Non-precious bifunctional electrocatalysts with high water splitting activity are of paramount importance in reducing energy losses in alkaline environments. Herein, we report NiCo–NiCoO₂ heterostructures embedded in N-doped carbon (NiCo–NiCoO₂@NC) as an efficient bifunctional catalyst for overall water splitting in alkaline media. Partially reduced nickel and cobalt interfaced with nickel cobalt oxide result from thermal decomposition of nickel cobalt double hydroxide precursors encapsulated in high surface area carbon. Bimetallic thin film NiCo–NiCoO₂@NC exhibits a synergy of two active metals – Ni and Co, achieving remarkably small onset potentials of 0 and 200 mV, and overpotentials of 94 and 318 mV at 10 mA cm⁻² for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, and achieving TOFs at η = 500 mV more than 2-fold of Pt/C in the HER and IrO₂ in the OER. Momentously, a water electrolyzer that achieves 20 mA cm⁻² at a voltage of 1.44 V is fabricated by utilizing cheap non-precious NiCo–NiCoO₂@NC as anode and cathode catalysts simultaneously. Thanks to the protective carbon coating and the mutual aid of Ni–Co alloy and NiCoO₂, NiCo–NiCoO₂@NC maintains 20 mA cm⁻² for at least 50 h in alkaline media without trivial activity losses. The prominent synergetic and structural effects have been extensively discussed to shed light on the overall water splitting electrocatalyst design.