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Versatile Strategy for Tuning ORR Activity of a Single Fe-N Site by Controlling Electron Withdrawing/Donating Properties of Carbon Plane Yeongdong Mun, Seonggyu Lee, Kyeounghak Kim, Seongbeen Kim, Seunghyun Lee, Jeong Woo Han, and Jinwoo Lee J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13543 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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Journal of the American Chemical Society

Versatile Strategy for Tuning ORR Activity of a Single Fe-N4 Site by Controlling Electron Withdrawing/Donating Properties of Carbon Plane. Yeongdong Mun†,§, Seonggyu Lee†,§, Kyeounghak Kim†,§, Seongbeen Kim†, Seunghyun Lee†, Jeong Woo Han*,†, and Jinwoo Lee*,‡ †Department

of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Nam-gu, Pohang 37673, Gyeongbuk, Republic of Korea. ‡Department

of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea. KEYWORDS: PEMFC, oxygen reduction reaction, non-precious metal catalysts, activity tuning, electron withdrawing/donating

ABSTRACT: Replacement of Pt-based oxygen reduction reaction (ORR) catalysts with non-precious metal catalysts (NPMCs) like Fe/N/C is one of the most important issues in the commercialization of proton exchange membrane fuel cells (PEMFCs). Despite numerous studies on Fe/N/C catalysts, a fundamental study on the development of a versatile strategy is still required for tuning the kinetic activity of a single Fe-N4 site. Herein, we report a new and intuitive design strategy for tuning and enhancing the kinetic activity of a single Fe-N4 site by controlling electron withdrawing/donating properties of a carbon plane with the incorporation of sulfur functionalities. The effect of electron-withdrawing/donating functionalities was elucidated by experimentation and theoretical calculations. Finally, the introduction of oxidized sulfur functionality decreases the d-band center of iron by withdrawing electrons, thereby facilitating ORR at the Fe-N4 site by lowering the intermediate adsorption energy. Furthermore, this strategy can enhance ORR activity without decrease in the stability of the catalyst. This simple and straightforward approach can be a cornerstone to develop optimum NPMCs for application in the cathodes of PEMFCs.

INTRODUCTION Low-temperature fuel cells employing hydrogen and oxygen are attractive energy conversion devices, which are expected to replace conventional power generators in vehicles. They have high energy conversion efficiencies and produce only environmentally benign products. Proton exchange membrane fuel cell (PEMFC) is the closest form to commercialization among the lowtemperature fuel cells. PEMFCs operate under acidic conditions and exhibit much higher performance than alkaline anion exchange membrane fuel cells (AEMFC) due to high proton mobility and high ionic conductivity of the proton exchange membranes (e.g. Nafion®).1-2 An important bottleneck in development of PEMFCs is the catalysis of oxygen reduction reaction (ORR) in the cathode. Since the kinetics of ORR are much slower under acidic conditions than under alkaline conditions,3 Ptbased catalysts are used to obtain a sufficiently high reaction rate. However, the high price of Pt becomes a big hurdle to overcome for the commercialization of

PEMFCs. To solve this problem, non-precious metal catalysts (NPMCs), containing third-row transition metals and N-doped carbon (M/N/C), have been studied in recent years.4-23 There had been debates in the active site of M/N/Cs but the M-Nx site, in which transition metal ion is coordinated to nitrogen functionalities in carbon basal plane, was elucidated as the main active site recently by the various state-of-the-art characterization methods.24-29 The research for M/N/Cs has been focused on active site characterization and appropriate nanostructure development.30-40 However, studies on controlling the kinetic activity of each M-Nx active site are rare. Although a well-designed strategy is required to develop M/N/Cs with high ORR activity under acidic conditions, many M/N/Cs have been developed by trial-and-error methods. The strategy to improve the kinetic activity of M/N/Cs under acidic conditions is of great importance for practical applications of NPMCs in PEMFCs.

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the rate of ORR is limited by the desorption step of the ORR product. In the two studies mentioned above, the Fe/N/C catalyst with large carbon plane exhibited low ORR activity, and that with small carbon plane exhibited high ORR activity. This trend of activity change is consistent with the fact that the ORR intermediate adsorb very strongly at the Fe-Nx site, as per the volcano plot shown in Figure 1c. Although several interesting results have been reported about the activity control of Fe-N4 site, the strategy cannot be generally employed for practical applications because it is difficult to control the carbon plane size independent of the formation of nanostructure and Fe-N4 active sites. Moreover, the decreased carbon plane size in the Fe/N/C catalyst to increase ORR activity would significantly lower the stability due to the increased number of carbon edge sites, which can be easily oxidized.46 Therefore, the strategy to increase the ORR kinetic activity of the single active site independent of the carbon plane size is strongly desired for real applications. Figure 1. (a) The effect of the size of carbon basal plane on Fe d-orbital in a Fe-N4 site (Brown arrows: electron donation effect of carbon plane to Fe-N4 active site; yellow arrows: repulsive force on Fe d-orbital derived from electron donation effect; grey arrows: increase of Fe d-orbital energy level by the repulsive force). (b) Relationship between intermolecular hardness and bond strength (blue bars: energy levels of orbitals which are involved in the bond between the active site and adsorbate).41 (c) Proposed position of the Fe-N4 site in a volcano plot for ORR based on the Sabatier principle. The catalysts located on the left branch (green area) of the plot have strong ORR intermediate adsorption strength, whereas the catalysts located on the right branch (yellow area) have weak adsorption strength. The adsorption strength of ORR intermediates at the Fe-N4 sites is generally strong, and therefore the Fe-N4 site would be located on the left branch of the volcano plot.41-43

To this end, the factor governing the kinetic activity of the single M-Nx active site must be understood. Two related studies on improving the kinetic activity of Fe-N4 sites have been reported.41-42 According to these reports, the size of the carbon plane, which incorporates the Fe-N4 sites, is the main factor governing the kinetic activity of the site. The electron density of delocalized π-band in the carbon plane interacts with the d-orbital of Fe ion in the Fe-N4 site (Figure 1a). A large carbon plane has an electron-rich π-band with high degree of delocalization. The high electron density of the π-band increases the dorbital energy level of the Fe-N4 sites. The adsorption energies of the ORR intermediates are affected by the energy level of the d-orbital in the Fe-N4 sites.44-45 High dorbital energy level induces a large intermolecular hardness factor (ηDA) of the bond between the ORR intermediates and the Fe-N4 sites, resulting in strong adsorption of the ORR intermediates (Figure 1b). Generally, adsorption strength of ORR intermediates at the Fe-N4 sites in Fe/N/C catalysts is too strong;41-43 thus,

In this paper, we report a new versatile strategy to control and enhance the ORR activity of the single Fe-N4 site in the catalyst. We incorporated electronwithdrawing/donating functionalities on the carbon plane by S-doping. These incorporated functionalities change the strength of the electronic effect, which is derived from the delocalized π-band of carbon plane to the d-orbital of the Fe ion in Fe-N4 site, resulting in the change of ORR activity of the Fe-N4 site. This strategy enables the intuitive design of highly active M/N/Cs for ORR, and the kinetic activity can be controlled without the decrease in stability because the size of carbon crystallite is not changed. This study opens a new avenue to research on strategies to control the single active site of M/N/Cs for ORR in fuel cells.

RESULTS AND DISCUSSION Before studying the electronic effect of S-doping, we synthesized Fe/N/C catalysts with pristine MSU-F-C, which is a mesoporous carbon with high surface area and large pore size (Figure S1),47 and commercial carbon blacks. We then compared their ORR activities with the carbon plane size to know where the catalysts are placed in the volcano plot. The synthesized catalysts are denoted as FeNC-A, where A signifies the carbon host. The size of the carbon plane was estimated from La size calculated from the areal ratio of the G and D peaks of the Raman spectra (Figure S2a, b).42 A reverse linear relationship was found between ORR activity and carbon plane size (Figure S2c, d), which confirms that FeNC-MSUFC is placed in the left side of volcano plot. In other words, electron density of the carbon plane in FeNC-MSUFC should be reduced to lower the adsorption energy of ORR intermediates and consequently accelerate ORR at the FeN4 sites. The synthesis of Fe/N/C catalysts with S-doped MSU-FC is schematically described in Figure 2. In this method, the formation of the carbon material, S-doping, and formation of the Fe-N4 active sites are separated. Dibenzyl


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Journal of the American Chemical Society disulfide (DBDS) and Fe(phen)3Cl2 were used as S-doping and Fe-N4 site precursors, respectively. Fe(phen)3Cl2 has multiple coordination bonds between Fe2+ and 1,10phenanthroline, which facilitates the generation of Fe-N4 sites after heat treatment. Through the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis, the atomically dispersed Fe ions were confirmed and homogeneous dispersion of iron and nitrogen in electron energy loss spectroscopy (EELS) mapping image supported the formation of Fe-N4 sites (Figure S3).

Figure 2. Schematic representation of the synthesis of Fe/N/C catalysts with S-doped MSU-F-C. In the figure, the black, light green, blue, red, and orange spheres represent carbon, nitrogen, iron, sulfur, and oxygen atoms, respectively. The doped sulfur existed in two different forms: thiophene-like S (C-S-C) and oxidized S (C-SOx). Thiophenelike S functionality gives electron-donating effect, and oxidized S functionality gives electron withdrawing effect to the Fe-N4 site. The oxygen functionality was excluded in the model.

form when the doping amount was higher (Figure S5). The N and O contents, measured by XPS, and Fe contents, measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), were similar in all the catalysts, which ensured that the effect of S-doping on ORR activity of the Fe/N/C catalysts could be investigated independently. Linear sweep voltammetry with rotating disk electrode was performed to measure the ORR activity of the catalysts in a typical three-electrode cell. ORR polarization curves of FeNC-S-MSUFCs show the effect of S-doping on the ORR activity of the Fe/N/C catalysts (Figure 3a). ORR activity increased with increasing S doping up to 0.24 at%, beyond which the activity decreased. Comparison with the ORR polarization curves of S-MSUFCs, which are composed of only S-doped carbons without any Fe/N/C precursor, revealed that the ORR activity is mainly due to the incorporation of Fe-N4 sites (Figure S6). Furthermore, the poisoning test using an electrolyte containing SCN- ions showed that the Fe-N4 site is the main active site for ORR in the catalysts (Figure 3b, Figure S7).48

Table 1. Elemental compositions (O, N, and S) obtained by XPS spectroscopy, and Fe contents measured by ICPAES. O (at. %)

N (at. %)

S (at. %)

Fe (at. %)

FeNCMSUFC

9.53

1.68

N/A

0.044

FeNC-SMSUFC-1

9.32

1.63

0.16

0.048

FeNC-SMSUFC-2

9.52

1.52

0.24

0.045

FeNC-SMSUFC-3

9.12

1.34

0.41

0.046

FeNC-SMSUFC-4

10.14

1.38

0.50

0.046

X-ray photoelectron spectroscopy (XPS) analysis was conducted to investigate the amount and chemical state of sulfur and other functionalities in the Fe/N/C catalysts (Figure S4, Table 1). The sulfur content obtained by XPS was 0.16–0.50 at%. It was confirmed that the doped sulfur existed in two different forms: thiophene-like S (C-S-C) and oxidized S (C-SOx). Upon varying the amount of Sdoping, the ratio between thiophene-like S and oxidized S was changed, and thiophene-like S became the dominant

Figure 3. (a) ORR polarization curves of FeNC-MSUFC, FeNC-S-MSUFC-1, FeNC-S-MSUFC-2, FeNC-S-MSUFC-3, and FeNC-S-MSUFC-4. (b) Poisoning test results with SCN- of FeNC-S-MSUFC-2. (c) Relationship between scan rate and capacitive current at 0.5 V vs. RHE of FeNC-MSUFC, FeNC-SMSUFC-1, FeNC-S-MSUFC-2, FeNC-S-MSUFC-3, and FeNCS-MSUFC-4. (d) Normalized kinetic current density by electrochemically effective area.

X-ray absorption spectroscopy (XAS) analysis of the Fe/N/C catalysts was conducted to confirm the formation of the Fe-N4 sites and to examine the difference between the sites. Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) signals of the catalysts were exactly overlapped and clearly showed the peak corresponding to Fe-N bonding near 1.4 Å (Figure S8). Furthermore, the geometric factors of Fe, such as coordination number and interatomic distance, in each catalyst, as obtained by EXAFS fitting, were also similar, and supported the formation of Fe-N4 sites with an axial oxygen ligand (Table S1).49 This indicates that Fe exists in



the same geometric state in all the catalysts. Since the total Fe content, measured by ICP-AES, was the same in all the catalysts, it can be concluded the number of Fe-N4 sites is not the main cause for the changes in ORR activity.

value calculated by the (002) peak in XRD were almost independent of S-doping. From the results of Raman spectroscopy and XRD analysis, it can be concluded that the change in carbon plane size is not a reason for the change in ORR activity of the Fe-N4 sites.

N2 physisorption analysis confirmed that all the catalysts had uniform BET surface areas and pore sizes (Figure S9, Table S2). Moreover, electrochemical effective surface area (EESA), calculated from electric double layer capacitance, were also in a uniform range (Figure 3c, Table S3), and kinetic current density normalized by EESA showed the same trend as the ORR polarization curves (Figure 3d). Therefore, the change in ORR activity of the catalysts could be attributed from the activity change in each Fe-N4 site rather than the change in the number of active sites or surface area of the catalysts.

The introduction of S-doping can be expected to exert an electron-withdrawing or electron-donating effect on the carbon plane; thus, the activity of the Fe-N4 sites can be controlled by tuning the strength of the electronic effect from the carbon plane to Fe ion. The lone pair of electrons in thiophene-like S makes it a strong electron donor to the carbon plane,50-51 and makes the carbon plane electron-rich. It lifts the energy of the d-orbital of Fe ion, and adsorption of ORR intermediate on the Fe-N4 sites becomes stronger. Since the Fe-N4 site of FeNCMSUFC is located on the strong adsorption side (left branch) in the volcano plot, the strong adsorption energy lowers the activity of the Fe-N4 site. In the XANES spectra of FeNC catalysts, FeNC-SMSUFC-2, which has the highest oxidized S/thiophenelike S ratio, showed the highest white line intensity because electron withdrawing property of oxidized S made electron transfer from the Fe-N4 site and generated more holes compared to pristine FeNC-MSUFC catalyst. In the same vein, FeNC-S-MSUFC-4, which has the lowest oxidized S/thiophene-like S ratio, showed the lowest white line intensity due to the electron donating property of thiophene-like S (Figure S10).

Figure 4. (a) Raman spectra and (b) XRD patterns of FeNCMSUFC, FeNC-S-MSUFC-1, FeNC-S-MSUFC-2, FeNC-SMSUFC-3, and FeNC-S-MSUFC-4. (c) La sizes and peak ratio of the G and D bands in the Raman spectra. (d) d-spacing values and Lc sizes obtained from XRD analyses.

The carbon plane size in the Fe/N/C catalysts is closely related to the kinetic activity of the Fe-N4 sites, as described earlier, and heteroatom doping into carbon might alter the carbon plane size; thus, we analyzed the characteristics of carbon in the catalysts using Raman spectroscopy and X-ray diffraction (XRD) (Figure 4). The Raman spectra showed that the shape, peak height, and area of the D and G bands, which are characteristic of conductive carbon materials, remained almost constant upon S-doping. The calculated La values of the catalysts were also similar. Furthermore, geometric factors like the thickness of carbon crystallite (Lc size) and the d-spacing

Figure 5. (a) Relationship between the ratio of oxidized S and thiophene-like S, and ORR activity. (b) Linear relationship between the amount of charge transfer from the functional groups and the d-band center. (c) Linear relationship between the adsorption energy of various intermediates and the d-band center.

Meanwhile, oxidized S functionalities like sulfone or sulfonyl group exert electron-withdrawing effect on the πelectron band in the carbon plane.52-53 Therefore, oxidized S functionalities lower the d-orbital energy of Fe ion, and consequently, weaken the adsorption of ORR intermediates at Fe-N4 sites. The oxidized S functionalities with electron-withdrawing property on the


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Journal of the American Chemical Society carbon plane of Fe/N/C enhanced the specific activity for ORR, while thiophene-like S functionalities with electrondonating property reduced the activity. This description is consistent with the change in ORR activity of Fe/N/C catalyst observed upon S-doping. Half-wave potentials of FeNC-S-MSUFC catalysts showed a similar tendency with the ratio of oxidized S and thiophene-like S (Figure 5a), indicating that the change in ORR activity of the Fe-N4 sites was due to the electronic effect of the S functionalities incorporated in carbon plane. To elucidate the effect of functional groups on tuning the ORR activity of the Fe/N/C catalysts, density functional theory (DFT) calculations were performed. We first examined the charge transfer between functionalized graphene and the Fe-N4 site. This can verify our hypothesis that the tendency of charge donation and withdrawal on thiophene-like S (-S) and oxidized S (-SO2) functional groups results in lower and higher ORR activity, respectively. At the -S-functionalized Fe/N/C catalyst, the electrons were transferred from functionalized graphene to Fe-N4 site, whereas electron transfer occurred in the reverse direction at the -SO2functionalized Fe/N/C catalyst. This implies that the thiophene-like S and oxidized S functional groups play opposite roles in determining the charge state of the FeN4 site. The DFT-calculated d-band center is up-shifted with increasing amount of charge transfer from the functional group to Fe-N4 site (Figure 5b). The up-shifted d-band center induces strong interaction between the FeN4 site and the adsorbate.54 Therefore, the adsorption energies of ORR intermediates were increased by increasing the d-band center at the -S-functionalized Fe/N/C catalyst (Figure 5c). In contrast, the -SO2 functional group in Fe/N/C catalyst lowered the d-band center of Fe ion by withdrawing electrons from the Fe-N4 site, resulting in weaker adsorption of ORR intermediates (Figure 5b, c).

To elucidate the roles of thiophene-like S (-S) and oxidized S (-SO2) functional groups on the ORR activity in detail, we investigated three representative ORR pathways that have been widely proposed as ORR mechanisms on Fe/N/C catalysts.6, 25, 55-56 The most stable adsorption structures and the corresponding energies of ORR intermediates are summarized in Figure S11 and Table S4, respectively. As shown in the free energy diagrams of ORR, all the pathways start with O2 adsorption (Figure 6). However, it differentiates into different pathways with regard to further reactions such as O2 dissociation (Figure 6a), O2 hydrogenation (Figure 6b), and OOH dissociation (Figure 6c). In the reaction pathway 1, oxygen adsorption, oxygen dissociation, and OH formation reactions are exothermic with highly negative reaction energies (Figure 6a). However, the H2O formation steps show different energy requirements for the -S-functionalized, -SO2functionalized, and pristine Fe/N/C catalysts. H2O formation is thermodynamically favorable at for the -SO2functionalized Fe/N/C catalyst with an exothermic reaction energy (-0.26 eV), while that by the -Sfunctionalized and pristine Fe/N/C catalysts are unfavorable with endothermic reaction energies of 0.43 eV and 0.25 eV, respectively. This means that ORR only occurs preferentially on the -SO2-functionalized Fe/N/C catalyst. The reaction pathways 2 and 3 also show that the different reaction energy requirements for H2O formation result in different ORR activities on the -S and -SO2functionalized Fe/N/C catalysts, implying that H2O formation is the rate determining step (RDS) for ORR on Fe/N/C catalysts, regardless of functional groups and reaction pathways. Based on previous Fe-Nx-C model structures,57 we constructed another type of Fe/N/C model, where all of carbon atoms connected to nitrogen atoms at the Fe-N4 site form C-C bonds by subtracting four hydrogen atoms (Figure S12d). However, the tendency was also consistent (Figure S12) with energetically the most stable Fe-N-C model in Figure 6. We found that the –SO2 functional group withdraws electrons from the Fe-N4 site, resulting in weaker interactions between the Fe/N/C catalyst and the ORR intermediates. Thus, it provides the optimal adsorption strength of ORR intermediates for facilitating ORR on Fe/N/C catalysts (Figure 6), which is in good agreement with our experimental results of higher ORR activity of the oxidized S functionalized Fe/N/C catalyst (Figure 3a).

Figure 6. (a-c) Three different Gibbs free energy diagrams (in eV) of ORR on pristine, –SO2-functionalized, and –Sfunctionalized Fe-N4-C catalysts. (d) The optimized structure of Fe-N4-C catalyst. The dark brown, light blue, and light brown spheres represent carbon, nitrogen, and iron atoms, respectively. The yellow spheres represent the positions of the functional groups.



activity was found to originate from the electronic effect of the sulfur functionality rather than the change in carbon crystallinity, the number of active sites, or the structural change of the catalytic site, as supported by various experimental characterizations and computational calculations. Furthermore, this strategy increased the kinetic activity without hampering the stability of the catalysts, which is also critical for practical applications. This simple and straightforward approach to control the ORR activity of a single Fe-N4 site can be a cornerstone for developing the optimum form of NPMCs for application in the cathode of PEMFCs.

ASSOCIATED CONTENT

Figure 7. ORR polarization curves of (a) FeNC-MSUFC, (b) FeNC-S-MSUFC-2, and (c) FeNC-KB600 obtained at the initial state and after 3000 potential cycles; (d) Changes in the kinetic activity of FeNC-MSUFC, FeNC-S-MSUFC-2, and FeNC-KB600 at 0.7 V (vs. RHE) after 3000 potential cycles.

The stabilities of FeNC-MSUFC, -S-MSUFC-2, and KB600 were compared after 3000 cycles of potential cycling between 0.9 and 1.1 V (Figure 7). FeNC-KB600 showed 2.5-times higher kinetic activity than FeNCMSUFC at the initial state due to its small carbon plane size, but it lost 64% of its initial kinetic activity after the stability test. In contrast, FeNC-S-MSUFC-2 only lost 19% of its initial activity, although its initial activity was similar to that of FeNC-KB600. The results show that small carbon plane size can improve the ORR activity of Fe-Nx/C sites, but reduce the stability due to the increased number of carbon edge sites. For FeNC-SMSUFC-2, the kinetic activity was increased without any change in the carbon plane size; therefore, the strategy developed by herein solved the weak point of previously reported strategies, i.e. stability of the catalysts. Furthermore, the morphology and sulfur functionalities of the FeNC-S-MSUFC-2 were also maintained after the stability test (Figure S13)

Supporting Information. Experimental details, materials characterization, supplementary data. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

ORCID Kyeounghak Kim: 0000-0003-1297-6038 Jeong Woo Han: 0000-0001-5676-5844 Jinwoo Lee: 0000-0001-6347-0446

Author Contributions §Y.M.,

S.L., and K.K. contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF2017R1A2B3004648 and NRF-2018R1A2B2002875). This work was also supported by the NRF funded by the Ministry of Science and ICT (NRF-2015M1A2A2056557 and NRF2018M1A2A2061987).

REFERENCES

CONCLUSION Improving the intrinsic kinetic activity of NPMCs is the most important prerequisite for substituting the expensive Pt-based catalysts in the cathode of PEMFCs and for contributing to the commercialization of PEMFCs. For this, fundamental consideration of the electronic and geometric structures of a specific catalytic site along with the understanding of reaction mechanism is strongly required, and a simple versatile strategy should be developed to enhance ORR activity of the site. In this work, ORR activity of a single Fe-N4 catalytic site was controlled by introducing electron-withdrawing/donating groups to the carbon basal plane by simple S doping. Electron-withdrawing oxidized S functionalities induced the increase of ORR activity of Fe-N4 sites, whereas electron-donating thiophene-like S functionalities induced the decrease of ORR activity. The change in ORR

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