Jinwei Chen

Study on stabilities and electrochemical behavior of V(V) electrolyte with acid additives for vanadium redox flow battery

Several acid compounds have been employed as additives of the V(V) electrolyte for vanadium redox flow battery (VRB) to improve its stability and electrochemical activity. Stability of the V(V) electrolyte with and without additives was investigated with ex-situ heating/cooling treatment at a wide temperature range of −5 °C to 60 °C. It was observed that methanesulfonic acid, boric acid, hydrochloric acid, trifluoroacetic acid, polyacrylic acid, oxalic acid, methacrylic acid and phosphotungstic acid could improve the stability of the V(V) electrolyte at a certain range of temperature. Their electrochemical behaviors in the V(V) electrolyte were further studied by cyclic voltammetry (CV), steady state polarization and electrochemical impedance spectroscopy (EIS). The results showed that the electrochemical activity, including the reversibility of electrode reaction, the diffusivity of V(V) species, the polarization resistance and the flexibility of charge transfer for the V(V) electrolyte with these additives were all improved compared with the pristine solution.

Enhanced photocatalytic performance: a β-Bi2O3 thin film by nanoporous surface

Beta-Bi2O3 film photoanodes with different surface structures were prepared by oxidizing bismuth films. The physical properties were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), UV–visible absorbance spectra and atomic force microscopy (AFM). XRD shows that all films are beta phase crystal structure except the thinnest 12 nm film. SEM and AFM characterizations indicate that a nanoporous surface structure is generated on the surface after the film is annealed for 3 h, while the films annealed for 1 h show a dense surface. The direct band gaps vary from 2.63 to 2.88 eV, with the film thickness decreasing from 500 to 12 nm. The nanoporous surface structure film exhibits better light harvesting ability and incident photon-to-electron conversion efficiency (IPCE) than the dense surface films. The IPCE (61% at 350 nm and 43% at 400 nm, 0.197 VNHE) is the highest ever reported. The photocurrent density reaches 0.45 mA cm−2 when illuminated with a bias of 1.23 VNHE in 0.5M Na2SO3.

Facile synthesis of flower-like platinum nanostructures as an efficient electrocatalyst for methanol electro-oxidation

This paper presents a facile approach for the synthesis of a novel Pt/graphene-nickel foam (Pt/GNF) electrode composed of flower-like Pt nanoparticles (NPs) and 3D graphene. The fabrication process involved the chemical vapor deposition of graphene onto Ni foam as a substrate and the subsequent growth of Pt NPs via a galvanic replacement reaction without using any seed and organic solvent. The surface morphology and composition of the prepared materials were characterized. Meanwhile, cyclic voltammetry and electrochemical impedance spectroscopy were employed to confirm their typical electrochemical characteristics. The as-prepared nanocomposites displayed enhanced catalytic activity and kinetics toward methanol electro-oxidation. Such an excellent performance can be ascribed to the high dispersion of flower-like Pt NPs and to the exposure of more sites provided by the flower-like structure. The improved stability, decreased charge transfer resistance, and enhanced reaction rate of the nanocomposites promise new opportunities for the development of direct methanol fuel cells.

Several ionic organic compounds as positive electrolyte additives for a vanadium redox flow battery

Several ionic organic compounds have been employed as additives of the V(V) electrolyte for a vanadium redox flow battery (VRB) to improve its stability and electrochemical activity. Stability of the V(V) electrolyte with and without additives was investigated with an ex situ heating/cooling treatment over a wide temperature range of −5 °C to 60 °C. It was found that cationic organic compounds could significantly improve the stability of the V(V) electrolyte over a wide range of temperatures. Their electrochemical behavior in the V(V) electrolyte was further studied by cyclic voltammetry (CV) and steady state polarization. The results showed that the electrochemical activity, including the reversibility of the electrode reaction, the diffusivity of V(V) species, polarization resistance of V(V) species, and the flexibility of charge transfer for the V(V) electrolyte with these additives were all improved compared with the pristine solution. The VRB employing the positive electrolyte with cationic organic compounds as additive exhibited excellent charge–discharge behavior with an average energy efficiency of more than 80% at a current density of 20 mA cm−2. XPS spectra illustrated that the addition of CHPTAC introduced more oxygen-containing and nitrogen-containing functional groups, which improved the electrochemical performance and cycling stability of the VRB.

Solvent effects on Pt-Ru/C catalyst for methanol electro-oxidation

Abstract Alloying degree, particle size and the level of dispersion are the key structural parameters of Pt-Ru/C catalyst in fuel cells. Solvent(s) used in the preparation process can affect the particle size and alloying degree of the object substance, which lead to a great positive impact on its properties. In this work, three types of solvents and their mixtures were used in preparation of the Pt-Ru/C catalysts by chemical reduction of metal precursors with sodium borohydride at room temperature. The structure of the catalysts was characterized by X-ray diffraction (XRD) and Transmission electron microscopy (TEM). The catalytic activity and stability for methanol electro-oxidation were studied by Cyclic Voltammetry (CV) and Chronoamperometry (CA). Pt-Ru/C catalyst prepared in H 2 O or binary solvents of H 2 O and isopropanol had large particle size and low alloying degree leading to low catalytic activity and less stability in methanol electro-oxidation. When tetrahydrofuran was added to the above solvent systems, Pt-Ru/C catalyst prepared had smaller particle size and higher alloying degree which resulted in better catalytic activity, lower onset and peak potentials, compared with the above catalysts. Moreover, the catalyst prepared in ternary solvents of isopropanol, water and tetrahydrofuran had the smallest particle size, and the high alloying degree and the dispersion kept unchanged. Therefore, this kind of catalyst showed the highest catalytic activity and good stability for methanol electro-oxidation.

Ultralow loading palladium nanocatalysts prepared by atomic layer deposition on three-dimensional graphite-coated nickel foam to enhance the ethanol electro-oxidation reaction

A novel three-dimensional graphite-coated nickel foam (GNF) was synthesized by the chemical vapor deposition (CVD) method, and palladium nanoparticles (Pd NPs) were successfully synthesized on a GNF support by metal atomic layer deposition (ALD) technology for the first time. The physicochemical properties of the as-prepared catalysts were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma-atomic emission spectrometry (ICP). Results showed that the Pd NPs with ultralow loading (below 50 μg cmPd−2) were uniformly dispersed on the GNF support, and the as-prepared catalysts presented the highest catalytic activity toward ethanol electro-oxidation (the peaking current density was about 39.97 mA cm−2) in alkaline media. In particular, it was found that the morphology and content of graphite of the GNF will greatly affect the dispersion of the ALD Pd NPs. When the CVD time for preparing the GNF was 10 min, the as-prepared catalyst presented a higher dispersity of Pd NPs and catalytic activity toward ethanol electro-oxidation than that of other as-prepared catalysts. The effect of the ALD cycle for Pd NPs growth and its performance was also investigated. When the cycle of ALD Pd was 450, the peaking current density of the as-prepared catalysts was about 2.64 times as high as that of commercial Pd/C to ethanol electro-oxidation. Herein, there is a promising application prospect for the prepared Pd/GNF nanocomposite as an electrocatalyst toward ethanol electro-oxidation in alkaline media.

Graphene oxide-assisted facile synthesis of platinum–tellurium nanocubes with enhanced catalytic activity for formic acid electro-oxidation

In order to obtain a loaded Pt-based catalyst with enhanced high activity and stability towards formic acid electro-oxidation (FAO), PtTe nanoparticles loaded on graphene oxide (GO) were fabricated by a facile and scalable method. XRD and HRTEM results show that the morphology of PtTe particles could be affected by the additive amount of GO and Te. It is observed that the supported PtTe particles are cubic. The XPS results show the change in the Pt electronic structure after the incorporation of Te, which impedes the chemisorption of the CO intermediate and promotes the dehydrogenation pathway of FAO. By electrochemical analysis, the performance towards FAO is greatly enhanced. The mass activity of PtTe/GO-67 is [Formula: see text] at 0.45 V (versus SCE), which is 11.5 times as high as that of Pt/C [Formula: see text] The incorporation of Te atoms and the content of GO are two major parameters for tuning the crystal structure and morphology and enhancing catalytic activity.

Efficient synthesis of nitrogen-doped carbon with flower-like tungsten nitride nanosheets for improving the oxygen reduction reactions

A novel three-dimensional (3D) electrocatalyst composite was successfully prepared through a facile hydrothermal method, consisting of tungsten nitride nanosheets with flower-like morphology (WN FNs) and nitrogen-doped carbon black (N–C). Highly crystalline WN FNs were observed to be uniformly distributed in N–C, and remarkably promoted the catalytic activities toward oxygen reduction reaction (ORR) of the N–C. Moreover, the electrochemical results proved that, WN FNs/N–C composite shows a significantly higher ORR activity with a 4-electron transfer pathway and limiting current density of 5.8 mA cm−2 in alkaline solutions. The stability test of the nanocomposite showed less degradation after 5 h and its methanol tolerance also was enhanced compared with Pt/C. The improved performance of the WN FNs/N–C composite can be attributed to its unique configuration and the increased exposure of active sites, which suggests that it could serve as an ideal candidate for developing high performance ORR catalysts.

Sulfonated poly(ether ether ketone)/poly(vinylidene fluoride)/graphene composite membrane for a vanadium redox flow battery

AbstractSulfonated poly(ether ether ketone)/poly(vinylidene fluoride)/graphene (S/P/G) composite membrane was prepared through a solution-casting method for a vanadium redox flow battery (VRB), and the weight ratio of high sulfonated poly(ether ether ketone) (SPEEK), polyvinylidene fluoride (PVDF), and graphene was optimized. The preferred S/P/G-7 composite membrane showed the lowest VO2+ permeability and highest ion selectivity compared with other four kinds of cation exchange membranes SPEEK75, heterogeneous PSSA-PE, Nafion 117, and recast Nafion (r-Nafion). The VRB with S/P/G-7 membrane exhibited the higher coulombic efficiency of ∼8% and energy efficiency of ∼4%, but lower capacity loss and self-discharge than that of VRB with Nafion117 membrane during cycling tests, which further indicated the promising prospects of S/P/G-7 composite membrane in VRB application. Graphical abstractᅟ

Controllable synthesis of two-dimensional tungsten nitride nanosheets as electrocatalysts for oxygen reduction reaction

A facile synthetic strategy was developed for in-situ preparation of two-dimensional (2D) highly crystalline tungsten nitride (WN) nanosheets with controllable morphology as oxygen reduction reaction (ORR) catalysts. The dependence of the crystal structure and morphology ofWN on K2SO4 content, pH, and pyrolysis temperature was thoroughly examined. The electrocatalytic performance of WN toward ORR in an alkaline electrolyte indicated that K+ plays an important role in the control of size and shape in the hydrothermal and nitridation process, thereby promoting the formation of plate-like WO3 and 2D WN nanosheets. The WN nanosheets, with largely exposed edge sites, provide abundant catalytic active sites and allow fast charge transfer. Furthermore, they exhibit high stability for ORR and methanol tolerance.摘要本文以钨酸钠和硫酸钾等试剂为原料, 采用水热结合氨气氮化的方法原位获得了一种二维的氮化钨(WN)晶体, 并将其作为氧还原 (ORR)反应的催化剂. 通过控制和优化制备条件(水热过程中硫酸钾含量、pH值以及氮化条件包括氮化温度和氮化时间), 实现了纳米WN 的可控制备.