Direct Electrochemical Protonation of Metal Oxide Particles

Oxygen Vacancy-Electron Polarons Featured InSnRuO2 Oxides: Orderly and Concerted In-Ov-Ru-O-Sn Substructures for Acidic Water Oxidation

Polymetallic oxides with extraordinary electrons/geometry structure ensembles, trimmed electron bands, and way-out coordination environments, built by an isomorphic substitution strategy, may create unique contributing to concertedly catalyze water oxidation, which is of great significance for proton exchange membrane water electrolysis (PEMWE). Herein, well-defined rutile InSnRuO2 oxides with density-controllable oxygen vacancy (Ov)-free electron polarons are firstly fabricated by in situ isomorphic substitution, using trivalent In species as Ov generators and the adjacent metal ions as electron donors to form orderly and concerted In-Ov-Ru-O-Sn substructures in the tetravalent oxides. For acidic water oxidation, the obtained InSnRuO2 displays an ultralow overpotential of 183 mV (versus RHE) and a mass activity (MA) of 103.02 A mgRu -1, respectively. For a long-term stability test of PEMWE, it can run at a low and unchangeable cell potential (1.56 V) for 200 h at 50 mA cm-2, far exceeding current IrO2||Pt/C assembly in 0.5 m H2SO4. Accelerated degradation testing results of PEMWE with pure water as the electrolyte show no significant increase in voltage even when the voltage is gradually increased from 1 to 5 A cm-2. The remarkably improved performance is associated with the concerted In-Ov-Ru-O-Sn substructures stabilized by the dense Ov-electron polarons, which synergistically activates band structure of oxygen species and adjacent Ru sites and then boosting the oxygen evolution kinetics. More importantly, the self-trapped Ov-electron polaron induces a decrease in the entropy and enthalpy, and efficiently hinder Ru atoms leaching by increasing the lattice atom diffusion energy barrier, achieves long-term stability of the oxide. This work may open a door to design next-generation Ru-based catalysts with polarons to create orderly and asymmetric substructures as active sites for efficient electrocatalysis in PEMWE application.

Mediating trade-off between activity and selectivity in alkynes semi-hydrogenation via a hydrophilic polar layer

As a crucial industrial process for the production of bulk and fine chemicals, semi-hydrogenation of alkynes faces the trade-off between activity and selectivity due to undesirable over-hydrogenation. By breaking the energy linear scaling relationships, we report an efficient additive-free WO3-based single-atom Pd catalytic system with a vertical size effect of hydrogen spillover. Hydrogen spillover induced hydrophilic polar layer (HPL) with limited thickness on WO3-based support exhibits unconventional size effect to Pd site, in which over-hydrogenation is greatly suppressed on Pd1 site due to the polar repulsive interaction between HPL and nonpolar C=C bonds, whereas this is invalid for Pd nanoparticles with higher altitudes. By further enhancing the HPL through Mo doping, activated Pd1/MoWO3 achieves recorded performance of 98.4% selectivity and 10200 h-1 activity for semi-hydrogenation of 2-methyl-3-butyn-2-ol, 26-fold increase in activity of Lindlar catalyst. This observed vertical size effect of hydrogen spillover offers broad potential in catalytic performance regulation.

Solid-state atomic hydrogen as a broad-spectrum RONS scavenger for accelerated diabetic wound healing

Hydrogen therapy shows great promise as a versatile treatment method for diseases associated with the overexpression of reactive oxygen and nitrogen species (RONS). However, developing an advanced hydrogen therapy platform that integrates controllable hydrogen release, efficient RONS elimination, and biodegradability remains a giant technical challenge. In this study, we demonstrate for the first time that the tungsten bronze phase H0.53WO3 (HWO) is an exceptionally ideal hydrogen carrier, with salient features including temperature-dependent highly-reductive atomic hydrogen release and broad-spectrum RONS scavenging capability distinct from that of molecular hydrogen. Moreover, its unique pH-responsive biodegradability ensures post-therapeutic clearance at pathological sites. Treatment with HWO of diabetic wounds in an animal model indicates that the solid-state atomic H promotes vascular formation by activating M2-type macrophage polarization and anti-inflammatory cytokine production, resulting in acceleration of chronic wound healing. Our findings significantly expand the basic categories of hydrogen therapeutic materials and pave the way for investigating more physical forms of hydrogen species as efficient RONS scavengers for clinical disease treatment.

Strain-Induced Uphill Hydrogen Distribution in Perovskite Oxide Films

Incorporating hydrogen into transition-metal oxides (TMOs) provides a facile and powerful way to manipulate the performances of TMOs, and thus numerous efforts have been invested in developing hydrogenation methods and exploring the property modulation via hydrogen doping. However, the distribution of hydrogen ions, which is a key factor in determining the physicochemical properties on a microscopic scale, has not been clearly illustrated. Here, focusing on prototypical perovskite oxide (NdNiO3 and La0.67Sr0.33MnO3) epitaxial films, we find that hydrogen distribution exhibits an anomalous "uphill" feature (against the concentration gradient) under tensile strain, namely, the proton concentration enhances upon getting farther from the hydrogen source. Distinctly, under a compressive strain state, hydrogen shows a normal distribution without uphill features. The epitaxial strain significantly influences the chemical lattice coupling and the energy profile as a function of the hydrogen doping position, thus dominating the hydrogen distribution. Furthermore, the strain-(H+) distribution relationship is maintained in different hydrogenation methods (metal-alkali treatment) which is first applied to perovskite oxides. The discovery of strain-dependent hydrogen distribution in oxides provides insights into tailoring the magnetoelectric and energy-conversion functionalities of TMOs via strain engineering.

Time-Domain View of Polaron Dynamics in Metal Oxide Photocatalysts

The polaron is a fundamental physical phenomenon in transition metal oxides (TMOs), and it has been studied extensively for decades. However, the implication of a polaron on photochemistry is still ambiguous. As such, understanding the fundamental properties and controlling the dynamics of polarons at the atomistic level is desired. In this Perspective, we seek to highlight the recent advances in studying small polarons in TMOs, with a particular focus on nonadiabatic molecular dynamics at the ab initio level, and discuss the implications for photocatalysis from the aspects of the structure, intrinsic physical properties, formation, migration, and recombination of small polarons. Finally, various methods were proposed to advance our understanding of manipulating the small-polaron dynamics, and strategies to design high-performance TMO-based photoelectrodes were discussed.

Carbonized Polymer Dots/Bi/β-Bi2O3 for Efficient Photosynthesis of H2O2 via Redox Dual Pathways

A novel heterojunction photocatalyst of carbonized polymer dots (CPDs)/Bi/β-Bi2O3 is successfully synthesized via a one-pot solvothermal method by adjusting the reaction temperature and time. As a solvent and carbon source, ethylene glycol not only supports the conversion of Bi3+ to β-Bi2O3 but also undergoes its polymerization, cross-linking, and carbonization to produce CPDs. In addition, partial Bi3+ is reduced to Bi by ethylene glycol. As a result, the CPDs and Bi are deposited in situ on the surface of β-Bi2O3 microspheres. There are four built-in electric fields in the CPDs/Bi/β-Bi2O3 system, namely, the n-type semiconductor β-Bi2O3/H2O interface, the p-type CPDs/H2O interface, the ohmic contact between Bi and β-Bi2O3, and the Schottky junction between Bi and CPDs. Under the action of four built-in electric fields, the Z-type charge separation mechanism is formed. It promotes the effective separation of the photogenerated electron-hole and greatly improves the yield of H2O2. Under irradiation for 2 h, the H2O2 production is 1590 μmol g-1 h-1. The solar energy to H2O2 conversion efficiency is 0.11%.

Tailoring a local acid-like microenvironment for efficient neutral hydrogen evolution

Electrochemical hydrogen evolution reaction in neutral media is listed as the most difficult challenges of energy catalysis due to the sluggish kinetics. Herein, the Ir-H_xWO₃ catalyst is readily synthesized and exhibits enhanced performance for neutral hydrogen evolution reaction. H_xWO₃ support is functioned as proton sponge to create a local acid-like microenvironment around Ir metal sites by spontaneous injection of protons to WO₃, as evidenced by spectroscopy and electrochemical analysis. Rationalize revitalized lattice-hydrogen species located in the interface are coupled with H_ad atoms on metallic Ir surfaces via thermodynamically favorable Volmer-Tafel steps, and thereby a fast kinetics. Elaborated Ir-H_xWO₃ demonstrates acid-like activity with a low overpotential of 20 mV at 10 mA cm^-2 and low Tafel slope of 28 mV dec^-1, which are even comparable to those in acidic environment. The concept exemplified in this work offer the possibilities for tailoring local reaction microenvironment to regulate catalytic activity and pathway.

Subsurface Engineering Induced Fermi Level De-pinning in Metal Oxide Semiconductors for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is a promising approach for renewable solar light conversion. However, surface Fermi level pinning (FLP), caused by surface trap states, severely restricts the PEC activities. Theoretical calculations indicate subsurface oxygen vacancy (sub-O_v ) could release the FLP and retain the active structure. A series of metal oxide semiconductors with sub-O_v were prepared through precisely regulated spin-coating and calcination. Etching X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and electron energy loss spectra (EELS) demonstrated O_v located at sub ∼2-5 nm region. Mott-Schottky and open circuit photovoltage results confirmed the surface trap states elimination and Fermi level de-pinning. Thus, superior PEC performances of 5.1, 3.4, and 2.1 mA cm^-2 at 1.23 V vs. RHE were achieved on BiVO₄ , Bi₂ O₃ , TiO₂ with outstanding stability for 72 h, outperforming most reported works under the identical conditions.

Thermally Accelerated Surface Polaron Hopping in Photoelectrochemical Water Splitting

Electron-hole separation is a main challenge that limits the energy efficiency of photoelectrochemical water splitting for hydrogen fuel production. Surface polaron states with an energy level distribution near the conduction band are highly efficient charge separation passageways to massively accept or transfer the photogenerated electrons. Here, we found that the charge separation via surface polaron states could be further enhanced by heating (<100 °C) to accelerate the electron mobility of surface polaron states. As a result of heating from 30 to 70 °C, the saturated photocurrent increased about 34.5% under 1 sun and 18.3% under 10 suns from heat-induced increase in electron flux of surface polaron states. The heat-sensitive surface-state electron transfer provides a new heat-photoelectricity coupling mechanism to guide the design of new photoanodes that are available for complementary multienergy systems with high energy efficiency.

Realizing Metastable Cobaltite Perovskite via Proton-Induced Filling of Oxygen Vacancy Channels

The interaction between transition-metal oxides (TMOs) and protons has become a key issue in magneto-ionics and proton-conducting fuel cells. Until now, most investigations on oxide-proton reactions rely on electrochemical tools, while the direct interplay between protons and oxides remains basically at simple dissolution of metal oxides by an acidic solution. In this work, we find classical TMO brownmillerite SrCoO_2.5 (B-SCO) films with ordered oxygen vacancy channels experiencing an interesting transition to a metastable perovskite phase (M-SCO) in a weak acidic solution. M-SCO exhibits a strong ferromagnetism (1.01 μ_B/Co, T_c > 200 K) and a greatly elevated electrical conductivity (∼10⁴ of pristine SrCoO_2.5), which is similar to the prototypical perovskite SrCoO₃. Besides, such M-SCO tends to transform back to B-SCO in a vacuum environment or heating at a relatively low temperature. Two possible mechanisms (H₂O addition/active oxygen filling) have been proposed to explain the phenomenon, and the control experiments demonstrate that the latter mechanism is the dominant process. Our work finds a new way to realize cobaltite perovskite with enhanced magnetoelectric properties and may deepen the understanding of oxide-proton interaction in an aqueous solution.

Liquid-Metal-Induced Hydrogen Insertion in Photoelectrodes for Enhanced Photoelectrochemical Water Oxidation

Fast charge separation and transfer (CST) is essential for achieving efficient solar conversion processes. This CST process requires not only a strong driving force but also a sufficient charge carrier concentration, which is not easily achievable with traditional methods. Herein, we report a rapid hydrogenation method enabled by gallium-based liquid metals (GBLMs) to modify the prototypical WO₃ photoelectrode to enhance the CST for a PEC process. Protons in solution are controllably embedded into the WO₃ photoanode accompanied by electron injection due to the strong reduction capability of GBLMs. This process dramatically increases the carrier concentration of the WO₃ photoanode, leading to improved charge separation and transfer. The hydrogenated WO₃ photoanode exhibits over a 229% improvement in photocurrent density with long-term stability. The effectiveness of GBLMs treatment in accelerating the CST process is further proved using other more general semiconductor photoelectrodes, including Nb₂O₅ and TiO₂.

Direct growth of three-dimensional nanoflower-like structures from flat metal surfaces

Here we report on a facile top-down approach for the direct growth of Co₃O₄ hierarchical nanoflowers from a bulk Co surface via chemical etching and thermal annealing. The effect of the annealing temperature was investigated, showing that amorphous Co₃O₄ was formed at 250 °C, while crystalline Co₃O₄ with notable oxygen vacancies was created at 550 °C. The formed 3D nanostructures exhibited excellent oxygen evolution reaction (OER) activities with a low overpotential of 0.34 V at 10 mA cm^-2 and high durability. The proposed novel approach was further demonstrated by the direct growth of 3D NiO and CuO nanostructures on Ni and Cu substrates.

Facile Removal of Bulk Oxygen Vacancy Defects in Metal Oxides Driven by Hydrogen-Dopant Evaporation

Oxygen vacancy is a common defect in metal oxides that causes appreciable damage to material properties and performance. Removing bulk defects of oxygen vacancy (V_O) typically needs harsh conditions such as high-temperature annealing. Supported by first-principles simulations, we propose an effective strategy of removing V_O bulk defects in metal oxides by evaporating hydrogen dopants. The hydrogen dopants not only lower the migration barrier of V_O but also push V_O away due to their repulsive interaction. The coevaporation mechanism was supported by a neural networks potential-based molecular dynamics simulation, which shows that the migration of hydrogen dopants from inside to surface at 400 K promotes the migration of V_O as well. Our proof-of-concept study suggests an alternative and efficient way of modulating oxygen vacancies in metal oxides via reversible hydrogen doping.