Operando Studies of Electrochemical Denitrogenation and Its Mitigation of N-Doped Carbon Catalysts in Alkaline Media

Abstract

N-doped carbons (NCs) have excellent electrocatalytic performance in oxygen reduction reaction, particularly in alkaline conditions, showing great promise of replacing commercial Pt/C catalysts in fuel cells and metal−air batteries. However, NCs are vulnerable when biased at high potentials, which suffer from denitrogenation and carbon corrosion. Such material degradation drastically undermines the activity, yet its dynamic evolution in response to the applied potentials is challenging to examine experimentally. In this work, we used differential electrochemical mass spectroscopy coupled with an optimized cell and observed the dynamic behaviors of NCs under operando conditions in KOH electrolyte. The corrosion of carbon occurred at ca. 1.2 V vs RHE, which was >0.3 V below the measured onset potential of water oxidation. Denitrogenation proceeded in parallel with carbon corrosion, releasing both NO and NO 2 . Combined with the ex situ characterizations and density-functional theory calculations, we identified that the pyridinic nitrogen moieties were particularly in peril. Three denitrogenation pathways were also proposed. Finally, we demonstrated that transferring the oxidation reaction sites to the well-deposited metal hydroxide with optimized loading was effective in suppressing the N leaching. This work showed the dynamic evolution of NC under potential bias and might cast light on understanding and mitigating NC deactivation for practical applications.

Co-Incorporated N-Doped Micro–Meso Porous Carbon as an Electrocatalyst for Oxygen Reduction Reaction and Zn–Air Battery

“…The nitrogen content was decreased from 4 to 2.1% after ADT due to corrosion and denitrogenation. 80,81 Whereas the surface cobalt content detected was less for both before and after ADT since it is trapped inside the carbon shell evident from the TEM image. Hence, we can infer that the Co(0) phase contributed more toward ORR since only negligible ORR activity loss was observed after ADT, though the surface nitrogen content was decreased.…”

Impact of Carbon Corrosion and Denitrogenation on the Deactivation of Fe–N–C Catalysts in Alkaline Media

“…45 Additionally, the oxidation of N pyridinic is thermodynamically more preferable than that of C pyridinic (C bonded with N pyridinic ). 24 So far, the oxidations are independent of the presence of O 2 as the oxidant is water. However, in O 2 , in addition to being oxidized to NO x at 1.5 V RHE (see Scheme 2D-ii), the N pyridinic on the armchair edge may also be converted into N pyridinic-COH/pyridonic during the step at 0.93 V RHE , where slow ORR occurs (see Scheme 2D-iii).…”

“…Compared to the degradation during regular fuel cell operation, the degradation during start–stop events is more harmful to the Fe–N–C catalysts. , During start–stop events, the C matrix and the N species may also undergo electrochemical oxidation, known as carbon corrosion (Reaction and ) and denitrogenation (Reaction ), respectively. Their onset potentials (defined as potentials at which CO x and NO x products are experimentally measured) are all reported to be around 1.2 V RHE at 25 °C. , Thus, they both occur during start–stop events, when the Fe–N–C electrodes experience potentials up to 1.5 V RHE . Note that the free energy barrier (Δ E b ) of denitrogenation of N-doped C (Reaction ) varies with the type of N species undergoing oxidation.…”

“…Their onset potentials (defined as potentials at which CO x and NO x products are experimentally measured) are all reported to be around 1.2 V RHE at 25 °C. 24,25 Thus, they both occur during start−stop events, when the Fe−N−C electrodes experience potentials up to 1.5 V RHE . 26 Note that the free energy barrier (ΔE b ) of denitrogenation of N-doped C (Reaction 3) varies with the type of N species undergoing oxidation.…”

Suppressing carbon corrosion via mechanically mixing transition metal phosphide clusters: a comparative in situ study in alkaline media

“…18,19 We and others also reported that different noble metal species can speed up the oxidation of the adjacent carbon atoms. 20,21 For example, Choi et al 22 compared the electrochemical corrosion behaviors of different carbon substrates and found that the presence of Pt nanoparticles promotes carbon corrosion to yield CO in alkaline electrolytes. The dominant partial oxidation of carbon to CO is also observed in acidic environments.…”