Fabrication of one-dimensional MnxCd1-xS@D-MoSeyS2-y heterostructure with enhanced visible-light photocatalytic hydrogen evolution

Developing a visible light photocatalysts for hydrogen (H₂) production to replacing fossil fuels is a huge challenge. Herein, one-dimensional Mn_xCd_1-xS@D-MoSe_yS_2-y heterostructure was prepared for improving the hydrogen generation. The molar ratio of Mn to Cd in Mn_xCd_1-xS solid solution was adjusted, which effectively enhances the photocatalytic H₂ evolution efficiency. MoSe_yS_2-y with defects (D-MoSe_yS_2-y) was synthesized by solvothermal method during the photocatalytic process as cocatalyst, which wrapping on the Mn_xCd_1-xS solid solution. Mn_xCd_1-xS@D-MoSe_yS_2-y shows improved photocatalytic H₂ evolution activity, which was endowed by suitable energy band structure, exposure of active sites, high carrier concentration and fast electrons transfer. The highest photocatalytic H₂ evolution rate of sample among prepared Mn_xCd_1-xS@D-MoSe_yS_2-y is 12.46 mmol/g/h, which is equivalent to 37 times that of CdS. This present study offers a insight to the preparation of CdS-based photocatalysts.     

Effect of mixed-valence of manganese on water oxidation activity of La1-xCaxMnO3 (0 ≤ x ≤ 1) solid solutions

The water oxidation reaction is one of the crucial process involves 4e^- and 4H⁺ transfer, which causes a high energy barrier and makes the water-splitting process a slow kinetic reaction. Herein, we have systematically observed the water oxidation activity of La_1-xCa_xMnO₃ (0≤ x ≤ 1) solid solutions by tuning the valence of Mn ion through selective substitution of La³⁺ with Ca²⁺. With increasing Ca concentration in La_1-xCa_xMnO₃ solid solutions, the water oxidation activity increases up to x = 0.5 and then decreases. Mixed-valence of Mn generated due to substitution of La³⁺ with Ca²⁺, which leads to the double exchange mechanism. This could results in high electrical conduction and a faster electron transfer rate because of the rapid charge exchange between Mn³⁺ and Mn⁴⁺ ions, which pinpoints the reason behind the enhanced oxygen evolution activity. Our findings will be a promising way to relate the Mn mixed-valence with its catalytic activity to design and development of efficient water oxidation catalysts.     

Enhancement of visible light-driven hydrogen production over zinc cadmium sulfide nanoparticles anchored on BiVO4 nanorods

Photocatalytic water splitting to produce H₂ is a promising technology for clean energy generation. However, the use of expensive noble metals, toxicity, low charge separation efficiency and wide band gap of semiconductors hampering the widespread commercialization. Herein, we showed the potential of combining BiVO₄ nanorods with ZnCdS forming a hetero-structure which extend the spectral responsive range, separate the charge carriers effectively and enhances photocatalytic activity compared to single-component materials. The two components of hetero-structure forms an interface contact which also mitigate the problems of lower conduction band position of BiVO₄ and fast recombination of charge carriers of ZnCdS. The BiVO₄–ZnCdS hetero-structure was studied through surface morphology, crystallization properties, elemental analysis and optical properties. Under visible light irradiation, the BiVO₄–ZnCdS heterostructure produced 152.5 μmol g^?1 h^?1 hydrogen from water splitting, which was much higher than that of the individual components and stability of the hydrogen production was observed in three consecutive cycles. The as-obtained heterostructure showed improved visible light harvesting ability, prolong life of charges carriers and charge separation efficiency and Z-scheme mechanism features which results in enhanced photocatalytic activity for water splitting.     

Characterization of (Y1-xCax)BaCo4-yZnyO7 as cathodes for intermediate temperature solid oxide fuel cells

The effects of Ca and Zn substitution, respectively, for Y and Co in (Y_1-xCa_x)BaCo_4-yZn_yO₇ (0.25 ≤ x ≤ 0.75 and 1.0 ≤ y ≤ 1.75) on the structure, high-temperature phase stability, thermal expansion coefficient (TEC), and electrochemical performance for intermediate temperature solid oxide fuel cells (IT-SOFC) have been investigated. The (Y_1-xCa_x)BaCo_4-yZn_yO₇ oxides crystallize in a trigonal P31c symmetry similar to YBaCo₄O₇. The substitution of Zn for Co improves the long-term phase stability at high temperatures, but at the expense of the electrochemical performance. In contrast, the substitution of Ca for Y is improves electrochemical performances, but deteriorates the long-term phase stability at high temperatures at high Ca contents (x = 0.75 and 1.0). Among the various chemical compositions investigated in the (Y_1-xCa_x)BaCo_4-yZn_yO₇ system, the (Y_0.5Ca_0.5)BaCo_2.5Zn_1.5O₇ composition offers a combination of good electrochemical performance and low TEC, while maintaining the phase stability at 600-800 °C for 120 h. The (Y_0.5Ca_0.5)BaCo_2.5Zn_1.5O₇ + GDC (50 : 50 wt. %) composite cathodes exhibit a maximum power density of ∼ 450 mW cm⁻² at 700 °C in anode-supported single SOFC.     

Charge carrier separation and enhanced PEC properties of BiVO4 based heterojunctions having ultrathin overlayers

The photoelectrochemical performance of a BiVO₄ photoanode is limited by its poor charge transport properties, despite other useful optical absorption properties. Modifying the surface charge transport properties by forming heterojunction of BiVO₄ with other metal oxides layers having ultralow thickness is a promising route, as it may facilitate charge separation/transport without affecting other properties of BiVO₄. In this study, the structural, optical and PEC properties of heterojunction of BiVO₄ having ultrathin overlayers of Fe₂O₃, MoO₃ and ZnO has been investigated. The electrochemical impedance (via electrochemical impedance spectra in PEC cell) and surface photovoltage (using KPFM) measurements indicates improved charge transport owing to staggered band alignment and favourable band bending in case of BiVO₄/MoO₃ heterojunction as compared to pristine BiVO₄, BiVO₄/Fe₂O₃ and BiVO₄/ZnO heterojunctions. Enhanced photocurrent density in BiVO₄/MoO₃ of ~0.22 mA/cm² at 1.23 V_RHE which is 6 times as compared to pristine BiVO₄ layers has been observed. The results of the present study show that by forming heterojunction with a suitable semiconductor material can be used to enhance the PEC response by modifying the surface charge transfer characteristics and there is a large possibility of using other semiconductor materials for further investigations and improvement.     

Improving electrochemical performance of LiCoPO4 via Mn substitution

Abstract

We report a Mn substitution strategy to tailor the morphology, size, surface of LiCo_1–xMn_xPO₄ (x?=?0·2, 0·5 and 0·8) materials using a facile organic acid mediated hydrothermal approach. The results show that the Mn substitution plays a profound role in reducing the particle size, stabilising the surface, and improving the diffusivity of LiCo_1–xMn_xPO₄ materials, thus leading to much enhanced electrochemical performance compared with LiCoPO₄. However, when excessive Mn (e.g. x?=?0·8) is present in the olivine crystal, the performance of LiCo_1–xMn_xPO₄ degrades, possibly due to Jahn–Teller lattice distortion. As a result, the LiCo_0·5Mn_0·5PO₄ displays the best electrochemical performance in terms of capacity delivery, cyclability and rate capability. Therefore, the Mn substitution in LiCoPO₄ could be an efficient way to achieve high energy density and long cycle life for high voltage materials.     

Storage and cycling performance of Cr-modified spinel at elevated temperatures

The influences of partial substitution of Mn in LiMn₂O₄ with Cr³⁺ and Li⁺ on their charge/discharge profiles were quite different: Cr³⁺ affected it only in the high-voltage region, while Li⁺ showed in the both high and low voltage regions. Either Cr³⁺ or Li⁺ doping significantly improved the storage and cycling performance of spinel LiMn₂O₄ at the elevated temperature, specially both doped spinel. Li_1.02Cr_0.1Mn₂O₄ shows very low rate of capacity rention, 0.1% per cycle, and maintained a steady discharge capacity of 114 mAh/g, 95% of the initial discharge capacity over 50 cycles at 50°C. The chemical analysis and X-ray diffraction measurement indicate that the capacity losses of LiMn₂O₄ is mainly due to the dissolution of Mn into electrolyte, further transformation to lithium-rich spinel Li_1+xMn₂O₄. The improvements in their electrochemical profiles for the Cr³⁺ and Li⁺ modified spinel is attributed to that the partial substitution of Mn stabilize its structure, thus minimizing the dissolution of Mn into electrolyte, as well as maintaining its original morphologies.     

Promoting photoelectrochemical hydrogen production performance by fabrication of Co1-XS decorating BiVO4 photoanode

Photoelectrochemical (PEC) water splitting is an effective way of converting solar energy into hydrogen (H₂) energy. However, the carriers’ transmission and the reaction kinetics of the photoelectrode are dilatory, which will influence the conversion efficiency of solar energy to H₂. In this work, a novel of BiVO₄/Co_1-XS photoanode was successfully fabricated through the successive ionic layer adsorption reaction. The photocurrent density of optimal sample BiVO₄/Co_1-XS (2.9 mA cm^?2 at 1.23 V_RHE) has reached up to 5 times that of pure BiVO₄, and the applied bias photon to current conversion efficiency increased from 0.04% (BiVO₄) to 0.4% (BiVO₄/Co_1-XS). The superior PEC performance of the BiVO₄/Co_1-XS photoanode is mainly related to the improved conductivities and reaction kinetics. The charge injection efficiency of BiVO₄/Co_1-XS grew to about 80%, and the charge separation efficiency was up to 34%, revealing that the decoration of Co_1-XS could significantly accelerate the transfer speed of photogenerated carriers from the electrode surface to the electrolyte. This work provided an efficient and simple scheme for improving the PEC performance of photoanode, through reasonable design and research.     

Monolithically-integrated BiVO4/p+-n GaAs1-xPx tandem photoanodes capable of unassisted solar water splitting

Monolithically-integrated tandem photoanodes were fabricated on substrates consisting of epitaxial n-GaAs_1-xP_x (x ? 0.32) grown on n⁺-GaAs wafers. A p⁺-n junction photovoltaic (PV) cell was first formed by zinc diffusion into the n-GaAs_0.68P_0.32 from a deposited ZnO coating. After diffusion the ZnO serves as a transparent electrical contact to the resulting p⁺-GaAs_0.68P_0.32 surface layer. Transparent, conducting SnO₂:F provides chemical and mechanical protection for the ZnO and the underlying PV cell, and it electrically connects this cell to a top BiVO₄ photocatalyst layer. In some photoanodes, a WO₃ thin film was interposed between the SnO₂:F and BiVO₄. All oxide coatings were produced by ultrasonic spray pyrolysis except WO₃, which was spin coated. Unassisted (unbiased) solar water splitting was achieved, with a solar-to-hydrogen efficiency approaching 2%, without addition of any co-catalyst to the BiVO₄ surface. This work can provide insights to other researchers regarding scalable, low cost approaches for the planar monolithic integration of oxide photoanode materials with PV cells to create new tandem devices.     

The charge transfer pathway of g-C3N4 decorated Au/Bi(VO4) composites for highly efficient photocatalytic hydrogen evolution

In this report, a novel g-C₃N₄/Au/BiVO₄ photocatalyst has been prepared successfully by assembling gold nanoparticles on the interface of super-thin porous g-C₃N₄ and BiVO₄, which exhibits outstanding photocatalytic performance toward hydrogen evolution and durable stability in the absence of cocatalyst. FESEM micrograph analysis suggested that the intimate contact between Au, BiVO₄, and g-C₃N₄ in the as-developed photocatalyst allows a smooth migration and separation of photogenerated charge carriers. In addition, the XRD, EDX and XPS analysis further confirmed the successful formation of the as-prepared g-C₃N₄/Au/BiVO₄ photocatalyst. The photocatalytic hydrogen production activity of the developed photocatalyst was evaluated under visible-light irradiation (λ > 420 nm) using methanol as a sacrificial reagent. By optimizing the 5-CN/Au/BiVO₄ composite shows the highest H₂ evolution rate (2986 μmolg⁻¹h⁻¹), which is 15 times higher than that of g-C₃N₄ (199 μmolg⁻¹h⁻¹) and 10 time better than bare BiVO₄ (297 μmolg⁻¹h⁻¹). The enhancement in photocatalytic activity is attributed to efficient separation of the photoexcited charges due to the anisotropic junction in the g-C₃N₄/Au/BiVO₄ system. The enhancement in photocatalytic activity is attributed to efficient separation of the photoexcited charges due to the anisotropic junction in the g-C₃N₄/Au/BiVO₄ system.     

Electrochemical hydrogen storage properties of mechanically alloyed Mg0.8Ti0.2-xMnxNi (x = 0, 0.025, 0.05, 0.1) type alloys

Mechanical alloying was used in the synthesis of Mg_0.8Ti_0.2-xMn_xNi (x = 0, 0.025, 0.05, 0.1) quaternary alloys to analyze the effect of Mn substitution for Ti on the electrochemical performance of MgNi alloys. The milling was carried out for 25 h. By adding a small amount of Mn (x = 0.025) to the Mg_0.8Ti_0.2Ni alloy, a completely amorphous structure was obtained. The maximum discharge capacity of the Mg_0.8Ti_0.175Mn_0.025Ni alloy was observed as 543 mAh g^?1 at the initial charge/discharge cycle. When x = 0 and x = 0.05, the discharging performances of Mg_0.8Ti_0.2-xMn_xNi alloys were approximately the same. However, when x = 0.1, the lowest initial discharge capacity (401 mAh g^?1) and discharge capacity performance were observed. The capacity retention rates of Mg_0.8Ti_0.175Mn_0.025Ni, Mg_0.8Ti_0.2Ni, Mg_0.8Ti_0.05Mn_0.05Ni, and Mg_0.8Ti_0.1Mn_0.1Ni alloys were 81%, 68%, %67, and 47%, respectively, at the 20th cycle.     

Self-assembly core-shell BixY1-xVO4@g-C3N4 as an S-scheme heterojunction photocatalyst for pure water splitting

The self-assembly core-shell Bi_xY_1-xVO₄@g-C₃N₄ (BYVO@PCN) photocatalyst was synthesized by in-situ polycondensation of melamine on the surface of Bi_xY_1-xVO₄. The formation mechanism of the core-shell structure was mainly attributed to the excessive unpaired O atoms existed on the surface of BYVO, which could absorb the intermediate products of polycondensation. In addition, the core-shell BYVO@PCN can achieve photocatalytic pure water splitting into H₂ and O₂ which is about 5 times higher than the Bi_xY_1-xVO₄. Compared with PCN, BYVO@PCN tackles the problem that little O₂ evolution in pure water splitting by PCN. Furthermore, BYVO@PCN forms an S-scheme heterojunction instead of a type Ⅱ heterojunction, which significantly accelerates the separation of charge carriers.     

Transport and electrochemical properties of the LiyCrxMn2−xO4 (0 < x < 0.5) cathode material

In this paper structural, electrical, electrochemical and thermal (DSC) characterization of series of manganese spinel samples with manganese substituted to different degree (x = 0–0.5) with chromium are presented. The conductivity and thermoelectric power measurements were performed in wide temperature range also versus oxygen partial pressure and for deintercalated samples. Electrochemical studies of these cathode materials were conducted in Li/Li⁺/Li_yCr_xMn_2−xO₄ type cells. Substitution of manganese with chromium causes disappearance of the phase transition characteristic of LiMn₂O₄ spinel. Studies of electrical properties reveal that Cr ions do not participate in charge transport at low temperatures. In the charge curves of Li/Li⁺/Li_yCr_xMn_2−xO₄ cells there are two visible plateaux, separated with distinct potential jump (∼0.5 V), which position on Li content perfectly matches the Mn³⁺ content in the doped cathode material. The lower plateau is related to the Mn³⁺ → Mn⁴⁺ oxidation, while the next of higher voltage, of the dopant Cr³⁺ → Cr⁴⁺ oxidation. The schematic diagrams of relative Mn–Cr electronic levels alignment are proposed.     

Rational design of W-doped BiVO4 photoanode coupled with FeOOH for highly efficient photoelectrochemical catalyzing water oxidation

Developments of promising photocatalyst for PEC water oxidation gain significant interest in the research field of PEC water splitting. The BiVO₄ has been envisioned as suitable photocatalyst material for the PEC water oxidation due to suitable bandgap with favorable band edge positions. Nevertheless, the poor electron-hole separation and low charge transfer efficiency of BiVO₄ yield sluggish surface catalysis reaction. Herein, facile electrodeposition and annealing techniques are proposed to fabricate W-doped BiVO₄ photoanode coupled with FeOOH (W–BiVO₄/FeOOH) for efficient photocatalytic water oxidation. This synthesis is simple, cost-effective and less time consuming. The doping concentration of W and deposition time of FeOOH are optimized to improve photocatalytic ability of BiVO₄. At 1.23 V vs. reversible hydrogen electrode (RHE) under 1 sun illumination, the W–BiVO₄/FeOOH photoanode exhibits a high photocurrent density of 2.2 mA/cm², which is seven folds higher than that of the pristine BiVO₄ photoanode (0.31 mA/cm² 1.23 V vs. RHE). The enhanced photocatalytic ability of W–BiVO₄/FeOOH photoanode is due to the enhanced charge transport properties and synergistic effects of W doping and FeOOH deposition. The excellent long-term stability with the photocurrent density retention of 90% after continuous light illumination for 1000 s is also achieved for the W–BiVO₄/FeOOH photoanode.     

Tailoring the activation behaviour and oxide resistant properties of TiFe alloys by doping with Mn

Ti–Fe alloy is the most investigated material for H₂ storage, however, the poor activation kinetics and surface oxide formation limits its practical application. Herein, Mn substituted Ti–Fe alloys are investigated for hydrogen storage application and the effect of air exposure on their performance is evaluated. The alloys were synthesized using arc melting method and characterized for structure, composition and morphology analysis. The XRD analysis confirmed the partial substitution of Fe by Mn in the TiFe_1-xMn_x alloys. The activation kinetics of the alloys are improved by Mn substitution, and the rate of reaction increased with Mn concentration. The desorption PCIs showed a distinct but dual plateau for the low Mn content and the slope of plateau increased with Mn content. The surface oxide layer formation upon air exposure was analysed by XPS technique. The combined XRD and XPS results illustrated a thin surface oxide layer formation. It was also observed that Mn acts as a sacrificial element to prevent the bulk oxidation of alloys. The overall study depicts synergetic effect of Mn addition on hydrogen absorption kinetics of TiFe_1-xMn_x alloys.     

Photoelectrochemical water splitting with 600 keV N2+ ion irradiated BiVO4 and BiVO4/Au photoanodes

Low energy N²⁺ ion beam with 600 keV energy has been used to irradiate BiVO₄ and Au nanoparticles loaded BiVO₄ (BiVO₄/Au) thin films deposited over fluorine doped tin oxide substrates via spray pyrolysis technique. Ion irradiation results in tailoring the optical, electrical, and morphological properties of the thin films and thence also responsible for changes in electrochemical properties. The scanning electron microscope images reveal the evolution of Au nanoparticles after irradiation at 2 × 10¹⁵ fluence to a nanourchins type of morphology. In consequence of morphological changes, the signature of surface plasmon resonance peak exhibited by Au nanoparticles in BiVO₄/Au shows improvement. An increase of approximately 92% in photocurrent density in comparison to pristine BiVO₄ has been found after irradiation in BiVO₄/Au photoanode at 2 × 10¹⁵ ions/cm² fluence. Moreover, irradiation also aids in improving photoelectrochemical response of BiVO₄ photoanodes without Au nanoparticles. The enhancement can be attributed to the notable changes in onset potential, charge separation, charge transfer resistance and optical properties.     

TiO2-rutile/anatase homojunction with enhanced charge separation for photoelectrochemical water splitting

The mismatched interfaces of heterojunction usually have lots of defects, deriving in recombination of generated electron-hole pairs. On the other hand, homojunction interfaces are considered to be beneficial to the separation of charge carriers due to the similar characteristics in two sides of homojunction. TiO₂ have rutile and anatase two typical photoactive phases in nature. In this work, TiO₂-rutile/anatase (TiO₂-R/A) homojunction photoanode is fabricated by in situ growth of anatase TiO₂ on TiO₂-R surface. By contrast with TiO₂-rutile/rutile (TiO₂-R/R) photoanode, TiO₂-R/A displays higher photocurrent density (1.70 mA cm^?2 at 0.6 V vs. SCE). Deep insight into the mechanism suggests that TiO₂-R/A homojunction has intense band bending and enhanced surface area, which facilitate the charge separation and transmission. This study offers some novel insights to design and fabricate semiconductors photoanodes for highly efficient photocatalytic reactions.     

Enhancement of hydrogen storage properties of Li12+xMg3-xSi4-ySny (x=y=0.48) phase by modification with LixZnO/La2O3-CNT composites

Two kinds of Li_xZnO-CNT and La₂O₃-CNT composite additives (CA) are employed to improve the hydrogen absorption/desorption processes of Li_12+xMg_3-xSi_4-ySn_y phase. The effect of the addition of composite additives on the improvement of hydrogen sorption kinetics of intermetallic phase is due to the following factors: CA are more stable than Li contain intermetallic; CA provides diffusion pathways to most of the grains inside the material, where the dissociation of H₂ molecules into H atoms takes place; CA surface has a relatively higher reactivity to H₂ dissociation. The substituted (Si atoms by Sn) and doped by La₂O₃-CNT composite Li_12+xMg_3-xSi_4-ySn_y alloy have the highest uptake and release capability (9.8 wt%). Li_xZnO-CNT give 9.3 wt% hydrogen uptake and release, which are still much better than undoped Li_12+xMg_3-xSi_4-ySn_y (8.9 wt%). Li_12+xMg_3-xSi_4-ySn_y is a maximally disordered intermetallic phase (MD-IP) with a cubic structure (I-43d, a = 10.7409(8) Å) related to Cu₁₅Si₄-type.     

Fabrication of BiVO4 photoanode catalyzed with bimetallic sulfide nanoparticles for enhanced photoelectrochemical water oxidation

The photoelectrochemical water oxidation ability of BiVO₄ is usually restrained by its low separation efficiency of the photogenerated charge and slow kinetics for water oxidation. Here, FeCoS₂ bimetallic sulfide nanoparticles are successfully coated on BiVO₄ photoanode to address these problems. The photocurrent density of FeCoS₂/BiVO₄ photoanode is 3.08 times that of BiVO₄ photoanode. Furthermore, the charge separation efficiency can reach 75% and the charge injection efficiency reaches nearly 78%. The onset potential of FeCoS₂/BiVO₄ shifts 300 mV negatively, while the incident photon-to-electron conversion efficiency value of FeCoS₂/BiVO₄ is 2.86 folds of BiVO₄. The enhancement benefits from the loading of FeCoS₂, which promotes the separation of photogenerated electron and hole, and inhibits the recombination of photogenerated charge and hole.     

2D/3D WO3/BiVO4 heterostructures for efficient photoelectrocatalytic water splitting

Developing novel photoanodes with high efficiency for photoelectrochemical (PEC) water splitting has become the key to solar energy conversion and storage realm. Herein, 3D worm-like bismuth vanadate (BiVO₄) is grafted on 2D thin tungsten trioxide (WO₃) underlayer by electrodeposition to form mixed–dimensional structured photoanode, resulting in significant improvement of the photocatalytic performance and the charge separation efficiency. Characterization results prove that the mixed–dimensional structured can boost the photocatalytic activity by suppressing back reaction and charge recombination of the bulk BiVO₄. Simultaneously, the electrical conductivity of photoanode can be increased by W⁶⁺ doping. Furthermore, a robust catalyst NiCo₂O_x is coated onto the surface of WO₃/BiVO₄ photoanode, exhibiting a desirable photocurrent of 3.85 mA cm^?2 at 1.23 V vs. RHE and an excellent stability over 3 h. Both the excellent photocurrent density and great operational stability of this 2D/3D WO₃/BiVO₄ photoanode make it a promising material for practical applications.