Electrochemical Characterization of Surface-Bound Redox Polymers Derived from 1,1'-Bis[((3- (triethoxysily 1)propyl)amino)carbonyl]cobaltocenium: Charge Transport, Anion Binding, and Use in Photoelectrochemical Hydrogen Generation

This paper describes the behavior of electrode-bound redox material derived from the hydrolysis of the -Si(OEt)₃ groups of l,l'-bis[((3-(triethoxysilyl)propyl)amino)carbonyl]cobaltocenium (I). Surfaces of the conventional electrodes SnO₂ and Pt derivatized with I have a reversible electrochemical response in H₂O/electrolyte; the E°' is pH independent at -0.62 V vs. SCE. The photoelectrochemical behavior of p-type Si photocathodes derivatized with I reveals that the photoreduction of the cobaltocenium derivative can be effected at an electrode potential ~500 mV more positive than on metallic electrodes, consistent with the known behavior of p-type Si photocathodes. When polymer from I is deposited on p-type Si and subsequently coated with a small amount of Rh or Pd (~10-7 mol/cm²), the photoelectrochemical generation of H₂ is possible with 632.8-nm (~15 mW/cm²) radiation and efficiencies in the vicinity of 2%. The polymer derived from I is more optically transparent and more durable at negative potentials than redox polymers derived from viologen monomers. Potential-step measurements and steady-state-current measurements for mediated redox processes show that the charge-transport rate for the polymer derived from I is about the same as for polymers from viologen monomers; As for other positively charged redox polymers, the material from I will electrostatically bind large transition-metal complex anions such as IrCl₆^2+/3-, Fe(CN)₆ ^3-/4-, and Mo(CN)₈ ^3-/4-. A quantitative study of the relative binding of Cl^- and Fe(CN)₆^3- has been done; ΔH° = +12 ± 0.5 kcal/mol, and ΔS° = +52 ± 2 cal/(mol K). Thus, entropy drives the displacement of Cl^- by Fe(CN)₆^3-. The redox potential of the Fe(CN)₆ ^3-/4- system is approximately 50 mV more negative in the polymer compared to the solution potential.