Two dimensional materials (graphene and MXenes) for supercapacitor applications

Cette thèse vise à étudier les propriétés électrochimiques de graphène et de MXenes utilisés en tant que matériaux d'électrodes pour supercondensateurs. La première partie concerne la synthèse du graphène et la préparation des films d'électrode. Après immersion dans un mélange contenant une concentration de 10wt% de mélange de liquides ionique ((PYP13)0.5(PYR14)0.5-TFSI) dans l'acétonitrile et séchage sous vide, un film de gel de graphène est obtenu, qui est ensuite caractérisé électrochimiquement dans l'électrolyte ((PYP13)0.5(PYR14)0.5-TFSI pur ; une capacité de 175F/g est alors obtenue. L'intérêt de cette méthode de synthèse est d'améliorer l'accessibilité de la surface du graphène en pré-intercalant entre les feuillets les liquides ioniques. L'utilisation de ce mélange eutectique de liquides ioniques (PYP13)0.5(PYR14)0.5-TFSI permet également d'augmenter considérablement la plage de température d'utilisation du système (de -40°C à 80°C), grâce à l'absence de solidification du mélange eutectique jsuque -60°C. Dans une deuxième partie, nous nous sommes intéressés à de nouveaux matériaux 2 Dimmensions, les MXènes, et plus particulièrement à la phase Ti3C2Tx. Des films de Ti3C2Tx. ont été préparés suivant le même protocole que précédemment, à la différence près que les feuillets de MXènes ont d'abord été pré-intercalés avec H2SO4. Les électrodes de Ti3C2Tx ainsi préparées montrent des capacités extrêmement élevées de 380 F/ g avec des capacités volumiques dépassant les 1500 F/cm3 dans l'électrolyte 3 M H2SO4. Ces performances surpassent tous les résultats rapportés pour les MXenes à ce jour ; ces capacités sont même comparables avec celles obtenues avec des matériaux pseudocapacitifs comme le RuO2. Pour terminer, les électrodes de Ti3C2Tx pré-intercalées avec du liquide ionique EMI-TFSI ont été étudiées dans l'électrolyte liquide ionique pur (EMI-TFSI). Des capacités atteignant 80 F/g ont tout d'abord été obtenues dans un domaine de potentiel de 3 V, ce qui constitue à ce jour les meilleurs résultats obtenus avec les MXenes en milieu liquide ionique pur. En plus de ces performances électrochimiques remarquables, le mécanisme de stockage des charges a également été étudié diffraction des RX in-situ. Les résultats ont montré que la distance entre deux feuillets de MXenes augmente lors de polarisations négatives du fait de l'effet stérique associé à l'insertion des cations EMI+. Différemment, la diminution de cette même distance inter-feuillets durant les polarisations positives a été attribuée à a) l'attraction électrostatique entre les anions TFSI intercalés et la surface Ti3C2Tx chargée positivement et/ou à b) l'effet stérique lors de la désinsertion des cations EMI+ présents. Cette thèse montre le potentiel prometteur de graphène et MXenes pré-intercalés avec des électrolytes de type liquides ioniques en tant que matériaux d'électrodes pour la réalisation de supercondensateurs de grande densité d'énergie fonctionnant en milieu aqueux ou organique.This thesis aims at studying the electrochemical properties of graphene and MXenes materials used as electrode in supercapacitor applications. The first part starts with the graphene synthesis and electrode films preparation. After immersion in a solution of 10wt% ((PIP13)0.5(PYR14)0.5-TFSI) in acetonitrile electrolyte and vacuum drying, a graphene gel film was obtained and electrochemically characterized in (PIP13)0.5(PYR14)0.5-TFSI ionic liquid mixture electrolyte. The combination of high-voltage electrolyte with fully accessible, high surface area graphene film enables to achieve high gravimetric capacitance up to 175 F/g in neat ionic liquid electrolyte. A large operation temperature range from -40 to 80 oC was achieved thanks to the use of (PIP13)0.5(PYR14)0.5-TFSI ionic liquid eutectic mixture which does not show any phase change down to -60°C. In a second part, we processed 2-Dimmensional Ti3C2Tx MXene materials into gel film using a similar approach that we did for graphene. Ti3C2Tx shows extremely high capacitance of 380 F/g and 1500 F/cm3 in 3 M H2SO4 electrolyte, which i) surpass all the reported results for MXenes so far and ii) show at least similar performance than pseudocapacitive materials such as RuO2. Besides, Ti3C2Tx MXene gel films were also studied in neat ionic liquid electrolyte (EMI-TFSI). A capacitance of 80 F/g was achieved with good rate performance, which is today the best performance obtained in neat ionic liquid for these materials. More interestingly, the charge storage mechanism was further studied by in- situ XRD technique. This in-situ study has evidenced two different charge storage mechanism. During negative polarization, the interlayer spacing in MXene flakes increases due to steric effect during EMI+ cation insertion. Differently, the decrease in the interlayer spacing during positive polarization was ascribed to i) electrostatic attraction between the intercalated TFSI- anions and positively-charged Ti3C2Tx surface and/or ii) a steric effect of EMI+ cations de-intercalation. This thesis presents the promising potential for using Graphene and MXenes as electrode materials for supercapacitor, and shed lights on further development of these materials

Electrochemical double layer capacitors: What is next beyond the corner?

This review summarizes some recent developments achieved in the fundamental understanding of ion confinement in microporous carbon supercapacitor electrodes. Combined with computational simulations, these advanced techniques provided new insights into the charge storage mechanism, providing guidelines for designing improved porous carbon structures with high-energy density. Also, innovative electrolytes have recently been proposed by introducing some redox-active moieties into electrolyte anions/cations, called biredox electrolytes, that combines redox contribution to double capacitance. This approach opens up new opportunities to develop high-energy supercapacitors and a new field of biredox electrolyte

Advanced analytical techniques to characterize materials for electrochemical capacitors

This review covers recent developments in advanced analytical techniques to characterize materials for electrochemical capacitors. For double layer capacitors, examples of the use of in situ X-ray photoelectron spectroscopy (XPS), pulsed electrochemical mass spectrometry (PEMS) technique, temperature-programmed desorption coupled with mass spectroscopy (TPD-MS) technique, in situ NMR spectroscopy, and in situ dilatometry measurement are presented, for studying carbon/electrolyte interface with a focus onto electrolyte ions confinement in nanopores and changes during ageing. For the pseudocapacitive system, in situ X-ray (neutron) diffraction or scattering, in situ dilatometry technique, cavity micro-electrode, in situ Raman spectroscopy, TPD-MS technique, and electrochemical quartz crystal microbalance (EQCM) technique have been employed for studying materials structure, electrochemical kinetic, interface interaction, and ions adsorption/desorption. These advanced analytical techniques probe insight into charge storage mechanisms, and guiding the fast development of supercapacitors

Ergodic Theory Over {\F}_2[[T]]

In cryptography and coding theory, it is important to study the pseudo-random sequences and the ergodic transformations. We already have the $1$-Lipshitz ergodic theory over ${\Z}_2$ established by V. Anashin and others. In this paper we present an ergodic theory over {\F}_2[[T]] and some ideas which might be very useful in applications

Graphene-Based Supercapacitors Using Eutectic Ionic Liquid Mixture Electrolyte

Compact graphene films were prepared and electrochemically tested at various temperatures in a eutectic of ionic liquid mixture (1:1 by weight or mole N-methyl-N-propylpiperidinium bis(fluorosulfonyl)imide and N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide) electrolyte. A large temperature window from -30 °C to 80 °C was achieved together with a large potential window of 3.5 V at room temperature and below. A maximum gravimetric capacitance of 175 F.g−1 (85 mAh.g−1) was obtained at 80 °C. 130 F.g−1 (63 mAh.g−1) and 100 F.g−1 (49 mAh.g−1) were still delivered at -20 °C and -30 °C respectively. Besides, a volumetric capacitance of 50 F.cm−3 was achieved with a thick graphene film (60 μm). The outstanding performance of such compact graphene film in the eutectic ionic liquid mixture electrolyte makes it a promising alternative to activated carbon for supercapacitor applications, especially under extreme temperature conditions

3D rGO aerogel with superior electrochemical performance for K – Ion battery

As one possible alternative metal to lithium in ion batteries, potassium has recently attracted considerable attention as a result of its geochemical abundance and low cost. In this work, a detailed study of the electrochemical properties of potassium ion storage was performed using reduced graphene oxide (rGO) aerogel as a negative electrode material. The influence of the nature of the electrolyte and the drying methods used were investigated in order to optimize the electrochemical performance of freeze-dried rGO in potassium-ion batteries (PIBs). Electrochemical impedance spectroscopy (EIS) were used to assess the performance of our rGO material in PIBs. Used as the negative electrode, freeze-dried rGO can deliver a high capacity of 267 mA h/g at C/3 rate together with 78% capacity retention during 100 cycles, combined with high rate capability (92 mA h/g at 6.7C). This set of results makes rGO aerogel a promising electrode material for PIBs

Tracking Ionic Rearrangements and Interpreting Dynamic Volumetric Changes in Two-Dimensional Metal Carbide Supercapacitors: A Molecular Dynamics Simulation Study

We present a molecular dynamics simulation study achieved on two‐dimensional (2D) Ti3C2Tx MXenes in the ionic liquid 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]+[TFSI]−) electrolyte. Our simulations reproduce the different patterns of volumetric change observed experimentally for both the negative and positive electrodes. The analysis of ionic fluxes and structure rearrangements in the 2D material provide an atomic scale insight into the charge and discharge processes in the layer pore and confirm the existence of two different charge‐storage mechanisms at the negative and positive electrodes. The ionic number variation and the structure rearrangement contribute to the dynamic volumetric changes of both electrodes: negative electrode expansion and positive electrode contraction

Comment to the letter to the editor from Costentin et al. Entitled “Ohmic drop correction in electrochemical techniques. Multiple potential step chrono-amperometry at the test bench”

Comment to the letter to the editor from Costentin et al. Entitled “Ohmic drop correction in electrochemical techniques. Multiple potential step chrono-amperometry at the test bench