Supporting Information Enhanced Electrocatalytic Performance for Oxygen Reduction via Active Interfaces of Layer-By-Layered Titanium Nitride / Titanium Carbonitride Structures Zhaoyu Jin, 1 Panpan Li, 1,2 Dan Xiao, 1,* Experimental Section Characterizations. Scanning electron microscopy (SEM) analysis was carried out on a field emission Hitachi S4800 microscope (Japan). The high-resolution transmission electron microscopy (HRTEM) images were recorded by a FEI Tecnai G2 F20 S (USA) with an accelerating voltage of 200 kv. X-ray diffraction (XRD) analysis was performed on a Fangyuan DX-1000 powder X-ray diffractometer (China) with Cu Ka radiation at 40 kv. X-ray photoelectron spectra (XPS) were acquired with Kratos AXIS ULTRA DLD Photoelectron Spectroscope (UK) with element carbon as internal standard (binding energy at 284.6eV). Electrochemical Measurements. Catalyst inks for electrochemical testing were prepared by adding 2 mg catalyst powders to a mixture of 500 μl distilled water / isopropyl alcohol (3:1, v/v) and 10μL Nafion solution (5% wt, Dupont, USA). After ultrasonical dispersion to homogeneous, 5μL fresh catalyst ink were cast onto a glassy carbon (GC) disk electrode (0.196 cm 2 geometrical areas, Pine Research Instrument, USA) and dried at room temperature. The typical catalyst loading was about 100 μg cm -2. All tests were carried out at room temperature on a computer-controlled Autolab
PGSTAT 12 potentiostat / galvanostat (Metrohm, Switzerland) in 0.1 M KOH assembled with a rotating system (Pine Research Instrument, USA) as working electrode, Hg /HgO (1 M KOH) as reference electrode and a Pt foil as counter electrode. Cyclic voltammograms (CVs) were recorded without rotation in solutions saturated with either N 2 or O 2 gas. Rotating disc electrode (RDE) measurements were collected in O 2 -saturated solutions with a rotation speed of 400-2500 rpm. The Koutecky-Levich equation was used to determine the number of transferred electrons during oxygen reduction reaction (ORR). This equation is expressed as 1/j=1/j k +1/(B ω 1/2 ), where B=0.62nFC O2 D O2 2/3 υ -1/6, j is the measured disk current density, j k is the kinetic current density, ωis the rotation speed, n is the electron transfer number, F is the faraday constant (96485 C / mol), C O2 is the concentration of dissolved oxygen in electrolyte (1.2 10-6 mol cm -3 ), D O2 is the diffusion coefficient of dissolved oxygen (1.73 10-5 cm 2 s -1 ), andυis the kinematic viscosity of the electrolyte (0.01 cm 2 s -1 ). s1-s3 In all measurements, we used Hg/HgO as the reference. It was calibrated with respect to reversible hydrogen electrode (RHE). The calibration was performed in the high purity H 2 saturated electrolyte with a Pt wire as the working electrode. CVs were run at a scan rate of 1 mv s -1, and the average of the two potentials at which the current crossed zero was taken to be the thermodynamic potential for the hydrogen electrode reactions. Single Cell Testing. Catalysts were tested in the fuel cell cathode to evaluate their activity under alkaline fuel cells (AFC) operating conditions. A suspension consisting of 20 wt % Pt/C catalysts (Alfa Aesar, Johnson-Matthey In. UK), 5 wt % Nafion
solution (DuPont, USA) (35wt% maintain in the dry catalyst layer), and distilled water / isopropyl alcohol (3:1, v/v) was used to prepare the anode by successive brush-painting to a carbon paper gas diffusion layer (geometric area of 5 cm -2, Hesen Electrical Co. Ltd. China) with the a Pt loading of 0.4 mg cm -2. The cathode of AFC was also prepared similarly as description of the anode fabrication. The Final loadings of TNTCNHS and Pt/C catalysts were 4 mg cm -2 and 2 mg cm -2 on the gas diffusion layer. The two carbon papers of the anode and cathode in single cell were then binded by hot melt adhesive (thickness of ~ 2 mm) and pressed tightly with an alkaline anion exchange membrane in the middle to form a sandwich-type cell. The polymer electrolyte membrane is fabricated according to the literature and the main properties for anion exchange were tested and reach the standard before constructing membrane electrode assembly. s4,s5 Afterwards, two stainless steel boards for gas flow were utilized to press the anode and cathode, respectively. Pure hydrogen and oxygen humidified at 50 C, were supplied to the anode and cathode at a flow rate of 200 ml min -1.The polarization curves (I-V) were recorded by a Keithley 2400 source meter (USA). Computation. The density function theory (DFT) calculations were performed through the Cambridge serial total package (CASTEP). s6 The energies (E ad ) of oxygen adsorption on various catalyst models, which is the rate determining step (RDS) of ORR process, are used as criteria to evaluate catalysts ORR activities. The crystal structures used in this study were acquired from The Inorganic Crystal Structure Database (ICSD). For simplifying the calculation, we studied the three adsorption
modes on (200) crystal plane of the TiCN and TiN after geometry optimization with 0.8 nm vacuum slab, which were O 2 @ TiCN, O 2 @ TiN, TiCN @ O 2 @ TiN, TiN @ O 2 @ TiN and TiCN @ O 2 @ TiCN. Then the most stable adsorption pattern and the corresponding adsorption energy E ad were obtained and used as a barometer for the ORR activity of each catalyst. E ad is defined as: E ad = E total (E catalyst + E O2 ). Where E total is the total energy of the system with adsorbed O 2, E catalyst and E O2 are the energies of the investigated substrates (TiN/TiCN, TiN/TiN or TiCN/TiCN hierarchical structures) and isolated O 2, respectively. The electron densities of Ti atoms in TiN and TiCN were obtained from the Mulliken charges of TiN and TiCN. Three adsorption modes and corresponding E ad are shown in Supporting Information.
Figure S1. (A) XRD pattern of TNTCNHS, where the JCPDS file numbers have been indicated. (B) XPS survey and high-resolution spectra of Ti 2p, N 1s and C 1s in TNTCNHS. The peak at 284.6eV for C 1s is internal standard in this investigation.
Figure S2. (A) and (B) are the SEM images of the broken hierarchical structure of blending TiN/TiCN nanocomposites (TNTCNNC).
Figure S3. K-L plots of TNTCNHS-2 corresponding Figure 3C at 0.6 V 0.3 V and calculated average electron transfer number.
Figure S4. (A) RDE polarization curves of TiN nanoparticl es at the rotating rate of 4000 2500 rpm in O 2 saturated 0.1 M KOH and (B) corresponding K-L plots at - 0.4 V - 0.7 V and calculated average electron transfer number.
Figure S5. (A) RDE polarization curves of TiN nanoparticl es at the rotating rate of 4000 2500 rpm in O 2 saturated 0.1 M KOH and (B) corresponding K-L plots at 0.6 V 0.3 V and calculated average electron transfer number.
Figure S6. (A) RDE polarization curves of TNTCNTF at the rotating rate of 400 2500 rpm in O 2 saturated 0.1 M KOH and (B) corresponding K-L plots at 0.6 V 0.3 V and calculated average electron transfer number.
Figure S7. (A) RDE polarization curves of TNTCNNC at the rotating rate of 400 2500 rpm in O 2 saturated 0.1 M KOH and (B) corresponding K-L plots at 0.6 V 0.7 V and calculated average electron transfer number.
Figure S8. (A) RDE polarization curves of platinum on carbon at the rotating rate of 4000 2500 rpm in O 2 saturated 0.1 M KOH and (B) corresponding K-L plots at 0.6 V 0.3 V and calculated average electron transfer number.
Figure S9. Nyquist plots of electrochemical impedance spectroscopy at 0.8 V of TNTCNHS, TiCN nanoparticles and TiN nanoparticles in O 2 saturated 0.1 M KOH. The scanning frequency range is from 10 5 Hz to 10-2 Hz.
Figure S10. Electrochemical capacitance measurements for determination of the TNTCNHS-1 and TNTCNHS-2 catalyst surface area. Cyclic voltammograms were taken in a potential range where no faradic processes were observed to measure the capacitive current from double layer charging.
Figure S11. XPS survey and high-resolution spectra of Ti 2p, O 1s The sample was collected from the electrode after ORR chronoamperometry at - 0.4 V with rotating rate of 400 rpm for 1 h. in TNTCNHS. activation by
Figure S12. The FFT patterns of the corresponding regions in the HRTEM image marked in yellow boxes.
Figure S13. Three adsorption modes of TNTCNHS (200) and oxygen molecule and other two modes on TiN / TiN and TiCN / TiCN for E ad calculation via DFT.
Figure S14. Schematic transport of electrolyte, reactants and products between external and interface. References: s1. Tham, M.J., Walker, R.D. & Gubbins, K.E. Diffusion of oxygen and hydrogen in aqueous potassium hydroxide solutions. J. Phys. Chem. 74, 1747-1751 (1970). s2. Guo, S.J., Zhang, S., Wu, L.H. & Sun, S.H. Co/CoO Nanoparticles assembled on graphene for electrochemical reduction of oxygen. Angew. Chem. Int. Ed. 51, 11770-11773 (2012). s3. Wang, S.Y., Yu, D.S., Dai, L.M., Chang, D.W. & Baek, J.B. Polyelectrolyte-Functionalized Graphene as Metal-Free Electrocatalysts for Oxygen Reduction. Acs Nano 5, 6202-6209 (2011). s4. Lin, X.C., et al. Alkaline polymer electrolytes containing pendant dimethylimidazolium groups for alkaline membrane fuel cells. J. Mater. Chem. A, 1, 7262-7269 (2013). s5. Lin, X.C., et al. Alkali resistant and conductive guanidinium-based anion-exchange membranes for alkaline polymer electrolyte fuel cells. J. Power Sources, 217, 373 380 (2012). s6. S. J. Clark et al. First principles methods using CASTEP. Zeitschrift fuer Kristallographie, 220, 567-570 (2005).