Perfluoroalkyl substances (PFAS), also called 'forever chemicals', are stable, persistent environmental contaminants increasingly found in concentrations exceeding regulatory limits in water sources. In this work, we undertake a computational approach using density functional theory to elucidate the electrochemical degradation of a category of PFAS called perfluoroalkyl carboxylic acids (PFCAs) on a Pd catalyst. We explore the pH and potential dependent minimum energy reaction pathways to fully mineralized products like fluoride ions and carbon dioxide, and hydrocarbon. We examine the degradation behavior of the smallest and the most recalcitrant PFCA, trifluoroacetic acid (TFA). While electrochemical adsorption of TFA on Pd is exothermic in oxidative conditions, we have identified that the degradation is limited by a high barrier for the first reaction step. The initial C-C bond scission between the carboxyl and the fluoroalkyl tail being 2.10 eV, while direct C-F scission has a barrier of 2.11 eV. The C–C scission pathway remains preferred, driven by the higher thermodynamic favorability associated with the labile C–C bond. Following the initial direct defluorination of TFA, subsequent C–F bond cleavages exhibited lower energy barriers, ranging from 0.3 to 1.4 eV. This trend suggests that once the initial barrier is overcome, further defluorination steps proceed more readily. To gain more insights on the factors influencing direct C–F scissions, we calculated the activation barriers for all partially fluorinated acetic acids (mono-: MFA, di-: DFA) and observed the trend: MFA < DFA < TFA, suggesting lower direct defluorination barriers for higher degree of hydrogenation. Finally, we compare the initial scission of longer chain PFCAs (one and two carbon extra over TFA) and find that the barrier difference among them is within 0.2 eV.
