A First Outlook of Sputtered FeWO₄ Thin Films for Micro-Supercapacitor Electrodes

Abstract

The next generation of Internet of Things devices requires micro-supercapacitors operating at high voltage which is difficult to achieve using symmetrical design. Thus, their fabrication in an asymmetric configuration is mandatory. While MnO2 is well-established as positive electrode, the scarcity of existing efficient materials able to be used at the negative side drives the research towards new promising materials. Since few years, a new class of oxide materials, named multicationic oxides, were demonstrated to be attractive solutions as bulk electrodes for electrochemical capacitor. Among them, the wolframite-type FeWO4 oxide was proposed as an interesting negative electrode material for asymmetric FeWO4/MnO2 electrochemical capacitors. The present paper reports for the first time on the successful thin film synthesis of such iron-tungstate oxide films by reactive DC magnetron sputtering, a deposition method widely used in the semiconductor industry to manufacture micro-devices. The pseudocapacitive behaviour documented at the bulk scale is preserved at the thin film level as well, and opens-up the possibility to use FeWO4 in the next generations of micro-supercapacitors.

FeWO4 Single Crystals: Structure, Oxidation States, and Magnetic and Transport Properties

“…In FeWO 4 , the tendency toward holelike transport deduced from Seebeck measurements ,, was explained theoretically by the presence of Fe vacancies creating “hole-like” Fe 3+ . These vacancies should play a crucial role in the pseudocapacitor properties of thin films but also in the magnetic properties of nanoparticles inducing a decrease of the Néel temperature . However, there is a lack of study combining structure refinement and magnetic and (di)­electric measurements in more stoichiometric crystals catching the intrinsic features of “FeWO 4 .” Thus, we have undertaken a study on FeWO 4 polycrystalline samples and single crystals, grown by chemical vapor transport ,, starting from the obtained polycrystalline material.…”

Wafer-Scale Performance Mapping of Magnetron-Sputtered Ternary Vanadium Tungsten Nitride for Microsupercapacitors

“…The surface capacitance of such a VWN z thin film is higher than that of WN but lower than that of VN using the same synthesis parameters (Figure e). However, this volumetric capacitance is one of the best reported so far for multicationic electrode materials for MSCs. − On the one hand, it is well known that VN is an excellent electrode material for MSCs with a very high capacity, but on the other hand, depending on the synthesis conditions, the capacitance retention is not at its best level depending on the nitrogen content in the total gas. For VN films made by the magnetron sputtering method, when the nitrogen content in the total gas is close to 17%, 80% of the initial capacitance is maintained during 50,000 cycles, while this capacitance retention is close to 100% during 150,000 cycles when the nitrogen amount in the total gas is set to ∼95% .…”

“…In the first part of this paper, it was rotated at a typical speed of 30 rpm (revolutions per minute) for the expected homogeneous films. Since vanadium and tungsten nitrides ,,,, are already known as high-performance electrode materials for MSCs, a combination of both performance is expected to have great potential for application in MSCs as electrodes. ,− We can also expect a better performance from this new VWN 3 material, i.e., an increase in the working potential window or surface capacitance compared to its constituent binary metal nitride (Figure b). The challenge is therefore to synthesize and stabilize the metastable sputtered VWN 3 film (see Figure c, which shows the predicted structural arrangement of VWN 3 , alongside the VN and WN structural projections).…”

“…18 Buvat et al succeeded in depositing FeWO 4 by a sputtering method. 19 Their film showed 3.5 mF•cm −2 (39 F•cm −3 ) and good capacitance retention after 10,000 cycles. Fe 2 WO 6 was synthesized by Espinosa-Angeles et al and showed a capacitance of 240 F•cm −3 with an initial value of 85% after 10,000 cycles.…”

Advances on Microsupercapacitors: Real Fast Miniaturized Devices toward Technological Dreams for Powering Embedded Electronics?

“…The coexistence of two different cations in a single crystal structure could improve the electrochemical performance as compared to their constituting binary metal oxides. − In that context, the group of Brousse has recently demonstrated that FeWO 4 is an interesting negative electrode for electrochemical capacitors . More especially, FeWO 4 was proposed as thin film electrode for MSC using deposition method compatible with semiconductor techniques to be integrated in a technological process on pilot production line. Besides FeWO 4 , Fe 2 WO 6 as a negative electrode was synthesized by a polyol-mediated method and showed 240 F cm –3 of volumetric capacitance with capacitance retention of 85% after 10 000 cycles (Figure a) …”