Solar Water Oxidation Research Articles

An effective strategy to promote hematite photoanode at low voltage bias via Zr4+/Al3+ codoping and CoOx OER co-catalyst

Herein, we report the surface treatment on Zr4+/Al3+ codoped α-Fe2O3 photoanode for high-performance photoelectrochemical water splitting. A high-temperature quenching exhibits the Zr4+/Al3+ codoping in α-Fe2O3 photoanode without damaging morphology. The presence of Zr4+/Al3+ codoping shows a cathodic shift in onset potential, but lack of increment in photocurrent reveals the major role of passivation and the minimum doping effect of aluminum. Additionally, CoOx cocatalyst exhibits increment in photocurrent with the greater cathodic shift in onset potential than the pristine α-Fe2O3 nanorods. The CoOx surface-reworked Zr4+/Al3+ codoped α-Fe2O3 photoanode displays the highest photocurrent of 1.5 mA/cm2 at 1.23 V vs. RHE (76% increment over the pristine α-Fe2O3) and 0.7 mA/cm2 at 1.0 V vs. RHE (102% increment over the pristine α-Fe2O3). The systematic characterization carried out using x-ray diffraction and scanning electron microscopy confirms that after Zr4+/Al3+ codoping, and surface treatment, the crystalline structure, and morphology of the photoanodes remains unchanged. X-ray photoelectron spectroscopy confirmed the existence of Zr4+/Al3+ codopants in the hematite nanostructure. The electrochemical properties of the photoanode suggest that Al3+ and Zr4+ codoping, as well as surface treatment with CoOx, cocatalyst lowers charge transfer resistance across the FTO/hematite interface, and hematite/electrolyte interface. This designs not only lowers onset potential but also offers the blueprint for the development of an efficient catalyst for solar water oxidation.

Energy band edge alignment of anisotropic BiVO4 to drive photoelectrochemical hydrogen evolution

Bismuth vanadate (BiVO4) has been highlighted as the most efficient photoanode light absorber for the solar light-driven water oxidation reaction despite the lack of fundamental understanding of its poor performance. Though the BiVO4 performance has been significantly improved, the underlying reasons for this performance are not well understood. Here, anisotropically grown BiVO4 is designed to achieve band edge engineering induced by coupled crystal facets for hydrogen evolution reaction (HER), which is not possible with isotropic BiVO4 powder suspension. HER performances depending on the exposure area of the crystal facets in the anisotropic BiVO4 are investigated using electronic conduction and in situ X-ray absorption spectroscopy during HER. Our results reveal that the nature of the exposed area of the optimized coupled crystal facets in a BiVO4 photoelectrode critically dominates the surface charge density and conductivity, efficiently driving the HER. Sequentially, coupling between different crystal facets of BiVO4 photoelectrode led energy band edge to be upward. The results show that the energy band edge alignment tuned by coupled crystal facet can be applicable to other inorganic photocatalyst/photoelectrodes to promote efficient artificial photosynthetic reactions.

Boosting the performance of hematite photoanodes for solar water oxidation by synergistic W-incorporation and Zr-passivation

Incorporating impurities into hematite is an effective approach to enhance the photoelectrochemical (PEC) water splitting performance of hematite photoanode. Here, we report an efficient W-incorporation in hematite (W-Fe2O3) to enhance the PEC performance by depositing a WO3 underlayer onto the FTO substrate. The W-incorporation dramatically enhances the photocurrent density of hematite by a factor of ∼2. Moreover, it can be well coupled with Zr to achieve a high photocurrent of 2.0 mA cm−2 at 1.23 V vs. RHE by simultaneously depositing a ZrO2 underlayer on the FTO substrate. A large cathodic shift of the onset potential up to 120 mV can also be obtained. The boosted PEC performance can be attributed to the synergistic effect of W and Zr in hematite, which can both improve the carrier density (by W-incorporation) and suppress the charge recombination (by Zr-passivation).

Epitaxial Growth of Iron Tungstate Nanosheets on One-Dimensional Photoanodes for Efficient Solar Water Oxidation

Type-II heterojunctions have been recognized as an effective structure to significantly improve the charge transport efficiency of photoelectrodes. Herein, a facile method was developed for in-situ epitaxial growth of Iron Tungstate (Fe2WO6) nanosheets on various one-dimensional nanowire arrays including Fe2O3 and WO3, generating the bespoke photoanodes. Photogenerated electron can be easily transported through the interface between Fe2WO6 and Fe2O3 (/WO3) because of lattice match and less Fermi-level pinning. The 2D Fe2WO6 nanosheets branching of 1D arrays in epitaxial fashion exhibited significantly enhanced solar water oxidation performance, which is clearly shown in improved light utilizations, enhanced charge separation, as well as decreased electrochemical impedance of the photoelectrode. This work advocates a new paradigm in interface design for optimizing the photoelectrochemical efficiency.

Electrolyte effects on undoped and Mo-doped BiVO4 film for photoelectrochemical water splitting

As the electrolyte in a photoelectrochemical system used for solar water splitting is closely associated with the charge transfer phenomenon, the fundamental study of the electrolyte is an increasingly crucial issue. In this paper, the relation between the photoelectrochemical performance and the variation of electrolyte composition is investigated in depth. Here, the potassium phosphate (KPi) containing different anions (CH3COO−, Cl− and NO3−) or cations (NH4+, K+ and Na+) was prepared at the electrolyte with the different pH values of 5, 7, 9, and 11 in order to investigate how anions or cations affect the PEC performance. Herein, we compared the differences in the undoped and Mo-doped BiVO4 films due to the different bulk/surface states, and found different morphological and crystalline features as well as remarkably different PEC performance. At first, the pH in the electrolyte significantly influences the solar water oxidation reaction, revealing that the electrolyte with a high pH grants better PEC activity, because the higher pH solution can provide more hydroxide ions (OH−) to react with holes to form hydroxyl radicals, which are recognized as important intermediates for PEC water oxidation in the presence of O2. Further, in the electrolyte containing the different cations, the NH4+ ions exhibit the enhanced PEC performance, due to its cation size which increases the ionic dissociation and viscosity. By contrast, the anion effect can be negligible in the used electrolyte. From this fundamental research, it can be known that the optimization of electrolyte is quite a vital parameter for advancing the PEC performance.

Z-scheme design of Ag@g-C3N4/ZnS photoanode device for efficient solar water oxidation: An organic-inorganic electronic interface

Abstract Herein, we demonstrate a design of a composite photoanode comprising a very thin layer of ZnS NPs onto a dense layer of Ag@g-C3N4 to form a dual absorber tandem device based on organic-inorganic electronic interface for photo water-splitting. The XRD pattern of Ag@g-C3N4 showed both the diffraction peaks of cubic Ag and main peak of g-C3N3. The XPS survey spectrum confirms the existence of the C, N, Ag, Zn, and S elements on the surface of the Ag@g-C3N4/ZnS photoelectrode. With incorporating Ag in g-C3N4 structure, the band gap decreased from 2.90 for g-C3N4 to 2.55 eV for Ag@g-C3N4 and also light absorption ability increased. The device based on the architecture of TiO2/Ag@g-C3N4/ZnS/Ni(OH)2 showed a photocurrent of about 0.1 and 0.2 mA cm−2 at potential of 1.23 and 1.7 V vs. RHE. The Ag@g-C3N4/ZnS photoanode exhibited a photocurrent turn-on potential of 0.45 V vs. RHE. In this photoanode device, Ag@g-C3N4 as a second absorber layer could absorb the photons of visible light which are transparent to ZnS layer and transforms into additional photovoltage. This architecture introduces a successful band alignment for high efficiency water-splitting through organic-inorganic junction.

Cascading Interfaces Enable n-Si Photoanodes for Efficient and Stable Solar Water Oxidation.

Interfaces with multifunctions for promoted solid/solid interfacial charge-transfer dynamics and accelerated solid/electrolyte interfacial water redox reaction kinetics are determinative for the photoelectrodes achieving high performances for photoelectrochemical (PEC) water splitting. In this work, well-designed cascading interfaces are introduced in the n-Si photoanode, which is effectively protected by an atomic layer-deposited CoO x thin layer for stabilizing the n-Si photoanode and then coated with an earth-abundant NiCuO x layer for catalyzing the water oxidation reaction. Furthermore, the formed n-Si/CoO x/NiCuO x triple junction could generate a large band bending to provide a considerable photovoltage for promoting the photoinduced charge-transfer and separation processes at the n-Si/CoO x/NiCuO x cascading interfaces. Moreover, at the NiCuO x/electrolyte interface, an in situ electrochemically formed NiCu(OH) x/NiOOH active layer facilitates the water oxidation reaction kinetics. This study demonstrates an alternative approach to stabilize and catalyze n-Si-based photoanodes with cascading interfaces for efficient solar water oxidation.

P-Type NiO modified BiVO4 photoanodes with enhanced charge separation and solar water oxidation kinetics

Bismuth vanadate is considered as a promising photoanode material for photoelectrochemical water splitting. However, the frequent bulk charge recombination and poor surface reaction kinetics restrict its performance in photoelectrochemical cell seriously. Herein, we developed a facile method to build a p-n heterojunction of NiO/BiVO4 with enhanced charge separation and surface reaction kinetics. The internal electric field of our heterojunction can improve the electron-hole separation efficiency from 51% to 65% at 0.6 VRHE. Moreover, the p-type NiO can significantly enhance the surface oxygen evolution reaction kinetics. Based on these synergistic effects, our NiO/BiVO4 photoanode achieved a high solar to hydrogen efficiency of 0.72% at 0.76 VRHE.

Gold-Sensitized Silicon/ZnO Core/Shell Nanowire Array for Solar Water Splitting.

Solar water splitting represents one of the most promising strategies in the quest for clean and renewable energy. However, low conversion efficiency, use of sacrificial agents, and external bias for current water splitting system limit its practical application. Here, a gold-sensitized Si/ZnOcore/shell nanowire photoelectrochemical (PEC) cell is reported for efficient solar water oxidation. We demonstrated gold-sensitized n-Si/n-ZnO nanowire arrays exhibited higher energy conversion efficiency than gold-sensitized p-Si/n-ZnO nanowire arrays due to the favorable energy-band alignment characteristics. Without any assistance from an external electrical source and sacrificial reagents, gold-sensitized n-Si/n-ZnO core/shell nanowire array photoanode achieved unbiased water splitting under simulated solar light illumination. This method opens a promising venue to cost-efficient production of solar fuels.

Crystallographic-Orientation-Dependent Charge Separation of BiVO4 for Solar Water Oxidation

Charge separation plays a crucial role in determining the solar-to-hydrogen conversion efficiency for photoelectrochemical water splitting. Of the factors that affect charge separation, the anisotropic charge transport property of semiconductors shows great potential in promoting charge separation, but it has received little attention. Herein, we report BiVO4 photoanodes with predominant [010] and [121] orientations and demonstrate a crystallographic-orientation-dependent charge separation of BiVO4 for solar water oxidation. We found that a [010]-orientated BiVO4 photoanode generated a photocurrent 2.9 times that of the [121]-orientated one, owing to the significantly improved charge separation. An in-depth investigation of the surface band bending by open-circuit potential and film conductivity by contacting atomic force microscopy reveals that the higher electron mobility along the [010] direction than that of [121] accounts for the improvement in charge separation. This work offers a fundamental insigh...

Boosting solar water oxidation activity and stability of BiVO4 photoanode through the Co-catalytic effect of CuCoO2

The poor surface charge separation and transfer properties as well as sluggish water oxidation kinetics jointly limit the performance of BiVO4 photoanode for water oxidation. In this work, p-type CuCoO2 with high-spin Co3+(Oh) was investigated as co-catalyst to synergistically improve the surface charge separation and transfer efficiencies as well as reaction kinetics of BiVO4 film for photoelectrochemical water oxidation. In comparison with the photocurrent on BiVO4 photoanode for water oxidation (1.21 mA/cm2 at 1.23 V vs. RHE), the CuCoO2-coupled BiVO4 (CuCoO2/BiVO4) photoanode exhibits a higher photocurrent density of 3.32 mA/cm2 at 1.23 V vs. RHE under AM 1.5G illumination. In addition, a significant improvement on the reaction stability is achieved on the CuCoO2/BiVO4 photoanode, about ∼79% water oxidation activity is retained on the CuCoO2/BiVO4 photoanode after operating at 0.8 V vs. RHE for 5 h, while only ∼9% activity is retained on the BiVO4 photoanode. The boosted water oxidation activity and stability on CuCoO2/BiVO4 photoanode could be attributed the synergistic effect that originated from CuCoO2-electrocatalysis and BiVO4-photocatalysis in thermodynamics and kinetics. Specifically, p-n heterojunctions are formed in the coupling interface between CuCoO2 (p-type) and BiVO4 (n-type), which thermodynamically improve the surface charge separation and transfer efficiencies of BiVO4 photoanode during water oxidation. Simultaneously, the high-spin Co3+(Oh) of CuCoO2 could act as active sites to accelerate the water oxidation of CuCoO2/BiVO4 photoanode in kinetics. In addition, Cu2+ active sites are formed for water oxidation through the oxidation reaction of photogenerated holes with the Cu+ of CuCoO2.

Dual Quantum Dot-Decorated Bismuth Vanadate Photoanodes for Highly Efficient Solar Water Oxidation.

Photo-induced charge separation and photon absorption play important roles in determining the performance of the photoelectrocatalytic water splitting process. In this work, we utilize dual quantum dots (QDs), consisting of BiVO4 and carbon, to fabricate a hybrid homojunction-based BiVO4 photoanode for efficient and stable solar water oxidation. Formation of homojunctions, by decorating as-prepared BiVO4 substrate with BiVO4 QDs, enhances the charge separation efficiency by 1.3 times. This enhancement originates from lattice match, which benefits charge transfer across the interface. Furthermore, the use of carbon QDs as a stable photosensitizer effectively extends the photon absorption limit from 520 nm to over 700 nm, yielding an incident photon-to-electron conversion efficiency of 6.0 %, even at 600 nm at 1.23 V versus RHE. Finally, a remarkable photocurrent density of 6.1 mA cm-2 at 1.23 V was recorded after depositing FeOOH/NiOOH as cocatalysts, thereby, reaching 82 % of the theoretical efficiency for BiVO4 .

Catalytic Multilayers for Efficient Solar Water Oxidation through Catalyst Loading and Surface-State Passivation of BiVO4 Photoanodes.

We studied the kinetics of photoelectrochemical (PEC) water oxidation using a model photoanode BiVO4 modified with various water oxidation catalysts (WOCs) by electrochemical impedance spectroscopy. In particular, we prepared BiVO4 photoanodes with catalytic multilayers (CMs), where cationic polyelectrolytes and anionic polyoxometalate (POM) WOCs were assembled in a desired amount at a nanoscale precision, and compared their performance with those with well-known WOCs such as cobalt phosphate (CoPi) and NiOOH. Our comparative kinetics analysis suggested that the deposition of the CMs improved the kinetics of both the photogenerated charge carrier separation/transport in bulk BiVO4 due to passivation of surface recombination centers and water oxidation at the electrode/electrolyte interface due to deposition of efficient molecular WOCs. On the contrary, the conventional WOCs were mostly effective in the former and less effective in the latter, which is consistent with previous reports. These findings explain why the CMs exhibit an outstanding performance. We also found that separated charge carriers can be efficiently transported to POM WOCs via a hopping mechanism due to the delicate architecture of the CMs, which is reminiscent of natural photosynthetic systems. We believe that this study can not only broaden our understanding on the underlying mechanism of PEC water oxidation but also provide insights for the design and fabrication of novel electrochemical and PEC devices, including efficient water oxidation photoanodes.

Synthesis and photoelectrochemical properties of CoOOH/phosphorus-doped hematite photoanodes for solar water oxidation

In order to improve the carrier mobility and restrain the charge recombination of hematite (α-Fe2O3) for water oxidation performance, a P-doped and CoOOH modified α-Fe2O3 electrode was synthesized using a hydrothermal method and photo-assisted electrodeposition process. The as-obtained CoOOH/P:Fe2O3 photoelectrode presented a significant enhancement in the PEC current density, conversion efficiency and stability for water oxidation with simulated light irradiation. The photocurrent density of CoOOH/P:Fe2O3 photoanode is up to 1.11 mA cm−2 at 1.23 V vs. RHE, which is 1.65, 2.02 and 2.78 times higher than those of P:Fe2O3, CoOOH/Fe2O3 and pristine α-Fe2O3, respectively. The significantly improved PEC performance is attributed to the enhanced conductivity in α-Fe2O3 electrode and improved interfacial charge transfer at the surface of α-Fe2O3, from the synergistic effects of P-doping and CoOOH modification. Our results proposed a new paradigm to construct hybrid photoanode materials with high activity, conversion efficiency and stability for PEC water oxidation.

Charge transfer modulation in layer-by-layer-assembled multilayered photoanodes for solar water oxidation

Two spatially separated charge transfer channels were constructed in multilayered photoanodes for efficient solar water oxidation under both simulated and visible light irradiation.

Insight into the Improvement Mechanism of Copper Oxide/BiVO4 Heterojunction Photoanodes for Solar Water Oxidation

N-type BiVO4 films coupled with p-type copper oxide nanoparticles were investigated as photoanodes for water oxidation. Compared with the BiVO4 film, the Cu2O/BiVO4, CuO/BiVO4 and CuxO/BiVO4 films all showed significantly improved photoelectrochemical water oxidation activity and stability. As expected, the radiative recombination of charge on the three kinds of copper oxide/BiVO4 films was decreased due to the p-n junction effect between copper oxide and BiVO4. However, our investigations indicated that the electrocatalytic water oxidation on the copper oxide could be initiated when the copper oxide/BiVO4 photoanodes were irradiated by solar light; therefore, the copper oxide-coupling resulted in a more obvious improvement in the water oxidation kinetics of BiVO4 than the thermodynamic improvement in the charge separation of BiVO4. Of the three kinds of copper oxide/BiVO4 films, the CuxO/BiVO4 film comprising BiVO4 film coupled with a combination of Cu2O/CuO nanoparticles showed the best solar water oxidation performance due to the better electrocatalytic activity for water oxidation of CuxO compared with Cu2O or CuO alone. The kinetic investigations and analyses in the present work provide comprehensive insight into the mechanism of the copper oxide/BiVO4 photoanodes for solar water oxidation.

Photoelectrochemical Driving and Simultaneous Synthesis of 3-pyridinecarboxylic Acid and Hydrogen in WO3 Photoanode-Based Cell

In conventional photoelectrochemical (PEC) cell for solar water splitting, its solar-to-hydrogen efficiency is limited by the sluggish oxygen evolution on its photoanode. 3-pyridinecarboxylic acid (3-PA) is an important organic in industry, but there are several disadvantages in its conventional synthesis approaches. Here, we reported that the oxidation of 3-methylpyridine (3-MP) into 3-PA as an alternative to the oxygen evolution on WO3 photoanode for improving the hydrogen-evolution efficiency and the product added-value of PEC cell. In a WO3 film photoanode-based dual-chamber PEC cell, 3-PA and hydrogen were simultaneously synthetized. Since the oxidation of 3-MP into 3-PA is more thermodynamically and kinetically favorable than water oxidation, higher efficiency of hydrogen evolution was achieved in the PEC cell with function of 3-PA synthesis relative to the conventional PEC cell for water splitting. Our investigations indicated that the oxidation of 3-MP into 3-PA on WO3 photoanode is co-initiated by the photogenerated holes and the H2O2 intermediate that originated from two-holes pathway of solar water oxidation on WO3. Additionally, the Cr2O72−/Cr3+ redox pair could mediate and promote the oxidation of 3-MP into 3-PA on WO3 photoanode, and thus the PEC efficiency of 3-PA synthesis can be improved by adding Cr3+ into the H2SO4-3-MP electrolyte.

Insights into the interfacial carrier behaviour of copper ferrite (CuFe2O4) photoanodes for solar water oxidation

A thorough photoelectrochemical investigation reveals the bulk and surface characteristics limiting the performance of CuFe2O4 photoanodes for solar water oxidation.

Correction: Boosting the solar water oxidation performance of a BiVO4 photoanode by crystallographic orientation control

Correction for ‘Boosting the solar water oxidation performance of a BiVO4 photoanode by crystallographic orientation control’ by Hyun Soo Han et al., Energy Environ. Sci., 2018, 11, 1299–1306.

Multilayered Fluorine Doped SnO2 Inverse Opal/WO3/BiVO4 Film for Solar Water Oxidation: Systematic Development and Defined Role of Each Layer

We developed the multilayered fluorine doped SnO2 (abbreviated as FTO) inverse opal (IO)/WO3/BiVO4 film by sol-gel based spin coating, subsequently followed by facile electrodeposition. Above FTO IO template, both WO3 and BiVO4 layers were prepared by the electrodeposition methods in the variation of charge amount of 200–1,000 mC cm−2 and of 300–1,500 mC cm−2, respectively. On increasing the loaded charge amount of WO3 under the constant loaded amount of BiVO4, the light absorption in the near 400 nm is increased to steadily enhance PEC performance. In contrast, the further loaded WO3 layer of 800–1,000 mC cm−2 on FTO IO film degrades PEC activity, due to the increase of the intrinsic material and interfacial resistances. Meanwhile, on increasing the electrodeposited charge amount of BiVO4 under the constant charge amount of WO3, the light absorption toward the visible wavelength is sharply increased, and gives rise to the enhancement of PEC performance; and conversely, the more loaded BiVO4 of 1,200–1,500 mC cm−2 induced the decay of PEC performance. The optimum condition in FTO IO/WO3 (600 mC cm−2)/BiVO4 (600 mC cm−2) film was well established, and the function or role of each layer was confirmed using photoluminescence, charge separation/transfer efficiency, and photovoltage-time spectra.