Supporting Information Intrinsically Conductive Perovskite Oxides with Enhanced Stability and Electrocatalytic Activity for Oxygen Reduction Reactions Xiaoming Ge,,# Yonghua Du,,# Bing Li, T. S. Andy Hor,, Melinda Sindoro, Yun Zong,*, Hua Zhang*, and Zhaolin Liu*, Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis #08-03, Republic of Singapore 138634 Institute of Chemical and Engineering Science (ICES), A*STAR (Agency for Science, Technology and Research), 1 Pesek Road, Jurong Island, Republic of Singapore 627833 Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Republic of Singapore 639798 # These authors contributed equally. Corresponding authors: * E-mail for Z. L.: zl-liu@imre.a-star.edu.sg. S1
* E-mail for H. Z.: HZhang@ntu.edu.sg. * E-mail for Y. Z.: y-zong@imre.a-star.edu.sg. S2
Experimental Materials. La(NO 3 ) 3 6H 2 O (99.99%), Sr(NO 3 ) 2 (99.995%), Mn(NO 3 ) 2 xh 2 O (99.99%), Ni(NO 3 ) 2 6H 2 O (99.999%), Ca(NO 3 ) 2 4H 2 O (99.98%), citric acid monohydrate (99.5%), Nafion 117 solution (5 wt%), K 3 Fe(CN) 6 (99%), K 4 Fe(CN) 6 3H 2 O (99.95%), KCl (99%), and H 2 O 2 (30 wt.%), 1.0 N standard KOH solution, ultra-high purity O 2 (99.999%) and N 2 (99.99%) were commercially available and used as received. 30 wt.% platinum supported on Vulcan XC-72 carbon (Pt/C) was from E-Tek. Aqueous solutions were prepared with ultrapure water (>18 MΩ cm). Synthesis of La 0.5 Sr 0.5 Mn 1-x Ni x O 3-δ. Lanthanum strontium manganese nickel oxides La 0.5 Sr 0.5 Mn 1-x Ni x O 3-δ (x=0, 0.1, 0.2, 0.3, 0.4, 0.6 and 0.8) were synthesized by a sol-gel method using nitrate salts and citric acid. We illustrated the synthesis route by taking the example of LSMN. 1.5 mmol of lanthanum nitrate, 1.5 mmol of strontium nitrate, 2.7 mmol of manganese nitrate, 0.3 mmol of nickel nitrate, and 12 mmol of citric acid were dissolved in 40 ml H 2 O. The mixed solution was heated at 80 C under magnetic stirring, until a viscous gel was formed. The gel was baked at 180 C for 16 h. The foam-like precursor was grinded in an agate mortar and was calcined at 950 C for 4 h. Electrical conductivity. A qualitative evaluation on the electrical conductivity was realized by cyclic voltammetry (CV) with catalyst-loaded glassy carbon electrode (GCE) as the working electrode. The catalyst loading was 1.2 mg cm -2. The electrolyte was N 2 -saturated 0.1 M KCl solution containing 5 mm K 3 Fe(CN) 6 and 5 mm K 4 Fe(CN) 6. The potential sweeping rate was 100 mv s -1. We employed the facile outer-sphere redox couple ferri/ferrocyanide, [Fe(CN) 6 ] 3 /4, to probe the electron transfer behavior. LSMN-loaded GCE exhibits well-defined Fe(CN) 6 ] 3 /4 redox peaks and notable currents in the CV voltammogram. As can be seen in Figure S8, S3
LSMN-loaded GCE shows the highest peak current among La 0.5 Sr 0.5 Mn 1-x Ni x O 3-δ. In contrast, smaller depressed redox peaks are observed for poor electronic conductors such as LaMnO 3 - loaded GCE. The sheet resistances of sintered La 0.5 Sr 0.5 Mn 1-x Ni x O 3-δ pellets were measured by a Loresta-GP MCP-T610 resistivity meter using a Loresta TFP probe (Mitsubishi Chemical Analytech) at room temperature. For each composition of La 0.5 Sr 0.5 Mn 1-x Ni x O 3-δ, the powders were compressed into pellet by a Φ 13 mm steel die (Cole Parmer) at 8 tons for 5 minutes. The green pellets are sintered at 1200 o C for 4h in ambient conditions. The electrical conductivity of LSMN was measured by impedance spectroscopy. The sintered pellet had a relative sintering density of 59 %. Both sides of the pellet were painted with silver paste (DuPont 5000) and dried at 120 o C in air. The effective conductivity (σ eff ) was calculated from σ eff = t/ra, where t is thickness, A is the area and R is the ohmic resistance. R was obtained from the equivalent circuit fitting of the impedance spectrum. The intrinsic conductivity (σ e ) was calculated according to a modified equation: 65 σ e = 0.493σ effτ 2 ε 1.57 β 0.72 (Equation S1) where τ is geometric tortuosity, ε volume fraction of pores, and β constrictivity. τ and β were taken to be 1.78 and 0.72, respectively. The intrinsic conductivity (σ e ) of LSMN was calculated to be 141 ms cm -1 based on the equivalent circuit fitting results of impedance spectrum as given in Figure S9. Characterization. Powder XRD patterns were recorded with a Bruker D8 Discover GADDS diffractometer with a Cu K α radiation. Surface morphology was characterized by a FESEM (JEOL JSM7600F) at an accelerating voltage of 5 KV. EDX spectrum was recorded by an AZtecSynergy system at an accelerating voltage of 15 KV. TEM images were taken by a Philips S4
CM300-FEG at an accelerating voltage of 300 kv. TOF-SIMS depth profiling was performed in dual beam mode using a TOF-SIMS IV instrument (IONTOF GmbH). XPS spectra were obtained with a Al Kα radiation (VG ESCALAB 200i-XL). Mn and Ni K edge XAS spectra were collected in transmission mode at XAFCA beamline, Singapore Synchrotron Light Source (SSLS). 64 Reference samples, i.e. LaMnO 3, CaMnO 3 and LaNiO 3, were prepared by a citric route. LaMnO 3 and CaMnO 3 were obtained after the calcination at 950 C for 4 h. LaNiO 3 was obtained after the calcination at 900 C for 8 h in O 2 atmosphere. Electrochemical measurement. Ag/AgCl reference electrode (3 M KCl), Pt sheet counter electrode, Φ 5mm GCE and RDE test rig were from Metrohm. RRDE test rig was from Pine Instruments. The Ag/AgCl electrode was converted to the reversible hydrogen electrode (RHE), E vs RHE = E vs Ag/AgCl + 0.976 V. To prepare a catalyst ink, 13 mg of catalyst and 65 µl 5 wt% Nafion solution were dispersed in 2.4 ml isopropanol over 1 h of sonication. An aliquot of catalysts ink was applied on GCE to yield a catalyst loading 0.5 mg cm -2. To prepare the catalyst ink from commercial Pt/C, 4 mg of catalyst and 13 µl 5 wt% Nafion solution were dispersed in 1 ml of 2.5:1 v/v water/isopropanol mixed solvent over 1 h sonication. 5 µl of the catalyst ink was pipetted onto a glassy carbon RDE, giving a catalyst loading of 0.1 mg cm -2. O 2 -saturated electrolyte was prepared by purging O 2 for at least 30 min prior to the start of each experiment. A flow of O 2 was maintained over the electrolyte during the experiments. The ORR polarization curves were recorded at a sweep rate of 5 mv s -1. The content of HO 2 generated during the ORR was determined by RRDE with an E7R9 AFE7R9GCPT tip (Pine Instruments). The Pt ring was set at 1.48 V. The collection efficiency of RRDE loaded with LSMN was calibrated in N 2 - saturated 0.1 M NaOH + 0.01M K 3 Fe(CN) 6 solution (Figure S19). All the potentials were compensated with the ohmic drop (R = 39.0 Ω) given by impedance spectroscopy. S5
Koutecky Levich (K L) plots were generated from capacitive-corrected and ir compensated rotating disk electrode (RDE) voltammograms. The least squares fitted slopes of K L plots were used to calculate the electron transfer number (n): 1 = 1 + 1 j j L 1 = + 1 j K 0.2nFC 0 (D 0 ) 2 3 v -1/6 ω 1/2 j K (Equation S2) where j K is the kinetic-limiting current density, ω is the angular velocity (rpm), C 0 is the bulk concentration of O 2, D 0 is the diffusion coefficient of O 2, and υ is the kinematic viscosity of the electrolyte. The yield of peroxide ions (HO 2 ), X HO2, and n at various potentials are monitored by rotating ring-disk electrode (RRDE), according to Eqs. S3 and S4: X HO2 = 2 I R N I D +I R (Equation S3) n = 4 I D I D +I R /N (Equation S4) where I R is the ring current, I D the disk current, and N the collection efficiency. S6
Figure S1. Thermogravimetric curves obtained from the gel precursors of La 0.5 Sr 0.5 MnO 3-δ (LSM), La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN), La 0.5 Sr 0.5 Mn 0.8 Ni 0.2 O 3-δ, and La 0.5 Sr 0.5 Mn 0.7 Ni 0.3 O 3-δ. S7
Figure S2. XRD patterns of La 0.5 Sr 0.5 MnO 3-δ (LSM), La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN), La 0.5 Sr 0.5 Mn 0.8 Ni 0.2 O 3-δ, La 0.5 Sr 0.5 Mn 0.6 Ni 0.4 O 3-δ, La 0.5 Sr 0.5 Mn 0.4 Ni 0.6 O 3-δ, and La 0.5 Sr 0.5 Mn 0.2 Ni 0.8 O 3-δ. S8
Figure S3. Energy-dispersive X-ray (EDX) spectroscopy of La 0.5 Sr 0.5 Mn 0.9x Ni 0.1 O 3-δ (LSMN). S9
Figure S4. Rietveld refinement of the XRD profile of La 0.5 Sr 0.5 MnO 3-δ (LSM). The black dots are the experimental pattern and the red line is the fitted pattern. The fitting process was implemented in TOPAS3. The fitted results are listed in table S1. S10
Figure S5. ir-compensated RDE oxygen reduction polarization curves of La 0.5 Sr 0.5 MnO 3-δ (LSM), La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN), La 0.5 Sr 0.5 Mn 0.8 Ni 0.2 O 3-δ, La 0.5 Sr 0.5 Mn 0.7 Ni 0.3 O 3-δ, La 0.5 Sr 0.5 Mn 0.6 Ni 0.4 O 3-δ, and La 0.5 Sr 0.5 Mn 0.4 Ni 0.6 O 3-δ (single-phase perovskite, solid line; mixed phases, dashed line). The inset magnifies the onset potential region of the polarization curves. The electrolyte is O 2 -saturated 0.1 M KOH solution. The rotating rate of RDE is 2000 rpm. S11
Figure S6. Field-emission scanning electron microscopy (FESEM) images of (A) La 0.5 Sr 0.5 MnO 3-δ (LSM), (B) La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN), (C) La 0.5 Sr 0.5 Mn 0.8 Ni 0.2 O 3-δ, (D) La 0.5 Sr 0.5 Mn 0.7 Ni 0.3 O 3-δ, (E) La 0.5 Sr 0.5 Mn 0.6 Ni 0.4 O 3-δ, (F) La 0.5 Sr 0.5 Mn 0.4 Ni 0.6 O 3-δ, (G) La 0.5 Sr 0.5 Mn 0.2 Ni 0.8 O 3-δ, and (H) LaMnO 3. FESEM samples were prepared by drip coating catalyst suspensions on Si wafers. The particle sizes of LSM, LSMN and La 0.5 Sr 0.5 Mn 0.8 Ni 0.2 O 3-δ are 35 65 nm, 40 70 nm, 30 55 nm, respectively (A to C). S12
Figure S7. Nitrogen adsorption/desorption isotherms of (A) La 0.5 Sr 0.5 MnO 3-δ (LSM) (B) La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN), (C) La 0.5 Sr 0.5 Mn 0.8 Ni 0.2 O 3-δ, and (D) LaMnO 3. Based on the isotherms, the BET surface areas of LSM, LSMN, La 0.5 Sr 0.5 Mn 0.8 Ni 0.2 O 3-δ and LaMnO 3 are calculated to be 2.05, 1.98, 3.25 and 0.24 m 2 g -1, respectively. Note that the BET surface area of LSMN is compatible to LSM and smaller than La 0.5 Sr 0.5 Mn 0.8 Ni 0.2 O 3-δ. S13
Figure S8. Cyclic voltammograms of La 0.5 Sr 0.5 MnO 3-δ (LSM), La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN), La 0.5 Sr 0.5 Mn 0.8 Ni 0.2 O 3-δ, La 0.5 Sr 0.5 Mn 0.7 Ni 0.3 O 3-δ, La 0.5 Sr 0.5 Mn 0.6 Ni 0.4 O 3-δ, La 0.5 Sr 0.5 Mn 0.4 Ni 0.6 O 3-δ, La 0.5 Sr 0.5 Mn 0.2 Ni 0.8 O 3-δ, LaMnO 3, and blank glassy carbon (GC) electrode in N 2 -saturated 0.1 M KCl solution containing 5 mm K 3 Fe(CN) 6 and 5 mm K 4 Fe(CN) 6. The potential sweeping rate is 100 mv s -1. Note that the higher redox peak currents signify the more conductive materials. LSMN shows the most pronounced redox peak currents, so that LSMN is the most conductive material of La 0.5 Sr 0.5 Mn 1-x Ni x O 3-δ. S14
Figure S9. Sheet resistance of La 0.5 Sr 0.5 Mn 1-x Ni x O 3-δ. The results indicate LSMN as the most conductive composition in the series of La 0.5 Sr 0.5 Mn 1-x Ni x O 3-δ. S15
Figure S10. Impedance spectroscopy from a sintered pellet of La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN) with a relative sintered density of 59%. The impedance spectroscopy was carried out in ambient conditions (T= 23 o C). The inset shows the equivalent circuit model that is used for the fitting. S16
Figure S11. Rietveld refinement of the XRD profile of La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN). The black dots are experimental pattern and the red line is the fitted pattern. The fitting process was implemented in TOPAS3. The fitted results are listed in table S2. S17
Figure S12. O K-edge X-ray absorption spectra of La 0.5 Sr 0.5 MnO 3-δ (LSM), LaMnO 3, La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN), and LaNiO 3, collected at 4B8B beamline of Beijing Synchrotron Light Source. S18
Figure S13. Time of flight secondary ion mass spectroscopy (TOF-SIMS) depth profiling of (A) La, (B) Sr, (C) Mn and (D) Ni of La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN). A 1 kev Ar was used for sputtering while a pulsed 25 kev Bi was used for analysis. LSMN powder suspensions were drip coated on Si wafer substrate and dried in air. The A-site rich LSMN surface is evident from the much higher signal counts from A-site cation (i.e. La and Sr) species than those of B-site cations (i.e. Mn and Ni) species. As shown in A and B, The La and Sr signals in the initial 160 s are relatively undercounted because of their relatively heavy-weight ion species. The more pronounced dip of La species as compared to Sr species in the initial 160s further implies a higher surface concentration of Sr than that of La. S19
Figure S14. (A) ir-compensated and capacitive-corrected ORR polarization curves and (B) the corresponding K L plots of commercial Pt/C. The electrolyte is O 2 -saturated 0.1 M KOH solution. The rotating rates are from 625 to 2500 rpm. At the rotating rate of 1600 rpm, E onset and E 1/2 of Pt/C are 0.99 V and 0.82 V, respectively. S20
Figure S15. Tafel plot of ir-compensated and capacitive-corrected ORR polarization curve of Pt/C under a rotating rate of 1600 rpm. The electrolyte is O 2 -saturated 0.1 M KOH solution. S21
Figure S16. Cyclic voltammogram of La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN) in stagnant O 2 -saturated 0.1 M KOH solution. S22
Figure S17. RRDE voltammogram of La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN). The inset shows the corresponding electron transfer numbers at various potentials. The electrolyte is O 2 -saturated 0.1 M KOH and the rotating rate of RRDE is 1600 rpm. S23
Figure S18. ir-compensated RDE curves of La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN) in O 2 -saturated 0.1 M KOH and in N 2 -saturated 0.1 M KOH + 84 mm H 2 O 2. The rotating rate of RDE is 1600 rpm. S24
Figure S19. Disk (Fe 3+ + e Fe 2+ ) and ring (Fe 2+ Fe 3+ + e ) currents for the determination of collection efficiency of RRDE loaded with La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN) catalyst. The electrolyte is N 2 -saturated 0.1 M KOH with 0.01 M K 3 Fe(CN) 6. The rotating rate of RRDE is 1600 rpm. The collection efficiency of RRDE obtained from 0.70 V to 1.20 V is (36.1 ± 0.5) %, close to the manufacturer s data of 37 %. S25
Table S1. Lattice settings and atomic positions after the Rietveld refinement of La 0.5 Sr 0.5 MnO 3-δ (LSM). Structure Trigonal/rhombohedral, R 3cH (167) a = 5.4386(14) Å, c = 13.3545(60) Å, V=342.08(23) Å 3, Z=6 Atoms x y z Occ. La1(+3) 0.000 0.000 0.250 0.500 Sr1(+2) 0.000 0.000 0.250 0.500 Mn1 (+3) 0.000 0.000 0.000 0.500 Mn2 (+4) 0.000 0.000 0.000 0.500 O1 (-2) 0.459 0.000 0.250 1.000 R-Bragg, 1.319; R exp, 10.80; R wp, 7.54; R p, 5.17; GOF, 0.70 S26
Table S2. Lattice settings and atomic positions after the Rietveld refinement of La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN). Structure Trigonal/rhombohedral, R 3cH (167) a = 5.4445(31) Å, c = 13.3782(94) Å, V=343.43(46) Å 3, Z=6 Atoms x y z Occ. La1(+3) 0.000 0.000 0.250 0.500 Sr1(+2) 0.000 0.000 0.250 0.500 Mn1 (+3) 0.000 0.000 0.000 0.270 Mn2 (+4) 0.000 0.000 0.000 0.630 Ni1 (+2) 0.000 0.000 0.000 0.070 Ni2 (+3) 0.000 0.000 0.000 0.030 O1 (-2) 0.550 0.000 0.250 1.000 R-Bragg, 1.145; R exp, 9.34; R wp, 7.03; R p, 5.03; GOF, 0.75 S27
Table S3. Coordination numbers, spin state, ionic radius, and the calculation of Goldschmidt tolerance factor (t) of perovskite La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN). La 3+ Sr 2+ Mn 3+ Mn 4+ Ni 2+ Ni 3+ O 2- Coordination 12 12 6 6 6 6 6 numbers Spin state N.A. N.A. H. S. N. A. N. A. L. S. N. A. Ionic radius 0.144 0.136 0.0645 0.053 0.069 0.056 0.140 (nm) Occupancy 100% 100% 48.0% 52.0% 88.0% 12.0% 100% percentage Goldschmidt 0.993 tolerance factor (t) S28
Table S4. Summary of fitted XPS results of Mn, Ni, O, La and Sr of La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN). Mn 2p 3/2 Ni 2p 3/2 O 1s Mn 3+ Mn 4+ Mn 3+ content Ni 2+ Ni 3+ Ni 2+ content Lattice Surface Surface (ev) (ev) (ev) (ev) (ev) (ev) phases (ev) 641.3 642.8 30.0% 854.8 855.5 69.2% 529.1 529.8 531.2 Main (ev) La 3d 5/2 Sr 3d 5/2 Valley Shake-up Lattice Surface (ev) (ev) (ev) (ev) Mn/Ni ratio Sr/La ratio (Mn+Ni)/(La+Sr) ratio O/(Mn+Ni+La+Sr) ratio 833.8 835.8 838.0 132.3 133.0 3.45 1.77 0.75 0.88 S29
Table S5. Mn 3+/4+ and Ni 2+/3+ redox couples of La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN). Mn Valence Mn 3+ (3d 4 ) Mn 4+ Ni Ni 2+ (3d 8 ) Ni 3+ (3d 7 ) (3d 3 ) spin state high spin N.A. arbitrary spin states low spin assignment t 3 1 2g e g t 3 0 2g e g t 6 2 2g e g t 6 1 2g e g Occupancy percentage (the 48.0 at.% 52.0 at.% 88.0 at.% 12.0 at.% bulk) Occupancy 30.1% 69.9% 69.2% 30.8% percentage (the surface) S30
Table S6. Electrocatalytic activity of state-of-the-art ORR catalysts. The electrolyte is O 2 - saturated 0.1 M KOH solution. The rotating rate of RDE is 1600rpm. Catalysts E onset vs RHE E 1/2 vs RHE Tafel slope (mv/decade) Ref. Precious metals/carbon Pt (30 wt.%)/vulcan carbon 0.99 0.82 76 This work Pd 2 NiAg 0.97 * 0.83 * 44 Au/rGO 0.85 ** 0.70 ** 45 FePd 3 /rgo 0.93 *** 0.75 *** 46 Non-metal doped carbons N-doped graphene naroribbon 0.98 ** 0.83 ** 47 N-doped graphene/carbon nanotube 1.08 *** 0.87 *** 48 B- and N-doped carbon 1.09 ** 0.93 ** 49 Carbon-transition metal hybrids Co/N/C 0.85 0.77 24 Fe/N/C 1.03 0.82 50 Fe 3 C/C 0.90 0.81 51 S31
Table S6. Continued. Catalysts E onset vs RHE E 1/2 vs RHE Tafel slope Ref. (mv/decade) Transition metal oxide/carbon Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 δ 0.73 Undefined 64 27 Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 δ /C 0.84 Undefined 65 FeCo 2 O 4 /NG 0.97 0.86 43 Co 3 O 4 /N-rmGO 0.93 0.82 42 52 La(Co 0.55 Mn 0.45 ) 0.99 O 3-δ 0.95 0.78 53 La 0.7 (Ba 0.5 Sr 0.5 ) 0.3 Co 0.8 Fe 0.2 O 3-δ 0.80 0.66 54 /carbon Ba 0.5 Sr 0.5 Co 0.2 Fe 0.8 O 3 δ /C 0.78 0.61 55 La 0.5 Sr 0.5 Mn 0.9 Ni 0.1 O 3-δ (LSMN) 1.02 0.80 68 this work *: converted from Ag/AgCl in 3 M KCl, E vs RHE = E vs Ag/AgCl + 0.209 V + 0.059 ph. **: converted from Ag/AgCl in saturated KCl, E vs RHE = E vs Ag/AgCl + 0.197 V + 0.059 ph. ***: converted from saturated camel electrode, E vs RHE = E vs SCE + 0.241 V + 0.059 ph. S32