A promising strategy to boost electrocatalytic performance is via assembly of heteronanostructured electrocatalysts that delivers the essential specific surface area and also active sites by lowering the reaction barrier. However, the challenges associated with the intricate designs and mechanisms remain underexplored. Therefore, the present study constructs a p-n junction in a freestanding MnCo$_2$O$_{4.5}$@Ni$_3$S$_2$ on Ni-Foam. The space-charge region's electrical characteristics is dramatically altered by the formed p-n junction, which enhances the electron transfer process for urea-assisted electrocatalytic activity. The optimal MnCo$_2$O$_{4.5}$@Ni$_3$S$_2$ electrocatalyst resulted in greater OER electroactivity, delivering an overpotential of only 240 mV than pure systems. Remarkably, upon employing as UOR electrode the required potential decreases to 30 mV. The impressive performance of the designed catalyst is attributed to the enhanced electrical conductivity, greater number of electrochemical active sites, and improved redox activity due to the junction interface formed between p-MnCo$_2$O$_{4.5}$ and n-Ni$_3$S$_2$. There is strong indication that the in-situ formed extreme-surface NiOOH starting from Ni3S2 boosts the electrocatalytic activity by lowering the generation potential of the surface reconstructive active species. In consideration, this work presents a high-performance p-n junction design for broad use, together with a viable and affordable UOR electrocatalyst.