Supercapacitors record breaking performance for self-powered wearables

The NSF Nanosystems Engineering Research Center (NERC) for Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) develops and employs nano-enabled energy harvesting, energy storage, nanodevices and sensors to create innovative battery-free, body-powered, and wearable health monitoring systems.

ASSIST’s vision is to achieve a comprehensive assessment of health and environment through wearables that enable multi-modal sensing. A key component of this vision is to make the wearables as hassle-free as possible by making them rely on energy harvested from the body (e.g. heat and motion) as well as the ambient (e.g. solar). While ASSIST harvesters are pushing the envelope in energy harvesting, it is still essential to store this energy to increase the energy budget – and hence, the functionality – of ASSIST sensors and electronics. Dr Clive Randall and Dr. Ramakrishnan Rajagopalan’s team at the Pennsylvania State University are pushing forward in innovative methodologies to create a low-leakage high energy density electrochemical supercapacitor that can handle long-term energy storage from the ASSIST harvesters.

As a concept, supercapacitors are high capacity electrochemical storage units with capacitance values greater than 1,000 farads operable at 2.7 volts; these units typically bridge the gap between electrolytic capacitors and rechargeable batteries. In relation to rechargeable batteries, supercapacitors have better abilities for multiple rapid discharge and recharge cycles and are relied upon for short-term energy storage or burst-mode power delivery but have much larger form factors and are subject to high leakage current volumes. While off the shelf supercapacitors serve as a benchmark standard for the research being completed at Penn State, to be sustainable for ASSIST Testbed related platforms, these supercaps must endure long-term energy storage and low leakage currents on a much smaller form factor. In addition, these storage devices must capture harvested energy from harvesting sources such as thermoelectric and piezoelectric while working on a low-power load. In typical, these specifications are highly deviated from the norm. Randall and Rajagopalan’s group of researchers at Penn State, including students Weiguo Qu, Danhao Ma, Jianfeng Wang and Seth Berbano, have been very successful in the creation and demonstration of extremely high energy density low leakage supercapacitors, with the energy density capabilities of up to 300 J/cc (joules per cubic centimeter) based on volume of electrode and separators. Their group focused their efforts on creation of three primary electrolyte based materials for their base of the supercaps: lithium ion, organic electrical double layered capacitors (EDLC), and solid state proton based capacitors. Each methodology has areas that can be improved, and their team have plans to do just that. With current state of the art commercial capacitors being based off either aqueous or non-aqueous electrolytes, the group hopes to push forward with innovation in each of the mentioned materials.

Dr. Rajagopalan and team’s current lithium ion supercap is made using high surface area nanoporous electrodes and a prelithiated graphite anode, and is capable of meeting the current Generation 1 requirements for ASSIST Testbeds. These supercaps have proven results, showing a gravimetric energy density of 98 Wh/kg and volumetric energy density of 70 Wh/l with cell voltage of 4.5V. In addition, these supercaps demonstrate high energy density of 300 J/cc and good self-discharge performance at 90% charge retained over a 2 month cycle. As research expands on this version of ASSIST supercap, the team plans to create novel nanoparticle cathode materials which could help push energy density to a magnitude of 500 J/cc to meet Generation 2 Testbed requirements.

ASSIST’s organic EDLCs are currently capable of being cycled upto 3.5V based on propylene carbonate based electrolyte, a significant improvement in cell voltage compared to the current EDLCs available commercially. EDLCs also demonstrated low leakage characteristics of 100 – 200 nA/cm2. The group intends to further increase the energy density of the electrolyte based system by improving the cell voltage to 4V, which would double the energy density of existing capacitors. They remain optimistic of improving the specific capacitance of existing carbon materials to further improve the energy density.

Using binderfree carbon nanotubes and highly ionically conducting solid state polyvinylalcohol based nanocomposite electrolytes, high power density low ESR flexible capacitors with time constant of ~ 3 – 4 ms was fabricated with leakage current density of 400 nA/cm2. Developing asymmetric electrode designs in aqueous electrolyte can significantly enhance the energy density of aqueous capacitors. When questioned about the high success rate of the supercapacitors created, Dr. Rajagopalan stated that the purity of the carbon materials being created at Penn State were largely to be attributed for such success. With oxygen concentrations as low as 1 to 2 atomic percent, this high purity carbon helps immensely with suppressing the decomposition reactions of the electrolytes in supercaps. With such purity in the carbon materials, this ASSIST research team has been able to achieve at least an order of magnitude improvement in leakage current for comparable capacitance values working at high rated voltage.

The advancement shown in the areas of supercapacitors by Dr. Randall and Rajagopalan’s team at Penn State makes the idea for a wearable, body energy powered health monitoring system one step closer to realisation.