Partially oxidized cobalt species in nitrogen-doped carbon nanotubes: Enhanced catalytic performance to water-splitting

It's still an ongoing research challenge to explore non-precious metal-based catalysts for substituting precious metal catalysts during full water electrocatalysis. Herein, we reported the partially oxidized cobalt species in nitrogen-doped carbon nanotubes hierarchical structures to produce dual-functionality towards oxygen/hydrogen evolution reactions. The in situ transformation of carbon nanotubes and well-exposed metal-oxide contributes to mass diffusion and greater electrolyte-accessible surface area. The as-synthesized catalyst displays low overpotentials of 287 mV and 171 mV for oxygen and hydrogen evolution reactions at 10 mA cm^?2 of current density with remarkable performance during long-term stability. Furthermore, when employed as cathode and anode, a respectable performance of 1.68 V demonstrated our catalyst as an efficient bifunctional material for conducting water-splitting operation.     

Low-temperature water-splitting by sodium redox reaction

Thermochemical water-splitting by sodium redox reactions was investigated from material science point of view as a future hydrogen production method. The reaction system consists of three separate reactions, which are hydrogen generation by NaOH-Na reaction, metal separation by thermolysis of Na₂O, and oxygen generation by hydrolysis of Na₂O₂. Although the current techniques of thermochemical water-splitting required a temperature higher than 800 °C for whole reaction cycle, the sodium system was able to be operated below only 400 °C by using nonequilibrium techniques to control the entropy of the chemical reactions. Therefore, this system should be recognized as a potential water-splitting technique that can widely utilize any heat sources in contrast to the conventional methods.     

Chemical aspects of the water-splitting thermochemical cycle based on sodium manganese ferrite

Water thermolysis by means of the sodium manganese ferrite cycle for sustainable hydrogen production is reviewed, with particular focus on known elementary chemical processes taking place on solid substrates in the 600–800 °C temperature range. For the purpose, in-situ high temperature x-ray diffraction technique has been utilized to observe structural transformations produced by both temperature and reactive environment. The water-splitting reaction and the regeneration of initial reactants are described as multi-step reactions, in which the role of carbon dioxide, through carbonation and de-carbonation reactions is highlighted. A thermodynamic phase stability diagram is reported for the system MnFe₂O₄/Na₂CO₃/CO₂.     

Photoelectrochemical hydrogen production from water/methanol decomposition using Ag/TiO2 nanocomposite thin films

Though less frequently studied for solar-hydrogen production, films are more convenient to use than powders and can be easily recycled. Anatase TiO₂ films decorated with Ag nanoparticles are synthesized by a rapid, simple, and inexpensive method. They are used to cleave water to produce H₂ under UV light in the presence of methanol as a hole scavenger. A simple and sensitive method is established here to monitor the time course of hydrogen production for ultralow amounts of TiO₂. The average hydrogen production rate of Ag/TiO₂ anatase films is 147.9 ± 35.5 μmol/h/g. Without silver, it decreases dramatically to 4.65 ± 0.39 μmol/h/g for anatase TiO₂ films and to 0.46 ± 0.66 μmol/h/g for amorphous TiO₂ films fabricated at room temperature. Our method can be used as a high through-put screening process in search of high efficiency heterogeneous photocatalysts for solar-hydrogen production from water-splitting.     

Simple solid-state synthesis and improved performance of Ni(OH)2-TiO2 nanocomposites for photocatalytic H2 production

To reduce the cost of the traditional noble metal-loaded photocatalysts for H₂ production, low-cost Ni(OH)₂-TiO₂ nanocomposites were designed and synthesized by a simple room-temperature solid-state chemical method (RSCM), which is a facile, low-cost and eco-friendly manipulation. Various testing methods and tools were used to characterize the crystal structure, elemental composition, morphology, light absorption ability, fluorescent performance, photocurrent density, and photocatalytic activity of the obtained nanocomposites. The results indicated that RSCM can be used to synthesize Ni(OH)₂-TiO₂ nanocomposites with a small size (50 nm) and good dispersity. Compared to pure TiO₂, the obtained nanocomposites displayed excellent photocatalytic performance for H₂ production by photocatalytic water-splitting. The amount of hydrogen needed for the optional nanocomposite NOT-1 was 9180 μmol/g, which is 29 times that of the commercial P25. The reason for the improved performance for photocatalytic hydrogen production is that the existing Ni(OH)₂ in nanocomposites promoted the separation between the photogenerated electron and holes.     

Enhanced visible-light-photoconversion efficiency of TiO2 nanotubes decorated by pulsed laser deposited CoNi nanoparticles

The pulsed laser deposition (PLD) technique has been used to decorate TiO₂ nanotubes (NTs) with cobalt-nickel (CoNi) nanoparticles (NPs). The TiO₂ NTs were produced beforehand through the controlled anodic oxidation of titanium substrates. The effect of the nature of the PLD background gas (Vacuum, O₂ and He) on the microstructure, composition and chemical bondings of the CoNi-NPs deposited onto the TiO₂-NTs has been investigated. We found that the PLD CoNi-NPs have a core/shell (oxide/metal) structure when deposited under vacuum, while they are fully oxidized when deposited under O₂. On the other hand, by varying the CoNi-NPs loading of the TiO₂-NTs (through the increase of the number of laser ablation pulses (N_LP)), we have systematically studied their photocatalytic effect by means of cyclic-voltammetry (CV) measurements under both AM1.5 simulated solar light and filtered visible light. We show that depositing CoNi-NPs on the substrate under vacuum and He increases the photo-electrochemical conversion effectiveness (PCE) by 600% (at N_LP = 10,000) in the visible light domain, while their overall PCE degrades with N_LP under solar illumination. In contrast, the fully oxidized CoNi-NPs (deposited under O₂) are found to be the most effective catalyst under sunlight with an overall increase of more than 50% of the PCE at the optimum loading around N_LP ~1000. Such catalytic enhancement is believed to result from both an enhanced light absorption by CoO (of which bandgap is of ~2.4 eV) and the formation of a heterojunction between NiO/CoO nanoparticles and TiO₂ nanotubes.     

Thermal decomposition of (NH4)2SO4 in presence of Mn3O4

The main objective of this work is to develop a hybrid water-splitting cycle that employs the photon component of sunlight for production of H₂ and its thermal (i.e. IR) component for generating oxygen. In this paper, (NH₄)₂SO₄ thermal decomposition in the presence of Mn₃O₄, as an oxygen evolving step, was systematically investigated using thermogravimetric/differential thermal analyses (TG/DTA), temperature programmed desorption (TPD) coupled with a mass spectrometer (MS), X-ray Diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS) techniques. Furthermore, thermolysis of ammonium sulfate, (NH₄)₂SO₄, in the presence of Mn₃O₄ was also investigated by conducting flow reactor experiments. The experimental results obtained indicate that at 200-450 °C, (NH₄)₂SO₄ decomposes forming NH₃ and H₂O and sulfur trioxide that in the presence of manganese oxide react to form manganese sulfate, MnSO₄. At still higher temperatures (800∼900 °C), MnSO₄ further decomposed forming SO₂ and O₂.     

TiO2 photocatalyst with single and dual noble metal co-catalysts for efficient water splitting and organic compound removal

Photocatalysts can be used both for air cleaning and solar energy harvesting through water splitting. However, pure TiO₂ photocatalysts are often inefficient and therefore co-catalysts are needed to improve the yield. To achieve this goal, we prepared TiO₂ and deposited Pt, Ir and Ru co-catalysts on its surface. Two base TiO₂ nanoparticles were used: P25 and rutile TiO₂ synthesized via hydrothermal method. Co-catalysts were deposited by wet impregnation technique using single element and a combination of two elements (Pt and Ir or Pt and Ru), followed by annealing in either air or H₂/Ar. Annealing in reducing atmosphere increased the photocatalytic activity of oxidation of isopropanol compared to annealing in air. We demonstrated a clear influence of the co-catalysts on the photocatalytic degradation of isopropanol and on electrochemical water-splitting reaction. The platinum-containing samples showed the best HER activity.     

Monoclinic zirconia-supported Fe3O4 for the two-step water-splitting thermochemical cycle at high thermal reduction temperatures of 1400–1600 °C

Two-step thermochemical water-splitting using monoclinic ZrO₂-supported Fe₃O₄ (Fe₃O₄/m-ZrO₂) for hydrogen production was examined at high thermal reduction temperatures of 1400–1600 °C. After thermal reduction of Fe₃O₄/m-ZrO₂, the reduced sample was quenched in liquid nitrogen, and was subsequently subjected to the water-decomposition step at 1000 °C. Quenching of the solid sample was conducted for analysis of the chemical reactions, such as phase transitions, occurring at high-temperature. The hydrogen productivity of Fe₃O₄ on a m-ZrO₂ support and the conversion of Fe₃O₄ to FeO were significantly enhanced with higher thermal reduction temperatures. The Fe₃O₄-to-FeO conversion reached 60% when the Fe₃O₄/m-ZrO₂ was thermally reduced at 1600 °C. The phase transition of m-ZrO₂ support to tetragonal ZrO₂ (t-ZrO₂) did not occur during the thermal reduction at 1400–1500 °C, but it did proceed slightly at 1600 °C. Fe ions from Fe₃O₄ did not enter the ZrO₂ lattice during high-temperature thermal reduction. Thus, the Fe₃O₄ loaded on a m-ZrO₂ support can continuously contribute as a Fe₃O₄–FeO redox reactant for thermochemical water-splitting at high-temperatures of 1400–1600 °C.     

Solar water splitting for hydrogen production with monolithic reactors

The present work proposes the exploitation of solar energy for the dissociation of water and production of hydrogen via an integrated thermo-chemical reactor/receiver system. The basic idea is the use of multi-channelled honeycomb ceramic supports coated with active redox reagent powders, in a configuration similar to that encountered in automobile exhaust catalytic aftertreatment.Iron-oxide-based redox materials were synthesized, capable to operate under a complete redox cycle: they could take oxygen from water producing pure hydrogen at reasonably low temperatures (800 °C) and could be regenerated at temperatures below 1300 °C. Ceramic honeycombs capable of achieving temperatures in that range when heated by concentrated solar radiation were manufactured and incorporated in a dedicated solar receiver/reactor. The operating conditions of the solar reactor were optimised to achieve adjustable, uniform temperatures up to 1300 °C throughout the honeycomb, making thus feasible the operation of the complete cycle by a single solar energy converter.