Reduction of carbon dioxide (CO2) released from the industries helps to reduce the greenhouse gases (GHGs) emissions to the atmosphere while producing value added chemicals and contributes to carbon fixation. Microbial electrosynthesis (MES) is a recent process which accomplishes this idea by using cathodic bacteria at the expense of minimum energy. In this study, enriched mixed homoacetogenic bacteria as cathodic biocatalyst for the reduction of CO2 with five different concentrations were evaluated to produce acetate at a constant potential. Increasing the carbon concentration showed improved acetate production rate and carbon conversion efficiency. A maximum acetate production rate of 142.2 mg L-1 day-1 and maximum carbon conversion efficiency of 84% were achieved respectively at 4.0 and 2.5 g HCO3- L-1. The changes in pH due to interactive reactions between bicarbonate (substrate) and acetate (products) were able to create buffering nature in the catholyte that control operating parameters of microbial electrosynthesis (MES) process such as pH and substrate specificity. Higher acetate production shifted catholyte pH towards acidic conditions which further triggered favorable conditions for the bioelectrochemical reduction of acetate to ethanol.