The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

Flexible solid-state supercapacitors (FSSCs) are frontrunners in energy storage device technology and have attracted extensive attention owing to recent significant breakthroughs in modern wearable electronics In this study, we review the state-of-the-art advancements in FSSCs to provide new insights on mechanisms, emerging electrode materials, flexible gel electrolytes and novel cell designs The review begins with a brief introduction on the fundamental understanding of charge storage mechanisms based on the structural properties of electrode materials The next sections briefly summarise the latest progress in flexible electrodes (ie, freestanding and substrate-supported, including textile, paper, metal foil/wire and polymer-based substrates) and flexible gel electrolytes (ie, aqueous, organic, ionic liquids and redox-active gels) Subsequently, a comprehensive summary of FSSC cell designs introduces some emerging electrode materials, including MXenes, metal nitrides, metal–organic frameworks (MOFs), polyoxometalates (POMs) and black phosphorus Some potential practical applications, such as the development of piezoelectric, photo-, shape-memory, self-healing, electrochromic and integrated sensor-supercapacitors are also discussed The final section highlights current challenges and future perspectives on research in this thriving field

Towards flexible solid-state supercapacitors for smart and wearable electronics

Design and fabrication of electrochemical energy storage systems with both high energy and power densities as well as long cycling life is of great importance. As one of these systems, Battery-supercapacitor hybrid device (BSH) is typically constructed with a high-capacity battery-type electrode and a high-rate capacitive electrode, which has attracted enormous attention due to its potential applications in future electric vehicles, smart electric grids, and even miniaturized electronic/optoelectronic devices, etc. With proper design, BSH will provide unique advantages such as high performance, cheapness, safety, and environmental friendliness. This review first addresses the fundamental scientific principle, structure, and possible classification of BSHs, and then reviews the recent advances on various existing and emerging BSHs such as Li-/Na-ion BSHs, acidic/alkaline BSHs, BSH with redox electrolytes, and BSH with pseudocapacitive electrode, with the focus on materials and electrochemical performances. Furthermore, recent progresses in BSH devices with specific functionalities of flexibility and transparency, etc. will be highlighted. Finally, the future developing trends and directions as well as the challenges will also be discussed; especially, two conceptual BSHs with aqueous high voltage window and integrated 3D electrode/electrolyte architecture will be proposed.

https://www.onlinelibrary.wiley.com/doi/epdf/10.1002/advs.201600539

Battery-Supercapacitor Hybrid Devices: Recent Progress and Future Prospects.

Increasing power and energy demands for next-generation portable and flexible electronics such as roll-up displays, photovoltaic cells, and wearable devices have stimulated intensive efforts to explore flexible, lightweight and environmentally friendly energy storage devices. Flexible solid-state supercapacitors (SCs) have attracted increasing interest because they can provide substantially higher specific/volumetric energy density compared to conventional capacitors. Additionally, flexible solid-state SCs are typically small in size, highly reliable, light-weight, easy to handle, and have a wide range of operation temperatures. In this regard, solid-state SCs hold great promise as new energy storage devices for flexible and wearable electronics. In this article, we review recent achievements in the design, fabrication and characterization of flexible solid-state SCs. Moreover, we also discuss the current challenges and future opportunities for the development of high-performance flexible solid-state SCs.

Flexible solid-state supercapacitors: design, fabrication and applications

Naturally derived cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) are emerging nanomaterials that display high strength, high surface area, and tunable surface chemistry, allowing for controlled interactions with polymers, nanoparticles, small molecules, and biological materials. Industrial production of nanocelluloses is increasing rapidly with several companies already producing on the tons-per-day scale, intensifying the quest for viable products across many sectors. While the hydrophilicity of the nanocellulose interface has posed a challenge to the use of CNCs and CNFs as reinforcing agents in conventional plastics, it is a significant benefit for creating reinforced or structured hydrogel composites (or, when dried, aerogels) exhibiting both mechanical reinforcement and a host of other desirable properties. In this context, this Review describes the quickly growing field of hydrogels and aerogels incorporating nanocelluloses; over 200 references are summarized in comprehensive tables ...

http://pubs.acs.org/doi/pdf/10.1021/acs.chemmater.7b00531

Review of Hydrogels and Aerogels Containing Nanocellulose

Efficient dual conductive network based on layered double hydroxide nanospheres and nanosheets anchored in N-carbon nanofibers for asymmetric supercapacitors

Amorphous Nanosheets Constructed Nickel Cobalt Hydroxysulfide Hollow Spheres as Cathode Materials for Hybrid Supercapacitors

Hydrogels are regarded as excellent substrates for flexible materials owing to their porous structure, softness, and flexibility. However, the requirement of enhancement and functionalization for a hydrogel is still an important issue. Cellulose nanomaterials present promising reinforcing potentials for hydrogels thanks to their high strength, good compatibility, and environmental friendliness. Inspired by the composition of the dermis, we use carboxymethyl cellulose nanofibrils (CMCNFs) and NaCl as additives to participate in the crosslinking polymerization of acrylamide (AM), fabricating PAM/CMCNFs/NaCl composite hydrogels with excellent mechanical properties, good resilience, and strain-sensitive conductivity. The interactions between CMCNFs and PAM chains contribute to mechanical improvement of composite hydrogels; 0.5 M NaCl in precursor solutions imparts further synergistic reinforcement and ionic conductivity. The compressive stress, tensile strength and elongation at break values of PAM-based hydrogels are increased with the incorporation of CMCNFs, and the optimal CMCNFs dosage is found at 4 wt% of AM. The resilience of composite hydrogels can be maintained at approximately or over 90% after five compression or stretching cycles. Moreover, the strain-sensitive conductivity of a composite hydrogel demonstrates their potential application in flexible sensors. In brief, this work provides a facile one-step approach to fabricate composite hydrogels suitable for application as flexible sensors. 

Bioinspired cellulose nanofibrils and
 NaCl
 composited polyacrylamide hydrogels with improved toughness, resilience, and strain‐sensitive conductivity

By non-solvent-induced phase separation, nitrate glycerol ether cellulose (NGEC) gels were formed in ternary NGEC/acetone/ethanol system. The rheological behaviors of NGEC gels were investigated using dynamic rheological measurements. The final compositions and morphologies of NGEC gels were influenced by the initial solvent/non-solvent (acetone/ethanol) ratios and NGEC concentrations. In addition, the effect of initial acetone/ethanol ratios on the characteristics of NGEC gels is different from the effect of NGEC concentrations. The critical strain of NGEC gels decreased almost with increasing NGEC concentration of gels by increasing initial acetone ratio, but it increased with increasing NGEC concentration of gels by increasing NGEC concentration beyond a certain concentration in 2/3 (w/w) acetone/ethanol solution. For all gels, the storage modulus (G′) and loss modulus (G′′) of NGEC gels rapidly increased with increasing NGEC concentration of gels. In addition, the curves for G′ and G′′ were temperature sensitive throughout the entire temperature sweep, which implied that the interactions between NGEC/solvent could be disrupted upon heating to higher temperatures.

Rheological characteristics of nitrate glycerol ether cellulose gel based on phase separation in ternary system

Interfacial interaction enhancement between biodegradable poly (butylene adipate-co-terephthalate) and microcrystalline cellulose based on covalent bond for improving puncture, tearing, and enzymatic degradation properties

Ziqiang Shao

Papers

Efficient dual conductive network based on layered double hydroxide nanospheres and nanosheets anchored in N-carbon nanofibers for asymmetric supercapacitors

Amorphous Nanosheets Constructed Nickel Cobalt Hydroxysulfide Hollow Spheres as Cathode Materials for Hybrid Supercapacitors

Bioinspired cellulose nanofibrils and NaCl composited polyacrylamide hydrogels with improved toughness, resilience, and strain‐sensitive conductivity

Rheological characteristics of nitrate glycerol ether cellulose gel based on phase separation in ternary system

Interfacial interaction enhancement between biodegradable poly (butylene adipate-co-terephthalate) and microcrystalline cellulose based on covalent bond for improving puncture, tearing, and enzymatic degradation properties