A facile method for reduced CoFe2O4 nanosheets with rich oxygen vacancies for efficient oxygen evolution reaction

The efficiency of electrochemical water splitting is greatly hindered by the thermodynamic uphill reaction of oxygen evolution reaction (OER). Thus, it is important to synthesize an active OER electrocatalysts with abundant active sites, favorable conductivity and good durability. Herein, a facile reduction method using NaBH₄ as readily available reductant has been developed to fabricate the reduced CoFe₂O₄ nanosheets (NS). The obtained reduced CoFe₂O₄ NS are rich in oxygen deficient sites, leading to more active sites as well as the enhanced conductivity than the pristine CoFe₂O₄ hollow nanosphere, which reaches the current density of 10 mA cm^?2 at the overpotential of 320 mV in 1 M KOH. Meanwhile, CoFe₂O₄ samples with three different morphology nanostructures including hollow nanospheres, bulk and nanoparticles have been provided to study the effect of different morphology on NaBH₄ reduction efficiency. As expected, after NaBH₄ reduction, CoFe₂O₄ hollow nanosphere with relatively higher surface area exhibits most obvious improvement for OER activity and also its corresponding reduced CoFe₂O₄ NS showed best OER performance than the reduced CoFe₂O₄ bulk as well as the reduced CoFe₂O₄ nanoparticles, implying the hollow nanospheres feature more accessible surface area than bulk and nanoparticles samples, thus greatly facilitate efficiency of NaBH₄ reduction treatment.     

Double doping of V and F on Co3O4 nanoneedles as efficient electrocatalyst for oxygen evolution

Rationally designing high-activity catalyst for oxygen evolution reaction (OER) is of primary importance due to its sluggish kinetic process in water splitting. Herein, we report a metallic (V) and nonmetallic (F) double doping in Co₃O₄ with nanoneedles structure, which is synthesized through facile oil bath and annealing. Electrochemical measurements show that the Co₃O₄ dopped with fluorine and vanadium (F_0.2-V-Co₃O₄-350) only needs a low overpotential of 320 mV to afford a current density of 10 mA cm^?2, which is superior to commercial RuO₂. The excellent electrocatalytic performance can be attributed to double doping of vanadium and fluorine which have strong electron absorption effect to optimize the density of electrons in Co₃O₄. Besides, nanoneedles structure can enlarge exposure of active sites. And its great durability is evaluated through 2000 cycles CV test. Furthermore, the optimal ratio of fluorine to vanadium and different annealing temperatures of the target catalyst are explored reasonably.     

Large-area manganese oxide nanorod arrays as efficient electrocatalyst for oxygen evolution reaction

Large-area manganese oxide nanorod arrays (MnO₂ NRAs) have been directly grown vertically on Ti foil with a uniform length and diameter by a simple electrochemical method without any templates. The deposition temperature is one of the most important parameters for formation MnO₂ NRAs and at 25 °C no MnO₂ NRAs can be obtained. The results show that MnO₂ has high activity and good stability for oxygen evolution reaction (OER) and the structure of nanorod arrays pronounced enhances MnO₂ activity. The onset potential of MnO₂ NRAs is lower than that of Pt foil and lower 401 mV than that of MnO₂ film, indicating that the structure of MnO₂ NRAs shows an easy OER for water split. The MnO₂ NRAs may be of great potential in electrochemical water split.     

Coupling molybdenum carbide nanoparticles with N-doped carbon nanosheets as a high-efficiency electrocatalyst for hydrogen evolution reaction

Searching for earth-abundant and high-efficiency electrocatalysts for the hydrogen evolution reaction (HER) is of critical importance for future energy conversion devices. To facilitate the HER on a nonprecious metal-based catalyst, integration of catalytically active nanoparticles with highly conductive carbon supports represents a promising strategy since the formed nanohybrid can offer available active sites and improved electron transfer capability. Herein, we demonstrate a feasible and scalable approach to fabricate well-dispersed Mo₂C nanoparticles firmly anchored on 2D ultrathin N-doped carbon nanosheets (denoted as Mo₂C@NC nanosheets) using inexpensive NaCl as recyclable templates. The adoption of NaCl template provides a 2D space for the one-step concurrent growth of Mo₂C nanoparticles and N-doped carbon nanosheets. Benefiting from the synergy between fine Mo₂C nanoparticles with high dispersity and N-doped C nanosheets, the resultant Mo₂C@NC nanosheets exhibit an outstanding HER performance with a low overpotential, a small Tafel slope and excellent stability under acidic medium, making them a promising noble-metal-free HER catalyst.     

Manganese oxide with different morphology as efficient electrocatalyst for oxygen evolution reaction

Three-dimensional (3D) manganese oxides consisted of tetragonal phase Mn₃O₄ and α-MnO₂ with different morphology have been directly grown vertically on Ti foil by a simple electrochemical method without any template and used as the catalysts for oxygen evolution reaction (OER). The results show that manganese oxides with different morphology show high activity and good stability for OER and the manganese oxide (MnO_x) nanowire arrays obtained at 70 °C show higher activity and better stability than MnO_x with cotton wool structure and MnO_x nanosheet arrays.     

Integrating Co3O4 nanoparticles with MnO2 nanosheets as bifunctional electrocatalysts for water splitting

Developing high-efficiency and low-cost electrocatalyst is significant for the application of water splitting technology. Herein, Co₃O₄ nanoparticles and MnO₂ nanosheets are separately synthesized and subsequently assembled into a unique 0/2-dimensional heterostructure via van der Waals interactions. The consequent composites expose abundant accessible active sites and expedite the reaction kinetics, which can be testified by the superiorities in Tafel slope, exchange current density and double-layer capacitance, only requiring overpotentials of 355 and 129 mV for oxygen and hydrogen evolution reactions in 1.0 M KOH at 10 mA cm^?2, respectively. Moreover, a cell voltage of 1.660 V can drive the electrolyzer at 10 mA cm^?2. Benefitted from robust integration, the original aggregation and restacking of individual materials have been overcome, thereby leading to superior elelctrocatalysis durability. This facile and universal strategy may inspire the researchers on the design and construction of advanced functional composites.     

Electrosynthesis of Mn-Fe oxide nanopetals on carbon paper as bi-functional electrocatalyst for oxygen reduction and oxygen evolution reaction

A layered binary Mn-Fe oxide as bi-functional electro-catalyst with nanopetals morphology is grown on porous carbon paper for the first time via one-step electrodeposition process. The electrocatalyst is characterized by X-ray diffraction, scanning electron microscopy (SEM) and energy dispersive spectroscopy analysis. SEM analysis demonstrates notable morphology viz. nanopetals of the Mn-Fe oxide grown on carbon paper. The electrocatalytic activity is checked for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline medium. Rotating disk electrode (RDE) voltammetry is carried out to study the ORR kinetics, which proves that ORR process follows four-electron pathway in alkaline medium. Oxygen evolution reaction study reveals that it has higher activity for OER with a lower onset potential of 1.6 V vs RHE and higher current density of 11.5 mA/cm² at 2.0 V vs RHE reference electrode.     

Free-standing carbon nanotubes as non-metal electrocatalyst for oxygen evolution reaction in water splitting

Oxygen evolution reaction (OER) has significant impact on the overall electrochemical water splitting. We introduce, for the first time, a facile approach towards the fabrication of versatile electrode composed of free-standing multiwalled carbon nanotubes (MWCNTs) as electrocatalyst for the water splitting reaction. Directly extracted MWCNTs as sheets from vertically grown arrays transferred over the glass substrate, are used without any post treatment as a working electrode for OER. Onset potential of 1.60 V was achieved for MWCNTs which is significantly reduced as compared to platinum based metal electrode (1.72 V) with excellent current density. No surface modification, metal-free nature, flexibility and low cost with excellent catalytic activity proved this material as a promising candidate for the replacement of metal based electrodes in electrochemical water splitting.     

Nickel foam-supported NiFe layered double hydroxides nanoflakes array as a greatly enhanced electrocatalyst for oxygen evolution reaction

In this work, nickel-iron layered double hydroxides nanoflakes are grown on nickel foam by a facile in-situ complexation precipitation method. The fabricated nickel-iron layered double hydroxides/nickel foam with special 3D structure with large electrochemical activated surface area is proposed as a greatly enhance electrode material for oxygen evolution reaction. The electrochemical properties of the as-fabricated nickel-iron layered double hydroxides/nickel foam electrode are evaluated using 1 mol L^?1 KOH as electrolyte. The obtained electrochemical results show that the fabricated nickel-iron layered double hydroxides/nickel foam electrode exhibits a low overpotential of 245 mV at current density of 10 mA cm^?2 with small Tafel slope of 27 mV dec^?1. Also, it displays a much longer durability of 20 h with very small decay of 0.02% as compared with 3D nickel foam, IrO₂ and the related catalysts reported. Therefore, this study indicates that the nickel-iron layered double hydroxides/nickel foam is a promising electrode material for oxygen evolution reaction due to its facile preparation method, low cost and environmentally friendly nature.     

Trimetallic NiFeCo selenides nanoparticles supported on carbon fiber cloth as efficient electrocatalyst for oxygen evolution reaction

Trimetallic NiFeCo selenides (NiFeCoSe_x) anchored on carbon fiber cloth (CFC) as efficient electrocatalyst for oxygen evolution reaction (OER) in alkaline medium have been synthesized via a facile two-step method. Firstly, trimetallic NiFeCo (oxy) hydroxides have been electrodeposited on CFC support (NiFeCo/CFC). Secondly, a solvothermal selenization process has been used to convert NiFeCo/CFC into NiFeCoSe_x/CFC using N, N-dimethylformamide (DMF) as solvent. The composition and homogeneous distribution of NiFeCoSe_x/CFC nanoparticles are determined by XRD, XPS, SEM elemental mapping and EDX images. Furthermore, SEM images reveal that NiFeCoSe_x/CFC has volcano-shaped morphology with rough surface and homogenously distributed on the surface of CFC, which may provide more active sites for OER. The electrochemical measurements show that trimetallic NiFeCoSe_x/CFC possesses the better electrocatalytic activity with the lower overpotential (150 mV at 10 mA cm^?2), Tafel slope (85 mV dec^?1), larger double-layer capacitance (200 mF cm^?2) and long-term stability than unary or binary metal selenides. The enhanced activity of NiFeCoSe_x/CFC may be attributed to the trimetallic NiFeCo selenides and selenides-CFC synergistic interaction. It may offer a promising way to design transition multimetallic selenides supported on conductive support as electrocatalysts for OER.     

Oxygen vacancy and p-n junction synergistically boosting photocatalytic hydrogen evolution in alkaline solution for a 2D lamellar Co3O4/K7HNb6O19

The exploration of efficient alkaline hydrogen evolution photocatalysts is very meaningful in overall water splitting due to the fact that the high O₂ evolution performance is usually obtained under alkaline conditions. Herein, we successfully prepared a kind of photocatalysts with excellent H₂ production activity in an alkaline environment by combining Lindqvist-type polyoxoniobate K₇HNb₆O₁₉ with Co₃O₄. The hydrogen production performance of the optimal sample could reach 5394.17 μmol g^?1 under highly alkaline conditions. The superior H₂ evolution activity is mainly endowed by p-n heterojunction creating an internal-built electric field on the interface of the two semiconductors, which realizes the effective spatial separation of electron-hole pairs. Simultaneously, the increased oxygen vacancies and lamellar structure of catalysts also respectively endow samples with a high H₂O adsorption capacity and efficient photoinduced carrier separation rate. The work will be instructive for designing high-efficiency and stable HER catalyst in alkaline conditions for practical applications.     

In situ synthesis of a novel organic-inorganic composite as a non-noble metal electrocatalyst for the oxygen evolution reaction

Offering new techniques for efficient design and fabrication of inexpensive and earth-abundant catalysts for the development of oxygen evolution electrodes is a fundamental approach to promote sustainable energy processes. Herein, we report the in situ synthesis of a novel organic-inorganic composite directly onto carbon paste electrode (CPE) surface, as a robust substrate to incorporate Nickel-Iron (Ni-Fe) metal ions without using any binders or energy consumer techniques. Polyoxometalate (POM) and o-Anisidine (oA) are composite components that can be easily combined on the electrode surface (oA-POM/CPE). Ascribed to the synergy of context and metal ions, the as-prepared electrode affords a high catalytic activity and stability towards oxygen evolution reaction (OER), and gained a current density of 10 mA cm^?2 at overpotential of 330 mV. Moreover, the distinct electrocatalytic activity is illustrated by varying the amount of Fe in immersion solution, which proves the change made in percentage ratio of Ni-Fe in immersion solution that consequently affects Ni-Fe percentage value on electrode surface. This represents the competition between metal cations in creating complex with composite. Collectively, this simple strategy provides a promising way for the development of effective and non-noble metal-based OER electrocatalysts.     

N-doped graphene as a bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions in an alkaline electrolyte

In this work, a nitrogen-doped graphene (NG) catalyst was prepared using a hydrothermal method with ammonia as the nitrogen precursor, which was followed by a freeze-dry process. The catalyst was characterized using X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscope, and X-ray photoelectron spectroscopy. The bifunctional catalytic activities for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) were investigated using cyclic voltammetry in an alkaline electrolyte. The results indicate that nitrogen is successfully doped in the NG catalyst, and the catalyst has a loose structure that was produced during the freeze-dry process. The catalyst exhibits an excellent ORR activity with an onset potential of −0.08 V and a high OER activity with an obvious OER current at 0.7 V. The rotating-disk-electrode test results indicate that the ORR process catalyzed by the NG catalyst involves a mix of the two-electron and four-electron transfer pathways. This work preliminarily explores the bifunctional catalytic properties for the ORR and the OER of nitrogen-doped graphene materials in alkaline electrolyte.     

Bipolar electrochemistry as a powerful technique for rapid synthesis of ultrathin graphdiyne nanosheets: Improvement of photoelectrocatalytic activity toward both hydrogen and oxygen evolution

The acetylenic carbon-rich nanostructures such as graphdiyne has been received increasing attentions due to its potential applications in energy conversion, photoelectronic devices, catalysis, sensing and biomedical areas. So, development of facile synthesis procedures for ultrathin graphdiyne nanostructures is a challenge. Here, a prompt and simple method is proposed for polycondensation of 1,3,5-triethynylbenezene and synthesis of graphdiyne-like nanosheets, using bipolar electrochemistry assisted by copper grid electrode in the ethanol/acetonitrile solvent. The large scale of graphdiyne can be achieved with a series of bipolar electrodes in a single bipolar cell. The prepared nanosheets are characterized by various techniques, such as SEM, TEM, Raman and XPS. The as prepared material shows a remarkable photocatalytic activity toward hydrogen evolution (25 μA cm^?2 at 0.6 V vs. RHE) as well as oxygen evolution (4.5 μA cm^?2 at 1.1 V vs. RHE) activity at low overpotentials. The proposed method promised as a rapid and simple process for synthesis of graphdiyne-like nanostructures with remarkable electrocatalytic activity at less than 150 min. Furthermore, the presented procedure can be developed as applicable method for preparation of other grphdiyne-like nanostructures for fabrication of sensing and biosensing devices, optical imaging and nanoparticles loading.     

Carbon supported nickel phosphide as efficient electrocatalyst for hydrogen and oxygen evolution reactions

Hydrogen production through water splitting is an efficient and green technology for fulfilling future energy demands. Carbon nanotubes (CNT) supported Ni₂P has been synthesized through a simpler hydrothermal method. Ni₂P/CNT has been employed as efficient electrocatalysts for hydrogen and oxygen evolution reactions in acidic and alkaline media respectively. The electrocatalyst has exhibited low overpotential of 137 and 360 mV for hydrogen and oxygen evolution reactions respectively at 10 mA cm^?2. Lower Tafel slopes, improved electrochemical active surface area, enhanced stability have also been observed. Advantages of carbon support in terms of activity and stability have been described by comparing with unsupported electrocatalyst.     

Facile synthesis of polyoxometalate-based composite with doped ternary NiCoFe cations as electrocatalyst for oxygen evolution reaction

The construction of efficient and low-cost electrocatalysts for oxygen evolution reactions (OER) to replace precious catalysts is a necessity to achieve economic production of hydrogen. Herein, we report an efficient tri-metallic electrocatalysts for the OER that is prepared by incorporate nickel, cobalt and iron cations on Triton X-100/phosphotungstic acid organic-inorganic composite without utilize any binders or energy consumer procedure. Considering to the synergy effect of simultaneous absorption of NiCoFe cations on composite substrate, the as-made tri-metallic catalyst exhibits excellent OER activity with a small overpotential of 210 and 330 mV at a current density of 10 and 100 mA cm^?2, respectively. Moreover, remarkable trends in electrocatalytic activity of mono-, bi- and tri-metallic electrocatalysts at low (10 mA) and high (100 mA) current density are observed. In addition, this new families of non-precious metal catalyst shows long-term durability in 1 M KOH.     

One-step synthesis of atomic Ru doped ultra-thin Co(OH)2 nanosheets for oxygen evolution reaction in different pH values

Herein, atomic Ru doped ultra-thin Co(OH)₂ nanosheet arrays are firstly synthesized by a one-step electrochemical deposition method. Importantly, the obtained electrocatalyst can display excellent activity for oxygen evolution reaction, which only needs 305 mV at 50 mA cm^?2 in 1 M KOH and 261 mV at 10 mA cm^?2 in 0.1 M KOH. Further mechanism studies disclose that the doping of Ru could reduce the thickness of nanosheets and contribute to the generation of active Co³⁺ sites by donating electrons from Co to Ru atoms via the Co–O–Ru bonds. This work paves a simple method to fabricate Co based nanosheet catalyst, which may be extended to the preparation of other highly active electrocatalysts.     

One-pot wet-chemical synthesis of uniform AuPtPd nanodendrites as efficient electrocatalyst for boosting hydrogen evolution and oxygen reduction reactions

Uniform trimetallic AuPtPd nanodendrites (NDs) were synthesized by a simple and quick method, using l-proline and ascorbic acid (AA) as eco-friendly structure-guiding agent and reducing agent, respectively. The obtained AuPtPd NDs displayed greatly enlarged electrochemically active surface area (27.65 m² g^?1_metal) relative to home-made AuPt nanocrystals (NCs, 21.76 m² g^?1_metal), AuPd NCs (3.91 m² g^?1_metal), Pt black (20.88 m² g^?1_metal) and Pd black (8.30 m² g^?1_metal). For hydrogen evolution and oxygen reduction reactions, AuPtPd NDs showed excellent catalytic performances relative to the referenced catalysts. These results reveal the practical applications of the as-obtained catalyst in energy storage and conversion.     

Porous nickel telluride nanostructures as bifunctional electrocatalyst towards hydrogen and oxygen evolution reaction

Electrochemical water splitting technology has attracted researchers for the development of next generation fuels. Herein, we report the synthesis of nanostructured porous hollow nickel telluride nanosheets and their use as bifunctional electrocatalyst towards hydrogen and oxygen evolution reaction, anticipating an enhanced performance owing to their 2D sheet like morphology, conductivity, porous nature providing larger catalytic surface for water splitting reaction. In this regard, nickel telluride nanostructures were synthesized via an anion-exchange-reaction between pre-synthesized nickel hydroxide hexagonal nanosheets and tellurium ions under hydrothermal conditions. The as-synthesized nanostructures were characterized for structural, morphological and compositional properties using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Nickel telluride modified electrodes were tested as bifunctional electrocatalyst under acidic and alkaline conditions, through linear sweep voltammetry and constant current chronopotentiometry methods. The modified electrodes revealed an onset potential of ?422 mV and 87.4 mV dec^?1 Tafel slope towards HER and overpotential of 679 mV and 151 mV dec^?1 Tafel slope towards OER. The lower onset potentials are complimented with excellent electrocatalytic stability.     

Highly active and durable cauliflower-like NiCo2O4 film for oxygen evolution with electrodeposited SiO2 as template

The sluggish kinetics of oxygen evolution reaction (OER) is the most challenging technical problem in water splitting. Here, we reported a new type of cauliflower-like NiCo₂O₄ (CF-NiCo₂O₄) film which was co-electrodeposited with silica for oxygen evolution reaction. Structure and morphology of the obtained films were characterized using X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Electrochemical properties of the CF-NiCo₂O₄ film were evaluated by quasi-steady state polarization curve, cyclic voltammetry and electrochemical impedance spectroscopy. Results show that the enhanced electrocatalytic activity of the CF-NiCo₂O₄ film toward OER is a synergistic effect of improved intrinsic catalytic activity and increased surface area.