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Two-dimensional ultrathin nickel hydroxide nanosheets with large surface area and excellent electronic properties have been considered as promising electrocatalysts for the oxygen evolution reactio...

Electrocatalytic water splitting to produce H2 plays an important role in the capture, conversion, and storage of renewable energy sources, such as solar energy and wind power. As the reductive half reaction of water splitting, H2 evolution reaction (HER) suffers from sluggish kinetics, and hence competent HER catalysts are needed. Despite being excellent HER catalysts, noble metal-based catalysts (i.e. Pt) are too expensive to be economically competitive. Therefore, low-cost catalysts composed of solely earth-abundant elements have attracted increasing attention these years, among which nickel-based HER catalysts, particularly nickel chalcogenides, are considered as promising candidates. Although many nickel chalcogenides, including NiS, NiS2, and Ni3S2, have been reported for hydrogen evolution, their intrinsic catalytic activities have never been investigated and compared in detail under the same conditions. Most of the previous investigations were limited to only one species of nickel chalcogenides under very unique conditions, rendering a fair comparison of their HER activities impossible. Herein, we report the preparation and characterization of three crystalline nickel sulfides, NiS, NiS2, and Ni3S2, with comparable crystal sizes and specific surface areas. Detailed electrochemical studies under strongly alkaline conditions coupled with theoretical computations were performed to probe their intrinsic HER activities, resulting in the order of Ni3S2 > NiS2 > NiS. The superior HER performance of Ni3S2 mainly stems from the combined effect of large electrochemically active surface area and high conductivity (metallic conductor vs. semiconductor).

The development of high-performance nonprecious electrocatalysts with both H2 and O2 evolution reaction (HER and OER) activities for overall water splitting is highly desirable but remains a grand challenge. Herein, we report a facile two-step method to synthesize three-dimensional hierarchically porous urchin-like Ni2P microsphere superstructures anchored on nickel foam (Ni2P/Ni/NF) as bifunctional electrocatalysts for overall water splitting. The Ni2P/Ni/NF catalysts were prepared by template-free electrodeposition of porous nickel microspheres on nickel foam followed by phosphidation. The hierarchically macroporous superstructures with 3D configuration can reduce ion transport resistance and facilitate the diffusion of gaseous products (H2 and O2). The optimal Ni2P/Ni/NF exhibited remarkable catalytic performance and outstanding stability for both the HER and OER in alkaline electrolyte (1.0 M KOH). For the HER, Ni2P/Ni/NF afforded a current density of 10 mA cm–2 at a low overpotential of only −98 mV. ...

Efficient, stable, and low-cost electrocatalysts are crucial for realizing large-scale water splitting. Herein, we report that electrodeposited nickel–phosphorous (Ni–P) films can act as efficient bifunctional electrocatalysts for overall water splitting. The as-prepared Ni–P films exhibit remarkable catalytic performance for both H2 and O2 evolution reactions (HER and OER) in alkaline media, achieving a current density of 10 mA cm−2 at overpotentials of −93 mV for HER and 344 mV for OER with Tafel slopes of 43 and 49 mV dec−1, respectively, rivaling the performance of Pt and IrO2. Various techniques were employed to probe the composition and morphology of the Ni–P films prior to and post catalysis, revealing the major composition of the as-prepared and post-HER films as metallic nickel and nickel phosphide, which partially transform to nickel oxides during OER. It was also found that the catalytic rate of OER catalyzed by Ni–P was first order in the activity of the hydroxide anion.

One of the challenges to realize large-scale water splitting is the lack of active and low-cost electrocatalysts for its two half reactions: H2 and O2 evolution reactions (HER and OER). Herein, we report that cobalt-phosphorous-derived films (Co-P) can act as bifunctional catalysts for overall water splitting. The as-prepared Co-P films exhibited remarkable catalytic performance for both HER and OER in alkaline media, with a current density of 10 mA cm(-2) at overpotentials of -94 mV for HER and 345 mV for OER and Tafel slopes of 42 and 47 mV/dec, respectively. They can be employed as catalysts on both anode and cathode for overall water splitting with 100% Faradaic efficiency, rivalling the integrated performance of Pt and IrO2. The major composition of the as-prepared and post-HER films are metallic cobalt and cobalt phosphide, which partially evolved to cobalt oxide during OER.

We report a planar nickel complex coordinated with a pincer-type carbene–pyridine–carbene ligand which exhibits high selectivity for electrocatalytic CO2reduction in the presence of H2O.

© 2015 American Chemical Society. The development of low-cost catalysts with oxygen reduction reaction (ORR) activity superior to that of Pt for fuel cells is highly desirable but remains challenging. Herein, we report a bimetal-organic framework (bi-MOF) self-adjusted synthesis of support-free porous Co-N-C nanopolyhedron electrocatalysts by pyrolysis of a Zn/Co bi-MOF without any post-treatments. The presence of initial Zn forms a spatial isolation of Co that suppresses its sintering during pyrolysis, and Zn evaporation also promotes the surface area of the resultant catalysts. The composition, morphology, and hence ORR activity of Co-N-C could be tuned by the Zn/Co ratio. The optimal Co-N-C exhibited remarkable ORR activity with a half-wave potential of 0.871 V versus the reversible hydrogen electrode (RHE) (30 mV more positive than that of commercial 20 wt % Pt/C) and a kinetic current density of 39.3 mA cm-2 at 0.80 V versus RHE (3.1 times that of Pt/C) in 0.1 M KOH, and excellent stability and methanol tolerance. It also demonstrated ORR activity comparable to and stability much higher than those of Pt/C in acidic and neutral electrolytes. Various characterization techniques, including X-ray absorption spectroscopy, revealed that the superior activity and strong stability of Co-N-C originated from the intense interaction between Co and N, the high content of ORR active pyridinic and pyrrolic N, and the large specific surface area.

© 2015 American Chemical Society. The design of active, robust, and nonprecious electrocatalysts with both H2and O2evolution reaction (HER and OER) activities for overall water splitting is highly desirable but remains a grand challenge. Herein, we report a facile two-step method to synthesize porous Co-P/NC nanopolyhedrons composed of CoPx(a mixture of CoP and Co2P) nanoparticles embedded in N-doped carbon matrices as electrocatalysts for overall water splitting. The Co-P/NC catalysts were prepared by direct carbonization of Co-based zeolitic imidazolate framework (ZIF-67) followed by phosphidation. Benefiting from the large specific surface area, controllable pore texture, and high nitrogen content of ZIF (a subclass of metal-organic frameworks), the optimal Co-P/NC showed high specific surface area of 183 m2g-1and large mesopores, and exhibited remarkable catalytic performance for both HER and OER in 1.0 M KOH, affording a current density of 10 mA cm-2at low overpotentials of -154 mV for HER and 319 mV for OER, respectively. Furthermore, a Co-P/NC-based alkaline electrolyzer approached 165 mA cm-2at 2.0 V, superior to that of Pt/IrO2couple, along with strong stability. Various characterization techniques including X-ray absorption spectroscopy (XAS) revealed that the superior activity and strong stability of Co-P/NC originated from its 3D interconnected mesoporosity with high specific surface area, high conductivity, and synergistic effect of CoPxencapsulated within N-doped carbon matrices.

Hollow cobalt sulfide nanoprisms obtained by a two-step, microwave-assisted synthesis within 15 min exhibit higher hydrogen evolution catalytic activity and better specific capacitance than its counterpart prepared by a traditional solvothermal method.

Along with rising concerns about fossil fuel depletion and associated climate change, there are research efforts worldwide to explore renewable and carbon-neutral energy resources. Among various candidates, solar energy stands out as the most attractive candidate because of the gigantic amount that is available. In order to capture and store the intermittent solar power in chemical forms, light-driven water splitting to produce H2 and O2 has been the focus of renewable energy catalysis for decades. This chapter highlights the recent advances of water-splitting catalysis, with a particular emphasis on the development of hybrid molecular–nanomaterial assemblies for H2 and O2 evolution reactions. Hybrid systems based on earth-abundant metals, such as iron, cobalt, and nickel, have been intensively studied for H2 evolution catalysis, while for O2 evolution, ruthenium and iridium catalysts have attracted much attention and the involved mechanistic steps are relatively well elucidated. Recent years have also witnessed the rapid advancement of molecular–nanomaterial assemblies incorporating more inexpensive transition metals, including manganese, iron, and cobalt, for effective O2 evolution catalysis. A diverse array of nanomaterial supports, such as polymer, oligomer, carbon nanotube, graphene, and nanostructured semiconductors, have been employed as catalyst supports. This chapter emphasizes the formation of hybrid catalytic systems with enhanced performance compared to homogeneous molecular counterparts. As clearly shown by many the examples discussed in this chapter, hybrid molecular–nanomaterial assemblies represent a unique category of catalytic systems for water splitting, synergistically integrating the advantages of both homogeneous and heterogeneous catalytic reactions.

A novel composite film based on carbon nanodots (CNDs) and chitosan was readily prepared and used as immobilization matrix to entrap a heme protein, hemoglobin (Hb) for direct electrochemistry and bioelectrocatalysis. A modified electrode was obtained by casting Hb–CNDs–chitosan composites on the glassy carbon (GC) electrode surface. Spectroscopic and electrochemical studies showed that Hb entrapped in the composite film remained in its native structures, and CNDs in the film can greatly facilitate DET between the protein and the GC electrode. The electron-transfer kinetics of Hb in composite film was qualitatively evaluated by using the Marcus theory, and the apparent heterogeneous electron-transfer rate constant ( k s ) was estimated to be 2.39(±0.03) s −1 with Laviron equations. The modified electrode showed excellent electrocatalytic behavior to the substrate, hydrogen peroxide (H 2 O 2 ). The linear current response for H 2 O 2 was from 1×10 −6 to 1.18×10 −4 M with a detection limit of 0.27(±0.02) μM at the signal-to-noise ratio of 3, and the apparent Michaelis–Menten constant was 0.067(±0.02) mM. These important features of CNDs–chitosan film have implied to be a promising platform for elaborating bioelectrochemical devices such as biosensors and biofuel cells.