We demonstrate stable hybrid electrochemical energy storage performance of a redox-active electrolyte, namely potassium ferricyanide in aqueous media in a supercapacitor-like setup. Challenging issues associated with such a system are a large leakage current and high self-discharge, both stemming from ion redox shuttling through the separator. The latter is effectively eliminated when using an ion exchange membrane instead of a porous separator. Other critical factors toward the optimization of a redox-active electrolyte system, especially electrolyte concentration and volume of electrolyte, have been studied by electrochemical methods. Finally, excellent long-term stability is demonstrated up to 10 000 charge/discharge cycles at 1.2 and 1.8 V, with a broad maximum stability window of up to 1.8 V cell voltage as determined via cyclic voltammetry. An energy capacity of 28.3 Wh/kg or 11.4 Wh/L has been obtained from such cells, taking the nonlinearity of the charge–discharge profile into account. The power performance of our cell has been determined to be 7.1 kW/kg (ca. 2.9 kW/L or 1.2 kW/m2). These ratings are higher compared to the same cell operated in aqueous sodium sulfate. This hybrid electrochemical energy storage system is believed to find a strong foothold in future advanced energy storage applications.
Capacitive technologies, such as capacitive deionization and energy harvesting based on mixing energy ("capmix" and "CO2 energy"), are characterized by intermittent operation: phases of ion electrosorption from the water are followed by system regeneration. From a system application point of view, continuous operation has many advantages, to optimize performance, to simplify system operation, and ultimately to lower costs. In our study, we investigate as a step towards second generation capacitive technologies the potential of continuous operation of capacitive deionization and energy harvesting devices, enabled by carbon flow electrodes using a suspension based on conventional activated carbon powders. We show how the water residence time and mass loading of carbon in the suspension influence system performance. The efficiency and kinetics of the continuous salt removal process can be improved by optimizing device operation, without using less common or highly elaborate novel materials. We demonstrate, for the first time, continuous energy generation via capacitive mixing technology using differences in water salinity, and differences in gas phase CO2 concentration. Using a novel design of cylindrical ion exchange membranes serving as flow channels, we continuously extract energy from available concentration differences that otherwise would remain unused. These results may contribute to establishing a sustainable energy strategy when implementing energy extraction for sources such as CO2-emissions from power plants based on fossil fuels.
Most efforts to improve the energy density of supercapacitors are currently dedicated to optimized porosity or hybrid devices employing pseudocapacitive elements. Little attention has been given to the effects of the low charge carrier density of carbon on the total material capacitance. To study the effect of graphitization on the differential capacitance, carbon onion (also known as onion‐like carbon) supercapacitors are chosen. The increase in density of states (DOS) related to the low density of charge carriers in carbon materials is an important effect that leads to a substantial increase in capacitance as the electrode potential is increased. Using carbon onions as a model, it is shown that this phenomenon cannot be related only to geometric aspects but must be the result of varying graphitization. This provides a new tool to significantly improve carbon supercapacitor performance, in addition to having significant consequences for the modeling community where carbons usually are approximated to be ideal metallic conductors. Data on the structure, composition, and phase content of carbon onions are presented and the correlation between electrochemical performance and electrical resistance and graphitization is shown. Highly graphitic carbons show a stronger degree of electrochemical doping, making them very attractive for enhancing the capacitance.
The effect of water contamination in 1 mol L-1 Et4NBF4 dissolved in acetonitrile or in propylene carbonate as electrolyte on the capacitance fading of activated carbon based electrochemical double layer capacitors has been investigated during constant voltage load tests at 2.75 V and 3.5 V cell voltage. For a cell voltage of 2.75 V, water contamination in the PC based electrolyte led to a fast initial cell capacitance decrease. This cell capacitance loss was predominantly due to a capacitance fading on the negative activated carbon electrode. For the AN based electrolyte a similar but less pronounced effect is observed at 2.75 V. For both electrolytes tested at 3.5 V, no influence of deliberately introduced water contamination on the capacitance loss was identified. The cause for capacitance fading at the higher cell voltage, therefore, appears to be independent on the water contamination. Post mortem X-ray photoelectron spectroscopy and surface area analysis of the negative electrode aged at 2.75 V did not provide evidence for a corresponding loss of surface area or electrode passivation. These results point toward water reduction on the negative electrode as the primary cause for capacitance loss of electrochemical double layer capacitors in water-contaminated electrolytes near nominal working voltage. The formation of trapped hydrogen within micropores of the activated carbon electrodes is proposed as the main aging mechanism in this case.
