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Molten Carbonates from Fuel Cells to New Energy Devices
17 Molten Carbonates from Fuel Cells to New Energy Devices Michel Cassir, Armelle Ringuede´, Virginie Lair Chimie ParisTech ENSCP, UMR CNRS 7575, Laboratory of Electrochemistry, Chemistry of Interfaces and Modelling for Energy, Paris, France
17.1
Introduction
The idea of using molten salts as electrolytes in energy production arises from the interest of a fuel cell consuming carbon directly to transform it into electricity and stems from the interest of a fuel to consume directly from coal while generating electricity [1]. Systems with a molten salt electrolyte, a carbon anode and an iron cathode, were tested early in the twentieth century. First tests in molten KOH environment provided promising results at a relatively low temperature of 400 C. Progressively, molten alkali carbonates with higher operating temperature were found more efficient. In the meantime, the interest for a direct exploitation of carbon decreased and the focus on hydrogen oxidation gave rise to the current MCFC in the early 1950s. In the past decades, the molten carbonate science has been mainly dedicated to the optimization of the MCFC, which has progressed significantly. It is still a major issue in the heart of a new vision of energy development. More recently, new topics related to other kinds of fuel cells (DCFC, composite SOFC/MCFC systems) and CO2 capture have emerged and are concentrating important research and technological efforts. Moreover, new fields of implementation, related to molten carbonates, are also present: geology, glasses, production of nanocarbon particles, etc. Considering other areas where this molten phase plays a major role, such as catalysis, waste destruction, and surface treatment, it can be confirmed that the field of molten carbonates is very rich and offers many opportunities for fundamental science and competitive applications. Whatever their specific application, molten carbonate activity is associated with more efficient energy systems with low production of CO2 and, in a prospective sense, with nonpolluting energy using the value of CO2.
17.2
Physicochemical Properties of Molten Carbonates
Different molten carbonates can be used either in binary or in ternary mixtures. Most of the time, the melt is made out of alkali or alkaline earth carbonates. General properties—such as chemical, physical, and electrochemical characteristics—of molten carbonates have been widely studied [1–3]. We will focus our interest on the most used mixture, molten Li2CO3-K2CO3 at the eutectic composition 62-38 mol%. Molten Salts Chemistry © 2013 Elsevier Inc. All rights reserved.
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17.2.1 Density Value The determination of the density of different carbonate systems has been carried out by different groups [4–7]. Despite the use of different apparatus, density values seem to be comparable with a quite good accuracy. The variations of density have been studied as a function of the composition of different alkali carbonate mixtures as well as as a function of temperature. The most notable aspect of the results concerning density is the linear relationship which exists between density and temperature even at points quite close to the melting point. For this reason, the empirical equation of this variation is given by r ¼ a b 103 T
(17.1)
where r is the density in g cm3, T the absolute temperature (Kelvin), and a and b are empiric constants. For instance, some density values are given in Table 17.1. Whatever the composition, density seems to follow a linear variation with temperature. To conclude, it seems that the density follows a direct molar additive relationship, which makes possible evaluating the density of an unknown mixture if the densities of the alkali carbonate pure salts are known.
17.2.2 Surface Tension of Different Molten Carbonates Surface tension is an important property of the surface of a liquid. It is what causes the surface of a portion of liquid to be attracted by another surface, such as that of another portion of liquid. Usually, the higher the surface tension is, the stronger the interactions between the molecules of the liquid are. The surface tension decreases with temperature because the interactions inside the liquid are lower than the heat moving forces. According to Ward and Janz [6] and later to Kojima et al. [8,9], the experimental measurement could be fitted by a linear law with a quite good correlation. For this reason, the variations of surface tension can be described by the following equation: g ¼ a bT
(17.2) 1
where g is the surface tension (mN m ), T the temperature (K), and a and b experimental constants. The surface tension varies linearly with temperature. The values of a and b for pure carbonates and one eutectic composition are given in Table 17.2. The main result is that the surface tension of Li-Y system (where Y is Na, K, Rb, or Cs) varies as follows: Li-Na > Li-K > Li-Rb > Li-Cs. Moreover, surface tensions of alkali binary carbonates appear lower than surface tension of alkaline earth carbonates, Li-X (where X is Ca, Sr, or Ba). The surface tension poorly Table 17.1 Density Values as a Function of Temperature and Composition for Pure Carbonates and Li-K (62-38 mol%) Carbonate Eutectic [7] Density (g cm3)
Alkali Carbonate
Temperature Range ( C)
a
b
Li2CO3
2.2365
0.4041
741-856
K2CO3
2.4295
0.4543
907-981
Li2CO3-K2CO3
2.3526
0.4532
570-945
Source: Parameters a and b are defined in Equation (17.5).
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Table 17.2 Surface Tension of Some Pure Carbonate Salts and Li-K (62-38 mol%) Carbonate Eutectic as a Function of Temperature [6,8]
Molten Carbonate
a
b
Temperature Range (K)
g (mN m1) 1173 K
g (mN m1) 1123 K
K2CO3
243.7
0.063681
1178-1283
169.0
No data
Li2CO3
285.2
0.0410
1118-1235
237.0
No data
Li2CO3-K2CO3
266.5
0.06637
794-1264
No data
198.6
varies along the column of the periodic table, but the tendency is Li-Ca < Li-Sr < Li-Ba. The evolution is then inversed along a column between alkali systems and alkaline earth systems.
17.2.3 Conductivity of Molten Carbonates Janz [5] was the first author to show that the conductivity values follow an Arrhenius’ law given by the following equation: Ea s ¼ s0 exp RT
(17.3)
where s is the conductivity of the molten carbonate (S cm1), s0 is a preexponential factor (S cm1), Ea is the apparent activation energy (J mol1), R is the gas constant (J mol1 K1), and T is the temperature (K). The equivalent conductance, L (S cm2 equiv.1), was calculated from the specific conductivity s and the molar volume Vm (cm3 equiv.1) according to the following equation: L ¼ sVm
(17.4)
Moreover, this equivalent conductance can be expressed by the Arrhenius equation EL L ¼ AL exp RT
(17.5)
The variations of the conductivity s found by Janz [5] were confirmed by Spedding [7] and Kojima [8,9] with a good accuracy. Janz et al. [5,6] determined the conductivity of pure Li, K, molten carbonates, and their eutectic mixtures. It seems that they all follow an Arrhenius’ law. However, the variation of the logarithm of the conductivity as a function of the reverse absolute temperature (1/T) shows a change in the slope at a certain temperature for (Li-K) at 720 C. Spedding [7] observed the same behavior. He reported that the activation energy of the Arrhenius equation (Ea) varied with the temperature. For this reason, a deviation is observed. Kojima et al. suggested fitting the experimental results not with an Arrhenius’ law but with a quadratic function: s ¼ a þ bT þ cT 2
(17.6)
where a (S cm1), b (S cm1 K1), and c (S cm1 K2) are constants, determined from experimental data. All these results allow estimating the conductivity of either binary or
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ternary systems in order to optimize the conductivity of the melt. It can be deduced that the cations bigger than Liþ and Kþ should decrease the conductivity of the molten carbonate Li2CO3-K2CO3 (62-38 mol%) [11].
17.2.4 Oxoacidity Concept Each molten carbonate salt presents a self-ionization equilibrium which can be described as followed: M2 CO3 ðlÞ ¼ M2 OðsÞ þ CO2 ðgÞ with M ¼ alkali ions. The dissociation constant associated to the previous reaction is defined as follows: K ¼
aðM2 OÞ PðCO2 Þ aðM2 CO3 Þ
(17.7)
where a(M2O) and a(M2CO3) are the activity of the corresponding components and P(CO2) is the partial pressure of carbon dioxide. This equilibrium represents the acidity level of the electrolyte due to the capability to gain or to release an oxide ion. As the anion O2 has an available electronic pair, it can associate with any electronic pair acceptor. Such an acceptor is commonly called “oxoacid.” In parallel, an “oxobase” is a component which can release an oxide anion O2. Thus, it is possible to define oxoacid/oxobase couple according to the following reaction: oxobase ¼ oxoacid þ O2 The acidity level of a molten carbonate is defined by the value of the activity of the oxide O2, according to the following equation: PO2 ¼ log a O2 The acidity domain is limited on the acid part by the saturation of CO2 (arbitrary chosen at 1 bar) and on the basic part by the precipitation of the most stable oxide M2O(s). In the case of Li/K and Li/Na systems, the most stable oxide, referring to its dissociation constant, is the lithium oxide: Li2 CO3 ðlÞ ¼ Li2 OðsÞ þ CO2 ðgÞ The dissociation constant associated to the previous reaction is defined as K ¼
aðLi2 OÞ PðCO2 Þ aðLi2 CO3 Þ
(17.8)
Then a(Li2CO3) in the mixture is no more equal to 1 but has a constant value for each eutectic. So, an apparent constant Kd can be defined as follows: Kd ¼ K aðLi2 CO3 Þ ¼ aðLi2 OÞ PðCO2 Þ
(17.9)
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This constant Kd is finally the key parameter, delimiting the oxoacidity domain in a molten carbonate electrolyte. Previous works showed that the Li/K eutectic is not an ideal mixture and the activity of Li2CO3 can be calculated according to literature data, considering that the molten salt is a simple regular solution [10]. Then, for the Li/K system, the activity coefficient can be calculated according to the following relation: RTlnðgLi Þ ¼ bxK 2
(17.10)
where gLi is the activity coefficient of the lithium carbonate salt, xK the mole fraction of the potassium carbonate salt, and b the interaction parameter (equal to 3.7 kcal). For the eutectic composition of Li2CO3-K2CO3, the pKd is equal to 5.07 with an ideal behavior and 5.52 with a regular solution behavior.
17.2.5 Electrochemical Stability Diagrams The establishment of potential-oxoacidity diagrams (also called Pourbaix diagrams) is useful to determine the electrochemical stability domain of the molten salt and the existence of the stable species at a given activity [12]. As in aqueous system, the vertical limits are fixed by the dissociation constant. In water, the oxidative limit is fixed by the oxidation of water into oxygen. In molten carbonate mixtures, the positive limits of the oxoacidity domain are defined by the oxidation of O(II) species. This means that carbonate or alkali oxides can be transformed into peroxide ions, superoxide or oxygen. Thermodynamically, the main redox couples to be considered in the oxidation side are Li2O/O2 (II/0), M2O/MO2 (II/I/2), where M ¼ alkali ions, and Li2O/Li2O2 (II/I). All the standard potentials of the different couples are referred to the Li2O/O2 system; therefore, its standard potential is equal to zero. It is possible to express the Nernst potential of each redox couple as a function of the carbon dioxide partial pressure. In molten carbonate mixtures, the limiting reduction reactions are due to the reduction of alkali ions to metals, CO2 to CO (if the medium is oxoacid) or to Cgraphite (if it is is oxobasic) and H2O to H2. Only the reduction of gases leads to a variation of the potential with the partial pressure of CO2. Nernst potential equations have been calculated according to a methodology described by Cassir et al. [12]. For instance, a potentialoxoacidity diagram of the eutectic Li-K (62-38 mol%) is presented in Figure 17.1.
17.3
Molten Carbonate Fuel Cell
17.3.1 Historical Approach and Present Status The first system resembling an MCFC was constructed in 1921, but in the 1930s, the effort of researchers was focused on high-temperature fuel cells with solid oxide electrolytes. The low conductivity of these materials conduced Broers (the Netherlands) in the 1950s to investigate again molten carbonates [13]. This author initiated the real development of MCFC using a nickel anode, a silver cathode, and a ternary eutectic Li2CO3 - Na2CO3 - K2CO3 with an MgO matrix. The use of LiAlO2 matrix (1965), stable and insoluble material in molten carbonates, as a new electrolyte support, allowed to increase significantly the lifetime of this fuel cell (from less than 1000 h for MgO to 12,000 h) [13]. An intense activity, due essentially to the possibility of using syngas proceeding from coal gasification, was observed in the 1960s. After a slowdown, in the early 1970s, MCFC device was in constant progress, especially since the use of lithiated nickel oxide cathodes and chromium-doped nickel anodes.
360
Molten Salts Chemistry 0.6 0.4
O2-/O22-
E(V/(Li2O/O2)
0.2 0
a(O22−) = 10−3
O2-/O2 P(O2)=1 bar P(O2)=10−1 bar P(O2)=10−2 bar
CO2 gas
−0.2
CO2/CO P(CO)=10−1 bar
−0.4
Li2O sat H2O/H2 P(H2O)/P(H2)=10 P(H2O)/P(H2)=1
−0.6
K
CO2/C graphite −0.8
−1
0
1
2
3
4
5
-log P(CO2)
Figure 17.1 Potential-oxoacidity diagram of the eutectic Li-K (62-38 mol%) at 650 C. This plot defines the limits of the molten carbonate system from the oxidative side and the reductive side along the dissociation constant of the system. Then it is possible to study every redox system in such a medium, as a function of CO2 partial pressure.
Power density improved from 10 to nearly 250 mW cm2 and lifetime reached about 35,000 h for 200-300 kW systems. In the past years, the molten carbonate fuel cell development has been very fast, with about 90 demonstration systems installed in the United States, Japan, Korea, Germany, Spain, and recently France. About 70 million kWh of electricity was produced, among them 40 million kWh directly for customers. Although MCFC represents a promising alternative for the decentralized production of electricity and/or cogeneration, problems of corrosion and elevated costs still delay the construction of perfectly operational and competitive systems for the market.
17.3.2 Principle and Electrochemical Reactions Figure 17.2 shows a general scheme of a state of the art of a single cell. The electrochemical reactions occurring at the electrodes are the following: At the anode: The fuel in the anode compartment is H2 or a mixture of H2 þ CO, resulting from natural gas conversion by reforming or thermal cracking. a. If hydrogen is the fuel:
H2 þ CO3 2 ! H2 O þ CO2 þ 2e
(17.11)
Kinetics of this reaction is considered more rapid than the oxygen reduction. However, this mechanism is also complex and involves a previous hydrogen adsorption: H2 , 2Hads. But the role of H2O and OH in the anodic process is still controversial. An interesting review has been published some years ago on the oxidation mechanism [14].
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Figure 17.2 General scheme of a molten carbonate fuel cell.
At the cathode: The oxidant is constituted by a mixture of air and CO2:
1 O2 þ CO2 þ 2e ! CO3 2 2
(17.12)
According to the literature, this global reduction process involves in fact reduced oxygen species, O2 2 , O2 , and hypothetically O. An overall representation, involving superoxide ions, is admitted by most of the authors [15–18]: O2 þ 3e , 2O2
(17.13)
Afterward, oxide ions are neutralized by CO2. Nevertheless, a three-electron transfer in a single step is unlikely and, as postulated by Appleby et al., this so-called superoxide mechanism could occur in successive steps [15,19]: O2 þ e , CO3 2
(17.14)
O2 2 þ e , O2 þ O
(17.15)
O þ e , O2
(17.16)
The global cell reaction is the following: 1 H2 þ O2 þ CO2 ðcathodeÞ ! H2 O þ CO2 ðanodeÞ 2
(17.17)
Even though carbonate ions participate in the reactions, the melt has an invariant composition. Otherwise, CO2 formed at the anode is recycled and consumed at the cathode. The emf (electromotive force) of the cell reaction is as follows: emf ¼ emf 0 þ
RT PðH2 ÞPðO2 Þ1=2 RT PðCO2 Þcathode þ ln ln PðH2 OÞ PðCO2 Þanode 2F 2F
(17.18)
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Molten Salts Chemistry
with emf 0, standard value of the electromotive force corresponding to the cell reaction (all activities are 1), and P(i): partial pressure of i gas. According to this relation, an increase of the pressure should favor the electrochemical process, but this evolution is limited by interfering reactions occurring at high pressures, such as the so-called Boudouard reaction: 2CO ! C þ CO2, or the methane formation by syngas: CO þ 3H2 ! CH4 þ H2O. b. If methane is the fuel, it is first transformed by vaporeforming into syngas:
CH4 þ H2 O ! CO þ 3H2
(17.19)
The oxidation reaction becomes H2 þ CO þ 2CO3 2 ! 3CO2 þ H2 O þ 2e
(17.20)
It should be outlined that different technological solutions are possible in order to use methane as fuel: external reforming as well as direct and indirect internal reforming.
17.3.3 Electrolyte Compositions and Matrix The most conventional molten salt constituted by Li2CO3/K2CO3 (62/38 mol%), is nowadays partially replaced by Li2CO3/Na2CO3 (52/48 mol%) with a good ionic conductivity, better cell performances, and lower effective volatility [20–23]. One of the main issues in the choice of additives in the carbonate eutectic and, especially to decrease the NiO solubility, is the control of the acidic properties of the electrolyte [20]. An oxobasic medium favors in principle the cathode resistance to corrosion, e.g., the addition of a few percent of alkali earth, Mg, Ca, Sr, and Ba [20,21,23–25], mostly to Li-Na eutectic, has a benefit effect by increasing the oxobasicity of the electrolyte what results in decreasing the solubility of NiO in the electrolyte [20,23–25]. Nevertheless, these additives have the inconvenient of reducing the conductivity of the salt and, thus, cell performance [2]. Ota et al. have shown that the logarithm of the solubility of the rare-earth metal oxides exhibits a linear relation with respect to the Coulomb force ratio between the rare earth and the alkaline metals [26]. Although Nd2O3 has the highest solubility in the carbonate eutectics, La2O3 is still the best additive with respect to solubility reduction. Experiments have also been realized in single cells with Li0.75Cs0.25 carbonate electrolytes showing higher cell potentials, lower cathode, and anode overpotentials than Li0.68K0.32 or Li0.52Na0.48 [27]. With respect to the classical eutectics mentioned, Li2CO3-Cs2CO3 mixtures have lower surface tension, greater oxygen solubility, and an increased electrode kinetics, but they also have a drawback because of their relative acidity which promotes NiO dissolution; however, this drawback may be compensated by using Li-rich Li2CO3-Cs2CO3 mixtures (NiO solubility being inversely proportional to Li content in the melt) and operating at slightly reduced temperatures [5,27]. An interesting patent developed by Hoffmann reports the improvement of an MCFC with an Li-K or Li-Na electrolyte modified by the addition of different proportions of Cs or Rb (from 0.05% to 25%) [28]. The carbonate eutectic is supported by a porous ceramic body constituted by thin particles of about 0.1 mm, isolating, chemically inert and insoluble in the electrolyte, g-LiAlO2. This stable crystalline structure of lithium aluminate is impregnated with the carbonate eutectic to form a paste structure. The matrix contains about 45 wt% of carbonate melt and 55 wt% of lithium aluminate and is manufactured as a thin plate by tape casting [29]. About 70% of the ohmic resistance of the cell is due to this matrix; therefore, it is necessary to reduce its
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thickness (about 0.3 mm) [30]. As g-LiAlO2 suffers stresses during operation, in particular, by thermal cycling, it is often mixed with other thicker particles of a-LiAlO2 and fibers of a-LiAlO2, in the following proportions: 55%-30%-15%, respectively [31]. Recently, Kim et al. have proposed to reinforce the a-LiAlO2 matrix by adding Al and Li2CO3 particles, which increased significantly its mechanical strength [32].
17.3.4 Electrode Materials Issues 17.3.4.1 Cathode The materials used for the MCFC cathode must have a high electrical conductivity, a high mechanical resistance, a sufficient porosity, and a low solubility in the molten carbonates. Oxidized nickel is the state of the art cathode material, representing a good compromise among the required properties. When porous metallic nickel is in contact with the carbonate melt at high temperature under an oxidizing atmosphere (air/CO2), a NiO layer is formed in situ. It is a p-type semiconductor containing crystal defects in its lattice. In these conditions, Liþ can be incorporated into the NiO lattice, which creates positives holes. As the conductivity of NiO is strongly dependent on the defects in the crystal, the presence of these incorporated species increases the cathode conductivity [33]. LixNi1xO, with a lithium content estimated at 0.2 at% [34], presents a relatively high solubility in the electrolyte which can provoke the formation of metallic nickel and short-circuit between the anode and the cathode. NiO dissolution increases as the oxoacidity or/and the potassium content of the melt is increased [35]. The situation can be summarized as follows. In acidic media, close to MCFC conditions, the reaction is as follows: NiO þ CO2 , Ni2þ þ CO3 2
(17.21)
Different substitute materials more stable in the carbonate melts have already been tested: – Ni recovered by thin oxide films, mostly corrosion-resistant cobalt-based oxides [36–40]; – single oxides: LiFeO2, Li2MnO3, La0.8Sr0.2CoO3, and LiCoO2 [40–42]; – NiO-based materials containing other oxides less soluble: alloys Ni-M (M ¼ Nb, Al, Ti, Co) and mixed oxides [29,42–45].
17.3.4.2 Anode A porous nicked tile is commonly used as MCFC anode, with pore diameters of 3-5 mm, a porosity of 55-70%, and a thickness from 0.5 to 0.8 mm. The structure of this electrode is stabilized from the loss of surface area, pore growth, and further sintering via the addition of about 2-10% chromium [46,47]. Nevertheless, only low amounts of chromium are recommended due to the formation of a surface layer of LiCrO2 reducing the wettability of molten carbonates and modifying the surface. Nowadays, one of the best solutions is to add aluminum to nickel, which allows a good protection of the anode and improves its mechanical resistance [19,48,49]. The search of new materials or other manufacturing techniques is also important for lowering the anode cost, which contributes to about 25% of the whole stack. Anode also plays the role of a gas barrier and electrolyte reservoir. Contrarily to the cathode, the electrolytic process at the anode is relatively independent of the electrolyte filling degree, which allows this electrode to compensate electrolyte losses in the matrix tile during operation. In order to avoid a rapid flow from the anode to the matrix, a thin layer of small pores of Ni-LiAlO2 is formed at the anode surface. This layer is also a barrier preventing gas
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crossover. Most of hydrocarbon fuels which can be used in MCFC devices contain impurities, which will significantly affect cell performances. Most of the time, these impurities have a poisoning effect on the catalytic properties of the anode, decreasing MCFC lifetime and damaging surrounding parts: electrolyte, seal, bipolar plates, etc. Among the most common impurities, the most harmful contaminants are the sulfides (H2S, COS, CS2), which react chemically and/or electrochemically with the electrolyte to form both sulfide and sulfate ions [50].
17.3.4.3 Interconnects Interconnects or bipolar plates separate the anode from the adjacent cathode in a fuel cell stack containing several individual cells. These corrugated stainless steel plates ensure the gas distribution and the separation of the anode and cathode gases. They are connected with the electrodes’ current collectors in the so-called active areas, ensuring the electrical contact in a stack. Wet seals, formed by the extension of the electrolyte/LiAlO2 tile pressed by two individual plates, prevent leakage of the reactant gases. A schematic representation of a stack with bipolar plates is represented in Figure 17.3. Hot corrosion affects the cathode, anode, and wet-seal areas, causing electrolyte losses and an increase in the ohmic resistance [20,51]. The interconnection plates must be resistant to corrosion in both oxidizing and fuel environments (from 550 to 750 C) and to carburization on the fuel side, good electrical conductors, protected by a passive film electrically conductive, formable in the desired shape and able to support the weight of the cells above it. Currently, steels 316L and 310S are being used.
17.3.5 Prospects MCFC materials have not suffered dramatic changes since the past 30 years; nevertheless, one must agree that several improvements are underway. Demonstrators (United States, Germany, Japan, Italy, and Korea) and researchers are responsible for a subtle evolution Figure 17.3 Schematic representation of a bipolar plate separating an anode from a cathode and a MCFC stack where single cells are separated by bipolar plates.
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by adding to known material dopants, additives, or coatings and optimizing their micro- or nanostructures. This allows explaining the important increase in the stacks lifetime (more than 35,000 h for a 300-kW system) and the performance increase. However, problems of corrosion and elevated costs still delay the construction of perfectly operational and competitive systems for the market. Research efforts are still necessary for optimizing the MCFC system (decrease in the cathode and bipolar plates solubility, control of the microstructure of both electrodes, wetting properties, and lower temperature operation).
17.4
New Topics
We will now focus on new topics related to other kinds of fuel cells (DCFC, composites solid oxide fuel cell, SOFC/MCFC systems) and CO2 capture, which are growing applications concentrating an important research and technological effort.
17.4.1 Composite Electrolytes In recent years, the enhancement of the conductivity of solid electrolytes by adding a molten salt phase has attracted a growing attention for innovative high-temperature fuel cells applications. In particular, doped-ceria oxides mixed with molten carbonates have been tested [52–59]. In such systems, one may roughly state that oxide ions ensure the conductivity in the oxide phase; meanwhile the conductivity is attributed mostly to carbonates in the carbonate phase. Nevertheless, the conductivity mechanisms are complex because these composites form highly disordered interfacial regions (“percolating conducting paths”) between the oxide phase and the carbonate phase [60]. An important research effort has been developed these last years for optimizing the composition, particle size, structure, and morphology of the composite. Li and Sun have developed an oxide/molten carbonate nano-SOFC, stable during 200 h at 650 C, with a power density of 140 mW cm2 [61]. Some authors showed that the use of Na2CO3 as the carbonate phase mixed with samarium-doped ceria improved significantly the performance with respect to Li-Na carbonate eutectic, power densities above 0.62 W cm2 were reached at 500 C [62,63]. Zhang analyzed the behavior of a co-doped ceria Ce0.8Gd0.05Y0.15O1.9 mixed with Li-Na carbonate, obtaining a power density of 670 mW cm2 at 550 C [64]. Liu, analyzing Ce0.8Sm0.1Nd0.1O1.9, mixed with Li-Na carbonates, demonstrated the beneficial effect on the cell performance of the number of oxygen transfer routes at the interface between doped-ceria and carbonates [65]. Many researches are in progress on the performance of mixed composites either in an SOFC or MCFC configuration.
17.4.2 Electrolyte or Fuel Carrier in DCFC The important resources of carbon and the high energy density of this fuel (energy produced per volume four times higher than for methane) explain the interest in oxidizing directly a carbon fuel in a fuel cell system. Moreover, apart from fossil fuels, carbon may also proceed from biomass renewable sources (farming products and wastes, forests, etc). Although some attempts have been made in the past to find a clean way to transform coal through a DCFC, it is only in recent years that promising prototypes have been realized. The concept of a direct conversion of carbon in a molten carbonate fuel cell system was first developed in 1976 [66], but more recent advances are due to Cooper et al. [67].
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Another attractive system is the hybrid direct carbon fuel cell, which combines a classical SOFC system and a reservoir at the anode side filled with a carbonate melt playing the role of fuel carrier and electrochemical mediator for carbon oxidation. Work is in progress and some promising results have been obtained, e.g., power densities above 120 mW cm2 [68–70]. History and achievements of DCFCs are thoroughly developed in a full chapter of this book (see Chapter 19).
17.4.3 Carbon Dioxide Capture and Valorization In an MCFC, CO2 is transported from the cathode to the anode stream while producing electricity from the fuel at the anode side; therefore, such a high-temperature device can act as a CO2 separator and concentrator. This greenhouse effect gas can be extracted from the flue gas of a combined cycle power plant while generating electricity, avoiding loss in plant efficiency and consequent increase in primary energy consumption. The benefit of using MCFC seems to be proven, but the effectiveness still has to be evaluated against other alternatives for CO2 emission mitigation (precombustion, oxy-fuel combustion, or postcombustion). As MCFCs may reach about 90% CO2 separation efficiency while producing electricity, the potential of this technology is huge but exceeds the present capacity for producing MCFCs at large scale. In the case of small power plants of about 15 MtonCO2/year, this device can find more immediate implementation [71,72].
17.4.4 Miscellaneous About 20 years ago, molten carbonate media have been tested in homogeneous catalysis, i.e., catalytic dimerization of methane [73,74]. The nuclear field has also been investigated through oxidation of UO2 to uranates [75,76]. There are also known applications of molten carbonate media in other fields not necessarily related to energy production. For instance, there are some attempts to use these media in waste treatments, e.g., oxidation of organic compounds (carbon tetrachloride, methane, propane), surface treatment of metals in extreme conditions (aeronautics, car industry, etc.), and in-depth syngas cleaning. Carbonate media are also awakening a growing interest in geological applications [77] and for the synthesis of carbon nanoparticles in molten carbonates for lithium-ion batteries applications [78].
17.5
Conclusion
This chapter constitutes a synthetic approach to the properties of molten carbonates science and applications in the field of molten carbonate fuel cells and other energy devices. The main challenges were outlined concerning MCFC, DCFC, and combined MCFC/SOFC electrolytes. The panorama of applications is even larger and could also be of particular interest in the recovery and transformation of CO2 in molten carbonates into valuable fuels such as CO and CH4. It was not our purpose to be exhaustive but to give the mainstream of molten carbonates potentialities. In fact, this chapter is an opening to new developments of molten salts such as carbonates in the strategic areas of energy and environment.
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[46] Frangini, S., Masci, A. (2004). Intermetallic FeAl based coatings deposited by the electrospark technique: corrosion behavior in molten (LiþK) carbonate. Surf. Coat. Technol., 184, 31–39. [47] Wendt, H., Bo¨hme, O., Leidich, F.U. Brenscheidt, T. (1993). Proceedings of the 3rd International Symposium on Carbonate Fuel Cell Technology, The Electrochemical Society Proceedings Series 93-3, The Electrochem. Soc. Inc., Pennington, NJ. [48] Perez, F.J., Duday, D., Hierro, M.P., Gomez, C., Agu¨ero, A., Garcia, M.C., Muela, R., Sanchez Pascual, A., Martinez, L. (2002). Hot corrosion study of coated separator plates of molten carbonate fuel cells by slurry aluminides. Surf. Coat. Technol., 161, 293–301. [49] Yoshiba, F., Morita, H., Yoshikawa, M., Mugikura, Y., Izaki, Y., Watanabe, T., Komoda, M., Masuda, M., Zaima, N. (2004). Improvement of electricity generating performance and life expectancy of MCFC stack by applying Li/Na carbonate electrolyte. Test results and analysis of 0.44 m2/10 kW- and 1.03 m2/10 kW-class stack. J. Power Sour., 128, 152–164. [50] Devianto, H., Yoon, S.P., Nam, S.W., Han, J., Lim, T.-H. (2006). The effect of a ceria coating on the H2S tolerance of a molten carbonate fuel cell. J. Power Sour., 159 (2006), 1147. [51] Yuh, C., Johnsen, R., Farooque, M., Maru, H. (1995). Status of carbonate fuel cell materials. J. Power Sour., 56, 1–10. [52] Zhu, B., Mat, M.D. (2006). Studies on dual phase ceria-based composites in Electrochemistry. Int. J. Electrochem. Sci., 1, 383–402. [53] Bode´n, A., Di, J., Lagergren, C., Lindbergh, G., Wang, C.H. (2007). Conductivity of SDC and (Li/ Na)2CO3 composite electrolytes in reducing and oxidising atmospheres. J. Power Sour., 172, 520–529. [54] Lapa, C.M., Figueiredo, F.M.L., de Souza, D.P.F., Song, L., Zhu, B., Marques, F.M.B. (2010). Synthesis and characterization of composite electrolytes based on samaria-doped ceria and Na/Li carbonates. Int. J. Hydrogen Energy, 35, 2953–2957. [55] Di, J., Chen, M., Wang, C., Zheng, J., Fan, L., Zhu, B. (2010). Samarium doped ceria– (Li/Na)2CO3 composite electrolyte and its electrochemical properties in low temperature solid oxide fuel cell. J. Power Sour., 195, 4695–4699. [56] Xia, C., Li, Y., Tian, Y., Liu, Q., Wang, Z., Jia, L., Zhao, Y., Li, Y. (2010). Intermediate temperature fuel cell with a doped ceria–carbonate composite electrolyte. J. Power Sour., 195, 3149–3154. [57] Zhang, L., Lan, R., Petit, C.T.G., Tao, S. (2010). Durability study of an intermediate temperature fuel cell based on an oxide–carbonate composite electrolyte. Int. J. Hydrogen Energy, 35, 6934–6940. [58] Benamira, M., Ringuede´, A., Albin, V., Vannier, R.-N., Hildebrandt, L., Lagergren, C., Cassir, M. (2011). Gadolinia-doped ceria mixed with alkali carbonates for solid oxide fuel cell applications: I. A thermal, structural and morphological insight. J. Power Sour., 196, 5546–5554. [59] Benamira, M., Ringuede´, A., Vannier, R.-N., Hildebrandt, L., Lagergren, C., Cassir, M. (2013). Gadolinia-doped ceria mixed with alkali carbonates for solid oxide fuel cell applications: II. An electrochemical insight. Int. J. Hydrogen Energy, 37, 19371–19379. [60] Zhu, B., Li, S., Mellander, B. (2008). Theoretical approach on ceria-based two-phase electrolytes for low temperature (300-600 C) solid oxide fuel cells. Electrochem. Commun., 10, 302–305. [61] Li, S., Sun, J. (2010). Electrochemical performances of NANOCOFC in MCFC environments. Int. J. Hydrogen Energy, 35, 2980–2985. [62] Raza, R., Wang, X., Ma, Y., Liu, X., Zhu, B. (2010). Improved ceria–carbonate composite electrolytes. Int. J. Hydrogen Energy, 35, 2684–2688. [63] Ma, Y., Wang, X., Raza, R., Muhammed, M., Zhu, B. (2010). Thermal stability study of SDC / Na2CO3 nanocomposite electrolyte for low-temperature SOFCs. Int. J. Hydrogen Energy, 35, 2580–2585. [64] Zhang, L., Lan, R., Xu, X., Tao, S., Jiang, Y., Kraft, A. (2009). A high performance intermediate temperature fuel cell based on a thick oxide–carbonate electrolyte. J. Power Sour., 194, 967–971. [65] Liu, W., Liu, Y., Li, B., Sparks, T.D., Wie, X., Pan, W. (2010). Ceria (Sm3þ, Nd3þ)/carbonates composite electrolytes with high electrical conductivity at low temperature. Compos. Sci. Technol., 70, 181–185. [66] Anbar, M., McMillen, D.F., Weaver, R.D., Jorgenden, P.J. (1976). Method and apparatus for electrochemical generation of power from carbonaceous fuels, US Patent Number, US 3970474, 1-5.
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[67] Cooper, J.L., Cherepy, N., Krueger, R.L., (2003). Tilted fuel cell apparatus, US Patent Number: US 2003/0017380 A1. [68] Pointon, K.D., Lakerman, B., Irvine, J.T.S., Bradley, J., Jain, S. (2006). The development of a carbon–air semi fuel cell. J. Power Sour., 162, 750–756. [69] Nabae, Y., Pointon, K.D., Irvine, J.T.S. (2008). Electrochemical oxidation of solid carbon in hybrid DCFC with solid oxide and molten carbonate binary electrolyte. Energy Environ. Sci., 1, 148–155. [70] Bradley, J., Irvine, J.T.S., Lakeman, J.B. (2004). Electricity producing fuel cell or battery device, Brit Patent Appl., GB0426879.3, WO2006061639-A2, 2006. [71] Suguira, K., Takei, K., Tanimoto, K., Miyazaki, Y. (2003). The carbon dioxide concentrator by using MCFC. J. Power Sour., 118, 218–227. [72] Wade, J.L., Lackner, K.S., West, A.C. (2007). Transport model for a high temperature, mixed conducting CO2 separation membrane. Solid State Ionics, 178, 1530–1540. [73] Tashjian, V., Cassir, M., Devynck, J., Rummel, W. (1994). Oxidative dimerization of methane in molten Li2CO3-Na2CO3 eutectic supported by lithium aluminate at 700-850 C. Appl. Catal., 108, 157–169. [74] Tashjian, V., Cassir, M., Devynck, J. (1995). Catalytic performance of supported molten carbonate towards the oxidative dimerization of methane. J. Catal., 152, 204–210. [75] Volkovich, V., Griffiths, T.R., Fray, D.J., Fields, M., Wilson, P.D. (1997). Increased oxidation of UO2 in molten alkali carbonate-based mixtures by increasing oxygen solubility and by controlled generation of superoxide ions, and evidence for a new sodium uranate. J. Chem. Soc., Faraday Trans., 93, 3819–3826. [76] Volkovich, V., Griffiths, T.R., Fray, D.J., Fields, M., Wilson, P.D. (1996). Oxidation of UO2 in molten alkali-metal carbonate mixtures: formation of uranates and diuranates. J. Chem. Soc., Faraday Trans., 92, 5059–5065. [77] Gaillard, F., Iacono-Marziano, G., Malki, M., Pichavant, M., Scaillet, B. (2008). Conductivity in the asthenosphere. Science, 322, 1363–1365. [78] Le Van, K., Groult, H., Lantelme, F., Dubois, M., Avignant, D., Tressaud, A., Komaba, S., Kumagai, N., Sigrist, S. (2009). Electrochemical formation of carbon nano-powders with various porosities in molten alkali carbonates. Electrochim. Acta, 54, 4566–4573.
Biographical Sketch: Michel Cassir, 60 years-old Affiliation Head of the Laboratoire d’Electrochimie, Chimie des Interfaces et Mode´lisation pour l’Energie (LECIME)- Chimie ParisTech. Member of the National Committee of CNRS (Sect. 13). E-mail: [email protected] Education and training 1977 Ph.D. in Analytical Chemistry (University of Paris VI) (speciality: XPS and Auger spectroscopy) 1993 Habilitation as a research director (University of Paris VI) (speciality: solution chemistry, separation techniques and electrochemistry applied to the understanding of chemical processes) Professional Since 2004 1987-2004 1977-1987
Experience Professor (Chimie-ParisTech) Associate Professor (Chimie-ParisTech) Professor at the National University of Mexico (UNAM)
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Publications 180 publications in peer reviewed journals and proceedings
▪ ▪ ▪ ▪ ▪
M. Cassir M. Cassir M. Cassir M. Cassir M. Cassir
et al, Electrochimica Acta, 1989, 34, 1563 et al, J. Electrochemical Society, 1993, 140, 3114 et al, J. Appl. Electrochem., 2000, 30, 1415 et al., J. Mater. Chem., 2010, 20, 8987. et al, Int. J. Hydrogen Energy, 2012, 37, 19345
Biographical Sketch: Armelle Ringuede, 42-years old Affiliation UMR CNRS-ENSCP-UPMC 7575 LECIME [email protected] Professional Since 2001 2009 2000-2001 1999-2000 1999
Experience & Education Research Fellow at CNRS (Charge´ de Recherche) Habilitation in Chemistry, University of Paris 6 Assistant Professor (ATER, INPGrenoble) Visiting scientist (European Training Network), Univerite´ d’Aveiro, Portugal PhD in Electrochemistry, Institut National Polytechnique de Grenoble
Publications 77 publications in peer reviewed journals and proceedings
▪ ▪ ▪ ▪ ▪
C. Mansour et al, J. Power Sources, 2006, 156, 23 A. Ringuede´ et al, J. Chem. Phys., 2006, 160, 789 M. Benamira et al, Int. J. Hydrogen Energy, 2012, 34, 19371 L. Z˙ivkovic et al., Electrochimica Acta., 2011, 56, 4638 M. J. Escudero et al, J. Power Sources., 2011, 196, 5546
Biographical Sketch: Virginie Lair, 36-years old Affiliation UMR CNRS-ENSCP-UPMC 7575 LECIME [email protected] Professional Since 2005 2004-2005 2003-2004 2003 2000
Experience & Education Associate Professor (Chimie ParisTech) Assistant Professor (ATER, Chimie ParisTech) Assistant Professor (ATER, Universite´ d’Evry val d’Essonne) PhD in Chemical Engineering, University of Paris 6 Master in Chemistry, University of Paris 6
Publications 26 publications in peer reviewed journals and proceedings
▪ ▪ ▪ ▪
V. Lair V. Lair V. Lair V. Lair
et al, et al, et al, et al,
Ionics, 2008, 14,555 Electrochimica Acta, 2010, 56, 784 Electrochimica Acta, 2011, 56, 4638 Int. J. Hydrogen Energy, 2012, 37, 19357
17.1
Introduction
The idea of using molten salts as electrolytes in energy production arises from the interest of a fuel cell consuming carbon directly to transform it into electricity and stems from the interest of a fuel to consume directly from coal while generating electricity [1]. Systems with a molten salt electrolyte, a carbon anode and an iron cathode, were tested early in the twentieth century. First tests in molten KOH environment provided promising results at a relatively low temperature of 400 C. Progressively, molten alkali carbonates with higher operating temperature were found more efficient. In the meantime, the interest for a direct exploitation of carbon decreased and the focus on hydrogen oxidation gave rise to the current MCFC in the early 1950s. In the past decades, the molten carbonate science has been mainly dedicated to the optimization of the MCFC, which has progressed significantly. It is still a major issue in the heart of a new vision of energy development. More recently, new topics related to other kinds of fuel cells (DCFC, composite SOFC/MCFC systems) and CO2 capture have emerged and are concentrating important research and technological efforts. Moreover, new fields of implementation, related to molten carbonates, are also present: geology, glasses, production of nanocarbon particles, etc. Considering other areas where this molten phase plays a major role, such as catalysis, waste destruction, and surface treatment, it can be confirmed that the field of molten carbonates is very rich and offers many opportunities for fundamental science and competitive applications. Whatever their specific application, molten carbonate activity is associated with more efficient energy systems with low production of CO2 and, in a prospective sense, with nonpolluting energy using the value of CO2.
17.2
Physicochemical Properties of Molten Carbonates
Different molten carbonates can be used either in binary or in ternary mixtures. Most of the time, the melt is made out of alkali or alkaline earth carbonates. General properties—such as chemical, physical, and electrochemical characteristics—of molten carbonates have been widely studied [1–3]. We will focus our interest on the most used mixture, molten Li2CO3-K2CO3 at the eutectic composition 62-38 mol%. Molten Salts Chemistry © 2013 Elsevier Inc. All rights reserved.
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17.2.1 Density Value The determination of the density of different carbonate systems has been carried out by different groups [4–7]. Despite the use of different apparatus, density values seem to be comparable with a quite good accuracy. The variations of density have been studied as a function of the composition of different alkali carbonate mixtures as well as as a function of temperature. The most notable aspect of the results concerning density is the linear relationship which exists between density and temperature even at points quite close to the melting point. For this reason, the empirical equation of this variation is given by r ¼ a b 103 T
(17.1)
where r is the density in g cm3, T the absolute temperature (Kelvin), and a and b are empiric constants. For instance, some density values are given in Table 17.1. Whatever the composition, density seems to follow a linear variation with temperature. To conclude, it seems that the density follows a direct molar additive relationship, which makes possible evaluating the density of an unknown mixture if the densities of the alkali carbonate pure salts are known.
17.2.2 Surface Tension of Different Molten Carbonates Surface tension is an important property of the surface of a liquid. It is what causes the surface of a portion of liquid to be attracted by another surface, such as that of another portion of liquid. Usually, the higher the surface tension is, the stronger the interactions between the molecules of the liquid are. The surface tension decreases with temperature because the interactions inside the liquid are lower than the heat moving forces. According to Ward and Janz [6] and later to Kojima et al. [8,9], the experimental measurement could be fitted by a linear law with a quite good correlation. For this reason, the variations of surface tension can be described by the following equation: g ¼ a bT
(17.2) 1
where g is the surface tension (mN m ), T the temperature (K), and a and b experimental constants. The surface tension varies linearly with temperature. The values of a and b for pure carbonates and one eutectic composition are given in Table 17.2. The main result is that the surface tension of Li-Y system (where Y is Na, K, Rb, or Cs) varies as follows: Li-Na > Li-K > Li-Rb > Li-Cs. Moreover, surface tensions of alkali binary carbonates appear lower than surface tension of alkaline earth carbonates, Li-X (where X is Ca, Sr, or Ba). The surface tension poorly Table 17.1 Density Values as a Function of Temperature and Composition for Pure Carbonates and Li-K (62-38 mol%) Carbonate Eutectic [7] Density (g cm3)
Alkali Carbonate
Temperature Range ( C)
a
b
Li2CO3
2.2365
0.4041
741-856
K2CO3
2.4295
0.4543
907-981
Li2CO3-K2CO3
2.3526
0.4532
570-945
Source: Parameters a and b are defined in Equation (17.5).
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Table 17.2 Surface Tension of Some Pure Carbonate Salts and Li-K (62-38 mol%) Carbonate Eutectic as a Function of Temperature [6,8]
Molten Carbonate
a
b
Temperature Range (K)
g (mN m1) 1173 K
g (mN m1) 1123 K
K2CO3
243.7
0.063681
1178-1283
169.0
No data
Li2CO3
285.2
0.0410
1118-1235
237.0
No data
Li2CO3-K2CO3
266.5
0.06637
794-1264
No data
198.6
varies along the column of the periodic table, but the tendency is Li-Ca < Li-Sr < Li-Ba. The evolution is then inversed along a column between alkali systems and alkaline earth systems.
17.2.3 Conductivity of Molten Carbonates Janz [5] was the first author to show that the conductivity values follow an Arrhenius’ law given by the following equation: Ea s ¼ s0 exp RT
(17.3)
where s is the conductivity of the molten carbonate (S cm1), s0 is a preexponential factor (S cm1), Ea is the apparent activation energy (J mol1), R is the gas constant (J mol1 K1), and T is the temperature (K). The equivalent conductance, L (S cm2 equiv.1), was calculated from the specific conductivity s and the molar volume Vm (cm3 equiv.1) according to the following equation: L ¼ sVm
(17.4)
Moreover, this equivalent conductance can be expressed by the Arrhenius equation EL L ¼ AL exp RT
(17.5)
The variations of the conductivity s found by Janz [5] were confirmed by Spedding [7] and Kojima [8,9] with a good accuracy. Janz et al. [5,6] determined the conductivity of pure Li, K, molten carbonates, and their eutectic mixtures. It seems that they all follow an Arrhenius’ law. However, the variation of the logarithm of the conductivity as a function of the reverse absolute temperature (1/T) shows a change in the slope at a certain temperature for (Li-K) at 720 C. Spedding [7] observed the same behavior. He reported that the activation energy of the Arrhenius equation (Ea) varied with the temperature. For this reason, a deviation is observed. Kojima et al. suggested fitting the experimental results not with an Arrhenius’ law but with a quadratic function: s ¼ a þ bT þ cT 2
(17.6)
where a (S cm1), b (S cm1 K1), and c (S cm1 K2) are constants, determined from experimental data. All these results allow estimating the conductivity of either binary or
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ternary systems in order to optimize the conductivity of the melt. It can be deduced that the cations bigger than Liþ and Kþ should decrease the conductivity of the molten carbonate Li2CO3-K2CO3 (62-38 mol%) [11].
17.2.4 Oxoacidity Concept Each molten carbonate salt presents a self-ionization equilibrium which can be described as followed: M2 CO3 ðlÞ ¼ M2 OðsÞ þ CO2 ðgÞ with M ¼ alkali ions. The dissociation constant associated to the previous reaction is defined as follows: K ¼
aðM2 OÞ PðCO2 Þ aðM2 CO3 Þ
(17.7)
where a(M2O) and a(M2CO3) are the activity of the corresponding components and P(CO2) is the partial pressure of carbon dioxide. This equilibrium represents the acidity level of the electrolyte due to the capability to gain or to release an oxide ion. As the anion O2 has an available electronic pair, it can associate with any electronic pair acceptor. Such an acceptor is commonly called “oxoacid.” In parallel, an “oxobase” is a component which can release an oxide anion O2. Thus, it is possible to define oxoacid/oxobase couple according to the following reaction: oxobase ¼ oxoacid þ O2 The acidity level of a molten carbonate is defined by the value of the activity of the oxide O2, according to the following equation: PO2 ¼ log a O2 The acidity domain is limited on the acid part by the saturation of CO2 (arbitrary chosen at 1 bar) and on the basic part by the precipitation of the most stable oxide M2O(s). In the case of Li/K and Li/Na systems, the most stable oxide, referring to its dissociation constant, is the lithium oxide: Li2 CO3 ðlÞ ¼ Li2 OðsÞ þ CO2 ðgÞ The dissociation constant associated to the previous reaction is defined as K ¼
aðLi2 OÞ PðCO2 Þ aðLi2 CO3 Þ
(17.8)
Then a(Li2CO3) in the mixture is no more equal to 1 but has a constant value for each eutectic. So, an apparent constant Kd can be defined as follows: Kd ¼ K aðLi2 CO3 Þ ¼ aðLi2 OÞ PðCO2 Þ
(17.9)
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This constant Kd is finally the key parameter, delimiting the oxoacidity domain in a molten carbonate electrolyte. Previous works showed that the Li/K eutectic is not an ideal mixture and the activity of Li2CO3 can be calculated according to literature data, considering that the molten salt is a simple regular solution [10]. Then, for the Li/K system, the activity coefficient can be calculated according to the following relation: RTlnðgLi Þ ¼ bxK 2
(17.10)
where gLi is the activity coefficient of the lithium carbonate salt, xK the mole fraction of the potassium carbonate salt, and b the interaction parameter (equal to 3.7 kcal). For the eutectic composition of Li2CO3-K2CO3, the pKd is equal to 5.07 with an ideal behavior and 5.52 with a regular solution behavior.
17.2.5 Electrochemical Stability Diagrams The establishment of potential-oxoacidity diagrams (also called Pourbaix diagrams) is useful to determine the electrochemical stability domain of the molten salt and the existence of the stable species at a given activity [12]. As in aqueous system, the vertical limits are fixed by the dissociation constant. In water, the oxidative limit is fixed by the oxidation of water into oxygen. In molten carbonate mixtures, the positive limits of the oxoacidity domain are defined by the oxidation of O(II) species. This means that carbonate or alkali oxides can be transformed into peroxide ions, superoxide or oxygen. Thermodynamically, the main redox couples to be considered in the oxidation side are Li2O/O2 (II/0), M2O/MO2 (II/I/2), where M ¼ alkali ions, and Li2O/Li2O2 (II/I). All the standard potentials of the different couples are referred to the Li2O/O2 system; therefore, its standard potential is equal to zero. It is possible to express the Nernst potential of each redox couple as a function of the carbon dioxide partial pressure. In molten carbonate mixtures, the limiting reduction reactions are due to the reduction of alkali ions to metals, CO2 to CO (if the medium is oxoacid) or to Cgraphite (if it is is oxobasic) and H2O to H2. Only the reduction of gases leads to a variation of the potential with the partial pressure of CO2. Nernst potential equations have been calculated according to a methodology described by Cassir et al. [12]. For instance, a potentialoxoacidity diagram of the eutectic Li-K (62-38 mol%) is presented in Figure 17.1.
17.3
Molten Carbonate Fuel Cell
17.3.1 Historical Approach and Present Status The first system resembling an MCFC was constructed in 1921, but in the 1930s, the effort of researchers was focused on high-temperature fuel cells with solid oxide electrolytes. The low conductivity of these materials conduced Broers (the Netherlands) in the 1950s to investigate again molten carbonates [13]. This author initiated the real development of MCFC using a nickel anode, a silver cathode, and a ternary eutectic Li2CO3 - Na2CO3 - K2CO3 with an MgO matrix. The use of LiAlO2 matrix (1965), stable and insoluble material in molten carbonates, as a new electrolyte support, allowed to increase significantly the lifetime of this fuel cell (from less than 1000 h for MgO to 12,000 h) [13]. An intense activity, due essentially to the possibility of using syngas proceeding from coal gasification, was observed in the 1960s. After a slowdown, in the early 1970s, MCFC device was in constant progress, especially since the use of lithiated nickel oxide cathodes and chromium-doped nickel anodes.
360
Molten Salts Chemistry 0.6 0.4
O2-/O22-
E(V/(Li2O/O2)
0.2 0
a(O22−) = 10−3
O2-/O2 P(O2)=1 bar P(O2)=10−1 bar P(O2)=10−2 bar
CO2 gas
−0.2
CO2/CO P(CO)=10−1 bar
−0.4
Li2O sat H2O/H2 P(H2O)/P(H2)=10 P(H2O)/P(H2)=1
−0.6
K
CO2/C graphite −0.8
−1
0
1
2
3
4
5
-log P(CO2)
Figure 17.1 Potential-oxoacidity diagram of the eutectic Li-K (62-38 mol%) at 650 C. This plot defines the limits of the molten carbonate system from the oxidative side and the reductive side along the dissociation constant of the system. Then it is possible to study every redox system in such a medium, as a function of CO2 partial pressure.
Power density improved from 10 to nearly 250 mW cm2 and lifetime reached about 35,000 h for 200-300 kW systems. In the past years, the molten carbonate fuel cell development has been very fast, with about 90 demonstration systems installed in the United States, Japan, Korea, Germany, Spain, and recently France. About 70 million kWh of electricity was produced, among them 40 million kWh directly for customers. Although MCFC represents a promising alternative for the decentralized production of electricity and/or cogeneration, problems of corrosion and elevated costs still delay the construction of perfectly operational and competitive systems for the market.
17.3.2 Principle and Electrochemical Reactions Figure 17.2 shows a general scheme of a state of the art of a single cell. The electrochemical reactions occurring at the electrodes are the following: At the anode: The fuel in the anode compartment is H2 or a mixture of H2 þ CO, resulting from natural gas conversion by reforming or thermal cracking. a. If hydrogen is the fuel:
H2 þ CO3 2 ! H2 O þ CO2 þ 2e
(17.11)
Kinetics of this reaction is considered more rapid than the oxygen reduction. However, this mechanism is also complex and involves a previous hydrogen adsorption: H2 , 2Hads. But the role of H2O and OH in the anodic process is still controversial. An interesting review has been published some years ago on the oxidation mechanism [14].
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Figure 17.2 General scheme of a molten carbonate fuel cell.
At the cathode: The oxidant is constituted by a mixture of air and CO2:
1 O2 þ CO2 þ 2e ! CO3 2 2
(17.12)
According to the literature, this global reduction process involves in fact reduced oxygen species, O2 2 , O2 , and hypothetically O. An overall representation, involving superoxide ions, is admitted by most of the authors [15–18]: O2 þ 3e , 2O2
(17.13)
Afterward, oxide ions are neutralized by CO2. Nevertheless, a three-electron transfer in a single step is unlikely and, as postulated by Appleby et al., this so-called superoxide mechanism could occur in successive steps [15,19]: O2 þ e , CO3 2
(17.14)
O2 2 þ e , O2 þ O
(17.15)
O þ e , O2
(17.16)
The global cell reaction is the following: 1 H2 þ O2 þ CO2 ðcathodeÞ ! H2 O þ CO2 ðanodeÞ 2
(17.17)
Even though carbonate ions participate in the reactions, the melt has an invariant composition. Otherwise, CO2 formed at the anode is recycled and consumed at the cathode. The emf (electromotive force) of the cell reaction is as follows: emf ¼ emf 0 þ
RT PðH2 ÞPðO2 Þ1=2 RT PðCO2 Þcathode þ ln ln PðH2 OÞ PðCO2 Þanode 2F 2F
(17.18)
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Molten Salts Chemistry
with emf 0, standard value of the electromotive force corresponding to the cell reaction (all activities are 1), and P(i): partial pressure of i gas. According to this relation, an increase of the pressure should favor the electrochemical process, but this evolution is limited by interfering reactions occurring at high pressures, such as the so-called Boudouard reaction: 2CO ! C þ CO2, or the methane formation by syngas: CO þ 3H2 ! CH4 þ H2O. b. If methane is the fuel, it is first transformed by vaporeforming into syngas:
CH4 þ H2 O ! CO þ 3H2
(17.19)
The oxidation reaction becomes H2 þ CO þ 2CO3 2 ! 3CO2 þ H2 O þ 2e
(17.20)
It should be outlined that different technological solutions are possible in order to use methane as fuel: external reforming as well as direct and indirect internal reforming.
17.3.3 Electrolyte Compositions and Matrix The most conventional molten salt constituted by Li2CO3/K2CO3 (62/38 mol%), is nowadays partially replaced by Li2CO3/Na2CO3 (52/48 mol%) with a good ionic conductivity, better cell performances, and lower effective volatility [20–23]. One of the main issues in the choice of additives in the carbonate eutectic and, especially to decrease the NiO solubility, is the control of the acidic properties of the electrolyte [20]. An oxobasic medium favors in principle the cathode resistance to corrosion, e.g., the addition of a few percent of alkali earth, Mg, Ca, Sr, and Ba [20,21,23–25], mostly to Li-Na eutectic, has a benefit effect by increasing the oxobasicity of the electrolyte what results in decreasing the solubility of NiO in the electrolyte [20,23–25]. Nevertheless, these additives have the inconvenient of reducing the conductivity of the salt and, thus, cell performance [2]. Ota et al. have shown that the logarithm of the solubility of the rare-earth metal oxides exhibits a linear relation with respect to the Coulomb force ratio between the rare earth and the alkaline metals [26]. Although Nd2O3 has the highest solubility in the carbonate eutectics, La2O3 is still the best additive with respect to solubility reduction. Experiments have also been realized in single cells with Li0.75Cs0.25 carbonate electrolytes showing higher cell potentials, lower cathode, and anode overpotentials than Li0.68K0.32 or Li0.52Na0.48 [27]. With respect to the classical eutectics mentioned, Li2CO3-Cs2CO3 mixtures have lower surface tension, greater oxygen solubility, and an increased electrode kinetics, but they also have a drawback because of their relative acidity which promotes NiO dissolution; however, this drawback may be compensated by using Li-rich Li2CO3-Cs2CO3 mixtures (NiO solubility being inversely proportional to Li content in the melt) and operating at slightly reduced temperatures [5,27]. An interesting patent developed by Hoffmann reports the improvement of an MCFC with an Li-K or Li-Na electrolyte modified by the addition of different proportions of Cs or Rb (from 0.05% to 25%) [28]. The carbonate eutectic is supported by a porous ceramic body constituted by thin particles of about 0.1 mm, isolating, chemically inert and insoluble in the electrolyte, g-LiAlO2. This stable crystalline structure of lithium aluminate is impregnated with the carbonate eutectic to form a paste structure. The matrix contains about 45 wt% of carbonate melt and 55 wt% of lithium aluminate and is manufactured as a thin plate by tape casting [29]. About 70% of the ohmic resistance of the cell is due to this matrix; therefore, it is necessary to reduce its
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363
thickness (about 0.3 mm) [30]. As g-LiAlO2 suffers stresses during operation, in particular, by thermal cycling, it is often mixed with other thicker particles of a-LiAlO2 and fibers of a-LiAlO2, in the following proportions: 55%-30%-15%, respectively [31]. Recently, Kim et al. have proposed to reinforce the a-LiAlO2 matrix by adding Al and Li2CO3 particles, which increased significantly its mechanical strength [32].
17.3.4 Electrode Materials Issues 17.3.4.1 Cathode The materials used for the MCFC cathode must have a high electrical conductivity, a high mechanical resistance, a sufficient porosity, and a low solubility in the molten carbonates. Oxidized nickel is the state of the art cathode material, representing a good compromise among the required properties. When porous metallic nickel is in contact with the carbonate melt at high temperature under an oxidizing atmosphere (air/CO2), a NiO layer is formed in situ. It is a p-type semiconductor containing crystal defects in its lattice. In these conditions, Liþ can be incorporated into the NiO lattice, which creates positives holes. As the conductivity of NiO is strongly dependent on the defects in the crystal, the presence of these incorporated species increases the cathode conductivity [33]. LixNi1xO, with a lithium content estimated at 0.2 at% [34], presents a relatively high solubility in the electrolyte which can provoke the formation of metallic nickel and short-circuit between the anode and the cathode. NiO dissolution increases as the oxoacidity or/and the potassium content of the melt is increased [35]. The situation can be summarized as follows. In acidic media, close to MCFC conditions, the reaction is as follows: NiO þ CO2 , Ni2þ þ CO3 2
(17.21)
Different substitute materials more stable in the carbonate melts have already been tested: – Ni recovered by thin oxide films, mostly corrosion-resistant cobalt-based oxides [36–40]; – single oxides: LiFeO2, Li2MnO3, La0.8Sr0.2CoO3, and LiCoO2 [40–42]; – NiO-based materials containing other oxides less soluble: alloys Ni-M (M ¼ Nb, Al, Ti, Co) and mixed oxides [29,42–45].
17.3.4.2 Anode A porous nicked tile is commonly used as MCFC anode, with pore diameters of 3-5 mm, a porosity of 55-70%, and a thickness from 0.5 to 0.8 mm. The structure of this electrode is stabilized from the loss of surface area, pore growth, and further sintering via the addition of about 2-10% chromium [46,47]. Nevertheless, only low amounts of chromium are recommended due to the formation of a surface layer of LiCrO2 reducing the wettability of molten carbonates and modifying the surface. Nowadays, one of the best solutions is to add aluminum to nickel, which allows a good protection of the anode and improves its mechanical resistance [19,48,49]. The search of new materials or other manufacturing techniques is also important for lowering the anode cost, which contributes to about 25% of the whole stack. Anode also plays the role of a gas barrier and electrolyte reservoir. Contrarily to the cathode, the electrolytic process at the anode is relatively independent of the electrolyte filling degree, which allows this electrode to compensate electrolyte losses in the matrix tile during operation. In order to avoid a rapid flow from the anode to the matrix, a thin layer of small pores of Ni-LiAlO2 is formed at the anode surface. This layer is also a barrier preventing gas
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crossover. Most of hydrocarbon fuels which can be used in MCFC devices contain impurities, which will significantly affect cell performances. Most of the time, these impurities have a poisoning effect on the catalytic properties of the anode, decreasing MCFC lifetime and damaging surrounding parts: electrolyte, seal, bipolar plates, etc. Among the most common impurities, the most harmful contaminants are the sulfides (H2S, COS, CS2), which react chemically and/or electrochemically with the electrolyte to form both sulfide and sulfate ions [50].
17.3.4.3 Interconnects Interconnects or bipolar plates separate the anode from the adjacent cathode in a fuel cell stack containing several individual cells. These corrugated stainless steel plates ensure the gas distribution and the separation of the anode and cathode gases. They are connected with the electrodes’ current collectors in the so-called active areas, ensuring the electrical contact in a stack. Wet seals, formed by the extension of the electrolyte/LiAlO2 tile pressed by two individual plates, prevent leakage of the reactant gases. A schematic representation of a stack with bipolar plates is represented in Figure 17.3. Hot corrosion affects the cathode, anode, and wet-seal areas, causing electrolyte losses and an increase in the ohmic resistance [20,51]. The interconnection plates must be resistant to corrosion in both oxidizing and fuel environments (from 550 to 750 C) and to carburization on the fuel side, good electrical conductors, protected by a passive film electrically conductive, formable in the desired shape and able to support the weight of the cells above it. Currently, steels 316L and 310S are being used.
17.3.5 Prospects MCFC materials have not suffered dramatic changes since the past 30 years; nevertheless, one must agree that several improvements are underway. Demonstrators (United States, Germany, Japan, Italy, and Korea) and researchers are responsible for a subtle evolution Figure 17.3 Schematic representation of a bipolar plate separating an anode from a cathode and a MCFC stack where single cells are separated by bipolar plates.
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by adding to known material dopants, additives, or coatings and optimizing their micro- or nanostructures. This allows explaining the important increase in the stacks lifetime (more than 35,000 h for a 300-kW system) and the performance increase. However, problems of corrosion and elevated costs still delay the construction of perfectly operational and competitive systems for the market. Research efforts are still necessary for optimizing the MCFC system (decrease in the cathode and bipolar plates solubility, control of the microstructure of both electrodes, wetting properties, and lower temperature operation).
17.4
New Topics
We will now focus on new topics related to other kinds of fuel cells (DCFC, composites solid oxide fuel cell, SOFC/MCFC systems) and CO2 capture, which are growing applications concentrating an important research and technological effort.
17.4.1 Composite Electrolytes In recent years, the enhancement of the conductivity of solid electrolytes by adding a molten salt phase has attracted a growing attention for innovative high-temperature fuel cells applications. In particular, doped-ceria oxides mixed with molten carbonates have been tested [52–59]. In such systems, one may roughly state that oxide ions ensure the conductivity in the oxide phase; meanwhile the conductivity is attributed mostly to carbonates in the carbonate phase. Nevertheless, the conductivity mechanisms are complex because these composites form highly disordered interfacial regions (“percolating conducting paths”) between the oxide phase and the carbonate phase [60]. An important research effort has been developed these last years for optimizing the composition, particle size, structure, and morphology of the composite. Li and Sun have developed an oxide/molten carbonate nano-SOFC, stable during 200 h at 650 C, with a power density of 140 mW cm2 [61]. Some authors showed that the use of Na2CO3 as the carbonate phase mixed with samarium-doped ceria improved significantly the performance with respect to Li-Na carbonate eutectic, power densities above 0.62 W cm2 were reached at 500 C [62,63]. Zhang analyzed the behavior of a co-doped ceria Ce0.8Gd0.05Y0.15O1.9 mixed with Li-Na carbonate, obtaining a power density of 670 mW cm2 at 550 C [64]. Liu, analyzing Ce0.8Sm0.1Nd0.1O1.9, mixed with Li-Na carbonates, demonstrated the beneficial effect on the cell performance of the number of oxygen transfer routes at the interface between doped-ceria and carbonates [65]. Many researches are in progress on the performance of mixed composites either in an SOFC or MCFC configuration.
17.4.2 Electrolyte or Fuel Carrier in DCFC The important resources of carbon and the high energy density of this fuel (energy produced per volume four times higher than for methane) explain the interest in oxidizing directly a carbon fuel in a fuel cell system. Moreover, apart from fossil fuels, carbon may also proceed from biomass renewable sources (farming products and wastes, forests, etc). Although some attempts have been made in the past to find a clean way to transform coal through a DCFC, it is only in recent years that promising prototypes have been realized. The concept of a direct conversion of carbon in a molten carbonate fuel cell system was first developed in 1976 [66], but more recent advances are due to Cooper et al. [67].
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Another attractive system is the hybrid direct carbon fuel cell, which combines a classical SOFC system and a reservoir at the anode side filled with a carbonate melt playing the role of fuel carrier and electrochemical mediator for carbon oxidation. Work is in progress and some promising results have been obtained, e.g., power densities above 120 mW cm2 [68–70]. History and achievements of DCFCs are thoroughly developed in a full chapter of this book (see Chapter 19).
17.4.3 Carbon Dioxide Capture and Valorization In an MCFC, CO2 is transported from the cathode to the anode stream while producing electricity from the fuel at the anode side; therefore, such a high-temperature device can act as a CO2 separator and concentrator. This greenhouse effect gas can be extracted from the flue gas of a combined cycle power plant while generating electricity, avoiding loss in plant efficiency and consequent increase in primary energy consumption. The benefit of using MCFC seems to be proven, but the effectiveness still has to be evaluated against other alternatives for CO2 emission mitigation (precombustion, oxy-fuel combustion, or postcombustion). As MCFCs may reach about 90% CO2 separation efficiency while producing electricity, the potential of this technology is huge but exceeds the present capacity for producing MCFCs at large scale. In the case of small power plants of about 15 MtonCO2/year, this device can find more immediate implementation [71,72].
17.4.4 Miscellaneous About 20 years ago, molten carbonate media have been tested in homogeneous catalysis, i.e., catalytic dimerization of methane [73,74]. The nuclear field has also been investigated through oxidation of UO2 to uranates [75,76]. There are also known applications of molten carbonate media in other fields not necessarily related to energy production. For instance, there are some attempts to use these media in waste treatments, e.g., oxidation of organic compounds (carbon tetrachloride, methane, propane), surface treatment of metals in extreme conditions (aeronautics, car industry, etc.), and in-depth syngas cleaning. Carbonate media are also awakening a growing interest in geological applications [77] and for the synthesis of carbon nanoparticles in molten carbonates for lithium-ion batteries applications [78].
17.5
Conclusion
This chapter constitutes a synthetic approach to the properties of molten carbonates science and applications in the field of molten carbonate fuel cells and other energy devices. The main challenges were outlined concerning MCFC, DCFC, and combined MCFC/SOFC electrolytes. The panorama of applications is even larger and could also be of particular interest in the recovery and transformation of CO2 in molten carbonates into valuable fuels such as CO and CH4. It was not our purpose to be exhaustive but to give the mainstream of molten carbonates potentialities. In fact, this chapter is an opening to new developments of molten salts such as carbonates in the strategic areas of energy and environment.
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Biographical Sketch: Michel Cassir, 60 years-old Affiliation Head of the Laboratoire d’Electrochimie, Chimie des Interfaces et Mode´lisation pour l’Energie (LECIME)- Chimie ParisTech. Member of the National Committee of CNRS (Sect. 13). E-mail: [email protected] Education and training 1977 Ph.D. in Analytical Chemistry (University of Paris VI) (speciality: XPS and Auger spectroscopy) 1993 Habilitation as a research director (University of Paris VI) (speciality: solution chemistry, separation techniques and electrochemistry applied to the understanding of chemical processes) Professional Since 2004 1987-2004 1977-1987
Experience Professor (Chimie-ParisTech) Associate Professor (Chimie-ParisTech) Professor at the National University of Mexico (UNAM)
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Publications 180 publications in peer reviewed journals and proceedings
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M. Cassir M. Cassir M. Cassir M. Cassir M. Cassir
et al, Electrochimica Acta, 1989, 34, 1563 et al, J. Electrochemical Society, 1993, 140, 3114 et al, J. Appl. Electrochem., 2000, 30, 1415 et al., J. Mater. Chem., 2010, 20, 8987. et al, Int. J. Hydrogen Energy, 2012, 37, 19345
Biographical Sketch: Armelle Ringuede, 42-years old Affiliation UMR CNRS-ENSCP-UPMC 7575 LECIME [email protected] Professional Since 2001 2009 2000-2001 1999-2000 1999
Experience & Education Research Fellow at CNRS (Charge´ de Recherche) Habilitation in Chemistry, University of Paris 6 Assistant Professor (ATER, INPGrenoble) Visiting scientist (European Training Network), Univerite´ d’Aveiro, Portugal PhD in Electrochemistry, Institut National Polytechnique de Grenoble
Publications 77 publications in peer reviewed journals and proceedings
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C. Mansour et al, J. Power Sources, 2006, 156, 23 A. Ringuede´ et al, J. Chem. Phys., 2006, 160, 789 M. Benamira et al, Int. J. Hydrogen Energy, 2012, 34, 19371 L. Z˙ivkovic et al., Electrochimica Acta., 2011, 56, 4638 M. J. Escudero et al, J. Power Sources., 2011, 196, 5546
Biographical Sketch: Virginie Lair, 36-years old Affiliation UMR CNRS-ENSCP-UPMC 7575 LECIME [email protected] Professional Since 2005 2004-2005 2003-2004 2003 2000
Experience & Education Associate Professor (Chimie ParisTech) Assistant Professor (ATER, Chimie ParisTech) Assistant Professor (ATER, Universite´ d’Evry val d’Essonne) PhD in Chemical Engineering, University of Paris 6 Master in Chemistry, University of Paris 6
Publications 26 publications in peer reviewed journals and proceedings
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V. Lair V. Lair V. Lair V. Lair
et al, et al, et al, et al,
Ionics, 2008, 14,555 Electrochimica Acta, 2010, 56, 784 Electrochimica Acta, 2011, 56, 4638 Int. J. Hydrogen Energy, 2012, 37, 19357