The reactor design for photoelectrochemical hydrogen production

The heat transfer and flow characteristics of a photoelectrochemical (PEC) hydrogen generation reactor are investigated numerically. Four different reactor designs are considered in this study. The solar irradiation is separated into short and long wavelength parts depending on the energy band gap of the photoelectrode used. While short wavelength part is used to generate electron and hole pairs, the long wavelength part is used to heat the system. Because the energy required for splitting water decreases as temperature is increased, heating the reactor by using the long wave energy increases the system efficiency. Thus, how the long wavelength energy is absorbed by the reactor is very important.The results show that more long wavelength energy kept inside the reactor can increase the solar-to-hydrogen efficiency, η_SH. For Fe₂O₃ photoelectrode, careful reactor design can increase η_SH by 11.0%. For design D under 4000 W/m² irradiation and a quantum efficiency of 30%, η_SH is found to be 14.1% and the hydrogen volume production rate is 166 L/m² h for Fe₂O₃. Effects of several parameters on the PEC hydrogen reactor are also discussed.     

Surface modification of semiconductor photoelectrode for improved solar cell performance

The investigation is focused on the synthesis of nanostructured TiO₂–CuO admixed photoelectrode and its use as a photoelectrode of high-efficiency PEC solar cells for hydrogen production. TiO₂, in the nanostructured form, has been prepared by hydrolysis of titanium(IV) isopropoxide solution. An improvement in the nanostructured TiO₂ photoelectrode carried out in the present work corresponds to admixing CuO to improve the spectral response. In the present study, photo-electrochemical (PEC) and hydrogen evolution characteristics of new types of ns-TiO₂–CuO admixed/Ti septum-based semiconductor septum photo-electrochemical (SC-SEP PEC) solar cell has been studied. The CuO admixed ns-TiO₂ exhibited a high photocurrent and photovoltage of 18.6 mA/cm² and 680 mV, respectively. The ns-TiO₂–CuO electrode exhibited a higher hydrogen gas evolution rate of 14.00 l/h m².     

Surface and bulk modification for advanced electrode design in photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting provides a prominent strategy for harnessing solar energy in the production of sustainable hydrogen fuel from water. Over the past few decades, extensive efforts have been devoted to develop advanced electrodes for efficient PEC water splitting. This review presents the recent progress in the development of efficient photoanodes through two major approaches: surface modification, including co-catalyst-loading, passivation, and defect engineering; and bulk modification, including hybridization, dopant engineering, and structural control. By virtue of bulk and surface modification a considerable improvement in PEC activity has been obtained so far. Photocurrent response of various anodes observed in the range of 0.063 mA cm⁻² – 8.5 mA cm⁻² (as listed in Table 1) require further improvement to upgrade the overall performance efficiency of PEC cells.This review also provides a systematic overview of the fundamentals of PEC water splitting, as well as the key challenges and notable achievements made so far in terms of electrode design and material modification. Finally, future research perspectives that will further advance this field are discussed. The contribution of this paper is to provide fundamental information about bulk and surface modifications, which will aid in the design of advanced electrodes for high-performance PEC cells.     

Design of a solar-driven methanol steam reforming receiver/reactor with a thermal storage medium and its performance analysis

A novel method of triple line focused on solar-powered receiver/reactor with a thermal storage medium for methanol steam reforming (MSR) hydrogen production is proposed in this paper. The photo-thermal-chemical energy conversion and coupling equations of the receiver/reactor are established, and the dynamic regularity between solar radiation and the hydrogen production characteristics is obtained by numerical simulation. The results show that a high solar radiation intensity helps to stabilize the duration of the reaction. For every 100 W m⁻² increase in the solar radiation intensity, the duration of the reaction maintained at the phase change point temperature of the phase change material (PCM) increases by approximately 11%. The daily hydrogen production performance of the system in Kunming (102°43′E and 25°02′N) during typical solar days is studied. The average annual total hydrogen production per unit of the lighting area is approximately 1300 m³. This research can guide similar issues related to solar thermochemical technology.     

Solar enriched methane production by steam reforming process: Reactor design

A novel hybrid plant for a mixture of methane and hydrogen (enriched methane) production from a steam reforming reactor whose heat duty is supplied by a molten salt stream heated up by a concentrating solar power (CSP) plant developed by ENEA is here presented. By this way, a hydrogen stream, mixed with natural gas, is produced from solar energy by a consolidated production method as the steam reforming process and by a pre-commercial technology as molten salts parabolic mirrors solar plant. After the hydrogen production plant, the residual heat stored in molten salt stream is used to produce electricity and the plant is co-generative (hydrogen + electricity).The heat-exchanger-shaped reactor is dimensioned by a design tool developed in MatLab environment. A reactor 3.5 m long and with a diameter of 2″ is the most efficient in terms of methane conversion (14.8%) and catalyst efficiency (4.7 Nm³/h of hydrogen produced per kg_cat).     

Influence of process parameters on direct solar-thermal hydrogen and graphite production via methane pyrolysis

Current hydrogen and carbon production technologies emit massive amounts of CO₂ that threaten Earth's climate stability. Here, a new solar-thermal methane pyrolysis process involving flow through a fibrous carbon medium to produce hydrogen gas and high-value graphitic carbon product is presented and experimentally quantified. A 10 kW_e solar simulator is used to instigate the methane decomposition reaction with direct irradiation in a custom solar reactor. From localized solar heating of fibrous medium, the process reaches steady-state thermal and chemical operation from room temperature within the first minute of irradiation. Additionally, no measurable carbon deposition occurs outside the fibrous medium, leaving the graphitic product in a form readily extractable from the solar reactor. Parametric variations of methane inlet flow rate (10–2000 sccm), solar power (0.92–2.49 kW) and peak flux (1.3–3.5 MW/m²), operating pressure (1.33–40 kPa), and medium thickness (0.36–9.6 mm) are presented, with methane conversion varying from 22% to 96%.     

Remarkable improvement of photoelectrochemical water splitting in pristine and black anodic TiO2 nanotubes by enhancing microstructural ordering and uniformity

Efficient photoelectrochemical (PEC) water splitting is crucial for future energy and sustainable world. We here report on the improvement of PEC activity of anodic TiO₂ nanotubes (TNTs) by enhancing tube ordering and subsequent electrochemical reduction. TNTs were prepared by two-step anodic oxidisation from an organic electrolyte containing fluoride ions. The effects of first-step anodisation time on the ordering of TNTs and subsequent electrolytic reduction were investigated on the PEC performance under simulated solar light spectrum. The photocurrent densities of TNTs anodised for 1 h, 4 h and subsequently reduced are about 25.12 μA cm⁻², 51.76 μA cm⁻² and 126.89 μA cm⁻², respectively, at 1.23 V vs RHE and their conversion efficiency of light to electrical energy achieved are about 0.016%, 0.04% and 0.08% respectively. Electrochemical impedance spectroscopy (ESI) curves revealed the improved PEC water splitting confirmed by sharper charge carrier separation and enhanced charge transfer in highly ordered pristine and black TNTs. This improvement of PEC in dopant-free TNT is at the first instance interpreted by enhancing TNT ordering and uniformity achieved by prolonging of the first-step anodisation time and its effect on the electronic band structure of TNTs. This significant effect on PEC performance of pristine TNT under visible light absorption takes place due to the induced surface defects and slower recombination rates of hole and electron. This demonstrates an efficient economic materials production appraoch for PEC hydrogen production.     

Measured effects of light intensity and catalyst concentration on photocatalytic hydrogen and oxygen production with zinc sulfide suspensions

In this paper, an experimental study is performed for hydrogen and oxygen production by new photo-catalytic and electro-catalytic water splitting systems. An effective method for hydrogen production by solar energy without consumption of additional reactants is a hybrid system which combines photo-chemical and electro-catalytic reactions. Experiments are performed in batch and dual cell quasi-steady operation with different light intensities and zinc sulfide photo-catalyst concentrations. The photo-reactor in batch operation achieves 6 mL h⁻¹ of hydrogen production with 3% w/v of catalyst. The hydrogen production rate corresponds to a quantum efficiency of 75% as measured through illumination of zinc sulfide suspensions in a dual cell reactor.     

An efficient tandem photoelectrochemical cell composed of FeOOH/TiO2/BiVO4 and Cu2O for self-driven solar water splitting

An integrated solar water splitting tandem cell without external bias was designed using a FeOOH modified TiO₂/BiVO₄ photoanode as a photoanode and p-Cu₂O as a photocathode in this study. An apparent photocurrent (0.37 mA/cm² at operating voltage of +0.36 V_RHE) for the tandem cell without applied bias was measured, which is corresponding to a photoconversion efficiency of 0.46%. Besides, the photocurrent of FeOOH modified TiO₂/BiVO₄–Cu₂O is much higher than the operating point given by pure BiVO₄ and Cu₂O photocathode (∼0.07 mA/cm² at +0.42 V_RHE). Then we established a FeOOH modified TiO₂/BiVO₄–Cu₂O two-electrode system and measured the current density-voltage curves under AM 1.5G illumination. The unassisted photocurrent density is 0.12 mA/cm⁻² and the corresponding amounts of hydrogen and oxygen evolved by the tandem PEC cell without bias are 2.36 μmol/cm² and 1.09 μmol/cm² after testing for 2.5 h. The photoelectrochemical (PEC) properties of the FeOOH modified TiO₂/BiVO₄ photoanode were further studied to demonstrate the electrons transport process of solar water splitting. This aspect provides a fundamental challenge to establish an unbiased and stabilized photoelectrochemical (PEC) solar water splitting tandem cell with higher solar-to-hydrogen efficiency.     

Modeling of a direct solar receiver reactor for decomposition of sulfuric acid in thermochemical hydrogen production cycles

Hydrogen production thermochemical cycles, based on the recirculation of sulfur-based compounds, are among the best suited processes to produce hydrogen using concentrated solar power. The sulfuric acid decomposition section is common to each sulfur-based cycle and represents one of the fundamental steps. A novel direct solar receiver-reactor concept is conceived, conceptually designed and simulated. A detailed transport phenomena model, including mass, energy and momentum balance expressions as well as suitable decomposition kinetics, is described adopting a finite volume approach. A single unit reactor is simulated with an inlet flow rate of 0.28 kg/s (corresponding to a production of approximately 11 kg_H2/h in a Hybrid Sulfur process) and a direct solar irradiation at a constant power of 143 kW/m². Results, obtained for the high temperature catalytic decomposition of SO₃ into SO₂ and O₂, demonstrate the effectiveness of the proposed concept, operating at pressures of 14 bar. A maximum temperature of 879 °C is achieved in the reactor body, with a corresponding average SO₂ mass fraction of 27.8%. The overall pressure drop value is 1.7 bar. The reactor allows the SO₃ decomposition into SO₂ and O₂ to be realized effectively, requiring an external high temperature solar power input of 123.6 kJ/molSO2 (i.e. 123.6 kJ/mol_H2).     

Highly-stable,bifunctional, binder-free & stand-alone photoelectrode (FexNi1-xO@a-CC) for natural waters splitting into hydrogen

Photoelectrochemical (PEC) water splitting is a promising approach to boost green hydrogen production. Herein, we prepared novel binder-free photoelectrode by direct growth of iron doped nickel oxide catalyst over activated carbon cloth (Fe_xNi_1-xO@a-CC) having band gap energy of 2.2 eV for overall water splitting. Fe_xNi_1-xO@a-CC photoelectrode had shown remarkable lower potential of only 1.36 V for oxygen evolution reaction (OER) to reach 10 mA cm^?2 current density using very low photonic intensity of 8.36 × 10^?4 E/L.s. For the first time, we also reported electrical efficiency required for PEC water splitting for 1 m³ of water that is equal to 0.09 kWh/m³. Fe_xNi_1-xO@a-CC photoelectrode also exhibits low potentials of 1.44 V (OER) and ?0.210 V (HER) at 10 mA cm^?2 to split sea water. Our results confirmed that designing Fe_xNi_1-xO@a-CC photoelectrode would be an innovative step to widen green energy conversion applications using natural waters (both sea and fresh water).     

Solar hydrogen production by the Hybrid Sulfur process

A conceptual design and economic analysis are presented for a hydrogen production plant based on the use of thermochemical water splitting combined with a solar central receiver. The reference design consists of a Hybrid Sulfur thermochemical process coupled to a solar plant, based on the particle receiver concept, for a yearly average hydrogen production rate of 100 tons per day. The Hybrid Sulfur plant has been designed on the basis of results obtained from a new flowsheet ASPEN Plus^® simulation, carrying out specific evaluations for the Sulfur dioxide Depolarized Electrolyzer, being developed and constructed at Savannah River National Laboratory, and for the sulfuric acid decomposition bayonet-based reactor, investigated at Sandia National Laboratory. Solar hydrogen production costs have been estimated considering two different scenarios in the medium to long term period, assuming the financing and economic guidelines from DOE’s H2A model and performing ad hoc detailed evaluations for unconventional equipment. A minimum hydrogen production specific cost of 3.19 $/kg (2005 US $) has been assessed for the long term period. The costs, so obtained, are strongly affected by some quantities, parameters and assumptions, influence of which has also been investigated and discussed.     

A pilot-scale solar reactor for the production of hydrogen and carbon black from methane splitting

A pilot-scale solar reactor was designed and operated at the 1 MW solar furnace of CNRS for H₂ and carbon black production from methane splitting. This constitutes the final objective of the SOLHYCARB EC project. The reaction of CH₄ dissociation produces H₂ and carbon nanoparticles without CO₂ emissions and with a solar upgrade of 8% of the high heating value of the products. The reactor was composed of 7 tubular reaction zones and of a graphite cavity-type solar receiver behaving as a black-body cavity. Temperature measurements around the cavity showed a homogeneous temperature distribution. The influence of temperature (1608K–1928K) and residence time (37–71 ms) on methane conversion, hydrogen yield, and carbon yield was especially stressed. For 900 g/h of CH₄ injected (50% molar, the rest being argon) at 1800K, this reactor produced 200 g/h H₂ (88% H₂ yield), 330 g/h CB (49% C yield) and 340 g/h C₂H₂ with a thermal efficiency of 15%. C₂H₂ was the most important by-product and its amount decreased by increasing the residence time. A 2D thermal model of the reactor was developed. It showed that the design of the reactor front face could be drastically improved to lower thermal losses. The optimised design could reach 77% of the ideal black-body absorption efficiency (86% at 1800K), i.e. 66%.     

Stable and highly efficient MoS2/Si NWs hybrid heterostructure for photoelectrocatalytic hydrogen evolution reaction

We report, the fabrication of molybdenum disulphide (MoS₂) wrapped silicon nanowires (Si NWs) for visible light driven water splitting applications. The morphological and elemental studies ensure the vertical alignment of Si NWs wrapped with 2D layered MoS₂. The photoelectrocatalytic (PEC) results evidence the significant enhancement in performance of MoS₂/Si NWs based hybrid photocathode with ~300 mV (under reversible hydrogen electrode (RHE)) anodic shift in onset potential as that of pristine Si NWs (+0.194 V vs. RHE), and the current density of −26.5 mA/cm² was achieved at the applied bias of 0 V vs. RHE. Further, the electrochemical impedance studies ensure the interface resistance-free charge transfer between Si NWs and electrolyte via 2D MoS₂ layer which provokes rapid hydrogen production. The wrapping of Si NWs with MoS₂ protects the superlative photocathode from harsh acid electrolyte environment. The overgrown MoS₂ triangular particles with active sulphur edge sites are found to eventually augment the solar hydrogen evolution rate. Further, the PEC performance of our MoS₂/Si NWs is also comparable with stable Pt/Si NWs photoelectrode. It is note-worthy that, MoS₂/Si NWs hybrid heterostructure would be a potential candidate in future large scale, low cost and day-to-day solar water splitting applications.     

Hydrogen production from solar energy powered supercritical cycle using carbon dioxide

A hydrogen production method is proposed, which utilizes solar energy powered thermodynamic cycle using supercritical carbon dioxide (CO₂) as working fluid for the combined production of hydrogen and thermal energy. The proposed system consists of evacuated solar collectors, power generating turbine, water electrolysis, heat recovery system, and feed pump. In the present study, an experimental prototype has been designed and constructed. The performance of the cycle is tested experimentally under different weather conditions. CO₂ is efficiently converted into supercritical state in the collector, the CO₂ temperature reaches about 190 °C in summer days, and even in winter days it can reach about 80 °C. Such a high-temperature realizes the combined production of electricity and thermal energy. Different from the electrochemical hydrogen production via solar battery-based water splitting on hand, which requires the use of solar batteries with high energy requirements, the generated electricity in the supercritical cycle can be directly used to produce hydrogen gas from water. The amount of hydrogen gas produced by using the electricity generated in the supercritical cycle is about 1035 g per day using an evacuated solar collector of 100.0 m² for per family house in summer conditions, and it is about 568.0 g even in winter days. Additionally, the estimated heat recovery efficiency is about 0.62. Such a high efficiency is sufficient to illustrate the cycle performance.     

Hydrogen production probability distributions for a PV-electrolyser system

In this study, we comprehensively analyze the probability distribution of the hydrogen production for PV assisted PEM electrolyser system. A case study is conducted using the experimental data taken from a recently installed system in Balikesir University, Turkey. A novel computational tool is developed in Matlab-Simulink for analyzing the data. The concept of probability density frequency is successfully applied in the analyses of the wind speed and the solar energy in literature. This study presents a method of applying this knowledge to solar energy assisted hydrogen production. The change in the probability distribution of the hydrogen production with the solar irradiation throughout a year is studied and illustrated. It is found that the maximum amount of hydrogen production occurs at between 600 and 650 W/m² of solar radiation. Annual hydrogen production is determined as 2.97 kg for per m² of PV system. Average hydrogen production efficiency of the studied PEM electrolyser is found to be 60.5% with 0.48 A/cm² of current density. The presented results of this study are expected to be valuable for the researchers working on renewable hydrogen production systems.     

Biological fermentative hydrogen and ethanol production using continuous stirred tank reactor

Hydrogen and ethanol are promising biofuels and have great potential to become alternatives to fossil fuels. The influence of organic loading rates (OLRs) on the production of fermentative hydrogen and ethanol were investigated in a continuous stirred tank reactor (CSTR) from fermentation using molasses as substrate. Four OLRs were examined, ranging from 8 to 32 kg/m³·d. The H₂ and ethanol production rate in CSTR initially increased with increasing OLR (from 8 to 24 kg/m³ d). The highest H₂ production rate (12.4 mmol/h l) and ethanol production rate (20.27 mmol/h l) were obtained in CSTR both operated at OLR = 24 kg/m³ d. However, the H₂ and ethanol production rate tended to decrease with an increase of OLR to 32 kg/m³ d. The liquid fermentation products were dominated by ethanol, accounting for 31-59% of total soluble metabolities. Linear regression results show that ethanol production rate (y) and H₂ production rate (x) were proportionately correlated which can be expressed as y = 0.5431x + 1.6816 (r² = 0.7617). The total energy conversion rate based on the heat values of H₂ and ethanol was calculated to assess the overall efficiency of energy conversion rate. The best energy conversion rate was 31.23 kJ/h l, occurred at OLR = 24 kg/m³ d.     

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.     

Fabrication of large area nanorod like structured CdS photoanode for solar H2 generation using spray pyrolysis technique

Large area nanorod like structured CdS films (9 × 9 cm²) were deposited on the FTO glass substrate using simple and economic spray pyrolysis deposition technique for photoelectrochemical (PEC) hydrogen production. With an intention of electrode scaling-up, the deposition area of photoanode was varied to evaluate its effect on the PEC hydrogen generation capability. High photocurrent of 5 mA has been achieved from the PEC active area of 37.5 cm². Its unit area (1 cm²) counterpart yielded Solar-to-Hydrogen (STH) conversion efficiency of 0.20% at a bias of 0.2 V vs Ag/AgCl using sacrificial reagents under solar simulator (AM1.5) with 80 mW/cm² irradiance. The 500 nm thick film exhibiting uniformly distributed nano-rod features yielded 3-times more photocurrent, as well as hydrogen evolution than other films. It exhibited an enhanced photo-activity as indicated by the higher IPCE values (5–9%) in the wavelength range of 450–550 nm. It exhibited superior optical properties (E_g ∼2.4 eV), and formation of high crystallinity hexagonal CdS phase with space group P63MC. The superior performance of the photoanode is attributed to the nanostructured morphology acquired under optimized spray pyrolysis conditions. Large area photoanodes showed unaltered photo-activity indicating the homogeneity in the film properties even in scaled-up version.