Engineering Interfacial Built-in Electric Field in Polymetallic Phosphide Heterostructures for Superior Supercapacitors and Electrocatalytic Hydrogen Evolution.

Herein, a patterned rod-like CoP@NiCoP core-shell heterostructure is designed to consist of CoP nanowires cross-linked with NiCoP nanosheets in tight strings. The interfacial interaction within the heterojunction between the two components generates a built-in electric field that adjusts the interfacial charge state and create more active sites, accelerating the charge transfer and improving supercapacitor and electrocatalytic performance. The unique core-shell structure suppresses the volume expansion during charging and discharging, achieving excellent stability. As a result, CoP@NiCoP exhibits a high specific capacitance of 2.9 F cm-2 at a current density of 3 mA cm-2 and a high ion diffusion rate (Dion is 2.95 × 10-14  cm2  s-1 ) during charging/discharging. The assembled asymmetric supercapacitor CoP@NiCoP//AC exhibits a high energy density of 42.2 Wh kg-1 at a power density of 126.5 W kg-1 and excellent stability with a capacitance retention rate of 83.8% after 10 000 cycles. Furthermore, the modulated effect induced by the interfacial interaction also endows the self-supported electrode with excellent electrocatalytic HER performance with an overpotential of 71 mV at 10 mA cm-2 . This research may provide a new perspective on the generation of built-in electric field through the rational design of heterogeneous structures for improving the electrochemical and electrocatalytical performance.

Boosting Electrocatalytic Ammonia Synthesis of Bio-Inspired Porous Mo-Doped Hematite via Nitrogen Activation.

Electrochemical N2 reduction reaction (NRR) emerges as a highly attractive alternative to the Haber-Bosch process for producing ammonia (NH3) under ambient circumstances. Currently, this technology still faces tremendous challenges due to the low ammonia production rate and low Faradaic efficiency, urgently prompting researchers to explore highly efficient electrocatalysts. Inspired by the Fe-Mo cofactor in nitrogenase, we report Mo-doped hematite (Fe2O3) porous nanospheres containing Fe-O-Mo subunits for enhanced activity and selectivity in the electrochemical reduction from N2 to NH3. Mo-doping induces the morphology change from a solid sphere to a porous sphere and enriches lattice defects, creating more active sites. It also regulates the electronic structures of Fe2O3 to accelerate charge transfer and enhance the intrinsic activity. As a consequence, Mo-doped Fe2O3 achieves effective N2 fixation with a high ammonia production rate of 21.3 ± 1.1 µg h-1 mgcat.-1 as well as a prominent Faradaic efficiency (FE) of 11.2 ± 0.6%, superior to the undoped Fe2O3 and other iron oxide catalysts. Density functional theory (DFT) calculations further unravel that the Mo-doping in Fe2O3 (110) narrows the band gap, promotes the N2 activation on the Mo site with an elongated N≡N bond length of 1.132 Å in the end-on configuration, and optimizes an associative distal pathway with a decreased energy barrier. Our results may pave the way toward enhancing the electrocatalytic NRR performance of iron-based materials by atomic-scale heteroatom doping.

Boosting Hydrogen Evolution Electrocatalysis via Regulating the Electronic Structure in a Crystalline-Amorphous CoP/CeOx p-n Heterojunction.

The modulation of the electronic structure is the effective access to achieve highly active electrocatalysts for the hydrogen evolution reaction (HER). Transition-metal phosphide-based heterostructures are very promising in enhancing HER performance but the facile fabrication and an in-depth study of the catalytic mechanisms still remain a challenge. In this work, the catalytically inactive n-type CeOx is successfully combined with p-type CoP to form the CoP/CeOx heterojunction. The crystalline-amorphous CoP/CeOx heterojunction is fabricated by the phosphorization of predesigned Co(OH)2/CeOx via the as-developed reduction-hydrolysis strategy. The p-n CoP/CeOx heterojunction with a strong built-in potential of 1.38 V enables the regulation of the electronic structure of active CoP within the space-charge region to enhance its intrinsic activity and facilitate the electron transfer. The functional CeOx entity and the negatively charged CoP can promote the water dissociation and optimize H adsorption, synergistically boosting the electrocatalytic HER output. As expected, the heterostructured CoP/CeOx-20:1 with the optimal ratio of Co/Ce shows significantly improved HER activity and favorable kinetics (overpotential of 118 mV at a current density of 10 mA cm-2 and Tafel slope of 77.26 mV dec-1). The present study may provide new insight into the integration of crystalline and amorphous entities into the p-n heterojunction as a highly efficient electrocatalyst for energy storage and conversion.

Superelasticity of Carbon Nanocoils from Atomistic Quantum Simulations.

A structural model of carbon nanocoils (CNCs) on the basis of carbon nanotubes (CNTs) was proposed. The Young's moduli and spring constants of CNCs were computed and compared with those of CNTs. Upon elongation and compression, CNCs exhibit superelastic properties that are manifested by the nearly invariant average bond lengths and the large maximum elastic strain limit. Analysis of bond angle distributions shows that the three-dimensional spiral structures of CNCs mainly account for their unique superelasticity.