Interplay between Ruthenium Sensitizer and Ruthenium Catalyst in Photoelectrochemical Cells with Different Water-Based Electrolytes

Popular ruthenium sensitizer (RuP) and ruthenium catalyst (RuOEC) are studied in the cosensitized configurations of a dye-sensitized photoelectrochemical cell with several frequently used electrolytes. Using transient absorption techniques, we have observed that the initial steps of electron transfer from the catalyst to the oxidized sensitizer occur on a very fast time scale (below 100 ps in most of the electrolytes), comparable to that of previously reported supramolecular systems. Electron injection from RuP to TiO2 occurs over a broad span of time scales ranging from tens of picoseconds to hundreds of nanoseconds. Its average rate constant strongly depends on the TiO2 conduction band potential, which varies in different electrolytes and does not follow a simple pH dependence. On the contrary, the excited state of the catalyst deactivates very fast, mostly within 1 ps in all electrolytes. Also, the ratios of initial to stable photocurrents and dark to stable photocurrents, recorded in the operating photoelectrochemical cell, have been found to depend on the conduction band potential. Both sensitizer and catalyst show different resistances to desorption in various environments, e.g., RuP is the most stable in Na2SO4 as well as HCl and RuOEC in phosphate buffer and ethylphosphonic acid. Thus, finding the optimum conditions ensuring the best stability of both compounds is required. The studies prove the potential of cosensitized configurations for efficient water-splitting systems and give more insight into further ways of optimization of such systems.