Investigating Water Splitting with CaFe₂O₄ Photocathodes by Electrochemical Impedance Spectroscopy

Artificial photosynthesis constitutes one of the most promising alternatives for harvesting solar energy in the form of fuels, such as hydrogen. Among the different devices that could be developed to achieve efficient water photosplitting, tandem photoelectrochemical cells show more flexibility and offer high theoretical conversion efficiency. The development of these cells depends on finding efficient and stable photoanodes and, particularly, photocathodes, which requires having reliable information on the mechanism of charge transfer at the semiconductor/solution interface. In this context, this work deals with the preparation of thin film calcium ferrite electrodes and their photoelectrochemical characterization for hydrogen generation by means of electrochemical impedance spectroscopy (EIS). A fully theoretical model that includes elementary steps for electron transfer to the electrolyte and surface recombination with photogenerated holes is presented. The model also takes into account the complexity of the semiconductor/solution interface by including the capacitances of the space charge region, the surface states and the Helmholtz layer (as a constant phase element). After illustrating the predicted Nyquist plots in a general manner, the experimental results for calcium ferrite electrodes at different applied potentials and under different illumination intensities are fitted to the model. The excellent agreement between the model and the experimental results is illustrated by the simultaneous fit of both Nyquist and Bode plots. The concordance between both theory and experiments allows us to conclude that a direct transfer of electrons from the conduction band to water prevails for hydrogen photogeneration on calcium ferrite electrodes and that most of the carrier recombination occurs in the material bulk. In more general vein, this study illustrates how the use of EIS may provide important clues about the behavior of photoelectrodes and the main strategies for their improvement

Modeling Pore-Scale Two-Phase Flow: How to Avoid Gas-Channeling Phenomena in Micropacked-Bed Reactors via Catalyst Wettability Modification

A model capable of providing a reliable estimation of two-phase flow dynamics and mass-transfer coefficients, is lacking for the design of micropacked-bed reactors via correlations, especially when the particle size of the bed is around 100 μm. In this work, we present a validation of the use of the phase field method for reproducing two-phase flow experiments found in the literature. This numerical simulation strategy sheds light on the impact of the micropacked-bed geometry and wettability on the formation of preferential gas channels. Counterintuitively, to homogenize the two-phase flow hydrodynamics and reduce radial mass-transfer limitations, solvent wettability of the support needs to be restricted, showing best performance when the contact angle ranges to 60° and capillary forces are still dominant. The tuning of gas–liquid–solid interactions by surface wettability modification opens a new window of opportunity for the design and scale-up of micropacked-bed reactors

Improving the Stability and Efficiency of CuO Photocathodes for Solar Hydrogen Production through Modification with Iron

Cupric oxide (CuO) is considered as a promising photocathode material for photo­(electro)­chemical water splitting because of its suitable band gap, low cost related to copper earth abundancy, and straightforward fabrication. The main challenge for the development of practical CuO-based photocathodes for solar hydrogen evolution is to enhance its stability against photocorrosion. In this work, stable and efficient CuO photocathodes have been developed by using a simple and cost-effective methodology. CuO films, composed of nanowires and prepared by chemical oxidation of electrodeposited Cu, develop relatively high photocurrents in 1 M NaOH. However, this photocurrent appears to be partly associated with photocorrosion of CuO. It is significant though that, even unprotected, a faradaic efficiency for hydrogen evolution of ∼45% is attained. The incorporation of iron through an impregnation method, followed by a high-temperature thermal treatment for promoting the external phase transition of the nanowires from CuO to ternary copper iron oxide, was found to provide an improved stability at the expense of photocurrent, which decreases to about one-third of its initial value. In contrast, a faradaic efficiency for hydrogen evolution of ∼100% is achieved even in the absence of co-catalysts, which is ascribable to the favorable band positions of CuO and the iron copper ternary oxide in the core–shell structure of the nanowires

Concentration-Dependent Photoredox Conversion of As(III)/As(V) on Illuminated Titanium Dioxide Electrodes

The photoconversion of As­(III) (arsenite) and As­(V) (arsenate) over a mesoporous TiO₂ electrode was investigated in a photoelectrochemical (PEC) cell for a wide range of concentrations (μM–mM), under nonbiased (open-circuit potential measurements) and biased (short-circuit current measurements) conditions. Not only As­(III) can be oxidized, but also As­(V) can be reduced in the anoxic condition under UV irradiation. However, the reversible nature of As­(III)/As­(V) photoconversion was not observed in the normal air-equilibrated condition because the dissolved O₂ is far more efficient as an electron acceptor than As­(V). Although As­(III) should be oxidized by holes, its presence did not increase the photooxidation current in a monotonous way: the photocurrent was reduced by the presence of As­(III) in the micromolar range but enhanced in the millimolar range. This abnormal concentration-dependent behavior is related with the fate of the intermediate As­(IV) species which can be either oxidized or reduced depending on the experimental conditions, combined with surface deactivation for the water photooxidation process. The lowering of the photooxidation current in the presence of micromolar As­(III) is ascribed to the role of As­(IV) as a charge recombination center. Being an electron acceptor, the addition of As­(V) consistently lowers the photocurrent in the entire concentration range. A global concentration-dependent mechanism is proposed accounting for all the PEC results and its relation with the photocatalytic oxidation mechanism is discussed

Preparation and Characterization of Nickel Oxide Photocathodes Sensitized with Colloidal Cadmium Selenide Quantum Dots

Quantum dot sensitized solar cells (QDSCs) are receiving a lot of attention as promising third generation solar cells, being virtually all of them based on sensitized photoanodes. Finding efficient QD-sensitized photocathodes would pave the way toward the implementation of tandem QDSCs. In this context, NiO photocathodes have been sensitized with colloidal CdSe quantum dots directly attached to the semiconductor oxide surface. The emission spectra indicate effective hole injection from the excited state of the quantum dots to the valence band of the NiO. A maximum incident current to photon conversion efficiency of 17% at 420 nm has been achieved. For the sake of comparison, other ways to prepare and anchor the QDs have been tested. Sensitization routes based on presynthesized colloidal quantum dots show better results than in situ growth techniques such as successive ionic layer adsorption and reaction. Electrochemical impedance measurements have identified transport resistance in NiO as one of the limiting factors in the performance of the system under study. Interestingly, surface treatments based on the deposition of very thin films of either SiO₂ or Al₂O₃ can diminish recombination at the NiO/CdSe/electrolyte interface. This work also identifies a number of possible routes for the improvement of this kind of electrodes, unveiling their potential use in tandem quantum dot solar cells

Improving the Photoelectrochemical Response of TiO₂ Nanotubes upon Decoration with Quantum-Sized Anatase Nanowires

TiO₂ nanotubes (NTs) have been widely used for a number of applications including solar cells, photo­(electro)­chromic devices, and photocatalysis. Their quasi-one-dimensional morphology has the advantage of a fast electron transport although they have a relatively reduced interfacial area compared with nanoparticulate films. In this study, vertically oriented, smooth TiO₂ NT arrays fabricated by anodization are decorated with ultrathin anatase nanowires (NWs). This facile modification, performed by chemical bath deposition, allows to create an advantageous self-organized structure that exhibits remarkable properties. On one hand, the huge increase in the electroactive interfacial area induces an improvement by 1 order of magnitude in the charge accumulation capacity. On the other hand, the modified NT arrays display larger photocurrents for water and oxalic acid oxidation than bare NTs. Their particular morphology enables a fast transfer of photogenerated holes but also efficient mass and electron transport. The importance of a proper band energy alignment for electron transfer from the NWs to the NTs is evidenced by comparing the behavior of these electrodes with that of NTs modified with rutile NWs. The NT-NW self-organized architecture allows for a precise design and control of the interfacial surface area, providing a material with particularly attractive properties for the applications mentioned above

Sensitization of TiO₂ with PbSe Quantum Dots by SILAR: How Mercaptophenol Improves Charge Separation

The use of PbSe quantum dots (QDs) as sensitizers for TiO₂ samples has been primarily hampered by limitations on charge injection. Herein, a novel successive ionic layer adsorption and reaction (SILAR) method, allowing for an intimate TiO₂/PbSe contact and a strong quantum confinement, is described. Photoelectrochemical experiments and transient absorption measurements reveal that charge separation indeed occurs when using either aqueous sulfite or <i>spiro</i>-OMeTAD as a hole conductor and that it can be further enhanced by attaching <i>p</i>-mercaptophenol (MPH) to the QD surface. These results suggest that MPH can promote an efficient funneling of the photogenerated holes from the PbSe to the hole scavenging medium, thereby increasing the yield of electron injection into TiO₂. In a more general vein, this work paves the way for the fabrication of PbSe-sensitized solar cells, emphasizing the importance of controlling the QD/hole scavenger interface to further boost their conversion efficiency

Toward Tandem Solar Cells for Water Splitting Using Polymer Electrolytes

Tandem photoelectrochemical cells, formed by two photoelectrodes with complementary light absorption, have been proposed to be a viable approach for obtaining clean hydrogen. This requires the development of new designs that allow for upscaling, which would be favored by the use of transparent polymer electrolyte membranes (PEMs) instead of conventional liquid electrolytes. This article focuses on the photoelectrochemical performance of a water-splitting tandem cell based on a phosphorus-modified α-Fe₂O₃ photoanode and on an iron-modified CuO photocathode, with the employment of an alkaline PEM. Such a photoelectrochemical cell works even in the absence of bias, although significant effort should be directed to the optimization of the photoelectrode/PEM interface. In addition, the results reveal that the employment of polymer electrolytes increases the stability of the device, especially in the case of the photocathode

Toward Antimony Selenide Sensitized Solar Cells: Efficient Charge Photogeneration at <i>spiro</i>-OMeTAD/Sb₂Se₃/Metal Oxide Heterojunctions

Photovoltaic devices comprising metal chalcogenide nanocrystals as light-harvesting components are emerging as a promising power-generation technology. Here, we report a strategy to evenly deposit Sb₂Se₃ nanoparticles on mesoporous TiO₂ as confirmed by Raman spectroscopy, energy-dispersive X-ray spectrometry, and transmission electron microscopy. Detailed study of the interfacial charge transfer dynamics by means of transient absorption spectroscopy provides evidence of electron injection across the Sb₂Se₃/TiO₂ interface upon illumination, which can be improved 3-fold by annealing at low temperatures. Following addition of the <i>spiro</i>-OMeTAD hole transporting material, regeneration yields exceeding 80% are achieved, and the lifetime of the charge separated species is found to be on the millisecond time scale (τ_50% ∼ 50 ms). These findings are discussed with respect to the design of solid-state Sb₂Se₃ sensitized solar cells