Photovoltaic electrolysis: Hydrogen and electricity from water and light

Two photovoltaic couples, consisting of n on p and p on n gallium arsenide, respectively, have been converted into a water splitting device. Light is allowed to fall on the p part of one couple, which is in contact with air, and on the n side platinum is plated, which contacts the solution. On the other couple, the n side is in contact with air, while on the p side ruthenium dioxide is plated, which is in contact with the solution. Such a device gives a performance (8% conversion efficiency of solar light to hydrogen) better than that of known photoelectrolysis devices operating without battery assistance. Comparison with a coupled photovoltaic-distant water electrolyzer shows, under certain circumstances, some advantages for the present device.     

Modeling photovoltaic-electrochemical water splitting devices for the production of hydrogen under real working conditions

Photoelectrochemical splitting of water is potentially a sustainable and affordable solution to produce hydrogen from sun light. Given the infancy stage of technology development, it is important to compare the different experimental concepts and identify the most promising routes. The performance of photoelectrochemical devices is typically measured and reported under ideal irradiation conditions, i.e. 1 sun. However, real-life operating conditions are very different, and are varying in time according to daily and seasonal cycles.In this work, we present an equivalent circuit model for computing the steady state performance of photoelectrochemical cells. The model allows for a computationally efficient, yet precise prediction of the system performance and a comparison of different devices working in real operating conditions. To this end, five different photoelectrochemical devices are modeled using experimental results from literature. The calculated performance shows good agreement with experimental data of the different devices. Furthermore, the model is extended to include the effect of illumination and tilt angle on the hydrogen production efficiency. The resulting model is used to compare the devices for different locations with high and low average illumination and different tilt angles. The results show that including real illumination data has a considerable impact on the efficiency of the PV-EC device. The yearly average solar-to-hydrogen efficiency is significantly lower than the ideal one. Moreover, it is dependent on the tilt angle, whose optimal value for European-like latitude is around 40°. Notably, we also show that the most performing device through the whole year might not necessarily be the one with highest sun-to-hydrogen efficiency for one-sun illumination.     

Hydrogen evolution under visible light illumination on the solid solution CdxZn1-xS prepared by ultrasound-assisted route

The complete solid solution Cd_xZn_1-xS (0 ≤ x ≤ 1) prepared by ultrasound-assisted route is used for the H₂ formation upon visible light illumination. The correlation of chemical and physical characterizations permits to assess the feasibility of the system for the photocatalytic hydrogen evolution. The compounds crystallize in a cubic structure (x < 0.5) and convert to hexagonal variety above 0.5 with a crystallite size (8–17 nm). All materials exhibit n-type conduction with an activation energy (0.22–0.05 eV). The optical transitions are directly allowed (3.10–2.30 eV) and appropriately matched to the sun spectrum while the conduction band, deriving from (Zn, Cd) ns orbital (∼-1 V_SCE), is positioned above the H₂O/H₂ potential (∼-0.68 V_SCE), allowing H₂-liberation under visible illumination. The photocatalyst dose, pH and SO₃²⁻ concentration are optimized. Under the favorable conditions, the H₂ liberation rate reaches 12 × 10⁻⁴ mL mg⁻¹ min⁻¹ with a quantum yield η(H₂) of 1.40%.     

Template synthesis of porous hierarchical Cu2ZnSnS4 nanostructures for photoelectrochemical water splitting

Environmentally friendly and low-cost Cu₂ZnSnS₄ (CZTS) is a promising light absorber for photoelectrochemical (PEC) hydrogen production from water splitting due to the earth-abundant elements, high absorption coefficient, and narrow bandgap. Herein, the hierarchical CZTS film with porous nanostructures was successfully synthesized by a template method. The hierarchical CZTS film was composed of flower-like particles, which were assembled with thin CZTS nanosheets. Macropores were generated owing to the aggregation of flower-like spheres, and mesopores were formed from the stacking of CZTS nanosheets. Compared to the dense CZTS film, the porous hierarchical CZTS film showed a much higher PEC property for water splitting. The improved performance could be attributed to three merits of the porous hierarchical morphology: enhanced light absorption, improved charge separation and transfer, and enlarged electrochemically active surface area. This study provides a useful idea to design efficient semiconductor photoelectrodes for water splitting with delicately controlled morphology.     

Enhanced solar-driven water splitting by ZnO/CdTe heterostructure thin films-based photocatalysts

Solar-driven hydrogen production by water splitting using a photocatalyst is considered the most effective approach to produce hydrogen. Hydrogen is the most suitable renewable energy source. The efficiency of hydrogen production is still low. The efficiency of hydrogen production through photocatalysis can be enhanced by preparing a suitable and efficient photocatalyst. In this work, ZnO thin films were deposited on CdTe thin films at 600 °C, 650 °C, and 700 °C temperatures to form ZnO/CdTe heterostructure thin films by chemical vapor deposition (CVD) as photoelectrodes for water splitting. The photoelectrochemical properties showed that ZnO/CdTe heterostructure thin films have better photocurrent response compared to pristine ZnO and CdTe thin films. EIS results showed that the charge transfer at the electrode-electrolyte interface for ZnO/CdTe heterostructure thin films is much better than that of the pristine ZnO film. The ZnO/CdTe-700 °C heterostructure thin film has a 112-fold higher rate of photocatalytic hydrogen generation than pure ZnO.     

Hydrogen production by a low-cost electrolyzer developed through the combination of alkaline water electrolysis and solar energy use

Electrolysis is a relatively simple process for obtaining hydrogen and can be combined with use of renewable energy sources, such as solar photovoltaic energy, for clean, sustainable gas production. This study designed a cylindrical electrolytic cell made of acrylic and 304 stainless steel electrodes to produce hydrogen. The electrolyte used was sodium hydroxide (NaOH 2–5 mol L^?1), and the direct current voltages applied were 2.0, 2.7, and 3.4 V. The maximum hydrogen production was achieved with 5.0 mol L^?1 NaOH and 3.4 V electric voltage. The system was connected to a photovoltaic panel of 20 W and exposed to solar radiation from 10 a.m. to 2 p.m. Approximately 2 L of hydrogen was produced within a period, and an average irradiance of 800.0 W m^?2 ± 60 W m^?2 was achieved. The system was stable throughout the tests.     

Analysis and assessment of a continuous-type hybrid photoelectrochemical system for hydrogen production

In this study, we conceptually develop and thermodynamically analyze a new continuous-type hybrid system for hydrogen production which photoelectrochemically splits water and performs chloralkali electrolysis. The system has a potential to produce hydrogen efficiently, at low cost, and in an environmentally benign way by maximizing the utilized solar spectrum and converting the byproducts into useful industrial commodities. Furthermore, by using electrodes as electron donors to drive photochemical hydrogen production, the hybrid system minimizes potential pollutant emissions. The products of the hybrid system are hydrogen, chlorine and sodium hydroxide, all of which are desired industrial commodities. The system production yield and efficiencies are investigated based on an operation temperature range of 20 °C–80 °C. A maximum energy efficiency of 42% is achieved between the temperatures of 40 °C and 50 °C.     

Promoting photoelectrochemical hydrogen production performance by fabrication of Co1-XS decorating BiVO4 photoanode

Photoelectrochemical (PEC) water splitting is an effective way of converting solar energy into hydrogen (H₂) energy. However, the carriers’ transmission and the reaction kinetics of the photoelectrode are dilatory, which will influence the conversion efficiency of solar energy to H₂. In this work, a novel of BiVO₄/Co_1-XS photoanode was successfully fabricated through the successive ionic layer adsorption reaction. The photocurrent density of optimal sample BiVO₄/Co_1-XS (2.9 mA cm^?2 at 1.23 V_RHE) has reached up to 5 times that of pure BiVO₄, and the applied bias photon to current conversion efficiency increased from 0.04% (BiVO₄) to 0.4% (BiVO₄/Co_1-XS). The superior PEC performance of the BiVO₄/Co_1-XS photoanode is mainly related to the improved conductivities and reaction kinetics. The charge injection efficiency of BiVO₄/Co_1-XS grew to about 80%, and the charge separation efficiency was up to 34%, revealing that the decoration of Co_1-XS could significantly accelerate the transfer speed of photogenerated carriers from the electrode surface to the electrolyte. This work provided an efficient and simple scheme for improving the PEC performance of photoanode, through reasonable design and research.     

Enhanced photoelectrochemical-response in highly ordered TiO2 nanotube-arrays anodized in boric acid containing electrolyte

We examine the photoelectrochemical properties of highly ordered titanium dioxide nanotube-array photoanodes, fabricated by anodization of titanium in a nitric acid/hydrofluoric acid electrolyte, with and without the addition of boric acid. Under UV–Vis illumination the photocurrent densities achieved with TiO₂ nanotube-arrays fabricated in the H₃BO₃–HNO₃–HF electrolyte are a factor of seven greater than the TiO₂ nanotube-array samples obtained in the commonly used HNO₃–HF electrolyte, indicating the ability to control the photoelectrochemical response of the highly ordered nanotube arrays by tailoring the electrolyte composition. For 560 nm long boric-acid fabricated nanotube arrays, a photoconversion efficiency of 7.9% is achieved upon a 320–400 nm illumination at an intensity of 98 mW/cm², with hydrogen generated by water photoelectrolysis at the power-time normalized rate of 1708-μmol/h W (42 ml/h W). The resulting nanotube-arrays demonstrate excellent photocorrosion stability, with no detectable degradation in photoconversion properties over 6 months of testing. While the titania bandgap is not suitable for high visible spectrum efficiencies, the high photoconversion efficiency achieved under UV illumination indicates the suitability of the highly ordered nanotube-array architecture for hydrogen generation by water photoelectrolysis.     

An analysis of hydrogen production from renewable electricity sources

Three aspects of producing hydrogen via renewable electricity sources are analyzed to determine the potential for solar and wind hydrogen production pathways: a renewable hydrogen resource assessment, a cost analysis of hydrogen production via electrolysis, and the annual energy requirements of producing hydrogen for refueling. The results indicate that ample resources exist to produce transportation fuel from wind and solar power. However, hydrogen prices are highly dependent on electricity prices. For renewables to produce hydrogen at $2 kg⁻¹, using electrolyzers available in 2004, electricity prices would have to be less than $0.01 kWh⁻¹. Additionally, energy requirements for hydrogen refueling stations are in excess of 20 GWh/year. It may be challenging for dedicated renewable systems at the filling station to meet such requirements. Therefore, while plentiful resources exist to provide clean electricity for the production of hydrogen for transportation fuel, challenges remain to identify optimum economic and technical configurations to provide renewable energy to distributed hydrogen refueling stations.     

Optimization hydrogen production over visible light-driven titania-supported bimetallic photocatalyst from water photosplitting in tandem photoelectrochemical cell

Solar hydrogen production was investigated over a Cu-Ni doped TiO₂ photocatalyst from water photosplitting in a tandem photoelectrochemical cell, which was made up by connecting a modified photoelectrochemical cell to dye solar cell in a series. A mathematical representation for preparation parameters for hydrogen production was successfully generated. Optimization of hydrogen production was conducted with varying preparation parameters of Cu-Ni doped TiO₂ photocatalyst including molar ratios of water, acetic acid and Cu to titanium tetraisopropoxide. The optimum preparation parameters of photocatalyst was obtained at molar ratios of water, acetic acid and Cu to titanium tetraisopropoxide of 32, 4.9, and 5.9, respectively. Physical and photoelectrochemical characterization revealed that low content of water and Cu decreased the charge transfer resistance and charge carrier recombination rate on Cu-Ni/TiO₂ surface. This is attributed to the better crystallinity and less degree of agglomeration which led to obtain optimum particle size at this condition. Maximum hydrogen production rate of 2.12 mL/cm². h was achieved under the optimum condition using the tandem photoelectrochemical cell in the aqueous KOH and glycerol solution under visible light irradiation (λ > 400 nm).     

Energetic and exergetic investigations of an innovative light-based hydrogen production reactor

In this study, it is aimed to thermodynamically study and experimentally test a continuous type hybrid photoelectrochemical hydrogen production system. The hybrid system considered in this study is capable of enhancing solar spectrum utilization via the combination of photocatalysis and PV/T. In addition, the system eliminates the electron donor requirement of photocatalysis by employing photoelectrodes. Which, as a result, risk of potentially harmful pollutant emissions is reduced. In this study, the present system is investigated in electrolysis operation under three different inlet mass flow rates (0.25, 0.50, and 0.75 g/s). The experimental results are compared to the thermodynamic model outputs. Parametric studies are conducted by changing the inlet mass flow rate from 0 to 1 g/s. The present experimental results suggest that the highest hydrogen production rate is observed at 0.75 g/s inlet mass flow rate, which is 2.43 mg/h. The highest energy and exergy efficiencies are calculated at 0.25 g/s, which are 36% and 32%, respectively. Furthermore, thermodynamic model outputs are confirmed to have a good agreement with the experimental results.     

A review on photoelectrochemical hydrogen production systems: Challenges and future directions

Water photolysis is a fundamental concept in which solar-driven water splitting is utilized to generate renewable hydrogen fuel using semiconductor-based electrochemical systems. The engineering design principles for each system configuration, including single, dual/tandem photoelectrodes, tandem photoelectrochemical-photovoltaic, and multi-junction designs are reviewed. Modeling and numerical simulation of photoelectrochemical processes based on up-to-date multi-scale analysis are presented and discussed. In addition, the achievements made in semiconductor photoelectrode materials and the rational engineering methods needed to improve the solar to hydrogen efficiency are demonstrated. Furthermore, some key accomplishments in different aspects, such as electron-hole recombination, stability, photocorrosion, energy band gap, and photocurrent density are discussed. Moreover, key points on the challenges, opportunities and future directions towards commercialization of viable photoelectrochemical reactors are discussed.     

半导体催化光解制氢技术研究

氢能是一种新型高效洁净能源。本文介绍了颇有应用前景的光分解水制氢技术的原理,重点介绍了半导体光催化分解水制氢反应机理和技术方法。     

Kinetics of reduction of water to hydrogen by visible light on alumina supported Pt–CdS photocatalysts

The kinetics of hydrogen production from photolysis of water on alumina supported Pt–CdS catalyst using visible light has been studied. An induction period with negligible rate was observed upon illumination of catalyst. The rate gradually increases and again falls down. The rate has been observed to be proportional to the sulfide ions adsorbed on the surface of CdS. The induction period has been related to the re-establishment of adsorption equilibrium of sulfide ions on the catalyst surface under illumination. The decrease in rate is due to the deactivation of catalyst by hydrogen. A power law type rate expression for hydrogen production has been proposed which takes into account the deactivation of the catalyst.     

Branched multiphase TiO2 with enhanced photoelectrochemical water splitting activity

An efficient hierarchical structure, nano-branch containing anatase TiO₂ nanofibers and rutile nanorods, was prepared via the combination of the electrospinning and hydrothermal processes. This novel configuration of TiO₂ multiphase possessed higher surface area, roughness, and fill factors compared with each single phase component prepared in the same condition, which significantly enhanced its light absorption. Our experimental results showed that within the interface of multiphase TiO₂, the heterojunction promoted the charge separation and improved the charge transfer rate, leading to higher efficiency for photoelectrochemical water splitting. The photocurrent density of the nano-branched TiO₂ electrode could reach 0.95 mA/cm², which was almost twice as large as that of the pristine TiO₂ nanorod. Our work provides a simple and feasible routine to synthesize complex TiO₂ nanoarchitectures, which lays a foundation for improving energy storage and conversion efficiency of TiO₂-based photoelectrodes.     

Hydrogen production by biomass gasification in supercritical water using concentrated solar energy: System development and proof of concept

A novel system of hydrogen production by biomass gasification in supercritical water using concentrated solar energy has been constructed, installed and tested at the State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF). The “proof of concept” tests for solar-thermal gasification of biomass in supercritical water (SCW) were successfully carried out. Biomass model compounds (glucose) and real biomass (corn meal, wheat stalk) were gasified continuously with the novel system to produce hydrogen-rich gas. The effect of direct normal solar irradiation (DNI) and catalyst on gasification of biomass was also investigated. The results showed that the maximal gasification efficiency (the mass of product gas/the mass of feedstock) in excess of 110% were reached, hydrogen fraction in the gas product also approached to 50%. The experimental results confirmed the feasibility of the system and the advantage of the process, which supports future work to address the technical issues and develop the technology of solar-thermal hydrogen production by gasification of biomass in supercritical water.     

Exergoeconomic analysis and optimization of a concentrated sunlight-driven integrated photoelectrochemical hydrogen and ammonia production system

The study presented here concerns a comprehensive investigation on exergoeconomic analysis and optimization of an integrated system for photoelectrochemical hydrogen and electrochemical ammonia production. The present integrated system consists of a solar concentrator, spectrum-splitting mirrors, a photoelectrochemical hydrogen production reactor, a photovoltaic module, an electrochemical ammonia production reactor and support mechanisms. Detailed thermodynamic and exergoeconomic analyses are initially conducted to determine the performance of the integrated system namely; efficiency and total cost rate. The obtained performance parameters are then optimized to yield the minimum cost rate and maximum efficiency under given constraints of the experimental system. The highest capital cost rates are observed in the photoelectrochemical hydrogen and electrochemical ammonia production reactors because of high procurement costs and electricity inputs. The optimized values for exergy efficiency of the integrated system range from 5% to 9.6%. The photovoltaic and photoelectrochemical cell areas and solar light illumination mainly affect the overall system efficiencies. The optimum efficiencies are found to be 8.7% and 5% for the multi-objective optimization of hydrogen production and integrated ammonia production system, respectively. When the exergy efficiency of the integrated system is maximized and the total cost rate is minimized at the same time, the total cost rate of the system is calculated to be about 0.2 $/h. The cost sensitivity analysis results of the present study show that the total cost rate of the system is mostly affected by the interest rate and lifetime of the system.     

Hydrogen generation from water and aluminum promoted by sodium stannate

A new process to obtain H₂ from H₂O using Al corrosion in Na₂SnO₃ solutions is described. Results showed an enhancement of H₂ production rates using Na₂SnO₃ instead of NaOH at the same pH. A side reaction of Al in Na₂SnO₃ solutions has been found, which consumes Al to produce metallic Sn. H₂ yield depends chiefly on Al/Na₂SnO₃ molar ratio for experiments with Na₂SnO₃ concentrations above 0.025 M, reaching higher yields with higher Al/Na₂SnO₃ ratios. The maximum H₂ production rates are proportional to the initially added Al mass. Two different shrinking core models for examining the kinetics of H₂ generation are verified and the activation energy (E_a) is 73 ± 6 kJ mol⁻¹, confirming a rate control by a chemical step. A mechanism of Al corrosion in Na₂SnO₃ solutions is proposed and compared with the mechanism in NaOH and NaAlO₂ solutions.     

Hydrogen production from water splitting over Eosin Y-sensitized mesoporous-assembled perovskite titanate nanocrystal photocatalysts under visible light irradiation

The photocatalytic water splitting is a promising process for producing H₂ from two abundant renewable sources of water and solar light, with the aid of a suitable photocatalyst. In this work, a combination of sensitizer addition and noble metal loading was employed to modify perovskite photocatalysts in order to achieve the enhancement of photocatalytic H₂ production under visible light irradiation. The dependence of the H₂ production on type of mesoporous-assembled perovskite titanate nanocrystal photocatalysts (MgTiO₃, CaTiO₃, and SrTiO₃), calcination temperature of photocatalyst, Pt loading, type and concentration of electron donor (diethanolamine, DEA; and triethanolamine, TEA), concentration of sensitizer (Eosin Y, E.Y.), photocatalyst dosage, and initial solution pH, was systematically studied. The experimental results showed that the 0.5 wt.% Pt-loaded mesoporous-assembled SrTiO₃ nanocrystal synthesized by a single-step sol-gel method and calcined at 650 °C exhibited the highest photocatalytic H₂ production activity from a 15 vol% DEA aqueous solution with dissolved 0.5 mM E.Y. Moreover, the optimum photocatalyst dosage and initial solution pH for the maximum photocatalytic H₂ production activity were found to be 6 g/l and 11.6, respectively.