Role of nitrogen doping at the surface of titanium nitride thin films towards capacitive charge storage enhancement

We recently reported an impressive cycling stability (over 20,000 cycles) of titanium nitride (TiN) electrodes with high specific capacitance. It is anticipated that nitrogen (β−N) doping in the oxidized surface of TiN film plays a crucial role in charge storage mechanism and stability of this material. In this work, we offer an evidence on the effect of β−N doping on improvement in specific capacitance of vacuum annealed TiN thin films. The annealing of the TiN films leads to the diffusion of the excess β−N from sub-surface to oxidized TiN film surface without further oxidation of the electrode surface. We demonstrate an increase in the TiN areal capacitance upon an increase in the amount of β−N dopant. The areal capacitance of the annealed films was enhanced by 3-fold (8.2 mF cm−2 in K2SO4 aqueous electrolyte) without sacrificing the cycling stability of the electrodes after more than 10,000 consecutive charge/discharge cycles. [Display omitted] •Reactive sputtering of TiN films was carried out with annealing at ≤550 °C.•Excess β−N in the annealed TiN surface causes 3-fold increase in areal capacitance.•The maximum areal capacitance was 8.2 mF cm−2 with no loss after 10,000 cycles.•β−N doping of oxidized TiN film improves charge storage of these electrodes in ECs.