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, Fdoped 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, Fdoping 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.

Automated summary

Doped carbon materials, especially F-doped carbon catalysts, have gained significant attention as metal-free catalysts for oxygen reduction reactions and as corrosion-resistant supports for Pt and non-Pt nanoparticles. F-doped carbon materials possess unique properties and greatly improve the activity and durability of the ORR compared to N-doped carbon materials. The high electronegativity of F-doping can efficiently modify the electronic band structure of carbon materials, favoring the ORR. This review systematically discusses the effect of F-doping/F co-doping on the electrocatalytic reduction of O2 in acidic and alkaline conditions, the stability and durability of fuel cell catalysts, and provides guidelines for working with F-doped carbons. Future research directions on F-doped carbons are also proposed to bridge the gap between laboratory-scale assessments and commercial applications.