7 Search Results

Cascade catalysis for photoelectrochemical CO₂ reduction (CO₂R) decouples the overall reaction into sequential steps occurring on separately optimized catalysts (for example, Ag and Cu) between which an intermediate species such as CO is transferred. A 3-terminal tandem (3TT) photovoltaic architecture advantageously holds two different catalytic regions at different potentials under a single illumination source, but its overall efficiency is low. Using a stochastic reaction-diffusion model, we have examined 3TT photocathode design principles, focusing on the coupling of surface chemistry and transport of CO both inside the boundary layer and outward, into the bulk electrolyte. We find that ensuring that themore » lateral diffusion distance within the boundary layer is short compared to the boundary layer thickness and controlling bulk flow are key, with interdigitated designs showing an overall conversion efficiency improvement by 2 orders of magnitude compared to the side-by-side case without flow. These findings can be adapted for other cascaded architectures.« less

Electrochemical carbon dioxide reduction (CO₂R) is a promising approach for the decentralized production of fuels such as ethylene (C₂H₄). However, the use of Cu, the most efficient metal CO₂R catalyst for the generation of C₂H₄ known to date, generally yields a product stream with poor selectivity. In an effort to increase selectivity, the reaction from CO₂ to C₂H₄ can be broken down into two steps using tandem CO₂R electrolyzers: formation of CO from CO₂ and subsequent reduction of CO to C₂H₄. Here, in this study, we present two novel tandem electrolyzer architectures that closely integrate two cathodes, one for COmore » generation and one for conversion to C₂H₄, while still enabling independent electrical control of the cathodic surfaces. Cathode segmentation in each of these designs also permits the controlled sequencing of mass flow of chemical intermediates in the order of Au to Cu cathode catalysts, in contrast to earlier work relying on uncontrolled, passive diffusion to facilitate the flow of chemical intermediates between catalysts. When comparing the performance of the newly developed electrolyzer cell designs with a dual electrolyzer system, we found that the dual electrolyzer system yields the highest C₂H₄ faradaic efficiencies (FEs) of 31% and C₂H₄ concentrations (∼8 mol %). However, a single Cu-containing electrolyzer outperformed all three tandem systems in terms of C₂H₄ FE (34%). Our findings, enabled by independent control of the two tandem cathode surfaces, indicate that tandem CO₂R systems need to be evaluated carefully by testing them at various relevant current densities.« less

Ni-rich cathode materials provide high energy density, but their structural and surface instability limits their cyclability and thermal stability. As one of the approaches to mitigate this problem, cathode materials comprising Ni-rich high-capacity core wrapped in Mn-rich multiple shells are produced successfully. In contrast to the conventional batch-type process for concentration-gradient materials, a digital-gradient cascade coprecipitation process described here achieves the improvements in productivity and quality consistency needed to move toward large-scale manufacturing. The core-multishell cathode materials produced in this manner not only have longer cycle life and improved rate performance compared to homogeneous Ni-rich cathode materials having the samemore » overall composition, but also show remarkably enhanced thermal stability and low impedance growth characteristics. In a novel attempt to determine the correlation between the mechanical properties of the core-multishell cathode particles and their electrochemical cyclabilities, their breaking force and elasticity were successfully measured using a statistical approach, which indicates that a cathode particle with stable surface composition as well as high breaking force has improved capacity retention and durability. These results guide the realization of long life and high thermal stability in Ni-rich cathode materials through heterogeneous particle engineering.« less

The role of baroclinicity, which arises from the misalignment of pressure and density gradients, is well-known in the vorticity equation, yet its role in the kinetic energy budget has never been obvious. Here, we show that baroclinicity appears naturally in the kinetic energy budget after carrying out the appropriate scale decomposition. Strain generation by pressure and density gradients, both barotropic and baroclinic, also results from our analysis. These two processes underlie the recently identified mechanism of “baropycnal work”, which can transfer energy across scales in variable density flows. As such, baropycnal work is markedly distinct from pressure-dilatation into which themore » former is implicitly lumped in Large Eddy Simulations. We provide numerical evidence from 1024³ direct numerical simulations of compressible turbulence. The data shows excellent pointwise agreement between baropycnal work and the nonlinear model we derive, supporting our interpretation of how it operates.« less

Substitution of O in the ZnS lattice is observed to result in a doublet consisting of two local vibrational modes (LVMs) and a second order harmonic of the LVMs. The first order doublet is attributed to vibrational modes of a defect with trigonal (C_3v) symmetry. The results of temperature dependent resonant Raman scattering and anti-Stokes scattering support this assignment. The substitution of O in the lattice also leads to the presence of LVM + nLO combination modes, which are interpreted in terms of the cascade model and resonance effects.

Numerous nuclear processes involve the deexcitation of a compound nucleus through the emission of several neutrons, gamma-rays and/or conversion electrons. The characteristics of such a deexcitation are commonly derived from a total statistical framework often called “Hauser–Feshbach” method. In this work, we highlight a numerical limitation of this kind of method in the case of the deexcitation of a high spin initial state. To circumvent this issue, an improved technique called the Fluctuating Structure Properties (FSP) method is presented. Two FSP algorithms are derived and benchmarked on the calculation of the total radiative width for a thermal neutron capture onmore » ²³⁸U. We compare the standard method with these FSP algorithms for the prediction of particle multiplicities in the deexcitation of a high spin level of ¹⁴³Ba. The gamma multiplicity turns out to be very sensitive to the numerical method. The bias between the two techniques can reach 1.5 γγ/cascade. Lastly, the uncertainty of these calculations coming from the lack of knowledge on nuclear structure is estimated via the FSP method.« less