Fabrication And Performance Of Flexible Biodegradable Supercapacitors

Biodegradable implantable electronics are of growing interest in flexible bioelectronics.One of the key problems is the availability of power supply.As a promising energy supply solution,biodegradable batteries and supercapacitors are being extensively studied.Ideally,biodegradable energy storage devices should have the following characteristics:small size,sufficient capacity,and mechanical flexibility required for compliance with soft biological tissues,biocompatibility,as well as full biodegradability.However,there is still a significant gap,specifically in terms of poor electrochemical performance（including low output voltage,low energy density and power density）and mechanical rigidity.Hence,this thesis first develops electrode materials with excellent electrochemical properties,good biodegradability and biocompatibility,selects biodegradable gel electrolytes with matching properties,and finally assembles them to obtain high-performance biodegradable batteries and supercapacitors.The main works are as follows:1.Mo O₃ based biodegradable supercapacitors:The water-soluble Mo foil is used as a collector,and an active material is formed in situ on its surface through a two-step process involving electrochemical oxidation and annealing.After electrochemical oxidation,a stacked oxide layer with microcracks is formed on the surface of the Mo foil,and the generated oxygen vacancies can serve as shallow energy level defects to increase the carrier concentration,which promotes electron transport to enhance the kinetics of the electrochemical reaction.Furthermore,the annealing treatment results in the formation of oxide layers with good crystallinity and high surface area,while retaining the microcracked structure,thus greatly enhancing the charge storage capacity of Mo O₃.Benefiting from a rational synthesis strategy,the Mo O₃ electrode exhibits a high area specific capacitance of 118.75 m F cm^-2.The capacitance retention of 88.09%after 5000 charge-discharge cycles was achieved,which is mainly due to the increased crystallinity of the oxide layer,significantly improving its long-term cycling stability.Flexible,biodegradable symmetrical supercapacitors are obtained by assembling with Mo O₃ electrodes and biocompatible gel electrolyte,achieving a high area specific capacitance of 114.37 m F cm^-2 and a capacitance retention of 73.6%after 2000 charge-discharge cycles.As an independent power source,the device exhibits excellent energy density(15.8μWh cm^-2)and power density(3.83 m W cm^-2)and to drive red LEDs and electronic watch,demonstrating that the biodegradable supercapacitors can supply power for low-power implantable electronic devices.2.Preparation and performances of heterostructured Mo O₃-Mo S₂ composite electrodes:2D heterostructured Mo O₃-Mo S₂ nanosheet arrays grown in situ on a Mo foil current collector to serve as fully biodegradable pseudocapacitive electrodes.The heterostructures of Mo O₃ coupled with Mo S₂ can establish a built-in electric field to facilitate charge transport,while the Mo O₃-Mo S₂ heterostructured interface facilitates electron mobility.The crystal distortion near the interface between Mo O₃ and Mo S₂may provide extra active sites to enhance electrochemical activity.In addition,in situ growth strategy guarantees good contact interface and structural stability between active materials and the current collector.Unique morphological and structural feature enable Mo O₃-Mo S₂ electrodes to achieve high area specific capacitance of 164.38 m F cm^-2.After 15,000 charge-discharge cycles,a 104%capacitance retention is achieved.In vitro degradation experiments and cell cytotoxicity tests further confirm a good biocompatibility and biodegradability of the Mo O₃-Mo S₂ electrode.Hence,Mo O₃-Mo S₂ composites can be used as a promising active material for high-performance and biodegradable energy storage devices.3.High-performance biodegradable energy storage devices enabled by Mo O₃-Mo S₂ composites:Using the Mo O₃-Mo S₂ composite as electrodes,different energy devices,that is,a symmetric supercapacitor,asymmetric Zn-ion hybrid supercapacitor,and Mg primary battery were assembled for different application scenarios.The in situ synthesis approach has led to intimate contact between active materials and the current collector,and an open nanosheet array-like microstructure of active materials,which endows the devices with outstanding electrochemical performances.Specifically,the Mo O₃-Mo S₂-based symmetrical supercapacitors achieve an area specific capacitance of 79 m F cm^-2 and a capacitance of 84.8%after 3000 cycles.In addition,the fabricated Zn-ion hybrid supercapacitors demonstrate a high areal capacitance of 181.86 m F cm^-2at 0.5 m A cm^-2 and an energy density of 30.56μWh cm^-2.The Mg primary batteries exhibit a stable high output voltage（～1.6 V）and long lifetime（～7 h）.The used materials in these devices have good biocompatible and biodegradable.A rational packaging strategy allows the devices to operate stably in body fluid and achieve a controlled lifetime.Demonstration experiments in vitro further confirm their potential applications as biodegradable energy devices for powering biodegradable medical electronic implants as well as eco-friendly electronics.