Electrosynthesis of iron, cobalt and zinc microcrystals and. magnetic enhancement of the oxygen reduction reaction

Transcription

1 M / [A m 2 kg -1 ] Electrosynthesis of iron, cobalt and zinc microcrystals and magnetic enhancement of the oxygen reduction reaction Lorena M. A. Monzon*, Karsten Rode, M. Venkatesan and J.M.D. Coey School of Physics, Trinity College, Dublin 2, Ireland. *aranzazl@tcd.ie 1 m Figure 1S. Overview of Cobalt particles produced with a low concentration of Co 2+. Dendritic structures are shown in detail in Figure 2c. BA BA ph=4.4 AA buffer ph= m AA B / [T] m Figure 2S. Magnetization loops and SEM images of Co grains deposited at -1.2 V,.2 C, ph of both baths is 4.4. Solution composition:.2 M Co SO 4, red corresponds to a bath containing.1 M of acetic acid and sodium acetate, while black contains.1 M boric acid. 1S

2 Fe (22) counts Fe (11) Au (222) Au (111) Si (4) M / [A m 2 kg -1 ] At -1.2 V the coercivity for the sample prepared in BA is H c = 37 mt with a remanecence ratio M r /M s = 16.6 %, while the sample prepared in AA, has a coercivity value of 3 mt and M r /M s = 2.5 %. 15 Fe-PANI fresh Fe-PANI aged H / [T] Figure 3S. Magnetization loops of iron cubes obtained from a 1 M FeSO 4,.1 M acetic acid bath. The red curve corresponds to a freshly-prepared sample, while the curve in green is the same sample taken 24 hours later. The inset shows a magnification of the hysteresis loop near zero field. Iron cubes exhibit 3 mt hysteresis degrees Figure 4S. XRD patterns of Iron cubes obtained from a 1 M FeSO 4,.1 M AA. 2S

3 counts Zn (1 1) Zn ( 4) Zn ( 2) Fe (11) counts Au (222) Au (111) Si (4) degrees Figure 5S. XRD patterns of Iron hemispheres obtained from a 1 M FeSO 4,.1 M BA. 1 5 Si (4) 1 4 Au (1 1 1) 1 3 Au (2 2 2) / [degrees] Figure 6S. XRD patterns of Zinc hexagons obtained from a 1 M ZnSO 4,.1 M acetic acid bath. The intensity ratio between the 2 and 11 reflexions is m Figure 7S. SEM image of cobalt particles extracted from PANI films. They were electrodeposited at -1.4 V vs. Ag/AgCl, from a.2 M CoSO 4,.1 M BA. Charge accumulated was.325 C. 3S

4 I / [ A] V vs Ag/AgCl / [V] Nafion-PANI Nafion-Au substrate bare Au substrate sputtered Au-PANI Nafion-sputtered Au-PANI Nafion-sputtered Au-Fe-PANI Nafion-sputtered Au-Co-PANI Nafion-sputtered Au-Zn-PANI Figure 8S. Cathodic scans for the ORR on Nafion-PANI (orange), Nafion-Au substrate (black), bare Au (grey), Au-PANI (magenta), Nafion-sputtered Au-PANI (cyan) and on substrates modified with Fe (green), Co (blue) and Zn (red) grains. The substrates containing the metallic grains were coated with 1 nm of Au, followed by Nafion. Scan rate 5 mv s -1. Geometric area: 1.13 cm 2. O 2 -saturated.5 M H 2 SO 4 solution. Figure 8S compares the cathodic response recorded at 5 mv s -1 for the ORR on various substrates. In orange is shown the scan obtained with a PANI film coated with Nafion, without Au sputtered on top. The current is very low given the insulating nature of PANI in this potential region and the poor electronic conductivity of Nafion. 1S The response recorded for bare gold coated with Nafion is also quite resistive (see curve black and grey) because gold is not a good catalyst for ORR and additional resistivity is added by the Nafion layer. The onset potentials, E onset, of these two substrates are around -.2 V vs Ag/AgCl. Nevertheless, when PANI is coated with a thin layer of Au there is a better ORR response than for Au or PANI alone, with a E onset of Nafion-Au-PANI at ~.5 V vs. Ag/AgCl. One possible explanation for the improved ORR E onset and the peak current could be due to an increased active surface area after Au coating. 2S Nevertheless, this still does not explain why the ORR is more hindered on bare gold than on Nafion-Au-PANI, where the added resistivity of Nafion should somehow counteract this effect. One plausible explanation for this extra enhancement could be attributed to the catalytic effect towards the ORR exhibited by several nitrogen-containing compounds. 3S-5S The mechanism proposed to 4S

5 I / [ A] explained this effect considers that the molecular orbitals in the nitrogen, or nitrogenmetal sites overlap with the ones in the O 2 molecule, therefore charge transfer takes place and the O-O bond is weakened. It would seem that during Au sputtering, some electron transfer from Au to PANI occurs, which leaves PANI able to suply electrons through its N sites. A similar effect has been reported during the heat treatment of PANI and vulcan carbon. 6S The next set of curves shown in Figure 6S (red, green and blue) are the ones containing metal particles. The response for all the transition metals is similar, but quite different to the response without the particles, i.e. Nafion-Au-PANI. The slight shift in the E onset and the peak current values could be given to an increase in the roughness of the electrode surface, which it is comparable for all three metallic grains, Zn, Fe and Co. -5 with M without without Au coating -2 with Au coating V vs Ag/AgCl / [V] Figure 9S. Cathodic scan for the ORR recorded with substrates containing cobalt hemispheres, with and without gold coating. Scan rate 5 mv s -1. Geometric area: 1.13 cm 2. O 2 -saturated.5 M H 2 SO 4 solution. References (1S) Easton, E.B.; Qi, Z.; Kaufman, A.; Pickup, P.G.; Electrochem. Solid-State Lett, 21, 4, A59-A61 (2S) Shim, J.H.; Kim, Y.S.; Kang, M.; Lee, C.; Lee, Y.; Phys. Chem. Chem. Phys., 212, 14, S

6 (3S) Matter, P. H.; Zhang, L.; Ozkan, U. S.; J. Catal., 26, 239, (4S) Li, Z. P.; Liu, B. H.; J. Appl. Electrochem., 21, 4, (5S) Chen, Z.; Higgins, D.; Yu, A.; Zhang, L.; Zhang, J.; Energy Environ. Sci., 211, 4, (6S) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P.; Science, 211, 332, S