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The Race for Wearable Energy

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Photovoltaics and Energy Storage Threads For Smart Fabrics

October 10th 2016

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Weaving two types of specially designed fibres with cotton yarn and conductive copper-coated threads, a team of researchers from China and Singapore has devised a smart fabric that can both harvest energy from light and store it as a supercapacitor would do.

As its main threads (used as flying shuttle), the textile combines fibre-shaped photo-anodes interlaced with counter electrodes (CEs) for the energy harvesting functionality, with TiN nanowire-based fibre supercapacitors (FSCs) for the energy storage part.

Published in the ACS Nano journal, the paper "Tailorable and Wearable Textile Devices for Solar Energy Harvesting and Simultaneous Storage" details the fabrication process of both types of specialty threads as commercially viable for production in any length.

The smart thing about this textile is that it can be woven with both threads side by side in any proportion, offering the right balance between energy harvesting and energy storage based on the end applications envisaged.

What's more, the textile can be tailored into any shape while still retaining its overall solar energy harvesting and storage capabilities. Two equal parts of a fabric cut in half providing each roughly half the energy harvesting and energy storage capacity of the integral fabric (as long as the threads' extremities are reconnected in some way to build up functional modules in series or in parallel). In fact, every individual “thread” (FSC, fiber-shaped DSSC photoanode and CE) can be cut while retaining its functionality.

To prove their idea, the researchers weaved a palm-size sample that could be fully charged to 1.2V in 17s by self-harvesting solar energy. It fully discharged in 78s at a discharge current of 0.1mA.

About these special threads

The TiN nanowire-based fibre supercapacitors (FSCs) were obtained starting with a Ti wire roughly 250μm in diameter. The titanium wire undergoes an alkali hydrothermal treatment and a further ion-exchange process to obtain H2Ti2O5·H2O nanowires on its surface, about 100nm in diameter. A nitridation process followed by a hydrothermal carbon-coating process yields carbon-coated TiN nanowires (NWs) about 80 to 100nm in diameter and 10μm long, grown uniformly and radially around the titanium wire.

Synthesis of the TiN NWs on Ti wire for the FSC electrode (a), and a panoramic view SEM image of the TiN/Ti wire and its cross-section (b).

The 1D porous structure it forms as a thread can simultaneously provide a fast electron transport channel and large surface area, supporting fast charge/discharge rates as well as boasting a high volumetric capacitance. Functional FSC devices were obtained by assembling two TiN/Ti electrodes into a symmetric structure with a KOH/PVA gel used simultaneously as an electrolyte and a separator. Such strings were then woven with cotton yarns using an industrial weaving loom with flying shuttle to produce the energy textile with predesigned patterns. The FSCs were tested under thousands of bending cycles, from 0° to 360°, maintaining about 98% of their original capacitance.

The fibre-shaped photo-anodes were obtained by radially growing ZnO NWs on a manganese-coated polymer wire, then sensitizing them with industry standard ruthenium-based dye N719. The fibre-shaped photo-anode thread was finally completed with a coating of copper iodide (CuI) as the hole-transfer layer.

The DSSC textile made up of photo-anode threads woven with Cu-coated polymer wire or cotton yarn as the counter electrodes (CE) (a); and a cross-sectional view of the photo-anode showing the radially grown ZnO nanowires.

Solar energy harvesting modules were obtained by weaving these fibre-shaped photo-anodes with Cu-coated polymer wire or cotton yarn as the counter electrodes.

In their paper, the researchers note that when connected in series, the open circuit voltage (Voc) of the DSSC textile increases linearly with the number of the photo-anode strings (the short-circuit current remains unchanged). As for the FSCs modules, they can be charged to several volts when connected in series.  

A sample of the energy textile with a close-up on a cut showing the DSSC and FSCs thread extremities (a). An equivalent circuit of connected DSSC and FSCs regions is shown on the right.

The threads exist, but how would you connect their loose ends in a piece of fabric to establish connections between the different domains and form complete circuits? We asked corresponding author Wenjie Mai, Department Chair / Professor in the department of physics at Jinan University (China).

"We simply use metal wires to connect all these fibres at their extremities" explained Mai, understanding that although this may not look perfect, it was good enough to show the idea on a working prototype.

Maybe some sort a welded seam process involving special conductive coatings could overlap and connect the conductive ends of these special threads during garment manufacture.

Hoping to industrialize and commercialize their smart fabric, the researchers filed a few patent applications and established contact with a few Chinese companies as potential industrial partners, Mai revealed.

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