In this study, a selective Sn based catalytic system that electrochemically reduces CO2 to formate has been developed based on a new substrate effect. Dendritic Sn catalysts were synthesised by hydrogen gas bubble assisted electrodeposition on Pt, Cu, Sn or In substrates and applied to electrocatalytic reduction of CO2. The four substrates exhibit a hydrogen evolution activity that follows the order Pt > Cu > Sn > In. However, the Cu supported dendritic Sn catalyst provides the best selectivity towards formate formation (67.3% at -0.95 V vs. RHE). A substrate induced local pH change is proposed as the origin of formate selectivity. This was confirmed by the bulk electrolysis results obtained from two electrolyte solutions with different buffer capacities. The high buffer capacity phosphate buffer electrolyte solution provides minimal local pH change while an electrolyte with a low buffer capacity such as NaHCO3 maximises this effect to enhance the selectivity towards formate. The strategy reported here does not only focus on the catalyst, but also takes into consideration the local chemical environment. Hence, this work provides an optimal approach to improving the catalytic performance of electrocatalysts for electrochemical CO2 reduction.