Rational design and facile synthesis of Ni-Co-Fe ternary LDH porous sheets for high-performance aqueous asymmetric supercapacitor

Factorial Design and Optimization of Trimetallic CoNiFe-LDH/Graphene Composites for Enhanced Oxygen Evolution Reaction

Layered double hydroxides (LDH) have exhibited promising applications as electrocatalysts in oxygen evolution reactions (OER). In this work, trimetallic LDHs (CoNiFe-LDH) were designed and grown on graphene (G) through a one-step hydrothermal approach to obtain a structure that promotes efficient charge transfer. A 2-level full-factorial design was utilized to evaluate the effects of varying the concentrations of Co (1.5, 3, and 4.5 mmol) and graphene (10, 30, and 50 mg) on the OER activity. The potential needed to deliver 10 mA cm-2 was chosen as the response parameter. The independent and dependent parameters were fitted to a linear model equation through ANOVA analysis. The computed p-values were below 0.05 signifying the statistical significance of the concentrations of cobalt and graphene and their interaction, suggesting a correlation with the OER activity. The OER experiments were conducted in triplicate using the Co[3]Ni[3]Fe[3]-LDH/G[30] (central point) to estimate variability (0.58%). Comparative analysis showed that Co[1.5]Ni[3]Fe[3]-LDH/G[10] achieved the lowest onset potential (1.54 V), potential at 10 mA cm-2 (1.58 V), and Tafel slope (58.4 mV dec-1), indicating that a low concentration of cobalt and graphene make an efficient electrocatalyst for OER. Furthermore, the optimized composite demonstrated favorable electronic properties, with a charge transfer resistance (RCT) of 188.1 Ω, and exhibited good stability, maintaining its catalytic activity with no significant loss over a 24-h period.

Construction of flexible MnCo2O4@FeCoNi-LDH electrode materials with nanoflower-like and hierarchical structure for high-performance asymmetric supercapacitor

In the realm of energy storage, flexible portable supercapacitors have been receiving increasing attention in the last few years. Nonetheless, the process of choosing appropriate flexible materials remains challenging. Herein, we successfully synthesized a flower-like MnCo2O4@FeCoNi-LDH/CC (MnCo@FCN/CC) hierarchically nanostructured electrode material by anchoring MnCo2O4 (MnCo) on a flexible carbon cloth (CC) substrate first and then loading FeCoNi-LDH nanosheets on MnCo2O4. The synthesized MnCo@FCN/CC material has numerous mesopores, huge specific surface area and multivalent metal ions, which makes MnCo@FCN/CC nanomaterial possess powerful electrochemical reaction kinetics and exceptional cycle stability. As a result, the electrode material exhibits a high specific capacitance (Cs) value of 2235F g-1 and maintains 88.6 % of the initial capacitance after 10,000 cycles. Significantly, a flexible asymmetric supercapacitor (ASC) constructed in the form of MnCo@FCN/CC//AC/CC has excellent energy density (51.66 Wh kg-1 at 890.81 W kg-1), and after 10,000 times of constant current charging and discharging, the capacitance retention rate still reaches 92.9 %. Therefore, the as-construct MnCo@FCN/CC//AC/CC high-performance flexible supercapacitors should envision broad commercial applications in flexible energy storage devices.

High-Performance Supercapacitors Using Hierarchical And Sulfur-Doped Trimetallic NiCo/NiMn Layered Double Hydroxides

A supercapacitor features high power density and long cycling life. However, its energy density is low. To ensemble a supercapacitor with high power- and energy-densities, the applied capacitor electrodes play the key roles. Herein, a high-performance capacitive electrode is designed and grown on a flexible carbon cloth (CC) substrate via a hydrothermal reaction and a subsequent ion exchange sulfuration process. It has a 3D heterostructure, consisting of sulfur-doped NiMn-layered double hydroxide (LDH) nanosheets (NMLS) and sulfur-doped NiCo-LDH nanowires (NCLS). The electrode with sheet-shaped NMLS and wire-shaped NCLS on their top (NMLS@NCLS/CC) increases the available surface area, providing more pseudocapacitive sites. It exhibits a gravimetric capacity of 555.2 C g-1 at a current density of 1 A g-1, the retention rate of 75.1% when the current density reaches up to 20 A g-1, as well as superior cyclic stability. The assembled asymmetric supercapacitor that is composed of a NMLS@NCLS/CC positive electrode and a sulfurized activated carbon negative electrode presents a maximum energy density of 24.2 Wh kg-1 and a maximum power density of 16000 W kg-1. In this study, a facile strategy for designing hierarchical LDH materials is demonstrated as well as their applications in advanced energy storage systems.

Hierarchical MoS2@NiFeCo-Mo(doped)-Layered Double Hydroxide Heterostructures as Efficient Alkaline Water Splitting (Photo)Electro-catalysts

Designing cost-effective electrocatalysts with fast reaction kinetics and high stability is an outstanding challenge in green hydrogen generation through overall water splitting (OWS). Layered double hydroxide (LDH) heterostructure materials are promising candidates to catalyze both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), the two OWS half-cell reactions. This work develops a facile hydrothermal route to synthesiz hierarchical heterostructure MoS2@NiFeCo-LDH and MoS2@NiFeCo-Mo(doped)-LDH electrocatalysts, which exhibit extremely good OER and HER performance as witnessed by their low IR-corrected overpotentials of 156 and 61 mV with at a current density of 10 mA cm-2 under light assistance. The MoS2@NiFeCo-Mo(doped)-LDH-MoS2@NiFeCo-LDH OWS cell achieves a low cell voltage of 1.46V at 10 mA cm-2 during light-assisted water electrolysis. Both materials exhibited exceptional stability under industrially relevant HER and OER conditions, maintaining a current density of 1 A cm-2 with minimal alterations in their potential and performance. The experimental and computational results demonstrate that doping the LDH matrix with high-valence Mo atoms and MoS2 quantum dots improves the electrocatalytic activity by 1) enhancing electron transfer, 2) making the electrocatalyst metallic, 3) increasing the number of active sites, 4) lowering the thermodynamic overpotential, and 5) changing the OER mechanism. Overall, this work develops a facile synthesis method to design highly active and stable MoS2@NiFeCo-Mo(doped)-LDH heterostructure electrocatalysts.

Role of synthetic process parameters of nano-sized cobalt/nickel oxide in controlling their structural characteristics and electrochemical energy performance as supercapacitor electrodes

The synthesis of nano-sized bimetallic Cobalt/Nickel oxides (Ni1.5Co1.5O4) with a 1:1 Co/Ni atomic ratio has been achieved using a surfactant-free co-precipitation/hydrothermal process. The growth mechanism of Cobalt/Nickel oxides Ni1.5Co1.5O4 is elucidated by tuning the synthesis process parameters, including co-precipitation pH and hydrothermal time. The formation of Cobalt/Nickel oxides Ni1.5Co1.5O4 oxide began with the nucleation of cobalt nickel hydroxide nanoplates through the co-precipitation process, followed by dissolution-recrystallization, stacked hexagonal nano-flakes, and a flower-like microstructure. The electrochemical performances of the oxides were evaluated, with the largest surface area observed at pH 9 being the main factor for the best super-capacitive performance. As hydrothermal time increased, the structural directing growth forward, resulted in the formation of a nano-flower structure with a larger surface area. The as-prepared cobalt nickel oxide exhibited a maximum specific capacitance value of 525.5 F g-1 at a current density of 1 A g-1 and energy and power densities of 88.2 WhKg-1 and 606 WKg-1, respectively.

2D-on-2D Al-Doped NiCo LDH Nanosheet Arrays for Fabricating High-Energy-Density, Wide Voltage Window, and Ultralong-Lifespan Supercapacitors

Battery-type electrode materials with high capacity, wide potential windows, and good cyclic stability are crucial to breaking through energy storage limitations and achieving high energy density. Herein, a novel 2D-on-2D Al-doped NiCo layered double hydroxide (NiCoAlx LDH) nanosheet arrays with high-mass-loading are grown on a carbon cloth (CC) substrate via a two-step hydro/solvothermal deposition strategy, and the effect of Al doping is employed to modify the deposition behavior, hierarchical morphology, phase stability, and multi-metallic synergistic effect. The optimized NiCoAl0.1 LDH electrode exhibits capacities of 5.43, 6.52, and 7.25 C cm-2 (9.87, 10.88, and 11.15 F cm-2) under 0-0.55, 0-0.60, and 0-0.65 V potential windows, respectively, illustrating clearly the importance of the wide potential window. The differentiated deposition strategy reduces the leaching level of Al3+ cations in alkaline solutions, ensuring excellent cyclic performance (108% capacity retention after 40 000 cycles). The as-assembled NiCoAl0.1 LDH//activated carbon cloth (ACC) hybrid supercapacitor delivers 3.11 C cm-2 at 0-2.0 V, a large energy density of 0.84 mWh cm-2 at a power density of 10.00 mW cm-2, and excellent cyclic stability with ≈135% capacity retention after 150 000 cycles.

Construction of porous flower-like Ru-doped CoNiFe layered double hydroxide for supercapacitors and oxygen evolution reaction catalysts

In recent years, ternary layered double hydroxide (LDH) has become a research hotspot for electrode materials and oxygen evolution reaction (OER) catalyst due to the enhanced synergistic effect between individual elements. However, the application of LDH is greatly limited by its low electrical conductivity and the disadvantage that nanosheets tend to accumulate and mask the active sites. Herein, a novel Ru-doped CoNiFe - LDH was prepared via a facile hydrothermal method. According to the density functional theory (DFT) calculations, the doping of Ru element could improve electron state density and band gaps of LDH and consequently boosted the electrochemical reaction kinetics as well as electrical conductivity. Furthermore, introduction of Ru atom induced the formation of porous flower-like structures in nanosheets. Compared to CoNiFe - LDH (28.9 m2/g), Ru-doped CoNiFe - LDH performed larger specific surface area of 53.1 m2/g, resulting in more electrochemically active sites. In these case, Ru-doped CoNiFe - LDH demonstrated better energy storage performance of 176.0 mAh/g at 1 A/g compared to original CoNiFe - LDH (78.9 mAh/g at 1 A/g). Besides, the assembled Ru-doped CoNiFe - LDH//activated carbon (AC) device delivered a maximum energy density of 36.4 W h kg-1 at the power density of 740.3 W kg-1 and an outstanding cycle life (78.7 % after 10,000 cycles). Meanwhile, Ru-doped CoNiFe - LDH exhibited lower overpotential (339 mV at 50 mA cm-2) and Tafel slope (93.2 mV dec-1). Therefore, this work provided novel and valuable insights into the rational doping of Ru elements for the controlled synthesis of supercapacitor electrode materials and OER catalysts.

Constructing Hierarchical CoGa2O4-S@NiCo-LDH Core-Shell Heterostructures with Crystalline/Amorphous/Crystalline Heterointerfaces for Flexible Asymmetric Supercapacitors

The rational design and construction of composite electrodes are crucial for overcoming the issues of poor working stability and slow ionic electron mobility of a single component. Nevertheless, it is a big challenge to construct core-shell heterostructures with crystalline/amorphous/crystalline heterointerfaces in straightforward and efficient methods. Here, we have successfully converted a portion of crystalline CoGa2O4 into the amorphous phase by employing a facile sulfidation process (denoted as CoGa2O4-S), followed by anchoring crystalline NiCo-layered double hydroxide (denoted as NiCo-LDH) nanoarrays onto hexagonal plates and nucleation points of CoGa2O4-S, synthesizing dual-type hexagonal and flower-like 3D CoGa2O4-S@NiCo-LDH core-shell heterostructures with crystalline/amorphous/crystalline heterointerfaces on carbon cloth. Furthermore, we further adjust the Ni/Co ratio in LDH, achieving precise and controllable core-shell heterostructures. Benefiting from the abundant crystalline/amorphous/crystalline heterointerfaces and synergistic effect among various components, the CoGa2O4-S@Ni2Co1-LDH electrode exhibits a specific capacity of 247.8 mAh·g-1 at 1 A·g-1 and good rate performance. A CoGa2O4-S@Ni2Co1-LDH//AC flexible asymmetric supercapacitor provides an energy density of 58.2 Wh·kg-1 at a power density of 850 W·kg-1 and exhibits an impressive capacitance retention of 105.7% after 10,000 cycles at 10 A·g-1. Our research provides profound insights into the design of other similar core-shell heterostructures.

A novel 3-dimensional graphene-based cobalt-manganese bimetallic layered double hydroxide:Formation mechanism and performance in photo-assisted permonosulfate-activated degradation of sulfamethoxazole in aqueous solution

Sulfamethoxazole (SMX) is a common antibiotic used mainly for bacterial treatment. In this study, a novel three-dimensional cobalt-manganese bimetallic layered double hydroxide graphene hydrogel (CoMn-LDHs/rGO) has been prepared for photo-assisted permonosulfate (PMS)-activated degradation of SMX in water. Compared with the CoMn-LDHs/rGO + PMS and CoMn-LDHs/rGO + Vis systems, the degradation effect of CoMn-LDHs/rGO + PMS + Vis system is the best, and the degradation effect of CoMn-LDHs/rGO system could reach more than 98% under the optimal conditions. After 10 cycles, the catalytic degradation performance of CoMn-LDHs/rGO system remained good, while effectively preventing the leaching of metal ions. Based on the synergistic effect of photocatalysis and PMS oxidation, electron spin resonance spectroscopy and quenching experiments showed that three active substances (•OH, •SO4- and O2•-) were involved in the degradation of SMX. Density functional theory and liquid chromatography-mass spectrometry (LC-MS) results further proposed the SMX degradation transformation calculation. As expected, the study of the reaction mechanism of 3D CoMn-LDHs/rGO assisted PMS activation under visible light provides an efficient and rapid method for the sustainable degradation of pollutants in water system.

Fabrication of Hierarchical MOF-Derived NiCo2S4@Mo-Doped Co-LDH Arrays for High-Energy-Density Asymmetric Supercapacitors

The rational fabrication of composite structures made of mixed components has shown great potential for boosting the energy density of supercapacitors. Herein, an elaborate hierarchical MOF-derived NiCo2S4@Mo-doped Co-LDH arrays hybrid electrode was fabricated through a step-wise method. By leveraging the synergistic effects of a uniform array of NiCo2S4 nanowires as the core and an MOF-derived porous shell, the NiCo2S4@Mo-doped Co-LDH hybrid electrode demonstrates an exceptional specific capacitance of 3049.3 F g-1 at 1 A g-1. Even at a higher current density of 20 A g-1, the capacitance remains high at 2458.8 F g-1. Moreover, the electrode exhibits remarkable cycling stability, with 91% of the initial capacitance maintained after 10,000 cycles at 10 A g-1. Additionally, the as-fabricated asymmetric supercapacitor (ASC) based on the NiCo2S4@Mo-doped Co-LDH electrode achieves an impressive energy density of 97.5 Wh kg-1 at a power density of 835.6 W kg-1. These findings provide a promising approach for the development of hybrid-structured electrodes, enabling the realization of high-energy-density asymmetric supercapacitors.

CoMoO4-CoP/NC heterostructure anchored on hollow polyhedral N-doped carbon skeleton for efficient water splitting

We report the synthesis and electrocatalytic properties of a CoMoO₄-CoP heterostructure anchored on a hollow polyhedral N-doped carbon skeleton (CoMoO₄-CoP/NC) for water-splitting applications. The preparation involved the anion exchange of MoO₄^2- to the organic ligand of ZIF-67, the self-hydrolysis of MoO₄^2-, and NaH₂PO₂ phosphating annealing. CoMoO₄ was found to enhance thermal stability and prevent active site agglomeration during annealing, while the hollow structure of CoMoO₄-CoP/NC provided a large specific surface area and high porosity that facilitated mass transport and charge transfer. The interfacial electron transfer from Co to Mo and P sites promoted the generation of electron-deficient Co sites and electron-enriched P sites, which accelerated water dissociation. CoMoO₄-CoP/NC exhibited excellent electrocatalytic activity for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1.0 M KOH solution, with overpotentials of 122 mV and 280 mV at 10 mA cm^-2, respectively. The CoMoO₄-CoP/NC‖CoMoO₄-CoP/NC two-electrode system only required an overall water splitting (OWS) cell voltage of 1.62 V to achieve 10 mA cm^-2 in an alkaline electrolytic cell. In addition, the material showed comparable activity to 20% Pt/C‖RuO₂ in a pure water home-made membrane electrode device, demonstrating potential for practical applications in proton exchange membrane (PEM) electrolyzers. Our results suggest that CoMoO₄-CoP/NC is a promising electrocatalyst for efficient and cost-effective water splitting.