Anthony G. Pandolfo

Ionic liquid–solvent mixtures as supercapacitor electrolytes for extreme temperature operation

Ionic liquid (IL)-based electrolytes containing molecular solvents were shown to be attractive for extreme temperature applications in electric double layer capacitors (EDLCs). In particular, the IL–butyronitrile (BuCN) mixture provides high capacitance (around 125 F g−1 at 500 mA g−1) independent of testing temperature, and superior performance at high current rates (reduced current dependence at high rates). Importantly, the IL–BuCN electrolyte can safely operate between −20 and + 80 °C, which overcomes the high temperature limitations of current commercial EDLCs. An additional advantage of IL–solvent mixtures is that the higher concentration of IL ions in the mixtures allows a greater specific capacitance (F g−1) to be achieved. The conductivity of the ionic liquid N-butyl-n-methylpyrrolidinium bis(trifluoromethane sulfonyl) imide (PYR14TFSI) could be increased from 2.48 mS cm−1 up to 45 mS cm−1 by mixing with an appropriate solvent. Importantly, these solvent mixtures also retain a wide electrochemical voltage window, in the range 4–6 V.

Role of Carbon Porosity and Ion Size in the Development of Ionic Liquid Based Supercapacitors

The role played by carbon porosity and electrolyte chemistry in the development of double-layer supercapacitors based on solvent-free ionic liquids ILs of a wide electrochemical stability window is investigated. Voltammetric studies performed in N-methyl-N-butyl-pyrrolidinium bis trifluoromethanesulfonyl imide PYR14TFSI , N-trimethyl-N-propylammonium bis trifluoromethanesulfonyl imide, and N-methyl-N-butyl-pyrrolidinium tris pentafluoroethyl trifluorophosphate ionic liquids and PYR14TFSI—tetraethyl ammonium bis trifluoromethanesulfonyl imide solutions demonstrate that the pore-to-ion size ratio and the porous electrode/IL interface properties may have a higher impact on the electrode electrical response than do the inherent IL bulk properties. The effect of carbon porosity on the electrode capacitance and charge storage capability in both the positive and negative potential domains is discussed in relation to the IL properties, and an estimation of the upper limits of the performance of IL based supercapacitors with carbons of optimized porosity is reported.

Detection of Oxygen Evolution from Nickel Hydroxide Electrodes Using Scanning Electrochemical Microscopy

An important parameter for judging the performance of the nickel hydroxide electrode in electrochemical energy storage devices is the difference between the oxygen evolution potential and the nickel hydroxide oxidation potential. Scanning electrochemical microscopy (SECM) has been investigated as an accurate method for the determination of the potential at which nickel hydroxide formulations evolve oxygen. Oxygen evolution from a variety of nickel hydroxide substrate electrodes in KOH was studied in situ by SECM. The SECM probe tip, positioned just above the substrate surface, was used to detect the oxygen evolved while cyclic voltammetry was also performed on the substrate, thus separating the detection of oxygen evolution from the nickel hydroxide oxidation process. A range of supporting substrates were investigated: conducting glass (indium tin oxide), glassy carbon, and nickel metal, with either nickel hydroxide formulations pasted or microparticles abrasively attached onto the support. Graphite, as a conductive additive to Ni(OH) 2 paste formulations, gave more positive oxygen-evolution overpotentials than similar formulations containing filamentous nickel as the conductive additive. Unmilled nickel hydroxide samples outperformed milled samples; higher rates of oxygen evolution and lower capacities were obtained from the milled nickel hydroxide samples as determined by SECM and high-rate charge/discharge cycling measurements.

Physical and chemical characteristics of densified low-rank coals

Abstract The conversion of Victorian brown coal into a hard densified product is described. Densified brown coal (DBC) is readily moulded or extruded into a convenient form and is characterized by reduced moisture content, greater bulk density and increased strength. Removal of water is initiated by a kneading process and continues during drying under ambient conditions. The loss of water is also accompanied by pellet shrinkage and an increase in crush strength. The development of crush strength is attributed to an increase in pellet density arising from greater coal compaction through the formation of a particulate gel network. The extent of compaction and the strength of the carbonaceous gel network depends on the pH and the nature of the acidic oxygenated functional groups in the coal. Derivatization of acidic functionalities, by O-methylation, results in a collapse of the coal network structure to give a product with inferior physical properties owing to the blockage of potential bonding sites. Acidic coals produce DBC of moderate density and strength, whereas base-treated coals produce very dense and stronger products. The improved physical properties of the base-treated DBCs is attributed in part to the presence of ionic associations between the coal particles to give a stronger three-dimensional network.

Increasing Cycle Life of Nickel Hydroxide Electrodes at High Currents

The effect of high current (>10C) cycling on the capacity of nickel hydroxide was investigated in aqueous KOH with the objective of increasing cycle life at high currents. Capacity loss at high voltages has been attributed to excess oxygen evolution which occurs when charging is conducted at high rates without voltage limits. Cyclic voltammetry of nickel hydroxide microparticles, attached to glassy carbon electrodes, enabled the separation of the oxidation process from oxygen evolution. With large pasted electrodes, due to their large uncompensated resistance (Ru), these processes are poorly resolved. An alternate method for the discrimination of oxygen evolution has been developed using Scanning Electrochemical Microscopy (SECM) to map the evolution of oxygen against potential. With no control of upper voltage, poor cycle-life was confirmed at 15C (40% capacity loss in 50 cycles). By contrast, voltage control based on SECM studies (Vmax = 1.45V) allowed thousands of cycles with little capacity loss.