www.asiachem.news December 2021 | 83 PECs (Figure 3c) intend to drive the cathode reaction (HER) in the dark because of the much faster rate generally achieved for the HER compared to the OER (oxygen evolution reaction). In other words, the OER must be considered as a bottleneck in water splitting so that the overpotential required for the OER must be drastically controlled by finely tuning the redox properties of the PS together with the catalytic performance of WOCs in our photoanodes. Why mesoporous TiO2 films? Why pyridyl anchors for stable adsorption? The advancement of our projects on the molecular-based PECs largely relies on the knowledge and experimental techniques gained from the studies on dye-sensitized solar cel ls (DSSCs).38 We assume that mesoporous TiO2 films possess extremely high specific surface area (ca. 80 m2/g),39 extremely larger than the apparent film area, and thereby permit the development of practically ef fective molecular-based PECs exhibiting relatively high hydrogen production capacity. Our initial effort was devoted to invent a new technique to produce tightly anchored molecular PECs which do not easily desorb the molecular components upon soakage into aqueous photolysis solutions. Both carboxylate and phosphonate anchors (Figure 4a,b) are widely adopted in making adsorption of polypyridyl ruthenium dyes and subcomponents in DSSCs, for they are relatively stable anchors in acetonitrile solvent adopted. However, these anchors rather easily dissociate from the TiO2 surfaces due to the hydrolysis in aqueous media when adopted for the artificial photosynthetic purposes, as described elsewhere.40 In general, such dyes are highly soluble in water, which also contributes to the rapid desorption of dyes from the TiO2 surfaces. To substantially suppress desorption of molecular components, we decided to utilize pyridyl anchors (Figure 4c). This pyridyl anchoring technique was evoked by a repor t on DSSCs which revealed improvement in cell performance based on the enhanced co-adsorption of two components using both carboxylate and pyridyl anchors.41 To test our idea, we initially developed a PEC consisting of a dye-anchored photoanode and a dark platinum cathode (Figure 5).42 The TiO 2based photoanode was modi f ied with [Ru(dmbpy)2(qpy)] 2+ (Ru-dmqpy; dmbpy = 4,4′-dimethyl-2,2′-bipyr idine, qpy = 4,4′:2′,2″:4″,4′′′-quar terpyridine) having two pyridyl anchors. By illuminating the photoanode by visible light in the presence of sacrificial Donor (EDTA), we could successfully demonstrate the stable adsorption of Ru-dmqpy over the TiO2 surfaces in aqueous media by monitoring the sustained evolution of H2 from the dark cathode. This behavior was also compared with the rapid deactivation of the PEC prepared by using a similar polypyridyl ruthenium dye possessing either carboxylate or phosphonate anchors instead of pyridyl anchors.42 Importantly, the H2 evolution was found to proceed even without applying any external bias, in line with our direction to avoid the Tandem type PECs (Figure 5). Platinum(II)-based HER catalyst anchored to the cathode. In spite of the superior zero-overpotential characteristics of the HER catalyzed by the platinum electrode, there has been a continued demand to develop comparably efficient non-precious metal HER catalysts for the sake of improving the cost effectiveness. However, as noted above, molecular catalysts capable of promoting the HER under the low driving-force conditions (150 meV by MV+•/MV2+) are quite Figure 3. Schematic representation of the PEC reported by Honda and Fujishima (a), tandem PECs (b), and PECs developed by the authors’ group (c).
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