Graphite intercalation with fluoroanions by chemical and electrochemical methods Public Deposited

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  • New acceptor-type graphite intercalation compounds (GICs) containing perfluoroalkyl anions have been synthesized by using both chemical and electrochemical methods and characterized by elemental and thermogravimetric analyses. Investigation into these graphite intercalation compounds can provide novel materials and a detailed understanding of their properties. GICs of composition Cx[FB(C₂F₅)₃]·δF are prepared for the first time by the intercalation of fluoro-tris(pentafluoroethyl)borate anion, [FB(C₂F₅)₃]⁻, under ambient conditions in aqueous (48 %) hydrofluoric acid containing the oxidant K₂[MnF₆].Powder-XRD data indicate that products are pure stage 2 and physical mixture of stage 2 and stage 3 after 1 h to 20 h reaction times. The calculated basal repeat distance, Ic, is 1.20 nm for stage 2 and 1.54-1.56 nm for stage 3 GICs, corresponding to gallery heights of di= 0.86-0.89 nm. In addition, stage 2 GIC of Cx[FB(C₂F₅)₃]·δCH₃NO₂ having di=0.84 nm is prepared by electrochemical oxidation of graphite in a nitromethane electrolyte. The elemental analyses of these complex GICs required that a new sample digestion protocol be developed. After digestion, the fluoride amounts in these GIC samples were analyzed by using ion-selective fluoride combination electrode. The method developed is able to provide fluoride anion content in GICs without interference from the decomposition products of [FB(C₂F₅)₃]- anion. For the boron analyses the same digestion procedure above is used and the B contents were determined by ICP-AES. For Cx[FB(C₂F₅)₃]·δF, both compositional parameters x and δ are obtained from the results of elemental B and F analyses. For the chemically prepared GICs at 1 h to 20 h, calculated x values were in the range of 51-56 and the calculated δ values increased with reaction time from approx. 0-2. Combining B analysis and TGA mass loss gives a composition of x=44 and δ= 0.37 for the electrochemically prepared GIC of Cx[FB(C₂F₅)₃]·δCH₃NO₂. Energy minimized structure for the isolated borate anion and powder XRD data show that the borate anions adopt a "lying-down" orientation where the long axes of [FB(C₂F₅)₃]- intercalate anions are parallel to the encasing graphene sheets. The same electrochemical synthesis strategy is also used for the preparation of a new acceptor-type GIC containing the cyclo-hexafluoropropane-1,3-bis(sulfonyl)amide anion, [CF₂(CF₂SO₂)₂N]⁻. The gallery heights of 0.85-0.86 nm are determined by powder X-ray diffraction for stage 2 and 3 products. These GICs are obtained by electrochemical oxidation of graphite in a nitromethane electrolyte. GICs containing the linear anion, [(CF₃SO₂)₂N]⁻ are also prepared in order to compare the gallery heights and the electron charge distributions that helps to understand the GIC stabilities within the graphene sheets. The compositions of GICs containing [CF₂(CF₂SO₂)₂N]⁻ are determined by thermogravimetric, fluorine and nitrogen elemental analyses. GICs of composition Cx[(C₂F₅)₃PF₃] are prepared for the first time by the intercalation of tris(pentafluoroethyl trifluorophosphate (FAP) anion, [(C₂F₅)₃PF₃]⁻ by electrochemical oxidation of graphite. Powder-XRD data indicate that products are of stages 2-4 with gallery heights of 0.82-0.86 nm. These GICs are characterized by the same methods using TGA and F ion selective probe analyses.
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