Cobalt based functional inorganic materials: Electrocatalytic water oxidation

Abstract

Water splitting is considered to be one of the most promising sources of sustainable energy, as it can produce hydrogen ( $$\hbox {H}_{2}$$ ) fuel. To have successful water splitting in a sustained manner, it is necessary to develop efficient and robust catalysts that can perform water oxidation, the bottleneck process of water splitting either electrochemically or photochemically. Here, we have presented a brief descriptive analysis of different aspects of designing such catalysts in connection with our recent works on the same field. The focus of the article is to discuss contemporary works in the field of designing cobalt-based heterogeneous water oxidation electrocatalysts. To the best of our knowledge, although cobalt is the most extensively studied 1st row transition metal for water oxidation catalysis reaction, no such report has been found where the simplest cobalt complex, [ $$\hbox {Co(H}_{2}\hbox {O})_{6}]^{2+}$$ , has been employed as a water oxidation catalyst. Not only that, reports of cobalt-based simple and small molecular catalysts are also not very frequent. With the help of our recent works, we have tried to detail here a wide aspect of the study on cobalt-based simple and small molecular catalysts, starting from the reasons behind the scarcity of such water oxidation catalysts, to development of new ideas addressing the challenges in utilization of such small cobalt complexes for water oxidation catalyst. Here, we have addressed the scope of encapsulation chemistry in designing robust and efficient heterogeneous water oxidation catalysts using cobalt-based small molecular guest species. With the help of structural insight, gained from the recent results, we published in the field of water oxidation catalysis; here, we try to formulate a general approach that can help to prepare water oxidation catalyst based on host-guest chemistry. The article critically evaluates our recent results in connection with the approach of addressing the problem. SYNOPSIS An efficient electrocatalytic water oxidation can be achieved by cobalt(II)-based functional materials that comprise of a mononuclear Co(II)-aqua coordination complex [ $$\hbox {Co}(\hbox {H}_{2}\hbox {O})_{4}\hbox {(DMF)}_{2}]^{2+}$$ , included in the confined space of a metal organic framework (MOF) material, zeolite-Y-supported $$\upalpha $$ - $$\hbox {Co(OH)}_{2}$$ nano-film and ZIF-8 MOF-encapsulated cobalt(II)-centred Keggin-type polyoxometalate. The relevant kinetic analyses have been discussed in details.