Supporting Information for Amorphous FeOOH Oxygen Evolution Reaction Catalyst for Photoelectrochemical Water Splitting

Transcription

1 Supporting Information for Amorphous FeOOH Oxygen Evolution Reaction Catalyst for Photoelectrochemical Water Splitting William D. Chemelewski 1,2, Heung-Chan Lee 2,3, Jung-Fu Lin 1,4, Allen J. Bard 1,2,3, C. Buddie Mullins 1,2,3,5* Affiliations: 1 Texas Materials Institute, University of Texas at Austin. 2 Center for Electrochemistry, University of Texas at Austin. 3 Department of Chemistry and Biochemistry, University of Texas at Austin. 4 Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin. 5 Department of Chemical Engineering, University of Texas at Austin. Figure S1. Image of experimental setup used for photoelectrochemical tests of a-si TJ cells. S1

2 Figure S2. Images of holder used to test TJ cells in air as photovoltaic devices. a) Perspective view, showing the ITO Cu tape contact on the left and the stainless steel contact tape on the right. Not seen is an O-ring that acts as an insulator between the front aluminum piece and the TJ ITO. b) Front view of holder, these images were used to determine the active area of each device, accounting for the area blocked by the Cu tape. Figure S3. a) Representative CV scan of FTO in Fe deposition bath, scan is from -0.4 to 0.2 to V. b) Representative current-time trace at -0.2 V vs Ag/AgCl of FTO in Fe deposition bath. S2

3 Figure S4. TEM selected area electron diffraction (SAED) and image data. a) Representative SAED for the particle show in b). c) Representative SAED for the particle shown in d). For both SAED patterns, definite diffraction spots are visible but the bright rings also show there is a significant amount of amorphous material surrounding the small crystallites. e) Representative high resolution image of one of these crystallites with fringes evident in the dark particle in the center of the image while the larger volume of material to the right of that particle is disordered. The streaking seen in the SAED patterns is an artifact of the CCD camera. S3

4 Figure S5. a) J-V traces of a-feooh and Co-B i in 1 M Na 2 CO 3 before and after ir-correction for the a-feooh film shown, the measured resistance was 20.5 Ω, while for the Co-B i filmthe measured resistance was 20 Ω. b) Comparison of the overpotential of Co-B i films in 1 M Na 2 CO 3 to overpotential in potassium borate buffer (0.5 M H 3 BO M KOH) as a function of thickness. Open circles are 1 ma/cm 2, closed are 10 ma/cm 2. c) J-V curve comparison adding in hematite (Fe 2 O 3, approximately 15 nm thick), all ir corrected in 1 M Na 2 CO 3. While hematite performs significantly better than FTO it is still about 100 mv worse than a-feooh and Co-B i, and has the added disadvantage of requiring high temperature annealing for synthesis. Figure S6. Chopped light response of 5 mc/cm 2 thick a-feooh in 1 M Na 2 CO 3. Scan rate is 25 mv/s. Photocurrent at 1.23 V vs RHE is about 0.2 µa/cm2, whereas the total absorption is equivalent to about 0.6 ma/cm2. This gives an approximate quantum efficiency of 0.03%, making a-feooh nearly useless as a standalone PEC material. S4

5 Figure S7. a) Representative current-time plots for photoelectrodeposition of Co-Bi (black) and a-feooh (red). Jumps in current before t = 1s are from unblocking the light showing that the current is indeed light dependent. Also note that a large amount of the Co-Bi current is from oxygen evolution, not due to Co 2+ oxidation. b) Cathodic (positive to negative) CVs of bare TJ and a-feooh/tj in 1 M Na 2 CO 3 with a scan rate of 50 mv/s. The a-feooh/tj trace is consistent from scan to scan while the bare TJ degrades significantly with each scan. The scan number is labeled next to each bare TJ trace. Figure S8. Stability tests of catalyst/tj cells in 1 M Na 2 CO 3 for a) 2.5 mc/cm 2 a-feooh and b) 43 mc/cm 2 Co-Bi (not all 43 mc/cm 2 is due to Co deposition). S5

6 Figure S9. a) Schematic of the experimental setup used to measure the a-feooh/tj/pt device in a standalone configuration. b) Gas chromatography trace of headspace after 15 minutes of illumination. The H 2 to O 2 ratio is 1.95 to 1. The N 2 is from air that was not fully excluded from the cell and syringe. The N 2 peak is used to adjust the amount of O 2 attributed to the device. S6