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.


Introduction
The evolution of electronics allows the development of tiny wireless sensor modules that can be installed anywhere, including buildings, the sea, and forests.Wireless sensor networks with such electronic devices apply to primary industries, including agriculture, fishing, and animal husbandry.The wireless sensor network retrieves environmental data for agriculture, such as solar irradiance, soil moisture, and soil temperature.Furthermore, data processing by artificial intelligence contributes to increasing the yield and quality of crops.With considerable demand for wireless sensor modules, their consumption

Transient Materials 2.1. Conductors
High electrical conductivity is essential for the current collectors of energy storage.With the progress of transient electronics, conductors such as Mg, Zn, Mo, W, and Fe, have been widely studied due to their biodegradability, processability, and high electrical conductivity. [7]8a] Pure metals show sufficiently small resistivity to be used as conductors.For electrochemical stability, the standard electrode potential is crucial for ionic devices to avoid undesirable redox reactions that lead to corrosion of the conductors.The Gibbs energy change in the reaction of an electrode, ΔG°, is written as ΔG°¼ ÀzFE° (1)   where z, F, and E°are the number of electrons consumed for the reaction, the Faraday constant, and the standard electrode potential, respectively. [8]The half-reaction of proton H þ in aqueous solutions is expressed as Given that the metals react with H þ in aqueous solutions, the metals with negative standard electrode potentials corrode by a cathodic reaction, and hydrogen generates by an anodic reaction according to the following reactions where E°m is the standard electrode potential of the metals.The ΔG°of the cell reaction is where E°a node and E°c athode are the standard electrode potentials of anodic and cathodic reactions, respectively.8c] The ecoresorbable metals (Mg, Zn, Mo, W, and Fe) dissolve into deionized water (DIW), phosphate-buffered saline (PBS) solutions, and simulated body fluids.Although pH, ions, and types of solutions influence the dissolution rate, it is high for metals with low standard electrode potentials (Table 1).For transient electronics, devices are designed to disappear after a prescribed time, and the selection of metals allows us to control such times.Mg and Zn dissolve quickly in DIW and simulated body fluids, which is favorable for developing transient devices that disappear when contacting liquids. [9]Mo and W are candidates for applications where long time and stable operations are required (Figure 2).

Metal Oxide
In addition to biodegradability and conductivity, transition metal oxides are beneficial to boost the capacitance of supercapacitors by reversible and rapid faradic reaction, intercalation of alkaline metal ions into the surface, and even bulk of the oxide.Upon immersion of the metal oxides into electrolytes containing small ions, such as Li þ and Na þ , the ions absorb and intercalate into the vdW gap.Such absorption and intercalation exhibit capacitive behavior known as intercalation pseudocapacitance. [10]ccording to the previous works on energy storage devices with metal oxides, TiO 2 , [11] Nb 2 O 5 , [12] V 2 O 5 , [13] MnO 2 , [14] WO 3 , [15] and MoO 3 [16] show pseudocapacitive behavior that is ascribed to intercalation of Li and Na ions.Based on biodegradation and Reproduced with permission. [55]11b,17] and cyclic voltammetry, [18] respectively, and such nano/microstructures allow to develop electrodes with a large surface area that boosts the capacitance of pseudocapacitors.

Aqueous Electrolytes
An electrolyte is composed of a medium that contains movable ions, which leads to electrical conduction.Although organic electrolytes show large potential windows that are beneficial for secondary batteries with high operating voltage, they are flammable and toxic substances that can arise safety and environmental pollution.Aqueous electrolytes are, meanwhile, promising candidates for transient energy storage due to their low toxicity against humans and environments, nonflammability, and abundance.Human's intake sodium chloride (NaCl) as a source of Na ions essential for living organisms to maintain cell activity.NaCl saline is benign for the environment and humans and widely used as an electrolyte.PBS solutions are also used as electrolytes for transient energy storage, comprise ions found in human bodies, and are widely used in biology.The composition of the PBS solution differs in its use.Typically, 10 mM PBS solution is prepared as follows: 8.00 g of NaCl, 0.20 g of KCl, 2.90 g of Na 2 HPO 4 •12H 2 O, and 0.20 g of KH 2 PO 4 are dissolved in 1 L of DIW.

Ionic Liquids (ILs)
While aqueous electrolytes show excellent ionic and electrical characteristics, water evaporation in the hydrogels is inevitable.The aqueous electrolytes further suffer from small potential water windows, leading to a low operation voltage of ionic devices.ILs comprising a pair of ions show extremely low vapor pressure, high ionic conductivity, and high electrochemical/thermal stability.In addition to such excellent characteristics, metal ion doping yields IL electrochemical behavior, for example, intercalation pseudocapacitance, redox pseudocapacitance, and electrochromism.ILs are widely adopted in sensing, energy storage, and transistor applications. [19]Cations and anions of imidazolium and bistriflimide derivatives, respectively, are widely used in ILs.However, due to their toxicity, [20] using such ionic liquids could adversely affect human bodies and the environment, hindering the sustainable application of ILs.Therefore, biodegradable ILs have been employed for transient applications.
An IL, tris(2-hydroxy-ethyl) methylammonium methylsulfate ([MTEOA][MeOSO 3 ]), is commercially available and classified as readily biodegradable.The capacitance and conductivity of [MTEOA][MeOSO 3 ] are 22 μF cm À1 at 1 Hz and 285 μS cm À1 at 100 kHz, respectively. [21][MTEOA][MeOSO 3 ] can disperse into polymer poly(vinyl alcohol) (PVA) to produce a transient ionic gel that disappears upon contact with water.Due to the low vapor pressure of the ILs, the ionic gel remained in an oven at 80 °C without a change in the ionic conductivity and capacitance.Further, thermal gravimetric analysis (TGA) found that [MTEOA][MeOSO 3 ] shows thermal stability up to 175 °C and decomposition at 350 °C, [22] which thermal robustness is beneficial for high-temperature operation of their devices.The hydrogel, meanwhile, cannot maintained those characteristics in 90 min.The transient behavior of the ionic gel was verified by soaking the ionic gel in DIW at 60 °C and disappearing in 16 h.
The National Institutes of Health classifies choline, present in food, drugs, and biological systems, [23] as a nutrient.Choline produces ILs with anions (nitrate, chloride/urea, and lactate), and these anions are environmentally benign and biodegradable for transient ionics.Nitrate exists in the soil and is essential for the nitrogen cycle to produce organic substances, and the IL [Ch][NO 3 ] is a promising candidate for an electrolyte of transient ionics.Although [Ch][NO 3 ] is a bio-derived material, it shows thermal stability at up to 260 °C and decomposition at 320 °C. [24]Pristine [Ch][NO 3 ] shows ionic conductivity of 4.85 mS cm À1 , [24] which is suitable for ionic devices.
Jia et al. reported ionic gel-incorporating silk ([Ch][NO 3 ]) and water. [25]The ionic gel contains water and is a hydrogel type.The ionic gel with a width ratio of silk ([Ch][NO 3 ] = 1:3) shows a 98% maximum fracture strain and ionic conductivity of 3.4 mS cm À1 .[Ch][NO 3 ] leached from the silk matrix in the biodegradation test due to its weak bound to the SF host matrix when the ionic gel was soaked in a PBS solution.The silk matrix did not degrade in the PBS solution, but a buffered protease XIV solution decomposed the silk matrix to reduce the mass of the ionic gel to approximately 0.
The IL 2-hydroxyethyl-trimethylammonium L-lactate ([Ch][Lac]) comprises choline and lactic acid, essential elements for human bodies.Lactate also exists in the human body and is one of the final products to derive energy from glucose after fermentation and respiration. [26][Ch][Lac] also shows high thermal stability up to 200 °C and decomposes at 250 °C, which characteristics are similar to that of [Ch][NO 3 ]. [27]Its ionic conductivity and potential windows are 163 μS cm À1 and 3 V. [28] Choline produces salt derivatives with alkaline metals Li, Na, and Ca, and [Ch][Lac] can accommodate salts [Li][Lac], [Na][Lac], and [Ca][Lac] by 10, 15, and 15 wt%, respectively. [28,29]The biodegradation of [Ch][Lac] and its mixtures of salt derivatives were investigated according to the Organization for Economic Co-operation and Development (OECD) guidelines for testing chemicals (OECD 301C modified Ministry of Economy, Trade and Industry (MITI) test (I)).According to a biochemical oxygen demand (BOD) test, the biodegradability of [Ch][Lac] (BD BOD_PIL ) reached 70%, and the BD BOD s curves for [Ch][Lac] with metal ions Li, Na, and Ca were more than 75% after the test, indicating that the alkaline metal ions and the density did not hinder the degradation process of active sludge.The biodegradability of the ionic gel comprising [Ch][Lac] and PVA (BD BOD_IG ) was 54% within 28 days due to the slow degradation of PVA under the OECD 301C modified MITI test (I).The biodegradability of the ILs was also evaluated using dissolved organic carbon (DOC) before and after incubation (Figure 3).As summarized in Table 2, the BD DOC of the ILs showed high biodegradation levels of over 92%.Based on the BOD and DOC tests, alkaline metal ions did not negatively influence the biodegradation of the ILs by the active sludge, and the ILs were classified as readily biodegradable.Choline-based ILs including choline maleate, choline propionate, and choline tiglate, are also promising candidates for the transient electrolytes. [30]rea is a nitrogen carrier and is used as a soil fertilizer.When choline chloride and urea are mixed, the melting point of the mixture decreases to exist as a liquid at room temperature.This new class of ILs is widely known as deep eutectic solvents (DESs), [31] and DESs are typically obtained by complexation of a Reproduced with permission. [29]Copyright 2023, Wiley-VCH.
quaternary ammonium salt with a metal salt or hydrogen bond donor.
[Ch][Cl][urea] shows a lethal aquatic concentration at 50% (LC 50 ) of 12 500 mg L À1 and is classified as "relatively harmless". [32]The biodegradability of [Ch][Cl][urea] was 85%, determined by the closed bottle test due to the carboxylic acid and amide groups, which showed excellent degradability.The DES, [Ch][Cl][urea], showed an ionic conductivity of 0.4623 AE 0.002 mS cm À1 [33] and thermal stability and decomposition at up to 150 °C and over 400 °C, [27] respectively, which characteristics are comparable with the biodegradable ionic liquids.
The above electrolytes are liquid and are challenging to manage.Such electrolytes are dispersed into polymers to form solidstate electrolytes, and naturally sourced polymers are widely used for gelation (Figure 4a).Agarose extracted from red algae seaweed is a polysaccharide comprising D-galactose and 3,6-anhydro-L-galactopyranose.Agarose can dissolve in hot water, and the solution solidifies at room temperature. [34]garose gels involve a three-dimensional matrix with a network and are widely used for the electrophoresis of DNA and affinity chromatography.Moon et al. reported a biodegradable electrolyte with agarose and saline, [35] and 1 g agarose could uptake 100 mL of NaCl solution (Figure 4b).
Other polysaccharides, alginates, are also natural polymers comprising linear copolymers of βdmannuronic acid (M) and αlguluronic acid (G) linked by one to four glycosidic bonds, forming polymeric blocks (GG, MM, and GM blocks). [36]he alginates are found in brown seaweeds, such as species of Ascophyllum, Durvillaea, Ecklonia, Laminaria, Lessonia, Macrocystis, Sargassum, and Turbinaria.Alginates have derivatives in the form of salts, for example, sodium alginate and calcium alginate.When a sodium alginate solution contacts with a solution with a polyvalent cation, Ca 2þ ions, the polyvalent cation crosslink the alginate chains, leading to the gelation of the sodium alginate solution (Figure 4c). [37]Sheng et al. reported a gel electrolyte comprising 0.8 g sodium alginate and 20 mL DIW.19b] Karami-Mosammam et al. developed degradable electrolytes using such gelation, [38] and 0.5 M CaCl 2 solution crosslinked solutions contain various sodium alginates (1%, 2%, and 3% w/v) and PBSs (pH 7.4, Table 2. Summary of the BOD and DOC tests on the ILs [Ch][Lac] and ionic gel (PVA). [28,29]
prepared from 3.35 g L À1 NaH 2 PO 4 and 0.85 g L À1 K 2 HPO 4 ) to obtain a uniform film with a thickness of 1 mm.Insects and arachnids, including silkworms, spiders, scorpions, and mites, produce silk for cocoons or hunting their prays. [39]Silk is made of two proteins, sericin and fibroin, and fibroin is a fibrous protein and structural component. [40]Silk fibroin is extracted from cocoons to fabricate silk films, microspheres, and hydrogels by degumming cocoons and dissolving fibroin fibers. [41]Zhao et al. reported a hydrogel including silk fibroin, acrylamide, acrylic acid, and CaCl 2 (Figure 4d).The chemically and physically crosslinked gel network enables mechanical robustness of up to 568.9 kPa and a large fracture strain of 1800%. [42]He et al. developed a hydrogel with excellent stretchability and mechanical strength, adopting silk fibroin, polyacrylamide, graphene oxide, and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate).The hydrogel shows a large fracture strain of approximately 900%. [43]hereas the above gels are sourced from natural products, a biodegradable polymer, PVA, is made from fossil fuel sources.The direct polymerization of the monomer vinyl alcohol cannot synthesize the PVA due to its instability.PVA synthesis involves the alkali hydrolysis of polyvinyl acetate in the methanol, referred to as the saponification process.The biocompatibility and biodegradability of PVA allow the development of wearable and implantable devices attached to human bodies. [44]Its hydrogel shows good adhesion and ionic conductivity and is widely used for electrolytes for electrochemical devices. [45]dditionally, the PVA hydrogel involves a self-healing capability without an external stimulus and healing agents, prepared using the freeze-thaw method. [46]Such characteristics are favorable for flexible and stretchable electronics that require high mechanical robustness.Further, PVA can dissolve in water, which is desirable for transient ionic devices that disappear upon contact with water (Figure 4e). [47]4.Encapsulation Encapsulation of the transient energy storage is crucial for their stable operation and long lifetime, protecting them from moisture and oxygen in the air.Lee et al. reported a biodegradable secondary battery packaged with the multilayered film of CMC, silicon, and aliphatic copolyester [poly(butylene adipate-co-terephthalate); PBAT].[48] The package showed a low oxygen and water transmission rate of less than 200 and 125 cm 3 m À2 day À1 , respectively.Polyanhydride is a hydrophobic material that can repel moisture, and Huang et al. reported that LED illumination retained for over 16 h with a primary battery packaged with polyanhydride and poly(lactide-co-glycolide) (PLGA).[49] A natural product, agarose, can be used as a packaging material.A pouch cell made of a %25 μm thick agarose layer enabled the stable operation of a Zn ion battery with an opencircuit voltage of 1.123 V and a discharge capacity of 157 mAh g À1 after 200 cycles at a current density of 50 mA g À1 .[50] The aforementioned encapsulation materials are flexible but not stretchable, and their mechanical characteristics may hinder to develop stretchable transient energy storage.A biodegradable elastomer poly(1,8-octanediol-co-citrate) (POC) [51] and its derivative, (poly(octamethylene maleate (anhydride) citrate)) (POMaC), [52] can be promising encapsulation for the stretchable energy storage.A POC film with a thickness of 300 μm showed a fracture strain of approximately 170%, and its encapsulation enabled repetitive stretching by 50%.The POC encapsulation allowed the stable operation of pseudocapacitors that was soaked in deionized water at 37 °C for 11 d.[19a] POMaC is composed of 1,8-octanediol, citric acid, and maleic anhydride, and three types of POMaC are synthesized via UV and (or) thermal cure, namely photo crosslinked POMaC (PPOMaC), ester bond crosslinked POMaC (EPOMaC), and ester bond crosslinked photo crosslinked POMaC (EPPOMaC).
Differences in molar ratios of maleic anhydride and citric acid tune Young's modulus and elongation.Prepared with the ratios of maleic anhydride: citric acid: 1,8-octanediol = 2:3:5, 3:2:5, 4:1:5, PPOMaC becomes stiff with an increase in the molar ratio of maleic anhydride (4:1:5) to raise Young's modulus and elongation to be 0.29 MPa and 194%, respectively.EPPOMaC involves UV and ester bond crosslinks; it is the stiffest among the three types of POMaC and showed Young's modulus and elongation of 1.52 MPa and 51%, respectively.An in vitro degradation study revealed that EPPOMaC exhibited a slow degradation rate, incubated in PBS solution at 37 °C.The EPPOMaC encapsulation protected tactile and strain sensors from infiltration of the PBS solution for 2 weeks.The tactile and strain sensors encapsulated with EPPOMaC maintained sensing capability for 3.5 weeks after being implanted into a mouse although the sensors adopted Mg electrodes that are subject to reacting with body fluids [4c] ; POMaC, therefore, possesses a superior barrier against moisture.

Supercapacitor/Pseudocapacitor
With transient conductors and electrolytes, transient energy storage has been developed (Table 3).Supercapacitors adopt the physical absorption and desorption of ions on electrodes with large surface areas upon voltage application.The absorption forms EDLs near the electrodes, and EDL capacitance (C EDL ) is written as where ε, S, and d are the permittivity, surface area, and thickness of the EDL, respectively.The d is typically the nanometric level.Supercapacitors deploy EDLs to achieve a large capacitance.Lee et al. proposed a fully biodegradable micro supercapacitor for transient electronics (Figure 5a). [53]NaCl/agarose gel, biodegradable metals Mo, and PLGA were used as electrolytes, electrodes, and substrates, respectively.Planar-type supercapacitors are flexible and have capacitances of 1.6 mF cm À2 at a current density of 0.15 mA cm À2 .Although PLGA showed slow degradation, the Mo electrode and NaCl/agarose gel dissolved into the PBS solutions (10 Â 10 À3 M, pH = 7.4) at 37 °C and (pH = 12) 65 °C, 9 d, and 24 h after soaking, respectively (Figure 5b).Metal electrodes show small surface areas that afford small capacitance.Migliorini et al. reported all-printed green micro-supercapacitors to address this issue using carbon (Figure 5c). [54]A spray coating of Au, carbon, and ionic gel ([Ch][Lac] and 2-hydroxyethyl cellulose) on a cellulose acetate sheet enables 7.6-9.0μm thick supercapacitors without substrate.The carbon/ionic gel supercapacitor quickly dissolved in distilled water within 1 d.The cellulose-choline lactate ionic gel involves no chemical bonds, allowing water molecules to pervade the polymeric matrix, unfolding the chains, and causing dissolution.Immersing a full device (cellulose acetate substrate, supercapacitor, and current collectors) inside a physiological PBS revealed that the gold current collectors fully disassembled from the substrate, leaving the ionic gel free to be permeated by the solution and completely dissolved within 24 h (Figure 5d).The supercapacitor can be charged at 1.6 V owing to the large potential window of the IL and shows a maximum capacitance of 1.0 mF cm À1 (Figure 5e).Yamada et al. developed a carbon/IL ([MTEOA][MeOSO 3 ]) composite to increase the capacitance of supercapacitors (Figure 5f,g). [55]ILs show cation-π interactions with carbon surfaces [56] that can disperse CNT bundles by the strong van der Waals force associated with the one-dimensional structure.Mixing [MTEOA][MeOSO 3 ] with carbon can yield a large accessible surface area for the ions.The supercapacitor shows the maximum areal and volumetric capacitances of 65 mF cm À2 and 2.2 F cm À3 , respectively, with a discharge current density of 0.2 mA cm À2 .The cycle test reveals that the capacitance retention is 77% after 10 000 charge and discharge cycles.
The supercapacitor was immersed in 0.01 M PBS, kept in an oven at 100 °C, and delaminated into the cellulose separator, carbon, and Mo electrodes in 1 d.The Mo electrode changed the color from glossy gray to dark gray after 18 d (Figure 5h); however, the Mo foil did not dissolve within 18 d.The dissolution rate of the Mo film was 0.017-0.02μm day À1 ; therefore, the 10 μm thick Mo film herein will completely dissolve in 500-588 d.
In addition to the electrical double-layer capacitance, Faradaic or redox reactions are employed for charge storage to boost the capacitance.19a] The Na ions intercalate into the vdW gaps of anodized Mo as follows.
The anodized Mo wire shows a capacitance of 18 mF cm À2 using a three-electrode cell, much larger than that of a pure Mo wire owing to the pseudocapacitive behavior.The wire pseudocapacitor shows a maximum areal capacitance of 4.15 mF cm À2 at 0.05 mA cm À2 , corresponding to a volumetric capacitance of 166 mF cm À3 at 2 mA cm À3 .The serpentinite structure and stretchable substrate yield the pseudocapacitor mechanical robustness to bending 600 times and stretching 1000 times.19b] The pseudocapacitor adopted MoO 3 , sodium alginate gel, and starch as electrodes, electrolytes, and encapsulation, respectively.Cyclic voltammetry on the Mo electrode produced flake MoO x on its surface, and annealing at 500 °C in the air oxidized the MoO x to MoO 3 .The pseudocapacitor showed an ultrahigh capacitance of 112.5 mF cm À2 at 1 mA cm À2 owing to the large surface area of the flaky MoO 3 structures and the pseudocapacitive behavior.The dissolution of the pseudocapacitor into PBS (pH 7.4) at 37 °C in vitro, 3 months after soaking, confirmed the degradation.The pseudocapacitor encapsulated with a starch film and PLA was implanted into rat body and fully degraded and bioresorbed into it 6 months after implantation.The implanted pseudocapacitor can illuminate a red LED, indicating the capability of biodegradable energy storage for implant devices.The aforementioned pseudocapacitors show excellent energy storage characteristics.However, the hydrogels suffer from water evaporation.19c] ILs can accommodate salts comprising alkaline metal ions with the same cations or anions as the ILs.6d, the Na ions intercalated into the vdW gaps of the MoO 3 electrodes to increase the areal and volumetric capacitances up to 1.5 mF cm À2 and 0.74 F cm À3 , respectively, at a discharge current density of 0.20 mA cm À2 .The capacitance of a pseudocapacitor without Na ions is 0.54 mF cm À2 at 0.20 mA cm À2 ; therefore, the intercalation of Na ions increases the capacitance by 200%.The degradation test using 10 mM PBS solutions at 37 °C revealed that the pseudocapacitor was dissolved, leaving a silk separator 101 after soaking (Figure 6e).Although the silk does not degrade in the PBS solution, the silk separator shows biodegradability and degrades with enzymes.The pseudocapacitor without a silk separator can show degradation behavior in 10 mM PBS solution.

Primary Battery
A primary battery adopts an irreversible electrochemical reaction at the cathode and anode designed to be used once.The difference in the standard electrode potentials of the cathode and anode determines the output voltage of the primary batteries.Mg (V SHE = À2.36V) is widely used as an anode for biodegradable primary batteries owing to its biodegradability, benignity, and stability in ambient air.Yin et al. reported a transient battery using Mg, X (X = Fe, W, or Mo), and PBS solutions as an anode, cathode, and electrolyte, respectively (Figure 7a). [59]The primary electrochemical reaction at the anode is with the following side reaction.[Lac] supercapacitor.Reproduced with permission. [54]Copyright 2021, Wiley-VCH.f ) A schematic and g) photograph of a supercapacitor with [MTEOA][MEOSO 3 ] and a carbon electrode.h) Decomposition into PBS solution.Reproduced with permission. [55]Copyright 2022, American Chemical Society.
The following reactions occur at the cathode The operating voltages were %0.75, %0.65, and %0.45 V for Fe, W, and Mo, respectively.The voltages were not sufficiently high to operate commercial electronic devices, and a stacked configuration of four Mg-Mo cells boosted the output voltage up to 1.5-1.6V at a constant current density of 0.1 mA cm À2 .The stacked Mg-Mo cells illuminated a conventional LED (threshold voltage = 1.6 V) and operated the radio circuit that generated a signal of %30 MHz.A signal analyzer connected by an antenna could capture the signal %2 cm away at À60 dBm.The battery was soaked in a PBS solution at 37 °C to investigate its transient behavior.The polyanhydride encasement degrades first to leave partially dissolved Mg and Mo foils after 11 d in PBS at 37 °C.Accelerating the dissolution by increasing the temperature to 85 °C eliminated the Mo foils after another 8 d (Figure 7b).
Although the Mg-X battery shows excellent biodegradability, it suffers from the hydrogen evolution reaction, affording a low operating voltage that hinders practical applications in transient power electronics.Jia et al. developed a transient air battery comprising a silk-IL composite, AZ31, and Au as a gel electrolyte, an anode, and a cathode, respectively. [25]The anodic reaction is the same as in Equation (8).The cathodic reaction is primarily The overall reaction is written as The open-circuit voltages were 1.58-1.45V after the cell was assembled.The cell capacity was 2.2 mAh cm À2 , with an operating voltage of %1.03 V (the middle point of the discharge curve) at a current density of 5 μA cm À2 .As for biodegradation, the overall encapsulated battery (170 μm thickness) nearly totally degraded after 45 d and was soaked in a buffered protease solution at 37 °C.The Au foil was physically fragmented in the solution because of the degradation of the silk substrate (Figure 7c).
A reaction at the cathode is crucial in increasing the operating voltage, and Huang et al. adopted the intercalation of metal ions into MoO 3 to achieve an output voltage of 1.6 V (Figure 7d). [49]he Mg-MoO 3 battery comprised Mg and Mo coated with MoO 3 and sodium alginate hydrogel with phosphates as an anode, cathode, and electrolyte.In addition to the chemical reaction in Equation ( 8), the following intercalation of multivalent (n þ ) metal ions (M nþ ) simultaneously occurs at the cathode.The Mg-MoO 3 battery yielded a voltage of up to 1.6 V, a prolonged operational lifetime (%1.5 V for 50 h and %0.6 V for 250 h, %13 d), and a high energy capacity (6.5 mWh cm À2 ).A degradation test on the battery in vitro revealed that most materials (Mg, alginate, and MoO 3 /PLGA) entirely dissolved within 9 d in the PBS solution, except Mo, which needed another 10 d to entirely disappear at an elevated temperature (85 °C).Mechanical characteristics, including flexibility and stretchability, are negligible for the practical batteries of wearable devices.Karami-Mosammam et al. fabricated sophisticated stretchable and biodegradable Mg-MoO 3 batteries incorporating kirigami electrodes and elastic encapsulation (Figure 7e). [38]The Mg and Mo electrodes with kirigami patterns on poly(glycerolsebacate) show large 300% and 400% strains, with <6% resistance increase, respectively.The stretchable cell with a strain of 20% shows a comparable discharge profile at 200 μA with an unstrained cell (Figure 7f ).Another kirigami pattern enabled the battery with a biaxial strain of 20% to deliver a current of 200 μA for 30 h.The degradation behavior of the completed cell was investigated by immersing the battery in the PBS solution (pH = 7.4).Most of the battery components dissolved in the PBS solution in week 10, and an accelerated degradation test at 85 °C from week 11 confirmed the complete dissolution of the battery and its residue in week 14.
Huang et al. developed a Mg-I battery that shows a high operating voltage ascribed to the following reaction at the cathode (Figure 7g). [6]2 þ 2e À ⇄ 2I À ð0.536 V vs SHEÞ (15)   The Mg-I battery comprises Mg foil, I 2 -carbon composite, IL, and the PBS solution as the anode, cathode, anolyte, and catholyte, respectively.I 2 has a higher standard electrode potential and enables a high operation voltage (%1.8 V) with excellent characteristics, including areal capacity (%9.8 mAh cm À2 ), areal energy density (%17.7 mWh cm À2 ), areal power density (%0.7 mW cm À2 ), volumetric energy density (%93.0 mWh cm À3 ), and volumetric power density (%3.8 mW cm À3 ).As shown in Figure 7h, degradation in the PBS solution at 37 °C occurs most rapidly via the hydrolysis of Mg (0.05-0.5 mm h À1 in physiological conditions) and polyanhydride (10 À2 μg day À1 in physiological conditions), followed by the dissolution of I 2 into ions and finally with comparatively slow hydrolysis of Mo (10 À4 -10 À3 μm h À1 at room temperature).
Although Mg involves low equilibrium potential and biodegradability, it frequently undergoes intense side reactions with aqueous electrolytes, limiting the operational lifetime, and causing extra hydrogen production.Zn represents an alternative Reproduced with permission. [59]Copyright 2014, Wiley-VCH.c) Photographs of the Mg-Au battery dissolution behavior in the PBS solution.Reproduced with permission. [25]Copyright 2017, American Chemical Society.d) The Mg-MoO 3 battery was operated under the PBS solution for 16 h.Reproduced with permission. [49]Copyright 2018, Wiley-VCH.Photographs of e) A stretchable Mg-MoO 3 with kirigami electrodes, f ) stretched and twisted.Reproduced with permission. [38]Copyright 2022, Wiley-VCH.g) The Mg-I 2 battery and h) its degradation in the PBS solution.Reproduced with permission. [6]opyright 2022, The Royal Society of Chemistry.i) Photographs of the Zn-Mo battery.j) Illumination of blue LEDs using four Zn-Mo batteries.Reproduced with permission. [60]Copyright 2023, American Chemical Society.
candidate anode material with suppressed adverse reactions and has a theoretical volume-specific capacity of up to 5822 mAh cm À3 , low equilibrium potential (À0.763 V vs a standard hydrogen electrode), and high hydrogen evolution potential.Huang et al. developed a Zn-Mo battery that showed complete biodegradation and long-term operation (Figure 7i). [60]The anode and cathode comprise chemically sintered Zn particles and a PLGA-Mo composite, respectively.The electrolyte is saline (0.9 wt% NaCl, pH = 7) or hydrogel (gelatin with 9 wt% NaCl).The primary reaction of the Zn anode is Zn ⇄ Zn 2þ þ 2e À ðÀ0.763V vs SHEÞ ( 16) The Zn-Mo battery shows an output voltage of %0.6 V and a power density of 6 μW cm À2 , with a discharge current density of 10 μA cm À2 .Four Zn-Mo batteries can illuminate blue LEDs (Figure 7i).The degradation process of a Zn-Mo battery in PBS at 37 °C was investigated.The electrodes gradually dissolve with time, and the PLGA substrate degrades through swelling, hydrolysis, and depolymerization.The noticeable disintegration of electrodes and substrate materials occurs around 25 d.The leftover traces of metallic electrodes were observed after 70 d, and the Zn-Mo batteries entirely disappeared after 85 d.

Secondary Battery
Primary batteries involve irreversible reactions at the anodes and cathodes, and their use is limited to one time.However, secondary batteries adopt reversible reactions for multiple charge and discharge cycles.Owing to their high energy-conversion efficiencies and superior energy densities, secondary batteries dominate the battery market and are used for energy storage, including electronic devices and electric vehicles.

Lithium-Ion Battery
Fu et al. developed transient lithium-ion batteries with components that dissolve upon immersion in an alkaline solution (Figure 8a,b). [61]The transient lithium-ion battery was packaged with a polycarbonate polymer and PVA to protect the battery package from corrosion and damage by moisture or water in the surrounding environment.Al electrodes are used as electrical conductors, and the cathodes and anodes comprise vanadium oxide (V 2 O 5 ), indium tin oxide, and LiAl alloy. 1 M LiPF 6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 vol%) was used as an electrolyte.When these components are integrated, the transient batteries show a high areal capacity of Reproduced with permission. [61]Copyright 2016, Wiley-VCH.d) A schematic and e) individual sodium-ion battery components.f ) Photographs of a sodium-ion battery on days 0 (left) and 120 (right) after the burial of the SIB in the plant soil.Reproduced with permission. [48]Copyright 2021, Wiley-VCH.g) A photograph of the zinc-ion battery and h) its galvanostatic charge-discharge profiles at 0.5 A g À1 i) Open-circuit voltage of the battery.Reproduced with permission. [50]Copyright 2022, Wiley-VCH.
%3 mAh cm À2 and a high working voltage above 2.0 V, comparable with conventional lithium-ion batteries.Upon immersion into 1.5 M KOH solution, the dissolution behavior of the full transient cell was similar to the dissolution of each component in KOH (Figure 8c).The packaging swelled fast and dissolved instantly upon meeting the KOH solution.Then, the Al and electrodes dissolved, generating numerous microbubbles.After 3 min, the polymers almost disappeared, leaving only remnants of Al and cathodes.At 5 min, the cell entirely dissolved in the KOH solution, leaving no trace of the material discernible to the naked eye.

Sodium-Ion Battery
While lithium-ion batteries show excellent performances, they need rare metals, including lithium, cobalt, and nickel, for anode or cathode materials.The sodium-ion batteries adopt carbon and Na ions that are abundant in the earth, which are favorable for transient energy storage for sustainable applications.Lee et al. developed a biodegradable sodium-ion battery with a cathode, anode, binder, separator, electrolyte, and packaging materials that were carefully designed and fabricated (Figure 8d). [48]a 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) (NFP) and pyroprotein-based carbon with binders of cellulose derivatives were selected to fabricate the biodegradable composite electrode (cellulose acetate for the cathode and carboxymethyl cellulose for the anode, respectively) and were coated on an Al current collector.Sodium perchlorate (NaClO 4 ; 1 M) in a propylene carbonate solution was selected as a biodegradable electrolyte (Figure 8e).The pouch-type biodegradable cell exhibited a discharge voltage plateau near %3 V.For the single-layered pouch cell, the reversible areal charge/discharge capacity of %0.12 mAh cm À1 could be maintained over 50 cycles; the capacity of the overall system could be increased using multiple pouch cells.The degraded sodium-ion battery did not adversely affect the growth of microorganisms, such as Basidiomycota and Ascomycota.The growth of larger organisms, such as plants (peperomia), was also unaffected (Figure 8f ).

Zinc-Ion Battery
Zinc-ion batteries with multivalent ions (Zn 2þ ) are a promising alternative to developing sustainable secondary batteries owing to their benignity and abundance in the earth and promise high gravimetric and volumetric energy densities for monovalent ions (Li and Na). [50]Mittal et al. reported a transient zinc-ion battery comprising a Zn foil, polydopamine-carbon composite, agarose-carboxymethyl cellulose hydrogel, and agarose film as anode, cathode, electrolyte, and encapsulation, respectively (Figure 8g). [62]The completed zinc-ion battery showed a capacity of 264 mAh g À1 in the first cycle and an open-circuit voltage of 1.123 V (Figure 8h,i).The battery maintained a discharge capacity of 110 mAh g À1 after 10 000 cycles at a current density of 1 A g À1 , demonstrating remarkable stability for a ZIB of transient components.A study of the degradation behavior was performed on different battery components, including the anode, hydrogel electrolyte, cathode, and packaging, over a composting period of 9 weeks.The fragmentation processes of the hydrogel and agarose packaging started at week 2 owing to water absorption and hydrolytic degradation reactions.The pouch cell continuously disintegrated at a rate of 0.89 wt% per week and lost 49.9 AE 2.9 wt% of its mass after 63 d.

Conclusion and Outlook
We reviewed the recent progress in transient energy storage, highlighting its materials, design, and performance.The material section described biodegradable conductors, electrolytes, and gels.Supercapacitors, pseudocapacitors, primary batteries, and secondary batteries were reviewed, focusing on transient materials, designs, and performance in energy storage.Although transient energy storage involves excellent biodegradation without harmful substances during decomposition, their performance must be comparable to commercially available devices for practical applications; therefore, continuous research and new insight into materials and designs are necessary.Emerging 2D materials, MXenes, [63] involve large surface area, metallic conductivity, and large density [64] ; further, their papers made of multilayer exfoliated MXenes can accommodate small ions [65] (Li, Na, K, Mg, and Al) in their interlayers to exhibit intercalation pseudocapacitive behaviors with high cycle stability over 10 000.
Transient supercapacitors and pseudocapacitors with MXenes are promising candidates for energy storage with large capacitance and high cycle stability.The transient secondary batteries suffer from low coulomb efficiency and small capacity due to selfdischarge at the anode.Recent works on secondary batteries revealed that coating (2,2,6,6,-tetramethylpiperidinyl-1-oxyl; [66] polydopamine; [67] and Fe 3 O 4 ) [68] on anode efficiently suppressed the self-discharging.Such coating with biodegradable materials enables the transient secondary batteries with high coulomb efficiency and large capacity.The disappearance of transient energy storage after a prescribed time is a critical characteristic for environmental sensing; nonetheless, their cycle stability is inevitable for practical applications.Table 4 summarizes the cycle stability of commercial and recent works on transient energy storage; the transient energy storage exhibited poor cycle stability by a few orders of magnitude smaller than those of commercial ones.High cycle stability that is comparable with commercial ones is a challenge for transient energy storage.In addition to the battery performance, further integration with electrical circuits is beneficial for compensating for their low capacities and reducing the power consumption of logic, radio frequency circuits, and sensors.The intermittent operation of sensor modules is promising to achieve a long lifetime of energy storage using timers or event-driven triggers using battery-less sensors [69] to wake up modules in the sleep mode.For rechargeable energy storage, including supercapacitors, pseudocapacitors, and secondary batteries, energy harvesting [70] is an option to expand their capacity.
Owing to the increase in the output power of energy harvesters, they can retrieve energy from energy sources that have never been used, for example, interior illumination, [71] vibration infrastructures, [72] and geothermal heat. [73]The development of energy harvesters is expected, and their materials, design, and performance should be considered for transient behavior.
With the development of electronics, the issue of electronic device waste is inevitable.Transient technology can address it, enabling disposable devices that degrade to benign materials in environments.Owing to their environmental benignity, such modules can be installed where protection from toxic substances is urgent, such as agriculture, animal husbandry, and marine products.Finally, fully transient modules integrated with energy storage, sensors, and transistors are crucial in environmental sensing to shed light on global challenges.80% capacity, equivalent full cycle.

Figure 1 .
Figure 1.Transient energy storage using biodegradable materials, individual components, and a schematic of the circulation of transient devices.Reproduced with permission.[55]Copyright 2022, American Chemical Society.

Figure 2 .
Figure 2. The transient behavior of the Mo electrode in deionized water (DIW).a-d) Optical and e-h) SEM images with cross-sectional views in the insets.Reproduced with permission. [8a] Copyright 2013, Wiley-VCH.

Figure 6 .
Figure 6.Transient pseudocapacitors.a) A schematic of a pseudocapacitor with anodized Mo serpentine electrodes and b) photographs of the transient behavior of individual components in deionized water (DIW).Reproduced with permission. [19a] Copyright 2023, Elsevier.c) A schematic of MoO 3 pseudocapacitors with ionic liquid (IL) and d) intercalation of Na ions into MoO 3 crystals.e) Dissolution test of MoO 3 pseudocapacitor soaked in PBS solution.Reproduced with permission. [19c] Copyright 2023, Wiley-VCH.

Figure 7 .
Figure 7. Transient primary battery.a) A photograph of the Mg-Mo battery and b) its dissolution behavior into the PBS solution.Reproduced with permission.[59]Copyright 2014, Wiley-VCH.c) Photographs of the Mg-Au battery dissolution behavior in the PBS solution.Reproduced with permission.[25]Copyright 2017, American Chemical Society.d) The Mg-MoO 3 battery was operated under the PBS solution for 16 h.Reproduced with permission.[49]Copyright 2018, Wiley-VCH.Photographs of e) A stretchable Mg-MoO 3 with kirigami electrodes, f ) stretched and twisted.Reproduced with permission.[38]Copyright 2022, Wiley-VCH.g) The Mg-I 2 battery and h) its degradation in the PBS solution.Reproduced with permission.[6]Copyright 2022, The Royal Society of Chemistry.i) Photographs of the Zn-Mo battery.j) Illumination of blue LEDs using four Zn-Mo batteries.Reproduced with permission.[60]Copyright 2023, American Chemical Society.

Figure 8 .
Figure 8. Transient secondary battery.a) A schematic and b) photograph of a lithium-ion battery and c) its dissolution into the alkaline solution.Reproduced with permission.[61]Copyright 2016, Wiley-VCH.d) A schematic and e) individual sodium-ion battery components.f ) Photographs of a sodium-ion battery on days 0 (left) and 120 (right) after the burial of the SIB in the plant soil.Reproduced with permission.[48]Copyright 2021, Wiley-VCH.g) A photograph of the zinc-ion battery and h) its galvanostatic charge-discharge profiles at 0.5 A g À1 i) Open-circuit voltage of the battery.Reproduced with permission.[50]Copyright 2022, Wiley-VCH.

Table 3 .
Summary of transient energy storage.

Table 4 .
Comparison of cycle stability in recent studies with commercial energy storage. a)