**4. Fiber-based electrodes**

*Science, Technology and Advanced Application of Supercapacitors*

conductive polymers. An asymmetrical device with high energy densities can be fabricated by carefully choosing the electrodes based on their working reduction potential ranges. Changing the crystallinity and nanostructure of the deposited material by controlling the deposition parameters and composition can also yield better properties. The alpha phase of the manganese oxide is more conductive and electrochemically stable than its beta and amorphous forms. Thus, a very fine nanorod-structured deposition of alpha phase manganese oxide can greatly increase the energy density. Another option for the device property improvement is exploring more electrochemically active materials such as lithium metal oxides, sulfur, etc. Thus, with the use of a polymer electrolyte matrix separator, a flexible battery device

*with permission from [47]. Copyright 2011 American Chemical Society.*

*(a) Schematic of electrodeposition of manganese oxide on graphene coated textile fibers; (b) half-cell cyclic voltammetry of manganese oxide—graphene—fabric electrode; (c) capacitance vs. scan rate curves. Reprinted* 

As demonstrated in the cyclic voltammetry curves in **Figure 4b** and **c**, there is still a very non-capacitive (rectangular) nature to the fabric devices. This is in part a contribution from the low loading of conductive material—CNT or graphene. The other active materials (pseudocapacitive or battery) are not very conductive in nature and highly rely upon the carbon-base material for their electron transfer requirements. Thus, there are heavy losses in terms of power density in such materials. This can be avoided with the use of metals as current collectors, however

The most promising advantage of the discussed dip coating is the low production cost and time consumption. However, further improvement of the carbon coating material needs to be realized to progress in this fabrication method. An alternative is a fully core to shell design made completely of conductive carbon material and coated active material. This would result in a 100% mass loading of

active material and would give more energy per unit gram of fabric.

**26**

can be fabricated.

**Figure 4.**

making the device non-flexible.

Electrodes in the form of yarns that can be readily stitched as a textile fabric has the potential to overcome many of the disadvantages discussed in the previous section. The research in fiber/yarn-based electrodes began with dip coating techniques and as discussed in the sections before, some challenges have emerged. The conductivity and active loading of the material was low, and the related research eventually moved towards more promising approaches. One of them is the possibility to realize a fully active material loaded fiber form.

A conductive carbon-based core that provides the strength, conductivity and flexibility with an outer shell of active material that secures high energy density has been explored. CNTs dispersed in acid and extruded in the form of a fibers which have been easily fabricated. These porous fibers act as a current collector and housing for the active material. Vertically aligned CNT arrays can be used to spin long and porous fibers. These are produced in a more pristine format and hence can be easily processed and react less with electrolytes. These full core-shell carbon structures have very high surface area ~1400 m2 /g, which gives the opportunity to take advantage of the ELDC characteristic of these fibers [48]. Novel fiber structures utilizing higher surface area graphene have also been fabricate. RGO based fibers that are highly porous and boasting surface area up to 2400 m2 /g have been reported, which results in a very high energy density of 400 F/g [49] (**Figure 5i a–c**). Hybrids involving both CNTs and graphene have been synthesized, utilizing the high surface area of both material to create a sponge like fiber material [48] (**Figure 5ii a–c**). Flexible fibers were successfully woven into a fabric and its performance remained consistent through exposure to bending stress. The major drawback of these high energy storage materials is their strength. Although their Young's modulus is appreciable, which allows them to be flexible, their tensile strain to failure is quite low. This makes them prone to breakage and unfit to survive as a self-supporting fabric patch. Addition of any active material to the fiber would make them too brittle to handle.

Carbon fibers, which are a very sturdy and high strength materials, haves been used as a substrate to house pseudocapacitive manganese oxide [50] (**Figure 5v a**-**c**). The resulting fiber proved to be quite strong and showed appreciable energy storage values. However, the electrical conductivity of the material is still a concern. Conventional carbon nanotube fibers have also been used as conductive substrate for housing manganese oxide [51] (**Figure 5iv a**–**c**). The resulting composite was wound like a spring and encapsulated in a gel polymer, showing no losses in energy storage properties for tensile strains of up to 100%.

A polymer core with carbon nanomaterials grown/deposited across the surface derives most of its strength from the elastic polymer and can sustain higher tensile strains. Active material can then be successfully decorated on the surface [52] (**Figure 5v a**–**c**). This however suffers from the loss of conductivity and requires an additional current collector in the form of a multiplied metal yarn structure.

Fibers made completely out of electrochemically active material such as poly(3,4 ethylenedioxythiophene) (PEDOT) have also been developed [53]. The biscrolled PEDOT with the CNT matrix makes the composite quite easy to scale up. Further, due to the high electrical contact between the current collector and active material, the device shows good performance at 1 mV/s scan rate. The greatly twisted fiber boasts very high active material loading and high volumetric capacitance.

There are some challenges to the development of these forms of devices. They have the potential to be composed completely of active material leading to unsurpassed energy storage capabilities, however, the loss in strength of the material needs to be countered. A fiber that possess both great tensile strain, strength and high loading of active material could pave way to the next generation of wearable devices. We believe that the key lies in highly densified CNT fibers with modified surfaces.

#### **Figure 5.**

*(a) SEM image of electrode base material; (b) cyclic voltammetry graph of the working device; (c) picture of full device. (i) RGO fiber reprinted with permission from [49]. Copyright 2014 American Chemical Society. (ii) CNT-graphene fiber reproduced from [48] with permission from Royal Society of Chemistry. (iii) Carbon fiber coated with manganese oxide reprinted with permission from [50]. Copyright 2012 American Chemical Society. (iv) CNT fiber coated with manganese oxide reprinted with permission from [51]. Copyright 2015 American Chemical Society. (v) GO-active material coated stainless steel fiber reprinted with permission from [52]. Copyright 2015 American Chemical Society.*

#### **5. Ionic liquid-based fabric capacitor**

Below are presented experimental results on ionic liquid-based fabric capacitors obtained by our team. A factor that is often ignored when it comes to wearable devices is the evaluation of their properties as a function of the weight of the materials. In these cases, most used is the area of the material or only the weight of the active material is taken into consideration. This becomes a limitation when scaling to industry production as bulky materials will not be suitable for powering

**29**

*Fabric-Integrated, Ionic Liquid-Based Supercapacitor as a Tunable and Flexible Power Source*

fabric-based devices. Most of literature data still relies on textile fibers to provide the structural and flexible skeleton for their electrodes. This requires the usage of two layers of fabrics (electrodes) that adds unnecessary weight to the full device which is mostly never reported. An aspect to consider is the synthesis approach. The more structured the active synthesized material, the higher the surface area and therefore gives a higher specific capacitance. A drawback of this is the cost—both monetary and time consuming—that scales exponentially with the structure complexity. The other major consideration that is overlooked is the practical feasibility of the devices fulfilling certain applications. Most wearable energy storage devices have adopted gel-based polymer electrolyte systems to create a flexible separator. However, they still rely upon aqueous electrolytes such as phosphoric acid, potassium hydroxide, etc. This caps the voltage window for conventional devices with high cyclability to a low value of 0.8 V. However, even LEDs require around 1.3 V to operate. The non-conventional asymmetrical devices can push the envelope to around 1.6 V, but beyond that they suffer from heavy capacitance losses. In this section, we hope to provide a solution based on our work to these practical challenges. Aqueous electrolytes, although inexpensive and stable, are limited by their low reduction potentials with most conventional carbon materials. Anything beyond voltage window of 1 V results in irrecoverable losses in both the electrode material and the electrolyte. This limitation comes mainly from the presence of water. Thus, a change from aqueous to organic solutions results in a drastic change in the voltage window stability range. Commonly explored solutions are ethanol with suitable salts such as lithium perchlorate dissolved in them. Devices made with this electrolyte can expand the operation window from 1 V to around 2.2 V. This window decreases with introduction of pseudocapacitive materials to around 1.2 V, which is much higher than the 0.8 V offered by aqueous electrolytes. A device with asymmetrical electrodes can increase this window up to 2.2 V. To achieve a higher voltage window more expensive and less explored materials need to be considered. The latter are known as ionic liquids (ILs). These are a highly reactive group of electrolytes that are a combination of an organic cation with a very electronegative anion. Some of them—1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4)—have a very large and stable working voltage window, going as high as 4.5 V. They find usage as catalysts for higher voltage reduction reactions, however there is an emerging interest in their utilization in energy storage applications. ILs are most commonly combined with polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) to cre-

*DOI: http://dx.doi.org/10.5772/intechopen.80693*

ate a gel polymer electrolyte separator matrix.

We introduce here a novel way to incorporate the IL EMIMBF4 into any fabric material (from stretchable polyester sports-wear to sturdy military fabric) that utilizes only the essential amount of fabric material. For our tests we procured normal Bucky paper (BP) from General Nano, LLC which is a matrix of single walled carbon nanotubes under no alignment. Our procedure includes using the fabric as a separator to house the gel electrolyte and eventually be sandwiched between symmetrical BP electrodes (**Figure 6a**). Initially, polyvinyl alcohol (PVA)-sulfuric acid gel matrix was used. The gel electrolyte (1 g of PVA in 10 ml 0.1 M sulfuric acid) was dropped into the fabric. The soaked fabric was then allowed to dry till it reached a gel-like consistency. The electrodes were then applied on top of the electrolyte and allowed to further dry until they became integrated into the fabric electrolyte structure. This device was tested electrochemically using a Potentiostat Interface 1000, Gamry Instruments and showed results comparable to conventional devices made without the fabric separator. A similar gel polymer concentration was utilized for the IL (3 ml EMIMBF4 in 10 ml acetone with 1 g PVDF). The same methodology was adopted to create the gel fabric. However, the electrochemical results with the device were not comparable with average values of conventional devices. This was

#### *Fabric-Integrated, Ionic Liquid-Based Supercapacitor as a Tunable and Flexible Power Source DOI: http://dx.doi.org/10.5772/intechopen.80693*

fabric-based devices. Most of literature data still relies on textile fibers to provide the structural and flexible skeleton for their electrodes. This requires the usage of two layers of fabrics (electrodes) that adds unnecessary weight to the full device which is mostly never reported. An aspect to consider is the synthesis approach. The more structured the active synthesized material, the higher the surface area and therefore gives a higher specific capacitance. A drawback of this is the cost—both monetary and time consuming—that scales exponentially with the structure complexity. The other major consideration that is overlooked is the practical feasibility of the devices fulfilling certain applications. Most wearable energy storage devices have adopted gel-based polymer electrolyte systems to create a flexible separator. However, they still rely upon aqueous electrolytes such as phosphoric acid, potassium hydroxide, etc. This caps the voltage window for conventional devices with high cyclability to a low value of 0.8 V. However, even LEDs require around 1.3 V to operate. The non-conventional asymmetrical devices can push the envelope to around 1.6 V, but beyond that they suffer from heavy capacitance losses. In this section, we hope to provide a solution based on our work to these practical challenges.

Aqueous electrolytes, although inexpensive and stable, are limited by their low reduction potentials with most conventional carbon materials. Anything beyond voltage window of 1 V results in irrecoverable losses in both the electrode material and the electrolyte. This limitation comes mainly from the presence of water. Thus, a change from aqueous to organic solutions results in a drastic change in the voltage window stability range. Commonly explored solutions are ethanol with suitable salts such as lithium perchlorate dissolved in them. Devices made with this electrolyte can expand the operation window from 1 V to around 2.2 V. This window decreases with introduction of pseudocapacitive materials to around 1.2 V, which is much higher than the 0.8 V offered by aqueous electrolytes. A device with asymmetrical electrodes can increase this window up to 2.2 V. To achieve a higher voltage window more expensive and less explored materials need to be considered. The latter are known as ionic liquids (ILs). These are a highly reactive group of electrolytes that are a combination of an organic cation with a very electronegative anion. Some of them—1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4)—have a very large and stable working voltage window, going as high as 4.5 V. They find usage as catalysts for higher voltage reduction reactions, however there is an emerging interest in their utilization in energy storage applications. ILs are most commonly combined with polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) to create a gel polymer electrolyte separator matrix.

We introduce here a novel way to incorporate the IL EMIMBF4 into any fabric material (from stretchable polyester sports-wear to sturdy military fabric) that utilizes only the essential amount of fabric material. For our tests we procured normal Bucky paper (BP) from General Nano, LLC which is a matrix of single walled carbon nanotubes under no alignment. Our procedure includes using the fabric as a separator to house the gel electrolyte and eventually be sandwiched between symmetrical BP electrodes (**Figure 6a**). Initially, polyvinyl alcohol (PVA)-sulfuric acid gel matrix was used. The gel electrolyte (1 g of PVA in 10 ml 0.1 M sulfuric acid) was dropped into the fabric. The soaked fabric was then allowed to dry till it reached a gel-like consistency. The electrodes were then applied on top of the electrolyte and allowed to further dry until they became integrated into the fabric electrolyte structure. This device was tested electrochemically using a Potentiostat Interface 1000, Gamry Instruments and showed results comparable to conventional devices made without the fabric separator. A similar gel polymer concentration was utilized for the IL (3 ml EMIMBF4 in 10 ml acetone with 1 g PVDF). The same methodology was adopted to create the gel fabric. However, the electrochemical results with the device were not comparable with average values of conventional devices. This was

*Science, Technology and Advanced Application of Supercapacitors*

**28**

**Figure 5.**

**5. Ionic liquid-based fabric capacitor**

*[52]. Copyright 2015 American Chemical Society.*

Below are presented experimental results on ionic liquid-based fabric capacitors obtained by our team. A factor that is often ignored when it comes to wearable devices is the evaluation of their properties as a function of the weight of the materials. In these cases, most used is the area of the material or only the weight of the active material is taken into consideration. This becomes a limitation when scaling to industry production as bulky materials will not be suitable for powering

*(a) SEM image of electrode base material; (b) cyclic voltammetry graph of the working device; (c) picture of full device. (i) RGO fiber reprinted with permission from [49]. Copyright 2014 American Chemical Society. (ii) CNT-graphene fiber reproduced from [48] with permission from Royal Society of Chemistry. (iii) Carbon fiber coated with manganese oxide reprinted with permission from [50]. Copyright 2012 American Chemical Society. (iv) CNT fiber coated with manganese oxide reprinted with permission from [51]. Copyright 2015 American Chemical Society. (v) GO-active material coated stainless steel fiber reprinted with permission from* 

#### **Figure 6.**

*(a) Schematic of creating gel fabric structure with Bucky paper electrode pressed on top; (b) cyclic voltammetry curves at 200 mV/s for normal BP electrode and BP electrode pre-soaked in IL solution; (c) different scan rate cyclic voltammetry curves of IL-soaked device; and (d) charge and discharge curves of the fabricated device obtained at different current densities.*

not an indication of the limit of the electrode as the sulfuric acid device gave appreciable values. We added a further step to the device fabrication, which included first soaking the electrode in a solution of the IL plus acetone, and then incorporating it into the fabric structure. This resulted in an increase of the device energy density (**Table 1** and **Figure 6b**–**d**). Since acetone is fast evaporating solvent, it did not allow for the electrode to be fully penetrated with the electrolyte before drying. The described soaking procedure ensured complete utilization of the electrode surface area and thus increased the stored energy.

The device can be steadily cycled between 0 and 3 V window range without fade in capacitance values. This is not the only major advantage of such a device. Since


#### **Table 1.**

*Specific capacitance and energy density values of the fabric device achieved with different polymer electrolytes and preparation techniques.*

**31**

**Figure 7.**

*Fabric-Integrated, Ionic Liquid-Based Supercapacitor as a Tunable and Flexible Power Source*

the fabric is being used as a separator and not as the electrode itself, it essentially reduces the weight of the device in half, as compared to other wearable energy storage devices that rely on a fabric skeleton backing for each electrode. The fabric can then be further sown in as a patch thus making the only additional weight contribution to the device that of the gel electrolyte. Another key advantage of the described system is the ability to be easily modified for various applications. Depending on the energy need, the area of the electrode used can be increased or decreased to give highly tunable capacitance values (**Figure 7a** and **b**). Since the application and fabrication of the gel fabric is easily controlled, the device can also be made into sections. Various electrodes can be applied next to each other as isolated devices on the same fabric. One patch of fabric could contain many isolated devices. This gives high tunability to the device. Three isolated devices were made in one patch and when connected in series it achieved a working voltage of 9 V. In a parallel wiring

*(a) Pictures of different size with increasing areas of electrode materials used for the fabric device; (b) corresponding cyclic voltammetry profiles at 200 mV/s scan rate; (c) picture of isolated devices in a single patch of fabric; (d) corresponding cyclic voltammetry profiles at 200 mV/s scan rate in parallel connections and (e)* 

*corresponding cyclic voltammetry profiles at 200 mV/s scan rate in series connections.*

*DOI: http://dx.doi.org/10.5772/intechopen.80693*

*Fabric-Integrated, Ionic Liquid-Based Supercapacitor as a Tunable and Flexible Power Source DOI: http://dx.doi.org/10.5772/intechopen.80693*

the fabric is being used as a separator and not as the electrode itself, it essentially reduces the weight of the device in half, as compared to other wearable energy storage devices that rely on a fabric skeleton backing for each electrode. The fabric can then be further sown in as a patch thus making the only additional weight contribution to the device that of the gel electrolyte. Another key advantage of the described system is the ability to be easily modified for various applications. Depending on the energy need, the area of the electrode used can be increased or decreased to give highly tunable capacitance values (**Figure 7a** and **b**). Since the application and fabrication of the gel fabric is easily controlled, the device can also be made into sections. Various electrodes can be applied next to each other as isolated devices on the same fabric. One patch of fabric could contain many isolated devices. This gives high tunability to the device. Three isolated devices were made in one patch and when connected in series it achieved a working voltage of 9 V. In a parallel wiring

#### **Figure 7.**

*Science, Technology and Advanced Application of Supercapacitors*

not an indication of the limit of the electrode as the sulfuric acid device gave appreciable values. We added a further step to the device fabrication, which included first soaking the electrode in a solution of the IL plus acetone, and then incorporating it into the fabric structure. This resulted in an increase of the device energy density (**Table 1** and **Figure 6b**–**d**). Since acetone is fast evaporating solvent, it did not allow for the electrode to be fully penetrated with the electrolyte before drying. The described soaking procedure ensured complete utilization of the electrode surface

*(a) Schematic of creating gel fabric structure with Bucky paper electrode pressed on top; (b) cyclic voltammetry curves at 200 mV/s for normal BP electrode and BP electrode pre-soaked in IL solution; (c) different scan rate cyclic voltammetry curves of IL-soaked device; and (d) charge and discharge curves of the fabricated device* 

The device can be steadily cycled between 0 and 3 V window range without fade in capacitance values. This is not the only major advantage of such a device. Since

**Gel electrolyte Specific capacitance (F/g) Energy density (Wh/kg)**

*Specific capacitance and energy density values of the fabric device achieved with different polymer electrolytes* 

PVA-sulfuric acid (1 V) 11.54 3.2 PVDF-ionic liquid (3 V) 61.66 19.53 Ionic liquid soak 137.6 48.62

area and thus increased the stored energy.

*obtained at different current densities.*

**30**

**Table 1.**

**Figure 6.**

*and preparation techniques.*

*(a) Pictures of different size with increasing areas of electrode materials used for the fabric device; (b) corresponding cyclic voltammetry profiles at 200 mV/s scan rate; (c) picture of isolated devices in a single patch of fabric; (d) corresponding cyclic voltammetry profiles at 200 mV/s scan rate in parallel connections and (e) corresponding cyclic voltammetry profiles at 200 mV/s scan rate in series connections.*

the device achieved 3 times the current output and energy density of an individual one (**Figure 7c**–**e**). The fabricated device is highly versatile in terms of how it can fit to different applications. Electrical contacts can be made by simply applying copper tapes with silver paint to the ends of the electrodes. These connections need not be pre-made, giving a further degree of freedom.

Textile fabrics are exposed to stress in the form of flexing, bending or wrinkling. This is unavoidable in clothing when the wearer executes any form of motion. Such an environment must be taken into consideration when making fabric integrated devices. Here we tested our fabric device by exposing them to 10,000 cycles, where in each cycle it was bent to an angle of 120° and brought back to normal shape (**Figure 8a**). This is a higher degree of bending than fabrics

#### **Figure 8.**

*(a) Picture of the degree of bending during each cycle of the stress test; (b) cyclic voltammetry curve of the device before and after bending; (c) picture of the water bath immersion tests; and (d) cycling curve of the device at 2 A/g current density.*

**33**

*Fabric-Integrated, Ionic Liquid-Based Supercapacitor as a Tunable and Flexible Power Source*

would have to usually sustain on a daily basis. The device showed no depreciation in terms of capacitance after the cycling (**Figure 8b**). Another consideration to be taken into account is the exposure to moisture in the form of sweat from the wearer. Our device was soaked in a bath of water and cycled. In this test no observable depreciation in energy density was noticed (**Figure 8c**). Finally, the device was cycled 8000 times at a current density of 2 A/g that resulted in a final capacitance of 102% (**Figure 8d**). This increased capacitance is due to the forcible opening of the pores of the BP electrode with constant cycling. The high retention rate is very promising and ensures that the electrolyte is not being degraded at

There still exists room for improvement for the described architecture of the fabric devices. Altering the electrodes by creating more energy dense materials is a quick way to further improve the properties of the device. Using pseudocapacitive materials like transition metal oxides or conductive polymers in an asymmetrical electrode arrangement has the potential to further enhance the properties. This will

The wearable energy storage devices have seen much development over the last decade. This evolution included starting from cotton coated electrodes that were not highly reproducible and moving to more reliable techniques such as screen printing using a polymer resist. Further, full active material loaded yarns have been realized. There are however drawbacks with each of the approaches which include either a limit to the active material content and subsequently the energy density, or a lack of strong materials that can sustain the wear and tear of fabrics that will potentially go into articles of clothing. The most promise is shown when using yarn-based devices that, if coupled with high strength, may provide the

In our work we designed an approach to making a tunable device that can accommodate a various range of practical applications including those requiring voltages as high as 9 V or high current ratings. The gel fabric separator fabricated and reported here utilizes just the essential amounts of fabric, which can be easily patched onto articles of clothing thus adding no extra weight. The final device we created exhibits an energy density of 48 Wh/kg at 1 A/g current density and can sustain high cycling without any noticeable losses in capacitance. The described fabrication approach is versatile and can be adopted for making various electrodes that can further improve the properties of the fabric energy

This work was partly funded by the following grants: DURIP-ONR N00014-15- 1-2473, AFOSR LRIR 16COR322, ARMY W911NF-16-2-0026, NASA NNX13AF46A, and NNC16CA17C. The financial support from the above-mentioned Government

The authors declare that there was no conflict of interest during this work.

*DOI: http://dx.doi.org/10.5772/intechopen.80693*

such high voltage ranges.

**6. Conclusion**

best properties.

storage devices.

**Acknowledgements**

**Conflict of interest**

institutions is highly appreciated.

be explored in our following works.

*Fabric-Integrated, Ionic Liquid-Based Supercapacitor as a Tunable and Flexible Power Source DOI: http://dx.doi.org/10.5772/intechopen.80693*

would have to usually sustain on a daily basis. The device showed no depreciation in terms of capacitance after the cycling (**Figure 8b**). Another consideration to be taken into account is the exposure to moisture in the form of sweat from the wearer. Our device was soaked in a bath of water and cycled. In this test no observable depreciation in energy density was noticed (**Figure 8c**). Finally, the device was cycled 8000 times at a current density of 2 A/g that resulted in a final capacitance of 102% (**Figure 8d**). This increased capacitance is due to the forcible opening of the pores of the BP electrode with constant cycling. The high retention rate is very promising and ensures that the electrolyte is not being degraded at such high voltage ranges.

There still exists room for improvement for the described architecture of the fabric devices. Altering the electrodes by creating more energy dense materials is a quick way to further improve the properties of the device. Using pseudocapacitive materials like transition metal oxides or conductive polymers in an asymmetrical electrode arrangement has the potential to further enhance the properties. This will be explored in our following works.

## **6. Conclusion**

*Science, Technology and Advanced Application of Supercapacitors*

pre-made, giving a further degree of freedom.

the device achieved 3 times the current output and energy density of an individual one (**Figure 7c**–**e**). The fabricated device is highly versatile in terms of how it can fit to different applications. Electrical contacts can be made by simply applying copper tapes with silver paint to the ends of the electrodes. These connections need not be

Textile fabrics are exposed to stress in the form of flexing, bending or wrinkling. This is unavoidable in clothing when the wearer executes any form of motion. Such an environment must be taken into consideration when making fabric integrated devices. Here we tested our fabric device by exposing them to 10,000 cycles, where in each cycle it was bent to an angle of 120° and brought back to normal shape (**Figure 8a**). This is a higher degree of bending than fabrics

**32**

**Figure 8.**

*device at 2 A/g current density.*

*(a) Picture of the degree of bending during each cycle of the stress test; (b) cyclic voltammetry curve of the device before and after bending; (c) picture of the water bath immersion tests; and (d) cycling curve of the* 

The wearable energy storage devices have seen much development over the last decade. This evolution included starting from cotton coated electrodes that were not highly reproducible and moving to more reliable techniques such as screen printing using a polymer resist. Further, full active material loaded yarns have been realized. There are however drawbacks with each of the approaches which include either a limit to the active material content and subsequently the energy density, or a lack of strong materials that can sustain the wear and tear of fabrics that will potentially go into articles of clothing. The most promise is shown when using yarn-based devices that, if coupled with high strength, may provide the best properties.

In our work we designed an approach to making a tunable device that can accommodate a various range of practical applications including those requiring voltages as high as 9 V or high current ratings. The gel fabric separator fabricated and reported here utilizes just the essential amounts of fabric, which can be easily patched onto articles of clothing thus adding no extra weight. The final device we created exhibits an energy density of 48 Wh/kg at 1 A/g current density and can sustain high cycling without any noticeable losses in capacitance. The described fabrication approach is versatile and can be adopted for making various electrodes that can further improve the properties of the fabric energy storage devices.

### **Acknowledgements**

This work was partly funded by the following grants: DURIP-ONR N00014-15- 1-2473, AFOSR LRIR 16COR322, ARMY W911NF-16-2-0026, NASA NNX13AF46A, and NNC16CA17C. The financial support from the above-mentioned Government institutions is highly appreciated.

### **Conflict of interest**

The authors declare that there was no conflict of interest during this work.
