Academic literature on the topic 'Dual-phase cobalt sulfide'

Nanocrystalline Ni – Co alloys are electrodeposited by direct (DC) and pulse current (PC) in an electrolyte solution which consisted of nickel sulfate, cobalt sulfate and boric acid. Electrodeposition parameters including current density, electrolyte pH and pulse times in a single electrolyte bath were changed. XRD pattern showed that the structure of the alloys depends on Co content and the synthesis parameter and changed from single phase structure (fcc) to dual phase structure (fcc + hcp). The Co content in the deposited alloys declined from 70 at.% to 50 at.% by increasing in direct current from 70 mA/cm2 to 115 mA/cm2 and also decreased from 75 at.% to 33 at.% with decrease in pH values from 4 to 2. By applying PC the Co content changed from 76 at.% to 41 at.%. Magnetic properties measurements showed the saturation magnetization (Ms) increased with increasing the Co content. There was no significant effect on coercivity values (Hc) with change in Co content and about 40 Oe was obtained for all samples. The grain size of deposited alloys obtained between 24–58 nm and 15–21 nm by applying DC and PC, respectively.

The transition metal-based layered double hydroxides (LDHs) have been extensively studied as promising electrode materials for battery-type supercapacitors owing to their excellent electrochemical activity and tunable chemical composition. In our work, we firstly built a novel three-dimensional composite with a core-shell structure by self-assembling NiCo-LDHs nanosheets on dual surface-modified halloysite nanotubes (D-HNTs) via in-situ electrodeposition and hydrothermal method (NiCo-LDH/D-HNTs). HNTs were pretreated by a dual surface modification for the first time including a carbonization process and cobalt doping to enhance electrical conductivity and chemical stability. This multicomponent hierarchical nanocomposite, NiCo-LDH/D-HNTs, has a specific capacity of 1401.4 C g-1 at 1 A g-1, a rate capability of 52.9% increasing the current density to 30 A g-1, and cycling stability of 80.8% after 2000 cycles. Moreover, using acetate anions (Ac-) as intercalating element, the NiCo-LDH nanosheets arraying on Ni Foam were prepared with the optimized amount of Ac- anions expanding the interlayer space of LDH nanosheets from 0.8 to 0.94 nm. An ultrahigh specific capacity of 1200 C g-1 at 1 A g-1 (690 C g-1 without Ac- anions), outstanding rate capability of 72.5% at 30 A g-1 and cycle stability of 79.90% after 4500 cycles were achieved. The enlarged interlayer spacing was beneficial for stabilizing the α-phase of LDH and accelerating the electron transport and electrolyte penetration in the electrochemical reaction. In addition, ionic surfactants like sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) were introduced to regulate the morphology of LDHs and alter the electrochemical performance. The original NiCo-LDHs shows microspheres formed by nanorods, and NiCo-LDHs-CTAB is similar, yet more tangled, while NiCo-LDHs-SDS is like microflowers consisting of interwoven nanosheets. NiCo-LDHs-SDS achieved better electrochemical performance (1003.6 C g-1 at 1 A g-1 and 73.84% after 3500 cycles) than NiCo-LDHs-CTAB (430.54 C g-1, 57.3%), and both higher than original NiCo-LDHs (278.5 C g-1, 54.3%), clearly showing the importance of morphology regulation in the synthesis of nanomaterials. Our work offers a promising strategy to synthesize functional nanomaterials with excellent electrochemical performance via integrating its unique layered structure and interlayer anion exchange characteristics.

Lithium-oxygen (Li-O2) batteries have attracted intensive attention in last decade, due to its high theoretical energy densities and environmental benignity that satisfy the need for large energy storage systems including electric vehicles. However, they are still in their infancy and several challenges remain to be addressed immediately. In addition to the degradation of anode and electrolyte, one of the biggest challenges is the structure and catalytic design for oxygen electrode to achieve high capacity and long cycle life. In this thesis, size-controlled polystyrene (PS) spheres were introduced to a polydopamine derived N-doped reduced graphene (N-rGO) to explore the impact of the pore size of carbon oxygen electrodes to the performance of Li-O2 batteries. The battery containing N-rGO with 170 nm pores revealed a high specific capacity of 16777 mA h g-1, which is one of the highest among the reported carbon-based Li-O2 batteries. Field emission scanning electron microscope (FESEM) of cathode morphologies before and after discharge/charge showed that the N-rGO with 170 nm pores could hold most discharge products at a cut-off capacity of 1000 mA h g-1 without deformation to achieve a long stable cycle life. Furthermore, cobalt sulfides with controlled phases being synthesized via thermal decomposition of Co(TU)4(NO3)2 were studied as the bi-functional catalysts towards both ORR and OER of Li-O2 batteries in order to improve specific capacity and cycling life. A dual-phase cobalt sulfide prepared at 900 °C (CoS-900) contains both Co9S8 and CoS exhibited excellent ORR and OER catalytic activities with a low overvoltage (1.25 V) for Li-O2 batteries. The designated CoS-900@NG cathode achieved large discharge capacity at 7410 mA h g-1 with 100% charge capacity recovery as well as a super long cycle life at 108 cycles for Li-O2 battery. The excellent Li-O2 batteries performance can be attributed to the generation of both crystalline and amorphous film-like Li2O2 that effectively improve ORR/OER kinetics of Li-O2 batteries. Finally, Co9S8 nanoparticles were anchored to N, S co-doped graphene to form leave-like Co9S8/N, S-GO composites through hydrothermal treatment. The composite was further optimized by adjusting cobalt sulfide precursor amount to achieve an improvement of battery performance. As a result, the Li-O2 battery with Co9S8/N, S-GO composite can achieve a 100% recoverable high discharge capacity at 4884 mA h g-1 and a stable cycle life. The thesis systematically explored the relationship between the structures of oxygen electrode and electrochemical performance of Li-O2 batteries, including surface structure, heteroatom doping and cobalt sulphide hybridization. The outcomes provide new perspectives for the future development of high-performance Li-O2 batteries by strategically designing ORR/OER catalysts.
Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering & Advanced Materials, 2020