Redox-active metal-organic frameworks for energy conversion and storage

Metal-organic frameworks (MOFs) are hybrid solids formed of organic and inorganic building blocks. While the nature of electron addition, removal, and transport is well known in organic and inorganic crystals, the behaviour of hybrid materials is poorly understood in comparison. We review progress over the past 5 years in the study of electroactive MOFs with redox activity promoted by different strategies: (i) redox-active metals; (ii) redox-active organic linkers; (iii) host-guest interactions; and (iv) charge-transfer frameworks. The properties and performance of materials are analysed with respect to emerging application areas including electrochemical energy storage (batteries and supercapacitors) and photo-/electrochemical reactions (solar cells, fuels and electrocatalysis). We further highlight the development of mixed-valence MOFs, which have been found to give rise to unprecedented charge transport in semiconducting and metallic hybrid frameworks.