-
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: [email protected]
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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2
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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3
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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4
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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5
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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6
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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7
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.
-
8
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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9
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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10
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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11
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
ativ
e Fr
ee E
negy
vs.
CHE
at U
= O
.0 V
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)
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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. This
represents the first direct
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