Transition-metal phosphides have been shown to be promising electrocatalysts in water for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). To maximize reactivity toward both entails limiting the catalyst size while maintaining reactivity and avoiding aggregation. Frame-like hollow nanostructures (nanoframes) provide the required open structure with sufficient channels into the interior volume. We demonstrate here the design and synthesis of CoP nanoframes (CoP NFs) by a strategy involving precipitation, chemical etching, and low-temperature phosphidation steps. It results in impressive bifunctional catalytic activities for both HER and OER and consequently enables a highly efficient water electrolyzer with a current density of 10 mA cm–2 driven by a cell voltage of only 1.65 V. The strategy has been generalized for the preparation of nanoframe Co dichalcogenides CoX2 NFs, with X = S, Se, and Te. The results of electrochemical measurements, supported by density functional theory calculations, show that HER catalytic activities for the series follow the sequence: CoP NFs > CoSe2 NFs > CoS2 NFs > CoTe2 NFs.
side, with the same iron group elements, their phosphide compounds recently have been intensively investigated as new earthabundant electrocatalysts that can potentially replace Pt to catalyze the HER. [23][24][25][26][27][28][29][30][31][32] In both cases, the electrocatalyst materials were typically electrodeposited or prepared by solid-state reaction or hydrothermal reaction followed by cast as an extraneous layer onto electrode substrate surfaces using electrical insulating binding agents such as Nafion. [15,33,34] These fabrication procedures increase the cost and energy consumption for catalyst preparation and also decrease catalyst performance stability.The spontaneous galvanic replacement reaction (GRR) is a classical single-step reaction. Driven by the different electrochemical potentials between two substances, the GRR causes the deposition of noble elements and the dissolution of other elements. [35] The electroless nature of the GRR offers the significant advantages of simplicity and zero energy consumption. However, most studies have been performed with noble metals, such as Pt, Pd, Au, and Ag, as substitution substances for electrocatalyst fabrication. [36][37][38][39] The fabrication of the OER and HER electrocatalysts with first-row transition metals using this method is desirable but has not been explored, which necessitates judicious choice of catalytically synergistic components with appropriate electrochemical potential difference.In this study, we prepared the NiFe-based electrocatalysts for water splitting by means of the GRR as the main step for fabrication of both OER and HER electrocatalysts, which can simplify the fabrication procedure of electrolyzers, substantially lower the production costs, and improve the catalytic performance. The fabrication process and the utilization of the electrode materials for the overall water splitting are illustrated in Scheme 1. The GRR-facilitated electrode fabrication was accomplished by simply immersing a piece of 3D iron foam (IF) into a solution containing Ni(II) cations, which required no complex instrumentation, only a beaker. The NiFe integrated electrode can be used immediately for the OER; otherwise, it is simply pretreated by cyclic voltammetry (CV) to oxidize the surface Fe and Ni, which eliminates the occurrence of corrosive microcell reactions during long-term storage of the electrode. A layered NiFe-based film of uniform nanosheets was formed on the iron A NiFe-based integrated electrode is fabricated by the spontaneous galvanic replacement reaction on an iron foam. Driven by the different electrochemical potentials between Ni and Fe, the dissolution of surface Fe occurs with electroless plating of Ni on iron foam with no need to access instrumentation and input energy. A facile cyclic voltammetry treatment is subsequently applied to convert the metallic NiFe to NiFeO x . A series of analytical methods indicates formation of a NiFeO x film of nanosheets on the iron foam surface. This hierarchically structured three dimensional electrode di...
Chemicals. Nickel(II) nitrate (Ni(NO 3 ) 2 •6H 2 O, 98%), iron(III) sulfate (Fe 2 (SO 4 ) 3 , 99%), iron(II) sulfate (FeSO 4 •7H 2 O, 99%), sodium carbonate (Na 2 CO 3 , > 99.8%), sodium bicarbonate (NaHCO 3 , 100.3%), and potasium hydroxide (KOH, 99%) were obtained from Fisher Scientific. Indium tin oxide (ITO) glass (11 Ω sq -1 ) was obtained from Delta Technologies, Limited. Nickel foam (NF, thickness ~ 0.5 mm, bulk density ~ 0.56 g/cm 3 ) was obtained from Shanxi Lizhiyuan Material of Battery Co. Ltd (China). All other reagents were analytical grade and used as received. All electrolyte solutions were prepared by Milli-Q ultrapure water (> 18 MΩ) unless stated otherwise. The concentrated Na 2 CO 3 solution of pH 10.8 is prepared by mixing Na 2 CO 3 and NaHCO 3 (c(CO 3 2-) + c(HCO 3 -) = 2 M). The pH for 1 M KOH is 13.6 according to literature reports. [1,2] Apparatus. UV-visible spectroscopy was recorded on a SPECORD ® 200 PLUS (analytikjena, Germany) diode-array spectrophotometer. A blank ITO was used as reference during the measurement. The average transmittance in wavelength between 400 -900 nm was S2 used to evaluate the transparency of the catalyst film. The decrease in transmittance is assumed to be proportional to the thickness of the catalyst film which is used to calculate the thickness of the catalyst film by 15-min and 30-min CPE by referring the thickness of the catalyst film by 60-min CPE determined by AFM measurement.Scanning electron microscope (SEM) images, energy dispersive X-ray analysis (EDX) data and EDX mapping images were obtained at Hitachi S-4800 (Hitachi, Japan) equipped with aHoriba EDX system (X-max, silicon drift X-Ray detector). SEM Images were obtained with an acceleration voltage of 3 kV and EDX mapping images and EDX spectra were obtained with acceleration voltages between 15 kV. The time for EDX mapping images is 15 min.After electrodeposition from CPE, the catalyst-coated electrode was rinsed with deionized water and dried in air before being loaded into the instrument.Transmission electron microscopy (TEM) images, high resolution TEM (HRTEM) images and selected area electron diffraction (SAED) pattern were obtained using JEM-2100, JEOL.The NiFeO x /ITO slides were rinsed gently with deionized water and dried in air. The NiFeO x catalyst was scraped from the ITO electrode substrate and dispersed in the absolute ethanol uniformly, and a drop of the mixture was dried on a carbon-coated copper grid for analysis.The catalyst film on the ITO and nickel foam was imaged through a bright field optical microscope (Olympus BX51) fitted with a Olympus DP27 camera. The magnification is 200 (20 × 10) or 1000 (100 × 10) times the original size.Because the catalyst film is very thin, the cross-sectional SEM images could not clearly dispaly the boundary between the catalyst film and the ITO substrate. Therefore, the thickness of the catalyst film was determined by an Atomic Force Microscope (AFM, CSPM-4000).The NiFeO x /ITO electrode was immobilized on the platform by a double side tape. Con...
Polytriphenylamine derivative, poly[N,N,N,N-tetraphenylphenylenediamine] (PDDP), with a high free radical density, has been synthesized and studied as a cathode material for organic free radical batteries for the first time. The chemical structure, morphology, and electrochemical properties of the prepared polymers were characterized by Raman spectra (RS), electron spin resonance (ESR), ultraviolet visible spectroscopy (UVVis), scanning electron microscopy (SEM), cyclic voltammograms (CV), and electrochemical impedance spectra (EIS), respectively. In addition, the charge-discharge properties of the prepared polymers were studied by galvanostatic charge-discharge testing. Compared to polytriphenylamine (PTPA), the fabricated lithium ion half-cells based on PDDP as the cathode exhibited two well-defined plateaus at two discharge voltages of 3.8 and 3.3 V vs. Li/Li + and an improved capacity of 129.1 mA h g À1 , which was very close to its theoretical capacity (130 mA h g À1 ). The excellent electrochemical performances of the PDDP electrode were due to its stable chemical structure and high free radical density, which makes the PDDP a promising free radical cathode material for organic lithium secondary batteries.
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