Molecular catalysis plays an essential role in both natural and artificial photosynthesis (AP). However, the field of molecular catalysis for AP has gradually declined in recent years because of doubt about the long-term stability of molecular-catalyst-based devices. This review summarizes the development history of molecular-catalyst-based AP, including the fundamentals of AP, molecular catalysts for water oxidation, proton reduction and CO2 reduction, and molecular-catalyst-based AP devices, and it provides an analysis of the advantages, challenges, and stability of molecular catalysts. With this review, we aim to highlight the following points: (i) an investigation on molecular catalysis is one of the most promising ways to obtain atom-efficient catalysts with outstanding intrinsic activities; (ii) effective heterogenization of molecular catalysts is currently the primary challenge for the application of molecular catalysis in AP devices; (iii) development of molecular catalysts is a promising way to solve the problems of catalysis involved in practical solar fuel production. In molecular-catalysis-based AP, much has been attained, but more challenges remain with regard to long-term stability and heterogenization techniques.

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Artificial photosynthesis: opportunities and challenges of molecular catalysts

A new family of tetra-anionic tetradentate amidate ligands, N1,N1′-(1,2-phenylene)bis(N2-methyloxalamide) (H4L1), and its derivatives containing electron-donating groups at the aromatic ring have been prepared and characterized, together with their corresponding anionic Cu(II) complexes, [(LY)Cu]2–. At pH 11.5, the latter undergoes a reversible metal-based III/II oxidation process at 0.56 V and a ligand-based pH-dependent electron-transfer process at 1.25 V, associated with a large electrocatalytic water oxidation wave (overpotential of 700 mV). Foot-of-the-wave analysis gives a catalytic rate constant of 3.6 s–1 at pH 11.5 and 12 s–1 at pH 12.5. As the electron-donating capacity at the aromatic ring increases, the overpotential is drastically reduced down to a record low of 170 mV. In addition, DFT calculations allow us to propose a complete catalytic cycle that uncovers an unprecedented pathway in which crucial O–O bond formation occurs in a two-step, one-electron process where the peroxo intermediate g...

Redox Non-innocent Ligand Controls Water Oxidation Overpotential in a New Family of Mononuclear Cu-Based Efficient Catalysts.

Developing anodic oxygen evolution reaction (OER) electrocatalysts with high catalytic activities is of great importance for effective water splitting. Compared with the water-oxidation electrocatalysts that are commonly utilized in alkaline conditions, the ones operating efficiently under neutral or near neutral conditions are more environmentally friendly with less corrosion issues. This review starts with a brief introduction of OER, the importance of OER in mild-pH media, as well as the fundamentals and performance parameters of OER electrocatalysts. Then, recent progress of the rational design of electrocatalysts for OER in mild-pH conditions is discussed. The chemical structures or components, synthetic approaches, and catalytic performances of the OER catalysts will be reviewed. Some interesting insights into the catalytic mechanism are also included and discussed. It concludes with a brief outlook on the possible remaining challenges and future trends of neutral or near-neutral OER electrocatalysts. It hopefully provides the readers with a distinct perspective of the history, present, and future of OER electrocatalysts at mild conditions.

Recent Advances in the Development of Water Oxidation Electrocatalysts at Mild pH

A dinuclear cobalt complex [Co2(OH)L1](ClO4)3 (1, L1=N[(CH2)2NHCH2(m-C6H4)CH2NH(CH2)2]3N) displays high selectivity and efficiency for the photocatalytic reduction of CO2 to CO in CH3CN/H2O (v/v=4:1) under a 450 nm LED light irradiation, with a light intensity of 100 mW cm−2. The selectivity reaches as high as 98 %, and the turnover numbers (TON) and turnover frequencies (TOF) reach as high as 16896 and 0.47 s−1, respectively, with the calculated quantum yield of 0.04 %. Such high activity can be attributed to the synergistic catalysis effect between two CoII ions within 1, which is strongly supported by the results of control experiments and DFT calculations.

A Dinuclear Cobalt Cryptate as a Homogeneous Photocatalyst for Highly Selective and Efficient Visible‑Light Driven CO 2 Reduction to CO in CH 3 CN/H 2 O Solution

The water-soluble cationic nickel(II) complex of meso-tetrakis(4-N-methylpyridyl)porphyrin (1) can electrocatalyze water oxidation to O2 in neutral aqueous solution (pH 7.0) with the onset of the catalytic wave appearing at ∼1.0 V (vs NHE). The homogeneous catalysis with 1 was verified. Catalyst 1 exhibited water oxidation activity in a pH range 2.0–8.0 and had a strict linear dependence of catalytic current on its concentration. After 10 h of constant potential electrolysis at 1.32 V (vs NHE), a negligible difference of the solution was observed by UV–vis. In addition, inspection of the working electrode by electrochemistry, scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDX) showed no sign of deposition of NiOx films. These results strongly argued that 1 is a real molecular electrocatalyst for water oxidation. The turnover frequency (TOF) for this process was 0.67 s–1 at 20 °C. On the basis of results from the kinetic isotope effect (KIE) and inhibition experiments, electr...

Electrocatalytic Water Oxidation by a Water-Soluble Nickel Porphyrin Complex at Neutral pH with Low Overpotential

Electrocatalytic water oxidation using the oxidatively robust 2,7-[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine ligand (BPMAN)-based dinuclear copper(II) complex, [Cu2(BPMAN)(μ-OH)](3+), has been investigated. This catalyst exhibits high reactivity and stability towards water oxidation in neutral aqueous solutions. DFT calculations suggest that the O-O bond formation takes place by an intramolecular direct coupling mechanism rather than by a nucleophilic attack of water on the high-oxidation-state Cu(IV)=O moiety.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.201411625

Electrocatalytic water oxidation by a dinuclear copper complex in a neutral aqueous solution.

Sustainable preparation of cellulose nanocrystals: state of the art and perspectives



In recent years, flexible all-solid-state supercapacitors have been widely used as the energy storage devices for various smart and wearable electronic devices. However, the design and fabrication of high-performance flexible supercapacitor electrodes is still challenging since most of the active materials used for supercapacitor electrodes lack the ability to form flexible and mechanically stable structures. Recently, cellulose nanomaterials (mainly include cellulose nanocrystals and cellulose nanofibrils) have gained extensive interests due to their large specific surface areas, versatile surface chemistry, high mechanical strength, and the ability to form mechanically stable structures (e.g., films, aerogels). These days, the design of flexible supercapacitor electrodes by combining cellulose nanomaterials with different active materials gradually attracted the attention of scholars. The main objective of this review is to give an overview of recent developments in the preparation of cellulose nanomaterials based flexible all-solid-state supercapacitor electrodes. The fabrication approach, structure characterization, and electrochemical performance of the invented cellulose nanomaterials based flexible supercapacitor are elaborated. Also, the current challenges and future outlook for the design and fabrication of cellulose nanomaterials based flexible all-solid-state supercapacitor are proposed.


Meng Gao

Papers

Electrocatalytic water oxidation by a dinuclear copper complex in a neutral aqueous solution.

Sustainable preparation of cellulose nanocrystals: state of the art and perspectives

Cellulose Nanomaterials Based Flexible Electrodes for All-Solid-State Supercapacitors