Alkaline decoupled electrolysis is an emerging safest approach for efficient green hydrogen production. Bifunctional electrocatalysts subjected to reverse polarity can be effectively used as the gas-evolving electrodes (GEE) in decoupled water electrolysis for generating both hydrogen and oxygen at different rates and at different times. Designing a bifunctional electrocatalyst with excellent activity and durability will successfully pave the way towards non-precious metal-based decoupled electrolysis systems. Here, for the first time, ternary NiFeMn layered hydroxide is presented as the bifunctional electrocatalyst associated with the β-Ni(OH)2 solid-state redox mediator (SRM) for independently generating hydrogen and oxygen in an alkaline solution without membranes. This electrocatalyst had shown low overpotentials in HER (230 mV at 10 mA/cm2) and OER (270 mV at 50 mA/cm2). In this work, a chemically synthesized hexagonal β-Ni(OH)2 nanosheet was employed as the solid-state redox mediator for fabricating the membrane-free electrolysis setup. Accompanying a bifunctional electrocatalyst in decoupled electrolysis, this device displayed a 1.91 V cell voltage at 10 mA/cm2. When considered together, the complexity and expense of the electrolyzer were decreased by using NiFeMn LH/NF as a bifunctional electrocatalyst. This approach can be considered for constructing a cost-effective and step-wise green hydrogen harvesting system of membrane-free water electrolyzer.