Porphyrins, phthalocyanines, and related covalent-organic frameworks in the photochemical and electrochemical water splitting: A review

In this review, the possibilities of photochemical and electrochemical water splitting using porphyrins, phthalocyanines, related macrocycles and their metal complexes are discussed. Significant efforts are being carried out in order to develop novel and cheap methods for water decomposition to generate inexhaustible greener energy sources (molecular hydrogen) instead of oil and gas. The processes of water splitting in heterogeneous and homogeneous media require novel photo- and electrocatalysts, in particular those on macrocycle basis. A high water splitting efficiency has been observed for porphyrins and phthalocyanines, showing their suitability both for HER and OER processes are available using these relative compounds for WS purposes. Porphyrins can be used in photocatalytic and electrocatalytic water splitting processes both in their free forms or as metal porphyrins/porphyrinates, mainly loaded on various inorganic materials such as Au NPs, C3N4, GaP, KTaO3, GaP, etc., as part of porphyrin-containing polymers or on electrode surface as modifying coatings, being sometimes better than commercial catalysts, such as Pt/C. Photocatalytic H2 evolution can be considerably depended on the shape of porphyrin particles; in particular, porphyrin nanowires can yield up to 20 times more H2 than its powder. In certain cases, both HER and OER can be driven in the same electrolyte using metal porphyrins. The capacity of metal porphyrins to generate ROS (in particular, singlet oxygen 1O2) under ultrasonic treatment is known and can be successfully used for H2O2 and O2 generation. Phthalocyanines can be used in similar processes in free form or as composites with inorganic or organic counterparts, being immobilized on nanocarbons, metal oxides and salts, or simply dissolved in water acting in a homogeneous medium. An additional use of ultrasound can lead to better results due to the synergistic effect of the sonochemical treatment and light. Covalent Organic Frameworks are also applied for WS purposes and are mainly based on a variety of N-containing ligands, such as triazine, β-ketoenamines and β-ketoamines, bipyridine, as well as some S-containing ligands. Metal complexes in WS processes are widely represented by ruthenium coordination compounds, being suitable for future commercial artificial photosynthesis, as well by copper and cobalt coordination compounds with phenantroline, porphyrazine and related derivatives, being used as electrocatalysts in the form of homogenous molecular films on GCE electrodes or as a part of a homogeneous photocatalytic water reduction system for HER processes. The analysis of the photo(electro)catalytic activity of these macrocycles indicated that the Covalent Organic Frameworks are highly promising catalysts for H2 evolution from water splitting.