Electrochemical Analysis of the Printed NiCo2O4 // Reduced Graphene Oxide Solid-State Hybrid Supercapacitors on Fabric Substrates for Wearable Applications

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The rapid developments of the wearable or body-integrated electronics and boom in the portable healthcare devices have increased the demand of the wearable energy storage devices [1-3]. Inkjet printing is an emerging technology to transfer the device components on the variety of the substrates at the specific locations. However, achieving the good electrochemical performance of the printed electrode materials on wearable substrates is still a major challenge [4, 5]. Here we report a printed, binder-free, flexible, and high-performance NiCo2O4// Reduced Graphene Oxide (rGO) battery- supercapacitor hybrid device on cotton fabric substrates. A thin layer of polydimethysiloxane (PDMS) polymer was applied over the textile fabric for surface planarization. The NiCo2O4 micro-flower arrays were successfully fabricated over this cloth substrates using printing and low-temperature heat treatment method and further treated with the UV irradiation. The crystalline and conducting NiCo2O4 micro-flower array layer acts as a self-supporting electrode, and its open space structure ensures the fast electron transport and full utilization of the electrode surface. The printed NiCo2O4 // rGO asymmetric device with the LiCl –PVA gel electrolyte displays good electrochemical performance within a potential window of 0 to 1.6 V. The fabricated device exhibited excellent electrochemical performance including high areal capacitance of 7.2 F/cm2 (2650 F/g) at a current density of 1 mA/cm2, good cycle life (89 % capacitance retention after 5000 charge-discharge cycles), high rate capability, high power density, high energy density, and low charge transfer resistance both in the normal and flexed conditions. The obtained results are comparable to the existing literature for the high temperature processed NiCo2O4 based energy storage devices, and therefore the reported method has a great potential for the direct fabrication of electrode materials over the temperature sensitive fabric substrates. These fabric based supercapacitors can be easily integrated with the various flexible and wearable devices and exhibit the great beneficial for the next generation of the wearable electronic devices. References Zeng, W., et al., Fiber‐based wearable electronics: a review of materials, fabrication, devices, and applications. Advanced Materials, 2014. 26(31): p. 5310-5336.Kim, J., et al., Advanced materials for printed wearable electrochemical devices: A review. Advanced Electronic Materials, 2017. 3(1): p. 1600260.Bandodkar, A.J., Wearable biofuel cells: Past, present and future. Journal of The Electrochemical Society, 2017. 164(3): p. H3007-H3014.Sundriyal, P. and S. Bhattacharya, Inkjet-Printed Electrodes on A4 Paper Substrates for Low-Cost, Disposable, and Flexible Asymmetric Supercapacitors. ACS applied materials & interfaces, 2017. 9(44): p. 38507-38521.Choi, K.-H., D.B. Ahn, and S.-Y. Lee, Current status and challenges in printed batteries: toward form factor-free, monolithic integrated power sources. ACS Energy Letters, 2017. 3(1): p. 220-236.