Electropolymerized dielectric polymer coats conductive surfaces like no other technique is capable of and thus various applications can be proposed, such as making a uniform dielectric coating for complex micro- or nanowires or even coating porous carbon. It is shown here that electrolytes containing nitrile-based anions, such as dicyanamide (DCA), tetracyanoborate (TCB), tricyanomethanide (TCM), or cyanate OCN are capable of producing thin polymeric layers when electropolymerized at positive voltages onto carbon, aluminum or other inert electrodes. Interestingly, reaction mechanisms are quite different, but all the polymers are inorganic nitriles, are insoluble in water or acetone, have no melting point and decompose only at temperatures > 300 ^oC. [1-3]

The advantage of preparing a dielectric polymer by electrochemical deposition method was indicated in our paper [2], where large surface area graphene-based electrodes in 1-butyl-1-methylpyrrolidinium dicyanamide (BMPDCA) ionic liquid were coated with a thin polydicyanamide dielectric layer and used as high voltage supercapacitor electrodes. The new capacitor technology offers more than order of magnitude higher energy densities, compared to commercial tantalum and aluminum capacitors in the 4-10 V cell voltage range [2]. The protective self-healing nanolayer formed is a new carbon-nitrogen material polyDCA with dielectric constant value of 6 and dielectric strength of 250 MV m^-1 (at 10 V). Good stability (>10000 cycles) and high efficiency (>99.8% coloumbic) have been demonstrated for the passivated reduced graphene oxide (RGO) electrodes.

In the second paper [3] it was demonstrated that TCB-based electrolytes could be used as safe supercapacitor/battery electrolytes as these are protected against overvoltage by two mechanisms: Firstly, voltage (E) more positive than 2.2 V (vs. Ag|AgCl reference electrode) kills the conductivity of single layer graphene and electrochemical reactions stop. Secondly, at E > 2.2 V tetracyanoborate anion polymerizes to a dielectric polymer polyTCB, which electrically isolates graphite or porous carbon grains from current collectors. Differently from polyDCA, polyTCB forms faster and without formation of gasses at anode.

Acknowledgments

Authors would like to thank Estonian Ministry of Education and Research (institutional research project IUT20-13) and European Regional Development Fund (The Centres of Excellence TK117 and, TK141) for financial support.

References

[1] T. Romann, E. Lust, O. Oll, Method of forming a dielectric through electrodeposition on an electrode for a capacitor, WO2016050761 A1, 2016.

[2] T. Romann, O. Oll, P. Pikma, K. Kirsimäe, E. Lust, 4-10 V capacitors with graphene-based electrodes and ionic liquid electrolyte, Journal of Power Sources 280 (2015) 606-611.

[3] T. Romann, E. Anderson, P. Pikma, H. Tamme, P. Möller, E. Lust, Reactions at graphene | tetracyanoborate ionic liquid interface - new safety mechanisms for supercapacitors and batteries, Electrochem. Commun. 74 (2017) 38-41.

Fig 1. Electron microscopy images of an unmodified RGO electrode (a) and with 18 nm thick polyDCA dielectric layer created at 5 V in BMPDCA (b) (scale bar is 1 μm ). c) Atomic force microscopy image of flat carbon film, coated with 38 nm thick polyDCA layer by applying 10 V in BMPDCA electrolyte (rectangular hole was scratched).