Coordination-adaptive Fe-N-C Catalysts Break the Scaling Relations for Oxygen Reduction Reaction

Fe-N-C catalysts, with atomically dispersed FeN4 sites, are hailed as the most promising non- precious metal catalysts towards oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs). However, the inherent linear scaling relations between oxygen-containing intermediates on the single-atom site fundamentally constraint further enhancements in ORR performance. Herein, we propose a self-adaptive reconstruction strategy to break the scaling relations, which is accomplished by engineering a coordination-adaptive FeN5 active site. Under ORR conditions, FeN5 sites undergo a reversible transformation cycle (FeN5-OOH*→FeN4- O*→FeN4-OH*→FeN5), enabling independent optimization of OH* adsorption energy, while minimally perturbing OOH* formation. This regulation effectively decouples the OOH*/OH* scaling relation. The resulting catalyst exhibits exceptional PEMFC performance with a peak power density of 1.17 W cm-2 and a current density of 32.6 mA cm-2 at 0.9 ViR-free, outperforming the FeN4-enriched Fe-N4-C counterpart (0.67 W cm-2). This work not only affords a highly competitive ORR electrocatalyst, but also paves up a new avenue to circumvent the inherent scaling relations of heterogeneous catalysis.

Keywords: Oxygen Reduction Reaction, Electrocatalyst, Scaling Relations, Proton Exchange Membrane Fuel Cells

1. Introduction

Proton exchange membrane fuel cells (PEMFCs) represent a highly promising next- generation energy conversion technology due to high efficiency and zero-carbon emissions.[1-4] Nevertheless, their large-scale deployment is still constrained by the sluggish oxygen reduction reaction (ORR) kinetics at the cathode.[5-8] Although platinum group metals (PGMs) have demonstrated efficiency in catalyzing ORR, their scarcity and high cost impede the commercial application.[9, 10] In this context, single atom catalysts (SACs) structured in transition metal- nitrogen-carbon (M-N-C) offer cost-effective alternatives with well-defined active site structure and appreciable catalytic activity.[11-13] Tremendous efforts have been dedicated to further improve their performance, e.g., morphology engineering, first-shell coordination regulation and long-range electronic modulation.[14-20] Despite significant progress achieved, considerable performance disparity between the state-of-the-art M-N-C SACs and Pt/C benchmark remains.

The fundamental limitation lies in the linear scaling relationships between the adsorption energies of multiple intermediates. To be specific, the ORR process in conventional SACs involves four proton-coupled electron transfer (PCET) steps, with OOH*, O*, and OH* as intermediates.[21, 22] According to bond-order conservation theory, the adsorption energies of these intermediates on the same metal site is correlated to one another, i.e., ΔGOOH* = ΔGOH* + 3.2 eV, ΔGO* = 2ΔGOH*.[23, 24] Such a linear scaling relationship restricts independent optimization of binding strength of each intermediate, thus imposing an inherent activity limitation with minimum thermodynamic overpotential of 0.37 V [(ΔGOOH*-ΔGOH*)/2-1.23=0.37 V].[25, 26] While dual-site designs have been proposed as a viable strategy to circumvent the scaling relation through either intermediate spillover-mediated cascade reaction mechanism or dissociative ORR pathway without *OOH formation, stringent requirements on the geometric and electronic configuration of the active site, including appropriate site proximity and intermediate transfer energy barrier, are necessitated to satisfy simultaneously.[25, 27-29] Therefore, developing innovative and feasible strategies to circumvent the OH*-OOH*/O* scaling relation is highly desirable but remains challenging.

Fortunately, recent studies have demonstrated that structure dynamics, such as metal-ligand coordination changes and metal atom re-arrangement, can effectively break the scaling relationship, which inspires us to engineer dynamic Fe–N coordination.[30, 31] This concept is realized by incorporating axial N coordination into the planar D4h-symmetry Fe–N4 configuration. Guided by prior studies on NH3 enabling axial-N installation and micropore/edge-N formation on graphitic carbons, we adopted a secondary NH3 annealing to install FeN5 sites.[32-36] The spontaneous breaking/reformation of the axial Fe–N bond enables dynamic structural regulation, allowing different intermediates to adsorb onto distinct site configurations. The atomic-scale structure dynamics were monitored by post-electrochemical X-ray adsorption spectroscopy (XAS), revealing reversible site transformation from FeN5 to FeN4 and eventually back to FeN5. This decouples their adsorption energy and thereby circumvents the inherent scaling relationship, yielding a minimum thermodynamic overpotential of 0.19 V (vs. 0.37 V for pristine FeN4). The electrochemical characterizations confirm the superior ORR performance of the designed FeN5-C catalyst as it achieves an 18-fold higher turnover frequency (TOF) than that of conventional FeN4- C catalyst. The reversible structure evolution ensures excellent stability, with an E1/2 loss of only 7 mV after 10,000 cycles testing. More importantly, the PEMFC assembled with FeN5-C cathode

delivers an impressive peak power density of 1.17 W cm-2 and a kinetic current density of 32.6 mA cm-2 at 0.9 ViR-free, approaching the U.S. DOE 2025 target of 44 mA cm-2 and surpassing most of the reported M-N-C electrocatalysts. Beyond providing a highly promising non-precious metal ORR electrocatalyst for fuel cells, this work also paves the way for overcoming the inherent linear scaling relations in heterogeneous catalysis.