When two fluids with different compositions are mixed, mixing energy is released. This holds true for both liquids and gases, though in the case of gases, no technology is yet available to harvest this energy source. Mixing the CO2 in combustion gases with air represents a source of energy with a total annual worldwide capacity of 1570 TWh. To harvest the mixing energy from CO2-containing gas emissions, we use pairs of porous electrodes, one selective for anions and the other selective for cations. We demonstrate that when an aqueous electrolyte, flushed with either CO2 or air, alternately flows between these selective porous electrodes, electrical energy is gained. The efficiency of this process reached 24% with deionized water as the aqueous electrolyte and 32% with a 0.25 M monoethanolamine (MEA) solution as the electrolyte. The highest average power density obtained with a MEA solution as the electrolyte was 4.5 mW/m2, significantly higher than that with water as the electrolyte (0.28 mW/m2).