Enhanced performance of molecular electrocatalysts for CO2 reduction in a flow cell following K+ addition

Electrocatalytic CO2 reduction is a key aspect of artificial photosynthesis systems designed to produce fuels. Although some molecular catalysts have good performance for CO2 reduction, these compounds also suffer from poor durability and energy efficiency. The present work demonstrates the improved CO2 reduction activity exhibited by molecular catalysts in a flow cell. These catalysts were composed of a cobalt-tetrapyridino-porphyrazine complex supported on carbon black together with potassium salt and were both stable and efficient. These systems were found to promote electrocatalytic CO2 reduction with a current density of 100 mA/cm2 and generated CO over at least 1 week with a selectivity of approximately 95%. The optimal catalyst gave a turnover number of 3,800,000 and an energy conversion efficiency of more than 62% even at 200 mA/cm2.

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.

Ni-modified β-FeOOH nanorod cocatalysts for oxygen evolution utilising photoexcited holes on N2p level in N-doped TiO2 electrode

Traditionally, N-doped TiO2 (N-TiO2) has been regarded as unsuitable for oxygen evolution reaction (OER) under visible light. Ni-modified β-FeOOH nanorod cocatalysts enabled to use N2p holes in N-TiO2 photoanode induced...

Operando X-ray absorption spectroscopy of hyperfine β-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 β-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 β-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 β-FeO_1−x(OH)_1+xCl_x with x = 0.12 (β-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, β-FeOOH transformed into ε-FeOOH, as in the case of α-FeOOH. However, the established pressure–temperature phase diagram of β-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 β-FeOOH, which are widely used as OER catalysts. Hence, ε-FeOOH is a potential candidate as a next-generation earth-abundant OER 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.

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

Solar-driven CO₂ reduction utilizing earth-abundant catalysts of a Mn-complex and Ni-modified β-FeOOH was demonstrated in a single-compartment reactor.