Youkui Zhang

Electronic Structure Reconfiguration toward Pyrite NiS2 via Engineered Heteroatom Defect Boosting Overall Water Splitting

Developing highly active and low-cost heterogeneous catalysts toward overall electrochemical water splitting is extremely desirable but still a challenge. Herein, we report pyrite NiS2 nanosheets doped with vanadium heteroatoms as bifunctional electrode materials for both hydrogen- and oxygen-evolution reaction (HER and OER). Notably, the electronic structure reconfiguration of pyrite NiS2 is observed from typical semiconductive characteristics to metallic characteristics by engineering vanadium (V) displacement defect, which is confirmed by both experimental temperature-dependent resistivity and theoretical density functional theory calculations. Furthermore, elaborate X-ray absorption spectroscopy measurements reveal that electronic structure reconfiguration of NiS2 is rooted in electron transfer from doped V to Ni sites, consequently enabling Ni sites to gain more electrons. The metallic V-doped NiS2 nanosheets exhibit extraordinary electrocatalytic performance with overpotentials of about 290 mV for OER and about 110 mV for HER at 10 mA cm-2 with long-term stability in 1 M KOH solutions, representing one of the best non-noble-metal bifunctional electrocatalysts to date. This work provides insights into electronic structure engineering from well-designed atomic defect metal sulfide.

Hierarchical 1T-MoS2 nanotubular structures for enhanced supercapacitive performance

Layered transition metal disulfides are currently being widely studied for advanced energy generation and storage applications. Here we report a facile template-assisted solvothermal strategy to obtain a hierarchical nanotubular structure consisting of ultrathin MoS2 nanosheets with a metallic 1T phase. Synchrotron radiation based X-ray absorption fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS) are used to investigate the structure and electronic properties of the 1T-MoS2, which are largely different from annealed samples. Its hierarchical structure makes the obtained nanotubular 1T-MoS2 an excellent electrode material for supercapacitors, with a high specific capacitance of 328.547 F g−1 at a current density of 1 A g−1 and 243.66 F g−1 at a current density of 15 A g−1. Moreover, the material displays excellent capacitance retention, retaining 98.4% capacity after 5000 cycles at a current density of 3 A g−1. Notably, a high specific capacitance of 250 F g−1 at 1 A g−1 is also achieved in a two-electrode symmetrical cell, suggesting its great potential for new-generation supercapacitors.

Atomic Iridium Incorporated in Cobalt Hydroxide for Efficient Oxygen Evolution Catalysis in Neutral Electrolyte

Developing highly efficient catalysts for oxygen evolution reaction (OER) in neutral media is extremely crucial for microbial electrolysis cells and electrochemical CO2 reduction. Herein, a facile one-step approach is developed to synthesize a new type of well-dispersed iridium (Ir) incorporated cobalt-based hydroxide nanosheets (nominated as CoIr) for OER. The Ir species as clusters and single atoms are incorporated into the defect-rich hydroxide nanosheets through the formation of rich Co-Ir species, as revealed by systematic synchrotron radiation based X-ray spectroscopic characterizations combining with high-angle annular dark-field scanning transmission electron microscopy measurement. The optimized CoIr with 9.7 wt% Ir content displays highly efficient OER catalytic performance with an overpotential of 373 mV to achieve the current density of 10 mA cm-2 in 1.0 m phosphate buffer solution, significantly outperforming the commercial IrO2 catalysts. Further characterizations toward the catalyst after undergoing OER process indicate that unique Co oxyhydroxide and high valence Ir species with low-coordination structure are formed due to the high oxidation potentials, which authentically contributes to superior OER performance. This work not only provides a state-of-the-art OER catalyst in neutral media but also unravels the root of the excellent performance based on efficient structural identifications.

Well‐Defined Cobalt Catalyst with N‐Doped Carbon Layers Enwrapping: The Correlation between Surface Atomic Structure and Electrocatalytic Property

Admittedly, the surface atomic structure of heterogenous catalysts toward the electrochemical oxygen reduction reaction (ORR) are accepted as the important features that can tune catalytic activity and even catalytic pathway. Herein, a surface engineering strategy to controllably synthesize a carbon-layer-wrapped cobalt-catalyst from 2D cobalt-based metal-organic frameworks is elaborately demonstrated. Combined with synchrotron radiation X-ray photoelectron spectroscopy, the soft X-ray absorption near-edge structure results confirmed that rich covalent interfacial Couf8ffNuf8ffC bonds are efficiently formed between cobalt nanoparticles and wrapped carbon-layers during the polydopamine-assisted pyrolysis process. The X-ray absorption fine structure and corresponding extended X-ray absorption fine structure spectra further reveal that the wrapped cobalt with Co-N coordinations shows distinct surface distortion and atomic environmental change of Co-based active sites. In contrast to the control sample without coating layers, the 800 °C-annealed cobalt catalyst with N-doped carbon layers enwrapping achieves significantly enhanced ORR activity with onset and half-wave potentials of 0.923 and 0.816 V (vs reversible hydrogen electrode), highlighting the important correlation between surface atomic structure and catalytic property.

Confined bimetallic phosphide within P, N co-doped carbon layers towards boosted bifunctional oxygen catalysis

Rational design of bifunctional oxygen electrocatalysts with high activity and low cost is crucial but still challenging for the development of rechargeable energy storage devices. In this work, a novel bimetallic phosphide confined within P, N co-doped carbon layers is achieved through a space-confinement phosphorization strategy. Electrochemical measurements demonstrate that the obtained bimetallic phosphide nanoparticle-containing hybrid can serve as a highly efficient bifunctional oxygen reduction and evolution reaction (ORR/OER) catalyst. It delivers a small potential difference of 0.75 V in 0.1 M KOH solution along with long-term catalytic durability, superior to most of the non-precious metal catalysts reported to date. The detailed synchrotron-based spectra results combined with structural characterizations reveal that the constructed hierarchical structure endows the confined bimetallic phosphide catalyst with abundant catalytic sites and stable spatial structure, thereby achieving remarkable electrocatalytic performance. This work opens up a facile way to design effective and durable bifunctional oxygen electrocatalysts for practical applications in renewable energy production, especially in rechargeable metal–air batteries and regenerative fuel cells.

Probing lattice vibration and surface electronic state in a layered (NH4)2V3O8 single crystal

Two-dimensional layered structure of a single crystal is regarded as an ideal feature for physical and chemical fundamental studies. Herein, we demonstrated a high-quality (NH4)2V3O8 single crystal with a layered tetragonal structure prepared via a hydrothermal method. The lattice vibrational behavior and surface electronic state of (NH4)2V3O8 layers were systematically investigated via polarized Raman scattering spectroscopy and ultraviolet photoelectron spectroscopy (UPS), respectively. It was found that all Raman peaks of (NH4)2V3O8 could be clearly identified as four active Raman modes through parallel and perpendicular polarization configurations in the backscattering geometry for (001) crystal surface. The UPS results indicated that the valence band maximum of (NH4)2V3O8 was mainly composed of localized vanadium 3d states, which was further confirmed by the density functional theory calculations.