In situ Electrostatic Modulation of Path Selectivity for the Oxygen

d International Iberian Nanotechnology Laboratory, Av, Mestre Jose Veiga, 4715-330 Braga, Portugal. e Max-Planck Institute for Chemical Energy Convers...
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In Situ Electrostatic Modulation of Path Selectivity for the Oxygen Reduction Reaction on Fe−N Doped Carbon Catalyst Kuang-Hsu Wu,*,† Wen Shi,† Dan Wang,†,§ Junyuan Xu,∥ Yuxiao Ding,⊥ Yangming Lin,⊥ Wei Qi,† Bingsen Zhang,† and Dangsheng Su*,†,‡ †

Shenyang National Laboratory, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning 110016, China Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Dalian, Liaoning 116023, China § Department of Science, Northeastern University, Shenyang, Liaoning 110004, China ∥ International Iberian Nanotechnology Laboratory, Av, Mestre Jose Veiga, 4715-330 Braga, Portugal ⊥ Max-Planck Institute for Chemical Energy Conversion, 45470 Mülheim, Germany ‡

S Supporting Information *

condition. We demonstrate that the kinetic selectivity for HO2− can be directed by cationic surfactant through intermolecular Coulombic interaction. The rationale of how a cationic surfactant is expected to interfere electrostatically with an intermediary η1-peroxo (endon) on a suggested Fe−N4+x active structure in Fe-NC catalyst,4,17,18 is depicted in Figure 1a. Cationic surfactant

T

he chemistry of molecular oxygen reduction is an important topic to catalyst design for chemical and energy conversion. How O2 is efficiently activated and selectively transformed has crucial implication to aerobic chemical conversions and fuel cell.1,2 In alkaline systems, O2 is reduced to form hydroperoxide (2e path) or hydroxide (4e path), depending on the catalyst used and system potential.3 Besides the state-of-the-art Pt-based catalysts for an efficient 4e oxygen reduction reaction (ORR), the Fe−N4+1 center (e.g., planar porphyrinic N4 with an imidazole N in the axial) in native enzymes such as cytochrome c oxidase can also deliver a selective 4e-ORR.4 This has inspired the incentives to search for “correct” electronic configurations to deliver an efficient 4eORR,4,5 whereas the others have striven ligand engineering to favor a selective 2e-ORR.6−8 Because the burst of interest in nonprecious metal composite and N doped nanocarbon as more practical fuel cell catalysts,9−12 the attention has been diverged to understanding the source of activity of the more complex chemistry of active sites in M−N doped carbons,13,14 especially for Fe−N doped carbons (Fe-NCs).15,16 Recent reports have revealed that the 4e-ORR is enabled by the electronic modulation of the Fe center by the binding ligands in the form of Fe−N4+x sites (wherein x = 1 or 2).17,18 As the ORR is governed by the thermodynamics but the selectivity is finally determined by the allowed molecular kinetics, nonelectronic factors relevant to the kinetics is also indispensable.19,20 The ORR on Fe-NC catalysts is known to proceed through reductive end-on binding.21 When hydroperoxide synthesis is concerned, one can imagine that the selectivity on Fe-NC catalyst may be tuned toward 2e-ORR by promoting the η1-peroxo cleavage from the Fe−N4+x center during the ORR. Applying an independent positive electric field near the catalyst surface in situ appears a sensible strategy for the purpose to “pull-off” the electron-rich peroxo for HO2− generation. It is of fundamental interest to examine the electrostatic effect for a novel selectivity tuning method, and as a route to reveal the ORR kinetic path. With the intention to study the electrostatic effect, we exploit ionic surfactant as a probe to create an electric field at the electrode double layer.22 Here, we report on the electrostatic modulation of the ORR selectivity in situ by cationic surfactant on an active Fe-NC (i.e., performs 4e-ORR) under reaction © 2017 American Chemical Society

Figure 1. (a) Electrostatic pulling of the η1-peroxo moiety of a Fe− N4+x active site by a cationic surfactant. (b) Proposed reaction scheme of O2 activation and reduction on a Fe−N4+x center under ORR potential, with the blue shades signifying the peroxo domains that are susceptible to a positive field.

tends to gather near electrode surface and enter the double layer to impose an electrostatic field on the peroxo−Fe−N4+x complex. The Coulombic attraction toward the positively charged headgroup could enforce the cleavage of the peroxo in situ from the Fe center. Figure 1b is a schematic diagram of the ORR process on the Fe−N4+x site in the active (FeII) form.1 A possible η2-peroxo (side-on) complex is also considered as it is suggested by an extended X-ray absorption fine structure (EXAFS) analysis.18 In both binding modes, the electron-rich peroxo (blue shades) projects out of the radial plane and are susceptible to an independent electric field. When a positive field is imposed, paths A and B should be kinetically promoted for a facilitated HO2− production. The electrostatic modulation of the ORR kinetics is studied on an active Fe-NC catalyst. The catalyst was prepared by Received: April 20, 2017 Revised: May 29, 2017 Published: May 30, 2017 4649

DOI: 10.1021/acs.chemmater.7b01619 Chem. Mater. 2017, 29, 4649−4653

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Chemistry of Materials following the exact procedure of our previous work.17 Briefly, the precursor p-phenylenediamine (p-PDA) was polymerized over carbon black (CB) and impregnated with FeCl3, followed by a pyrolysis at 900 °C under an Ar atmosphere. A range of precursor (p-PDA and FeCl3) mass ratio was also examined for an optimal performance. The Fe-NC catalyst was characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Further experimental detail is provided in the Supporting Information (SI). The Fe-NC catalyst was first studied for the composition and morphology. A survey by XPS (Table S1, SI) and staircase voltammetry on rotating ring-disk electrode (SCV-RRDE) (Figure S1, SI) confirms that the optimal catalyst exhibits excellent ORR activity (Eonset = 0.0 V (vs Hg/HgO), with an average electron-transfer number (nav) above 3.8; see also Figure S2 in the SI) and contains both N and Fe elements for which the N content (4.4 at. %) is similar to that of the Fe-free NC (4.6 at. %) and the Fe content is 0.27 at. %. The TEM images in Figure 2a,b show that the Fe-NC catalyst is in the

see Figure S1 and S2, SI). This perhaps suggests that the N group is not truly active or not in a “correct” configuration. On the other side, the Fe 2p spectrum in Figure 2d reveals the presence of FeIII species in the Fe-NC catalyst. The 2p3/2 feature located at 711.1 eV signifies the FeIII species in FeIII− N4+x. Any FeII species shall appear at 709.0−709.5 eV.23,24 The absence of FeII species should be due to the rather thick carbon layer and the rich surface FeIII−N4+x sites. The above information suggests that the morphology and chemical properties of the active Fe-NC catalyst conforms to what is reported in the literature.17 On the basis of the structure−activity correlation, we are able to investigate the role of ionic surfactant in modulating the ORR kinetics. Cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were respectively used as representative cationic and anionic surfactant for the study. Figure 3a,b shows the SCV-RRDE profiles of the Fe-NC

Figure 3. SCV-RRDE profiles (1600 rpm) of the Fe-NC catalyst with and without (a) CTAB and (b) SDS in O2-saturated 0.1 M KOH. (c) CV (50 mV s−1) and (d) 2e/4e selectivity profiles of the Fe-NC catalyst in the presence of CTAB.

catalyst in an O2-saturated 0.1 M KOH electrolyte, in the absence and presence of the surfactants. According to our previous work on in situ charge-screening by ionic surfactant, a change in ORR Eonset can result when there is a strong Coulombic interaction with the active site under reaction condition.22 However, no shift in the Eonset was found upon addition of SDS, despite the active Fe center is known to carry a positive charge. This may be explained by the cage-like octahedral coordination by electron-rich ligands that only allows O2 entrance on demand.1,17 Hence, the charge-screening method should be used with caution for metal−ligand types of active site. Another remarkable distinction of the two surfactant systems is the prominent change in the ring (jR) and disk (jD) current densities upon addition of CTAB, as compared to the almost unaffected profile with SDS. The rise of jR and the reduction of jD is strong evidence to a facilitated O2 reduction to HO2−. This indicates that the positively charged surfactant interferes with the ORR occurred on the catalyst. Provided that cationic surfactants in no way can covalently interact with the active structure or the substrate, their involvement in the ORR must be of a physical origin, very likely through intermolecular electrostatic interaction, near the electrode surface. The percentage generation of hydroperoxide (%HO2−) and the nav values are evaluated to offer intuitive inferences on the effect of ionic surfactants to the ORR on the Fe−N4+x site; the

Figure 2. (a) TEM micrographs of the Fe-NC catalyst and (b) embedded FeS NPs in the catalyst; inset shows the diffraction fringes of FeS (112) plane. (c) XPS N 1s spectra of the Fe-NC catalyst and Fe-free NC base material and (d) the Fe 2p spectrum of the Fe-NC catalyst.

form of carbon nanoparticles (NPs) with few occurrences of crystalline FeS NPs wrapped by a graphitic carbon layer of 3− 10 nm thickness; the XRD patterns of the catalyst is provided in Figure S3 (SI). The chemical states of N and Fe in the catalyst were examined by high-resolution XPS. Figure 2c is the N 1s spectra of the Fe-NC and Fe-free NC. The spectra is deconvoluted into five N species (N1−N5) and they can be assigned to pyridinic N (N1 or Npy, 398.5 eV), “pyrrolic” N (N2, 399.9 eV), graphitic N (N3, 401.0 eV), pyridinic N-oxide (N4, 402.9 eV) and NOx (N5, 404.9 eV) by conventional assignment.19 It is clear that the N functional group distribution in the Fe-NC catalyst is distinct to that of Fe-free NC, of which the loss in graphitic N is compensated by the increased content of “pyrrolic” N and Npy. This is due to the ligand stabilization of the N groups by Fe during the pyrolysis. Another intriguing fact is that the metal-free NC is dominated by the supposedly active graphitic N,19 but the material does not exhibit decent activity for 4e-ORR (Eonset = −0.17 V (vs Hg/HgO), with an nav of 2.9; 4650

DOI: 10.1021/acs.chemmater.7b01619 Chem. Mater. 2017, 29, 4649−4653

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Chemistry of Materials profiles are given in Figure S4, SI. Considering the %HO2− (Figure S4a), the addition of 1.6 mM CTAB significantly increases the HO2− generation up to 4 folds (from 9.3% up to 36.7% at −0.67 V), whereas no change is observed in the % HO2− profile upon the addition of SDS. In another case, the addition of CTAB drops the nav values from 3.8 to 3.3, and the addition of SDS again does not result in a change (Figure S4b). These results basically reflect the role of cationic surfactant in facilitating the HO2− generation in situ under the reaction. Interestingly, a peak feature emerged at ca. −0.70 V in the presence of cationic surfactant (also reflected in the jR profile in Figure 3a) is an indication of a switched reaction path. The presence of a peak feature under hydrodynamic condition in such a narrow potential range (−0.6 to −0.8 V) means that either a special intermediate configuration stable within the potential range is responsive to CTAB or there is another potential-activated species beyond −0.70 V that functions to promote HO2− reduction to OH−, unaffected by CTAB. The former would appear an unusual phenomenon because the HO2− production is already enhanced from −0.2 V; it is difficult to imagine another peculiar intermediate configuration with a specific potential dependence. The latter, however, is rather possible because there is a fair amount of FeS NPs present in the catalyst, although the majority is embedded in a thick carbon layer (3−10 nm). Cyclic voltammetry (CV) was employed to clarify their participation in the ORR. In Figure 3c, a redox couple associated with the FeII/FeIII pair of oxidized FeS (i.e., FeOx) can be found apart from the ORR,25 suggesting the Fe species are electrochemically active. The neat voltammogram in the presence of CTAB also suggests the surfactant does not involve in the reaction. The FeIII reduction peak is centered at −1.0 V, although its initiation can be tracked from near −0.70 V (see the dashed line). It is reminded that FeIII needs to be reduced to be active for HO2− reduction as only FeII species is the ORR-active form in FeOx NPs.1 The catalytic role of FeOx is exemplified by a supported Fe2O3 catalyst on nanocarbon, with the results in Figure S5 and discussion in the SI. Hence, the peak feature in the %HO2− profile with CTAB is not a surprising result because the decline in the %HO2− beyond −0.70 V (hilltop) is contributed by the catalytic role of electrochemically activated FeOx.26 The kinetic path selectivity for 2e-ORR (1/γ = kd/ke2, wherein k values are the first-order reaction rate constants) and 4e-ORR (γ = ke2/kd) is displayed in Figure 3d, based on the analytical solution to the ORR electrokinetics.27 Note that this selectivity is a direct presentation of the relative rate of a product-determining path over the other (as depicted in the simplified scheme) in an unbounded scale, and is distinguished from %HO2− and nav values. For the selectivity for 2e-ORR (1/ γ), the profile follows those described in the previous section. Markedly, the 1/γ value rose from 0.1 to close to 0.6 upon addition of 1.6 mM CTAB (−0.4 to −0.7 V), meaning that the relative rate for 4e-ORR decreases from 10 times the rate of 2eORR down to ∼1.7 times of the rate. The drop in 2e-ORR selectivity in the region beyond −0.70 V is, again, attributable to the promoted HO2− reduction by FeOx (path C2, Figure 1b). On the other side, the selectivity for 4e-ORR (γ) in the range of 0.0 to −0.2 V is significantly reduced from ∼220 down to 80 upon the addition of CTAB. It is noted that no OH− is released (of jR) from 0.0 to −0.2 V when the ORR (of jD) is substantial; the delayed HO2− release can be read from Figure 3a,b. This could be due to either nonspecific surfactant coverage of active

site or an inhibitory role of cationic surfactant preventing the direct reduction of O2 to OH− via breaking the O−O bond. When nonspecific coverage is concerned for the O2 transport near the electrode (i.e., a loss in effective surface area), one can expect a plain drop in jD across the reaction potential range. However, there appears a weak transition point in the jD at ca. −0.2 V upon addition of CTAB (the slight change in slope in Figure 3a), right at the point where HO2− release has started. In other words, it is likely that cationic surfactant has an inhibitory role to dissociative 4e-ORR and the surfactant adsorption is not significant to the process, despite of its probable coverage. Because surfactant adsorption to catalyst surface appears an important subject of concern, electrochemical impedance spectroscopy (EIS) was employed to resolve the double layer structure. Figure 4a shows the Nyquist plots of the Fe-NC

Figure 4. (a) Nyquist plots of the Fe-NC catalyst recorded at +0.05 V (vs Hg/HgO) in O2-saturated 0.1 M KOH, with and without surfactants. (b) Schematic diagrams of the corresponding double layer structure at electrode surface with the ionic surfactants. Red, blue and gray spheres represent cations (N+), anions (O) and neutral (C) species, respectively.

catalyst at a nonreaction potential. In simple terms, the first semicircle in high frequency region provides the electrical properties of the electrode and the interface (e.g., the chargetransfer resistance and double layer capacitance), whereas the slope of the inclining tail in low frequency region offers diffusional resistance as described by Warburg impedance.28 In the presence of CTAB, the gradual emergence of another semicircle indicates the formation of a soft, resistive layer or interface on top of the electrode surface. This strongly endorses that a layer of adsorbed CTAB is developed at inner part of the double layer. In contrast, a trivial change was observed for the case of SDS. This suggests that SDS does not form an adsorbed layer and is probably not in direct contact with the electrode. Interestingly, the linear tails at low frequency region share the same gradient for all cases, which suggests that the diffusion capability of ions near the electro-active interface is unaffected by the surfactant adsorption. This phenomenon is unlike what was observed for nonmetal active sites on N doped nanocarbon.22 Perhaps the ion flux toward the electrode surface is insensitive to the soft adsorbed layer and the surfactants only act on the intermediate structure and do not block the active 4651

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Chemistry of Materials sites (i.e., ligand-protected Fe centers), leaving the ion flux and O2 transport to the sites unhindered. A schematic diagram of the double layer structure is illustrated in Figure 4b. As an electrode potential is given at more negative than the measured open-circuit potential (Eoc = +0.12 V, vs Hg/HgO), the electrode surface should be negatively biased. For the case with CTAB, the R4N+ head groups penetrate the double layer and form an adsorbed layer on electrode surface, whereas the tail can distort the double layer and result in a positively biased layer for in situ kinetics modulation. On the other side, SDS is repelled by the negatively biased surface and can only stay near the outer layer without packing and therefore no effect is expected. The above results and discussion allow us to elucidate how cationic surfactant is involved during the course of the ORR on Fe−N4+x active site, in correspondence to Figure 1b. The reaction starts with reductive O2 binding to an active FeII−N4+x site to form a η1-peroxo or η2-peroxo complex. Under a reducing potential, electrons should localize at about the peroxo groups (charge tends to localize at terminals), making them susceptible to the positive field imposed by a layer of cationic surfactant. At a low biased potential (0.0 to −0.2 V, Figure 3a), η2-peroxo complex may be dominated for dissociative O2 reduction (path C1) and hence no HO2− is produced. Here the positive field can retard the O−O bond cleavage by stabilizing the electron density on the peroxo and weakening the Fe−O bond relative to the O−O bond. At this stage, the overpotential is not significant enough to break the Fe−O bond to release HO2−. At a medium potential (−0.2 V to −0.4 V), the η1-peroxo complex should be predominant of a promoted path A kinetics, the η1-peroxo groups are stripped off through Coulombic pulling by the positive field of the surfactant layer and hence result in a facilitated HO2− release (path B). This process should be in competition with dissociative O2 reduction (path C1). At a higher potential (−0.4 to −0.7 V), O2 binding should directly arrive at η1-peroxo complex, making the electrostatic modulation more effective for path B. Beyond −0.7 V, the exposed FeOx begins to take part in the mechanism of the FeIII reduction by promoting the HO2− reduction (path C2). Throughout the process, the mass transport of O2 is not hindered by surfactant adsorption. To this end, it is clear that cationic surfactant takes an important part in the ORR on Fe− N4+x active structure and can modulate the ORR selectivity via intermolecular electrostatic interaction. In conclusion, cationic surfactant can modulate the ORR selectivity on a Fe-NC catalyst by kinetic promotion through Coulombic interaction with the peroxo−Fe−N4+x complexes. Our hypothesis of electrostatic stripping of the peroxo group by cationic surfactant for an improved HO2− yield is successfully validated by experimental results and the mechanism is elucidated with a solved double layer structure. This work offers a fundamental understanding for the ORR at double layer and brings forward the use of a molecular probe for in situ process engineering of an electrocatalytic reaction. It is anticipated to inspire rational design of functional catalysts that would enable multifacet control of catalytic reactions.

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Experimental detail, composition analysis; XPS spectra of Fe-NCs; nav and %HO2− profiles of Fe-NC and NC; XRD patterns of Fe-NC and NC; nav and %HO2− profiles of Fe-NC in the presence of CTAB and SDS (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected] (Dr. K. H. Wu). *Email: [email protected] (Prof. D. Su). ORCID

Kuang-Hsu Wu: 0000-0002-7670-7948 Wei Qi: 0000-0003-1553-7508 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K. H. Wu thanks the International Postdoctoral Exchange Scholar Fellowship and PIFI Postdoctoral Research Fellowship (2017PM0002) from the Chinese Academy of Sciences (CAS). The authors also acknowledge the financial support from NSFC (91645114 and 2161101164) and Shenyang National Laboratory for Materials Science, CAS.

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ABBREVIATIONS Fe-NC, Fe−N doped carbon; NP, nanoparticle; p-PDA, pphenylenediamine; CTAB, cetyltrimethylammonium bromide; SDS, sodium dodecyl sulfate; ORR, oxygen reduction reaction; CV, cyclic voltammetry; EIS, electrochemical impedance spectroscopy; EXAFS, extended X-ray absorption fine structure; SCV, staircase voltammetry; RRDE, rotating ringdisk electrode; XRD, X-ray diffraction; XPS, X-ray photoelectron spectroscopy

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01619. 4652

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DOI: 10.1021/acs.chemmater.7b01619 Chem. Mater. 2017, 29, 4649−4653