Unlocking High Capacity and Reversible Alkaline Iron Redox using Silicate-Sodium Hydroxide Hybrid Electrolytes.

Alkaline iron (Fe) batteries are attractive due to the high abundance, low cost, and multiple valent states of Fe but show limited columbic efficiency and storage capacity when forming electrochemically inert Fe3O4 on discharging and parasitic H2 on charging. Herein, sodium silicate is found to promote Fe(OH)2/FeOOH against Fe(OH)2/Fe3O4 conversions. Electrochemical experiments, operando X-ray characterization, and atomistic simulations reveal that improved Fe(OH)2/FeOOH conversion originates from (i) strong interaction between sodium silicate and iron oxide and (ii) silicate-induced strengthening of hydrogen-bond networks in electrolytes that inhibits water transport. Furthermore, the silicate additive suppresses hydrogen evolution by impairing energetics of water dissociation and hydroxyl de-sorption on iron surfaces. This new silicate-assisted redox chemistry mitigates H2 and Fe3O4formation, improving storage capacity (199 mAh g-1 in half-cells) and coulombic efficiency (94% after 400 full-cell cycles), paving a path to realizing green battery systems built from earth-abundant materials.

Revealing the Nature of Binary-Phase on Structural Stability of Sodium Layered Oxide Cathodes.

The emergence of layered sodium transition metal oxides featuring a multiphase structure presents a promising approach for cathode materials in sodium-ion batteries, showcasing notably improved energy storage capacity. However, the advancement of cathodes with multiphase structures faces obstacles due to the limited understanding of the integrated structural effects. Herein, the integrated structural effects by an in-depth structure-chemistry analysis in the developed layered cathode system NaxCu0.1Co0.1Ni0.25Mn0.4Ti0.15O2 with purposely designed P2/O3 phase integration, are comprehended. The results affirm that integrated phase ratio plays a pivotal role in electrochemical/structural stability, particularly at high voltage and with the incorporation of anionic redox. In contrast to previous reports advocating solely for the enhanced electrochemical performance in biphasic structures, it is demonstrated that an inappropriate composite structure is more destructive than a single-phase design. The in situ X-ray diffraction results, coupled with density functional theory computations further confirm that the biphasic structure with P2:O3 = 4:6 shows suppressed irreversible phase transition at high desodiated states and thus exhibits optimized electrochemical performance. These fundamental discoveries provide clues to the design of high-performance layered oxide cathodes for next-generation SIBs.

Boosting the interfacial superionic conduction of halide solid electrolytes for all-solid-state batteries.

Designing highly conductive and (electro)chemical stable inorganic solid electrolytes using cost-effective materials is crucial for developing all-solid-state batteries. Here, we report halide nanocomposite solid electrolytes (HNSEs) ZrO2(-ACl)-A2ZrCl6 (A = Li or Na) that demonstrate improved ionic conductivities at 30 °C, from 0.40 to 1.3 mS cm-1 and from 0.011 to 0.11 mS cm-1 for Li+ and Na+, respectively, compared to A2ZrCl6, and improved compatibility with sulfide solid electrolytes. The mechanochemical method employing Li2O for the HNSEs synthesis enables the formation of nanostructured networks that promote interfacial superionic conduction. Via density functional theory calculations combined with synchrotron X-ray and 6Li nuclear magnetic resonance measurements and analyses, we demonstrate that interfacial oxygen-substituted compounds are responsible for the boosted interfacial conduction mechanism. Compared to state-of-the-art Li2ZrCl6, the fluorinated ZrO2-2Li2ZrCl5F HNSE shows improved high-voltage stability and interfacial compatibility with Li6PS5Cl and layered lithium transition metal oxide-based positive electrodes without detrimentally affecting Li+ conductivity. We also report the assembly and testing of a Li-In||LiNi0.88Co0.11Mn0.01O2 all-solid-state lab-scale cell operating at 30 °C and 70 MPa and capable of delivering a specific discharge of 115 mAh g-1 after almost 2000 cycles at 400 mA g-1.

Revitalizing Iron Redox by Anion-Insertion-Assisted Ferro- and Ferri-Hydroxides Conversion at Low Alkalinity.

Iron hydroxides are desirable alkaline battery electrodes for low cost and environmental beneficence. However, hydrogen evolution on charging and Fe3O4 formation on discharging cause low storage capacity and poor cycling life. We report that green rust (GR) (Fe2+4Fe3+2 (HO-)12SO4), formed via sulfate insertion, promotes Fe(OH)2/FeOOH conversion and shows a discharge capacity of ∼211 mAh g-1 in half-cells and Coulombic efficiency of 93% after 300 cycles in full-cells. Theoretical calculations show that Fe(OH)2/FeOOH conversion is facilitated by intercalated sulfate anions. Classical molecular dynamics simulations reveal that electrolyte alkalinity strongly impacts the energetics of sulfate solvation, and low alkalinity ensures fast transport of sulfate ions. Anion-insertion-assisted Fe(OH)2/FeOOH conversion, also achieved with Cl- ion, paves a pathway toward efficient utilization of Fe-based electrodes for sustainable applications.

High-Capacity Aqueous Storage in Vanadate Cathodes Promoted by the Zn-Ion and Proton Intercalation and Conversion-Intercalation of Vanadyl Ions.

Aqueous Zn-ion batteries (AZIBs) are promising alternatives to lithium-ion batteries in stationary storage. However, limited storage capacity and cyclic life impede their large-scale implementation. We report reversible electrochemical insertion of multi-ions into sodium vanadate (NaV3O8) cathode materials for AZIBs, achieving a maximum storage capacity of 450 mAh g-1 at 0.05 A g-1 and a capacity retention of 82% after 500 cycles at 0.4 A g-1. In addition to Zn2+ and H+ insertion, in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) collectively provide explicit evidence on vanadyl ions (VO2+) conversion-intercalation at the NaV3O8 cathode, showing the deintercalation of VO2+ from NaV3O8 and the consequent conversion of VO2+ into V2O5 on charging, and vice versa on discharging. Our study is the first to report on the cation conversion-intercalation mechanism in AZIBs. This reversible multi-ion storage mechanism provides a design principle for developing high-capacity aqueous electrode materials by engaging both the intercalation and conversion of charge carriers.

Potentiodynamics of the Zinc and Proton Storage in Disordered Sodium Vanadate for Aqueous Zn-Ion Batteries.

A rechargeable Zn-ion battery is a promising aqueous system, where coinsertion of Zn2+ and H+ could address the obstacles of the sluggish ionic transport in cathode materials imposed by multivalent battery chemistry. However, there is a lack of fundamental understanding of this dual-ion transport, especially the potentiodynamics of the storage process. Here, a quantitative analysis of Zn2+ and H+ transport in a disordered sodium vanadate (NaV3O8) cathode material has been reported. Collectively, synchrotron X-ray analysis shows that both Zn2+ and H+ storages follow an intercalation storage mechanism in NaV3O8 and proceed in a sequential manner, where intercalations of 0.26 Zn2+ followed by 0.24 H+ per vanadium atom occur during discharging, while reverse dynamics happens during charging. Such a unique and synergistic dual-ion sequential storage favors a high capacity (265 mA h g-1) and an energy density (221 W h kg-1) based on the NaV3O8 cathode and a great cycling life (a capacity retention of 78% after 2000 cycles) in Zn/NaV3O8 full cells.

Structure and dynamics of the molten alkali-chloride salts from an X-ray, simulation, and rate theory perspective.

Molten salts are of great interest as alternative solvents, electrolytes, and heat transfer fluids in many emerging technologies. The macroscopic properties of molten salts are ultimately controlled by their structure and ion dynamics at the microscopic level and it is therefore vital to develop an understanding of these at the atomistic scale. Herein, we present high-energy X-ray scattering experiments combined with classical and ab initio molecular dynamics simulations to elucidate structural and dynamical correlations across the family of alkali-chlorides. Computed structure functions and transport properties are in reasonably good agreement with experiments providing confidence in our analysis of microscopic properties based on simulations. For these systems, we also survey different rate theory models of anion exchange dynamics in order to gain a more sophisticated understanding of the short-time correlations that are likely to influence transport properties such as conductivity. The anion exchange process occurs on the picoseconds time scale at 1100 K and the rate increases in the order KCl < NaCl < LiCl, which is in stark contrast to the ion pair dissociation trend in aqueous solutions. Consistent with the trend we observe for conductivity, the cationic size/mass, as well as other factors specific to each type of rate theory, appear to play important roles in the anion exchange rate trend.

Characterizing electronic and atomic structures for amorphous and molecular metal oxide catalysts at functional interfaces by combining soft X-ray spectroscopy and high-energy X-ray scattering.

Amorphous thin film materials and heterogenized molecular catalysts supported on electrode and other functional interfaces are widely investigated as promising catalyst formats for applications in solar and electrochemical fuels catalysis. However the amorphous character of these catalysts and the complexity of the interfacial architectures that merge charge transport properties of electrode and semiconductor supports with discrete sites for multi-step catalysis poses challenges for probing mechanisms that activate and tune sites for catalysis. This minireview discusses advances in soft X-ray spectroscopy and high-energy X-ray scattering that provide opportunities to resolve interfacial electronic and atomic structures, respectively, that are linked to catalysis. This review discusses how these techniques can be partnered with advances in nanostructured interface synthesis for combined soft X-ray spectroscopy and high-energy X-ray scattering analyses of thin film and heterogenized molecular catalysts. These combined approaches enable opportunities for the characterization of both electronic and atomic structures underlying fundamental catalytic function, and that can be applied under conditions relevant to device applications.

Elucidating Ionic Correlations Beyond Simple Charge Alternation in Molten MgCl2-KCl Mixtures.

The development of technologies for nuclear reactors based on molten salts has seen a big resurgence. The success of thermodynamic models for these hinges in part on our ability to predict at the atomistic level the behavior of pure salts and their mixtures under a range of conditions. In this letter, we present high-energy X-ray scattering experiments and molecular dynamics simulations that describe the molten structure of mixtures of MgCl2 and KCl. As one would expect, KCl is a prototypical salt in which structure is governed by simple charge alternation. In contrast, MgCl2 and its mixtures with KCl display more complex correlations including intermediate-range order and the formation of Cl--decorated Mg2+ chains. A thorough computational analysis suggests that intermediate-range order beyond charge alternation may be traced to correlations between these chains. An analysis of the coordination structure for Mg2+ ions paints a more complex picture than previously understood, with multiple accessible states of distinct geometries.

Microfluidic electrochemical cell for in situ structural characterization of amorphous thin-film catalysts using high-energy X-ray scattering.

Porous, high-surface-area electrode architectures are described that allow structural characterization of interfacial amorphous thin films with high spatial resolution under device-relevant functional electrochemical conditions using high-energy X-ray (>50 keV) scattering and pair distribution function (PDF) analysis. Porous electrodes were fabricated from glass-capillary array membranes coated with conformal transparent conductive oxide layers, consisting of either a 40 nm-50 nm crystalline indium tin oxide or a 100 nm-150 nm-thick amorphous indium zinc oxide deposited by atomic layer deposition. These porous electrodes solve the problem of insufficient interaction volumes for catalyst thin films in two-dimensional working electrode designs and provide sufficiently low scattering backgrounds to enable high-resolution signal collection from interfacial thin-film catalysts. For example, PDF measurements were readily obtained with 0.2 Šspatial resolution for amorphous cobalt oxide films with thicknesses down to 60 nm when deposited on a porous electrode with 40 µm-diameter pores. This level of resolution resolves the cobaltate domain size and structure, the presence of defect sites assigned to the domain edges, and the changes in fine structure upon redox state change that are relevant to quantitative structure-function modeling. The results suggest the opportunity to leverage the porous, electrode architectures for PDF analysis of nanometre-scale surface-supported molecular catalysts. In addition, a compact 3D-printed electrochemical cell in a three-electrode configuration is described which is designed to allow for simultaneous X-ray transmission and electrolyte flow through the porous working electrode.

Resolution of Electronic and Structural Factors Underlying Oxygen-Evolving Performance in Amorphous Cobalt Oxide Catalysts.

Non-noble-metal, thin-film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron-proton charge transport properties. Here, we use a combination of L edge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, whereas CoPi contains approximately 17% tetrahedral Co(II), with the remainder in octahedral coordination. Oxygen-mediated 4 p-3 d hybridization through Co-O-Co bonding was detected by RXES and the intersite dd excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO2-like structure, whereas CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance oxygen evolving catalysis performance.

Water Oxidation by Size-Selected Co27 Clusters Supported on Fe2 O3.

The complexity of the water oxidation reaction makes understanding the role of individual catalytic sites critical to improving the process. Here, size-selected 27-atom cobalt clusters (Co27 ) deposited on hematite (Fe2 O3 ) anodes were tested for water oxidation activity. The uniformity of these anodes allows measurement of the activity of catalytic sites of well-defined nuclearity and known density. Grazing incidence X-ray absorption near-edge spectroscopy (GIXANES) characterization of the anodes before and after electrochemical cycling demonstrates that these Co27 clusters are stable to dissolution even in the harsh water oxidation electrochemical environment. They are also stable under illumination at the equivalent of 0.4 suns irradiation. The clusters show turnover rates for water oxidation that are comparable or higher than those reported for Pd- and Co-based materials or for hematite. The support for the Co27 clusters is Fe2 O3 grown by atomic layer deposition on a Si chip. We have chosen to deposit a Fe2 O3 layer that is only a few unit cells thick (2 nm), to remove complications related to exciton diffusion. We find that the electrocatalytic and the photoelectrocatalytic activity of the Co27 /Fe2 O3 material is significantly improved when the samples are annealed (with the clusters already deposited). Given that the support is thin and that the cluster deposition density is equivalent to approximately 5 % of an atomic monolayer, we suggest that annealing may significantly improve the exciton diffusion from the support to the catalytic moiety.

Solution Structures of Highly Active Molecular Ir Water-Oxidation Catalysts from Density Functional Theory Combined with High-Energy X-ray Scattering and EXAFS Spectroscopy.

The solution structures of highly active Ir water-oxidation catalysts are elucidated by combining density functional theory, high-energy X-ray scattering (HEXS), and extended X-ray absorption fine structure (EXAFS) spectroscopy. We find that the catalysts are Ir dimers with mono-µ-O cores and terminal anionic ligands, generated in situ through partial oxidation of a common catalyst precursor. The proposed structures are supported by (1)H and (17)O NMR, EPR, resonance Raman and UV-vis spectra, electrophoresis, etc. Our findings are particularly valuable to understand the mechanism of water oxidation by highly reactive Ir catalysts. Importantly, our DFT-EXAFS-HEXS methodology provides a new in situ technique for characterization of active species in catalytic systems.

Oxyanion induced variations in domain structure for amorphous cobalt oxide oxygen evolving catalysts, resolved by X-ray pair distribution function analysis.

Amorphous thin film oxygen evolving catalysts, OECs, of first-row transition metals show promise to serve as self-assembling photoanode materials in solar-driven, photoelectrochemical `artificial leaf' devices. This report demonstrates the ability to use high-energy X-ray scattering and atomic pair distribution function analysis, PDF, to resolve structure in amorphous metal oxide catalyst films. The analysis is applied here to resolve domain structure differences induced by oxyanion substitution during the electrochemical assembly of amorphous cobalt oxide catalyst films, Co-OEC. PDF patterns for Co-OEC films formed using phosphate, Pi, methylphosphate, MPi, and borate, Bi, electrolyte buffers show that the resulting domains vary in size following the sequence Pi < MPi < Bi. The increases in domain size for CoMPi and CoBi were found to be correlated with increases in the contributions from bilayer and trilayer stacked domains having structures intermediate between those of the LiCoOO and CoO(OH) mineral forms. The lattice structures and offset stacking of adjacent layers in the partially stacked CoMPi and CoBi domains were best matched to those in the LiCoOO layered structure. The results demonstrate the ability of PDF analysis to elucidate features of domain size, structure, defect content and mesoscale organization for amorphous metal oxide catalysts that are not readily accessed by other X-ray techniques. PDF structure analysis is shown to provide a way to characterize domain structures in different forms of amorphous oxide catalysts, and hence provide an opportunity to investigate correlations between domain structure and catalytic activity.

Size- and support-dependent evolution of the oxidation state and structure by oxidation of subnanometer cobalt clusters.

Size-selected subnanometer cobalt clusters with 4, 7, and 27 cobalt atoms supported on amorphous alumina and ultrananocrystalline diamond (UNCD) surfaces were oxidized after exposure to ambient air. Grazing incidence X-ray absorption near-edge spectroscopy (GIXANES) and near-edge X-ray absorption fine structure (NEXAFS) were used to characterize the clusters revealed a strong dependency of the oxidation state and structure of the clusters on the surface. A dominant Co(2+) phase was identified in all samples. However, XANES analysis of cobalt clusters on UNCD showed that ∼10% fraction of a Co(0) phase was identified for all three cluster sizes and about 30 and 12% fraction of a Co(3+) phase in 4, 7, and 27 atom clusters, respectively. In the alumina-supported clusters, the dominating Co(2+) component was attributed to a cobalt aluminate, indicative of a very strong binding to the support. NEXAFS showed that in addition to strong binding of the clusters to alumina, their structure to a great extent follows the tetrahedral morphology of the support. All supported clusters were found to be resistant to agglomeration when exposed to reactive gases at elevated temperatures and atmospheric pressure.

Anomalously increased oxygen reduction reaction activity with accelerated durability test cycles for platinum on thiolated carbon nanotubes.

We report an anomalous phenomenon in Pt supported on thiolated multi-walled carbon nanotubes (Pt-S-MWNT): oxygen reduction reaction (ORR) activity increases with accelerated durability test (ADT) cycles. Sub-nanometer-sized Pt clusters on S-MWNT were gradually agglomerated to an optimal size with ADT cycles, and finally showed increased ORR activity after the ADT.

Size-dependent subnanometer Pd cluster (Pd4, Pd6, and Pd17) water oxidation electrocatalysis.

Water oxidation is a key catalytic step for electrical fuel generation. Recently, significant progress has been made in synthesizing electrocatalytic materials with reduced overpotentials and increased turnover rates, both key parameters enabling commercial use in electrolysis or solar to fuels applications. The complexity of both the catalytic materials and the water oxidation reaction makes understanding the catalytic site critical to improving the process. Here we study water oxidation in alkaline conditions using size-selected clusters of Pd to probe the relationship between cluster size and the water oxidation reaction. We find that Pd4 shows no reaction, while Pd6 and Pd17 deposited clusters are among the most active (in terms of turnover rate per Pd atom) catalysts known. Theoretical calculations suggest that this striking difference may be a demonstration that bridging Pd-Pd sites (which are only present in three-dimensional clusters) are active for the oxygen evolution reaction in Pd6O6. The ability to experimentally synthesize size-specific clusters allows direct comparison to this theory. The support electrode for these investigations is ultrananocrystalline diamond (UNCD). This material is thin enough to be electrically conducting and is chemically/electrochemically very stable. Even under the harsh experimental conditions (basic, high potential) typically employed for water oxidation catalysts, UNCD demonstrates a very wide potential electrochemical working window and shows only minor evidence of reaction. The system (soft-landed Pd4, Pd6, or Pd17 clusters on a UNCD Si-coated electrode) shows stable electrochemical potentials over several cycles, and synchrotron studies of the electrodes show no evidence for evolution or dissolution of either the electrode material or the clusters.

Size-selected clusters as heterogeneous model catalysts under applied reaction conditions.

First results of investigations are presented, where size-selected metal clusters generated in ultra high vacuum (UHV) are transferred to ambient conditions and tested for suitable electrochemical applications. As demonstrated, the transfer allows for the characterization of clusters by transmission electron microscopy (TEM) as well as catalytic measurements, which is exemplified by the application of electrochemical measurements. It is demonstrated that well known electrochemical processes on the carbon supported Pt clusters are detected, and thus Pt clusters can be characterized with respect to their accessible surface area, an essential requirement for the study of catalytic processes. Furthermore, as an example for an important electrocatalytic process, it is shown that the oxygen reduction reaction can be probed on the cluster samples featuring a detrimental particle size effect, previously reported for industrial catalysts as well.