(Invited) Vanadium and Tungsten Nitride Thin Film Electrodes for Micro-Supercapacitors

Abstract

Electrochemical Energy Storage systems have to be miniaturized to get autonomous millimeter scale sensors for Internet of Things (IoT) applications[1,2]. One attractive solution to power such small sensors deals with the use of micro-supercapacitors (MSC). Unfortunately, the low technological readiness level of the MSC limits the large scale deployment of smart miniaturized sensors. Deposition and micromachining techniques widely developed in the microelectronic industry have to be selected to produce MSC on large scale substrates. Among the existing thin film deposition technique, magnetron sputtering (MS) is a versatile tool to synthesize binder free electrode that can be easily transferred to pilot production line and to study the intrinsic properties of the electrode material. In this talk, we will show our recent results dealing with vanadium (VN) and tungsten nitride (WN) thin films deposited by MS on silicon wafers. During the last ten years, transition metal nitride[3–5] have been investigated as an attractive pseudocapacitive electrode. In the frame of this study, the nitride thin films have been proposed as an efficient bi-functional material acting not only as the pseudocapacitive electrode but also as a suitable current collector. Structural, electrical and electrochemical properties have finely tuned according to the synthesis parameters to reach the best performance. Advanced characterization technique such as operando X-Ray Absorption Spectroscopy (XAS) was performed on these nitride layers to monitor the change of the oxidation state occurring at the metal centers at the V K-edge and W L3-edge when cycling in different electrolytes (KOH, LiOH…). Operando XAS spectra were acquired at ROCK beamline (SOLEIL synchrotron, France) using the quick-XAS channel-cut Si (111) monochromator with a frequency oscillations of 2 Hz, corresponding to 15 spectra for a whole cycle. Surface analysis (X-Ray Photoelectron Spectroscopy – XPS) coupled with Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) were used to evaluate the chemical composition and to study the change of the oxidation state of the transition metal at different etched depth. Finally, based on the optimized nitride layers, symmetric MSC in parallel plates or interdigitated topologies were fabricated using microfabrication technique and tested in aqueous electrolytes.