The hydrous ruthenium oxide has been formed by a sol-gel process. The precursor was obtained by mixing aqueous solutions of RuC13 9 xH20 and alkalis. The hydrous ruthenium oxide powder was obtained by annealing the precursor at low temperatures. The crystalline structure and the electrochemical properties of the powder have been studied as a function of the annealing temperature. At lower annealing temperatures the powder is in an amorphous phase with a high specific capacitance. Specific capacitance as high as 720 F/g was measured for the powder formed at 150~ When the annealing temperature exceeded 175~ the crystalline phase was formed, and the specific capacitance dropped rapidly. The surface area of the powder and the resistivity of the pellet made from these powders have also been studied. The specific surface area and the resistivity decreased as the annealing temperature increased. A capacitor was made with electrodes comprised of hydrous ruthenium oxide and HzSO4 electrolyte. The energy density of 96 J/g (or 26.7 Wh/kg), based on electrode material only, was measured for the cell using hydrous ruthenium oxide electrodes. It was also found that hydrous ruthenium oxide is stable in H2SO4 electrolyte.
The hydrous form of ruthenium oxide (RuO2. xH20) has been demonstrated to be an excellent electrode material for electrochemical capacitors. This material, as prepared by a sol-gel process at low temperatures, is amorphous and electrically conductive. The specific capacitance is over 720 F/g. This value is at least two times higher than the highest value ever reported for such materials. The charge storage mechanism is believed to involve bulk electrochemical protonation of the oxide. This discovery opens a new avenue of research in the field of high energy density electrochemical capacitors.
The oxidative stability
and initial oxidation-induced decomposition
reactions of common electrolyte solvents for batteries and electrical
double layer capacitors were investigated using quantum chemistry
(QC) calculations. The investigated electrolytes consisted of linear
(DMC, EMC) and cyclic carbonate (EC, PC, VC), sulfone (TMS), sulfonate,
and alkyl phosphate solvents paired with BF4
–, PF6
–, bis(fluorosulfonyl)imide (FSI–), difluoro-(oxalato)borate (DFOB–), dicyanotriazolate (DCTA–), and B(CN)4
– anions. Most QC calculations were performed using
the M05-2X, LC-ωPBE density functional and compared with the
G4MP2 results where feasible. The calculated oxidation potentials
were compared with previous and new experimental data. The intrinsic
oxidation potential of most solvent molecules was found to be higher
than experimental values for electrolytes even after the solvation
contribution was included in the QC calculations via a polarized continuum
model. The presence of BF4
–, PF6
–, B(CN)4
–, and FSI– anions near the solvents was found to significantly
decrease the oxidative stability of many solvents due to the spontaneous
or low barrier (for FSI–) H- and F-abstraction reaction
that followed the initial electron removal step. Such spontaneous
H-abstraction reactions were not observed for the solvent complexes
with DCTA– or DFOB– anions or
for VC/anion, TMP/PF6
– complexes. Spontaneous
H-transfer reactions were also found for dimers of the oxidized carbonates
(EC, DMC), alkyl phosphates (TMP), while low barrier H-transfer was
found for dimers of sulfones (TMS and EMS). These reactions resulted
in a significant decrease of the dimer oxidation potential compared
to the oxidation potential of the isolated solvent molecules. The
presence of anions or an explicitly included solvent molecule next
to the oxidized solvent molecules also reduced the barriers for the
oxidation-induced decomposition reaction and often changed the decomposition
products. When a Li+ cation polarized the solvent in the
EC
n
/LiBF4 and EC
n
/LiPF6 complexes, the complex oxidation
potential was 0.3–0.6 eV higher than the oxidation potential
of EC
n
/BF4
– and EC
n
/PF6
–.
Lithium ethylene dicarbonate ((CH2OCO2Li)2) was chemically synthesized and its Fourier transform infrared (FTIR) spectrum was obtained and compared with that of surface films formed on Ni after cyclic voltammetry (CV) in 1.2 M lithium hexafluorophosphate (LiPF6)/ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7, w/w) electrolyte and on metallic lithium cleaved in-situ in the same electrolyte. By comparison of IR experimental spectra with that of the synthesized compound, we established that the title compound is the predominant surface species in both instances. Detailed analysis of the IR spectrum utilizing quantum chemical (Hartree-Fock) calculations indicates that intermolecular association through O...Li...O interactions is very important in this compound. It is likely that the title compound in the passivation layer has a highly associated structure, but the exact intermolecular conformation could not be established on the basis of analysis of the IR spectrum.
We scrutinized the conventional practice of measuring an electrolyte stability window. It is shown that misleading values might be generated by this practice. Thus, we recommend that to obtain a real stability window, the working electrode material should simulate the electrodes used in a real device. Further, in applications that have a high-surface-area electrode, a new quantification of a stability window is proposed. The electrochemical stability values of various nonaqueous electrolytes that are derived this way should reflect the actual operation limits of these electrolytes in real-life devices.
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