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A Co complex, [CoII(bibpy)L] (1-L, L is an axial ligand) with an equatorial N4 ligand of bibpy2− (H2bibpy = 6,6’-bis-(1H-benzimidazol-2-yl)-2,2’-bipyridine) has been newly synthesized to study electrocatalytic CO2 reduction in a homogeneous DMF solution. Cyclic voltammogram (CV) of 1-L in DMF under Ar exhibited two reversible redox responses at -1.42 V (1st step) and -2.32 V (2nd step) vs. the ferrocene/ferrocenium (Fc/Fc+) redox potential, which could be assigned to 1-L/1− (with release of L) and 1−/12− pairs, respectively. The CV measurement with adding 4.0% water suggests that water replaces the axial L ligand on 1-L, but no longer interacts with 1− and 12− because of requiring no axial ligand. In CV of 1-L in the CO2-saturated DMF solution, a relatively weak reduction wave appeared at -1.70 V of the peak reduction potential (Ep1’) in the 1’st step, which is attributed to the reduction of 1-L with the concerted oxidative coordination of CO2 onto the axial site to form [1-CO2]−. CV of 1-L exhibited the significant catalytic current below the onset potential (Eon) of -1.9 V for CO2 reduction. The electrocatalytic CO2 reduction proceeded stably in the presence of 2.0% water as a proton source. The Eon value increased linearly from -1.98 to -1.89 V with the water content increase from 0 to 4.0%. The hypothetical [1-CO2]−/[1-CO2]2− reduction (2’nd step) involved in the catalytic CO2 reduction might be proton-coupled redox process of [1-CO2]−/[1-CO2H]−. The bulk electrolysis by 1-L in the 2.0% water-containing DMF solution at -2.32 V afforded H2 with 3.0% Faraday efficiency (FE) and HCO2H with 68% FE. The selectivity (RHCO2H) for HCO2H production was 96%, defined as the molar fraction of HCO2H in all the products. The overpotential (η) for CO2 reduction to HCO2H was 0.62 V (at -2.07 V vs Fc/Fc+). These performances of 1-L are among those of the hitherto-reported state-of-the-art Co-complexes for electrocatalytic CO2 reduction to HCO2H in homogenous solutions.

A [Ru(bpy) 3 ] 2+ (bpy = 2,2′-bipyridine)/WO 3 hybrid (denoted as Ru-WO 3 ) film was prepared as a base layer on an indium tin oxide electrode by electrodeposition from a colloidal solution containing peroxotungstic acid, [Ru(bpy) 3 ] 2+ and poly(sodium 4-styrenesulfonate). A ruthenium purple (RP, Fe III 4 [Ru II (CN) 6 ] 3 , denoted as Fe III -Ru II ) layer was electrodeposited on a neat WO 3 film or a Ru-WO 3 film from an aqueous RP colloid solution to yield a WO 3 /RP bilayer film or a Ru-WO 3 /RP bilayer film, respectively. The spectrocyclic voltammetry measurement reveals that Fe II -Ru II is oxidized to Fe III -Ru II by a geared reaction of [Ru(bpy) 3 ] 2+/3+ and Fe III -Ru II is reduced by a geared reaction of H x WO 3 /WO 3 in the Ru-WO 3 /RP film. These geared reactions produced electrochromic hysteresis of the RP layer. However, the absorbance change in the hysteresis was smaller than that for the Ru-WO 3 /Prussian blue bilayer film reported previously, resulting from the lower electroactivities of any redox component for the Ru-WO 3 /RP film. The lower electroactivities could be explained by the specific interface between the Ru-WO 3 and RP layers. It might contribute to either an increase of the interfacial resistance between the Ru-WO 3 and RP layers, or formation of the physically precise interface between the layers to make it difficult for counter ions to be transported in the interfacial liquid phase involved in the redox reactions in the film. The specific interface at the Ru-WO 3 and RP layers could be formed possibly by the electrostatic interaction between [Ru(bpy) 3 ] 2+ and terminal [Ru(CN) 6 ] 4− moieties of RP. It could be suggested by the decreased redox potential of [Ru(bpy) 3 ] 2+ in the Ru-WO 3 layer from 1.03 to 0.61 V by formation of the RP layer.

Charge transport (CT) in a Nafion membrane containing μ-oxobis[aquabis(2,2′-bipyridine)ruthenium(III)] complex, [(bpy)2(H2O)RuORu(H2O)(bpy)2]4+ (bpy=2,2′-bipyridine, abbreviated to RuIIIORuIII) was investigated by potential-step chronocoulospectrometry (PSCCS). Electrochemical reduction of RuIIIORuIII in the membrane occurred irreversibly to form [Ru(bpy)2(OH2)2]2+ monomer. The CT by reduction of RuIIIORuIII in the membrane was suggested to take place by physical displacement of the complex, which is quite different from the mechanism in the CT by oxidation of RuIIIORuIII in the same membrane in which charge is transported by charge hopping based on reversible redox reaction between RuIIIORuIII and RuIIIORuIV. The fractions of the electrochemically reacted complex in the membrane for the oxidative CT was dependent on the complex concentration, and the yield was low (maximum fraction=0.42 at 0.87 M) relative to the reductive CT. By contrast, the fraction for the reductive CT was independent of the concentration over 0.12 M and close to unity. The different concentration dependence of the fraction was discussed related to the difference in the CT mechanism.

Electrocatalytic water oxidation was carried out using a basal-plane pyrolytic graphite (BPG) electrode coated with electrodeposited Au particles adsorbing Ru-red ([(NH3)5Ru-O-Ru(NH3)4-O-Ru(NH3)5]6+). The amount of O2 evolved, VO2 (mol h−1), increased remarkably by adsorbing the complex onto Au particles. The plots of VO2 versus the complex amount on Au particles gave an almost straight line at its low surface coverage, but deviated downward with an increase in its amount, which is ascribed to the bimolecular decomposition. The intrinsic turnover number (TN) of the complex is larger than that of the Au particle by three orders of magnitude. The catalytic activity of the complex was analyzed in terms of its intrinsic TN, k (h−1) and critical decomposition distance, rd (nm) based on intermolecular distance distribution to obtain k=21.4 h−1 and rd=1.11 nm.

Charge transfer in a Nafion membrane incorporating a trinuclear ruthenium complex, Ru-red ([(NH3)5Ru(μ-O)Ru(NH3)4(μ-O)Ru(NH3)5]6+), was studied using potential-step chronoamperospectrometry (PSCAS). Reversible oxidation of Ru-red (RuIII–RuIV–RuIII) to Ru-brown (RuIV–RuIII–RuIV) was observed under a potential-step from 0 to 0.5 V vs. SCE in the PSCAS measurement. The charge transport rate by the oxidation of Ru-red to Ru-brown in the membrane depended on the complex concentration and was analyzed by both the physical displacement of the complexes and charge hopping between them. It was found that the charge transport takes place in the membrane by both the physical displacement and charge hopping. The charge transfer distance was analyzed based on a intermolecular distance distribution between the redox centers, and the real charge hopping distance was estimated to be 1.1 nm.

Charge transport in a Nafion membrane incorporating a μ-oxobis[aqabis(2,2′-bipyridine)ruthenium(III)] complex, [(bpy)2(H2O)Ru–O–Ru(H2O)(bpy)2]4+ [abbreviated to RuIII(OH2)–O–RuIII(OH2)], was studied by using a potential-step chronocoulospectrometry (PSCCS). The oxidation of RuIII(OH2)–O–RuIII(OH2) to RuIII(OH2)–O–RuIV(OH) was observed on a potential-step from 0 V vs. SCE to 1.0 V in the PSCCS measurement. The charge transport rate, vCT, is remarkably depended on the complex concentration. The analysis showed that the charge propagation takes place by charge hopping, whose rate constant is dependent on the complex concentration. Charge hopping distance between the complexes in the membrane was analyzed to be 1.09 nm based on an intermolecular distance distribution between them.

Redox reaction and charge transport of trinuclear ruthenium complex, Ru-red ([(NH3)5Ru(μ-O)Ru(NH3)4(μ-O)Ru(NH3)5]6+) were investigated in a Nafion (Nf) membrane as well as in a homogeneous aqueous solution using a potential-step chronoamperospectrometry (PSCAS) in addition to a cyclic voltammetry (CV). It has been shown in CV that Ru-red is oxidized to Ru-brown and then oxidatively transforms to stable redox species in the membrane during repetitive cyclic scans. In a potential-step from 0 to 0.5 V (vs SCE) in PSCAS, a reversible oxidation of Ru-red to Ru-brown was observed in both the membrane and solution. The charge transport based on Ru-red/Ru-brown took place in the membrane by both physical displacement of the complex and charge hopping between them, while in the aqueous solution charge was transported only by physical displacement of the complex. In a potential-step from 0.5 to 1.5 V (vs SCE), an irreversible oxidative transformation of Ru-brown takes place to produce the species which has almost no absorption in the visible region. The transformed species were stable and redox active in the membrane.