Hydrodynamics and Oxygen Bubble Characterization of Catalytic Cells Used in Artificial Photosynthesis by Means of CFD

Miniaturized cells can be used in photo-electrochemistry to perform water splitting. The geometry, process variables and removal of oxygen bubbles in these cells need to be optimized. Bubbles tend to remain attached to the catalytic surface, thus blocking the reaction, and they therefore need to be dragged out of the cell. Computational Fluid Dynamics simulations have been carried out to assess the design of miniaturized cells and their results have been compared with experimental results. It has been found that low liquid inlet velocities (~0.1 m/s) favor the homogeneous distribution of the flow. Moderate velocities (0.5–1 m/s) favor preferred paths. High velocities (~2 m/s) lead to turbulent behavior of the flow, but avoid bubble coalescence and help to drag the bubbles. Gravity has a limited effect at this velocity. Finally, channeled cells have also been analyzed and they allow a good flow distribution, but part of the catalytic area could be lost. The here presented results can be used as guidelines for the optimum design of photocatalytic cells for the water splitting reaction for the production of solar fuels, such as H2 or other CO2 reduction products (i.e., CO, CH4, among others)

Syngas Production from Electrochemical Reduction of CO2: Current Status and Prospective Implementation

The CO2 that comes from the use of fossil fuels accounts for about 65% of the global greenhouse gas emission, and it plays a critical role in global climate changes. Among the different strategies that have been considered to address the storage and reutilization of CO2, the transformation of CO2 into chemicals or fuels with a high added-value has been considered a winning approach. This transformation is able to reduce the carbon emission and induce a “fuel switching” that exploits renewable energy sources. The aim of this brief review is to gather and critically analyse the main efforts that have been made and achievements that have been made in the electrochemical reduction of CO2 for the production of CO. The main focus is on the prospective of exploiting the intrinsic nature of the electrolysis process, in which CO2 reduction and H2 evolution reactions can be combined, into a competitive approach, to produce syngas. Several well-established processes already exist for the generation of fuels and fine-chemicals from H2/CO mixtures of different ratios. Hence, the different kinds of electrocatalysts and electrochemical reactors that have been used for the CO and H2 evolution reactions have been analysed, as well as the main factors that influence the performance of the system from the thermodynamic, kinetic and mass transport points of view

Electro-oxidation of phenol over electrodeposited MnOx nanostructures and the role of a TiO2 nanotubes interlayer

More and more attention has recently been paid to the electrochemical treatment of wastewater for the degradation of refractory organics, such as phenol and its derivatives. The electrodeposition of different types of manganese oxides (MnOx) over two substrates, namely metallic titanium and titania nanotubes (TiO2-NTs), is reported herein. X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS) analyses have confirmed the formation of different oxidation states of the manganese, while Field Emission Scanning Electronic Microscopy (FESEM) analysis has helped to point out the evolutions in the morphology of the samples, which depends on the electrodeposition parameters and calcination conditions. Moreover, cross section FESEM images have demonstrated the penetration of manganese oxides inside the NTs for anodically deposited samples. The electrochemical properties of the electrodes have been investigated by means of cyclic voltammetry (CV) and linear sweep voltammetry (LSV), both of which have shown that both calcination and electrodeposition over TiO2-NTs lead to more stable electrodes that exhibited a marked increase in the current density. The activity of the proposed nanostructured samples toward phenol degradation has been investigated. The cathodically electrodeposited manganese oxides (α-MnO2) have been found to be the most active phase, with a phenol conversion of 26.8%. The anodically electrodeposited manganese oxides (α-Mn2O3), instead, have shown higher stability, with a final working potential of 2.9 V vs. RHE. The TiO2-NTs interlayer has contributed, in all cases, to a decrease of about 1–1.5 V in the final (reached) potential, after a reaction time of 5 h. Electrochemical impedance spectroscopy (EIS) and accelerated life time tests have confirmed the beneficial effect of TiO2-NTs, which contributes by improving both the charge transfer properties (kinetics of reaction) and the adhesion of MnOx films

A model for electrode effects based on adsorption theory

A model to describe the electrode effects based on the adsorption theory is proposed. We assume that the coverage (i.e by gas bubbles, electrodeposition of compounds, etc) of the electrodes is governed by a kinetics equation where the adsorption term is proportional to the bulk current density, and the desorption term to the actual coverage. The adsorption can take place only on the uncovered part of the electrode. We show that the coverage is responsible for a variation of the interface properties of the electrode. The time dependence of the electric response of the cell, submitted to an external voltage, is determined by solving the differential equation for the coverage. We show that two regimes are expected. One, in the limit of small time, controlled by the charging of the surface interface, and one related to the coverage. The theoretical predictions are in reasonable agreement with the experimental data concerning the time dependence of the current and the current-voltage characteristics of a home-made photo-electrolyzer constituted by a BiVO4 photoanode and a Pt cathode. Moreover, a normalized current-voltage curve was obtained, which fit also literature data based on (i) electrolysis on cylindrical stainless-steel electrodes in NaOH electrolyte and (ii) electrolytic plasma nitrocarburizing of AISI 1020 steel discs in an Urea-based aqueous solution, demonstrating the versatility and broad range of application of the here proposed model

Comparison of photocatalytic and transport properties of TiO2 and ZnO nanostructures for solar-driven water splitting

Titanium dioxide (TiO2) and zinc oxide (ZnO) nanostructures have been widely used as photo-catalysts due to their low-cost, high surface area, robustness, abundance and non-toxicity. In this work, four TiO2 and ZnO - based nanostructures, i.e. TiO2 nanoparticles (TiO2 NPs), TiO2 nanotubes (TiO2 NTs), ZnO nanowires (ZnO NWs) and ZnO@TiO2 core-shell structures, specifically prepared with a fixed thickness of about 1.5 μm, are compared for the solar-driven water splitting reaction, under AM1.5G simulated sunlight. A complete characterization of these photo-electrodes in their structural and photo-electrochemical properties was carried out. Both TiO2 NPs and NTs showed photo-current saturation reaching 0.02 and 0.12 mA/cm2, respectively, for potential values of about 0.3 and 0.6 V vs. RHE. In contrast, the ZnO NWs and the ZnO@TiO2 core-shell samples evidence a linear increase of the photocurrent with the applied potential, reaching 0.45 and 0.63 mA/cm2 at 1.7 V vs. RHE, respectively. However, under concentrated light conditions, the TiO2 NTs demonstrate a higher increase of the performance with respect to the ZnO@TiO2 core-shells. Such material dependent behaviours are discussed in relation with the different charge transport mechanisms and interfacial reaction kinetics, which were investigated through electrochemical impedance spectroscopy. The role of key parameters such as electronic properties, specific surface area and photo-catalytic activity on the performance of these materials are discussed. Moreover, proper optimization strategies are analyzed in view of increasing the efficiency of the best performing TiO2 and ZnO - based nanostructures, toward their practical application in a solar water splitting device

BiVO4 as photocatalyst for solar fuels production through water splitting: A short review

As solar energy is the most abundant energy source, it has been widely exploited for thermal and electrical power generation. However, owing to the greater convenience of chemical energy storage, such as H2, compared to electricity, solar fuels have been considered as one of the most promising technological concepts due to their potential higher efficiency and environmental suitability. In this context, the photocatalytic water splitting into O2 and solar fuels (e.g. H2) is a topic of current interest. Furthermore, the development of photocatalysts that can utilize the whole electromagnetic spectrum is preferable in order to enhance the overall water splitting efficiency. Direct photocatalytic water splitting is a challenging problem because the water oxidation (WO) reaction is thermodynamically uphill. Hence, several WO photocatalysts have been developed and assessed over the last few decades, and it has been reported that BiVO4 is one of the most active O2 evolution photocatalysts. In this review, a first introduction regarding the solar fuel production and the water oxidation reaction is reported. Subsequently, the crystal and electronic structures as well as the optical properties that are closely related to the photoelectrochemical properties of BiVO4 are described. Finally, the monoclinic BiVO4 synthesis methods and the optimization methods to improve the performances of BiVO4 are discussed. The information gained from this analysis contributes to the better understanding of the main parameters affecting the activity and will ultimately lead to the optimized synthesis of a more efficient BiVO4 photocatalytic material

Role of the electrode morphology on the optimal thickness of BiVO4 anodes for photoelectrochemical water splitting cells

Photoelectrochemical (PEC) water splitting is one of the most promising technologies able to exploit renewable resources such as water and sunlight for the sustainable production of energy carriers (i.e. H2, solar fuels). Recently, it was demonstrated that the currents through a PEC water splitting cell present a maximum for a specific thickness of the photocatalytic anode film (J. Electroanal. Chem. 2017, 788, 61–65 ), which is related to the attenuation of the light in the semiconductor layer and to the applied bias potential. In this work, it is shown that this non-monotonic behavior is not only due to the trade-off between the absorption length of light and thickness of a photocatalyst (e.g. BiVO4) film, but mainly to the porous nature of the semiconducting material. A theoretical analysis of such behavior for an ideal compact medium shows a very broad maximum, in contrast to the experimental values that present steeper slopes both at low and large thicknesses (e.g. for films prepared by dip-coating method). There have been taken into account, subsequently, the loss of useful current at grain boundaries, due to an increased number of recombination centers, the reduction of specific area (less porosity) as the grains increase in size when increasing the number of dip-coatings, the possible non-uniform coverage of the substrate by the BiVO4 for a low number of coatings, and the partial blocking of pores with oxygen bubbles under operation. For each of these considerations, a normalized current plot in terms of the number of coatings has shown a remarkably good concordance to the experimental data