Two types of PdCu2 nanoparticles were prepared through one-pot synthesis and a two-step reducing process, named as PdCu2-1 and PdCu2-2, respectively. The morphology and structure of as-prepared samples were investigated by transmission electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and inductively coupled plasma-optical emission spectrometry. Results showed that more Pd atoms were buried in the inside of PdCu2-1, whereas more available Pd sites were distributed on the surface of PdCu2-2. The electrochemical measurements indicated that both PdCu2-1 and PdCu2-2 nanoparticles showed a higher electrocatalytic activity than that for pure Pd nanoparticles. In particular, PdCu2-2 predictably exhibited a better stability and durability as well as a lower onset potential and a higher catalytic current density than that of PdCu2-1 toward ethanol oxidation in alkaline media. On the basis of these studies, the formation mechanisms of both the PdCu2 catalysts and the relationship between their structure and properties were discussed in this paper.Keywords: electrocatalysis; Pd-based catalysts; PdCu2; solution-phase synthesis; structural regulation;

The electrocapacitive behavior of MnFe2O4-based supercapacitors has been studied by a series of electrochemical techniques, including circle voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy. The size of the used MnFe2O4 colloidal nanocrystal assemblies (CNAs) was in the range of 230–950 nm, formed by the in situ self-assembly of primary MnFe2O4 nanoparticles with different sizes. Electrochemical measurements showed that the electrochemical performances of MnFe2O4-based supercapacitors were related to the structure of MnFe2O4 CNAs. MnFe2O4 CNAs with the size of 420 nm, composed of 16 nm nanoparticles, displayed the highest capacitance of about 88.4 F/g at the current density of 0.01 A/g, which, respectively, decreased to 55.8 and 20.2 F/g for CNAs with size of 230 and 950 nm, assembled by 21 and 43 nm nanoparticles. Electrochemical stability data showed that 420 nm MnFe2O4 CNAs had the best capacitance retention of 59.4% with the current density increased from 0.01 to 2 A/g and the best capacitance retention of 69.2% after 2000 cycles among all the samples under the current density of 0.2 A/g. The structure–property relationship of MnFe2O4 CNAs was analyzed and discussed based on the experimental data.

•Carbon spheres with uniform pore size were prepared by spray-drying assisted method.•Porous carbon acted as sacrificial template for growing carbon modified porous MnO2.•The morphology of the porous MnO2 spheres can be tuned by changing hydrothermal time.•The in-situ formation of carbon modified MnO2 spheres are monitored with microscopy.•Carbon modified porous MnO2 spheres show excellent performance for supercapacitors.Carbon modified MnO2 (CMMO) spheres have been fabricated through a facile low temperature (60 °C) hydrothermal method using mesoporous carbon spheres as reductive agent and sacrificial template and KMnO4 as manganese source. CMMO spheres with novel nanostructures such as flower-like and sea urchin-like are obtained by controlling the reaction time. The roles of mesoporous carbon in directing the growth of the CMMO spheres and controlling their morphologies have been investigated. The CMMO spheres are characterized by XRD, XPS, SEM, TEM, Raman spectra, TGA and N2 adsorption-desorption technique and electrochemical measurement. The resulted samples possess unique morphologies and regular pores, and their properties changed as reaction time proceed. The peseudocapacitive behaviors of the as-prepared samples are tested in two-electrode supercapacitors using 2 mol L−1 KOH aqueous solutions as electrolyte. A high gravimetric capacitance of 344 F g−1 at 1 A g−1 and the capacity retaining of 75% after 5000 cycles are achieved on the electrode prepared with one of the CMMO samples. The other CMMO samples also possess excellent electrochemical performance in comparison with the pristine mesoporous carbon (p-MC). Such superior electrochemical performance makes the porous CMMO spheres to be promising materials in the application of pseudocapacitors.Download high-res image (369KB)Download full-size image

•LixFe0.2Mn0.8PO4/C composite fibers were synthesized based on the electrospinning method.•The content of lithium of LixFe0.2Mn0.8PO4/C fibers has a strong effect on their electrochemical properties.•The molecular structure of carbon sources plays an important role in determining the electrochemical properties of composite fibers.LixFe0.2Mn0.8PO4/C (LixFMP/C, x = 1.2, 1.1, 1.05) composite fibers were successfully prepared by stabilization and calcination of the electrospun fibers from the precursor solution. The structural and morphological characterizations revealed that the LixFe0.2Mn0.8PO4 with high purity was evenly coated with an amorphous carbon layer. Experimental data testified the well-crystalline structure of composite fibers based on the results of the X-ray diffraction (XRD) and selected area electron diffractions (SAED). The galvanostatic charge–discharge measurements indicated that Li1.2Fe0.2Mn0.8PO4 displayed the highest capacity of 174 mA h g−1 at 0.05 C and the best cycling stability. The charge-transfer impedance of LixFe0.2Mn0.8PO4/C was decreased negatively with the content of lithium. It was found that the molecular structure of carbon sources and calcination procedure played key factor in determining the electrochemical properties of the composite fibers. These results suggested that electrospinning should be a promising method for fabricating crystalline LFMP/C composite fibers as electrode materials for lithium-ion battery.

Submicrometer MnFe2O4 colloidal nanocrystal assemblies (CNAs) have been synthesized controllably by using a solvothermal method through simply adjusting synthetic reagents. The size and microstructure of MnFe2O4 CNAs were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results showed that MnFe2O4 CNAs were well-separated and uniform with the size scales ranging from 230 nm to 950 nm, which were composed of primary crystalline nanoparticles with the sizes ranging from 16 nm to 43 nm. Room-temperature magnetic measurement results showed that MnFe2O4 CNAs were weakly ferromagnetic with small remnant saturation and coercivity values. The magnetization saturation values of CNAs were increased with the increase of the size of primary nanoparticles. Electrochemical measurements showed that the size of primary nanoparticles of MnFe2O4 CNAs had an important effect on the electrochemical reduction of H2O2. However, the electrocatalytic activity of MnFe2O4 CNAs for oxygen reduction reaction closely correlated with both the crystal size and self-assembly of primary nanoparticles. Based on the experimental results, the formation mechanisms of MnFe2O4 CNAs as well as the relationship between their structures and properties have been analyzed and discussed.

A one-step high-temperature solvothermal process (can be used up to 400 °C) has been explored for the preparation of Fe3O4/graphene composites. The influence of high temperature (>230 °C) on the structure, morphology and electrochemical properties of the resulting Fe3O4/graphene composites was investigated by XRD, SEM, TEM and N2 adsorption–desorption measurements. Electrochemical performances of the as-prepared Fe3O4/graphene composites at different temperatures were evaluated in coin-type cells as anode materials for lithium-ion batteries. In comparison with the traditional solvothermal method (<240 °C), the high-temperature method does not require an additional calcination process yet it still could result in Fe3O4/graphene composites with pure phase and excellent electrochemical properties. A preferred solvothermal temperature of 280 °C has been deduced based on a series of control experiments. The Fe3O4/graphene composite derived at 280 °C exhibited a high reversible capacity of 907 mA h g−1 at 0.1 C (92.6 mA g−1) even after 65 cycles, showing outstanding cycle stability. It also exhibited a high rate capability of 410 mA h g−1 at 2 C (1852 mA g−1). The role of the graphene substrates in improving the electrochemical properties of the composite is discussed based on the morphology, structure, phase and electrochemical property studies.

•The capacitance of MnFe2O4 colloidal nanocrystal clusters can be adjusted by the electrolytes.•The assembly of colloidal nanocrystals should be important for their electrocapacitive features.•The size of hydrated ions plays the key roles in determining the electrocapacitive features.•The capacitance of the electrode was the largest when using the potassium hydroxide electrolyte.The electrochemical performances of symmetric supercapacitors assembled by MnFe2O4 colloidal nanocrystal clusters (CNCs) in aqueous electrolytes were investigated by using cyclic voltammetry, galvanostatic charge–discharge, cycle stability and electrochemical impedance spectroscopy. Results showed that the capacitance of MnFe2O4 CNCs can be easily adjusted by the controlled electrolytes. It was found that the specific capacitances of CNCs-based electrodes were 97.1, 93.9, 74.2 and 47.4 F g−1 for the electrolytes (2 M) containing KOH, NaOH, LiOH and Na2SO4, respectively, at the current density of 0.1 A g−1. The capacitance of the electrode was increased from 56.9 to 152.5 F g−1 with aqueous KOH electrolytes changed from 0.5 M to 6 M. The MnFe2O4 CNCs-based supercapacitor using aqueous KOH (6 M) electrolyte displayed the best cycle stability among all the supercapacitors. Based on the experimental results, the enhancement mechanism of electrochemical performances for the CNCs-based supercapacitors was proposed.

Palladium (Pd) nanocrystals have been synthesized by using formic acid as the reducing agent at room temperature. When the concentration of formic acid was increased continuously, the size of Pd nanocrystals first decreased to a minimum and then increased slightly again. The products have been investigated by a series of techniques, including X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), UV–vis absorption, and electrochemical measurements. The formation of Pd nanocrystals is proposed to be closely related to the dynamical imbalance of the growth and dissolution rate of Pd nanocrystals associated with the adsorption of formate ions onto the surface of the intermediates. It is found that small Pd nanocrystals showed blue-shifted adsorption peaks compared with large ones. Pd nanocrystals with the smallest size display the highest electrocatalytic activity for the electrooxidation of formic acid and ethanol on the basis of cyclic voltammetry and chronoamperometric data. It is suggested that both the electrochemical active surface area and the small size effect are the key roles in determining the electrocatalytic performances of Pd nanocrystals. A “dissolution–deposition–aggregation” process is proposed to explain the variation of the electrocatalytic activity during the electrocatalysis according to the HRTEM characterization.

Nanoparticles (NPs) and colloidal nanocrystal clusters (CNCs) of ZnFe2O4 were synthesized by using a solvothermal method in a controlled manner through simply adjusting the solvents. When a glycerol/water mixture was used as the solvent, ZnFe2O4 NPs were obtained. However, using ethylene glycol solvent yielded well-dispersed ZnFe2O4 CNCs. X-ray diffraction (XRD) and transmission electron microscopy (TEM) data confirmed that the ZnFe2O4 NPs were a single crystalline phase with tunable sizes ranging from 12 to 20 nm, while the ZnFe2O4 CNCs of submicrometer size consisted of single-crystalline nanosheets. Magnetic measurement results showed that the ZnFe2O4 NPs were ferromagnetic with a very small hysteresis loop at room temperature. However, CNCs displayed a superparamagnetic behavior due to preferred orientations of the nanosheets. Electrochemical sensing properties showed that both the size of the NPs and the structure of the CNCs had a great influence on their electrochemical properties in the reduction of H2O2. Based on the experimental results, the formation mechanisms of both the ZnFe2O4 CNCs and NPs as well as their structure–property relationship were discussed.

Microporous carbon materials were prepared by carbonization of sulfuric acid-pretreated sucrose. The pore size and specific surface area of the samples were measured to be in the ranges of 0.7–1.2 nm and 178–603 m2/g, respectively. The pore parameters were found to depend strongly on the carbonization temperature. Raman spectra showed that the intensity of the G band was stronger than that of the D band for the samples obtained with carbonization temperatures above 800 °C. It was also found that the sample carbonized at 800 °C displayed the highest specific surface area with a main pore size of about 0.75 nm. This sample exhibited the highest specific capacitance (232 F/g) at a current density of 0.1 A/g and lowest electrical resistance based on the results of cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy (EIS). All samples displayed a good cycle performance behavior evaluated using both three-electrode and two-electrode cells. Based on the experimental results, the formation mechanism of the carbon materials as well as the relationship between the pore structure and their electrochemical properties were analyzed and discussed.Graphical abstractHighlights► The microporous carbons have been prepared from sucrose directly by the carbonization method. ► The specific surface area and pore size of microporous carbons can be easily adjusted. ► The carbonization temperature is the only tunable parameter in fabricating the carbon materials. ► The microporous carbon with the highest surface area shows the best electrochemical performance.

Hollow spheres and colloidal nanocrystal clusters (CNCs) of MnFe2O4 with similar submicron scales have been synthesized controllably by a solvothermal method through simply adjusting the synthesis microenvironment. Morphology and microstructure of the products were investigated by the power X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). Magnetic property measurements at room temperature showed that MnFe2O4 hollow spheres were ferromagnetic with a very small hysteresis loop while MnFe2O4 CNCs showed superparamagnetic behavior. Magnetization saturation values were about 76.5 and 63.4 emu/g for hollow spheres and CNCs, respectively. The band gaps for hollow spheres and CNCs were 1.68 and 1.74 eV, respectively, based on the results of diffuse reflectance spectra (DRS). MnFe2O4 hollow spheres showed higher photocatalytic activity under visible light to methylene blue than that of CNCs. Both of the samples also displayed an excellent recycle performance. Formation mechanisms of MnFe2O4 hollow spheres and CNCs, and the relationship between their structure and properties, have been studied based on the experimental results.Graphical abstractHighlights► Hollow spheres and colloidal nanocrystal clusters (CNCs) of MnFe2O4 are fabricated controllably. ► Submicron-scale hollow spheres show ferromagnetic behavior with a very small hysteresis loop. ► Submicron-scale CNCs of MnFe2O4 display superparamagnetic behavior. ► Hollow spheres and CNCs show high photocatalytic activity under visible light to methylene blue.

Two kinds of Pd nanoparticle aggregates with different electrocatalytic activity are developed in a facile hydrothermal synthesis method. The average size of Pd nanoparticles synthesized from the mixed solvent containing water and acetone (Pd–M) is ∼9 nm smaller than that of Pd samples synthesized from aqueous systems (Pd–A) with the size of ∼14 nm. Nanoparticles of Pd–M and Pd–A show the crystalline nature based on the XRD and HRTEM results. It is found that Pd–M display a remarkably high electrooxidation activity per unit mass toward formic acid and ethanol, which is 8–12 times as high as that of Pd–A. The electrooxidation activity per unit area of Pd–M is also higher than that of Pd–A. The formation mechanisms of Pd–A and Pd–M as well as the relationship between their microstructures and electrocatalytic activity have been discussed based on the experimental results.Graphical abstractHighlights► Pd nanocrystal aggregates show high electrocatalytic activity toward formic acid and ethanol. ► Pd nanocrystals with controlled size are prepared through hydrothermal process. ► Addition of acetone into the synthesis systems leads to the formation of smaller Pd nanocrystals. ► Smaller Pd nanocrystals show much higher catalytic activity than that of larger Pd nanocrystals.