The electrocatalytic transformation of CO₂ into valuable chemicals or fuels represents a compelling strategy toward a carbon-neutral future. Yet, this ambition is often constrained by the persistent challenges of selectivity, energy efficiency, and long-term operational resilience. Herein, we report a meticulously engineered CuNi alloy catalyst anchored on carbon nanosheets (CuNi/CNs), in which the electronic structure is delicately tailored to amplify CO₂ adsorption and catalytic conversion. The optimized CuNi/CNs catalyst exhibits highly desired performance in the electroreduction of CO₂ to CO, achieving a Faradaic efficiency (FE_CO) exceeding 95% across a wide voltage window. Impressively, it sustains an industrially relevant current density of 268.4 mA·cm⁻² and maintains an FE_CO above 90% over 142-hours of continuous operation. In-situ spectroscopic investigations, coupled with theoretical simulations, reveal that the introduction of Cu modulates the Ni active sites, benefiting from fast interfacial electron transfer and the formation of Ni-CO intermediates, and thus suppressing competing oxygenate species. We further design an Al-CO₂ battery featuring CuNi/CNs as the cathode, which delivers a peak power density of 27.23 mW·cm⁻² at 80.58 mA·cm⁻², alongside a stable discharge at 50 mA·cm⁻² for 35 h. This work not only presents a high-efficiency and enduring electrocatalyst for CO₂ conversion, but also pioneers its integration into functional energy-storage, charting a promising avenue toward sustainable, carbon-neutral energy technologies.