Alternatives to Fluoropolymers for Motion-Based Energy Harvesting: Perspectives on Piezoelectricity, Triboelectricity, Ferroelectrets, and Flexoelectricity.

Fluoropolymers, including polytetrafluoroethylene (PTFE, Teflon), polyvinylidene difluoride (PVDF), and fluorine kautschuk materials (FKMs, Viton) are critical polymers for applications ranging from non-stick coatings, corrosion resistant seals, semiconductor manufacturing, membranes, and energy harvesting technologies. However, the synthesis of these fluoropolymers requires the use of per- and polyfluorinated alkyl substances (PFAS) known colloquially as "forever chemicals," and as such there is a pressing need to develop alternative technologies that can serve the end-use of fluoropolymers without the environmental cost of using PFAS. Further, fluoropolymers themselves fall under the PFAS umbrella. Here, alternative mechanical-to-electrical energy harvesting polymers are reviewed and benchmarked against the leading fluoropolymer energy harvesters. These alternative technologies include nonfluoropolymer piezoelectric polymers, triboelectric nanogenerators (TENGs), ferroelectric elastomers, and flexoelectric polymers. A vision towards sustainable, non-fluoropolymer-based energy harvesting is provided.

Understanding and Controlling Electrostatic Discharge in Triboelectric Nanogenerators.

Triboelectric nanogenerators (TENGs) have been widely used to harness various forms of mechanical energy for conversion to electrical energy. However, the contentious challenge in characterising TENGs is the lack of standard protocols for assessing mechanical-to-electrical energy conversion processes. Herein, macroscopic signal analysis is used to identify three key charging events within triboelectric signals: charge induction (CI), contact electrification (CE), and electrostatic discharge (ESD). By considering two phases of motion during contact-separation (approach and departure of the contact materials), CI arising from the motion of bound surface charge (varying electric field) between opposing contact materials is shown to dominate the measured displacement current signal, rather than the process of CE itself. Furthermore, the conventional signal (i. e., voltage, current, charge) interpretation of CE and CI during approach and departure phases is re-assessed, to indicate that the sudden spike of current often observed immediately prior to contact (or after separation) arises from polarity inverting electrostatic discharge (ESD). This aspect of the measured triboelectric effect, which is often ignored, is crucial for the design of TENGs and hence, techniques to enhance the understanding and control over the stochastic occurrence of ESDs is explored. The methods proposed for the deconvolution of the macroscopic signal components of TENGs, and mitigation of ESD occurrences, will allow for precise quantification of the associated charging events. The applications of this study will template the design and development of future super-TENGs with optimised energy conversion capabilities.

Fluorescent Nanocarbons: From Synthesis and Structure to Cancer Imaging and Therapy.

Nanocarbons are emerging at the forefront of nanoscience, with diverse carbon nanoforms emerging over the past two decades. Early cancer diagnosis and therapy, driven by advanced chemistry techniques, play a pivotal role in mitigating mortality rates associated with cancer. Nanocarbons, with an attractive combination of well-defined architectures, biocompatibility, and nanoscale dimension, offer an incredibly versatile platform for cancer imaging and therapy. This paper aims to review the underlying principles regarding the controllable synthesis, fluorescence origins, cellular toxicity, and surface functionalization routes of several classes of nanocarbons: carbon nanodots, nanodiamonds, carbon nanoonions, and carbon nanohorns. This review also highlights recent breakthroughs regarding the green synthesis of different nanocarbons from renewable sources. It also presents a comprehensive and unified overview of the latest cancer-related applications of nanocarbons and how they can be designed to interface with biological systems and work as cancer diagnostics and therapeutic tools. The commercial status for large-scale manufacturing of nanocarbons is also presented. Finally, it proposes future research opportunities aimed at engendering modifiable and high-performance nanocarbons for emerging applications across medical industries. This work is envisioned as a cornerstone to guide interdisciplinary teams in crafting fluorescent nanocarbons with tailored attributes that can revolutionize cancer diagnostics and therapy.

The Impact of Activating Agents on Non-Enzymatic Nucleic Acid Extension Reactions.

Non-enzymatic template-directed primer extension is increasingly being studied for the production of RNA and DNA. These reactions benefit from producing RNA or DNA in an aqueous, protecting group free system, without the need for expensive enzymes. However, these primer extension reactions suffer from a lack of fidelity, low reaction rates, low overall yields, and short primer extension lengths. This review outlines a detailed mechanistic pathway for non-enzymatic template-directed primer extension and presents a review of the thermodynamic driving forces involved in entropic templating. Through the lens of entropic templating, the rate and fidelity of a reaction are shown to be intrinsically linked to the reactivity of the activating agent used. Thus, a strategy is discussed for the optimization of non-enzymatic template-directed primer extension, providing a path towards cost-effective in vitro synthesis of RNA and DNA.

Tribovoltaic Performance of TiO2 Thin Films: Crystallinity, Contact Metal, and Thermoelectric Effects.

Tribovoltaic devices are attracting increasing attention as motion-based energy harvesters due to the high local current densities that can be generated. However, while these tribovoltaic devices are being developed, debate remains surrounding their fundamental mechanism. Here, we fabricate thin films from one of the world's most common oxides, TiO2, and compare the tribovoltaic performance under contact with metals of varying work functions, contact areas, and applied pressure. The resultant current density shows little correlation with the work function of the contact metal and a strong correlation with the contact area. Considering other effects at the metal-semiconductor interface, the thermoelectric coefficients of different metals were calculated, which showed a clear correlation with the tribovoltaic current density. On the microscale, molybdenum showed the highest current density of 192 mA cm-2. This work shows the need to consider a variety of mechanisms to understand the tribovoltaic effect and design future exemplar tribovoltaic devices.

Electrospinning Triboelectric Laminates: A Pathway for Scaling Energy Harvesters.

Herein, a new paradigm of triboelectric polymers-the triboelectric laminate-a volumetric material with electromechanical response comparable to the benchmark soft piezoelectric material polyvinylidene difluoride is reported. The electromechanical response in the triboelectric laminate arises from aligned dipoles, generated from the orientation of contact electrification in the laminates bulk volume. The dipoles form between sequential bilayers consisting of two different electrospun polymer fibers of different diameter. The loose interface between the fiber bilayers ensures friction and triboelectric charging between two polymers. The electric output from the electrospun triboelectric laminate increases with increasing density of the bilayers. This system design has clear benefits over other flexible devices for mechanical energy harvesting as it does not require any poling procedures, and the electromechanical response is stable over 24 h of continuous operation. Moreover, the electromechanically responsive electrospun laminate can be made from all types of polymers, thus providing ample room for further improvements or functionalities such as stretchability, biodegradability, or biocompatibility. The concept of a triboelectric laminate can be introduced into existing triboelectric nanogenerator form factors, to dramatically increase charge harvesting of a variety of devices.

Energy Interplay in Materials: Unlocking Next-Generation Synchronous Multisource Energy Conversion with Layered 2D Crystals.

Layered 2D crystals have unique properties and rich chemical and electronic diversity, with over 6000 2D crystals known and, in principle, millions of different stacked hybrid 2D crystals accessible. This diversity provides unique combinations of properties that can profoundly affect the future of energy conversion and harvesting devices. Notably, this includes catalysts, photovoltaics, superconductors, solar-fuel generators, and piezoelectric devices that will receive broad commercial uptake in the near future. However, the unique properties of layered 2D crystals are not limited to individual applications and they can achieve exceptional performance in multiple energy conversion applications synchronously. This synchronous multisource energy conversion (SMEC) has yet to be fully realized but offers a real game-changer in how devices will be produced and utilized in the future. This perspective highlights the energy interplay in materials and its impact on energy conversion, how SMEC devices can be realized, particularly through layered 2D crystals, and provides a vision of the future of effective environmental energy harvesting devices with layered 2D crystals.

Doped 2D SnS materials derived from liquid metal-solution for tunable optoelectronic devices.

Gas-liquid reaction phenomena on liquid-metal solvents can be used to form intriguing 2D materials with large lateral dimensions, where the free energies of formation determine the final product. A vast selection of elements can be incorporated into the liquid metal-based nanostructures, offering a versatile platform for fabricating novel optoelectronic devices. While conventional doping techniques of semiconductors present several challenges for 2D materials. Liquid metals provide a facile route for obtaining doped 2D semiconductors. In this work, we successfully demonstrate that the doping of 2D SnS can be realized in a glove box containing a diluted H2S gas. Low melting point elements such as Bi and In are alloyed with base liquid Sn in varying concentrations, resulting in the doping of 2D SnS layers incorporating Bi and In sulphides. Optoelectronic properties for photodetectors and piezoelectronics can be fine-tuned through the controlled introduction of selective migration doping. The structural modification of 2D SnS results in a 22.6% enhancement of the d11 piezoelectric coefficient. In addition, photodetector response times have increased by several orders of magnitude. Doping methods using liquid metals have significantly changed the photodiode and piezoelectric device performances, providing a powerful approach to tune optoelectronic device outputs.

How to Build a Microplastics-Free Environment: Strategies for Microplastics Degradation and Plastics Recycling.

Microplastics are an emergent yet critical issue for the environment because of high degradation resistance and bioaccumulation. Unfortunately, the current technologies to remove, recycle, or degrade microplastics are insufficient for complete elimination. In addition, the fragmentation and degradation of mismanaged plastic wastes in environment have recently been identified as a significant source of microplastics. Thus, the developments of effective microplastics removal methods, as well as, plastics recycling strategies are crucial to build a microplastics-free environment. Herein, this review comprehensively summarizes the current technologies for eliminating microplastics from the environment and highlights two key aspects to achieve this goal: 1) Catalytic degradation of microplastics into environmentally friendly organics (carbon dioxide and water); 2) catalytic recycling and upcycling plastic wastes into monomers, fuels, and valorized chemicals. The mechanisms, catalysts, feasibility, and challenges of these methods are also discussed. Novel catalytic methods such as, photocatalysis, advanced oxidation process, and biotechnology are promising and eco-friendly candidates to transform microplastics and plastic wastes into environmentally benign and valuable products. In the future, more effort is encouraged to develop eco-friendly methods for the catalytic conversion of plastics into valuable products with high efficiency, high product selectivity, and low cost under mild conditions.

A bright future for engineering piezoelectric 2D crystals.

The piezoelectric effect, mechanical-to-electrical and electrical-to-mechanical energy conversion, is highly beneficial for functional and responsive electronic devices. To fully exploit this property, miniaturization of piezoelectric materials is the subject of intense research. Indeed, select atomically thin 2D materials strongly exhibit the piezoelectric effect. The family of 2D crystals consists of over 7000 chemically distinct members that can be further manipulated in terms of strain, functionalization, elemental substitution (i.e. Janus 2D crystals), and defect engineering to induce a piezoelectric response. Additionally, most 2D crystals can stack with other similar or dissimilar 2D crystals to form a much greater number of complex 2D heterostructures whose properties are quite different to those of the individual constituents. The unprecedented flexibility in tailoring 2D crystal properties, coupled with their minimal thickness, make these emerging highly attractive for advanced piezoelectric applications that include pressure sensing, piezocatalysis, piezotronics, and energy harvesting. This review summarizes literature on piezoelectricity, particularly out-of-plane piezoelectricity, in the vast family of 2D materials as well as their heterostructures. It also describes methods to induce, enhance, and control the piezoelectric properties. The volume of data and role of machine learning in predicting piezoelectricity is discussed in detail, and a prospective outlook on the 2D piezoelectric field is provided.

Spatially isolated redox processes enabled by ambipolar charge transport in multi-walled carbon nanotube mats.

This work demonstrates a simple dual-well device which enables spatially isolated solutions to undergo complementary redox reactions. The device functions by the ambipolar transport of charge carriers between two spatially isolated poly(dimethylsiloxane) (PDMS) microwells through an underlying multi-walled carbon nanotube (MWCNT) mat. This MWCNT mat enables charge carriers, produced from the decomposition of an analyte in one solution, to drive a redox reaction in a spatially isolated second colorimetric read-out solution via a potential difference between the wells. As proof-of-concept a visible colorimetric read-out was shown using an enzyme, cytochrome c (reduced in 16 h), and the visualizing reagent 3,3',5,5'-tetramethylbenzidine (TMB) (oxidized in 2.5 h) for the detection of dithionite and hydrogen peroxide, respectively, without any external energy input. We discuss the origin of this phenomenon and highlight the ability of MWCNTs to accept and transport both electrons and holes efficiently between spatially isolated solutions giving rise to a highly versatile sensor suitable for use in simple, low-cost point-of-care diagnostics.

Organic/Inorganic Hybrid Fibers: Controllable Architectures for Electrochemical Energy Applications.

Organic/inorganic hybrid fibers (OIHFs) are intriguing materials, possessing an intrinsic high specific surface area and flexibility coupled to unique anisotropic properties, diverse chemical compositions, and controllable hybrid architectures. During the last decade, advanced OIHFs with exceptional properties for electrochemical energy applications, including possessing interconnected networks, abundant active sites, and short ion diffusion length have emerged. Here, a comprehensive overview of the controllable architectures and electrochemical energy applications of OIHFs is presented. After a brief introduction, the controllable construction of OIHFs is described in detail through precise tailoring of the overall, interior, and interface structures. Additionally, several important electrochemical energy applications including rechargeable batteries (lithium-ion batteries, sodium-ion batteries, and lithium-sulfur batteries), supercapacitors (sandwich-shaped supercapacitors and fiber-shaped supercapacitors), and electrocatalysts (oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction) are presented. The current state of the field and challenges are discussed, and a vision of the future directions to exploit OIHFs for electrochemical energy devices is provided.

Polycrystalline diamond coating on 3D printed titanium scaffolds: Surface characterisation and foreign body response.

Titanium-based implants are the leading material for orthopaedic surgery, due to their strength, versatility, fabrication via additive manufacturing and invoked biological response. However, the interface between the implant and the host tissue requires improvement to better integrate the implant material and mitigate foreign body response. The interface can be manipulated by changing the surface energy, chemistry, and topography of the Titanium-based implant. Recently, polycrystalline diamond (PCD) has emerged as an exciting coating material for 3D printed titanium scaffolds showing enhanced mammalian cell functions while inhibiting bacterial attachment in vitro. In this study, we performed in-depth characterisation of PCD coatings investigating the surface topography, thickness, surface energy, and compared its foreign body response in vivo with uncoated titanium scaffold. Coating PCD onto titanium scaffolds resulted in a similar microscale surface roughness (RMS(PCD-coated) = 24 µm; RMS(SLM-Ti) = 28 µm), increased nanoscale roughness (RMS(PCD-coated) = 35 nm; RMS(SLM-Ti) = 66 nm) and a considerable decrease in surface free energy (E(PCD-coated) = 4 mN m-1; E(SLM-Ti) = 16 mN m-1). These surface property changes were supported by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy as corresponding to observed surface chemistry changes induced by the coating. The underlying mechanism of how the diamond coatings chemical and physical properties changes the wettability of implants was examined. In vivo, the coated scaffolds induced similar level of fibrous encapsulation with uncoated scaffolds. This study thus provides further insight into the physicochemical characteristics of PCD coatings, adding evidence to the promising potential of PCD-coatings of medical implants.

Probing Contact Electrification: A Cohesively Sticky Problem.

Contact electrification and the triboelectric effect are complex processes for mechanical-to-electrical energy conversion, particularly for highly deformable polymers. While generating relatively low power density, contact electrification can occur at the contact-separation interface between nearly any two polymer surfaces. This ubiquitousness of surfaces enables contact electrification to be an important phenomenon to understand energy conversion and harvesting applications. The mechanism of charge generation between polymeric materials remains ambiguous, with electron transfer, material (also known as mass) transfer, and adsorbed chemical species transfer (including induced ionization of water and other molecules) all being proposed as the primary source of the measured charge. Often, all sources of charge, except electron transfer, are dismissed in the case of triboelectric energy harvesters, leading to the generation of the "triboelectric series", governed by the ability of a polymer to lose, or accept, an electron. Here, this sole focus on electron transfer is challenged through rigorous experiments, measuring charge density in polymer-polymer (196 polymer combinations), polymer-glass (14 polymers), and polymer-liquid metal (14 polymers) systems. Through the investigation of these interfaces, clear evidence of material transfer via heterolytic bond cleavage is provided. Based on these results, a generalized model considering the cohesive energy density of polymers as the critical parameter for polymer contact electrification is discussed. This discussion clearly shows that material transfer must be accounted for when discussing the source of charge generated by polymeric mechanical energy harvesters. Thus, a correlated physical property to understand the triboelectric series is provided.

Interfacial piezoelectric polarization locking in printable Ti3C2Tx MXene-fluoropolymer composites.

Piezoelectric fluoropolymers convert mechanical energy to electricity and are ideal for sustainably providing power to electronic devices. To convert mechanical energy, a net polarization must be induced in the fluoropolymer, which is currently achieved via an energy-intensive electrical poling process. Eliminating this process will enable the low-energy production of efficient energy harvesters. Here, by combining molecular dynamics simulations, piezoresponse force microscopy, and electrodynamic measurements, we reveal a hitherto unseen polarization locking phenomena of poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) perpendicular to the basal plane of two-dimensional (2D) Ti3C2Tx MXene nanosheets. This polarization locking, driven by strong electrostatic interactions enabled exceptional energy harvesting performance, with a measured piezoelectric charge coefficient, d33, of -52.0 picocoulombs per newton, significantly higher than electrically poled PVDF-TrFE (approximately -38 picocoulombs per newton). This study provides a new fundamental and low-energy input mechanism of poling fluoropolymers, which enables new levels of performance in electromechanical technologies.

Grapevine waste in sustainable hybrid particleboard production.

Grapevine prunings are an ideal sustainable additive to wood-based composites, providing a powerful resource stream, reducing the demand of logging of forests. Valorisation of grapevine prunings into commercial products further reduces greenhouse gas emissions caused by waste vegetation break-down. Particleboard is a ubiquitous wood-based composite with over 97 million m3 produced annually from soft wood. Agricultural crop waste is ideal for incorporation into particleboard, however maintaining the density and mechanical properties with these additives is imperative. Herein, mixed grapevine/pine cores comprising of 100%, 25% and 10% grapevine particles were produced and the mechanical and density properties of 16 mm thick moisture resistant particleboards were evaluated. Hybrid particleboards based on 10% grapevine and 90% pine showed great promise, surpassing global industry standards for key mechanical properties. Hybrid particleboards displayed a higher surface density and a steeper vertical density gradient than the 100% pine control boards, due to grapevine particles filling voids between the pine, thereby improving the mechanical properties. This work forms a foundation for the continued study of agricultural waste into wood-based composites.

Heterogeneous Single-Atom Catalysts for Electrochemical CO2 Reduction Reaction.

The electrochemical CO2 reduction reaction (CO2 RR) is of great importance to tackle the rising CO2 concentration in the atmosphere. The CO2 RR can be driven by renewable energy sources, producing precious chemicals and fuels, with the implementation of this process largely relying on the development of low-cost and efficient electrocatalysts. Recently, a range of heterogeneous and potentially low-cost single-atom catalysts (SACs) containing non-precious metals coordinated to earth-abundant elements have emerged as promising candidates for the CO2 RR. Unfortunately, the real catalytically active centers and the key factors that govern the catalytic performance of these SACs remain ambiguous. Here, this ambiguity is addressed by developing a fundamental understanding of the CO2 RR-to-CO process on SACs, as CO accounts for the major product from CO2 RR on SACs. The reaction mechanism, the rate-determining steps, and the key factors that control the activity and selectivity are analyzed from both experimental and theoretical studies. Then, the synthesis, characterization, and the CO2 RR performance of SACs are discussed. Finally, the challenges and future pathways are highlighted in the hope of guiding the design of the SACs to promote and understand the CO2 RR on SACs.

Measuring Piezoelectric Output-Fact or Friction?

Piezoelectric polymers are emerging as exceptionally promising materials for energy harvesting. While the theoretical figures of merit for piezoelectric polymers are comparable to ceramics, the measurement techniques need to be retrofitted to account for the different mechanical properties of the softer polymeric materials. Here, how contact electrification, including friction and contact separation, is often mistaken for piezoelectric charge is examined, and a perspective for how to separate these effects is provided. The state of the literature is assessed, and recommendations are made for clear and simple guidelines in reporting, for both sample geometry and testing methods, to enable accurate determination of piezoelectric figures of merit in polymers. Such improvements will allow an understanding of what types of material manipulation are required in order to enhance the piezoelectric output from polymers and enable the next generation of polymer energy harvester design.

Fe/Co-based Bimetallic MOF-derived Co3 Fe7 @NCNTFs Bifunctional Electrocatalyst for High-Efficiency Overall Water Splitting.

Electrocatalytic water splitting to produce hydrogen and oxygen is regarded as one of the most promising methods to generate clean and sustainable energy for replacing fossil fuels. However, the design and development of an efficient bifunctional catalyst for simultaneous generation of hydrogen and oxygen remains extremely challenging yet is critical for the practical implementation of water electrolysis. Here, we report a facile method to fabricate novel N-doped carbon nanotube frameworks (NCNTFs) by the pyrolysis of a bimetallic metal organic framework (MIL-88-Fe/Co). The resultant electrocatalyst, Co3 Fe7 @NCNTFs, exhibits excellent oxygen evolution reaction (OER) activity, achieving 10 mA/cm2 at a low overpotential of just 264 mV in 1 M KOH solution, and 197 mV for the hydrogen evolution reaction. The high electrocatalytic activity arises from the synergistic effect between the chemistry of the Co3 Fe7 and the NCNTs coupled to the novel framework structure. The remarkable electrocatalytic performance of our bifunctional electrocatalyst provides a promising pathway to high-performance overall water splitting and electrochemical energy devices.