Supplementary Information: Cobalt phosphide-based nanoparticles as bifunctional electrocatalysts for alkaline water splitting Julian A. Vigil, Timothy N. Lambert* and Benjamin Christensen Department of Materials, Devices & Energy Technologies, Sandia National Laboratories, Albuquerque, New Mexico, 87185, USA *Fax: 505 844 7786; Tel: 505 284 6967; e-mail: tnlambe@sandia.gov. Abbreviations List: BET – Brunauer–Emmett–Teller (surface area theory) CPP – Cobalt phosphide-based catalyst CPP-2 – Cobalt phosphide-based catalyst (after second phosphidation) CV – Cyclic voltammetry ECSA – Electrochemically active surface area HER – Hydrogen evolution reaction ICP – Inductively coupled plasma MS – Mass spectroscopy NCPP – Nickel-modified cobalt phosphide-based catalyst NCPP-2 – Nickel-modified cobalt phosphide-based catalyst (after second phosphidation) OER – Oxygen evolution reaction RDE – Rotating disk electrode RHE – Reversible hydrogen electrode SAED – selected area electron diffraction STEM – Scanning transmission electron microscopy XPS – X-ray photoelectron spectroscopy XRD – X-ray diffraction b – Tafel slope υ – Scan rate

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2016

Experimental General

Cobalt acetate tetrahydrate [Co(OOCCH3)2·4H2O] and sodium hypophosphite

monohydrate (Na2PO2·H2O) were purchased from Alfa Aesar. Nafion® perflourinated

resin solution (5 wt. % in lower aliphatic alcohols and water), potassium hydroxide

(KOH) and ammonium hydroxide in water (NH4OH, 28%) were purchased from Sigma-

Aldrich. Ethyl alcohol (200 proof) was purchased from Pharmco-Aaper. Nickle chloride

hexahydrate (NiCl2·6 H2O) was purchased from Fisher Scientific. Reagents were used as

received, with no further purification.

Materials Characterization

Scanning Transmission Electron Microscopy (STEM). Scanning transmission

electron microscopy (STEM) was performed on an FEI Company Titan G2 80-200

operated at 200kV, equipped with a spherical aberration corrector on the probe-forming

optics and four silicon-drift X-ray detectors. Both bright-field and high-angle annular

dark field images were acquired with a sub-0.136 nm probe with a current of 200pA and

convergence angle of 18 mrad. TEM, SAED, STEM, and X-ray microanalysis data were

also acquired with an FEI Company Tecnai F30-ST operated at 300kV. The X-ray

spectral image (full spectrum from each pixel in an array) data were analyzed with

Sandia’s automated eXpert Spectral Image (AXSIA) multivariate statistical analysis

software1-2 to both filter noise and extract the relevant chemical components with no a

priori input as to what elements may be present. SAED patterns were analyzed to extract

inter-planar spacing for comparison to data from the Powder Diffraction Files (PDF) of

the relevant chemical phases.

Powder X-ray diffraction (XRD): Diffraction spectra were collected using a

PANalytical X’Pert PRO powder diffractometer connected to the X’Pert Data Collector

software. Ten or thirty minute scans were run with a power of 45 kV and 40 mA. Bulk

powders were placed directly on to holders for analysis. The spectrum was analyzed

using MDI Jade 9 software with the ICPDD database. Background subtracted spectra are

shown. Background subtraction, whole pattern fitting, and peak finding was done using

the built-in funcitons in the MDI Jade 9 software package.

X-ray photoelectron spectroscopy (XPS): Samples were analyzed via XPS at

pressures less than 5 x 10-9 Torr. XPS was performed using a Kratos Axis Ultra DLD

instrument using monochromatic Al Kα (1486.7 eV) source. The analysis area was an

elliptical spot size of 300 x 700 microns. Several locations on each sample were

analyzed to obtain a representative sampling. Survey spectra were recorded with an 80

eV pass energy, 500 meV step sizes, and 100 ms dwell times. High resolution spectra

were recorded with a 20 eV pass energy, 50 meV step sizes, and 100 ms dwell times.

Charge neutralization was used for all samples to reduce any potential differential

charging effects. Data processing was performed with CasaXPS Version 2.3.15. High

resolution core-level peaks were compared by normalizing counts for each respective

core-level.

Inductively coupled plasma mass spectrometry (ICP-MS): Co3O4, NixCo3-xO4,

CPP and NCPP powders were digested in concentrated (~70%) HNO3. Digested sample

solutions were diluted to appropriate detection range (0.1-1 ppm) with 1% HNO3. Blank

1% HNO3 and 0.1, 1, 5 and 10 ppm combined Ni and Co in 1% HNO3 were prepared as

standards for calibration. Measurements were taken by a Perkin Elmer Elan 6100 model

ICP Mass Spectrometer. Intensities for each sample were averaged from eight

measurements and repeated in triplicate. Elan Instrument Control software was used to

control the auto sampler and spectrometer, as well as perform calibration prior to sample

measurement.

Brunauer–Emmett–Teller surface area (BET): BET surface area was calculated

from gas adsorption data obtained for CPP and NCPP using a Micromeritics ASAP 2020

Surface Area & Porosity Analyzer and the ASAP 2020 V4.03 software. CPP and NCPP

nanoparticulate powders were degassed for 1200 min. after an initial ramp period of 5

mm Hg s-1 evacuation to 10 µm Hg and 10 °C s-1 heating to 100 °C. The sample tube with

a filler rod was submerged in liquid N2 at 77.35 K for analysis. Relative pressure (p/p°)

was varied from 0.01 to 1 with an equilibration interval 10 s, using N2 as the adsorptive

gas.

CPP Synthesis

Co3O4 nanoparticles were synthesized by adapting established procedures.3-4 In a

typical procedure, cobalt acetate (0.5 g, 2.01 mmol) was mixed with deionized water (25

mL) and stirred in air for ten minutes. Ammonium hydroxide (28% in H2O, 2.5 mL, 18

mmol) was then added and stirred for an additional ten minutes. The solution was

transferred to a 48.0 mL stainless steel autoclave, sealed and placed in a furnace at 175

°C for 24 h. The autoclave was then removed, and the slurry transferred to 10 mL

centrifuge tubes. The particles were centrifuged, washed and redistributed repeatedly

using water and ethanol. The suspension was then dried using a rotary evaporator to

isolate the Co3O4 nanoparticles and then dried in vacuo overnight. A small amount of

CoO (PDF #01-075-0393) was sometimes observed along with the Co3O4 (PDF #01-071-

4921). Figure S6 shows a representative XRD for this case. Whole pattern matching

using Jade 9.0 software gave an approximate phase distribution of 96% Co3O4 and 4%

CoO.

The Co3O4 nanoparticles were mixed with Na2PO2·H2O by mortar and pestle to

ensure intimate mixing and then transferred to a crucible. The crucible was transferred to

an Ar-filled glovebox after evacuation in the antechamber. A furnace in the air-free

environment was used to heat the mixture in the crucible to 300 °C rapidly (~ 55 °C min-

1), holding at 300 °C for two hours, and then gently cooling to room temperature. Once

cool, the CPP catalyst was removed from the furnace and glove box, re-dispersed in

water, and washed and dried by the same method outlined above. Whole pattern

matching using Jade 9.0 software gave an approximate phase distribution of 80% CoP

and 20% Co2P. CPP-2 was prepared by subjecting the CPP to a second phosphidation

cycle and workup identical to that described above. Whole pattern matching, again using

Jade 9.0 software, gave an approximate phase distribution of 87% CoP and 13% Co2P.

NCPP Synthesis

Co3O4 particles (100 mg, 0.42 mmol) were combined in a flask with NiCl2·6H2O

(200 mg, 0.84 mmol) and DI H2O (100 mL).5 The suspension was then stirred for 36

hours. The suspension was centrifuged, washed and re-dispersed repeatedly as outlined

above, however the final solvent dispersion was in THF. A rotary evaporator was used to

isolate the NixCo3-xO4 nanoparticles which were then dried overnight in vacuo. Any CoO

originally present in the Co3O4 spinel was retained in the NixCo3-xO4. Whole pattern

matching using Jade 9.0 software gave a similar approximate phase distribution of 97%

Co3O4 and 3% CoO. The NixCo3-xO4 nanoparticles were then mixed with sodium

hypophosphite, heated, washed and isolated by the same methods outlined above to

isolate the NCPP catalyst. Whole pattern matching using Jade 9.0 software gave an

approximate phase distribution of 81% CoP and 19% Co2P. NCPP-2 was prepared by

subjecting the NCPP to a second phosphidation cycle and workup identical to that

described above. Whole pattern matching, again using Jade 9.0 software, gave an

approximate phase distribution of 89% CoP and 11% Co2P.

Electrocatalytic Measurements

2.5 mg of catalyst powder was added to 700 µL DI H2O, 250 µg EtOH and 50 µg

Nafion solution and placed in the bath sonicator for 20 minutes to prepare the desired

catalyst ink. Prior to drop coating, electrodes were polished with aqueous alumina slurry

and rinsed with DI H2O and EtOH. A 5 µL aliquot of the sonicated ink was drop-cast on

to the surface of a glassy carbon rotating disk electrode (Bioanalytical   Systems,   Inc.  

(BASi), geometric area = 0.0788 cm2) and allowed to dry overnight. The active catalyst

loading was thus 158 µg cm-2.

Hydrogen evolution (HER) and oxygen evolution (OER) were assessed by cyclic

voltammetry (CV) in a rotating disk electrode (RDE) set-up. The RDE-2 (BASi) unit was

connected to a Versastat 4 (Princeton Applied Research, PAR) potentiostat operated by

the VersaStudio (PAR) software suite. The three-electrode cell consisted of the above

mentioned glassy carbon rotating disk working electrode, a Pt wire counter electrode and

a Ag/AgCl (3 M NaCl) reference electrode. Potential was scanned from 1.23 V vs the

reversible hydrogen electrode (RHE) to 1.8 V vs RHE to assess OER activity, and 0.1 V

vs RHE to -0.6 V vs RHE to assess HER activity. The OER CV was repeated two times

and the second was chosen for analysis, as the first scan often exhibited capacitive

behavior in the pre-onset region, which obscured OER onset and current. OER stability

was assessed by a galvanostatic experiment for two hours at constant current of 0.788

mA (jgeo = 10 mA cm2). HER stability was assessed by reptitive CV scans in the potential

window beginning as above and ending when the current reached -0.788 mA (jgeo = 10

mA cm2) for 100 times. All experiments were performed using N2 purged (20 minutes)

and blanketed atmosphere, with a rotation rate of 1600 RPM and a scan rate (except

galvanostatic OER stability) of 10 mV s-1. Electrochemical impedance spectroscopy

(EIS) was assessed under galvanostatic conditions, at OER and HER current density

values of 10 mA/cm2 and -10 mA/cm2, respectively. Spectra were collected from 104 to 1

Hz while collecting 10 points per decade. The EIS experiments were carried out under

identical electrolytic conditions as the above electrocatalytic experiments: 1 M KOH

under purging/blanketing of N2 gas. Spectra were fit to an equivalent circuit using ZView

software.

The Tafel slope (b) was calculated in the potential window ± 10 mV relative to

the onset potential. The onset potential was calculated by the tangential method,6-7 i.e. the

intersection of extrapolated tangent lines in the pre- and post-reaction onset regions of the

CV curve. This method likely underestimated the onset values but has proven more

reliable. Catalyst activity was compared and normalized at 1.6 V vs. RHE for the OER

and -0.15 V vs. RHE. These values were chosen as potentials in which both NCPP and

CPP catalysts had passed the onset overpotential, but the more active catalyst (NCPP)

had not yet reached 10 mA cm-2 activity.

Additional Tables and Figures

Figure S1. XRD patterns of the Co3O4 (orange) and Ni0.15Co2.85O4 (light blue). Whole pattern matching using Jade 9.0 software gave a similar approximate phase distribution of 97% Co3O4 (PDF # 01-071-4921) and 3% CoO (PDF# 01-075-0393) for Ni0.15Co2.85O4 and 96% Co3O4 and 4% CoO for Co3O4, indicating phase retention during Ni insertion. Only major index lines are provided, with (specific assignments) also provided for the Co3O4 indices.

Figure S2. TEM images and EDS mapping of Co3O4 and Ni0.15Co2.85O4 nanoparticles.

Co3O4

Ni0.15Co2.85O4

Table S1. ICP-MS elemental analysis of Co3O4, NixCo3-xO4, CPP and NCPP digested powders.

Co concentration

(mmol L-1)

Ni concentration

(mmol L-1)

Co : Ni

Co3O4 1.47 x 10-2 8.60 x 10-5 171:1

NixCo3-xO4 8.66 x 10-3 4.21 x 10-4 20.5:1

CPP 1.99 x 10-4 2.20 x 10-5 91.0:1

NCPP 2.65 x 10-3 1.38 x 10-4 19.3:1

Figure S3. TEM images and EDS mapping of CPP nanorods.

CPP  

CPP  

Figure S4. a) Bright field images, b) SAED [Major reflections are provided] and c) EDS spectra of NCPP nanoparticles.

d  (Å),  Index  assignments 2.83,  (011)  CoP 2.49,  (111)  CoP 1.89,  (211)  CoP 1.74,  (103)  CoP,  (002/230)  Co2P  

a

a a a

b

c

Figure S5. a) Additional Bright field image showing some nanorod-like features observed for NCPP; b) SAED of area shown in a, many reflections are present, major assignments are indicated in bold, some minor co-assignments may be coincidental but evidence of some oxide was observed; c) HAADF image of NCPP nanoparticle cluster and d) EDS mapping of NCPP.

d  (Å),  Index  assignment 2.81,  (011)  CoP 2.46,  (111)  CoP,  (311)  Co3O4,  (111)  CoO 2.20,  (121)  Co2P,  (211)  Co2P 2.13,  (211)  Co2P,  (200)  CoO 1.95,  (112)  CoP 1.88  (211)  CoP,  (202)  CoP 1.74  (103)  CoP,  (002)(230)  Co2P 1.63  (422)  Co3O4 1.51  (220)  CoO

a b

c d d

d d

Figure S6. XRD patterns of the CPP (red) and NCPP (purple). Whole pattern matching using Jade 9.0 software gave nearly identical approximate phase distribution of 81% CoP and 19% Co2P for NCPP and 80% CoP and 20% Co2P for CPP. The minor oxide CoO also appears in the CPP pattern however was not quantifiable in whole pattern matching. Only major index lines are provided, with (specific assignments) also provided for the CoP indices.

Figure S7. a) XRD of twice-phosphidized CPP (CPP-2, red) and NCPP (NCPP-2, purple); Only major index lines are provided, with (specific assignments) also provided for the CoP (black) and Co2P (blue) indices. Co3O4 and CoO are included to demonstrate the lack of oxide; b) OER activity comparison of CPP and NCPP (solid) vs. CPP-2 and NCPP-2 (dashed) in 1 M KOH; (c) HER activity comparison of CPP and NCPP (solid) vs. CPP-2 and NCPP-2 (dashed) in 1 M KOH.

Figure S8. Sample STEM mapping image and spectra indicating no core-shell structure for small nanorod like area of NCPP. The X-ray spectral image (full spectrum from each pixel in an array) data were analyzed with Sandia’s Automated eXpert Spectral Image (AXSIA) multivariate statistical analysis software to both filter noise and extract the relevant chemical components with no a priori input as to what elements may be present. Only one component could be resolved from the data. Both smaller rod like and spherical particles were imaged and found to be consistent.  

P

Co Co

Co Ni

O

Figure S9. Exchange current density extrapolation from the Tafel plots for 20% Pt/C (black), 20% Ir/C (grey), NCPP (purple), CPP (red), Ni0.15Co2.85O4 (blue) and Co3O4 (orange). OER (left) shows 20% Ir/C (10-4.69 mA cm-2), NCPP (10-3.88 mA cm-2), CPP (10-4.82 mA cm-2), Ni0.15Co2.85O4 (10-4.51 mA cm-2) and Co3O4 (10-4.84 mA cm-2). HER (right) shows 20% Pt/C (10-0.05 mA cm-2), NCPP (10-1.78 mA cm-2), CPP (10-2.24 mA cm-

2), Ni0.15Co2.85O4 (10-2.33 mA cm-2) and Co3O4 (10-4.49 mA cm-2).

Table S2. Comparison of selected state-of-the-art Ni-based and Co-based HER/OER bifunctional electrocatalysts tested in alkaline media.

Electrode

preparation/ catalyst loading

(mg cm-2)

Electrolyte

HER

Tafel slope (mV dec-1)

HER

η-10 mA cm-2

(V)

OER

Tafel slope (mV dec-1)

OER

η 10 mA cm-2

(V)

NCPP [this work] Particles on GC/ 0.158

1 M KOH 63 0.18 82 0.36

CPP [this work] Particles on GC/ 0.158

1 M KOH 60 0.20 83 0.43

Co@N-C8 Particles on C/ NR

1 M KOH NR 0.21 108 0.42

Co3O4 NCs9 Particles on C/ 0.35

1 M KOH 101 >0.3 116 0.32‡

CoOx@CN10 Particles on C/ 0.42

1 M KOH 115 0.23 NR 0.41‡

PCPTF11 Electrodeposited film on Au/ 0.1

1 M KOH 53 0.375‡ NR 0.30‡

Co-P12 Electrodeposited film on Cu/ 1.0

1 M KOH 42 0.094 47 0.35

CoP/Au7 Electrodeposited film on Au/ 0.14

1 M KOH NR 0.20 67 0.34

Co9S8@MoS2/ CNF13

Particles on C/ 0.21

1 M KOH NR NR 61 0.43

Ni2P nanoparticles14

Ni foam/ 0.14 (OER), 1.8

(HER)

1 M KOH NR 0.22 59 0.29

2-cycle NiFeOx15 Particles on C/

1.6 1 M KOH NR 0.084 31.5 <0.3

NiSe/NF16 Particles on Ni

foam/ 2.8 1 M KOH 120 0.096 64 <0.3

Ni3S2/NF17 Particles on Ni foam/ 1.6

1 M KOH NR 0.22 NR 0.26

Ni3S2/AT-Ni foam18

Particles on Ni foam/ NR

1 M KOH 104 0.20 163 0.22*

Ni5P419 Particles on Ni

foil/ NR 1 M KOH 53 0.15 40 0.33‡

η = Overpotential; b = Tafel slope; NR = not reported; NA = not attained; C = carbon support; GC = Glassy carbon electrode. ‡Values have been approximated from figures in the cited texts. *OER onset current obscured by redox behavior.

Table S3. Comparison of selected state-of-the-art Ni-based and Co-based OER electrocatalysts tested in alkaline media.

Electrode

preparation/ catalyst loading

(mg cm-2)

Electrolyte

OER

Tafel slope (mV dec-1)

OER

η10 mA cm-2

(V)

NCPP [this work] Particles on GC/ 0.158

1 M KOH 82 0.36

CPP [this work] Particles on GC/ 0.158

1 M KOH 83 0.43

CoP NR20 Particles on GC/ 0.71

1 M KOH

84 0.49

CoP NP20 Particles on GC/ 0.71

1 M KOH 96 0.52

CoP hollow polyhedrons21

Particles on GC/ 0.1

1 M KOH 57 0.4

CoxOy/NC22 Particles on GC/ 0.21

0.1 M KOH

NR 0.43

Ni0.6Co2.4O46 Film on Ni/

0.127 0.1 M KOH

NR 0.53

NiCo LDH23 Sheets on carbon paper/ 0.17

1 M KOH 40 0.37

CoCo-NS24 Particles on GC/

0.07 1 M KOH 45 0.35

NiCo-NS24 Particles on GC/ 0.07

1 M KOH 41 0.33

NiFe-NS24 Particles on GC/ 0.07

1 M KOH 40 0.3

η = Overpotential; b = Tafel slope; NR = not reported; NA = not attained; C = carbon support; GC = Glassy carbon electrode.

Table S4. Comparison of selected state-of-the-art Ni-based and Co-based HER electrocatalysts tested in alkaline media.

Electrode

preparation/ catalyst loading

(mg cm-2)

Electrolyte

HER

Tafel slope (mV dec-1)

HER

η - 10mA cm-2

(V)

NCPP [this work] Particles on GC/ 0.158

1 M KOH 63 0.18

CPP [this work] Particles on GC/ 0.158

1 M KOH 60 0.20

CoP/CC25 Array on carbon cloth/ 0.92

1 M KOH 129 0.21

Ni2P26 Particles on Ti/ 1.0

1 M KOH NR 0.18‡

N-CO@G27 Particles on C/ 0.285

0.1 M NaOH

98 0.33‡

η = Overpotential; b = Tafel slope; NR = not reported; NA = not attained; C = carbon support; GC = Glassy carbon electrode. ‡Values have been approximated from figures in the cited texts.

Figure S10. Determination of the electrochemically active surface area (ECSA) for CPP (red) and NCPP (purple). CV scans in a non-Faradaic region were done between 0.22 V and 0.32 V vs. RHE to observe scan rate dependence (top). Current increases with subsequent scans, as the scan rate (υ) was changed from 10 to 100 mV s-1 in increments of 10. The ECSA was calculated using the slope of the scan rate-current density plot (bottom), as explained below. The anodic and cathodic scan current density values were chosen at a potential 10% from the switching potential in a given scan, i.e. 0.31 V and 0.23 V for anodic and cathodic scans, respectively.7

Slope = (ǀmanodic scansǀ + ǀmcathodic scansǀ)/2

CDL = Slope • 0.0788 cm2

ECSA = CDL/ 0.00004 F cm-2

RF = ECSA/ 0.0788 cm2

Table S5. Physical and electrochemical surface area and number of active sites, CPP and NCPP catalysts.

ECSA (m2 g-1 )

Roughness factor

BET surface area (m2 g-1)

Number of active sites (moles)

CPP

0.57 0.89

9.89 8.08 x 10-9

NCPP

12.28 19.47 24.4 1.71 x 10-8

Table S6. OER and HER activity scaling of the CPP and NCPP catalysts.

OER geometric

current density at 1.6 V vs

RHE (mA cm-2)

HER geometric

current density at -0.15 V vs

RHE (mA cm-2)

OER TOF at 1.6 V

vs RHE (Hz)

HER TOF at -0.15 V vs RHE

(Hz)

OER geometric

mass activity at 1.6 V vs.

RHE (mA mg-1)

HER geometric

mass activity at -0.15 V vs

RHE (mA mg-1)

CPP

2.1 -2.2 0.055 0.108 13.3 -13.9

NCPP

10.5 -4.2 0.125 0.100 66.4 -26.6

OER ECSA-

specific activity at 1.6

V vs RHE (mA cm-2)

HER ECSA-specific

activity at -0.15 V vs RHE

(mA cm-2)

OER BET-specific

activity at 1.6 V vs RHE (mA cm-2)

HER BET-specific

activity at -0.15 V vs RHE

(mA cm-2) CPP

2.33 -2.44 0.134 -0.141

NCPP

0.541 -0.216 0.272 -0.109

Figure S11. Static CV scans in 1 M phosphate buffer solution, CPP (red) and NCPP (purple) catalysts. The total charge for CPP and NCPP are 1.56 and 3.30 mC, respectively. The number of active sites (N) and turnover frequency (TOF) were calculated according to a literature procedure (Q = charge, F = Faraday’s constant, I = current, n = number of electrons transferred):28-29

N = Q / 2F

TOF = I / nFN

Figure S12. OER and HER (left, right, respectively) TOF vs potential for the CPP (red) and NCPP (purple) catalysts.

Figure S13. Static CV scans in the OER onset region, CPP (red) and NCPP (purple) catalysts, cycle 1 (solid) vs cycle 20 (dashed). Cycle 1 shows no obvious redox in the potential window preceding water oxidation as to be expected for Ni if present at the surface of the electrocatalysts. Furthermore, cycle 20 shows an increase in capacitive behavior prior to water oxidation; however there is no discernable Ni 2+ to 3+ redox peak at 1.38 V vs. RHE.14 The lower surface area CPP also exhibits an increase in capacitive current from cycle 1 to cycle 20.

Figure S14. Galvanostatic Electrochemical Impedance Spectroscopy (EIS) spectra of a), c) CPP and b), d) NCPP. Spectra were collected prior to testing (green) and after 1 hour (blue) of constant OER @-10 mA/cm2 or HER @10 mA/cm2. Modeling to a modified Randles circuit yielded charge transfer resistance (RCT) values of: HER (initial): 48.6 Ω (CPP) vs 48.1 Ω (NCPP) HER (after 1 hour): 50.5 Ω (CPP) vs 49.1 Ω (NCPP) OER (initial): 36.9 Ω (CPP) vs 35.1 Ω (NCPP) OER (after 1 hour): 45.7 Ω (CPP) vs 34.6 Ω (NCPP)

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