Ion Dynamics at the Single Wall Carbon Nanotube Based Composite

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Ion Dynamics at the Single Wall Carbon Nanotube Based Composite Electrode/Electrolyte Interface: Influence of the Cation Size and the Electrolyte pH Freddy Escobar-Teran, Hubert Perrot, and Ozlem Sel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11672 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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The Journal of Physical Chemistry

Ion Dynamics at the Single Wall Carbon Nanotube Based Composite Electrode/Electrolyte Interface: Influence of the Cation Size and the Electrolyte pH F. Escobar-Terana,b, H. Perrota,*, O. Sela,* aSorbonne

Universite, CNRS, Laboratoire Interfaces et Systemes Electrochimiques, LISE, F-

75005, Paris, France. bFacultad

de Ciencia e Ingeniería de Alimentos, Univ. Tecnica de Ambato, Avenida Los

Chasquis y río Payamino s/n, Ambato, Ecuador. Email: * [email protected] and [email protected] Abstract: Electrochemical quartz crystal microbalance (EQCM) and ac-electrogravimetry methods were employed to study ion dynamics in single wall carbon nanotube (SWCNT) based electrodes in various aqueous electrolytes and different electrolyte pH. The pH dependence of the electroadsorption phenomena studied in NaCl at pH 2, pH 7 and pH 10 indicated that low and high pH values amplify the anion and cation contribution, respectively; which means that in the same potential range, the pH of the aqueous electrolyte is adjustable to preferentially electroadsorb cations or anions. The cation size dependence of the electrodeposition phenomena was also studied by changing the electrolyte cation from Li+, Na+ to K+. The results indicated that Li+ and Na+ species are more tightly bonded to their water molecules in their hydration shell compared to the potassium species, i.e. dehydration of K+ is easier than that of Na+ and Li+. K+ ions due their completely dehydrated state, probably become equivalent to partially dehydrated Na+.nH2O, in terms of kinetics. Li+.nH2O ions dynamics is slower since it is much more difficult for them to get rid of their hydration shell making their transfer the slowest among the other cations. Our electrogravimetric results clearly indicate that the dehydration affinity and the ions size play a role in the electroadsorption dynamics of the species on the SWCNT based composite electrodes. 1 ACS Paragon Plus Environment

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1. INTRODUCTION The efficiency of the advanced energy storage systems such as batteries and supercapacitors is certainly related to the development of the components constituting these electrochemical devices. Among them, electrochemical double layer capacitors (EDLC) store charge at the electrode/electrolyte interface based on a reversible adsorption of ions.1, 2 The EDLCs have been launched using porous carbon electrodes. The physical properties such as large specific surface area and a suitable pore size of these materials are considered to be optimized to be able to further improve the performance of these energy storage devices.3, 4 A marked improvement in performance has been obtained thanks to recent progress in the development of nanostructured materials with high specific surface area and sophisticated structures.5 Nevertheless, these goals concerning the materials development, even if they are reached are not sufficient to obtain perfect devices with high performance. Therefore, it is highly significant to meticulously study and understand the electrochemical mechanism related to the ionic transfer between the electrode and the electrolyte and corroborate these comprehension with the required materials specifications. Carbon nanotubes have been extensively studied as carbon materials for the electrochemical storage of energy in capacitors due to their particular characteristics such as high electrical conductivity, unique pore structure, chemical reactivity and their surface area where the charges are continuously distributed.5,6 However, the investigation of the charging mechanisms, i.e.; ion dynamics at the interfaces is experimentally difficult because there are not many suitable electrochemical or physico-chemical methods that allow a direct access to such information. Electrochemical quartz crystal microbalance (EQCM) has been used in previous studies employing carbon based materials, because it provides substantial information concerning the mass changes of the electrodes during charging/discharging.7-18. The basic interpretation of the EQCM is that the mass changes of an electrode deposited on the gold electrode of a quartz

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resonator, m , are related to changes in the resonance frequency of the quartz crystal, f m , by means of Sauerbrey equation (Equation 1):19

 2 fo2  Sn   q q 

fm   k s xm   

  m  

(1)

where q is the quartz density (2.648 g.cm-3), μq is the shear modulus of a shear AT quartz crystal (2.947 x 1011 g cm-1 s-2), f0 is the fundamental resonant frequency of the quartz (Hz), S is the active surface on the quartz corresponding to the metal electrode deposited on it (cm2), n is the overtone number and ks is the theoretical sensitivity factor (Hz g-1 cm2). The interest for EQCM based methods has continuously increased in the energy storage community (supercapacitors and batteries) and gained wide applicability to investigate the ion dynamics in porous materials. Over the past decade, Levi, M. D., et al. have extensively studied the effect of specific adsorption of ions and their size on the charge-compensation mechanism in carbon micropores.7-12 Following these developments, EQCM was employed to study the charge mechanism and the solvation effect at the electrode/electrolyte interface13 and to study the effect of the electrolyte concentration and compositional changes in porous materials.14 It has been combined with other characterization methods such as with nuclear magnetic resonance (NMR) to access to a full description of the electric double layer.15 These developments are summarized in recent review papers.20-22 In EQCM, mass and charge variations measured simultaneously during the electrode cycling allow the derivation of the global mass per mole of electrons (MPE) that is exchanged between the electrode and the electrolyte. If only one species is exchanged, then the MPE corresponds to its molar mass. If multiple ion transfer occurs, using Faraday’s law to interpret classical EQCM data reaches its limitations.23 To discriminate/identify the species involved in the charge compensation, Donnan-type electrical double layer models were incorporated into the gravimetric EQCM equations.24 Besides, the complex character of the electrode mass changes 3 ACS Paragon Plus Environment


and the viscoelastic properties of the electrodes (and also possibly formed solid electrolyte interface (SEI) layer) can be investigated using multiharmonic EQCM with dissipation monitoring (EQCM-D)25 offering opportunities to identify the impact of several parameters (nature of the electrolytes, ions, binder…) on the structure change of the electrodes.25,26 As an extension to these developments in the use of EQCM based methods in the energy storage domain, here, an alternative electrochemical and gravimetric method, specifically the electrochemical impedance spectroscopy (EIS) coupled to a fast quartz crystal microbalance (QCM) also called ac-electrogravimetry, was used to study the capacitive charge storage behavior of carbon based electrodes as a function of the electrolyte properties. Acelectrogravimetry methodology or electrogravimetric impedance was largely used for characterizing redox materials, such as Prussian Blue27,28 or conducting polymers such as polypyrrole or polyaniline29-33. Recently, it has been employed to study the capacitive charge storage behavior such as in carbon nanotubes16,17 and in reduced graphene oxide34, as well as pseudocapacitive metal oxide based electrodes23,35. It has been demonstrated that highly relevant and complementary information to the classical EQCM can be obtained: (i) kinetics and identification of species transferred between the electrode and the electrolyte, (ii) separation of the different contributions related to the charged and non-charged species involved in the electrochemical processes, (iii) identification of species transferred in opposite flux directions provided that their kinetics are sufficiently different (iv) variation of the relative concentrations of the species inside the examined material. Therefore, the ac-electrogravimetry methodology has been used here as a complementary tool to EQCM to study the capacitive charge storage behavior of SWCNT electrodes in different aqueous electrolyte and various electrolyte pH.


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2. EXPERIMENTAL METHODS Thin film preparation. Single Wall CNT (755117-1G, length: 300-2300 nm and diameter: 0.7-1.1 nm), acquired in Sigma Aldrich Company was used to prepare nanostructured composite electrodes. The preparation of CNT films was carried out according to the method described in the literature.9,16,36 CNTs were deposited by the "drop-casting" method on a gold electrode (effective surface area of 0.20 cm2) of a quartz crystal resonator (9 MHz -AWS, Valencia, Spain), from a solution of carbon containing 90% (9 mg) CNT powder and 10% (1mg) PVDF-HFP (Poly(vinylidene fluoride-hexafluoropropylene)) polymer binder in 10 ml of N-methyl-2-pyrrolidone. Around 8 μl portion of this solution was deposited on the gold electrode of the QCM. Then, the carbon films were subjected to a heat treatment at 120 °C for 30 minutes, with a heating rate of ~ 5 °C min-1 to evaporate the residual solvent and improve the adhesion properties of the films on gold electrode. The deposited mass was estimated by measuring, Δfm, before and after the deposition and by converting it to the mass change (Δm), using the Sauerbrey equation,19 (Δfm=−ks.Δm where ks is the experimental calibration constant (16.3 × 107 Hz/g·cm−2)). The details of the estimation of the experimental sensitivity factor are previously given.37 Morphological and physical characterizations. Prior to the thin film preparation, the SWCNT powders were characterized by N2 sorption, X-ray diffraction (XRD) and highresolution transmission electron microscopy (HR-TEM). Nitrogen adsorption-desorption isotherms of the samples were recorded at 77 K (temperature of liquid N2) using a BELSORPmax high-performance surface area and porosity analysis instrument. Samples were previously degassed at 250 °C and 0.397 µPa. The specific surface area and pore size distribution (PSD) were calculated according to the BET and BJH method. The crystal structure of the samples was analyzed by using an Empyrean Panalytical X-ray diffractometer (Cu Kα radiation, λ = 1.541 84 Å) operated at 60 kV. High Resolution Transmission Electron Microscopy (HR-TEM)



analyses were performed using a JEOL 2010 UHR microscope operating at 200 kV equipped with a TCD camera. The samples were scraped off the substrate and ultrasonicated in ethanol before being placed on copper TEM grids. The surface morphology of the SWCNT composite thin films deposited on the gold electrode of the quartz resonators were investigated by field emission gun scanning electron microscopy (FEG-SEM) (Zeiss, Supra 55). Before the analysis, the samples were fixed onto an aluminum stub with a conductive carbon tape and sputter-coated with gold (“JEOL JFC-1300 Auto fine coater”). Electrochemical and electrogravimetric characterizations. EQCM measurements were performed in aqueous solutions of LiCl, NaCl and KCl (at different pH values) in a threeelectrode configuration. A lab-made QCM device (Miller oscillator) was used to measure frequency shift (∆f) of the quartz crystals. Modified quartz resonator was used as the working electrode. Platinum grid and Ag/AgCl (3M KCl) was used as counter and reference electrode, respectively. The gravimetric regime was assured by keeping film thickness acoustically thin (

Ion Dynamics at the Single Wall Carbon Nanotube Based Composite

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