Porous nickel electrodes were fabricated by the phase inversion tape casting method. The as-prepared nickel electrode contained finger-like straight open pores with an average diameter of ∼100 μm and smaller pores with an average diameter of 1–3 μm in the walls. When used as an anode for the oxygen evolution reaction (OER), the porous nickel electrode shows high catalytic activity, reaching a catalytic current density of 10 mA cm−2 under an overpotential of only 300 mV in 1.0 M KOH. It also exhibited excellent performance for the hydrogen evolution reaction (HER), resulting in a current density of 10 mA cm−2 under an overpotential of 125 mV in the same electrolyte. For both OER and HER, the electrode shows great catalytic stability and much better catalytic performance than commercial nickel foam. Moreover, an alkaline electrolyzer using identical porous nickel electrodes as both the anode and cathode required a cell voltage of only 1.65 V to reach 10 mA cm−2 for overall water splitting. The improved electrocatalytic performance of the electrode can be attributed to its unique dual-pore structure.

Metal-free elemental photocatalysts for hydrogen (H2) evolution are more advantageous than the traditional metal-based inorganic photocatalysts since the nonmetal elements are generally cheaper, more earth-abundant, and environmentally friendly. Black phosphorus (BP) has been attracting increasing attention in recent years based on its anisotropic 2D layered structure with tunable bandgap in the range of 0.3–2.0 eV; however, the application of BP for photocatalytic H2 evolution has been scarcely reported experimentally although being theoretically predicted. Herein, for the first time, the visible light photocatalytic H2 evolution of BP nanosheets prepared via a facile solid-state mechanochemical method by ball-milling bulk BP is reported. Without using any noble metal cocatalyst, the visible light photocatalytic hydrogen evolution rate of BP nanosheets reaches 512 µmol h−1 g−1, which is ≈18 times higher than that of the bulk BP, and is comparable or even higher than that of graphitic carbon nitrides (g-C3N4).

AbstractThe development of efficient water-oxidation electrocatalysts based on inexpensive and earth-abundant materials is significant to enable water splitting as a future renewable energy source. Herein, the synthesis of novel FeNiP solid-solution nanoplate (FeNiP-NP) arrays and their use as an active catalyst for high-performance water-oxidation catalysis are reported. The as-prepared FeNiP-NP catalyst on a 3D nickel foam substrate exhibits excellent electrochemical performance with a very low overpotential of only 180 mV to reach a current density of 10 mA cm−2 and an onset overpotential of 120 mV in 1.0 m KOH for the oxygen evolution reaction (OER). The slope of the Tafel plot is as low as 76.0 mV dec−1. Furthermore, the long-term electrochemical stability of the FeNiP-NP electrode is investigated by cyclic voltammetry (CV) at 1.10–1.55 V versus reversible hydrogen electrode (RHE), demonstrating very stable performance with negligible loss in activity after 1000 CV cycles. This present FeNiP-NP solid solution is thought to represent the best OER catalytic activity among the non-noble metal catalysts reported so far.

•CdS/ZnO/ZnS heterojunctions with a molecular co-catalyst showed a synergistic effect.•ZnS was formed in situ on CdS/ZnO to increase the charge transfer process.•The photogenerated electrons are transferred to both ZnO and the molecular co-catalyst.•ZnO, ZnS, and a cobalt co-catalyst together promoted solar H2 production.Photocatalytic hydrogen production is considered to be a promising solution to the global energy crisis and to environmental pollution caused by fossil fuel consumption. In the present study, a core/shell cadmium sulfide/zinc oxide (ZnO/CdS) semiconductor heterojunction photocatalyst is used with a cobalt–salen molecular co-catalyst for highly enhanced photocatalytic activity. CdS nanorods were synthesized using a simple solvothermal method and a ZnO shell was grown by a solution deposition method. Under optimum conditions, the system exhibited a H2 evolution rate of 725 µmol h−1 mg−1 with a turnover number of ∼102,700 and excellent stability over 50 h in the presence of Na2S/Na2SO3 as the electron donor under visible light. The highest apparent quantum yield of the system was 44% under monochromatic 420 nm light. The formation of ZnS during photocatalysis was proved due to surface dissolution of ZnO in alkaline sulfide solution. ZnS can enhance the photocatalytic activity of ZnO/CdS nanorods by providing increased charge transfer interfaces. The molecular cobalt co-catalyst also contributed to the enhanced activity by accepting the photogenerated electrons from the semiconductor photosensitizer. The proposed mechanism suggests that the photogenerated electrons in CdS are transferred not only to ZnO but also to the molecular co-catalysts, leading to highly improved photocatalytic activity for H2 production.Download high-res image (89KB)Download full-size image

Hydrogen is essential to many industrial processes and could play an important role as an ideal clean energy carrier for future energy supply. Herein, we report for the first time the growth of crystalline Cu3P phosphide nanosheets on conductive nickel foam (Cu3P@NF) for electrocatalytic and visible light-driven overall water splitting. Our results show that the Cu3P@NF electrode can be used as an efficient Janus catalyst for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). For OER catalysis, a current density of 10 mA/cm2 requires an overpotential of only ∼320 mV and the slope of the Tafel plot is as low as 54 mV/dec in 1.0 M KOH. For HER catalysis, the overpotential is only ∼105 mV to achieve a catalytic current density of 10 mA cm–2. Moreover, overall water splitting can be achieved in a water electrolyzer based on the Cu3P@NF electrode, which showed a catalytic current density of 10 mA/cm2 under an applied voltage of ∼1.67 V. The same current density can also be obtained using a silicon solar cell under ∼1.70 V for both the HER and the OER. This new Janus Cu3P@NF electrode is made of inexpensive and nonprecious metal-based materials, which opens new possibilities based on copper to exploit overall water splitting for hydrogen production. To the best of our knowledge, such high performance of a copper-based water oxidation and overall water splitting catalyst has not been reported to date.Keywords: copper; hydrogen production; Janus catalyst; water oxidation; water splitting;

•The Co7, Ni7 and CoxNi7-x were first used as water oxidation electrocatalysts.•The CoxNi7-x requires low onset overpotential of only ∼270 mV.•The CoxNi7-x has a high Faradaic efficiency under an overpotential of 470 mV.Herein we report a comparative study for a series of cheap, robust, and efficient water oxidation electrocatalysts made up of the heptanuclear disk clusters ([M7(OH)6L6](ClO4)2·solvent (M = Co, Ni or mixed Co-Ni) and L = 2-ethoxy-6-(N-methyl-iminomethyl)phenolate) for renewable energy production. Experiments performed on the three complexes immobilized on conductive substrates reveal the catalytic efficiency in the order Co-Ni > Ni > Co based on threshold and overpotentials as well as electro-impedance parameters. Remarkably, the Co-Ni mixed metal cluster shows low onset overpotential of only 270 mV and a small Tafel slop of 53.4 mV/dec. Moreover, the Faradaic efficiency exceeds 90% for the Co-Ni cluster. Their characteristics are major improvements on those reported for transition metal complexes. These complexes could be proposed as promising electrocatalysts for water oxidation reaction.Heptanuclear Co-Ni mixed metal clusters as new electrocatalysts showed a low onset overpotential of only ∼270 mV for water oxidation reaction.Download high-res image (129KB)Download full-size image

Photocatalytic splitting of water to hydrogen (H2) has attracted much attention because of its potential to address the concerns of air pollution and energy shortage. In this present study, we report that noble-metal-free cobalt nitride (Co3N) can be used as an efficient cocatalyst on CdS nanorods (CdS NRs) for photocatalytic H2 production in water under visible light irradiation. Photoluminescence (PL) spectra and photoelectrochemical measurements indicated that the loading of Co3N onto CdS NRs can facilitate the separation and transfer of photogenerated charge carriers, leading to an enhanced photocatalytic activity for H2 production. The H2 production rate reached ∼137.33 μmol h−1 mg−1 (λ > 420 nm) and the apparent quantum yield (AQY) was ∼14.9% at 450 nm.

Photocatalytic hydrogen production from water in a noble-metal-free system has attracted much attention in recent years. Herein we report on the use of core/shell cadmium sulfide/graphitic carbon nitride (CdS/g-C3N4) heterojunction nanorods modified by nickel hydroxide (Ni(OH)2) as a highly efficient photocatalyst for visible light-driven hydrogen production from water. Due to efficient separation of the photoexcited charge carriers in the CdS/g-C3N4 core/shell nanorods and the synergistic effect of Ni(OH)2, the optimal hydrogen evolution rate over Ni(OH)2–CdS/g-C3N4 is 115.18 μmol h−1 mg−1 under visible light irradiation (λ > 420 nm), which is ∼26 times higher than the CdS/g-C3N4 nanorod composite without Ni(OH)2 and ∼7 times better than the 0.5 wt% Pt–CdS/g-C3N4 nanorod composite. The apparent quantum efficiency is ∼16.7% at an excitation of 450 nm. During photocatalysis, no degradation of Ni(OH)2 was observed based on the XPS data, indicating that it is a robust cocatalyst. Moreover, the present photocatalyst showed excellent photocatalytic stability for hydrogen production and the turnover number (TON) reached ∼24600 over 90 hours.

Photocatalytic hydrogen production using solar energy offers a clean and sustainable pathway for future energy supply. Herein, we report nickel nitride (Ni3N) as a novel cocatalyst on cadmium sulfide nanorod (CdS NR) semiconductors to enhance photocatalytic hydrogen production in water. The Ni3N cocatalyst was grown by a facile in situ growth method. Under optimal conditions, the hydrogen production rate can be improved more than 10 times by introducing an appropriate amount of Ni3N on CdS NRs. PL spectra and photoelectrochemical measurements suggest that faster interfacial charge transfer between Ni3N and CdS may be the key factor for the enhanced photocatalytic activity.

Developing highly active electrocatalysts for the hydrogen evolution reaction (HER) is crucial to construct an efficient water-splitting device. In this present study, a novel ternary cobalt–nickel phosphide nanosheet with nanowire edges on 3D nickel foam (CoNiP@NF) was first synthesized and used as an excellent cathode for the HER over a wide pH range from 0 to 14. The cathode showed high-performance catalytic activity to produce hydrogen in aqueous solution, with quite low overpotentials of only 60 mV (0.5 M H2SO4, pH ∼ 0.26), 120 mV (1.0 M KPi, pH ∼ 7), and 155 mV (1.0 M KOH, pH ∼ 14) to reach a current density of 10 mA cm−2. This high performance is probably due to the combination of the advantages of both one-dimensional nanowire and two-dimensional nanosheet materials, which can effectively improve the electrochemical performance for the HER. To the best of our knowledge, the present ternary CoNiP material is among the best noble-metal-free electrocatalysts for hydrogen evolution in water.

Efficient hydrogen (H2) production is considered to be a key pathway for future clean energy supply. Herein we report that a photocatalyst made of core–shell amorphous cobalt phosphide (CoPx) integrated with cadmium sulfide nanorods (CdS NRs) gives exceptional performance of photocatalytic H2 production under visible light. Under optimal conditions, the CoPx/CdS NRs photocatalyst allows an H2 evolution rate of ∼500 μmol h−1 mg−1 based on the photocatalyst (λ > 420 nm) and the apparent quantum yield was ∼35% in aqueous solutions (λ = 450 nm). The turnover numbers (TONs) reached ∼630000 per mole of cobalt in 70 hours with a TOF of ∼9000 h−1. Such high performance of an artificial photosynthetic H2 production system using a cobalt-based cocatalyst has, to the best of our knowledge, not been reported to date.

Highly efficient, visible-light-induced hydrogen (H2) production via water splitting can be achieved without the help of a cocatalyst by using a noble-metal-free core–shell photocatalyst, in which zinc sulfide (ZnS) nanoparticles as the protective shell are anchored on the surface of cadmium sulfide nanorods (CdS NRs). Due to the close interfacial contact of component semiconductors, the electronic structure of CdS is strongly coupled with that of ZnS nanoparticles, leading to efficient transfer of charge carriers between them and the improvement of the CdS photostability. The CdS/ZnS NR photocatalyst showed much higher catalytic activity for H2 production than CdS NRs and ZnS under visible light irradiation (λ > 420 nm), which is probably due to fast transfer of the photogenerated charge carriers and/or electron tunneling in the one-dimensional core–shell nanorod structure. Under optimal conditions, the highest hydrogen evolution rate reached 239 μmol h−1 mg−1, which is much greater than ZnS and CdS NRs and also among the best cocatalyst-free photocatalysts for H2 production. The average apparent quantum yield can be achieved as ∼16.8% after 8 h of irradiation (monochromatic light at 420 nm ± 5 nm). A possible mechanism for the photocatalytic reaction based on CdS/ZnS NRs is also discussed.

Developing efficient water oxidation catalysts made up of earth-abundant elements has attracted much attention as a step toward for future clean energy production. Herein we report a simple one-step method to generate a low cost copper oxide catalyst film in situ from a copper(II) ethylenediamine complex. The resulting catalyst has excellent activity toward the oxygen evolution reaction in alkaline solutions. A catalytic current density of 1.0 mA cm−2 and 10 mA cm−2 for the catalyst film requires the overpotentials of only ∼370 mV and ∼475 mV in 1.0 M KOH, respectively. This catalytic performance shows that the new catalyst is one of the best Cu-based heterogeneous OER catalysts to date.

•Noble-metal-free copper-based thin films for electrocatalytic water oxidation.•Facile and direct electrodeposition of copper oxide thin films on conductive electrode.•The onset overpotential for water oxidation is low (η ∼330 mV).•The catalyst films show high Faradaic efficiency and low slope of the Tafel plot.•Inexpensive starting materials (a simple copper salt).Catalysts made of earth-abundant elements for water splitting have attracted increasing attention in recent years. Herein we report that inexpensive cheap copper oxide thin film material can be used as an excellent electrocatalyst precursor for the oxygen evolution reaction (OER). Cuprous oxide (Cu2O) thin films were facilely electrodeposited on conductive fluorine doped tin oxide (FTO) substrates from a simple Cu(II) salt solution under a very low applied potential (-0.17 V or -0.23 V). Two kinds of morphologies (dendritic branching and cluster-like) of Cu2O were obtained just by altering the deposition potentials. Both Cu2O films can be used for OER, and the dendritic branching Cu2O material exhibits better performance. Under optimal conditions, OER was achieved under an onset potential at ∼0.92 V (vs. Ag/AgCl) in 0.1 M borate solution at pH 9.2. A catalytic current density of ∼0.1 mA/cm2 required a low overpotential of ∼430 mV using the electrodeposited Cu2O material under an optimal condition. The slope of the Tafel plot is ∼59.9 mV/dec and the Faradaic efficiency was close to 93%. The samples were well characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS).

Copper hydroxide (Cu(OH)2) is a quite inexpensive and abundant material, but the literature contains no reports of using it as a stable water oxidation catalyst (WOC). In this study, we report for the first time that Cu(OH)2 material synthesized from a simple copper salt can be used as a WOC with good activity and stability. Under optimal conditions using Cu(OH)2 as the electrocatalyst, a catalytic current density of 0.1 mA/cm2 can be achieved under an applied potential of ∼1.05 V relative to Ag/AgCl at pH 9.2. The slope of the Tafel plot is 78 mV/dec. The Tafel plot indicates that a current density of ∼0.1 mA/cm2 requires an overpotential of 550 mV. The Faradaic efficiency was measured to be ∼95%. The as-synthesized Cu(OH)2 material was characterized by X-ray powder diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy.Keywords: Catalyst; Copper hydroxide; Electrocatalysis; Water oxidation;

Photocatalytic hydrogen production via water splitting has attracted much attention for future clean energy application. Herein we report a noble-metal-free photocatalytic hydrogen production system containing a simple bidentate cobalt Schiff base complex as the molecular cocatalyst, CdS nanorods as the photosensitizer, and ascorbic acid as the electron donor. The system shows highly enhanced photocatalytic activity compared to pure CdS NRs under visible light (λ > 420 nm). Under optimal conditions, the turnover numbers (TONs) for hydrogen production reached ∼15200 after 12 hours of irradiation, and an apparent quantum yield of ∼27% was achieved at 420 nm monochromatic light. Steady-state photoluminescence (PL) spectra indicated efficient charge transfer between the excited CdS NRs and the cobalt cocatalyst for improved hydrogen production. Spectroscopic studies of the photocatalytic reaction revealed the reduction of the Co(II) complex to Co(I) species, which are probably active intermediates for hydrogen evolution. On the basis of the spectroscopic studies, we propose a reaction mechanism for hydrogen production in the present photocatalytic system.

Developing efficient water oxidation catalysts made of earth-abundant elements is a demanding challenge that should be met to fulfill the promise of water splitting for clean energy. Herein we report an annealing approach to synthesize binder-free, self-supported heterogeneous copper oxide (CuO) on conductive electrodes for oxygen evolution reaction (OER), producing electrodes with excellent electrocatalytic properties such as high efficiency, low overpotential, and good stability. The catalysts were grown in situ on fluorine-doped tin oxide (FTO) by electrodeposition from a simple Cu(II) salt solution, followed by annealing at a high temperature. Under optimal conditions, the CuO-based OER catalyst shows an onset potential of <0.58 V (vs Ag/AgCl) in 1.0 M KOH at pH 13.6. From the Tafel plot, the required overpotentials for current densities of 0.1 and 1.0 mA/cm2 are only 360 and 430 mV, respectively. The structure and the presence of a CuO motif in the catalyst have been identified by high-energy X-ray diffraction (HE-XRD), Cu K-edge X-ray absorption (XAS) spectra including X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS). To the best of our knowledge, this represents the best catalytic activity for CuO-based OER catalysts to date.

Photocatalytic hydrogen evolution via water splitting is an attractive scientific and technological goal to address the increasing global demand for clean energy and to reduce the climate change impact of CO2 emission. Although tremendous efforts have been made, hydrogen production by a robust and highly efficient system driven by visible light still remains a significant challenge. Herein we report that nickel phosphide, as a cocatalyst to form a well-designed integrated photocatalyst with one-dimensional semiconductor nanorods, highly improves the efficiency and durability for photogeneration of hydrogen in water. The highest rate for hydrogen production reached ∼1200 μmol h−1 mg−1 based on the photocatalyst. The turnover number (TON) reached ∼3270000 in 90 hours with a turnover frequency (TOF) of 36400 for Ni2P, and the apparent quantum yield was ∼41% at 450 nm. The photoinduced charge transfer process was further confirmed by steady-state photoluminescence spectra and time-resolved photoluminescence spectra. Such extraordinary performance of a noble-metal-free artificial photosynthetic hydrogen production system has, to our knowledge, not been reported to date.

The production of hydrogen through water splitting via electrolysis/photocatalysis seems a promising and appealing pathway for clean energy conversion and storage. Herein we report for the first time that a series of water-soluble copper complexes can be used as catalyst precursors to generate the copper-based bifunctional catalyst composite for both hydrogen production and water oxidation reactions. Under an applied cathodic potential, a thin catalyst film was grown on a fluorine-doped tin oxide (FTO) electrode, accompanied by the production of a large amount of hydrogen gas bubbles. Scanning electron microscopy shows the presence of nanoparticulate material on the FTO. Powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results indicated that the materials consist of amorphous cuprous oxide mixed copper hydroxide (H2–CuCat), which can catalyze water reduction in a copper-free aqueous solution (pH = 9.2) under a low overpotential. Remarkably, under an applied anodic potential, the material can also efficiently catalyze water oxidation to evolve oxygen. The present robust, bifunctional, switchable, and noble-metal-free catalytic material has potential applications in solar water-splitting devices.Keywords: bifunctional; copper; electrocatalysis; hydrogen production; noble-metal-free electrocatalyst; water oxidation

A noble-metal-free, efficient, and robust catalyst made of Earth-abundant elements for water oxidation is vital to achieve practical water splitting for future clean energy production. Herein, we report the synthesis of multilayer covalent cobalt porphyrin framework on multiwalled carbon nanotubes ((CoP)n-MWCNTs) to produce a highly active electrocatalyst for water oxidation. A linear sweep voltammetry curve showed that a catalytic current density of 1.0 mA/cm2 can be achieved under a potential of only 1.52 V (vs RHE, corresponding to an overpotential of only 0.29 V) in alkaline solution at pH 13.6. Such an onset potential is much lower than that of cobalt porphyrin monomer (CoP-TIPS) and pure MWCNTs. In addition, the chronopotentiometry data confirmed its excellent catalytic activity and suggested that the (CoP)n-MWCNTs catalyst has good durability for water oxidation catalysis. A Tafel slope of 60.8 mV per decade was obtained by bulk electrolysis measurement, and the Faradaic efficiency of oxygen production was >86%.

The generation of hydrogen (H2) through photocatalytic water splitting by employing various cocatalysts has attracted much attention. Herein we report for the first time that metallic molybdenum phosphide (MoP), as a highly active cocatalyst, can significantly enhance photocatalytic H2 production from water. A series of MoP/CdS nanorod (NR) hybrids were facilely prepared. The optimal amount of MoP led to a maximal H2 evolution rate of 163.2 μmol h−1 mg−1 under visible light illumination (λ > 420 nm), which is more than 20 times higher than that of freshly prepared CdS NRs. This work demonstrated that the suitable Fermi level alignment of MoP and CdS is responsible for the high photocatalytic activity of H2 production under visible light in the present system, as evidenced by both experimental and theoretical results.

An efficient photocatalytic system is highly demanded for the production of hydrogen fuel through water splitting. Herein, we report an artificial photocatalytic system made of low-cost materials for high-performance H2 production from water. The new system contains semiconductors (CdS nanorods) as the photosensitizer, a cobalt–salen complex as the H2 evolution cocatalyst, and Na2S and Na2SO3 as sacrificial electron donors. Under optimal conditions, the highest hydrogen evolution turnover number reached 64700 after 37 hours and the rate was 106 μmol h−1 mg−1, which is much higher than when using CdS NRs and also is among the best for photocatalytic systems using molecular cocatalysts for H2 production. The highest apparent quantum yield (AQY) was ∼29% at 420 nm. Steady state photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) decay spectra revealed that the system allows effective electron transfer from the excited CdS NRs to the cobalt–salen complex for highly efficient H2 production.

The design and preparation of highly active catalysts for the hydrogen evolution reaction (HER) is very important for water splitting. Herein, we report a highly active HER catalyst, which is synthesized by loading nanostructured nickel phosphide (Ni2P) on three-dimensional few-layer graphene/nickel foam (G@NF). G@NF was successfully prepared by a chemical vapor deposition process in the presence of methane at high temperature. Compared with nickel phosphide, G@NF, as well as commercial platinum, the Ni2P–G@NF catalyst exhibited very high activity in electrocatalytic H2 production from water (∼7 mV overpotential in alkaline solutions, pH ∼ 14; and ∼30 mV overpotential in acidic solutions, pH ∼ 0). The high catalytic activity of Ni2P–G@NF is attributed to the excellent performance of Ni2P, the large 3D framework which facilitates proton accessibility and electron transfer, and the high surface area.

Developing efficient photocatalysts made of earth-abundant elements for hydrogen (H2) production from water is considered to be a key pathway for future clean energy supply. Herein we report for the first time that p-type copper phosphide (Cu3P) can be an efficient promoter to improve photocatalytic H2 production from water when loaded on n-type cadmium sulphide nanorods (CdS NRs). The formation of a p–n junction in Cu3P/CdS NRs leads to fast charge transfer and enhanced photocatalytic activity under visible light irradiation. Under optimal conditions, the H2 evolution rate was as high as ∼200 μmol h−1 mg−1 (λ > 420 nm) and the apparent quantum yield at λ = 450 nm was ∼25% in water.

We report here a Cu(0)-based catalyst free of noble metals for the electrocatalytic hydrogen evolution reaction in neutral water with an onset overpotential of only 70 mV. This is the lowest reported value among Cu(0)-based catalysts for the hydrogen evolution reaction in neutral water.

For the first time, noble-metal-free nickel phosphide (Ni2P) was used as an excellent catalyst precursor for water oxidation catalysis. The lowest onset potential was observed at ∼1.54 V (vs. RHE) and a Tafel slope of 60 mV dec−1 was obtained in alkaline solution (pH = 13.6).

For the first time, ferrous phosphide (Fe2P) is used as an active noble metal-free cocatalyst for photocatalytic H2 production under visible light in water. The rate of H2 production can be enhanced by more than 30 times through loading of Fe2P on CdS nanorod surfaces. Efficient charge transfer between CdS and Fe2P might be the key factor for the high photocatalytic activity.

•Noble-metal-free cobalt porphyrin-based catalyst for water oxidation.•Carbon support improves catalytic current density.•Heat-treatment enhances the catalytic activity.•High Faradaic efficiency and turnover frequency.Design and preparation of highly active catalysts for the oxygen evolution reaction (OER) is important to achieve efficient water splitting. Herein, we report on the use of pyrolyzed cobalt porphyrin/carbon materials as noble-metal-free electrocatalysts for the catalytic OER. The cobalt porphyrin/carbon electrocatalysts were prepared by loading a cobalt porphyrin complex on a carbon support, followed by heat treatment under argon atmosphere. Cyclic voltammetry showed that the maximum catalytic activity was obtained after heat treatment at 1000 °C. The turnover frequency was ∼0.078 s−1 based on cobalt under an applied anodic potential of 1.31 V (vs NHE) at pH = 9.2. The catalyst was further characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and powder X-ray diffraction (XRD). These results indicate that the pyrolyzed cobalt porphyrin/carbon nanomaterial is a promising catalyst for OER.Pyrolyzed cobalt porphyrin/carbon materials as noble-metal-free electrocatalysts for the catalytic OER with high performance.

Water oxidation is an important half-reaction to achieve overall water splitting. In this present study, we show that a series of molecular cobalt–salen complexes can serve as catalyst precursors to form nanostructured and amorphous cobalt-based thin films during electrodeposition, which can catalyze the water oxidation reaction at low overpotentials. Cyclic voltammetry and bulk electrolysis using the cobalt-based film electrodes demonstrated obvious catalytic currents in 0.1 M KBi solution at pH 9.2. The onset catalytic potentials of the catalyst films are at ∼0.84 V (vs Ag/AgCl) with a film made by electrodeposition of cobalt–salen complex 2 on FTO and at ∼0.85 V for complex 4. Oxygen gas bubbles were clearly seen on the FTO electrode when the applied potential was above the onset potential. The Tafel plots using a catalyst film made of complex 4 showed that appreciable catalytic current was observed starting at η = 0.26 V for the film (a current density of 0.01 mA/cm2 required η = 290 mV), accompanied by a Faradaic efficiency >93% at 1.2 V. The catalyst film was further characterized by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS).

Herein we report a facile and direct synthesis of porous NiCo2O4 nanowire arrays (NWAs) with robust mechanical adhesion to conductive electrodes by a simple two-step method. Upon complete pyrolysis of the cobalt–nickel-hydroxide precursor, high-quality crystalline NiCo2O4 is achieved. The porous NiCo2O4 nanowires were found to be highly active for catalytic water oxidation when serving as the working electrodes without any external materials (binder and/or carbon black), as evidenced by exhibiting higher catalytic current density for water oxidation compared to precious metal oxide catalysts such as iridium oxide (IrO2) under the same conditions and appreciable catalytic wave at ∼1.52 V (vs. RHE). The optimal performance of the as-synthesized NiCo2O4 nanowires showed a current density of 10 mA cm−2 under an overpotential of only 0.46 V and 20 mA cm−2 under an overpotential of 0.72 V, corresponding to a Faradaic efficiency of nearly 100%. The atomic-scale analysis of the NiCo2O4 nanowires was further conducted by spherical-aberration-corrected transmission electron microscopy (TEM). The highly exposed high-index facets and one-dimensional (1D) configuration of the as-synthesized porous NiCo2O4 nanowires may be responsible for the high catalytic performance of water oxidation, which exhibit excellent activity and unique advantages for catalytic water splitting.

We report one dimensional (1D) MoS2 nanosheet/porous TiO2 nanowire hybrid nanostructures synthesized by a simple hydrothermal method, leading to an enhanced specific surface area (66 m2 g−1). These 1D hybrid nanostructures as co-catalysts exhibit high activity in visible light photocatalytic hydrogen evolution reaction (HER) with an enhanced hydrogen generation rate of 16.7 mmol h−1 g−1.

In this study, we report for the first time on the use of a water-soluble BF2-annulated cobaloxime, Co(dmgBF2)2(OH2)2 (Co-DMB, dmgBF2 = difluoroboryl-dimethylglyoxime), as a catalyst precursor for electrocatalytic water oxidation. Oxygen gas bubbles were clearly produced on the FTO electrode at a low overpotential under neutral pH conditions containing Co-DMB. Interestingly, stable green films were produced under these conditions. The current densities can reach to >5 mA/cm2 at 1.1 V and >10 mA/cm2 at 1.5 V (vs Ag/AgCl). The morphologies of the films showed nanoribbon structures, which were characterized by scanning electron microscope (SEM), energy-dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS).Keywords: cobalt oxide; electrodeposition; oxygen production; water splitting

Herein, we report electrodeposited nickel-based thin film (NiOx) on multiwalled carbon nanotubes (MWCNTs) as a highly efficient bifunctional catalyst for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Under reductive conditions (−1.2 V vs Ag/AgCl), the hydrogen evolution catalyst (H2-NiOx) was facilely deposited on MWCNTs. The resulting film demonstrates good catalytic activity for hydrogen production in a near-neutral aqueous solution at low overpotential. When switched to oxidative conditions (+1.1 V vs Ag/AgCl), the amorphous H2-NiOx film onto MWCNTs can be transformed into another amorphous material (O2-NiOx) to efficiently catalyze OER. The NiOx-MWCNTs catalyst was further characterized by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS). The results show that the content of oxygen in the O2-NiOx-MWCNTs film is higher than that in the H2-NiOx-MWCNTs film. The NiOx-MWCNTs catalyst has good catalytic stability, and the film is reversible when the potentials are switched between the reductive conditions and oxidative conditions. The Faradaic efficiencies of hydrogen and oxygen production are >95%.Keywords: bifunctional; electrocatalyst; hydrogen production; nickel; oxygen evolution; water splitting

Reversible mechanochromic luminescence in cationic platinum(II) terpyridyl complexes is described. The complexes [Pt(Nttpy)Cl]X2 (Nttpy = 4′-(p-nicotinamide-N-methylphenyl)-2,2′:6′,2″-terpyridine, X = PF6 (1), SbF6 (2), Cl (3), ClO4 (4), OTf (5), BF4 (6)) exhibit different colors under ambient light in the solid state, going from red to orange to yellow. All of these complexes are brightly luminescent at both room temperature and 77 K. Upon gentle grinding, the yellow complexes (4–6) turn orange and exhibit bright red luminescence. The red luminescence can be changed back to yellow by the addition of a few drops of acetonitrile to the sample. Crystallographic studies of the yellow and red forms of complex 5 suggest that the mechanochromic response is likely the result of a change in intermolecular Pt···Pt distances upon grinding.

•Nanostructured copper oxide films have been electrodeposited from copper(II) complexes.•The films are used for electrocatalytic water oxidation with good catalytic activities.•The first observation of copper oxide nanoparticles as an active catalystIn this report we show that nanostructured copper oxide thin films electrodeposited from copper(II) complexes can catalyze the oxygen evolution reaction. Cyclic voltammetry and bulk electrolysis with copper oxide film electrode in alkaline aqueous solutions showed significant catalytic currents. The catalyst film was characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray analysis and X-ray diffraction. The results identify that nanostructured copper oxide is an active electrocatalyst for water oxidation.

Six cobalt and manganese corrole complexes were synthesized and examined as single-site catalysts for water splitting. The simple cobalt corrole [Co(tpfc)(py)2] (1, tpfc = 5,10,15-tris(pentafluorophenyl)corrole, py = pyridine) catalyzed both water oxidation and proton reduction efficiently. By coating complex 1 onto indium tin oxide (ITO) electrodes, the turnover frequency for electrocatalytic water oxidation was 0.20 s−1 at 1.4 V (vs. Ag/AgCl, pH = 7), and it was 1010 s−1 for proton reduction at −1.0 V (vs. Ag/AgCl, pH = 0.5). The stability of 1 for catalytic oxygen evolution and hydrogen production was evaluated by electrochemical, UV-vis and mass measurements, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX), which confirmed that 1 was the real molecular catalyst. Titration and UV-vis experiments showed that the pyridine group on Co dissociated at the beginning of catalysis, which was critical to subsequent activation of water. A proton-coupled electron transfer process was involved based on the pH dependence of the water oxidation reaction catalyzed by 1. As for manganese corroles 2–6, although their oxidizing powers were comparable to that of 1, they were not as stable as 1 and underwent decomposition at the electrode. Density functional theory (DFT) calculations indicated that water oxidation by 1 was feasible through a proposed catalytic cycle. The formation of an O–O bond was suggested to be the rate-determining step, and the calculated activation barrier of 18.1 kcal mol−1 was in good agreement with that obtained from experiments.

Catalysts play very important roles in artificial photosynthesis for solar energy conversion. In this present study, two water-insoluble cobalt porphyrin complexes, cobalt(II) meso-tetraphenylporphyrin (CoP-1) and cobalt(II) 5,10,15,20-tetrakis-(4-bromophenyl)porphyrin (CoP-2), were synthesized and coated as thin films on the FTO working electrode. The films showed good activities for electrocatalytic water oxidation in aqueous solutions at pH 9.2. The Faradaic efficiencies of both films approached to ∼100%, measured using a fluorescence-based oxygen sensor. The turnover frequencies were close to 0.50 s−1 and 0.40 s−1 for CoP-1 and CoP-2, respectively, under an applied anodic potential of 1.3 V (vs. Ag/AgCl) at pH 9.2. Importantly, no cobalt oxide particles were observed on the working electrode after catalysis. The stability of the catalyst films was further evaluated by UV-vis spectroscopy, inhibition measurements, mass spectrometry, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The pH dependence of water oxidation on CoP-1 and CoP-2 suggested a proton-coupled electron transfer (PCET) mechanism. The catalyst films could be recycled and showed almost unchanged catalytic activities when they were reused in new electrocatalytic studies of water oxidation.

•Noble metal-free nickel oxide (NiOx) for efficient water oxidation.•Carbon nanotubes supported NiOx improves catalytic current density.•NiOx is facilely electrodeposited with porous, honeycomb-like structure.•High Faradaic efficiency.•Low overpotentials for water oxidation.Herein we report for the first time to use multi-walled carbon nanotubes (MWCNTs) supported porous nickel oxide (NiOx) as non-precious electrocatalysts for oxidation of water at low overpotentials. The nickel oxide catalyst was facilely electrodeposited on MWCNTs in a 0.1 M KBi buffered solution at pH 9.2 containing 0.1 mM Ni2+ with an applied anodic potential. The current density of bulk electrolysis is 2.2 mA/cm2 at +1.1 V (vs Ag/AgCl) using NiOx-MWCNTs as the working electrode at pH 9.2, which is much higher than that in a system containing no MWCNTs. Tafel plot indicates that the present NiOx-MWCNTs catalyst requires the overpotential of only 200 mV to catalyze the water oxidation reaction at pH 9.2. The Faradaic efficiency of >95% has been achieved at +1.1 V. The highly porous character of the NiOx catalyst materials were further studied by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray analysis (EDX).Highly porous NiOx was facilely electrodeposited on multi-walled carbon nanotubes for catalytic water oxidation at low overpotentials under the weak basic condition.

•Cobalt oxide (CoOx) as cocatalyst on TiO2/CdS for photocatalytic H2 production.•Improved visible light-driven H2 production from water at room temperature.•Cobalt oxide is earth-abundant materials for catalytic reactions.•CoOx-loaded TiO2/CdS composite shows good photocatalytic activity and stability.In this present paper, cobalt oxide (CoOx) is studied as an effective cocatalyst in a photocatalytic hydrogen production system. CoOx-loaded titanium dioxide/cadmium sulfide (TiO2/CdS) semiconductor composites were prepared by a simple solvothermal method and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS), and X-ray photoelectron spectroscopy (XPS). Photocatalytic hydrogen production was studied using the as-synthesized photocatalysts in aqueous solution containing sodium sulfide (Na2S)/sodium sulfite (Na2SO3) as hole scavengers under visible light irradiation (λ > 400 nm). The optimal cobalt content in CoOx-loaded TiO2/CdS composite is determined to be 2.1 wt% and the corresponding rate of hydrogen evolution is 660 μmol g−1 h−1, which is about 7 times higher than TiO2/CdS and CdS photocatalysts under the same condition. Visible light-driven photocurrents of the semiconductor composites were further measured on a photoelectrochemical electrode, revealing that the photocorrosion of CdS can be prevented due to the presence of TiO2–CoOx.Cobalt oxide (CoOx) is studied as an effective cocatalyst for photocatalytic hydrogen production on TiO2/CdS semiconductor heterojunction, which exhibits the rate of hydrogen evolution as high as 660 μmol g−1 h−1.

Herein we report the synthesis of two luminescent branched tetranuclear alkynylplatinum(II) complexes {[Pt(tBu3-tpy)]2[Ar(CC)4][Pt(tBu3-tpy)]}2(PF6)4 (I and II; tBu3-tpy = 4,4′,4″-tri-tertbutyl-2,2′:6′,2″-terpyridine; Ar = [(C6H4)2(CCCC)]). Two terminal alkyne ligands were synthesized by oxidative coupling in the air, and subsequently reacted with a chloroplatinum(II) terpyridine complex. Their photophysical properties have been studied by both steady-state spectroscopy and transient spectroscopy. The final two platinum(II) complexes were found to show long-lived excited states in solution at room temperature. The longest lifetime of ∼17 μs for the twisted branched molecule was observed in toluene at room temperature. Application of the two complexes as a light-harvesting chromophore in photocatalytic hydrogen production was further examined.

A series of nickel hydroxide-modified cadmium sulfide/reduced graphene oxide (Ni(OH)2-CdS/rGO) nanocomposites were synthesized and characterized. The photocatalytic activity of the as-prepared Ni(OH)2-CdS/rGO materials for hydrogen production from water under visible light irradiation (λ > 420 nm) was investigated. The results demonstrated that Ni(OH)2 is an efficient cocatalyst for photocatalytic hydrogen production and rGO can significantly enhance the rate of photocatalysis. The optimal Ni(OH)2 loading was found to be 1.0 wt %, giving a rate for hydrogen production of 4731 μmol·h–1·g–1, which is nearly 10 times higher than that of CdS/rGO photocatalyst under the same condition. This work demonstrated the synergetic effect of Ni(OH)2 and rGO to enhance catalytic activity for visible light-driven hydrogen production.

The series of luminescent bimetallic alkynylplatinum(II) terpyridyl complexes [(tBu3-tpy)PtC≡C(ArC≡C)nPt(tBu3-tpy)](PF6)2 (I–IV; tBu3-tpy = 4,4′,4″-tritertbutyl-2,2′:6′,2″-terpyridine, Ar = C6H2(n-hexyloxy)2-2,5), in which two platinum(II) centers are linked by p-phenylene ethynylene oligomers, were synthesized and characterized. Their electronic absorption and luminescent behaviors were studied by steady-state spectroscopy and transient spectroscopy. Concentration-dependent emission data show that all of the complexes form self-assembled aggregates in certain concentration ranges. Complex IV, bearing a longer p-phenylene ethynylene oligomer, exhibits higher propensity in comparison to I–III to form aggregates at a concentration of >1 × 10–5 M, probably resulting from stronger metal–metal interactions and π–π stacking. All four of the complexes displayed biexponential decays with lifetimes in the range of nanoseconds in dilute solutions.

Limited control over charge recombination between photogenerated charge carriers largely hinders the progress in photocatalysis. Here, we introduce metal nanoparticles (Cr, Ag) to the surface of MoS2 nanosheets by simple synthetic means creating a hybrid metal–MoS2 nanosheet system with well-defined metal/semiconductor interfaces. We demonstrate that this hybrid nanosheet structure is capable of decoupling light absorption, primarily in MoS2, and carrier separation, across the metal–MoS2 heterostructure leading to drastic quenching of recombination between photogenerated carriers in MoS2, as proven by absorptance, photoluminescence, and ultrafast pump-probe spectroscopy. The photocatalytic activity in the hybrid system is also improved, which further shows excellent stability against photocorrosion.Keywords: carrier recombination; hybrid nanosheets; interface; MoS2; photocatalysis; pump probe

Cobaloximes are usually used as molecular hydrogen evolution reaction (HER) catalysts. Herein we report for the first time the use of molecular cobaloximes as catalyst precursors for water oxidation when anodic potentials were applied. Highly active thin catalyst films were deposited at +1.5 V and +1.1 V (vs. Ag/AgCl) in 0.1 M borate buffer solution at pH = 9.2 containing 1 mM cobaloximes. Four catalyst films (CoOx-1–CoOx-4) were synthesized from four different cobaloximes. The current densities of CoOx-1 were up to ∼5.5 mA cm−2 and ∼2.6 mA cm−2 when the applied potentials were +1.5 V and +1.1 V, respectively, which were higher than the current densities of CoOx-2, CoOx-3 and CoOx-4 under the same conditions. Scanning electron microscopy (SEM) images reveal that the nanometer-sized particles of CoOx-1 possibly contribute to its high activity while the other three catalysts have micrometer-sized amorphous materials on the surface of FTO. X-ray photoelectron spectroscopy (XPS) data displayed the valence state of the cobalt element as Co(II) or Co(III) oxide species. The morphological stability of the CoOx-1 catalyst was further studied using SEM.

Nature utilizes solar energy to extract electrons and release protons from water, a process called photosynthetic water oxidation or oxygen evolution. This sunlight-driven reaction is vital to the planet because it directly produces dioxygen and couples with photosystem I to generate the reducing equivalents for the reduction of carbon dioxide to carbohydrates (also known as CO2 fixation). Inspired by this natural process, people are intensely interested in water splitting using sunlight to convert and store solar energy into chemical energy, which is believed to be able to ultimately solve the energy problem that we are facing. Water splitting can be separated into two half reactions, namely water oxidation and water reduction, and they can be studied individually. Catalysts are very helpful in both reactions. Recent progress in finding new highly efficient water oxidation catalysts (WOCs) has shed light on this complicated four-electron/four-proton reaction and made it possible to catalyze water oxidation using mononuclear metal complexes. This article focuses on molecular catalysts that are able to perform catalytic water oxidation at single metal sites. Different series of catalysts (or precatalysts) made of ruthenium, iridium and earth abundant elements (iron, cobalt, and manganese) that can be applied in chemical, electrochemical and photochemical (light-driven) water oxidation are summarized, and their catalytic mechanisms are discussed in detail. Finally, the future outlook and perspective to design and develop catalysts that are efficient, cheap and stable are presented.

Facile deposition of a water-splitting catalyst on low-cost electrode materials could be attractive for hydrogen production from water and solar energy conversion. Herein we describe fast electrodeposition of cobalt-based water oxidation catalyst (Co-WOC) on simple graphite electrode for water splitting. The deposition process is quite fast, which reaches a plateau in less than 75 min and the final current density is ∼1.8 mA/cm2 under the applied potential of 1.31 V at pH = 7.0. The scanning electron microscopy (SEM) study shows the formation of nanometer-sized particles (10–100 nm) on the surface of the electrode after only 2 min and micrometer-sized particles (2–5 μm) after 90 min of electrolysis. X-ray photoelectron spectroscopy (XPS) data demonstrate the as-synthesized ex-situ catalyst mainly contains Co2+ and Co3+ species incorporating a substantial amount of phosphate anions. These experiments suggest that cost-efficient cobalt oxide materials on graphite exhibit alluring ability for water splitting, which might provide a novel method to fabricate low-cost devices for electrochemical energy storage.Direct deposition of cobalt oxide based water oxidation catalyst on low-cost and tin-free graphite materials has been achieved. The assynthesized electrocatalyst shows current density of >3.0 mA/cm2 at about 600 mV overpotential with good durability for catalytic water splitting.Download full-size image

Developing an efficient, robust, and noble-metal-free catalyst for the oxygen reduction reaction (ORR) is crucial for the large-scale commercialization of fuel cells and metal–air batteries. Herein, we report a structurally well-defined iron porphyrin-based conjugated network on carbon nanotubes ((FeP)n-CNTs) as a novel electrocatalyst for the ORR. Its superior electrocatalytic activity toward the ORR is demonstrated by the high-performance catalytic activity of (FeP)n-CNTs with a positive ORR onset potential and half-wave potential (E1/2 ∼ 0.76 V vs. RHE) values as well as outstanding durability and methanol tolerance in alkaline media. In addition, the low H2O2 yield illustrates that the ORR occurs mainly via the direct four-electron (4e−) pathway, and testing shows that the small amount of produced H2O2 can be rapidly consumed through both electrochemical reduction and oxidation. Our results demonstrate that the as-prepared (FeP)n-CNT catalyst is a promising noble-metal-free catalyst for potential applications in fuel cells and metal–air batteries.

In this present study, a series of cobalt porphyrin-based conjugated mesoporous polymers (CoP-nph-CMP, n = 2, 3, 4) were fabricated as catalyst precursors to generate bifunctional catalysts via pyrolysis (CoP-nph-CMP-800, n = 2, 3, 4) for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Among these catalysts, CoP-2ph-CMP-800 exhibited the best catalytic activity with quite a low overpotential for both the OER (η = 370 mV for 10 mA cm−2) and the HER (η = 360 mV for 10 mA cm−2). Moreover, their excellent bifunctional catalytic performance was also explored in the overall water splitting test (η = 760 mV for 10 mA cm−2).

Photocatalytic splitting of water to hydrogen (H2) has attracted much attention because of its potential to address the concerns of air pollution and energy shortage. In this present study, we report that noble-metal-free cobalt nitride (Co3N) can be used as an efficient cocatalyst on CdS nanorods (CdS NRs) for photocatalytic H2 production in water under visible light irradiation. Photoluminescence (PL) spectra and photoelectrochemical measurements indicated that the loading of Co3N onto CdS NRs can facilitate the separation and transfer of photogenerated charge carriers, leading to an enhanced photocatalytic activity for H2 production. The H2 production rate reached ∼137.33 μmol h−1 mg−1 (λ > 420 nm) and the apparent quantum yield (AQY) was ∼14.9% at 450 nm.

Herein we report a facile and direct synthesis of porous NiCo2O4 nanowire arrays (NWAs) with robust mechanical adhesion to conductive electrodes by a simple two-step method. Upon complete pyrolysis of the cobalt–nickel-hydroxide precursor, high-quality crystalline NiCo2O4 is achieved. The porous NiCo2O4 nanowires were found to be highly active for catalytic water oxidation when serving as the working electrodes without any external materials (binder and/or carbon black), as evidenced by exhibiting higher catalytic current density for water oxidation compared to precious metal oxide catalysts such as iridium oxide (IrO2) under the same conditions and appreciable catalytic wave at ∼1.52 V (vs. RHE). The optimal performance of the as-synthesized NiCo2O4 nanowires showed a current density of 10 mA cm−2 under an overpotential of only 0.46 V and 20 mA cm−2 under an overpotential of 0.72 V, corresponding to a Faradaic efficiency of nearly 100%. The atomic-scale analysis of the NiCo2O4 nanowires was further conducted by spherical-aberration-corrected transmission electron microscopy (TEM). The highly exposed high-index facets and one-dimensional (1D) configuration of the as-synthesized porous NiCo2O4 nanowires may be responsible for the high catalytic performance of water oxidation, which exhibit excellent activity and unique advantages for catalytic water splitting.

An efficient photocatalytic system is highly demanded for the production of hydrogen fuel through water splitting. Herein, we report an artificial photocatalytic system made of low-cost materials for high-performance H2 production from water. The new system contains semiconductors (CdS nanorods) as the photosensitizer, a cobalt–salen complex as the H2 evolution cocatalyst, and Na2S and Na2SO3 as sacrificial electron donors. Under optimal conditions, the highest hydrogen evolution turnover number reached 64700 after 37 hours and the rate was 106 μmol h−1 mg−1, which is much higher than when using CdS NRs and also is among the best for photocatalytic systems using molecular cocatalysts for H2 production. The highest apparent quantum yield (AQY) was ∼29% at 420 nm. Steady state photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) decay spectra revealed that the system allows effective electron transfer from the excited CdS NRs to the cobalt–salen complex for highly efficient H2 production.

The generation of hydrogen (H2) through photocatalytic water splitting by employing various cocatalysts has attracted much attention. Herein we report for the first time that metallic molybdenum phosphide (MoP), as a highly active cocatalyst, can significantly enhance photocatalytic H2 production from water. A series of MoP/CdS nanorod (NR) hybrids were facilely prepared. The optimal amount of MoP led to a maximal H2 evolution rate of 163.2 μmol h−1 mg−1 under visible light illumination (λ > 420 nm), which is more than 20 times higher than that of freshly prepared CdS NRs. This work demonstrated that the suitable Fermi level alignment of MoP and CdS is responsible for the high photocatalytic activity of H2 production under visible light in the present system, as evidenced by both experimental and theoretical results.

Developing efficient photocatalysts made of earth-abundant elements for hydrogen (H2) production from water is considered to be a key pathway for future clean energy supply. Herein we report for the first time that p-type copper phosphide (Cu3P) can be an efficient promoter to improve photocatalytic H2 production from water when loaded on n-type cadmium sulphide nanorods (CdS NRs). The formation of a p–n junction in Cu3P/CdS NRs leads to fast charge transfer and enhanced photocatalytic activity under visible light irradiation. Under optimal conditions, the H2 evolution rate was as high as ∼200 μmol h−1 mg−1 (λ > 420 nm) and the apparent quantum yield at λ = 450 nm was ∼25% in water.

Highly efficient, visible-light-induced hydrogen (H2) production via water splitting can be achieved without the help of a cocatalyst by using a noble-metal-free core–shell photocatalyst, in which zinc sulfide (ZnS) nanoparticles as the protective shell are anchored on the surface of cadmium sulfide nanorods (CdS NRs). Due to the close interfacial contact of component semiconductors, the electronic structure of CdS is strongly coupled with that of ZnS nanoparticles, leading to efficient transfer of charge carriers between them and the improvement of the CdS photostability. The CdS/ZnS NR photocatalyst showed much higher catalytic activity for H2 production than CdS NRs and ZnS under visible light irradiation (λ > 420 nm), which is probably due to fast transfer of the photogenerated charge carriers and/or electron tunneling in the one-dimensional core–shell nanorod structure. Under optimal conditions, the highest hydrogen evolution rate reached 239 μmol h−1 mg−1, which is much greater than ZnS and CdS NRs and also among the best cocatalyst-free photocatalysts for H2 production. The average apparent quantum yield can be achieved as ∼16.8% after 8 h of irradiation (monochromatic light at 420 nm ± 5 nm). A possible mechanism for the photocatalytic reaction based on CdS/ZnS NRs is also discussed.

Photocatalytic hydrogen production using solar energy offers a clean and sustainable pathway for future energy supply. Herein, we report nickel nitride (Ni3N) as a novel cocatalyst on cadmium sulfide nanorod (CdS NR) semiconductors to enhance photocatalytic hydrogen production in water. The Ni3N cocatalyst was grown by a facile in situ growth method. Under optimal conditions, the hydrogen production rate can be improved more than 10 times by introducing an appropriate amount of Ni3N on CdS NRs. PL spectra and photoelectrochemical measurements suggest that faster interfacial charge transfer between Ni3N and CdS may be the key factor for the enhanced photocatalytic activity.

Photocatalytic hydrogen production via water splitting has attracted much attention for future clean energy application. Herein we report a noble-metal-free photocatalytic hydrogen production system containing a simple bidentate cobalt Schiff base complex as the molecular cocatalyst, CdS nanorods as the photosensitizer, and ascorbic acid as the electron donor. The system shows highly enhanced photocatalytic activity compared to pure CdS NRs under visible light (λ > 420 nm). Under optimal conditions, the turnover numbers (TONs) for hydrogen production reached ∼15200 after 12 hours of irradiation, and an apparent quantum yield of ∼27% was achieved at 420 nm monochromatic light. Steady-state photoluminescence (PL) spectra indicated efficient charge transfer between the excited CdS NRs and the cobalt cocatalyst for improved hydrogen production. Spectroscopic studies of the photocatalytic reaction revealed the reduction of the Co(II) complex to Co(I) species, which are probably active intermediates for hydrogen evolution. On the basis of the spectroscopic studies, we propose a reaction mechanism for hydrogen production in the present photocatalytic system.

We report one dimensional (1D) MoS2 nanosheet/porous TiO2 nanowire hybrid nanostructures synthesized by a simple hydrothermal method, leading to an enhanced specific surface area (66 m2 g−1). These 1D hybrid nanostructures as co-catalysts exhibit high activity in visible light photocatalytic hydrogen evolution reaction (HER) with an enhanced hydrogen generation rate of 16.7 mmol h−1 g−1.

We report here a Cu(0)-based catalyst free of noble metals for the electrocatalytic hydrogen evolution reaction in neutral water with an onset overpotential of only 70 mV. This is the lowest reported value among Cu(0)-based catalysts for the hydrogen evolution reaction in neutral water.

Developing efficient water oxidation catalysts made up of earth-abundant elements has attracted much attention as a step toward for future clean energy production. Herein we report a simple one-step method to generate a low cost copper oxide catalyst film in situ from a copper(II) ethylenediamine complex. The resulting catalyst has excellent activity toward the oxygen evolution reaction in alkaline solutions. A catalytic current density of 1.0 mA cm−2 and 10 mA cm−2 for the catalyst film requires the overpotentials of only ∼370 mV and ∼475 mV in 1.0 M KOH, respectively. This catalytic performance shows that the new catalyst is one of the best Cu-based heterogeneous OER catalysts to date.

For the first time, ferrous phosphide (Fe2P) is used as an active noble metal-free cocatalyst for photocatalytic H2 production under visible light in water. The rate of H2 production can be enhanced by more than 30 times through loading of Fe2P on CdS nanorod surfaces. Efficient charge transfer between CdS and Fe2P might be the key factor for the high photocatalytic activity.

Porous nickel electrodes were fabricated by the phase inversion tape casting method. The as-prepared nickel electrode contained finger-like straight open pores with an average diameter of ∼100 μm and smaller pores with an average diameter of 1–3 μm in the walls. When used as an anode for the oxygen evolution reaction (OER), the porous nickel electrode shows high catalytic activity, reaching a catalytic current density of 10 mA cm−2 under an overpotential of only 300 mV in 1.0 M KOH. It also exhibited excellent performance for the hydrogen evolution reaction (HER), resulting in a current density of 10 mA cm−2 under an overpotential of 125 mV in the same electrolyte. For both OER and HER, the electrode shows great catalytic stability and much better catalytic performance than commercial nickel foam. Moreover, an alkaline electrolyzer using identical porous nickel electrodes as both the anode and cathode required a cell voltage of only 1.65 V to reach 10 mA cm−2 for overall water splitting. The improved electrocatalytic performance of the electrode can be attributed to its unique dual-pore structure.

For the first time, noble-metal-free nickel phosphide (Ni2P) was used as an excellent catalyst precursor for water oxidation catalysis. The lowest onset potential was observed at ∼1.54 V (vs. RHE) and a Tafel slope of 60 mV dec−1 was obtained in alkaline solution (pH = 13.6).

The design and preparation of highly active catalysts for the hydrogen evolution reaction (HER) is very important for water splitting. Herein, we report a highly active HER catalyst, which is synthesized by loading nanostructured nickel phosphide (Ni2P) on three-dimensional few-layer graphene/nickel foam (G@NF). G@NF was successfully prepared by a chemical vapor deposition process in the presence of methane at high temperature. Compared with nickel phosphide, G@NF, as well as commercial platinum, the Ni2P–G@NF catalyst exhibited very high activity in electrocatalytic H2 production from water (∼7 mV overpotential in alkaline solutions, pH ∼ 14; and ∼30 mV overpotential in acidic solutions, pH ∼ 0). The high catalytic activity of Ni2P–G@NF is attributed to the excellent performance of Ni2P, the large 3D framework which facilitates proton accessibility and electron transfer, and the high surface area.

Efficient hydrogen (H2) production is considered to be a key pathway for future clean energy supply. Herein we report that a photocatalyst made of core–shell amorphous cobalt phosphide (CoPx) integrated with cadmium sulfide nanorods (CdS NRs) gives exceptional performance of photocatalytic H2 production under visible light. Under optimal conditions, the CoPx/CdS NRs photocatalyst allows an H2 evolution rate of ∼500 μmol h−1 mg−1 based on the photocatalyst (λ > 420 nm) and the apparent quantum yield was ∼35% in aqueous solutions (λ = 450 nm). The turnover numbers (TONs) reached ∼630000 per mole of cobalt in 70 hours with a TOF of ∼9000 h−1. Such high performance of an artificial photosynthetic H2 production system using a cobalt-based cocatalyst has, to the best of our knowledge, not been reported to date.

Developing highly active electrocatalysts for the hydrogen evolution reaction (HER) is crucial to construct an efficient water-splitting device. In this present study, a novel ternary cobalt–nickel phosphide nanosheet with nanowire edges on 3D nickel foam (CoNiP@NF) was first synthesized and used as an excellent cathode for the HER over a wide pH range from 0 to 14. The cathode showed high-performance catalytic activity to produce hydrogen in aqueous solution, with quite low overpotentials of only 60 mV (0.5 M H2SO4, pH ∼ 0.26), 120 mV (1.0 M KPi, pH ∼ 7), and 155 mV (1.0 M KOH, pH ∼ 14) to reach a current density of 10 mA cm−2. This high performance is probably due to the combination of the advantages of both one-dimensional nanowire and two-dimensional nanosheet materials, which can effectively improve the electrochemical performance for the HER. To the best of our knowledge, the present ternary CoNiP material is among the best noble-metal-free electrocatalysts for hydrogen evolution in water.

Six cobalt and manganese corrole complexes were synthesized and examined as single-site catalysts for water splitting. The simple cobalt corrole [Co(tpfc)(py)2] (1, tpfc = 5,10,15-tris(pentafluorophenyl)corrole, py = pyridine) catalyzed both water oxidation and proton reduction efficiently. By coating complex 1 onto indium tin oxide (ITO) electrodes, the turnover frequency for electrocatalytic water oxidation was 0.20 s−1 at 1.4 V (vs. Ag/AgCl, pH = 7), and it was 1010 s−1 for proton reduction at −1.0 V (vs. Ag/AgCl, pH = 0.5). The stability of 1 for catalytic oxygen evolution and hydrogen production was evaluated by electrochemical, UV-vis and mass measurements, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX), which confirmed that 1 was the real molecular catalyst. Titration and UV-vis experiments showed that the pyridine group on Co dissociated at the beginning of catalysis, which was critical to subsequent activation of water. A proton-coupled electron transfer process was involved based on the pH dependence of the water oxidation reaction catalyzed by 1. As for manganese corroles 2–6, although their oxidizing powers were comparable to that of 1, they were not as stable as 1 and underwent decomposition at the electrode. Density functional theory (DFT) calculations indicated that water oxidation by 1 was feasible through a proposed catalytic cycle. The formation of an O–O bond was suggested to be the rate-determining step, and the calculated activation barrier of 18.1 kcal mol−1 was in good agreement with that obtained from experiments.

Cobaloximes are usually used as molecular hydrogen evolution reaction (HER) catalysts. Herein we report for the first time the use of molecular cobaloximes as catalyst precursors for water oxidation when anodic potentials were applied. Highly active thin catalyst films were deposited at +1.5 V and +1.1 V (vs. Ag/AgCl) in 0.1 M borate buffer solution at pH = 9.2 containing 1 mM cobaloximes. Four catalyst films (CoOx-1–CoOx-4) were synthesized from four different cobaloximes. The current densities of CoOx-1 were up to ∼5.5 mA cm−2 and ∼2.6 mA cm−2 when the applied potentials were +1.5 V and +1.1 V, respectively, which were higher than the current densities of CoOx-2, CoOx-3 and CoOx-4 under the same conditions. Scanning electron microscopy (SEM) images reveal that the nanometer-sized particles of CoOx-1 possibly contribute to its high activity while the other three catalysts have micrometer-sized amorphous materials on the surface of FTO. X-ray photoelectron spectroscopy (XPS) data displayed the valence state of the cobalt element as Co(II) or Co(III) oxide species. The morphological stability of the CoOx-1 catalyst was further studied using SEM.