Herein, we employ a partial phase conversion strategy to transform cobalt/nickel carbonate hydroxide (Co_xNi_yCH) nanosheet arrays in an Fe-containing KOH electrolyte. The optimized sample exhibits remarkable electrocatalytic activity (η₅₀ = 256 mV, 51 mV dec⁻¹) and good stability (18 h) with negligible morphology change in a 1 M KOH solution, outperforming most reported cobalt–nickel-based electrocatalysts. From our research, it is found that the Fe impurity in the KOH electrolyte plays an important role in achieving high OER performance. The heterostructure caused by the partial phase transformation of Co_xNi_yCH can enhance the conductivity and stability along with providing more effective active sites.