Autonomous Chemistry Enabling Environment-Adaptive Electrochemical Energy Storage Devices

Open AccessCCS ChemistryMINI REVIEWS7 Jul 2022Autonomous Chemistry Enabling Environment-Adaptive Electrochemical Energy Storage Devices Zhisheng Lv, Wenlong Li, Jiaqi Wei, Fanny Ho, Jie Cao and Xiaodong Chen Zhisheng Lv Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 138634 Singapore , Wenlong Li Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 138634 Singapore , Jiaqi Wei Innovative Centre for Flexible Devices (iFLEX), Max Planck—NTU Joint Lab for Artificial Senses, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore , Fanny Ho Innovative Centre for Flexible Devices (iFLEX), Max Planck—NTU Joint Lab for Artificial Senses, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore , Jie Cao Innovative Centre for Flexible Devices (iFLEX), Max Planck—NTU Joint Lab for Artificial Senses, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore and Xiaodong Chen *Corresponding author: E-mail Address: email protected Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 138634 Singapore Innovative Centre for Flexible Devices (iFLEX), Max Planck—NTU Joint Lab for Artificial Senses, School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore https://doi.org/10.31635/ccschem.022.202202153 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Next-generation electronics that are fused into the human body can play a key role in future intelligent communication, smart healthcare, and human enhancement applications. As a promising energy supply component for smart biointegrated electronics, environment-adaptive electrochemical energy storage (EES) devices with complementary adaptability and functions have garnered huge interest in the past decade. Owing to the advancements in autonomous chemistry, which regulate the constitutional dynamic networks in materials, EES devices have witnessed higher freedom of autonomous adaptability in terms of mechano-adaptable, biocompatibility, and stimuli-response properties for biointegrated and smart applications. In this mini-review, we summarize the recent progress in emerging environment-adaptive EES devices enabled by the constitutional dynamic network of mechanical adaptable materials, biocompatible materials, and stimuli-responsive supramolecular polymer materials. Finally, the challenges and perspectives of autonomous chemistry on the environment-adaptive EES devices are discussed. Download figure Download PowerPoint Introduction Advancements made in the merging of human consciousness and machine capabilities have driven the emergence of smart electronics.1–6 The fusion of electronics into human beings to form biointegrated systems has motivated the development of electronics with a higher degree of adaptability in ever-changing environments. To intimately integrate electronics with the human body, electronics are becoming more flexible and stretchable to accommodate the arbitrary shapes of the human body and mechanical deformation during human motion. Alongside mechanical adaptability, biointegrated electronics are also required to be biocompatible so as to operate in physiological environments, such as near-body, on-skin, and in vivo environments. Moreover, by combining with other smart functions, such as biometric sensing and stimuli-responsive actuating, smart biointegrated electronics can allow humans to share their senses and engage in collaborative work with machines to extend the limitation of the human body and machines. Progress in such adaptability of electronics and emerging symbiosis between humans and machines has unlocked exciting opportunities in fabricating biointegrated electronics for smart healthcare and human enhancement applications. To seamlessly power the emerging environment-adaptative electronics, in the last decade, the indispensable electrochemical energy storage (EES) devices, mainly supercapacitors and batteries, have witnessed complementary changes in mechanical adaptability, biocompatibility, and smart functionalities for biointegrated applications.7–9 The environment-adaptive transformations in the EES devices would not be realized without the participation of autonomous chemistry. Autonomous chemistry is an adaptive and self-evolved chemical system that manipulates the covalent and noncovalent constitutional dynamic networks to autonomously adapt and respond to internal and external stimuli, such as light, temperature, pH, and biological enzymes (Figure 1). The molecular chemistry governing the dynamic covalent bonds (Figure 1a) and supramolecular chemistry implementing the intermolecular interactions (Figure 1b) create building blocks of constitutional dynamic chemistry networks.10–14 These dynamic chemistry networks extend and combine with molecules of various sizes to allow for variation and adaptation through autonomous component selection and response to internal and external environmental stimuli (Figure 1c). The introduction of autonomous chemistry into materials science opens up new perspectives on self-adaptive materials. These autonomously adaptive materials integrated with device technologies provide great opportunities for the development of environment-adaptive EES devices. Figure 1 | The emergence of autonomous chemistry toward adaptive and self-evolved chemical systems. (a) Schematic representation of the dynamic reversible and irreversible molecular interactions for the formation of the crystalline and amorphous covalent organic framework. Adapted with permission from ref 14. Copyright 2019 American Chemical Society. (b) Schematic representation of the noncovalent constitutional dynamic network for supramolecular interactions. (c) Autonomous chemistry with covalent and noncovalent constitutional dynamic networks to construct adaptive and self-evolved chemical systems. Download figure Download PowerPoint Autonomous chemistry allows for regulating the functions of environment-adaptive EES devices. By using autonomously adaptive materials, autonomously operating chemical systems in devices can adapt and respond to environmental changes (Figure 2a–f, h). Since the first transformation from the original rigid prototype into flexible devices in 2007, environment-adaptive EES devices have ushered in a new era.7,15 With enhanced flexibility of electrode materials via flexible and stretchable molecular and supramolecular design, intrinsically flexible and stretchable electrodes can endow the mechano-adaptable EES devices with adaptability to arbitrary shapes of the human body.16–22 Accompanying the enhanced mechanical adaptability, the molecular designs of biodegradable and bioresorbable materials enable EES devices to be autonomously biocompatible with the human body for skin-mounted, ingestible, and implantable applications.23–26 Furthermore, the incorporation of stimuli-responsive alloys and supramolecular polymers has spurred EES devices towards smart transformations (e.g., self-healing, shape memory, thermal protection, and electrochromism), providing environment-adaptive EES devices with self-responsive and self-protective functions under external mechanical, thermal, and/or electrical stimuli.27–30 The design and synthesis of autonomous molecular and supramolecular networks with autonomous operation in mechano-adaptable materials, biocompatible materials, and stimuli-responsive supramolecular polymer materials have enabled the development of EES devices with programmable environment-adaptive functions, offering new opportunities to power the next-generation smart and biointegrated electronics.31,32 Figure 2 | The transformation of traditional EES devices into environment-adaptive EES devices enabled by autonomous chemistry. Autonomous chemistry modulates the constitutional dynamic (a) rigid and elastic networks, (b) biological regulatory networks, and (c) the stimulus-response networks for environment-adaptive EES with (d–f) desired functionalities. (a) Chemical structure and illustration of stretchable and adhesive water-dispersible PUs with both soft and hard segments. Reproduced with permission from ref 22. Copyright 2021 Wiley-VCH. (b) Colorimetric detection of glucose using cascade enzyme reaction systems with glucose oxidase and horseradish peroxidase. Adaptable with permission from ref 26. Copyright 2019 Springer Nature Limited. (c) Stimuli-responsive polymers designed with different kinds of topological structures and morphologies. (d) The representative mechanical deformations. (e) The biofriendly interfaces with the human body that require biocompatible properties. (f) The stimulus-responsive smart functions. (g) Schematic device structure of conventional EES devices. (h) The schematic device structure of the environment-adaptive EES devices. Download figure Download PowerPoint Herein, we review recent important advances in environment-adaptive EES devices that are enabled by autonomous chemistry. Specifically, electrodes, electrolytes, and devices associated with constitutional dynamic networks of mechano-adaptable materials, biocompatible materials, and stimuli-responsive supramolecular polymers will be discussed. The emerging mechano-adaptable, biocompatible, and smart EES devices for environment-adaptive applications will also be emphasized. Finally, this review concludes with an outlook on the challenges and opportunities in this fast-developing field. Constitutional Dynamic Networks for Mechano-Adaptable EES Devices The basic mechanical deformations of EES devices involve bending and stretching to cover the curved surface of human skin and soft tissues. Conventional EES devices with packaging consist of two electrodes separated by an electrically insulating separator immersed in a liquid electrolyte (Figure 2g). However, the maximum elastic strains these components in conventional EES devices, including electrodes, separators, and packaging, are typically less than 1%, which cannot match the stretchability (>50%) required for biointegrated applications.33,34 Clearly, improving the tensile strains of these components is essential to designing mechano-adaptable EES devices. To make the components in EES devices stretchable, two strategies have been exploited, including directly replacing inelastic materials with elastic alternatives and constructing structural nanomaterial networks for stretchable electrodes. The inextensible components—electrolytes, separators, and packaging—can be directly replaced by their stretchable alternatives. For example, silicone rubbers such as polydimethylsiloxane and Ecoflex are frequently adopted as stretchable packaging materials, and gel electrolytes with stretchable molecular design can serve as both electrolytes and stretchable separators to eliminate the necessity of poorly elastic separators and possible leakage of liquid electrolyte. As for electrodes, the available inelastic materials can be reinforced with conductive nanomaterials (e.g., graphene, carbon nanotubes (CNTs), and conductive polymers) that have higher tensile strains. However, the tensile strain of composite electrodes is still less than 10%.35,36 In this regard, directly replacing electrode materials with elastic alternatives is not enough. As a complementary method, a stretchable structural design has been developed to improve the stretchability of the electrode. Based on thin and porous nanomaterials, more bendable electrodes cooperate with elastic polymer substrates and proper electrode layouts, such as wave-like wrinkled structures, coiled and braided structures, and kirigami structures, which are highly stretchable and can accommodate tensile strain greater than 100%.16,20,37–42 Instead of using structural designs, the molecular design of intrinsically stretchable polymers is an effective way to realize flexible and stretchable electrodes and electrolytes for mechano-adaptable EES devices. Common conjugated polymers, including polypyrrole (PPy), polyaniline (PANI), and poly(3,4-ethylenedioxythiophene) (PEDOT), when doped into a conducting state, are promising pseudocapacitive materials for flexible supercapacitors.21,43,44 However, the doped conductive polymers usually possess a close-packed and laminated structure that is not capable of releasing mechanical stresses induced by ion exchange in the charging/discharging process and mechanical deformation. This leads to the deterioration of both electrochemical and mechanical performances of as-fabricated devices during mechanical deformation.21 To address this issue, partially doping anion into a conductive polymer matrix has been applied to prevent the overstacking of polymer chains. Taking the electropolymerization of PPy as an example, a porous PPy/black phosphorus oxide (BPO) composite electrode was prepared through a two-step electropolymerization method by utilizing partially oxidized black phosphorus (BP) as dopant.21 The BPO surface was partially doped into the PPy matrix, while the unoxidized BP surface helped to form micropores during the deposition of PPy/BPO composites on the CNT films. As a comparison, the doped PPy composite electrode showed densely packed structures when the anion dopants (SO42− and graphene oxides (GO)) with fully covered oxygen functional groups were employed for the electropolymerization process. Owing to the porous structures to buffer the strain under deformation, the as-prepared stretchable supercapacitors with the porous PPy/BPO-CNT electrode exhibited 97% capacitance retention when being stretched to 2400%, which is superior to that of dense stacking PPy/SO42− and PPy/GO electrode-enabled stretchable supercapacitors (78% and 87% respectively). Besides tuning the dopant in conductive polymers, the additional template and solvent precursors also affect the polymerization process, thus affecting the resultant morphology and stretchability of the conductive polymer-based electrodes. With the template of ice crystals, anisotropic polyvinyl alcohol (PVA)/PANI hybrid hydrogels were synthesized through a cryopolymerization strategy (Figure 3a).43 During the freezing process, the 3D-ordered honeycomb structure of PVA was formed along the growing direction of vertically aligned ice crystals. Followed by cryopolymerization, the polymerization of PANI nanofiber was confined within the boundaries between PVA cell walls and ice crystals. The honeycomb structures with the PVA and PANI interpenetrating networks enabled isotropic PVA/PANI hybrid hydrogels with maximum elongation up to 416%. The as-fabricated supercapacitors delivered 85% capacitance under 200% tensile strain, demonstrating an exciting avenue to synthesize conductive polymer hybrid hydrogels as intrinsically stretchable electrodes for mechano-adaptable EES devices. Additionally, adding dimethyl sulfoxide (DMSO) into an aqueous PEDOT:polystyrene sulfonate (PSS) solution for dry annealing and rehydration processes can lead to a well-controlled phase separation to form interconnected networks of PEDOT:PSS nanofibrils in the as-prepared pure PEDOT:PSS hydrogels. In contrast, fragmented PEDOT:PSS microgel was obtained from the PEDOT:PSS aqueous solution without DMSO due to the separation between the soft PSS-rich domain and the rigid PEDOT-rich domain under the same drying and swelling process.44 The pure PEDOT:PSS hydrogels (20 vol % DMSO in the preparation) can be used as intrinsically stretchable electrodes for supercapacitors and reach a maximum stretchability up to 35% in phosphate-buffered saline (PBS) solution, which closely matches the stretchability of biological tissues (∼20% for neural tissues and ∼50% for skin).35 Figure 3 | Molecular design of flexible and stretchable polymers for mechano-adaptable EES devices. (a) The fabrication of the anisotropic hybrid porous PVA/PANI hydrogels for stretchable supercapacitors with diverse shapes under stretching and compression. Reproduced with permission from ref 43. Copyright 2019 Springer Nature Limited. (b) The synthesis process of VSNPs-PAM electrolytes for stretchable and compressible supercapacitors by cross-linking the VSNPs from vinyltriethoxysilane with acrylamide monomers at the presence of the ammonium persulfate initiator and phosphoric acid. Reproduced with permission from ref 45. Copyright 2017 Wiley-VCH. (c) Illustration of the VSNPs cross-linking PAM network for superstretchability and high compressibility. Reproduced with permission from ref 45. Copyright 2017 Wiley-VCH. (d) The chemical structure of the lithium-ion conductor (LIC) and diagram showing the LIC polymer electrolyte upon stretching. The orange squares represent hydrogen-bonding UPy moieties, black wires are poly(propylene glycol)-pol(ethylene glycol)-poly(propylene glycol) (PPG-PEG-PPG) chains, and the blue circles are lithium ions. Reproduced with permission from ref 46. Copyright 2019 Springer Nature Limited. (e) The application of the supramolecular LIC for stretchable LIBs. Reproduced with permission from ref 46. Copyright 2019 Springer Nature Limited. Download figure Download PowerPoint Apart from the stretchable electrodes, the molecular design of supramolecular dynamic polymers with both mechanically robust and ionically conductive segments is also used to fabricate intrinsically stretchable electrolytes for mechano-adaptable EES devices.45 Pure polyacrylamide (PAM) hydrogels with weak hydrogen bonds are hard to make ultrastretchable, which limits their applications as stretchable electrolytes for stretchable supercapacitors. To enhance the toughness and stretchability of the PAM hydrogel, the hydrogel skeleton was reinforced by the strong covalent bonding between the PAM chains and vinyl hybrid silica nanoparticles (VSNPs) (Figure 3b).45 The VSNPs-PAM hydrogels with ionically conductive PAM polyelectrolyte matrix and stress-buffering of VSNP cross-linkers enabled stretchable supercapacitors to possess intrinsic superstretchability (up to 1000% strain) and compressibility (up to 50% strain) without degradation of their initial capacitance (Figure 3c).45 Similarly, the crosslinking of hairy nanoparticles helped strengthen the stretchable hydrogel electrolytes. The stretchable supramolecular lithium-ion conductor with mechanically reinforced hydrogen bonds was designed to serve as the polymer electrolyte for stretchable lithium-ion batteries (LIBs) (Figure 3d).46 To decouple mechanical robustness from ionic conductivity in low-Tg (the glass transition temperature) polymer electrolytes, a copolymer was created by introducing a dynamic bonded ureido-pyrimidinone (UPy) backbone into low-Tg polyether backbone, wherein the low-Tg polyether backbone provided the polymer electrolyte with high ionic conductivity (1.2 ± 0.21 × 10−4 S cm−1), and the Upy group enhanced the mechanical toughness of the polymer electrolyte (29.3 ± 0.21 × 10−4 MJ m−3, three times higher than reported polymer electrolytes) by dynamic hydrogen bonds. The as-prepared polymer electrolyte-based stretchable LIBs, with a capacity density of 1.1 mAh cm−2 function well to power light-emitting diodes even when stretched up to 70%, suggesting the promising application of tough ion-conducting polymers for conformable EES devices (Figure 3e). Constitutional Dynamic Networks for Biocompatible EES Devices The boom in environment-adaptive electronics for real-time in vivo health monitoring and diagnosis has stimulated the development of EES devices with not only mechano-adaptability but also biocompatibility, so as to adapt to biological environments and intimately integrate onto/into essential organs of the human body.47 However, developing biocompatible EES devices that avoid the likelihood of infection remains a great challenge, especially in the exploration of nontoxic and biodegradable devices. To safely use biocompatible EES devices for in vivo applications, the original corrosive and toxic materials in EES devices should be replaced with nontoxic and biodegradable alternatives that can be autonomously dissolved and resorbed or disposed of by the body through biochemical processes like metabolization and bioabsorption. Such biodegradable materials include inorganic materials and organic polymers. Inorganic metals (e.g., Mg, Mo, and Li) that can react and dissolve in aqueous solutions are suitable materials to fabricate biodegradable electrodes for EES devices. Likewise, biodegradable polymers that can undergo chemical or enzymatical hydrolysis and/or oxidation are suitable as well.48–52 A representative biodegradable battery system is shown in Figure 4a, wherein the primary Mg–Mo battery is packaged with polyanhydride materials to provide a constant current density of 0.1 mA cm−2 at a voltage of 1.6 V for around 6 h.49 Because all the constituent materials in the battery are water-soluble, the Mg–Mo battery is fully degradable after 11 days in PBS at 37 °C followed by another 8 days in PBS at 85 °C. The biodegradation of polyanhydride in the Mg–Mo battery stems from the fact that its ester bonds are susceptible to hydrolysis. Other hydrolytically degradable moieties as shown in Figure 4b, like the amide, thioester, and imine, can serve as the synthetic polymer backbone for eco-friendly degradation.48 Complementary to hydrolysis, oxidation is another way to biologically degrade polymers. Polymers designed with oxidizable moieties, such as ethers, alcohols, and phenols, are susceptible to oxidative cleavage (Figure 4b).53 Currently, previous reports about the degradation of biocompatible EES devices are mainly limited to hydrolysis of electrode materials in physiological conditions, and the mechanism for the oxidation of polymers in biodegradable EES devices has yet to be thoroughly explored. Figure 4 | Biodegradable and biocompatible materials for biocompatible EES devices. (a) The dissolution of the biodegradable Mg–Mo battery with degradable inorganic metallic electrodes and an organic polyanhydride spacer. Reproduced with permission from ref 49. Copyright 2014 Wiley-VCH. (b) The chemical structures of moieties tend to be hydrolyzed and oxidized, and the red marks indicate the hydrolyzation and oxidation sites. Images adapted with permission from ref 48. Copyright 2018 American Chemical Society. (c) The biophilized graphene oxide (bGO)-Mb electrode for biocompatible supercapacitors. Upper: the synthesis of bGO with the negative charge by absorbing cationized bovine serum albumin (cBSA) ion of cBSA onto GO sheet. Down: dose-dependent toxicity in (COS-7) and MEF cells coincubated with GO and bGO/Mb as measured by the intracellular dehydrogenases activity. Reproduced with permission from ref 56. Copyright 2017 Wiley-VCH. (d) An implantable NAD/BQ/CNT yarn supercapacitor. Top: Fabrication scheme of NAD/BQ/CNT yarn electrode and the reversible redox reaction of NAD with the assistance of BQs (oxidized and reduced forms of NAD abbreviated as NAD+ and reduced nicotinamide adenine dinucleotide NADH, respectively). Down: Implantation of the yarn supercapacitors into the abdominal cavity of a mouse and the capacitance retention of the implantable supercapacitors on the day of surgery, 3 and 14 days after implantation. Reproduced with permission from ref 58. Copyright 2021 Wiley-VCH. Download figure Download PowerPoint As the EES devices become incorporated into the digestive system, biocompatible EES devices that can be ingested by individuals also emerge, with emphasis on biodegradable, bioresorbable, and noncytotoxic characteristics of the devices. Conventional active materials, such as MnO2, which is a constituent material of supercapacitors, pose a threat to the human body (causing abdominal pain and nausea) when ingested. Moreover, the toxicity of many nanomaterials is still unknown.24,54 In this case, naturally derived materials (e.g., biochar, cellulose, silk, and collagen) provide alternatives to fabricate biocompatible and edible devices due to their intrinsic nontoxicity and enzymatic degradability.48,55 Edible supercapacitors with naturally-derived food materials, including active charcoal as the electrode material, egg whites as the edible and as the current can be in to power a red and dissolved in the This that naturally derived materials are promising nontoxic materials to biocompatible EES As for the implantable EES devices, of the to degradable devices has only degradation in or stimulated such as PBS that the of in vivo degradation is different from that of in degradation due to the between electrode materials and biological components (e.g., and To improve the properties and the for implantable applications, the used active materials for supercapacitors, such as CNT and have been with functional and conductive A biocompatible with aligned by oxygen was to operate in biological such as and The has that the synthesized with oxygen including and and for cell and than biophilized reduced graphene oxide as the electrode for nontoxic supercapacitors (Figure toxicity of materials (up to high of to mouse and cell was in the biocompatible The maximum nontoxic was times higher than that of GO electrodes. Besides surface and the conductive composites also for implantation. A of CNT electrodes, was into the abdominal cavity of a infection was during the implantation. Moreover, the implantable capacitance retention after 8 days of implantation. In energy the redox like the nicotinamide adenine dinucleotide to the energy storage by this redox system, an implantable CNT yarn was by CNT electrodes with NAD and Figure The biocompatible NAD/BQ/CNT yarn electrodes into the abdominal cavity of a exhibited the in vivo electrical of a even after 14 days (Figure Constitutional Dynamic Networks for EES Devices With the of electronics into soft and smart EES devices with and to respond to external stimuli have as a new of biointegrated devices. In kinds of smart EES devices have been including self-healing, thermal and EES devices. These emerging smart EES devices cannot be realized