The hydrogenation of CO2 into high-value fuels is a potentially effective approach to reduce anthropogenic dependence on fossil fuels and effects of climate change. In this study, we evaluated the hydrogenation of CO2 into CO and CH4 under the electrochemical promotion of catalysis (EPOC) effect through experimental and computational studies using Ru nanoparticles. Ru nanoparticles (1-2 nm) supported on yttria-stabilized zirconia (YSZ) solid electrolyte were evaluated at 250 °C at atmospheric pressure. Under positive polarization, the methanation reaction was promoted and the competitive reverse water gas shift (RWGS) reaction was impeded. On the other hand, negative polarization resulted in suppressing permanently the methanation reaction and minimally affecting the RWGS reaction. To qualitatively rationalize the tuning of selectivity via EPOC, Density Functional Theory (DFT) computations were used to model the EPOC effect induced on the Ru(0001) surface. DFT computations uncovered that electric field effects together with a change in surface electrochemical potential between intermediates are responsible for the contrasting influence of EPOC on the CH4 and CO formation over Ru catalysts.