Highly selective CO2 electrolysis in aqueous media by a water-soluble cobalt dimethyl-bipyridine complex.

A water-soluble Co complex with dimethyl-bipyridine ligands reduced CO2 to CO electrochemically with almost 100% selectivity at -0.80 V vs. NHE in an aqueous medium (pH 6.8) without an organic solvent. The reaction overpotential was 270 mV. A possible CO formation mechanism was discussed based on experiments and calculations.

Ni-modified ß-FeOOH nanorod cocatalysts for oxygen evolution utilising photoexcited holes on a N 2p level in a N-doped TiO2 electrode.

Traditionally, N-doped TiO2 (N-TiO2) has been regarded as unsuitable for the oxygen evolution reaction (OER) under visible light. Ni-modified ß-FeOOH nanorod cocatalysts enabled to use N 2p holes in the N-TiO2 photoanode induced by 400-500 nm visible light photons for the OER, enhancing anodic photocurrent of N-TiO2 with 13-fold with 100% faradaic efficiency.

Photocatalytic CO2 Reduction Using Water as an Electron Donor under Visible Light Irradiation by Z-Scheme and Photoelectrochemical Systems over (CuGa)0.5ZnS2 in the Presence of Basic Additives.

We demonstrated photocatalytic CO2 reduction using water as an electron donor under visible light irradiation by a Z-scheme photocatalyst and a photoelectrochemical cell using bare (CuGa)0.5ZnS2 prepared by a flux method as a CO2-reducing photocatalyst. The Z-scheme system employing the bare (CuGa)0.5ZnS2 photocatalyst and RGO-(CoOx/BiVO4) as an O2-evolving photocatalyst produced CO of a CO2 reduction product accompanied by H2 and O2 in a simple suspension system without any additives under visible light irradiation and 1 atm of CO2. When a basic salt (i.e., NaHCO3, NaOH, etc.) was added into the reactant solution (H2O + CO2), the CO formation rate and the CO selectivity increased. The same effect of the basic salt was observed for sacrificial CO2 reduction using SO32- as an electron donor over the bare (CuGa)0.5ZnS2 photocatalyst. The selectivity for the CO formation of the Z-schematic CO2 reduction reached 10-20% in the presence of the basic salt even in an aqueous solution and without loading any cocatalysts on the (CuGa)0.5ZnS2 metal sulfide photocatalyst. It is notable that CO was obtained accompanied by reasonable O2 evolution, indicating that water was an electron donor for the CO2 reduction. Moreover, the present Z-scheme system also showed activity for solar CO2 reduction using water as an electron donor. The bare (CuGa)0.5ZnS2 powder loaded on an FTO glass was also used as a photocathode for CO2 reduction under visible light irradiation. CO and H2 were obtained on the photocathode with 20% and 80% Faradaic efficiencies at 0.1 V vs RHE, respectively.

Solar-Driven CO2 Reduction Using a Semiconductor/Molecule Hybrid Photosystem: From Photocatalysts to a Monolithic Artificial Leaf.

The synthesis of organic chemicals from H2O and CO2 using solar energy is important for recycling CO2 through cyclical use of chemical ingredients produced from CO2 or molecular energy carriers based on CO2. Similar to photosynthesis in plants, the CO2 molecules are reduced by electrons and protons, which are extracted from H2O molecules, to produce O2. This reaction is uphill; therefore, the solar energy is stored as the chemical bonding energy in the organic molecules. This artificial photosynthetic technology mimicking green vegetation should be implemented as a self-standing system for on-site direct solar energy storage that supports CO2 recycling in a circular economy. Herein, we explain our interdisciplinary fusion methodology to develop hybrid photocatalysts and photoelectrodes for an artificial photosynthetic system for the CO2 reduction reaction (CO2RR) in aqueous solutions. The key factor for the system is the integration of uniquely different functions of molecular transition-metal complexes and solid semiconductors. A metal complex catalyst and a semiconductor appropriate for a CO2RR and visible-light absorption, respectively, are linked, and they function complementary way to catalyze CO2RR under visible-light irradiation as a particulate photocatalyst dispersion in solution. It has also been proven that Ru complexes with bipyridine ligands can catalyze a CO2RR as photocathodes when they are linked with various semiconductor surfaces, such as those of doped tantalum oxides, doped iron oxides, indium phosphides, copper-based sulfides, selenides, silicon, and others. These photocathodes can produce formate and carbon monoxide using electrons and protons extracted from water through potential-matched connections with photoanodes such as TiO2 or SrTiO3 for oxygen evolution reactions (OERs). Benefiting from the very low overpotential of an aqueous CO2RR at metal complexes approaching the theoretical lower limit, the semiconductor/molecule hybrid system demonstrates a single tablet-formed monolithic electrode called "artificial leaf." This single electrode device can generate formate (HCOO-) from H2O and CO2 in a water-filled single-compartment reactor without requiring a separation membrane under unassisted or bias-free conditions, either electrically or chemically. The reaction proceeds with a stoichiometric electron/hole ratio and stores solar energy with a solar-to-chemical energy conversion efficiency of 4.6%, which exceeds that of plants. In this Account, the key results that marked our milestones in technological progress of the semiconductor/molecule hybrid photosystem are concisely explained. These results include design, proof of the principle, and understanding of the phenomena by time-resolved spectroscopies, synchrotron radiation analyses, and DFT calculations. These results enable us to address challenges toward further scientific progress and the social implementation, including the use of earth-abundant elements and the scale-up of the solar-driven CO2RR system.

Particulate photocatalytic reactors with spectrum-splitting function for artificial photosynthesis.

We have applied spectrum splitting, which is the most reliable way for highly efficient solar energy utilization, to particulate photocatalytic reactors. We have elucidated that the spectrum splitting is feasible using plural cells/compartments, in which photocatalyst particles of different bandgaps are suspended respectively, arranged optically in series. When the particles are sufficiently small (≤20 nm in diameter), high-energy photons are absorbed in the wide-gap cell/compartment on the solar illumination side while low-energy photons reach the backside narrow-gap cell/compartment with being scarcely diffuse-reflected. We have proposed two concrete configurations of the reactors: wide-gap cell/narrow-gap Z-scheme cell (WG/Z), and wide-gap cell/two-compartment cell of middle-gap and narrow-gap (WG/MG-NG), based on the previous configuration of a two-compartment cell of wide-gap and narrow-gap (WG-NG). We have constructed a new model of the carrier supply process from the semiconductor photocatalysts to the active sites, and calculated the practical upper limits of the carrier supply rates and solar-to-chemical conversion efficiencies. The spectrum-splitting reactors can yield higher efficiencies of artificial photosynthetic H2 and CO production by up to 1.5-1.6 times than the conventional Z-scheme reactors. The newly proposed WG/Z reactor widens the room of the material developments and improves the robustness against solar spectrum variation, and hence would be a promising practical solution, although the efficiency is slightly lower than that for the ideal WG-NG reactor. The WG/MG-NG reactor yields the highest efficiency among the three configurations, with high spectral robustness.

Study of Excited States and Electron Transfer of Semiconductor-Metal-Complex Hybrid Photocatalysts for CO2 Reduction by Using Picosecond Time-Resolved Spectroscopies.

A semiconductor-metal-complex hybrid photocatalyst was previously reported for CO2 reduction; this photocatalyst is composed of nitrogen-doped Ta2 O5 as a semiconductor photosensitizer and a Ru complex as a CO2 reduction catalyst, operating under visible light (>400 nm), with high selectivity for HCOOH formation of more than 75 %. The electron transfer from a photoactive semiconductor to the metal-complex catalyst is a key process for photocatalytic CO2 reduction with hybrid photocatalysts. Herein, the excited-state dynamics of several hybrid photocatalysts are described by using time-resolved emission and infrared absorption spectroscopies to understand the mechanism of electron transfer from a semiconductor to the metal-complex catalyst. The results show that electron transfer from the semiconductor to the metal-complex catalyst does not occur directly upon photoexcitation, but that the photoexcited electron transfers to a new excited state. On the basis of the present results and previous reports, it is suggested that the excited state is a charge-transfer state located between shallow defects of the semiconductor and the metal-complex catalyst.

Electrochemical CO2 reduction over nanoparticles derived from an oxidized Cu-Ni intermetallic alloy.

Oxide-derived Cu-Ni (3-32 at%-Ni) alloy nanoparticles with a size of 10 nm enhance selectivity for ethylene and ethanol formation over oxide-derived Cu nanoparticles by electrochemical CO2 reduction. X-ray absorption spectroscopy measurements suggest that Ni (generally recognized as an element to avoid) is in a mixed phase of oxidized and metallic states.

Operando X-ray absorption spectroscopy of hyperfine ß-FeOOH nanorods modified with amorphous Ni(OH)2 under electrocatalytic water oxidation conditions.

Operando X-ray absorption spectroscopy was employed to study an active electrocatalyst, hyperfine ß-FeOOH nanorods (∅ 3 × 15 nm) surface-modified with amorphous Ni hydroxide. The nearest neighbor structure and valence of Fe3+ ions did not change under water oxidation conditions, while changes in the nearest neighbor ordering of Ni2+ ions and a reversible transition to Ni3+ were observed in accordance with the electrical bias for the reaction.

High-pressure synthesis of ε-FeOOH from ß-FeOOH and its application to the water oxidation catalyst.

Research on materials under extreme conditions such as high pressures provides new insights into the evolution and dynamics of the earth and space sciences, but recently, this research has focused on applications as functional materials. In this contribution, we examined high-pressure/high-temperature phases of ß-FeO1-x (OH)1+x Cl x with x = 0.12 (ß-FeOOH) and their catalytic activities of water oxidation, i.e., oxygen evolution reaction (OER). Under pressures above 6 GPa and temperatures of 100-700 °C, ß-FeOOH transformed into ε-FeOOH, as in the case of α-FeOOH. However, the established pressure-temperature phase diagram of ß-FeOOH differs from that of α-FeOOH, probably owing to its open framework structure and partial occupation of Cl- ions. The OER activities of ε-FeOOH strongly depended on the FeOOH sources, synthesis conditions, and composite electrodes. Nevertheless, one of the ε-FeOOH samples exhibited a low OER overpotential compared with α-FeOOH and its parent ß-FeOOH, which are widely used as OER catalysts. Hence, ε-FeOOH is a potential candidate as a next-generation earth-abundant OER catalyst.

First principles calculations of surface dependent electronic structures: a study on ß-FeOOH and γ-FeOOH.

We report a theoretical study on iron oxyhydroxide (FeOOH). The FeOOH surface is expected to act as an efficient electrochemical catalyst for the oxygen evolution reaction (OER), because it is based on iron, an element of the fourth highest Clarke number. Experimentally, the OER activity of ß-FeOOH is known to be higher than that of γ-FeOOH. However, the details of the OER mechanism and the surface reactivities of the FeOOH polymorphs have not yet been fully understood. We performed first-principles calculations of bulk and surfaces of ß-FeOOH and γ-FeOOH using density functional theory, to investigate their electronic structures and catalytic activities. The calculations suggest that depending on the surface indices, several surfaces may be favored for catalytic activities.

Molecular Catalysts Immobilized on Semiconductor Photosensitizers for Proton Reduction toward Visible-Light-Driven Overall Water Splitting.

Photocatalytic or photoelectrochemical hydrogen production by water splitting is one of the key reactions for the development of an energy supply that enables a clean energy system for a future sustainable society. Utilization of solar photon energy for the uphill water splitting reaction is a promising technology, and therefore many systems using semiconductor photocatalysts and semiconductor photoelectrodes for the reaction producing hydrogen and dioxygen in a 2:1 stoichiometric ratio have been reported. In these systems, molecular catalysts are also considered to be feasible; recently, systems based on molecular catalysts conjugated with semiconductor photosensitizers have been used for photoinduced hydrogen generation by proton reduction. Additionally, there are reports that the so-called Z-scheme (two-step photoexcitation) mechanism realizes the solar-driven uphill reaction by overall water splitting. Although the number of these reports is still small compared to those of all-inorganic systems, the advantages of molecular cocatalysts and its immobilization on a semiconductor are attractive. This Minireview provides a brief overview of approaches and recent research progress toward molecular catalysts immobilized on semiconductor photocatalysts and photoelectrodes for solar-driven hydrogen production with the stoichiometric uphill reaction of hydrogen and oxygen generation.

Solar-driven CO2 to CO reduction utilizing H2O as an electron donor by earth-abundant Mn-bipyridine complex and Ni-modified Fe-oxyhydroxide catalysts activated in a single-compartment reactor.

Photoelectrochemical CO2 to CO reduction was demonstrated with 3.4% solar-to-chemical conversion efficiency using polycrystalline silicon photovoltaic cells connected with earth-abundant catalysts: a manganese complex polymer for CO2 reduction and iron oxyhydroxide modified with a nickel compound for water oxidation. The system operated around neutral pH in a single-compartment reactor.

Z-Schematic and visible-light-driven CO2 reduction using H2O as an electron donor by a particulate mixture of a Ru-complex/(CuGa)1-xZn2xS2 hybrid catalyst, BiVO4 and an electron mediator.

Visible-light-driven Z-schematic CO2 reduction using H2O as an electron donor was achieved using a simple mixture of a metal-sulfide/molecular hybrid photocatalyst for CO2 reduction, a water oxidation photocatalyst and a redox-shuttle electron mediator. This is the first demonstration of a highly selective particulate CO2 reduction system accompanying O2 generation utilizing a semiconductor/molecular hybrid photocatalyst.

Efficient Catalytic Electrode for CO2 Reduction Realized by Physisorbing Ni(cyclam) Molecules with Hydrophobicity Based on Hansen's Theory.

An electrochemical electrode physisorbed with Ni(cyclam) complex molecules containing tetraphenylborate ions (BPh4(-)) as counteranions shows catalytic activity for the reduction reaction of CO2 to CO in an aqueous electrolyte, superior to that of an electrode physisorbed with conventional [Ni(cyclam)]Cl2 complex molecules. The BPh4(-)-containing Ni(cyclam) is inferred as having high hydrophobicity based on its Hansen solubility parameter (HSP), with an interaction sphere excluding HSPs of water in a three-dimensional vector space. The high hydrophobicity of BPh4(-)-containing Ni(cyclam) molecules inhibits their dissolution into aqueous electrolyte and retains their immobilization onto the electrode surface, which we believe to result in the improved catalytic activity of the electrode physisorbed with them. HSP analysis also provides an optimized mixing ratio of solvents dissolving BPh4(-)-containing Ni(cyclam) molecules.

Nitrogen and transition-metal codoped titania nanotube arrays for visible-light-sensitive photoelectrochemical water oxidation.

Vertically aligned titanium dioxide nanotube (TNT) arrays codoped with nitrogen and 3d transition metals were successfully fabricated using anodization and nitridation processes. The codoping of N and Fe yielded the highest visible-light-induced photoelectrochemical water oxidation due to bandgap narrowing of impurity levels by N and Fe.

Selective CO2 conversion to formate conjugated with H2O oxidation utilizing semiconductor/complex hybrid photocatalysts.

Photoelectrochemical reduction of CO(2) to HCOO(-) (formate) over p-type InP/Ru complex polymer hybrid photocatalyst was highly enhanced by introducing an anchoring complex into the polymer. By functionally combining the hybrid photocatalyst with TiO(2) for water oxidation, selective photoreduction of CO(2) to HCOO(-) was achieved in aqueous media, in which H(2)O was used as both an electron donor and a proton source. The so-called Z-scheme (or two-step photoexcitation) system operated with no external electrical bias. The selectivity for HCOO(-) production was >70%, and the conversion efficiency of solar energy to chemical energy was 0.03-0.04%.

Direct assembly synthesis of metal complex-semiconductor hybrid photocatalysts anchored by phosphonate for highly efficient CO2 reduction.

Hybrid photocatalysts consisting of a ruthenium complex and p-type photoactive N-doped Ta(2)O(5) anchored with an organic group were successfully synthesized by a direct assembly method. The photocatalyst anchored by phosphonate exhibited excellent photoconversion activity of CO(2) to formic acid under visible-light irradiation with respect to the reaction rate and stability.