Enhancing Ni/Co Activity by Neighboring Pt Atoms in NiCoP/MXene Electrocatalyst for Alkaline Hydrogen Evolution.

Density functional theory (DFT) calculations demonstrate neighboring Pt atoms can enhance the metal activity of NiCoP for hydrogen evolution reaction (HER). However, it remains a great challenge to link Pt and NiCoP. Herein, we introduced curvature of bowl-like structure to construct Pt/NiCoP interface by adding a minimal 1 ‰-molar-ratio Pt. The as-prepared sample only requires an overpotential of 26.5 and 181.6 mV to accordingly achieve the current density of 10 and 500 mA cm-2 in 1 M KOH. The water dissociation energy barrier (Ea) has a ~43 % decrease compared with NiCoP counterpart. It also shows an ultrahigh stability with a small degradation rate of 10.6 µV h-1 at harsh conditions (500 mA cm-2 and 50 °C) after 3000 hrs. X-ray photoelectron spectroscopy (XPS), soft X-ray absorption spectroscopy (sXAS), and X-ray absorption fine structure (XAFS) verify the interface electron transfer lowers the valence state of Co/Ni and activates them. DFT calculations also confirm the catalytic transition step of NiCoP can change from Heyrovsky (2.71 eV) to Tafel step (0.51 eV) in the neighborhood of Pt, in accord with the result of the improved Hads at the interface disclosed by in situ electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) tests.

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.

Oxygen-doped NiCoP derived from Ni-MOFs for high performance asymmetric supercapacitor.

Oxygen doping strategy is one of the most effective methods to improve the electrochemical properties of nickel-cobalt phosphide (NiCoP)-based capacitors by adjusting its inherent electronic structure. In this paper, O-doped NiCoP microspheres derived from porous nanostructured nickel metal-organic frameworks (Ni-MOFs) were constructed through solvothermal method followed by phosphorization treatment. The O-doping concentration has a siginificant influence on the rate performance and cycle stability. The optimized O-doped NiCoP electrode material shows a specific capacitance of 632.4 F-g-1at 1 A-g-1and a high retention rate of 56.9% at 20 A g-1. The corresponding NiCoP-based asymmetric supercapacitor exhibits a high energy density of 30.1 Wh kg-1when the power density is 800.9 W kg-1, and can still maintain 82.1% of the initial capacity after 10 000 cycles at 5 A g-1.

Bi-Interlayer Strategy for Modulating NiCoP-Based Heterostructure toward High-Performance Aqueous Energy Storage Devices.

Nickel-cobalt (NiCo) phosphides (NCPs) possess high electrochemical activity, which makes them promising candidates for electrode materials in aqueous energy storage devices, such as supercapacitors and zinc (Zn) batteries. However, the actual specific capacitance and rate capability of NCPs require further improvement, which can be achieved through reasonable heterostructural design and loading conditions of active materials on substrates. Herein, novel hierarchical Bi-NCP heterogeneous structures with built-in electric fields consisting of bismuth (Bi) interlayers (electrodeposited on carbon cloth (CC)) are designed and fabricated to ensure the formation of uniform high-load layered active materials for efficient charge and ion transport. The resulting CC/Bi-NCP electrodes show a uniform, continuous, and high mass loading (>3.5 mg) with a superior capacitance reaching 1200 F g-1 at 1 A g-1 and 4129 mF cm-2 at 1 mA cm-2 combined with high-rate capability and durable cyclic stability. Moreover, assembled hybrid supercapacitors (HSCs), supercapatteries, and alkaline Zn-ion (AZBs) batteries constructed using these electrodes deliver high energy densities of 64.4, 81.8, and 319.1 Wh kg-1, respectively. Overall, the constructed NCPs with excellent aqueous energy storage performance have the potential for the development of novel transition metal-based heterostructure electrodes for advanced energy devices.

Construction of iron-doped nickel cobalt phosphide nanoparticles via solvothermal phosphidization and their application in alkaline oxygen evolution.

Multi-metallic phosphides offer the possibility to combine the strategies of surface reconstruction, electronic interaction and mechanistic pathway tuning to achieve high electrocatalytic oxygen evolution activity. Here, iron-doped nickel cobalt phosphide nanoparticles (FexCoyNi2-x-yP) with the crystalline NiCoP phase are for the first time synthesized by the solvothermal phosphidization method via the reaction between metal-organic frameworks and white phosphorus. When used to electrochemically catalyze oxygen evolution reaction (OER), the Fe0.4Co0.8Ni0.8P supported by nickel foam requires only 248 mV overpotential to achieve 10 mA cm-2 current densities, and is robust towards the long-term OER in 1 M KOH. The higher number of electrochemically active sites can account for the good OER activity, along with the improved intrinsic activity which is caused by the electron interaction that optimizes the adsorption energy of hydroxyl intermediates, and that increases the acidity of high-valent metal centers. The OER mechanistic pathway involves both adsorbate and lattice oxygen. Surface conversion is observed after OER in alkaline solution, and metal phosphide layer transforms to metal oxides and (oxy)hydroxides.

Enhancing Electrochemical Non-Enzymatic Dopamine Sensing Based on Bimetallic Nickel/Cobalt Phosphide Nanosheets.

In this study, the successful synthesis of bimetallic nickel/cobalt phosphide nanosheets (Ni-Co-P NSs) via the hydrothermal method and the subsequent high-temperature phosphorization process were both confirmed. Ni-Co-P NSs exhibited excellent electrocatalytic activity for the electrochemical non-enzymatic DA sensing. The surface morphologies and physicochemical properties of Ni-Co-P NSs were characterized by atomic force microscopy (AFM), field-emission scanning (FESEM), field-emission transmission electron microscopy (FETEM), and X-ray diffraction (XRD). Further, the electrochemical performance was evaluated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The metallic nature of phosphide and the synergistic effect of Ni/Co atoms in Ni-Co-P NSs provided abundant catalytic active sites for the electrochemical redox reaction of DA, which exhibited a remarkable consequence with a wide linear range from 0.3~50 µM, a high sensitivity of 2.033 µA µM-1 cm-2, a low limit of detection of 0.016 µM, and anti-interference ability. As a result, the proposed Ni-Co-P NSs can be considered an ideal electrode material for the electrochemical non-enzymatic DA sensing.

Interface Engineering via Ti3C2Tx MXene Enabled Highly Efficient Bifunctional NiCoP Array Catalysts for Alkaline Water Splitting.

Developing a non-noble metal-based bifunctional electrocatalyst with high efficiency and stability for overall water splitting is desirable for renewable energy systems. We developed a novel method to fabricate a heterostructured electrocatalyst, comprising a NiCoP nanoneedle array grown on Ti3C2Tx MXene-coated Ni foam (NCP-MX/NF) using a dip-coating hydrothermal method, followed by phosphorization. Due to the abundance of active sites, enhanced electronic kinetics, and sufficient electrolyte accessibility resulting from the synergistic effects of NCP and MXene, NCP-MX/NF bifunctional alkaline catalysts afford superb electrocatalytic performance, with a low overpotential (72 mV at 10 mA cm-2 for HER and 303 mV at 50 mA cm-2 for OER), a low Tafel slope (49.2 mV dec-1 for HER and 69.5 mV dec-1 for OER), and long-term stability. Moreover, the overall water splitting performance of NCP-MX/NF, which requires potentials as low as 1.54 and 1.76 V at a current density of 10 and 50 mA cm-2, respectively, exceeded the performance of the Pt/C∥IrO2 couple in terms of overall water splitting. Density functional theory (DFT) calculations for the NCP/Ti3C2O2 interface model predicted the catalytic contribution to interfacial formation by analyzing the electronic redistribution at the interface. This contribution was also evaluated by calculating the adsorption energetics of the descriptor molecules (H2O and the H and OER intermediates).

Integrated electrochemical microfluidic sensor with hierarchically porous nanoarrays modified graphene fiber microelectrode for bioassay.

The development of high-efficient biosensing systems for rapid and sensitive detection of disease-related biomarkers in human samples is of great significance for disease diagnosis and treatment in clinical practice. In this work, we develop an integrated electrochemical microfluidic sensing platform based on freestanding graphene fiber (GF) microelectrode for bioassay. In order to improve the electrocatalytic activity of GF microelectrode, it has been modified by unique 3D well-ordered hierarchically porous nickel-cobalt phosphide (NiCoP) nanosheet arrays (NSAs). Benefiting from the excellent electrochemical properties and structural merits, the resultant NiCoP-NSAs modified GF microelectrode shows excellent sensing performances towards neurotransmitter dopamine (DA), with a high sensitivity of 5.56 µA cm-2 µM-1, a low detection limit of 14 nM, as well as good selectivity, reproducibility and stability. Furthermore, in virtue of the miniaturized size and good mechanical properties, the nanohybrid GF microelectrode can be embedded into a home-made microfluidic chip to construct an integrated electrochemical microfluidic sensing device, which has been used for sensitive analysis of DA in minimal volume of human serum and urine samples, and in situ tracking DA released from neuroblastoma cells SHSY-5Y under the stimulation for physio-pathological and pharmacological study of nervous system-related diseases.

Three-Dimensional Hierarchical NixCo1-xO/NiyCo2-yP@C Hybrids on Nickel Foam for Excellent Supercapacitors.

Active materials and special structures of the electrode have decisive influence on the electrochemical properties of supercapacitors. Herein, three-dimensional (3D) hierarchical NixCo1-xO/NiyCo2-yP@C (denoted as NiCoOP@C) hybrids have been successfully prepared by a phosphorization treatment of hierarchical NixCo1-xO@C grown on nickel foam. The resulting NiCoOP@C hybrids exhibit an outstanding specific capacitance and cycle performance because they couple the merits of the superior cycling stability of NixCo1-xO, the high specific capacitance of NiyCo2-yP, the mechanical stability of carbon layer, and the 3D hierarchical structure. The specific capacitance of 2638 F g-1 can be obtained at the current density of 1 A g-1, and even at the current density of 20 A g-1, the NiCoOP@C electrode still possesses a specific capacitance of 1144 F g-1. After 3000 cycles at 10 A g-1, 84% of the initial specific capacitance is still remained. In addition, an asymmetric ultracapacitor (ASC) is assembled through using NiCoOP@C hybrids as anode and activated carbon as cathode. The as-prepared ASC obtains a maximum energy density of 39.4 Wh kg-1 at a power density of 394 W kg-1 and still holds 21 Wh kg-1 at 7500 W kg-1.