Much effort has been devoted to photocatalytic production of hydrogen peroxide (H 2 O 2 )a sa na lternative to fossil fuels.F roma ne conomic point of view,r eductive synthesis of H 2 O 2 from O 2 coupled with the oxidative synthesis of value-added products is particularly interesting.W eh erein report application of MIL-125-NH 2 ,aphotoactive metalorganic framework (MOF), to ab enzylalcohol/water twophase system that realized photocatalytic production and spontaneous separation of H 2 O 2 and benzaldehyde.H ydrophobization of the MOF enabled its separation from the aqueous phase.T his resulted in enhanced photocatalytic efficiency and enabled application of various aqueous solutions including extremely low pH solution which is favorable for H 2 O 2 production but fatal to MOF structure.I na ddition, ah ighly concentrated H 2 O 2 solution was obtained by simply reducing the volume of the aqueous phase.
A transparent Ta3N5 photoanode is a promising
candidate for the front-side photoelectrode in a photoelectrochemical
(PEC) cell with tandem configuration (tandem cell), which can potentially
provide high solar-to-hydrogen (STH) energy conversion efficiency.
This study focuses in particular on the semiconductor properties and
interfacial design of transparent Ta3N5 photoanodes
fabricated on insulating quartz substrates (Ta3N5/SiO2), typically the geometric area of 1 × 1 cm2 in contact with indium on its edge. This material utilizes
the self-conductivity of Ta3N5 to make the PEC
system operational, and the electrode would strongly reflect the intrinsic
nature of Ta3N5 without a back contact that
is commonly introduced. First, PEC measurements using acetonitrile
(ACN)/H2O mixed solution were made to elucidate the intrinsic
photoresponse in the presence of tris(2,2′-bipyridine)ruthenium(II)
bis(hexafluorophosphate) (Ru(bpy)3(PF6)2) without water contact which avoids a multielectron-transfer
oxygen evolution reaction (OER) and photoinduced self-oxidation. The
potential difference between the onset potential of Ru2+ PEC oxidation by Ta3N5/SiO2 and
the redox potential of Ru2+/3+ in the nonaqueous environment
was about 0.7 V. While a stable photoanodic response was observed
for Ta3N5/SiO2 in the nonaqueous
phase, the addition of a small quantity of water into this nonaqueous
system led to the immediate deactivation of Ta3N5/SiO2 photoanode under illumination by self-photooxidation
to form TaO
x
at the solid/water interface.
In aqueous phase, flatband potentials estimated from Mott–Schottky
analysis varied with solution pH (constant potential against reversible
hydrogen electrode (RHE)). Photoelectrode modification by a transparent
NiFeO
x
layer was attempted. The complete
coverage of the Ta3N5 surface with transparent
NiFeO
x
electrocatalysts, achieved by an
optimized spin-coating protocol with controlled Ni–Fe precursors,
allowed for the successful protection of Ta3N5 and demonstrated an extremely stable photocurrent for hours without
any additional protective layers. The stability of the resultant NiFeO
x
/Ta3N5/SiO2 was limited not by Ta3N5 but mainly by a NiFeO
x
electrocatalyst due to Fe dissolution with
time.
Photocatalytic H2O2 production via two-electron reduction of O2 was realized by visible-light irradiation of a metal-organic framework, MIL-125-NH2, in the presence of TEOA and benzylalcohol. Deposition of NiO nanoparticles onto MIL-125-NH2 dramatically enhanced the catalytic activity. Further studies suggested that fast disproportionation of the O2˙- intermediate to H2O2 resulted in the enhancement.
A semitransparent Ta3N5 photoanode is designed for efficient and durable solar water splitting. The Ta3N5-CuInSe2 tandem device exhibits an initial and stabilized solar-to-hydrogen efficiency of ∼9% (highest for metal oxides/nitrides) and 4%, respectively.
The photocatalytic activity of a cluster-alkylated MOF for H2O2 production far exceeded that of a linker-alkylated MOF in a benzyl alcohol/water two-phase system.
The development of an efficient conversion system to transform solar energy into chemical energy, such as renewable hydrogen, is a promising way to overcome energy problems. Photoelectrochemical (PEC) water splitting is a promising means of obtaining renewable hydrogen directly from water utilizing sunlight. Recent reports have demonstrated that a PEC cell with a tandem configuration (tandem cell) has the potential to realize a high solar-to-hydrogen (STH) energy conversion efficiency by solar water splitting. However, there are still many obstacles to the development of practical and cost-effective tandem cells. In particular, development of efficient photoanodes for the oxygen evolution reaction (OER) is a prerequisite for improving the STH efficiency. Herein, recent progress in developing (semi)transparent photoanodes for the OER, such as Fe 2 O 3 , BiVO 4 , and Ta 3 N 5 , is described based on the topics of preparation methods, semiconductor properties, and PEC performance. In addition, the strategies for enhancing the STH efficiency of tandem cells consisting of (semi) transparent photoanodes conjugated with photovoltaic (PV)-based cathodes are summarized. This Review is expected to provide guidelines for the future development of tandem cells capable of highly efficient and stable water splitting.
Much effort has been devoted to photocatalytic production of hydrogen peroxide (H2O2) as an alternative to fossil fuels. From an economic point of view, reductive synthesis of H2O2 from O2 coupled with the oxidative synthesis of value‐added products is particularly interesting. We herein report application of MIL‐125‐NH2, a photoactive metal–organic framework (MOF), to a benzylalcohol/water two‐phase system that realized photocatalytic production and spontaneous separation of H2O2 and benzaldehyde. Hydrophobization of the MOF enabled its separation from the aqueous phase. This resulted in enhanced photocatalytic efficiency and enabled application of various aqueous solutions including extremely low pH solution which is favorable for H2O2 production but fatal to MOF structure. In addition, a highly concentrated H2O2 solution was obtained by simply reducing the volume of the aqueous phase.
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