Molecular aspects, electrochemical properties and water oxidation catalysis on a nanoporous TiO2 electrode anchoring a mononuclear ruthenium(II) aquo complex

Efficient Electrocatalytic Water Oxidation by a Dinuclear Ruthenium(II) Complex with Vicinal Aquo and Hydroxo Groups Adsorbed on a TiO2 Electrode

“…The peak potential at 0.73 V was pH-independent in the pH range of 6.0–9.0 on the Pourbaix diagram (Figure ). This can be explained mainly by the non-proton-coupled two-electron oxidation process of the Ru 2 II (OH) 2 /Ru 2 III (OH) 2 redox pair due to the pH buffering ability of the TiO 2 surface. , Alternatively, the possible process of two-electron oxidation and kinetically uncoupled proton release of Ru 2 II (OH)­(OH 2 ) to Ru 2 III (OH) 2 can be involved.…”

“…For the CV of the Ru 2 (OH)-(OH 2 )/TiO 2 and Ru 2 (μ-Cl)/TiO 2 electrodes (Figure 4), the charge transport through the TiO 2 film occurs via electron hopping between complexes adsorbed on the TiO 2 surface, as shown in the previous reports. 62,66,67 The significant catalytic current for water oxidation was observed above 1.1 V versus SCE, and the catalytic current density reached 5.1 mA cm −2 at 1.6 V for the Ru 2 (OH)(OH 2 )/TiO 2 electrode. The catalytic current at 1.6 V was higher than those for the Ru 2 (μ-Cl)/TiO 2 and bare TiO 2 electrodes by factors of 9.5 and 36, respectively, indicating that Ru 2 (OH)(OH 2 ) (or Ru 2 (OH) 2 ) works efficiently for electrocatalytic water oxidation compared with Ru 2 (μ-Cl).…”

“…These absorption characteristics are close to those of Ru 2 (OH)(OH 2 ) (peak at 500 nm and shoulder at 651 nm) in an aqueous NaHCO 3 buffer solution. However, very recently, we reported that a mononuclear Ru II aquo complex was adsorbed as a deprotonated hydroxo complex form on TiO 2 62 and other metal oxide surfaces 63 even at a pH of 7.0 of the electrolyte solution due to the pH buffering ability offered by the TiO 2 surface, although the pK a values of most Ru II aquo complexes with polypyridyl ligands are located in the range of 9−11. 15,20,64,65 Furthermore, as taking the little absorption spectral difference between Ru 2 (OH)(OH 2 ) and Ru 2 (OH) 2 (Figure S13) into account, either Ru 2 (OH)(OH 2 ) or Ru 2 (OH) 2 is supposed to be adsorbed on the TiO 2 surface from the absorbance change of the electrode before and after the complex adsorption, assuming the molar absorption coefficients (ε/M −1 cm −1 ) in respective solutions (see Experimental Section).…”

Strategies for Electrochemically Sustainable H2 Production in Acid

“…[ 180 ] Other graphitic‐type carbons can also be used as electrode substrates for immobilization. To enable stronger immobilization of molecular catalysts, covalent linkages are usually needed, which requires delicate surface functionalization of electrode substrates/supports to provide the groups or defects (e.g., COOH, SH, OH, PO 3 H 2 , NH 2 , and pyrene) [ 147 , 181 , 182 , 183 ] for anchoring. Typically, [Ru(C 8 Otpy)(H 2 dcbpy)(OH 2 )] 2+ is anchored on the nanoporous TiO 2 surface via the interaction between carboxylic groups and surface oxides.…”

“…Typically, [Ru(C 8 Otpy)(H 2 dcbpy)(OH 2 )] 2+ is anchored on the nanoporous TiO 2 surface via the interaction between carboxylic groups and surface oxides. [ 181 ] This complex is stable during OER with a high current of 0.39 mA cm –2 and FE of 83%. In general, immobilization should be strong enough to retard the activity loss of molecule during electrocatalysis, and it should not alter the physical and chemical nature of molecular catalysts or isolate them, which limits their original electrocatalytic function.…”