The multistep proton-coupled electron transfer (PCET) processes are beneficial for products distribution and selectivity of the electrocatalytic CO2 reduction reaction (CO2RR), which are affected by the nature of the catalyst and electrolyte at electrode-electrolyte interface. Polyoxometalates (POMs) are electron regulators of PCET processes, which can catalyze CO2RR effectively. Accordingly, the commercial indium electrodes are combined in this work with a series of Keggin-type POMs (PVnMo(12-n)O40)(n+3)-, n = 1, 2, 3) to process CO2RR with Faradaic efficiency toward ethanol reaching 93.4% at-0.3 V (vs. RHE). The cyclic voltammetry and X-ray photoelectron spectroscopy results reveal the activation of CO2 molecules by the first PCET process of the VV/IV in POM. Subsequently, the PCET process of MoVI/V results the oxidation of the electrode, causing the loss of In0 active sites. Electrochemical in-situ infrared spectroscopy confirms the weak adsorption of *CO at the later stage of electrolysis due to the oxidation of the In0 active sites. The indium electrode in PV3Mo9 system retains more In0 active sites owing to the highest V-substitution ratio, thereby ensuring a high adsorption ratio of *CO and C-C coupling. In sum, the regulation of the interface microenvironment by POM electrolyte additives can be used to boost the performance of CO2RR.