Spinel nano-ferrites as low-cost (photo)electrocatalysts with unique electronic properties in solar energy conversion systems

Spinel nano-ferrites (MFe₂O₄, M: Co²⁺, Ni²⁺, …) are excellent low-cost candidates as (photo) electrocatalysts in solar-to-hydrogen energy conversion systems owing to their high dual electrochemical and photochemical activities. Their crystal structure consists of alternating tetrahedral and octahedral units that accommodate Fe³⁺/Fe²⁺ ions and another divalent ion in their interstices that give them unique electronic properties for (photo) electrocatalysis. Herein, the role of nano-ferrites as oxygen evolution reaction (OER) catalyst and counter-electrode catalyst in photovoltaic-electrolysis (PV-E) and photoelectrochemical (PEC) water splitting systems are highlighted. In particular, the recent advances on OER (photo) electrocatalysts based on spinel nano-ferrites and their design strategies such as doping, oxygen/cation defect engineering, as well as supporting and compositing with other materials are discussed and summarized. Finally, their advantages and performance limitations were presented, and future perspectives towards their improvement are suggested. Overall, this report is a critical review of the performance of spinel nano-ferrites as truly low-cost candidate catalysts with a wide natural abundance and easy preparation toward clean hydrogen generation.