Printed Supercapacitors for Energy Harvesting Applications

Name of the conference: Advanced Energy Materials

Date of publication: 10 - 12 September 2018

Abstract: Harvesting devices extracting energy from various ambient environment sources (electromagnetic/RF, thermal, mechanical, solar, triboelectric) have started to be developed since the year 2000 leading to self-powered devices. Printed and flexible electronics are gaining increasing attention as alternative fabrication method of electronics increasing the productions speeds, decreasing the manufacturing costs and establishing low temperature fabrication processes. Usually, flexible and wearable electronics require at least one energy harvesting unit to ensure longer operation or even self-powered capability. The harvested energy may be stored in batteries, but their finite life-time and low power densities can be a major problem when they have to be embedded in permanent structures and when high power demands are required. Supercapacitors (SCs), store energy mainly through electrical double layer (EDL) effect or fast/redox reactions at the electrode involved during the charge/discharge pro-cess. As a result, they can sustain millions of cycles, can provide fast charge/discharge rates and subsequently high power density, while keeping a reasonable energy density which makes them ideal for energy harvesting applications. In this work, we consider an inkjet-printed, flexible and transparent SC to be integrated with wearable electronic devices. The SC is based on nickel oxide (NiO) interdigitated co-planar electrodes to utilise the pseudocapacitive capability of NiO and boost the performance of the storage device beyond the EDL storage mechanism.The SC is prepared by first using inkjet printing (Dimatix DMP 2800) to pattern the current collector of the device on polyethylene terephthalate (PET) plastic substrate. Silver nanoparticle ink is used as the collector material. For the formation of the active NiO electrode material, two approaches are followed. In the first approach, approximately 5µm of nickel is electroplated on the current collector which is then oxidized to produce a mesoporous NiO film. In the second approach, a NiO nanoparticle ink (<50nm nanoparticles) is formulated and deposited using inkjet printing on top of the current collector followed by annealing in air at 150°C for 3 hours. In both approaches, the devices are encapsulated with another PET film on top and a thermoplastic sealing film sandwiched in between. A saturated sodium perchlorate aqueous solution (SSPAS) with the widest voltage window of around 3V is used to fill the device. One of the key elements and challenges for this co-planar SC design is the interdigitated fingers with small gaps which can minimize the traveling distance of the ions. The realized SC is consisted of 100 interdigitated fingers 100µm wide each with 20µm gaps.

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