Electrocatalytic hydrodechlorination (EHDC) is a promising environmental technology that can produce highly active atomic hydrogen (H-ad*) to detoxify chlorinated hydrocarbon pollutants in water via the hydrogenolysis of C-Cl bonds. However, its efficiency is hampered by hydrogen evolution reaction (HER) in aqueous media which competes with EHDC by consuming H-ad*. Here we report that, by controlling the solution pH, we can facilely tune the relative kinetics of EHDC of 2,4-dichlorophenol (2,4-DCP) and HER over palladium nanoparticles (Pd NPs) for an optimized EHDC reaction. The batch EHDC experiments with different starting pHs find that both the EHDC efficiency (conversion rate of 2,4-DCP to phenol) and H-ad* utilization efficiency (HUE%, the molar percentage of H-ad* used by EHDC) present a volcano-shaped relationship vs. starting solution pH, with the peak EHDC efficiency of 66.4% and HUE% of 35-40% achieved at pHs of 2.12 and 3.49, respectively. The mechanism study finds that this volcano relationship originates from the dual effect of pH on both the H-ad* production rate and the adsorption behavior of 2,4-DCP over Pd NPs. The overfast generation of H-ad* at very low pHs and the poor adsorption of 2,4-DCP on electrode under alkaline conditions will both favor the HER over EHDC. A solution condition of weak acid is thus ideal to optimize the EHDC performance. This work presents an efficient approach to improve the EHDC efficiency and energy selectivity by tuning the solution pH, which should advance the EHDC application in environmental remediation.