We report here electrochemical capacitors using an aqueous electrolyte based on the concept of "water-in-salt" with the aim to improve the energy density by increasing the voltage of the cell. A "water-in-salt" consists of a highly concentrated aqueous LiTFSI solution in which both volume and mass of LiTFSI are greater than those of water. With activated carbon supercapacitor electrodes (PICA) and 31 m "water-in-salt" electrolytes (m stands for molality), we were able to reach a cell voltage of 2.4 V whereas it is difficult to exceed 1.6 V in conventional aqueous devices because of water splitting. Moreover, it was observed that the specific capacitance of the cell is improved using "water-in-salt" electrolytes. In these conditions, an energy density of 30 Wh kg −1 was obtained which is at least three times greater than for conventional aqueous devices and in the same order of magnitude than for redox enhanced capacitors. Interestingly, fair stability, over 2000 cycles, was obtained for the 7 m electrolyte. Up to 90 sec chargingdischarging rate, this latter electrolyte offers the best compromise between voltage, power and energy densities and stability. This study demonstrates the feasibility of water-in-salt as an electrolyte for supercapacitors and points out the most suited compositions for these electrolytes.

A route to break molecular nitrogen (N 2 ) under mild conditions is demonstrated by N 2 gas cracking on, and incorporation into, lanthanide films. Successful growth of lanthanide nitride thin films, made by evaporation of lanthanides in a partial N 2 atmosphere at room temperature and pressure as low as 10 –4 Torr, is confirmed using X-ray diffraction. In situ conductance measurements of pure lanthanide thin films exposed to N 2 gas show an immediate surface reaction and a slower bulk reaction. Finally, we report partial reversal of the nitrogen incorporation in a lanthanide nitride by cycling vacuum and nitrogen conditions in the sample chamber.

Nowadays commercial supercapacitors are based on purely capacitive storage at the porous carbons that are used for the electrodes. However, the limits that capacitive storage imposes on energy density calls to investigate new materials to improve the capacitance of the device. This new type of electrodes (e.g., RuO, MnO…) involves pseudo-capacitive faradaic redox processes with the solid material. Ion exchange with solid materials is, however, much slower than the adsorption process in capacitive storage and inevitably leads to significant loss of power. Faradaic process in the liquid state, in contrast can be similarly fast as capacitive processes due to the fast ion transport. Designing new devices with liquid like dynamics and improved specific capacitance is challenging. We present a new approach to increase the specific capacitance using biredox ionic liquids, where redox moieties are tethered to the electrolyte ions, allowing high redox concentrations and significant pseudo-capacitive storage in the liquid state. Anions and cations are functionalized with anthraquinone (AQ) and 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) moieties, respectively. Glassy carbon, carbon-onion, and commercial activated carbon electrodes that exhibit different double layer structures and thus different diffusion dynamics were used to simultaneously study the electrochemical response of biredox ionic liquids at the positive and negative electrode.