Photoelectrochemical Solar Energy Conversion Research Articles

Rational design of nanostructured BiVO4/FeOx photoanode coupling with 2D Co(OH)2 cocatalyst for enhanced photoelectrochemical water splitting

Efficient charge transport and separation is of prime significance to achieve high efficiency of photoelectrochemical (PEC) solar energy conversion. In this study, we designed a novel ternary nanostructured BiVO4/FeOx/Co(OH)2 composite as photoelectrode for enhanced PEC water splitting. Compared with pristine BiVO4, the fabricated BiVO4/FeOx/Co(OH)2 photoelectrode produced a doubled increase in photocurrent (from 0.36 to 0.72 mA cm−2 at 1.23 V versus RHE) with a negative shift of 390 mV in the initial potential (from 0.72 to 0.33 V). In addition, the conversion efficiency and photostability for PEC water oxidation were also improved. The elevated PEC properties of ternary composite photoelectrode were originated from the synergy of heterojunction construction and 2D cocatalyst loading that provides effective pathway for promoting charge separation as well as sufficient sites for photoelectrochemical reaction. These findings could provide an exemplary PEC model for enhanced solar hydrogen evolution of BiVO4 photoanode.

(Invited) Solar Flow Batteries: Integrated Photoelectrochemical Solar Energy Conversion and Redox Flow Battery Systems

Due to the intermittent nature of sunlight, practical solar energy utilization systems demand both efficient solar energy conversion and inexpensive large scale energy storage. We have developed hybrid solar-charged storage devices called solar flow batteries (SFBs) that integrate photoelectrochemical solar cells with redox flow batteries (RFBs). In these devices, photoexcited carriers collected at the semiconductor-liquid electrolyte interface convert the redox couples to charge up the RFB without external electric bias; which can be discharged to generate the electricity when needed. By matching various silicon solar cells and tandem III-V solar cells (Chem 2018, 4, 2644) with various organic redox couples and optimizing the SFB device design, we improved their round trip solar-to-output electricity efficiency (SOEE) and eventually achieved a record SOEE of 20% with a 500-hour lifetime using high performance tandem perovskite/silicon solar cells (Nature Mater. 2020, 19, 1326). The design principles and the quantitative analysis model for voltage matching solar cells with RFBs (Acc. Chem. Res. 2020, 53, 2611) light up the pathways for achieving even better performance and stability yet maintaining a low cost, which would make these SFBs practical for standalone solar energy conversion and storage systems in remote off-grid locations.

Phase engineering of 1 T-MoS2 on BiVO4 photoanode with p–n Junctions: Establishing high speed charges transport channels for efficient photoelectrochemical water splitting

Photoelectrochemical (PEC) conversion of solar energy into chemical energy urgently requires attractive photoelectrodes to advance its practical application. Herein, we demonstrated a simple dopant electron strategy for phase control engineering and prepared a hybrid photoelectrode of 1 T-MoS2 nanosheets sensitized monoclinic BiVO4 (denoted as 1 T-MoS2@BiVO4) with p–n junction. The internal p–n heterojunction in the composite photoanode contributes to its superior charges transport. Further, the inserted 1 T-MoS2 layer at the BiVO4/traditional co-catalysts (OECs) (OECs: FeOOH and NiOOH) interface can profoundly improve the PEC water splitting and solar conversion efficiency of BiVO4-based photoanodes. The corresponding photocurrent density of FeOOH@1T-MoS2@BiVO4 photoelectrode (4.02 mA cm−2) is 3.5 times as high as that of the pure BiVO4 electrode (1.14 mA cm−2) at 1.23 V vs reversible hydrogen electrode (RHE), accompanied by a photoconversion efficiency of 1.28% at 0.68 vs RHE. Theoretical integrated with experimental studies reveal that the insertion of an 1 T-MoS2 layer between BiVO4 and OECs facilitates the establishment of high speed transport channels for holes and electrons. Subsequently, copper phthalocyanine (CuPc) as an unconventional hole-transporting organic material could maintain the photocurrent density of the composite photoanode for stability test over 8 h. Importantly, this study sheds light on the rational design of phase-control-based devices for solar-to-chemical energy conversion, and also implies a prospective nexus between BiVO4 and traditional or unconventional co-catalysts.

Highly conducting flake-like CuS nanostructured counter electrode for photoelectrochemical solar energy conversion

Fabrication of platinum (Pt) free, low cost, environmentally friendly counter electrode is highly attractive for electrochemical solar energy conversion. In this report, highly conducting flake-like CuS nanostructured counter electrode has been prepared for photosensing study and semiconductor sensitized solar cell application. Flake-like CuS nanostructures were synthesized by sulfuration of CuO nanorods grown by hydrothermal process. Scanning electron microscopy and atomic force microscopy study confirmed the growth of flake-like CuS nanostructures on glass substrates. CuS nanoflakes were highly crystalline with hexagonal phase. The various electronic states of Cu and S in CuS nanoflakes were studied using X-ray photoelectron spectroscopy. Chemically grown CuS films were highly conducting and highest conducting film was obtained at 85°C growth temperature with sheet resistance ∼20.02±0.02Ω/sq. Photoelectrochemical solar cell (PEC) was fabricated using CdS decorated TiO2 nanorods as working electrode and CuS film as counter electrode. The short circuit current density, open circuit voltage and power conversion efficiency of the PEC device were found to be ∼4.8 A/m2, ∼0.38 V and ∼0.17%, respectively.

Triclinic-Phase Bismuth Chromate: A Promising Candidate for Photocatalytic Water Splitting with Broad Spectrum Ranges.

Photocatalytic water splitting for solar energy conversion remains challenged by the lack of novel semiconductor photocatalysts with paramount parameters including wide light-harvesting ranges and suitable band structures. Here, a novel triclinic-phase bismuth chromate (Bi2 CrO6 ) acting as a semiconductor photocatalyst candidate is reported. Triclinic Bi2 CrO6 exhibits a broad absorption range of ≈650nm with a direct bandgap of 1.86eV and shows a suitable band structure for water splitting. Theoretical simulations of triclinic Bi2 CrO6 reveal a high charge mobility,possibly owing to the strong hybridized covalent bonds, large elastic modulus, and small carrier effective mass. The triclinic Bi2 CrO6 is demonstrated to work well toward photocatalytic water oxidation and hydrogen production reactions under visible light and match well with its absorption ranges. In particular, it exhibits decent photocatalytic water oxidation performance in the presence of various electron scavengers. Furthermore, the visible-light-driven Z-scheme overall water splitting system is fabricated by coupling triclinic Bi2 CrO6 as the oxygen evolution photocatalyst with SrTiO3 :Rh as the hydrogen evolution photocatalyst, giving a stable overall water splitting with stoichiometric evolution of H2 and O2 . This work presents a promising semiconductor material enabling wide-range light harvesting for photocatalytic and photo-electrochemical solar energy conversion.

Surface active oxygen engineering of photoanodes to boost photoelectrochemical water and alcohol oxidation coupled with hydrogen production

Photoelectrochemical conversion of solar energy into fuels (or chemicals) is in urgent need of efficient photoanodes to drive its practical applications. Nevertheless, the development of high performance photoanode usually suffers from slow charge transfer and sluggish surface catalytic kinetics. Here, we report a surface active oxygen engineering strategy through the modification and activation of NiCo layered double hydroxide (NiCo-LDH), where the activated NiCo-LDH (NiCo-LDH-Act) captures the photogenerated holes and acts as cocatalyst for surface oxidation reactions. The as-developed BiVO4/NiCo-LDH-Act achieves more than 3-fold higher photocurrent density than that of pristine BiVO4. Experimental integrated with theoretical studies further testify the bifunctional of NiCo-LDH-Act for hole transport and catalytic reactions. Moreover, the adsorbed ·OH originated from active oxygen enables efficient oxidation of glycerol to attain 1,3-dihydroxyacetone (DHA) with a productivity of 20.5 μmol cm–2 h–1 at 1.4 V vs. RHE with simultaneously producing hydrogen at the cathode.

Toward Ideal Metal–Organic Framework Thin-Film Growth via Automated Layer-by-Layer Deposition: Examples Based on Perylene Diimide Linkers

Photoactive metal–organic framework (MOF) thin films offer an opportunity for translating the advantages of periodic, crystalline, and tailorable light-harvesting materials directly into devices such as those for photoelectrochemical solar energy conversion. In this study, we report the fabrication of light-absorbing perylene-diimide-containing pillared-paddlewheel MOF thin films using an automated layer-by-layer (LbL) deposition technique. Our focus here is on optimizing the growth of representative chromophoric MOFs as oriented films of uniform and predefined thickness. Growth was examined as a function of metal identity, pillaring ligand composition, and supporting-surface chemical functionality. Application of atomic force microscopy (AFM) and complementary techniques revealed that the surface-supported MOFs initially display island-type film growth (Volmer–Weber growth), resulting in comparatively rough films. Further growth is accompanied by the merging of islands, resulting in films that, depending on experimental details, can be remarkably smooth (i.e., roughness on the order of ±1 nm (one structural repeat unit in the pillaring direction)). These details include the use of 1,4-diazabicyclo[2.2.2]octane (DABCO) as a MOF pillar and ALD-grown zinc oxide as a film support (ALD = atomic layer deposition). Also helpful for mitigating island-type growth, at least in part, is the replacement of Zn2+ by Cu2+ as the metal component of the MOF. Notably, each of these adjustments entails replacing weaker chemical bonds with stronger ones.

Unveiling charge dynamics of Co3S4 nanowalls/CdS nanospheres n-n heterojunction for efficient photoelectrochemical Cr(VI) detoxification and N2 fixation

Photoelectrochemical (PEC) conversion of solar energy is a sustainable and ecological approach as it offers green energy carriers, renewable fuels as well as environmental remediation. Herein, a polyaniline-coated anodized stainless-steel mesh (PANI-ASSM) was supported n-n heterojunction of CdS nanosphere (NSs)-Co3S4 nanowalls (NWs) by a simple step by step electrochemical deposition method. The PEC performance of the developed photoanode was then assessed toward Cr(VI) detoxification and N2 fixation. The PL, UV-Vis, electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS), cyclic voltammetry (CV), transient photocurrent (TPC), chronopotentiometry, and Bode phase tests also proved the excellent PEC performance and significant stability of this catalyst. UV–vis, Urbach energy (UE), Mott-Schottky (M-S), and linear sweep voltammetry (LSV) results coupled to the Robert Mulliken method confirmed the formation of robust n-n CdS-NSs-Co3S4-NWs heterojunction with viable band edge potential for Cr(VI) detoxification and N2 fixation; while PANI only acted as a powerful bridge to transfer the photo-generated electrons to ASSM support. The possible mechanism was investigated by experimental and electrochemical analysis which suggested a 3-electron-coupled Cr(VI) and 6-electron-coupled N2 reduction pathway. Such a PEC exhibited considerable electrocatalytic performance due to effective electron-hole separation with an ammonia yield and Cr(VI) reduction of 12.33 µg/mol.h NH3 and 90.61%, respectively. This work presented a new paradigm for the illustrative design of promising photoanodes for renewable fuel production and detoxification performance.

Low Temperature Open-Air Plasma Deposition of SrTiO3 Films for Solar Energy Harvesting: Impact of Precursors on the Properties and Performances.

Strontium titanate (STO) is a well-known oxide used in a wide variety of applications due to its excellent stability and optoelectronic properties. However, its integration in photoelectrocatalytic devices is limited by the lack of fast and scalable methods to produce robust films at a low temperature and atmospheric pressure. Herein, we report an atmospheric pressure plasma-enhanced chemical vapor deposition (AP-PECVD) approach for the synthesis of STO crystalline films and their applications for photoelectrochemical solar energy conversion. The film crystallinity, which plays a determinant role in the photoelectrochemical performance, was linked to the selected strontium precursor and injection method. Through thermal stability studies of the precursors [Sr(dpm), Sr(ipo), Sr(acac), and Ti(ipo)] and analysis of the solution droplet size, it was demonstrated that the closer thermal decomposition behavior and superior miscibility of the Sr(dpm) and Ti(ipo) precursors led to more homogeneous and crystalline films with the highest photoelectrochemical performance (16.5 μA cm-2 at 1.23 V vs RHE under 100 mW cm-2), which can be further improved by a factor of 3.4 using thermal annealing at 500 °C. Evidence of the impact of a strontium precursor on the properties of STO films is provided through thermogravimetric analysis, X-ray diffraction, energy-dispersive system, UV-vis, X-ray photoelectron spectroscopy, HIM-SIMS, and photoelectrochemical analysis.

Single Nanoflake Photoelectrochemistry Reveals Intrananoflake Doping Heterogeneity That Explains Ensemble-Level Photoelectrochemical Behavior.

Transition metal dichalcogenide (TMD) nanoflake thin films are attractive electrode materials for photoelectrochemical (PEC) solar energy conversion and sensing applications, but their photocurrent quantum yields are generally lower than those of bulk TMD electrodes. The poor PEC performance has been primarily attributed to enhanced charge carrier recombination at exposed defect and edge sites introduced by the exfoliation process. Here, a single nanoflake PEC approach reveals how an alternative effect, doping heterogeneity, limits ensemble-level PEC performance. Photocurrent mapping and local photocurrent-potential (i-E) measurements of MoS2 nanoflakes exfoliated from naturally occurring bulk crystals revealed the presence of n- and p-type domains within the same nanoflake. Interestingly, the n- and p-type domains in the natural MoS2 nanoflakes were equally efficient for iodide oxidation and tri-iodide reduction (IQE values exceed 80%). At the single domain-level, the natural MoS2 nanoflakes were nearly as efficient as nanoflakes exfoliated from synthetic n-type MoS2 crystals. Single domain-level i-E measurements explain why natural MoS2 nanoflakes exhibit an n-type to p-type photocurrent switching effect in ensemble-level measurements: the n- and p-type diode currents from individual domains oppose each other upon illuminating the entire nanoflake, resulting in zero photocurrent at the switching potential. The doping heterogeneity effect is likely due to nonideal stoichiometry, where p-type domains are S-rich according to XPS measurements. Although this doping heterogeneity effect limits photoanode or photocathode performance, these findings open the possibility to synthesize efficient TMD nanoflake photocatalysts with well-defined lateral p- and n-type domains for enhanced charge separation.

Enhancing Photoelectrochemical Energy Storage by Large-Area CdS-Coated Nickel Nanoantenna Arrays

The integration of thin films made up of periodic plasmonic nanostructures and semiconductors holds great potential to develop efficient technologies for photoelectrochemical solar energy conversion and storage. However, to date, only periodic nanoantenna arrays made up of Au have been explored, posing severe limitations in terms of scalability and costs. Here, we show that nickel nanopillar arrays can support complex electromagnetic resonances that strongly enhance the photoelectrochemical response of CdS thin films. By controlling the pitch size and diameter of the nanopillars, we obtain broadband light absorption from the ultraviolet (UV) to the near-infrared (NIR) wavelength range, thus achieving large photocurrent enhancements compared to a planar Ni/CdS sample and in line with those generated by previously reported Au nanostructures. The photocurrent enhancement is attributed to photonic modes in the UV and hybrid cavity-plasmonic modes in the visible and NIR ranges, which give rise to efficient energy transfer and hot carrier injection between metallic structures, the semiconductor, and the electrolyte. The developed nanopillar arrays are promising candidates for photoelectrochemical devices fully exploiting the solar spectrum and using Earth-abundant raw materials.

WO3/BiVO4 Photoanodes: Facets Matching at the Heterojunction and BiVO4 Layer Thickness Effects

Photoelectrochemical solar energy conversion offers a way to directly store light into energy-rich chemicals. Photoanodes based on the WO3/BiVO4 heterojunction are most effective mainly thanks to the efficient separation of photogenerated charges. The WO3/BiVO4 interfacial space region in the heterojunction is investigated here with the increasing thickness of the BiVO4 layer over a WO3 scaffold. On the basis of X-ray diffraction analysis results, density functional theory simulations show a BiVO4 growth over the WO3 layer along the BiVO4 {010} face, driven by the formation of a stable interface with new covalent bonds, with a favorable band alignment and band bending between the two oxides. This crystal facet phase matching allows a smooth transition between the electronic states of the two oxides and may be a key factor ensuring the high efficiency attained with this heterojunction. The photoelectrochemical activity of the WO3/BiVO4 photoanodes depends on both the irradiation wavelength and the thickness of the visible-light-absorbing BiVO4 layer, a 75 nm thick BiVO4 layer on WO3 being best performing.

Adaptive response by an electrolyte: resilience to electron losses in a dye-sensitized porous photoanode.

Photovoltage and photocurrents below theoretical limits in dye-sensitized photoelectrochemical solar energy conversion systems are usually attributed to electron loss processes such as dye–electron and electrolyte–electron recombination reactions within the porous photoanode. Whether recombination is a major loss mechanism is examined here, using a multiscale reaction–diffusion computational model to evaluate system characteristics. The dye-sensitized solar cell with an I−/I3− redox couple is chosen as a simple, representative model system because of the extensive information available for it. Two photoanode architectures with dye excitation frequencies spanning 1–25 s−1 are examined, assuming two distinct recombination mechanisms. The simulation results show that although electrolyte–electron reactions are very efficient, they do not significantly impact photoanode performance within the system as defined. This is because the solution-phase electrolyte chemistry plays a key role in mitigating electron losses through coupled reactions that produce I− within the photoanode pores, thereby cycling the electrolyte species without requiring that all electrolyte reduction reactions take place at the more distantly located cathode. This is a functionally adaptive response of the chemistry that may be partly responsible for the great success of this redox couple for dye-sensitized solar cells. The simulation results provide predictions that can be tested experimentally.

Optimization of Solar Water Battery for Efficient Photoelectrochemical Solar Energy Conversion and Storage

Optimization of Solar Water Battery for Efficient Photoelectrochemical Solar Energy Conversion and Storage

TiO2 Nanotube Arrays on Flexible Kapton Substrates for Photo-Electrochemical Solar Energy Conversion

This work reports the growth of stable TiO2 nanotube arrays on flexible Kapton substrates by electrochemical anodization of a sputtered Ti (titanium) film. Although such nanotubes are conventionally fabricated on Ti foils, obtaining these on polymer-based flexible substrates remained a challenge because of higher annealing temperature not compatible with thermal stability of the substrates. Here, we demonstrate the fabrication of TiO2 nanotubes (1.5 μm long and 80 nm diameter) by anodization of the Ti film deposited using the RF sputtering technique at two different substrate temperatures (room temperature and 300 °C). Nanoindentation and nanoscratch techniques reveal better adhesion of the Ti film with an underlying Kapton substrate for 300 °C deposition temperature. Such investigations reveal a more than twofold enhancement of the “rear pileup” for the Ti film deposited at elevated temperature compared to that at room temperature. The amorphous TiO2 nanotubes are crystallized at 220 °C for 3 h using a solvothermal technique that allows crystallization at temperatures much lower than the annealing temperature. Application of these nanotubes for photo-electrochemical water splitting reveals a photocurrent density of 18 μA/cm2 under AM 1.5 G conditions. Furthermore, the charge density and flat band potential (VFB) are calculated from Mott–Schottky analysis, showing features comparable to the TiO2 nanotubes on the Ti foil crystallized through thermal annealing. The present work establishes a scalable approach for developing TiO2 nanotube arrays on the flexible substrate and its use for photo-electrochemical solar energy conversion.

Functions of MnOx in NaCl Aqueous Solution for Artificial Photosynthesis.

SummaryPhotoelectrochemical water splitting has been intensively investigated as artificial photosynthesis technology to convert solar energy into chemical energy. The use of seawater and salted water has advantages for minimum environmental burden; however, the oxidation of Cl− ion to hypochlorous acid (HClO), which has toxicity and heavy corrosiveness, should occur at the anode, along with the oxygen evolution. Here, O2 and HClO production in aqueous solution containing Cl− on photoanodes modified with various metal oxides was investigated. The modification of MnOx resulted in the promotion of the O2 evolution reaction (OER) specifically without HClO production over a wide range of conditions. The results will contribute not only to the practical application of artificial photosynthesis using salted water but also to the elucidation of substantial function of manganese as the element for OER center in natural photosynthesis.

Electronic Structure and Interface Energetics of CuBi2O4 Photoelectrodes.

CuBi2O4 exhibits significant potential for the photoelectrochemical (PEC) conversion of solar energy into chemical fuels, owing to its extended visible-light absorption and positive flat band potential vs the reversible hydrogen electrode. A detailed understanding of the fundamental electronic structure and its correlation with PEC activity is of significant importance to address limiting factors, such as poor charge carrier mobility and stability under PEC conditions. In this study, the electronic structure of CuBi2O4 has been studied by a combination of hard X-ray photoemission spectroscopy, resonant photoemission spectroscopy, and X-ray absorption spectroscopy (XAS) and compared with density functional theory (DFT) calculations. The photoemission study indicates that there is a strong Bi 6s–O 2p hybrid electronic state at 2.3 eV below the Fermi level, whereas the valence band maximum (VBM) has a predominant Cu 3d–O 2p hybrid character. XAS at the O K-edge supported by DFT calculations provides a good description of the conduction band, indicating that the conduction band minimum is composed of unoccupied Cu 3d–O 2p states. The combined experimental and theoretical results suggest that the low charge carrier mobility for CuBi2O4 derives from an intrinsic charge localization at the VBM. Also, the low-energy visible-light absorption in CuBi2O4 may result from a direct but forbidden Cu d–d electronic transition, leading to a low absorption coefficient. Additionally, the ionization potential of CuBi2O4 is higher than that of the related binary oxide CuO or that of NiO, which is commonly used as a hole transport/extraction layer in photoelectrodes. This work provides a solid electronic basis for topical materials science approaches to increase the charge transport and improve the photoelectrochemical properties of CuBi2O4-based photoelectrodes.

Magnetron sputtered copper bismuth oxide photocathodes for solar water reduction

There is an urgent need for new materials that can catalyze or drive the photoelectrochemical (PEC) conversion of solar energy into chemical energy, i.e. solar fuels. Copper bismuth oxide (CBO) is a promising photocathode material for the photochemical reduction of water. Here, we systematically control the stoichiometry of CBO thin films prepared by reactive, direct-current magnetron co-sputtering from metallic Bi and Cu targets. The intrinsic photophysical and PEC material properties are investigated and evaluated in order to determine the optimum composition for hydrogen formation. Changing the stoichiometry of the films reveals a dramatic change in the optical band gap and crystal structure of CBO. The largest photocurrent density was achieved for a copper-to-bismuth ion ratio of 0.53, close to the CuBi2O4 stoichiometry, which yielded Jph = − 0.48 mA cm−2 at 0 VRHE (RHE = reversible hydrogen electrode). This is the highest value to date for the photochemical reduction of water with CuBi2O4 without an externally applied bias. The absorbed photon-to-current efficiency and the photostability of the films in neutral and alkaline electrolytes were also investigated.

Synthesis and Computational and Experimental Investigations of a para-Nicotinic Acid-Bridged Dirhenium(I) Dimer Complex.

The Re(I) dimer complex, [fac(CO)3(phen)Re1-N(py)COORe2(phen)fac(CO)3]+ (py = pyridine; phen = 1,10-phenanthroline), contains two different Re(I) centers 9.3 Å apart, one with a nitrogen donor and the other with an acetate donor from the bridging isonicotinate ligand. The complexes were characterized by 1H NMR, UV–vis, fluorescence, and IR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. The absorption and emission properties of the dimer dominated by charge transfer transitions are analyzed with respect to those of the monomers, [fac(CO)3(phen)Re-N(pyCOOCH3)]+ and [fac(CO)3(phen)ReOOCCH3]. Spectral comparison of these three complexes results in the unexpected finding that the dimer emission (575 nm) occurs near that of the nicotinate-containing monomer (580 nm) rather than near the lower energy-emitting state (650 nm) of the acetate-containing monomer. Density functional theory (DFT) calculations elucidate this unusual emission behavior. The geometries of the dimer and two monomers are optimized in the singlet ground and lowest-energy triplet excited states (LLTS’s) to interpret absorption and emission behaviors, respectively. The singlet excited states calculated using time-dependent DFT correlate well with the absorption spectra in the lowest-energy and other major electronic transitions. The energy gaps and low-lying singlet excited states of the dimer are close to those of the acetate-containing monomer. The lowest-energy Franck–Condon triplet excited state of the dimer arising from electronic transitions localized on the acetate moiety is unstable. The next higher Franck–Condon triplet excited state arises from long-range charge transfer transition, and its energy is close to that of the nicotinate-containing monomer. Optimization of the dimer LLTS yields a stable state based on a long-range charge transfer transition involving occupied orbitals partially localized on the bridging nicotinate moiety. The LLTS energies of the dimer and nicotinate-containing monomer are in very good agreement as are the emission energies of these complexes. The correlated spectroscopic and computational results corroborate to the understanding of charge transfer states and transitions toward the development of photosensitive compounds for photoelectrochemical solar energy conversion cells.

Photoelectrocatalytic production of solar fuels with semiconductor oxides: materials, activity and modeling.

Oxide photoelectrochemistry has been under continuous development over the last half century. These decades have witnessed the use of electrodes of different nature (from single crystals to nanoparticulate films), new electrode materials (including ternary and multinary transition metal oxides), and different strategies for improving their efficiency and stability (e.g. doping or protective layers). Although the very high initial expectations for using oxide electrodes in solar energy conversion were not fully met, substantial efforts have been devoted to reach an in-depth understanding of the processes limiting their functioning, providing firm bases for further developments. In this article, we review our main contributions in this field; in particular, we focus on the water photooxidation (i.e. oxygen evolution reaction), water photoreduction (i.e. hydrogen evolution reaction) and full water splitting processes (in a tandem cell) with binary and ternary oxides, including metal hydroxides as co-catalysts. We emphasize the importance of modeling and obtaining mechanistic insights and we conclude with a reflection on the main issues to be tackled in this field, which in our opinion should experience major advances in the coming years.