Materials for Electrical Energy Storage: ionic liquid thin films on HOPG and VN surfaces

2019-03-09 10:39:38 adman 8

An ever increasing demand for less reliance on fossil fuels has stimulated search for more effective, affordable and environmentally friendly energy storage technologies.Among those batteries andsupercapacitors (SC) are considered to be ones of the most efficient options. Thebatteries are energy efficient while the SCs are power efficient, the latter possess long cycle-life but they are not that energy efficient. The difference stems from the charge storage mechanisms which in SCs, in contrast to batteries, are surface related: charge accumulation in electrical double layer (EDL) and Faradaic (redox) reactions at the electrolyte-electrode interface. Thus the surface science approach can substantially contribute in developing new materials and establishing new charge storage mechanisms for the electrical energy storage systems like SCs and SC-battery hybrids. Currently, commercially available SC make use of activated carbon (specific surface area ~1000 m2g-1and specific surface capacitance ~0.01mF cm-2) as the electrode material and water based electrolytes (potential window limited to 1.23 V)have almost reached their capacitance limits. Therefore, alternatives - nanosized metal oxides and nitrides, working through different mechanisms, often in nanocomposites with nanostructured carbons such as nanotubes and graphene so as to enhance the electrical conductivity of electrodes - have been in the focus of recent research.

In this talk we report on synthesis of pure oxygen-free thin films of VN which are capable of delivering impressively high surface capacitance up to ~3 mF cm-2in aqueous electrolytes. Combining electrochemical testing with X-ray photoelectron spectroscopy (XPS) has revealed that redox reactions play no or little role in the electrochemical response of pure VN, in contrast to the common wisdom stemming from the electrochemical response of oxygen-containing films. An alternative charge storage mechanism –space charge accumulationin a subsurface layer of ~100 nm – was put forward to explain the experimentally observed anomalous capacitance of the VN thin films.

Recently, room temperature ionic liquids (IL) (electrochemical window up to 6 V, thermo stable up to ~300oC) have been introduced as an alternative to aqueous (electrochemical window 1.2 V) and organic (electrochemical window 2.5-2.8 V, flammable) electrolytes[1]. Taking advantage of the extremely low vapor pressure of the IL (<10-9mbar) we have realizedin-situelectrochemical X-ray photoelectron spectroscopy (in-situ EC XPS) to probe interface between the solid electrodes (HOPG, VN) and the IL electrolyte ([EMI] [FSI]) underin operandoconditions. Dramatic morphological transformation of the ionic liquid thin films reversibly controlled by electrode potential was observed in the XPS and AFM measurements andexplained as electrical field induced reduction of the liquid-solid transition temperature of the ionic liquid film when the thickness of the ionic liquid film is comparable with the charge screening length in the ionic liquid.