(Invited) Evolution of Supercapacitors Towards Climate Change–Promoting Effective Solar/Regenerative Energy Utilization
Authors: Naoi Katsuhiko , Naoi Wako 
Published: December 2020

1. Microcurrents for Macroefficiency: Effective harvesting of solar energy under low-sunlight conditions

Under low-sunlight conditions—dawn, dusk, and occlusion by clouds—the electric current produced by a photovoltaic cell (PVC) drops to a microscopic trickle, which conventional solar-power systems simply discard. Here we show that these tiny currents may in fact be effectively captured and used for substantial energy harvesting, significantly improving overall system efficiency. Our system involves an innovative mechanism, enabled by sensors and an AI-powered microcontroller, for automatically routing PVC currents through conventional (standard-current) or modified (microscopic-current) circuits. A decisive role in the design of our system is played by the rapid pace of global climate change; as we show, changes in the magnitude of sunlight-intensity variations over just the past ten years are an essential driver of the surprisingly large efficiency gains enabled by our innovation. In this talk we introduce the basic ideas of our technique, present results of a prototype study achieving 17% improvement in power-system efficiency, and discuss a variety of promising applications.
2. Supercharging Solar Cells with The Next Generation Supercapacitors

Li-ion based hybrid supercapacitors and their functional materials are being vigorously researched in hopes to improve their capacity/voltage and therefore their energy density(Figure 1). Transition metal oxides are among the most popular materials utilized in this purpose. Thanks to high voltage and associated high energy density, they are tuned as both high energy and high-power materials. We have developed “Nanohybrid” capacitor (NHC) based on the single-nanocrystalline Li4Ti5O12 (LTO, 5-20 nm) negative and active carbon positive electrodes, showing ultrafast charge-discharge capability up to 300C (=12 s) with a 3-hold energy density of EDLC.1) To further increase the energy density of NHC, currently we are expanding the search for alternative materials of LTO negative electrode, which possess i) higher capacity such as TiO2(B)2) with 2-hold theoretical capacity (= 335 mAh g-1) compared to LTO, and ii) lower reaction potential (higher cell voltage) such as Li3VO4 (LVO)3) and Y2Ti2O5S2 (YTOS). The present talk will mainly describe ultrafast/stable pseudocapacitive electrochemistry of LVO and YTOS as promising negative electrodes for replacing NHC, which can achieve higher cell voltage from 3.85 to 4.4 V. The volumetric energy density for the hybrid (YTOS//AC) will reach up to 4-fold of EDLC. The talk will also cover the 3rd generation “SuperRedox” capacitor which further replace the positive AC electrode by ultrafast nanocrystalline Li3V2(PO4)3, whereby the energy density will be 6-fold of EDLC.

References

1) Naoi, et al., ACC. Chem. Research, 46(5), 1075 (2013).
2) Naoi, E. Iwama, W. Naoi, P. Simon et al., Adv. Mater., 28, 6751 (2016).
3) Iwama, K. Kisu, W. Naoi, P. Rozier, P. Simon, K. Naoi et al., ACS Nano, 10(5), 5398 (2016).

Figure 1