Supporting Information



Supporting Information

Selective Adsorption of Multivalent Ions Into TiC-derived Nanoporous Carbon

Sergey Sigalov, 1 Mikhael D. Levi*,1 Gregory Salitra, 1 Doron Aurbach,1, Alar Jänes,2 Enn Lust, 2 Ion C. Halalay3

Supporting Information

1. Background on the Combined EQCM Plus CV Study of Nanoporous Carbon Electrodes

A. Summary of Previous Work

We have substantiated the use of conventional EQCM and then achieved a considerable advance through the use of a more general quartz crystal impedance approach to the study of ionic and solvent fluxes during the dynamic charging of nanoporous carbon electrodes in several recent publications. This approach includes consideration of width of the resonance in addition to the resonance frequency shift. First, in a communication to Nature Materials [1], we have presented convincing evidence that a thin µm-size composite carbon coating can be rigidly attached to an Au- or Pt-plated quartz crystal. This allowed us to measure changes in the resonance frequency and the motional resistance of the coating as function of the electrode potential in a variety of aqueous electrolyte solutions. We carried out EQCM measurements using a Maxtek RQCM system and a 5 MHz one inch size quartz crystal. Special attention was paid to verify operation of the coated crystal in the linear response regime and to determining the limit of linearity between changes in the resonance frequency of the crystal and coatings mass changes, i.e., the limit of applicability for Sauerbrey’s equation, eqn. (1). The systems studied in our former and present publications are characterized small changes in the resonance peak width of the coated crystal and therefore a linear response, based on Sauerbrey's equation, obtains for changes in resonant frequency with mass:

Δfr = -Cf Δm, (1)

where Cf = 0.056 Hz/ng/cm2 is the sensitivity factor of the AT cut 5 MHz Maxtek crystal, and Δm is the change in mass of the coating per unit area.

Mass changes during cyclic voltammetry (CV) scans were transformed into ion population changes as a function of the carbon electrode charge, by taking into account the value and sign of the charge during the capacitive charging of the carbon electrodes. Experimentally determined population changes with electrode charge were then compared with the changes expected from Faraday’s law. An unexpected result revealed by these plots indicated that all investigated ions, with the exception of the large tetraalkylammonium (TAA) cations, enter carbon nanopores (having a width smaller than 1 nm) in more or less desolvated form. This provides a reliable basis for tracing ionic fluxes into nanoporous carbons and for distinguishing between fluxes of principal counter-ions, co-ions and solvent molecules. Bulky TAA cations were found to stick to carbon nanopore walls, so that the Cl- counter-ions play a major role in the charge compensation mechanism in carbon nanopores.

This approach was then extended to the characterization of ionic fluxes in electrolytes typical for Li-ion batteries and carbon supercapacitors, based on organic electrolyte solutions. Partial desolvation of highly solvated Li+ ions in a dipolar aprotic solvent (PC) has been discovered. [2]

The surface of nanoporous carbon displays typical amphoteric features: the carbon electrode, having pure C-C bonded atoms, is charged as an inorganic semiconductor with storage of electronic charge in the space charge region of the carbon nanopores which have only a few graphene layers thick walls. [3] However, some types of carbons may contain a lot of various surface functionalities of either basic or acidic character, depending on their synthesis and storage conditions, which may lead to charge storage in fixed functional groups, or sometimes even dark injection of electronic charge carriers into the conduction or valence band of nanoporous carbons through these functional groups. [4] The carbon surface can be protonated not only by bulk H+ ions from acidified electrolyte solutions, but also by protons from dissociated water molecules. [4] We have clearly demonstrated [3] that, even in an aqueous CsCl solution slightly acidified with HCl, the flux of Cs+ cations at negatively charged surface disappears, so that the negative charging of carbon surface is effected by desorption of Cl- anions rather than adsorption of Cs+ cations.

In a recent full-length paper, [5] taking as example a series of simple ammonium and tetraalkylammonium cations in aqeous chloride electrolyte solutions, we have further developed the principles of EQCM analysis of nanoporous carbons and discussed the relationship between the pzc of the carbon electrodes obtained from minima in the related CV curves, the potential of maximum electronic resistance (obtained with in-situ use of measurements with interdigitated microelectrodes) usually associated with flat-band potentials, and the characteristic EQCM parameter called the potential of zero mass change (pzmc). The pzmc separates the increase and the decrease of the charged electrode mass, distinguishing between mass ingress and egress. It allows, in combination with CV and Faraday law of electrolysis, the separation between fluxes of individual ions and solvent molecules.

Very recently, by using a quartz crystal impedance (admittance) station based on an Agilent network analyzer, we have at our disposal a powerful means for understanding the nature of the electrical impedance responses from carbon coated quartz crystals. In our practice we use nanoporous carbons with particle sizes varying from a few nm to several microns. Coatings on the quartz crystal can be not only chemically heterogeneous (they consist of carbon particles and PVdF binder), but may exhibit a significant surface roughness. In order to characterize such complicated non-homogeneous coatings we have performed their hydrodynamic spectroscopic characterization in a series of liquids with different velocity decay length scales (see our full length paper in Analytical Chemistry). [6] An appropriate modeling of the quartz crystal impedance response from such structurally non-homogeneous electrodes allows the estimation of important structural parameters such as the effective thickness of the coating, its internal porosity and external roughness, the contribution of carbon particles agglomerates (bumps) and of homogeneous layers of smaller particles mixed with PVdF binder to the resonance frequency shift and the width of the resonance as a function of agglomerates size, the coverage of the crystal surface, and the varying extent of internal porosity. Semi-quantitative estimations showed that it is possible to determine the extent of internal porosity in the carbon particles and compare it with that obtained from the DFT analysis of N2 adsorption isotherms. [6] This study was initiated to understand the nature of EQCM response from non-homogeneous nanoporous and non-porous carbon particles without using electrochemistry. At the present time we are extending the use of quartz crystal impedance measurements for the characterization of charging process in nanoporous carbon electrodes to the cases when large changes in the width of the resonance occur while scanning the electrode potential.

B. Examples of EQCM Data Analyses

The following two examples provide details of EQCM data analysis and interpretation.

Example #1: Composite coating consisting of 90 % nanoporous carbon YP-17 from Kuraray and 10 % PVdF binder. The coating produces a 1.5 kHz shift in the resonance frequency measured in air (loading of ~30 mg of active carbon mass per 1 inch2 of quartz crystal, an effective coating thickness of 0.6 - 0.7 µm estimated with the impedance model). Electrolyte solution: 0.025 and 0.1 M CsCl aqueous solutions.

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Figure S1. Panels A and B, respectively: CV curves and the accompanying changes in resonant frequency and motional resistance as functions of potential for two aqueous CsCl solutions with different salt concentrations (as indicated). Panel C shows the potential dependences of the electronic conductivity of carbon, measured with the use of carbon-coated interdigitated microelectrodes. Panel D displays the plots of ion populations as a function of carbon electrode charge calculated as indicated in the text. The dotted lines are constructed in accordance with Faraday's law for single-charged anions and cations at Q > 0 and Q < 0, respectively. Three separate regions of considerably different ions population-to-charge ratios are characteristic for the ionic and solvent fluxes involved in the charging of the carbon electrode, as discussed in detail in the text.

Figure S1A shows that the increase in concentration of the electrolyte solution from 0.025 to 0.1 M results in a shift of the shallow CV minimum from -0.11 to -0.04 V. This minimum results from a combination of the spatial electronic charge carriers distribution in the carbon nanopores walls [7] and the ionic charge depletion in the diffuse electric double layers near the electrode surface. The involvement of the semiconducting properties of YP-17 carbon in its electrochemical behavior becomes evident from the minimum in the electronic conductivity (Figure S1C) located between -0.5 to -0.6 V, which is closer to the potential of the minimum in the CV for the 0.1 M CsCl solution. We always use the potential of the electronic conductivity minimum for determining the approximate location of the pzc in cases when concentrated electrolyte solutions are used and the minima in CV curves are absent.

Note that, theoretically, an ideal coincidence between the electrode's pzc and the potential of the electronic conductivity minimum is expected for purely intrinsic semiconductors or semiconductors doped with extremely low amounts of acceptor or donor impurities. [8] The conductivity of YP-17 carbon in aqueous solution changes only by a few percent with potential (Figure S1 C) and the shift between the potential of the CV minimum and the conductivity minimum is only several tens of mV. There exists only a very limited number of literature sources that discuss the electronic conductivity of charged nanoporous carbons. The most detailed characterization has been carried out by B. Kastening et al. [8] with a typical semiconducting nanoporous carbon from Kureha, containing a large percent of acceptor impurity. For most electrolytic systems, the potential of the conductivity minimum is several hundred millivolt more negative than the potential of zero charge, reflecting the fact that the charge carriers in this carbon are holes. From a comparison of our results for YP-17 carbon and those of Kastening et al. [8], we can conclude that YP-17 carbon contains a large number of intrinsic charge carriers whereas the concentration of acceptor impurities is rather small compared to the Kureha activated carbon.

The issue of specific adsorption of ions is discussed in the next example, based on our recently published EQCM data on the adsorption of halogenide anions on carbon electrodes. [9]

Example #2: Composite coating consisting of 90 % nanoporous carbon YP-17 from Kuraray and 10 % PVdF binder. Electrolyte solution: 0.025 M aqueous solutions of KF, KCl, KBr and KI.

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Figure S2. Panels A and B, respectively: CV curves and the accompanying changes in the resonant frequency as a function of potential for 0.025 M KF, KCl, KBr and KI solutions (as indicated). The Δf vs. E curves for the different salts were shifted by 20 Hz for better visibility of the maxima. Panel D shows the potential dependence of the normalized electronic resistance (with respect to the resistance maximum at the related pzc), measured with carbon-coated interdigitated microelectrodes. Panel C displays the plots of ion populations as a function of carbon electrode charge, calculated as explained in the text. The dotted lines are constructed using Faraday's law for single-charged anions and cations at Q > 0 and Q < 0, respectively. The values of pzc’s obtained from the related CV minima, the potentials of the maxima in the R/Rmax curves, and the potentials of zero mass changes (pzmc’s) are listed in the Table 1.

Table 1. The pzc values obtained from CV minima, the potentials at R/Rmax = 1 (i.e., at resistivity maxima obtained from in-situ conductance measurements) and pzmc values obtained from the potential dependences of the resonance frequency (or mass). All potentials were measured with respect to a Ag/AgCl reference electrode.

|Salt in aqueous 0.025 M |pzc from CV min / V |E at Rmax / V |pzmc / V |

|solution | | | |

|KF |-0.05 |-0.10 |-0.03 |

|KCl |-0.05 |-0.10 |-0.03 |

|KBr |-0.07 |-0.14 |-0.07 |

|KI |-0.12 |-0.15 |-0.13 |

The CVs for the series of solutions with KF, KCl, KBr and KI salts display a gradual disappearance of the broad minima and shift of the potentials of the CV minima towards more negative values due to increasing anions adsorption. The specific adsorption of anions at the pzc of the carbon electrode is compensated by the electrostatic adsorption of equal number of counter-ions, i.e. cations (e.g. for 1:1 electrolytes, as in the present case). Whereas the change in resonance frequency is very similar for the negatively charged carbon surface in the case of K+ counter-ions for all the investigated electrolytes, it is very different for anions, especially for those specifically adsorbed on the carbon surface, like Br- and I- anions. This is in contrast to the potential dependence of nanoporous carbon resistance remaining practically independent of the nature of anion. From Table 1 and Figure S2B, it is seen that the pzmc, characterizing the ionic the electric double layer of carbon, shifts gradually to more negative values in the series F- ...I- remaining closely to the pzc obtained from the minima of the CV curves.

From the standpoint of EQCM analysis, the case of specific adsorption of anions can be regarded as a particular case of enhanced cation-anion mixing near the pzc, which breaks down starting from moderately positively or negatively charged carbon surfaces. An increase in negative charge density results in anions desorption, whereas the increase in positive charge density forces the co-ions (i.e. the cations) to be desorbed from the electrode surface. For this reason, the specific adsorption of ions does not complicate the analysis of the mass-to-charge ratio apart from the pzc region, i.e. for a moderately or heavily charged carbon surface, as was demonstrated previously in the analysis of the plots shown in Figure S2C. [5]

Finally, in the worse case of "sticky" bulky TAA cations, the pzmc may considerably differ from the pzc, since the pzc is not dependent on the mobility of ions, whereas the pzmc directly reflects the different mobilities of the ions: the ions which are more mobile inside the carbon nanopores play a decisive role in the charge compensation mechanism inside the pores even under unfavorable electric field conditions, i.e. playing the role of the co-ions; see ref. [5] for examples. Note also that, for the sake of simplicity, when analyzing the mass-per-charge ratio we sometimes calculate the effective surface charges from the pzmc rather than the pzc (e.g., in the present communication). This allows for the ion population Γ to always remain positive, whereas a rigorous account of a pzmc which is much more negative than the pzc (e.g., for sticky TAA cations) will make Γ negative in the potential interval situated between pzc and pzmc. This approximation does not affect the analysis of the mass-per-charge ratio as was demonstrated in detail in a previous publication. [5]

2. Preparation of the Carbon Coatings on the Quartz Crystals

Nanoporous carbon films were spray coated onto the quartz crystal surface from a slurry composed of the film components (90 wt% carbon powder, and 10 wt% PVdF binder) and N-methyl pyrrolidone (see Fig. S3). Prior to the coating, the crystal is placed into a stainless steel mask (frame), so that only the Pt or Au electrodes of the crystal are exposed to the spray from a home-made spray gun (pulverizer). The crystal and mask are

Figure S3. Schematic of the process for coating of the AT cut 5 MHz Maxtek 1 inch diameter quartz crystal with a slurry containing a mixture of 90 wt% carbon particles and 10 wt% PVdF binder in N-methylpyrrolidone.

then placed onto a hot plate at a temperature of 120° C. Pure Ar or N2 gasses enter a home-made spray head under the elevated pressure. The slurry is continuously mixed in a

beaker using a magnetic stirrer. The crystals coated with composite carbon deposits are then dried in an oven at 120° C for 30 min. After cooling to room temperature, the coated crystal is placed into a Maxtek crystal holder linked through a rubber o-ring to an electrochemical cell made of glass. Prior to contact of the crystal with solution in the electrochemical cell, the electrolyte solution is deaerated with pure argon. During the measurements the dearation is stopped and an argon flux is directed over the surface of the solution, in order to protect it from contamination by atmospheric oxygen.

3. Details Regarding the TiC-Derived Carbon

A. Synthesis Procedure

The porous TiC derived carbon (also abbreviated as “Ti-CDC” in the text) was synthesized from TiC (Alfa Aesar, 99.5%, metals basis) by the chlorination method. TiC with an average particle size of 1–3 μm was loaded into the silica tube rector and reacted with a flow of chlorine gas (99.99% purity) for 2 h in a tube furnace at 950 °C. The by-product titanium tetrachloride (TiCl4) was carried away by the stream of the excess chlorine (Cl2). The reaction then was flushed with the argon gas (99.999% purity) at 950 °C for 1 h, in order to remove the excess Cl2 and any residues of gaseous by-products from carbon. The resulting carbon powder was then treated with hydrogen at 800 °C for 1 h. The reactor was flushed with a slow stream of argon during the heating to the reaction temperature and cooling to room temperature.

B. Morphology and Structure

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C. Figure S5. Powder XRD patterns for Ti-C carbon. The R ratio introduced by J. Dahn (Y. Liu, J. X. Xue, T. Zheng, J. R. Dahn, Carbon 1996, 34, 193; compare also with Fig. 1 in H. Wang, Q. Gao, and J. Hu, J. Am. Chem. Soc. 2009, 131, 7016–7022) was found to be close to 1, pointing to the entirely amorphous character of this carbon.

.

D. Porosity

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Figure S6. N2 adsorption isotherm. The specific (BET) surface area and specific total pore volume of TiC derived carbon powders were determined to be 1420 m2g-1 and 0.52 cm3g-1, respectively.

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Figure S7. A plot of cumulative pore volume versus pore width calculated from the related adsorption isotherm using the DFT method. The size of the TBA+ cation and the cross-sectional dimensions along the different orientations of the Co(CN)63- anion are also indicated.

4. Additional Information on the Combined CV and EQCM Study of Ion Adsorption into Nanoporous Carbons.

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Figure S8. (a) CV at a scan rate 20 mVs-1 and (b) the related ΔΓ vs. σ plot for TiC-CDC electrode in 0.025 M NH4Cl. The location of pzc = pzmc is indicated. Three adsorption domains with considerably different ΔΓ/σ ratios are marked in blue in panel b. The slope of the ΔΓ vs. σ plot within domain II is equal to the theoretical one obtained from Faraday's law (dashed black lines in panel b).

References

[1] Levi MD, Salitra G, Levy N, Aurbach D, Maier J. Nat Mater 2009; 8: 872-5.

[2] Levi MD, Levy N, Sigalov S, Salitra G, Aurbach D, Maier J Am Chem Soc 2010; 132:13220–13222.

[3] Sigalov S, Salitra G, Levi MD, Aurbach D, Maier J. Electrochem Commun 2010; 12:1718-1721.

[4] Muller M, Kastening B J Electroanal Chem 1994; 374: 149-158.

[5] Levi MD, Sigalov S, Salitra G, Aurbach D, Maier ChemPhysChem 2011; 12:854-862.

[6] Daikhin L, Sigalov S, Levi MD, Salitra G, Aurbach D Analyt Chem 2011; 83: 9614–9621.

[7] Hahn M, Barbieri O, Kötz R Grundlagen und Anwendungen der Elektrochemischen Oberflächentechnik, GDCH-Monogrphie, pp. 229-239, Bd. 32, 2004, 2005 Gesellschaft Deutscher Chemiker, Frankfurt am Main.

[8] Kastening B, Hahn M, Kremeskiitter J J Electroanal Chem 1994; 374: 159-166.

[9] Levi MD, Salitra G, Sigalov S, Elazari R, Aurbach D J Phys Chem Lett 2011; 2: 120-124.

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* [1] Mikhael D. Levi, Sergey Sigalov, GregoriySalitra, Doron Aurbach

Department of Chemistry Bar-Ilan University, Ramat-Gan, 52900, Israel

Fax: +972-3-7384053

E-mail: aurbach@mail.biu.ac.il

[2] Alar Jänes, Enn Lust

Institute of Physical Chemistry, University of Tartu, 51014 Tartu, Estonia

[3] Ion C. Halalay, General Motors Global R&D, Warren, MI 48090, USA

[((] This work was supported by GM.

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