Mechanism Of H2 Evolution Research Articles

Novel noble-metal-free NiCo2O4/CdIn2S4 S-scheme heterojunction photocatalyst with redox center for highly efficient photocatalytic H2 evolution

The exploration of efficient and noble-metal-free photocatalysts for photocatalytic H2 production is still a hot topic. Herein, a novel NiCo2O4/CdIn2S4 S-scheme heterojunction composites were successfully synthesized. Morphology characterization manifests that CdIn2S4 nanoparticles are tightly bound to the rod NiCo2O4 to form S-scheme heterojunction. Besides, X-ray photoelectron spectroscopy (XPS) results verify the existence of co-existing cobalt and nickel ions with multivalent state, which could provide H2 evolution active site and form redox centers. The S-scheme heterojunction and redox centers prompt the transfer of photogenerated e− from CdIn2S4 to NiCo2O4 for improving carrier separation efficiency, which is confirmed by a series of electrochemical experiments and photoluminescence (PL). The specific surface area and transmission electron microscope (TEM) results show that NiCo2O4 possesses a high specific surface area and porous structure, which could provide more active sites and facilitate H2O adsorption for further reduction reaction. The optimal NiCo2O4/CdIn2S4 composites possess maximum H2 evolution rate of 624 μmol·g−1·h−1, which is about 3.2 times than CdIn2S4-1 %Pt (198 μmol·g−1·h−1). After three cycles, the activity of NiCo2O4/CdIn2S4 shows no significant weaken. Moreover, the possible H2 evolution mechanism of NiCo2O4/CdIn2S4 S-scheme heterojunction composites is discussed. This research offers an idea for bimetallic oxide (NiCo2O4 etc.) modified sulfide to improve photocatalytic hydrogen production.

One step co-deposition of Ni0.96S and Ag2S on TiO2 nanosheets for enhanced photocatalytic H2 generation with superior stability

Introduction of co-catalysts to construct heterojunctions is one of the important strategies to improve the photocatalytic hydrogen production activity of semiconductor catalysts. Here, Ni0.96S and Ag2S co-decorated TiO2 composites were prepared by a simple one-step solvothermal method, whose photocatalytic H2 production activities were evaluated and related H2 evolution mechanism was explored. The 5%Ni0.96S-8%Ag2S/TiO2 ternary composite exhibited much enhanced photocatalytic activity with the photocatalytic hydrogen evolution rate (HER) of 23.67 mmol·h−1·g−1, which is 131.5 fold higher than that of the pure TiO2. Even higher HER, 29.45 mmol·h−1·g−1, was obtained in the presence of 40 mL glycerol. The photocatalytic HERs of the 5%Ni0.96S-8%Ag2S/TiO2 catalyst under visible light (λ > 420 nm) and simulated solar light (AM1.5) were 353.78 and 935.24 µmol·h−1·g−1, respectively. The apparent quantum efficiency (365 nm) is as high as 13.9%. Remarkably, the catalyst exhibited excellent photocatalytic cycling stability and durability, beneficial to practice applications. The improved photocatalytic activity of the composite can be attributed to much improved charge carrier transfer/separation, optimized energy band structure and enhanced visible light absorption caused by synergistic effect of Ni0.96S and Ag2S.

Fabrication of CZS NRs/NiSx NPs hybrids with abundant S-vacancies for signally promoting photocatalytic hydrogen production

The cocatalysts play a key role in photocatalytic technology in remarkably improving the efficiency of H2 evolution. Herein, S vacancies-rich Cd0.6Zn0.4S nanorods (CZS NRs) were prepared via solvothermal method. Then, as-obtained CZS NRs were modified with Ni–Co PBA-derived NiSx nanoparticles (NPs) cocatalyst using grinding method. The optimized CZS/NiSx-N-1.5 hybrids (CZS/NiSx-N, where N represents nickel nitrate which was used as nickel source) exhibited outstanding stability and activity, the average rate of H2 was up to 192.82 mmolg−1h−1 in deionized water medium, which was 201.4% higher than that of CZS. Besides, NiSx-A and NiSx-C cocatalysts with different particle sizes were obtained using nickel acetate (A) and nickel chloride (C) as nickel source, respectively. The CZS/NiSx-C-1.5 and CZS/NiSx-A-1.5 exhibited the H2 evolution rate of 180.70 and 201.93 mmolg−1h−1 in deionized water medium, respectively. The as-prepared materials also exhibited the excellent H2 evolution performance in natural seawater medium. Besides, the photocatalytic mechanism of H2 evolution over CZS NRs/NiSs NPs was also discussed. This work develops a novel H2 evolution cocatalyst and unveils the relationship between the loading amount/particle size of cocatalyst on the H2 evolution rate.

The enhancement of CdS ultrathin nanosheets photocatalytic activity for water splitting via activating the (001) polar facet by hydrogenation and its charge separation mechanism

The photocatalytic H2 evolution activity of the CdS nanosheets with exposed {001} facets were significantly improved by hydrogenation. Photocatalytic H2 evolution mechanism of charge separation driven by Es between CdS {001} polar faces are proposed.

Experimental insights and DFT analysis of metal-free DNA nanocatalyst with enhanced hydrogen evolution via phosphate-mediated proton acceptance

Although platinum is widely recognized as a benchmark catalyst for realizing highly efficient hydrogen evolution reactions, its practical application is hindered by the scarcity and high cost of Pt. In this regard, metal-free organic catalysts are considered vital alternatives for producing H2 in green energy applications. In this study, we prepare binder-free electrocatalysts by combining salmon DNA with activated carbon (AC) (cacao pods) for H2 production. The AC-DNA (0.025 M DNA) sample requires an overpotential (η) of 106 mV to generate a current density of 10 mA cm−2, with a Tafel slope of 96 mV dec−1, in 0.5 M H2SO4. The phosphate-mediated proton acceptance of DNA facilitates the hydrogen evolution reaction (HER) in the presence of AC, resulting in excellent durability over 40 h at 10 mA cm−2 (η10 = 106 mV) and 100 mA cm−2 (η100 = 271 mV). In addition, the electrocatalyst exhibits a faradaic efficiency of 96.9%. The proton acceptance facilitated by the phosphate group in DNA achieves outstanding performance with a turnover frequency of 2.76 s−1 and an exchange current density of 2.08 × 10−3 A cm−2. Theoretical calculations support the in-depth H2 evolution mechanism at the DNA-anchored AC samples via proton capturing of phosphate groups during water splitting.

Facile synthesis of noble-metal-free MoP as an efficient catalyst for H2 evolution from formaldehyde solution

A series of MoP were synthesized by a facile solid reaction and used as catalysts for H2 evolution from alkaline HCHO solution. It was found that the as-prepared MoP catalysts exhibited high H2 evolution activity and stability. The calcination temperature had a great influence on the structure, morphology and H2 evolution activity of MoP. When the calcination temperature was 400 °C, the MoP-400 exhibited the highest H2 evolution activity. The concentration of HCHO and reaction temperature in the reaction system also affected the H2 evolution efficiency. The H2 evolution performance of MoP was better than or comparable to those of the reported noble metal catalysts. The possible mechanism of H2 evolution from HCHO was discussed.

Interfacial engineering of Ti3C2 MXene/CdIn2S4 Schottky heterojunctions for boosting visible-light H2 evolution and Cr(VI) reduction

Developing efficient heterojunction photocatalysts that have a high charge carrier separation rate and improved light-harvesting capacity is a crucial step in solving energy crisis and reducing environmental pollution. Herein, we synthesized few-layered Ti3C2 MXene sheets (MXs) by a manual shaking process, and combined with CdIn2S4 (CIS) to construct novel Ti3C2 MXene/CdIn2S4 (MXCIS) Schottky heterojunction through a solvothermal method. The strong interface between two-dimensional (2D) Ti3C2 MXene and 2D CIS nanoplates led to enhanced light-harvesting capacity and promoted charge separation rate. Additionally, the presence of S vacancies on the MXCIS surface helped to trap free electrons. The optimal sample, 5-MXCIS (with 5 wt% MXs loading), exhibited outstanding performance for photocatalytic hydrogen (H2) evolution and Cr(VI) reduction under visible light due to the synergistic effect of enhanced light-harvesting capacity and charge separation rate. The charge transfer kinetics was thoroughly studied using multiple techniques. The reactive species of •O2−, •OH and h+ were generated in 5-MXCIS system, and e− and •O2− radicals were found to be the main contributors to Cr(VI) photoreduction. Based on the characterization results, a possible photocatalytic mechanism for H2 evolution and Cr(VI) reduction was proposed. On the whole, this work provides new insights into the design of 2D/2D MXene-based Schottky heterojunction photocatalysts for boosting photocatalytic efficiency.

Dual S-scheme ZnO–g-C3N4–CuO heterosystem: a potential photocatalyst for H2 evolution and wastewater treatment

A ZnO–g-C3N4–CuO catalyst prepared by an ecofriendly solution combustion process is used for H2 evolution. The mechanism of H2 evolution over ZnO–g-C3N4–CuO is described under visible light illumination.

Current Scenario of MXene-Based Nanomaterials for Wastewater Remediation: A Review

Rapid urban and industrial sectors generate massive amounts of wastewater, creating severe ecological disruption and harming living organisms. The number of harmful pollutants such as dyes, heavy metals, antibiotics, phenolic compounds, and volatile and several organic chemicals discharged into aquatic systems varies depending on the effluent composition of various sectors. MXene-based composites with unique characteristics were spotlighted as newly developed nanomaterials specifically for environmental-related applications. Therefore, this review broadly discusses the properties, basic principles of MXene, and synthesis routes for developing different MXene-based nanomaterials. The most current strategies on the energy and environmental applications of MXene-based nanomaterials, particularly in photocatalysis, adsorption, and water splitting, were deeply explored for the remediation of different pollutants and hydrogen (H2) evolution from wastewater. The detailed mechanism for H2 evolution and the remediation of industrial pollutants via photocatalysis and adsorption processes was elaborated. The multi-roles of MXene-based nanomaterials with their regeneration possibilities were emphasized. Several essential aspects, including the economic, toxicity and ecological power of MXene-based nanomaterials, were also discussed regarding their opportunity for industrialization. Finally, the perspectives and challenges behind newly developed MXene and MXene-based nanomaterials for environmental pollution were reviewed.

In-situ constructing cobalt incorporated nitrogen-doped carbon/CdS heterojunction with efficient interfacial charge transfer for photocatalytic hydrogen evolution

Designing an efficient heterojunction interface is an effective way to promote the electrons' transfer and improve the photocatalytic H2 evolution performance. In this work, a novel hollow hybrid system of Co@NC/CdS has been fabricated and constructed. CdS nanospheres are anchored on the hollow-structured cobalt incorporated nitrogen-doped carbon (Co@NC) through a one-pot in-situ chemical deposition approach, forming an intimate interface and establishing an excellent channel to improve the electrons transfer and charge carriers separation between CdS and Co@NC cocatalyst, which immensely promotes the photocatalytic activity. The rate of photocatalytic H2 evolution over hollow structured Co@NC/CdS heterojunction can be achieved 8.2 mmol g−1 h−1, which is about 45 times of pristine CdS nanospheres. The photocatalytic H2 evolution mechanism has been investigated by the techniques of photoluminescence (PL) spectra, photocurrent-time (i-t) curves, electrochemical impedance spectroscopy (EIS) etc. This work aims to provide a new way in developing of high-performance advanced 3D heterojunction for photocatalytic hydrogen evolution.

Rationalizing hydrogen evolution mechanism on the slab of Zn-reduced 2H–MoS2 monolayer by density functional theory calculations

In this paper, the H2 evolution mechanism and activity on Zn-reduced 2H–MoS2 were explored by density functional theory (DFT) calculations based on the possible active sites including nZn-MoS2 and nZn-S vacancy (SV) (n = 1, 2, 3), which were built by replacing one, two, three adjacent Mo atoms in MoS2 and at SV by Zn, respectively. The calculations indicate that the amount of Zn incorporation can significantly affect the H2 evolution activities and mechanisms of Zn-doped MoS2 and Zn-doped SV and Zn-doped SV is more favorable to the production of H2. The formation of 3Zn–MoS2 and 3Zn-SV is less favorable to H2 evolution reactions (HER) due to their too high or low ΔGH. The replacement of one Mo in MoS2 by Zn can reduce the ΔGH of S atom to −0.18 eV, and S bonded by one Zn (SA) in Zn–MoS2 cannot catalyze HER. In 2Zn–MoS2, S bonded by two Zn atoms (SB) catalyzes HER by the Heyrovsky-step-controlled Volmer-Heyrovsky mechanism, and the rate determining step (RDS) has a barrier of 47.8 kcal/mol. Moreover, Zn doping is beneficial for generating Zn-doped SV. Zn-SV catalyzes HER via the same mechanism with 2Zn–MoS2, and the barrier of RDS is 30.7 kcal/mol. After replacing two adjacent Mo atoms of SV with Zn, the resulting 2Zn-SV follows the Volmer-Heyrovsky mechanism to catalyze HER, and the RDS is the Volmer step with a barrier of 27.2 kcal/mol.

Mechanistic insights into H2 evolution via water splitting at the expense of B2(OH)4: a theoretical study.

H2 has been comprehensively deemed a promising potential candidate to replace traditional fossil fuel-based energy. Typically, the hydrolysis of most hydrogen-rich boron hydrides (e.g. NaBH4, NH3BH3 and Me2NHBH3) catalyzed by nanomaterials generates H2 with only one H atom supplied by water and the other one by a hydrogen-rich boron hydride. Interestingly, both H atoms of produced H2 are provided by water upon hydrolysis of B2(OH)4. Herein, the catalytic mechanisms of H2 evolution upon water splitting at the expense of B2(OH)4 in its hydrolysis reactions catalyzed by acid, base or metal nanoparticles have been investigated by density functional theory (DFT) calculations. By computational studies, the mechanisms of catalysis by base and metal nanoparticles are basically the same as those speculated from our previous experiments. The previously proposed acid catalytic mechanism has been overturned, however. This study not only provides important insights into the catalytic mechanism for water splitting at the expense of B2(OH)4, but also opens up an exciting opportunity to use water to store H2.

Unsaturated selenium-enriched MoSe2+x amorphous nanoclusters: One-step photoinduced co-reduction route and its boosted photocatalytic H2-evolution activity for TiO2

Maximumly increasing the number of active sites is crucial to improve the H2-evolution efficiency of cocatalyst. Herein, a rich-active-site regulation strategy by the synergism of unsaturated selenium enrichment and amorphization is developed to precisely construct unsaturated selenium-enriched MoSe2+x amorphous nanoclusters onto the TiO2 via a mild photoinduced co-reduction route by using MoCl5 and dibenzyl diselenide as the precursors. The resulting a-MoSe2+x exposes more unsaturated Se atoms (46.5%) because of its unsaturated Se-enriched configuration, highly irregular arrangement, and ultrasmall size (0.3 −1 nm). Photocatalytic tests show that the a-MoSe2+x/TiO2(3 wt%) achieves an optimal H2-evolution rate (4984.46 μmol h−1 g−1, AQE = 23.90%), with the 4.51-fold enhancement relative to that of c-MoSe2/TiO2. Hence, a rich unsaturated Se-mediated H2-evolution mechanism is proposed, namely, the abundant unsaturated Se atoms as the active sites can not only provide sufficient proton-adsorption-centers to enrich H+, but also present an outstanding catalytic efficiency to fleetly convert H+ to H2.

Photochemical H2 Evolution from Bis(diphosphine)nickel Hydrides Enables Low-Overpotential Electrocatalysis.

Molecules capable of both harvesting light and forming new chemical bonds hold promise for applications in the generation of solar fuels, but such first-row transition metal photoelectrocatalysts are lacking. Here we report nickel photoelectrocatalysts for H2 evolution, leveraging visible-light-driven photochemical H2 evolution from bis(diphosphine)nickel hydride complexes. A suite of experimental and theoretical analyses, including time-resolved spectroscopy and continuous irradiation quantum yield measurements, led to a proposed mechanism of H2 evolution involving a short-lived singlet excited state that undergoes homolysis of the Ni-H bond. Thermodynamic analyses provide a basis for understanding and predicting the observed photoelectrocatalytic H2 evolution by a 3d transition metal based catalyst. Of particular note is the dramatic change in the electrochemical overpotential: in the dark, the nickel complexes require strong acids and therefore high overpotentials for electrocatalysis; but under illumination, the use of weaker acids at the same applied potential results in a more than 500 mV improvement in electrochemical overpotential. New insight into first-row transition metal hydride photochemistry thus enables photoelectrocatalytic H2 evolution without electrochemical overpotential (at the thermodynamic potential or 0 mV overpotential). This catalyst system does not require sacrificial chemical reductants or light-harvesting semiconductor materials and produces H2 at rates similar to molecular catalysts attached to silicon.

The distribution effect of sulfur vacancy in 2H–MoS2 monolayer on its H2 generation mechanism from density functional theory

In this paper, the effects of the sulfur vacancy (VS) distribution in 2H–MoS2 monolayer on the H2 evolution mechanism and activity are researched by density functional theory (DFT) calculations. The calculation results reveal that the H2 generation on VS follows the Volmer-Heyrovsky mechanism with the Heyrovsky reaction as the rate determined step (RDS) with an energy barrier of 18.5 kcal/mol. When two VS are separated by one S atom, the H2 evolution on VS remains the Heyrovsky-step-determined Volmer-Heyrovsky mechanism and the RDS energy barrier increases to 20.0 kcal/mol. Removing adjacent S of VS causes that the Tafel-step-determined Volmer-Tafel mechanism works for HER and the barrier of RDS increases to 21.4 kcal/mol. Though the barriers of RDS ascend when the concentration of VS is enhanced by these two strategies, the Tafel step can take place at a lower potential and more VS are exposed. Therefore, two adjacent or next-near VS may obtain better H2 generation performance. Three adjacent or spaced out VS by one S may be less favorable for HER.

The E2 state of FeMoco: Hydride Formation versus Fe Reduction and a Mechanism for H2 Evolution.

The iron‐molybdenum cofactor (FeMoco) is responsible for dinitrogen reduction in Mo nitrogenase. Unlike the resting state, E0, reduced states of FeMoco are much less well characterized. The E2 state has been proposed to contain a hydride but direct spectroscopic evidence is still lacking. The E2 state can, however, relax back the E0 state via a H2 side‐reaction, implying a hydride intermediate prior to H2 formation. This E2→E0 pathway is one of the primary mechanisms for H2 formation under low‐electron flux conditions. In this study we present an exploration of the energy surface of the E2 state. Utilizing both cluster‐continuum and QM/MM calculations, we explore various classes of E2 models: including terminal hydrides, bridging hydrides with a closed or open sulfide‐bridge, as well as models without. Importantly, we find the hemilability of a protonated belt‐sulfide to strongly influence the stability of hydrides. Surprisingly, non‐hydride models are found to be almost equally favorable as hydride models. While the cluster‐continuum calculations suggest multiple possibilities, QM/MM suggests only two models as contenders for the E2 state. These models feature either i) a bridging hydride between Fe2 and Fe6 and an open sulfide‐bridge with terminal SH on Fe6 (E2‐hyd) or ii) a double belt‐sulfide protonated, reduced cofactor without a hydride (E2‐nonhyd). We suggest both models as contenders for the E2 redox state and further calculate a mechanism for H2 evolution. The changes in electronic structure of FeMoco during the proposed redox‐state cycle, E0→E1→E2→E0, are discussed.

Low-Temperature Direct Electrochemical Methanol Reforming Enabled by CO-Immune Mo-Based Hydrogen Evolution Catalysts.

Hydrogen storage in the form of intermediate artificial fuels such as methanol is important for future chemical and energy applications, and the electrochemical regeneration of hydrogen from methanol is thermodynamically favorable compared to direct water splitting. However, CO produced from methanol oxidation can adsorb to H2 -evolution catalysts and drastically reduce activity. In this study, we explore the origins of CO immunity in Mo-containing H2 -evolution catalysts. Unlike conventional catalysts such as Pt or Ni, Mo-based catalysts display remarkable immunity to CO poisoning. The origin of this behavior in NiMo appears to arise from the apparent inability of CO to bind Mo under electrocatalytic conditions, with mechanistic consequences for the H2 -evolution reaction (HER) in these systems. This specific property of Mo-based HER catalysts makes them ideal in environments where poisons might be present.

Facile preparation of Zn x Cd1-x S/ZnS heterostructures with enhanced photocatalytic hydrogen evolution under visible light.

Hydrogen evolution from water using solar energy is regarded as a most promising process, thus, exploring efficient photocatalysts for water splitting is highly desirable. To avoid the rapid recombination of photogenerated electrons and holes in CdZnS semiconductors, ZnxCd1−xS/ZnS composites were synthesized via a one-step hydrothermal method and then annealed at 400 °C for 60 min under argon flow. ZnxCd1−xS/ZnS composites are composed of ZnS nanosheets decorated with ZnxCd1−xS nanorods, and TEM and UV-vis absorption spectra confirm the formation of the heterostructure between ZnxCd1−xS nanorods and ZnS nanosheets. Because of the well-matched band alignment, stronger optical absorption and larger carrier density, Zn0.2Cd0.8S/ZnS has the highest hydrogen production, with a photocatalytic hydrogen production rate up to 16.7 mmol g−1 h−1 under visible light irradiation. Moreover, the photocatalyst also exhibits high stability and good reusability for hydrogen production reaction. The facile and efficient approach for ZnS based heterostructures could be extended to other metal compound materials.

Photoinduced synthesis of ultrasmall amorphous NiWSx nanodots for boosting photocatalytic H2-evolution activity of TiO2

It is a great challenge to explore new, abundant and cost-effective H2-evolution cocatalysts for realizing highly efficient photocatalytic water splitting using renewable solar energy. In this study, the ultrasmall amorphous NiWSx nanodots (ca. 2 nm) as a new H2-evolution cocatalyst of bimetallic sulfide were evenly anchored onto TiO2 surface to prepare NiWSx nanodot-modified TiO2 (NiWSx-ND/TiO2) photocatalysts through a facile photodeposition strategy. The photocatalytic results showed that the amorphous NiWSx nanodots could obviously accelerate the hydrogen-production activity of TiO2. Especially, the as-prepared NiWSx-ND/TiO2 (3 wt%) photocatalyst manifested the maximum H2-generation efficiency of 4580 μmol h−1 g−1, corresponding to an apparent quantum efficiency (AQE) of 13%. Based on the present results, a H2-evolution mechanism of NiWSx cocatalyst is proposed, namely, the ultrasmall amorphous NiWSx nanodots first act as a capturer to speedily capture photoexcited electrons, and then play as an electron-transport medium to promote the hydrogen-generation activity. Considering its mild preparation, inexpensive cost, and admirable efficiency, the amorphous NiWSx nanodots can become a promising cocatalyst for potential various applications in photocatalytic field.

Formic acid fractionation towards highly efficient cellulose-derived PdAg bimetallic catalyst for H2 evolution

The present work, in which cellulose isolated from formic acid fractionation (FAC) is decorated with polyetherimide (PEI) to attain highly efficient cellulose-derived PdAgbimetallic catalyst (PdAg-PEI-FAC), has been investigated, and the catalyst properties are characterized by XRD, XPS, BET, ICP-AES and HAADF-STEM. The as-obtained Pd3.75Ag3.75-PEI-FAC exhibits excellent catalytic performance for H2 evolution from a sodium formate-free formic acid (FA) aqueous medium at ambient temperature and the turnover frequency (TOF) reaches a high value of 2875 h−1, which is superior to most of the previously reported Pd-based heterogeneous catalysts supported on a carbon matrix in the literature. The remarkable catalytic activities of PdAg-PEI-FAC result from high dispersion Pd and synergistic effects between the PdAg bimetallic system. Furthermore, the amide (-NH) group in PEI coated on cellulose acting as a proton scavenger efficiently improves the catalytic property of catalyst. In addition, the critical factors affecting H2 release, such as FA concentration, reaction temperature, PdAg compositions and support matrix type, are also evaluated. Based on the experimental results, the probable three-step mechanism of H2 evolution from FA over Pd3.75Ag3.75-PEI-FAC is proposed. In the end, the activation energy (Ea) of Pd3.75Ag3.75-PEI-FAC catalyst is calculated to 53.97 kJ mol-1, and this catalyst shows unique robustness and satisfactory re-usability with no loss of catalytic activity after five recycles. The findings in this work provide a novel routine from lignocellulose fractionation towards cellulose-derived catalyst for H2 evolution.