Xiaoyan Chai

In situ coating of nitrogen-doped graphene-like nanosheets on silicon as a stable anode for high-performance lithium-ion batteries

Carbon coating is an effective approach to improve the cycling stability of silicon (Si) anodes for lithium-ion batteries. In this research, we report a facile one-step carbon-thermal method to coat Si nanoparticles with nitrogen-doped (N-doped) graphene-like nanosheets derived from a liquid-polyacrylonitrile (LPAN) precursor. The coated Si anode displays an initial coulombic efficiency of 82%, which is about three times greater than its pristine counterpart, as well as superior cycling stability. The performance improvement is a result of the N-doped graphene-like nanosheet conformal coating, which not only creates an electrically conductive network for the electrode, but also provides a buffering matrix to accommodate the volume change of Si during charging and discharging processes.

Facile fabrication of a 3D network composed of N-doped carbon-coated core–shell metal oxides/phosphides for highly efficient water splitting

Development of robust, bifunctional, and non-precious catalysts for oxygen and hydrogen evolution reactions (OER and HER) is a prerequisite to realizing the overall splitting of water. This, however, remains a great challenge. In this context, we fabricated a novel three-dimensional (3D) network comprising N-doped carbon-coated core–shell NiFeOx@NiFe–P (denoted as NC–NiFeOx@NiFe–P) by two-pot high-temperature phosphorization and surface oxidation of a NiFe-Prussian blue analogue/polyvinylpyrrolidone (denoted as NiFe–PBAs/PVP) hybrid precursor. The as-synthesized NC–NiFeOx@NiFe–P catalyst demonstrated exceptional performance for both OER and HER, offering a current density of 10 mA cm−2 (a metric related to solar fuel) at small overpotentials of 285 mV for the OER and 237 mV for the HER in 1 M KOH, respectively. As expected, a NC–NiFeOx@NiFe–P based alkaline electrolyzer with durability of 20 h was manufactured to achieve 10 mA cm−2 at a voltage of 1.59 V, outperforming most non-precious metal-based electrolyzers. The exceptional performance could be attributed to the unique 3D network composed of core–shell NiFeOx@NiFe–P and highly conductive N-doped carbon (NC), which provided a large amount of highly active sites for both OER and HER and favored fast electron transport during electrocatalytic processes.

Boosting Electrochemical Hydrogen Evolution of Porous Metal Phosphides Nanosheets by Coating Defective TiO2 Overlayers

Nonprecious transition metal phosphides (TMPs) have emerged as robust electrocalysts for the hydrogen evolution reaction (HER). However, the TMPs suffer from low activity for water dissociation, which greatly limits the efficiency for alkaline HER. Here, a facile yet robust strategy is reported to boost the HER of metal phosphides by coating defective TiO2 overlayers. The oxygen vacancies (Ov ) on defective TiO2 overlayers are found to possess high activity for adsorption and dissociation of water, thereby significantly promoting the initial Volmer step of HER to generate the reactive hydrogen atoms. Moreover, the porous (Co, Ni)2 P (i.e., Co2 P and Ni2 P) nanosheets provide enough active sites for adsorption and recombination of reactive hydrogen atoms to produce hydrogen gas. The catalytic synergy of (Co, Ni)2 P and Ov coupled with the hierarchically porous structure renders the porous (Co, Ni)2 P@0.1TiO2 nanosheet arrays excellent electrocatalysts for HER, showing a small overpotential (92 mV) to yield a current density of 10 mA cm-2 , a small Tafel slope (49 mV dec-1 ), and an outstanding stability. This work demonstrates a surface decoration route for enhancing the activity of nonprecious metal-based electrocatalysts for HER.

A Core-Shell-Structured Silver Nanowire/Nitrogen-Doped Carbon Catalyst for Enhanced and Multifunctional Electrofixation of CO2

Numerous catalysts have been successfully introduced for CO2 fixation in aqueous or organic systems. However, a single catalyst showing activity in both solvent types is still rare, to the best of our knowledge. We developed a core-shell-structured AgNW/NC700 composite using a Ag nanowire (NW) core encapsulated by a N-doped carbon (NC) shell at 700 °C. Through control experiments and density functional theory calculations, it was confirmed that Ag nanowires acted as the active sites for CO2 fixation and the uniformly coating of N-doped carbon created a CO2 -rich environment around the Ag nanowires, which could significantly improve the catalytic activity of Ag nanowires for electrochemical CO2 fixation. Under mild conditions, up to 96 % faradaic efficiency of CO, 95 % yield of Ibuprofen and 92 % yield of propylene carbonate could be obtained in the electrochemical CO2 direct reduction, carboxylation and cycloaddition, respectively, using the same AgNWs/NC700 catalyst. These results might provide an alternative strategy for efficient electrochemical fixation of CO2 .

Composition Tailoring via N and S Co‐doping and Structure Tuning by Constructing Hierarchical Pores: Metal‐Free Catalysts for High‐Performance Electrochemical Reduction of CO2

A facile route to scalable production of N and S co-doped, hierarchically porous carbon nanofiber (NSHCF) membranes (ca. 400 cm2 membrane in a single process) is reported. As-synthesized NSHCF membranes are flexible and free-standing, allowing their direct use as cathodes for efficient electrochemical CO2 reduction reaction (CO2 RR). Notably, CO with 94 % Faradaic efficiency and -103 mA cm-2 current density are readily achieved with only about 1.2 mg catalyst loading, which are among the best results ever obtained by metal-free CO2 RR catalysts. On the basis of control experiments and DFT calculations, such outstanding CO Faradaic efficiency can be attributed to the co-doped pyridinic N and carbon-bonded S atoms, which effectively decrease the Gibbs free energy of key *COOH intermediate. Furthermore, hierarchically porous structures of NSHCF membranes impart a much higher density of accessible active sites for CO2 RR, leading to the ultra-high current density.