Visualization of cation diffusion at the TiO2 interface in dye sensitized photoelectrosynthesis cells (DSPEC)

Time-resolved, UV-vis spectroscopic measurements of Li+ diffusion in mesoscopic TiO2 photoanodes were conducted in dye sensitized photoelectrosynthesis cells (DSPECs) under operating conditions. In these experiments the spectral response of TiO2 derivatized with [Ru(bpy)(2)(4,4'-((HO)(2)PO)(2)bpy)](2+) (RuP, where bpy is 2,2'-bipyridine, (4,4'-((HO)(2)PO)(2)bpy) is [2,2'-bipyridine]-4,4'-diphosphonic acid) arises from electric field (Stark) effects on the metal-to-ligand-charge transfer (MLCT) absorption spectrum of RuP, which is screened by cation intercalation. These results verify that Li+ diffusion is coupled to electron injection and to electron recombination/extraction at the TiO2 interface. Li+ doping levels depend on the competition between dynamics of its intercalation and electron recombination/transport. For a DSPEC operating in aqueous solution at pH 4.5, the observed rate constants for Li+ intercalation and release were 0.22 s(-1) and 0.014 s(-1), respectively. Both processes were considerably slower in the more viscous solvent propylene carbonate with Li+ release rate constants <2 x 10(-4) s(-1). Accumulation of Li+ under these conditions shifts conduction band/trap states to less negative potentials, increasing electron lifetime in TiO2.