Hydrothermal Synthesis of a rGO Nanosheet Enwrapped NiFe Nanoalloy for Superior Electrocatalytic Oxygen Evolution Reactions

Multifunctionality exploration of NiCo2O4-rGO nanocomposites: photochemical water oxidation, methanol electro-oxidation and asymmetric supercapacitor applications

Different weight percentages of NiCo₂O₄-rGO nanocomposites were prepared via a facile hydrothermal method. The prepared nanocomposites were structurally and morphologically characterized by X-ray diffraction, Raman spectroscopy, and electron microscopy. The structural studies show the formation of rGO-NiCo₂O₄ nanocomposites by embedment of porous NiCo₂O₄ rods on rGO sheets. The effect of the NiCo₂O₄ content on photochemical water oxidation was investigated. It revealed that the catalysts NiCo₂O₄-rGO with 1 : 26 ratio (NCO26) and 1 : 13 ratio (NCO13) are efficient in generating oxygen under light illumination. It proves that NCO26 works far more effectively as a photocatalyst compared to NCO13. Methanol electro-oxidation of the NCO26 nanocomposite shows a current density of 24 mA cm^-2 at a potential of 0.45 V in cyclic voltammetry and maintains the current for 3600 s at 0.45 V in chronoamperometry. An onset potential of 0.344 V was observed for 0.5 M methanol oxidation. The specific capacitance values were found to be 354.75 F g^-1 and 375.32 F g^-1 at 1 mV s^-1 and 1 A g^-1, respectively, for NCO26 in supercapacitor studies. The charge stored via capacitive and diffusion-controlled processes was determined using Power's law and Trasatti plot. An asymmetric supercapacitor device shows a specific capacitance of 122.2 F g^-1 at a current density of 1 A g^-1 and exhibits a retention of 74.3% after 5000 cycles. An energy density of 67.89 W h kg^-1 and a power density of 1 kW kg^-1 at a current density of 1 A g^-1 are observed.

Defect-Driven Enhancement of Electrochemical Oxygen Evolution on Fe-Co-Al Ternary Hydroxides

Efficient, abundant and low-cost catalysts for the oxygen evolution reaction (OER) are required for energy conversion and storage. In this study, a doping-etching route has been developed to access defect rich Fe-Co-Al (Fe-Co-Al-AE) ternary hydroxide nanosheets for superior electrochemical oxygen evolution. After partial etching of Al, ultrathin Fe₃ Co₂ Al₂ -AE electrocatalysts with a rich pore structure are obtained with a shift of the cobalt valence state towards higher valence (Co²⁺ →Co³⁺ ), along with a substantial improvement in the catalytic performance. Fe₃ Co₂ Al₂ -AE shows a notably lower overpotential of only 284 mV at a current density of 10 mA cm^-2 and double the OER mass activity of the etching-free Fe₃ Co₂ Al₂ with an overpotential of 350 mV. Density functional theory shows the leaching of Al changes the rate-determining step of the OER from conversion of *OOH into O₂ on Fe₃ Co₂ Al₂ to formation of OOH from *O on the Al-defective catalysts. This work demonstrates an effective route to design and synthesize transition metal electrocatalysts and provides a promising alternative for the further development of oxygen evolution catalysts.

Three-Dimensional Graphene-Supported Ni3Fe/Co9S8 Composites: Rational Design and Active for Oxygen Reversible Electrocatalysis

The development of low-cost and efficient electrocatalysts with a bicomponent active surface for reversible oxygen electrode reactions is highly desirable and challenging. Herein, we develop an effective calcination-hydrothermal approach to fabricate graphene aerogel-anchored Ni₃Fe-Co₉S₈ bifunctional electrocatalyst (Ni₃Fe-Co₉S₈/rGO). The mutually beneficial Ni₃Fe-Co₉S₈ bifunctional active components efficiently balance the performance of oxygen reduction and oxygen evolution reactions (ORR/OER), in which Co₉S₈ promotes the ORR and Ni₃Fe facilitates the OER. This balance behavior has an obvious advantage over that of monocomponent Ni₃Fe/rGO and Co₉S₈/rGO catalysts. Meanwhile, the additional synergy between porous rGO aerogels and Ni₃Fe-Co₉S₈ endows the composite with more exposed active sites, faster electrons/ions transport rate, and better structural stability. Benefiting from the reasonable material selection and structural design, the Ni₃Fe-Co₉S₈/rGO exhibits not only outstanding ORR activity with the high onset- and half-wave potentials ( E_onset = 0.91 V and E_1/2 = 0.80 V) but also satisfactory OER activity with a low overpotential at 10 mA cm^-2 (0.39 V). Moreover, rechargeable Zn-air cells equipped with Ni₃Fe-Co₉S₈/rGO exhibit excellent rechargeability and a fast dynamic response.

Controllable electrodeposition of binary metal films from deep eutectic solvent as an efficient and durable catalyst for the oxygen evolution reaction

In this work, we present an easy and scalable electrodeposition protocol that operates in a deep eutectic solvent, used to prepare self-supported Ni–Fe alloy films directly grown on copper foils.

Effect of Intrinsic Properties of Anions on the Electrocatalytic Activity of NiCo2O4 and NiCo2O x S4-x Grown by Chemical Bath Deposition

Electrochemical water (H₂O) splitting is one of the most promising technologies for energy storage by hydrogen (H₂) generation but suffers from the requirement of high overpotential in the anodic half-reaction (oxygen evolution), which is a four-electron process. Though transition-metal oxides and oxysulfides are increasingly researched and used as oxygen evolution electrocatalysts, the bases of their differential activities are not properly understood. In this article, we have synthesized NiCo₂O₄ and NiCo₂O _x S_4-x by a chemical bath deposition technique, and the latter has shown better oxygen evolution performance, both in terms of stability and activity, under alkaline conditions. Comprehensive analysis through time-dependent cyclic voltammetry, microscopy, and elemental analysis reveal that the higher activity of NiCo₂O _x S_4-x may be attributed to the lower metal-sulfur bond energy that facilitates the activation process to form the active metal hydroxide/oxyhydroxide species, higher electrochemically active surface area, higher pore diameter and rugged morphology that prevents corrosion. This work provides significant insights on the advantages of sulfur-containing materials as electrochemical precatalysts over their oxide counterparts for oxygen evolution reaction.

Mixed NiO/NiCo2O4 Nanocrystals Grown from the Skeleton of a 3D Porous Nickel Network as Efficient Electrocatalysts for Oxygen Evolution Reactions

Mixed NiO/NiCo₂O₄ nanocrystals grown in situ from the skeleton of a 3D porous nickel network (3DPNN) were prepared with a simple hydrothermal method followed by a low temperature calcination, exhibiting outstanding electrocatalytic efficiencies toward oxygen evolution reactions (OER). The 3DPNN was prepared with a novel leaven dough method and served as both the nickel source for growth of the mixed NiO/NiCo₂O₄ nanocrystals and the charge transport highway to accelerate the sluggish kinetics of the OER. The mixed NiO/NiCo₂O₄ nanocrystals exhibited pronounced synergistic effects to achieve a high mass activity of 200 A g^-1 at the catalyst mass loading of 0.5 mg cm^-2, largely outperforming the corresponding single component nanocrystal systems, NiO (5.87) and NiCo₂O₄ (9.35). The NiO/NiCo₂O₄@3DPNN composite electrocatalyst achieved a low overpotential of 264 mV at the current density of 10 mA cm^-2 and 389 mV at the practically high current density of 250 mA cm^-2, which compete favorably among the top tier of previously reported OER electrocatalysts. Moreover, it exhibited good stability even at the high current density of 250 mA cm^-2, showing only 9.40% increase in working applied potential after a continuous 12 h operation. The present work demonstrates a new design for highly efficient OER catalysts with in situ growth of mixed oxide nanocrystals of pronounced synergistic effects.

A freestanding SiO2 ultrathin membrane with NiCu nanoparticles embedded on its double surfaces for catalyzing nitro-amination

A 2D ultrathin-structured NiCu–SiO₂ nanocomposite formed by assembling NiCu nanoparticles on the double surfaces of a freestanding SiO₂ membrane achieves excellent catalytic performance for nitro-amination reactions.

Architecting a Mesoporous N-Doped Graphitic Carbon Framework Encapsulating CoTe2 as an Efficient Oxygen Evolution Electrocatalyst

To improve the efficiency of cobalt-based catalysts for water electrolysis, tremendous efforts have been dedicated to tuning the composition, morphology, size, and structure of the materials. We report here a facile preparation of orthorhombic CoTe₂ nanocrystals embedded in an N-doped graphitic carbon matrix to form a 3D architecture with a size of ∼500 nm and abundant mesopores of ∼4 nm for the oxygen evolution reaction (OER). The hybrid electrocatalyst delivers a small overpotential of 300 mV at 10 mA cm^-2, which is much lower than that for pristine CoTe₂ powder. After cycling for 2000 cycles or driving continual OER for 20 h, only a slight loss is observed. The mesoporous 3D architecture and the strong interaction between N-doped graphitic carbon and CoTe₂ are responsible for the enhancement of the electrocatalytic performance.