Self-Adjusting Activity Induced by Intrinsic Reaction

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Self-Adjusting Activity Induced by Intrinsic Reaction Intermediate in Fe-N-C Single-Atom Catalysts Yu Wang, Yu-Jia Tang, and Kun Zhou J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Journal of the American Chemical Society

Self-Adjusting Activity Induced by Intrinsic Reaction Intermediate in Fe-N-C Single-Atom Catalysts Yu Wang,† Yu-Jia Tang,‡ and Kun Zhou*,†,‡ Environmental Process Modelling Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, 1 CleanTech Loop, Singapore 637141, Singapore. ‡ School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. †

Supporting Information Placeholder ABSTRACT: Fe-N-C single-atom catalysts (SACs) exhibit high activity for oxygen reduction reaction (ORR). However, it remains controversial how the active center mediates catalysis, and the predicted potential deviates from experimental results, hindering development of ideal SACs. Here, using first-principles calculations, we present a microkinetic model for ORR on Fe-N-C SACs, disclosing a selfadjusting mechanism induced by its intrinsic intermediate. The modelling results show that the single-atom Fe site of the FeN4 center of Fe-N-C is covered with an intermediate OH* from 0.28 to 1.00 V. Remarkably, such OH* becomes part of the active moiety, Fe(OH)N4, and can optimize intermediate bindings on the Fe site, exhibiting a theoretical half-wave potential of ~0.88 V. Partial current density analysis reveals the dominating associative path over the dissociative ones. In addition, ORR on Mn-N-C and Co-N-C SACs is unveiled. This work demonstrates the necessity of assessing the effect of intrinsic intermediates in single-atom catalysis and provides practical guidance for rational design of high-performance SACs.

Electrocatalysis of oxygen reduction reaction (ORR) is essential for fuel cell technology, which is promising for facing the growing energy demands.1,2 While fuel cell technology has come a long way, its critical dependency on Pt-based catalysts makes such devices cost-prohibitive for large-scale commercialization.3,4 Accordingly, it is imperative to develop an efficient non-precious ORR catalyst. Following a pioneering report,5 a class of catalysts named metal-nitrogen-carbon (MN-C), where the metal part is typically earth-abundant Fe,6‒9 Co,10,11 or Mn,12 has been greatly developed in recent years. Also known as single-atom-catalysts (SACs), the metal atoms of M-N-C are isolated with single-atom (SA) level distribution, endowing them with distinctive features compared to other forms.6‒12 Apart from high ORR activity, M-N-C SACs exhibit good stability and feasibility of low-cost scalable synthesis,13‒15 with Fe-N-C ranking among one of the most promising candidates as alternatives to Pt.6‒9 The ORR origin of Fe-N-C model catalyst has been intensively studied using modern quantum mechanical methods17‒22 and characterizations (e.g., X-ray absorption and Mössbauer spectroscopy).7,22‒25 A carbon-hosted FeN4 moiety

is typically located in the proposed active center, of which active site is SA Fe. Indeed, with computational hydrogen electrode (CHE) model,26 SA Fe is suggested for launching the ORR process.16,17 How SA Fe boosts ORR remains controversial. It seems that the well-established SA Fe is not as active for ORR as Pt if we consider that the theoretical limiting-potential (UTL) of Fe-N-C SACs based on the FeN4 center (0.25−0.43 V)16‒18 is much smaller than the theoretical values of Pt (111) (0.79 V)27 and even N-doped graphene (0.50−0.78 V)28,29 using CHE modelling; such UTL of Fe-N-C deviates from its experimental half-wave potential by ~0.4 V.6‒9 Besides, while several ORR mechanisms (e.g., associative and dissociative paths) have been explored,16,18 which path is more accountable for ORR remains unclear. More importantly, although new active states of Fe-N-C SACs (e.g., FeON4 and Fe(OH)N4) were proposed using in situ characterizations22,23 and CHE modelling,7 a good understanding of the real active state and dynamic intermediate conversions of Fe-N-C during the catalytic process is still lacking. The discrepancy about the UTL and the ambiguities about the reaction mechanism and intermediate dynamics seriously impede rational design of ideal SACs. Herein, we address these critical issues using microkinetic simulations and first-principles calculations. We find that the SA Fe site of FeN4 of Fe-N-C SACs is preferentially covered with an intrinsic intermediate OH*. Such OH* becomes part of the Fe center with the formation of Fe(OH)N4 to improve intermediate binding, exhibiting a self-adjusting mechanism. In contrast to the inactive FeN4 center, the Fe(OH)N4 center exhibits an impressive activity, which rationalizes the high ORR activity of Fe-N-C SACs.7 Note that both FeN4 and Fe(OH)N4 centers belong to Fe-N-C SACs, of which the latter is the real active center. An FeN4 moiety-embedded graphene slab is adopted to represent the possible active center of Fe-N-C SACs (Figure S1). We focus on the associative and OOH* dissociative paths, while O2* dissociation is out of consideration because of its considerable energy barrier Eb (1.16 eV) (Figure S2). Compared to dissociative steps, the Eb of proton transfer is small (< 0.17 eV) (Figure S3). To determine the underlying mechanism while considering the dynamic conversions of intermediates, we present a microkinetic model for ORR on

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Fe-N-C in acidic medium. The related steps and rate equations are listed in equations S1-S16 (see Supporting Information). The intermediate coverages are estimated by numerically solving rate equations at steady state. There are two abundant intermediates (O*Fe and OH*Fe, Figure 1) and two nonabundant intermediates (OOH*Fe and O2*Fe, Figure S4) on the SA Fe site of FeN4. Above 1.00 V the Fe site is dominated by O*Fe. With the decrease of electrode potential (U), the covered O*Fe is gradually replaced by OH*Fe, which covers the Fe site over a wide range of U from 0.28 to 1.00 V. The Fe sites are cooccupied by O* Fe and OH*Fe between 0.78 and 1.00 V. OH*Fe is gradually removed to produce H2O at U < 0.51 V, and below 0.28 V most of Fe sites are adsorbate-free.

Figure 2. Free energy diagrams of ORR along (a) associative and (b) OOH* dissociative paths on Fe(OH)N4 at 0 and 0.76 V. The potential-limiting step is highlighted by a red dashed box. Compared with the FeN4 center (Figure S8), the ∆G values along the associative path on the Fe(OH)N4 center are more favorable (Figure 2a). The potential-limiting step of Fe(OH)N4 is still OH* removal, while its UTL increases from 0.40 V to 0.76 V. O2* and OOH* dissociations on Fe(OH)N4 are investigated (Figure S9); the Eb of OOH* dissociation is 0.83 eV and that of O2* dissociation (1.18 eV) remains large. Although its UTL is 0.76 V, the OOH* dissociative route is slightly energetically unfavorable given that OOH* dissociation to O*+OH* is uphill by 0.16 eV (Figure 2b).

Figure 1. Coverages of the most abundant ORR intermediates on the SA Fe site of the FeN4 center as a function of U with the inset being the geometry of OH*Fe. These microkinetic analyses demonstrate that O*Fe, OH*Fe and *Fe are in quasi-equilibrium during ORR. Especially, the wide-potential coverage of OH*Fe implies that a complete ORR cycle is unrealizable on the FeN4 center at U > 0.51 V. This explains why Fe-N-C SACs based on the FeN4 center theoretically appear inactive. The undesirable performance of the FeN4 center is also reflected from its theoretical ORR polarization curve obtained from microkinetic simulations (Figure S5), which shows a low half-wave potential (~0.51 V). The coverage of OH*Fe also indicates that the SA Fe site of the FeN4 center at low potential is not adsorbent-free but bonds an OH* to form a new center Fe(OH)N4. Because of the coordination of OH*Fe, the central Fe moves away from the N4plane with a buckling of 0.29 Å (Figure 1, inset). It rationalizes the observed out-of-plane Fe atom (FeXN4 moiety) at low potential revealed by in situ X-ray absorption spectroscopy,22,23 and suggests that the ligand X is OH* rather than O*. Note that the O-H axis in the most stable OH*Fe configuration is tilted16‒21 relative to rather than perpendicular7 to the FeN4-plane. We also study ORR on the FeN4 center using the DFT(PBE)+U approach.31 The DFT+U brings much weak intermediate adsorptions (Table S1), which is against the strong binding feature of SA Fe, and contradicts the fact that the pentacoordinate FeXN4 moiety should be formed at low potential (Figure S6).22,23 Therefore, we do not take the DFT+U approach in this work, following the previous M-N-C SACs studies.7,16‒21,31,32 We next study ORR over the Fe(OH)N4 center (Figure S7). The interactions between SA Fe and intermediates become weak after involving OH*Fe (Table S2). For example, the adsorption free energy of OH* (∆GOH*) of Fe(OH)N4 is 0.76 eV, larger than that of FeN4 (0.40 eV), indicating an adsorption improvement rather than poisoning. The ∆GOH* of Fe(OH)N4, albeit still not as positive as that of Pt(111) (0.79 eV),27 is sufficient to drive OH* removal.

The theoretical ORR polarization curve of the Fe(OH)N4 center is then simulated (Figure 3a). Remarkably, Fe-N-C SACs based on the Fe(OH)N4 center exhibits an impressive onset potential of ~0.95 V (Table S3); its theoretical halfwave potential (at the current density of −3 mA cm−2) is ~0.88 V, a value much larger than that based on the FeN4 center (~0.51 V) and comparable to theoretical value of Pt (111) (~0.86 V).33,34 The predicted half-wave potential of Fe-N-C SACs based on the Fe(OH)N4 center agrees well with the bestperforming Fe-N-C SACs at acidic medium (~0.8 V).7

Figure 3. (a) Simulated ORR polarization curves of Fe-N-C SACs based on FeN4 and Fe(OH)N4 centers. (b) Intermediate coverages of Fe(OH)N4 as a function of U, and (c) partial current density. The intermediate coverages of Fe(OH)N4 are further evaluated (Figure 3b). In contrast to FeN4, OH*Fe is the only abundant intermediate and removed below 0.92 V. The OH*Fe coverage reveals that the central Fe atom moves back into the N4-plane at high potential due to formation of symmetrical Fe(OH)2N4 (Figure S7d). It rationalizes the previous in situ X-

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Journal of the American Chemical Society ray absorption spectroscopy, which revealed that the Fe atoms are located within the N4-plane at high potential.22,23 No abundant intermediate (θ * C = ~1 and θOH * C = ~0) is found on the C site (Figure 3b, right), indicating that once OOH*Fe is formed, it will be converted to O*Fe+H2O. Therefore, the associative path should dominate ORR on Fe-N-C SACs. This is further deduced from partial current density analysis (Figure 3c), which displays zero partial current density for OOH* dissociative route.

O2* protonation (Figrue 5b). In contrast, the microkinetic analysis reveals that the theoretical half-wave potential of CoN-C is ~0.85 V (Figure 5c), a value more consistent with the experimental value (~0.80 V),12 and ORR on Co-N-C is limited by both O2* protonation and OH* removal (Figure 5d). Thus, the microkinetic analysis could be a good way to unveil nature of M-N-C SACs. In addition, the charge effect was recently proposed to be important among two-dimensional catalysts.35 Such an effect would be considered in future study, although these M-N-C SACs are not a two-dimensional system in a strict way.

Figure 4. Mechanistic scheme for ORR on Fe-N-C SACs. Invoking above results, ORR on Fe-N-C SACs is schematically summarized in Figure 4. In brief, the SA Fe site is first covered by OH* to form a pentacoordinate Fe(OH)N4 center, followed by a complete cycle efficiently realized on this “new” center. Note that the OH* covers the Fe site of FeN4 over a wide range of U from 0.28 to 1.00 V, although part of OH* can gradually react with proton‒electron to form H2O below 0.51 V; that is, Fe(OH)N4 acts as the active center at U > 0.28 V. Moreover, reaction/adsorption on both sides of Fe-N-C SACs is reasonable, as revealed by previous in situ X-ray absorption spectroscopy (hexacoordinate FeX2N4).22,23 The electronic density distributions of FeN4 and Fe(OH)N4 are investigated to understand why ORR on the Fe site can be improved by OH*. Bader charge population analysis shows that the net charge of Fe in Fe(OH)N4 (1.32 |e|) is larger than that in FeN4 (1.12 |e|). The increased valency of Fe in Fe(OH)N4 would weaken its ability to bond with adsorbates, indicating an optimization of electronic structure, like a selfadjustment. The more positively charged Fe can be further seen from the deformation electronic density (Figure S10), where more electrons are extracted from the 3d orbitals of the Fe of Fe(OH)N4 to the O atom due to Fe-O bond formation. The unique self-adjusting mechanism in Fe-N-C SACs has enabled alternative strategies for improving the ORR activity. Apart from simply increasing the density of SA Fe, one promising strategy is to further optimize the electronic structure of SA Fe by utilizing an appropriate ligand to replace OH*, strain engineering or a unique substrate. For example, functionalization using S, Cl, and other ligands may enhance the ORR activity of Fe-N-C SACs. Furthermore, the identification of non-trivial OH* in Fe-N-C SACs brings a new perspective to the evaluation of reaction mechanisms of other M-N-C SACs (and may not be limited to ORR), as they share a similar MN4 center. We extend our modelling into ORR on Mn-N-C and Co-N-C SACs and find that Mn-N-C possesses a similar self-adjusting mechanism as Fe-N-C (Figure 5a and Figure S11). Interestingly, the CHE modelling alone delivers an inaccurate activity assessment to Co-N-C: its UTL is only 0.48 V limited by

Figure 5. (a) Simulated polarization curves of Mn-N-C SACs based on MnN4 and Mn(OH)N4 centers. (b) Calculated free energy diagram, (c) polarization curve and (d) coverages of the most abundant intermediates of Co-N-C SACs. We have provided theoretical insights into ORR on the FeN-C model catalyst. Using microkinetic simulations, we demonstrate the dynamic coverages of intermediates on the SA Fe site and reveal that the intrinsic intermediate OH* is involved in the active structure. Such OH* can optimize the electronic structure of SA Fe to boost ORR, exhibiting a selfadjusting mechanism. The partial current density analysis shows the dominating associative path. Besides, the origin of ORR on Mn-N-C and Co-N-C SACs is disclosed. This work rationalizes experimental results, demonstrates the necessity of assessing the effect of intrinsic intermediates in single-atom catalytic process and provides practical guidance for rational design of high-performance SACs.

ASSOCIATED CONTENT Supporting Information Computational methods, microkinetic model, structure of FeN4 center, O2* and OOH* dissociation on FeN4 center, schematic of proton transfer on FeN4 and Fe(OH)N4 centers, coverages of intermediates on FeN4 center, theoretical polarization curve of FeN4 center, intermediate coverages of FeN4 center using DFT+U, geometries of intermediates on Fe(OH)N4 center, free energy diagram of ORR on FeN4, O2* and OOH* dissociation on Fe(OH)N4, deformation electronic density, free energy diagrams and intermediate

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coverages for ORR on Mn-N-C SACs, and solvated proton structures.

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge financial support from the Nanyang Environment and Water Research Institute (Core Fund), Nanyang Technological University, Singapore.

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