Selective adsorption of multivalent ions into TiC-derived nanoporous carbon

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Selective adsorption of multivalent ions into TiC-derived nanoporous carbon Sergey Sigalov a, Mikhael D. Levi Enn Lust b, Ion C. Halalay c a b c

a,*

, Gregory Salitra a, Doron Aurbach a, Alar Ja¨nes b,

Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel Institute of Physical Chemistry, University of Tartu, 51014 Tartu, Estonia Electrochemical Energy Research Lab, General Motors Global R&D, Warren, MI 48090, USA

A R T I C L E I N F O

A B S T R A C T

Article history:

A TiC-derived carbon (TiC-CDC) was prepared, and the adsorption of large hexacyanocobal-

Received 5 March 2012

tate and tetrabutylammonium ions of approximately same size was examined on this

Accepted 2 April 2012

carbon. While selectively absorbing these large ions, it rejects smaller chloride and ammo-

Available online 13 April 2012

nium ions in mixed electrolyte solutions. The result demonstrates the important role of electrostatic repulsive forces, space-efficient charge packing and hydrophobic ion interactions with the pore walls of TiC-CDC, similar to what is known for a variety of biological membranes. Ó 2012 Elsevier Ltd. All rights reserved.

Carbon-based supercapacitors accumulate electric charge in the double layers formed by charged carbon surface and the oppositely charged layer of electrolytic ions. Until recently, the sole method for determining the ionic content in the microporous volume of carbon electrodes as a function of their specific charge density (per unit electrode mass) was based on the direct determination of concentration changes in the electrolyte solutions in contact with carbon [1]. Although accurate, its incompatibility with the dynamic conditions extant during carbon electrode charging is a major drawback. The electrochemical quartz crystal microbalance method (EQCM) was recently demonstrated to be an indispensable tool for the assessment of compositional and concentration changes in porous carbon electrodes under dynamic charging conditions [2]. We have studied an experimental TiC derived carbon (henceforth abbreviated as TiC-CDC), with a highly disordered structure, [3,4] using EQCM, cyclic voltammetry (CV) and resistivity measurements (see the Supporting Information (SI) for experimental details, a summary of our recently published EQCM papers, as well as instructive examples of analyses of EQCM, CV and resistivity data for activated carbon electrodes). Aqueous solutions of three-ion binary salt mixtures with a common ion were used throughout our work. (Mixed electrolyte solutions have been recently used to reveal pronounced ion-size effect during adsorption on carbon [5]). Reference and test ions were selected to provide considerable differences in size, charge, solvation

ability and specific interactions with the TiC-CDC nanopore walls. Our data analysis relies on examining the mass changes per unit surface area (Dm) of the carbon during ion adsorption measured by EQCM as a function of electrode charge density (r) measured during voltammetric scans. r is obtained from the CVs with the potential of zero charge (pzc) as integration constant and referred to 1 cm2 of geometric surface area of carbon coating. A parameter called potential of zero mass change (pzmc) separates the increase and decrease in the electrode mass measured by EQCM during potentiodynamic scans, which correspond to adsorption or desorption of cations and anions, respectively at r < 0 and r > 0. It was verified experimentally that pzc ffi pzmc during the permeation of ions with similar mobilities into carbon pores [6]. Identification of the permselective behavior of a porous electrode in a mixed electrolyte solution is based on comparing the experimental plots of changes in the equivalent population density DC = hzii Dm/hMii inside the pores vs. the surface charge density r against the theoretical line derived from Faraday’s law: DC = r/F. F is Faraday’s constant and Mi is the molecular (or atomic) weight of specific ions, while hMii = x1M1 + x2M2 and hzii = x1z1 + x2z2 are, respectively, the mass and charge of the effective ion from the solution, with xk the mole fractions of the two salts (ideal mixture approximation). Note that the slope of the molar population density change DC/zi vs. r/F is inverse proportional to the ionic charge number zi. Let xS and xP be, respectively, the mole fraction of

* Corresponding author. E-mail address: [email protected] (M.D. Levi). 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.04.002

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the test ion in bulk solution and inside the pore. Non-selective ion adsorption holds for xP = xS, while xP > xS means selective adsorption from its mixture with a reference ion of the same sign. Three characteristic adsorption domains can be observed for TiC-CDC as a function of r (see Fig. S8b; also Fig. S1C for the previously reported YP-17 activated carbon). We limit our analysis to domain II, corresponding to moderate r values, when r is compensated by the charge of the adsorbed counter-ions (with sign opposite to r), whereas co-ions (having same sign as r) are electrostatically excluded from the pores. This represents permselective behavior. We first address the issue of selective electro-adsorption (further abbreviated as adsorption) of the Co(CN)63 (hexacyanocobaltate, abbreviated as HCC3) anion versus Cl as reference anion in mixed electrolyte solutions. HCC3 consists of a central Co3+ coordinated octahedrally by CN with sizes of 0.89, 0.60, and 0.71 nm along the four-, three-, and twofold symmetry axes, respectively. [7] (Note that HCC3 is electrochemically inactive over the potential window of the carbon electrode). We applied this analysis to data for HCC3-containing solutions of varying ionic strengths, in both binary and mixed solutions with the Cl anion (with K+ as common cation), collected under various regimes of cycling of the TiC-CDC (see Fig. 1a and b for CVs and DC/zi vs. r plots). Note that, for all HCC3-containing solutions, the experimental DC/zi vs. r slopes coincide (at r > 0) with the theoretical one for hzii = 3 and hMii = 215 g mol1, independent of cycling regime. As defined above, this implies perfect selectivity for TiC-CDC with respect to HCC3 in a mixture with Cl. By the same methodology we find that TiC-CDC selectively adsorbs divalent Ba2+ from a mixture with NH4+ ions (Fig. 2a), while it only slightly prefers monovalent Cs+ over NH4+ (Fig. 2b). This strikingly different behavior points to the deci-

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sive role played by the ion valence in determining the selective permeation of ions into carbon nanopores. We chose tetrabutylammonium (TBA+) as the next test ion, since, while its size (0.82 nm) is similar to that of HCC3, it is much larger of K+ and Cl (0.14 and 0.36 nm, respectively) [8]. CVs and the DC vs. r plots for TiC-CDC in TBACl solutions (Fig. 3a and b) clearly show that, in contrast to the large HCC3- as well as the smaller NH4+ and K+ ions, a characteristic triangular shape of the CV is obtained at r < 0, unlike the butterfly-like CVs for either NH4Cl (dotted purple line, Fig. 3a) and K3Co(CN)6 (black solid curve, Fig. 1a). This type of EQCM response is related to the unique membrane properties of TiCCDC in contact with TBA+. In particular, note that (i) the very large value for (pzc  pzmc) = 0.4 V implies specific adsorption of the TBA+ cation onto the carbon surface (with pzc determined as 0.33 V), and (ii) Cl co-ions desorption from the carbon surface during a negative potential scan is dominant in the range of potentials between pzc and pzmc (Fig. 3b); and, (iii) for the middle part of the potential domain the experimental DC/r ratio was found equal to the theoretical one. Since this is observed for Cl desorption during an increase in the negative surface charge density, this behavior represents reverse permselectivity, i.e. permselectivity supported by desorption of co-ions rather than by adsorption of TBA+ counter-ions. A slight mass increase occurs only at more negative r values due to adsorption of low mobility TBA+ ions into the nanopores. This corresponds to an extensive mixing of adsorbed cations and desorbed anions, as indicated by the small DC/r ratio. Continuous cycling of the carbon electrode results in a stronger cation–anion mixing compared to that performed over a gradually increasing voltage range (black and red solid curves, Fig. 3a), exemplifying the effect of ionic mobility on the adsorption of ions into TiC-CDC. These data suggest that disordered TiC-CDC nanopore walls are ‘‘sticky’’ for TBA+ cations. As soon as a TBA+ cation

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Fig. 1 – (a) CVs at 20 mVs1 and (b) related molar population density changes DC/zi inside pores vs. r plots for a TiC-CDC electrode in: 0.0025 M K3Co(CN)6 + 0.0225 M KCl (blue), ionic strength I = 0.0375; 0.0125 M K3Co(CN)6 + 0.0125 M KCl (brown), I = 0.0875; 0.025 M K3Co(CN)6 (black), I = 0.15; 0.025 M K3Co(CN)6 + 0.025 M KCl (red), I = 0.175. Open circles in panel (b) correspond to unipolar scans (i.e., charged in a single direction from the pzc, here positively), in 0.025 M K3Co(CN)6 with CVs shown with dotted red lines in panel (a). Dotted green curves are experimental data for 0.025 M K3Co(CN)6 + 0.025 M KCl, analyzed with shown values of hzii and hMii equal to 35.5, 125.3 and 215.0 g mol1 for hzii = 1, 2 and 3, respectively. The data for all K3Co(CN)6 solutions overlaps at r > 0 and coincides with the theoretical line for hzii = 3 and hMii = 215 g mol1 (selective adsorption of Co(CN)63). Dashed lines are theoretical lines for data shown with dotted lines of same color.

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Fig. 2 – DC vs. r plots for TiC-CDC in two mixed salt solutions: (a) 0.0025 M [(1  x) NH4Cl + (x/2) BaCl2] and (b) 0.0025 M [(1  x) NH4Cl + x CsCl]; x as listed inside panels. For solutions with Ba2+ agreement with the theoretical slope (dashed lines) obtains forhzii = 2 and hMii = 137.3 g mol1 (selective adsorption of Ba2+). For solutions with Cs+ agreement obtained with 0.0025 M [(1  x) NH4Cl + yCsCl] and following parameters: x = 0.11, y = 0.14,hMii = 34.1 g mol1 and x = 0.20, y = 0.27, hMii = 49 g mol1 (y P x means a slight selectivity for Cs+ over NH4+).

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σ Fig. 3 – (a) CVs for TiC-CDC in aqueous 0.025 M TBACl (dashed black lines – unipolar CVs with increasing cut-off potentials; solid black and dotted purple line – full range CVs for 0.025 M solutions of TBACl and NH4Cl, respectively); (b) DC vs. r plots for cycling regimes marked by the same colors as in panel (a). Dashed black lines in panel (b) are theoretical plots. When calculating DC, we used zi and Mi for TBA+ and Cl at r < 0 and r > 0, respectively. In order to determine r, both pzmc and pzc were used as integration constants (bottom and right-middle parts of panel b, respectively).

Fig. 4 – Graphics illustrating the origin of the differences in the extent and kinetics of the adsorption of the bulky TBA+ cations and Co(CN)63 anions (a and b, respectively) into narrow pores with highly disordered wall surfaces.

enters a pore, it sticks to the hydrophobic pore wall composed of randomly stacked single graphene layers (Fig. 4a and

Fig. S2), thus preventing penetration of other TBA+ cations into the pores, even for high negative values of r. This provides a

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clue for understanding the unusual membrane properties of TiC-CDC in the TBACl solution: nanopores wider than 1.1– 1.2 nm, containing both TBA+ counter- and Cl- co-ions, may contribute to the negative charging of the carbon surface via Cl- anion desorption. The selective adsorption of multivalent ions (HCC3, Ba2+) into TiC-CDC can be described by a ‘‘knock-on’’ mechanism as previously proposed for the highly selective permeation of doubly charged Ca2+ and Ba2+ cations into the ionic channels of biological membranes [9] (see Fig. 4b): the strong electrostatic repulsion between multivalent ions is the likely cause for their facile permeation into nanopores with disordered wall surfaces. Two more factors may also be relevant: (i) the space-efficient packing of multivalent ions; (ii) desolvation energy gain due to the water-structure breaking ability of larger ions (the latter was demonstrated for both biological membranes [9] and amorphous carbon [10]). We have discovered and explored novel features of ion dynamics related to the selective adsorption of ions into TiCCDC. Our study of permselectivity and its failure in several binary electrolyte solutions under different cycling conditions, together with the selective adsorption of ions from their mixtures, indicates that the these effects can be controlled by factors others than the mere ion size. We emphasized the role of electrostatic repulsive forces, space-efficient charge packing and hydrophobic interactions with structurally disordered pore walls, which demonstrate the close similarity of ion transport mechanisms in amorphous carbon and in biological membranes.

Acknowledgments This work was supported by General Motors. The authors thank Dr. Mark F. Mathias for insightful comments and suggestions.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2012.04.002.

R E F E R E N C E S

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