Promoting Atomically Dispersed MnN₄ Sites via Sulfur Doping for Oxygen Reduction: Unveiling Intrinsic Activity and Degradation in Fuel Cells

Abstract

Carbon supported and nitrogen coordinated single Mn site (Mn–N–C) catalysts are the most desirable platinum group metal (PGM)-free cathode catalysts for proton-exchange membrane fuel cells (PEMFCs) due to their insignificant Fenton reactions (vs. Fe), earth abundances (vs. Co), and encouraging activity and stability. However, current Mn–N–C catalysts suffer from high overpotential due to low intrinsic activity and less dense MnN4 sites. Herein, we present a sulfur-doped Mn–N–C catalyst (Mn–N–C–S) synthesized through an effective adsorption-pyrolysis process. Using electron microscopy and X-ray absorption spectroscopy (XAS) techniques, we verify the uniform dispersion of MnN4 sites and confirm the effect of S doping on the Mn–N coordination. The Mn–N–C–S catalyst exhibits a favorable oxygen reduction reaction (ORR) activity in acidic media relative to the S-free Mn–N–C catalyst. The corresponding membrane electrode assembly (MEA) generates enhanced performance with a peak power density of 500 mW cm–2 under a realistic H2/air environment. The constant voltage tests of fuel cells confirm the much-enhanced stability of the Mn–N–C–S catalyst compared to the Fe–N–C and Fe–N–C–S catalysts. The electron microscopy and Fourier transform XAS analyses provide insights into catalyst degradation associated with Mn oxidation and agglomeration. The theoretical calculation elucidates that the promoted ORR activity is mainly attributed to the spatial effect stemmed from the repulsive interaction between the ORR intermediates and adjacent S dopants.

Bimetal Organic Framework Derived Atomically Dispersed Mn and N Codoped Porous Carbon for Efficient Oxygen Reduction

“…The molar ratio of Zn/Mn was determined as 38/1 according to the inductively coupled plasma‐atomic emission spectrometry (ICP‐MS) results, significantly higher than the theoretical Zn/Mn molar ratio (15/1). It implies that Mn ions cannot easily coordinate with N to form Mn‐N 4 moieties and thus a very low content of Mn sites were introduced, which is consistent with the reported literatures [11,18] …”

“…In recent years, Mn and N co‐doped carbon electrocatalysts (Mn‐N−C) have attracted much attention [11] . Unlike the Fe‐N−C and Co‐N−C electrocatalysts, Mn metal was reported to be inactive for the Fenton reaction and barely reacts with hydrogen peroxide [8,12] .…”

Atomically dispersed Mn–N4 electrocatalyst with high oxygen reduction reaction catalytic activity from metal–organic framework ZIF-8 by minimal-water-assisted mechanochemical synthesis

“…Recently, several groups have reported the successful preparation of Mn-N-C catalysts using different organic precursors. 12,[25][26][27][28][29] Wu and co-workers reported a novel two-step doping and adsorption synthetic strategy, which successfully generated Mn-N-C catalyst with promising catalytic activity in acidic solution. 12 The ZIF-8 precursors used in the study were synthesized in organic solvents, e.g.…”

Active site engineering of single-atom carbonaceous electrocatalysts for the oxygen reduction reaction

“…3f). Moreover, the above dopants could as well regulate the electronic structure and improve the intrinsic activity of single Co-N x , 130 Cu-N x 59 and Mn-N x 131,132 and Mn-N-C electrocatalysts, respectively. ref.…”