High capacitance B/C/N composites for capacitor electrodes synthesized by a simple method

The B/C/N composites were synthesized by a very simple method, that is, carbonization at HTT = 800-1200 °C of the precursor prepared by drying a solution mixture of polyacrylamide and boric acid, followed by boiling in water to remove borate by-products. The amount of insoluble B species in the composite increased linearly from 4.8 to 18.6 mass% with raising HTT. The XRD and FT-IR revealed that turbostratic h-BN started to form at around 1000 °C as a by-product. By XPS, major B and N components in the composite were B-N bond, C-B-O type B, pyridinic N, pyrrolic N, and quaternary N. A fraction for B-N bond including h-BN in the total B or N components increased with raising HTT and it exceeded 50 at% between 900 and 1000 °C. It was suggested that in the composites formed at HTT > 1000 °C the amounts of h-BN increased, leading to reduction in other B and N components. The S_BET was almost unchanged up to 1000 °C, 410-420 m² g⁻¹. Large and broad redox peaks arisen from plural reactions appeared in the cyclic voltammogram (CV) measured in 1 mol dm⁻³ H₂SO₄ for the composites formed at HTT ≤ 1000 °C. These peaks disappeared in 1 mol dm⁻³ solutions of Na₂SO₄ and Li₂SO₄. By comparing CV with that for C/N composite formed from PAA by the MgO template method, the pseudo-capacitance owing to reactions of B-N and C-B-O components with protons was found to be added to commonly observed pseudo-capacitance for nitrogen-doped carbons. The capacitances for the composites formed at 850-950 °C exceeded 300 F g⁻¹ at 2 mV s⁻¹ in the acid electrolyte and the retention at 50 mV s⁻¹ was 78-80%. The shape of CV in the neutral electrolytes was trapezoid and the current density increased with lowering potential, suggesting adsorption and desorption of Na⁺ and Li⁺ ions. This was considered to be due to doped nitrogen, indicating the development of pseudo-capacitance. The capacitance per S_BET was 0.33-0.74 F m⁻² and 0.17-0.32 F m⁻², larger for lower HTT, in the acid and neutral electrolytes, respectively.     

Laminar burning velocities and flame instabilities of butanol isomers-air mixtures

Laminar burning velocities and flame instabilities of the butanol-air premixed flames and its isomers are investigated using the spherically expanding flame with central ignition at initial temperature of 428 K and initial pressures of 0.10 MPa, 0.25 MPa, 0.50 MPa and 0.75 MPa. Laminar burning velocities and sensitivity factor of n-butanol-air mixtures are computed using a newly developed kinetic mechanism. Unstretched laminar burning velocity, adiabatic temperature, Lewis number, Markstein length, critical flame radius and Peclet number are obtained over a wide range of equivalence ratios. Effect of molecular structure on laminar burning velocity of the isomers of butanol is analyzed from the aspect of CH bond dissociation energy. Study indicates that although adiabatic flame temperatures of the isomers of butanol are the same, laminar burning velocities give an obvious difference among the isomers of butanol. This indicates that molecular structure has a large influence on laminar burning velocities of the isomers of butanol. Branching (CH3) will decrease laminar burning velocity. Hydroxyl functional group (OH) attaching to the terminal carbon atoms gives higher laminar burning velocity compared to that attaching to the inner carbon atoms. Calculated dissociation bond energies show that terminal CH bonds have larger bond energies than that of inner CH bonds. n-Butanol, no branching and with hydroxyl functional group (OH) attaching to the terminal carbon atom, gives the largest laminar burning velocity. tert-Butanol, with highly branching and hydroxyl functional group (OH) attaching to the inner carbon atom, gives the lowest laminar burning velocity. Laminar burning velocities of iso-butanol and sec-butanol are between those of n-butanol and tert-butanol. The instant of transition to cellularity is experimentally determined for the isomers of butanol and subsequently interpreted on the basis of hydrodynamic and diffusion-thermal instabilities. Little effect on flame instability is observed for the isomers of butanol. Critical flame radii are the same for the isomers of butanol. Peclet number decreases with the increase in equivalence ratio.