This article presents a novel nanoscale nickel oxide/nickel (NiO/Ni) heterostructure attached to carbon nanotubes (CNTs) as an efficient and stable electrocatalyst for the hydrogen evolution reaction (HER). The structure is formed by the thermal decomposition of nickel hydroxide precursors on oxidized CNTs, resulting in a partial reduction of nickel and a heterostructure with NiO and Ni. This heterostructure exhibits HER catalytic activity comparable to platinum (Pt), a commonly used but expensive catalyst. The NiO/Ni-CNT catalyst achieves a current density of 10 mA cm⁻² at an overpotential of less than 100 mV in alkaline solutions, and a high-performance electrolyzer using this catalyst achieves a current density of ~20 mA cm⁻² at 1.5 V. The catalyst is cost-effective, non-precious, and environmentally friendly, making it a promising alternative to Pt-based catalysts for water splitting applications. The NiO/Ni heterostructure is synthesized by low-temperature hydrolysis of nickel salts onto oxidized CNTs, followed by low-pressure thermal annealing. The resulting structure consists of NiO nanoparticles on a Ni core, with a core-shell-like morphology. The NiO/Ni interface is believed to be synergistically active for HER, with NiO facilitating the adsorption of OH⁻ and Ni promoting H adsorption. The catalyst's performance is attributed to the nanoscale NiO/Ni interfaces, which enhance the catalytic activity and stability. The NiO/Ni-CNT catalyst is used in a water electrolyzer with a NiFe layered double hydroxide (NiFe LDH) as the oxygen evolution catalyst. The electrolyzer operates at a low voltage of ~1.5 V and can be powered by a single-cell alkaline battery. At higher temperatures (~60°C), the electrolyzer achieves a lower voltage of ~1.42 V at 20 mA cm⁻² and a higher current density of 100 mA cm⁻² at ~1.45 V, demonstrating improved kinetics and thermodynamics. The study highlights the importance of substrate-precursor interactions in controlling the morphology and catalytic activity of materials. The NiO/Ni heterostructure is a promising candidate for future water-splitting devices due to its high activity, stability, and cost-effectiveness. The research provides insights into the design of efficient and sustainable electrocatalysts for hydrogen production.This article presents a novel nanoscale nickel oxide/nickel (NiO/Ni) heterostructure attached to carbon nanotubes (CNTs) as an efficient and stable electrocatalyst for the hydrogen evolution reaction (HER). The structure is formed by the thermal decomposition of nickel hydroxide precursors on oxidized CNTs, resulting in a partial reduction of nickel and a heterostructure with NiO and Ni. This heterostructure exhibits HER catalytic activity comparable to platinum (Pt), a commonly used but expensive catalyst. The NiO/Ni-CNT catalyst achieves a current density of 10 mA cm⁻² at an overpotential of less than 100 mV in alkaline solutions, and a high-performance electrolyzer using this catalyst achieves a current density of ~20 mA cm⁻² at 1.5 V. The catalyst is cost-effective, non-precious, and environmentally friendly, making it a promising alternative to Pt-based catalysts for water splitting applications. The NiO/Ni heterostructure is synthesized by low-temperature hydrolysis of nickel salts onto oxidized CNTs, followed by low-pressure thermal annealing. The resulting structure consists of NiO nanoparticles on a Ni core, with a core-shell-like morphology. The NiO/Ni interface is believed to be synergistically active for HER, with NiO facilitating the adsorption of OH⁻ and Ni promoting H adsorption. The catalyst's performance is attributed to the nanoscale NiO/Ni interfaces, which enhance the catalytic activity and stability. The NiO/Ni-CNT catalyst is used in a water electrolyzer with a NiFe layered double hydroxide (NiFe LDH) as the oxygen evolution catalyst. The electrolyzer operates at a low voltage of ~1.5 V and can be powered by a single-cell alkaline battery. At higher temperatures (~60°C), the electrolyzer achieves a lower voltage of ~1.42 V at 20 mA cm⁻² and a higher current density of 100 mA cm⁻² at ~1.45 V, demonstrating improved kinetics and thermodynamics. The study highlights the importance of substrate-precursor interactions in controlling the morphology and catalytic activity of materials. The NiO/Ni heterostructure is a promising candidate for future water-splitting devices due to its high activity, stability, and cost-effectiveness. The research provides insights into the design of efficient and sustainable electrocatalysts for hydrogen production.