Supporting Information Copyright Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, 2010 Fluoride-Modulated Cobalt Catalysts for Electrochemical Oxidation of Water under Non-Alkaline Conditions James B. Gerken, Elizabeth C. Landis, Robert J. Hamers, and Shannon S. Stahl* [a] cssc_201000161_sm_miscellaneous_information.pdf
Table of Contents General Experimental Conditions and Apparatus Apparatus for Prolonged Electrolysis in Fluoride Buffer Apparatus for Faradaic Efficiency Determination in Fluoride Buffer Prolonged Electrolysis in Fluoride Buffers With and Without Dissolved Cobalt Faradaic Efficiency of Oxygen Production from Fluoride-Buffered Solutions Current and Charge During Deposition From Fluoride Buffer XPS Data for Co-Oxide Electrodeposited Films Page S2 S3 S4 S5 S6 S7 S8 S1
General Experimental Conditions and Apparatus. Cyclic voltammetry 1 was conducted using an Ag/AgCl reference and a platinum wire counter electrode at a scan rate of 50 mv/s unless otherwise noted. The reference potential was calibrated by a ferroin internal standard following experiments in ferroin-free buffer. Potentials are ir corrected and reported relative to the normal hydrogen electrode (NHE). Electrolyte solutions were made with KF in deionized water with the ph adjusted by the addition of KHF 2 or NaOH as needed. Safety Note: Acidic fluoride buffers contain significant concentrations of hydrofluoric acid and should be handled using precautions appropriate for HF solutions. Solution ph measurements were obtained via indicator paper or a phsensitive electrode that was calibrated against ph 4.0 or ph 7.0 buffer. Glassy carbon anodes were polished before use; FTO and ITO anodes were discarded after use. ITO anodes were 8-12 Ω/sq on a glass substrate and were used as received. X-ray photoelectron spectroscopy (XPS) experiments were conducted on an ultrahigh vacuum system with a monochromatic Al Kα source with an analyzer resolution of 0.050 ev. All spectra were collected with a 45 photoelectron takeoff angles. All spectra were referenced to the bulk carbon peak 285.0 ev. Atomic area ratios for core-level spectra were calculated by fitting the baseline corrected data to Voigt functions and normalizing those areas with their atomic sensitivity factors (C=0.296, Co2p=2.427, F=1.000, O=0.711). 2 Scanning electron microscopy (SEM) of the films was collected on a Leo Supra55 VP microscope. All images were taken at a 3 kev electron accelerating voltage using the standard in-lens secondary electron detector. It was found to be essential to operate in glass-free equipment to preclude etching of the glass and consequent loss of fluoride from the solution. A plastic divided cell was constructed of a polypropylene cup with a polyethylene tube capped with a porous frit of polyethylene to form the counter-electrode compartment. A similar technique was used to construct fluoride-compatible reference electrodes. Convenient tubes for reference electrodes were obtained from disposable 1 ml syringe barrels. These had the frit press-fitted in place and were then fitted with silver chloride coated silver wire electrodes and filled with sodium chloride solution in the customary manner. It was also necessary to paint the glass portions of the ITO and FTO electrodes with a solution of hard vacuum wax in methylene chloride, which was then allowed to dry. This wax coating was removed before ex situ characterization of the electrodeposited films. 3 Prolonged electrolysis and Faradaic efficiency experiments required more intricate apparatus as shown below. 1 R. S. Nicholson, I. Shain, Anal. Chem. 1964, 36, 706-723. 2 J. F. Moulder, W. F. Stickle, P. E. Sobol, K. D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Eden Prairie, MN, 1992. 3 Paraffin wax has a long history as a material for electrochemistry in HF and its solutions: G. Gore Phil. Trans. Roy. Soc. London 1869, 159, 173-200. S2
Apparatus for Prolonged Electrolysis in Fluoride Buffer It was found to be necessary to have the FTO electrode contact the fluoride solution only at the front face, with no air-liquid interface. To facilitate this, the cell shown below (Figure S1) was constructed. It has a port for reproducible positioning of the reference electrode, a counterelectrode contained in largediameter polyethylene tubing capped with a polyethylene frit, and an aperture against which the working electrode can be held. The seal between the FTO anode and the polyethylene cell is constructed of layers of parafilm against the cell topped with teflon tape, which in combination produce a watertight, electrochemically inert seal against the anode. A hole punched in the seal establishes the active electrode area. Due to the nature of the electrolyte, secondary containment as shown is prudent. Figure S1. Fluoride-compatible electrochemical cell. The catalyst deposit on the region of the anode in contact with the electrolyte solution is visible. S3
Apparatus for Faradaic Efficiency Determination in Fluoride Buffer Faradaic efficiency was determined by measurement of oxygen produced in a sealed and purged divided cell. To facilitate this, the cell shown below (Figure S2) was constructed. It has a pair of compartments separated by a polymer frit located in the cross-tube which is epoxied to the electrode compartments. The inner surface of the joint is sealed with paraffin wax to produce an inert surface. The working electrode compartment cap is sealed with teflon tape on its screw threads and is pierced for septa which are in turn pierced by needles for inert gas purging, the electrode lead and oxygen sensor. The oxygen sensor tip is held within a 12 ga. needle and sealed by a gasketed Luer fitting. The reference electrode is sealed in its aperture by teflon tape. Due to the nature of the electrolyte, secondary containment as shown is prudent. Figure S2. Fluoride-compatible electrochemical cell. The oxygen sensor tip is incorporated within the large-gauge needle. S4
Prolonged Electrolysis in Fluoride Buffers With and Without Dissolved Cobalt. Catalyst electrodeposition was performed as below from 40 ml of either a 1.0 or 0.1 M ph 3.7 fluoride buffer containing 1 mm CoSO 4. The 0.1 M fluoride buffer had 0.9 M KBF 4 added to maintain constant ionic strength. The electrodeposition was performed at 1.48 V vs. NHE (overpotential of 0.44 V) in a divided cell with stirring of the electrolyte. Following deposition, electrolysis was performed at 1.58 V vs. NHE without ir compensation in either the same solution or a fresh buffer solution identical to the deposition solution except for the absence of added cobalt. The current density was monitored as a function of time and is shown below (Figure S3). 0-2 -4 j ( ma/cm 2 ) -6-8 -10 1.0 M F- w/co 1.0 M F- wo/co 0.1 M F- w/co 0.1 M F- wo/co -12 0 2 4 6 8 10 12 14 16 T (h) Figure S3. Current density at 1.58 V vs NHE (overpotential of 0.54 V) of catalyst-coated FTO electrodes during electrolysis in fluoride buffer at ph 3.7 (1.0 M F - with 1 mm CoSO 4, 1.0 M F - without CoSO 4 -, 0.1 M F - with 1 mm CoSO 4 - -, 0.1 M F - without CoSO 4 ). In cobalt-free electrolysis solutions, dissolution of the catalyst deposit was noted during the initial equilibration period. S5
Faradaic Efficiency of Oxygen Production from Fluoride-Buffered Solutions. Electrolysis was performed in 25 ml of a 1.0 M ph 3.9 fluoride buffer containing 1 mm CoSO 4. The catholyte was a similar solution lacking cobalt. The divided cell shown above was sealed and rigorously purged with nitrogen. The purging was ceased and the background leak-up rate was measured via a fluorescent oxygen sensor in the cell headspace. Subsequently, electrolysis was performed at 1.63 V vs. NHE (overpotential of 0.64 V) with stirring of the electrolyte without ir compensation. The current was monitored as a function of time as was the response of the oxygen sensor. The oxygen sensor output was corrected for leak-up and converted into a total µmol of oxygen produced in the headspace and solution volume via a Henry's Law approximation of dissolved oxygen in the anolyte. Integration of the current gave the charge passed, which had 1 e - /Co subtracted from it (vide infra) and was then converted to a theoretical µmol of oxygen produced (Figure S4a). Similar electrolysis at a pre-formed catalyst in phosphate buffer at ph 7 gave similar results (Figure S4b). 20 ph 3.9 Fluoride 11 ph 7 Phosphate 15 9 umol oxygen 10 5 0 Faradaic umol O2 Detected umol O2 umol oxygen 7 5 3 Faradaic umol O2 Detected umol O2-5 1-10 -1000 0 1000 2000 3000 4000 5000 t (s) -1-250 250 750 1250 1750 a b Figure S4. Theoretical (- -) and observed ( ) oxygen amounts as functions of time for electrolyses performed in fluoride (a) and phosphate (b) buffers. t (s) S6
Current and Charge During Deposition From Fluoride Buffer. Current vs. time was monitored during electrodeposition from 40 ml of a ph 3.5 1.0 M fluoride buffer containing 1 mm CoSO 4 (Figure S5). The electrodeposition was performed at 1.48 V vs. NHE (overpotential of 0.44 V) in a divided cell with stirring of the electrolyte. To obtain the amount of charge passed, the current was integrated. Figure S5. Current density at 1.48 V vs NHE (overpotential of 0.44 V) of an FTO electrode during deposition from 1.0 M fluoride buffer at ph 3.5 with 1 mm CoSO 4. Charge passed in Coulombs is shown on the second vertical axis with the amount equivalent to 1 e - /Co II in the initial solution indicated. S7
XPS spectra of Co films Figure S6. XPS survey spectra of the following materials: a. Film deposited from 0.1 M aqueous fluoride, showing bulk Co, F, O and C peaks. b. Film deposited from 1.0 M aqueous fluoride with similar bulk composition. c. FTO background with bulk O, Sn, and C peaks but no significant Co. Figure S7. High resolution Co(2p) (a, d), F(1s) (b, e), and O(1s) (c, f) spectra XPS spectra of films deposited from 0.1 M fluoride (a-c) and 1.0 M fluoride (d-f). S8