Cation-Hydroxide-Water Co-Adsorption Inhibits the. Alkaline Hydrogen Oxidation Reaction

Transcription

1 Supporting Information Cation-Hydroxide-Water Co-Adsorption Inhibits the Alkaline Hydrogen Oxidation Reaction Hoon Taek Chung [a], Ulises Martinez [a], Ivana Matanovic [b,c] and Yu Seung Kim* [a]. [a] MPA-11: Materials Synthesis and Integrated Devices, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States [b] Department of Chemical and Biological Engineering, Center for Micro-Engineered Materials (CMEM), The University of New Mexico, Albuquerque, New Mexico 87131, United States [c] T-1: Physics and Chemistry of Materials, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States S1

2 1. Experimetal Materials M aqueous TMAOH solution was prepared from high-purity 25% TMAOH solution from ACROS. 0.1 M HClO 4 solution was made with perchloric acid (70%, GFS Chemicals Inc.). Rotating disk electrode measurement (RDE). - RDE measurements were performed using a CHI Electrochemical Station (Model 760D) in a standard three-electrode cell at a room temperature, 25 ± 1 C. A platinized Pt wire served as a counter electrode and a Hg/HgO (Radiometer Analytical Inc.) in 1.0 M KOH as a reference electrode for alkaline electrolytes and a Ag/AgCl (Bioanalytical Systems Inc.) in 3.0 M NaCl as a reference electrode for acidic electrolyte. All potentials initially measured vs. the Hg/HgO electrode (or Ag/AgCl electrode) were converted to a reversible hydrogen electrode (RHE) scale by measuring HOR/hydrogen evolution reaction (HER) currents on the Pt polycrystalline electrode in the same electrolyte, whereby the potential at zero current corresponds to 0.0 V vs. RHE. After the electrolytes were saturated with pure hydrogen, polarization plots were recorded between ca. 1.0 and 1.2 V vs. RHE at a sweep rate of 5 mv s - 1 and rotation rate of 900 rpm. The HOR/HER voltammograms were presented without correction for reactant diffusion limitations because the electrolyte resistance of the acid and alkaline solution is negligible (see impedance data in Fig. 4). The cyclic voltammogram (CV) at 25 C was performed between 0.05 and 1.1 V vs. RHE at a scan rate of 50 mv s -1 at 900 rpm in saturated nitrogen conditions. Before measuring the initial CVs, Pt polycrystalline electrode was cycled in the potential range of V vs. RHE to clean the Pt electrode. The ac impedance was measured from 100 to 0.1 khz with a voltage perturbation of 5 mv and rotation rate of 900 and 2500 rpm in the hydrogen saturated electrolytes. Infrared Measurement. Infrared Reflection Adsorption Spectroscopy (IRRAS) experiments were performed at room temperature using a Nicolet 6700 FT-IR spectrometer equipped with a Mercury Cadmium Telluride (MCT) detector cooled with liquid nitrogen. Experimental setup is described previously. 1,2 For each spectrum, 128 interferograms were added together at a resolution of 8 cm - 1 with unpolarized light. Absorbance units of the spectra are defined as A = -log(r/r 0), where R and R 0 represent reflected IR intensities corresponding to the sample and reference single beam spectrum, respectively. Thus, a positive peak in the resulting spectrum indicates a production of species, while a negative peak indicates consumption or decrease in concentration of a species compared to the reference spectrum. The reference spectrum for the chronoamperometry study was collected at 0.1 V vs. RHE prior to the addition of the cation solution. The same polished platinum electrode used for the electrochemical measurements were used for the IR studies. A ZnSe hemisphere was used as the IR window, and the working electrode was pressed against S2

3 the window, creating a thin solution layer with a thickness of a few micrometers. The incident angle of the IR radiation passing through the ZnSe window was Argon was used to purge the electrolyte while dry air was used to purge the spectrometer and chamber, reducing the spectral interference from ambient CO 2 and water vapor. NMR Measurement 1 H NMR spectra were recorded using a Bruker 600MHz AVII spectrometer equipped with a cryoprobe with z-gradients at 298K. For samples in 90% water and 5% deuterium oxide, solvent suppression was performed using presaturation of the water line as a 54Hz constant wave RF field applied at the center frequency of the water signal during the 5 sec interscan delay period. All NMR spectra were recorded at 25 C and were processed with MestReNova 8.1 (Mestrelab Research SL) software. TMAOH was prepared as 0.1M solutions in DI water. 2 ml of the solution was added to 5 ml vials. 5% D 2O (0.1 ml) was added to a vial. Then 0.45 ml of the solution was transferred to NMR tubes and NMR spectra were recorded. For TMAOH stability test, the 5% D 2O NMR solution was transferred back into its vial. 50 mg of Pt black was added to the vial, and the vial was stirred with a magnetic stir bar at room temperature for 2 h. After 2 h the solution was centrifuged and then filtered through a cotton plug. Then 0.45 ml of a solution was transferred to NMR tubes and NMR spectrum was recorded. 2. Impurity analysis of TMAOH solution It is important to know the impurity level of TMAOH solution to confirm that our observations do not result from the presence of side-contaminants in the electrolyte solution. The concentration of alkali and other metal contaminants in the as-received 25% TMAOH solution is extremely low (< 10 part per trillion) compared to high purity NaOH or KOH, which normally has to 0.005% metal impurity. 4 The organic impurity level in the TMAOH solution is also extremely low. 1 H NMR analysis of diluted 0.1 M TMAOH solutions did not detect any organic impurity in the nano-mol level (Figure S2). References 1. Martinez, U.; Asazawa, K.; Halevi, B.; Falase, A.; Kiefer, B.; Serov, A.; Padilla, M.; Olson, T.; Datye, A.; Tanaka, H.; Atanassov, P. Aerosol-derived Ni1 xznx Electrocatalysts for Direct Hydrazine Fuel Cells. Phys. Chem. Chem. Phys. 2012, 14, Konopka, D. A.; Li, M.; Artyushkova, K.; Marinkovic, N.; Sasaki, K.; Adzic, R.; Ward, T. L.; Atanassov, P. Platinum Supported on NbRuyOz as Electrocatalyst for Ethanol Oxidation in Acid and Alkaline Fuel Cells. J. Phys. Chem. C 2011, 115, Faguy, P. W.; Marinkovic, N. S. Design and Performance of a New Infrared Reflection Accessory for Spectroelectrochemical Studies. Appl. Spectrosc. 1996, 50, S3

4 4. Ong, K. Determination of Impurities in Semiconductor-Grade TMAH with NexION 300S/350S ICP-MS. Application Note ICP Mass Spectrometry, PerkinElmer, Inc. Singapole, avialable from S4

5 Current density /ma cm -2 geo a) HClO 4 NaOH at 0.1 V vs. RHE Time /min Current density/ma cm b) initial after 120 min at 0.1 V Potential/V vs. RHE Figure S1. (a) Chronoamperometry at 0.1 V for the Pt/C in 0.1 M HClO4, and NaOH solutions; HOR voltammograms were performed at 25 C, rotating speed: 900 rpm; scan rate: 5 mv s -1. (b) Cyclic voltammograms of Pt measured in nitrogen saturated 0.1 M NaOH before and after 0.1 V exposure for 120 min. S5

6 Figure S2. 1 H NMR spectrum of TMAOH, a 0.1M aqueous solution in 5% D2O, before (a) and after (b) treatment with Pt black at room temperature for 2 h. No changes were observed after 2 h. S6

7 ~100 khz ~ 10 khz reference pre-condition Z''/ohm ~ 1 khz 50 Hz 100 Hz 20 Hz 10 Hz (0.1, 0.2, 0.3, 0.4, 1.0) Hz Z'/ohm Figure S3. Impedance measured at segmented frequencies after 1.2 V for 30 seconds preconditioning. Measurement starts from low frequency, i.e., 0.1 Hz, to high frequency, i.e., 100 khz. Semicircle at low frequencies is decreased. This indicates the 2 nd semicircle at low frequencies is due to TMA + adsorption. Increase in semicircle at high frequencies is due to TMA + adsorption during test. Impedance was measured at 0.01 V vs. RHE at 900 rpm. S7