Bioderived Ionic Liquids with Alkaline Metal Ions for Transient Ionics

Choline lactate, an ionic liquid composed of bioderived materials, offers an opportunity to develop biodegradable electrochemical devices. Although ionic liquids possess large potential windows, high conductivity, and are nonvolatile, they do not exhibit electrochemical characteristics such as intercalation pseudocapacitance, redox pseudocapacitance, and electrochromism. Herein, bioderived ionic liquids are developed, including metal ions, Li, Na, and Ca, to yield ionic liquid with electrochemical behavior. Differential scanning calorimetry results reveal that the ionic liquids remained in liquid state from 230.42 to 373.15 K. The conductivities of the ionic liquids with metal are lower than those of the pristine ionic liquid, whereas the capacitance change negligibly. A protocol of the Organization for Economic Co-operation and Development 301C modified MITI test (I) confirms that the pristine ionic liquid and ionic liquids with metal are readily biodegradable. Additionally, an ionic gel comprising the ionic liquid and poly(vinyl alcohol) is biodegradable. An electrochromic device is developed using an ionic liquid containing Li ions. The device successfully changes color at −2.5 V, demonstrating the intercalation of Li ions into the WO3 crystal. The results suggest that the electrochemically active ionic liquids have potential for the development of environmentally benign devices, sustainable electronics, and bioresorbable/implantable devices.journal articl

Nanoporous Cu Prepared through Dealloying by Selectively Etching an Alkaline Metal with Saline

A facile, benign, and rapid dealloying method was employed to synthesize nanoporous Cu using an ammonium chloride (NH4Cl) saline solution and Mg as an etchant and a sacrificial metal, respectively. The large difference between the standard electrode potentials of Mg and Cu (−2.36 and +0.34 V, respectively) resulted in rapid etching at 10 nm/min and 303 K. Dealloying for 15 min yielded a nanoporous Cu foil, exhibiting a specific surface area of 24.1 m2/g. The raw materials Mg and NH4Cl are naturally abundant; Mg is classified as a biodegradable metal, while NH4Cl is recognized as a safe material by the United States Food and Drug Administration. No harmful byproducts were produced during dealloying. Therefore, the proposed dealloying method is environmentally benign and addresses the issue of the hazardous chemical waste produced via conventional electrochemical dealloying methods.journal articl

Biodegradable Mg–Mo2C MXene Air Batteries for Transient Energy Storage

Primary batteries are the fundamental power sources in small electronic gadgets and bio/ecoresorbable batteries. They are fabricated from benign and biodegradable materials and are of interest in environmental sensing and implants because of their low toxicity toward the environment and human body during decomposition. However, current bio/ecoresorbable batteries suffer from low operating voltages and output powers because of the occurrence of undesired hydrogen evolution reactions (HERs) at cathodes. Herein, Mo2C MXene was used as a cathode to achieve high operating voltage and areal power. Mo2C provides energy barriers for HERs in alkaline solutions, and such barriers suppress HERs and allow the oxygen reduction reaction to dominate at the cathode. The fabricated battery exhibits an operating voltage and areal power of 1.4 V and 0.92 mW cm–2, respectively. Degradation tests show that the full cell completely degrades within 123 days, leaving only Mo fragments from the electrode and biodegradable encapsulation. This study provides insights into bio/ecoresorbable batteries with high power and operating voltage, which can be used for environmental sensing.journal articl

Recent Progress in Transient Energy Storage using Biodegradable Materials

With the development of wireless sensor networks, electrical waste that remains in the environment is an inevitable issue in achieving sustainability and progress in electronics. Transient electronics that disappear after a prescribed time are of interest in electronics and material sciences. Such devices comprise naturally sourced materials that degrade without harmful or toxic substances during biodegradation. Although there are reports on transient electronic devices, including transistors, sensors, and radio frequency circuits, insufficient research has been conducted on the energy storage essential for operating transient devices. This review highlights the recent progress in developing transient energy storage. First, materials for transient energy storage, including conductors, electrolytes, and gels, are introduced. Second, transient supercapacitors, pseudocapacitors, primary batteries, and secondary batteries, are described and summarized. Finally, this review concludes and discusses the prospect of transient energy storage. The continuous progress of transient batteries and integration with transient devices are promising for sustainable electronics in the future.journal articl

A Transient Pseudo-Capacitor Using a Bioderived Ionic Liquid with Na Ions

A pseudo-capacitor with transient behavior is applied in implantable, disposable, and bioresorbable devices, incorporating an Na ion-doped bioderived ionic liquid, molybdenum trioxide (MoO3)-covered molybdenum foil, and silk sheet as the electrolyte, electrode, and separator, respectively. Sodium lactate is dissolved in choline lactate as a source of Na ions. The Experimental results reveal that the Na ions are intercalated into the van der Waals gaps in MoO3, and the pseudo-capacitor shows an areal capacitance (1.5 mF cm−2) that is three times larger than that without the Na ion. The fast ion diffusion of the electrolyte and the low resistance of the MoO3 and Mo interface result in an equivalent series resistance of 96 Ω. A cycle test indicates that the pseudo-capacitor exhibited a high capacitance retention of 82.8% after 10 000 cycles. The transient behavior is confirmed by the dissolution of the pseudo-capacitor into phosphate-buffered saline solution after 101 days. Potential applications of transient pseudo-capacitors include electronics without the need for device retrieval after use, including smart agriculture, implantable, and wearable devices.journal articl

Battery-less luminance sensor biomimicking human sensory nervous system

With the evolution of materials science and microfabrication processes, energy harvesters have become sophisticated, achieving power outputs in the range of several milliwatts, and have become a promising alternative to conventional batteries. Although their output power is insufficient to continuously operate a wireless sensor module, energy harvesters can operate small integrated circuits, including timers, watches, and ring oscillators. In this study, we emulated the human sensory nervous system to develop a battery-less sensor with a built-in analog-to-digital converter. The human sensory nervous system comprises a sophisticated sensing mechanism that digitalizes external stimuli by pulse-density modulation. To mimic this behavior, we integrated a ring oscillator with photovoltaics, allowing it to function as a luminance sensor with an event-driven operation. The oscillation frequency of the ring oscillator changes with respect to the operating voltage; hence, the output voltage of the photovoltaic modulates the frequency by more than two orders of magnitude. The sensor exhibits oscillation frequencies of 10 kHz and 7.7 MHz corresponding to luminance levels of 25–25 000 lx. Its response times are 40 μs and 15 ms when the light source is turned on and off, respectively. Battery-less sensors expand the opportunities for the application of energy harvesters in biomedical, wearable, and environmental sensing.journal articl

An Autonomous Power Management System with Event-driven Energy Harvester Switch

This paper presents a new use of piezoelectric PVDF film as an event-driven sensor switch that activates the power-management circuit for a subsequent wireless sensor ZigBee such that it would not consume extra power during the sleep mode. The circuit is designed to pick up a mechanical vibration with the PVDF film and control the self-holding circuit to keep flowing the drive current to the load until a reset pulse signal is received. The developed power management circuit can supply load current upwards of 33 mA at 3.3 Vdc for load resistance ranging between 100 Ω and 1 MΩ.17th International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS 2017) 14-17 November 2017, Kanazawa, Japanjournal articl

Temperature-Compensated Pure Silicon Cantilever Resonator with Coupled Torsional Structure at Anchor

Combined with a coupled torsional structure at the anchor, a temperature compensated cantilever resonator was developed to reduce frequency dependence. The device shows the frequency dependence of 856 ppm with the temperature ranging from 25°C to 80°C. FEM simulation indicates that the coupled torsional structure at the anchor allows us to reduce temperature dependence of resonators to be 170 ppm with temperature ranging from −40°C to 80°C, optimizing wafer dopant, density, and mode.2023 22nd International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers), 25-29 June 2023, Kyoto, Japanjournal articl

A Water Dissolvable Electrolyte with an Ionic Liquid for Eco‐Friendly Electronics

A water-dissolvable electrolyte is developed by combining an ionic liquid (IL) with poly(vinyl alcohol) (PVA), which decays over time by contact with water. An IL generally consists of two species of ions (anion and cation), and forms an electrical double layer (EDL) of a large electrostatic capacitance due to the ions accumulated in the vicinity of a conductive electrode when voltage is applied. In a similar manner, the ionic gel developed in this work forms an EDL due to the ions suspended in the conjugated polymer network while maintaining the gel form. Test measurements show a large capacitance of 13 µF cm−2 within the potential window of the IL. The ionic gel shows an electrical conductance of 20 µS cm−1 due to the ionic conduction, which depends on the weight ratio of the IL with respect to the polymer. The developed ionic gel dissolves into water in 16 h. Potential application includes the electrolyte in disposable electronics such as distributed sensors and energy harvesters that are supposed to be harmless to environment.journal articl

Event Driven Time-logging System based on Continuous Operation of Real Time Clock towards Perpetual Electronics

This paper presents a novel time logging system using a real time clock (RTC) powered by a vibrational energy harvester to record the time of events when piezoelectric PVDF film as an event-driven sensor is activated. The power-management circuit normally remains in the sleep mode to cut power supply to the micro-processor and the ferroelectric random access memory. On the other hand, the RTC alone is powered by a battery to keep the time by seamless operation, which lowers the standby power consumption as small as 231 nW. As the PVDF film generates power, the developed power management circuit can supply load current upwards of 8 mA at 3.3 Vdc for external circuit to record the time of events with the energy consumption of 467 µJ.18th International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications, 4–7 December, 2018, Daytona Beach, Florida, USAjournal articl