Effect of chlorine-repulsive molecular fragments on photocatalytic seawater splitting performance of BiVO4 for oxygen evolution

Photocatalysis to produce clean energy by splitting seawater have great practical importance for dealing with the energy crisis. However, seawater contains many Cl^? ions whose oxidation competes with the oxygen evolution reaction. In this work, we present a photocatalyst modified with S-containing molecular fragments on the surface to improve its efficiency in the oxygen evolution reaction in seawater splitting. We found that the oxygen evolution performance of BiVO₄ modified with S-containing molecular fragments was 1.7 times higher than the unmodified material. Based on this finding, the modification didn't affect the light absorption and charge separation efficiency of BiVO₄, but lead to a marked (83.6%) decrease of the effective chlorine concentration in the reactor after photocatalytic reaction. The results indicate that the surface modification with S-containing molecular fragments is an effective method to repel chlorine. This work provides a useful reference to improve the efficiency of photocatalysts in overall seawater splitting.     

Novel twin reactor for separate evolution of hydrogen and oxygen in photocatalytic water splitting

Photocatalytic water splitting with separate H₂ and O₂ evolution is crucial because it eliminates the explosion potential and hydrogen-purification cost. A novel twin reactor was designed to separate the evolution of hydrogen and oxygen in photocatalytic water splitting under visible light. A modified Nafion membrane was employed to segregate the two photocatalysts in the twin reactor so that hydrogen and oxygen can be evolved separately. Conventional Z-scheme catalysts, Pt/SrTiO₃:Rh and WO₃, were used as hydrogen-photocatalyst and oxygen-photocatalyst, respectively. Fe²⁺ and Fe³⁺ were added in the reaction solution as electron-transfer mediator. The ratio of evolved H₂ and O₂ was in agreement with the stoichiometric ratio (2:1) of hydrogen and oxygen of water. An average hydrogen generation rate of 1.59 μmol/g-h was achieved in the twin-reactor system, which was twice as much as that in the conventional Z-scheme system. The improved H₂ yield was due to the prevention of the water-splitting backward reaction in the twin reactor.     

A novel dual-bed photocatalytic water splitting system for hydrogen production

A novel dual-bed system was designed to produce hydrogen through photocatalytic water splitting. The system was comprised of a photocatalytic reaction bed and a regeneration bed. Aqueous KI solution and Pt-loaded TiO₂ constituted the photocatalytic reaction bed where hydrogen was produced; meanwhile the hole scavenger iodide ion was oxidized into I_2. The effluent containing I₂ from the photocatalytic bed entered the regeneration bed and passed through a Cu₂O layer where I₂ was reduced to I⁻. The regeneration bed effluent was then recycled to the photocatalytic reaction bed. Since the hole scavenger KI in the photocatalytic bed was constantly kept at a high level through the continuous reduction of I₂ in the regeneration bed, steady production of hydrogen was achieved in the dual-bed system for a much longer period as compared to a single-bed system without regeneration.     

Monometallic,bimetallic, and trimetallic chalcogenide-based electrodes for electrocatalytic hydrogen evolution reaction

Cu₂CoSnS₄, Cu₂SnS₃, Cu₂CoS₄, Co₂SnS₃, Cu₂S, CoS₂, and SnS₂ were synthesized using a one-step solvent-free solid-phase approach. The surface structure, morphology, and composition were characterized using an X-ray diffractometer (XRD), Fourier-Transform Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and X-ray Photoelectron Spectroscopy (XPS). The characterizations reveal pure phase formation and porous morphology. Further, the Hydrogen evolution reaction was performed using Cu₂CoSnS₄, Cu₂SnS₃, Cu₂CoS₄, Co₂SnS₃, Cu₂S, CoS₂, and SnS₂-based electrodes. Amid all electrocatalysts, Cu₂CoSnS₄ shows an excellent hydrogen evolution reaction with a low overpotential of ?192.1 mV at ?10 mA/cm² in 0.5 M H₂SO₄. And higher current density. Cu₂CoSnS₄ also shows a lower Tafel slope of 98.6 mV/dec and charge transfer resistance than mono and bimetallic chalcogenide-based electrodes. The Cu₂CoSnS₄ exhibit very good stability for ～22 h at ?10 mA/cm² current density in 0.5 M H₂SO₄.     

Preparation and photocatalytic activity of PANI-CdS composites for hydrogen evolution

In this paper, a series of PANI-CdS (PANI is the abbreviation of Polyaniline) composites were synthesized by changing the mass of PANI (molar ratio of PANI and CdS : 0.5, 0.7, 1, 1.5, 2). The PANI-CdS composites were characterized by XRD patterns, IR spectra and elemental analysis. The activities of as-prepared composites were also tested with regard to hydrogen evolution. It was found that PANI-CdS composites showed good activity for hydrogen evolution. With increment of PANI, the activity for hydrogen evolution of PANI-CdS reduced. But compared with pure CdS, PANI-CdS had a much higher activity for hydrogen evolution.     

CuInS2-based photocatalysts for photocatalytic hydrogen evolution via water splitting

The realization of efficient photocatalytic hydrogen evolution (PHE) significantly depends on the development of durable and effective semiconductor photocatalysts. Copper indium sulfide (CuInS₂) is an emerging ternary chalcogenide semiconductor material for solar-to-chemical energy application, because it possesses a suitable bandgap, environment-friendly elements, and a low melting point. CuInS₂-based semiconductor photocatalysts have been investigated for PHE via water splitting, but current PHE performance still has difficulty in meeting commercial application requirements and needs to be further improved. In this review, the basic semiconductor properties of CuInS₂, including its crystal and band structures, are introduced, and its PHE mechanism is discussed in detail. The PHE performance of CuInS₂-based photocatalysts is systematically discussed, with a focus on morphology, engineered structure, and heterojunction construction. Finally, issues and challenges currently encountered in the PHE application of CuInS₂-based photocatalysts and their possible solutions are presented.     

Polyoxometalate-based catalysts for photocatalytic,chemical catalytic and electrocatalytic water oxidation

Complex [Fe₁₁ (H₂O)₁₄(OH)₂(W₃O₁₀)₂(α-SbW₉O₃₃)₆]^27?(2) has been studied independently under photocatalytic or chemical catalytic or electrocatalytic conditions by our group. (1) Under the optimal photocatalytic conditions (photoirradiation at λ = 420 nm, [Ru(bpy)₃](ClO₄)₂ as the photosensor, Na₂S₂O₈ as the oxidant in borate buffer (pH = 10.0)], the turnover number (TON) can reach as high as 1815, the initial quantum yield and the initial turnover frequency (TOF) for the first 60 s was 47% and 6.3 s^?1, respectively. (2) We report herein that 2 are highly active homogeneous water oxidation catalysts when [Ru(bpy)₃]³⁺ is used as the terminal oxidant, with a turnover number (TON) of higher than 78. (3) The current densities achieved during cyclic voltammetry (CV) show that 2 exhibit no electrocatalytic activity at pH 10.0. This study discusses the catalytic behaviors and mechanism of 2 in different experimental conditions (driving forces) for water oxidation.     

Increased photocatalytic hydrogen evolution and stability over nano-sheet g-C3N4 hybridized CdS core@shell structure

A novel visible-light-active CdS@g-C₃N₄ photocatalyst was synthesized via a chemisorption method. This core@shell structure catalyst exhibited enhanced photocatalytic H₂ production activity under visible-light (λ ≥ 420 nm) irradiation. The nano-sheet g-C₃N₄ was successfully coated on CdS nanoparticles with intimate contact. When the content of g-C₃N₄ in the hybridized composite is 3 wt. %, the hydrogen-production rate of the CdS@g-C₃N₄ is 2.5 and 2.2 times faster than pure CdS and bulk g-C₃N₄, respectively. Superior stability was also observed in the cyclic runs. The improvement in stability and activity result from the ability of the π-conjugated g-C₃N₄ material in transporting photo-induced holes. The core@shell structure promoted separation of the photo-generated electron-hole pair and accelerated the emigration speed of the hole from the valence band of CdS. This effect also results in a greatly improved amount of hydrogen production. The possible mechanism for the photocatalytic activity and stability of CdS@g-C₃N₄ are tentatively proposed.     

Three-dimensional structure of WS2/graphene/Ni as a binder-free electrocatalytic electrode for highly effective and stable hydrogen evolution reaction

Tungsten disulfide (WS₂) has attracted much attention as the promising electrocatalyst for hydrogen evolution reaction (HER). Herein, the three-dimensional (3D) structure electrode composed of WS₂ and graphene/Ni foam has been demonstrated as the binder-free electrode for highly effective and stable HER. The overpotential of 3D WS₂/graphene/Ni is 87 mV at 10 mA cm^?2, and the current density is 119.1 mA cm^?2 at 250 mV overpotential, indicating very high HER activity. Moreover, the current density of 3D WS₂/graphene/Ni at 250 mV only decreases from 119.1 to 110.1 mA cm^?2 even after 3000 cycles, indicating a good stability. The high HER performance of 3D WS₂/graphene/Ni binder-free electrode is superior than mostly previously reported WS₂-based catalysts, which is attributed to the unique graphene-based porous and conductive 3D structure, the high loading of WS₂ catalysts and the robust contact between WS₂ and 3D graphene/Ni backbones. This work is expected to be beneficial to the fundamental understanding of both the electrocatalytic mechanisms and, more significantly, the potential applications in hydrogen economy for WS₂.     

In situ growth of nickel phosphide nanoparticles on inner wall of graphitic carbon nitride tubes for efficient photocatalytic hydrogen evolution

A facile two-step approach is employed to prepare novel Ni₂P@CNT hybrid photocatalyst, which is assembled by nickel phosphide (Ni₂P) nanoparticles on the inner wall of graphitic carbon nitride tube (CNT). This unique microstructure endows Ni₂P@CNT with close interfacial interaction, promotes efficient separation of photoexcited charge carriers and provides enriched sites for photocatalytic reaction. Moreover, the hybrid system is found to exhibit more superior photocatalytic hydrogen evolution activity than pure CNT and Pt-decorated CNT (Pt@CNT). As a consequence, the work illustrates the essential role of experimental process on the final morphology and performance, which is expected to pave a new method to construct various kind of excellent photocatalyst.     

Guanidine carbonate assisted supramolecular self-assembly for synthesizing porous g-C3N4 for enhanced photocatalytic hydrogen evolution

Graphitic carbon nitride (g-C₃N₄) is taken as one of the most promising polymer semiconductor photocatalysts for energy conversion. However, the photocatalytic activity of g-C₃N₄ is usually impeded by the low light absorption and fast recombination of photogenerated carriers. Herein, three-dimensional porous g-C₃N₄ with controllable morphology are synthesized by thermal polycondensation of supramolecular preorganization assembly of melamine, cyanuric acid and guanidine carbonate (1:1:x, x means the ratio of guanidine carbonate). By adjusting the amount of guanidine carbonate in the assembly, the precursors’ morphology can be changed from microrods to polyhedrons, which affects the g-C₃N₄ structure accordingly. The optimized hollow porous polyhedral g-C₃N₄ shows the enhanced light absorption and improved photogenerated carriers separation efficiency, thus exhibiting a 7.7-fold hydrogen evolution activity and 9-fold apparent quantum efficiency (AQE) higher than microtube without addition of guanidine carbonate. This work paves a complementary way towards synthesizing highly efficient photocatalysts through the guanidine carbonate-assisted supramolecular assembly.     

Chlorine-free alkaline seawater electrolysis for hydrogen production

A new process for chlorine-free seawater electrolysis is proposed in this study. The first step of the process is separation of Mg²⁺ and Ca²⁺ ions from seawater by nanofiltration. Next, the NF permeate is dosed into the electrochemical system. There it is completely split into hydrogen and oxygen gases and NaCl precipitate. The electrochemical system comprises an electrochemical cell operated at elevated temperatures (e.g. ≥ 50 °C) and a settling tank filled with aqueous NaOH solution (20–40 %wt) that operates at lower temperatures (e.g. 20–30 °C). High concentration of hydroxide ions in the electrolyzed solution prevents anodic chlorine evolution, while the accumulated NaCl precipitates in the settling tank. Batch electrolysis tests, performed in NaCl-saturated NaOH solutions, showed absolutely no chlorine formation on Ni200 and Ti/IrO₂RuO₂TiO₂ anodes at [NaOH] > 100 g/kgH₂O. Three long-term operations (9, 12 and 30 days) of the electrochemical system showed no Cl₂ or chlorate (ClO₃^?) production on both electrodes operated at current densities of 93–467 mA/cm². The Ni200 anode was corroded in the continuous operation that resulted in formation of nickel oxide on the anode surface. On the other hand, the system was successfully operated at 467 mA/cm² with Ti/IrO₂RuO₂TiO₂ electrodes in NaCl-saturated solution of NaOH (30 %wt) for 12 days. During this period no formation of Cl₂ and ClO₃^? has been observed and precipitation of NaCl occurred only in the settling tank. The performance of the system was stable during the operation as indicated by the insignificant fluctuations in the applied cell potentials and measured constant concentrations of NaOH_(aq) and NaCl_(aq) in the electrolyte solution. During 12 days of operation at ≈ 470 mA/cm² about 1.2 m³ of H₂ and ≈150 g of solid NaCl were produced in the system. Electrical energy demand of the electrolysis cell was 5.6–6.7 kWh/m³H₂ for the current density range of 187–467 mA/cm².     

Recent advances in cobalt disulfide for electrochemical hydrogen evolution reaction

Hydrogen production by water electrolysis is the most promising green hydrogen supply method in the future. Electrocatalytic hydrogen evolution reaction (HER), an essential step in water electrolysis, has received continuous interest for a long time. Noble metal-based electrocatalysts exhibit excellent performance for HER, while their high price, limited reserves, and insufficient durability limit their large-scale applications. Transition metal sulfides (TMSs) have been extensively studied as potential alternative catalysts, among which cobalt disulfide (CoS₂) stands out due to its unique structure, low price, and good electrical conductivity. Although remarkable progress has been made, the catalytic activity and stability of CoS₂ electrode materials themselves are still insufficient for large-scale industrial applications, so effective improvement of the HER catalytic performance of CoS₂ remains the focus of research. In this review, we briefly outline the reaction mechanism of HER, focusing on strategies to improve the catalytic performance of CoS₂, including morphology engineering, carbon materials combination, heteroatom doping, and heterostructure construction. Furthermore, the key challenges and opportunities for CoS₂ electrode materials as an electrocatalytic material for HER are discussed.     

A hydrothermal route for constructing reduced graphene oxide/TiO2 nanocomposites: Enhanced photocatalytic activity for hydrogen evolution

A series of reduced graphene oxide/TiO₂ (RGO/TiO₂) nanowire microsphere composites were synthesized with a facile one-step hydrothermal method using TiCl₃ and graphene oxide (GO) as the starting materials, during which the formation of TiO₂ and the reduction of GO occur simultaneously. The obtained nanocomposites were characterized with X-ray diffraction, field emission scanning electron microscope, transmission electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy, respectively. UV–vis absorption spectra showed that the absorption edges of TiO₂ were extended into visible light region with the addition of RGO. The photocatalytic activities of the samples with and without Pt as cocatalysts were evaluated by hydrogen evolution from water photo-splitting under UV–vis light illumination. Enhanced photocatalytic properties were observed for the as-prepared RGO/TiO₂ nanocomposites. The amount of hydrogen evolution from the optimized photocatalyst reached to 43.8 μmol h⁻¹, which was about 1.6 times as high as that of bare TiO₂. The results shown here indicate a convenient and applicable approach to further exploitation of high activity materials for photocatalytic water splitting applications.     

A ZIF-67-derived lamellar CoP@C cocatalyst for promoting photocatalytic hydrogen evolution from water

Photocatalytic hydrogen evolution from water is one of the top issues to achieve green hydrogen energy and utilize solar energy. Construction of cocatalyst is a major part for efficient photocatalysts. Lamellar flower-like CoP@C cocatalyst is synthesized via the phosphating of cobalt precursor derived from metal-organic framework ZIF-67. Different from usual phosphating of ZIF-67 directly, a typical solvothermal treatment of ZIF-67 contributes to tuning the formation of C nanodots on the lamellar CoP. CoP@C as cocatalyst exhibits a remarkable role of improving photocatalytic activity for hydrogen evolution. CoP@C/CdS composite shows a photocatalytic hydrogen evolution rate of 164.4 mmol g^?1 h^?1, which is much higher than those of pure CdS and other CoP/CdS photocatalysts. The heterojunction and interaction are verified between CoP@C and CdS. Light absorption and photoelectric properties of CoP@C/CdS are enhanced accompanying with strong reduction ability. A type-Ⅱ transfer path of photoelectrons is underway in CoP@C/CdS photocatalyst, accelerating the separation of electron-hole pairs and the transfer of carriers, and further resulting in the promoted photocatalytic performance. This work provides a suitable way to achieve carbon nanodots involved metal compound cocatalysts for efficient hydrogen production.     

Enhancing the electrocatalytic water splitting efficiency for amorphous MoSx

Amorphous molybdenum sulfide (MoS_x) materials have been considered as cheap and promising catalysts for hydrogen evolution reaction (HER). In this contribution, we report that the amorphous MoS_x catalysts prepared by the low temperature thermolysis of the (NH₄)₂MoS₄ precursors on carbon clothes (catalyst loading: 3.2 mg/cm²) exhibit a Tefal slope of 50.5 mV/dec and a high exchange current density of 1.5 × 10⁻³ mA/cm² in 0.5 M H₂SO₄ solutions. Spectroscopic studies of the amorphous MoS_x catalysts show that the increase of HER efficiency is positively correlated to the concentration of S₂²⁻ species, providing strong evidence to support the argument that S₂²⁻ is an active species for electrocatalytic HER. Additionally, the method for preparing catalysts is simple, scalable and applicable for large-scale production.     

The enhanced electrocatalytic activity and stability of NiW films electrodeposited under super gravity field for hydrogen evolution reaction

NiW films as cathode materials for hydrogen evolution reaction (HER) were electrodeposited under various gravity conditions. The morphologies of NiW films were characterized by SEM. Tafel curves and electrochemical impedance spectroscopy (EIS) were determined to study the electrocatalytic activity and stability of NiW films for HER in 10% NaOH solution. The results indicated that NiW films electrodeposited under normal gravity condition consisted of cellular grains and microcracks. Both surface morphologies and electrocatalytic activities of NiW films were not improved obviously by adjusting solution composition and current density. However, NiW films with fine grains and fewer microcracks were electrodeposited under super gravity field. The electrocatalytic activities of NiW films for HER increased with gravity coefficient (G) value. Meanwhile, NiW films showed excellent stability. During shut-down electrolysis, corrosion resistances of NiW film electrodeposited under G value of 256 were always higher than those of NiW film electrodeposited under normal gravity condition.     

Facile hydrothermal synthesis of combined MoSe2PS nanostructures on nickel foam with superior electrocatalytic properties for hydrogen evolution reaction

Producing an efficient and inexpensive electrocatalyst for use in the water electrolysis process is the most efficient and logical way to industrialize this method to produce hydrogen as a clean and alternative fuel for fossil fuels. In this study, combined and unique MoSe₂PS nanostructures are synthesized on nickel foam by three steps hydrothermal process. Microstructural observations reveal the unique morphology of the petals covered by the elongated nano-blades. A high electrocatalytic performance is attained with this nanostructure in hydrogen evolution reaction, so that the 90 mV overpotential is achieved at a current density of ?10 mA/cm². The near-platinum activity is due to the unique and combined nanostructure due to the synergistic properties of S and P on MoSe₂ as well as the high electrochemical active sites in the specimen. Additionally, excellent stability of the synthesized electrocatalyst is observed in the alkaline medium for 30 h, which confirms its potential application in relevant industries such as fuel cells and transportation.     

Efficient electrocatalytic and photocatalytic hydrogen evolution using a linear trimeric thiolato complex of nickel

Increasing interest has been paid to the development of earth-abundant metal complexes as promising surrogates of noble-metal platinum and biological catalysts hydrogenases for catalyzing the hydrogen evolution reaction. In this study, we report on a molecular H₂-evolving catalyst based on a linear trimeric thiolato complex of nickel Ni₃(L^N2S2)₂ (L^N2S2 = N,N′-dimethyl-N-N′-bis(2-mecaptoethyl)-ethylenediaminato). Electrochemical studies showed that the trinuclear nickel complex Ni₃(L^N2S2)₂ can electrocatalyze hydrogen evolution from weakly acidic solutions with remarkable turnover frequencies (3495 s^?1 at ?1.98 V and 715 s^?1 at ?1.58 V vs SCE). An efficient noble-metal-free homogeneous photocatalytic system for hydrogen generation from water working under visible light irradiation was further constructed by using the target nickel complex as photocatalyst, fluorescein (Fl) as photosensitizer (PS), and triethylamine (TEA) as sacrificial electron donor. Our studies showed that Ni₃(L^N2S2)₂ can be used in purely aqueous solution and gave a turnover number (TON, vs catalyst) for H₂ evolution of 790, corresponding to a TOF 60 h^?1. The results show that multinuclear nickel(II) complexes are a promising new direction for molecular catalysts for the electro- and photoreduction of protons.     

Comparative study on MoS2 and WS2 for electrocatalytic water splitting

Replacing Pt by earth abundant catalysts is one of the most important tasks toward potential large-scale HER applications. Among many potential candidates, low cost and earth abundant transition metal dichalcogenides such as MoS₂ and WS₂ have been promising as good H₂ evolution electrocatalysts when they are engineered into the structures with active sites. In this work, we have performed systematic studies on the catalytic reactivity of both MoS₂ and WS₂ materials produced by one-step and scalable thermolysis from (NH₄)₂WS₄ and (NH₄)₂MoS₄ precursors respectively. Structural analysis shows that these materials prepared at a higher thermolysis temperature exhibit higher crystallinity. The H₂ evolution electrocatalysts efficiency for the MoS₂ prepared at a lower temperature is higher than those at higher temperatures, where amorphous MoS₂ or S₂²⁻${{\text{S}}_2}^{2-}$ species instead of crystalline MoS₂ is the main active site. By contrast, crystalline WS₂ prepared at high temperature is identified to be the key reaction site. Both catalysts display excellent efficiency and durability as an electrocatalyst operating in acidic electrolytes. This work provides fundamental insights for further design and preparation of emergent metal dichalcogenide catalysts, beneficial for the development in clean energy.