Monolayer Iron and Iron-Rich Hydroxide Nanosheets Exfoliated from High-Quality Green Rust for Enhanced Electrocatalytic Oxygen Evolution Reaction

Ultrathin nanosheets of Ni-, Co-, and Fe-based (oxy)hydroxides exhibit promising catalytic activity for the oxygen evolution reaction (OER) in water electrolysis under alkaline conditions. It has been revealed that Fe may play a crucial role in the catalytic process of Ni- or Co-based catalysts. However, it lacks effective methods to prepare and study pure Fe hydroxide nanosheets. In the present work, we report a topochemical synthesis of mixed-valent Fe²⁺-Fe³⁺ layered double hydroxides (LDHs), i.e., Green Rust (GR), featured with much higher quality and crystallinity compared to the traditionally synthesized ones. Monolayer Fe²⁺-Fe³⁺ LDH nanosheets with a thickness of ∼0.8 nm were derived from the GR product. In addition, a series of Ni-bearing Fe-rich LDH nanosheets were successfully prepared by the same method. The OER catalytic performance of the obtained Fe-rich LDH nanosheets achieved a small overpotential with η = 290 mV@10 mA cm^-2 in 1 M KOH, which either outperforms or is comparable to the most active Ni-Fe LDH catalysts. The monolayer Fe²⁺-Fe³⁺ LDH nanosheets may be ideal for exploring the fundamental physicochemical properties of iron hydroxides from a molecular-scale perspective as well as serving as a building block for the assembly with other functional materials to obtain hybrid nanocatalysts.