This work presents a new route to suppress grain growth and tune the sensitivity and selectivity of nanocrystalline SnO2 fibers. Unloaded and Pd‐loaded SnO2 nanofiber mats are synthesized by ...electrospinning followed by hot‐pressing at 80 °C and calcination at 450 or 600 °C. The chemical composition and microstructure evolution as a function of Pd‐loading and calcination temperature are examined using EDS, XPS, XRD, SEM, and HRTEM. Highly porous fibrillar morphology with nanocrystalline fibers comprising SnO2 crystallites decorated with tiny PdO crystallites is observed. The grain size of the SnO2 crystallites in the layers that are calcined at 600 °C decreases with increasing Pd concentration from about 15 nm in the unloaded specimen to about 7 nm in the 40 mol% Pd‐loaded specimen, indicating that Pd‐loading could effectively suppress the SnO2 grain growth during the calcination step. The Pd‐loaded SnO2 sensors have 4 orders of magnitude higher resistivity and exhibit significantly enhanced sensitivity to H2 and lower sensitivity to NO2 compared to their unloaded counterparts. These observations are attributed to enhanced electron depletion at the surface of the PdO‐decorated SnO2 crystallites and catalytic effect of PdO in promoting the oxidation of H2 into H2O. These phenomena appear to have a much larger effect on the sensitivity of the Pd‐loaded sensors than the reduction in grain size.
Nanocrystalline SnO2 fibers are fabricated by electrospinning using Pd as a grain growth inhibitor and a catalyst for enhancing oxidation reactions. Pd‐loaded or unloaded (pristine) SnO2 fibers can be used as gas sensors capable of detecting trace concentrations as low as several parts per billion (ppb) of H2 and NO2, respectively.
This review reports on the most updated technological aspects of Li–air battery cathode materials. It provides the reader with recent developments, alongside critical views. The requirements for ...air‐cathodes, as well as the classification and characterization of carbon‐based and carbon‐free air cathodes, are listed. The effects of two major substituent groups of materials, namely carbon and advanced materials (metals, metal‐oxides, metal‐carbides, and metal‐nitrides) aimed at replacing carbon, are discussed in terms of their chemical and electrochemical stability. The report covers aspects of surface chemistry and structure influence on the electrolyte and discharge products stability. The review also reports on the efforts to suppress side reactions and deterioration of the polymeric binders (if a composite electrode is being considered). This is recognized as a means to enhance Li–air battery performance. The report concludes with an outlook and perspective, providing the readers with some insight on other factors and their impact on the long road toward a viable air‐cathode suitable for Li–air battery operations.
This review reports on the most updated technological aspects of Li–air battery cathode materials. It discusses the most recent materials developments alongside critical views and provides an outlook and perspective. It provides the readers with some insight on the factors and their impact on the long road toward a viable air cathode suitable for Li–air battery operations.
We report on the heterogeneous sensitization of metal–organic framework (MOF)-driven metal-embedded metal oxide (M@MO) complex catalysts onto semiconductor metal oxide (SMO) nanofibers (NFs) via ...electrospinning for markedly enhanced chemical gas sensing. ZIF-8-derived Pd-loaded ZnO nanocubes (Pd@ZnO) were sensitized on both the interior and the exterior of WO3 NFs, resulting in the formation of multiheterojunction Pd–ZnO and ZnO–WO3 interfaces. The Pd@ZnO loaded WO3 NFs were found to exhibit unparalleled toluene sensitivity (R air /R gas = 4.37 to 100 ppb), fast gas response speed (∼20 s) and superior cross-selectivity against other interfering gases. These results demonstrate that MOF-derived M@MO complex catalysts can be functionalized within an electrospun nanofiber scaffold, thereby creating multiheterojunctions, essential for improving catalytic sensor sensitization.
Zinc–ion batteries (ZIBs) have drawn much attention for next‐generation energy storage for smart and wearable electronics due to their high theoretical gravimetric/volumetric energy capacities, ...safety from explosive hazards, and cost‐effectiveness. However, current state‐of‐the‐art ZIBs lack the energy capacity necessary to facilitate smart functionalities for intelligent electronics. In this work, a “π‐bridge spacer”‐embedded electron donor–acceptor polymer cathode combined with a Zn2+–ion‐conducting electrolyte is proposed for a smart and flexible ZIB to provide high electrochromic–electrochemical performances. The π‐bridge spacer endows the polymeric skeleton with improved physical ion accessibility and sensitive charge transfer through the cycles, providing extremely stable cyclability with high specific capacity (110 mAh g−1) at very fast rates (8 A g−1) and large coloration efficiency (79.8 cm2 C−1) under severe mechanical deformation over a long period. These results are markedly outstanding compared to the topological analogue without the π‐bridge spacer (80 mAh g−1 at current density of 8 A g−1, 63.0 cm2 C−1). The design to incorporate a π‐bridge spacer realizes notable electrochromism behaviors and high electrochemical performance, which sheds light on the rational development of multifunctional flexible‐ZIBs with color visualization properties for widespread usage in powering smart electronics.
A “π‐bridge spacer”‐embedded electron donor–acceptor polymer cathode combined with Zn2+–ion‐conducting gel electrolyte is proposed for a smart and flexible Zn–ion batteries to provide high electrochromic–electrochemical performances. The π‐bridge spacer endows the polymeric skeleton with improved physical ion accessibility and sensitive charge transfer through the charging–discharging cycles, providing extremely stable cyclability with high specific capacity.
Solar energy has seen 180 years of development since the discovery of the photovoltaic effect, having achieved the most successful commercialization in the energy‐harvesting fields. Despite its long ...history, even the most state‐of‐the‐art photovoltaics remain confined to solid‐state devices, limiting spatial and light utilization efficiencies. Herein, a liquid‐state photoenergy harvester based on a photoacid (PA), a chemical that releases protons upon light irradiation and recombines with them in the dark through a fully reversible reaction, is demonstrated. Asymmetric light exposure on a PA solution contained in a transparent tube generates a pH gradient (ΔpH = 2) along the exposed and dark regions, which charges the Nernst potential up to 0.7 V across the two electrodes embedded at each end, as if a capacitor. Owing to the reversibility of PAs, a PA‐driven liquid‐state photoenergy harvester (PLPH) generates capacitive currents up to 0.72 mA m−2 on an irradiation. Notably, the transparent nature of the PLPH enables vertical stacking up to 25 units, which multiplies the light‐harvesting efficiencies by over 1000%. This unique approach provides a new route to harness solar energy with a form‐factor‐free design that maximizes spatial and light‐use efficiencies.
A new way of harvesting solar energy is demonstrated by using a photoacid solution that generates a Nernst potential and capacitive current upon asymmetric light exposure. Such transparent liquid‐state devices enable form‐factor‐free design and vertical stacking that maximizes spatial and light‐use efficiencies.
Edges of 2D transition metal dichalcogenides (TMDs) are well known as highly reactive sites, thus researchers have attempted to maximize the edge site density of 2D TMDs. In this work, metal‐organic ...framework (MOF) templates are introduced to synthesize few‐layered WS2 nanoplates (a lateral dimension of ≈10 nm) confined in Co, N‐doped hollow carbon nanocages (WS2_Co‐N‐HCNCs), for highly sensitive NO2 gas sensors. WS2 precursors are assembled in the surface cavity of Co‐based zeolite imidazole framework (ZIF‐67) and subsequent pyrolysis produced WS2_Co‐N‐HCNCs. During the pyrolysis, the carbonized ZIF‐67 are doped by Co and N elements, and the growth of WS2 is effectively suppressed, creating few‐layered WS2 nanoplates functionalized Co‐N‐HCNCs. The WS2_Co‐N‐HCNCs exhibit outstanding NO2 sensing characteristics at room temperature, in terms of response (48.2% to 5 ppm), selectivity, response and recovery speed, and detection limit (100 ppb). These results are attributed to the enhanced adsorption and desorption kinetics of NO2 on abundant WS2 edges, confined in the gas permeable HCNCs. This work opens up an efficient way for the facile synthesis of edge abundant few‐layered TMDs combined with porous carbon matrix via MOF templating route, for applications relying on highly active sites.
Few‐layered WS2 nanoplates confined in Co, N‐doped hollow carbon nanocages (WS2_Co‐N‐HCNCs) are synthesized using metal‐organic framework (MOF) templating. The porous MOF suppresses the growth of WS2, creating few‐layered WS2 nanoplates in Co‐N‐HCNCs. The WS2_Co‐N‐HCNCs exhibit highly sensitive NO2 sensing characteristics at room temperature, in terms of response, selectivity, response/recovery speed, and detection limits.
Polyimide (PI) nanofibers with an average diameter of 300 nm, possessing superior electrolyte wettability and thermal stability, are synthesized by electrospinning a poly(amic acid) (PAA) solution ...followed by an imidization process. The large pore volume of the PI nanofiber mat can facilitate faster Li+-ion transport and greater rate capability, but it also cause an irreversible increase in cell impedance during long term cycling. To overcome these problems, thin Al2O3 over-layers are coated on both sides of a PI separator via a dip-coating process. The Al2O3-coated PI separators exhibit enhanced capacity, cyclability (95.53% retention after 200 cycles at 1 C), and rate capabilities (78.91% at 10 C) compared to the bare PI separator (68.65% at 10 C) and a commercial polypropylene separator (18.25% at 10 C) with a limited increase of cell impedance.
The global energy crisis caused by the overconsumption of nonrenewable fuels has prompted researchers to develop alternative strategies for producing electrical energy. In this review, a fascinating ...strategy that simply utilizes water, an abundant natural substance throughout the globe and even in air as moisture, as a power source is introduced. The concept of the hydrovoltaic electricity generator (HEG) proposed herein involves generating an electrical potential gradient by exposing the two ends of the HEG device to dissimilar physicochemical environments, which leads to the production of an electrical current through the active material. HEGs, with a large variety of viable active materials, have much potential for expansion toward diverse applications including permanent and/or emergency power sources. In this review, representative HEGs that generate electricity by the mechanisms of diffusion, streaming, and capacitance as case studies for building a fundamental understanding of the electricity generation process are discussed. In particular, by comparing the use and absence of hygroscopic materials, HEG mechanism studies to establish active material design principles are meticulously elucidated. The review with future perspectives on electrode design using conducting nanomaterials, considerations for high performance device construction, and potential impacts of the HEG technology in improving the livelihoods are reviewed.
This review discusses the design and applications of hydrovoltaic electricity generators through an in‐depth discussion of published case studies. Upon a concise introduction of their operating principles, strategic considerations for the development of high‐performance HEGs, including the choice of hydrovoltaic active materials, hygroscopic materials, electrode materials, and device structures are provided.
Transition metal dichalcogenides (TMDs) have attracted significant interest as gas‐sensing materials due to their unique crystal structure and surface. However, there are still issues when it comes ...to expanding the types of sensing gases for the TMD gas sensors. To extend gas‐sensing selectivity for the TMD gas sensors in this study, a monolayer (ML) 2D metal–organic framework (MOF) is introduced on top of the PtSe2 gas sensor, thereby tuning the major sensing analyte of PtSe2 from NO2 to H2S. Density functional theory calculations elucidate that the metal species of ML MOFs are attributed to the tuned selectivity of the analytes, based on the difference in binding energies. It is also demonstrated that ML MOF maintained the high responsivity of the pristine PtSe2 even at a low concentration of gas (200 ppb). This is further confirmed through the molecular dynamics simulations, which reveal that the ML feature of the ML MOF is highly essential to preserve the intrinsic ultra‐low limit detection properties of pristine PtSe2.
The monolayer 2D metal–organic framework is introduced to tune the gas‐sensing selectivity of PtSe2, one of the most promising gas‐sensing materials in transition metal dichalcogenides. The tuning mechanism is revealed by density functional theory calculations. The monolayer metal–organic framework also preserves ultra‐low detection limit of PtSe2, and it is elucidated by molecular dynamics simulation.
The humidity dependence of the gas‐sensing characteristics in SnO2‐based sensors, one of the greatest obstacles in gas‐sensor applications, is reduced to a negligible level by NiO doping. In a dry ...atmosphere, undoped hierarchical SnO2 nanostructures prepared by the self‐assembly of crystalline nanosheets show a high CO response and a rapid response speed. However, the gas response, response/recovery speeds, and resistance in air are deteriorated or changed significantly in a humid atmosphere. When hierarchical SnO2 nanostructures are doped with 0.64–1.27 wt% NiO, all of the gas‐sensing characteristics remain similar, even after changing the atmosphere from a dry to wet one. According to diffuse‐reflectance Fourier transform IR measurements, it is found that the most of the water‐driven species are predominantly absorbed not by the SnO2 but by the NiO, and thus the electrochemical interaction between the humidity and the SnO2 sensor surface is totally blocked. NiO‐doped hierarchical SnO2 sensors exhibit an exceptionally fast response speed (1.6 s), a fast recovery speed (2.8 s) and a superior gas response (Ra/Rg = 2.8 at 50 ppm CO (Ra: resistance in air, Rg: resistance in gas)) even in a 25% r.h. atmosphere. The doping of hierarchical SnO2 nanostructures with NiO is a very‐promising approach to reduce the dependence of the gas‐sensing characteristics on humidity without sacrificing the high gas response, the ultrafast response and the ultrafast recovery.
The humidity dependence of the gas‐sensing characteristics in a SnO2‐based sensor is reduced to a negligible level by doping with NiO. NiO‐doped‐SnO2 hierarchical nanostructures show highly sensitive, ultrafast response and recovery speeds with little dependence on humidity. The NiO plays the roles of a humidity absorber and a catalytic promoter.
