Unveiling the performance of ultrathin bimetallic CoxNi1−x(OH)2 nanosheets for pseudocapacitors and oxygen evolution reaction

The rational design of highly efficient and stable electrodes is necessary for energy storage and electrocatalysis. Herein, we developed a nanometre thin bimetallic ultrathin Co_xNi_1−x(OH)₂ nanosheet with a large lateral size by the ionic layer epitaxy (ILE) technique as an efficient bifunctional electrode material for pseudocapacitors and the oxygen evolution reaction. Its electrochemical performance was readily tuned by controlling the Co/Ni ratio. The nanosheet with a 1 : 3 Co : Ni ratio (termed Co₁Ni₃-NS) showed an excellent volumetric (areal) capacitance of 3783 F cm⁻³ (3 mF cm⁻²) at 0.3 mA cm⁻² with 336 mW h cm⁻³ energy density at 256 W cm⁻³ power density and excellent stability, substantially outperforming other monometallic and bimetallic NSs. Moreover, as an electrocatalyst, Co₁Ni₃-NS delivered a lower overpotential (η₁₀ = 318 mV) and Tafel slope (61 mV dec⁻¹) in an alkaline environment. In situ Raman spectroscopy was employed to demonstrate the dynamic structural evolution of the catalyst during the OER process. Furthermore, DFT investigations further revealed that Co₁Ni₃-NS is a promising electrode with higher quantum capacitance and lower overpotential compared to other Co/Ni ratios. These findings pave a new way for controlled synthesis of highly efficient, bimetallic, and bifunctional electrode materials for pseudocapacitors and the OER.