UC San Diego

Energy storage systems, including batteries, fuel cells, and supercapacitors, plays an essential role in various applications, from mass energy storage to transient energy buffering to assist power. Among these systems, supercapacitors have attracted widespread attention due to their high-power density and substantial cycle life. However, their practical applications are still limited by their low energy. To address this limitation, hybrid supercapacitors have been introduced. With the development of lithium-ion batteries, hybrid supercapacitor utilizing monovalent metal ions, such as lithium and sodium ions, have emerged. However, the actual anode capacities of lithium and sodium ion capacitors are lower than their theoretical capacities (Li: 3,860 mAh/g / Na: 1,166 mAh/g) due to the avoidance of direct plating/stripping. Additionally, although monovalent metal ions offer higher theoretical capacities, their charge density is lower than that of divalent metal ions, such as zinc and magnesium. Among the divalent metal ions, zinc is preferred due to its reduction potential (-0.76 V vs. SHE). Thus, zinc ion hybrid supercapacitors have been introduced. However, the dendritic zinc growth and low cathode capacity pose significant obstacles to practical applications. Moreover, low thermal stability, resulting from low ion conductivity in cold environments and high self-discharge at high temperatures, significantly degrade device performance. This study investigates strategies to enhance the energy and cycle lifetime of zinc ion hybrid supercapacitors across a wide temperature range. The dissertation is divided into three parts. The first part discusses the suppression of dendritic zinc growth on anode to enhance the cycle life. In this study, we demonstrated the effectiveness of a zinc-copper dual-ion electrolyte in promoting uniform zinc plating by reducing the zinc nuclei radius. The zinc ion capacitor utilizing the zinc-copper dual-ion electrolyte achieved a density of 41 W/kg with a negative-to-positive (n/p) electrode capacity ratio of 3.10. At an n/p ratio of 5.93, the device exhibited a remarkable cycle life of 22,000 full charge-discharge cycles, equivalent to 557 hours of discharge. The cumulative capacity reached ~581 ampere hour per gram, surpassing the benchmarks of lithium and sodium ion capacitors. These results highlight the promise of the dual-ion electrolyte for delivering high- performance, low-maintenance electrochemical energy supplies. The second part of the dissertation focuses on improving cathode capacity with extended cycle life. In this work, we investigated the use of protic additive to mitigate capacity fading and increase the utilization emeraldine salt (ES) polyaniline (PANI) cathode in a non-aqueous electrolyte. The protic additive, propylene glycol, characterized by its hydrogen-bonding capabilities, stabilizes the PS PANI and promotes reversible redox reactions. This facilitates high capacity and an extended cycle lifetime for applications in zinc ion hybrid supercapacitors. The use of this protic non-aqueous electrolyte in a PANI-zinc device resulted in an energy density of 255 Wh/kg at a power density of 1.8 kW/kg and a robust cycle lifetime of 3,850 charge/discharge cycles. At a high current density of 6.5 mA/cm2, the PANI reached a capacity of 257 mAh/g, equivalent to 87% of its theoretical capacity (294 mAh/g). These results showcase the effectiveness of the protic additive in improving both capacity and cycle life in electrochemical supercapacitors. In addition, we developed a gel polymer electrolyte (GPE) consisting of polyacrylonitrile (PAN) mixed with an ion exchange resin filler to broaden the operational temperature range and minimize self-discharge characteristics in supercapacitor. The inclusion of the ion exchange resin filler effectively lowered the self-discharge rate and impurity diffusion coefficients in the supercapacitors, thereby minimizing energy loss in hot environments. In cold settings, the device exhibited an energy density of 94 uWh/cm2 at an output power density of 0.4 mW/cm2 in a -20°C chamber. The combination of our improved GPE increased the stored energy and extended the operational time of a motor driven from a cold start by a supercapacitor. This demonstrates a high-performance design suitable for harsh environments. During the training for high thermal stability supercapacitor research, the research of short-wave infrared (SWIR) organic photodetectors with heterojunction bilayer was conducted. The utilization of a heterojunction bilayer in SWIR organic photodetectors improved the external quantum efficiency and detectivity, enabling the photomultiplication effect due to the electron blocking capability of CuSCN layer. Despite the enhanced external quantum efficiency (560%), the noise current was efficiently reduced by the heterojunction bilayer, resulting in improved detectivity (3.5x10^9 Jones).