Oxygen reduction electrochemistry at F doped carbons: A review on the effect of highly polarized C-F bonding in catalysis and stability of fuel cell catalysts

Doped carbon materials, particularly N-doped carbon catalysts, have drawn considerable attention in recent years as metal-free catalysts for oxygen reduction reactions (ORR) and as a carbon corrosive resistance support for Pt and non-Pt nanoparticles. While nitrogen-doped carbons (N-doped carbons) were once the standard, F-doped carbons (F-doped carbons) have recently overtaken their popularity. This is because F doping gives carbon materials unique properties that not only differ from the N-doped carbons but also significantly improves the ORR activity and especially the durability. Being the highest electronegative element of the periodic table, F-doping can efficiently modify the electronic band structure of the carbon materials favoring for ORR. The edge F doping to the carbon is found to improve carbon corrosion resistance more than any other heteroatom doped catalyst discovered previously, including N doped carbons, both in highly acidic, alkaline pH conditions and high oxidative potentials that exists in the fuel cell including start-up and shut-down conditions. In this review, the fundamental understanding of effect of F-doping/F co-doping on the electrocatalytic reduction of O₂ into H₂O and OH⁻ in acidic and alkaline pH conditions, effect of F doping on stability and durability of fuel cell catalysts, careful considerations/guidelines one needs to know before working with F doped carbons (F doping advantages vs. poisoning effect on Pt or M/F-C (M = transition metal) catalysts, are being reviewed systematically. Finally, several strategies for future research directions on F-doped carbons were proposed to bridge the gap between laboratory-scale assessment to commercial aspects.