Proton-Coupled Electron Transfer
Proton coupled electron transfer (PCET) is a redox process in which electron transfer is accompanied by proton transfer, either in a concerted step or stepwise. Quinone/hydroquinone couples are classic examples, undergoing two electron, two proton reactions in water, and the local pH at the electrode and the identity of the available proton acceptor or donor strongly shape their voltammetry.
Quinone PCET as an electrochemical proton pump (2015)
Watkins et al. studied hydroquinone/benzoquinone and 2,6-dimethylbenzoquinone/hydroquinone in sulfate and bicarbonate electrolytes and attributed extra oxidation peaks in weakly buffered media to local pH drops that change the proton acceptor from water to sulfate or bicarbonate, rather than to a change away from PCET (Watkins et al. 2015). The oxidation peak shifted with pH while the reduction peak did not (peak separation about 206 mV at pH 7.5 rising to about 499 mV at pH 6.5), and 1.0 M NaHCO3 buffered the system enough to give single peaks. They then used the PCET reaction to pump protons between two electrodes and drive CO2 separation. Summary: watkins2015.
Diagnosing PCET versus non-PCET rate-determining steps in the ORR (2025)
Bender and co-workers used pH-dependent kinetics to tell whether the kinetically relevant ORR step involves PCET (Bender et al. 2025). A Nernstian (pH-independent on the RHE scale) response on Pt, with orders of about 1 in H and -1 in OH- measured at fixed SHE potential, indicates a PCET step (the *O2 to *OOH transition), while zero-order H+ and OH- dependence on Au indicates non-PCET O2 adsorption. In alkaline media the proton for the PCET steps comes from water dissociation rather than hydronium. Microkinetic modeling showed that Tafel slopes alone cannot distinguish these cases. Summary: bender2025.