Solar Hydrogen Generation Research Articles

Highly efficient visible-light-induced photoactivity of the CdS–Mn/MoS2/CdTe/TiO2 quaternary photocatalyst for label-free immunoassay of tris-(2,3-dibromopropyl) isocyanurate and enhanced solar hydrogen generation

A novel visible-light-induced quaternary photocatalyst for TBC detection and solar hydrogen generation was successfully prepared.

Solar Thermochemical Hydrogen (STCH) Processes

In order for hydrogen to play a key role in the move towards renewable energy, CO2-neutral hydrogen must be generated efficiently, inexpensively, and at a large scale. This article focuses on thermochemical water-splitting cycles, especially when coupled with solar energy. There are hundreds of possible thermochemical cycles, but only a few have been considered commercially viable. Commonalities among thermochemical cycles are a series of reactions that split water at lower temperatures (∼500–1000°C) than thermal dissociation (>2500°C), with other species recycled in the system. Thermochemical cycles can be direct (all chemical steps) or hybrid (i.e., a combination of chemical and electrochemical steps) processes. If concentrated solar energy is used for the thermal dissociation step, all thermochemical cycles can be classed as solar thermochemical hydrogen (STCH) processes. This article discusses two cycles: 1) a direct thermochemical cycle (zinc oxide cycle), and 2) a hybrid cycle (hybrid sulfur cycle). The latter combines a chemical thermal dissociation step with an electrochemical step. As two-step cycles, their appeal is in their relative simplicity. Although probably several years away, the outlook for solar thermochemical hydrogen generation is strong, especially with continued advances in materials, and system design and optimization.

Efficient solar light-driven H2 production: post-synthetic encapsulation of a Cu2O co-catalyst in a metal–organic framework (MOF) for boosting the effective charge carrier separation

The development of new and efficient catalytic systems for solar light-driven hydrogen generation is one of the prime focuses of contemporary chemical sciences.

Lab scale optimization of various factors for photocatalytic hydrogen generation over low cost Cu0.02Ti0.98O2-δ photocatalyst under UV/Visible irradiation and sunlight

We have demonstrated earlier that maximum H2 generated @ 1.167 l/h/m2 over Cu0.02Ti0.98O2-δ photocatalyst with apparent quantum efficiency, AQE of 7.5% and solar fuel efficiency, SFE of 3.9% under sunlight. With an aim to scale-up the solar photocatalytic hydrogen process to pilot plant, optimization studies at lab scale as well as in upscaled photoreactors were performed over Cu0.02Ti0.98O2-δ, photocatalyst under UV/visible and sunlight. Cu0.02Ti0.98O2-δ was synthesized by facile sol-gel route and characterized by relevant techniques. Several operational parameters were investigated in order to finalize the conditions which are most favourable for photocatalytic hydrogen yield. Factors such as photocatalyst loadings, v/v concentration of sacrificial reagent, replacement of methanol by industrial waste glycerol, role of different configuration of light source with reactor, effect of stirring during the photocatalytic reaction, effect of fluctuations of solar flux at hourly basis, illumination area on hydrogen yield were studied. Contribution of each factor in determining the hydrogen yield was quantified. Relative standard deviation in hydrogen yield as a function of each factor was estimated. Our findings suggest that in addition to catalyst loadings and sacrificial reagent, improved dispersion of photocatalyst obtained by stirring the reaction mixture in horizontal geometry resulted in enhanced H2 yield. Hydrogen yield obtained at lab scale can be appropriately extrapolated with respect to illumination area instead of weight of photocatalyst. A relative standard deviation (RSD) of ± 3.82% and ± 4.53% in H2 yield was calculated for sunny and cloudy days in time zone of 10.30–16.30 h IST. Deviation of H2 yield was more on cloudy days and beyond 16:30 h. These studies have provided a daily window of 11:00–15:00 h to be utilized throughout the year for a commercial scaled up process, prohibiting the illumination during less productive hours of the day for shaping the improved economics of solar hydrogen generation. Our results obtained at lab scale would be useful to perform sunlight driven scaled –up photocatalytic process using low cost visible light efficient photocatalyst, Cu0.02Ti0.98O2-δ.

Photocurrent Enhancement by a Rapid Thermal Treatment of Nanodisk-Shaped SnS Photocathodes.

Photocathodes made from the earth-abundant, ecofriendly mineral tin monosulfide (SnS) can be promising candidates for p/n-type photoelectrochemical cells because they meet the strict requirements of energy band edges for each individual photoelectrode. Herein we fabricated SnS-based cell that exhibited a prolonged photocurrent for 3 h at -0.3 V vs the reversible hydrogen electrode (RHE) in a 0.1 M HCl electrolyte. An enhancement of the cathodic photocurrent from 2 to 6 mA cm-2 is observed through a rapid thermal treatment. Mott-Schottky analysis of SnS samples revealed an anodic shift of 0.7 V in the flat band potential under light illumination. Incident photon-to-current conversion efficiency (IPCE) analysis indicates that an efficient charge transfer appropriate for solar hydrogen generation occurs at the -0.3 V vs RHE potential. This work shows that SnS is a promising material for photocathode in PEC cells and its performance can be enhanced via simple postannealing.

Pt – g-C3N4 – (Au/TiO2): Electronically integrated nanocomposite for solar hydrogen generation

A potential nanocomposite photocatalyst was designed by integrating Pt nanoclusters (co-catalyst and electron sink) with graphitic carbon nitride (g-C3N4 (gcn)) (charge diffusion) and 0.5 wt % Au containing Au-TiO2 (AuT) (plasmonic on semiconductor) for solar water splitting (SWS). Variety of Pt-gcn-AuTiO2 compositions has been evaluated for SWS under one sun conditions. Complexity of the photocatalyst was increased systematically from Au-TiO2, gcn-TiO2 to Pt-gcn-Au-TiO2 to explore the influence of different combinations. Electronic integration of charge separation/diffusion component (gcn) with light absorbing sensitizer components (Au and gcn), and co-catalyst (Pt) seems to be the critical factor to improve hydrogen yield (HY) or overall efficiency. Although addition of gcn increase the HY of composites, there is no SWS activity observed on bare TiO2 or gcn. Au or Pt on gcn enhances the charge separation effectively and interface between Au and/or Pt with gcn works as the Schottky barrier. A monodispersion of Au over TiO2 and Pt nanoclusters over gcn/AuTiO2 composite lead to the maximum solar hydrogen yield (1.52 mmol/h g) with an apparent quantum yield (AQY) of 7.5%. Photoelectron and photoluminescence spectral studies confirm the electron transfer from Au to gcn, and Au and/or gcn to titania. A thorough physico-chemical investigation of various composites underscores the electronic integration aspects of the nanocomposite towards storage of electrons in the Pt co-catalyst and hence an effective charge separation and an increase in AQY.

A Study on Doped Heterojunctions in TiO2 Nanotubes: An Efficient Photocatalyst for Solar Water Splitting

The two important factors that affect sunlight assisted water splitting ability of TiO2 are its charge recombination and large band gap. We report the first demonstration of nitrogen doped triphase (anatase-rutile-brookite) TiO2 nanotubes as sun light active photocatalyst for water splitting with high quantum efficiency. Nitrogen doped triphase TiO2 nanotubes, corresponding to different nitrogen concentrations, are synthesized electrochemically. Increase in nitrogen concentration in triphase TiO2 nanotubes is found to induce brookite to anatase phase transformation. The variation in density of intra-band states (Ti3+ and N 2p states) with increase in nitrogen doping are found to be critical in tuning the photocatalytic activity of TiO2 nanotubes. The presence of bulk heterojunctions in single nanotube of different nitrogen doped TiO2 samples is confirmed from HRTEM analysis. The most active nitrogen doped triphase TiO2 nanotubes are found to be 12 times efficient compared to pristine triphase TiO2, for solar hydrogen generation. The band alignment and charge transfer pathways in nitrogen doped TiO2 with triphase heterojunctions are delineated. Bulk heterojunctions among the three phases present in the nanotubes with intra-band defect states is shown to enhance the photocatalytic activity tremendously. Our study also confirms the theory that three phase system is efficient in photocatalysis compared to two phase system.

Discovery of Manganese-Based Solar Fuel Photoanodes via Integration of Electronic Structure Calculations, Pourbaix Stability Modeling, and High-Throughput Experiments

The solar photoelectrochemical generation of hydrogen and carbon-containing fuels comprises a critical energy technology for establishing sustainable energy resources. The photoanode, which is responsible for solar-driven oxygen evolution, has persistently limited technology advancement due to the lack of materials that exhibit both the requisite electronic properties and operational stability. Efforts to extend the lifetime of solar fuel devices increasingly focus on mitigating corrosion in the highly oxidizing oxygen evolution environment, motivating our development of a photoanode discovery pipeline that combines electronic structure calculations, Pourbaix stability screening, and high-throughput experiments. By applying the pipeline to ternary metal oxides containing manganese, we identify a promising class of corrosion-resistant materials and discover five oxygen evolution photoanodes, including the first demonstration of photoelectrocatalysis with Mn-based ternary oxides and the introduction of alka...

Improved Stability and Photoelectrochemical Performance of Al2O3 Passivated CdS/1D Zr:Fe2O3 Heterojunctions for Solar Hydrogen Generation

Al2O3 passivated CdS sensitized 1D Zr:Fe2O3 nanorod arrays have been synthesized on fluorine-doped tin oxide (FTO) by hydrothermal method and dip coating methods. The photoelectrochemical study of CdS/1D Zr:Fe2O3 photoelectrode exhibits the current density of 4.2 mA.cm-2 at 0 V (vs. Ag/AgCl) which is 2.8 time higher than the bare 1D Zr:Fe2O3. The reduced recombination and the effective transport of photogenerated holes in CdS facilitates the enhancement in photoelectrochemical performance. Furthermore, the photocurrent and stability of the CdS/Zr:Fe2O3 nanorods was significantly enhanced by Al2O3 compared to bare CdS/Zr:Fe2O3 heterojunction due to its ability to act as an effective hole transport as well as photocorrosion protecting layer. The chemical analyses and nanostructural study of the Al2O3 coated CdS/1D Zr:Fe2O3 photoelectrodes confirms the presence of Al2O3 on surface of CdS and 1D Zr:Fe2O3 nanorods embedded in the CdS flakes. These remarkable enhancements in light energy harvesting, improvement in charge transport and stability directly suggest the usefulness of photoelectrodes for solar hydrogen generation. KEYWORDS: Al2O3 passivation; Zr:Fe2O3 nanorod arrays; CdS Sensitizer; Hydrogen generation ACKNOWLEDGMENT This research was supported by the BK21 plus program, the Korean National Research Foundation (Nano-Material Fundamental Technology Development, 2016M3A7B4909370).

Unbiased Photoelectrochemical Solar Hydrogen Generation on Highly Stable, Ultrathin Organic Linker/Metal Ions-Treated InGaN Nanorods on Metal

InGaN-based nanostructures have recently been recognized as promising materials for efficient solar hydrogen generation. This is due to their chemical stability, adjustable optoelectronic properties, suitable band edge alignment, and large surface-to-volume ratio. The inherent high density of surface trapping states and the lack of compatible conductive substrates, however, hindered their use as stable photo-catalysts. We have designed, synthesized and tested an efficient photocatalytic system using stable InGaN-based nanorods (NRs) grown on an all-metal stack substrate (Ti-Mo) for a better electron transfer process. In addition, we have applied a bifunctional ultrathin thiol-based organic surface treatment using 1,2-ethanedithiol (EDT), in which sulfur atoms protected the surface from oxidation. This treatment has dual functions, it passivates the surface (by the removal of dangling bonds) and creates ligands for linking Ir-metal ions as oxygen evolution centers on top of the semiconductor. This treatment when applied to InGaN NRs resulted in a photo-catalyst that achieved 3.5% solar-to-hydrogen (STH) efficiency, in pure water (pH~7, buffer solution) under simulated one-sun (AM1.5G) illumination and without electrical bias. Over the tested period, a steady increase of the gas evolution rate was observed from which a turnover frequency of 0.23 s-1 was calculated. The novel growth of InGaN-based NRs on a metal as well as the versatile surface functionalization techniques (EDT-Ir) have a high potential for making stable photo-catalysts with adjustable band gaps and band edges to harvest sun light.

Photothermal effect of infrared light to enhance solar catalytic hydrogen generation

Higher reaction temperature can induce the better solar catalytic hydrogen (H2) production performance on noble-metal loaded photocatalyst. Here, the thermal effect generated by the infrared light was used to enhance the solar catalytic hydrogen production from methanol-water solution. The light to hydrogen (LTH) conversion efficiency of Pt/TiO2 reached to 0.36% under full spectrum light source. The results indicated that infrared part of solar light can be utilized as thermal source to enhance the photocatalytic hydrogen generation.

Techno-economic modeling and optimization of solar-driven high-temperature electrolysis systems

We present a techno-economic analysis of solar-driven hightemperature electrolysis systems used for the production of hydrogen and synthesis gas. We consider different strategies for the incorporation of solar energy, distinguished by the use of differing technologies to provide solar power and heat: i) thermal approaches (system 1) using concentrated solar technologies to provide heat and to generate electricity through thermodynamic cycles, ii) electrical approaches (system 2) using photovoltaic technologies to provide electricity and to generate heat through electrical heaters, and iii) hybrid approaches (system 3) utilizing concentrated solar technologies and photovoltaics to provide heat and electricity. We find that system 3 generates hydrogen at a high efficiency (ηSTF = 9.9%, slightly lower than the best performing system 1 with 10.6%) and at a low cost (Cfuel = $4.9/kg, lowest cost of all three systems) at reference conditions, providing evidence for the competitiveness of this hybrid approach for scaled solar hydrogen generation. Sensitivity analysis indicates an optimal working temperature for system 3 of 1350 K, which balances the increased thermal receiver losses with the reduced electrolysis cell potential when increasing the temperature. Lower working pressure always favors high system efficiency and low cost. The working current densities for thermoneutral voltage were determined for various temperature and pressure combinations, and trends for efficient and cost-effective thermoneutral operation were identified. The water conversion extent was optimized to avoid mass transport limitations in the electrodes while ensuring large fuel generation rates. For synthesis gas production, a H2/CO molar ratio of 2 can be achieved by tuning the inlet feeding molar ratio of CO2/H2O, temperature, and pressure. This study introduces a flexible simulation framework of solar-driven high-temperature electrolysis systems allowing for the assessment of competing solar integration approaches and for the guidance of the operational conditions maximizing efficiency and minimizing cost, providing pathways for scalable solar fuel processing.

Hydrogen concentrator demonstrator module with 19.8% solar-to-hydrogen conversion efficiency according to the higher heating value

Renewable hydrogen is a key element to a sustainable energy system of the future. Therefore solar hydrogen generation has been investigated by various research groups in recent years. The patented concept of the Hydrogen Concentrator (HyCon), which combines III-V multi-junction solar cells with polymer electrolyte membrane electrolysis, has been constantly developed over the last years. In this work, a unique weatherproof HyCon module with an area of 8 × 90.7 cm2 was build and characterized in an outdoor measurement for over two month. During this measurement period, the module showed a stable operation regardless of the water volume flow. The module works under natural convection without any circulation pumps at a suitable maximum temperature of 60–70 °C. The HyCon module consists of eight individual units (HyCon cells), each combining a photovoltaic and an electrolysis cell. Some of the HyCon cells reach a solar-to-hydrogen conversion efficiency of 20% according to the higher heating value at high current densities of 0.8 A/cm2. On the module level a maximum efficiency of 19.8% is reached. To the best of our knowledge this is the highest conversion efficiency so far achieved at such high current densities using a dual junction solar cell.

Fabrication of A/R-TiO2 composite for enhanced photoelectrochemical performance: Solar hydrogen generation and dye degradation

Anatase/rutile TiO2 nanorods composites were prepared by a facile hydrothermal method followed by dip coating method using titanium isopropoxide in acetic acid and ethanol solvent. The effects of the titanium isopropoxide precursor concentration, on the formation of dip coated anatase/rutile TiO2 nanorods composite were systematically explored. The growth of anatase on rutile TiO2 nanorods can be controlled by varying the titanium isopropoxide concentration. The morphological study reveals that anatase TiO2 nanograins formed on the surface of rutile TiO2 nanorod arrays through dip coating method. Photoelectrochemical analyses showed that the enhancement of the photocatalytic activities of the samples is affected by the anatase nanograins present on the rutile TiO2 nanorods, which can induce the separation of electrons and holes. To interpret the photoelectrochemical behaviors, the prepared photoelectrodes were applied in photoelectrochemical solar hydrogen generation and orange II dye degradation. The optimized photocurrent density of 1.8mAcm−2 and the 625μmolhydrogen generation was observed for 10mM anatase/rutile TiO2 NRs composites. Additionally, 96% removal of the orange II dye was achieved within 5h during oxidative degradation under solar light irradiation. One of the benefits of high specific surface area and the efficient photogenerated charge transport in the anatase/rutile TiO2 nanorod composite improves the photoelectrochemical hydrogen generation and orange dye degradation compared to the rutile TiO2. Thus, our strategy provides a promising, stable, and low cost alternative to existing photocatalysts and is expected to attract considerable attention for industrial applications.

Possibly scalable solar hydrogen generation with quasi-artificial leaf approach

Any solar energy harvesting technology must provide a net positive energy balance, and artificial leaf concept provided a platform for solar water splitting (SWS) towards that. However, device stability, high photocurrent generation, and scalability are the major challenges. A wireless device based on quasi-artificial leaf concept (QuAL), comprising Au on porous TiO2 electrode sensitized by PbS and CdS quantum dots (QD), was demonstrated to show sustainable solar hydrogen (490 ± 25 µmol/h (corresponds to 12 ml H2 h−1) from ~2 mg of photoanode material coated over 1 cm2 area with aqueous hole (S2−/SO32−) scavenger. A linear extrapolation of the above results could lead to hydrogen production of 6 L/h.g over an area of ~23 × 23 cm2. Under one sun conditions, 4.3 mA/cm2 photocurrent generation, 5.6% power conversion efficiency, and spontaneous H2 generation were observed at no applied potential (see S1). A direct coupling of all components within themselves enhances the light absorption in the entire visible and NIR region and charge utilization. Thin film approach, as in DSSC, combined with porous titania enables networking of all the components of the device, and efficiently converts solar to chemical energy in a sustainable manner.

Remarkable enhancement in solar hydrogen generation from MoS2-RGO/ZnO composite photocatalyst by constructing a robust electron transport pathway

Exploiting an efficient and noble-metal-free cocatalyst is highly desired for photocatalytic H2 evolution from water. Here, we synthesized the MoS2-RGO/ZnO composite photocatalysts via a facile two-step solvothermal process. Through optimizing of the MoS2 and reduced graphene oxide (RGO) cocatalyst component proportion, the 0.75wt%5G95M/ZnO composite photocatalyst shows the highest H2 production rate of 288.4μmolg−1h−1 when the amount of MoS2-RGO is 0.75wt% and the weight ratio of RGO to MoS2 is 5:95, which is about 14.1 times higher than the pure ZnO. More importantly, the ternary MoS2-RGO/ZnO composite photocatalyst exhibits much higher H2 evolution activity than the Pt loaded ZnO under the same condition. The significant improvement in the H2 evolution activity is ascribed to the positive synergetic effect between MoS2 and RGO, which acts as a hydrogen evolution cocatalyst and a robust electron transport pathway, respectively. This study suggests that MoS2-RGO can serves as an efficient and noble-metal-free cocatalyst to improve the H2 production performance of ZnO by suppressing the electron-hole pairs recombination.

Ultrafast Spectroscopy Reveals Electron-Transfer Cascade That Improves Hydrogen Evolution with Carbon Nitride Photocatalysts.

Solar hydrogen generation from water represents a compelling component of a future sustainable energy portfolio. Recently, chemically robust heptazine-based polymers known as graphitic carbon nitrides (g-C3N4) have emerged as promising photocatalysts for hydrogen evolution using visible light while withstanding harsh chemical environments. However, since g-C3N4 electron-transfer dynamics are poorly understood, rational design rules for improving activity remain unclear. Here, we use visible and near-infrared femtosecond transient absorption (TA) spectroscopy to reveal an electron-transfer cascade that correlates with a near-doubling in photocatalytic activity from 2050 to 3810 μmol h-1 g-1 when we infuse a suspension of bulk g-C3N4 with 10% mass loading of chemically exfoliated carbon nitride. TA spectroscopy indicates that exfoliated carbon nitride quenches photogenerated electrons on g-C3N4 at rates approaching the molecular diffusion limit. The TA signal for photogenerated electrons on g-C3N4 decays with a time constant of 1/ke' = 660 ps in the mixture versus 1/ke = 4.1 ns in g-C3N4 alone. Our TA measurements suggest that the charge generation efficiency in g-C3N4 is greater than 65%. Exfoliated carbon nitride, which liberates only trace hydrogen levels when photoexcited directly, does not appear to independently sustain appreciable long-lived charge generation. Thus, the activity enhancement in the two-component infusion evidently results from a cooperative effect in which charge is generated on g-C3N4, followed by electron transfer to exfoliated carbon nitride containing photocatalytic chain terminations. This correlation between electron transfer and photocatalytic activity provides new insight into structural modifications for controlling charge separation dynamics and activity of carbon-based photocatalysts.

Direct solar photocatalytic hydrogen generation with CPC photoreactors: System development

A new compound parabolic concentrator (CPC) for solar photocatalytic hydrogen production has been designed and estimated in the present study. Parameters of the CPC, i.e., the orientation, the acceptance angle and the absorber tube diameter are designed according to the local meteorological conditions. A north-south orientated, adjustable, single axis tracking truncated CPC with the concentration ratio of 4.22 is designed as one unit. An experimental system including 76 units with the total light area of 103.7m2 is developed and installed. The units have four different angles with inclination being 25°, 35°, 45° and 55°, respectively. The system is operated under the natural circulation condition for most of the time, with high pressure gas perturbing the sedimentary photocatalysts from time to time. NiS-CdxZn1−xS is used as the photocatalyst. Preliminary results show that the mean hydrogen productivity and cumulative hydrogen generation are 7.14NL/h and 42.84NL/day of the horizontal angle 25° row on a typical meteorological day. The average energy conversion efficiency is 0.087%. The daily hydrogen generation of the whole system is accumulated to be 160.34NL.

Unbiased photocatalytic hydrogen generation from pure water on stable Ir-treated In0.33Ga0.67N nanorods

InGaN-based nanostructures have recently been recognized as promising materials for efficient solar hydrogen generation. This is due to their chemical stability, adjustable optoelectronic properties, suitable band edge alignment, and large surface-to-volume ratio. The inherent high density of surface trapping states and the lack of compatible conductive substrates, however, hindered their use as stable photo-catalysts. We have designed, synthesized and tested an efficient photocatalytic system using stable In0.33Ga0.67N-based nanorods (NRs) grown on an all-metal stack substrate (Ti-Mo) for a better electron transfer process. In addition, we have applied a bifunctional ultrathin thiol-based organic surface treatment using 1,2-ethanedithiol (EDT), in which sulfur atoms protected the surface from oxidation. This treatment has dual functions, it passivates the surface (by the removal of dangling bonds) and creates ligands for linking Ir-metal ions as oxygen evolution centers on top of the semiconductor. This treatment when applied to In0.33Ga0.67N NRs resulted in a photo-catalyst that achieved 3.5% solar-to-hydrogen (STH) efficiency, in pure water (pH~7, buffer solution) under simulated one-sun (AM1.5G) illumination and without electrical bias. Over the tested period, a steady increase of the gas evolution rate was observed from which a turnover frequency of 0.23s-1 was calculated. The novel growth of InGaN-based NRs on a metal as well as the versatile surface functionalization techniques (EDT-Ir) have a high potential for making stable photo-catalysts with adjustable band gaps and band edges to harvest sun light.

Solar hydrogen generation: Exceeding 100% efficiency

Multiple exciton generation, in which two electron–hole pairs are generated from the absorption of one high-energy photon, has been demonstrated to improve efficiency in quantum-dot-based solar cells. Now, a photoelectrochemical system using PbS quantum dots is shown to drive hydrogen evolution with external quantum efficiency over 100%.