Showing papers in "Journal of Physical Chemistry Letters in 2013"
TL;DR: In this paper, the authors discussed the steps that have led to this discovery, and the future of this rapidly advancing concept have been considered, and it is likely that the next few years of solar research will advance this technology to the very highest efficiencies while retaining the very lowest cost and embodied energy.
Abstract: Over the last 12 months, we have witnessed an unexpected breakthrough and rapid evolution in the field of emerging photovoltaics, with the realization of highly efficient solid-state hybrid solar cells based on organometal trihalide perovskite absorbers. In this Perspective, the steps that have led to this discovery are discussed, and the future of this rapidly advancing concept have been considered. It is likely that the next few years of solar research will advance this technology to the very highest efficiencies while retaining the very lowest cost and embodied energy. Provided that the stability of the perovskite-based technology can be proven, we will witness the emergence of a contender for ultimately low-cost solar power.
2,506 citations
TL;DR: In this article, the perovskite CH3NH3PbX3 (X = Cl, Br, or I) sensitizer made a breakthrough in solid-state mescoscopic solar cells, where the first record efficiency of around 10% was reported in 2012 using submicrometer-thick TiO2 film sensitized with CH 3NH3pbI3. A rapid increase in efficiency approaching 14% followed shortly.
Abstract: Recently, perovskite CH3NH3PbI3 sensitizer has attracted great attention due to its superb light-harvesting characteristics. Organometallic or organic materials were mostly used as sensitizers for solid-state dye-sensitized solar cells at early stages. Inorganic nanocrystals have lately received attention as light harvesters due to their high light-absorbing properties. Metal chalcogenides have been investigated with solid-state dye-sensitized solar cells; however, the best power conversion efficiency was reported to be around 6%. CH3NH3PbX3 (X = Cl, Br, or I) perovskite sensitizer made a breakthrough in solid-state mescoscopic solar cells, where the first record efficiency of around 10% was reported in 2012 using submicrometer-thick TiO2 film sensitized with CH3NH3PbI3. A rapid increase in efficiency approaching 14% followed shortly. In this Perspective, recent progress in perovskite-sensitized solid-state mesoscopic solar cells is reviewed. On the basis of the recent achievements, a power conversion eff...
1,275 citations
TL;DR: In this paper, the authors show that the band gap of three-dimensional hybrid perovskites is dominated by a giant spin-orbit coupling (SOC) in the conduction-band (CB).
Abstract: Three-dimensional (3D) hybrid perovskites CH3NH3PbX3 (X = Br, I) have recently been suggested as new key materials for dye-sensitized solar cells (DSSC) leading to a new class of hybrid semiconductor photovoltaic cells (HSPC). Thanks to density functional theory calculations, we show that the band gap of these compounds is dominated by a giant spin–orbit coupling (SOC) in the conduction-band (CB). At room temperature, direct and isotropic optical transitions are associated to a spin–orbit split-off band related to the triply degenerated CB of the cubic lattice without SOC. Due to the strong SOC, the electronic states involved in the optical absorption are only slightly perturbed by local distortions of the lattice. In addition, band offset calculations confirm that CH3NH3PbX3/TiO2 is a reference material for driving electrons toward the electrode in HSPC. Two-dimensional (2D) hybrids are also suggested to reach further flexibility for light conversion efficiency. Our study affords the basic concepts to re...
1,027 citations
TL;DR: The recent advances as well as future prospects of quantum dot solar cells discussed in this perspective provide the basis for consideration as "The Next Big Thing" in photovoltaics.
Abstract: The recent surge in the utilization of semiconductor nanostructures for solar energy conversion has led to the development of high-efficiency solar cells. Some of these recent advances are in the areas of synthesis of new semiconductor materials and the ability to tune the electronic properties through size, shape, and composition and to assemble quantum dots as hybrid assemblies. In addition, processes such as hot electron injection, multiple exciton generation (MEG), plasmonic effects, and energy-transfer-coupled electron transfer are gaining momentum to overcome the efficiency limitations of energy capture and conversion. The recent advances as well as future prospects of quantum dot solar cells discussed in this perspective provide the basis for consideration as “The Next Big Thing” in photovoltaics.
763 citations
TL;DR: The first theoretical estimation of effective masses of photocarriers in CH3NH3PbI3 is reported, consistent with the long-range ambipolar transport property and with the larger diffusion constant for electrons compared with that for holes in the perovskite, which enable efficient photovoltaic conversion.
Abstract: Methylammonium lead iodide perovskite (CH3NH3PbI3) plays an important role in light absorption and carrier transport in efficient organic-inorganic perovskite solar cells. In this Letter, we report the first theoretical estimation of effective masses of photocarriers in CH3NH3PbI3. Effective masses of photogenerated electrons and holes were estimated to be me* = 0.23m0 and mh* = 0.29m0, respectively, including spin-orbit coupling effects. This result is consistent with the long-range ambipolar transport property and with the larger diffusion constant for electrons compared with that for holes in the perovskite, which enable efficient photovoltaic conversion.
651 citations
TL;DR: Inkjet-printed, high conductivity graphene patterns that are suitable for flexible electronics and attain low resistivity while showing uniform morphology, compatibility with flexible substrates, and excellent tolerance to bending stresses are demonstrated.
Abstract: The ability to print high conductivity, conformal, and flexible electrodes is an important technological challenge in printed electronics, especially for large-area formats with low cost considerations. In this Letter, we demonstrate inkjet-printed, high conductivity graphene patterns that are suitable for flexible electronics. The ink is prepared by solution-phase exfoliation of graphene using an environmentally benign solvent, ethanol, and a stabilizing polymer, ethyl cellulose. The inkjet-printed graphene features attain low resistivity of 4 mΩ·cm after a thermal anneal at 250 °C for 30 min while showing uniform morphology, compatibility with flexible substrates, and excellent tolerance to bending stresses.
573 citations
TL;DR: A model based on density functional theory calculations is developed to describe trends in catalytic activity for CO2 electroreduction to CO in terms of the adsorption energy of the reaction intermediates, CO and COOH and shows that the strong scaling between adsorbed CO and adsorbates on metal surfaces is responsible for the persistent overpotential.
Abstract: We develop a model based on density functional theory calculations to describe trends in catalytic activity for CO2 electroreduction to CO in terms of the adsorption energy of the reaction intermediates, CO and COOH The model is applied to metal surfaces as well as the active site in the CODH enzymes and shows that the strong scaling between adsorbed CO and adsorbed COOH on metal surfaces is responsible for the persistent overpotential The active site of the CODH enzyme is not subject to these scaling relations and optimizes the relative binding energies of these adsorbates, allowing for an essentially reversible process with a low overpotential
563 citations
TL;DR: This Perspective focuses on the challenges and prospects associated with the current researching results of these lithium-rich layered cathode materials and their average/local structures, reaction mechanisms, and electrochemical properties.
Abstract: Lithium-rich layered oxide materials xLi2MnO3·(1-x)LiMO2 (M = Mn, Ni, Co, Fe, Cr, etc.) have attracted much attention for the use of cathode materials in lithium-ion batteries in recent years. However, there are many issues still unclear (the structure and reaction mechanism are ambiguous until now), and numerous scientific challenges (low initial Coulombic efficiency, poor rate capability, and voltage degradation during cycling) of these materials that must be overcome to realize their utilization in commercial lithium-ion batteries. This Perspective focuses on the challenges and prospects associated with the current researching results of these lithium-rich layered cathode materials. Specifically, their average/local structures, reaction mechanisms, and electrochemical properties are discussed.
524 citations
TL;DR: Judicious selection of the perovskite lead halide-based absorber, matching organic hole conductor, and contacts, a cell with a ∼ 1.3 V open circuit voltage was made and provides a general guideline for additional improvement of cell performance.
Abstract: Mesoscopic solar cells, based on solution-processed organic-inorganic perovskite absorbers, are a promising avenue for converting solar to electrical energy. We used solution-processed organic-inorganic lead halide perovskite absorbers, in conjunction with organic hole conductors, to form high voltage solar cells. There is a dire need for low-cost cells of this type, to drive electrochemical reactions or as the high photon energy cell in a system with spectral splitting. These perovskite materials, although spin-coated from solution, form highly crystalline materials. Their simple synthesis, along with high chemical versatility, allows tuning their electronic and optical properties. By judicious selection of the perovskite lead halide-based absorber, matching organic hole conductor, and contacts, a cell with a ∼ 1.3 V open circuit voltage was made. While further study is needed, this achievement provides a general guideline for additional improvement of cell performance.
500 citations
TL;DR: The recent significant advances on the design and applications of graphene-based photocatalytic composites are summarized and the rational designs for high-performance photoc atalysts using graphene- based materials are described.
Abstract: Graphene-based photocatalysts have gained increasing interest as a viable alternate to increase photocatalytic H2 production performance in converting solar energy into chemical energy. The use of graphene to enhance the efficiency of photocatalysts has been proved due to its unique two-dimensional conjugated structure and electronic properties. In this Perspective, we have summarized the recent significant advances on the design and applications of graphene-based photocatalytic composites. The rational designs for high-performance photocatalysts using graphene-based materials are described. The applications of the new materials in photocatalytic hydrogen evolution are presented. Finally, the ongoing challenges and opportunities for the future development of graphene-based photocatalysts are also proposed.
493 citations
TL;DR: It is shown that the nature of the HTM is essential for charge recombination and elucidates that finding an optimal HTM for the perovskite solar cell includes controlling the perOVskite/HTM interaction.
Abstract: We report on perovskite (CH3NH3)PbI3-sensitized solid-state solar cells using spiro-OMeTAD, poly(3-hexylthiophene-2,5-diyl) (P3HT) and 4-(diethylamino)benzaldehyde diphenylhydrazone (DEH) as hole transport materials (HTMs) with a light to electricity power conversion efficiency of 8.5%, 4.5%, and 1.6%, respectively, under AM 1.5G illumination of 1000 W/m(2) intensity. Photoinduced absorption spectroscopy (PIA) shows that hole transfer occurs from the (CH3NH3)PbI3 to HTMs after excitation of (CH3NH3)PbI3. The electron lifetime (τe) in these devices are in the order Spiro-OMeTAD > P3HT > DEH, while the charge transport time (ttr) is rather similar. The difference in τe can therefore explain the lower efficiency of the devices based on P3HT and DEH. This report shows that the nature of the HTM is essential for charge recombination and elucidates that finding an optimal HTM for the perovskite solar cell includes controlling the perovskite/HTM interaction. Design routes for new HTMs are suggested.
TL;DR: In this paper, the authors unraveled the origin of the poor carrier transport properties of BiVO4, a promising metal oxide photoanode for solar water splitting, and showed that Tungsten doping is strongly decreasing the carrier mobility by introducing intermediate-depth donor defects as carrier traps.
Abstract: We unravel for the first time the origin of the poor carrier transport properties of BiVO4, a promising metal oxide photoanode for solar water splitting. Time-resolved microwave conductivity (TRMC) measurements reveal an (extrapolated) carrier mobility of ∼4 × 10–2 cm2 V–1 s–1 for undoped BiVO4 under ∼1 sun illumination conditions, which is unusually low for a photoanode material. The poor carrier mobility is compensated by an unexpectedly long carrier lifetime of 40 ns. This translates to a relatively long diffusion length of 70 nm, consistent with the high quantum efficiencies reported for BiVO4 photoanodes. Tungsten (W) doping is found to strongly decrease the carrier mobility by introducing intermediate-depth donor defects as carrier traps. At the same time, the increased carrier density improves the overall photoresponse, which confirms that bulk electronic conductivity is one of the main performance bottlenecks for BiVO4.
TL;DR: In this paper, the authors investigate cathodes for the Li-S cell comprised of sulfur-imbibed robust spherical carbon shells with tailored porosity that exhibit excellent cycling stability, showing how sulfur fraction (underutilization) and sulfide precipit...
Abstract: Among the most challenging issues in electrochemical energy storage is developing insightful in situ probes of redox processes for a working cell. This is particularly true for cells that operate on the basis of chemical transformations such as Li–S and Li–O2, where the factors that govern capacity and cycling stability are difficult to access owing to the amorphous nature of the intermediate species. Here, we investigate cathodes for the Li–S cell comprised of sulfur-imbibed robust spherical carbon shells with tailored porosity that exhibit excellent cycling stability. Their highly regular nanoscale dimensions and thin carbon shells allow highly uniform electrochemical response and further enable direct monitoring of sulfur speciation within the cell over the entire redox range by operando X-ray absorption spectroscopy on the S K-edge. The results reveal the first detailed evidence of the mechanisms of sulfur redox chemistry on cycling, showing how sulfur fraction (under-utilization) and sulfide precipit...
TL;DR: In this paper, it was shown that the rate of abstraction of hydrogen by peroxy radicals is largely determined by the thermochemistry of the nascent alkyl radicals and thus is highly influenced by neighboring substituents.
Abstract: We present a hypothesis that autoxidation (inter- and intramolecular hydrogen abstraction by peroxy radicals) plays an important role in the oxidation of organic compounds in the atmosphere, particularly organic matter associated with aerosol. In the laboratory, we determine the rate of this process at room temperature for a model system, 3-pentanone. We employ ab initio calculations to investigate H-shifts within a broader group of substituted organic compounds. We show that the rate of abstraction of hydrogen by peroxy radicals is largely determined by the thermochemistry of the nascent alkyl radicals and thus is highly influenced by neighboring substituents. As a result, autoxidation rates increase rapidly as oxygen-containing functional groups (carbonyl, hydroxy, and hydroperoxy) are added to organic compounds. This mechanism is consistent with formation of the multifunctional hydroperoxides and carbonyls often found in atmospheric aerosol particles.
TL;DR: The current understanding of VFBs from materials to stacks is reported, describing the factors that affect materials' performance from microstructures to the mechanism and new materials development.
Abstract: The vanadium flow battery (VFB) as one kind of energy storage technique that has enormous impact on the stabilization and smooth output of renewable energy. Key materials like membranes, electrode, and electrolytes will finally determine the performance of VFBs. In this Perspective, we report on the current understanding of VFBs from materials to stacks, describing the factors that affect materials’ performance from microstructures to the mechanism and new materials development. Moreover, new models for VFB stacks as well as structural design will be summarized as well. Finally, the challenges, the overall cost evaluation, and future research directions will be briefly proposed.
TL;DR: In this article, a review of recent developments in the exploration of the structure and composition of multimetallic core-shell nanoparticles is presented, addressing current achievements and challenges in the preparation as well as microscopic and spectroscopic characterization of coreshell nanocatalysts.
Abstract: Pt-based core–shell nanoparticles have emerged as a promising generation of highly active electrocatalysts to accelerate the sluggish kinetics of oxygen reduction reaction (ORR) in fuel cell systems. Their electronic and structural properties can be easily tailored by modifying the Pt shell thickness, core composition, diameter, and shape; this results in significant improvements of activity and durability over state-of-the-art pure Pt catalysts. Prompted by the relevance of efficient and robust ORR catalysts for electrochemical energy conversion, this Perspective reviews several concepts and selected recent developments in the exploration of the structure and composition of core–shell nanoparticles. Addressing current achievements and challenges in the preparation as well as microscopic and spectroscopic characterization of core–shell nanocatalysts, a concise account of our understanding is provided on how the surface and subsurface structure of multimetallic core–shell nanoparticles affect their reactiv...
TL;DR: This Letter combines many quantitative techniques, including a newly developed peroxide titration, to assign and quantify decomposition pathways occurring in cells employing a variety of solvents and cathodes and finds that Li2O2-induced electrolyte solvent and salt instabilities account for nearly all efficiency losses upon discharge.
Abstract: Li–air batteries have generated enormous interest as potential high specific energy alternatives to existing energy storage devices. However, Li–air batteries suffer from poor rechargeability caused by the instability of organic electrolytes and carbon cathodes. To understand and address this poor rechargeability, it is essential to elucidate the efficiency in which O2 is converted to Li2O2 (the desired discharge product) during discharge and the efficiency in which Li2O2 is oxidized back to O2 during charge. In this Letter, we combine many quantitative techniques, including a newly developed peroxide titration, to assign and quantify decomposition pathways occurring in cells employing a variety of solvents and cathodes. We find that Li2O2-induced electrolyte solvent and salt instabilities account for nearly all efficiency losses upon discharge, whereas both cathode and electrolyte instabilities are observed upon charge at high potentials.
TL;DR: This work found that UiO-66, a prototypical Zr-MOF with high porosity, exhibits unusually high shear stability, which provides important guidelines for developing new porous MOFs targeting at high mechanical stabilities.
Abstract: Metal–organic frameworks (MOFs) with high porosity usually exhibit weak mechanical stabilities, in particular, rather low stabilities against shear stress. This limitation remains one of the bottlenecks for certain applications of porous MOFs, such as gas storage or separation that requires dense packing of the MOF powders under mechanical compression without collapsing the pores. We found that UiO-66, a prototypical Zr-MOF with high porosity, exhibits unusually high shear stability. Its minimal shear modulus (Gmin = 13.7 GPa) is an order of magnitude higher than those of other benchmark highly porous MOFs (e.g., MOF-5, ZIF-8, HKUST-1), approaching that of zeolites. Our analysis clearly shows that the exceptional mechanical stability of UiO-66 is due to its high framework connections (i.e., the high degree of coordination of Zr–O metal centers to the organic linkers). Our work thus provides important guidelines for developing new porous MOFs targeting at high mechanical stabilities.
TL;DR: It is shown here that the carrier mobility in the novel sp-sp(2) hybridization planar 6,6,12-graphyne sheet should be even larger than that in the graphene sheet, due to the sp- Sp2 hybridization forming the triple bonds in graphyne, the electron-phonon scattering is reduced.
Abstract: We show here that the carrier mobility in the novel sp–sp2 hybridization planar 6,6,12-graphyne sheet should be even larger than that in the graphene sheet. Both graphyne and graphene exhibit a Dirac cone structure near the Fermi surface. However, due to the sp–sp2 hybridization forming the triple bonds in graphyne, the electron–phonon scattering is reduced compared with that of graphene. The carrier mobility is calculated at the first-principles level by using the Boltzmann transport equation coupled with the deformation potential theory. The intrinsic mobility of the 6,6,12-graphyne is 4.29 × 105 cm2 V–1 s–1 for holes and 5.41 × 105 cm2 V–1 s–1 for electrons at room temperature, which is found to be larger than that of graphene (∼ 3 × 105 cm2 V–1 s–1).
TL;DR: It is reported that transition-metal dichalcogenides such as MoS2 and WS2 can be decorated with gold nanoparticles by a spontaneous redox reaction with hexachloroauric acid in water, showing significantly enhanced electrocatalytic performance toward hydrogen evolution reactions.
Abstract: Here, we report that transition-metal dichalcogenides such as MoS2 and WS2 can be decorated with gold nanoparticles by a spontaneous redox reaction with hexachloroauric acid in water. The resulting gold nanoparticles tend to grow at defective sites, and therefore, selective decorations at the edges and the line defects in the basal planes of bulk single crystals were observed. The lithium intercalation–exfoliation process makes the basal planes of chemically exfoliated MoS2 and WS2 sheets much more defective than their single-crystalline counterparts, leading to a more uniform and higher-density deposition of gold nanoparticles. Due to the greatly improved charge transport between adjacent sheets, the resulting MoS2/Au and WS2/Au hybrids show significantly enhanced electrocatalytic performance toward hydrogen evolution reactions.
TL;DR: This study shows that the ORR kinetics are much faster than OER kinetics and OER occurs in two distinct stages upon Li-air battery charging, and provides insights into bridging current two schools of thought on the OER mechanism.
Abstract: Understanding the reaction mechanism of nonaqueous oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is key to increase the low round-trip efficiency and power capability of rechargeable Li-air batteries. Here we show that the ORR kinetics are much faster than OER kinetics and OER occurs in two distinct stages upon Li-air battery charging. The first OER stage occurs at low overpotentials (<400 mV) with a slopping voltage profile, whose kinetics are relatively insensitive to charge rates and catalysts. This OER stage could be attributed to the delithiation of the outer part of Li2O2 forming lithium-deficient Li2–xO2, which is chemically disproportionate to evolve O2. The second stage takes place at high overpotentials (400–1200 mV), whose kinetics are sensitive to discharge/charge rates and catalysts, which can be attributed to the oxidation of bulk Li2O2 particles. Our study provides insights into bridging current two schools of thought on the OER mechanism.
TL;DR: It is found that certain structural defects in graphene can bind Li stably, yet a more efficacious approach is through substitution doping with boron (B), and the layered C3B compound stands out as a promising Li storage medium.
Abstract: Nanomaterials are anticipated to be promising storage media, owing to their high surface-to-mass ratio. The high hydrogen capacity achieved by using graphene has reinforced this opinion and motivated investigations of the possibility to use it to store another important energy carrier - lithium (Li). While the first-principles computations show that the Li capacity of pristine graphene, limited by Li clustering and phase separation, is lower than that offered by Li intercalation in graphite, we explore the feasibility of modifying graphene for better Li storage. It is found that certain structural defects in graphene can bind Li stably, yet a more efficacious approach is through substitution doping with boron (B). In particular, the layered C3B compound stands out as a promising Li storage medium. The monolayer C3B has a capacity of 714 mAh/g (as Li1.25C3B), and the capacity of stacked C3B is 857 mAh/g (as Li1.5C3B), which is about twice as large as graphite's 372 mAh/g (as LiC6). Our results help clarify the mechanism of Li storage in low-dimensional materials, and shed light on the rational design of nanoarchitectures for energy storage.
TL;DR: In this article, the effect of TiO2 film thickness on the charge transport, recombination, and device characteristics of perovskite (CH3NH3)PbI3 sensitized solar cells using iodide-based electrolytes was investigated.
Abstract: We report on the effect of TiO2 film thickness on the charge transport, recombination, and device characteristics of perovskite (CH3NH3)PbI3 sensitized solar cells using iodide-based electrolytes. (CH3NH3)PbI3 is relatively stable in a nonpolar solvent (e.g., ethyl acetate) with a low iodide concentration (e.g., 80 mM). Frequency-resolved modulated photocurrent/photovoltage spectroscopies show that increasing TiO2 film thickness from 1.8 to 8.3 μm has little effect on transport but increases recombination by more than 10-fold, reducing the electron diffusion length from 16.9 to 5.5 μm, which can be explained by the higher degree of iodide depletion within the TiO2 pores for thicker films. The changes of the charge-collection and light-absorption properties of (CH3NH3)PbI3 sensitized cells with varying TiO2 film thickness strongly affect the photocurrent density, photovoltage, fill factor, and solar conversion efficiency. Developing alternative, compatible redox electrolytes is important for (CH3NH3)PbI3 o...
TL;DR: The synthesis and activity of a phase-pure nanocrystal perovskite catalyst that is highly active for the OER and ORR is reported and a new OER cycle is proposed that unifies theory and the unique surface properties of LaNiO3.
Abstract: Perovskites are of great interest as replacements for precious metals and oxides used in bifunctional air electrodes involving the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, we report the synthesis and activity of a phase-pure nanocrystal perovskite catalyst that is highly active for the OER and ORR. The OER mass activity of LaNiO3, synthesized by the calcination of a rapidly dried nanoparticle dispersion and supported on nitrogen-doped carbon, is demonstrated to be nearly 3-fold that of 6 nm IrO2 and exhibits no hysteresis during oxygen evolution. Moreover, strong OER/ORR bifunctionality is shown by the low total overpotential (1.02 V) between the reactions, on par or better than that of noble metal catalysts such as Pt (1.16 V) and Ir (0.92 V). These results are examined in the context of surface hydroxylation, and a new OER cycle is proposed that unifies theory and the unique surface properties of LaNiO3.
TL;DR: The importance of the specific surface area, conductivity, and heteroatom-doping of graphene sheets and the micro/nanostructures of their electrodes for controlling the performances of graphene-based ECs is highlighted.
Abstract: Electrochemical capacitors (ECs) have been widely applied in electronics, electric vehicles, aircrafts, energy storage devices, uninterrupted or emergency power supplies, and so on. An ideal EC should have high energy and/or powder density, good rate capability, and long cycling life. Recently, graphene, graphene derivatives, and their composites have been explored as the electrode materials of ECs to satisfy these requirements. In this Perspective, we review the recent development in synthesizing graphene materials for ECs and discuss the strategies of fabricating graphene-based macroscopic electrodes. Particularly, we highlight the importance of the specific surface area, conductivity, and heteroatom-doping of graphene sheets and the micro/nanostructures of their electrodes for controlling the performances of graphene-based ECs.
TL;DR: The discharge product formed in Li-O2 batteries on electrodes composed of carpets of aligned carbon nanotubes was characterized to show copious nucleation of equiaxed Li2O2 particles precedes growth of discs and toroids at high discharge rates.
Abstract: Li-O2 batteries, wherein solid Li2O2 is formed at the porous air cathode during discharge, are candidates for high gravimetric energy (3212 Wh/kgLi2O2) storage for electric vehicles. Understanding and controlling the nucleation and morphological evolution of Li2O2 particles upon discharge is key to achieving high volumetric energy densities. Scanning and transmission electron microscopy were used to characterize the discharge product formed in Li-O2 batteries on electrodes composed of carpets of aligned carbon nanotubes. At low discharge rates, Li2O2 particles form first as stacked thin plates, ∼10 nm in thickness, which spontaneously splay so that secondary nucleation of new plates eventually leads to the development of a particle with a toroidal shape. Li2O2 crystallites have large (001) crystal faces consistent with the theoretical Wulff shape and appear to grow by a layer-by-layer mechanism. In contrast, at high discharge rates, copious nucleation of equiaxed Li2O2 particles precedes growth of discs and toroids.
TL;DR: In this paper, the authors investigated the hydrophobicity and flexibility of ZIF-8 by a detailed adsorption and diffusion study of a series of probe molecules including ethanol, 1-butanol, water, hexane isomers, xylene isomers and 1,2,4-trimethylbenzene.
Abstract: The framework hydrophobicity and flexibility of ZIF-8 are investigated by a detailed adsorption and diffusion study of a series of probe molecules including ethanol, 1-butanol, water, hexane isomers, xylene isomers, and 1,2,4-trimethylbenzene. The prospects for using ZIF-8 in biofuel recovery and hydrocarbon separations are discussed in terms of adsorption or kinetic selectivities. ZIF-8 shows extremely low water vapor uptakes and is especially suitable for vapor phase butanol-based biofuel recovery. The extraordinary framework flexibility of ZIF-8 is demonstrated by the adsorption of hydrocarbon molecules that are much larger than its nominal pore size, such as m-xylene, o-xylene and 1,2,4-trimethylbenzene. The calculation of corrected diffusion coefficients reveals an interesting spectrum of promising kinetic hydrocarbon separations by ZIF-8. These findings confirm that a molecular sieving effect tends to occur in the sorbate molecular size range of 4–6 A rather than around the nominal ZIF-8 pore size o...
TL;DR: Results from transient photocurrent spectroscopy suggest that two separate loss mechanisms are indeed occurring and can be addressed separately, and calls into question the definition of a catalyst when coupled to a semiconductor photoelectrode.
Abstract: The photoelectrochemical reduction of water or CO2 is a promising route to sustainable solar fuels but hinges on the identification of a stable photoanode for water oxidation. Semiconductor oxides like Fe2O3 and BiVO4 have been gaining significant attention as promising materials. However, they exhibit a major drawback of a large required overpotential for solar water oxidation. In this Perspective, recent efforts to characterize and reduce the overpotential are critically examined. The accumulation of photogenerated holes at the semiconductor–liquid interface, recently observed with multiple techniques, is rationalized with surface state models. Transient absorption spectroscopy and electrochemical impedance spectroscopy suggest that surface treatments designed to either passivate surface traps or increase reaction rates (as catalysts) actually perform identically. This calls into question the definition of a catalyst when coupled to a semiconductor photoelectrode. In contrast, results from transient pho...
TL;DR: Scales for Quantitative Analysis of Emission Spectra E ffective representation of data plays a vital role in scientific communication as discussed by the authors, and a different representation of the same raw data may facilitate better quantitative analysis.
Abstract: Scales for Quantitative Analysis of Emission Spectra E ffective representation of data plays a vital role in scientific communication. Moreover, a different representation of the same raw data may facilitate better quantitative analysis. Often, signal data are recorded and presented as a function of the electromagnetic spectrum. Commonly, spectra are recorded in wavelength units, but presentation of data as a function of energy provides better physical insight. The inverse relationship between the two quantities presents some challenges and has sometimes led to incorrect conclusions, such as the assertion that the spectral response of the human eye is optimized to match the solar spectrum. Clearly, proper presentation of spectral data is critical for enlightening scientific investigation.