Abstract
Solar hydrogen (H2) evolution from water utilizing covalent organic frameworks (COFs) as heterogeneous photosensitizers has gathered significant momentum by virtue of the COFs’ predictive structural design, long-range ordering, tunable porosity, and excellent light-harvesting ability. However, most photocatalytic systems involve rare and expensive platinum as the co-catalyst for water reduction, which appears to be the bottleneck in the development of economical and environmentally benign solar H2 production systems. Herein, we report a simple, efficient, and low-cost all-in-one photocatalytic H2 evolution system composed of a thiazolo[5,4-d]thiazole-linked COF (TpDTz) as the photoabsorber and an earth-abundant, noble-metal-free nickel-thiolate hexameric cluster co-catalyst assembled in situ in water, together with triethanolamine (TEoA) as the sacrificial electron donor. The high crystallinity, porosity, photochemical stability, and light absorption ability of the TpDTz COF enables excellent long-term H2 production over 70 h with a maximum rate of 941 μmol h–1 g–1, turnover number TONNi > 103, and total projected TONNi > 443 until complete catalyst depletion. The high H2 evolution rate and TON, coupled with long-term photocatalytic operation of this hybrid system in water, surpass those of many previously known organic dyes, carbon nitride, and COF-sensitized photocatalytic H2O reduction systems. Furthermore, we gather unique insights into the reaction mechanism, enabled by a specifically designed continuous-flow system for non-invasive, direct H2 production rate monitoring, providing higher accuracy in quantification compared to the existing batch measurement methods. Overall, the results presented here open the door toward the rational design of robust and efficient earth-abundant COF–molecular co-catalyst hybrid systems for sustainable solar H2 production in water.
Highlights
The conversion and storage of solar energy in the form of chemical bonds in “solar fuels” like H2 through light-driven water reduction has evolved into a key technology over the past decade due to the fast depletion of fossil energy sources and rapid global climate change.[1]
covalent organic frameworks (COFs) with a similar pore size was synthesized as a reference,[19] with the DTz linker replaced with the linear terphenyl linker
We report the first COF photosensitizer and noble-metal-free molecular co-catalyst photocatalytic system for sustained solar H2 production from water
Summary
The conversion and storage of solar energy in the form of chemical bonds in “solar fuels” like H2 through light-driven water reduction has evolved into a key technology over the past decade due to the fast depletion of fossil energy sources and rapid global climate change.[1]. This in situ assembly strategy is different from those of most other Ni(II) and Co(II) cocatalyst complexes, featuring arduous ex situ synthesis and purification of a water-soluble analogue, adding to the cost-effectiveness of the NiME cluster co-catalyst approach.[12,15] This cluster has been shown to be a potent H2 evolution co-catalyst producing H2 immediately after light illumination in the presence of a PS and SED, and does not require any photodeposition, nor does it show an activation time, contrary to Pt-based photocatalytic systems.[10]. Our reaction modeling results suggest that as long as a slow catalyst activation time is observed, the RLS of the system is seemingly the electron transfer from the COF to the NiME complex While this outcome is fully consistent with the assumed outer-sphere electron-transfer process, it reinforces the idea of studying the kinetics of such processes in more detail as this will be crucial to improve the HER rate by rational design of the COF−co-catalyst interface
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