Enhanced H2 Generation Research Articles

Graphene‐Rich G‐Co‐Ni Nanomatrix: An Optimized Heterogeneous Catalyst for Hydrogen Generation Based on Morphology‐Performance Mapping

AbstractGraphene‐rich bi‐metallic nano‐hybrid G1Co0.3Ni0.15 consisting of novel nano‐matrix structure, a low‐cost catalyst was formulated for enhanced H2 generation from hydrolysis of NaBH4. The formulation was based on the optimization of the performance of G−Co‐Ni nano‐catalysts by tuning their morphology. Three types of catalysts namely metal‐rich G1Co1.5Ni1.5, 1:1 Graphene‐Metal G1Co0.6Ni0.4, and Graphene‐rich G1Co0.3Ni0.15 were synthesized. These catalysts were characterized using SEM with EDAX, HR‐TEM, XRD and CV analysis and tested for H2 generation.Based on morphology, three major nano‐hybrid structure models namely nano‐ensembles, unique nano‐alloy structure, nano‐matrix were identified over the whole composition range. HRTEM analysis revealed that increase in wt% of graphene (from 25–70) transforms dispersed nano‐ensembles to the connected nano‐matrix. Among the three catalyst, Graphene‐rich G1Co0.3Ni0.15, exhibits the highest catalytic activity (TOF=16.5 min−1,ECSA= 286.5 m2/g)and electrochemical stability. The activation energy of the catalytic hydrolysis using G1Co0.3Ni0.15 catalyst is 23±1 kJ mol−1.

Photo-assisted self-optimizing of charge-carriers transport channel in the recrystallized multi-heterojunction nanofibers for highly efficient photocatalytic H2 generation

Rational arrangement of nanosized semiconductor components in the multi-heterojunction photocatalyst can lead to the formation of a high-speed transport channel for charge-carriers separation and migration, which provides a promising way to achieve excellent photocatalytic efficiency for solar fuels generation. Herein, we develop a photo-assisted self-optimization strategy to re-build the charge-carriers transport channel in the copper species nanocrystals/TiO2 electrospun nanofibers with the organic hydrogen-carrier molecules as the photo-reactants. As a result of the dynamics difference of the excitons migration during the photocatalytic process, the binary CuO/TiO2 heterojunction nanofibers are recrystallized to form the quaternary Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers that exhibits a higher rate constant (∼1.7×108S−1) for the interfacial electron-transfer than the former nanofibers (∼0.7×108S−1) due to the improved charge-carriers transport channel. In this way, a 40-fold enhanced H2 generation rate was observed on the recrystallized nanofibers photocatalyst during the photocatalytic decomposition of formic acid as compared to the pure TiO2 nanofibers. Our work presents an available paradigm to skillfully use a transient photo-physicochemical process to mildly engineer a high-quality charge-carriers transport channel in the hetero-nanophotocatalyst for realizing an optimal photocatalytic performance.

BiVO4 nanowires decorated with CdS nanoparticles as Z-scheme photocatalyst with enhanced H2 generation

Highly efficient photocatalysts require to be achieved via an efficient charge separation action and responded under visible light. Here, we demonstrate a novel Z-scheme heterojunction photocatalyst composed of two visible light responding semiconductors without use of electron mediator. The as-prepared CdS nanoparticles decorated BiVO4 nanowires (CdS/BiVO4 NWs) exhibit broader light absorption region and Z-scheme charge separation mechanism for photocatalytic H2 generation under visible light. As a results, the optimizing CdS/BiVO4 NWs in 1:2wt ratio shows more than two times increase in photocatalytic H2 generation rate compared to bare CdS. Moreover, the Z-scheme CdS/BiVO4 NWs lead to main oxidation sites at BiVO4 NWs that successfully avoid photo-corrosion of CdS during photocatalysis.

Fabrication of photostable ternary CdS/MoS2/MWCNTs hybrid photocatalysts with enhanced H2 generation activity

This article reports the synthesis and photocatalytic H2-generation of pristine and CdS-based hybrid photocatalysts. Acid-treated-multi-walled carbon nanotubes (a-MWCNTs) and colloids of the exfoliated two-dimensional MoS2 nanosheets were used for the preparation of binary CdS/a-MWCNTs and ternary CdS/MoS2/a-MWCNTs nanocomposites. Controlled exfoliation of as-synthesized bulk marigold flower-like MoS2 to few-layer MoS2 nanosheets shows a strong absorption and photoluminescence, indicating an indirect to direct bandgap transition. The visible-light photocatalytic H2-evolution performance of the ternary nanocomposites was superior to that of the binary and pure photocatalysts. Transient photocurrent results revealed that the ternary nanocomposite exhibits a maximum photocurrent density of 11.12mA/cm2, which is 12 times higher than that of pristine CdS. This enhanced H2-generation performance and photocurrent response is attributed to the CdS/MoS2 heterojunction, which increases charge-separation efficiency of the composite. Furthermore, the presence of a-MWCNTs, as a co-catalyst/support, accounts for this excellent performance involving electron transfer from photoexcited CdS to MoS2 edges through a-MWCNTs; this transfer results in enhanced charge separation. This enhanced charge-separation efficiency and photostability was demonstrated using a suitable mechanism, which was evident from photocurrent measurements and photoluminescence data. In such ternary hybrids, metal co-catalysts showed H2-evolution activity with the following order: Pd>Ni>Au>Ag>Cu. This work further highlights the role of MoS2/CNTs p-n junction embedded in CdS which serves as a promising heterojunction that promoted hydrogen production.

TiO2 Nanotubes: Nitrogen-Ion Implantation at Low Dose Provides Noble-Metal-Free Photocatalytic H2 -Evolution Activity.

Low-dose nitrogen implantation induces an ion and damage profile in TiO2 nanotubes that leads to "co-catalytic" activity for photocatalytic H2 -evolution (without the use of any noble metal). Ion implantation with adequate parameters creates this active zone limited to the top part of the tubes. The coupling of this top layer and the underlying non-implanted part of the nanotubes additionally contributes to an efficient carrier separation and thus to a significantly enhanced H2 generation.

TiO2 Nanotubes: Nitrogen‐Ion Implantation at Low Dose Provides Noble‐Metal‐Free Photocatalytic H2‐Evolution Activity

AbstractLow‐dose nitrogen implantation induces an ion and damage profile in TiO2 nanotubes that leads to “co‐catalytic” activity for photocatalytic H2‐evolution (without the use of any noble metal). Ion implantation with adequate parameters creates this active zone limited to the top part of the tubes. The coupling of this top layer and the underlying non‐implanted part of the nanotubes additionally contributes to an efficient carrier separation and thus to a significantly enhanced H2 generation.

Origin of H2 Evolution in LIBs: H2O Reduction vs. Electrolyte Oxidation

Gassing in lithium-ion batteries (LIBs) is a serious challenge, especially at high voltage and elevated temperature. In this study, we use On-line Electrochemical Mass Spectrometry (OEMS) and a two-compartment cell with a newly developed aluminum edge-seal to elucidate the origin of H2 evolution in LIBs. We demonstrate that the new sealing is entirely impermeable for gaseous and liquid species, thus allowing us to measure the true H2 evolution from H2O reduction at a graphite electrode, without interference from the lithium counter-electrode. We further report that graphite//NMC full-cells without any diffusion barrier between anode and cathode show enhanced H2 generation, especially for high charging potentials and at elevated temperature. We propose that the diffusion of protic electrolyte oxidation species (R-H+) from the cathode to the anode and their subsequent reduction is the origin of enhanced H2 gassing. To prove this hypothesis, methanesulfonic acid is added to the electrolyte as a chemical source of protons. At the negative graphite electrode, all H+ can be quantitatively reduced to H2. By the use of the electrolyte additives vinylene carbonate (VC) and lithium bis(oxalato) borate (LiBOB), less H2 evolution is observed, since the reduction of both H2O and R-H+ is hindered by a more effective SEI on graphite. Finally, we demonstrate that the Al-sealed diffusion barrier between anode and cathode can stop the diffusion of oxidation products to the anode and therefore essentially eliminates the generation of H2 caused by high cathode potentials.

A Novel Integrated Hydrothermal Liquefaction and Solar Catalytic Reforming Method for Enhanced Hydrogen Generation from Biomass

Short energy intensive hydrothermal liquefaction (HTL) of biomass in the presence of Ni salt catalyst selectively generates H2 in the product gas and biocrude mainly containing C1-C3 acids (formic, lactic, propionic, acetic), HMF and furfural. The H2 mass balance indicated that only 3.12 vol% H2 in biomass (cotton) was released as product gas; 48.7 vol% was captured in the C1-C3 acids while the remainder H2 was trapped in oxygenated compounds and char. Continuing HTL after 120 minutes caused no further increase in gas phase H2 yields. To enhance the H2 yields with minimal energy input, solar photocatalytic reforming (PR) of the biocrude with Pt/TiO2 catalyst was investigated. Photocatalysis of activated carbon (AC) treated biocrude generated an additional H2, 17.82 wt%. H2 yields from photoreforming of simulated biocrude acid mixture and actual biocrude were compared. Enhanced H2 generation was observed with integrated HTL-PR of biomass.

Oxygen vacancies and intense luminescence in manganese loaded Zno microflowers for visible light water splitting.

ZnO nanorods and Mn/ZnO microflowers with nano-sized petals exhibit singly ionized oxygen vacancies, V. This is strongly supported by a green photoluminescence emission at 2.22 eV and an EPR g value of 1.953, both of which are suppressed greatly after annealing in an oxygen atmosphere. A strong red emission observed during exposure to X-rays reveals the presence of F(+) centres as a consequence of the V. Mn/ZnO displayed enhanced H2 generation with visible light exposure, when compared to pure ZnO and annealed Mn/ZnO in the visible region, which directly correlated with the oxygen vacancy concentration. There is an interesting correlation between the intensities of the EPR lines at the g-value of 1.953 due to the oxygen vacancies, the intensity of light emitted from the exposure to X-rays, the intensity of the photoluminescence due to oxygen vacancies and the quantity of H2 produced by the photocatalytic effect when comparing the three different nanomaterials, viz. pure ZnO, Mn/ZnO before and after annealing, all having been made exactly by the same methodologies.

Dye-sensitized Pt@TiO2 core-shell nanostructures for the efficient photocatalytic generation of hydrogen.

Pt@TiO2 core–shell nanostructures were prepared through a hydrothermal method. The dye-sensitization of these Pt@TiO2 core–shell structures allows for a high photocatalytic activity for the generation of hydrogen from proton reduction under visible-light irradiation. When the dyes and TiO2 were co-excited through the combination of two irradiation beams with different wavelengths, a synergic effect was observed, which led to a greatly enhanced H2 generation yield. This is attributed to the rational spatial distribution of the three components (dye, TiO2, Pt), and the vectored transport of photogenerated electrons from the dye to the Pt particles via the TiO2 particle bridge.

Metal‐Free Catalysis of Ammonia–Borane Dehydrogenation/Regeneration for a Highly Efficient and Facilely Recyclable Hydrogen‐Storage Material

As easy as ABC: Mesoporous graphitic carbon nitride (MGCN; g-C3N4) is utilized to support ammonia borane (AB) on the basis of its accessible nanoporous structure and basic properties. A high loading of uniformly dispersed AB nanoparticles into the MGCN is possible giving greatly enhanced H2 generation from AB, and facile regeneration cycles by a hydrazine hydrogenation process, even at room temperature.

Three‐Dimensional CdS–Titanate Composite Nanomaterials for Enhanced Visible‐Light‐Driven Hydrogen Evolution

3D Hierarchical echinus-like titanate spheres (HETSs), serving as the supporting material for CdS nanoparticles, are synthesized via a fast one-step hydrothermal method. The obtained 3D CdS-HETS composite materials show enhanced H2 generation under visible light irradiation.