γ-NiOOH electrocatalyst derived from a nickel dithiooxamide chelate polymer for oxygen evolution reaction in alkaline solutions

Molecular NiII Complexes as Bifunctional Electrocatalysts for O2 Evolution and Urea Electro-Oxidation Reaction

Developing resilient and robust electrocatalysts devoid of noble metals is vital for facilitating the generation of O2/H2 from water electrolysis, particularly in catalyzing oxygen evolution reaction (OER) and urea oxidation reaction (UOR), respectively. Nickel-based catalysts have attracted as part of global efforts to produce hydrogen from urea-rich wastewater due to their high reaction rates and favorable long-term stability. Two new mononuclear NiII-embedded complexes, namely, [C26H38NiN4O4] (Complex 1) and [C24H32NiF2N4O2] (Complex 2), are explored as a bifunctional catalyst for the OER and UOR herein. Complexes 1 and 2 crystallize in triclinic and monoclinic with space groupP ¯ $p\overbar$ 1 (2) & P 21/c (14), respectively. The OER outcomes of Complex 1 exhibit excellent performance, featuring a lower overpotential of 220 mV and reduced Tafel slope value of 82 mV dec-1 at a benchmarking current density of 10 mA cm-2 in 1 M KOH compared to Complex 2. Additionally, the results of UOR indicate that Complex 1 only requires 1.30 V potential to achieve the same current density which is significantly lower than other counterparts and most reported materials. Chronopotentiometry analysis reveals that Complex 1 is stable up to a longer period of 100 and 20 h in 1 M KOH and alkaline urea electrolyte, respectively.

Lattice Distortions Promoting the In-Depth Reconstruction of Ni-Based Electrocatalysts with Enriched Oxygen Vacancies for the Electrochemical Oxidation of 5-Hydroxymethylfurfural toward 2,5-Furandicarboxylic Acid

The electrocatalytic 5-hydroxymethylfurfural (HMF) oxidation reaction (HMFOR) toward 2,5-furandicarboxylic acid (FDCA) has been considered a promising approach for the substitution of the energy-consuming and hazardous oxygen evolution reaction and for the valorization of renewable biomass. However, it is limited by the susceptibility of HMF to the oxidative environment and requires efficient electrocatalysts. Herein, a NiMo complex (NiMo-N) is provided as the precatalyst for the HMFOR, exhibiting favorable performances with a current density of 450 mA·cm-2 achieved at an anodic potential of 1.4 V vs RHE (similarly hereinafter) with 50 mmol/L (mM) HMF and over 95% HMF conversion and FDCA FE for at least five cycles. Combined with quasi situ and in situ analysis, it is confirmed that the extensive lattice distortions in the precatalyst facilitate the in-depth reconstruction, increasing the accessible Ni sites and defective oxygen vacancies (Ov), which would promptly convert to high-valence Ni and active O species during the reaction. The improved performance is then attributed to the incorporation of the improved chemisorption and dehydrogenation ability of HMF by the as-evolved active sites.

Dual-site OER mechanism exploration through regulating asymmetric multi-site NiOOH

Asymmetric nickel oxyhydroxide (NiOOH) possesses multi-OH and O active sites on different surfaces, (001) and (001̄), which possibly causes a complicated catalytic process. Density functional theory (DFT) calculations reveal that the unconventional dual-site mechanism (UDSM) of the oxygen evolution reaction (OER) on NiOOH (001) and (001̄) exhibits significantly lower overpotentials of 0.80 and 0.77 V, compared to 1.24 and 1.62 V for the single-site mechanism (SSM), respectively. Through chemical doping or heterojunction modifications, the constructed NiOOH@FeOOH (001̄) heterojunction reduces the thermodynamic overpotential to 0.49 V from original 0.77 V undergoing the UDSM. Although Fe/Co-doping or physical compression yield similar or slightly higher overpotentials and are not conductive to facilitating the OER process by the UDSM, all dual-site paths exhibit obviously lower overpotentials than the SSM for pristine and regulated NiOOH (001) and (001̄) from the whole viewpoint. This work identifies a more reasonable and efficient dual-site OER mechanism, which is expected to help the rational design of highly-efficient electrocatalysts.

High electrochemical performance of nickel cobaltite@biomass carbon composite (NiCoO@BC) derived from the bark of Anacardium occidentale for supercapacitor application

Biomass carbon-based materials are highly promising for supercapacitor (SC) electrodes due to their availability, environment-friendliness, and low cost. Herein, an easy energy-saving hydrothermal process was used to produce NiCo2O4/NiOOH (NiCoO) composites with biomass carbon (BC) derived from the bark of Anacardium occidentale (AO) at different synthesis time durations (2 h, 4 h, 8 h, 16 h). The structural and morphological properties of the samples were analysed using XRD, Raman spectroscopy, XPS, SEM, TEM and BET, and the results exhibit the presence of carbon inserted into the nickel-cobalt hydroxide matrix. The NiCoO@BC composite synthesized in 4 h (NiCoO@BC(4 h)) displays a good specific capacitance of 475 F g-1 at 0.5 A g-1 and a low equivalent series resistance (ESR) value of 0.36 Ω. It shows a good coulombic efficiency of 98% and retains 86% of the capacitance after 4000 cycles. The asymmetric supercapacitor (ASC) device (NiCoO@BC(4 h)//AC) assembled using activated carbon (AC) as a negative electrode displays 20 W h kg-1 energy density and 900 W kg-1 power density at 1 A g-1. The stability test shows a good coulombic efficiency of 99% and 78% capacitance retention after 15 000 cycles. These findings imply that NiCoO@BC composites have outstanding electrochemical properties, making them suitable as SC electrode materials.

Improvement of photocatalytic degradation of methyl orange by impregnation of natural clay with nickel: optimization using the Box-Behnken design (BBD)

In this research work, the photocatalytic degradation of methyl orange dye was studied on nickel oxide supported on a natural Moroccan clay (Ni/NC). These catalysts have been prepared by dry impregnation of a nickel nitrate solution with different weight percentages (5, 10, 20% NiO). Experimental responses were obtained by a Box-Behnken (BBD) experimental design by varying the catalyst mass, solution pH, and initial dye concentration at three levels (low, medium, and high). The prepared catalysts were characterized using powder X-ray diffraction (XRD) to assess crystallinity and structure, Fourier transform infrared spectroscopy (FTIR) to detect different functional groups, scanning electron microscopy (SEM) combined with energy dispersive X-ray (EDX) analysis to study the surface morphology, and the optical characteristics of the catalysts were studied using absorption and diffuse reflectance measurements in the UV-visible range. The photocatalytic activity of the catalysts was evaluated in aqueous solutions under UV irradiation. ANOVA (analysis of variance) test is employed to recognize the significant factors and their interactions and then give the model equation for the percent dye degradation. The optimal values of the studied factors were determined by numerical optimization, and the results showed that about 100% degradation of the methyl orange dye could be achieved under the following optimal conditions, which are pH = 4.38, catalyst concentration of 0.99 g/L, and initial dye concentration of 30.42 mg/L.