Chromium doped copper vanadate photoanodes for water splitting

Solar hydrogen obtained from photoelectrochemical water splitting offers a versatile approach towards the substitution of fossil fuels by decentralized and sustainable resources, like water and sun. In the present study we have investigated the Chromium doped Copper Vanadate (Cr:Cu3V2O8) as a candidate photoanode for photoelectrochemical water splitting. We have synthetized this material through a simple aqueous precipitation reaction, which easily allows compositional modifications. We have studied the effect of extrinsic doping with substitutional atoms like Chromium on the optical and photoelectrochemical properties. The main limiting factor for performance is related to the high bulk recombination, which is partially overcome by 0.75 at.% Chromium doping, with a five-fold enhancement of the charge separation efficiency at 1.23 V vs RHE. Despite this remarkable milestone, significant further improvement is needed for the technological exploitation of this material

The Role of Underlayers and Overlayers in Thin Film BiVO4 Photoanodes for Solar Water Splitting

This is the pre-peer reviewed version of the following article: The Role of Underlayers and Overlayers in Thin Film BiVO4 Photoanodes for Solar Water Splitting, which has been published in final form at https://doi.org/10.1002/admi.201900299. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.Light‐driven water splitting with metal oxide semiconductor materials to produce H2 constitutes one of the most promising energy conversion technologies built on solar power. BiVO4 stands out as one of the most attractive metal oxides with reported photocurrents close to its theoretical maximum of 7.5 mA cm−2 at 1 sun illumination. The present work addresses the state‐of‐the‐art strategies to enhance the performance of this material for water oxidation by heterostructuring with different underlayer (SnO2 and WO3) and overlayer (NiOOH/FeOOH, Co–Pi, Co–Fe Prussian Blue derivative) materials, with particular emphasis on the physico‐chemical mechanisms responsible for the reported enhancements

Intensity-Modulated Photocurrent Spectroscopy for Solar Energy Conversion Devices: What Does a Negative Value Mean?

Small perturbation techniques constitute a wide family of tools for the characterization of solar energy conversion devices such as photovoltaic cells and photoelectrochemical (PEC) cells for solar fuel production. Two main small perturbation methods frequently used in the area of solar energy conversion materials are impedance spectroscopy (IS) and intensity-modulated photocurrent spectroscopy (IMPS). The first one consists of applying a small voltage perturbation and measuring modulated extracted current. The second one consists of applying the perturbation to the illumination and measuring the modulated extracted current

Cobalt Hexacyanoferrate on BiVO4 Photoanodes for Robust Water Splitting

The efficient integration of photoactive and catalytic materials is key to promoting photoelectrochemical water splitting as a sustainable energy technology built on solar power. Here, we report highly stable water splitting photoanodes from BiVO4 photoactive cores decorated with CoFe Prussian blue-type electrocatalysts (CoFe-PB). This combination decreases the onset potential of BiVO4 by,similar to 0.8 V (down to 0.3 V vs reversible hydrogen electrode (RHE)) and increases the photovoltage by 0.45 V. The presence of the catalyst also leads to a remarkable 6-fold enhancement of the photocurrent at 1.23 V versus RHE, while keeping the light-harvesting ability of BiVO4. Structural and mechanistic studies indicate that CoFe-PB effectively acts as a true catalyst on BiVO4. This mechanism, stemming from the adequate alignment of the energy levels, as showed by density functional theory calculations, allows CoFe-PB to outperform all previous catalyst/BiVO4 junctions and, in addition, leads to noteworthy long-term stability. A bare 10-15% decrease in photocurrent was observed after more than 50 h of operation under light irradiation

TiO2 Nanotubes for Solar Water Splitting: Vacuum Annealing and Zr Doping Enhance Water Oxidation Kinetics

Herein, we report the cooperative effect of Zr doping and vacuum annealing on the carrier dynamics and interfacial kinetics of anodized TiO2 nanotubes for light-driven water oxidation. After evaluation of different Zr loads and different annealing conditions, it was found that both Zr doping and vacuum annealing lead to a significantly enhanced light harvesting efficiency and photoelectrochemical performance. The substitution of Zr4+ by Ti4+ species leads to a higher density of surface defects such as oxygen vacancies, facilitating electron trapping on Zr4+, which reduced the charge recombination and hence boosted the charge transfer kinetics. More importantly, vacuum annealing promoted the presence of surface defects. Furthermore, the mechanistic study through impedance spectroscopy revealed that both charge transfer and surface conductivity are significantly enhanced due the presence of an oxygen-deficient TiO2 surface. These results represent an important step forward in the optimization of nanostructured TiO2-based photoelectrodes, with high potential in photocatalytic applications, including solar fuel production

Enhancing the Optical Absorption and Interfacial Properties of BiVO4 with Ag3PO4 Nanoparticles for Efficient Water Splitting

Photoelectrochemical water splitting using semiconductor materials has emerged as a promising approach to produce hydrogen (H2) from renewable resources such as sunlight and water. In the present study, Ag3PO4 nanoparticles were electrodeposited on BiVO4 photoanodes for water splitting. A remarkable water oxidation photocurrent of 2.3 mA·cm–2 at 1.23 V versus reversible hydrogen electrode with ∼100% Faradaic efficiency was obtained, which constitutes a notable increase compared to the pristine BiVO4 photoanode. It is demonstrated that the enhancement of optical absorption (above-band gap absorbance) and the decrease of surface losses after the optimized deposition of Ag/Ag3PO4 nanoparticles are responsible for this notable performance. Remarkably, this heterostructure shows promising stability, demonstrating 25% decrease of photocurrent after 24 h continuous operation. This approach may open new avenues for technologically exploitable water oxidation photoanodes based on metal oxides

Direct observation of the chemical transformations in BiVO4 photoanodes upon prolonged light-aging treatments

Altres ajuts: ICN2 is funded by the CERCA Programme/Generalitat de Catalunya.Exposing BiVO photoanodes to light-aging treatments is known to produce a significant photocurrent enhancement. Until now, the interpretation given to this phenomenon is associated to the formation of oxygen vacancies and little is reported about chemical changes in the material. Herein, the chemical segregation of Bi species toward the surface upon light-aging treatment is demonstrated, which takes place with the concomitant formation of intra-bandgap states associated to the oxygen vacancies. It is further demonstrated that these intra-bandgap states are photoactive and generate photocurrent under infrared excitation. These results highlight the importance of understanding light-induced effects while employing multinary metal oxide photoelectrodes

Photocatalytic and Photoelectrochemical Degradation of Organic Compounds with All-Inorganic Metal Halide Perovskite Quantum Dots

Inspired by the outstanding optoelectronic properties reported for all-inorganic halide perovskite quantum dots (QDs), we have evaluated the potential of these materials toward the photocatalytic and photoelectrochemical degradation of organic compounds, taking the oxidation of 2-mercaptobenzothiazole (MBT) as a proof-of-concept. First, we determined electrochemically the energy levels of dispersions of perovskite QDs with different band gaps induced by the different ratios between halides (Br and I) and metallic cations (Pb and Sn). Then, we selected CsPbBr3 QDs to demonstrate the photocatalytic and photoelectrochemical oxidation of MBT, confirming that hole injection takes place from CsPbBr3 QDs to MBT, resulting in the total degradation of MBT as evidenced by electrospray mass spectrometry analyses. Although the stability and toxicity of these QDs are major issues to address in the near future, the results obtained in the present study open promising perspectives for the implementation of solar-driven catalytic strategies based on these fascinating materials

Switchable All Inorganic Halide Perovskite Nanocrystalline Photoelectrodes for Solar-Driven Organic Transformations

All inorganic halide perovskite nanocrystals (NCs) are considered as fascinating materials for a wide range of optoelectronic applications encompassing photovoltaics, lasing, sensing, and photocatalysis due to their outstanding optoelectronic properties. Herein, it is demonstrated that the photoelectrochemical behavior of CsPbBr3 NC films can be tailored through engineering the selective contacts and accepting species in the electrolyte. This concept has been successfully applied to the photoelectrochemical oxidation of benzyl alcohol (BzOH) to benzyl aldehyde (BzCHO) and the reverse photoelectrochemical reduction of BzCHO to BzOH, demonstrating that CsPbBr3 NCs activate both reactions with photocurrents up to 40 μA cm 2 toward BzCHO production and 5 μA cm 2 for the reverse reaction at 0.15 V versus normal hydrogen electrode. The obtained results highlight the huge potential and versatility of halide perovskite NCs for photoelectrocatalytic applications, validating the implementation of these materials for a wide range of solar-driven complex organic transformations, and emphasizing the urgent need for stabilization strategies to move beyond the proof-of-concept stage to relevant technological developments

A metal-organic framework converted catalyst that boosts photo-electrochemical water splitting

Realization of photo-electrochemical water splitting to generate H2 alternative fuel requires the facilitation of the kinetically-sluggish oxygen evolution reaction (OER) occurring at the photoanode. To do so, there is a need to develop new methods to assemble suitable OER co-catalysts at the semiconductor–solution interface. Although Metal–Organic Frameworks (MOFs) are frequently used as precursor materials to synthesize high surface area, effective OER electrocatalysts, until now their utilization as co-catalysts in a working photo-electrochemical cell (PEC) has remained underexplored. As a proof-of-concept, here we provide a simple route for modification of BiVO4-based photoanodes with highly-active porous cobalt-oxide co-catalysts, converted from a cobalt–imidazolium MOF (ZIF-67). Photo-electrochemical and impedance spectroscopy analysis reveal that the co-catalyst significantly accelerates photoanodic OER (rather than serving as a surface passivation layer), and thus greatly improves the overall PEC performance. Hence, given the chemical flexibility of MOFs, this work provides a new tool-kit for designing efficient water splitting PECs