Photocatalytic Systems for CO2 Reduction: Metal-Complex Photocatalysts and Their Hybrids with Photofunctional Solid Materials.

ConspectusPhotocatalytic CO2 reduction is a critical objective in the field of artificial photosynthesis because it can potentially make a total solution for global warming and shortage of energy and carbon resources. We have successfully developed various highly efficient, stable, and selective photocatalytic systems for CO2 reduction using transition metal complexes as both photosensitizers and catalysts. The molecular architectures for constructing selective and efficient photocatalytic systems for CO2 reduction are discussed herein. As a typical example, a mixed system of a ring-shaped Re(I) trinuclear complex as a photosensitizer and fac-[Re(bpy)(CO)3{OC2H4N(C2H4OH)2}] as a catalyst selectively photocatalyzed CO2 reduction to CO with the highest quantum yield of 82% and a turnover number (TON) of over 600. Not only rare and noble metals but also earth abundant ones, such as Mn(I), Cu(I), and Fe(II) can be used as central metal cations. In the case using a Cu(I) dinuclear complex as a photosensitizer and fac-Mn(bpy)(CO)3Br as a catalyst, the total formation quantum yield of CO and HCOOH from CO2 was 57% and TONCO+HCOOH exceeded 1300.Efficient supramolecular photocatalysts for CO2 reduction, in which photosensitizer and catalyst units are connected through a bridging ligand, were developed for removing a diffusion control on collisions between a photosensitizer and a catalyst. Supramolecular photocatalysts, in which [Ru(N∧N)3]2+-type photosensitizer and Re(I) or Ru(II) catalyst units are connected to each other with an alkyl chain, efficiently and selectively photocatalyzed CO2 reduction in solutions. Mechanistic studies using time-resolved IR and electrochemical measurements provided molecular architecture for constructing efficient supramolecular photocatalysts. A Ru(II)-Re(I) supramolecular photocatalyst constructed according to this molecular architecture efficiently photocatalyzed CO2 reduction even when it was fixed on solid materials. Harnessing this property of the supramolecular photocatalysts, two types of hybrid photocatalytic systems were developed, namely, photocatalysts with light-harvesting capabilities and photoelectrochemical systems for CO2 reduction.Introduction of light-harvesting capabilities into molecular photocatalytic systems should be important because the intensity of solar light shone on the earth's surface is relatively low. Periodic mesoporous organosilica, in which methyl acridone groups are embedded in the silica framework as light harvesters, was combined with a Ru(II)-Re(I) supramolecular photocatalyst with phosphonic acid anchoring groups. In this hybrid, the photons absorbed by approximately 40 methyl acridone groups were transferred to one Ru(II) photosensitizer unit, and then, the photocatalytic CO2 reduction commenced.To use water as an abundant electron donor, we developed hybrid photocatalytic systems combining metal-complex photocatalysts with semiconductor photocatalysts that display high photooxidation powers, in which two photons are sequentially absorbed by the metal-complex photosensitizer and the semiconductor, resulting in both high oxidation and reduction power. Various types of dye-sensitized molecular photocathodes comprising the p-type semiconductor electrodes and the supramolecular photocatalysts were developed. Full photoelectrochemical cells combining these dye-sensitized molecular photocathodes and n-type semiconductor photoanodes achieved CO2 reduction using only visible light as the energy source and water as the reductant. Drastic improvement of dye-sensitized molecular photocathodes is reported.The results presented in this Account clearly indicate that we can construct very efficient, selective, and durable photocatalytic systems constructed with the metal-complex photosensitizers and catalysts. The supramolecular-photocatalyst architecture in which the photosensitizer and the catalyst are connected to each other is useful especially on the surface of solid owing to rapid electron transfer from the photosensitizer to the catalyst. On basis of these findings, we successfully constructed hybrid systems of the supramolecular photocatalysts with photoactive solid materials. These hybridizations can add new functions to the metal-complex photocatalytic systems, such as water oxidation and light harvesting.

A Molecular Z-Scheme Artificial Photosynthetic System Under the Bias-Free Condition for CO2 Reduction Coupled with Two-electron Water Oxidation: Photocatalytic Production of CO/HCOOH and H2 O2.

Bio-inspired molecular-engineered systems have been extensively investigated for the half-reactions of H2 O oxidation or CO2 reduction with sacrificial electron donors/acceptors. However, there has yet to be reported a device for dye-sensitized molecular photoanodes coupled with molecular photocathodes in an aqueous solution without the use of sacrificial reagents. Herein, we will report the integration of SnIV - or AlIII -tetrapyridylporphyrin (SnTPyP or AlTPyP) decorated tin oxide particles (SnTPyP/SnO2 or AlTPyP/SnO2 ) photoanode with the dye-sensitized molecular photocathode on nickel oxide particles containing [Ru(diimine)3 ]2+ as the light-harvesting unit and [Ru(diimine)(CO)2 Cl2 ] as the catalyst unit covalently connected and fixed within poly-pyrrole layer (RuCAT-RuC2 -PolyPyr-PRu/NiO). The simultaneous irradiation of the two photoelectrodes with visible light resulted in H2 O2 on the anode and CO, HCOOH, and H2 on the cathode with high Faradaic efficiencies in purely aqueous conditions without any applied bias is the first example of artificial photosynthesis with only two-electron redox reactions.

Earth-Abundant Molecular Z-Scheme Photoelectrochemical Cell for Overall Water-Splitting.

A push-pull organic dye and a cobaloxime catalyst were successfully cografted on NiO and CuGaO2 to form efficient molecular photocathodes for H2 production with >80% Faradaic efficiency. CuGaO2 is emerging as a more effective p-type semiconductor in photoelectrochemical cells and yields a photocathode with 4-fold higher photocurrent densities and 400 mV more positive onset photocurrent potential compared to the one based on NiO. Such an optimized CuGaO2 photocathode was combined with a TaON|CoO x photoanode in a photoelectrochemical cell. Operated in this Z-scheme configuration, the two photoelectrodes produced H2 and O2 from water with 87% and 88% Faradaic efficiency, respectively, at pH 7 under visible light and in the absence of an applied bias, equating to a solar to hydrogen conversion efficiency of 5.4 × 10-3%. This is, to the best of our knowledge, the highest efficiency reported so far for a molecular-based noble metal-free water splitting Z-scheme.

Solar-Driven Photoelectrochemical Water Oxidation over an n-Type Lead-Titanium Oxyfluoride Anode.

Mixed-anion compounds (e.g., oxynitrides and oxysulfides) are potential candidates as photoanodes for visible-light water oxidation, but most of them suffer from oxidative degradation by photogenerated holes, leading to low stability. Here we show an exceptional example of a stable, mixed-anion water-oxidation photoanode that consists of an oxyfluoride, Pb2Ti2O5.4F1.2, having a band gap of ca. 2.4 eV. Pb2Ti2O5.4F1.2 particles, which were coated on a transparent conductive glass (FTO) support and were subject to postdeposition of a TiO2 overlayer, generated an anodic photocurrent upon band gap photoexcitation of Pb2Ti2O5.4F1.2 (λ <520 nm) with a rather negative photocurrent onset potential of ca. -0.6 V vs NHE, which was independent of the pH of the electrolyte solution. Stable photoanodic current was observed even without loading a water oxidation promoter such as CoOx. Nevertheless, loading CoOx onto the TiO2/Pb2Ti2O5.4F1.2/FTO electrode further improved the anodic photoresponse by a factor of 2-3. Under AM1.5G simulated sunlight (100 mW cm-2), stable water oxidation to form O2 was achieved using the optimized Pb2Ti2O5.4F1.2 photoanode in the presence of an applied potential smaller than 1.23 V, giving a Faradaic efficiency of 93% and almost no sign of deactivation during 4 h of operation. This study presents the first example of photoelectrochemical water splitting driven by visible-light excitation of an oxyfluoride that stably works, even without a water oxidation promoter, which is distinct from ordinary mixed-anion photoanodes that usually require a water oxidation promoter.

Photoelectrochemical Reduction of CO2 Coupled to Water Oxidation Using a Photocathode with a Ru(II)-Re(I) Complex Photocatalyst and a CoOx/TaON Photoanode.

Photoelectrochemical CO2 reduction activity of a hybrid photocathode, based on a Ru(II)-Re(I) supramolecular metal complex photocatalyst immobilized on a NiO electrode (NiO-RuRe), was confirmed in an aqueous electrolyte solution. Under half-reaction conditions, the NiO-RuRe photocathode generated CO with high selectivity, and its turnover number for CO formation reached 32 based on the amount of immobilized RuRe. A photoelectrochemical cell comprising a NiO-RuRe photocathode and a CoOx/TaON photoanode showed activity for visible-light-driven CO2 reduction using water as a reductant to generate CO and O2, with the assistance of an external electrical (0.3 V) and chemical (0.10 V) bias produced by a pH difference. This is the first example of a molecular and semiconductor photocatalyst hybrid-constructed photoelectrochemical cell for visible-light-driven CO2 reduction using water as a reductant.

A Photoelectrochemical Solar Cell Consisting of a Cadmium Sulfide Photoanode and a Ruthenium-2,2'-Bipyridine Redox Shuttle in a Non-aqueous Electrolyte.

A photoelectrochemical (PEC) cell consisting of an n-type CdS single-crystal electrode and a Pt counter electrode with the ruthenium-2,2'-bipyridine complex [Ru(bpy)3](2+/3+) as the redox shuttle in a non-aqueous electrolyte was studied to obtain a higher open-circuit voltage (V(OC)) than the onset voltage for water splitting. A V(OC) of 1.48 V and a short-circuit current (I(SC)) of 3.88 mA cm(-2) were obtained under irradiation by a 300 W Xe lamp with 420-800 nm visible light. This relatively high voltage was presumably due to the difference between the Fermi level of photo-irradiated n-type CdS and the redox potential of the Ru complex at the Pt electrode. The smooth redox reaction of the Ru complex with one-electron transfer was thought to have contributed to the high V(OC) and I(SC). The obtained V(OC) was more than the onset voltage of water electrolysis for hydrogen and oxygen generation, suggesting prospects for application in water electrolysis.

Precise analyses of photoelectrochemical reactions on particulate Zn0.25Cd0.75Se photoanodes in nonaqueous electrolytes using Ru bipyridyl complexes as a probe.

Recombination of photoexcited carriers at interface states is generally believed to strongly govern the photoelectrochemical (PEC) performance of semiconductors in electrolytes. Sacrificial reagents (e.g., methanol or Na2SO3) are often used to assess the ideal PEC performance of photoanodes in cases of minimised interfacial recombination kinetics as well as accelerated surface reaction kinetics. However, varying the sacrificial reagents in the electrolyte means simultaneously changing the equilibrium potential and the number of electrons required to perform the sacrificial reaction, and thus the thermodynamic and kinetic aspects of the PEC reactions cannot be distinguished. In the present study, we propose an alternative methodology to experimentally evaluate the energy levels of interfacial recombination centres that can reduce PEC performance. We prepare nonaqueous electrolytes containing three different Ru complexes with different bipyridyl ligands; redox reactions of Ru complexes represent one-electron processes with similar charge transfer rates and diffusion coefficients. Therefore, the Ru complexes can serve as a probe to isolate and evaluate only the thermodynamic aspects of PEC reactions. Recombination centres at the interface between a nonaqueous electrolyte and a Zn0.25Cd0.75Se particulate photoanode are elucidated using this method as a model case. The energy level at which photocorrosion proceeds is also determined.

Stable hydrogen evolution from CdS-modified CuGaSe2 photoelectrode under visible-light irradiation.

The photoelectrochemical properties of CuGaSe2 modified by deposition of a thin CdS layer were investigated. The CdS layer formed a p-n junction on the surface of the electrode, improving its photoelectrochemical properties. There was an optimal CdS thickness because of the balance between the charge separation effect and light absorption by CdS. CdS-deposited CuGaSe2 showed high stability under the observed reaction conditions and evolved hydrogen continuously for more than 10 days.

Supramolecular photocatalysts fixed on the inside of the polypyrrole layer in dye sensitized molecular photocathodes: application to photocatalytic CO2 reduction coupled with water oxidation.

The development of systems for photocatalytic CO2 reduction with water as a reductant and solar light as an energy source is one of the most important milestones on the way to artificial photosynthesis. Although such reduction can be performed using dye-sensitized molecular photocathodes comprising metal complexes as redox photosensitizers and catalyst units fixed on a p-type semiconductor electrode, the performance of the corresponding photoelectrochemical cells remains low, e.g., their highest incident photon-to-current conversion efficiency (IPCE) equals 1.2%. Herein, we report a novel dye-sensitized molecular photocathode for photocatalytic CO2 reduction in water featuring a polypyrrole layer, [Ru(diimine)3]2+ as a redox photosensitizer unit, and Ru(diimine)(CO)2Cl2 as the catalyst unit and reveal that the incorporation of the polypyrrole network significantly improves reactivity and durability relative to those of previously reported dye-sensitized molecular photocathodes. The irradiation of the novel photocathode with visible light under low applied bias stably induces the photocatalytic reduction of CO2 to CO and HCOOH with high faradaic efficiency and selectivity (even in aqueous solution), and the highest IPCE is determined as 4.7%. The novel photocathode is coupled with n-type semiconductor photoanodes (CoO x /BiVO4 and RhO x /TaON) to construct full cells that photocatalytically reduce CO2 using water as the reductant upon visible light irradiation as the only energy input at zero bias. The artificial Z-scheme photoelectrochemical cell with the dye-sensitized molecular photocathode achieves the highest energy conversion efficiency of 8.3 × 10-2% under the irradiation of both electrodes with visible light, while a solar to chemical conversion efficiency of 4.2 × 10-2% is achieved for a tandem-type cell using a solar light simulator (AM 1.5, 100 mW cm-2).

Photoelectrochemical CO2 Reduction Using a Ru(II)-Re(I) Supramolecular Photocatalyst Connected to a Vinyl Polymer on a NiO Electrode.

A Ru(II)-Re(I) supramolecular photocatalyst and a Ru(II) redox photosensitizer were both deposited successfully on a NiO electrode by using methyl phosphonic acid anchoring groups and the electrochemical polymerization of the ligand vinyl groups of the complexes. This new molecular photocathode, poly-RuRe/NiO, adsorbed a larger amount of the metal complexes compared to one using only methyl phosphonic acid anchor groups, and the stability of the complexes on the NiO electrode were much improved. The poly-RuRe/NiO acted as a photocathode for the photocatalytic reduction of CO2 at E = -0.7 V vs Ag/AgCl under visible-light irradiation in an aqueous solution. The poly-RuRe/NiO produced approximately 2.5 times more CO, and its total Faradaic efficiency of the reduction products improved from 57 to 85%.

Electrocatalytic reduction of low concentration CO2.

Utilization of low concentration CO2 contained in the exhaust gases from various industries and thermal power stations without the need for energy-consuming concentration processes should be an important technology for solving global warming and the shortage of fossil resources. Here we report the direct electrocatalytic reduction of low concentration CO2 by a Re(i)-complex catalyst that possesses CO2-capturing ability in the presence of triethanolamine. The reaction rate and faradaic efficiency of CO2 reduction were almost the same when using Ar gas containing 10% CO2 or when using pure CO2 gas, and the selectivity of CO formation was very high (98% at 10% CO2). At a concentration of 1% CO2, the Re(i) complex still behaved as a good electrocatalyst; 94% selectivity of CO formation and 85% faradaic efficiency were achieved, and the rate of CO formation was 67% compared to that when using pure CO2 gas. The electrocatalysis was due to the efficient insertion of CO2 into the Re(i)-O bond in fac-[Re(dmb)(CO)3{OC2H4N(C2H4OH)2}] (dmb = 4,4'-dimethyl-2,2'-bipyridine).

Hybrid photocathode consisting of a CuGaO2 p-type semiconductor and a Ru(ii)-Re(i) supramolecular photocatalyst: non-biased visible-light-driven CO2 reduction with water oxidation.

A CuGaO2 p-type semiconductor electrode was successfully employed for constructing a new hybrid photocathode with a Ru(ii)-Re(i) supramolecular photocatalyst (RuRe/CuGaO2). The RuRe/CuGaO2 photocathode displayed photoelectrochemical activity for the conversion of CO2 to CO in an aqueous electrolyte solution with a positive onset potential of +0.3 V vs. Ag/AgCl, which is 0.4 V more positive in comparison to a previously reported hybrid photocathode that used a NiO electrode instead of CuGaO2. A photoelectrochemical cell comprising this RuRe/CuGaO2 photocathode and a CoO x /TaON photoanode enabled the visible-light-driven catalytic reduction of CO2 using water as a reductant to give CO and O2 without applying any external bias. This is the first self-driven photoelectrochemical cell constructed with the molecular photocatalyst to achieve the reduction of CO2 by only using visible light as the energy source and water as a reductant.

A Z-scheme photocatalyst constructed with an yttrium-tantalum oxynitride and a binuclear Ru(ii) complex for visible-light CO2 reduction.

An yttrium-tantalum oxynitride having a band gap of 2.1 eV (absorbing visible light at <580 nm) was applicable as a semiconductor component of a Z-scheme CO2 reduction system operable under visible light, in combination with a binuclear Ru(ii) complex that has strong absorption in the visible region (<600 nm). Excitation of this system with visible light under a CO2 atmosphere induced photocatalytic formation of formic acid with very high selectivity (>99%).