## 3 Results and Discussion
When the potential shifts negatively beyond \(-\)0.6 V, the cathodic current increases with a characteristic hump peak, indicating significant CO\({}_{2}\)RR activation at sufficiently negative potentials. Notably, Ni-MOF exhibits higher current density in both Ar- and CO\({}_{2}\)-saturated NaHCO\({}_{3}\) at low potentials compared to ZIF-L and the heterojunction, demonstrating its superior HER activity but poor CO\({}_{2}\)RR performance. Importantly, the Ni-MOF@ZIF-L heterojunction initiates CO\({}_{2}\)-to-HCOOH conversion at \(-\)0.5 V (vs. RHE), a significantly lower onset potential than pure MOFs (Figure 6b). The Faradaic efficiency (FE) of the heterojunction reaches 81.3% for HCOOH at \(-\)0.5 V and rapidly increases to 93.9% at \(-\)1.1 V, confirming its high activity and selectivity for CO\({}_{2}\)RR. Competitive HER is effectively suppressed with FEs below 20%, attributable to the material's hydrophobic properties. The Tafel slope of Ni-MOF@ZIF-L (101 mV dec\({}^{-1}\)) is substantially lower than those of pure Ni-MOF (157 mV dec\({}^{-1}\)) and ZIF-L (231 mV dec\({}^{-1}\)) (Figure 6c), indicating enhanced reaction kinetics for HCOOH production. This improvement likely stems from optimized charge transfer and increased catalytic surface area during CO\({}_{2}\)RR. Furthermore, the heterojunction maintains stable operation for over 1600 min at 10 mA cm\({}^{-2}\) with >90% HCOOH selectivity (Figure 6d), demonstrating exceptional electrocatalytic durability. Figure 5: (**a**) DFT-optimized heterojunction slab model. (**b**) Spatial charge difference distribution (isosurface: 0.0003; blue: charge reduction; yellow: charge accumulation). (**c**) Plane-averaged charge density difference along the z direction normal to the heterostructure. (**d**) Charge difference contribution decomposed to specific atoms. To further investigate the effect of the hydrophobicity on the electrocatalytic activity, the SEM and contact angle measurement of Ni-MOF@ZIF-L at different growth times were performed. As shown in Figure 7a, SEM images demonstrated the Ni-MOF can be coated by ZIF-L at different levels, assembling into Ni-MOF@ZIF-L 0.5 h, Ni-MOF@ZIF-L 1 h, and Ni-MOF@ZIF-L 2 h. After 2 h, it was found that ZIF-L completely covered the Ni-MOF, which leads to poor electron transfer capability and hinders the electrocatalytic process (Figure 7c,d). Moreover, the contact angle measurement further illustrates that pure Ni-MOF is hydrophilic, with a deposited water droplet sitting at a contact angle of 22\({}^{\circ}\). After ZIF-L coating, these heterojunctions show significantly enhanced hydrophobicity, with drastically increased contact angles of 88\({}^{\circ}\), 121\({}^{\circ}\), 135\({}^{\circ}\), and 153\({}^{\circ}\) (Figure 7b). This falls into the regime of hydrophobicity, where trapped gases are expected at both micro- and nanoscales [45]. The formate and H\({}_{2}\) Faradaic efficiencies of pure Ni-MOF are only 23% and 81%, respectively, demonstrating that hydrophilicity restrains CO\({}_{2}\)RR activity. When Ni-MOF is partially covered by hydrophobic ZIF-L, the formate Faradaic efficiency shows gradual enhancement: Ni-MOF@ZIF-L 0.5 h, Ni-MOF@ZIF-L 1 h, Ni-MOF@ZIF-L 2 h, and Ni-MOF@ZIF-L 4 h exhibit Faradaic efficiencies of 58.6%, 91.9%, 82.7%, and 78.6%, respectively (Figure 7e). A significant decrease in formate Faradaic efficiency is observed for the Ni-MOF@ZIF-L 4 h catalyst, which can be attributed to ZIF-L's lower conductivity compared to Ni-MOF. The high ZIF-L surface coverage likely compromises electrical conductivity. These results demonstrate that the correlation between ZIF coverage and Faradaic efficiency follows a \"volcano curve\" due to the competing effects of hydrophobicity and electronic conductivity [46]. Furthermore, investigation of the structure-reactivity relationship reveals that Ni-MOF (Figure 7f), which is initially susceptible to water molecule attack, undergoes surface property modification upon ZIF-L coating.