Cobalt-coordinated covalent organic framework as a versatile water oxidation catalyst in dye-sensitized photoelectrochemical cells for solar water splitting

doi:10.1016/S1872-2067(26)65196-0

Abstract

Abstract:

The development of efficient water oxidation catalysts is crucial for advancing dye-sensitized photoelectrochemical cells toward practical solar-driven water splitting. Herein, we synthesize a cobalt-coordinated covalent organic framework, TFPP-Co-COF, and integrate it with photosensitizers to construct high-performance photoanodes. The optimized electrode achieves an initial photocurrent density of 800 μA/cm² at 0 V vs. Ag/AgCl, maintaining 700 μA/cm² after 200 s with a Faradaic efficiency of 88.8%. Even under unbiased conditions (0 V vs. NHE), it delivers 450 μA/cm² (71.6% FE) and retains 400 μA/cm² over 25 min. Mechanistic studies confirm efficient electron transfer from TFPP-Co-COF to the photosensitizer. Theoretical calculations further reveal that water oxidation proceeds through a water nucleophilic attack pathway at cobalt sites, exhibiting a lower energy barrier than metal-free alternatives. The superior performance over amorphous analogs originates from enhanced crystallinity, high surface area, and abundant cobalt active sites that collectively promote charge separation and catalytic activity. TFPP-Co-COF also demonstrates excellent compatibility with pure organic dyes C4BTP and DT1, highlighting its versatility. This work pioneers COF application in DSPECs, providing a promising pathway for efficient solar fuel production.

Key words: Dye-sensitized photoelectrochemical cells, Covalent organic frameworks, Photosensitizers, Water oxidation catalysts, Solar water splitting

Zizhan Liang, Xiaolin Wang, Zhilin Hong, Xinao Li, Xinbang Peng, Lilin Tan, Limin Xiao, Yecheng Zhou, Junmin Liu. Cobalt-coordinated covalent organic framework as a versatile water oxidation catalyst in dye-sensitized photoelectrochemical cells for solar water splitting[J]. Chinese Journal of Catalysis, 2027, 92: 248-261.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)65196-0

https://www.cjcatal.com/EN/Y2027/V92/I1/248

Figures/Tables 6

Scheme 1. Schematic diagram of DSPEC device based on OPPs (middle) and chemical structures of dyes (left) and WOCs (right).

Fig. 1. (a) The synthetic route of TFPP-Co-COF under solvothermal conditions. (b) FT-IR spectra of TFPP-Co-COF, TFPP-Co-polymer and TFPP-COF. (c) Experimental, Pawley-refined, and simulated PXRD patterns of TFPP-Co-COF (Rp: unweighted-profile factor; Rwp: weighted-profile factor). (d) Solid-state UV-vis spectra of TFPP-Co-COF, TFPP-Co-polymer and TFPP-COF. (e) Mott-Schottky plot of TFPP-Co-COF. (f) Energy level diagrams of N719, OPPs and the oxygen evolution oxidation potential of water. (g) The nitrogen adsorption isotherm measured at 77 K and (h) the pore size distribution curves of TFPP-Co-COF.

Fig. 2. (a,b) SEM images of TFPP-Co-COF at different magnifications. STEM (c) and AC-TEM (d) images of TFPP-Co-COF. (e) SEM image of FTO|TiO2|N719+TFPP-Co-COF. (f) Profile analysis of electrode structure (Blue area represents FTO glass, yellow area represents TiO2 attached with dye molecules and purple area represents OPPs).

Fig. 3. (a) Schematic diagram of preparation process of sensitized electrode. (b) LSV curves of the N719-sensitized electrodes corresponding to different OPPs under positive scan direction. (c) I-t curves of photoanodes corresponding to the different OPPs with a bias of 0 V vs. Ag/AgCl. (d) DPV curves of FTO|TiO2|N719+TFPP-Co-COF, FTO|TiO2|N719 and FTO|TiO2|TFPP-Co-COF. EIS Nyquist plots of photoanodes in the dark with a bias of -0.6 V vs. Ag/AgCl (R1: series resistance; R2: recombination resistance; CPE: constant phase element) (e) and under irradiation with a bias of 0.2 V vs. Ag/AgCl (R1: series resistance; R2: charge-transfer resistance; CPE: constant phase element) (f). (g) I-t curves of photoanodes corresponding to the different OPPs with a bias of 0 V vs. NHE. (h) Long-term illumination test of electrodes corresponding to different OPPs under illumination without bias voltage in a dual-electrode system. Faradaic efficiency test results of FTO|TiO2|N719+TFPP-Co-COF in three-electrode system at 0 V vs. Ag/AgCl (i) and in dual-electrode system without bias voltage (j).

Fig. 4. (a) Energy level diagram of various dyes matching with TFPP-Co-COF. (b) Open circuit voltage-time curves of electrodes sensitized by different dyes. I-t curves of photoanodes sensitized by different dyes with a bias of 0 V vs. Ag/AgCl (c) and with a bias of 0 V vs. NHE (d). (e) Long-term illumination test of electrodes sensitized by different dyes under illumination without bias voltage in a dual-electrode system. (f) IPCE curves of different dye-sensitized electrodes loaded with TFPP-Co-COF.

Fig. 5. (a) Solid-state FL spectra of N719, TFPP-Co-COF and their complexes. (b) Fluorescence intensity-titration volume curves of fluorescence quenching experiment of OPPs. (c) Mechanism of water oxidation catalyzed by Co active site on TFPP-Co-COF. (d) XPS spectra of Co on TFPP-Co-COF before and after illumination. (e) Gibbs free energy diagrams for oxygen evolution at Co active sites of TFPP-Co-COF. (f) Gibbs free energy diagrams for direct oxygen evolution at C1 active sites of TFPP-COF.

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