www.sciencemag.org/content/357/6350/479/suppl/DC1

Supplementary Materials for

Direct atomic-level insight into the active sites of a high-performance PGM-free ORR catalyst

Hoon T. Chung, David A. Cullen, Drew Higgins, Brian T. Sneed, Edward F. Holby,

Karren L. More, Piotr Zelenay*

*Corresponding author. Email: zelenay@lanl.gov

Published 4 August 2017, Science 357, 479 (2017) DOI: 10.1126/science.aan2255

This PDF file includes:

Materials and Methods Figs. S1 to S13 References

Other Supplementary Materials for this manuscript includes the following: (available at www.sciencemag.org/content/357/6350/479/suppl/DC1)

DFT Data File S1 (PDF)

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Materials and Methods Catalyst synthesis

Aniline was added to 1.5 M HCl solution stirred by a magnetic bar at room temperature on a hot plate, followed by addding cyanamide and FeCl3 as an iron precursor. Once FeCl3 was dissolved, (NH4)2S2O8 (ammonium persulfate, APS) was added as oxidant to the solution to catalyze aniline polymerization. The solution was stirred at room temperature for 4 hours to allow full polymerization of aniline. Carbon (Cabot, Black Pearls 2000), pretreated with 70% nitric acid at 80°C for 8 hours, was ultrasonically dispersed, then mixed with the dispersion containing cyanamide and polyaniline (PANI). The temperature of the hot plate was then increased to 80 °C and the solution dried while stirring until it became tar-like. The subsequent heat treatment was performed at 900 °C in nitrogen atmosphere for 1 hour. After the initial pyrolysis in nitrogen at 900 °C, the resultant expanded into a foam-like structure. The product was ground in a mortar, pre-leached in 0.5 M H2SO4 at 80-90 °C for 8 hours, and washed with ample amount of de-ionized (DI) water. After drying at 90 °C in vacuum oven overnight, the powder was heat-treated again at 900 °C in N2 atmosphere for 3 hours to obtain the final product. Rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) measurements

Rotating ring-disk electrode (RRDE) measurements were performed in a standard three-electrode cell using a CHI Electrochemical Station (Model 750b) and a 5.61 mm diameter glassy carbon disk (disk geometric area 0.247 cm2). To avoid any potential contamination of the catalyst by platinum, all experiments with PGM-free catalysts were carried out using a graphite rod as the counter electrode. Potentials were measured vs. the Ag/AgCl electrode in 3.0 M NaCl and then converted to the reversible hydrogen electrode (RHE) scale. The catalyst ink was prepared by ultrasonically blending for 1 hour 5 mg of catalyst and ~ 10 mg of 5% Nafion® suspension in alcohol (Ion Power, Inc.) in 1.0 mL of isopropyl alcohol (IPA). 30 µl of the thus-prepared catalyst ink was deposited onto the disk yielding an approximate catalyst loading of 0.6 mg cm-2. After drying, the catalyst was activated by ca. 20 cyclic voltammetry (CV) scans in oxygen-saturated electrolyte at a scan rate of 100 mV s-1 and in the potential range of 0.0-1.0 V vs. RHE until a reproducible CV was obtained. Oxygen reduction reaction (ORR) measurements were performed at room temperature, 25±1°C, in O2-saturated 0.5 M H2SO4 at a rotation rate of 900 rpm. ORR polarization plots were recorded under steady-state conditions, starting at 1.0 V vs. RHE down to 0.0 V vs. RHE, using a 0.02 V step and a potential hold time of 25 s. Ring collection efficiency was 36 % as determined using an Fe(CN)64-/3- redox couple. The ring potential was set to 1.3 V vs. RHE in RRDE testing. Fuel cell testing

Catalyst ink for 35 wt% Nafion® electrodes was made by ultrasonically mixing the catalyst, IPA, DI water, and 5% Nafion® suspension in alcohols at a 1:12:12:11 weight ratio for 3 hours. Catalyst inks for electrodes with 50 and 60 wt% contents of Nafion® were prepared by increasing the ionomer content in the suspension while keeping the IPA and DI water volumes the same. The inks were applied to the membrane by brushing until a cathode catalyst of ~ 4.0 mg cm-2 was reached. A commercial Pt-catalyzed gas

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diffusion electrode (GDE, 2.0 mgPt cm-2, Johson Mattthey) was used at the anode. The cathode and anode were hot-pressed onto a Nafion® 211 membrane at 125°C for 3 minutes. The geometric surface area of the MEA was 5.0 cm2. Fuel cell testing was carried out in a cell with single-serpentine flow channels. Pure hydrogen and air/oxygen humidified at 80 °C, were supplied to the anode and cathode at a flow rate of 200 mL min-1. The backpressures at both electrodes were set at values assuring 1.0 bar partial pressure of gases (sum of partial pressures of oxygen and nitrogen in the case of air). Polarization plots were recorded using fuel cell test stations (Fuel Cell Technologies Inc.) in a voltage-control mode. Physical characterization

Catalyst morphology was characterized by scanning electron microscopy (SEM) on a Hitachi S-5400 instrument. Aberration-corrected scanning transmission electron microscopy (STEM) was performed using a Nion UltraSTEM 100 operated at 60 kV and equipped with a Gatan Enfina electron energy loss spectrometer. Aberration-corrected STEM images were acquired using a high-angle annular dark-field (HAADF) detector with a 54-200 mrad collection semi-angle. Surface area and pore size distribution of the samples was measured by Quantachrome Autosorb-iQ using N2. X-ray diffraction (XRD, Siemens, Diffraktometer D5000, Cu Kα) and Raman spectroscopy (Bruker Senterra, 532 nm laser) were used to study the structure and disorders of the catalysts. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Axis DLD Ultra X-ray photoelectron spectrometer using an Al Kα line source monochromatic operating at 150 W. Cross-sections of epoxy-embedded catalyst layers for STEM imaging and energy-dispersive x-ray (EDX) spectroscopy elemental mapping were prepared by ultramicrotomy using a Leica UltraCut UC7 at room temperature. DFT Calculations

Density functional theory calculations were performed using Vienna Ab Initio Simulation Package (VASP).37-40 The generalized gradient approximation (GGA), as parameterized by Perdew, Burke, and Ernzerhof (PBE)41, 42 was used for the exchange and correlation functional. Ions were relaxed using a conjugate gradient algorithm until the Hellmann-Feynman forces on each ion were less than 0.01 eV/Å. The electronic self-consistency loop was converged to within 1×10-5 eV using a residual minimization method direct inversion in the iterative subspace (RMM-DIIS) algorithm. Γ-point centered Monkhorst-Pack k-point meshes of 1×1×1 were utilized for sampling of the Brillouin zone for initial relaxation followed by subsequent relaxation on a 5×5×1 mesh (bulk) or 5×1×1 (zig-zag edge). A plane-wave kinetic energy cutoff of 400 eV was utilized for all simulations. Spin-polarization was included. Vacuum spacing between periodic images of ~10 Å was utilized to reduce self-interactions between these images. Van der Waals interactions were included using the DFT-D2 method of Grimme.43 Computational hydrogen electrode (CHE) and thermodynamic limiting potential formulations were utilized following References 35 and 44. Relative shifts in free energy for intermediate states were taken directly from calculated DFT free energies (in the limit that the smearing of band-structure goes to zero). References 45 and 46 showed relative balancing of additional simulated effects for ORR pathway (including water solvation, zero-point energy, vibrational entropy, and interfacial/potential effects) and discussed the suitability

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of such calculations to discern relative activity. This approach does not explicitly include the kinetics between intermediate states, though activity descriptors calculated in this fashion have proven to capture important shifts in electrocatalytic activity with atomic scale structure (see Reference 47 for further discourse on the topic). Due to the assumed isolated nature of PGM-free active site structures, no consideration of elastic surface interactions due to fractional surface coverage were considered. Calculated thermodynamic limiting potentials, Ul, were utilized for relative comparison of the ORR activity of atomic-scale structures. Calculated free energy reaction pathways necessary for calculation of Ul values are shown in Fig. S13 and relaxed atomic structures of the structures with and without reaction intermediates are provided separately in DFT structure files in VASP input/output file format (CONTCAR). It is the hope of the authors that future publications include such structure files in order to improve reproducibility as well as to accelerate future research geared toward studying additional physicochemical effects necessary for accurately producing catalyst turn over frequency values in silico.

The choice of using an OH ligand as shown in Fig. 4 and described in Fig. S13 is based on the calculated thermochemistry of the ORR pathway considered (Fig. S13). For the bulk-hosted and edge-hosted FeN4 sites considered without ligands, the final protonation step is calculated to be potential determining, i.e., the OH ligand is strongly bound on these sties. The final protonation step is predicted to be endothermic at potentials above 0.04 and 0.08 V vs. CHE respectively, which includes most relevant potentials considered. That is, at all relevant potentials, the OH bound intermediate state is thermodynamically lower energy than the completed ORR state. Thus, it is calculated that the OH-bound structure spontaneously forms in situ under potential and is thermodynamically persistent.

On an extended metal surface such strong OH binding would be considered OH poisoning as the strongly bound OH would block active sites from facilitating ORR. In the more three-dimensional structures discussed here, however, this strongly bound OH can act instead as a spontaneously formed modifying ligand since the “other side” of the structure that does not have the bound OH ligand/site blocked can act as the now-ligand-modified active site. The possibility of a spontaneously formed and persistent OH ligand is discussed in detail in previous publications (References 35 and 36 in the main text) for Fe2N5 structures at graphene edges. This manuscript extends this reasoning to FeN4 structures. Due to the effects of the OH ligand, it is calculated that the ligated structures have a significantly modified ORR reaction pathways (improved thermodynamic limiting potentials for edge structures but not for bulk structures). These calculations further support the hypothesis that edge-hosted FeN4 active sites can have different (improved) ORR activity than their bulk-hosted counter parts, even if their direct detection remains elusive.

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Figures

Fig. S1. TGA of (CM+PANI)-Fe-C vs. PANI-Fe-C.

Fig. S2. (a) HAADF-STEM image of a microtomed cross-section of (CM+PANI)-Fe-C catalyst electrode with 35 wt.% Nafion® content. “C” designates catalyst regions containing both macropores and micropores, “P” large pores in the electrode, and “D” dense catalyst regions. (b) Fluorine EDX spectroscopy map from the same area shown in (a). (c) Porous and (d) dense phases in (CM+PANI)-Fe-C catalysts within catalyst layer showing ionomer infiltration into porous regions but not into dense

h

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Fig. S4. iR-corrected (a) H2-air and (b) H2-O2 fuel cell polarization plots. Cathode: ca. 4.0 mg cm-2 (CM+PANI)-Fe-C; gas flow 200 mL min.-1; 100% RH; 1.0 bar partial pressure; anode: 2.0 mgPt cm-2 Pt/C; H2 200 mL min.-1; 100% RH; 1.0 bar partial pressure; membrane: Nafion® 211; cell: 80°C; 5 cm2 electrode area.

Fig. S3. (CM+PANI)-Fe-C catalyst RDE cycling durability test in nitrogen between 0.2 and 1.0 V in 0.5 M H2SO4 (catalyst loading, 0.6 mg cm-2). (a) Plots of potential vs. mass transport-corrected kinetic current density. (b) Change in potential as a function of cycle number.

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Fig. S5. XPS results of (CM+PANI)-Fe-C for N1s peak; peak fitting indicates presence of pyridinic-, pyrrolic-, and graphitic-nitrogen bonded species.

Binding energy (eV)396398400402404406408

Inte

nsity

(a.u

.)

measuredpyridinic Npyrrolic Ngraphitic NNO(1)NO(2)NO(3)envolope

Fig. S6. HAADF-STEM images of the primary fibrous carbon phase of (CM+PANI)-Fe-C catalysts. Carbon particles consisted of randomly oriented, intertwined, turbostratic graphitic grains/domains as shown in (a). The bright atoms in (b) are Fe atoms (with some Si), which are associated primarily with exposed edges of the intertwined graphite grain/domain in this phase. Due to their instability under the electron beam during STEM, EELS spectrum imaging of individual Fe atoms was not feasible, but the EEL spectrum in (c) acquired of the closely spaced atoms in the green box shown in (b) confirms the atoms are Fe. N is also detected in this area.

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Fig. S7. Raman spectrum of (CM+PANI)-Fe-C catalyst.

Fig. S8. X-ray diffraction pattern of (CM+PANI)-Fe-C catalyst.

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Fig. S9. BF-STEM and corresponding HAADF-STEM images of the phases in (CM+PANI)-Fe-C catalysts. Large, isolated Fe and FeS particles are circled in yellow.

Fig. S10. HAADF-STEM images of the few-layer graphene phase of the (CM+PANI)-Fe-C catalysts. Individual Fe atoms (spots exhibiting bright contrast) were routinely observed on or within the layers of the graphene sheets.

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Fig. S11. HAADF-STEM images with EEL spectrum images of a single Fe atom associated with the few-layer graphene sheet phase. The presence of N around the Fe atom indicates their association within the graphene. This represents the first direct observation of the proposed Fe-N active site in such PGM-free catalyst.

Fig. S12. HAADF-STEM of a single layer graphene region found within a few-layer graphene sheet. Most of the Fe was observed at the step-sites between single and double (or multi-) layer graphene. A single Fe atom was also observed in a substitutional site within the graphene lattice.

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Fig. S13. Reaction pathways/system energy graph at 0.00 V vs. CHE for bulk-hosted and zig-zag-edge-hosted FeN4 structures with and without OH ligand. Reaction coordinates represent: (1) * + O2 + 4 H+ + 4 e-; (2) *OO + 4 H+ + 4 e-; (3) *OOH + 3 H+ + 3 e-; (4) *O + H2O + 2 H+ + 2 e-; (5) *OH + H2O + H+ + e-; (6) * + 2 H2O. * represents either a free site or intermediate bound site. Note that the Bulk + OH structure does not exothermically bind O2 and reaction coordinate (3) has a spontaneously dissociated OOH (*O + OH). Reaction coordinate (5) for bulk and zig-zag edge occur at 0.04 eV and 0.08 eV respectively indicating that a spontaneously formed and persistent OH ligand should occur at potentials greater than 0.04 V and 0.08 V vs. CHE respectively (these are also the limiting potentials for bulk and zig-zag edge structures respectively). This spontaneous OH ligand modifies the zig-zag-edge-hosted FeN4 structure, leading to the calculated limiting potential of 0.80 V vs. CHE. It is interesting to note that if the bulk-hosted FeN4 site with OH ligand did evolve H2O from the bound O (from the spontaneous dissociation of OOH, perhaps in a multi-site reaction), that the protonation of that bound O would likely define the potential determining step with an Ul value of 0.70 V. This further suggests that even with an altered reaction pathway, bulk-hosted FeN4 structures are catalytically inferior to their edge-hosted counterparts.

0

1

2

3

4

5

6

1 2 3 4 5 6Rel

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e Fr

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negy

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Reaction Coordinate

Ideal Bulk Bulk+OH Zig-Zag Edge Zig-Zag Edge+OH

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2Op of 1.0 bar and a high O2 flow rate of 260 ml min–1 cm–2 (12) and,

independently, at a high2O

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37. G. Kresse, J. Furthmuller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6, 15–50 (1996). doi:10.1016/0927-0256(96)00008-0

38. G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B Condens. Matter 54, 11169–11186 (1996). doi:10.1103/PhysRevB.54.11169 Medline

39. G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals. Phys. Rev. B 47, 558–561 (1993). doi:10.1103/PhysRevB.47.558 Medline

40. G. Kresse, J. Hafner, Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium. Phys. Rev. B 49, 14251–14269 (1994). doi:10.1103/PhysRevB.49.14251 Medline

41. J. P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868 (1996). doi:10.1103/PhysRevLett.77.3865 Medline

42. J. P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple. Phys. Rev. Lett. 78, 1396–1396 (1997). doi:10.1103/PhysRevLett.78.1396 Medline

43. S. Grimme, Semiempirical GGA-type density functional constructed with a long-range dispersion correction. J. Comput. Chem. 27, 1787–1799 (2006). doi:10.1002/jcc.20495 Medline

44. J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, T. Bligaard, H. Jónsson, Origin of the overpotential for oxygen reduction at a fuel-cell cathode. J. Phys. Chem. B 108, 17886–17892 (2004). doi:10.1021/jp047349j

45. A. B. Anderson, Insights into electrocatalysis. Phys. Chem. Chem. Phys. 14, 1330–1338 (2012). doi:10.1039/C2CP23616H Medline

46. A. Lyalin, A. Nakayama, K. Uosaki, T. Taketsugu, Theoretical predictions for hexagonal BN based nanomaterials as electrocatalysts for the oxygen reduction reaction. Phys. Chem. Chem. Phys. 15, 2809–2820 (2013). doi:10.1039/c2cp42907a Medline

47. T. Bligaard, J. K. Nørskov, S. Dahl, J. Matthiesen, C. H. Christensen, J. Sehested, The Brønsted-Evans-Polanyi relation and the volcano curve in heterogeneous catalysis. J. Catal. 224, 206–217 (2004). doi:10.1016/j.jcat.2004.02.034

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Materials and MethodsFig. S1. TGA of (CM+PANI)-Fe-C vs. PANI-Fe-C.Fig. S3. (CM+PANI)-Fe-C catalyst RDE cycling durability test in nitrogen between 0.2 and 1.0 V in 0.5 M H2SO4 (catalyst loading, 0.6 mg cm-2). (a) Plots of potential vs. mass transport-corrected kinetic current density. (b) Change in potential as a fu...Fig. S4. iR-corrected (a) H2-air and (b) H2-O2 fuel cell polarization plots. Cathode: ca. 4.0 mg cm-2 (CM+PANI)-Fe-C; gas flow 200 mL min.-1; 100% RH; 1.0 bar partial pressure; anode: 2.0 mgPt cm-2 Pt/C; H2 200 mL min.-1; 100% RH; 1.0 bar partial pres...Fig. S5. XPS results of (CM+PANI)-Fe-C for N1s peak; peak fitting indicates presence of pyridinic-, pyrrolic-, and graphitic-nitrogen bonded species.Fig. S11. HAADF-STEM images with EEL spectrum images of a single Fe atom associated with the few-layer graphene sheet phase. The presence of N around the Fe atom indicates their association within the graphene. 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