ABSTRACT The electrochemical CO2 reduction reaction (CO2RR) has caught researchers’ attention over the past decades because of its potential for sustainable energy storage and conversion. In the previous research, copper has been discovered as a valuable catalyst based on its excellence in converting CO2 to higher-order hydrocarbons. Different from most of the work focusing on the room temperature (~25 oC), we investigate CO2RR on polycrystalline copper electrode under lower (5oC, 15oC) and higher (35oC, 45oC) temperatures across a range of potentials to observe the shift in the reaction selectivity toward each major gaseous product, CH4, C2H4, CO, and H2. By making Arrhenius plots, we can analyze the thermodynamic parameter (activation energy, E_a) per product at each studied potential and discuss the underlying mechanism with the change of the reaction temperature. This temperature dependent experiment also involves a custom-designed electrochemical cell, which can be simply applied to the electrochemical test on other potential CO2RR catalysts. Therefore, using this custom design with the fundamental understanding of the temperature dependent CO2RR mechanism, we select Mn3O4 nanocrystals (NCs) to keep studying the temperature effect of CO2RR as well as explore the efficient post-synthetic surface cleaning methods, among thermal, alkylation, Lewis-acid, and base treatment, by comparing electrochemical analysis and physical characterizations. From our results, even though Mn3O4 NCs are not good materials for CO2RR, we could conclude that alkylation treatment is the most efficient way to remove the surface ligands from the Arrhenius analysis.