Deciphering Local Microstrain-Induced Optimization of Asymmetric Fe Single Atomic Sites for Efficient Oxygen Reduction

Abstract

Disrupting the symmetric electron distribution of porphyrin-like Fe single-atom catalysts has been considered as an effective way to harvest high intrinsic activity. Understanding the catalytic performance governed by geometric microstrains is highly desirable for further optimization of such efficient sites. Here, we decipher the crucial role of local microstrain in boosting intrinsic activity and durability of asymmetric Fe single-atom catalysts (Fe–N3S1) by replacing one N atom with S atom. The high-curvature hollow carbon nanosphere substrate introduces 1.3% local compressive strain to Fe–N bonds and 1.5% tensile strain to Fe–S bonds, downshifting the d-band center and accelerating the kinetics of *OH reduction. Consequently, highly curved Fe–N3S1 sites anchored on hollow carbon nanosphere (FeNS-HNS-20) exhibit negligible current loss, a high half-wave potential of 0.922 V vs. RHE and turnover frequency of 6.2 e−1 s−1 site−1, which are 53 mV more positive and 1.7 times that of flat Fe–N–S counterpart, respectively. More importantly, multiple operando spectroscopies monitored the dynamic optimization of strained Fe–N3S1 sites into Fe–N3 sites, further mitigating the overadsorption of *OH intermediates. This work not only sheds new light on local microstrain-induced catalytic enhancement, but also provides a plausible direction for optimizing efficient asymmetric sites via geometric configurations.

1. Introduction

Oxygen reduction reaction (ORR) plays a pivotal role in next-generation energy conversion and storage configurations, such as metal–air batteries and fuel cells. Currently, Pt-group metals (PGM) act as the most efficient electrocatalysts for sluggish cathodic ORR due to their moderate activity [1–3]. However, the modest catalytic durability, terrestrial scarcity and high cost of PGM have hampered the development of such advanced devices. Among various PGM-free electrocatalysts, single-atom catalysts (SACs) have emerged as promising alternatives to PGM in the field of ORR [4, 5]. Accordingly, SACs have become the hottest frontier in energy conversion systems in recent decades [6–8]. Their unique electronic and geometric structures enable maximum atom utilization, optimized adsorption behaviors and tunable catalytic performance. In particular, porphyrin-like Fe–N–C moieties have attracted significant research interest in oxygen-related catalysis due to their prominent activity [9, 10]. Nevertheless, the strong adsorption strength of oxygenated intermediates on symmetric Fe–N4 sites leads to a blocked ORR process and increased reaction barriers [11, 12]. Numerous efforts have been developed to address this dilemma by exploiting efficient Fe SACs with asymmetric coordination structure.

The introduction of the secondary atoms into the first coordination shell of Fe–N–C structure could disrupt symmetry of center atoms and achieve satisfactory oxygen activations. Intensive research has been carried out to incorporate heteroatoms (P, S, B, etc.) into the carbon matrix of Fe SACs [13, 14]. Thereinto, the introduction of S atoms to form Fe–N3S1 sites is prone to result in an improvement of intrinsic ORR activity due to the fact that the asymmetric coordination structure could alleviate overadsorption of oxygenated intermediates on active sites [15, 16]. Despite tremendous efforts, the general try-and-error modes for constructing SACs lack the efficiency to further improve the activity of single-atom catalysts. Most of previous work focused on the fundamental coordinate structure, and the understanding of catalytic behavior relies heavily on flat graphene-based models and experiments instead of real geometric configurations [17, 18]. Compared with flat geometry, high-curvature surface tends to create no-planar curved structure and introduce unique microstrain effect, which might be conducive to boosting catalytic activity [19]. Further optimization of such efficient single atomic sites depends strongly on a comprehensive understanding of the structure–performance relationship and catalytic behavior, especially the dynamic mechanism. This understanding, however, still remains unclear due in part to a lack of quantitative and in-depth exploitation [20, 21].

Herein, we unveil the local microstrain-induced optimization for asymmetric Fe–N3S1 sites on high-curvature hollow carbon nanospheres, demonstrating enhanced ORR activity and durability through strain engineering.