Photocatalytic hydrogen evolution performance of metal ferrites /polypyrrole nanocomposites

The combination of inorganic (e.g., ferrite nanoparticles) and organic (e.g., conducting polymers) materials in the fabrication of heterojunctions or composites is an attractive scheme in the field of photocatalysis. We took the advantage of this phenomenon by fabricating MFerrite (M = Co, Ni, and Zn) @polypyrrole (MFerrite@Ppy) nanocomposites with a varying weight percentage of Ppy for the hydrogen production through photocatalytic water splitting under visible light irradiation. The structural, spectral, morphological, compositional, and optical features of the as-prepared nanocomposites were analyzed in full depth. The average crystallite sizes were estimated to be 30–40 nm from the XRD patterns which were further validated by TEM images from which a core-shell structure of the composites can be inferred. Likewise, the SEM images revealed spherical Ppy particles with a diameter in the range of 100–300 nm. From a photocatalytic viewpoint, CoFerrite@30Ppy is endowed with some peculiar characteristics including but not limited to strong light-harvesting ability (ranging between 300 and 650 nm), narrow optical band gap (as low as 1.6 eV), and higher photoluminescence (PL) lifetime (6.41 ns) which justify why it stands out among all composites in terms of photocatalysis. Under 8 h illumination of simulated visible light and using triethanolamine (TEOA) as a hole scavenger and Eosin-Y (EY) as a dye sensitizer, the photocatalytic hydrogen evolution (HER) amount for CoFerrite@30Ppy was found to be 10.44 mmol g^?1, far greater than any other composite catalysts in this study. From the PL spectra, it can be pointed out that sensitization of CoFerrite with 30 wt % Ppy conduces to simultaneous deceleration of the electron-hole recombination process and acceleration of the transference of excitons within the system.     

Ni2P(O)–Fe2P(O)/CeOx as high effective bifunctional catalyst for overall water splitting

The development of cost-effective bifunctional catalysts with excellent performance and good stability is of great significance for overall water splitting. In this work, NiFe layered double hydroxides (LDHs) nanosheets are prepared on nickel foam by hydrothermal method, and then Ni₂P(O)–Fe₂P(O)/CeO_x nanosheets are in situ synthesized by electrodeposition and phosphating on NiFe LDHs. The obtained self-supporting Ni₂P(O)–Fe₂P(O)/CeO_x exhibit excellent catalytic performances in alkaline solution due to more active sites and fast electron transport. When the current density is 10 mA cm^?2, the overpotential of hydrogen evolution reaction and oxygen evolution reaction are 75 mV and 268 mV, respectively. In addition, driven by two Ni₂P(O)–Fe₂P(O)/CeO_x electrodes, the alkaline battery can reach 1.45 V at 10 mA cm^?2.     

Swapping catalytic active sites from cationic Ni to anionic F in Fe–F–Ni3S2 enables more efficient alkaline oxygen evolution reaction and urea oxidation reaction

Electrolysis of water for producing hydrogen instead of traditional fossil fuels is one of the most promising methods to alleviate environmental pollution and energy crisis. In this work, Fe and F ion co-doped Ni₃S₂ nanoarrays grown on Ni foam substrate were prepared by typical hydrothermal and sulfuration processes for the first time. Density functional theory (DFT) calculation demonstrate that the adsorption energy of the material to water is greatly enhanced due to the doping of F and Fe, which is conducive to the formation of intermediate species and the improvement of electrochemical performance of the electrode. The adsorption energy of anions (F and S) and cations (Fe and Ni) to water in each material was also calculated, and the results showed that F ion showed the most optimal adsorption energy of water, which proved that the doping of F and Fe was beneficial to improve the electrochemical performance of the electrode. It is worth noting that the surface of Fe–F–Ni₃S₂ material will undergo reconstruction during the process of water oxidation reaction and urea oxidation reaction, and amorphous oxides or hydroxides in situ would be formed on the surface of electrode, which are the real active species.     

Highly stable γ-NiOOH/ZnCdS photocatalyst for efficient hydrogen evolution

It is extremely desirable to develop high hydrogen evolution activity and stable visible-light-driven photocatalysts. The sluggish oxidation process and holes accumulation are the main obstacles to high catalysis activity and photo-stability. An efficient γ-NiOOH/ZnCdS photocatalyst was prepared by in-situ hydrothermal method. The γ-NiOOH nanosheets distribute on ZnCdS nanospheres surface and accelerate holes transfer. The hydrogen evolution rate is up to 48.60 mmol g^?1 h^?1 under visible-light illumination (λ = 400–780 nm), about 10.8 times of pure ZnCdS (4.50 mmol g^?1 h^?1) and 1.8 times of general β-NiOOH modified ZnCdS (27.40 mmol g^?1 h^?1). And apparent quantum yield of γ-NiOOH/ZCS-100 is up to 18.23% (400 nm). The carrier lifetime extends from 5.50 ns (ZnCdS) to 6.10 ns (γ-NiOOH/ZCS), examined by steady photoluminescence and time-resolved photoluminescence. Moreover, the γ-NiOOH/ZCS photocatalyst has exhibited excellent photo-stability even after one-year of storage. The γ-NiOOH nanosheets can be an excellent co-catalyst on accelerating both holes transfer and oxidation process for high photo-stability and photo-activity.     

Enhanced oxygen evolution reaction kinetics through biochar-based nickel-iron phosphides nanocages in water electrolysis for hydrogen production

Highly-efficient and stable non-noble metal electrocatalysts for overcoming the sluggish kinetics of oxygen evolution reaction (OER) is urgent for water electrolysis. Biomass-derived biochar has been considered as promising carbon material because of its advantages such as low-cost, renewable, simple preparation, rich structure, and easy to obtain heteroatom by in-situ doping. Herein, Ni₂P–Fe₂P bimetallic phosphide spherical nanocages encapsulated in N/P-doped pine needles biochar is prepared via a simple two-step pyrolysis method. Benefiting from the maximum synergistic effects of bimetallic phosphide and biochar, high conductivity of biochar encapsulation, highly exposed active sites of Ni₂P–Fe₂P spherical nanocages, rapid mass transfer in porous channels with large specific surface area, and the promotion in adsorption of reaction intermediates by high-level heteroatom doping, the (Ni_0.75Fe_0.25)₂P@NP/C demonstrates excellent OER activity with an overpotential of 250 mV and a Tafel slope of 48 mV/dec at 10 mA/cm² in 1 M KOH. Also it exhibits a long-term durability in 10 h electrolysis and its activity even improves during the electrocatalytic process. The present work provides a favorable strategy for the inexpensive synthesis of biochar-based transition metal electrocatalysts toward OER, and improves the water electrolysis for hydrogen production.     

Phase composition and property evolution of (Yb1−xHox)2Si2O7 solid solution as environmental/thermal barrier coating candidates

RE disilicates are good candidates as environmental/thermal barrier coating for SiC_f/SiC composite in harsh gas turbine engines. We designed (Yb_1?xHo_x)₂Si₂O₇ solid solutions and studied mechanical properties, thermal properties, and water vapor resistance. Powders with different compositions were synthesized by pressureless sintering, and bulk samples were prepared by Spark Plasma Sintering (SPS). Polymorphic changes with temperature and composition of the solid solutions were examined. Through doping Ho into Yb₂Si₂O₇, water vapor corrosion resistance is significantly promoted, and thermal expansion coefficient is maintained close to that of Si-based ceramics. Compared with host disilicates, thermal conductivity of solid solutions are decreased, and mechanical properties, including Vickers hardness and fracture toughness, are increased. A two-phase domain is found at (Yb_1/2Ho_1/2)₂Si₂O₇, and the γ to δ phase transition of Ho₂Si₂O₇ is observed during SPS. Among all samples, γ-(Yb_1/3Ho_2/3)₂Si₂O₇ possesses superior high temperature stability, and excellent water vapor resistance, indicating its performance as environmental/thermal barrier coating.     

CNT-interconnected iron-doped NiP2/Ni2P heterostructural nanoflowers as high-efficiency electrocatalyst for oxygen evolution reaction

Oxygen evolution reaction (OER) plays a decisive role in electrolytic water splitting. However, it is still challengeable to develop low-cost and efficient OER electrocatalysts. Herein, we present a combination strategy via heteroatom doping, hetero-interface engineering and introducing conductive skeleton to synthesize a hybrid OER catalyst of CNT-interconnected iron-doped NiP₂/Ni₂P (Fe-(NiP₂/Ni₂P)@CNT) heterostructural nanoflowers by a simple hydrothermal reaction and subsequent phosphorization process. The optimized Fe-(NiP₂/Ni₂P)@CNT catalyst delivers an ultralow Tafel slope of 46.1 mV dec^?1 and overpotential of 254 mV to obtain 10 mA cm^?2, which are even better than those of commercial OER catalyst RuO₂. The excellent OER performance is mainly attributed to its unique nanoarchitecture and the synergistic effects: the nanoflowers constructed by a 2D-like nanosheets guarantee large specific area and abundant active sites; the highly conductive CNT skeleton and the electronic modulation by the heterostructural NiP₂/Ni₂P interface and the hetero-atom doping can improve the catalytic activity; porous nanostructure benefits electrolyte penetration and gas release; most importantly, the rough surface and rich defects caused by phosphorization process can further enhance the OER performance. This work provides a deep insight to boost catalytic performance by heteroatom doping and interface engineering for water splitting.     

Self-construction of pea-like Cu/Cu2S Mott-Schottky electrocatalyst for the oxygen evolution reaction

The speed of the oxygen evolution reaction seriously affects the hydrogen production efficiency of water electrolysis. Hence it is crucial to develop efficient and durable OER electrocatalysts. Construction of heterojunction catalysts is also one of the strategies to develop efficient catalysts. In this paper, a pea-like Cu/Cu₂S–C3 Mott?Schottky electrocatalyst was self-constructed by vapor deposition, while CF (copper foam) was used as substrate material and copper source, and thiourea was served as sulfur source. The built-in electric field is formed at the metal-semiconductor interface, which endows it with promising electrocatalytic performance. As the working electrode, the overpotentials of Cu/Cu₂S–C3 required to reach the current density of 10 and 50 mA cm^?2 were about 170 and 335 mV. The impact of the Mott-Schottky structure on the catalyst was also reflected in stability. The i-t tests of the sample Cu/Cu₂S–C3 were carried out under 10 and 60 mA cm^?2 and performed well.     

Spin-state regulating of cobalt assisted by iron doping and coordination for enhanced oxygen evolution reaction

Customizing catalysts from the electronic structure, such as spin state, is an effective but challenging strategy for oxygen evolution reaction (OER). Herein, an ultrafine Co–Fe material highly dispersed on nitrogen carbide matrix is fabricated by coordination polymer and self-templating method to scrutinize the impact of spin state of Co on OER through Fe doping. The optimized catalyst shows boosted OER performance, which only requires overpotential of 333 mV at 10 mA cm^?2, outperforming other control samples and commercial RuO₂. The elevated local spin states of Co by Fe doping lead to charge transfer acceleration and fast generation of oxygenated intermediates, which is proved to account for the OER elevation. In addition, the long-term stability of Co–Fe material is guaranteed by the strong coordination of Co/Fe to the melamine-formaldehyde resin, which is used to adsorb metal ions, contributing to the high dispersion of active sites during the OER process.     

Role of functionalized graphene quantum dots in hydrogen evolution reaction: A density functional theory study

Rapid advances in the field of catalysis require a microscopic understanding of the catalytic mechanisms. However, in recent times, experimental insights in this field have fallen short of expectations. Furthermore, experimental searches of novel catalytic materials are expensive and time-consuming, with no guarantees of success. As a result, density functional theory (DFT) can be quite advantageous in advancing this field because of the microscopic insights it provides and thus can guide experimental searches of novel catalysts. Several recent works have demonstrated that low-dimensional materials can be very efficient catalysts. Graphene quantum dots (GQDs) have gained much attention in past years due to their unique properties like low toxicity, chemical inertness, biocompatibility, crystallinity, etc. These properties of GQDs which are due to quantum confinement and edge effects facilitate their applications in various fields like sensing, photoelectronics, catalysis, and many more. Furthermore, the properties of GQDs can be enhanced by doping and functionalization. In order to understand the effects of functionalization by oxygen and boron based groups on the catalytic properties relevant to the hydrogen-evolution reaction (HER), we perform a systematic study of GQDs functionalized with the oxygen (O), borinic acid (BC₂O), and boronic acid (BCO₂). All calculations that included geometry optimization, electronic and adsorption mechanism, were carried out using the Gaussian16 package, employing the hybrid functional B3LYP, and the basis set 6-31G(d,p). With the variation in functionalization groups in GQDs, we observe significant changes in their electronic properties. The adsorption energy E_ads of hydrogen over O-GQD, BC₂O-GQD, and BCO₂-GQD is ?0.059 eV, ?0.031 eV and ?0.032 eV respectively. Accordingly, Gibbs free energy (ΔG) of hydrogen adsorption is extraordinarily near the ideal value (0 eV) for all the three types of functionalized GQDs. Thus, the present work suggests pathways for experimental realization of low-cost and multifunctional GQDs based catalysts for clean and renewable hydrogen energy production.