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. 2021 Apr 28;7(18):eabg2580.
doi: 10.1126/sciadv.abg2580. Print 2021 Apr.

Exposing unsaturated Cu1-O2 sites in nanoscale Cu-MOF for efficient electrocatalytic hydrogen evolution

Weiren Cheng  1 , Huabin Zhang  1 , Deyan Luan  1 , Xiong Wen David Lou  2
Affiliations

Exposing unsaturated Cu1-O2 sites in nanoscale Cu-MOF for efficient electrocatalytic hydrogen evolution

Weiren Cheng et al. Sci Adv. .

Abstract

Conductive metal-organic framework (MOF) materials have been recently considered as effective electrocatalysts. However, they usually suffer from two major drawbacks, poor electrochemical stability and low electrocatalytic activity in bulk form. Here, we have developed a rational strategy to fabricate a promising electrocatalyst composed of a nanoscale conductive copper-based MOF (Cu-MOF) layer fully supported over synergetic iron hydr(oxy)oxide [Fe(OH) x ] nanoboxes. Owing to the highly exposed active centers, enhanced charge transfer, and robust hollow nanostructure, the obtained Fe(OH) x @Cu-MOF nanoboxes exhibit superior activity and stability for the electrocatalytic hydrogen evolution reaction (HER). Specifically, it needs an overpotential of 112 mV to reach a current density of 10 mA cm-2 with a small Tafel slope of 76 mV dec-1 X-ray absorption fine structure spectroscopy combined with density functional theory calculations unravels that the highly exposed coordinatively unsaturated Cu1-O2 centers could effectively accelerate the formation of key *H intermediates toward fast HER kinetics.

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Figures

Fig. 1
Fig. 1. Schematic illustration of the synthetic process for Fe(OH)x@Cu-MOF NB.
(i) Growth of a conductive Cu-MOF layer over Cu2O nanocube via the reaction of locally dissolved Cu ions with organic ligands during the solvothermal process. (ii) Conversion to Fe(OH)x@Cu-MOF NB through a subsequent redox-etching process, where the exact value of subscript x in Fe(OH)x is undetermined but rather suggests the amorphous nature of the inner Fe(OH)x layer in Fe(OH)x@Cu-MOF NB.
Fig. 2
Fig. 2. Morphological and structural characterizations.
(A and B) FESEM images of Cu2O@Cu-MOF nanocubes. (C and D) TEM images of Cu2O@Cu-MOF nanocubes. (E and F) FESEM images of Fe(OH)x@Cu-MOF NBs. (G and H) TEM images of Fe(OH)x@Cu-MOF NBs. (I) HAADF-STEM image and corresponding elemental mapping images of Fe(OH)x@Cu-MOF NBs.
Fig. 3
Fig. 3. XAFS and XPS characterizations.
(A and B) The k3χ(k) oscillation curves (A) and the Fourier transform curves (B) of Cu K-edge EXAFS spectra for CuO, Cu2O, Cu-MOF NPs, and Fe(OH)x@Cu-MOF NBs. (C and D) Cu 2p XPS spectrum (C) and Fe 2p XPS spectrum (D) of Fe(OH)x@Cu-MOF NBs. a.u., arbitrary units.
Fig. 4
Fig. 4. Electrocatalytic performance.
(A and B) LSV plots (A) and the corresponding Tafel slopes (B) of Pt/C, Fe(OH)x NBs, Cu-MOF NPs, Fe(OH)x + Cu-MOF, and Fe(OH)x@Cu-MOF NBs. (C) Half of the capacitive current density (ΔJ/2) at 0.25 V versus RHE as a function of the scan rate for Cu-MOF NPs, Fe(OH)x + Cu-MOF, and Fe(OH)x@Cu-MOF NBs. (D) I-t curve for Fe(OH)x@Cu-MOF NBs and Pt/C catalysts, where the applied potentials are −0.112 and −0.068 V versus RHE for Fe(OH)x@Cu-MOF NBs and Pt/C catalysts, respectively.
Fig. 5
Fig. 5. DFT simulation.
(A) Crystal structure of unsaturated Cu-MOF [Cu3(HHTP)2] viewed along the c axis and the electron density difference plots of corresponding Cu1-O4 and Cu1-O2 centers, where yellow and green contours represent electron accumulation and depression, respectively. (B and C) Calculated partial density of states (PDOS) of Cu-MOF with (B) and without (C) Cu1-O2 centers. (D) Calculated free energy change of adsorbed *H on Cu sites of Cu1-O4 and Cu1-O2 centers.
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