V. Ruiz

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.

Hybrid energy storage: high voltage aqueous supercapacitors based on activated carbon–phosphotungstate hybrid materials

The use of hybrid electrodes prepared from phosphotungstic acid (H3PW12O40, PW12) and activated carbon (AC) allows the fabrication of highly durable aqueous supercapacitors operating at a superior voltage (1.6 V) thanks to the high overpotential of PW12 towards H2 generation. The combination of the double-layer capacitance plus the redox activity of the polyoxometalate clusters leads to an intrinsic increase in specific capacitance (from 185 F g−1 for AC to 254 F g−1 for AC–PW12) and a 60% increase in the operating voltage (compared to conventional AC supercapacitors with aqueous electrolytes, e.g. 1 M sulphuric acid). The hybrid energy storage mechanism and the increased operating voltage converge to yield improved specific energy and power. Moreover, the hybrid AC–PW12 electrode material showed an outstanding stability even after 30 000 cycles (0 to 1.6 V) with 98% retention of the initial capacitance, much superior to the stability of the parent supercapacitor based on plain AC under the same conditions.

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.

Novel hybrid micro-supercapacitor based on conducting polymer coated silicon nanowires for electrochemical energy storage

The development of a novel hybrid symmetric micro-supercapacitor based on poly(3,4-ethylenedioxythiophene) coated silicon nanowires using an ionic liquid (N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide) as an electrolyte has been demonstrated. The hybrid supercapacitor device was able to deliver a specific energy of 10 W h kg−1 and a maximal power density of 85 kW kg−1 at a cell voltage of 1.5 V. The hybrid device exhibited long lifetime and an outstanding electrochemical stability retaining 80% of the initial capacitance after thousands of galvanostatic charge–discharge cycles at a high current density of 1 mA cm−2. The improvement of the capacitive properties compared with the bare SiNWs was attributed to the pseudo-capacitive behavior induced by the conducting polymer coating.

Power Supply Based on Carbon Ultracapacitors for Remote Supervision Systems

The capacity to store energy in ultracapacitors or double-layer capacitors is about two orders of magnitude greater than that of ordinary capacitors. Also, their capability to receive and transfer power is greater than that of batteries in the same order of magnitude. These characteristics, together with their long life cycle (> 500 000) and their lack of maintenance work, make them a suitable choice in replacing batteries in pulsed or discontinued power consumption systems. This paper tries to evaluate the possibility to replace batteries with ultracapacitors in Global System for Mobile Communications (GSM)-based remote supervision systems using photovoltaic panels as main power source.

SiNWs-based electrochemical double layer micro-supercapacitors with wide voltage window (4 V) and long cycling stability using a protic ionic liquid electrolyte*

The present work reports the use and application of a novel protic ionic liquid (triethylammonium bis(trifluoromethylsulfonyl)imide; NEt3H TFSI) as an electrolyte for symmetric planar micro-supercapacitors based on silicon nanowire electrodes. The excellent performance of the device has been successfully demonstrated using cyclic voltammetry, galvanostatic charge-discharge cycles and electrochemical impedance spectroscopy. The electrochemical characterization of this system exhibits a wide operative voltage of 4 V as well as an outstanding long cycling stability after millions of galvanostatic cycles at a high current density of 2 mA cm−2. In addition, the electrochemical double layer micro-supercapacitor was able to deliver a high power density of 4 mW cm−2 in a very short time pulses (a few ms). Our results could be of interest to develop prospective on-chip micro-supercapacitors using protic ionic liquids as electrolytes with high performance in terms of power and energy densities.

The Effect of Charging and Discharging Lithium Iron Phosphate-graphite Cells at Different Temperatures on Degradation

The effect of charging and discharging lithium iron phosphate-graphite cells at different temperatures on their degradation is evaluated systematically. The degradation of the cells is assessed by using 10 charging and discharging temperature permutations ranging from -20 °C to 30 °C. This allows an analysis of the effect of charge and discharge temperatures on aging, and their associations. A total of 100 charge/discharge cycles were carried out. Every 25 cycles a reference cycle was performed to assess the reversible and irreversible capacity degradation. A multi-factor analysis of variance was used, and the experimental results were fitted showing: i) a quadratic relationship between the rate of degradation and the temperature of charge, ii) a linear relationship with the temperature of discharge, and iii) a correlation between the temperature of charge and discharge. It was found that the temperature combination for charging at +30 °C and discharging at -5 °C led to the highest rate of degradation. On the other hand, the cycling in a temperature range from -20 °C to 15 °C (with various combinations of temperatures of charge and discharge), led to a much lower degradation. Additionally, when the temperature of charge is 15 °C, it was found that the degradation rate is nondependent on the temperature of discharge.