The study reports the development of nanoscale nickel oxide/nickel (NiO/Ni) heterostructures on carbon nanotube (CNT) sidewalls as highly effective electrocatalysts for hydrogen evolution reaction (HER). These heterostructures exhibit activity comparable to platinum, a key material for water splitting in hydrogen production. The NiO/Ni-CNT catalyst is synthesized through a low-temperature hydrolysis of nickel salts, followed by thermal annealing, which results in a partially reduced nickel interface with nickel oxide. This structure impairs complete reduction and Ostwald ripening of nickel, maintaining its catalytic activity. The NiO/Ni-CNT catalyst demonstrates high HER activity in various alkaline solutions, achieving a current density of 20 mA cm⁻² at a voltage of 1.5 V. The catalyst's performance is attributed to the synergistic effect of NiO and Ni, with the NiO shell facilitating the formation and desorption of hydroxyl ions, while the Ni core facilitates the Volmer process. The study also fabricates a water electrolyzer using this catalyst, which can be powered by a single-cell alkaline battery, showcasing its practical potential for hydrogen production.The study reports the development of nanoscale nickel oxide/nickel (NiO/Ni) heterostructures on carbon nanotube (CNT) sidewalls as highly effective electrocatalysts for hydrogen evolution reaction (HER). These heterostructures exhibit activity comparable to platinum, a key material for water splitting in hydrogen production. The NiO/Ni-CNT catalyst is synthesized through a low-temperature hydrolysis of nickel salts, followed by thermal annealing, which results in a partially reduced nickel interface with nickel oxide. This structure impairs complete reduction and Ostwald ripening of nickel, maintaining its catalytic activity. The NiO/Ni-CNT catalyst demonstrates high HER activity in various alkaline solutions, achieving a current density of 20 mA cm⁻² at a voltage of 1.5 V. The catalyst's performance is attributed to the synergistic effect of NiO and Ni, with the NiO shell facilitating the formation and desorption of hydroxyl ions, while the Ni core facilitates the Volmer process. The study also fabricates a water electrolyzer using this catalyst, which can be powered by a single-cell alkaline battery, showcasing its practical potential for hydrogen production.