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- Mixed metal sulfides are increasingly being investigated because of their prospective applications for electrochemical energy storage and conversion. Their high electronic conductivity and high density of redox sites result in significant improvement of their electrochemical properties. Herein, the composition-dependent supercapacitive and water splitting performance of a series of Ni(1−x)CuxCo2S4 (0.2 ≤ x ≤ 0.8) solid solutions prepared via solvent-less pyrolysis of a mixture of respective metal ethyl xanthate precursors is reported. The use of xanthate precursors resulted in the formation of surface clean nanomaterials at low-temperature. Their structural, compositional, and morphological features were examined by p-XRD, SEM, and EDX analyses. Both supercapacitive and electrocatalytic (HER, OER) properties of the synthesized materials significantly vary with composition (Ni/Cu molar content). However, the optimal composition depends on the application. The highest specific capacitance of 770 F g−1 at a current density of 1 A g−1 was achieved for Ni0.6Cu0.4Co2S4 (NCCS-2). This electrode exhibits capacitance retention (CR) of 67% at 30 A g−1, which is higher than that observed for pristine NiCo2S4 (838 F g−1 at 1 A g−1, 47% CR at 30 A g−1). On the contrary, Ni0.4Cu0.6Co2S4 (NCCS-3) exhibits the lowest overpotential of 124 mV to deliver a current density of 10 mA cm−2. Finally, the best OER activity with an overpotential of 268 mV at 10 mA cm−2 was displayed by Ni0.8Cu0.2Co2S4 (NCCS-1). The prepared electrodes exhibit high stability, as well as durability.
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- Transition metal sulfides have been investigated as promising bifunctional materials for catalytic energy generation and energy storage applications. Although various strategies such as tuning the size, phase or defects and composition engineering have led to catalytic enhancement, there still remains the requirement for better performance for practical applications. In this study, we have used a potentially scalable solventless route for phase selective synthesis of α-NiS or β-NiS. Both phases were doped with different transition metals (Cu, Co and Fe) for enhanced catalytic performance. Interestingly, besides commonly observed thermal assisted phase transition, dopant (Co, Cu, and Fe) induced α- to β-phase transition or vice versa was also observed which has rarely been reported for NiS. The effect of dopants on the crystal structure and electrocatalytic activity has been investigated. The best supercapacitive behavior was observed for Co-doped α-NiS which showed a specific capacitance of 1586 F g−1 at a current density of 0.5 A g−1 and a high rate capability. On the other hand, Fe-doped α-NiS displayed the best electrocatalytic activity for both the OER (266 mV at 10 mA cm−2) and the HER (146 mV at 10 mA cm−2), with Tafel slopes of 79 and 113 mV dec−1 respectively. The Fe-doped α-NiS catalyst was also used as both the anode and cathode in an electrolyzer, in which an overpotential of about 410 mV at 10 mA cm−2 was observed. The prepared electrodes demonstrate outstanding stability and flexibility.
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- Ternary metal sulfides are currently in the spotlight as promising electroactive materials for high-performance energy storage and/or conversion technologies. Extensive research on metal sulfides has indicated that, amongst other factors, the electrochemical properties of the materials are strongly influenced by the synthetic protocol employed. Herein, we report the electrochemical performance of uncapped NiCo2S4 and CuCo2S4 ternary systems prepared via solventless thermolysis of the respective metal ethyl xanthate precursors at 200 and 300 °C. The structural, morphological and compositional properties of the synthesized nanoparticles were examined by powder X-ray diffraction (p-XRD), transmission electron microscopy (TEM), high-resolution TEM, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDX) techniques. Electrochemical studies indicate that NiCo2S4 nanoparticles synthesized at 300 °C exhibit superior energy storage characteristics with a high specific capacitance of ca. 2650 F g−1 at 1 mV s−1, as compared to CuCo2S4 nanoparticles, which showcased a specific capacitance of ca. 1700 F g−1 at the same scan rate. At a current density of 0.5 A g−1, NiCo2S4 and CuCo2S4 nanoparticles displayed specific capacitances of 1201 and 475 F g−1, respectively. In contrast, CuCo2S4 nanoparticles presented a higher electrocatalytic activity with low overpotentials of 269 mV for oxygen evolution reaction (OER), and 224 mV for the hydrogen evolution reaction (HER), at 10 mA cm−2. The stability of the catalysts was examined for 2000 cycles in which a negligible change in both OER and HER activities was observed.
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- Straightforward synthetic routes to the preparation of transition metal phosphides or their chalcogenide analogues are highly desired due to their widespread applications, including catalysis. We report a facile and simple route for the preparation of a pure phase nickel phosphide (Ni2P) and phase transformations in the nickel sulfide (NiS) system through a solvent-less synthetic protocol. Decomposition of different sulfur-based complexes (dithiocarbamate, xanthate, and dithiophosphonate) of nickel(II) was investigated in the presence and absence of triphenylphosphine (TPP). The optimization of reaction parameters (nature of precursor, ratio of TPP, temperature, and time) indicated that phosphorus- and sulfur-containing inorganic dithiophosphonate complexes and TPP (1:1 mole ratio) produced pure nickel phosphide, whereas different phases of nickel sulfide were obtained from dithiocarbamate and xanthate precursors in the presence or absence of TPP. A plausible explanation of the sulfide or phosphide phase formation is suggested, and the performance of Ni2P was investigated as an electrocatalyst for supercapacitance and overall water-splitting reactions. The performance of Ni2P with the surface free of any capping agents is not well explored, as common synthetic methods are solution-based routes; therefore, the electrocatalytic performance was also compared with metal phosphides, prepared by other routes. The highest specific capacitance of 367 F/g was observed at 1 A/g, and the maximum energy and power density of Ni2P were calculated to be 17.9 Wh/kg and 6951 W/kg, respectively. The prepared nickel phosphide required overpotentials of 174 and 316 mV along with Tafel slopes of 115 and 95 mV/dec to achieve a current density of 10 mA/cm2 for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively.
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- Cost-effective and readily available catalysts applicable for electrochemical conversion technologies are highly desired. Herein, we report the synthesis of dithiophosphonate complexes of the type [Ni{S2P(OH)(4-CH3OC6H4)}2] (1), [Co{S2P(OC4H9)(4-CH3OC6H4)}3] (2) and [Fe{S2P(OH)(4-CH3OC6H4)}3] (3) and employed them to prepare Ni2P, Co-Ni2P and Fe-Ni2P nanoparticles. Ni2P was formed by a facile hot injection method by decomposing complex 1 in tri-octylphosphine oxide/tri-n-octylphosphine at 300 °C. The prepared Ni2P was doped with Co and Fe employing complexes 2 and 3, respectively, under similar experimental conditions. Doping Ni2P with Co and Fe demonstrated synergistic improvement of Ni2P performance as an electrocatalyst in supercapcitance, hydrogen evolution and oxygen evolution reactions in alkaline medium. Cobalt doping improved the Ni2P charge storage capacity with a supercapacitance of 864 F g−1 at 1 A g−1 current density. Fe doped Ni2P recorded the lowest overpotential of 259 mV to achieve a current density of 10 mA cm−2 and a Tafel slope of 80 mV dec−1 for OER, better than the undoped Ni2P and the benchmark IrO2. Likewise, Fe-doped Ni2P electrode required the lowest overpotential of 68 mV with a Tafel slope of 110 mV dec−1 to attain the same current density for HER. All catalysts showed excellent stability in supercapacitance and overall water splitting reactions, indicating their practical use in energy conversion technologies.
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