Understanding the effects of pH and potential on proton-coupled electron transfer (PCET) steps and optimizing the reaction activity are of key importance for efficient applications of energy conversion processes. In this work, we develop a simple theory to optimize the electrocatalytic PCET reactions, in which electron and proton transfer take place in sequential steps, by combining an energy level diagram and micro-kinetic theories. Our theory suggests matching conditions on PCET energetics (redox potentials and pK(a)'s) and reaction environments (potential and pH) in order to achieve maximum kinetics. Consequently, a descriptor representing deviation from the ideal condition, i.e. vertical bar Delta U-re vertical bar+ 2.30k(B)T/e vertical bar Delta pK(a)vertical bar, is proposed to measure catalyst (in)activity. To test our theory, we further investigate CO2 electroreduction on a model catalyst both computationally and experimentally, and show that the activity for CO2 reduction on FePc (Iron Phthalocyanine) molecule is higher than H-2 evolution at near neutral condition. Our theory not only identifies the individual contribution of electron and proton transfer to overpotentials but also provides simple guidelines for optimizing the experimental conditions and searching for efficient catalysts. (C) 2019 Elsevier Inc. All rights reserved.