Ludmila A. Monyakina

Intercalation reactions and carbide formation in graphite-lithium system

In the present work we have investigated a lithium-graphite system by DTA, DSC, heat flow calorimetric, and volumetric methods in a “piston-cylinder apparatus” under high pressures (23 kbar). For the first time, we have determined the enthalpies (ΔHform) of LiC6 (1 stage) and LiC12 (2 stage) formation by the direct calorimetric method at 455 K. It was stated that the values are −13.9 ± 1.2 kJ/mol Li and −24.8 ± 1 kJ/mol Li, respectively. We have defined the enthalpies and temperatures of (3D) → (2D) transitions in LiC6 (T = 711 ± 5 K, ΔHph.tr. = 1.1 ± 0.3 kJ/mol Li) and in LiC12 (T = 472 ± 5K, ΔHph.tr = 1.45 ± 0.2 kJ/mol Li). The rhombohedral modification of Li2C2 was explored at T = 293–1000 K. Only one phase transition at T = 716 ± 6 K (ΔHph.tr. = 5.56 ± 0.09 kJ/mol Li2C2) was detected. The compressibility of Li2C2 was investigated at pressures up to 23 kbar and at room temperature. The change of molar volume (ΔVV0, %) was −2.07 ± 0.07 (10 kbar) and −3.46 ± 0.14 (22 kbar). We studied the carbide formation in Li-GICs at ambient and high pressure (P = 60 kbar) as well.

The choice of oxidizers for graphite hydrogenosulfate chemical synthesis

Abstract The features of chemical oxidation of graphite in H2SO4 are discussed. Measurements of currentless potentials of K2Cr2O7, KMnO4, (NH4)2S2O8 and complex salt Ce(SO4)2·4(NH4)2SO4·2H2O solutions in 96% H2SO4 were carried out. Correlation of the redox potentials of oxidizers to the stage number of graphite hydrogenosulfate (GHS) is established. It is proposed to use the standard redox potential scale to choose the oxidizers for the formation of a given stage GHS. On the basis of this approach, numerous oxidizers for GHS formation are considered. For the first time, ozone and Ce(IY) sulfate were applied for GHS production. The stage 1 GHS is obtained in accordance with standard potentials of these oxidizers.

Chemical synthesis of graphite hydrogenosulfate: Calorimetry and potentiometry studies

In the present work dynamics of sulfuric acid intercalation into graphite by chemical oxidation were investigated by means of the calorimeter and potentiometer techniques. Phase composition of intermediate and end-products of the reaction was investigated by X-ray diffraction.

Investigation of the Graphite-H2SO4-gaseous oxidizer (Cl2, O3, SO3) system

Abstract The intercalation of H 2 SO 4 into graphite during chemical oxidiation by gaseous oxidizers (Cl 2 , O 3 , SO 3 ) was investigated by X-ray and potentiometric methods. It is shown that stage 2 (oxidizer, Cl 2 ) and stage 1 (oxidizer, O 3 ) are formed in 94% H 2 SO 4 . It was established that the rate of the stage 2 GHS formation depends on the Cl 2 ultraviolet radiation, high pressure Cl 2 ( P = 40–70 atm ) and temperature. As expected the rate of the stage 1 GHS formation (oxidizer, O 3 ) was increased with content of O 3 in a O 3 -O 2 gaseous mixture. The graphite-oleum system was investigated in a wide range of oleum concentrations (5–57% of free SO 3 ) by potentiometric and calorimetric methods. It was established that the potentials of GIC formation with oleum are different from potentials of GHS formation when salt-like, Cl 2 or O 3 are used as oxidizers. In addition, the heat flow curves have a monotonous behaviour. These factors were explained by the co-intercalation of SO 3 with anions of sulphuric acid into graphite.

Potentials of graphite nitrate formation during spontaneous and electrochemical graphite intercalation

Earlier and new results on spontaneous and electrochemical intercalation reactions of highly oriented pyrolytic graphite and nitric acid in a wide range of HNO3 concentrations are summarized and analyzed. The oxidizing capacity of the solution is shown to determine the extent of intercalation and the stage index of the resulting graphite intercalation compound (GIC), as in the graphite–H2SO4 system. The final potential of graphite nitrate after spontaneous intercalation coincides with the potential EAg/AgCl in HNO3 . During the reaction of graphite with an HNO3 solution, the potential of graphite nitrate varies monotonically, in contrast to the steplike variation inE in the C–H2SO4–oxidant system. The behaviors of Brönsted acids which can and cannot spontaneously intercalate into graphite are compared. It is shown that the anodic polarization of graphite in HNO3 offers the possibility of controlling the potential and stage index of the resulting GIC by varying the HNO3 concentration and current. During anodic polarization in 75–98% HNO3 at I= 30–100 μA, the interplay between the spontaneous and electrochemical oxidation processes leads to the formation of stage II graphite nitrate, irrespective of the charge passed, and notably reduces the intercalation rate. This effect is interpreted in terms of the intercalation mechanism and sorption processes. The data on the anodic polarization of graphite at small currents point to fundamental differences in electrochemical behavior between the intercalants that can (HNO3) and cannot (H2SO4) spontaneously intercalate into graphite. The concentration ranges and potentials of the formation of stage I and II graphite nitrates via anodic oxidation in HNO3 are determined. The electric current is shown to influence the potential of formation of the stage I GIC: the minimal potential of graphite nitrate formation in 98% HNO3 is EAg/AgCl = 1.34 V (I= 500 μA). The potentialities of the spontaneous and electrochemical intercalation reactions for the controlled synthesis of graphite nitrate with a particular stage index are compared.

Synthesis and Analysis of the Behaviour of Graphite Nitrate in H2O, CH3COOH and their Mixtures

Abstract The behaviour of 2–4 stages graphite nitrate in different mediums: H2O, CH3COOH and their mixtures was analysed by potentiometry, calorimetry and X-ray on highly oriented pyrolytic graphite. The intercalation of 75–98.6% HNO3 into dispersible graphite at the different mass ratio of reagents, and the influence of chemical treatment of graphite nitrate by CH3COOH solutions on properties of hydrolyzed samples (oxidized graphite) were studied.

Calorimetric and Potentiometry Investigations of the Acceptor Compounds Intercalations into Graphite

Abstract In the work the intercalation of H2SO4 (from 98% to 76%) into graphite (oxidizer—(NH4)2S2O8) was investigated by potentiometry. It was established that the redox potential of oxidizer solution depends on the concentration of H2SO4. The criterion for the formation of graphite hydrogenosulfate1 (GHS) was corroborated by the experiment. It was shown that stage of GHS is determined by the redox potential of the oxidizer solution. The system C-K2Cr2O7–94% H2SO4 was investigated by calorimeter and potentiometer methods in situ. The “overoxidation” of stage 1 GHS was observed during the calorimetric investigation (the second exo—effect) but not seen on the potentiometry curve. It was proposed that more high oxidation of graphite and the formation of ion—covalent bonds take place during “overoxidation”.