Efficient solar hydrogen production by photocatalytic water splitting: From fundamental study to pilot demonstration

Photocatalytic water splitting with solar light is one of the most promising technologies for solar hydrogen production. From a systematic point of view, whether it is photocatalyst and reaction system development or the reactor-related design, the essentials could be summarized as: photon transfer limitations and mass transfer limitations (in the case of liquid phase reactions). Optimization of these two issues are therefore given special attention throughout our study. In this review, the state of the art for the research of photocatalytic hydrogen production, both outcomes and challenges in this field, were briefly reviewed. Research progress of our lab, from fundamental study of photocatalyst preparation to reactor configuration and pilot level demonstration, were introduced, showing the complete process of our effort for this technology to be economic viable in the near future. Our systematic and continuous study in this field lead to the development of a Compound Parabolic Concentrator (CPC) based photocatalytic hydrogen production solar rector for the first time. We have demonstrated the feasibility for efficient photocatalytic hydrogen production under direct solar light. The exiting challenges and difficulties for this technology to proceed from successful laboratory photocatalysis set-up up to an industrially relevant scale are also proposed. These issues have been the object of our research and would also be the direction of our study in future.     

Plasma-chemical reduction of iron oxide photoanodes for efficient solar hydrogen production

We demonstrate the effect of hydrogen plasma treatment on hematite films as a simple and effective strategy for modifying the existing substrate to improve significantly the band edge positions and photoelectrochemical (PEC) performance. Plasma treated hematite films were consist of mixed phases (Fe₃O₄:α-Fe₂O₃) which was confirmed by XPS and Raman analysis, treated films also showed higher absorption cross-section and were found to be a promising photoelectrode material. The treated samples showed enhance photocurrent densities with maximum of 3.5 mA/cm² at 1.8 V/RHE and the photocurrent onset potentials were shifted from 1.68 V_RHE (untreated) to 1.28 V_RHE (treated). Hydrogen plasma treatment under non-equilibrium conditions induced a valence dynamics among Fe centers in the sub-surface region that was sustained by the incorporation of hydrogen in the hematite lattice as supported by the density functional theory calculations.     

Metal-organic framework derived Co3O4/TiO2 heterostructure nanoarrays for promote photoelectrochemical water splitting

In this work, we proposed a simple and new method to fabricate Metal-organic frameworks (MOFs) derived Co₃O₄ modified TiO₂ nanorods (NRs) photoelectrode by immersion and anneal treatment. The positively charged Co-MOF (ZIF-67) was adsorbed on the negatively charged TiO₂ NRs by electrostatic interaction, and then annealed in air to obtain the Co₃O₄/TiO₂ photoelectrodes. The photoelectrochemical (PEC) performance of the Co₃O₄/TiO₂ photoelectrodes has been significantly improved compared with the pure TiO₂, the best photocurrent density of Co₃O₄/TiO₂ photoelectrode could reach 1.04 mA/cm² (1.23 V vs RHE) which was almost 1.65 times than that of pure TiO₂. On the Co₃O₄/TiO₂ photoelectrodes, the significant improvement in PEC performance could be attributed to the constructed p-n heterostructure, which can promote charge transfer within the system and improve the efficiency of electron/hole separation. Meanwhile, under the action of the MOFs-derived Co₃O₄, the number of active sites increases significantly and visibly improve the photoresponse performance.     

Enhanced photoelectrochemical water splitting via engineered surface defects of BiPO4 nanorod photoanodes

Herein, we report on the defect engineering of BiPO₄ nanorods (NRs) via a facile room-temperature template-free co-precipitation method, followed by hydrogen treatment. The hydrogen treatment temperature determined the type of induced defects in the fabricated BiPO₄ NRs and consequently their photocatalytic performance. Upon varying the annealing temperature, the x-ray diffraction (XRD) analysis showed phase transformation and x-ray photoelectron spectroscopy (XPS) analysis revealed variation in the oxygen vacancy content. At moderate treatment temperatures (200–300 °C), shallow defects were predominant, which extended the optical activity of the material to the visible region and increased the photocurrent 3 times when compared to that of bare BiPO₄ NRs. However, treatment at higher temperatures completely altered the crystalline structure, destructed the morphology of the BiPO₄ NRs, and severely affected the photoelectrochemical performance.     

Solar thermochemical ZnO/ZnSO4 water splitting cycle for hydrogen production

In this paper, solar reactor efficiency analysis of the solar thermochemical two-step zinc oxide–zinc sulfate (ZnO–ZnSO₄) water splitting cycle. In step-1, the ZnSO₄ is thermally decomposed into ZnO, SO₂, and O₂ using solar energy input. In step-2, the ZnO is re-oxidized into ZnSO₄ via water splitting reaction producing H₂. The ZnSO₄ is recycled back to the solar reactor and hence can be re-used in multiple cycles. The equilibrium compositions associated with the thermal reduction and water-splitting steps are identified by performing HSC simulations. The effect of Ar towards decreasing the required thermal reduction temperature is also explored. The total solar energy input and the re-radiation losses from the ZnO–ZnSO₄ water splitting cycle are estimated. Likewise, the amount of heat energy released by different coolers and water splitting reactor is also determined. Thermodynamic calculations indicate that the cycle (η_cycle) and solar-to-fuel energy conversion efficiency (η_{solar-to-fuel}) of the ZnO–ZnSO₄ water splitting cycle are equal to 40.6% and 48.9% (without heat recuperation). These efficiency values are higher than previously investigated thermochemical water splitting cycles and can be increased further by employing heat recuperation.     

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.     

Refined Z-scheme charge transfer in facet-selective BiVO4/Au/CdS heterostructure for solar overall water splitting

Artificial Z-scheme systems that mimic natural photosynthesis are well applicable to photocatalytic overall water splitting for hydrogen (H₂) production free of electricity. However, it commonly confronts low efficiency with huge challenge of steering charge transfer between H₂ evolution photocatalyst (HEP) and oxygen evolution photocatalyst (OEP). Here we report an all-solid-state Z-scheme system with facet-selective construction that favors charge spatial separation toward HEP and OEP for high efficient solar overall water splitting. Based on the spontaneous separation of photogenerated electrons and holes on the different crystal facets of BiVO₄ decahedra, we successively implemented the selective depositions of Au and CdS nanoparticles (NPs) onto the electron-rich {010} facets, to fortify the Z-scheme charge transfer between BiVO₄ and CdS across Au mediators upon two-step photoexcitation. In-situ photoelectron dynamics ascertains Z-scheme model of resultant BiVO₄/Au/CdS, which enables an impressive overall water splitting with stoichiometric H₂ and O₂ evolution rates of 281 and 138 μmol g^?1 h^?1, respectively, under 1 sun irradiation (100 mW cm^?2, AM 1.5G) without using any sacrificial agents and external bias. This work not only presents a refined Z-scheme overall water splitting system, but also gains insights into photo-induced charge transfer dynamics.     

One-pot sulfurized synthesis of ZnIn2S4/S,N-codoped carbon composites for solar light driven water splitting

To enhance the intrinsic active sites and to suppress the recombination of charge carriers, ZnIn₂S₄ modified with S,N-codoped carbon (ZIS/SN-C) composites were prepared for solar light driven water splitting via the one-pot sulfurized route. Compared with g-C₃N₄ and S-doped g-C₃N₄, the combined effect between S,N-codoped carbon and ZnIn₂S₄ can greatly enhance the photocatalytic activity of ZIS/SN-C. The optimal 4-ZIS/SN-C with the Zn(II) content of 13.78% and the calculated In/Zn molar ratio of 2.03:1 presents the H₂ evolution rate of 2937.1 μmol g^?1 h^?1, which is 2.98 and 23.42 times higher than that of one-pot sulfurized ZnIn₂S₄ and S-doped g-C₃N₄, respectively. However, long-term photo-corrosion induces to the declined durability of 4-ZIS/SN/C for water splitting after three cycles. S vacancies of ZnIn₂S₄ serve as the efficient active sites of H₂ evolution reaction, and S, N-codoped carbon acts as the photo-induced electrons trapper. The one-pot sulfurized approach is thus a potential strategy to fabricate metal sulfide-based photocatalysts.     

Photoelectrochemical water splitting with nanocrystalline Zn1−xRuxO thin films

Thin films of nanocrystalline Zn_1−xRu_xO are deposited on ITO substrate by sol–gel. XRD and EDX analysis indicated dominant evolution of wurtzite ZnO with crystallite size in the range 26–43 nm. With no evidence of phase segregation, Ru insertion in the host lattice is probably indicated by distortion in lattice parameters and concomitant rise in microstrain and dislocation density. SEM images indicated homogenous and continuous growth of nanocrystallites. AFM images confirmed pillar like growth of crystallites along c-axis. Ru incorporation (1, 3, 5 and 7% at.) made film surface rougher, nevertheless roughness decreased with rise in Ru concentration. Ru incorporation at low concentrations significantly improved PEC response of films.     

Experimental and first-principles studies of BiVO4/BiV1-xMnxO4-y n-n+ homojunction for efficient charge carrier separation in sunlight induced water splitting

Though bismuth vanadate (BiVO₄) is extensively used as a photoactive material, its performance in harnessing solar energy is limited by ineffective separation of photo-excited charge carriers. We demonstrate here a concept of n-n⁺ homojunction of BiVO₄/BiV_1-xMn_xO_4-y, which improves its charge separation efficiency. Using first-principles theoretical calculations, we determine the effect of Mn substitution on oxygen vacancy formation energies and associated changes in the electronic structure of BiVO₄. Showing that Mn substitution pushes the Fermi level of BiVO₄ towards its conduction band, we predict that the associated enhanced bending of bands at the homojunction (BiVO₄/BiV_1-xMn_xO_4-y) facilitates efficient separation of charge carriers. With Mott-Schottky experiments, we verify the increased band bending at the n-n⁺ homojunction, and show that the maximum photocurrent density measured in a sample with n-n⁺ homojunction is ten times higher than that obtained of the pristine sample. Secondly, Mn substitution in BiVO₄ also reduces the oxygen vacancy formation energy, promoting higher concentration of O-vacancies, further enhancing the photoelectrochemical response.     

TiO2 photoanodes for electrically enhanced water splitting

Photoelectrochemical properties of self-organized TiO₂ nanotubes formed by electrochemical anodization of titanium sheets and their working mechanism are investigated. Formation and growth of self-organized nanotubes were carried out by Titanium anodization in acid electrolyte containing fluorides. Annealing of the samples was performed in order to increase the crystallinity of the material. Also some results obtained with samples annealed in Nitrogen atmosphere are presented. Electrochemical Impedance Spectroscopy was used to give an interpretation of the main charge transfer processes that occur at the nanotube/electrolyte interphase. The use of glycerol as hole scavenger was considered in order to improve the photoelectrochemical performance of the samples.     

Preparation of carbon dots/TiO2 electrodes and their photoelectrochemical activities for water splitting

Carbon dots with various functional groups can be employed as the potential sensitizer. In this study, carbon dots are obtained by electrochemical ablation of graphite rods in alkaline electrolyte. The better preparation condition is the applied potential of 40 V and the ablation time of 5 h. TiO₂ nanotube arrays and TiO₂ nanoparticles photoelectrodes are sensitized by the as-prepared carbon dots through using impregnation method. Carbon dots/TiO₂ nanotube arrays electrodes exhibit greater photoelectrochemical hydrogen production activities than carbon dots/TiO₂ nanoparticles electrodes. It is because more carbon dots can be well combined with TiO₂ nanotube arrays. Based on the IPCE values in visible light region, the role of carbon dots on TiO₂ nanotube arrays electrode depends on the up-converted PL behaviors from their surface states and the alkaline electrolyte. The results provide insight into carbon dots that serve as sensitizer of TiO₂ photoelectrode in water splitting system of alkaline solution.     

An overview of photocells and photoreactors for photoelectrochemical water splitting

Solar hydrogen production from direct photoelectrochemical (PEC) water splitting is the ultimate goal for a sustainable, renewable and clean hydrogen economy. While there are numerous studies on solving the two main photoelectrode (PE) material issues i.e. efficiency and stability, there is no standard photocell or photoreactor used in the study. The main requirement for the photocell or photoreactor is to allow maximum light to reach the PE. This paper presents an overview of the PE configurations and the possible photocell and photoreactor design for hydrogen production by PEC water splitting.     

Efficient suppression of surface charge recombination by CoP-Modified nanoporous BiVO4 for photoelectrochemical water splitting

BiVO₄ is a promising photoanode material for water splitting due to its substantial absorption of solar light as well as favorable band edge positions. However, the poor water oxidation kinetics of BiVO₄ results in its insufficient photocurrent density. Herein, we demonstrate the use of CoP nanoparticles for facile surface modification of nanoporous BiVO₄ photoanode in potassium borate buffer solution (pH 9.0), which can generate a tremendous cathodic shift of ~430 mV in the onset potential for photoelectrochemical water oxidation. In addition, a remarkable photocurrent density of 4.1 mA cm^?2 is achieved at 1.23 V vs. RHE under AM 1.5G illumination. The photoelectrochemical measurement using sodium sulfite as a hole scavenger clearly shows that the greatly improved performances are attributed to the efficient suppression of interfacial charge recombination through loading of CoP catalyst. Moreover, the maximum surface charge injection yield can reach >81% at 1.23 V vs. RHE and the maximum IPCE of CoP/BiVO₄ can reach 75.8% at 420 nm, suggesting the potential application of CoP-modified BiVO₄ photoanode for overall solar water splitting in cost-effective tandem photoelectrochemical cells.     

Photon-absorption-based explanation of ultrasonic-assisted solar photochemical splitting of water to improve hydrogen production

The ultrasonic-assisted solar photochemical splitting of water had been explored in recent years to enhance hydrogen production efficiency. In this study, a photon-absorption-based study was conducted to investigate the mechanism of the ultrasonic-assisted solar photochemical splitting of water. An elaborate test bench for temperature-controlled, ultrasonic-assisted solar photochemical water splitting was designed, set up, and tested. A comparison of the hydrogen production between the ultrasonic-assisted and conventional solar photochemical splitting of water was carried out. The effective nanoparticle size before and after ultrasonic vibration, as well as after solar photocatalysis, was analyzed. Furthermore, the spectral absorptivity of the nanofluids before and after ultrasonic vibration, as well as after solar photocatalysis, was investigated by both experimental and numerical methods. The investigation indicated that the improved particle dispersion in the solution prepared by ultrasonication allowed the absorbance of more incoming sunlight. The amount of hydrogen produced by the ultrasonic-assisted hydrogen production was 3.45 times that of conventional solar photochemical splitting of water without pre-ultrasonicated. Besides, an effective spectral absorptivity coefficient was proposed as a modified measure of spectral absorptivity. In addition, the optimal particle diameter was optimized using the Monte Carlo ray tracing method to identify the best light absorption performance.     

Nanotube enhanced photoresponse of carbon modified (CM)-n-TiO2 for efficient water splitting

Carbon modified (CM)-n-TiO₂ nanotube arrays were successfully synthesized by anodization of Ti metal sheet in fluoride solution and subsequent annealing in air and natural gas flame oxidation. Both nanotube structure and carbon doping contributed to the enhancement of photoresponse of n-TiO₂. About two fold increase in photocurrent density was observed at undoped n-TiO₂ nanotube film compared to that at its undoped n-TiO₂ flat thin film. Also, about eight fold increase in photocurrent density was observed at carbon modified (CM)-n-TiO₂ nanotube film compared to that at undoped n-TiO₂ flat thin film. The sample prepared by anodization at 20 V cell voltage for 20 h followed by annealing in air at 500 °C for 1 h and natural gas flame oxidation at 820 °C for 18 min produced highest photocurrent density. It was found that the bandgap of n-TiO₂ was reduced to 2.84 eV and an additional intragap band was introduced in the gap at 1.30 eV above the valence band. The bandgap reduction and the new intragap band formation in CM-n-TiO₂ extended its utilization of solar energy up to the visible to infrared region.     

Electrophoretic behavior of solvothermal synthesized anion replaced Cu2ZnSn(SxSe1-x)4 films for photoelectrochemical water splitting

Quaternary Cu₂ZnSn(S_xSe_1-x)₄ (CZT(S_xSe_1-x)₄) compounds have drawn a great deal of attention for being used in the fabrication of optoelectronic devices such as solar cells, photocatalysts, and photoelectrochemical water splitting. However, one major challenge facing the utilization of this material is to reduce the production cost of synthesis and fabrication of high quality CZT(S_xSe_1-x)₄ films. In the present study, a facile and beneficial solvothermal route has been reported for synthesis of CZT(S_xSe_1-x)₄ compounds. The process of electrophoretic deposition (EPD) of synthesized CZT(S_xSe_1-x)₄ nanoparticles, is systematically compared with each other in order to obtain high quality films with appropriate porosity. The XRD patterns, EDS and Raman spectra confirm the formation of CZT(S_xSe_1-x)₄ phases with no trace of impurities and appreciable crystallinity and also with near stoichiometry composition in all the samples. The obtained particle size for CZTS, CZTSSe and CZTSe samples was in the range of 50–100 nm and also for some agglomerate particles was in the range of 500 nm to 2 μm. Based on the obtained results for thin films prepared using EPD in the present study, the best EPD parameters for each CZTS, CZTSe and CZTSSe samples with 120 V and 5 min as applied voltage and deposition time were reported as the best samples. The obtained photocurrent-potential and current-time curves of CZT(S_xSe_1-x)₄ thin film samples demonstrate that the photocurrents of each CZTS, CZTSe, CZTSSe thin films, are different in the range of ?2.1 to ?6 mA/cm² and also the CZTS and CZTSe samples show a detectable current under the exposure of sunlight that can have an appropriate stability for 3000 s but the CZTSSe sample showed a stable photocurrent just for 2000 s. According to the mentioned results in this study, the CZTS and CZTSe samples can potentially be suitable candidates for further applications.     

Light intensity effects on photocatalytic water splitting with a titania catalyst

Photocatalytic water splitting is a process which could potentially lead to commercially viable solar hydrogen production. In order to evaluate if solar concentration could be used to increase the feasibility of the process, the effect of light intensity on photocatalytic water splitting was examined.     

A comparative thermodynamic analysis of samarium and erbium oxide based solar thermochemical water splitting cycles

This paper reports a thermodynamic comparison between the samarium and erbium oxide based solar thermochemical water splitting cycles. These cycles are a two-step process in which the metal oxide is first thermally reduced into the pure metal, and the produced metal can be used to split water to produce H₂. The metal oxides can be reused for multiple cycles without consumption. The effect of water splitting temperature on various thermodynamic parameters which are essential to design the solar reactor system for the production of H₂ via water splitting reaction using the samarium and erbium oxides is studied in detail. The total amount of solar energy needed for the thermal reduction of samarium and erbium oxides is estimated. The amount of heat energy released by the water splitting reactor is calculated. Also, the cycle and solar-to-fuel energy conversion efficiency for both cycles are determined by employing heat recuperation. Obtained results indicate that the efficiencies associated with these cycles are comparable to the previously studies thermochemical cycles. It is observed that higher water splitting temperature favors towards higher efficiencies. At constant thermal reduction temperature = 2280 K, by employing 50% heat recuperation, the solar-to-fuel energy conversion efficiency for the samarium cycle (30.98%) is observed to be higher than erbium cycle (28.19%).