Understanding the Internal Conversion Efficiency of BiVO4/SnO2 Photoanodes for Solar Water Splitting: An Experimental and Computational Analysis.

This work aims to understand the spin-coating growth process of BiVO4 photoanodes from a photon absorption and conversion perspective. BiVO4 layers with thicknesses ranging from 7 to 48 nm and the role of a thin (<5 nm) SnO2 hole-blocking layer have been studied. The internal absorbed photon-to-current efficiency (APCE) is found to be nonconstant, following a specific dependence of the internal charge separation and extraction on the increasing thickness. This APCE variation with BiVO4 thickness is key for precise computational simulation of light propagation in BiVO4 based on the transfer matrix method. Results are used for accurate incident photon-to-current efficiency (IPCE) prediction and will help in computational modeling of BiVO4 and other metal oxide photoanodes. This establishes a method to obtain the sample's thickness by knowing its IPCE, accounting for the change in the internal APCE conversion. Moreover, an improvement in fill factor and photogenerated voltage is attributed to the intermediate SnO2 hole-blocking layer, which was shown to have a negligible optical effect but to enhance charge separation and extraction for the lower energetic wavelengths. A Mott-Schottky analysis was used to confirm a photovoltage shift of 90 mV of the flat-band potential.

Facing Seawater Splitting Challenges by Regeneration with Ni-Mo-Fe Bifunctional Electrocatalyst for Hydrogen and Oxygen Evolution.

Hydrogen, produced by water splitting, has been proposed as one of the main green energy vectors of the future if produced from renewable energy sources. However, to substitute fossil fuels, large amounts of pure water are necessary, scarce in many world regions. In this work, we fabricate efficient and earth-abundant electrodes, study the challenges of using real seawater, and propose an electrode regeneration method to face undesired salt deposition. Ni-Mo-Fe trimetallic electrocatalyst is deposited on non-expensive graphitic carbon felts both for hydrogen (HER) and oxygen evolution reactions (OER) in seawater and alkaline seawater. Cl- pitting and the chlorine oxidation reaction are suppressed on these substrates and alkalinized electrolyte. Precipitations on the electrodes, mainly CaCO3 , originating from seawater-dissolved components have been studied, and a simple regeneration technique is proposed to rapidly dissolve undesired deposited CaCO3 in acidified seawater. Under alkaline conditions, Ni-Mo-Fe-based catalyst is found to reconfigure, under cathodic bias, into Ni-Mo-Fe alloy with a cubic crystalline structure and Ni : Fe(OH)2 redeposits whereas, under anodic bias, it is transformed into a follicular Ni:FeOOH structure. High productivities over 300 mA cm-2 and voltages down to 1.59 V@10 mA cm-2 for the overall water splitting reaction have been shown, and electrodes are found stable for over 24 h without decay in alkaline seawater conditions and with energy efficiency higher than 61.5 % which makes seawater splitting promising and economically feasible.

Insight into the Degradation Mechanisms of Atomic Layer Deposited TiO2 as Photoanode Protective Layer.

Around 100 nm thick TiO2 layers deposited by atomic layer deposition (ALD) have been investigated as anticorrosion protective films for silicon-based photoanodes decorated with 5 nm NiFe catalyst in highly alkaline electrolyte. Completely amorphous layers presented high resistivity; meanwhile, the ones synthesized at 300 °C, having a fully anatase crystalline TiO2 structure, introduced insignificant resistance, showing direct correlation between crystallization degree and electrical conductivity. The conductivity through crystalline TiO2 layers has been found not to be homogeneous, presenting preferential conduction paths attributed to grain boundaries and defects within the crystalline structure. A correlation between the conductivity atomic force microscopy measurements and grain interstitials can be seen, supported by high-resolution transmission electron microscopy cross-sectional images presenting defective regions in crystalline TiO2 grains. It was found that the conduction mechanism goes through the injection of electrons coming from water oxidation from the electrocatalyst into the TiO2 conduction band. Then, electrons are transported to the Si/SiOx/TiO2 interface where electrons recombine with holes given by the p+n-Si junction. No evidences of intra-band-gap states in TiO2 responsible of conductivity have been detected. Stability measurements of fully crystalline samples over 480 h in anodic polarization show a continuous current decay. Electrochemical impedance spectroscopy allows to identify that the main cause of deactivation is associated with the loss of TiO2 electrical conductivity, corresponding to a self-passivation mechanism. This is proposed to reflect the effect of OH- ions diffusing in the TiO2 structure in anodic conditions by the electric field. This fact proves that a modification takes place in the defective zone of the layer, blocking the ability to transfer electrical charge through the layer. According to this mechanism, a regeneration of the degradation process is demonstrated possible based on ultraviolet illumination, which contributes to change the occupancy of TiO2 electronic states and to recover the defective zone's conductivity. These findings confirm the connection between the structural properties of the ALD-deposited polycrystalline layer and the degradation mechanisms and thus highlight main concerns toward fabricating long-lasting metal-oxide protective layers for frontal illuminated photoelectrodes.

Role of Bismuth in the Electrokinetics of Silicon Photocathodes for Solar Rechargeable Vanadium Redox Flow Batteries.

The ability of crystalline silicon to photoassist the V3+ /V2+ cathodic reaction under simulated solar irradiation, combined with the effect of bismuth have led to important electrochemical improvements. Besides the photovoltage supplied by the photovoltaics, additional decrease in the onset potentials, high reversibility of the V3+ /V2+ redox pair, and improvement in the electrokinetics were attained thanks to the addition of bismuth. In fact, Bi0 deposition has shown to slightly decrease the photocurrent, but the significant enhancement in the charge transfer, reflected in the overall electrochemical performance clearly justifies its use as additive in a photoassisted system for maximizing the efficiency of solar charge to battery.

Turning Earth Abundant Kesterite-Based Solar Cells Into Efficient Protected Water-Splitting Photocathodes.

CZTS/Se kesterite-based solar cells have been protected by conformal atomic layer deposition (ALD)-deposited TiO2 demonstrating its feasibility as powerful photocathodes for water splitting in highly acidic conditions (pH < 1), achieving stability with no detected degradation and with current density levels similar to photovoltaic productivities. The ALD has allowed low deposition temperatures of 200 °C for TiO2, preventing significant variations to the kesterite structure and CdS heterojunction, except for the pure-sulfide stoichiometry, which was studied by Raman spectroscopy. The measured photocurrent at 0 V vs reversible hydrogen electrode, 37 mA·cm-2, is the highest reported to date, and the associated half-cell solar-to-hydrogen efficiency reached 7%, being amongst the largest presented for kesterite-based photocathodes, corroborating the possibility of using them as abundant low-cost alternative photoabsorbers as their efficiencies are improved toward those of chalcopyrites. An electrical circuit has been proposed to model the photocathode, which comprises the photon absorption, charge transfer through the protective layer, and catalytic performance, which paves the way to the design of highly efficient photoelectrodes.

Charge Transfer Characterization of ALD-Grown TiO2 Protective Layers in Silicon Photocathodes.

A critical parameter for the implementation of standard high-efficiency photovoltaic absorber materials for photoelectrochemical water splitting is its proper protection from chemical corrosion while remaining transparent and highly conductive. Atomic layer deposited (ALD) TiO2 layers fulfill material requirements while conformally protecting the underlying photoabsorber. Nanoscale conductivity of ALD TiO2 protective layers on silicon-based photocathodes has been analyzed, proving that the conduction path is through the columnar crystalline structure of TiO2. Deposition temperature has been explored from 100 to 300 °C, and a temperature threshold is found to be mandatory for an efficient charge transfer, as a consequence of layer crystallization between 100 and 200 °C. Completely crystallized TiO2 is demonstrated to be mandatory for long-term stability, as seen in the 300 h continuous operation test.