Photocatalytic Energy Storage over Surface-modified WO3 Using V5+/V4+ Redox Mediator

Abstract

Photocatalytic V 5+ reduction accompanied by O 2 evolution over WO 3 is reported for the first time. Water oxidation reaction utilizing VO 2 + ion as an electron acceptor proceeded photocatalytically over WO 3 . The activity of WO 3 was remarkably enhanced by the presence of Fe 3+/2+ ion at the ion-exchange site on WO 3 surface, and its apparent quantum yield (AQY) was 22% at 420 nm. Moreover, the AQY was raised to about 78% when a small amount of formic acid was added to reactant solution.Recently, photocatalytic water splitting utilizing two kinds of photocatalysts and a reversible redox mediator, which are similar to the photosynthesis mechanism (Z-scheme reaction) where two photoexcitation centers and many redox mediators are leveraged, has been attracting attention for solar energy conversion and storage. 17 When the redox potential of mediator is located between the water reduction and oxidation potentials (from 0 to 1.23 V vs. RHE), the redox-oxidant (Ox) reduction and water oxidation (eq 1) and redox-reductant (Red) oxidation and water reduction (eq 2) are endergonic reactions (Gibbs free energy, ¦G > 0), respectively.Moreover, photocatalysis of eq 1 is also useful for photocatalysiselectrolysis hybrid systems as practical water-splitting systems, where the reaction of eq 2 proceeds by electrolysis with a low applied bias ( Figure S1). 1,14 In this hybrid system, a simple and wide photocatalytic device, which is one of the most important features for effective solar energy utilization, can be easily made owing to stable storage of solar energy in the redox aqueous solution without a H 2 gas-tight cover over the photocatalytic device. Pure H 2 can be accumulated at the counter electrode chamber of electrolyzer anytime with low applied bias (nearly ¦G of eq 2). Additionally, electric power can be generated by a fuel cell using redox-reductant such as Fe 2+ . 8,9 Some reversible redox mediators such as Fe 3+/2+ , 1,2,4 IO 3 ¹ / I ¹ , 3,6,7 and NO 3 ¹ /NO 2 ¹ 5 have been reported for endergonic photocatalysis. We reported recently that Fe 3+ reduction and O 2 evolution for eq 1 reaction took place effectively over Csmodified WO 3 photocatalyst.1 However, the variety of reversible redox mediators is very limited and they have some problems, that is, pH range restriction, light shielding, solubility, stability, redox potential, and so on. Therefore, further development of candidate redox mediators has a great significance in view of the combination with various types of photocatalysts.In this study, we revealed for the first time that V 5+/4+ redox mediator can function as a redox reagent for the photocatalytic eq 1 reaction with a high quantum yield. Moreover, we clarified the activity of surface-modified WO 3 photocatalysts was significantly improved by coupling of V 5+/4+ with Fe 3+/2+ redox mediators. Although the effect of multiple redox combination on photocatalysis has been expected as a model reaction of artificial photosynthesis, the real demonstration of its positive effect has been difficult so far...

Photocatalytic water splitting under visible light utilizing I3−/I− and IO3−/I− redox mediators by Z-scheme system using surface treated PtOx/WO3 as O2 evolution photocatalyst

“…It is suggested that the reaction sites created after the H + -exchange treatment are not stable thermally around 573 K. This behavior was almost the same compared with the results when Fe 3+ or VO 2 + ions were used as the electron acceptors. 23,24 Therefore, it is thought that the high activity may be caused by construction of an ion-exchangeable site, which can efficiently oxidize water. In the case of Fe 3+ and VO 2 + reduction using H-Cs-WO 3 , it is thought that H 3 O + ions preferentially occupy the majority of the surface ion-exchangeable sites because the reactant solutions are acidic (pH 2.3 and 0.7, respectively) and contain none of the other monovalent ion.…”

High-efficiency water oxidation and energy storage utilizing various reversible redox mediators under visible light over surface-modified WO3

“…Therefore, techniques that can improve the quantum efficiency (QE) of the photocatalytic reactions for eqn (1) and ( 2) are important for highefficiency water splitting. I 3 À /I À (+0.55 V vs. RHE), 17,19 Fe 3+ /Fe 2+ (+0.77 V vs. RHE), 21,22 VO 2 + /VO 2+ (+1.00 V vs. RHE), 23 and IO 3 À /I À (+1.09 V vs. RHE) 10,[12][13][14][15]18,24 have been reported as reversible redox ions that utilize eqn (1) and (2). With respect to stability, cost, and redox potential, Fe 3+ /Fe 2+ is an excellent redox mediator.…”

Surface modification by loading alkaline hydroxides to enhance the photoactivity of WO3

“…One may think that photodegradation of organic materials on KOH loaded WO 3 in the gas phase proceeds through a similar mechanism. Corresponding to two last reported papers, 36,37 ion exchange and two-step photo excitation via ions are the main causes for promotion of photoactivity. Actually ion exchange and two-step photo excitation are impossible in gas phase, this experiment, and they are not promising to explain promotion of photoactivity.…”

“…36 Also, it was reported that surface treatment using Cs + and Fe +2 ions on WO 3 can proceed VO 2 + reduction and water oxidation. 37 Since photoactivity was enhanced for Fe +3 and VO 2 + but it was decreased for Ag + , it can be understood that the photo-oxidation-reduction process on WO 3 and H-Cs-WO 3 depends on a lot of factors and the reason for improvement is not trivial. One may think that photodegradation of organic materials on KOH loaded WO 3 in the gas phase proceeds through a similar mechanism.…”