Jaemin Kim, Xi Yin, Kai-Chieh Tsao, Shaohua Fang and Hong Yang * Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, 114 Roger Adams Laboratory, MC-712, 600 South Mathews Avenue, Urbana, Illinois 61801, United States *: Corresponding author: hy66@illinois.edu (HY) S1
Experimental Details: Synthesis of CaMnO3. The stoichiometric amounts of CaCO3 (Alfa Aesar, 99.95%; 1.00 g) and Mn(NO3)2 4H2O (Alfa Aesar, 98%, 2.51 g) were mixed in nitric acid aqueous solution (1.4 ml of HNO3 (68 70 %) in 100 ml Millipore water), followed by addition of citric acid (Fisher Chemical, 99%, 8.41 g). The mixed solution was heated to 80 ºC in ~10 min and cured for ~5 h for gelation. The water was evaporated for overnight until yellow gel was formed. The water from the gel was fully removed at 120 ºC for at least 4 h in a vacuum oven (VWR symphony, ~10 mmhg) and the gel was further heated to 600 ºC at a rate of 2 C/min and maintained at that temperature for 6 h to remove the organic compounds. The dark grey colored product was then heated to 900 ºC at a rate of 2 C/min, and maintained at this temperature for 12 h. Synthesis of Ca2Mn2O5. The as-made CaMnO3 was reduced at 350 ºC with a forming gas of 5% H2 in Ar for 3 h. Preparation of Na + -Nafion. The proton in the Nafion solution was replaced by sodium cation through addition of sodium hydroxide solution. In a typical procedure, 1 ml of Nafion 117 solution (Sigma- Aldrich, ~5 %) was mixed with 0.5 ml of 0.1 M NaOH solution. The ph value of the final solution was ca. 8-9. Preparation of Carbon-Supported Catalyst Inks. Vulcan Carbon XC-72 was used as the conducting support for metal oxides. In a standard procedure, 35 mg of metal oxide was mixed with 7 mg of carbon black in 7 ml of tetrahydrofuran with 10 µl of Na + -Nafion, followed by sonication for 30 min to obtain a homogeneous solution. Electrocatalytic Measurement. A three-electrode cell configuration was used. The working electrode was a glassy carbon rotating disk electrode (RDE) with an area of 0.196 cm 2. A platinum wire (0.5 mm diameter) connected to a platinum foil (1 cm 2 area) was used as the counter electrode. HydroFlex hydrogen electrode was used as the reference in a separate compartment. 10 µl of the catalyst ink was dropped on the RDE and slowly dried to make a thin film working electrode. The reference electrode was calibrated in H2 (S. J. Smith, 99.999%)-saturated 0.1 M KOH (Sigma-Aldrich, 99.99%) solution before the tests. Oxygen evolution reaction (OER) currents were measured in the same KOH solution by purging with O2 (S. J. Smith, 99.999%) for at least 30 min before the measurement. Cyclic voltammogram (CV) was recorded between 1.1 V and 1.8 V at a scan rate of 10 mv/s and a rotating speed of 1600 rpm. The OER currents were took the average of positive and negative scans to remove capacitance effect. The rotating ring disk electrode (RRDE) currents were measured identically with RDE tests except Ar (S. J. Smith, 99.999%) was used as the purging gas instead of O2. The resistance of the electrolyte was measured using the catalyst-loaded RDE in 0.1 M KOH solution, and determined to be 50 Ω by ir compensation method. Characterization. The structure and crystal phase analyses were conducted by powder X-ray diffractometer (Siemens-Bruker D5000 XRD) with Cu X-ray source. The scan rate was 1 2θ/min. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) was carried out by JEOL 2100 Cryo microscope at an acceleration voltage of 200 kv. The TEM specimen was prepared by dispersing a suspension in ethanol on a carbon-coated copper grid. Scanning electron microscopy (SEM) S2
was carried out on a Hitachi S4700 microscope at 15 kv. The SEM specimen was prepared by putting the powders on carbon tape on a SEM stub. Tables: Table S1. Refined Parameters for Ca2Mn2O5 and CaMnO3 Ca2Mn2O5 x y z Ca 0.2990 0.3610 0.5000 Mn 0.2826 0.1201 000 O1 0.2875 0.1027 0.5000 O2 800 0.2824 000 O3 000 000 000 a = 5.4302(7) Å, b = 10.2322(1) Å, c = 3.7443(2) Å, space group Pbam, Rwp = 10.40 %, Rp = 16.39 % CaMnO3 x y z Ca 333 0.2500 0.9943 Mn 000 000 0.5000 O1 0.4898 0.2500 659 O2 0.2872 335 0.7121 a = 5.2807(7) Å, b = 7.4524(5) Å, c = 5.2667(4) Å, space group Pnma, Rwp = 19.18 %, Rp = 10.68 % R p = y i (obs) y i (calc), R y i (obs) wp = ( w i(y i (obs) y i (calc)) 2 ) 1 2, w i (y i (obs)) 2 where w i = 1 y i, y i (obs) = observed intensity at step i, y i (calc) = calculated intensity at step i. The Rietveld fitting was carried out using the Reflex module in Materials Studio 6.1 (Accelrys Inc.). S3
Figures: Figure S1. XRD pattern and Rietveld refinement plot of pure perovskite CaMnO3 (Orthorhombic, Pnma). Color code in the Rietveld refinement plot: experimental data (red), simulation (blue), background (orange), observed reflections (green), and difference (black). Figure S2. SEM image of CaMnO3 showing the submicron-sized uniform particles, and HRTEM and FFT (inset) images of CaMnO3. The inset in is the magnified image. S4
I 4.0 3.0 2.0 1.0 1st scan of Ca 2 5th scan of Ca 2 9th scan of Ca 2 10th scan of Ca 2 I 0.8 0.6 0.4 0.2 1st scan of CaMnO 3 5th scan of CaMnO 3 9th scan of CaMnO 3 10th scan of CaMnO 3 1.0 1.2 1.4 1.6 1.8 E - ir vs. RHE (V) 1.0 1.2 1.4 1.6 1.8 E - ir vs. RHE (V) Figure S3. Cyclic voltammograms of Ca2Mn2O5 and CaMnO3, respectively. The catalysts became stable after 9 th scan. -5-0.1-0.10 I ring -0.2-0.3-0.4 I disk 5.0 4.0 Ca 2 at 1.8 V 3.0 Ca 2 at 1.6 V 2.0 1.0 I ring -0.15-0.20-0.25-0.30 I disk 1.5 CaMnO 3 at 1.8 V 1.0 CaMnO 3 at 1.6 V 0.5 Figure S4. ORR amperometric (i-t) curves at Pt-ring electrode of RRDE for Ca2Mn2O5 and CaMnO3, respectively. The insets are the corresponding amperometric i-t curves at glassy carbon disk electrode. S5
1.8 1.8 E-iR vs. RHE (V) 1.7 1.6 1.5 1.4 1st scan Ca 2 5th scan Ca 2 10th scan Ca 2 1 10 100 I m (A/g) E-iR vs. RHE (V) 1.7 1.6 1.5 1st scan CaMnO 3 5th scan CaMnO 3 10th scan CaMnO 3 1 10 I m (A/g) Figure S5. Tafel plots for Ca2Mn2O5 and CaMnO3 after multiple cycles. The Tafel slope of Ca2Mn2O5 catalyst was determined to be 149 mv/dec for the 1 st scan, 146 mv/dec for the 5 th scan, and 174 mv/dec for the 10 th scan. The Tafel slope of CaMnO3 catalyst was determined to be 197 mv/dec for the 1 st scan, 186 mv/dec for the 5 th scan, and 194 mv/dec for the 10 th scan. S6