The performance of these NCs in electrochemical CO2RR shows remarkable variation despite similarities in their structural architecture or thiolate-capped ligand surfaces. The disparity in selectivity and efficiency of CO2RR products is assumed to stem from differences in the proton transfer mechanisms inherent to these NCs. Notably, neither structure contains lattice hydride, thereby precluding the straightforward utilization of hydrides during the reduction process. This absence necessitates an alternative hydrogen acquisition pathway, where the generated hydrogen must be successfully adsorbed on the cluster surface prior to transfer. To investigate the reaction mechanism and the origin of the selectivity differences among the NCs, density functional theory (DFT) calculations are performed using a simplified cluster system (see details in the SI). The theoretical analysis highlights that the free energy change for *H adsorption is lowest (-0.16 eV) for Cu11ABT NC compared to Cu11PTT and Cu18PTT NCs, consistent with this pronounced preference for the HER (Figure ). The difference in *H adsorption energy between Cu11 NCs variants can be attributed to the unique *H adsorption sites dictated by its confined structural architectures and stronger electronic interactions with favorable localized electron density. This structural confinement optimizes the overlap between the active Cu sites and *H intermediates, reducing the HER energy barrier and driving the reaction efficiently. This trend continues with Cu18PTT NC, where the level of confinement is reduced, yet remains present despite differences in structural organization (Figure ). Thus, the experimental HER, which serves as the primary competitive side reaction during CO2RR, follows the order: Cu11ABT > Cu18PTT > Cu11PTT NC which is also consistent with the computational results. To further elucidate the differences in CO2RR efficiency and selectivity between Cu11PTT NC and Cu18PTT NC, we explored two dominant reaction pathways: one leading to HCOOH and the other to CO. The variation in structural architecture impacts CO2 adsorption sites, influencing the energy profiles of the pathways (Figure ). For both NCs, the energy diagrams favor the production of HCOOH over CO, which is driven by stronger intermediate stabilization and favorable lower limiting potentials of HCOO* to *HCOOH than the *COOH to *CO. Specifically, Cu11PTT NC shows a significantly lower free energy (-0.57 eV) for *HCOO formation compared to Cu18PTT NC (-0.30 eV), highlighting its superior thermodynamic preference for HCOOH production.