Porphyrin and macrocycle derivatives for electrochemical water splitting
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oduction Water splitting into its elemental units, H2 and O2, is a potential strategy for producing hydrogen to realize a hydrogen-based society and solve current energy problems. For decades, the development of catalytic water splitting has been hampered by the fact that effective catalysts for splitting water are based on expensive and rare noble metals. Current research efforts on alternative water-splitting electrocatalysts suggest replacing noble metals by transition metals. Because these transition metals are not chemically stable in alkaline and acidic aqueous media under open-circuit conditions, an alternative solution is to use molecular complexes.1,2 Alternative catalysts based on molecular complexes have been reported as cathodic or anodic materials for water electrolysis. Numerous complexes containing transition-metallic electroactive centers and macrocyclic ligands have been reported as electrocatalysts for splitting water.3 Porphyrin and macrocycle derivative ligands, having a regulatable functional group and large π-conjugated electron system, can strongly bind metal ions through four N atoms.4,5 The resulting four-coordinate square-planar geometry of the metal center is rigid and stable. This feature gives metal porphyrin and macrocycle derivatives sufficient stability in both acidic and alkaline solutions.6,7 In addition, negatively charged porphyrin and macrocycle derivative molecules are effective in stabilizing positively charged
metal ions.8,9 The catalytic activity of these metalloporphyrins is influenced not only by the intrinsic properties of the central metals, but also by the chemical environment of their active centers. All of these above factors make porphyrin and macrocycle derivatives attractive for water electrolysis.10 These molecular catalysts can be used either dissolved in the electrolyte (homogeneous phase) or functionalized on the surface of a solid, but electrochemically inert, supporting electrode (heterogeneous phase) for applications. In the following, we review recent advances in porphyrin and macrocycle derivative electrocatalysts for splitting water, focusing on substituent effects and structures of these molecular catalysts, and summarizing the basic principles of designing molecular catalysts for electrocatalytic water splitting.
Homogeneous macrocycle molecule catalysts Homogeneous electrocatalysts have the potential to efficiently split water, owing to the possibility of predicting the mechanistic pathways as well as the high reproducibility of the complexes and results.11 As previously mentioned, porphyrin and macrocycle derivatives have the advantages of a stable structure and high designability;12,13 therefore, they have become one of the most important homogeneous electrolysis water catalysts. More importantly, porphyrins can interact strongly with most metals through four N atoms to form corresponding
Qi Li, The Key Laboratory for Special Functional Materials of MOE, School of Materials Science and Engineering, National and Local Joint Engineering Research Cente
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