Neelam Singh

Direct laser writing of micro-supercapacitors on hydrated graphite oxide films

Microscale supercapacitors provide an important complement to batteries in a variety of applications, including portable electronics. Although they can be manufactured using a number of printing and lithography techniques, continued improvements in cost, scalability and form factor are required to realize their full potential. Here, we demonstrate the scalable fabrication of a new type of all-carbon, monolithic supercapacitor by laser reduction and patterning of graphite oxide films. We pattern both in-plane and conventional electrodes consisting of reduced graphite oxide with micrometre resolution, between which graphite oxide serves as a solid electrolyte. The substantial amounts of trapped water in the graphite oxide makes it simultaneously a good ionic conductor and an electrical insulator, allowing it to serve as both an electrolyte and an electrode separator with ion transport characteristics similar to that observed for Nafion membranes. The resulting micro-supercapacitor devices show good cyclic stability, and energy storage capacities comparable to existing thin-film supercapacitors.

Hybrid supercapacitor-battery materials for fast electrochemical charge storage

High energy and high power electrochemical energy storage devices rely on different fundamental working principles - bulk vs. surface ion diffusion and electron conduction. Meeting both characteristics within a single or a pair of materials has been under intense investigations yet, severely hindered by intrinsic materials limitations. Here, we provide a solution to this issue and present an approach to design high energy and high power battery electrodes by hybridizing a nitroxide-polymer redox supercapacitor (PTMA) with a Li-ion battery material (LiFePO4). The PTMA constituent dominates the hybrid battery charge process and postpones the LiFePO4 voltage rise by virtue of its ultra-fast electrochemical response and higher working potential. We detail on a unique sequential charging mechanism in the hybrid electrode: PTMA undergoes oxidation to form high-potential redox species, which subsequently relax and charge the LiFePO4 by an internal charge transfer process. A rate capability equivalent to full battery recharge in less than 5 minutes is demonstrated. As a result of hybrids components synergy, enhanced power and energy density as well as superior cycling stability are obtained, otherwise difficult to achieve from separate constituents.

Direct Synthesis of Lithium-Intercalated Graphene for Electrochemical Energy Storage Application

A novel approach for bulk synthesis of lithium-intercalated graphene sheets through the reduction of exfoliated graphene oxide in liquid ammonia and lithium metal is reported. It is demonstrated here that as-synthesized lithiated graphite oxide sheets (Li-RGO) can be directly used as an electrode material in lithium batteries. The electrochemical studies on Li-RGO electrodes show a significant enhancement in the specific capacity of the lithium battery over commercially available graphite electrodes. Partial intercalation of lithium ions in between graphene layers makes this material a good candidate for electrochemical energy storage applications.

Roll up nanowire battery from silicon chips

Here we report an approach to roll out Li-ion battery components from silicon chips by a continuous and repeatable etch-infiltrate-peel cycle. Vertically aligned silicon nanowires etched from recycled silicon wafers are captured in a polymer matrix that operates as Li+ gel-electrolyte and electrode separator and peeled off to make multiple battery devices out of a single wafer. Porous, electrically interconnected copper nanoshells are conformally deposited around the silicon nanowires to stabilize the electrodes over extended cycles and provide efficient current collection. Using the above developed process we demonstrate an operational full cell 3.4 V lithium-polymer silicon nanowire (LIPOSIL) battery which is mechanically flexible and scalable to large dimensions.

Carbon Redox-Polymer-Gel Hybrid Supercapacitors

Energy storage devices that provide high specific power without compromising on specific energy are highly desirable for many electric-powered applications. Here, we demonstrate that polymer organic radical gel materials support fast bulk-redox charge storage, commensurate to surface double layer ion exchange at carbon electrodes. When integrated with a carbon-based electrical double layer capacitor, nearly ideal electrode properties such as high electrical and ionic conductivity, fast bulk redox and surface charge storage as well as excellent cycling stability are attained. Such hybrid carbon redox-polymer-gel electrodes support unprecedented discharge rate of 1,000C with 50% of the nominal capacity delivered in less than 2 seconds. Devices made with such electrodes hold the potential for battery-scale energy storage while attaining supercapacitor-like power performances.

Creating supersolvophobic nanocomposite materials

This paper illustrates two techniques that enhance the supersolvophobicity of inherently hydrophilic polymeric thin films. The first technique involves creating a perfluoro-functionalized carbon nanotube based “ink” that can be sprayed on virtually any surface, including polymeric thin films, to greatly enhance the supersolvophobicity. Our results show contact angles greater than 150° with 30 wt% monoethanolamine (MEA) on polysulfone (PSF) and polyimide films that have been treated with the CNT-based ink. The second method involves synthesizing a homogeneous, composite solution consisting of polymer (both PSF & polyimide) and perfluoro-functionalized CNTs (fCNTs). By designing a methodology for the fabrication of fCNT–polymer composite solutions, the supersolvophobicity is not only limited to the surface, but is present within the composite, thereby extending the proposed technique to a range of geometries and length scales. The ratio of polymer : fCNT was varied to locate an upper limit at which films maintain supersolvophobicity. The low density of fCNTs makes them a better alternative to conventional fluorine-based polymeric filler materials (i.e. PTFE, PVDF).