Bell shape vs volcano shape pH dependent kinetics of the electrochemical oxidation of formic acid and formate, intrinsic kinetics or local pH shift?

The interfacial pH at the electrode/electrolyte interface (pH(s)) and reaction kinetics of electrochemical oxidation of formic acid in the presence or absence of buffers are carried out. The transform from bell shape to volcano shape pH dependence of formic acid oxidation reaction (FOR) is clarified by considering both pH(s) drift and buffer effect. The Diagram of pHs as a function of FOR current density (j) in solutions with bulk pH (pH(b)) from 0 to 14 with/without buffers are constructed by numeral calculations, which are compared with experimental results of FOR on Pt and Au. Our study reveals that i) In solutions with 5< pH(b) <10 free of buffer or with limit buffer capability, there is significant pH(s) drop and drift of the thermodynamic equilibrium potential (E-eq); ii) FOR on both Pt and Au has a bell shape intrinsic pH dependence with maximum activity close to the pKa of HCOOH; iii) The observed volcano shape pH dependence of FOR activity with a plateau at 5 < pH(b) < 10 on top of the volcano is a coincidence related to pHs drift. (C) 2020 Elsevier Ltd. All rights reserved.