Supporting Information For:

Transcription

1 For: Size Dependent Activity of Co 3 O 4 Nanoparticle Anodes for Alkaline Water Electrolysis Arthur J. Esswein, Meredith J. McMurdo, Phillip N. Ross, Alexis T. Bell,* and T. Don Tilley* Contribution from the Departments of Chemistry and Chemical Engineering, University of California, Berkeley, Berkeley, California 94720, and the Chemical Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California, bell@cchem.berkeley.edu, tdtilley@berkeley.edu Table of Contents Page Cyclic Voltammograms the Co 3 O 4 Anodes 2 Mass Spectrographs of the Evolved Gases 3 4 Tafel Representation of the Anode Polarization Data 5 TOF per surface Co atom calculation 6 1

2 Figure S1 Figure S1. Cyclic voltammagrams of 1 cm 2 Ni foam working electrodes in 1.0 M KOH (ph 14) collected at 50 mv/s with a constant 1mg/cm 2 loading of Co 3 O 4 nanoparticles: A) Bare Ni foam, B) 46.9 nm Co 3 O 4, C) 21.1 nm Co 3 O 4, D) 5.9 nm Co 3 O 4. 2

3 Figure S2 S3 Figure S2. Mass Spectrograph of a headspace sample of a 1.0 M KOH (ph 14) solution biased at 800 mv vs. Ag/AgCl using a 1 cm 2 Ni foam anode loaded with 1 mg/cm 2 of 5.9 nm Co 3 O 4 nanoparticles. The peaks at m/z = 2 and 32 unequivocally identify H 2 and O 2 as the electrolysis products. The peak at m/z = 28 originates from a small leak of ambient air into the system over the course of the experiment. Peaks in the m/z =16-18 region correspond to residual water, and the signal at ~ m/z = 40 is an artifact of the correction for the Ar carrier gas background, these signals persist in all traces shown below. Figure S3. Expansion of Figure S2 concentrating on the region associated with oxygen and nitrogen. 3

4 Figure S4 S5 Figure S4. Mass Spectrograph of a headspace sample of a 1.0 M KOH (ph 14) solution containing 8.45% total 18 O content biased at 800 mv vs. Ag/AgCl using a 1 cm 2 Ni foam anode loaded with 1 mg/cm 2 of 5.9 Co 3 O 4 nanoparticles. The peaks at m/z = 2, 32, 34, and 36 unequivocally identify H 2 and O 2 as the electrolysis products. Figure S5. Expansion of Figure S4 concentrating on the region associated with oxygen and nitrogen. Statistical distribution of the 18 O label into the evolved oxygen predicts that approximately 83.8%, 15.4%, and 0.71% of the total evolved oxygen will be composed of 32 O 2, 34 O 2, and 36 O 2 respectively. The observed integrated intensity ratio of 32 O 2 : 34 O 2 of 5.74 corresponds well to theoretically predicted ratio of The increase in favor of 32 O 2 can be explained by the small leak of ambient air into the reaction vessel. A similar analysis for the 36 O 2 peak is complicated by low signal intensity. 4

5 Figure S6 Figure S6. Log of the current density against electrochemical overpotential for the Ni foam anodes used in this study: Bare Ni foam ( ), 46.9 nm Co 3 O 4 nanoparticles ( ), 21.1 nm Co 3 O 4 nanoparticles ( ), 5.9 nm Co 3 O 4 nanoparticles ( ). The dotted line marks the potential at which each of the anodes achieves a current density of 10 ma/cm 2. All catalyst loadings are 1 mg/cm 2 and all data collected at scanrates of 1 mv/s in 1.0 M KOH (ph 14) solution and are referenced to Ag/AgCl. The overpotential is calculated using the thermodynamic potential at 206 mv for water oxidation at ph 14 vs. Ag/AgCl. 5

6 TOF per Surface Co Atom Calculation Details concerning the calculation of turnover frequency per suface cobalt atom of the Co 3 O 4 nanoparticles are provided below: The number of surface cobalt atoms in the Co 3 O 4 spinel nanoparticles was calculated by assuming that the 100 crystal face is exposed in all cases (valid for the cubic nanoparticles synthesized in this report). On this face of the spinel unit cell 2 cobalt atoms are fully occupied, and 4 are half occupied, giving four total cobalt atoms. The unit cell edge lengths are Å, and thus the density of surface cobalt atoms is surface cobalt atoms/m 2. Using 1.0 mg of the 5.9 nm Co 3 O 4 nanoparticles (surface area m 2 /g) gives m 2 of total surface area, and thus surface Co atoms total in this sample. Turnover frequency calculations are straightforward from this point on. For example, operating at a current density of 50 ma/cm 2 and an electrode area of 1 cm 2 translates to moles of electrons/s. Since four electrons are required for water oxidation, this becomes moles of O 2 /s (assuming 100% Faradaic efficiency). Converting to molecules gives the reaction velocity as molecules of O 2 /s. The turnover number is then ( molecules of O 2 /s)/( surface Co atoms) giving molecules O 2 s 1 surface Co atom 1 or ~ 0.12 molecules O 2 s 1 surface Co atom 1. At 10 ma/cm 2 the turnover frequency is molecules O 2 s 1 surface Co atom 1. 6