Supplemental Information. Hard-Magnet L1 0 -CoPt Nanoparticles. Advance Fuel Cell Catalysis

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1 JOUL, Volume 3 Supplemental Information Hard-Magnet L1 0 -CoPt Nanoparticles Advance Fuel Cell Catalysis Junrui Li, Shubham Sharma, Xiaoming Liu, Yung-Tin Pan, Jacob S. Spendelow, Miaofang Chi, Yukai Jia, Peng Zhang, David A. Cullen, Zheng Xi, Honghong Lin, Zhouyang Yin, Bo Shen, Michelle Muzzio, Chao Yu, Yu Seung Kim, Andrew A. Peterson, Karren L. More, Huiyuan Zhu, and Shouheng Sun

2 This file includes: Figs. S1 to S18 Tables S1 to S6 Methods section

3 Fig. S1. A, TEM image of as-synthesized CoPt NPs in cashew -like shape. B, Size distribution of CoPt NPs (8.86±0.75 nm).

4 Fig. S2. A, HR-TEM image of as-synthesized CoPt NPs. B & C, inverse FFT images of the regions indicated in A (B shows the top square region and C shows the bottom one), showing the d-spacings which are respectively corresponding to the (111) facet and (200) facet in A1-structured CoPt.

5 Fig. S3. A, TEM images to monitor the growth of CoPt NPs at different reaction stages. Nucleation occurs at ~170⁰C and NPs can be isolated at >190⁰C. The samples were obtained by taking a small amount of reaction solution into a glass syringe and quickly injecting into ethanol to quench the NP growth. NPs were washed several times with ethanol to remove non-reacted reagents and solid samples were prepared for TEM and ICP test. NPs obtained at 190⁰C have spherical shape with a size of 2-3 nm and are Pt-rich. The NPs gradually grow into cashew -like shape with increasing amount of Co at 200⁰C. B, Pt/Co ratios obtained at different stages in A, indicating that Pt nucleates first and then catalyzes the deposition of Co.

6 Fig. S4. HR-TEM images of CoPt NPs during the synthesis at 190 C (A) and 200 C (B), respectively. Lattice fringe spacings measured from the images are smaller than those of pure Pt (0.197 nm and nm), indicating the formation of CoPt alloy.

7 Intensity (a.u.) Experimental data Refined result 88% ordering degree Experimental data Refined result 55% ordering degree Fig. S ( ) Refined results of XRD patterns and the experiment results of C-CoPt NPs annealed at 650 C for 6 h (blue) and 1 h (red), respectively, by using the standard PDF card (ICSD ) of L10-CoPt. The idealized XRD pattern has 100% L10-ordering. The good match between the blue dotted plot and the blue curve indicates a high L10-ordering degree of the 6 h annealed C-CoPt (88%) and the poor match in red curves indicates a lower L10-ordering degree of the 1 h annealed C-CoPt (55%). The L10-ordering degree is evaluated by calculating the ratio of I001/I111.

8 Fig. S6. A, STEM image of L10-CoPt/Pt NPs with 2-3 atomic layers of Pt shell over L10-CoPt core. B and C, Enlarged STEM images of the areas in the dashed squares in A, showing clearly the characteristic layered structure of L10-CoPt core and 2-3 atomic layers of Pt shell (pointed by yellow arrows).

9 50 45 L1 0 -CoPt A1-CoPt Co composition (%) Fig. S7. Time (h) ICP results of Co composition changes of L10-CoPt and A1-CoPt NPs from 0 h to 24 h treated in 0.1 HClO4 at 60⁰C in air, where the Co composition of L10-CoPt changed from 49% to 44% in the 24 h of etching whereas the Co composition of A1-CoPt dropped from 48% to 15% after 7 h of etching.

10 Fig. S8. EXAFS spectra of Pt L3 edge and Co K edge of L10-CoPt (A and C) and L10-CoPt/Pt (B and D), in which the black dotted plots are experimental data and the red curves are fitting results.

11 Table S1. EXAFS fitting results of the L10-CoPt and L10-CoPt/Pt NPs shown in Fig. S8. CN stands for coordination number; R is the bond length; σ 2 parameter is the Debye-Waller factor and reflects the structural disorder in the sample, including both dynamic disorder from thermal motion of the atoms and static disorder from packing defects, amorphousness, and the like. The ΔE0 parameter is the absorption edge energy shift; it represents the difference between the experimental E0 energy and that calculated for the structural model used to fit the spectra. System bond CN R(Å) σ 2 (Å 2 )*10-3 ΔE0(eV) Pt-Co 4.3(5) 2.62(1) 5(1) 7.5(7) L10-CoPt Pt-Pt 5(1) 2.72(1) 5(1) 7.5(7) Co-Co 1.3(6) 2.64(4) 8(1) 4(2) Co-Pt 7(2) 2.62(1) 8(1) 4(2) Pt-Co 4.3(5) 2.652(4) 4(2) 8.8(7) L10-CoPt/Pt Pt-Pt 5(1) 2.69(2) 4(2) 8.8(7) Co-Co 1.8(2) 2.65(1) 7.3(4) 6.6(7) Co-Pt 6.3(7) 2.652(4) 7.3(4) 6.6(7)

12 Fig. S9. ORR polarization curves of comm Pt/C (A) and etched A1-CoPt (B) at BOL and EOL (30,000 cycles). CV curves of comm Pt/C (C) and etched A1-CoPt (D) at BOL and EOL (30,000 cycles). MA and MA loss measured at 0.9 V of comm Pt/C (E) and etched A1-CoPt (F) at BOL and EOL (30,000 cycles). Notes: CV curves were collected in N2-saturated 0.1 M HClO4 with a scanning rate of 100 mv/s at room temperature. ORR polarization curves were collected by using LSV method in O2-saturated 0.1 M HClO4 with a scanning rate of 10 mv/s at room temperature. The durability test of all catalysts were conducted by cycling the potential between 0.6 and 1.0 V at a scanning rate of 100 mv/s at 60 C in 0.1 M HClO4 bubbled with O2.

13 Table S2. ECSA, SA, MA and half-wave potential of L10-CoPt/Pt at BOL and EOL in the liquid half-cell test. Catalyst Number of cycles ECSA (m 2 /g) SA (ma/cm 2 ) MA (A/mgPt) Half-wave potential (V) L10-CoPt/Pt

14 Current (A) Fig. S10. Potential vs. RHE (V) CO stripping curve of L10-CoPt/Pt catalyst obtained in 0.1 M HClO4 bubbled with CO at a scan rate of 20 mv/s.

15 Fig. S11. A, HR-TEM image of L10-CoPt/Pt catalyst at BOL, and the composition is Co43Pt57. B, HR-TEM image of L10- CoPt/Pt catalyst at EOL (30,000 ADT cycles) tested in liquid half-cell, and the composition is Co39Pt61. Energydispersive X-ray (EDX) elemental mapping of L10-CoPt/Pt NPs at BOL (C) and at EOL (30,000 ADT cycles) (D) respectively tested in liquid half-cell, in which Co is colored green and Pt is colored red.

16 Fig. S12. A & B, HR-TEM image of L10-CoPt/Pt NPs at BOL and EOL (30,000 cycles) after ADT at 60⁰C. C & D, inverse FFT images of the regions indicated in A and B (the lattice spacing of nm, and nm correspond to the (002), (200) and (111) facet of L10-CoPt, respectively).

17 Fig. S13. A and B, TEM images of comm Pt/C at BOL (A) and EOL (30,000 cycles) after ADT in liquid half-cell at 60⁰C (B), showing the obvious size growth and aggregation of NPs during ADT.

18 Specific Activity (ma/cm 2 ) BOL EOL Fig. S14. Specific activity of L10-CoPt/Pt at BOL and EOL in the MEA.

19 Voltage (V) BOL- 20 % RH EOL- 20 % RH BOL- 100 % RH EOL- 100 % RH Current Density (A cm -2 ) Fig. S15. H2-air fuel cell polarization curves in linear scale recorded on MEAs assembled with L10-CoPt/Pt (8 wt% Pt content, and the Pt loading is mgpt/cm 2 ) at 80 C, with 150 kpaabs H2/air, under 20%/100% relative humidity (RH), and a gas flow rate of 500/1000 sccm.

20 Table S3. Reaction steps via associative and dissociative mechanisms involved in the DFT calculation. Mechanism Reaction steps Associative mechanism O2 (g) + * O2 * (A1) O2 * + H + + e - HOO * (A2) HOO * + H + + e - H2O(l) + O * (A3) O * + H + + e - HO * (A4) HO * + H + + e - H2O(l) + * (A5) Dissociative mechanism ½ O2 (g) + * O * (D1) O * + H + + e - HO * (D2) HO * + H + + e - H2O(l) + * (D3)

21 Table S4. ORR overpotentials (η) on L10-CoPt/Ptx (x =1-3 Pt overlayers) (111) surface and unstrained Pt (111) surface, which were calculated by using η = Δ GPLS/e, where Δ GPLS is the change in free energy for the potential limiting step (D3 for dissociative mechanism and A2 for associative mechanism). System Overpotential (V): Dissociative Overpotential (V): Associative L10-CoPt/Pt L10-CoPt/Pt L10-CoPt/Pt Unstrained Platinum

22 Table S5. ORR overpotentials (η) on L10-FePt/Ptx (111) and L10-CoPt/Ptx) (111) surfaces (x =2-3 Pt overlayer), (the potential-limiting step is D3 for dissociative mechanism and A2 for associative mechanism, respectively). System Overpotential (V): Dissociative Overpotential (V): Associative L10-CoPt/Pt L10-FePt/Pt L10-CoPt/Pt L10-FePt/Pt

23 Table S6. Pt-Pt distances and the calculated elastic strains (ea and eb) calculated along the in-plain a [0-11] and b [-110] directions (see Figure S15) of Pt (111) surfaces in L10-MPt/Ptx systems (x = 2-3 Pt overlayers.). ea/b = (d a/b da/b)/da/b, where ea/b is the in-plane elastic strain along a/b direction, d a/b is the Pt-Pt distance along a/b direction on the Pt (111) surface in the Pt overlayers of L10-MPt/Ptx and da/b is the Pt-Pt distance of the corresponding value from unstrained Pt (111) surface. Note that negative sign for strains indicates compressive strain. System Pt-Pt distance along a direction [0-11] (nm) Pt-Pt distance along b direction [-110] (nm) L10-CoPt/Ptx (111) L10-FePt/Ptx (111) Pt (111) System In-plane strain (ea) In-plane strain (eb) L10-CoPt/Ptx (111) -4.50% -4.25% L10-FePt/Ptx (111) -3.47% -2.23%

24 Fig. S16. a and b directions on Pt (111) models. The adsorbate is O2 *. Note that the Pt-Pt distances (d) along a and b directions on unstrained Pt (111) surface are equal (da = db), while d a d b on Pt (111) surfaces of L10-FePt/Ptx and L10-CoPt/Ptx.

25 Fig. S17. A. ORR polarization curves of L10-CoPt/Pt and L10-FePt/Pt in O2-saturated 0.1 M HClO4 obtained at a scanning rate of 10 mv/s at room temperature. B. Mass activity of L10-FePt/Pt and L10-CoPt/Pt measured at 0.9 V.

26 Fig. S18. A. H2-air fuel cell polarization curves in linear scale recorded on MEAs assembled with L10-FePt/Pt (8 wt% Pt content, and the Pt loading is mgpt/cm 2 ) at 80 C, with 150 kpaabs H2/air, under 100% relative humidity (RH), and a gas flow rate of 500/1000 sccm. B. Mass activities of L10-CoPt/Pt and L10-FePt/Pt tested via the current DOE/fuel cell technologies team (FCTT) protocol (potential step from 0.6 V to 0.9 V and 15 min hold, current averaged during last 1 min), and by measuring the current at 0.9 V (ir-free) in 150 kpaabs H2/O2 (80 C, 100% RH, 500/1000 sccm) with correction for measured H2 cross-over. The horizontal lines indicate the DOE 2020 target requirement for mass activity at BOL and EOL (30,000 ADT cycles), respectively.

27 Methods Chemicals and Materials. All chemicals were used without further purifications. Platinum (II) acetylacetonate (Pt(acac)2) (98%) was purchased from Strem Chemicals. Oleylamine (OAm) (>70%) and cobalt(ii) acetylacetonate (Co(acac)2) (97 %), hexane (98.5%), isopropanol (99.5%), ethanol (100%) and Nafion (5% in a mixture of 2-propanol and water) were purchased from Sigma-Aldrich. The commercial Pt/C catalyst (product code: , 20% mass loading on carbon, Pt particle diameter at nm) was purchased from Fuel Cell Store. Commercial TKK TEC10E20E Pt catalyst (mass loading of 20%) was obtained from Tanaka. Ketjen-300J carbon was obtained from Lion Specialty Chemicals Co., Ltd. Synthesis of A1-CoPt NPs. The synthesis was modified from our previous publication mmol Pt(acac)2, 0.56 mmol Co(acac)2 and 20 ml OAm were mixed and de-gassed at 100 C for 30 min. The reaction solution was then heated to 300 C at a rate of 5 C/min and kept at this temperature for 60 min before it was cooled to room temperature. Hexane (10 ml) and ethanol (70 ml) were added to the reaction solution to precipitate the product. After centrifugation (8500 rpm, 8 min) and washed three times with hexane and ethanol, A1-CoPt NPs was collected and dispersed in hexane for further use. Preparation of carbon-supported L10-CoPt/Pt, etched A1-CoPt and L10-FePt/Pt. Ketjen-300J carbon was suspended in hexane under sonication for 20 min, then A1-CoPt NPs dispersion was dropwise added into the suspension (Pt loading ratio can be tuned from 8 wt% to 22 wt%). After 1 h sonication, the C-supported CoPt (C-CoPt) NPs were dried and annealed under 95% Ar + 5% H2 at 650 C to convert A1-structure into L10- structure. A1-CoPt NPs were converted into L10-CoPt NPs after annealing for 1 h and 6 h, respectively. 6 h annealing resulted in the full ordered L10-CoPt. L10-CoPt was treated in 0.1 M HClO4 at 60 C in air for 24 h, followed by annealing under 95% Ar + 5% H2 at 400 C for 2 h, giving L10-CoPt/Pt catalyst for further study. C-CoPt (A1-structured) was treated with the same procedure to remove most of the Co and be converted into etched A1-CoPt as a control. As a control group, L10-FePt/Pt catalyst with 2~3 atomic layers of Pt was prepared as reported. 2 Characterizations. X-ray diffraction (XRD) patterns of C-NPs were collected by using Bruker AXS D8- Advanced diffractometer with Cu Kα radiation (λ = Å). Rietveld refinement of XRD data was conducted by using MAUD (Material Analysis Using Diffraction) program and hypothesizing the crystal structure is L10- CoPt (ICSD ). The intensity of (111) peak was kept the same in the computerized XRD patterns to the experiment results. The ordering degree of L10-CoPt annealed at different time were determined by comparing the intensity ratio of peak (110)/ peak (111). A pure L10-CoPt crystal phase (100 % ordering degree) has a I110/I111 ratio of and the corresponding value for a pure A1-CoPt crystal phase is zero. Transmission electron microscopy (TEM) images were obtained on a Philips CM20 operating at 200 kv. JEOL electron microscope was used for high-resolution TEM (HR-TEM) imaging analyses. Magnetic hysteresis loop was measured on a Quantum Design Superconducting Quantum Interface Device (SQUID) with a field up to 70 koe. The inductively coupled plasma-atomic emission spectroscopy (ICP-AES) results were obtained on a JY2000 Ultrace ICP atomic emission spectrometer equipped with a JY AS 421 autosampler and 2400g/mm holographic grating. X-ray absorption spectroscopy (XAS) measurements were performed using the Sector 20-BM beamline of the Advanced Photon Source at Argonne National Laboratory. Sample powders were packed between Kapton windows and the extended X-ray Absorption Fine Structure (EXAFS) data were collected at room temperature using a multi-element X-ray fluorescence detector. The end-station was equipped with a double-crystal Si (111) monochromator for wavelength selection, which was detuned to 80% in order to help reject higher harmonics of the desired energy; a toroidal focusing mirror was also employed to further enhance harmonic rejection. Data processing and EXAFS fitting were performed using WinXAS software 2 in conjunction with scattering path amplitude and phase functions calculated using the computer codefeff8. 3, 4 Catalyst ink preparation and electrochemical measurements. Catalyst ink for electrochemical study was prepared by mixing 1 mg carbon-supported catalyst with 800 µl ultrapure water, 200 µl 2-propanol and 10 µl Nafion solution (5 wt%) and sonicating for 1 h. 20 µl of catalyst ink was then deposited onto the glassy carbon rotating disk electrode (5 mm in diameter) and dried via a spin-assisted drying method (700 rpm) for further study. An Autolab 302 potentiostat was used for electrochemical measurement with Ag/AgCl (4 M KCl)

28 as a reference electrode and graphite bar as a counter electrode. Cyclic voltammetry (CV) and linear scanning voltammetry (LSV) methods were used to evaluate the catalyst. All the potentials are referred to RHE. The catalyst was first subject to CV scanning between 0 and 1.2 V at 100 mv/s in N2-saturated 0.1 M HClO4 until a stable CV curve was obtained (typically 50 cycles). ORR polarization curves were recorded via LSV method at a scan rate of 10 mv/s in O2-saturated 0.1 M HClO4 with GC-RDE rotating at 1600 rpm at room temperature. Accelerated durability tests (ADT) of the catalysts for ORR were conducted by cycling the potential between 0.6 and 1.0 V at 100 mv/s at 60 C in 0.1 M HClO4 bubbled with O2. CO stripping test was conducted by first bubbling CO into 0.1 M HClO4 electrolyte and holding potential at 0.1 V for 30 min, followed by bubbling N2 into the electrolyte for 30 min. Then voltammetry curve was collected by scanning from 0.1 V to 1.0 V at a scanning rate of 20 mv/s. MEA Preparation and Fuel Cell Testing. Catalysts were incorporated into MEAs by direct spraying of a water/n-propanol based ink onto a Nafion 211 membrane. MEA break-in was conducted by holding at 0.6 V overnight and performing voltage steps. MEAs were prepared with a Pt loading of mgpt/cm 2. H2-air fuel cell testing was carried out in a single cell using a commercial fuel cell test system (Fuel Cell Technologies Inc.). The MEA was sandwiched between two graphite plates with straight parallel flow channels machined in them. The cell was operated at 80 C, with 150 kpaabs H2/air or H2/O2, and a gas flow rate of 500/1000 sccm for anode/cathode, respectively. Catalyst mass activity was measured via the current DOE/FCTT protocol (potential step from 0.6 V to 0.9 V and 15 min hold, current averaged during last 1 min) and by measuring the current at 0.9 V (ir-free) in 150 kpaabs H2/O2 (80 C, 100% RH, 500/1000 sccm) with correction for measured H2 crossover. The electrochemical active surface area was obtained by calculating HUPD charge in CV curves between V ( V background subtracted); assuming a value 210 µc/cm 2 for the adsorption of a hydrogen monolayer on Pt. (CV curves were obtained under 150 kpaabs H2/N2, 30 C, >100% RH, 500/1000 sccm) The potential cycling accelerated durability test (ADT) was conducted by using trapezoidal wave method from 0.6 V to 0.95 V with 0.5 s rise time and 2.5 s hold time (150 kpaabs H2/N2, 80 C, 100%RH, 200/200 sccm). Computational methods. A DFT-based calculator GPAW along with atomic simulation environment (ASE) is employed for all the calculations. 5, 6 The software utilizes plane wave basis set, the revised Perdew-Burke- Ernzerhof (RPBE) density functional. 7, 8 A 4 x 3 unit cell with four layers and 15 Å vacuum is used for modeling the system. All the calculations were carried out by keeping the bottom layer fixed, while the top three layers and the adsorbates were allowed to relax. The RPBE equilibrated bulk lattice constants used for modeling the surfaces are (3.99 Å) for Pt, (a= 3.82 Å, c = 3.80 Å) for L10-CoPt and (a= 3.90 Å, c = 3.80 Å) for L10-FePt. All the geometry optimizations were carried out using a plane wave cutoff of 450 ev and 4 x 4 x 1 k-point mesh. The lattice constants were calculated using a 7 x 7 x 7 k-point mesh. The calculations involving Fe and Co are spinpolarized. We study the ORR on native Pt (111), L10-CoPt/Ptx (111) and L10-FePt/Ptx (111) surfaces via both dissociative and associative mechanism. 1, 9, 10 (Table S2) The steps involved in both the mechanisms except the formation of O2* (A1) and formation of O* (D1) are potential dependent since they involve a proton-electron pair transfer. We study the intermediates O*, HO*, HOO*, and O2* adsorbed at their energetically most preferential site. Here, x represents the number of Pt overlayers and * represents an adsorption site on the surface. The free energies at 300K were calculated considering the contributions for zero-point energy, entropy, and solvent. The solvent stabilizations of 0.0, -0.60, and ev were used for O*, HO*, HOO*, respectively. 9 The bond energy of O2 is derived from the RPBE free energies of H2O and H2 molecules to accurately calculate the adsorption free energy of O2*. We used the experimental reaction free energy (2.46 ev) for the reaction H2O ½ O2 + H2. Computational hydrogen electrode model was used to obtain the reaction free energy of the reaction A* + (H+ + e-) HA* as that of A* + ½ H2 HA*. 19 A term ΔGU = -eu was added posterior to adjust the reaction potential with respect to 0 V. SUPPLEMENTAL REFERENCES 1. Yu, Y.; Yang, W.; Sun, X.; Zhu, W.; Li, X.-Z.; Sellmyer, D. J.; and Sun, S. (2014). Monodisperse MPt (M = Fe, Co, Ni, Cu, Zn) Nanoparticles Prepared from a Facile Oleylamine Reduction of Metal Salts. Nano Lett., 14,

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