Nano Energy 78 (2020) 105128

Available online 24 July 20202211-2855/© 2020 Elsevier Ltd. All rights reserved.

Axial ligand effect on the stability of Fe–N–C electrocatalysts for acidic oxygen reduction reaction

Feiteng Wang a, Yipeng Zhou a, Sen Lin b, Lijun Yang a,*, Zheng Hu a, Daiqian Xie a

a Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210023, China b State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350002, China

A R T I C L E I N F O

Keywords: Oxygen reduction reaction Stability Fe–N–C electrocatalysts First-principle modeling Potential dependent kinetic study

A B S T R A C T

Iron and nitrogen co-doped carbons (Fe–N–C) have comparable activity to Pt-based catalysts for oxygen reduction reaction (ORR), but with much poorer durability in acidic electrolytes. Recently, regulating the co-ordination environment of Fe center (in-plane or axially) to boost the ORR activity of Fe–N–C has attracted many interests, and the axial OH ligand is even regarded as a necessary part of a highly-active structure. However, the influence of these regulations on the stability is still not clear. Herein, we performed kinetic and thermodynamic calculations based on density functional theory with explicit consideration of electrode potential to study the OH axial ligand effect on the stability of Fe–N–C electrocatalysts. We found that although the OH ligand can enhance the ORR onset potential to some extent, it substantially increases the H2O2 selectivity, pushing ORR diverted to the 2e- + 2e-pathway. In the latter 2e-process (H2O2 reduction), harmful hydroxyl radicals could be produced upon H2O2 dissociation. Therefore, from the perspective of catalysts’ stability, OH ligand coordination on the metal center is not a good way to develop stable ORR catalysts.

1. Introduction

Replacing precious platinum with earth-abundant elements in oxy-gen reduction reaction (ORR) electrocatalysts is critical for the wide commercialization of proton exchange membrane fuel cells (PEMFCs) [1–4]. Among the various reported platinum group metal-free (PGM-free) ORR electrocatalysts, heat-treated transition metal-nitrogen-carbon (TM-N-C, TM = Fe, Co, Mn) catalysts are the most promising candidates, in which the Fe–N–C usually possesses the best activity [5–9]. The ORR proceeds on the metal centers of TM-N-C electrocatalysts, in which the four N coordinated metal centers (TMN4) have long been accepted as the active sites of ORR [2,10]. Recently, there have been quite a few reports regulating the in-plane or axial coordination of the metal centers to boost the activity (Fig. 1a) [11–15]. Among them, the axial ligands decoration is more controllable and easily implemented, which has been realized by coating iron phthalocyanine on the carbon support, or in-situ grafting ligands on the metal centers, all showing quite impressive activity enhancement [11–15]. A recent study even suggested that the axial OH ligand should be a necessary part of a highly-active FeN4 structure since the Fe center spontaneously bonds with an intermediate *OH from 0.28 to 1.00 V vs.

RHE [16]. Although there are plenty of evidences supporting the beneficial

effects of axial ligand on the promotion of ORR activity, some re-searchers worry that it might potentially hamper the catalysts’ long- term stability in harsh electrochemical conditions [17]. Actually, low stability in acidic electrolyte is now the biggest issue lying in front of the TM-N-C electrocatalysts, since their life span is only dozens of hours, significantly less than the commercial requirement of several thousand hours. Currently, there are several hypotheses for the degradation of TM-N-C catalysts in acidic electrolyte: 1) demetallation, 2) carbon corrosion by H2O2 associated radicals, 3) anion adsorption or proton-ation, and 4) active site flooding [18–26]. For Fe–N–C, the mechanism 2 (carbon oxidation by H2O2 associated radicals) plays a more profound role for the irreversible degradation, which might induce demetallation, structural deterioration, and increased hydrophily for micropore flooding [27–29]. Given the crucial role of H2O2 on the catalysts’ sta-bility, it is of great significance to figure out whether the axial ligand could change the selectivity of H2O2 and the production of radicals. However, direct in-situ observation of the on-site H2O2 selectivity is difficult since only the H2O2 diffused out of the catalyst layer could be possibly detected. Therefore, density functional theory (DFT)

* Corresponding author. E-mail address: lijunyang@nju.edu.cn (L. Yang).

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https://doi.org/10.1016/j.nanoen.2020.105128 Received 30 April 2020; Received in revised form 8 June 2020; Accepted 22 June 2020

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calculation becomes an indispensable tool for this topic [30]. Previous theoretical studies of the H2O2 selectivity are normally

based on the thermodynamic data calculated from the charge neutral Computational Hydrogen Electrode (CHE) model [31]. There is one ki-netic study to model the dissociation of *OOH on FeN4, the key inter-mediate for H2O2 production [32]. But the electrode potential and H-bonding of solution were not considered, whose impacts on the modeling results have recently been well addressed in a series of pub-lications [33–35]. Therefore, we employed a DFT method with explicit consideration of electrode potential to study the kinetics of H2O2 pro-duction and reduction. The proton transfer and bond dissociation were modeled in local H-bond network by adding water molecules around the adsorbate and combining implicit continuum solvation model (COSMO) in the system [36]. To ensure the modeling is on the right track, we started from a well-known experimental fact that Fe–N–C and Co–N–C catalysts have very different H2O2 yields (Fe–N–C < Co–N–C) and sta-bilities (Fe–N–C < Co–N–C), though they both have similar TMN4-like active structures [2,6]. Such comparative studies may help us deduce the key message about the stability issue of Fe–N–C.

Our results suggest that although OH ligand can enhance the ORR onset potential to some extent, it substantially increase the H2O2 selectivity, pushing ORR diverted to the 2e- + 2e-pathway. In the latter 2e-process (H2O2 reduction), harmful hydroxyl radicals could be pro-duced upon H2O2 dissociation. Therefore, from the perspective of cat-alysts’ stability, OH ligand coordination on the metal center is not a good way to develop promising ORR catalysts. We also revealed that the much increased H2O2 selectivity upon OH ligand coordination comes from the strengthened O–O bond of *OOH on the metal center, which is the ORR intermediate just before H2O2. The Fe–N–C catalysts with dual metal sites can facilely dissociate O–O bond of *OOH and suppress H2O2 production, thus might be a feasible route to develop stable ORR electrocatalysts.

2. Results and discussion

2.1. Explicit consideration of electrode potential

The key to this study is to make the kinetic barriers calculations closer to the practical situation, which require the electrode potential and H-bond to be introduced properly. The most widely used theoretical approach to study ORR process is the charge neutral CHE method, in which the effect of electrode potential is implicitly accounted for by adding constant values to the reaction free energies. This is based on the hypothesis that the binding energies of the intermediates would not vary with potential. Recently, it has been pointed out that the charge states of 2D models (like the models used for TM-N-C electrocatalysts) vary significantly with respect to potential, and this apparently changes the bonding states of ORR intermediates [34]. Herein, the CHE method was adapted by tuning the charge of each model according to the targeted potential, instead of keeping the system charge neutral (Supplementary Material computational method) [37]. In this way, the effects of po-tential were introduced explicitly (Fig. S1, Table S1), and the trends of reaction energy and reaction barriers, with respect to potential, could be obtained. The local H-bond network was accounted by adding water molecules combined with the implicit continuum solvation model (COSMO). One of the H2O molecules was replaced by H3O+ as the proton source.

2.2. Axial ligand effect on ORR activity

The effect of axial OH ligand on the thermodynamic activity is examined with the models shown in the lower-right part of Fig. 1a. On TM-N-C electrocatalysts, ORR normally follows the associative mecha-nism, where O2 does not dissociate before it is hydrogenated (Fig. 1b). The free energy profiles of ORR based on this associative mechanism are plotted in Fig. 1c. The indicator of activity, i.e., the thermodynamic onset potentials clearly get improved upon OH coordination. Such

Fig. 1. OH ligand coordination on the metal center of TM-N-C electrocatalysts affects the ORR process. (a) The schematic illustration of in-plane (left) and axial (right) regulation of metal centers of TM-N-C electrocatalysts [11–15]. (b) Schematic illustration of the overall ORR process. (c) The thermodynamic activity of these four models. The U here with different colors represents the theoretical limiting potential (thermodynamic onset potential). (d) The d-band centers of these four models.

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improvements come from the fine-tuned adsorption strength of the ORR intermediates, which all are lowered to different extents compared to the cases without OH ligand. The weakened adsorptions result from the filled d-band of the metal centers due to OH coordination, as demon-strated by the down-shifted d-band centers for the cases with OH ligands (Fig. 1d).

From the viewpoint of thermodynamics, the weakened adsorption is beneficial to ORR since it can mitigate the issues of too strong binding of *OH on the metal center. However, at the same time, the interactions between *OOH and metal center is also decreased, which can influence the O–O bond breaking process and thus the on-site H2O2 selectivity.

2.3. H2O2 selectivity

It has been suggested that the selectivity of H2O2 production is determined by the two competing branch reactions [38].

*OOH + H+ + e− → *H2O2 (1)

*OOH + H+ + e− → *O + H2O (2)

The branch reaction of equation (1) occurs through the hydrogena-tion of *OOH at proximal O, which make ORR follow the 2 e− + 2 e−

and/or 2 e− pathway. The branch reaction of equation (2) proceeds by O–O bond breaking and spontaneous proton transfer, which would finally lead to the 4 e− ORR pathway. From solely thermodynamic

analysis, it is clear that the *OOH dissociation (equation (2)) is more favorable than H2O2 formation (equation (1)), due to more heat released in the former reaction. For example, even for the least case of Co(OH)N4, the reaction of equation (1) still releases more heat than that of equation (2) by 0.93, 0.38, 0.15 eV at the electrode potential of 0.2, 0.5, 0.8 V. In the following, the reaction kinetics are presented to find out whether or not the thermodynamics-based analysis underestimates the selectivity of H2O2.

For the adsorbed *OOH, the proximal O of *OOH is defined as Oa, and the distal O is defined as Ob (Fig. 2a–b). For equation (1), we have the hydrogen atom in H3O+ gradually approaching proximal Oa to obtain the energy barriers. For equation (2), the barriers were obtained from the energy profiles with Oa-Ob bond getting gradually elongated. Traditionally, equation (2) is also believed to be a proton delivery dictated process, in which the protonation at the distal Ob leads to O–O bond cleavage and production of water. However, the simulation of distal Ob protonation does not support this mechanism. Ob becomes reluctant to accept the proton at U = 0.50/0.80 V, and the proton transfers to a nearby H2O, forming a new H3O+ and leaving the Oa-Ob bond intact. Therefore, we used the Oa-Ob bond length as the reaction coordinate, and the targeted *O + H2O motif was successfully obtained in the entire potential range. The above kinetic behaviours were observed on all Fe and Co based moieties, indicating that the branch pathway to *O + H2O is kinetically determined by the bond breaking of

Fig. 2. Energy profiles of *OOH reduction under different electrode potentials. (a) Schematic representation of the reaction from *OOH to *H2O2. (b) Sche-matic representation of the reaction from *OOH to *O + H2O. The reaction energies and kinetic barriers for *OOH reduction at 0.20/0.50/0.80 V vs. RHE on (c) FeN4, (d) Fe(OH)N4, (e) CoN4, and (f) Co(OH)N4.

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Oa-Ob, while the H2O2 formation is kinetically determined by the pro-tonation of proximal Oa. The corresponding barriers were calculated and plotted in Fig. 2c–f.

As shown in Fig. 2, the reaction energies decrease and the kinetic barriers increase as the electrode potential increases, which is consistent with the experimentally observed transition from the diffusion- dominant region (low potential and high current density) to the kinetics-dominant region (high potential and low current density). From the perspective of electron transfer number, the FeN4 site is an excellent 4 e− active site with a low barrier for the *O + H2O pathway in the entire potential range (Fig. 2c). When we have Fe coordinated with an axial OH ligand, the increased coordination of Fe(OH)N4 would strengthen the O–O bond of *OOH and change H2O2 selectivity. The barriers for O–O bond breaking to form *O + H2O on Fe(OH)N4 become comparable with those to H2O2 at 0.20/0.50/0.80 V, as shown in Fig. 2d and Figs. S2–S3. It means the selectivity to H2O2 is no longer negligible, and it could even surpass that to *O + H2O, although the formation of *H2O2 on Fe(OH)N4 releases significantly less heat. For the Co–N–C system, the same trend is also found, except the selectivity of H2O2 formation is much higher than Fe–N–C (Fig. 2e–f, Figs. S4–S5).

According to a recent microkinetic study, FeN4 sites are covered by OH from 0.28 to 1.00 V vs. RHE, and would transform into Fe(OH)N4 during ORR [16]. Therefore, the actual H2O2 yield in Fe–N–C electro-catalysts should be rather high. But the commonly observed H2O2 yield of Fe–N–C from experimental rotating ring disk electrode (RRDE) tests is rather low. This discrepancy comes from the intrinsic issue of RRDE test. The H2O2 molecules must diffuse out of the catalysts layer and then reach the ring electrode to be detected. Most of the H2O2 molecules could be further reduced (equations (3)–(5)) either on the sites they were generated or the other sites within the catalyst layer, leaving only a little trace detected by RRDE.

*H2O2 → *O + H2O (3)

*H2O2 + e− → *OH + OH− (4)

*H2O2 → *OH + •OH (5)

Equation (3) is the disproportionation of *H2O2 (Figs. S6 and S7, Table S2). Equations (4) and (5) are the dissociation of the *H2O2 with the difference of whether the electron transfer occurs upon the disso-ciation (Figs. S8 and S9). Here, we calculated the barriers of dispro-portionation and dissociation of H2O2 disregarding the dissociated products. The potential dependence of the dissociated products will be discussed in the next section. As shown in Fig. S10, the thermodynamic and kinetic results indicate the activity for H2O2 dissociation/dispro-portionation is in the order of FeN4 > Fe(OH)N4 ~ CoN4 > Co(OH)N4. Apparently, OH ligand increases the barriers and decreases the H2O2 reduction activity.

Combined with previous results of H2O2 selectivity, these data can well explain the experimental observations that the percentage of H2O2 yield varied significantly as the Fe–N–C loading changed (more than 40% for the loadings decreased from 0.8 to 0.16 mg cm− 2) [39]. Low catalyst loading would lead to more facile release of H2O2 molecule into bulk electrolyte before getting further reduced. Also, Co–N–C has much higher H2O2 yield than the Fe–N–C counterpart due to the high selec-tivity and low consumption of H2O2. This is why Co–N–C can be used as the electrocatalysts for H2O2 production while Fe–N–C cannot [40,41].

2.4. •OH radical formation

In the last part, the influence of OH axial ligand on the generation of hydroxyl radical (•OH) upon H2O2 dissociation are investigated, which is currently linked to the catalysts’ degradation. There have been evi-dences about •OH formation during the ORR process on Pt surface [42]. An in situ fluorescence spectroelectrochemistry reveals that •OH is the common intermediate for the electrochemical conversion between

oxygen and water [43]. For Fe–N–C, the dissolved Fe2+ ions were believed as the primary source to produce •OH via Fenton reactions with H2O2 [44]. Recently, a speculation has emerged that the Fe–N–C active sites could also induce Fenton-like reactions to generate •OH [27]. This is critical since it concerns whether the stability of Fe–N–C could be greatly enhanced just by removing the dissolved Fe ions from electrolyte to avoid Fenton reactions. However, there is no targeted investigation on this issue. According to equations (4) and (5), the kinetic selectivity of •OH radicals on a specific site could be obtained by calculating the barriers of OH− and •OH from *H2O2 under different potentials. How-ever, the modeling of •OH radical is still a great challenge for the present theoretical framework that only tackles the ground states system [45]. Here, we qualitatively estimate the production of radicals by comparing the free energies of forming OH− or •OH from *H2O2. The thermody-namic formulas used in the free energy calculations are shown in equations (10) and (11) (see Supporting Information Note1 for detailed equation derivation).

ΔG(OH− ) = G(*OH) + G(OH− ) – G(*H2O2) + eU (6)

ΔG(•OH) = ΔG(OH− ) + 1.9eV – eU (7)

where G (*OH) and G (*H2O2) are the free energies of the adsorbed *OH and *H2O2, respectively (Figs. S11–S12). G (OH− ) equals G (H2O) – 0.5G (H2) + 0.83eV. U is the electrode potential vs. RHE. The calculated ΔG (•OH) and ΔG (OH− ) vs. potential on different sites are plotted in Fig. 3.

In Fig. 3, one can see that the OH ligand does not change the trends on both Fe–N–C and Co–N–C, but we can get two interesting features about the generation of harmful •OH. The first one is that all curves go upward as electrode potential increases and the ones of ΔG (OH− ) are steeper in slope than that of ΔG (•OH) on all sites, indicating stronger potential dependence for the former. This is reasonable since OH− needs one electron transferred, while •OH does not. This makes the differences between ΔG (OH− ) and ΔG (•OH) smaller and smaller as potential increasing, and there could more chance to generate •OH at high po-tential. The second feature is that at high potential region, the •OH radical formation on Fe–N–C is obviously less endothermic than Co–N–C, also indicating higher chance to produce •OH. Thus, there exist Fenton-like reactions on the Fe–N–C active sites with the presence of H2O2. This is might be one of the reasons for the lower stability of Fe–N–C than Co–N–C for acidic ORR, deserving further investigations.

Based on the above analysis, we found that even though axial coor-dination could promote the ORR onset potential of TM-N-C, it hampers the catalysts’ stability. The axial ligand on single metal site would in-crease the H2O2 selectivity and •OH radicals could be produced at high potential (Fig. 4a). According to the preceding results, the key to improving stability during ORR is the facile dissociation of O–O bond in *OOH, which can supress the production of H2O2 and promote 4e− ORR. However, for single-atom active sites, facile O–O bond dissociation means strong interaction with *OOH, which would lead to difficult OH removal (the last step of ORR). Therefore, it’s natural to think to distribute the strong interaction to two sites, i.e., dual metal sites cata-lysts. The required interaction on each site is not that strong, and would not hinder the OH removal. Therefore, the dual-metal sites-based TM-N- C catalysts should be a good solution to achieve this idea and alleviate the stability issue (Figs. 4b and S13). Recent experiments about the TM- N-C catalysts with dual metal sites also demonstrated much improved ORR stability in acidic electrolytes [17,46,47].

3. Conclusions

In summary, a comparative study was performed on the typical TM- N4 sites (TM = Fe, Co) with and without OH ligand aiming at clarifying the axial ligand effect on the stability of the Fe–N–C catalysts. The ki-netics study revealed that the OH ligand coordination can significantly increase H2O2 production selectivity, and lower H2O2 reduction activity on both Fe–N–C and Co–N–C. The thermodynamic analysis indicates a

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higher chance to produce harmful •OH radical on the active sites of Fe–N–C than Co–N–C at high potential range, suggesting some Fenton- like reactions occurred on Fe-based active sites. We also found that the dual metal sites in TM-N-C catalysts can facilitate direct O–O bond breaking of *OOH, thus lowering the H2O2 yield and meanwhile increasing the stability of the catalysts. Therefore, our work provides useful insight that creating dual metal based active sites is a feasible strategy to obtain highly stable TM-N-C ORR electrocatalysts.

CRediT authorship contribution statement

Feiteng Wang: Conceptualization, Writing - original draft. Yipeng Zhou: Writing - original draft. Sen Lin: Conceptualization, Writing - original draft. Lijun Yang: Conceptualization, Writing - review & edit-ing. Zheng Hu: Supervision. Daiqian Xie: Conceptualization, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge support from the National Key Research and Development Program of China (2017YFA0206500); National Natural Science Foundation of China (21573107, 21733006, 21832003, 21673040 and 21973013). The DFT calculations in this paper have been done on the supercomputing system in the Supercomputing Center of Nanjing University.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.

org/10.1016/j.nanoen.2020.105128.

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Fig. 3. The free energies to produce OH¡ and •OH from H2O2 vs. electrode potentials on different sites. (a) Fe–N–C and (b) Co–N–C.

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Feiteng Wang received his B.S. degree from the Department of Chemistry at Hebei North University in 2016. He is currently a Ph. D. student at Nanjing University. His research focuses on the theoretical study of oxygen electrocatalysis on the transition-metal-nitrogen-carbon catalysts under the supervi-sion of Prof. Daiqian Xie.

Yipeng Zhou received B.S. degree (2015) from the Department of Chemistry at Sichuan University. He is currently a Ph. D. student at Nanjing University. His research focuses on defect properties of perovskite materials and catalytic dynamics on metal surfaces under the supervision of Prof. Daiqian Xie.

Sen Lin received his B.S. degree in Chemistry from Sichuan University in 2006 and Ph.D. in Physical Chemistry from Nanjing University in 2011. He is now a professor in State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, P. R. China. He was a visiting scholar at University of New Mexico in 2009–2010 and 2017–2018. His research focuses on the theoretical studies of heterogeneous catalysis.

Lijun Yang received his Ph.D. in solid mechanics from Harbin Institute of Technology in 2006, and gradually converged to chemistry after two post-doc periods in IMEC Belgium and Nanjing University. Now he is an associate professor in Nanjing University and mainly focuses on the theoretical understanding of the mechanisms in energy conversion and storage systems, such as fuel cells, supercapacitors and lithium batteries.

Zheng Hu received his BS (1985) and Ph.D. (1991) degrees in physics from Nanjing University. After two-year’s postdoctoral research in Department of Chemistry, he became an associate professor in 1993, and subsequently acquired the professor position in 1999, and Cheung Kong Scholar professor in 2007. He is the owner of the NSFC fund for outstanding young sci-entists of China (2005). Hu is engaged in the research field of physical chemistry and materials chemistry addressing the growth mechanism, materials design and energy applications of a range of nano-/mesostructured materials, especially the carbon-based materials, group III nitrides and transition metal oxides.

Daiqian Xie received his B.S. degree from Sichuan University in 1983 and Ph. D. degree in Physical Chemistry from Jilin University in 1988. He was a postdoctoral fellow at Jilin Uni-versity from 1988 to 1991. He worked in Sichuan University from 1991 to 2001. He was a visiting professor at University of New Mexico, Duke University, National University of Singapore, and Nanyang Technological University. Now he is a professor of Chemistry at Nanjing University. His research fo-cuses on the construction of reliable molecular potential energy surface, quantum state resolved reaction dynamics, and disso-ciative chemisorption of molecules on metal surfaces.

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