www.asiachem.news December 2021 | 85 filling into the CB causes a negative shift in the Fermi level of TiO2, which is known as a Burstein-Moss shift. We thus concluded that the origin of EMF in our PEC arises from the upward shift in the Fermi level of TiO2 at the photoanode, leading to promote the transfer of electrons to the cathode. This interpretation was further supported by the rational correlation between the H2 evolution and the EMF (Figure 7g), for the rise in EMF (ca. 20 μV) and the H2 production both concomitantly take place only within the light-on period (Figure 7g). These observations well rationalized the fact that the solar H2 production occurs at the cathode even under bias-free conditions. Splitting water by two TiO2 electrodes anchored with molecular catalysts. In spired by our previous finding in the water oxidation activity of cobalt porphyrins,47 the anode photochemically driving water oxidation was initially designed to be given by co-adsorption of Ru-dpqpy (PS) and a cobalt porphyrin WOC possessing a pyridyl anchor (CoP-py; see Figure 8a). However, our study revealed that the FTO/TiO2/Ru-dpqpy/ CoP-py electrode does not show any desirable photocatalytic performance, which we assumed to be due to the lack of sufficient driving force for water oxidation when driven by the Ru(III)/Ru(II) couple of Ru-dpqpy. We thus postulated that the idealized PEC (Figure 3c) is only achievable with an appropriate choice of the PS having a sufficiently higher driving force for the water oxidation catalyzed by CoP-py, which is now in progress. In order to precisely understand the driving force required to promote the water oxidation by CoP-py, we decided to examine the electrolysis performance by the set of the FTO/TiO2/ CoP-py and FTO/TiO2/PtP-py electrodes when adopted in water splitting in the dark (Figure 8).48 As a result, this molecular-catalyst-anchored water electrolyzer was found to promote simultaneous generation of H2 and O2 in a 2:1 molar ratio with a nearly quantitative Faradaic efficiency. The electrocatalytic performances of the anode and cathode were separately examined as a function of pH using the standard three-electrode configuration electrochemical cell (Figure 8b). The cathode exhibited its pH-independent characteristics, consistent with the pH-dependent shift of the flatband potential (see above) which exactly coincides with that of the equilibrium potential for water reduction: E(H2/2H+) = -0.059pH. On the other hand, the anode showed a decrease in the onset overpotential with increasing pH, consistent with the shift in water oxidation potential: E(H2/2H+) = 1.23-0.059pH. The smallest onset potential was observed to be minimized at pH=9.0 with the value of ca. 1.00 V vs. SCE (i.e., 1.77 V vs. RHE), indicative of the onset overpotential of 540 meV for water oxidation with the CoP-py-anchored anode. As for the PtP-py-anchored cathode, the onset potential for HER was observed to be located more positive than 0.83 V vs. SCE (i.e., -0.06 V vs. RHE) at pH=9.0, revealing that the onset overpotential is even smaller than 60 meV. We finally adopted a two-electrode configuration electrochemical cell by sweeping the anode potential versus the cathode potential which is shorted connected to the reference terminal. This setup allowed us to more clearly evaluate the water electrolysis performance of our TiO2-electrodebased electrolyzer. The large current derived from overall water splitting was thus observed with the applied potential of 1.8 V (Figure 8c). The minimum overall potential required to trigger the water decomposition was determined as ca. 1.75 V by conducting the in-situ quantification of the H2 and O2 evolved under various applied potentials. The result indicated that our molecular-catalyst-anchored water electrolyzer start splitting water with addition of 520 meV or more to the theoretical potential (1230 meV). This overall required overpotential (520 meV) corresponds to the sum of driving forces required to drive the HER and OER. Interestingly, this value is even less than the sum of the values independently determined for the anode and cathode using the three-electrode system (600 meV; see above). During 1 h of controlled potential electrolysis (CPE) using the applied potential of 2.2 V vs. cathode, corresponding to ca. 1.0 V of applied overall overpotential, this water electrolyzer produced H2 and O2 in a 2:1 molar ratio (5.9 ± 0.8 and 3.1 ± 0.3 μmol, respectively) with a nearly quantitative Faradaic efficiency (90 ± 6% and 94 ± 4%, respectively) (Figure 8d). The TONs based on the amounts of PtP-py and CoP-py adsorbed over the individual FTO/ TiO2 electrode were estimated to be 59 ± 8 and 31 ± 3, respectively. An interesting observation for this water electrolyzer is that H2 production continues to occur until satisfying the quantitative Faradaic efficiency even after stopping the 1 h of CPE, while such a delay response is not observed for O2 production (Figure 8d). The delayed action in the cathode was rationally interpreted by the fact that the electrons transferred from the anode are once Figure 6. (a) Schematic representation of molecular-based PEC cell for solar H2 production reported by authors′ group.43,44 (b) Schematic diagram and photograph of the molecular-based PEC consisting of the FTO/TiO2/Ru-dpqpy photoanode and the FTO/ TiO2/PtP-py cathode.
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