Nanostructured cobalt/copper catalysts for efficient electrochemical carbon dioxide reduction

The search for an efficient and stable catalyst for the electrochemical reduction of CO2 to value-added chemicals is especially critical for lowering the atmospheric CO2 concentration. In this study, self-supported cobalt/copper nanostructured catalysts were designed, where the influences of the elemental composition and acid-etching on their efficiency towards the CO2 reduction reaction were studied. The developed Co/Cu catalysts showed superb catalytic activity with a low onset potential at -0.2 V vs. RHE. Gas and liquid product analysis revealed that formate and CO were the main products. It was observed that lower reductive potentials were favourable for formate production, while higher reductive potentials were more favourable for CO formation. In situ electrochemical FTIR studies were further conducted to gain insight into the CO2 reduction mechanism. The novel synthetic procedure reported in this study leads to promising electrocatalysts with high efficiencies for the conversion of CO2 into valuable products.

Highly Oxidized Oxide Surface toward Optimum Oxygen Evolution Reaction by Termination Engineering

The oxygen evolution reaction (OER) is a critical step for sustainable fuel production through electrochemistry process. Maximizing active sites of nanocatalyst with enhanced intrinsic activity, especially the activation of lattice oxygen, is gradually recognized as the primary incentive. Since the surface reconfiguration to oxyhydroxide is unavoidable for oxygen-activated transition metal oxides, developing a surface termination like oxyhydroxide in oxides is highly desirable. In this work, we demonstrate an unusual surface termination of (111)-facet Co₃O₄ nanosheet that is exclusively containing edge-sharing octahedral Co³⁺ similar to CoOOH that can perform at approximately 40 times higher current density at 1.63 V (vs RHE) than commercial RuO₂. It is found that this surface termination has an oxidized oxygen state in contrast to standard Co-O systems, which can serve as active site independently, breaking the scaling relationship limit. This work forwards the applications of oxide electrocatalysts in the energy conversion field by surface termination engineering.

Enhanced electrocatalytic performance for oxygen evolution reaction via active interfaces of Co3O4arrays@FeOx/Carbon cloth heterostructure by plasma-enhanced atomic layer deposition

Oxygen evolution reaction (OER) is a necessary procedure in various devices including water splitting and rechargeable metal-air batteries but required a higher potential to improve oxygen evolution efficiency due to its slow reaction kinetics. In order to solve this problem, a heterostructured electrocatalyst (Co₃O₄@FeO_x/CC) is synthesized by deposition of iron oxides (FeO_x) on carbon cloth (CC) via plasma-enhanced atomic layer deposition, then growth of the cobalt oxide (Co₃O₄) nanosheet arrays. The deposition cycle of FeO_xon the CC strongly influences thein situgrowth and distribution of Co₃O₄nanosheets and electronic conductivity of the electrocatalyst. Owing to the high accessible and electroactive areas and improved electrical conductivity, the free-standing electrode of Co₃O₄@FeO_x/CC with 100 deposition cycles of FeO_xexhibits excellent electrocatalytic performance for OER with a low overpotential of 314.0 mV at 10 mA cm^-2and a small Tafel slope of 29.2 mV dec^-1in alkaline solution, which is much better than that of Co₃O₄/CC (448 mV), and even commercial RuO₂(380 mV). This design and optimization strategy shows a promising way to synthesize ideally designed catalytic architectures for application in energy storage and conversion.

Unravelling faradaic electrochemical efficiencies over Fe/Co spinel metal oxides using surface spectroscopy and microscopy techniques

Cobalt and iron metal-based oxide catalysts play a significant role in energy devices. To unravel some interesting parameters, we have synthesized metal oxides of cobalt and iron (i.e. Fe₂O₃, Co₃O₄, Co₂FeO₄ and CoFe₂O₄), and measured the effect of the valence band structure, morphology, size and defects in the nanoparticles towards the electrocatalytic hydrogen evolution reaction (HER), the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). The compositional variations in the cobalt and iron precursors significantly alter the particle size from 60 to <10 nm and simultaneously the shape of the particles (cubic and spherical). The Tauc plot obtained from the solution phase ultraviolet (UV) spectra of the nanoparticles showed band gaps of 2.2, 2.3, 2.5 and 2.8 eV for Fe₂O₃, Co₃O₄, Co₂FeO₄ and CoFe₂O₄, respectively. Further, the valence band structure and work function analysis using ultraviolet photoelectron spectroscopy (UPS) and core level X-ray photoelectron spectroscopy (XPS) analyses provided better structural insight into metal oxide catalysts. In the Co₃O₄ system, the valence band structure favors the HER and Fe₂O₃ favors the OER. The composites Co₂FeO₄ and CoFe₂O₄ show a significant change in their core level (O 1s, Co 2p and Fe 2p spectra) and valence band structure. Co₃O₄ shows an overpotential of 370 mV against 416 mV for Fe₂O₃ at a current density of 2 mA cm^-2 for the HER. Similarly, Fe₂O₃ shows an overpotential of 410 mV against the 435 mV for Co₃O₄ at a current density of 10 mA cm^-2 for the OER. However, for the ORR, Co₃O₄ shows 70 mV improvement in the half-wave potential against Fe₂O₃. The composites (Co₂FeO₄ and CoFe₂O₄) display better performance compared to their respective parent oxide systems (i.e., Co₃O₄ and Fe₂O₃, respectively) in terms of the ORR half-wave potential, which can be attributed to the presence of the oxygen vacancies over the surface in these systems. This was further corroborated in density functional theory (DFT) simulations, wherein the oxygen vacancy formation on the surface of CoFe₂O₄(001) was calculated to be significantly lower (∼50 kJ mol^-1) compared to Co₃O₄ (001). The band diagram of the nanoparticles constructed from the various spectroscopic measurements with work function and band gap provides in-depth understanding of the electrocatalytic process.

Direct growth of three-dimensional nanoflower-like structures from flat metal surfaces

Here we report on a facile top-down approach for the direct growth of Co₃O₄ hierarchical nanoflowers from a bulk Co surface via chemical etching and thermal annealing. The effect of the annealing temperature was investigated, showing that amorphous Co₃O₄ was formed at 250 °C, while crystalline Co₃O₄ with notable oxygen vacancies was created at 550 °C. The formed 3D nanostructures exhibited excellent oxygen evolution reaction (OER) activities with a low overpotential of 0.34 V at 10 mA cm^-2 and high durability. The proposed novel approach was further demonstrated by the direct growth of 3D NiO and CuO nanostructures on Ni and Cu substrates.

Morphology-oxygen evolution activity relationship of iridium(iv) oxide nanomaterials

This work demonstrated shape tuning of IrO2 nanoparticles to nanocube and nanorods in molten salt and demonstrated the exemplary performance of IrO2 nanorods as an electrocatalyst for oxygen evolution reaction even surpassing commercial IrO2.

Regulating oxygen vacancies in Co3O4by combining solution reduction and Ni2+ impregnation for oxygen evolution reaction

Oxygen vacancies are considered to be an important factor to influence the electronic structure and charge transport of electrocatalysts in the field of energy chemistry. Various strategies focused on oxygen vacancy engineering are proved to be efficient for further improving the electrocatalytic performance of Co₃O₄. Herein, an optimal Co₃O₄with rich oxygen vacancies have been synthesized via a two-step process combining solution reduction and Ni²⁺impregnation. The as-prepared electrocatalyst exhibits an enhanced oxygen evolution performance with the overpotential of 330 mV at the current density of 10 mA cm^-2in alkaline condition, which is 84 mV lower than that of pristine one. With the increasing of oxygen vacancies, the charge transfer efficiency and surface active area are relatively enhanced reflected by the Tafel slope and double-layer capacitance measurement. These results indicate that combination of solution reduction and heteroatom doping can be a valid way for efficient metal oxides-based electrocatalyst development by constructing higher concentration of oxygen vacancy.

Development of Three-Dimensional Nickel-Cobalt Oxide Nanoflowers for Superior Photocatalytic Degradation of Food Colorant Dyes: Catalyst Properties and Reaction Kinetic Study

In this work, we present three-dimensional flower-like nickel-cobalt oxide (F-NCO) nanosheets developed in a facile, eco-friendly hydrothermal route to apply as photocatalysts for food colorant Allura Red AC dye removal under light illumination. Using Brunauer-Emmett-Teller analysis, it was found that the F-NCO nanosheets displayed a surface area of ∼53.65 m²/g and a Barrett-Joyner-Halenda pore size of ∼14 nm, which was also confirmed by the calculated crystallite size of ∼15 nm using powder X-ray diffraction (XRD) analysis. From Williamson-Hall analysis of XRD spectra, F-NCO nanosheets revealed a crystal-lattice strain of ∼3.42 × 10^-3 and a dislocation density of ∼4.397 × 10¹⁵ lines/m² in the crystal structure. Transmission electron microscopy analysis revealed that F-NCO nanosheets accumulated to form flower-like nanostructures of <100 nm length with a d-spacing of ∼2.6 Å, which is attributed to the (311) crystallographic plane (α = γ = β = 90°, a = b = c = 8.110 Å, JCPDS No. 00-020-0781) of the cubic phase. The F-NCO nanosheets exhibited an excellent photocatalytic efficiency of ∼94.75% in ∼10 min with sodium borohydride under UV light. The Langmuir-Hinshelwood model determined pseudo-first-order reaction kinetics of dye degradation using the ln[AtA0]versus time plot. The kinetic study produced a first-order rate constant (k) of ∼0.219 min^-1, resulting in ∼3.16 min half-life (t_1/2) for the F-NCO-catalyzed degradation reaction. Thus outstanding photocatalytic performance of F-NCO nanosheets would display their huge potential for organic-pollutant removal from water with exceptional recyclability for wide research applications in the future.

Highly porous Co-doped NiO nanorods: facile hydrothermal synthesis and electrocatalytic oxygen evolution properties

Highly porous 3d transition metal oxide nanostructures are opening up the exciting area of oxygen evolution reaction (OER) catalysts in alkaline medium thanks to their good thermal and chemical stability, excellent physiochemical properties, high specific surface area and abundant nanopores. In this paper, highly porous Co-doped NiO nanorods were successfully synthesized by a simple hydrothermal method. The porous rod-like nanostructures were preserved with the added cobalt dopant ranging from 1 to 5 at% but were broken into aggregated nanoparticles at higher concentrations of additional cobalt. The catalytic activity of Co-doped NiO nanostructures for OER in an alkaline medium was assayed. The 5%Co-NiO sample showed a drastically enhanced activity. This result could originate from the combination of advantageous characteristics of highly porous NiO nanorods such as large surface area and high porosity as well as the important role of Co dopant that could provide more catalytic active sites, leading to an enhanced catalytic activity of the nanocatalyst.

Cobalt coordination controlled carbon nanospheres formation and inclusion of amorphous Co3O4 and AuNPs: strongly enhanced oxygen evolution reaction with excellent mass activity

Carbon nanospheres integrated with AuNPs and amorphous Co3O4 were fabricated by making use of cobalt coordination with AuNP surface ligands, which exhibited an enhanced oxygen evolution reaction (OER) with excellent mass activity. Co2+ coordination with AuNP surface functional molecules significantly influenced the nanostructure formation and OER activity. Nanospheres of carbon with an optimum concentration of AuNPs and Co3O4 (2) showed strong OER activity. 2 exhibited a high current density (358 mA cm-2 at an applied potential of 1.59 V) and required a low overpotential (256 mV) to generate a geometric current density (10 mA cm-2) compared to commercial RuO2 (363 mV). Importantly, 2 showed high mass activity (1352.5 mA mg-1), 14 times higher than RuO2 (93.87 mA mg-1). The low Tafel slope (52.4 mV dec-1) and charge transfer resistance along with large double layer capacitance (Cdl = 20.1) of 2 suggest strong electronic communication between the catalyst and the electrode surface and facilitated fast charge transport. Chronoamperometric studies confirmed the excellent stability of the catalyst. The present work demonstrates that the electrocatalytic activity of earth-abundant amorphous metal oxides can be strongly enhanced by integrating metallic nanoparticles (NPs) and optimizing nanostructures.

Sacrificial ZnO nanorods drive N and O dual-doped carbon towards trifunctional electrocatalysts for Zn–air batteries and self-powered water splitting devices

Integrated energy systems (IES) have attracted increasing attention in recent years.

Direct Oxidative Azo Coupling of Anilines Using a Self-Assembled Flower-like CuCo2O4 Material as a Catalyst under Aerobic Conditions

Herein, we report the synthesis of a self-assembled flower-like CuCo₂O₄ material by the oxalate decomposition method. The crystalline structure and morphology of the material have been analyzed by powder X-ray diffraction, Raman spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray measurement techniques. The self-assembled flower-like CuCo₂O₄ material showed remarkable catalytic activity in the direct aerobic oxidative azo coupling of anilines under oxidant and other additive-free reaction conditions. The mechanistic insight of CuCo₂O₄ in the oxidative azo coupling reaction has been established by density functional theory calculations, which disclosed that the absorption and dissociation of areal oxygen preferentially take place at the Cu site and dissociation of aniline takes place at the Co site. Thus, the Cu and Co sites of CuCo₂O₄ exert a cooperative effect on the direct oxidative azo coupling reactions through the selective activation of anilines and aerobic oxygen. The CuCo₂O₄ material was recovered from the reaction mixture and reused for at least eight runs without appreciable loss of catalytic activity.

Selective Synthesis of Monodisperse CoO Nanooctahedra as Catalysts for Electrochemical Water Oxidation

Thermal decomposition is a promising route for the synthesis of metal oxide nanoparticles because size and morphology can be tuned by minute control of the reaction variables. We synthesized CoO nanooctahedra with diameters of ∼48 nm and a narrow size distribution. Full control over nanoparticle size and morphology could be obtained by controlling the reaction time, surfactant ratio, and reactant concentrations. We show that the particle size does not increase monotonically with time or surfactant concentration but passes through minima or maxima. We unravel the critical role of the surfactants in nucleation and growth and rationalize the observed experimental trends in accordance with simulation experiments. The as-synthesized CoO nanooctahedra exhibit superior electrocatalytic activity with long-term stability during oxygen evolution. The morphology of the CoO particles controls the electrocatalytic reaction through the distinct surface sites involved in the oxygen evolution reaction.

Direct MOCVD Growth of Iron Oxide on Three-Dimensional Nickel Foam as Electrode for the Oxygen Evolution Reaction

Iron oxide thin films were grown directly on three-dimensional nickel foam via metalorganic chemical vapor deposition (MOCVD) in the temperature range of 250-450 °C using Fe(CO)₅ as precursor. Iron oxide (α-Fe₂ O₃ ) films were formed at low substrate temperatures (250-350 °C), whereas the additional growth of an underlying NiO film occurred at substrate temperatures above 350 °C. The electrochemical activities of the as-formed binder-free and noble metal-free electrodes were tested for the oxygen evolution reaction (OER) in alkaline media. An overpotential reduced by 250 mV at a current density of 50 mA cm^-2 and a lower Tafel slope of 55 mV dec^-1 compared to bare nickel foam were found for the best-performing electrocatalyst, while the long-term stability of the as-formed electrodes was proven by chronopotentiometry. The surface morphology of the iron oxide films was characterized by scanning electron microscopy, whereas the crystallographic phase as well as the elemental composition were determined by X-ray diffraction, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry in the pre- and the post-catalytic state.

Two-Dimensional Earth-Abundant Transition Metal Oxides Nanomaterials: Synthesis and Application in Electrochemical Oxygen Evolution Reaction

Development of a universal synthetic strategy for two-dimensional (2D) Earth-abundant transition metal oxides nanomaterials is highly vital toward numerous electrochemical applications. Herein, a facile and general synthesis of highly ordered two-dimensional metal oxides nanomaterials includes Co₃O₄, NiO, CuO, and Fe₃O₄ nanosheets as an electrocatalyst for oxygen evolution reaction (OER) is demonstrated. Among the synthesized 2D transition metal oxides, the Co₃O₄ nanosheet exhibits smallest overpotential (η) of ∼384.0 mV at a current density of 10.0 mA cm^-2 and Tafel slope of ∼52.0 mV dec^-1, highest mass activity of ∼112.3 A g^-1 at the overpotential of ∼384.0 mV, and high turn over frequency (TOF) of 0.099 s^-1, which is relatively favorable with state-of-the-art RuO₂ catalyst. The present synthetic approach may unlock a brand new pathway to prepare shape-controlled Earth-abundant transition metal oxides nanomaterials for electrocatalytic OER.

Electrocatalytic water oxidation reaction promoted by cobalt-Prussian blue and its thermal decomposition product under mild conditions

Water oxidation studies with Co-Prussian blue and Co₃O₄. Figure adapted from ‘Under the Wave off Kanagawa’ (Kanagawa oki nami ura), also known as ‘The Great Wave’, from the series ‘Thirty-six Views of Mount Fuji’ (‘Fugaku sanjūrokkei’) by K. Hokusai.

Composite NiCoO2/NiCo2O4 inverse opals for the oxygen evolution reaction in an alkaline electrolyte

The inverse opals exhibit a 3D ordered macroporous framework, which provides an excessive surface area and facile mass transport. A conformal NiCoO_x functional coating further renders these materials with increased reactivity in OER catalysis.

Electrodeposition of sulfur-engineered amorphous nickel hydroxides on MIL-53(Fe) nanosheets to accelerate the oxygen evolution reaction

Exploring Earth-abundant electrocatalysts that are highly efficient, low cost, and stable for the oxygen evolution reaction (OER) are critical to energy storage and water splitting. Metal-organic frameworks (MOFs) have been regarded as superior electrocatalysts due to their atomically dispersed metal ions. Currently, MOFs have been widely studied as templates to fabricate electrocatalysts through thermal annealing. Here, we report a novel synthetic approach to fabricate a Ni-S/MIL-53(Fe) electrode by electrodepositing sulfur-engineered amorphous nickel hydroxides on MIL-53(Fe) nanosheets. The obtained binder-free, self-supported Ni-S/MIL-53(Fe) shows high OER activity with overpotentials of 256 and 298 mV to achieve 10 and 100 mA cm-2, respectively. Moreover, it also exhibits excellent electrochemical stability with no obvious degradation at 100 mA cm-2 for at least 40 h. The new findings may pave a new avenue for designing and fabricating low-cost catalysts with high efficiency for electrochemical applications.

Two-Dimensional Zeolitic Imidazolate Framework-L-Derived Iron-Cobalt Oxide Nanoparticle-Composed Nanosheet Array for Water Oxidation

Rational design of various functional nanomaterials using MOFs as a template provides an effective strategy to synthesize electrocatalysts for water splitting. In this work, we reported that an iron-cobalt oxide with 2D well-aligned nanoflakes assembling on carbon cloth (Fe-Co₃O₄ NS/CC), fabricated by an anion-exchange reaction followed by an annealing process, could serve as a high-performance oxygen-evolving catalyst. Specifically, the zeolitic imidazolate framework-L-Co nanosheet array (ZIF-L-Co NS/CC) was synthesized through a facile ambient liquid-phase deposition reaction, and then reacted with [Fe(CN)₆]^3- ions as precursors during the anion-exchange reaction at room temperature. Finally, the Fe-Co₃O₄ NS/CC was obtained via annealing treatment. On account of the compositional and structural superiority, this 3D monolithic anode exhibited outstanding electrocatalytic performance with a low overpotential of 290 mV to obtain a geometrical current density of 10 mA cm^-2 and good durability for water oxidation in base.

Sulfur vacancy-rich N-doped MoS2 nanoflowers for highly boosting electrocatalytic N2 fixation to NH3 under ambient conditions

Sulfur vacancy-rich N-doped MoS₂ nanoflowers act as highly effective electrochemical catalysts for efficient nitrogen reduction.