Interrogation of 3d Transition Bimetallic Nanocrystal Nucleation and Growth Using In Situ Electron Microscope and Synchrotron X-ray Techniques.

Understanding the nucleation and growth mechanism of 3d transition bimetallic nanocrystals (NCs) is crucial to developing NCs with tailored nanostructures and properties. However, it remains a significant challenge due to the complexity of 3d bimetallic NCs formation and their sensitivity to oxygen. Here, by combining in situ electron microscopy and synchrotron X-ray techniques, we elucidate the nucleation and growth pathways of Fe-Ni NCs. Interestingly, the formation of Fe-Ni NCs emerges from the assimilation of Fe into Ni clusters together with the reduction of Fe-Ni oxides. Subsequently, these NCs undergo solid-state phase transitions, resulting in two distinct solid solutions, ultimately dominated by γ-Fe3Ni2. Furthermore, we deconvolve the interplays between local coordination and electronic state concerning the growth temperature. We directly visualize the oxidation-state distributions of Fe and Ni at the nanoscale and investigate their changes. This work may reshape and enhance the understanding of nucleation and growth in atomic crystallization.

Pourbaix Machine Learning Framework Identifies Acidic Water Oxidation Catalysts Exhibiting Suppressed Ruthenium Dissolution.

The demand for green hydrogen has raised concerns over the availability of iridium used in oxygen evolution reaction catalysts. We identify catalysts with the aid of a machine learning-aided computational pipeline trained on more than 36,000 mixed metal oxides. The pipeline accurately predicts Pourbaix decomposition energy (Gpbx) from unrelaxed structures with a mean absolute error of 77 meV per atom, enabling us to screen 2070 new metallic oxides with respect to their prospective stability under acidic conditions. The search identifies Ru0.6Cr0.2Ti0.2O2 as a candidate having the promise of increased durability: experimentally, we find that it provides an overpotential of 267 mV at 100 mA cm-2 and that it operates at this current density for over 200 h and exhibits a rate of overpotential increase of 25 µV h-1. Surface density functional theory calculations reveal that Ti increases metal-oxygen covalency, a potential route to increased stability, while Cr lowers the energy barrier of the HOO* formation rate-determining step, increasing activity compared to RuO2 and reducing overpotential by 40 mV at 100 mA cm-2 while maintaining stability. In situ X-ray absorption spectroscopy and ex situ ptychography-scanning transmission X-ray microscopy show the evolution of a metastable structure during the reaction, slowing Ru mass dissolution by 20× and suppressing lattice oxygen participation by >60% compared to RuO2.

Water induced ultrathin Mo2C nanosheets with high-density grain boundaries for enhanced hydrogen evolution.

Grain boundary controlling is an effective approach for manipulating the electronic structure of electrocatalysts to improve their hydrogen evolution reaction performance. However, probing the direct effect of grain boundaries as highly active catalytic hot spots is very challenging. Herein, we demonstrate a general water-assisted carbothermal reaction strategy for the construction of ultrathin Mo2C nanosheets with high-density grain boundaries supported on N-doped graphene. The polycrystalline Mo2C nanosheets are connected with N-doped graphene through Mo-C bonds, which affords an ultra-high density of active sites, giving excellent hydrogen evolution activity and superior electrocatalytic stability. Theoretical calculations reveal that the dz2 orbital energy level of Mo atoms is controlled by the MoC3 pyramid configuration, which plays a vital role in governing the hydrogen evolution activity. The dz2 orbital energy level of metal atoms exhibits an intrinsic relationship with the catalyst activity and is regarded as a descriptor for predicting the hydrogen evolution activity.

Chemically coupling SnO2 quantum dots and MXene for efficient CO2 electroreduction to formate and Zn-CO2 battery.

Electrochemical conversion of CO2 into formate is a promising strategy for mitigating the energy and environmental crisis, but simultaneously achieving high selectivity and activity of electrocatalysts remains challenging. Here, we report low-dimensional SnO2 quantum dots chemically coupled with ultrathin Ti3C2Tx MXene nanosheets (SnO2/MXene) that boost the CO2 conversion. The coupling structure is well visualized and verified by high-resolution electron tomography together with nanoscale scanning transmission X-ray microscopy and ptychography imaging. The catalyst achieves a large partial current density of -57.8 mA cm-2 and high Faradaic efficiency of 94% for formate formation. Additionally, the SnO2/MXene cathode shows excellent Zn-CO2 battery performance, with a maximum power density of 4.28 mW cm-2, an open-circuit voltage of 0.83 V, and superior rechargeability of 60 h. In situ X-ray absorption spectroscopy analysis and first-principles calculations reveal that this remarkable performance is attributed to the unique and stable structure of the SnO2/MXene, which can significantly reduce the reaction energy of CO2 hydrogenation to formate by increasing the surface coverage of adsorbed hydrogen.

Surface chemical heterogeneous distribution in over-lithiated Li1+xCoO2 electrodes.

In commercial Li-ion batteries, the internal short circuits or over-lithiation often cause structural transformation in electrodes and may lead to safety risks. Herein, we investigate the over-discharged mechanism of LiCoO2/graphite pouch cells, especially spatially resolving the morphological, surface phase, and local electronic structure of LiCoO2 electrode. With synchrotron-based X-ray techniques and Raman mapping, together with spectroscopy simulations, we demonstrate that over-lithiation reaction is a surface effect, accompanied by Co reduction and surface structure transformation to Li2CoO2/Co3O4/CoO/Li2O-like phases. This surface chemical distribution variation is relevant to the depth and exposed crystalline planes of LiCoO2 particles, and the distribution of binder/conductive additives. Theoretical calculations confirm that Li2CoO2-phase has lower electronic/ionic conductivity than LiCoO2-phase, further revealing the critical effect of distribution of conductive additives on the surface chemical heterogeneity evolution. Our findings on such surface phenomena are non-trivial and highlight the capability of synchrotron-based X-ray techniques for studying the spatial chemical phase heterogeneity.

X-Ray Spectromicroscopy Investigation of Heterogeneous Sodiation in Hard Carbon Nanosheets with Vertically Oriented (002) Planes.

Hard carbon (HC) is a promising anode material for sodium-ion batteries, but the performance remains unsatisfactory and the sodiation mechanism in HC is one of the most debated topics. Here, from self-assembled cellulose nanocrystal sheets with crystallographic texture, unique HC nanosheets with vertically oriented (002) planes are fabricated and used as a model HC to investigate the sodiation mechanisms using synchrotron scanning transmission X-ray microscopy (STXM) coupled with analytical transmission electron microscopy (TEM). The model HC simplifies the 3D sodiation in a typical HC particle into a 2D sodiation, which facilitates the visualization of phase transformation at different states of charge. The results for the first time unveil that the sodiation in HC initiates heterogeneously, with multiple propagation fronts proceeding simultaneously, eventually merging into larger aggregates. The spatial correlation between the preferential adsorption and nucleation sites suggests that the heterogeneous nucleation is driven by the local Na-ion concentration, which is determined by defects or heteroatoms that have strong binding to Na ions. By identifying intercalation as the dominant sodium storage mechanism in the model HC, the findings highlight the importance of engineering the graphene layer orientation and the structural heterogeneity of edge sites to enhance the performances.

Soft X-ray Ptychography Chemical Imaging of Degradation in a Composite Surface-Reconstructed Li-Rich Cathode.

The capability in spatially resolving the interactions between components in lithium (Li)-ion battery cathodes, especially correlating chemistry and electronic structure, is challenging but critical for a better understanding of complex degradation mechanisms for rational developments. X-ray spectro-ptychography and conventional synchrotron-based scanning transmission X-ray microscopy image stacks are the most powerful probes for studying the distribution and chemical state of cations in degraded Li-rich cathodes. Herein, we propose a chemical approach with a spatial resolution of around 5.6 nm to imaging degradation heterogeneities and interplay among components in degraded Li-rich cathodes. Through the chemical imaging reconstruction of the degraded Li-rich cathodes, fluorine (F) ions incorporated into the lattice during charging/discharging processes are proved and strongly correlate with the manganese (Mn) dissolution and oxygen loss within the secondary particles and impact the electronic structure. Otherwise, the electrode-electrolyte interphase component, scattered LiF particles (100-500 nm) along with the MnF2 layer, is also visualized between the primary particles inside the secondary particles of the degraded cathodes. The results provide direct visual evidence for the Li-rich cathode degradation mechanisms and demonstrate that the low-energy ptychography technique offers a superior approach for high-resolution battery material characterization.

Nanoscale assembling of graphene oxide with electrophoretic deposition leads to superior percolation network in Li-ion electrodes: TiNb2O7/rGO composite anodes.

In this paper, electrophoretic deposition (EPD) is shown to promote nanoscale assembling of graphene oxide (GO) enabling the fabrication of highly homogeneous, robust, and capacity fade resistant composite titanium niobate (TiNb2O7, TNO)/rGO anodes upon reductive annealing. Control tests revealed that EPD is superior to conventional PVDF-based casting in maximizing the performance benefits from using reduced GO in Li-ion electrode fabrication as is the case of TNO that is plagued with conductivity and capacity fading problems. In this particular study, we show that there is a synergy developed between GO and EPD with the former (1) stabilizing the EPD suspension, (2) acting as a flexible binder net that affords mechanical integrity during the volume expansion of TNO, (3) serving as a conductive filler, and (4) contributing to Li-ion storage via pseudocapacitance. As a consequence, a superior percolation network is developed. Thus while both EPD- and PVDF- built TNO/rGO composite anodes exhibited high initial capacities (∼350 and 318 mA h g-1) at 0.5 C cycling, respectively, their cycling behaviour was quite different with the latter experiencing high internal polarization and extended degradation. Post-mortem PEEM-XANES analysis clearly demonstrated a highly homogeneous mesostructure in the case of the EPD-built TNO/rGO anode vs. a highly segregated and dis-jointed rGO and TNO component clustering in the PVDF-built electrode.

Highly Selective Electrocatalytic Reduction of CO2 into Methane on Cu-Bi Nanoalloys.

Methane (CH4), the main component of natural gas, is one of the most valuable products facilitating energy storage via electricity conversion. However, the poor selectivity and high overpotential for CH4 formation with metallic Cu catalysts prevent realistic applications. Introducing a second element to tune the electronic state of Cu has been widely used as an effective method to improve catalytic performance, but achieving high selectivity and activity toward CH4 remains challenging. Here, we successfully synthesized Cu-Bi NPs, which exhibit a CH4 Faradaic efficiency (FE) as high as 70.6% at -1.2 V versus reversible hydrogen electrode (RHE). The FE of Cu-Bi NPs has increased by approximately 25-fold compared with that of Cu NPs. DFT calculations showed that alloying Cu with Bi significantly decreases the formation energy of *COH formation, the rate-determining step, which explains the improved performance. Further analysis showed that Cu that has been partially oxidized because of electron withdrawal by Bi is the most possible active site.

Curcumin-Induced DNA Demethylation in Human Gastric Cancer Cells Is Mediated by the DNA-Damage Response Pathway.

Curcumin, a natural polyphenol antioxidant extracted from the root of turmeric (Curcuma longa), can induce apoptosis and DNA demethylation in several types of cancer cells. However, the mechanism of its anticancer potentials and DNA demethylation effects and the potential relationships between these outcomes have not been clearly elucidated. In the present study, the effects of curcumin on the proliferation, colony formation, and migration of human gastric cancer cells (hGCCs) were explored. Reactive oxygen species (ROS) levels, mitochondrial damage, DNA damage, and apoptosis of curcumin-treated hGCCs were analyzed. Changes in the expression of several genes related to DNA damage repair, the p53 pathway, cell cycle, and DNA methylation following curcumin treatment were also evaluated. We observed that curcumin inhibited the proliferation, colony formation, and migration of hGCCs in a dose- and time-dependent fashion. A high concentration of curcumin elevated ROS levels and triggered mitochondrial damage, DNA damage, and apoptosis of hGCCs. Further, curcumin-induced DNA demethylation of hGCCs was mediated by the damaged DNA repair-p53-p21/GADD45A-cyclin/CDK-Rb/E2F-DNMT1 axis. We propose that the anticancer effect of curcumin could largely be attributed to its prooxidative effect at high concentrations and ROS elevation in cancer cells. Moreover, we present a novel mechanism by which curcumin induces DNA demethylation of hGCCs, suggesting the need to further investigate the demethylation mechanisms of other DNA hypomethylating drugs.

O-coordinated W-Mo dual-atom catalyst for pH-universal electrocatalytic hydrogen evolution.

Single-atom catalysts (SACs) maximize the utility efficiency of metal atoms and offer great potential for hydrogen evolution reaction (HER). Bimetal atom catalysts are an appealing strategy in virtue of the synergistic interaction of neighboring metal atoms, which can further improve the intrinsic HER activity beyond SACs. However, the rational design of these systems remains conceptually challenging and requires in-depth research both experimentally and theoretically. Here, we develop a dual-atom catalyst (DAC) consisting of O-coordinated W-Mo heterodimer embedded in N-doped graphene (W1Mo1-NG), which is synthesized by controllable self-assembly and nitridation processes. In W1Mo1-NG, the O-bridged W-Mo atoms are anchored in NG vacancies through oxygen atoms with W─O─Mo─O─C configuration, resulting in stable and finely distribution. The W1Mo1-NG DAC enables Pt-like activity and ultrahigh stability for HER in pH-universal electrolyte. The electron delocalization of W─O─Mo─O─C configuration provides optimal adsorption strength of H and boosts the HER kinetics, thereby notably promoting the intrinsic activity.

Correlative imaging of ionic transport and electronic structure in nano Li0.5FePO4 electrodes.

Phase separation and electronic structure variation of Li0.5FePO4, both in the bulk and surface, under concurrent lithiation, has been tracked by synchrotron X-ray microscopies. Oxygen K-edge XANES along with DFT calculations reveal unusual electronic structure varition which is attributable to the observed lithium gradient and interparticle transport. The new insights will benefit the future design of advanced batteries.

Engineering of Nitrogen Coordinated Single Cobalt Atom Moieties for Oxygen Electroreduction.

The nitrogen coordinated single cobalt atoms embedded in carbon matrix, i.e., Co/N/C material, is cost-efficient and free from iron-ion induced Fenton reagent, which has been thus considered as a promising candidate to replace the well-accepted Pt-based and Fe/N/C materials for oxygen reduction reaction (ORR). Recently, the pyrolysis of metal-organic framework (MOF) precursors has been investigated to achieve well-defined Co/N/C catalysts with high ORR activity. However, the relationships among the composition/structure of MOF precursor, the derived catalysts, and ORR performance have been rarely touched in specialty, while the regulations to achieve single-atom Co/N/C catalysts derived from MOF are confusing. Herein, we engineer several Co-doped MOF (zeolitic imidazolate frameworks, to be specific) precursors with different compositions and structures by tuning synthesis protocols (e.g., ratios, cobalt sources, and reaction time) and investigate the derived catalysts and their ORR properties. The regulations to single-atom Co/N/C are revealed in this work. The superior ORR activity and durability of the optimized Co/N/C catalysts are revealed and attributed to the well-defined Co-Nx moieties and their stable nanostructures.

Identification of the Solid Electrolyte Interface on the Si/C Composite Anode with FEC as the Additive.

Silicon-based anodes have the potential to be used in next-generation lithium ion batteries owing to their higher lithium storage capacity. However, the large volume change during the charge/discharge process and the repeated formation of a new solid electrolyte interface (SEI) on the re-exposed Si surface should be overcome to achieve a better electrochemical performance. Fluoroethylene carbonate (FEC) has been widely used as an electrolyte additive for Si-based anodes, but the intrinsical mechanism in performance improvement is not clear yet. Here, we combined solid-state NMR, X-ray photoelectron spectroscopy, and X-ray photoemission electron microscopy to characterize the composition, structure, and inhomogeneity of the SEI on Si/C composite anodes with or without the FEC additive. Similar species are observed with two electrolytes, but a denser SEI formed with FEC, which could prevent the small molecules (i.e., LiPF6, P-O, and Li-O species) from penetrating to the surface of the Si/C anode. The hydrolysis of LiPF6 leading to Li xPO yF z and further to Li3PO4 could also be partially suppressed by the denser SEI formed with FEC. In addition, a large amount of LiF could protect the cracking and pulverization of Si particles. This study demonstrates a deeper understanding of the SEI formed with FEC, which could be a guide for optimizing the Si-based anodes for lithium ion batteries.

Surface heterogeneity in Li0.5CoO2 within a porous composite electrode.

Surface phase heterogeneity within a fully charged LiCoO2 (LCO) composite electrode before and after thermal decomposition has been shown to strongly depend on the distribution of the binder and carbon additives, but be independent of the crystalline facet and size of LCO, as revealed by X-ray photoemission electron microscopy (X-PEEM). The obtained new insights deepen the understanding of thermal stability in a real composite electrode, which would be valuable for further development of long-life and safe cathodes for lithium-ion batteries.

Unexpected phase separation in Li1-xNi0.5Mn1.5O4 within a porous composite electrode.

The Ni oxidation state in Li1-xNi0.5Mn1.5O4 (LNMO) within a composite electrode mapped by soft X-ray scanning transmission X-ray microscopy (STXM) has shown unexpected distinct variations in phase separation among and within individual battery particles, which has been experimentally correlated to both their morphology and interface structure for the first time. The obtained new insights will assist the further development of long-life high voltage LNMO batteries.

Carbon Nanosheets Containing Discrete Co-Nx-By-C Active Sites for Efficient Oxygen Electrocatalysis and Rechargeable Zn-Air Batteries.

Structural and compositional engineering of atomic-scaled metal-N-C catalysts is important yet challenging in boosting their performance for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Here, boron (B)-doped Co-N-C active sites confined in hierarchical porous carbon sheets (denoted as Co-N,B-CSs) were obtained by a soft template self-assembly pyrolysis method. Significantly, the introduced B element gives an electron-deficient site that can activate the electron transfer around the Co-N-C sites, strengthen the interaction with oxygenated species, and thus accelerate reaction kinetics in the 4e- processed ORR and OER. As a result, the catalyst showed Pt-like ORR performance with a half-wave potential (E1/2) of 0.83 V versus (vs) RHE, a limiting current density of about 5.66 mA cm-2, and higher durability (almost no decay after 5000 cycles) than Pt/C catalysts. Moreover, a rechargeable Zn-air battery device comprising this Co-N,B-CSs catalyst shows superior performance with an open-circuit potential of ∼1.4 V, a peak power density of ∼100.4 mW cm-2, as well as excellent durability (128 cycles for 14 h of operation). DFT calculations further demonstrated that the coupling of Co-Nx active sites with B atoms prefers to adsorb an O2 molecule in side-on mode and accelerates ORR kinetics.

Revealing the Role of Poly(vinylidene fluoride) Binder in Si/Graphite Composite Anode for Li-Ion Batteries.

The conventional polyvinylidene fluoride (PVDF) binder works well with the graphite anode, but when combined with silicon in composites to increase the energy density of Li-ion batteries, it results in severe capacity fade. Herein, by using scanning electron microscopy and energy-dispersive X-ray spectroscopy analyses, we reveal that this failure stems from the loss of connectivity between the silicon (or its agglomerates), graphite, and PVDF binder because of the mechanical stresses experienced during battery cycling. More importantly, we reveal for the first time that the PVDF binder undergoes chemical decomposition during the cycling of not only the composite but also the Si-only or even graphite-only electrodes despite the excellent battery performance of the latter. Through X-ray photoemission electron microscopy and X-ray photoelectron spectroscopy techniques, LiF was identified as the predominant decomposition product. We show that the distribution of LiF in the electrodes due to the differences in the interactions between PVDF and either Si or graphite could correlate with the performance of the battery. This study shows that the most suitable binder for the composite electrode is a polymer with a good chemical interaction with both graphite and silicon.

Chemical Imaging of Nanoscale Interfacial Inhomogeneity in LiFePO4 Composite Electrodes from a Cycled Large-Format Battery.

The nanoscale interfacial inhomogeneity in a cycled large-format LiFePO4 (LFP) composite electrode has been studied by X-ray photoemission electron microscopy at single particle spatial resolution with a probe depth of ∼5 nm. The loss of active lithium in cycled LFP causes the coexsitence of fully delithiated LFP (FePO4) and partially delithiated LFP (Li0.6FePO4 or Li0.8FePO4) as a function of the extent of lithium loss. The distribution of various lithium loss phases along with local agglomeration of LFP and degradation of binder and carbon black are correlatively visualized. This is the first experimental exploration of chemical interplay between components in the composite electrode from a large-format battery, and implications on the LFP degradation in this battery are discussed.

Ultrahigh Mass Activity for Carbon Dioxide Reduction Enabled by Gold-Iron Core-Shell Nanoparticles.

Wide application of carbon dioxide (CO2) electrochemical energy storage requires catalysts with high mass activity. Alloy catalysts can achieve superior performance to single metals while reducing the cost by finely tuning the composition and morphology. We used in silico quantum mechanics rapid screening to identify Au-Fe as a candidate improving CO2 reduction and then synthesized and tested it experimentally. The synthesized Au-Fe alloy catalyst evolves quickly into a stable Au-Fe core-shell nanoparticle (AuFe-CSNP) after leaching out surface Fe. This AuFe-CSNP exhibits exclusive CO selectivity, long-term stability, nearly a 100-fold increase in mass activity toward CO2 reduction compared with Au NP, and 0.2 V lower in overpotential. Calculations show that surface defects due to Fe leaching contribute significantly to decrease the overpotential.