interpretation comes from the dependence of the current density on flow rate and the sharp fall off in cell voltage above a critical current density that increases with flow. In the low current segment, reaction (4) occurs in the interior and reaction (2) occurs on the exterior surface to a small degree. The total current is the sum of the two partial currents (Fig. 9). The short segment on the Tafel curve at currents below the intersection of LCS and HCS curves appears quasi-linear; since the LCS is linear, then the sum of the two segments also appears linear.
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Fig. 9 shows a reconstruction of the two-segmented polarization curve for graphite as modeled by Li [25]. The Tafel segment (HCS) is computed for La ¼ 30 nm, an intermediate ordered graphite. The HCS assumes the relation derived by Li, with T ¼ 1023 K, jo ¼ 1 mA/cm2, and b ¼ 0.5: j jo ¼ 4ð1 bÞF RT ln
hact
(12)
[For simplicity, we neglected to include Li’s factor for reduced carbon activity for this structure.] The resistance of the LCS (11 U-cm2) is typical of many of the curves of Fig. 6, depends on porosity and specific area, and cannot be predicted from the model. The segment originates at the open circuit potential (1.12 V vs. SHE) which is fixed by the relative concentrations of CO and CO2 under conditions of Boudouard equilibrium at zero current, as calculated by Hemmes and Cassir [6]. Finally, the progressive shift from the low- to the high current segment should produce a continuous increase in hcoul with current density, as first reported by Hauser [12] and Weaver [15].
5.1.4.
Boudouard reaction
CO2 produced throughout the discharge might be expected to react with carbon at the surface or interior to produce CO (reverse Boudouard reaction). This interpretation predicts that it can and does to the extent defined and limited by the linear combination of equations (2) and (4). To assume an additional degree of freedom by introducing independent Boudouard corrosion equation at the same reacting interface would overconstrain the system. The kinetics of the Boudouard reaction are quite complex and the topic is reviewed by Laurendeau [33], Calo and Perkins [34], and Hemmes and Cassir [6]. Our model does not require explicit inclusion of Boudouard kinetics. Table 1 gives the equilibrium constant (Keq ¼ [CO]2/[CO2]), mole fraction of CO2
Table 1 e Boudouard equilibrium data, mole fraction CO2 and hcoul, B assuming Boudouard gas composition; measured hcoul for Kentucky No. 9 char at 0.1 A/cm2 (Fig. 4b). T, C
Keq, atm
Mole fraction CO2
hcoul, B
hcoul (Fig. 4b)
650 700 750
0.288 0.914 2.59
0.586 0.397 0.229
0.54 0.43 0.34
0.995 0.986 w0.95
and coulombic efficiency corresponding to this mole fraction together with measured efficiency for Kentucky No. 9 char [17]. Clearly, even in the low current region, off-gas composition is not determined by the Boudouard equilibrium except at open circuit. Hauser concluded that the net four-electron transfer reaction he observed consisted of sequential reactions beginning with (4) (Fig. 2, steps [1]e [5]) and leading to production of CO2 via reaction (2) (steps [1]e [9]). Hauser’s model did not specify whether the CO intermediate remained chemically or physically adsorbed on the surface, or alternatively briefly entered the gaseous or solution phase. In both the modified Alcoa model and the Hauser model, the adsorption of O2- onto the surface at the -CsCO site is the ratedetermining step and requires considerable overpotential (75e150 mV) to overcome charge repulsion between -CsCO and an adjacent adsorbed O2. In the extreme cases of higly ordered carbons (such as studied by Hauser) only the HCS is observed and the i-V behavior is that of Butler-Volmer kinetics. High coulombic efficiencies (CO2) and low voltaic efficiencies are expected and found. At the other extreme of high surface-area carbon materials, the local pore current density remains low, and low coulombic efficiencies and high apparent voltage efficiencies are expected and found. Perhaps the least efficient configuration for DCFC is a melt-saturated particle bed of highly porous carbon and low pore current density. The most efficient fuel morphology for carbon fuel cells might be found with highly disordered carbon that is compressed into an electrode of low porosity.
6.
Fig. 9 e Reconstruction of low and high current segments of graphitic carbon. HCS, La [ 30 nm, T [ 1023 K, io [ 1 mA/cm2. Li model [25]; LCS resistance is 11 U-cm2. The curve originates at an open circuit potential (L1.12 V vs the standard potential of the C/CO2 electrode) and is determined by the Boudouard equilibrium gas composition at open current.
Conclusions
While a very large number of carbon materials have been characterized physically and electro chemically, there is still insufficient data pertaining to a single carbon material to fully verify this interpretation. In particular, simultaneous measurements of off-gas composition and anode polarization need to be undertaken within an operating cell using an oxygen electrode. The separate dependences of current density on flow rate and specific area need to be measured for each of many materials. The apparent dependence of potential on flow rate might be investigated with a rotating disk to infer transport mechanism. The oxygen electrode is needed as a counter electrode to assure the transference of carbonate (or oxide ion, with YSZ or ceria separator) into the anolyte that underlies the equations for current efficiency in terms of measured CO2 mole fraction or CO/CO2 mole ratios.
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We have also neglected the important role of electrode wetting by the melt, as investigated by Cao [24] and by Zhu [21]. These phenomena are examined further and modeled by one of us, in publications concurrent with this communication [32,a,b,c,]. The structure-based kinetic model of Li [25] should be combined with porous electrode theory [30] for development of reaction distribution computational models and tested on a few well-characterized carbon samples under well-defined experimental conditions. A rate equation describing the branching pathway of Fig. 2 needs to be evaluated for reasonable values of the electrochemical factors and rate constants of individual steps. To achieve both high coulombic- and voltage efficiencies, it appears necessary to start with highly disordered carbon materials with high specific electronic conductivity. Next, the proportion of current flowing to the exterior electrode surface might be increased by either of two methods: (1) The mesopore volume may be blocked by, for example, filling the pores with a solid, off-eutectic salt composition that melts at a temperature higher than that of cell operation. The salt will dissolve into the eutectic melt as it becomes exposed by the progressive retraction of the electrode during oxidation. The off-eutectic constituents of different feed-stocks can be balanced on either side of the eutectic composition, so that solution of the plug results in an invariant melt composition. Apparently, Weaver inadvertently achieved pore blockage by repeatedly filling the pore structure with fluid coal tar and pitch as part of a multistep pyrolysis process used in preparing his electrodes [18]. Pore-blocking techniques can be integrated with anode briquette or plate formation at minimal cost. Aluminum smelter anodes are manufactured using binders that reduce porosity and increase conductivity. (2) The other means by which 4-electron transfer might be achieved would be to use fine particles of dense, low-porosity disordered carbon which tend to increase the proportion of the current flowing to the exterior of the particle relative to the interior because of the high surface-area to volume ratio. In this approach, it is necessary curtail Boudouard corrosion in the bulk of the slurry–difficult challenges to flow engineering and the chemistry of reaction inhibition.
[4]
[5]
[6]
[7] [8]
[9]
[10] [11]
[12]
[13] [14]
[15]
[16]
[17]
[18]
Acknowledgments [19]
We gratefully acknowledge partial support for participation in the Carbonate Fuel Cell Conferences in Paris and Naples, from respectively, E´cole Nationale Supe´rieure de Chimie de Paris, and ENEA.
references
[1] Liebhafsky HA, Cairns EJ. Fuel cells and fuel batteries. New York: John Wiley & Sons; 1968. [2] Cao D, Sun Y, Wang G. Direct carbon fuel cell: fundamentals and recent developments. J Power Sources 2007;167:250e7. [3] Cooper JF. Direct conversion of coal derived carbon in fuel cells. In: Basu S, editor. Recent trends in fuel cell science and
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technology. New Dehli, India: Anamaya Publishers; 2005 [Chapter 10]. Cooper JF, Selman JR. Prospects for conversion of elemental carbon from fossil or renewable sources in fuel cells. Global challenges of electrochemistry symposium. Electrochemical Soc; Oct 2011. Coleman DH, White RE. Linear Algebra used to determine independent half cell equations. J Electrochem Soc 1996; 143(6):1781. Hemmes K, Cassir M. A theoretical study of the carbon/ carbonate/ hydroxide electrochemical system in a direct carbon fuel cell. ASME Proc fuel cell science, engineering, and technology; June 2004. No. 2497. Frank W, Aluminum Haupin W. Ullmann’s encyclopedia of industrial chemistry. 5th ed., vol A1. Weinheim: FRG: VCH; 1985. Vutetakis DG, Skidmore DR, Byker HJ. Electrochemical oxidation of molten carbonate-coal slurries. J Electrochem Soc 1987;134(12):3027. Cooper JF. Determination of efficiency in direct carbon fuel cells. Proc Solid Fuels Symposium. October 2011. ECS Trans, 2012, in press. Baur E, Ehrenberg H. Z Elektrochem 1912;18:1002; Baur E, Peterson A, Fuelleman G. Z Elektrochem 1916;22:409. Tamaru S, Kamada M. Brennstoffketen, deren arbeitstemperature unterhalb 600C liegt. Z Elektrochem 1935;41:93. Hauser VE. A study of carbon anode polarization in fused carbonate fuel cells. Ph.D. thesis. Oregon State University, 1964 June. Cooper JF, Selman JR. Electrochemical oxidation of carbon: a review. ECS Trans 2009;19(14):15. Weaver RD, Leach SC, Bayce AE, Nanis L. Direct electrochemical generation of electricity from coal. Report to US. DOE under Contract EY-76-C-03e0115. Menlo Park CA: Stanford Research Institute; 1979. Weaver RD, Nanis L. Electrochemical oxidation of carbon in molten carbonate coal-air fuel cells. Proc. 3rd Int Symp on Molten Salts. Electrochem Soc; 1981:81e9; Weaver RD. Direct electrochemical generation of electricity from coal. Menlo Park CA: Stanford Research Institute; July 1976. Quarterly report No. 1. Weaver RD, Tietz L, Cubicciotti D. Direct use of coal in a fuel cellefeasibility investigation. Report EPA-650/2-75-040. Menlo Park CA: Stanford Research Institute; June 1975. Weaver RD, Yasuda M, Bayce AE, Nanis L. Direct electrochemical generation of electricity from coal. Figure 5, SAN-15-105-1. Menlo Park CA: SRI International; May 1977. Weaver RD, Leach SC, Bayce AE, Nanis L. Direct elctrochemical generation of electricity from coal. Report SAN-0115/105e1. Menlo Park CA: SRI International; May 16 1977 to February 15 1979. Morita H, Mugikura Y, Izaki Y, Watanabe T, Abe T. Proc Electrochem Soc 1997;97(4):151. Vutetakis DG, Electrochemical oxidation of carbonaceous materials dispersed in molten salt, Ph.D. dissertation. Columbus OH: Ohio State University; 1985. Li X, Zhu Z, Chen J, De Marco R, Dicks A, Bradley J, et al. Surface modification of carbon fuels for direct carbon fuel cells. J Power Sources 2009;186:1. Li X, Zhu Z, De Marco R, Bradley J, Dicks A. Evaluation of raw coals as fuels for direct carbon fuel cells. J Power Sources 2010;195:4051e8. Cherepy NJ, Krueger R, Fiet K, Jankowski A, Cooper JF. Direct conversion of carbon fuels in a molten carbonate fuel cell. J Electrochem Soc 2005;152(1):A80. Cao D, Wang G, Wang C, Wang J, Lu T. Enhancement of electrooxidation activity of activated carbon for direct carbon fuel cell. Int J Hydrogen Energy 2010;35:1778e82.
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[25] Li H, Liu Q, Li Y. A carbon in molten carbonate anode model for a direct carbon fuel cell. Electrochim Acta 2010;55:1958. [26] Peelen WHA, Olivry M, Au SF, Fehribach JD, Hemmes K. Electrochemical oxidation of carbon in a 62/38 mol% Li/K carbonate melt. J Appl Electrochem 2000;30:1389. [27] Li X, Zhu Z, De Marco R, Bradley J, Dicks A. Carbon nanofibers syntheized by catalytic decomposition of methane and their electrochemical performance in a direct carbon fuel cell. Energy & Fuels 2009;23:3721e31. [28] Jia L, Tian Y, Liu Q, Xia C, Yu J, Wang Z, et al. A direct carbon fuel cell with (molten carbonate)/(doped ceria) composite electrolyte. J Power Sources 2010;195:5581e6. [29] Li X, Zhu ZH, De Marco R, Dicks A, Bradley J, Liu S, et al. Factors that determine the performance of carbon fuels in the direct carbon fuel cell. Ind Eng Chem Res 2008;47(23):9670e7. [30] Newman J, Thomas-Alyea KE. Electrochemical systems. New Jersey: John Wiley & Sons; 2004.
[31] Thonstad J. The electrode reaction on the C, CO2 electrode in cryolite-alumina melts. I. Steady state measurements; II: impedance measurements. Electochim Acta 1970;15:1569. [32] aChen CC, Maruyama T, Hsieh PH, Selman JR. Wetting behavior of carbon in molten carbonate. submitted for publication, J Electrochem Soc, 2011.bChen C, Maruyama T, Hsieh, PH, Selman JR. Carbon corrosion in direct carbon fuel cells. Submitted to Electrochim Acta, 2011.cChen C, Selman JR. Anode modeling of a molten carbonate based direct carbon fuel cell. Submitted to J Power Sources, 2011. [33] Laurendeau NM. Heterogeneous kinetics of coal char gasification and combustion. Prog Energy Combust Sci 1988; 4:221e71. [34] Calo JM, Perkins MT. A heterogeneous surface model for the "steady-state" kinetics of the Boudouard reaction. Carbon 1987;25(3):395.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Analysis of the carbon anode in direct carbon conversion fuel cells John F. Cooper a,*, J. Robert Selman b a b
John F. Cooper Consulting, LLC, 1971 Arrowhead Drive, Oakland, CA 94611, USA Dept of Chem and Biol Eng, Illinois Institute of Technology, 10 W. 33rd St., Chicago, Il 60616, USA
article info
abstract
Article history:
The total electrochemical efficiency of a direct carbon fuel cell with molten carbonate
Received 1 July 2011
electrolyte is dominated by the product of coulombic efficiency (electron yield (n) per carbon
Received in revised form
atom, divided by 4) and voltaic efficiency (ratio of cell voltage to theoretical voltage). The
5 March 2012
voltaic efficiency is acceptably high (70e80%) for many atomically-disordered carbon
Accepted 17 March 2012
materials. High coulombic efficiency is more difficult to achieve but ranges from below 50%
Available online 24 April 2012
at low current densities in porous material to 100% in certain monolithic and particulate carbon anodes at high current densities where substantially pure CO2 is the product gas.
Keywords:
We find evidence for two competing anode reactions associated with distinct low- and high
Direct carbon fuel cell
polarization segments, respectively: (1) a charge-transfer controlled, linearepolarization
Carbon anode efficiency
reaction occurring predominately within pores, proportional to specific area, and tending
Carbon anode in molten carbonate
toward low efficiency by co-production of CO and CO2; and (2) a flow-dependent reaction
Reaction distribution
occurring on the exterior surface of the anode, requiring > 100 mV polarization and tending
in carbon anodes
to produce CO2. Based on this interpretation, high electrochemical efficiency of a carbon fuel cell is expected with anodes made of atomically disordered ("turbostratic") carbon that have negligible porosity, or with anodes of disordered carbon for which interior pores are intentionally blocked with an impervious solid material, such as an inert salt or readily carbonized pitch. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
A century-old goal of electrochemistry has been a fuel cell that could generate electricity from elemental carbon derived from fossil or biological resources. This would avoid the thermal inefficiencies of heat engines and the pollution associated with combustion. Early research on this type of fuel cell (which we call "direct carbon fuel cell", or DCFC) has been reviewed by Liebhafsky and Cairns [1]. Later work is
summarized by Cao [2] and by Cooper [3]. In the last five years, there has been a resurgence of interest in the DCFC that is driven by increased emphasis on the efficiency of use of carbon fuels [4]. Electrochemical conversion of carbon is found in a variety of concepts. For example, these include: (1) designs resembling the gas-fueled molten carbonate fuel cell (MCFC) but with a solid carbon anode; (2) solid oxide fuel cells in which carbon particles are directly fed into the anode channel, (3) cells with carbon anodes in combination with
* Corresponding author. Tel./fax: þ1 510 339 9401. E-mail address: [email protected] (J.F. Cooper). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.03.095
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parallel solid oxide and molten salt electrolyte, and (4) systems in which gasification of carbon to CO is integrated with electrochemical oxidation or combustion of CO. In general, direct carbon fuel cell (DCFC) refers to the galvanic oxidation of solid carbon with direct transfer of electrons from the solid carbon to a current collector, as depicted in Fig. 1. Such fuel cells operate at elevated temperatures (650e800 C) to overcome sluggish kinetics of reactions leading to the CO2 product and make use of molten carbonate and/or solid oxide electrolytes. In this communication, discussion of DCFC is limited to carbonate-based systems. Efficient operation requires balanced cathodic and anodic half reactions to produce a CO2 reaction product by means of the transfer of four electrons per unit of reaction: O2 þ 2CO2 þ 4 e ¼ 2CO2 3
Cathode reaction
(1)
¼ 3CO2 þ 4 e C þ 2CO2 3
Anode reaction
(2)
C þ O2 ¼ CO2
Net cell reaction; n ¼ 4
(3)
One of many possible alternative reactions yielding 2 electrons per mole of carbon may be represented: C þ CO2 3 ¼ CO2 þ CO þ 2e
Anode reaction; n ¼ 2
(4)
Following Coleman and White [5], Hemmes and Cassir [6] made use of matrix algebra techniques to determine the minimum number of chemical equations (three) necessary to describe the complex DCFC system under constraints of equilibrium. The carbonate will dissociate at equilibrium according to: 2 CO2 3 ¼ CO2 þ O
(5)
Assuming that CO and CO2 concentration in the off gas are independent variables and that the anode system is at equilibrium, then reactions (2) and (4) along with (5) fully describe the anode system, as will any linear combinations of these three. For example, subtracting reaction (2) from twice reaction (4) yields the familiar Boudouard reaction that may occur in the gas phase contacting the carbon: C þ CO2 ¼ 2CO
(6)
This equilibrium together with (2) also fully describes the anode half cell. However, in anticipation of the reaction pathway described in Fig. 2, we regard the net anode reaction as a combination of the dissociation reaction (5) and anode
Fig. 1 e Schematic of a direct carbon fuel cell. Electrons from anodic oxidation of carbon pass directly from elemental carbon to the current collector.
Fig. 2 e Reaction pathway for our model is related to that accepted for the Hall cell anode in cryolite-alumina melts, except the formation of the free oxide ion results from carbonate decomposition rather than by aluminooxyfluoride decomposition. In both models, step [6] is assumed to be rate-determining.
reactions (2) and (4). To add the Boudouard equilibrium (6) to a description already defined by (2), (4) and (5) over-constrains the system of equations. Still, Boudouard corrosion of carbon (forward direction of (6)) may occur at other locations such as parts of the fuel mix that are physically isolated from the electrochemical reaction interface or out of electrical contact with the current collector. Such parasitic losses increase strongly with temperature. Moreover, chemical equations are not the same as chemical reactions even at equilibrium [5]. Equations (2) and (4) must yet be shown to describe the anode reactions. It is the purpose of this communication to re-examine published data on the carbon/carbonate anode in order to explain why some carbon materials and configurations are reported to have extraordinarily high electrochemical efficiency, while other studies seem to find unacceptable levels of polarization or CO production for efficient fuel cell operation. Our approach begins by noting that most polarization curves show a distinct inflection that separate low- and high potential regions having different slopes and curvatures. We will use reaction pathway models from the literature to interpret the separate dependences of low- and high potential regions on melt convection and specific area (total pore area/electrode volume) that we report here. Finally, we will suggest that the inflection signifies a shift in the predominant location of the anodic reaction from the interior of the porous anode to the exterior. Then we will outline a possible route to improve fuel efficiency.
1.1.
Thermodynamic basis and definition of efficiency
An enduring interest in carbon fuel cells derives from uniquely favorable thermodynamics associated with the cell reaction (3). Not only is the entropy change of reaction nearly zero, but pure carbon reactant and the CO2 product occupy separate phases at unit activity. The carbon activity is not degraded by the CO2 reaction product (as hydrogen fuel activity may be decreased by dilution with product steam in SOFC). This indicates the possibility of full utilization of the carbon fuel in a single pass through the cell. The total efficiency h total of a carbon fuel cell is defined as the ratio of electrical energy output to the heat of reaction, typically referenced to the standard enthalpy of the combustion reaction, C þ O2 ¼ CO2, DH ¼ 393.5 kJ/mol-C
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(94.05 kcal/mol). Total efficiency may be formally resolved into a product of partial efficiency factors: htotal ¼ hutil hcoul hvðiÞ hnernst htheor htheor ¼
DG+ TDS y1:007 T ¼ 650 750C DHT
(7)
(8)
where hvðiÞ hNernst ¼
VðiÞ EN VðiÞ ¼ + DG+o =nF EN V
(9)
and hutil ¼ 1
(10)
Theoretical efficiency, htheor, is the ratio of Gibbs free energy to the enthalpy of reaction under standard conditions (3). The coulombic efficiency, hcoul, is defined as the ratio of integrated cell current to that calculated for 4-electron transfer per atom of carbon. The Nernst efficiency hNernst is that fraction of the theoretical potential of the cell available when the gas composition corresponds to operational partial pressures of oxygen and carbon dioxide at operating current. Typically, hNernst w0.91 for the average gas compositions in the DCFC at ambient total pressure, where cathode input gas molar composition is 1/7 O2, 2/7 CO2 and 4/7 N2. The voltage efficiency hV(i) is defined as the ratio of the current-dependent cell voltage V(i) to the Nernst potential, EN ¼ 0.93 V. However, we combined the Nernst and voltage efficiencies into a single factor V(i)/V where V ¼ DGo /nF ¼ 1.022 V and n ¼ 4. F is the Faraday constant, 96500 C/equiv. Utilization efficiency hutil represents the fraction of carbon that may be consumed in a single refueling operation and is taken as 1.0. With these definitions, total efficiency (7) reduces to a simple product of a coulombic efficiency and voltaic efficiency: htotal zhcoul
VðiÞ V+
(11)
2. Mechanisms for the carbon electrode in the hall process: Background and possible relevance The production of substantially pure CO2 from carbon/ carbonate electrodes is often reported (see section 4) and may involve kinetic inhibitions of reactions that otherwise would lead to CO as favored by thermodynamics. While cryolite/ alumina electrolytes used in Hall smelters have different acid/ base properties than carbonates, Hall cell reactions may still be relevant to C/CO2 3 anodes. The accepted reaction mechanism for the carbon anode in Hall-Heroult aluminum smelters consists of a series of 1electron oxidation steps preceded by adsorption of oxide ions. We refer to this mechanism as the "Alcoa model" [7]. This model is used to explain the formation of pure CO2 even at temperatures of over 1100 C in carbon/cryolite-alumina half cells [31]. The anode reaction sequence is initiated by the formation of oxide ions from the decomposition aluminum ¼ O2 þ Al2OF4). (cf. oxyfluoride decomposition (Al2O2F2 4 Fig. 2). Adsorption of oxide occurs twice in the sequence. The
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second adsorption constitutes the rate-determining step for the 4-electron net process and forms a eCOO2- group that is then rapidly oxidized to CO2. The Alcoa model is related to the mechanism that we propose (Fig. 2) in important respects. The decomposition of alumino-oxyfluoride to form O2- is replaced, in our model, by 2 dissociation of carbonate, CO2 3 ¼ CO2 þ O , as suggested by Cherepy et al. [23]. Vutetakis cites the dissociation constant (K973 K ¼ PCO2 xO2/xCO32) to be 6.99 108 atm [8]. The Alcoa model assumes step 6 to be rate-determining, and it is logical but un-proven that step 6 should be rate-determining in the carbonate melt. The second adsorption step of charged O2- ion adjacent to an electronegative -CsCO site is hindered by charge repulsion and may require considerable overpotential. The first five steps in our mechanism have the net overall reaction as reaction (4), leading to a mole fraction of 0.5 for the CO2 in the off-gas and hcoul ¼ 0.5. If the sequence continues after a second O2- adsorption at an active site -CsCO to form eCOO2- then a net four electron transfer occurs according to (2) and the coulombic efficiency should approach 100%.
3. Literature relevant to determination of efficiency Coulombic efficiency may be determined from weight loss of the carbon anode or by analysis of the off-gas composition for relative amounts of CO and CO2. We prefer to use gas analysis from which the apparent valence for carbon oxidation may be inferred from a mass balance on the cell, as described elsewhere [9,13]. Gas analysis avoids problems of accurate weight determinations of porous electrodes; moreover, electrodes may disintegrate in the cell and are difficult to clean. Coulombic efficiency could also be inferred from assumed reactions by comparing the moles of gas evolved per unit charge passed (e.g., for reaction (2) this ratio is 3/4 ), but this approach requires adoption of a certain reaction stoichiometry without proof of its validity.
3.1.
Rigid anodes
Early work by Baur [10] concluded that a practical cell voltage would require very high operating temperature (w900 C), where the predominant product would be CO and not CO2. Tamaru and Kamada used both weight loss and gas analysis to conclude that CO2 was the predominant product of the electrolysis of charcoal anodes contained in a porous magnesia crucible separator [11]. The electrolyte consisted of mixed alkali carbonates in some cases with addition of halides (Cl or F). Apparent coulombic efficiencies of 1.1e1.2 were found at 600e650 C from gas composition and the open circuit potential approached 1.0 V above 750 C, but experimental conditions were not fully reported. Hauser studied the anodic oxidation of very dense graphite rods from 650 to 880 C [12]. He found predominantly CO2 at 50e100 mA/cm2 and coulombic efficiencies of >95% in the range 650e800 C. Salient results appear in Fig. 3A, where we have converted reported mole ratios (x ¼ [CO]/[CO2]) to coulombic efficiency using the equation, hcoul ¼ (1/2)(xþ2)/ (2xþ1) (cf. equation (10) in Cooper and Selman [13].) Plots of
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Fig. 3 e A. Ratios [CO]/[CO2] reported by Hauser are reduced to coulombic efficiency by this author. Data for 870 C on right axis. B Anode polarization, graphed against log(current density), exhibits Tafel behavior. After Hauser [12].
overpotential (ref. Au/(0.5 O2, 0.5 CO2) against log (i) were linear, suggesting Butler-Volmer kinetics (Fig. 3B). Hauser noted a slight shift of the Tafel slope at high temperatures and current densities < 5 mA/cm2. Sluggish kinetics of the dense, highly ordered graphite severely limited power and therefore practical applications. The coulombic efficiency at 870 C is low at current densities below 50 mA/cm2, possibly indicating a larger contribution from Boudouard corrosion, C þ CO2 ¼ 2CO at low overall currents. Weaver and colleagues measured anode polarizations on a number of rigid or compacted carbon anodes. Rates ranged from 0.1 mA/cm2 for dense, monolithic graphite to 100 mA/cm2 for bituminous coal chars at a fixed anode potential of 0.8 V vs. Au/(2/3 CO2, 1/3 O2) [14]. Weaver and Nanis showed that the proportion of CO2 in the off-gas increases linearly with current density for anodes of pyrolyzed Pocahontas and Peabody coal at 709e712 C [15,16]. (Fig. 4A) Contamination of the electrolyte with up to 20% coal ash had no observable effect on anode polarization. Rods and plates of pyrolyzed Kentucky No. 9 bituminous coal were fabricated by extruding particles together with coal tar or pitch followed by pyrolysis. Electrolysis yielded CO2 with coulombic efficiencies reported to be above 98% in half cell experiments in the range 650e725 C, at an overpotential of 0.2 V [17] (Fig. 4B). In the latter experiments using anodes fabricated from pyrolyzed Kentucky No. 9 coal, anode potential and coulombic efficiency data were measured for the same anode. Cooper
Fig. 4 e A. The coulombic efficiency is given for rigid electrodes made of bituminous coal chars and for FC-12, a graphitized carbon (Pure Carbon Company, St. Marys, PA) at temperatures of 709e712 C. ([15,16]). B Current efficiency and mole percentage CO2 are given at 100 mA/ cm2 as function of temperature for Kentucky No. 9 char [17]. Curve-fitting is for 3-rd order polynomial linear regression.
and Selman [13] combined polarization and coulombic data for this electrode with air electrode polarization data for a NiO(Li) cathode [18] to estimate the total efficiency from cell performance as a function of superficial power density, showing 80% maximum efficiency and peak power density in the range of 0.9e1.1 kW/m2. Power curves are reproduced from [13] in Fig. 5.
3.2.
Slurry anodes
Vutetakis [20] and more recently Zhu et al. [21] measured anode potentials on particles of diverse morphology in stirred electrolyte (Li,K,Na)2CO3, using gold or graphite current collector rods. Zhu confirmed Vutetakis’ key result, namely that CO2 product predominates at 700 C at 0.1 A/cm2. Both measured the differential increase of CO2 and CO in the offgas upon stepping current from zero, then subtracting the background "current-off" gas evolution of CO2 and CO. From research using slurry anodes, Zhu found a dependence of rate on various anode pretreatments that promote wetting, and
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4. Analysis: segmentation of anode polarization curves and dependence on experimental conditions
Fig. 5 e Depicted is the dependence of total efficiency on power density for Kentucky No. 9 char [13], as derived from electrode polarization, inter-electrode IR drop, and the dependence of coulombic efficiency on temperature [17e19]. Cathode polarization based on data from Morita [19].
We find that the polarization curve of anodes of diverse substances and configurations often breaks into two distinct segments. A linear, low-current segment (which we call the "LCS") is followed by a high current segment ("HCS") which is curved and has a lower differential resistance. Examples shown in Fig. 6A and B cover a wide range of diverse transport conditions, anode configurations (rigid, particle bed, slurry), and anode morphologies (dense to porous; ordered graphitic to highly disordered). Therefore the segmentation cannot be an artifact of the experimental technique. Segmentation is
investigated of raw coal as well as nanofibers produced by catalytic decomposition of methane [21,22,27,29]. Net efficiency was degraded by Boudouard corrosion occurring in the bulk of the stirred slurry out of electrical contact with the current collector.
3.3.
Dependence of rate on atomic-level disorder
Weaver [14] noted the 3-orders of magnitude difference between rates at fixed polarization of graphite and disordered bituminous chars. Cherepy et al. [23] conducted tests in cells combining a packed bed anode, porous ceramic separator, and gas diffusion cathode (NiO[Li]) to produce cells with rates to 150 mW/cm2 using reactive carbons. This work confirmed a dependence of rate on crystallographic factor (a measure of atomic disorder combining graphene plane spacing and the scale of basal plane micro-crystallinity, La). Electronic conductivity was found to be important, but total area and purity were deemed to be of secondary importance. Cao also found strong effects of surface functional groups and fluoride treatments of charcoal. HF treatment of the carbon reportedly removed metal oxide impurities and increased the degree of disorder at the thus uncovered reacting interface. The increased disorder led to 100% increase in rate while the mesopore area increased by only 5e7% [24]. A strong dependence of rate on atomic-level disorder was derived from physical and electrochemical factors by Yongdan Li and coworkers [25]. This model defines a reduced electrochemical activity associated with highly ordered structures. They developed a physical model based on the known microstructure of graphitic carbon and scaled the model using polarization and complex impedance measurements on dense graphite anodes reported by Peelen [26]. This model gives a quantitative explanation for the dominant role of atomic disorder in reaction rates as observed by Weaver. The model is based on Butler-Volmer kinetics and the pathway of Fig. 2, but does not assign a rate-determining step.
Fig. 6 e A. Polarization curves showing an inflection point (arrow) separating low- and high potential regions in electrolysis of carbon slurries at 700 C. 6A-a is an disordered carbon black (Koppers Continex N220, o.1 mm particles) of low La [29]. 6A-b, activated carbon (Calgon BPC 0.25e0.45 mm) [21] 6A-c and -d are low and high rank raw coal, respectively [22]. B Traces a0, a1, a2 and a3 show polarization of Lignite-derived Darco activated carbon without stirring and with stirring at 600-, 800- and 1000 rpm, respectively [8,20]. 6B-b1 and 6B-b2 are cell voltages taken from a loosely packed bed of carbon particles, at 600 and 650 C respectively [28]. Curve 6B-c represents polarization of a carbon nanofiber produced by catalytic decomposition of methane at 600 C [27].
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most pronounced in the case of stirred-slurry anodes, particle beds at low temperatures, and rigid anodes at high temperatures (above 800 C). Many examples can be found in the publications of Vutetakis [8,20] and Zhu, Dicks et al. [20,21,27,29]. An example of an inflection in a rigid high density rod of graphite is found at 750 C by Peelen [26]. The sharpness of transition between segments rules out that it is merely the transition from low-current linear behavior to Tafel behavior in Butler-Volmer kinetics. Fig. 6A and B compare polarization in stirred slurries (600 rpm) at 700 C. Fig. 6A shows behavior of various carbons: 6A-a: carbon black (Koppers Continex N220, o.1 mm particles) of low La ¼ 2.9 nm and low conductivity, 1.5 S/cm [29]. 6A-b: Activated carbon (Calgon BPC 0.25e0.45 mm) [21]. 6 A -c and 6A-d are raw coals: Newland (low rank, 52% C) and Germancreek (high rank, 73% C), respectively [22]. Fig. 6B-a shows behavior of an activated carbon without stirring (a0) and at 600-(a1), 800- (a2) and 1000 rpm (a3), showing dependence on flow rate of the high potential branch, but not of the low potential branch; the carbon is a 5% slurry of lignite-derived Darco activated carbon (Vutetakis [8,20],). 6B-b shows cell voltage (not anode potential) of an unstirred carbon/air cell reacting fine activated carbon particles at 600 C (b1) and 650 C (b2) in a cell with a composite electrolyte (Li et al. [28]). 6B-c is for a carbon nanofiber produced by decomposition of methane at 600 C on NieCueAl2O3 catalyst [27]. In most cases, the inflections occur at a polarization of at least 150 mV relative to the standard potential of C/CO23 but a polarization as high as 600e700 mV are found at low temperatures in packed particle beds. In some cases the dependence of the polarization curves on convective transport or specific area can be inferred from published data. For Calgon activated carbon, the slope of the low-current segment (LCS) increases with specific area (pore area to volume ratio), such that current at fixed potential is directly proportional to this parameter. The current shows little dependence on specific area within the high current segment. (Fig. 7; after Zhu et al. [29]). Samples were prepared by progressively grinding an granular starting material (AC-L) with an initial specific area of 70 cm1. The rapid falloff of potential at high current density for the stock material alone may be an artifact of the preparation and the tendency of the material to fracture along large pores [Zhu, private discussions]. On another carbon black material (Koppers, Continex, 0.1e0.2 mm size) tested by Zhu [29], the current at a fixed overpotential (0.5 V) on the low current segment (LCS) shows weak dependence on external flow rate while the high current segment (HCS) shows a strong dependence on rotation rate. Moreover, anode potential falls off sharply at a critical current density on the HCS that increases with the stirring rate. There is insufficient data to determine a transport law, and a particle impact or diffusion model cannot be ruled out. (Fig. 8). A linear dependence of current density on impellor rotational rate may be found by plotting the data for Darco activated carbon (Fig. 6B, curves a0ea3.)
4.1.
Fig. 7 e A. Dependence of low-polarization segment on current density and on specific area, for samples of activated carbon (Calgon BPL, 4DA6 mm). Specific area (cmL1): AC-L, 84; AC, 131; AC-M, 181; and AC-S, 214. Reproduced with permission, from reference [29]. B Dependence of low-polarization segment on current density and on specific area, for indicated anode potential, for granulated activated carbon (Calgon BPL, 4DA6 mm). After Zhu [29].
often quasi-linear polarization segment at low current density and a segment of lower differential resistance at high current density. The break is most pronounced with slurry anodes of fine particles, and rigid porous anodes at high temperatures (see Peelen [26]). In packed bed anodes, the phenomenon is restricted to low temperatures. Vutetakis [20] and Zhu [29] noted that the low current segment is dominated by activation polarization; Zhu suggests that ohmic polarization and mass transport (concentration polarization) control rate at high current densities.
5.
Interpretation
5.1.
Low and high current segments
Summary
Our examination of scores of polarization curves brought us to the conclusion that nearly all show clear breaks between an
We interpret the typical segmentation of the carbon-anode polarization curve as a reflection of the changing relative contributions of interior reactions (at the carbon/electrolyte
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equilibrium expression ([CO2]/[CO]2 ¼ KB) and fixes anode potential by substitution of this expression into the Nernst equation for either reaction (2) or (4). As the current increases, the concentration of dissolved CO2 increases within the electrolyte-filled voids and its diffusion to the exterior surface of the anode is impeded by the tortuous path. The concentration can increase to supersaturated levels because of the excess pressure needed to induce bubble formation in pores of small dimensions. The increase of [CO2] depresses the oxide concentration (from dissociation equilibrium, 2 CO2 3 ¼ CO2 þ O ). This impedes the adsorption reaction (step [2], Fig. 2). Ionic resistance within the porous electrode may also increase if bubbles are formed in macro voids and block access to the mesopore surface. In summary, the potential rises with current density in the low current segment because of the progressive depletion of available sites for initial oxide adsorption, because of depletion of oxide by reaction, and depletion of oxide because of the shift in equilibrium (reaction [1], Fig. 2). Pore blockage by gas bubbles may also play a role, but there is no evidence of this.
5.1.2.
Fig. 8 e A. Dependence of anode potential on impellor rotation rate. (Koppers Continex 0.1e0.2 mm size). Reproduced with permission, from reference [29]. B Dependence of high polarization segment on current density and on stirring impeller rate for a sample of granular carbon black (Koppers Continex 0.1e0.2 mm size); after Zhu [29].
interface within in the pores) and exterior reactions (taking place on the outer surface of the electrode). This shift in predominate reaction site from the porous interior to the exterior surface follows the overall increase of anode potential. This increase in potential may be caused by the saturation of available sites for initial oxide adsorption (Rxn [2], Fig. 2). It is possibly enhanced by the depletion of free oxide within the pores (Rxn [1], Fig. 2) or by blockage of the larger pores with gas bubbles. At some critical overpotential, an exponential rise in the adsorption of an oxide ion on the carbon substrate near to an existing CseCO site shifts the predominate reaction from the 2-electron reaction (steps [2e5] in Fig. 2) to the 4-electron net transfer, i.e., reactions (steps [1]e [9]). The high current segment follows the Butler-Volmer model as derived by Y. Li and coworkers [25].
5.1.1.
Low currents
In the low current segment (LCS), current penetrates effectively into the porous structure and the electrode reacts with production of both CO and CO2, as described by the net stoichiometry (4). As both carbon oxides are present, coulombic efficiency is less than unity. Under open circuit conditions, the Boudouard equilibrium determines the ratio through the
High currents
As the potential increases and approaches that of the intersection of the LCS and HCS, progressively more of the CO that is produced by reaction (4) (steps [1]- [5], Fig. 2) remains electrosorbed on the surface and is converted to CO2 according to reaction (2). On the HCS, the rate-determining step [6] controls the overall reaction rate (steps [1] through [9]). Since the differential resistance of the high current segment is roughly an order of magnitude lower than that of the low current segment, reaction (2) quickly dominates the stoichiometry of the carbon oxidation as the total current is increased. Therefore the coulombic efficiency rapidly approaches unity as the potential is raised beyond that of the intersection of the two curves, and the onset of 4-electron transfer appears as a quasi-threshold effect.
5.1.3.
Support for the model
Where the low current density net reaction (2) predominates, the rate is proportional to the aggregate area of the pores and to specific area (units cm2/cm3). This is observed. To the extent that exterior surface reactions predominate, the reaction tends towards high overpotential and a 4-electron net transfer per mole (hcoul / 100%). Coulombic efficiency generally increases with current density (Figs. 3, 4). Because a soluble liquid product is formed (supersaturated CO2) or consumed (oxide concentration at the interface) in a way which directly controls rate, one expects that the HCS reaction(s) should be ultimately limited by solution-side mass-transport. This also is observed. Support for this interpretation comes from the dependence of the current density on flow rate and the sharp fall off in cell voltage above a critical current density that increases with flow. In the low current segment, reaction (4) occurs in the interior and reaction (2) occurs on the exterior surface to a small degree. The total current is the sum of the two partial currents (Fig. 9). The short segment on the Tafel curve at currents below the intersection of LCS and HCS curves appears quasi-linear; since the LCS is linear, then the sum of the two segments also appears linear.
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Fig. 9 shows a reconstruction of the two-segmented polarization curve for graphite as modeled by Li [25]. The Tafel segment (HCS) is computed for La ¼ 30 nm, an intermediate ordered graphite. The HCS assumes the relation derived by Li, with T ¼ 1023 K, jo ¼ 1 mA/cm2, and b ¼ 0.5: j jo ¼ 4ð1 bÞF RT ln
hact
(12)
[For simplicity, we neglected to include Li’s factor for reduced carbon activity for this structure.] The resistance of the LCS (11 U-cm2) is typical of many of the curves of Fig. 6, depends on porosity and specific area, and cannot be predicted from the model. The segment originates at the open circuit potential (1.12 V vs. SHE) which is fixed by the relative concentrations of CO and CO2 under conditions of Boudouard equilibrium at zero current, as calculated by Hemmes and Cassir [6]. Finally, the progressive shift from the low- to the high current segment should produce a continuous increase in hcoul with current density, as first reported by Hauser [12] and Weaver [15].
5.1.4.
Boudouard reaction
CO2 produced throughout the discharge might be expected to react with carbon at the surface or interior to produce CO (reverse Boudouard reaction). This interpretation predicts that it can and does to the extent defined and limited by the linear combination of equations (2) and (4). To assume an additional degree of freedom by introducing independent Boudouard corrosion equation at the same reacting interface would overconstrain the system. The kinetics of the Boudouard reaction are quite complex and the topic is reviewed by Laurendeau [33], Calo and Perkins [34], and Hemmes and Cassir [6]. Our model does not require explicit inclusion of Boudouard kinetics. Table 1 gives the equilibrium constant (Keq ¼ [CO]2/[CO2]), mole fraction of CO2
Table 1 e Boudouard equilibrium data, mole fraction CO2 and hcoul, B assuming Boudouard gas composition; measured hcoul for Kentucky No. 9 char at 0.1 A/cm2 (Fig. 4b). T, C
Keq, atm
Mole fraction CO2
hcoul, B
hcoul (Fig. 4b)
650 700 750
0.288 0.914 2.59
0.586 0.397 0.229
0.54 0.43 0.34
0.995 0.986 w0.95
and coulombic efficiency corresponding to this mole fraction together with measured efficiency for Kentucky No. 9 char [17]. Clearly, even in the low current region, off-gas composition is not determined by the Boudouard equilibrium except at open circuit. Hauser concluded that the net four-electron transfer reaction he observed consisted of sequential reactions beginning with (4) (Fig. 2, steps [1]e [5]) and leading to production of CO2 via reaction (2) (steps [1]e [9]). Hauser’s model did not specify whether the CO intermediate remained chemically or physically adsorbed on the surface, or alternatively briefly entered the gaseous or solution phase. In both the modified Alcoa model and the Hauser model, the adsorption of O2- onto the surface at the -CsCO site is the ratedetermining step and requires considerable overpotential (75e150 mV) to overcome charge repulsion between -CsCO and an adjacent adsorbed O2. In the extreme cases of higly ordered carbons (such as studied by Hauser) only the HCS is observed and the i-V behavior is that of Butler-Volmer kinetics. High coulombic efficiencies (CO2) and low voltaic efficiencies are expected and found. At the other extreme of high surface-area carbon materials, the local pore current density remains low, and low coulombic efficiencies and high apparent voltage efficiencies are expected and found. Perhaps the least efficient configuration for DCFC is a melt-saturated particle bed of highly porous carbon and low pore current density. The most efficient fuel morphology for carbon fuel cells might be found with highly disordered carbon that is compressed into an electrode of low porosity.
6.
Fig. 9 e Reconstruction of low and high current segments of graphitic carbon. HCS, La [ 30 nm, T [ 1023 K, io [ 1 mA/cm2. Li model [25]; LCS resistance is 11 U-cm2. The curve originates at an open circuit potential (L1.12 V vs the standard potential of the C/CO2 electrode) and is determined by the Boudouard equilibrium gas composition at open current.
Conclusions
While a very large number of carbon materials have been characterized physically and electro chemically, there is still insufficient data pertaining to a single carbon material to fully verify this interpretation. In particular, simultaneous measurements of off-gas composition and anode polarization need to be undertaken within an operating cell using an oxygen electrode. The separate dependences of current density on flow rate and specific area need to be measured for each of many materials. The apparent dependence of potential on flow rate might be investigated with a rotating disk to infer transport mechanism. The oxygen electrode is needed as a counter electrode to assure the transference of carbonate (or oxide ion, with YSZ or ceria separator) into the anolyte that underlies the equations for current efficiency in terms of measured CO2 mole fraction or CO/CO2 mole ratios.
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We have also neglected the important role of electrode wetting by the melt, as investigated by Cao [24] and by Zhu [21]. These phenomena are examined further and modeled by one of us, in publications concurrent with this communication [32,a,b,c,]. The structure-based kinetic model of Li [25] should be combined with porous electrode theory [30] for development of reaction distribution computational models and tested on a few well-characterized carbon samples under well-defined experimental conditions. A rate equation describing the branching pathway of Fig. 2 needs to be evaluated for reasonable values of the electrochemical factors and rate constants of individual steps. To achieve both high coulombic- and voltage efficiencies, it appears necessary to start with highly disordered carbon materials with high specific electronic conductivity. Next, the proportion of current flowing to the exterior electrode surface might be increased by either of two methods: (1) The mesopore volume may be blocked by, for example, filling the pores with a solid, off-eutectic salt composition that melts at a temperature higher than that of cell operation. The salt will dissolve into the eutectic melt as it becomes exposed by the progressive retraction of the electrode during oxidation. The off-eutectic constituents of different feed-stocks can be balanced on either side of the eutectic composition, so that solution of the plug results in an invariant melt composition. Apparently, Weaver inadvertently achieved pore blockage by repeatedly filling the pore structure with fluid coal tar and pitch as part of a multistep pyrolysis process used in preparing his electrodes [18]. Pore-blocking techniques can be integrated with anode briquette or plate formation at minimal cost. Aluminum smelter anodes are manufactured using binders that reduce porosity and increase conductivity. (2) The other means by which 4-electron transfer might be achieved would be to use fine particles of dense, low-porosity disordered carbon which tend to increase the proportion of the current flowing to the exterior of the particle relative to the interior because of the high surface-area to volume ratio. In this approach, it is necessary curtail Boudouard corrosion in the bulk of the slurry–difficult challenges to flow engineering and the chemistry of reaction inhibition.
[4]
[5]
[6]
[7] [8]
[9]
[10] [11]
[12]
[13] [14]
[15]
[16]
[17]
[18]
Acknowledgments [19]
We gratefully acknowledge partial support for participation in the Carbonate Fuel Cell Conferences in Paris and Naples, from respectively, E´cole Nationale Supe´rieure de Chimie de Paris, and ENEA.
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