Relationships between Electrolyte Concentration and the

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Relationships between electrolyte concentration and the supercapacitive swing adsorption of CO2 Shan Zhu, Jiajie Li, Allison Toth, and Kai M. Landskron ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 6, 2019

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ACS Applied Materials & Interfaces

Relationships between electrolyte concentration and the supercapacitive swing adsorption of CO2 Shan Zhu, Jiajie Li, Allison Toth, Kai Landskron* Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, PA 18015, USA.

ABSTRACT.

We quantitatively investigate the influence of the NaCl electrolyte concentration on the adsorptive and energetic characteristics of Supercapacitive Swing Adsorption (SSA) for the separation of CO2 from a simulated flue gas mixture containing 15% CO2 and 85% N2. The investigated concentrations were that of de-ionized water, 0.010, 0.10, 1.0, 3.0, and 5.0 M NaCl. We find that the energetic metrics strongly improve with increasing NaCl concentration, while the adsorptive metrics improve by a comparatively small degree. The CO2 adsorption capacity increases up to 1.0 M NaCl and then remains constant. The adsorption rate remains near constant for all concentrations, except that it is somewhat smaller for deionized water. The charge efficiency also remains near constant for all experiments with 30 min potentiostatic holding steps, but near doubles for pure water when the potential holding step is doubled because the chemical adsorption equilibrium is reached only after 60 min. The results can be most satisfactorily explained by assuming that both ionic and non-ionic adsorption mechanisms contribute to the SSA effect.

KEYWORDS. CO2 adsorption, CO2 separation, supercapacitive swing adsorption, electro-adsorption, porous carbon, electrolyte concentration, mechanism

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1. Introduction Supercapacitive Swing Adsorption (SSA) is a new approach to gas separations that we develop as an alternative to currently practiced gas separation methods that use pressure and temperature changes to achieve gas separation, for example temperature and pressure swing adsorption (TSA and PSA, respectively), as well as amine-scrubbing.1–3 The pressure and temperature changes in these processes lead to a large energy consumption which is a major cost driver.4–8 In some cases, the cost increase inhibits the deployment of a technology, for example the carbon dioxide capture from coal-fired power plants. For these reasons approaches to gas separation that do not require temperature and pressure changes are desirable. SSA does not require pressure or temperature changes, but relies on the reversible and selective adsorption via capacitive charge and discharge of supercapacitor electrodes using electrical energy. Electrical energy is a more expensive energy form compared to thermal energy when thermal energy is derived from waste heat. However, a supercapacitor is an energy storage device, and thus the energy used for charging the capacitor is not lost and a large fraction of the energy can be recovered. In commercial supercapacitors the energy recovery is typically 80-95% depending on the exact electrochemical characteristics of the capacitor and the charge-discharge conditions.9 Thus, it seems possible that SSA can offset the greater expense of electric energy by a more effective use of the energy. The fact that PSA technologies, which use electrical energy to produce pressure changes via compressors, are commercially deployed shows that electric-energy-driven gas separation processes are economically viable.10 SSA only requires environmentally friendly and non-toxic activated carbon materials and aqueous NaCl solutions as electrolytes which is an advantage over amine-based gas separations. These substances are also quite inexpensive. Since supercapacitors can be charged within seconds to minutes, SSA also has potential time advantages to PSA, amine-scrubbing, and TSA which usually require minutes to days, except rotaryvalve, fast-cycle PSA which can achieve about one cycle per second.11 Supercapacitive Swing Adsorption may be explained by three mechanisms: a) Adsorption of gas molecules at the gas-solid interface, b) adsorption of gas molecules at the liquid-solid interface, and c) adsorption of ionized gas molecules at the liquid-solid interface. The last mechanism is only possible in ACS Paragon Plus Environment

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case the gas molecule is ionizable. For example, CO2 is an ionizable gas because it can form HCO3- and CO32- ions in an aqueous electrolyte, while N2 is not. The three mechanisms are schematically illustrated in Figure 1 with CO2 as the gas. In the gas-solid mechanism (Figure 1a) the gas molecules adsorb to pores that are not infiltrated by the electrolyte. These pores exist due to the fact that not all pores are hydrophilic enough to be wetted by the electrolyte.12 We have recently shown via contrast-matched neutron scattering experiments that only ~20% of the pore volume of uncharged BPL activated carbon is actually infiltrated by an aqueous 1 M NaCl solution.13 The adsorptivity of the pore surfaces changes upon charging because the proximity of neighbored electrolyte-infiltrated pores and the delocalization of the charges within the extended π-system of the carbon. The delocalization leads to a shift of the Fermi level of the non-infiltrated pores which changes the affinity of the pore surfaces to the gas. The non-ionic liquid-solid mechanism (Figure 1b) occurs in the infiltrated pores, and assumes that the solubility of the gas molecule in the electrical double-layer is different compared to the bulk solution. This may be because the double layer has a different chemical composition compared to the bulk solution, and because of the extremely strong electric field gradient in a double layer which is in the order of billions of Volts per meter.14 The ionic liquid-solid mechanism (Figure 1c) is due to the Coulomb attractions between the ionized gas molecules and the charged pore walls.

Figure 1. a) Gas-solid mechanism (illustrated for adsorption at the negative electrode), b) molecular liquid-solid mechanism (illustrated for adsorption at the negative electrode), c) ionic liquid-solid mechanism (can take place at the positive electrode only).


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In our initial work we demonstrated the concept of SSA using a simple gas adsorption cell with blockshaped electrodes and a 1 M NaCl solution as electrolyte.1 We showed the selective and reversible gas adsorption of CO2 from a 15%CO2/85%N2 gas mixture via reversible pressure changes in the cell, and the gas-chromatographic analysis of the gas mixture at the end of each charge and discharge step. The chosen gas mixture acted as a flue gas simulant. However, this gas adsorption cell was not suitable for gas separation from continuous gas streams and the electrode design was highly sub-optimized with regards to its internal resistance and adsorption kinetics. We then demonstrated a first SSA module that allowed for gas separations from gas streams.2 This SSA module was based on a coin type supercapacitor with thin electrode sheets that reduced electrical resistance, and a gas diffusion layer as well as serpentine gas flow channels that enabled gas flow through the module and increased the contact area between the gas and the electrode. We then simplified the module by replacing the gas flow channels by a radial gas flow system (Figure S1), scaled it, developed quantitative energetic and adsorptive metrics for the SSA process, and investigated the impact of the different charge-discharge technique on these performance metrics, and identified the most effective method.3 Herein, we quantitatively determine the influence of the NaCl concentration in the aqueous electrolyte on the adsorptive and energetic characteristics of SSA. The investigated concentrations were that of deionized water, 0.010, 0.10, 1.0, 3.0, and 5.0 M NaCl. Higher concentrations could not be investigated due to the limited solubility of NaCl in water. The NaCl concentration of the electrolyte has potential influence on the energetics of SSA modules because the ionic conductivity and dielectric constant of an electrolyte changes with concentration. The dielectric constant has an influence on the capacitance and thus the energy stored in the capacitor. The conductivity will influence the resistance of the capacitor, and thus its energy efficiency. In addition, the NaCl concentration has potential impact on the adsorptive metrics because the changes in the capacitance may change the number of gas molecules that can be adsorbed. 2. Experimental 2.1 Reagents and solutions ACS Paragon Plus Environment

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The NaCl used in this study was ACS grade (99+% purity) purchased from Fisher Scientific. Laboratory grade water (>18 M.cm) was used for the preparation of electrolyte solutions. The investigated electrolyte concentrations were that of de-ionized water, 0.010, 0.10, 1.0, 3.0, and 5.0 M NaCl. The conductivities of these electrolytes are known from literature and shown in Figure S2. 2.2 SSA electrode and module The SSA electrodes and module were prepared using the method reported in our previous publication.2,3 Details are described in Supporting Information section S1. The SSA module was charged and discharged using the “GCD+Pstat” method,3 comprising galvanostatic charges and discharges (GCD) between 0 and -1 V with 30 min potentiostatic (Pstat) holding steps at 0 V (after discharge) and -1 V (after charge). 2.3 SSA performance metrics The SSA energetic metrics were calculated from the voltage curves, and include the electrical resistance, the specific capacitance (C, F.g-1), the Coulombic efficiency (ηc, %), the energy efficiency (ηe, %), the energy loss (∆𝐸, J), the energy consumption (EC, kJ.mol-1), and the time-energy efficiency (TEE, mol.kJ-1.s-1). The adsorptive metrics are the adsorption capacity (AC, mol.kg-1), the charge efficiency (𝛬, dimensionless), and the adsorption rate (AR, mol.kg-1.s-1). The electrical resistance of the SSA module was determined using voltage drop method15 as 𝑅=

𝑉𝑃𝑠𝑡𝑎𝑡 ― 𝑉𝑑𝑟𝑜𝑝 𝐼

,

(1)

where 𝑉𝑃𝑠𝑡𝑎𝑡 (V) is the voltage at potentiostatic step (either 0 or -1 V), 𝑉𝑑𝑟𝑜𝑝 (V) is the voltage at the very beginning of the galvanostatic charge or discharge step, and I (A) is the constant current (50 mA in this study). In this work, the charge efficiency is used instead of the electron efficiency proposed in our previous publication.3 The charge efficiency is the ratio of adsorbed CO2 over the charge stored in the electrodes during electrical charging, as 𝛬=

𝑛𝑎, 𝐶𝑂2 𝑛𝑐

=

𝑛𝑎, 𝐶𝑂2 𝑄𝑐

×𝐹

,

(2)


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where 𝑛𝑎, 𝐶𝑂2 (mol) is the amount of adsorbed CO2, 𝑛𝑐 (mol) and 𝑄𝑐 (C) is the amount of capacitively stored charges, and F (96 485 C.mol-1) is Faraday constant. The definition and calculation of 𝑛𝑎, 𝐶𝑂2, 𝑛𝑐, 𝑄𝑐, and all the other performance metrics are detailed in our previous paper3 and in Section S2. 3. Results The voltage responses and CO2 concentration changes in the effluent gas leaving the SSA module during the charge-discharge experiments at NaCl concentrations varying between that of de-ionized water and 5.0 M are shown in Figure 2. The charge-discharge curves (black lines in Figure 2) for all electrolyte concentrations are similar: The voltage increases linearly with time until the final voltage is reached, remains constant during the potential holding steps, and then decreases linearly during the discharge steps. However, voltage drops of different magnitudes are observed at the beginning of each galvanostatic charge and discharge step. These voltage drops are caused by the internal electrical resistance, which are quantified in Table 1 (line 3-5) and graphed in Figure S4a. As expected, the internal resistance tends to increase with decreasing electrolyte concentrations, and is the highest for deionized water. The voltage drops in Figure 2a and b are asymmetric for the charge and discharge steps, with the internal resistance at -1 V being substantially higher than at 0 V. These effects occur because for de-ionized water and 0.010 M NaCl there is a substantial depletion of free ions in the bulk electrolyte.16 According to Table 1 the internal resistance of the charged SSA module (at -1 V) is 2.9 times compared to 0 V for de-ionized water as the electrolyte, and 4.8 times for 0.010 M NaCl as the electrolyte. The higher value for 0.010 M compared to water can be explained by the fact that in the case of de-ionized water HCO3- ions can be replenished via the diffusion and hydrolysis of additional CO2 from the gas phase into the electrolyte (equations 3-8), while this is not possible for the adsorbed Na+ and Cl- ions in the 0.010 M NaCl solution (equations 9-10). 𝐻𝑐𝑐

𝐶𝑂2(𝑔) 𝐻2𝐶𝑂3∗ 𝐾1

，

𝐻2𝐶𝑂3∗ 𝐻𝐶𝑂3― + 𝐻 +

,

with Hcc=0.83

(3)

with K1=10-6.4 M

(4)


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ACS Applied Materials & Interfaces 𝐾2

𝐻𝐶𝑂3― 𝐶𝑂23 ― + 𝐻 +

,

with K2=10-10.3 M

(5)

where 𝐻2𝐶𝑂3∗ stands for the dissolved CO2 in water in the form of CO2(aq) and H2CO3; Hcc is defined as aqueous concentration over gaseous concentration calculated from reference,17–19 and K1 and K2 values are taken from reference.20 𝐻𝐶𝑂3― ↔(𝐻𝐶𝑂3― )𝑎𝑑𝑠

,

(6)

𝐶𝑂23 ― ↔(𝐶𝑂23 ― )𝑎𝑑𝑠 ,

(7)

𝐻 + ↔(𝐻 + )𝑎𝑑𝑠

.

(8)

𝐶𝑙 ― ↔(𝐶𝑙 ― )𝑎𝑑𝑠

,

(9)

𝑁𝑎 + ↔(𝑁𝑎 + )𝑎𝑑𝑠 .

(10)


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ACS Applied Materials & Interfaces (a) Deionized water Voltage CO2 concentration

-1.0 -50

0

27 -0.2

24 21

-0.4

18 15

-0.6

12 9

-0.8

6 3

-1.0 -50

0

50

0 100 150 200 250 300 350

Time (min)

Time (min)

(c) 0.1 M NaCl Voltage 0.0

(d) 1 M NaCl 33

CO2 concentration

Voltage

30

33

CO2 concentration

0.0

30

27 21

-0.4

18 15

-0.6

12 9

-0.8

6

27 -0.2 Voltage (V)

24

CO2 concentration (%)

Voltage (V)

-0.2

100

200

300

400

24 21

-0.4

18 15

-0.6

12 9

-0.8

6

3

-1.0 0

3

-1.0

0 500

0

100

Time (min)

200

300

0 500

400

Time (min)

(e) 3 M NaCl Voltage

CO2 concentration

(f) 5 M NaCl

33

0.0

Voltage

30

0.0

CO2 concentration

27 21

-0.4

18 15

-0.6

12 9

-0.8

6 3

-1.0 100

200

300

400

500

0

33 30 27

-0.2 Voltage (V)

24


-0.2

0

30 CO2 concentration (%)

-0.8

33


-0.6

CO2 concentration

0.0

24 21

-0.4

18 15

-0.6

12 9

-0.8

6


-0.4

(b) 0.01 M NaCl Voltage

Voltage (V)

Voltage (V)

-0.2

33 30 27 24 21 18 15 12 9 6 3 0 50 100 150 200 250 300 350 400


0.0

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3

-1.0 0

Time (min)

100

200

300

400

500

0

Time (min)

Figure 2. Voltage response and CO2 concentration in the effluent gas of SSA modules using (a) de-ionized water, (b) 0.010 M, (c) 0.10 M, (d) 1.0 M, (e) 3.0 M and (f) 5.0 M NaCl as electrolytes. The dotted blue lines denote the 15% CO2 concentration. The first cycles were not included in metrics calculation.


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Table 1. Performance metrics of SSA modules using NaCl solution of different concentration as electrolyte.

NaCl concentration Resistance (Ω.cm2) at 0 V at -1 V Specific capacitance (F.g-1) Coulombic efficiency (%) Energy efficiency (%) Energy loss (J) Charge time (min) Adsorption capacity (mmol.kg-1) Charge efficiency (-) Adsorption rate (µmol.kg-1.s-1) Energy consumption (kJ.mol-1) Time-energy efficiency (µmol.kJ-1.s-1)

Deionize d water

0.010 M

0.10 M

1.0 M

3.0 M

5.0 M

179.6±8.1 513.1±8.9 19.8±0.5 93.6±1.6 4.5±0.1 25.0±0.5 33.6±0.1

55.2±0.1 264.3±0.2 34.0±0.4 93.5±0.6 19.2±0.4 31.7±0.5 35.4±0.1

23.4±0.9 29.0±0.4 64.1±0.3 93.0±1.0 60.2±2.5 11.6±1.2 42.4±0.0

12.4±0.0 12.5±0.1 77.7±0.8 86.8±0.9 60.3±1.0 11.6±0.5 42.9±0.0

7.1±0.0 6.8±0.1 77.8±1.0 91.7±3.1 71.7±4.2 9.1±1.9 45.9±0.2

8.2±0.3 8.6±0.1 96.8±1.9 92.0±1.0 78.8±1.9 6.6±0.8 46.7±0.3

32.8±2.0

47.6±1.3

48.6±3.6

62.4±3.3

63.2±6.4

62.0±10.3

0.13±0.01 0.13±0.00 0.12±0.01 0.12±0.01 0.14±0.01 0.11±0.02 16.3±1.0

22.4±0.6

19.1±1.4

24.3±1.3

23.0±2.3

21.7±3.7

628±40

712±22

200±26

202±14

118±27

97±37

0.8±0.1

0.7±0.0

2.0±0.3

1.9±0.1

3.1±0.7

3.6±0.6

The fact that there remains a significant difference in electrical resistance at 0 V and -1 V respectively for the experiment with de-ionized water indicates that the replenishment is incomplete, most likely because the dissolution and hydrolysis of additional CO2 (equation 3-5) is kinetically slower than the charging process. The differences between SSA internal resistance at 0 and -1 V become much smaller for 0.10 M NaCl and are negligible for NaCl concentrations at and above 1.0 M. For NaCl solutions ≥1.0 M, the free ion concentration change in the solution upon electrical charging becomes negligible, leading to a negligible change in resistance.21,22 Overall, SSA internal resistance decreased with increasing NaCl concentration from that of de-ionized water to 3.0 M, and slightly increased from 3.0 to 5.0 M NaCl. The slightly higher internal resistance in the SSA module with 5.0 M NaCl could be caused by strong solvation effects (i.e. many of the water molecules are hydrated water) and/or local NaCl crystallization in electrode pores.23 The energetic metrics for the different electrolytes were calculated from the galvanostatic chargedischarge curves in Figure 2 and listed in Table 1. The specific capacitance monotonically increased with 9 ACS Paragon Plus Environment


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increasing NaCl concentration. The specific capacitance with the 5.0 M NaCl electrolyte is 2.8 times of 0.010 M NaCl, and 4.9 times of de-ionized water. In de-ionized water, the specific capacitance is only 20 F.g-1 (Table 1). This is plausible given that there are no Cl- and Na+ ions and the concentration of proton, bicarbonate, and carbonate ions in pure water is very small (equation 3-6). As discussed above, the adsorption of (bi)carbonate and proton ions to the electrodes leads to ion depletion in the bulk solution, which should move the equilibrium of Equation 3-5 to the right, and lead to the dissolution and hydrolysis of additional CO2 from the gas phase. In principle, this process could lead to similar capacitances as observed for the NaCl solutions, but this is experimentally not observed. The explanation may be again that the ions are only partly replenished because that the dissolution of CO2 gas in water (Equation 3) is kinetically slow compared to the charging process.16 In this regards the results for the capacitance are consistent with the internal resistance measured for the voltage drops at 0 and -1 V, respectively. Similarly, the energy efficiency increased with increasing electrolyte concentration, while energy loss tends to decrease with increasing concentration. The lowest energy efficiency was measured for deionized water (4.5%, Table 1). These results are consistent with SSA internal resistance results, as higher internal resistance leads to more energy loss. The Coulombic efficiency was high for all concentrations, even for de-ionized water (94%), and varied between 86% and 94% (Table 1) showing that the chargedischarge process is highly reversible. The blue lines in Figure 2 show the CO2 concentration in the effluent gas leaving the SSA module. For all concentrations, the SSA module adsorbed CO2 during electrical charging, and desorbed CO2 upon discharging. At the end of each potentiostatic holding step, the CO2 concentration returned to around 15% for concentrations of 0.10 M and higher, indicating the completion of the adsorption and desorption half cycles. For de-ionized water and 0.010 M NaCl the CO2 concentration did not quite return to 15% indicating that the thermodynamic equilibrium has not yet been reached. This is most likely because the charging and discharging times are substantially shorter in de-ionized water and 0.010 M NaCl (due to lower capacitance) reducing the overall time available to establish the thermodynamic adsorption equilibrium (Table 1). ACS Paragon Plus Environment

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The SSA adsorption capacity tends to increase with NaCl concentration (Table 1 and Figure S4b). There is a significant increase (1.9 times) from 32 mmol.kg-1 (in de-ionized water) to 62 mmol.kg-1 (in 1.0 M NaCl). At higher NaCl concentrations the adsorption capacity remains practically constant (62-63 mmol.kg-1). The electrolyte concentration also impacts the repeatability of CO2 adsorption. For experiments with 3.0 and 5.0 M NaCl electrolytes shown in Figure 2e and f, the peaks and troughs of CO2 concentration curves become significantly smaller with increasing number of charge-discharge cycles, indicating decreasing CO2 adsorption capacities. This is not the case for experiments in deionized water and 0.010 M NaCl. The charge efficiency (Table 1) changed less than 20% among all the investigated concentrations, and the differences are within experimental errors. For each accumulated charge in the electrodes, about 0.13 CO2 molecules were adsorbed onto the carbon electrode regardless the NaCl concentration. The CO2 adsorption rate in experiments using different NaCl concentrations between 0.010 and 5.0 M varied insignificantly within error margins between 19 and 24 µmol kg-1 s-1, suggesting an insignificant impact of electrolyte concentration on adsorption rates. When de-ionized water was used, a somewhat smaller rate of 16 µmol kg-1 s-1 was measured. The smaller value may be because the CO2 adsorption capacity is the smallest for the experiments in water. In contrast, the two energy-related metrics, energy consumption and time-energy efficiency, changed dramatically with NaCl concentration (Table 1 and Figure S4c and d). The energy consumption decreased monotonically with increasing NaCl concentration, except that for de-ionized water the energy consumption was somewhat lower than for 0.010 M NaCl (Table 1). The 5.0 M NaCl electrolyte gave the lowest energy consumption of 97 kJ.mol-1, which is only 14% of that with 0.010 M NaCl (712 kJ.mol-1), and 15% of that in water (628 kJ.mol-1). The energy consumption for 5.0 M NaCl (97 kJ.mol-1) is comparable to the energy consumption reported for amine-scrubbing, pressure and temperature swing adsorption, as well as membrane separations.24,25 Time-energy efficiency increased with increasing NaCl concentration, except that the time-energy efficiency was slightly lower for 0.010 M NaCl compared to water. The highest time-energy efficiency was also found for the SSA experiment using 5.0 M NaCl (3.6 ACS Paragon Plus Environment

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µmol.kJ-1.s-1), which is 5.5 times of that with 0.010 M NaCl (0.7 µmol.kJ-1.s-1) and 4.5 times of that with de-ionized water (0.8 µmol.kJ-1.s-1). The trend for the time-energy efficiency follows that of the energy consumption, which is because the adsorption rates and the adsorption capacity change relatively little with NaCl concentration compared to the energy consumption (Table 1). 4. Discussion The observation that the charge efficiency is near constant regardless the NaCl concentration and the sorption capacity increases with NaCl concentration up to a concentration of 1.0 M (and then remains near constant) is mechanistically interpretable. The fact that capacitance is created in pure water, and that CO2 can be reversibly adsorbed in pure water argues that the ionic liquid-solid mechanism (Figure 1c) is in action. However, if SSA occurred solely via the ionic liquid-solid mechanism, then the charge efficiency and the sorption capacity should decrease with increasing electrolyte concentration because of the competition between Cl- and HCO3- as well as H+ and Na+ in the two double layers. The fact that this is not experimentally observed suggests that there are also non-ionic contributions and/or that either H+ or HCO3- have a much higher affinity to charged pore surfaces compared to Na+/Cl-. The hydrated radius and ionic potential of H+ ions are 78% and 1.3 times, respectively, of Na+ ions; while both the diffusion coefficient and electrical mobility of H+ ions are 7 times of Na+ ions (Section S4). This means that compared to Na+ ions, H+ ions are of smaller size, higher charge density, and move much faster with and without electrical field.22,26–28 As a result, the presence of Na+ ions may not significantly inhibit the electrosorption of H+ ions (Equation 8) explaining the similar charge-efficiency at all concentrations. For a complete picture, not only the affinity of the cations for the negative electrode, but also the affinity of the anions for the positive electrode needs to be taken into consideration. For anions, based on the K2 value in Equation 3, the concentration of CO32- ions is three orders of magnitude less than the concentration of HCO3- ions in a near neutral solution.1 We therefore neglect the CO32- ions here, and discuss only the electrosorption of HCO3- and Cl- ions. Compared to HCO3- ions, the Cl- ions are smaller in size, and have a higher ionic potential, diffusion coefficient, and electrical mobility (Section S4). Consequently, the electrosorption of Cl- ions (Equation 9) to the pore surfaces is thermodynamically more ACS Paragon Plus Environment

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favorable and faster than that of HCO3- ions (Equation 6).22,26–28 The preferential adsorption of Cl- ions to the electrical double layer (EDL) argues that the Cl- ions compete with the HCO3- at the positive electrode, which would influence the charge efficiency. However, the constant removal of protons from the bulk solution could lead to a higher concentration of HCO3- in the bulk solution, and for this reason the sorption capacity may not be reduced despite the predominant adsorption of Cl- ions to the double layer. Another factor to be taken into account is that for the experiments in deionized water and 0.010 M NaCl electrolyte the chemical equilibrium for the gas adsorption was not reached. This means that if the potentiostatic holding step had been longer, then more CO2 could have been adsorbed and this could have influenced the charge-efficiency. We therefore decided to perform an additional experiment with pure water in which the holding steps were increased from 30 to 60 minutes (Section S5). Indeed, in these experiments the CO2 concentration returned to ca. 15% at the end of each half-cycle showing that the additional time was sufficient for the thermodynamic adsorption equilibrium to establish (Figure S5). The sorption capacity was found to be increased by 48% from 32.8 to 48.4 mmol.kg-1 (Table S2) and the measured specific capacitance somewhat decreased by 21% (15.7 F.g-1 compared to 19.8 F.g-1) which lead to a significant increase in the charge efficiency from 0.13 to 0.20. These results would argue that there may be indeed competition of ions at the electrodes via the ionic liquid-solid mechanism. However, the fact that the sorption capacity is still significantly below those measured at NaCl concentrations ≥ 1 M NaCl (62-63 mmol.g-1) suggests that the other two mechanisms also contribute to the sorption capacity: in the non-ionic liquid-solid mechanism (Figure 1b) the CO2 adsorption capacity would be increased at higher NaCl concentrations because the increased number of Na+/Cl- would increase the solubility of molecular CO2 in the electrical double-layer. In the case of the gas-solid mechanism (Figure 1a) the greater change in the Fermi-level of the carbon due to higher capacitance would lead to a larger adsorption on the surface of non-infiltrated pores. Overall, the assumption of a combination of non-ionic and ionic mechanism can most satisfactorily explain the experimental results, however more research beyond the scope of this work will be needed for quantitative conclusions.


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The longer potentiostatic holding steps also have an impact on the energy consumption, the adsorption rate, and the time-electron efficiency. The energy consumption is somewhat reduced (591 vs. 628 kJ.mol1)

because the increase in adsorption capacity is greater than the increase of the energy loss due to the

additional holding time. However, the adsorption rate is decreased (12.7 vs. 16.3 µmol.kg-1.s-1) because during the additional holding time less than proportional amounts of CO2 are adsorbed. As a consequence, the time-energy efficiency is reduced from 0.8 to 0.5 µmol.kJ-1.s-1. At high concentration (3.0 and 5.0 M NaCl solutions), the SSA CO2 adsorption capacities decrease with increasing charge-discharge cycle numbers as discussed in Section 3 (Figure 2e and f). This phenomena could be explained by an “ion pumping” mechanism. As the NaCl concentration increases, the availabilities of both Cl- and Na+ ions increase. As discussed above, upon applying a cell voltage in the CO2 adsorption step, the H+ ions are preferentially adsorbed into in the EDLs in the micropores of the porous carbon electrodes,22,26–28 while the Na+ ions lag behind and move much slower into the micropores. In the SSA modules using 3.0 and 5.0 M NaCl solutions, the Na+ ions are very abundant in macropores. With increasing number of charge-discharge cycles, more and more Na+ ions are slowly driven into the micropores in the carbon electrode (ion pumping) upon charging, but incompletely released upon short circuit discharging. The presence of the Na+ ions may sterically hinder the electroadsorption of H+ ions into the micropores, explaining the somewhat decreased CO2 adsorption capacities at higher cycle numbers. 5. Conclusion In summary, we report on the relationships between the NaCl electrolyte concentration (deionized water to 5.0 M) on the adsorptive and energetic metrics of Supercapactive Swing Adsorption. We find that the energetic metrics are strongly improved with increasing NaCl concentration, in particular the energy consumption and the time-energy efficiency. The adsorptive metrics change by a comparatively small degree. The adsorption capacity tends to increase up to 1.0 M NaCl and then remains constant. The adsorption rate remains near constant for all concentrations, and is somewhat smaller for pure water, presumably because of the larger resistance that hinders the charging. The charge efficiency also remains ACS Paragon Plus Environment

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near constant for all experiments with 30 min potentiostatic holding steps, but near doubles for pure water when the potential holding step is doubled because the chemical adsorption equilibrium is reached only after 60 min. The results can be most satisfactorily explained by assuming that both ionic and non-ion adsorption mechanisms contribute to SSA. Further studies are required to (1) investigate SSA performance using other salts and solvents as electrolyte, (2) explore CO2/N2 gas mixtures with different CO2/N2 ratios, (3) explore SSA-based separations of gas mixtures other than CO2/N2, and (4) investigate different carbon materials as adsorbing electrodes. ASSOCIATED CONTENT Supporting Information SSA module and operation; SSA performance metrics; SSA performance; ion properties; and SSA test with deionized water and extended potentiostatic holding time.

AUTHOR INFORMATION Corresponding Author * (Kai Landskron) E-mail: [email protected]. Phone: +1 610 758 5788.

ACKNOWLEDGMENT This work was funded by the US National Science Foundation under award number CBET-1566201. REFERENCE (1)

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Phys. 2015, 17 (43), 28666–28673.

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Relationships between Electrolyte Concentration and the

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