Titanium dioxide-coated copper electrodes for hydrogen production by water splitting

Pt is the most commonly used electrode and catalyst materials for H₂ production via water splitting as it provides the highest Gibbs free energy of H₂ adsorption (ΔG_H) an d overpotential. However, as Pt catalysts are expensive and difficult to mass-produce, several efforts have been made to identify suitable substitutes. Although Cu provides lower ΔG_H and overpotential than Pt, it exhibits better catalytic performance than other catalysts and is suitable for H₂ production. However, corrosion of Cu may affect its stability of Cu electrode. To overcome this limitation, we have coated a layer of carbon on the copper electrode and then synthesized titanium dioxide-(TiO₂-) on the C/Cu electrode for water splitting application. Carbon black (CB) has excellent electrical conductivity and stable resistance for effective working as an electrochemical catalyst, and TiO₂ has diverse applications because of its low-cost, non-toxic, and corrosion-resistant characteristics. In this study, TiO₂ was synthesized on C/Cu electrodes under UV irradiation for different durations. The optimum irradiation duration was determined to be 15 min via surface and electrochemical analyses. To identify the potential applications of this TiO₂–C/Cu electrode, we used artificial wastewater as the electrolyte. The synthesized TiO₂–C/Cu electrode exhibited better stability than C/Cu electrode. Further, H₂ production with TiO₂–C/Cu electrode was higher than that with C/Cu electrode at the same current density. We also investigated the effect of TiO₂–C/Cu electrode on decomposition of formaldehyde.     

Electrochemical high temperature technology for hydrogen production or direct electricity generation

Solid oxide electrolyte cells have been developed for hydrogen production from water vapour with high overall efficiency. On the basis of experimental data plant concepts have been designed for autothermal electrolysis operation with low temperature (∼150°C) steam input at a mean cell voltage of ∼1.3 V. The production of modules of series connected cells is under way in order to demonstrate the vapour electrolysis in a small, but complete plant for ∼3.5 kW hydrogen output. The actual state of the art of this technology will be described. It has been demonstrated that the solid oxide cells can be operated also in the reverse mode acting as fuel cells for hydrogen as well as for carbon monoxide. Calculations of overall performance of high temperature fuel cell plants show that, e.g. for electricity generation from natural gas, efficiencies of about 60% can be achieved with the additional advantage of extremely low NO_x emission.     

Experimental study of direct solar photocatalytic water splitting for hydrogen production under natural circulation conditions

Photocatalytic water splitting for hydrogen production provides a promising route for the future hydrogen economy, being operational in the visible light domain with a potential use of solar radiation. An outdoor pilot demonstration of CPC-based photoreactors has been designed, installed and tested at the State Key Laboratory of Multiphase Flow in Power Engineering to assess its effectiveness in solar photocatalytic hydrogen production. Nine sets of CPC-based photoreactors, each of which is 3.6 m² in area and 23 L in volume, are connected, controlled and operated in parallel. The high efficiency photocatalyst (Cd_1-xZn_xS), low concentration sacrifice agents (Na₂S and Na₂SO₃) and deionized water are the raw materials of the pilot system. Two operation models, viz. the natural circulation model and the gas disturbance model, are proposed considering the expense and the efficiency. From our observations, the slurry temperature inside the tubes rises by 20–30 °C from the ambient. The slurry velocity can reach 1.2 m/s in the gas disturbance model, but is as low as 3.5 cm/s in the natural circulation model. The average hydrogen productivity is 184.30 mL/min and accumulated to be 10.321 L/h in the natural circulation model, with the average solar radiation, photocatalyst concentration and sacrifice agents' concentration being 803.8 W/m², 2.77 g/L and 0.1 mol/L, respectively.     

Experimental study on direct solar photocatalytic water splitting for hydrogen production using surface uniform concentrators

In this paper, a direct solar photocatalytic water splitting system with surface uniform concentrators (SUCs) is designed. Parameters influencing hydrogen production rates and energy conversion, viz. sacrificial agent concentration, catalyst concentration and circulation speed, are analyzed under typical days. It is found that the system with SUCs has better performances with higher sacrificial agent, higher catalyst concentration and lower speed: double and triple concentration of the sacrificial agent will improve the energy conversion efficiency by 4.52% and 19.35%, respectively; double and triple the photocatalyst concentration will improve the energy conversion efficiency by 81.32% and 200.00%, respectively; energy conversion efficiency under valve-half-closed and the valve-closed conditions are improve by 21.82% and 118.18% comparing with the valve-open condition.     

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.     

Pulsed current water splitting electrochemical cycle for hydrogen production

A pulsed current 3 D MnO₂ electrode water splitting electrochemical cycle is being proposed for hydrogen production. In 3D MnO₂ electrochemical cycle, the reactions take place at the solid/liquid and solid/gas two phase boundaries. Also, this electrochemical cycle should be able to generate hydrogen and oxygen gas separately at different periods of time. Here, we applied an interrupted pulsed current to reduce the overpotential caused by diffusion layers in conventional direct current electrolysis. The pulsed current, which disturbs the formation of the ion diffusion layer in the vicinity of the electrodes, is observed to be effective above 50 Hz. The best electrolysis performance was recorded at a current density of 0.2 A cm^?2, and the observed cell voltage was 1.69 V at 25 °C for a pulse frequency of 500 Hz, which is less than the corresponding conventional alkaline electrolysis.     

A novel composite photocatalyst for water splitting hydrogen production

A novel composite photocatalyst, Pt-TiO_2−xN_x-WO₃ was synthesized by the template method and characterized by X-ray diffraction (XRD), ultraviolet–visible light diffusion spectroscopy (UV–vis), and element analysis. XRD spectra indicated that the photocatalyst was in anatase form and the diagnostic peak for WO₃ existed. Combined with the XRD spectra and the results of elemental analysis, the formula of the composite photocatalyst was determined to be Pt-TiO_2−xN_x-WO₃. UV–vis spectra showed the absorption edge was red-shifted to around 750 nm. Under the irradiation of ultraviolet, and with Na₂S/Na₂SO₃ as the sacrificial reagent, the composite photocatalyst showed higher hydrogen production activity than anatase TiO₂, and under irradiation with visible light (λ > 400 nm) it showed higher hydrogen production activity than TiO_2−xN_x while the anatase TiO₂ showed negligible activity. An explanation was put forward for the mechanism of the red-shift of the absorption edge and the hydrogen production activity improvement.     

Thermally oxidized iron oxide nanoarchitectures for hydrogen production by solar-induced water splitting

Thermally oxidized iron oxide (α-Fe₂O₃, Hematite) nanostructures are investigated as photoanodes that convert solar energy into hydrogen by splitting water. α-Fe₂O₃ is stable for water photo-oxidation, it has a favorable band gap energy and is a non-toxic common material. However, α-Fe₂O₃ photoanodes suffer from high loss due to electron-hole recombination; therefore nanoarchitectures with high aspect ratio that allows photons to be absorbed close to the photoanode/electrolyte interface are preferred. The thermal oxidation of iron is a simple way to produce nanostructured iron oxide electrodes. Different morphologies, aspect ratios, and oxide thicknesses result depending on the process parameters. Nanorod structures were obtained by annealing iron foils in oxygen rich atmosphere, whereas annealing in oxygen lean atmosphere resulted in nanocoral-like morphology. The nanorod-structured photoanodes achieved moderate photocurrent density of 0.9 mA/cm² while the nanocoral morphology achieved 2.6 mA/cm² (both at 1.8 V vs. the reversible hydrogen electrode). The effect of the oxidation process and oxide layer on performance is discussed.     

Solar hydrogen production from the thermal splitting of methane in a high temperature solar chemical reactor

This study addresses the single-step thermal decomposition (pyrolysis) of methane without catalysts. The process co-produces hydrogen-rich gas and high-grade carbon black (CB) from concentrated solar energy and methane. It is an unconventional route for potentially cost effective hydrogen production from solar energy without emitting carbon dioxide since solid carbon is sequestered.A high temperature solar chemical reactor has been designed to study the thermal splitting of methane for hydrogen generation. It features a nozzle-type graphite receiver which absorbs the solar power and transfers the heat to the flow of reactant at a temperature that allows dissociation. Theoretical and experimental investigations have been performed to study the performances of the solar reactor. The experimental set-up and effect of operating conditions are described in this paper. In addition, simulation results are presented to interpret the experimental results and to improve the solar reactor concept. The temperature, geometry of the graphite nozzle, gas flow rates, and CH₄ mole fraction have a strong effect on the final chemical conversion of methane. Numerical simulations have shown that a simple tubular receiver is not enough efficient to heat the bulk gas in the central zone, thus limiting the chemical conversion. In that case, the reaction takes place only within a thin region located near the hot graphite wall. The maximum CH₄ conversion (98%) was obtained with an improved nozzle, which allows a more efficient gas heating due to its higher heat exchange area.     

Effect of carbon coating on Cu electrodes for hydrogen production by water splitting

In recent years, fossil fuel depletion has been increasing, which leads to environmental issues. Hydrogen energy is considered a promising renewable energy to replace fossil fuels because it is a sustainable, clean, and green energy source. Among hydrogen production methods, water splitting has the highest reliability and is used the most often. Platinum is normally used as water splitting catalyst and an electrode. However, there has been much effort to replace it as such owing to its high cost. Copper (Cu) is not used as water splitting catalyst or an electrode, despite its high current density, because of its corrosive properties. In this study, carbon was coated onto a Cu substrate and a hydrogen production experiment was carried out with 0.1 M Na₂SO₄ and 0.1 M H₂SO₄ electrolytes. As a result, the carbon coating decreased oxidation rate of the Cu electrode and effected stability in short-term hydrogen evolution experiment. This indicates the possibility of carbon-Cu electrode with other catalytic materials.     

新型太阳能制氢系统的分析与研究进展

本文对太阳能分解水制氢系统进行了理论和应用方面的分析,重点介绍了光解水的光热电化学分析。对近几年利用太阳能光解水制氢的进展进行了概述,并指出了它们目前存在的缺点。介绍了通过光热化学循环进行太阳能分解水制氢的新途径,并对未来的研究方向进行了展望。     

Kinetics of hydrogen absorption by scandium at high temperature

Hydrogen absorption by Sc has been investigated over wide ranges of temperature (790-1280 K) and pressure (10-150 mbar). The absorbed quantities of H were in agreement with those expected from p-x-T isotherms, available in the literature, only for temperatures higher than 1000 K, where the absorption curves could be fitted to a Johnson-Mehl-Avrami type of relationships. The gas-solid reaction was first-order and the reaction-rate-constant k exhibited an Arrhenius type of temperature dependence with an associated activation energy of 2.1 ± 0.1 eV (202 ± 10 kJ/mol·H₂). The step controlling the absorption rate turned out not to be H diffusion in the bulk. Namely, the values of the apparent H diffusion coefficient deduced from absorption data were found to be some orders of magnitude smaller than expected from extrapolations of lower temperature anelastic and spin-lattice relaxation data. The absorption rate appears to be governed by H penetration through a sub-surface Sc layer containing a high concentration of interstitial oxygen, originated from the decomposition of surface oxides occurring between 900 K and 1000 K.     

Revisiting solar hydrogen production through photovoltaic-electrocatalytic and photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting is regarded as a promising way for solar hydrogen production, while the fast development of photovoltaic-electrolysis (PV-EC) has pushed PEC research into an embarrassed situation. In this paper, a comparison of PEC and PV-EC in terms of efficiency, cost, and stability is conducted and briefly discussed. It is suggested that the PEC should target on high solar-to-hydrogen efficiency based on cheap semiconductors in order to maintain its role in the technological race of sustainable hydrogen production.     

A novel dual-bed photocatalytic water splitting system for hydrogen production

A novel dual-bed system was designed to produce hydrogen through photocatalytic water splitting. The system was comprised of a photocatalytic reaction bed and a regeneration bed. Aqueous KI solution and Pt-loaded TiO₂ constituted the photocatalytic reaction bed where hydrogen was produced; meanwhile the hole scavenger iodide ion was oxidized into I_2. The effluent containing I₂ from the photocatalytic bed entered the regeneration bed and passed through a Cu₂O layer where I₂ was reduced to I⁻. The regeneration bed effluent was then recycled to the photocatalytic reaction bed. Since the hole scavenger KI in the photocatalytic bed was constantly kept at a high level through the continuous reduction of I₂ in the regeneration bed, steady production of hydrogen was achieved in the dual-bed system for a much longer period as compared to a single-bed system without regeneration.     

Heat management for hydrogen production by high temperature steam electrolysis

Many research and development projects throughout the world are devoted to sustainable hydrogen production processes. Low-temperature electrolysis, when consuming electricity produced without greenhouse gas emissions, is a sustainable process, though having limited efficiency.     

Hydrogen production by high temperature electrolysis of water vapour

Introducing hydrogen as a future energy carrier an important condition is a high efficient and low cost process for hydrogen production. Total efficiencies of conventional water electrolysis plants are limited to 25–28% with respect to the primary heat necessary for power generation. Significant improvements of this efficiency can be achieved by steam electrolyzing at high temperatures.The thermodynamic advantages of this high-temperature electrolysis process are shown and the technological tasks of the required development work and the state of the art are described in detail.Significant improvements of the components for construction and interconnection of electrolysis cells have been achieved. Typical measured cell characteristics are adiabatic cell voltages of 1.3 V at 0.4 A/cm² current density.Industrial engineering studies of the whole process have been performed indicating total efficiencies of about 40–50%, depending on the primary energy source.     

Assessment of sustainable high temperature hydrogen production technologies

Apart from being a major feedstock for chemical production, hydrogen is also a very promising energy carrier for the future energy. Currently hydrogen is predominantly produced via fossil routes, but as green energy sources are gaining a larger role in the energy mix, novel and green production routes are emerging. The most abundant renewable hydrogen sources are water and biomass, which allow several possible processing routes, such as electrolysis, thermochemical cycles and gasification. By introducing heat to the process the required electricity demand can be reduced (high temperature electrolysis) or practically eliminated (thermochemical cycles). Each renewable hydrogen production route has its own strength and weaknesses; the choice of the most suitable method is always dependent on the economical potentials and the location. The aim of this paper is to evaluate the different high temperature, renewable hydrogen production technologies.     

Photocatalytic hydrogen production by water splitting over Au/Al-SrTiO3

Visible light water splitting activity of Au-Al/SrTiO₃ was tested in this work. Al/SrTiO₃ was synthesized via solid state reaction while Au loading was done with homogenous deposition precipitation method. The effects of Au loading and Al doping were investigated in 10, 20 and 30% aqueous solutions of methanol, ethanol, and isopropyl alcohol. The methanol was performed better over 0.25%Au-1.0%Al/SrTiO₃ at 20% alcohol concentration while the isopropyl alcohol resulted in better performance over the same catalyst at 30% concentration; the latter was also the best result obtained in this work with the hydrogen evolution rate of 347 μmol/h.gcat. Ethanol showed lower performance than other two alcohols. It was found from UV–vis analysis that Al doping increased the band energy of SrTiO₃. XRD and XPS analyses clearly showed that the dominant structure was SrTiO₃ in all samples. Au was found to be generally loaded as 30–40 nm particles by SEM.