Promoting photoelectrochemical hydrogen evolution activity of CuBi2O4 photocathode through ramping rate control

P-type CuBi₂O₄ represents a very promising photocathode material with visible light response for dual absorber photoelectrochemical (PEC) water splitting device. However, its photocurrent reported is limited by its inefficient utilization of photogenerated carriers, which greatly hinders its practical application. Herein, CuBi₂O₄ films were prepared by a simple spin-coating method and four distinct ramping rates were applied to investigate the influence of annealing rate on its hydrogen production activity. Among four different ramping rates, the CuBi₂O₄ film annealed with the rate of 2 °C/min successfully achieved the highest photocurrent of 0.68 mA/cm² at 0.25 V_RHE, corresponding to 38%–83% improvement when compared to those prepared with other rates. To reveal the working mechanism of this simple but effective heating control, structural and electrochemical characterizations were performed. It was found that, the electronic structure of the best performing CuBi₂O₄ film was notably modified with the largest amount of low valence Cu⁺ species. Meanwhile, the highest carrier concentration was measured through electrode impedance investigation.     

Hierarchical NiS decorated CuO@ZnFe2O4 nanoarrays as advanced photocathodes for hydrogen evolution reaction

Rationally designed architecture and smart components of catalysts can greatly accelerate the hydrogen evolution reaction in photoelectrochemical water splitting. Herein, hierarchical NiS quantum dots decorated CuO nanowires@ZnFe₂O₄ nanosheets core/shell nanoarrays were prepared by a viable multi-step synthesis approach. First, CuO nanowire arrays were prepared through the thermal treatment of copper mesh. Then, CuO@ZnO core/shell nanowire arrays were prepared via an impregnation-calcination process. Next, the CuO nanowire arrays with different ZnFe₂O₄ nanosheet contents were prepared through wet chemical reaction and subsequent thermal treatment. The further NiS quantum dots decoration was realized through a chemical bath deposition. The CuO nanowire arrays covered with ZnFe₂O₄ porous nanosheets not only offer abundant active sites to react with the electrolyte but also improve visible light utilization. Moreover, the hierarchical nanoarray structure provides a direct electron transport pathway with a graded interface for better charge flow. As a result, remarkably enhanced photoelectrochemical performance and excellent cycling stability were obtained for the CuO@ZnFe₂O₄ nanoarray photocathodes due to the synergistic effects of ideal components and hierarchical nanoarray structure. Additionally, the further NiS decoration makes CuO@ZnFe₂O₄ exhibit a significantly enhanced photocathodic current density.     

CuO/CuBi2O4 bilayered heterojunction as an efficient photocathode for photoelectrochemical hydrogen evolution reaction

The poor photostability and low photoactivity are two bottlenecks limiting the application of CuO and CuBi₂O₄ as competitive candidates for photoelectrochemical (PEC) hydrogen evolution reaction (HER). To overcome the bottlenecks, we constructed a novel CuO/CuBi₂O₄ bilayered structure for PEC HER. The underlying CuO layer functions as the main photoabsorber, while the outside CuBi₂O₄ layer acts as a protection shield. After further decorated with the NiO_x electrocatalysts, the CuO/CuBi₂O₄/NiO_x photocathode exhibits a high photoactivity and remarkable photostability. We ascribe this excellent performance to the following factors: (1) the bilayer structure improves light harvesting efficiency, (2) the outside CuBi₂O₄ enhances the photostability, (3) the favorable band alignment increases charge separation efficiency, and (4) the presence of NiO_x facilitates the charges transfer at the interface. Therefore, this work not only sets a new benchmark efficiency for the CuO and CuBi₂O₄ heterojunction, but also provides principles for designing layered heterojunction for PEC water splitting.     

Cobalt as a sacrificial metal to increase the photoelectrochemical stability of CuBi2O4 films for water splitting

CuBi₂O₄ is an excellent photocathode candidate in water-splitting photoelectrochemical cells. However, its poor photoelectrochemical stability caused by the reduction of Cu²⁺ to Cu metal limits its use. Here, we show a strategy to decrease the reduction of Cu²⁺ to Cu using cobalt as a sacrificial metal. Co-doped CuBi₂O₄ films were prepared by spray pyrolysis using Co²⁺ salt as the precursor. Co²⁺ ions replace Cu²⁺ in the CuBi₂O₄ structure, and subsequent heat treatment at 500 °C leads to partial oxidation of Co²⁺ to Co³⁺. As the reduction potential of Co³⁺/Co²⁺ is higher than that of Cu²⁺/Cu, Cu²⁺ reduction can be minimized. Comparatively, about 72% of the photocurrent produced by the CuBi₂O₄ film is lost in the first few minutes of illumination. In the Co-doped CuBi₂O₄ film, the photocurrent drops by less than 7%. Thus, the Co-doping can increase the CuBi₂O₄ photostability and be helpful for the fabrication of more stable photocathodes.     

Enhancement of photoelectrochemical hydrogen production by using a novel ternary Ag2CrO4/GO/MnFe2O4 photocatalyst

Novel Ag₂CrO₄/GO/MnFe₂O₄ photocatalysts synthesized by precipitation method and fabricated on indium tin oxide surface by electrophoretic deposition technique. The obtained Ag₂CrO₄/GO/MnFe₂O₄ composite has been verified by microscopic study, chemical and structural analyses. Also, the ternary composite showed higher photocurrent response and weaker photoluminescence spectrum than MnFe₂O₄, GO, Ag₂CrO₄ and GO/MnFe₂O₄. Photo-cathodes prepared using semiconductor photocatalyst powders were investigated for visible-light-driven photoelectrochemical hydrogen evolution. The best result of the photoelectrochemical hydrogen production in the presence of Ag₂CrO₄/GO/MnFe₂O₄ composite was determined as 446.93 μmol.cm⁻² in 90 min under visible light illumination. So, the enhanced hydrogen producing over the ternary composite was obtained. This study presents the development of stable visible light active photocatalytic materials and the design of efficient for the advancement of hydrogen production via photoelectrochemical water splitting in the future.     

Effects of grain size and deuteration conditions on the microstructure of erbium deuteride films

Erbium is widely considered as a promising tritium storage material due to its high capacity and stability. In this work, the influences of the grain size of original erbium and the deuteration conditions on the microstructure of erbium deuteride film were studied by SEM, FIB-TEM and XRD. The results indicate that the erbium films with small grains transformed into large grains after deuteration, whereas those with large grains changed to smaller grains. The lattice expansion after the loading of deuterium was measured, which confirmed that the films possess small grains present a weaker expansion (7.7%) than those with large grains (10.8%). All of the as-prepared erbium deuteride films exhibited a strong (111) preferred orientation and the film with smaller change of grain size during deuteration presents much stronger preferential growth. It is demonstrated that the deuteration temperature and pressure have a significant effect on the grain size of erbium deuteride films, which the higher deuteration temperature and pressure would induce a bigger change of grain size. In final, the underlying cause of the effect of grain size and deuteration conditions is discussed based on the thermodynamic and kinetics. The higher reaction rate under high deuteration temperature and pressure would prefer to increase the internal stress and the transformation of grain size.     

Enhanced photoelectrocatalytic hydrogen production performance of porous MoS2/PPy/ZnO film under visible light irradiation

Photoelectrochemical (PEC) water splitting provides a “green” approach for hydrogen production. However, the design and fabrication of high-efficient catalysts are the bottleneck for PEC water splitting owing to the involved thermodynamic and kinetic challenges. Herein, we report a new strategy for constructing a porous MoS₂/PPy/ZnO thin film photocatalyst with large specific surface area and excellent conductivity to achieve photoelectrochemical water splitting under visible light irradiation. Porous PPy/ZnO was synthesized via template-assisted electrodeposition, and MoS₂ was further electrodeposited to construct porous MoS₂/PPy/ZnO thin film photocatalyst. The hydrogen evolution rate of MoS₂/PPy/ZnO exhibits about 3.5-fold increase to 40.22 μmol cm⁻² h⁻¹ under visible light irradiation. The enhancement for photoelectrochemical hydrogen production is not only ascribed to enlarged specific surface area of the porous structure, but also attributed to the synergistic effects of MoS₂ and porous PPy/ZnO, which could dramatically improve its visible light absorption capacity and enhance the separation and transfer of photogenerated charges. Thus, more abundant photogenerated electrons and holes participate in photoelectrochemical process, which significantly enhances its photoelectrochemical hydrogen production performance.     

Effect of MgO addition and grain size on the electrical properties of Ce0.9Gd0.1O1.95 electrolyte for IT-SOFCs

The aim of this study is to investigate the effect of grain size on the electrical properties of Ce_0.9Gd_0.1O_1.95-x mol% MgO (GDC-xMgO) and to evaluate them as electrolytes for use in intermediate-temperature solid oxide fuel cells (IT-SOFCs). For this purpose, GDC-xMgO (x = 0–15) electrolytes were synthesized by the glycine-nitrate process and sintered at different temperatures. Impedance spectroscopy measurements revealed that for each composition, the grain-boundary resistivity decreased with decreasing grain size for the samples with grain size of >0.4 μm. Much too small grain sizes (0.2 < d_g < 0.3 μm) produced an increase in grain-boundary resistivity. The addition of MgO could weaken the influence of grain sizes on the grain-boundary resistivity. The interfacial polarization resistances could be decreased by adding MgO to GDC. The GDC-1MgO sample sintered at 1200 °C exhibited the highest total conductivity of 8.11 × 10^?2 S cm^?1 at 800 °C. The maximum power density of the GDC-1MgO-based cell was 0.73 W cm^?2 at 800 °C, which was much higher than that of the GDC-based cell. The results indicated that the GDC-1MgO was a potential electrolyte for IT-SOFCs.     

Influence of electrolytes on the photovoltaic performance of organic dye-sensitized nanocrystalline TiO2 solar cells

We have studied the influence of electrolytes on the photovoltaic performance of mercurochrome-sensitized nanocrystalline TiO₂ solar cells using LiI, LiBr, and tetraalkylammonium iodides as the electrolyte. Short-circuit photocurrent density (J_sc) and open-circuit photovoltage (V_oc) depended strongly on the electrolyte. J_sc of 3.42 mA cm⁻² and V_oc of 0.52 V were obtained for the LiI electrolyte and J_sc of 2.10 mA cm⁻² and V_oc of 0.86 V were obtained for the Pr₄NI electrolyte. This difference in photovoltaic performance was due to the change in the conduction band level of the TiO₂ electrode. Large V_oc of 0.99 V was obtained for the LiBr electrolyte due to the large energy gap between the conduction band level of TiO₂ and the Br/Br₂ redox potential. Solar cell performance also depended strongly on organic solvent, suggesting that the physical properties of solvents such as Li ion conductivity and donor number affect photovoltaic performance.     

Influence of reduction temperature on Ni particle size and catalytic performance of Ni/Mg(Al)O catalyst for CO2 reforming of CH4

Hydrotalcite-derived Ni/Mg(Al)O is promising for CH₄–CO₂ reforming. However, the catalysts reported so far suffer from sever coking at low temperatures. In this work, we demonstrate that a significant improvement of coke-resistance of Ni/Mg(Al)O can be achieved by fine tuning the Ni particle size through adjusting the reduction condition of catalyst. Ni particles having average size within 4.0–7.1 nm are in situ generated by reducing the catalyst at a selected temperature within 923–1073 K. Controllability of Ni particle size is related to the formation of Mg(Ni,Al)O solid solution upon hydrotalcite decomposition. It is found that the catalyst reduced at 973 K exhibits high activity, stability, and coke-resistance even at reaction temperature as low as 773 K. In contrast, the catalyst reduced at 923 K has low activity and deactivates due to Ni oxidation, while those reduced at 1023 and 1073 K suffer from sintering and severe coking. STEM and O₂-TPO reveal that coke deposition is directly proportional to the Ni particle size but becomes negligible at a size below 6.2 nm. It is evidenced that a critical size of about 6 nm is required to inhibit coking effectively. CO₂ temperature-programmed surface reaction indicates that the deposited carbon on small Ni particles can be easily removed by the CO₂ activated at the Ni–Mg(Al)O interfaces, accounting for the better resistance to coking.     

Effect of grain size on hydrogen embrittlement in stable austenitic high-Mn TWIP and high-N stainless steels

Two stable austenitic steels, 20Cr-11Ni-5Mn-0.3N (wt%) stainless steel (STS) and 18Mn-1.5Al-0.6C (wt%) twinning-induced plasticity steel (TWIP), were investigated to understand the effect of grain size on hydrogen embrittlement (HE). Grain refinement promoted HE in the STS but suppressed HE in the TWIP. These opposite effects occurred because the steel composition affected deformation mechanism. Cr-N pair enhanced short-range ordering (SRO) in STS, which promoted planar slip and delayed mechanical twinning. In contrast, TWIP exhibited mechanical twinning which was more active in coarser grains. Final dislocation density after tensile deformation was increased by grain refinement in STS, but was decreased in TWIP. The damaging effects of hydrogen on strain energy at interfaces and on interfacial bonding strength were controlled by dislocation density; therefore, increase in dislocation density led to increase in susceptibility to HE.     

Influence of grain boundary misorientation on hydrogen embrittlement in bi-crystal nickel

Computational techniques and tools have been developed to understand hydrogen embrittlement and hydrogen induced intergranular cracking based on grain boundary (GB) engineering with the help of computational materials engineering. This study can help to optimize GB misorientation configurations by identifying the cases that would improve the material properties increasing resistance to hydrogen embrittlement. In order to understand and optimize, it is important to understand the influence of misorientation angle on the atomic clustered hydrogen distribution under the impact of dilatational stress distributions. In this study, a number of bi-crystal models with tilt grain boundary (TGB) misorientation angles (θ) ranging between 0°≤ θ ≤ 90° were developed, with rotation performed about the [001] axis, using numerical microstructural finite element analysis. Subsequently, local stress and strain concentrations generated along the TGB (due to the difference in individual neighbouring crystals elastic anisotropy response as functions of misorientation angles) were evaluated when bi-crystals were subjected to overall uniform applied traction. Finally, the hydrogen distribution and segregations as a function of misorientation angles were studied. In real nickel, as opposed to the numerical model, geometrically necessary dislocations are generated due to GB misorientation. The generated dislocation motion along TGBs in response to dilatational mismatch varies depending on the misorientation angles. These generated dislocation motions affect the stress, strain and hydrogen distribution. Hydrogen segregates along these dislocations acting as traps and since the dislocation distribution varies depending on misorientation angles the hydrogen traps are also influenced by misorientation angles. From the results of numerical modelling it has been observed that the local stress, strain and hydrogen distributions are inhomogeneous, affected by the misorientation angles, orientations of neighbouring crystal and boundary conditions. In real material, as opposed to the numerical model, the clustered atomic hydrogens are segregated in traps near to the TGB due to the influence of dislocations developed under the effects of applied mechanical stress. The numerical model predicts maximum hydrogen concentrations are accumulated on the TGB with misorientation angles ranging between 15°< θ < 45°. This investigation reinforces the importance of GB engineering for designing and optimizing these materials to decrease hydrogen segregation arising from TGB misorientation angles.     

Effect of pigment concentration and particle size of TiO2 support on performance of chemochromic hydrogen tape sensor

A chemochromic hydrogen tape sensor has been developed to detect hydrogen leaks using titania (TiO₂) supported palladium oxide (PdO) pigments encapsulated within a silicone matrix. This study has been carried out to investigate the effects of pigment (PdO-TiO₂) concentration and particle size of TiO₂ support on detection performance in terms of color contrast of a chemochromic hydrogen tape sensor. The irreversible hydrogen tape sensors were tested with different concentration from 0.2 wt% to 10.0 wt%. Several pigments were synthesized using three different TiO₂ support with particle sizes ranging from 100 nm to 5 μm. The experimental results exhibit that the color of the pigment with 0.2 wt% shows distinctive color change in minimum and the optimal pigment concentration for silicone matrix type irreversible tape sensor is 3.0 wt%. In addition, TEM analysis revealed that the PdO particles become larger and agglomerated as increasing the particle size of TiO₂ support. The pigment with Aldrich TiO₂ particle size ≤5 μm has a good performance than smaller one.     

Influence of convection and grain movement on globular equiaxed solidification

A two-phase volume averaging model was used to study convection and grain movement, and their influence on the globular equiaxed solidification. Both liquid and solid phases were treated as separate interpenetrating continua. The mass, momentum, species and enthalpy conservation equations for each phase and a grain transport equation were coupled. An ingot casting (Al-4 wt.% Cu) with near globular solidification morphology was simulated. Case studies with different modeling assumptions such as with and without grain movement, and with slip and non-slip boundary conditions for solid phase were presented and compared. Understanding of grain evolution and macrosegregation formation in globular equiaxed solidification was improved.     

S-scheme heterojunction of CuBi2O4 supported Na doped P25 for enhanced photocatalytic H2 evolution

Rational design and construction of Step-scheme (S-scheme) photocatalyst have attracted considerable attention in the photocatalytic field. In this study, a series of Step-scheme (S-scheme) CuBi₂O₄/Na doped P25 photocatalysts (CBO/Na–P25) were synthesized through a facile hydrothermal method. CBO/Na–P25 photocatalyst without co-catalyst exhibited good photocatalytic performance for hydrogen evolution, and the H₂ yield rate of the 7.5%CBO/Na–P25 sample achieved 2695.73 μmol/g_cat/h after 4 h of illumination, which was more than 37.8, 13.7 and above 100 times those of pure P25, Na–P25 and CuBi₂O₄. The calculated apparent band gap of P25 decreased by Na doping and the construction of CuBi₂O₄/Na–P25 heterojunction contributed to efficient interfacial charge separation. Meanwhile, the S-scheme carrier transfer mechanism for the CuBi₂O₄/Na–P25 heterojunction photocatalyst was verified by the experimental results under different light wavelengths. This work demonstrated that the P25 based S-scheme heterojunction is a promising photocatalyst for hydrogen evolution.     

Elaboration,physical and photo-electrochemical properties of NiCr2O4. Application to hydrogen production under visible irradiation

The physical and photoelectrochemical characterization of NiCr₂O₄, prepared by sol gel route, were investigated to be applied for the H₂ production. The thermal gravimetry (TG) indicates that the single phase is formed above 530 °C as confirmed by X-ray diffraction (XRD). The Nano powder crystallizes in a tetragonal structure with lattice constants: a = 8.3276 Å and c = 8.5542 Å and a particle size of 63 nm, smaller than that obtained by Transmission Electronic Microscopy (TEM) analysis; the latter gives sizes between 80 and 150 nm, indicating crystallites agglomeration. The variation of the dielectric constant (ε) with temperature gives a relative value of 26 at 10 kHz. A direct optical transition at 1.79 eV is determined from the diffuse reflectance spectroscopy assigned to Cr³⁺ octahedrally coordinated. The thermal variation of the conductivity shows that 3d-electrons are localized and the data are modelled by a lattice-polaron hopping with an activation energy of 0.17 eV. The dependence of the interfacial capacitance on the potential (C⁻² - E) indicates p-type behavior with a flat band potential (E_fb) of −0.23 VSCE and holes density (N_A) of 5.88 × 10¹⁶ cm⁻³. The potential of the conduction band (−1.85 V_SCE) is below the H₂O/H₂ level (∼-1.2 V_SCE), allowing a spontaneous H₂-release under visible light. The O₂ evolution occurs at high over-voltage as shown from the intensity-potential (J-E) characteristic in Na₂SO₄ solution (0.1 M) and a hole scavenger was used to preclude the photo corrosion. The NiCr₂O₄ mass, pH and the hole scavenger (S₂O₃²⁻ and NO₂⁻) were optimized. The H₂ volume reached 65 μmol with an evolution rate of 8.6 μmol g⁻¹ min⁻¹, liberated under optimized conditions {1.2 g catalyst L⁻¹, pH ∼9 with thiosulfate S₂O₃²⁻ [10⁻³ M]}.     

Influence of Pr substitution on defects,transport, and grain boundary properties of acceptor-doped BaZrO3

We report on effects of partially substituting Zr with the multivalent Pr on the conductivity characteristics of acceptor (Gd) doped BaZrO₃-based materials. BaZr_0.6Pr_0.3Gd_0.1O_3−δ was sintered 96% dense at 1550 °C with grains of 1–4 μm. The electrical conductivity was characterised by impedance spectroscopy and EMF transport number measurements as a function of temperature and the partial pressures of oxygen and water vapour. H₂O/D₂O exchanges were applied to further verify proton conduction. The material is mainly a mixed proton–electron conductor: the p-type electronic conductivity is ∼0.004 and ∼0.05 S/cm in wet O₂ at 500 and 900 °C, respectively, while the protonic conductivity is ∼10⁻⁴ S/cm and ∼10⁻³ S/cm. The material is expectedly a pure proton conductor at sufficiently low temperatures and wet conditions. The specific grain boundary conductivity is essentially equal for the material with or without Pr, but the overall resistance is significantly lower for the former. We propose that replacing Pr on the Zr site reduces the grain boundary contribution due to an increased grain size after otherwise equal sintering conditions.     

Enhanced photoelectrochemical performance of 2D core-shell WO3/CuWO4 uniform heterojunction via in situ synthesis and modification of Co-Pi co-catalyst

In order to enhance the photoelectrochemical (PEC) performance of tungsten oxide (WO₃), it is critical to overcome the problems of narrow visible light absorption range and low carrier separation efficiency. In this work, we firstly prepared the 2D plate-like WO₃/CuWO₄ uniform core-shell heterojunction through in-situ synthesis method. After modification with the amorphous Co-Pi co-catalyst, the ternary uniform core-shell structure photoanode achieved a photocurrent of 1.4 mA/cm² at 1.23 V vs. RHE, which was about 6.67 and 1.75 times higher than that of pristine WO₃ and 2D uniform core-shell heterojunction, respectively. Furthermore, the onset potential of 2D WO₃/CuWO₄/Co-Pi core-shell heterojunction occurred a negatively shifts of about 20 mV. Experiments illuminated that the enhanced PEC performance of WO₃/CuWO₄/Co-Pi photoanode was attributed to the broader light absorption, reduced carrier transfer barrier and increased carrier separation efficiency. The work provides a strategy of maximizing the advantages of core-shell heterojunction and co-catalyst to achieve effective PEC performance.     

Pluronic-assisted modifications of FeOOH precursor facilitate morphology adjustment and performance enhancement of Fe2O3 photoanodes

Morphology regulation and surface modification are crucial strategies to improving the photoelectrochemical water oxidation performance of Fe₂O₃ photoanodes. In this study, Pluronic F127-assisted synthesis and post-treatment were adopted to achieve surface modification of FeOOH nanorods prepared by hydrothermal technique, thereby adjusting the morphology and surface properties of Fe₂O₃ photoanodes after calcination. Although the morphology of FeOOH barely changed, the creation of porous nanorods through F127-assisted synthesis and morphological change from worm-like nanorods into nanoplates by F127-assisted post-treatment were realized, and the electrochemically active surface area, crystallinity, number of surface disorders, and photoabsorption property were affected. Furthermore, relatively high intensity of lattice defects and low-valent ferrous ions (Fe²⁺) were generated after F127-assisted synthesis, and charge transfer from the surface states was increased. Consequently, Fe₂O₃ photoanode subjected to F127-assisted synthesis exhibited a reduction in the onset potential by 60 mV. The photocurrent density of Fe₂O₃ increased by 77% at 1.23 V versus reversible hydrogen electrode following a synergistic effect of F127-assisted synthesis and post-treatment.     

Effects of grain boundaries on cell performance of poly-silicon thin film solar cells by 2-D simulation

The effect of grain boundaries on the performance of poly-Si thin film solar cells was studied theoretically using a 2-D simulation assuming the presence of either rectangular-shaped or graded width grain boundaries in the i-layer of p/i/n structure of solar cells. The grain boundary had an adverse effect mainly on V_oc. J_sc gradually increased and saturated with increasing solar cell thickness in cells without grain boundaries, whereas it reached a maximum for an i-layer thickness of 5 μm in polycrystalline silicon cells. The calculation using the graded width model showed that the efficiency of the p⁺/p⁻/n⁺ structure was better than that of the p⁺/n⁻/n⁺ structure. A slight p-type doping of the i-layer was found to be effective in improving cell performance.