  Alkaline water electrolysis is considered a commercially viable option for
large-scale hydrogen production. However, this process still faces challenges
due to the high voltage (>1.65 V at 10 mA cm$^{-2}$) and its limited stability
at higher current densities due to the inefficient electron transport kinetics.
Herein, a novel cobalt based metallic heterostructure
(Co$_3$Mo3N/Co$_4$N/Co/Co) is designed for application for water electrolysis.
Operando Raman experiments reveal that the formation of Co$_3$Mo3N/Co$_4$N/Co
heterointerface boosts the free water adsorption and dissociation, resulting in
a surplus of protons available for subsequent hydrogen production. Furthermore,
the altered electronic structure of Co$_3$Mo3N/Co$_4$N/Co heterointerface
optimizes the {\Delta}GH of nitrogen atoms at the interface. This synergistic
effect between interfacial nitrogen atoms and metal phase cobalt creates highly
efficient hydrogen evolution reaction (HER) active sites, thereby enhancing the
overall performance. Additionally, the heterostructure exhibits a rapid OH-
adsorption rate, coupled with a strong adsorption strength, leading to improved
oxygen evolution reaction (OER) performance. Crucially, the metallic
heterojunction facilitates fast electron transport, expediting the
aforementioned reaction steps and ultimately improving the overall efficiency
of water splitting. The water electrolyzer with Co$_3$Mo3N/Co$_4$N/Co/Co as a
catalyst exhibits outstanding performance, requiring an impressively low cell
voltage of 1.58 V at 10 mA cm$^{-2}$ and maintaining approximately 100%
retention over a remarkable 100 h duration at 200 mA cm$^{-2}$. This
performance significantly exceeds that of the commercial Pt/C || RuO2
electrolyzer.
