Electrodes modified with synthetic clay minerals: Evidence of direct electron transfer from structural iron sites in the clay lattice

Abstract--Hydrothermal treatment of a mixture of silicic acid, cobalt chloride, sodium dithionite and sodium hydroxide at 250 ~ under 500 psi of argon produced a pink solid. X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and electron diffraction data showed the product o be a well-crystallized smectite. SEM/EDX analysis gave a unit cell formula of [(Si8.05)(Co558)O20(OH)4]Na066. Heating the same mixture at 150 ~ without argon gave a less well ordered smectite of composition [(Si7.93)(COs.92)O20(OH)4]Nao.42. Two peaks were observed in the cyclic voltammograms of electrodes mod-ified with films of these two clays recorded for the blank electrolytes in the absence of any adsorbed electroactive species. The first peak was attributed to the oxidation of a small fraction of the Co 2+ sites within the clay lattices to Co 3+. The second peak was assigned to further oxidation of these Co 3+ to Co 4+