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Fast electrochemical CO2 transport through a dense metal-carbonate membrane: A new mechanistic insight
Journal of Membrane Science 468 (2014) 373–379
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Fast electrochemical CO2 transport through a dense metal-carbonate membrane: A new mechanistic insight Lingling Zhang a, Jingjing Tong b, Yunhui Gong a, Minfang Han b, Siwei Wang a, Kevin Huang a,n a b
Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA School of Chemical & Environment Engineering, China University of Mining & Technology, Beijing 100083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2014 Received in revised form 12 June 2014 Accepted 14 June 2014 Available online 21 June 2014
We here report a mechanistic investigation into the electrochemical transport of CO2 through a dense metalcarbonate membrane. The investigation studied a total of six possible charge-transfer mechanisms involving CO3 2 , O2 2 , O2 , CO4 2 and CO5 2 as the active surface species, and derived the corresponding flux equation from each mechanism. The experimental CO2 flux densities measured under a range of chemical gradients were used to verify each mechanism by plotting the flux density against the corresponding flux expression of each mechanism. The results showed that CO4 2 scheme is the best mechanism to describe the electrochemical CO2 transport through the metal-carbonate membranes. A new transport model was also shown to describe the CO4 2 migration and charge-transfer. & 2014 Elsevier B.V. All rights reserved.
Keywords: CO2 capture Membrane Electrochemical Charge-transfer Flux density
1. Introduction The global-warming and climate-change catastrophe caused by the atmospheric release of greenhouse gas CO2 from power/ chemical plants operated on fossil fuels and billions of cars powered by the internal combustion engine has become one of the greatest challenges in human history. A realistic near-term solution to mitigate the adverse “greenhouse gas” effect without significantly changing our life style is to stabilize the current CO2 concentration in the atmosphere by capturing CO2 at the point sources of emission and storing it geologically. The state-of-the-art technologies for carbon capture at power/chemical plants are primarily based on the principles of physiochemical adsorption/ absorption. However, these technologies are expensive, cumbersome and energy intensive [1–7], making their implementation into the existing plants very difficult. Development of the next-generation cost-effective, energy-efficient and robust carbon capture technologies needs to be urgently pursued. Recently, we demonstrated a new mixed electron and carbonate-ion conducting (MECC) silver-carbonate duel-phase membrane for carbon capture [8–11]. Fig. 1 illustrates the working principle of the membranes using a syngas as the capture gas and a flue gas as the feed gas. With electrochemical principles, the new
n
Corresponding author. Tel.: þ 1 803 777 0204. E-mail address: [email protected] (K. Huang).
http://dx.doi.org/10.1016/j.memsci.2014.06.028 0376-7388/& 2014 Elsevier B.V. All rights reserved.
membrane is able to transport CO2 from a CO2-source to a capture reservoir selectively, efficiently and continuously. Moreover, the high operating temperature of the membrane (500–650 1C) makes it more compatible with high-temperature process-streams commonly encountered in combustion processes than the conventional solvents/sorbents based technologies, demonstrating its potential to be a cost-effective and energy-efficient CO2 capture technology. So far, we have obtained a CO2 permeation flux as high as 0.8 mL cm 2 min 1 at 650 1C from this type of membrane, the highest value among all metal-carbonate systems reported [8,9,12]. However a thorough fundamental understanding of the profoundly high CO2 permeation flux is still lacking. The present study is aimed to fill this scientific gap by investigating a number of charge-transfer mechanisms and rate-limiting steps possibly involved in the electrochemical CO2 transport through the membrane.
2. Experimental procedure 2.1. Synthesis of silver-carbonate membranes A two-step approach was employed to synthesize the dualphase silver-carbonate membranes. The first step is to fabricate a porous silver matrix by intimately mixing silver powders (99.9%, Alfa Aesar) with carbon black (Alfa Aesar) as a pore former in a ratio of 60:40 (vol%), ball-milling (Mix/Mill 8000M, Spex Sample
374
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Prep) and uniaxially pressing the mixture at 200 MPa into pellets (∅20 mm) using a static press, and finally sintering at 650 1C in air for 2 h. The microstructure of thus formed silver porous network is shown in Fig. 2(a). The second step is to fabricate the silvercarbonate composite by immersing the porous silver pellet into a carbonate melt containing a eutectic mixture of Li2CO3 (Z99%, Alfa Aesar) and K2CO3 ( Z99%, Alfa Aesar) in 62:38 (mol%) ratio. The details about this procedure can also be found in our previous work [8–10]. The dense microstructure of the formed composite is shown in Fig. 2(b). The obtained membrane was finally polished on both surfaces using ethanol as a medium to remove the residual carbonate left after infiltration, followed by gas tightness check using a homemade leak-check device before it was finally assembled into a permeation cell. The effective permeation area of the membrane was 0.92 cm2.
2.2. CO2 flux measurement The configurations of CO2 permeation cell used in this study have been previously described in detail [8–10]. The button-type membrane was first sealed to a supporting alumina tube with a commercial silver paste as the sealant. A second short alumina tube was then bonded to the top surface of the sample to shield the feed gas. To study the effect of CO2 concentration on flux density, the flow rates of CO2 and O2 were systematically varied in the range of 3–35 and 20–70 mL min 1, respectively, while constantly keeping their ratio as 2:1. While varying the concentrations of CO2 and O2, the flow rate of N2 served as the balance to keep the
total gas flow (CO2 þ O2 þN2) rate at 120 mL min 1 for all variations. The actual flow rates of CO2, O2 and N2 used for the study are given in Table 1. A high-purity helium (99.999%) at a flow rate of 50 mL min 1 was used as the sweeping gas, the composition of which was analyzed by an on-line micro-GC (Varian 490-GC, Varian). To ensure the accuracy, the GC was pre-calibrated with four standard gas compositions for each gas of interest (CO2, O2, and N2) prior to the measurement. The final CO2 flux density was calculated out from an averaged gas composition of 10 successive readings by the GC. For all the gas flows, commercial mass flow controllers (SmartTrak, 50 Series) specifically calibrated for the gas under use were employed to control the mass flow rates. At each concentration, approximately 30 min were given to allow the cell to reach steady state before sampling. The N2 concentration leaked through membrane during the entire period of measurement was very low, averaging o0.01% (sometimes not even detectable), proving silver sealing is an excellent method to achieve gastight cell.
3. Electrochemical CO2 transport through a metal-carbonate membrane under various surface charge-transfer mechanisms 3.1. Bulk-diffusion controlled CO2 transport flux density For the bulk-diffusion controlled transport through a homogenous membrane containing multiple charged particles, the partial flux density Ji of the charged particle i can be written by in a generalized form: Ji ¼
Di C i σ σ ∇ηi ¼ i 2 ∇ηi ¼ i 2 ð∇μi þ zi F∇ϕÞ RT ðzi FÞ ðzi FÞ
ð1 1Þ
where Di, Ci, σi and zi represent the self-diffusivity, concentration, conductivity and charge of species i, respectively; ηi and μi are electrochemical and chemical potentials of species i, respectively; ϕ is the static potential; ∇ is a symbol for gradient; R, F and T have their usual meanings. If the metal-carbonate membrane is a homogenous, binary, mixed carbonate-ion and electron conductor, Table 1 Mass flow rates (mL min 1) of CO2, O2 and N2 used to make different concentrations of the feeding gas.
Fig. 1. The working principles of CO2-selective electrochemical metal-carbonate separation membrane. A syngas H2 þ CO is used as the capturing gas to show the principles. The captured CO2 can then be easily separated from a mixture of CO2 and H2O.
CO2 O2 N2
Gas-1
Gas-2
Gas-3
Gas-4
Gas-5
Gas-6
Gas-7
5 10 105
10 20 90
15 30 75
20 40 60
25 50 45
30 60 30
35 70 15
Fig. 2. SEM-BSE images of (a) a porous Ag network created by carbon black pore-former and (b) silver-carbonate membrane. The darker area is the carbonate phase while the brighter area is the Ag phase.
L. Zhang et al. / Journal of Membrane Science 468 (2014) 373–379
Eq. (1-1) can be simplified into σ 1 σ 2 ∇μ1 z1 =z2 ∇μ2 J1 ¼ σ1 þ σ2 z2 F 2
ð1 2Þ
1
where CO3 2 ¼ 1 and e ¼2. Considering CO2 þ1=2O2 þ2e ¼ CO3 2 as the global electrochemical reaction occurring at the feed-surface for the CO2 transport (a reverse reaction can be written for the permeate surface), one has ! σ CO3 2 σ e 3 1 J CO3 2 ¼ J CO2 ¼ 2 ∇ ln P CO2 þ ∇ ln P O2 2 8F σ CO3 2 þσ e ð1 3Þ The flux density J CO2 at steady-state is then obtained by integrating through the thickness of the membrane: ! Z P}CO ;P}O σ CO3 2 σ e 2 2 3 1 J CO2 ¼ 2 d ln P CO2 þ d ln P O2 2 σ CO3 2 þ σ e 8F L P ′CO ;P ′O 2
2
ð1 4Þ P }CO2 ;
P }O2
P ′CO2 ;
P ′O2
and represent the partial pressures of Note that CO2 and O2 at the permeate and feed side, respectively. Since σ e ⪢σ CO3 2 for a metal-carbonate membrane, Eq. (1-4) can be further simplified into Z P}CO ;P}O 2 2 3 1 J CO2 ¼ 2 σ CO3 2 d ln P CO2 þ d ln P O2 ð1 5Þ 2 8F L P ′CO2 ;P ′O2 For a dual-phase such as silver-carbonate membrane studied herein, however, the microstructural and volumetric effects on J CO2 must be taken into account. A simple correction to the flux Eq. (1-5) for a dual-phase membrane can be rationalized by multiplying J CO2 of Eq. (1-5) by microstructural factor ε/τ and volumetric fraction φ, yielding Z P ′CO ;P ′O 2 2 ε 3 1 J CO2 ¼ φ ð1 6Þ ðσ CO3 2 Þ d ln P CO2 þ d ln P O2 2 τ 8F L 2 P }CO ;P }O 2
2
Here ε and τ are porosity and tortuosity of the porous silver network, respectively, and φ is the volumetric fraction of the carbonate phase. This simple correction approach has been previously confirmed to be effective by the authors and other groups [13–15]. Eq. (1-6) then serves as the new flux equation for CO2 transport through a dual-phase metal-carbonate membrane to be discussed in the following. 3.2. Case study In the following case studies, only single rate-limiting step is assumed for the sake of simplicity. These single-step rate-limiting mechanisms can be applied as the foundation to more complex multiple-step rate-limiting processes. 3.2.1. Case-1: Very fast surface reaction Assuming that the surface electrochemical reaction CO2 þ 1=2O2 þ 2e ¼ CO3 2 is so fast that the formation of CO3 2 is instantaneous, the conductivity of CO3 2 , σ CO2 , which is directly 3 proportional to the surface concentration of CO3 2 or [CO3 2 ]s according to the Nernst–Einstein equation σ CO3 2 ¼ ½CO3 2 s quCO3 2 , becomes a constant irrespective of P CO2 and P O2 . Under a steady-state condition, Eq. (1-6) can then be integrated into ε 3RT }1=2 ′1=2 J CO2 ¼ φ 2 σ CO3 2 ðlnðP }CO2 P O2 Þ lnðP ′CO2 P O2 ÞÞ ð2 1Þ τ 8F L }1=2
This expression implies that a plot of J CO2 vs ðlnðP }CO2 P O2 Þ ′1=2 lnðP ′CO2 P O2 ÞÞ would yield a straight-line if the surface reaction is indeed fast enough to make [CO3 2 ]s a constant.
375
3.2.2. Case-2: CO3 2 as a rate-limiting active surface species If the global electrochemical reaction CO2 þ1=2O2 þ 2e ¼ CO3 2 is the only and rate-limiting step involved in the charge transfer, [CO3 2 ]s can then be expressed as a function of P CO2 and P O2 in the form of ½CO3 2 s ¼ k2 P CO2 P O2
1=2
ð2 2Þ
where k2 is the rate constant of the reaction. According to the Nernst–Einstein equation, σ CO3 2 can be written by σ CO3 2 ¼ ½CO3 2 s quCO3 2 ¼ K 2 P CO2 P O2
1=2
ð2–3Þ
here q and uCO3 2 are the charge and drift mobility of CO3 2 , respectively; K2 is a constant summarizing k2, q and uCO3 2 . Substitution of Eq. (2-3) into Eq. (1-6) followed by integration across the membrane thickness yields the following stead-state flux density of CO2: ε 3RT }1=2 ′1=2 φ J CO2 ¼ ð2 4Þ K 2 ðP }CO2 P O2 P ′CO2 P O2 Þ τ 8F 2 L }1=2
This relationship suggests that a plot of J CO2 vs ðP }CO2 P O2 ′1=2 P ′CO2 P O2 Þ would yield a straight-line if CO3 2 is indeed a ratelimiting active surface species. 3.2.3. Case-3: Peroxide-ion O2 2 as a rate-limiting active surface species Davé et al. [16] has previously indicated that the physical solubility of oxygen in molten alkali carbonates is insignificant because oxygen readily reacts with the melt to form electroactive species such as peroxide ion O2 2 and/or superoxide O2 under oxygen-rich environments. Early research in molten carbonate fuel cells (MCFCs) has also identified peroxide-ions (O2 2 ) and superoxide-ions (O2 ) as the active intermediate species [17– 20]; these intermediate species play an important role in CO2 and O2 reduction at the cathode of MCFCs. For the case of O2 2 , the rate-limiting step in the surface reaction is considered by 1 O2 þCO3 2 ¼ O2 2 þ CO2 2
ð3 1Þ
which is followed by the subsequent fast elementary chargetransfer steps of O2 2 þ e ¼ ðO Þ þ O2
ð3 2Þ
ðO Þ þ CO2 þe ¼ ðO Þ þ CO3 2
ð3 3Þ
O2 þCO2 ¼ CO3 2
ð3 4Þ
2
where e , O and (O ) represent electron, oxide-ion and transient oxygen species, respectively. Summation of Eqs. (3-1)– (3–4) leads to the overall O2 and CO2 reduction reaction: 1=2O2 þ CO2 þ 2e ¼ CO3 2 . The surface concentration of O2 2 , [O2 2 ]s, involved in the rate-limiting step (3-1) can, therefore, be expressed as a function of P CO2 and P O2 by 1 ½O2 2 s ¼ k3 P CO P 2 O2
1=2
ð3 5Þ
where k3 is the rate-constant of reaction (3-1). Since [O2 2 ]s determines [CO3 2 ]s necessary for the bulk transport of CO2, [CO3 2 ]s and therefore σ CO3 2 should also follow the same dependence on P CO2 and P O2 : 1 σ CO2 ¼ K 3 P CO P 2 O2
1=2
3
ð3 6Þ
Substitution of Eq. (3-6) into Eq. (1-6), followed by integration over the membrane thickness yields steady-state flux density of CO2: ε 3RT 1 }1=2 1 ′1=2 J CO2 ¼ P O2 P ′CO P O2 Þ ð3 7Þ φK 3 ðP }CO 2 2 τ 8F 2 L
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L. Zhang et al. / Journal of Membrane Science 468 (2014) 373–379 }1=2
′1=2
1 1 This relationship infers that a plot of J CO2 vs ðP }CO P O2 P ′CO P O2 Þ 2 2 would yield a straight-line if O2 2 is indeed a rate-limiting active surface species.
3.2.6. Case-6: Superoxycarbonate-ion CO5 2 as a rate-limiting surface active species Parallel to the CO4 2 mechanism is the superoxycarbonate-ion CO5 2 counterpart with the following rate-limiting step:
3.2.4. Case-4: Superoxide-ion O2 as a rate-limiting active surface species A parallel mechanism to peroxide-ion proposed in prior research is the involvement of superoxide-ion O2 through the following rate-limiting step:
O2 þ CO3 2 ¼ CO5 2
4 1 1 O2 þ CO3 2 ¼ O2 þ CO2 3 2 2
ð4 1Þ
followed by the subsequent fast elementary charge-transfer steps: O2
þe
¼ O2
2
ð4 2Þ
O2 þ e ¼ ðO Þ þ O2
ðO Þ þCO2 þ e 2
O
¼ CO3
þ CO2 ¼ CO3
ð4 3Þ
2
ð4 4Þ
2
ð4–5Þ
The overall reaction remains as 1=2O2 þ CO2 þ 2e ¼ CO3 2 . The surface concentration of superoxide-ion O2 , [O2 ]s, follows the dependence of P CO2 and P O2 in the form of 1=2 3=4
½O2 s ¼ k4 P CO2 P O2
ð6 1Þ
The subsequent fast charge-transfer steps that lead to the overall reaction 1=2O2 þ CO2 þ 2e ¼ CO3 2 when combined with reaction (6-1) are written by CO5 2 þ2e ¼ CO3 2 þ 2ðO Þ
ð6 2Þ
ðO Þ þ e ¼ O2
ð6 3Þ
O2 þ CO2 ¼ CO3 2
ð6 4Þ
From reaction (6-1), the dependence of the surface concentration of CO5 2 , [CO5 2 ]s, on P CO2 and P O2 is as follows: ½CO5 2 s ¼ k6 P 0CO2 P 1O2
ð6 5Þ
which ultimately leads to a steady state flux density of CO2: ε 3RT φK 6 ðP }O2 ln P }CO2 P ′O2 ln P ′CO2 Þ J CO2 ¼ ð6 6Þ τ 8F 2 L This relationship implies that a plot of J CO2 vs ðP }O2 ln P }CO2 P ′O2 ln P ′CO2 Þ would yield a straight-line if CO5 2 is indeed a ratelimiting active surface species.
ð4 6Þ 2
Since [O2 ]s determines [CO3 ]s, thus σ CO3 2 , it leads to ε 3RT 1 }1=2 1 ′1=2 φK 4 ðP }CO J CO2 ¼ P O2 P ′CO P O2 Þ ð4 7Þ 2 2 τ 8F 2 L }1=2
1 P O2 This relationship implies that a plot of J CO2 vs ðP }CO 2 ′ 1 ′1=2 P CO2 P O2 Þ would yield a straight-line if O2 is indeed a ratelimiting active surface species.
3.2.5. Case-5: Peroxycarbonate-ion CO4 2 as a rate-limiting surface active species We have recently argued that the active surface species in the reduction of CO2 and O2 could be peroxycarbonate-ion CO4 2 and/ or superoxycarbonate-ion CO5 2 based on our DFT modeling and in-situ Raman spectroscopic studies [21,22]. For the CO4 2 mechanism, the rate-limiting step can be described by 1 O2 þ CO3 2 ¼ CO4 2 2
ð5 1Þ
The subsequent fast elementary charge-transfer steps that lead to the overall reaction of 1=2O2 þ CO2 þ2e ¼ CO3 2 when combined with reaction (5-1) are represented by CO4 2 þ e ¼ CO3 2 þðO Þ
ð5 2Þ
ðO Þ þe ¼ O2
ð5 3Þ
O2 þ CO2 ¼ CO3 2
ð5 4Þ
From reaction (5-1), the dependence of the surface concentration of CO4 2 , [CO4 2 ]s, on P CO2 and P O2 is as follows: ½CO4 2 s ¼ k5 P 0CO2 P O2
1=2
ð5 5Þ
which ultimately leads to a steady state flux density of CO2: ε 3RT }1=2 ′1=2 J CO2 ¼ ð5 6Þ φK 5 ðP O2 ln P }CO2 P O2 ln P ′CO2 Þ τ 8F 2 L }1=2
This relationship suggests that a plot of J CO2 vs ðP O2 ln P }CO2 ′1=2 P O2 ln P ′CO2 Þ would yield a straight-line if CO4 2 is indeed a ratelimiting active surface species.
3.3. Experimental results vs mechanisms The flux densities of CO2 and O2 measured at 600 1C and under different chemical gradients of CO2 and O2 from a 1.23-mm-thick silver-carbonate membrane are plotted in Fig. 3 against each mechanism proposed above. For all the measurements, the ratio of J CO2 =J O2 follows closely 2:1, confirming the overall reaction of CO2 transport through the silver-carbonate membrane is indeed 1=2O2 þ CO2 þ 2e ¼ CO3 2 . However, the actual rate of CO2 transport (i.e. the magnitude of J CO2 ; J O2 ) is determined by the slowest step or the rate-limiting step among all the elementary reactions listed above. The degree of linearity of J CO2 vs the analytical expression derived from one rate-limiting mechanism can suggest the likelihood of the dominating mechanism. However, we do acknowledge that the identification may not be exclusive due to the phenomenological nature of the study. A sole elucidation of the mechanism would require in-situ surface analysis of the membrane under operating condition. A quick glance of Fig. 3 easily rules out the first three mechanisms: very fast surface reaction, CO3 2 and O2 2 as the intermediate surface rate-limiting species because of their poorer linearity. The other three mechanisms, viz. O2 , CO4 2 and CO5 2 , all show a relatively good linearity; but a close examination indicates that CO4 2 mechanism yields the best linearity with a R-square value 0.990, whereas the other two have lower R-square value 0.976 and 0.977 for O2 and CO5 2 , respectively. Therefore, we conclude that the best rate-limiting mechanism describing the CO2 transport through a metal-carbonate membrane follows the formation of CO4 2 . In fact, the existence of CO4 2 has been experimentally observed by in-situ Raman spectroscopy [22], and theoretically confirmed by DFT modeling [23]. 3.4. A new CO2 transport model The above discussion, while extensive, remains phenomenological. To fully understand how CO2 is transported through the membrane via the CO4 2 mechanism, a physical model is needed. We herein propose a new transport model centered on CO4 2 as the rate-limiting surface active species. Fig. 4(a) illustrates
L. Zhang et al. / Journal of Membrane Science 468 (2014) 373–379
}1=2
′1=2
377
}1=2
′1=2
}1=2
′1=2
1 1 Fig. 3. The CO2 and O2 flux densities of silver-carbonate as a function of (a)lnðP }CO2 P O2 Þ lnðP ′CO2 P O2 Þ; (b) P }CO2 P O2 P ′CO2 P O2 ; (c) P }CO P O2 P ′CO P O2 ; (d) 2 2 }1=2 ′1=2 1 }1=2 1 ′1=2 P }CO P O2 P ′CO P O2 ; (e) P O2 ln P }CO2 P O2 ln P ′CO2 ; (f) P }O2 ln P }CO2 P ′O2 ln P ′CO2 . 2 2
schematically each elementary step involved in the chargetransfer process. In general, molten carbonate (MC) can provide a “soft” surface for O2 molecules to stick on [18,19,23], and further allow O2 to chemically dissolve into MC as CO4 2 through Eq. (51). The formed CO4 2 can then migrate in the medium of MC through a cooperative “cogwheel” (or “paddle-wheel”) mechanism to the MC/silver surface, where it is reduced by electrons to CO3 2 and (O ) (Eq. (5-2)). A schematic of the “cogwheel” mechanism similar to those described for ionic conductions in Li2SO4 and LaSrGaO4 phases [24,25] is shown in Fig. 4(b), which involves the breaking and reforming of O–CO3 2 bond within and between CO4 2 . Since the O in CO4 2 is weakly bonded, the migration of CO4 2 in MC is expected to be fast. The formed transient species (O ) as previously reported by Nicholson and White [16,26] is further reduced by electrons available at the MC/silver surface to
O2 (Eq. (5-3)); the latter readily reacts with CO2 to form CO3 2 via reaction (5-4) to complete the final transport of CO2 through the membrane. The CO5 2 mechanism could be viewed as a derivative of the CO4 2 mechanism since a restructuring of CO5 2 within CO3 2 may result in the formation of CO4 2 via the reaction CO5 2 þ CO23 ¼ 2CO4 2 .
4. Conclusions The dependence of CO2 and O2 flux densities of a silvercarbonate membrane on chemical gradients was studied at 600 1C over a range of feed-gas concentrations. Six rate-limiting mechanisms involving very fast surface reaction and CO3 2 , O2 2 ,
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L. Zhang et al. / Journal of Membrane Science 468 (2014) 373–379
Fig. 4. (a) A 3D representation of the new charge-transfer model for the silver-carbonate membrane; (b) CO4 2 migration through a cooperative “cogwheel” mechanism.
O2 , CO4 2 and CO5 2 active surface species were proposed. The validity of each mechanism was closely examined by plotting the measured CO2 flux density against the flux equations derived from each mechanism. Based on the degree of linearity, CO4 2 scheme was concluded to be the best mechanism to describe the CO2 transport through metal-carbonate membranes. A transport model encompassing CO4 2 as the rate-limiting species and migrating via a cooperative “cogwheel” mechanism was also proposed.
Acknowledgment Financial support from NSF (CBET-1340269, CBET-124706) and U.S. Army Research Office (W911NF-10-R-006 and W911NF-13-10158) are greatly appreciated.
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[22] L.J. Chen, C.J. Lin, J. Zuo, L.C. Song, C.M. Huang, First spectroscopic observation of peroxocarbonate/peroxodicarbonate in molten carbonate, J. Phys. Chem. B 108 (2004) 7553–7556. [23] Y. Gong, X. Li, L. Zhang, W. Tharp, C. Qin, K. Huang, Molten carbonates as an effective oxygen reduction catalyst for 550–650 1C solid oxide fuel cells, J. Electrochem. Soc. 160 (9) (2013) F958–F964. [24] R. Kaber, L. Nilsson, N.H. Andersen, A. Lundén, J.O. Thomas, A single-crystal neutron diffraction study of the structure of the high-temperature rotor phase of lithium sulphate, J. Phys.: Condens. Matter 4 (1992) 1925. [25] E. Kendrick, J. Kendrick, K.S. Knight, M. Saiful Islam, P.R. Slater, Cooperative mechanisms of fast-ion conduction in gallium-based oxides with tetrahedral moieties, Nat. Mater. 6 (2007) 871. [26] A.J. Appleby, S.B. Nicholson, Reduction of oxygen in lithium-potassium carbonate melts, J. Electroanal. Chem. 112 (1980) 71–76.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Fast electrochemical CO2 transport through a dense metal-carbonate membrane: A new mechanistic insight Lingling Zhang a, Jingjing Tong b, Yunhui Gong a, Minfang Han b, Siwei Wang a, Kevin Huang a,n a b
Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA School of Chemical & Environment Engineering, China University of Mining & Technology, Beijing 100083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2014 Received in revised form 12 June 2014 Accepted 14 June 2014 Available online 21 June 2014
We here report a mechanistic investigation into the electrochemical transport of CO2 through a dense metalcarbonate membrane. The investigation studied a total of six possible charge-transfer mechanisms involving CO3 2 , O2 2 , O2 , CO4 2 and CO5 2 as the active surface species, and derived the corresponding flux equation from each mechanism. The experimental CO2 flux densities measured under a range of chemical gradients were used to verify each mechanism by plotting the flux density against the corresponding flux expression of each mechanism. The results showed that CO4 2 scheme is the best mechanism to describe the electrochemical CO2 transport through the metal-carbonate membranes. A new transport model was also shown to describe the CO4 2 migration and charge-transfer. & 2014 Elsevier B.V. All rights reserved.
Keywords: CO2 capture Membrane Electrochemical Charge-transfer Flux density
1. Introduction The global-warming and climate-change catastrophe caused by the atmospheric release of greenhouse gas CO2 from power/ chemical plants operated on fossil fuels and billions of cars powered by the internal combustion engine has become one of the greatest challenges in human history. A realistic near-term solution to mitigate the adverse “greenhouse gas” effect without significantly changing our life style is to stabilize the current CO2 concentration in the atmosphere by capturing CO2 at the point sources of emission and storing it geologically. The state-of-the-art technologies for carbon capture at power/chemical plants are primarily based on the principles of physiochemical adsorption/ absorption. However, these technologies are expensive, cumbersome and energy intensive [1–7], making their implementation into the existing plants very difficult. Development of the next-generation cost-effective, energy-efficient and robust carbon capture technologies needs to be urgently pursued. Recently, we demonstrated a new mixed electron and carbonate-ion conducting (MECC) silver-carbonate duel-phase membrane for carbon capture [8–11]. Fig. 1 illustrates the working principle of the membranes using a syngas as the capture gas and a flue gas as the feed gas. With electrochemical principles, the new
n
Corresponding author. Tel.: þ 1 803 777 0204. E-mail address: [email protected] (K. Huang).
http://dx.doi.org/10.1016/j.memsci.2014.06.028 0376-7388/& 2014 Elsevier B.V. All rights reserved.
membrane is able to transport CO2 from a CO2-source to a capture reservoir selectively, efficiently and continuously. Moreover, the high operating temperature of the membrane (500–650 1C) makes it more compatible with high-temperature process-streams commonly encountered in combustion processes than the conventional solvents/sorbents based technologies, demonstrating its potential to be a cost-effective and energy-efficient CO2 capture technology. So far, we have obtained a CO2 permeation flux as high as 0.8 mL cm 2 min 1 at 650 1C from this type of membrane, the highest value among all metal-carbonate systems reported [8,9,12]. However a thorough fundamental understanding of the profoundly high CO2 permeation flux is still lacking. The present study is aimed to fill this scientific gap by investigating a number of charge-transfer mechanisms and rate-limiting steps possibly involved in the electrochemical CO2 transport through the membrane.
2. Experimental procedure 2.1. Synthesis of silver-carbonate membranes A two-step approach was employed to synthesize the dualphase silver-carbonate membranes. The first step is to fabricate a porous silver matrix by intimately mixing silver powders (99.9%, Alfa Aesar) with carbon black (Alfa Aesar) as a pore former in a ratio of 60:40 (vol%), ball-milling (Mix/Mill 8000M, Spex Sample
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L. Zhang et al. / Journal of Membrane Science 468 (2014) 373–379
Prep) and uniaxially pressing the mixture at 200 MPa into pellets (∅20 mm) using a static press, and finally sintering at 650 1C in air for 2 h. The microstructure of thus formed silver porous network is shown in Fig. 2(a). The second step is to fabricate the silvercarbonate composite by immersing the porous silver pellet into a carbonate melt containing a eutectic mixture of Li2CO3 (Z99%, Alfa Aesar) and K2CO3 ( Z99%, Alfa Aesar) in 62:38 (mol%) ratio. The details about this procedure can also be found in our previous work [8–10]. The dense microstructure of the formed composite is shown in Fig. 2(b). The obtained membrane was finally polished on both surfaces using ethanol as a medium to remove the residual carbonate left after infiltration, followed by gas tightness check using a homemade leak-check device before it was finally assembled into a permeation cell. The effective permeation area of the membrane was 0.92 cm2.
2.2. CO2 flux measurement The configurations of CO2 permeation cell used in this study have been previously described in detail [8–10]. The button-type membrane was first sealed to a supporting alumina tube with a commercial silver paste as the sealant. A second short alumina tube was then bonded to the top surface of the sample to shield the feed gas. To study the effect of CO2 concentration on flux density, the flow rates of CO2 and O2 were systematically varied in the range of 3–35 and 20–70 mL min 1, respectively, while constantly keeping their ratio as 2:1. While varying the concentrations of CO2 and O2, the flow rate of N2 served as the balance to keep the
total gas flow (CO2 þ O2 þN2) rate at 120 mL min 1 for all variations. The actual flow rates of CO2, O2 and N2 used for the study are given in Table 1. A high-purity helium (99.999%) at a flow rate of 50 mL min 1 was used as the sweeping gas, the composition of which was analyzed by an on-line micro-GC (Varian 490-GC, Varian). To ensure the accuracy, the GC was pre-calibrated with four standard gas compositions for each gas of interest (CO2, O2, and N2) prior to the measurement. The final CO2 flux density was calculated out from an averaged gas composition of 10 successive readings by the GC. For all the gas flows, commercial mass flow controllers (SmartTrak, 50 Series) specifically calibrated for the gas under use were employed to control the mass flow rates. At each concentration, approximately 30 min were given to allow the cell to reach steady state before sampling. The N2 concentration leaked through membrane during the entire period of measurement was very low, averaging o0.01% (sometimes not even detectable), proving silver sealing is an excellent method to achieve gastight cell.
3. Electrochemical CO2 transport through a metal-carbonate membrane under various surface charge-transfer mechanisms 3.1. Bulk-diffusion controlled CO2 transport flux density For the bulk-diffusion controlled transport through a homogenous membrane containing multiple charged particles, the partial flux density Ji of the charged particle i can be written by in a generalized form: Ji ¼
Di C i σ σ ∇ηi ¼ i 2 ∇ηi ¼ i 2 ð∇μi þ zi F∇ϕÞ RT ðzi FÞ ðzi FÞ
ð1 1Þ
where Di, Ci, σi and zi represent the self-diffusivity, concentration, conductivity and charge of species i, respectively; ηi and μi are electrochemical and chemical potentials of species i, respectively; ϕ is the static potential; ∇ is a symbol for gradient; R, F and T have their usual meanings. If the metal-carbonate membrane is a homogenous, binary, mixed carbonate-ion and electron conductor, Table 1 Mass flow rates (mL min 1) of CO2, O2 and N2 used to make different concentrations of the feeding gas.
Fig. 1. The working principles of CO2-selective electrochemical metal-carbonate separation membrane. A syngas H2 þ CO is used as the capturing gas to show the principles. The captured CO2 can then be easily separated from a mixture of CO2 and H2O.
CO2 O2 N2
Gas-1
Gas-2
Gas-3
Gas-4
Gas-5
Gas-6
Gas-7
5 10 105
10 20 90
15 30 75
20 40 60
25 50 45
30 60 30
35 70 15
Fig. 2. SEM-BSE images of (a) a porous Ag network created by carbon black pore-former and (b) silver-carbonate membrane. The darker area is the carbonate phase while the brighter area is the Ag phase.
L. Zhang et al. / Journal of Membrane Science 468 (2014) 373–379
Eq. (1-1) can be simplified into σ 1 σ 2 ∇μ1 z1 =z2 ∇μ2 J1 ¼ σ1 þ σ2 z2 F 2
ð1 2Þ
1
where CO3 2 ¼ 1 and e ¼2. Considering CO2 þ1=2O2 þ2e ¼ CO3 2 as the global electrochemical reaction occurring at the feed-surface for the CO2 transport (a reverse reaction can be written for the permeate surface), one has ! σ CO3 2 σ e 3 1 J CO3 2 ¼ J CO2 ¼ 2 ∇ ln P CO2 þ ∇ ln P O2 2 8F σ CO3 2 þσ e ð1 3Þ The flux density J CO2 at steady-state is then obtained by integrating through the thickness of the membrane: ! Z P}CO ;P}O σ CO3 2 σ e 2 2 3 1 J CO2 ¼ 2 d ln P CO2 þ d ln P O2 2 σ CO3 2 þ σ e 8F L P ′CO ;P ′O 2
2
ð1 4Þ P }CO2 ;
P }O2
P ′CO2 ;
P ′O2
and represent the partial pressures of Note that CO2 and O2 at the permeate and feed side, respectively. Since σ e ⪢σ CO3 2 for a metal-carbonate membrane, Eq. (1-4) can be further simplified into Z P}CO ;P}O 2 2 3 1 J CO2 ¼ 2 σ CO3 2 d ln P CO2 þ d ln P O2 ð1 5Þ 2 8F L P ′CO2 ;P ′O2 For a dual-phase such as silver-carbonate membrane studied herein, however, the microstructural and volumetric effects on J CO2 must be taken into account. A simple correction to the flux Eq. (1-5) for a dual-phase membrane can be rationalized by multiplying J CO2 of Eq. (1-5) by microstructural factor ε/τ and volumetric fraction φ, yielding Z P ′CO ;P ′O 2 2 ε 3 1 J CO2 ¼ φ ð1 6Þ ðσ CO3 2 Þ d ln P CO2 þ d ln P O2 2 τ 8F L 2 P }CO ;P }O 2
2
Here ε and τ are porosity and tortuosity of the porous silver network, respectively, and φ is the volumetric fraction of the carbonate phase. This simple correction approach has been previously confirmed to be effective by the authors and other groups [13–15]. Eq. (1-6) then serves as the new flux equation for CO2 transport through a dual-phase metal-carbonate membrane to be discussed in the following. 3.2. Case study In the following case studies, only single rate-limiting step is assumed for the sake of simplicity. These single-step rate-limiting mechanisms can be applied as the foundation to more complex multiple-step rate-limiting processes. 3.2.1. Case-1: Very fast surface reaction Assuming that the surface electrochemical reaction CO2 þ 1=2O2 þ 2e ¼ CO3 2 is so fast that the formation of CO3 2 is instantaneous, the conductivity of CO3 2 , σ CO2 , which is directly 3 proportional to the surface concentration of CO3 2 or [CO3 2 ]s according to the Nernst–Einstein equation σ CO3 2 ¼ ½CO3 2 s quCO3 2 , becomes a constant irrespective of P CO2 and P O2 . Under a steady-state condition, Eq. (1-6) can then be integrated into ε 3RT }1=2 ′1=2 J CO2 ¼ φ 2 σ CO3 2 ðlnðP }CO2 P O2 Þ lnðP ′CO2 P O2 ÞÞ ð2 1Þ τ 8F L }1=2
This expression implies that a plot of J CO2 vs ðlnðP }CO2 P O2 Þ ′1=2 lnðP ′CO2 P O2 ÞÞ would yield a straight-line if the surface reaction is indeed fast enough to make [CO3 2 ]s a constant.
375
3.2.2. Case-2: CO3 2 as a rate-limiting active surface species If the global electrochemical reaction CO2 þ1=2O2 þ 2e ¼ CO3 2 is the only and rate-limiting step involved in the charge transfer, [CO3 2 ]s can then be expressed as a function of P CO2 and P O2 in the form of ½CO3 2 s ¼ k2 P CO2 P O2
1=2
ð2 2Þ
where k2 is the rate constant of the reaction. According to the Nernst–Einstein equation, σ CO3 2 can be written by σ CO3 2 ¼ ½CO3 2 s quCO3 2 ¼ K 2 P CO2 P O2
1=2
ð2–3Þ
here q and uCO3 2 are the charge and drift mobility of CO3 2 , respectively; K2 is a constant summarizing k2, q and uCO3 2 . Substitution of Eq. (2-3) into Eq. (1-6) followed by integration across the membrane thickness yields the following stead-state flux density of CO2: ε 3RT }1=2 ′1=2 φ J CO2 ¼ ð2 4Þ K 2 ðP }CO2 P O2 P ′CO2 P O2 Þ τ 8F 2 L }1=2
This relationship suggests that a plot of J CO2 vs ðP }CO2 P O2 ′1=2 P ′CO2 P O2 Þ would yield a straight-line if CO3 2 is indeed a ratelimiting active surface species. 3.2.3. Case-3: Peroxide-ion O2 2 as a rate-limiting active surface species Davé et al. [16] has previously indicated that the physical solubility of oxygen in molten alkali carbonates is insignificant because oxygen readily reacts with the melt to form electroactive species such as peroxide ion O2 2 and/or superoxide O2 under oxygen-rich environments. Early research in molten carbonate fuel cells (MCFCs) has also identified peroxide-ions (O2 2 ) and superoxide-ions (O2 ) as the active intermediate species [17– 20]; these intermediate species play an important role in CO2 and O2 reduction at the cathode of MCFCs. For the case of O2 2 , the rate-limiting step in the surface reaction is considered by 1 O2 þCO3 2 ¼ O2 2 þ CO2 2
ð3 1Þ
which is followed by the subsequent fast elementary chargetransfer steps of O2 2 þ e ¼ ðO Þ þ O2
ð3 2Þ
ðO Þ þ CO2 þe ¼ ðO Þ þ CO3 2
ð3 3Þ
O2 þCO2 ¼ CO3 2
ð3 4Þ
2
where e , O and (O ) represent electron, oxide-ion and transient oxygen species, respectively. Summation of Eqs. (3-1)– (3–4) leads to the overall O2 and CO2 reduction reaction: 1=2O2 þ CO2 þ 2e ¼ CO3 2 . The surface concentration of O2 2 , [O2 2 ]s, involved in the rate-limiting step (3-1) can, therefore, be expressed as a function of P CO2 and P O2 by 1 ½O2 2 s ¼ k3 P CO P 2 O2
1=2
ð3 5Þ
where k3 is the rate-constant of reaction (3-1). Since [O2 2 ]s determines [CO3 2 ]s necessary for the bulk transport of CO2, [CO3 2 ]s and therefore σ CO3 2 should also follow the same dependence on P CO2 and P O2 : 1 σ CO2 ¼ K 3 P CO P 2 O2
1=2
3
ð3 6Þ
Substitution of Eq. (3-6) into Eq. (1-6), followed by integration over the membrane thickness yields steady-state flux density of CO2: ε 3RT 1 }1=2 1 ′1=2 J CO2 ¼ P O2 P ′CO P O2 Þ ð3 7Þ φK 3 ðP }CO 2 2 τ 8F 2 L
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L. Zhang et al. / Journal of Membrane Science 468 (2014) 373–379 }1=2
′1=2
1 1 This relationship infers that a plot of J CO2 vs ðP }CO P O2 P ′CO P O2 Þ 2 2 would yield a straight-line if O2 2 is indeed a rate-limiting active surface species.
3.2.6. Case-6: Superoxycarbonate-ion CO5 2 as a rate-limiting surface active species Parallel to the CO4 2 mechanism is the superoxycarbonate-ion CO5 2 counterpart with the following rate-limiting step:
3.2.4. Case-4: Superoxide-ion O2 as a rate-limiting active surface species A parallel mechanism to peroxide-ion proposed in prior research is the involvement of superoxide-ion O2 through the following rate-limiting step:
O2 þ CO3 2 ¼ CO5 2
4 1 1 O2 þ CO3 2 ¼ O2 þ CO2 3 2 2
ð4 1Þ
followed by the subsequent fast elementary charge-transfer steps: O2
þe
¼ O2
2
ð4 2Þ
O2 þ e ¼ ðO Þ þ O2
ðO Þ þCO2 þ e 2
O
¼ CO3
þ CO2 ¼ CO3
ð4 3Þ
2
ð4 4Þ
2
ð4–5Þ
The overall reaction remains as 1=2O2 þ CO2 þ 2e ¼ CO3 2 . The surface concentration of superoxide-ion O2 , [O2 ]s, follows the dependence of P CO2 and P O2 in the form of 1=2 3=4
½O2 s ¼ k4 P CO2 P O2
ð6 1Þ
The subsequent fast charge-transfer steps that lead to the overall reaction 1=2O2 þ CO2 þ 2e ¼ CO3 2 when combined with reaction (6-1) are written by CO5 2 þ2e ¼ CO3 2 þ 2ðO Þ
ð6 2Þ
ðO Þ þ e ¼ O2
ð6 3Þ
O2 þ CO2 ¼ CO3 2
ð6 4Þ
From reaction (6-1), the dependence of the surface concentration of CO5 2 , [CO5 2 ]s, on P CO2 and P O2 is as follows: ½CO5 2 s ¼ k6 P 0CO2 P 1O2
ð6 5Þ
which ultimately leads to a steady state flux density of CO2: ε 3RT φK 6 ðP }O2 ln P }CO2 P ′O2 ln P ′CO2 Þ J CO2 ¼ ð6 6Þ τ 8F 2 L This relationship implies that a plot of J CO2 vs ðP }O2 ln P }CO2 P ′O2 ln P ′CO2 Þ would yield a straight-line if CO5 2 is indeed a ratelimiting active surface species.
ð4 6Þ 2
Since [O2 ]s determines [CO3 ]s, thus σ CO3 2 , it leads to ε 3RT 1 }1=2 1 ′1=2 φK 4 ðP }CO J CO2 ¼ P O2 P ′CO P O2 Þ ð4 7Þ 2 2 τ 8F 2 L }1=2
1 P O2 This relationship implies that a plot of J CO2 vs ðP }CO 2 ′ 1 ′1=2 P CO2 P O2 Þ would yield a straight-line if O2 is indeed a ratelimiting active surface species.
3.2.5. Case-5: Peroxycarbonate-ion CO4 2 as a rate-limiting surface active species We have recently argued that the active surface species in the reduction of CO2 and O2 could be peroxycarbonate-ion CO4 2 and/ or superoxycarbonate-ion CO5 2 based on our DFT modeling and in-situ Raman spectroscopic studies [21,22]. For the CO4 2 mechanism, the rate-limiting step can be described by 1 O2 þ CO3 2 ¼ CO4 2 2
ð5 1Þ
The subsequent fast elementary charge-transfer steps that lead to the overall reaction of 1=2O2 þ CO2 þ2e ¼ CO3 2 when combined with reaction (5-1) are represented by CO4 2 þ e ¼ CO3 2 þðO Þ
ð5 2Þ
ðO Þ þe ¼ O2
ð5 3Þ
O2 þ CO2 ¼ CO3 2
ð5 4Þ
From reaction (5-1), the dependence of the surface concentration of CO4 2 , [CO4 2 ]s, on P CO2 and P O2 is as follows: ½CO4 2 s ¼ k5 P 0CO2 P O2
1=2
ð5 5Þ
which ultimately leads to a steady state flux density of CO2: ε 3RT }1=2 ′1=2 J CO2 ¼ ð5 6Þ φK 5 ðP O2 ln P }CO2 P O2 ln P ′CO2 Þ τ 8F 2 L }1=2
This relationship suggests that a plot of J CO2 vs ðP O2 ln P }CO2 ′1=2 P O2 ln P ′CO2 Þ would yield a straight-line if CO4 2 is indeed a ratelimiting active surface species.
3.3. Experimental results vs mechanisms The flux densities of CO2 and O2 measured at 600 1C and under different chemical gradients of CO2 and O2 from a 1.23-mm-thick silver-carbonate membrane are plotted in Fig. 3 against each mechanism proposed above. For all the measurements, the ratio of J CO2 =J O2 follows closely 2:1, confirming the overall reaction of CO2 transport through the silver-carbonate membrane is indeed 1=2O2 þ CO2 þ 2e ¼ CO3 2 . However, the actual rate of CO2 transport (i.e. the magnitude of J CO2 ; J O2 ) is determined by the slowest step or the rate-limiting step among all the elementary reactions listed above. The degree of linearity of J CO2 vs the analytical expression derived from one rate-limiting mechanism can suggest the likelihood of the dominating mechanism. However, we do acknowledge that the identification may not be exclusive due to the phenomenological nature of the study. A sole elucidation of the mechanism would require in-situ surface analysis of the membrane under operating condition. A quick glance of Fig. 3 easily rules out the first three mechanisms: very fast surface reaction, CO3 2 and O2 2 as the intermediate surface rate-limiting species because of their poorer linearity. The other three mechanisms, viz. O2 , CO4 2 and CO5 2 , all show a relatively good linearity; but a close examination indicates that CO4 2 mechanism yields the best linearity with a R-square value 0.990, whereas the other two have lower R-square value 0.976 and 0.977 for O2 and CO5 2 , respectively. Therefore, we conclude that the best rate-limiting mechanism describing the CO2 transport through a metal-carbonate membrane follows the formation of CO4 2 . In fact, the existence of CO4 2 has been experimentally observed by in-situ Raman spectroscopy [22], and theoretically confirmed by DFT modeling [23]. 3.4. A new CO2 transport model The above discussion, while extensive, remains phenomenological. To fully understand how CO2 is transported through the membrane via the CO4 2 mechanism, a physical model is needed. We herein propose a new transport model centered on CO4 2 as the rate-limiting surface active species. Fig. 4(a) illustrates
L. Zhang et al. / Journal of Membrane Science 468 (2014) 373–379
}1=2
′1=2
377
}1=2
′1=2
}1=2
′1=2
1 1 Fig. 3. The CO2 and O2 flux densities of silver-carbonate as a function of (a)lnðP }CO2 P O2 Þ lnðP ′CO2 P O2 Þ; (b) P }CO2 P O2 P ′CO2 P O2 ; (c) P }CO P O2 P ′CO P O2 ; (d) 2 2 }1=2 ′1=2 1 }1=2 1 ′1=2 P }CO P O2 P ′CO P O2 ; (e) P O2 ln P }CO2 P O2 ln P ′CO2 ; (f) P }O2 ln P }CO2 P ′O2 ln P ′CO2 . 2 2
schematically each elementary step involved in the chargetransfer process. In general, molten carbonate (MC) can provide a “soft” surface for O2 molecules to stick on [18,19,23], and further allow O2 to chemically dissolve into MC as CO4 2 through Eq. (51). The formed CO4 2 can then migrate in the medium of MC through a cooperative “cogwheel” (or “paddle-wheel”) mechanism to the MC/silver surface, where it is reduced by electrons to CO3 2 and (O ) (Eq. (5-2)). A schematic of the “cogwheel” mechanism similar to those described for ionic conductions in Li2SO4 and LaSrGaO4 phases [24,25] is shown in Fig. 4(b), which involves the breaking and reforming of O–CO3 2 bond within and between CO4 2 . Since the O in CO4 2 is weakly bonded, the migration of CO4 2 in MC is expected to be fast. The formed transient species (O ) as previously reported by Nicholson and White [16,26] is further reduced by electrons available at the MC/silver surface to
O2 (Eq. (5-3)); the latter readily reacts with CO2 to form CO3 2 via reaction (5-4) to complete the final transport of CO2 through the membrane. The CO5 2 mechanism could be viewed as a derivative of the CO4 2 mechanism since a restructuring of CO5 2 within CO3 2 may result in the formation of CO4 2 via the reaction CO5 2 þ CO23 ¼ 2CO4 2 .
4. Conclusions The dependence of CO2 and O2 flux densities of a silvercarbonate membrane on chemical gradients was studied at 600 1C over a range of feed-gas concentrations. Six rate-limiting mechanisms involving very fast surface reaction and CO3 2 , O2 2 ,
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L. Zhang et al. / Journal of Membrane Science 468 (2014) 373–379
Fig. 4. (a) A 3D representation of the new charge-transfer model for the silver-carbonate membrane; (b) CO4 2 migration through a cooperative “cogwheel” mechanism.
O2 , CO4 2 and CO5 2 active surface species were proposed. The validity of each mechanism was closely examined by plotting the measured CO2 flux density against the flux equations derived from each mechanism. Based on the degree of linearity, CO4 2 scheme was concluded to be the best mechanism to describe the CO2 transport through metal-carbonate membranes. A transport model encompassing CO4 2 as the rate-limiting species and migrating via a cooperative “cogwheel” mechanism was also proposed.
Acknowledgment Financial support from NSF (CBET-1340269, CBET-124706) and U.S. Army Research Office (W911NF-10-R-006 and W911NF-13-10158) are greatly appreciated.
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