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.     

An ultra-thin NiOOH layer loading on BiVO4 photoanode for highly efficient photoelectrochemical water oxidation

Monoclinic bismuth vanadate has been widely used as a promising n-type semiconductor for photoelectrochemical (PEC) water decomposition due to its high reserves, good stability in neutral solutions, and relatively narrow band gap. Here, we developed a simple method to prepare a thin NiOOH layer on the surface of BiVO₄ nanorod arrays. The heterostructured photoanode shows great enhancement for the photocurrent density of 2.7 mA cm⁻² at 1.23 V vs. RHE, which is ~2.3 times higher than that of pristine BiVO₄ electrode, due to NiOOH as an efficient oxygen-releasing catalyst with abundant oxygen vacancies. The NiOOH/BiVO₄ photoanodes are systematically studied with X-ray diffraction, Raman, X-ray photoelectron spectra, scanning electron microscopy, transmission electron microscopy, and UV–vis diffuse-reflectance spectrum. The heterostructured photoanode shows excellent PEC activity, which can provide a promising and easy strategy to prepare such photoanode with high-efficient oxygen evolution co-catalysts.     

Recent trends and outlooks on engineering of BiVO4 photoanodes toward efficient photoelectrochemical water splitting and CO2 reduction: A comprehensive review

Photoelectrochemical (PEC) water splitting, and carbon dioxide (CO₂) utilization devices have attracted immense attention as sustainable technologies for the generation of hydrogen (H₂) fuel and value-added chemicals feedstock. Among numerous semiconductors, bismuth vanadate (BiVO₄) has emerged as a promising photoanode owing to its fascinating features such as high chemical stability, straddling band alignment with water redox levels, eco-friendly, and cost-effectiveness. However, sluggish oxidation kinetics, photo-corrosive nature, low electronic conductivity, and short carrier diffusion length limit its commercialization on the PEC horizon. To mitigate these inadequacies, several strategies have emerged such as novel heterojunctions, doping with unique materials, interface modulation, morphology, facet orientation, co-catalyst loading for surface engineering, etc. to realize the outstanding cost-to-efficiency ratios and long-term stability of PEC devices. The review highlights the recent advancement in BiVO₄-based photoanodes in last five years (2018–2022) and their utilization in the single absorber and unexplored tandem PEC systems towards boosted water splitting and CO₂ reduction. A discussion on theoretical studies of BiVO₄-based PEC systems elucidates the microscopic mechanism of promotion effect of the bulk/interface/surface strategies on surface catalysis as well as interfacial charge transfer in boosting oxidation kinetics. Moreover, this review addresses the versatility of the BiVO₄-based photoanode for the novel yet commercially viable PEC applications. This review will provide a broad avenue in designing highly durable, and scalable BiVO₄-based systems toward various PEC energy conversion devices.     

Photoelectrochemical water splitting with 600 keV N2+ ion irradiated BiVO4 and BiVO4/Au photoanodes

Low energy N²⁺ ion beam with 600 keV energy has been used to irradiate BiVO₄ and Au nanoparticles loaded BiVO₄ (BiVO₄/Au) thin films deposited over fluorine doped tin oxide substrates via spray pyrolysis technique. Ion irradiation results in tailoring the optical, electrical, and morphological properties of the thin films and thence also responsible for changes in electrochemical properties. The scanning electron microscope images reveal the evolution of Au nanoparticles after irradiation at 2 × 10¹⁵ fluence to a nanourchins type of morphology. In consequence of morphological changes, the signature of surface plasmon resonance peak exhibited by Au nanoparticles in BiVO₄/Au shows improvement. An increase of approximately 92% in photocurrent density in comparison to pristine BiVO₄ has been found after irradiation in BiVO₄/Au photoanode at 2 × 10¹⁵ ions/cm² fluence. Moreover, irradiation also aids in improving photoelectrochemical response of BiVO₄ photoanodes without Au nanoparticles. The enhancement can be attributed to the notable changes in onset potential, charge separation, charge transfer resistance and optical properties.     

The co-decorated TiO2 nanorod array photoanodes by CdS/CdSe to promote photoelectrochemical water splitting

A cascade structure of TiO₂/CdS/CdSe semiconductor heterojunction is synthesized using a three-step technique of facile hydrothermal growth for the enhancement of the photoelectrochemical performances. The optical and photoelectrochemical properties controlled by the deposition processing parameters have been investigated. It is shown that the patterns of semiconductor heterojunction enlarge the absorption range of solar spectra, and improve the properties of the photogenerated charge carriers describing separation and transportation, and reduce the interface resistance between the photoelectrode and electrolyte comparing with the pure TiO₂ and CdS-decorated TiO₂ nanorod array photoanodes. The higher photocurrent density and photoconversion efficiency are up to 4.23 mA cm⁻² and 4.2%, which are the 4.1 and 25.3 times superior than that of the pure TiO₂ photoanode. The hydrothermal growth time increment of CdSe yields greater photoelectrochemical water splitting performances. The underlying physics mechanisms have been discussed based on forming a type-Ⅱ energy band alignment structure.     

rGO decorated semiconductor heterojunction of BiVO4/NiO to enhance PEC water splitting efficiency

Monoclinic phase of BiVO₄ is a promising photoanode material for photoelectrochemical (PEC) water splitting, but its sluggish water oxidation kinetics and frequent bulk charge recombination greatly reduce its efficiency of PEC water splitting. A novel BiVO₄/NiO/rGO photoanode was very simply prepared by electrodeposition, solution immersion and spin coating methods, in particular, the solution immersion method to loading NiO has never been reported in PEC research. Compared with BiVO₄, the photocurrent density of the ternary photoanode reaches 1.52 mA/cm² at 1.23 V vs RHE, which is 2.41 and 1.39 times higher than that of pure BiVO₄ and binary BiVO₄/NiO photoanode, respectively. The onset potential of the ternary photoanode shows a significant cathodic shift of 130 mV compared with the BiVO₄ photoanode. Moreover, the measured incident photon-to-current efficiency (IPCE) value reaches 50.52% at λ = 420 nm. The improvement is attributed to the type-II heterojunction formation that enhances the separation efficiency of electron/hole and the rGO decoration that accelerates the electron transfer and provides more active sites for gas adsorption.     

Plasmonic Bi nanoparticle decorated BiVO4/rGO as an efficient photoanode for photoelectrochemical water splitting

We report the application of plasmonic Bi nanoparticles supported rGO/BiVO₄ anode for photoelectrochemical (PEC) water splitting. Nearly, 2.5 times higher activity was observed for Bi-rGO/BiVO₄ composite than pristine BiVO₄. Typical results indicated that Bi-rGO/BiVO₄ exhibits the highest current density of 6.05 mA/cm² at 1.23 V, whereas Bi–BiVO₄ showed the current density of only 3.56 mA/cm². This enhancement in PEC activity on introduction of Bi-rGO is due to the surface plasmonic behavior of BiNPs, which improves the absorption of radiation thereby reduces the charge recombination. Further, the composite electrode showed good solar to hydrogen conversion efficiency, appreciable incident photon-to-current efficiency and low charge transfer resistance. Hence, Bi-rGO/BiVO₄ provides an opportunity to realize PEC water splitting.     

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.     

Thermochemical H2 production via solar driven hybrid SrO/SrSO4 water splitting cycle

This article reports the thermodynamic efficiency analysis of the strontium oxide – strontium sulfate (SrO-SrS) water splitting cycle by applying the principles of the second law of thermodynamics and by utilizing the commercially available HSC Chemistry software. Initially, the thermodynamic equilibrium compositions allied with a) the thermal reduction of SrSO₄, b) H₂ production via water splitting reaction (through SrO re-oxidation) are recognized. Moreover, the temperatures desirable for performing the thermal reduction and the water splitting steps are determined. The consequence of the molar flow rate of Ar on the thermal reduction of SrSO₄ is also examined in detail. The effect of the thermal reduction and water splitting temperatures on the total solar energy input mandatory to run the cycle, re-radiation shortfalls from the cycle, heat energy emitted by the coolers and the water splitting reactor, and the cycle and the solar-to-fuel energy conversion efficiency (with heat recuperation) is scrutinized in detail. The attained outcomes specify that the cycle and the solar-to-fuel energy conversion efficiency up to 18.9 and 22.8% can be accomplished if the thermal reduction and the water splitting steps are conducted at 2380 and 1400 K (with 30% heat recuperation).     

Effects of molecular modifications for water splitting enhancement of BiVO4

Combined organic (molecular adsorption) and inorganic (TiO₂ passivation) modifications for enhancing water splitting efficiency of porous bismuth vanadate electrodes are tested. The catalytic activity of BiVO₄ is increased after adsorption of a newly prepared ruthenium catalyst. TiO₂ passivation and sensitization with RuP dye does not show straightforward improvements to the complex photocatalytic behaviour depending on the configuration of the (two- or three-electrode) photoelectrochemical cell, type of the experiment and sample aging. The time constant for electron transport in BiVO₄ electrodes (in the range of seconds, revealed by electrochemical impedance measurements) was found to correlate with the stable photocurrent of the cells. The femtosecond transient absorption studies confirm the negligible effects of RuP on the population of the photoexcited carriers in BiVO₄. The transient absorption studies also show that the processes responsible for the differences in photocurrents of the modified BiVO₄ samples occur on a time scale longer than the first nanoseconds.     

Photoanode of LDH catalyst decorated semiconductor heterojunction of BiVO4/CdS to enhance PEC water splitting efficiency

BiVO₄ is an ideal photoanode material for solar-driven photoelectrochemical (PEC) water splitting but it easily suffers from the recombination of photogenerated electrons and holes due to its low carrier mobility thus cause low efficiency of PEC water splitting. Herein, the BiVO₄/CdS/NiCo-LDH photoanode was prepared by combining methods of metal organic decomposition, chemical and electrodeposition. The photoanode photocurrent density reaches 2.72 mA cm⁻² at 1.23 V (vs. RHE), which is 3.6 folds of pure BiVO₄ photoanode and onset potential shifts 450 mV toward cathodic. The incident photon-to-electron conversion efficiency (IPCE) value is 2.86 folds of BiVO₄, the calculated photon–to–current efficiency (ABPE) is 1.24% at 0.62 V (vs. RHE). The obtained results are higher than that of most BiVO₄ based photoanodes published so far. The enhancement benefits from increase of visible light absorption capacity, enhancement of separation efficiency of photoexcited electron-hole and fast transfer of holes accumulated on electrode/electrolyte surface for water oxidation, which has been confirmed by calculating carrier density and carrier transport rate.     

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.     

Morphology control of the hematite photoanodes for photoelectrochemical water splitting

The morphology of hematite photoanode is a significant relevant factor in its photoelectrochemical (PEC) performance. Hematite nanowires and nanocubes as well as nanorods with intentional Sn doping were prepared by hydrothermal processes containing disparate additives. The band-gap decreases in the sequence of nanowires, nanorods and nanocubes. Compared with nanorods, nanowires show higher carrier density but a lower light absorbance. With both inhibited bulk and surface charge recombination, nanowires achieve an enhanced photocurrent. Meanwhile, it is more complicated for the charge conversion in the hematite nanocubes. Light absorption is limited due to the compact arrangement of nanocubes. Besides, nanocubes show a highly oriented (104) plane which is unfavorable to the charge conductivity. Despite the negative factors hindering its PEC performance, the extremely high carrier density in the nanocubes benefits to the distinctly enhanced photocurrent collected from the hematite samples annealed at 550 and 650 °C respectively. However, the superiority of hematite nanocubes annealed under 800 °C is restricted by the high onset potential. Still, attributed to the high surface charge transfer efficiency, the hematite nanocubes achieve the highest photocurrent among the samples at biases above 1.3 V. Electrodes made of hematite nanorods, nanowires and nanocubes annealed at 800 °C achieve a photocurrent of 1.01, 1.30 and 1.40 mA cm⁻² at 1.6 V vs. RHE, respectively.     

A novel three-step GeO2/GeO thermochemical water splitting cycle for solar hydrogen production

This investigation reports the thermodynamic exploration of a novel three-step GeO₂/GeO water splitting (WS) cycle. The thermodynamic computations were performed by using the data obtained from HSC Chemistry thermodynamic software. Numerous process parameters allied with the GeO₂/GeO WS cycle were estimated by drifting the thermal reduction ($T_H$) and water splitting temperature ($T_L$). The entire analysis was divided into two section: a) equilibrium analysis and b) efficiency analysis. The equilibrium analysis was useful to determine the $T_H$ and $T_L$ required for the initiation of the thermal reduction (TR) of GeO₂ and re-oxidation of GeO via WS reaction. Furthermore, the influence of $P_{O_2}$ on the $T_H$ required for the comprehensive dissociation of GeO₂ into GeO and O₂ was also studied. The efficiency analysis was conducted by drifting the $T_H$ and $T_L$ in the range of 2080 to 1280 K and 500–1000 K, respectively. Obtained results indicate that the minimum ${\dot{Q}}_{solar-cycle}=624.3\text{kW}$ and maximum ${\eta }_{solar-to-fuel}=45.7\text{\%}$ in case of the GeO₂/GeO WS cycle can be attained when the TR of GeO₂ was carried out at 1280 K and the WS reaction was performed at 1000 K. This ${\eta }_{solar-to-fuel}=45.7\text{\%}$ was observed to be higher than the SnO₂/SnO WS cycle (39.3%) and lower than the ZnO/Zn WS cycle (49.3%). The ${\dot{Q}}_{solar-cycle}$ can be further decreased to 463.9 kW and the ${\eta }_{solar-to-fuel}$ can be upsurged up to 61.5% by applying 50% heat recuperation.     

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.     

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.     

Nanostructured Ni:BiVO4 photoanode in photoelectrochemical water splitting for hydrogen generation

Photoelectrochemical water splitting using bismuth vanadate (BiVO₄) is drawing attention but on account of presence of high charge recombination and poor water oxidation kinetics its performance is restricted. Present study attempts to understand the role of dopant Ni on BiVO₄ in a) reducing the charge recombination and b) to improve water oxidation kinetics. Ni doped BiVO₄ thin films are prepared via electrodeposition method and photoelectrochemical properties are investigated in 0.1 M phosphate buffer solution with and without sodium sulfite hole scavenger. Photocurrent density of 1.36 mA/cm² at 1.23 V vs. RHE has been obtained using 1.5% Ni doped BiVO₄. This sample also offered lower flat band potential, high open circuit potential and applied bias photon-to-current conversion efficiency. Addition of hole scavenger significantly increases the photoelectrochemical performance. Ni as a dopant therefore can play an important role in not only suppressing the electron-hole pair recombination but also in offering significantly enhanced photoelectrochemical response.