Abstract
Oxygen-anion redox in lithium-rich layered oxides can boost the capacity of lithium-ion battery cathodes. However, the over-oxidation of oxygen at highly charged states aggravates ...irreversible structure changes and deteriorates cycle performance. Here, we investigate the mechanism of surface degradation caused by oxygen oxidation and the kinetics of surface reconstruction. Considering Li
2
MnO
3
, we show through density functional theory calculations that a high energy orbital (lO
2
p
’
) at under-coordinated surface oxygen prefers over-oxidation over bulk oxygen, and that surface oxygen release is then kinetically favored during charging. We use a simple strategy of turning under-coordinated surface oxygen into polyanionic (SO
4
)
2−
, and show that these groups stabilize the surface of Li
2
MnO
3
by depressing gas release and side reactions with the electrolyte. Experimental validation on Li
1.2
Ni
0.2
Mn
0.6
O
2
shows that sulfur deposition enhances stability of the cathode with 99.0% capacity remaining (194 mA h g
−1
) after 100 cycles at 1 C. Our work reveals a promising surface treatment to address the instability of highly charged layered cathode materials.
Electric vehicles and grid storage devices have potentialto become feasible alternatives to current technology, but only if scientists can develop energy storage materials that offer high capacity ...and high rate capabilities. Chemists have studied anatase, rutile, brookite and TiO2(B) (bronze) in both bulk and nanostructured forms as potential Li-ion battery anodes. In most cases, the specific capacity and rate of lithiation and delithiation increases as the materials are nanostructured. Scientists have explained these enhancements in terms of higher surface areas, shorter Li+ diffusion paths and different surface energies for nanostructured materials allowing for more facile lithiation and delithiation. Of the most studied polymorphs, nanostructured TiO2(B) has the highest capacity with promising high rate capabilities. TiO2(B) is able to accommodate 1 Li+ per Ti, giving a capacity of 335 mAh/g for nanotubular and nanoparticulate TiO2(B). The TiO2(B) polymorph, discovered in 1980 by Marchand and co-workers, has been the focus of many recent studies regarding high power and high capacity anode materials with potential applications for electric vehicles and grid storage. This is due to the material’s stability over multiple cycles, safer lithiation potential relative to graphite, reasonable capacity, high rate capability, nontoxicity, and low cost (Bruce, P. G.; Scrosati, B.; Tarascon, J.-M. Nanomaterials for Rechargeable Lithium Batteries. Angew. Chem., Int. Ed. 2008, 47, 2930–2946). One of the most interesting properties of TiO2(B) is that both bulk and nanostructured forms lithiate and delithiate through a surface redox or pseudocapacitive charging mechanism, giving rise to stable high rate charge/discharge capabilities in the case of nanostructured TiO2(B). When other polymorphs of TiO2 are nanostructured, they still mainly intercalate lithium through a bulk diffusion-controlled mechanism. TiO2(B) has a unique open crystal structure and low energy Li+ pathways from surface to subsurface sites, which many chemists believe to contribute to the pseudocapacitive charging. Several disadvantages exist as well. TiO2(B), and titania in general, suffers from poor electronic and ionic conductivity. Nanostructured TiO2(B) also exhibits significant irreversible capacity loss (ICL) upon first discharge (lithiation). Nanostructuring TiO2(B) can help alleviate problems with poor ionic conductivity by shortening lithium diffusion pathways. Unfortunately, this also increases the likelihood of severe first discharge ICL due to reactive Ti–OH and Ti–O surface sites that can cause unwanted electrolyte degradation and irreversible trapping of Li+. Nanostructuring also results in lowered volumetric energy density, which could be a considerable problem for mobile applications. We will also discuss these problems and proposed solutions. Scientists have synthesized TiO2(B) in a variety of nanostructures including nanowires, nanotubes, nanoparticles, mesoporous-ordered nanostructures, and nanosheets. Many of these structures exhibit enhanced Li+ diffusion kinetics and increased specific capacities compared to bulk material, and thus warrant investigation on how nanostructuring influences lithiation behavior. This Account will focus on these influences from both experimental and theoretical perspectives. We will discuss the surface charging mechanism that gives rise to the increased lithiation and delithiation kinetics for TiO2(B), along with the influence of dimensional confinement of the nanoarchitectures, and how nanostructuring can change the lithiation mechanism considerably.
Abstract
Designing active and stable electrocatalysts with economic efficiency for acidic oxygen evolution reaction is essential for developing proton exchange membrane water electrolyzers. Herein, ...we report on a cobalt oxide incorporated with iridium single atoms (Ir-Co
3
O
4
), prepared by a mechanochemical approach. Operando X-ray absorption spectroscopy reveals that Ir atoms are partially oxidized to active Ir
>4+
during the reaction, meanwhile Ir and Co atoms with their bridged electrophilic O ligands acting as active sites, are jointly responsible for the enhanced performance. Theoretical calculations further disclose the isolated Ir atoms can effectively boost the electronic conductivity and optimize the energy barrier. As a result, Ir-Co
3
O
4
exhibits significantly higher mass activity and turnover frequency than those of benchmark IrO
2
in acidic conditions. Moreover, the catalyst preparation can be easily scaled up to gram-level per batch. The present approach highlights the concept of constructing single noble metal atoms incorporated cost-effective metal oxides catalysts for practical applications.
An algorithm is presented for carrying out decomposition of electronic charge density into atomic contributions. As suggested by Bader R. Bader, Atoms in Molecules: A Quantum Theory, Oxford ...University Press, New York, 1990, space is divided up into atomic regions where the dividing surfaces are at a minimum in the charge density, i.e. the gradient of the charge density is zero along the surface normal. Instead of explicitly finding and representing the dividing surfaces, which is a challenging task, our algorithm assigns each point on a regular (
x,
y,
z) grid to one of the regions by following a steepest ascent path on the grid. The computational work required to analyze a given charge density grid is approximately 50 arithmetic operations per grid point. The work scales linearly with the number of grid points and is essentially independent of the number of atoms in the system. The algorithm is robust and insensitive to the topology of molecular bonding. In addition to two test problems involving a water molecule and NaCl crystal, the algorithm has been used to estimate the electrical activity of a cluster of boron atoms in a silicon crystal. The highly stable three-atom boron cluster, B
3I is found to have a charge of −1.5
e, which suggests approximately 50% reduction in electrical activity as compared with three substitutional boron atoms.
We use density functional theory (DFT) to study CO-adsorption-induced Pd surface segregation in Au/Pd bimetallic surfaces, dynamics of Pd–Au swapping, effect of defects on the swapping rate, ...CO-induced Pd clustering, and the reaction mechanism of CO oxidation. The strong CO-philic nature of Pd atoms supplies a driving force for the preferential surface segregation of Pd atoms and Pd cluster formation. Surface vacancies are found to dramatically accelerate the rate of Pd–Au swapping. We find that Pd clusters consisting of at least four Pd atoms prefer to bind O2 rather than CO. These clusters facilitate the rapid dissociation of O2 and supply reactive oxygen species for CO oxidation. Our findings suggest that geometric, electronic, and dynamic effects should be considered in the function of bimetallic alloys or nanoparticles whose components asymmetrically interact with reacting molecules.
Sodium is globally available, which makes a sodium-ion rechargeable battery preferable to a lithium-ion battery for large-scale storage of electrical energy, provided a host cathode for Na can be ...found that provides the necessary capacity, voltage, and cycle life at the prescribed charge/discharge rate. Low-cost hexacyanometallates are promising cathodes because of their ease of synthesis and rigid open framework that enables fast Na+ insertion and extraction. Here we report an intriguing effect of interstitial H2O on the structure and electrochemical properties of sodium manganese(II) hexacyanoferrates(II) with the nominal composition Na2MnFe(CN)6·zH2O (Na2−δMnHFC). The newly discovered dehydrated Na2−δMnHFC phase exhibits superior electrochemical performance compared to other reported Na-ion cathode materials; it delivers at 3.5 V a reversible capacity of 150 mAh g–1 in a sodium half cell and 140 mAh g–1 in a full cell with a hard-carbon anode. At a charge/discharge rate of 20 C, the half-cell capacity is 120 mAh g–1, and at 0.7 C, the cell exhibits 75% capacity retention after 500 cycles.
Developing cost‐effective, high‐performance nitrogen reduction reaction (NRR) electrocatalysts is required for the production of green and low‐cost ammonia under ambient conditions. Here, a strategy ...is proposed to adjust the reaction preference of noble metals by tuning the size and local chemical environment of the active sites. This proof‐of‐concept model is realized by single ruthenium atoms distributed in a matrix of graphitic carbon nitride (Ru SAs/g‐C3N4). This model is compared, in terms of the NRR activity, to bulk Ru. The as‐synthesized Ru SAs/g‐C3N4 exhibits excellent catalytic activity and selectivity with an NH3 yield rate of 23.0 µg mgcat−1 h−1 and a Faradaic efficiency as high as 8.3% at a low overpotential (0.05 V vs the reversible hydrogen electrode), which is far better than that of the bulk Ru counterpart. Moreover, the Ru SAs/g‐C3N4 displays a high stability during five recycling tests and a 12 h potentiostatic test. Density functional theory calculations reveal that compared to bulk Ru surfaces, Ru SAs/g‐C3N4 has more facile reaction thermodynamics, and the enhanced NRR performance of Ru SAs/g‐C3N4 originates from a tuning of the d‐electron energies from that of the bulk to a single‐atom, causing an up‐shift of the d‐band center toward the Fermi level.
Ru SAs/g‐C3N4 exhibits excellent catalytic activity and selectivity, with an NH3 yield rate of 23.0 µg mgcat−1 h−1 and a Faradaic efficiency as high as 8.3% at 0.05 V versus reversible hydrogen electrode, which is far better than that of the bulk Ru counterpart. This is because Ru SAs/g‐C3N4 has stronger N2 adsorption and less H poisoning at the reactive sites.
The active sites of heterogeneous catalysts can be difficult to identify and understand, and, hence, the introduction of active sites into catalysts to tailor their function is challenging. During ...the past two decades, scaling relationships have been established for important heterogeneous catalytic reactions. More specifically, a physical or chemical property of the reaction system, termed as a reactivity descriptor, scales with another property often in a linear manner, which can describe and/or predict the catalytic performance. In this Review, we describe scaling relationships and reactivity descriptors for heterogeneous catalysis, including electronic descriptors represented by d-band theory, structural descriptors, which can be directly applied to catalyst design, and, ultimately, universal descriptors. The prediction of trends in catalytic performance using reactivity descriptors can enable the rational design of catalysts and the efficient screening of high-throughput catalysts. Finally, we outline methods to break scaling relationships and, hence, to break the constraint that active sites pose on the catalytic performance.Recently, scaling relationships have been established between certain physical or chemical properties of heterogeneous catalytic reactions. These properties, or reactivity descriptors, can describe and predict catalytic performance, and thus enable the rational design of new catalysts.
The mechanisms of ethanol (EtOH) decomposition via C–C or C–O bond cleavage on alloy surfaces are currently not well understood. In this study, we model EtOH decomposition on close-packed Pd–Au ...catalytic surfaces using density functional theory (DFT) calculations and derived Brønsted–Evans–Polanyi (BEP) relationships. Three characteristic Pd–Au surfaces are considered, Pd1Au2(111), Pd2Au1(111), and a Pd monolayer (ML), PdML(111), on a Au substrate. We show that, on close-packed Pd–Au surfaces, the C–C bond is easier to cleave than C–O, indicating that the formation of CH4 and CO is favored as the products of EtOH decomposition. Interestingly, we find that, though the C–C and C–O activation barriers on PdML(111) are generally lower than those on the other two surfaces, it is less active for EtOH decomposition due to a slow release of H2 and possible carbon coking. Pd2Au1(111), on the other hand, has a higher theoretical reaction rate due to facile H2 evolution from the surface and less carbon coking. A comparison of the surface d-band with the activation energy barriers shows that there is a trade-off between the barriers for C–C bond cleavage and H2 association, with Pd2Au1(111) having the best performance. Temperature-programmed desorption experiments of EtOH on Pd/Au surfaces show significant C–C bond cleavage and both CH4 and CO production on surfaces with Pd–Au interface sites. Furthermore, neither Auger electron spectroscopy nor EtOH reflection–adsorption infrared spectroscopy provide evidence of C–O bond cleavage. Finally, the experimental reaction rate for methane production from C–C bond cleavage was higher on surfaces with more Au present due to minimal carbon contamination and the promotion of product desorption. This combined theoretical and experimental study shows that, though Au is catalytically inactive for EtOH decomposition, it can dramatically promote the surface activity for EtOH steam reforming due to the existence of active Pd–Au surface ensemble sites.
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