TiC-carbide derived carbon electrolyte adsorption study by ways of X-ray scattering analysis
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Understanding ion adsorption in nanoporous carbon electrodes is of great importance for designing the next-generation of high energy density electrical double-layer capacitors. In this work, X-ray scattering is used for investigating the impregnation of nanoporous carbons with electrolytes in the absence of applied potential. We are able to show that interactions between the carbon surface and electrolytes allow adsorption to take place in sub-nanopores, thus confirming experimentally for the first time the results predicted by molecular dynamic simulations.
KeywordsIon adsorption Supercapacitors Porous carbon Ionic liquids X-ray scattering
Using carbide derived carbons (CDCs) with controlled, fine-tuned, narrow pore size distribution in the microporous range [9, 10], it was demonstrated that the maximum capacitance was achieved when the ion size was close to the carbon pore size [9, 11, 12]. Such result, defying the conventional wisdom that pores twice larger as the ion size were needed for optimizing the charge storage, was attributed to a shortening of the ion–wall pore distance .
Important efforts were directed towards the understanding of the ion adsorption and transport mechanisms in such confined pores. The capacitance increase was attributed to the absence of the over-screening effect [13, 14] and to a large electronic screening by the pore wall of like–like ions repulsive interactions . Additionally, both the ion desolvation and the local charge stored at the carbon electrode increase in highly confined pores . Still using modelling, it was also proposed that the electrolyte could wet—and access—small pores (<1 nm) even though no potential was applied at the electrode, suggesting that ion adsorption in these small pores was not an electrosorption-driven process . However, to our knowledge apart adsorption in activated carbon of neutral molecule such as water , there is no experimental evidence of such spontaneous adsorption of charged species without applied potential.
X-ray scattering is a powerful technique for investigating both atomic ordering and electronic density contrast evolution linked with pore filling. Usually, the range of distances to be analysed is so different that, for technical reasons, two different experimental set up must be used: small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS). SAXS brings information about phenomena impacting large distance range such as pore filling, while WAXS is used to study short (interatomic) distances. However, the high capacitance observed in porous carbons being associated with the presence of micropores with size close to interatomic distances (between 0.6 and 1 nm), these sizes are small enough to generate “typical SAXS” patterns in the lowest part of the angular range normally used to investigate interatomic distances. That makes the use of the WAXS technique sufficient to investigate both pore filling and pore wall modifications. Moreover, the access to low and high angles parts in a single experiment allows an easy evaluation of the ratio between the different components (carbon, electrolyte, sample holder) useful to separate their contributions.
The scope of this paper is then to report an original use of the X-ray scattering technique to investigate the spontaneous adsorption of ions in pores of the same size. A preliminary study confirming that the use of WAXS technique allows determining both atomic ordering and pore filling using a single experiment is first presented. Then, the effectiveness of selected CDC impregnation with different electrolytes is studied giving for the first time experimental evidences of spontaneous adsorption of ions without applied potential.
Microporous carbon, purchased from Y-Carbon Company, was prepared by chlorination of titanium carbide as described elsewhere [9, 18]. The control of the annealing temperature allows selecting pore size of 0.66 and 0.9 nm as confirmed by BET technique . The electrolytes (propylene carbonate from Aldrich, TEABF4 from Acros organics and RTIL from Solvionic) are used as received and stored under argon atmosphere in glovebox with <0.1 ppm content of water and O2. Samples for the X-ray scattering experiments were prepared by filling Lindemann glass capillaries of 1.5 mm diameter with carbon and electrolyte. Capillaries were centrifuged to ensure carbon being appropriately wet by the electrolyte.
Small-angle X-ray scattering data were recorded using an INEL SAXS device in transmission geometry with point focus using a sealed source at Cu Kα wavelength (λ = 0.154056 nm) and an image plate detector. The X-ray beam passes through Kirkpatrick–Baez mirrors, ensuring low divergence and spot-like shaping of the X-ray beam. Beam-size is adjusted using three consecutive cross-slits systems. The sample to detector distance was calibrated using a silver behenate reference sample. The collected intensity was corrected from capillary contribution and radial averaging was performed using the Fit2D software .
Wide angle X-ray scattering data were recorded in Debye–Scherrer geometry using a sealed source at Mo Kα wavelength (λ = 0.071069 nm) and a solid-state detector. This combination makes instrument background and absorption corrections close to negligible in our case. Data were thus only corrected for polarisation and capillary contribution.
Results and discussion
Validation of the “SWAXS” technique
Parameters used in the excluded volume chain model
Radius of gyration
v (Flory exponent)
Study of the CDC spontaneous impregnation
Once the approach was validated, the impregnation of CDC using different electrolytes selected for their respective behaviour was carried out. TiC CDC with pore size of 0.66 and 0.90 nm were used since these materials are known to show high capacitance . To check their behaviour without any electric field imposed, different electrolytes such as 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI, BF4) and 1 M tetraethylammonium tetrafluoroborate (TEA, BF4) in propylene carbonate (PC) are selected to offer a wide range of ion size in combination to the presence or absence of solvent.
The X-ray scattering spectra of the different samples (electrolytes, dry carbon and impregnated carbons) are collected and corrected as detailed in the experimental section. In all scattering spectra presented in the following figures, as expected from the study described above, the typical SAXS signal can be observed in the lowest scattering angle area of the measured WAXS data. The low-angle part (below 7°) will then be used as a conventional SAXS signal to follow the pore filling, while larger scattering angle domain (above 7°) will be used to follow atomic ordering evolution of CDC and ions during impregnation. As a general comment, apart the intensity of the signal directly related to the amount of CDC, there is no evolution of the peaks characteristics of the structure of the CDC. This clearly indicates that whatever the electrolyte used, and despite small ion-C distances, no significant change of the CDC walls’ constitution can be detected during the impregnation.
These results indicate the existence of an interaction (whatever physical or chemical bonding) between carbon and PC molecules at the origin of the extra energy which would be necessary to remove adsorbed PC molecules. Unfortunately, this spontaneous pore loading with neutral and small molecules prevents studying of ion adsorption separately from the solvent contribution. To resolve this ambiguity, solvent-free electrolytes such as Room Temperature Ionic Liquids (RTIL) have been investigated. EMI, BF4 was selected thanks to its favourable X-ray signature, showing only small contributions at small scattering angles. Ion size of BF4 − is 0.48 nm and EMI+ is an anisotropic ion 0.76 nm long and 0.43 nm wide.
The structural peaks of carbon observed in the high-angle part remain unchanged after impregnation. That shows that despite interaction between adsorbed ions and pore walls, the structural arrangement of carbon is not significantly modified or that such modification should concern only atoms located near the surface.
The reported results show that the filling of pores with size close to the one of adsorbed ions induces the shortening of the distances of interest, allowing the use of both SAXS and WAXS techniques using a single SWAXS experimental set up. The study of the evolution of the SAXS part of this SWAXS signal gives for the first time experimental evidences of spontaneous (without applied potential) filling of CDC pores without steric limitation. The study of the WAXS part of the SWAXS signal shows no significant changes in the structural arrangement of carbon. This is in agreement with whatever a localized interaction near the pore surface or the absence of strong chemical bonding between adsorbed species and pore walls. Despite that, an interaction should exist as indicated by the difficulties encountered to remove adsorbed species. The next step currently in progress is to develop cells for in situ experiments to study the behaviour of ions while a polarisation is applied between electrodes.
L.T. was supported by a PhD grant from Délégation Générale de l’Armement (DGA). P.S. would like to thank the European Research Council for supporting the research (Advanced Grant No. ERC-2011AdG, Project No. 291543—IONACES). Pr Y. Gogotsi is thanked for the supervision of the preparation of selected microporous carbon.
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