Maik R. J. Scherer

We report the first successful application of an ordered bicontinuous double-gyroid vanadium pentoxide network in an electrochromic supercapacitor. The freestanding vanadia network was fabricated by electrodeposition into a voided block copolymer template that had self-assembled into the double-gyroid morphology. The highly ordered structure with 11.0 nm wide struts and a high specific surface to bulk volume ratio of 161.4 μm(-1) is ideal for fast and efficient lithium ion intercalation/extraction and faradaic surface reactions, which are essential for high energy and high power density electrochemical energy storage devices. Supercapacitors made from such gyroid-structured vanadia electrodes exhibit a high specific capacitance of 155 F g(-1) and show a strong electrochromic color change from green/gray to yellow, indicating the capacitor's charge condition. The nanostructuring approach and utilizing an electrode material that has intrinsic electrochemical color-change properties are concepts that can be readily extended to other electrochromic intercalation compounds.

Manufacturing V2O5 in a 3D periodic highly interconnected gyroid structure on the 10 nm length scale is shown to lead to a significant electrochromic performance enhancement. The structured devices surpass previous inorganic electrochromic materials in all relevant parameters: the switching speed, coloration contrast, and composite coloration efficiency. In particular, the 85 ms switching speed lies within a factor of two of video rate. Enhanced ion intercalation into the gyroid morphology can be extended to other transition-metal oxides and is therefore promising for lithium ion batteries, supercapacitors, and sensors. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Ion intercalation processes into metal oxide porous materials benefit from a high surface-to-volume ratio, while electronic charge transport requires a continuous network morphology. Detailed control over structure formation on the 10 nm length scale is therefore an effective strategy to enhance performance in electrochromic devices, supercapacitors, and batteries. Here we demonstrate the transformation of nickel patterned in a three-dimensional, highly interconnected, periodic nanomorphology into a self-supporting nickel oxide array with hollow struts. The oxidation of nickel gives rise to the nanoscale Kirkendall effect, which substantially increases the surface area of the NiO gyroid framework, without sacrificing its connectivity. Applicable to a vast range of electroplatable metals, this is a versatile route to high surface area metal oxides/chalcogenides which is especially suitable for various thin film applications. Nanostructured NiO electrodes showed substantially enhanced electrochromic performance, combining fast switching speeds with high coloration contrast.

Gyroid-structured calcite crystals are grown by templating though self-assembled copolymer films. The remarkable triply periodic minimal surface is perfectly replicated on the nanometer scale, while single crystallinity is maintained. This is a wholly synthetic route to a crystal morphology found in biological systems, only on a smaller length scale.