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Improved reversibility in lithium-oxygen battery: Understanding elementary reactions and surface charge engineering of metal alloy catalyst

Most Li-O2 batteries suffer from sluggish kinetics during oxygen evolution reactions (OERs). To overcome this drawback, we take the lesson from other catalysis researches that showed improved catalytic activities by employing metal alloy catalysts. Such research effort has led us to find Pt3Co nanoparticles as an effective OER catalyst in Li-O2 batteries. The superior catalytic activity was reflected in the substantially decreased overpotentials and improved cycling/rate performance compared to those of other catalysts. Density functional theory calculations suggested that the low OER overpotentials are associated with the reduced adsorption strength of LiO2 on the outermost Pt catalytic sites. Also, the alloy catalyst generates amorphous Li2O2 conformally coated around the catalyst and thus facilitates easier decomposition and higher reversibility. This investigation conveys an important message that understanding elementary reactions and surface charge engineering of air-catalysts are one of the most effective approaches in resolving the chronic sluggish charging kinetics in Li-O2 batteries.

Ordered supramolecular gels based on graphene oxide and tetracationic cyclophanes.

A new strategy to form ordered hierarchical supramolecular gels that incorporate graphene oxide (GO) sheets and cationic rigid macrocyles under mild conditions via self-assembly is demonstrated. These ordered gels are stabilized by a series of non-covalent - donor-acceptor, π-π stacking, cation-π - interactions. These theoretical studies indicate that cationic macrocycles are positioned in between GO layers with a substantial binding energy.

Balancing activity, stability and conductivity of nanoporous core-shell iridium/iridium oxide oxygen evolution catalysts

The selection of oxide materials for catalyzing the oxygen evolution reaction in acid-based electrolyzers must be guided by the proper balance between activity, stability and conductivity—a challenging mission of great importance for delivering affordable and environmentally friendly hydrogen. Here we report that the highly conductive nanoporous architecture of an iridium oxide shell on a metallic iridium core, formed through the fast dealloying of osmium from an Ir25Os75 alloy, exhibits an exceptional balance between oxygen evolution activity and stability as quantified by the activity-stability factor. On the basis of this metric, the nanoporous Ir/IrO2 morphology of dealloyed Ir25Os75 shows a factor of ~30 improvement in activity-stability factor relative to conventional iridium-based oxide materials, and an ~8 times improvement over dealloyed Ir25Os75 nanoparticles due to optimized stability and conductivity, respectively. We propose that the activity-stability factor is a key “metric” for determining the technological relevance of oxide-based anodic water electrolyzer catalysts.Production of affordable, clean hydrogen relies on efficient oxygen evolution, but improving catalytic performance for the reaction in acidic media is challenging. Here the authors show how tuning the nanoporous morphology of iridium/iridium oxide leads to an improvement in activity/stability, compared with conventional iridium-based oxides.

Importance of Ligand Effects Breaking the Scaling Relation for Core–Shell Oxygen Reduction Catalysts

Tremendous recent efforts have been made toward developing highly active oxygen reduction reaction (ORR) catalysts with a minimized usage of noble metal Pt by using Pt alloys and core–Pt shell structures. A main computational framework for such a goal has been the search for a new material with the *OH binding slightly weaker than Pt based on the conventional volcano relation of ORR activity versus *OH binding energy. In this work, by using carbides and nitrides as core materials, we demonstrate that the conventional scaling relation between *OH and *O can be completely broken owing to a significant ligand–Pt orbital interactions in the core–Pt shell structure, and in such cases, the usual catalyst design strategy of tuning the *OH binding energy of Pt to a weaker leg of the volcano can mislead the prediction. In these cases, one needs to consider all reaction intermediates to appropriately predict the activity of ORR catalysts. We additionally show that, although the transition metal nitrides and carbides studied here as core materials all induce an undesired tensile strain to the Pt overlayers with a stronger *OH binding, a proper tuning of the ligand (core) effects in the Pt1 and Pt2 overlayers core–shell configurations can lead to an activity comparable to or slightly better than Pt.

On the structure of Si(100) surface: Importance of higher order correlations for buckled dimer

We revisit a dangling theoretical question of whether the surface reconstruction of the Si(100) surface would energetically favor the symmetric or buckled dimers on the intrinsic potential energy surfaces at 0 K. This seemingly simple question is still unanswered definitively since all existing density functional based calculations predict the dimers to be buckled, while most wavefunction based correlated treatments prefer the symmetric configurations. Here, we use the doubly hybrid density functional (DHDF) geometry optimizations, in particular, XYGJ-OS, complete active space self-consistent field theory, multi-reference perturbation theory, multi-reference configuration interaction (MRCI), MRCI with the Davidson correction (MRCI + Q), multi-reference average quadratic CC (MRAQCC), and multi-reference average coupled pair functional (MRACPF) methods to address this question. The symmetric dimers are still shown to be lower in energy than the buckled dimers when using the CASPT2 method on the DHDF optimized geometries, consistent with the previous results using B3LYP geometries [Y. Jung, Y. Shao, M. S. Gordon, D. J. Doren, and M. Head-Gordon, J. Chem. Phys. 119, 10917 (2003)]. Interestingly, however, the MRCI + Q, MRAQCC, and MRACPF results (which give a more refined description of electron correlation effects) suggest that the buckled dimer is marginally more stable than its symmetric counterpart. The present study underlines the significance of having an accurate description of the electron-electron correlation as well as proper multi-reference wave functions when exploring the extremely delicate potential energy surfaces of the reconstructed Si(100) surface.

Boosting hot electron flux and catalytic activity at metal–oxide interfaces of PtCo bimetallic nanoparticles

Despite numerous studies, the origin of the enhanced catalytic performance of bimetallic nanoparticles (NPs) remains elusive because of the ever-changing surface structures, compositions, and oxidation states of NPs under reaction conditions. An effective strategy for obtaining critical clues for the phenomenon is real-time quantitative detection of hot electrons induced by a chemical reaction on the catalysts. Here, we investigate hot electrons excited on PtCo bimetallic NPs during H2 oxidation by measuring the chemicurrent on a catalytic nanodiode while changing the Pt composition of the NPs. We reveal that the presence of a CoO/Pt interface enables efficient transport of electrons and higher catalytic activity for PtCo NPs. These results are consistent with theoretical calculations suggesting that lower activation energy and higher exothermicity are required for the reaction at the CoO/Pt interface.The real-time quantitative detection of hot electrons provides critical clues to understand the origin of the enhanced catalytic performance of bimetallic nanoparticles (NPs). Here, the authors investigate hot electrons generated on bimetallic PtCo NPs during H2 oxidation by measuring the chemicurrent on a catalytic nanodiode.

Unexpected solution phase formation of hollow PtSn alloy nanoparticles from Sn deposition on Pt dendritic structures

Hollow nanoparticles with a high surface-to-volume ratio have found various applications in catalysis, sensors, and energy storage, and thus new synthetic routes to these structures are of great interest. One of the best-known synthetic routes to hollow nanostructures is the utilization of the Kirkendall effect, which, however, is not useful for systems with a slow diffusing-out core such as Pt and fast diffusing-in surface elements such as Sn. Herein, we report a counterintuitive formation of hollow PtSn nanostructures by reacting dendritic Pt nanostructures with Sn precursors.

Ultralow Overpotential of Hydrogen Evolution Reaction using Fe-Doped Defective Graphene: A Density Functional Study

We report great promises of single (transition metal) atom catalysts (SACs) anchored to single‐ and double‐ vacancy sites of the defective graphene structures for hydrogen evolution reaction (HER). Among 120 candidates with all 3d, 4d, and 5d‐block transition metals considered, the inexpensive Fe‐based SAC catalyst shows a theoretical overpotential of 5 mV, the lowest value reported to date theoretically or experimentally. With the help of various electronic structural analysis, we reveal that the key to the observed superior HER activity of Fe‐based SAC is the enhanced ionicity of bonding between hydrogen and the corresponding SAC and the lack of ensemble effect that causes the *H binding weaker to be nearly at the top of the HER activity volcano.

Probing surface oxide formations on SiO2-supported platinum nanocatalysts under CO oxidation

Formations of an ultrathin oxide layer on noble metal catalysts affect the characteristics of fundamental molecular behaviours such as adsorption, diffusion, and desorption on their surfaces. That is directly correlated to enhancement of catalytic activity under operating conditions because the kinetics of catalytic reactions are also simultaneously influenced. Especially, a sub-monolayered surface oxide is known as having a key role for improving catalytic activity, but revealing its existence in catalysis is challenging due to their fast chemical conversion. Herein, we report the first evidence of surface oxide formations on platinum (Pt) nanocatalysts under CO oxidation probed with a diffuse reflectance infrared Fourier transform (DRIFT) technique. Spectroscopic information demonstrates that the abrupt blue shift of adsorbed CO molecules vibrational frequencies of CO stretching mode on the reduced Pt nanocatalyst surface is initiated prior to aggressive CO conversion to CO2 gas molecules. Site-specific replacements of the adsorbed CO molecule with dissociative oxygen occur just before the ignition temperature that is supposed to be an important reaction step for CO oxidation over a Pt nanocatalyst. Density functional theory (DFT) calculation results support this phenomenon as a function of relative atomic fractions between CO and O on a Pt model surface and consistently show a similar trend with experimental evidence.