The physical mechanisms which may contribute to the energy and entropy of mixing in oxide systems are identified and discussed. Ionic size, magnetism and electrostatics can all contribute to the configurational energy dependence of transition-metal oxides. While the many sources of substitutional disorder make configurational entropy an essential contribution to the free energy of oxides, electronic and magnetic entropy may be of the same order of magnitude. This is illustrated with some first-principles results on LiCoO2 and LiMnO2.

We have carried out a periodic Kohn-Sham density functional theory investigation of the pathways by which carbon-carbon bonds could be formed during the electrochemical reduction of CO2 on Cu(100) using a model that includes the effects of the electrochemical potential, solvent, and electrolyte. The electrochemical potential was set by relating the applied potential to the Fermi energy and then calculating the number of electrons required by the simulation cell for that specific Fermi energy. The solvent was included as a continuum dielectric, and the electrolyte was described using a linearized Poisson-Boltzmann model. The calculated potential of zero charge for a variety of surfaces agrees with experiment to within a mean average error of 0.09 V, thereby validating the assumptions of the model. Analysis of the mechanism for C-C bond formation revealed that at low-applied potential, C-C bond formation occurs through a CO dimer. However, at high applied potentials, a large activation barrier blocks this pathway; therefore, C-C bond formation occurs through reaction of adsorbed CHO and CO. Rate parameters determined from our calculations were used to simulate the kinetics of ethene formation during the electrochemical reduction of CO over a Cu(100) surface. An excellent match was observed between previously reported measurements of the partial current for ethene formation as a function of applied voltage and the variation in the partial current for C-C bond formation predicted by our microkinetic model. The electrochemical model reported here is simple, fairly easy to implement, and involves only a small increase in computational cost over calculations neglecting the effects of the electrolyte and the applied field. Therefore, it can be used to study the effects of applied potential and electrolyte composition on the energetics of surface reactions for a wide variety of electrochemical reactions.

In the original version of this article, when equations were referenced within the main text, the numbering was offset by 2. For instance, when eq 1 was referenced, it was incorrectly referred to as eq 3. When eq 16 was referenced, it was incorrectly referenced as eq 18, and so on and so forth for all other references to the equations in the main text. The correct equation referencing is shown below for all instances:n.

The electrochemical reduction of carbon dioxide is sensitive to electrolyte polarization, which causes gradients in pH and the concentration of carbon dioxide to form near the cathode surface. It is desirable to measure the concentration of reaction-relevant species in the immediate vicinity of the cathode because the intrinsic kinetics of carbon dioxide reduction depend on the composition of the local reaction environment. Meeting this objective has proven difficult because conventional analytical methods only sample products from the bulk electrolyte. In this study, we describe the use of differential electrochemical mass spectrometry to measure the concentration of carbon dioxide and reaction products in the immediate vicinity of the cathode surface. This capability is achieved by coating the electrocatalyst directly onto the pervaporation membrane used to transfer volatile species into the mass spectrometer, thereby enabling species to be sampled directly from the electrode-electrolyte interface. This approach has been used to investigate hydrogen evolution and carbon dioxide reduction over Ag and Cu. We find that the measured CO₂ reduction activity of Ag agrees well with what is measured by gas chromatography of the effluent from an H-cell operated with the same catalyst and electrolyte. A distinct advantage of our approach is that it enables observation of the depletion of carbon dioxide near the cathode surface due to reaction with hydroxyl anions evolved at the cathode surface, something that cannot be done using conventional analytical techniques. We also demonstrate that the influence of this relatively slow chemical reaction can be minimized by evaluating electrocatalytic activity during a rapid potential sweep, thereby enabling measurement of the intrinsic kinetics. For CO₂ reduction over Cu, nine products can be observed simultaneously in real time. A notable finding is that the abundance of aldehydes relative to alcohols near the cathode surface is much higher than that observed in the bulk electrolyte. It is also observed that for increasingly cathodic potentials the relative abundance of ethanol increases at the expense of propionaldehyde. These findings suggest that acetaldehyde is a precursor to ethanol and propionaldehyde and that propionaldehyde is a precursor to n-propanol.

The behavior and role of hydrogen is investigated by using Pt–Ga nano-alloy formation as a probe reaction.

The aim of this study was to investigate the influence of Si/Al ratio on the locations of exchangeable cations in H-MFI and on the monomolecular cracking and dehydrogenation reactions of n-butane. On the basis of UV-visible spectroscopic analysis of Co(II) exchanged into MFI, it was inferred that the fraction of Co(II) (and, by extension, Brønsted protons) located at channel intersections relative to straight and sinusoidal channels increases with increasing Al content. Concurrently, turnover frequencies for all monomolecular reactions, and the selectivities to dehydrogenation versus cracking and to terminal cracking versus central cracking, generally increased. The changes in selectivity with Al content are consistent with the finding that the transition-state geometry for dehydrogenation is bulky and resembles a product state, and should therefore exhibit a stronger preference to occur at channel intersections relative to cracking. Increases in turnover frequencies are attributed partly to increases in intrinsic activation entropies that compensate for concurrent increases in intrinsic activation energies, most strongly for dehydrogenation and terminal cracking, resulting in increased selectivity to these reactions at higher Al content. This interpretation contrasts with the view that intrinsic activation barriers are constant. It is also observed that isobutene inhibits the rate of n-butane dehydrogenation. Theoretical calculations indicate that this effect originates from adsorption of isobutene at the channel intersections. Because cracking reaction rates are not affected by the presence of isobutene, this result suggests that the preference of dehydrogenation to occur at channel intersections is much stronger than the preference for cracking to occur at these locations.

This work quantifies the performance of gas-diffusion electrodes using multiphysics modeling and provides design guidance.

Racial disparity in academia is a widely acknowledged problem. The quantitative understanding of racial-based systemic inequalities is an important step towards a more equitable research system. However, because of the lack of robust information on authors’ race, few large-scale analyses have been performed on this topic. Algorithmic approaches offer one solution, using known information about authors, such as their names, to infer their perceived race. As with any other algorithm, the process of racial inference can generate biases if it is not carefully considered. The goal of this article is to assess the extent to which algorithmic bias is introduced using different approaches for name-based racial inference. We use information from the U.S. Census and mortgage applications to infer the race of U.S. affiliated authors in the Web of Science. We estimate the effects of using given and family names, thresholds or continuous distributions, and imputation. Our results demonstrate that the validity of name-based inference varies by race/ethnicity and that threshold approaches underestimate Black authors and overestimate White authors. We conclude with recommendations to avoid potential biases. This article lays the foundation for more systematic and less-biased investigations into racial disparities in science.

The characterization of native point defects in ZnO is still a question of debate. For example, experimental evidence for ZnO with an excess of Zn is inconclusive as to whether the dominant defects are metal interstitials or oxygen vacancies. This information is essential to understand the behavior of the material and to tailor its numerous technological applications. We use the first-principles pseudopotential method to determine the electronic structure, atomic geometry, and formation energy of native point defects in ZnO. Interstitials, vacancies, and antisites in their relevant charge states are considered and the effects of dopants are also discussed. The results show that both the Zn and O vacancies are the relevant defects in ZnO. We also propose a possible transition mechanism and defect center responsible for the experimentally observed green luminescence.

There is increasing interest in the possibility of photoelectrochemical (PEC) reduction of CO<sub>2</sub> to C<sub>2+</sub> products; however, the criteria for maximizing PEC solar-to-C<sub>2+</sub> (STC<sub>2+</sub>) rates are not well understood. We report here a continuum-scale model of PEC CO<sub>2</sub> reduction (CO<sub>2</sub>R) on Cu in 0.1 M CsHCO<sub>3</sub> and use it to optimize the design and operating conditions for generating C<sub>2+</sub>products. Furthermore, we demonstrate that the potential-dependent product distribution of CO<sub>2</sub>R on Cu requires operating near the potential that maximizes C<sub>2+</sub> generation rates ($V$<sub>id</sub>), unlike PEC water splitting, which desires operation at the maximum photocurrent density. Because of this requirement, the criterion for a high STC<sub>2+</sub> rate includes high-photocurrent semiconductors with photovoltages near $V$<sub>id</sub> and low series resistance. The STC<sub>2+</sub> rate in these systems is enhanced by optimal CO<sub>2</sub> transport and exhibits low sensitivity to dirunal solar irradiance variations.

Electrochemical CO 2 reduction (CO 2 R) is a promising technology that could enable electricity generated from intermittent renewable sources to be stored in the form of carbon-neutral fuels and chemical precursors. Currently, metallic copper (Cu) is the only known electrocatalyst capable of reducing CO 2 to hydrocarbons and alcohols. However, polycrystalline Cu produces up to 16 different reaction products at an applied potential of -1 V vs RHE, with hydrogen, methane, and ethene accounting for the majority of the charge passed. This lack of selectivity has motivated considerable interest in discovering ways to modify the Cu surface that enhance the reaction selectivity to a single desired product. Of particular interest are C 2 products, such as ethanol and ethene, due to their high market value and fuel potential. Prior research efforts have identified several under coordinated Cu single crystal electrodes that exhibit an enhanced selectivity for C 2+ products compared to polycrystalline Cu; however, such electrocatalysts are not scalable and are likely unstable. The selectivity of ethene relative to methane has also been enhanced by nanostructuring the Cu surface by electrochemical cycling and by the electrodeposition of Cu (I) oxide thin-films. However, these electrocatalysts are generally more selective for the formation of hydrogen than hydrocarbons. We note that all reports in the current literature observe a higher selectivity for hydrocarbons than oxygenates over metallic Cu and that there are currently no known methods of enhancing the oxygenate selectivity. Carbon monoxide (CO) reduction has been identified both experimentally and theoretically as the overpotential-determining step in the reduction of CO 2 to hydrocarbons and alcohols over polycrystalline Cu. CO is known to cover a substantial portion of the Cu surface at steady state, resulting in the suppression of the hydrogen evolution reaction (HER) by competitive adsorption for active surface sites. Interestingly, the reaction selectivity observed during CO 2 R over polycrystalline Cu has been reported to change dramatically at potentials negative of -1 V vs RHE, with hydrogen and methane rapidly increasing at the expense of C 2+ products. The onset potential of this selectivity shift agrees well with the calculated onset potential of CO 2 depletion due to concentration polarization within the hydrodynamic boundary layer at the Cu surface. The local depletion of CO 2 results in a lower steady state coverage of adsorbed CO on the Cu surface, reducing the rate of C-C coupling and enhancing the methanation of CO. This analysis suggests that higher CO coverages on Cu would result in an increase in the rate of C-C coupling between CO-derived intermediates, such as formyl, at the expense of C-H bond formation. Based on the aforementioned trends in the current literature, we hypothesized that a phase segregated bimetallic alloy of Cu with a CO-generating metal would enhance the selectivity to oxygenated multi-carbon products at the expense of hydrogen and hydrocarbons. We expect this selectivity shift to occur because the supply of additional CO by spillover will enhance the steady state coverage of CO adsorbed to the Cu surface. While several different metals have been identified as being CO selective, such as Au, Ag, and Zn, only Ag is completely immiscible with Cu and exclusively produces CO at applied potentials where Cu is capable of reducing >95% of the CO that it produces. The reaction selectivity observed during CO 2 R over the phase segregated CuAg bimetallic electrodes has led to the conclusion that they possess a reaction selectivity unlike either metallic constituent. The individual proficiencies of each metal were utilized synergistically to sequentially reduce CO 2 , first to CO over Ag and then to hydrocarbons and alcohols over Cu. The supply of additional CO to Cu by spillover from Ag increases the surface coverage of this key reaction intermediate, resulting in a selectivity shift that favors the formation of multi-carbon oxygenates at the expense of hydrogen and hydrocarbons. This effect is so pronounced that these CuAg bimetallic electrodes are currently the only electrocatalyst yet discovered that is more selective for the formation of multi-carbon oxygenates than hydrocarbons. The selectivity of oxygenates relative to hydrocarbons was found to scale with the relative distribution of Cu and Ag facet terminations present at the electrode surface, in agreement with the CO spillover hypothesis. By tuning the surface composition of the CuAg bimetallic electrode, the selectivity to multi-carbon oxygenates relative to hydrocarbons was increased by a factor of ~6 as compared to polycrystalline Cu.