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. 2018 Feb 28;140(8):2985-2994.
doi: 10.1021/jacs.7b13077. Epub 2018 Feb 20.

Development of a UiO-Type Thin Film Electrocatalysis Platform with Redox-Active Linkers

Ben A Johnson  1 , Asamanjoy Bhunia  1 , Honghan Fei  2 , Seth M Cohen  2 , Sascha Ott  1
Affiliations

Development of a UiO-Type Thin Film Electrocatalysis Platform with Redox-Active Linkers

Ben A Johnson et al. J Am Chem Soc. .

Abstract

Metal-organic frameworks (MOFs) as electrocatalysis scaffolds are appealing due to the large concentration of catalytic units that can be assembled in three dimensions. To harness the full potential of these materials, charge transport to the redox catalysts within the MOF has to be ensured. Herein, we report the first electroactive MOF with the UiO/PIZOF topology (Zr(dcphOH-NDI)), i.e., one of the most widely used MOFs for catalyst incorporation, by using redox-active naphthalene diimide-based linkers (dcphOH-NDI). Hydroxyl groups were included on the dcphOH-NDI linker to facilitate proton transport through the material. Potentiometric titrations of Zr(dcphOH-NDI) show the proton-responsive behavior via the -OH groups on the linkers and the bridging Zr-μ3-OH of the secondary building units with pKa values of 6.10 and 3.45, respectively. When grown directly onto transparent conductive fluorine-doped tin oxide (FTO), 1 μm thin films of Zr(dcphOH-NDI)@FTO could be achieved. Zr(dcphOH-NDI)@FTO displays reversible electrochromic behavior as a result of the sequential one-electron reductions of the redox-active NDI linkers. Importantly, 97% of the NDI sites are electrochemically active at applied potentials. Charge propagation through the thin film proceeds through a linker-to-linker hopping mechanism that is charge-balanced by electrolyte transport, giving rise to cyclic voltammograms of the thin films that show characteristics of a diffusion-controlled process. The equivalent diffusion coefficient, De, that contains contributions from both phenomena was measured directly by UV/vis spectroelectrochemistry. Using KPF6 as electrolyte, De was determined to be De(KPF6) = (5.4 ± 1.1) × 10-11 cm2 s-1, while an increase in countercation size to n-Bu4N+ led to a significant decrease of De by about 1 order of magnitude (De(n-Bu4NPF6) = (4.0 ± 2.5) × 10-12 cm2 s-1).

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1
MOF design strategy for developing a redox-active framework featuring UiO-topology with proton responsive functionalities.
Figure 2
Figure 2
Characterization of bulk microcrystalline powder Zr(dcphOH-NDI) by: (a) PXRD, (b) SEM imaging, (c) FTIR-ATR spectroscopy, and (d) TGA from 25-800 °C.
Figure 3
Figure 3
(a) N2 sorption isotherm at 77 K of Zr(dcphOH-NDI) and (b) corresponding pore width distribution.
Figure 4
Figure 4
Acid-base titration curve for Zr(dcphOH-NDI) (red) and first derivative (blue).
Figure 5
Figure 5
(a) PXRD of Zr(dcphOH-NDI)@FTO films.(b) SEM image of Zr(dcphOH-NDI)@FTO. XPS core-level spectra Zr(dcphOH-NDI)@FTO films (red) and SAM-modified FTO (black) showing the (c) Zr3d and (d) N1s peaks.
Figure 6
Figure 6
CVs of Zr(dcphOH-NDI)@FTO thin films at (a) 50 mV s−1 and (b) multiple scans at 100 mV s−1 showing increasing current density on progressive scans.
Figure 7
Figure 7
Scan rate dependent CVs of Zr(dcphOH-NDI)@FTO upon: (a) reversing the scan after the first reduction and (b) after the second reduction at scan rates from 5 to 100 mV s−1. Plots of jpc vs. ν for (c) the first reduction wave and (d) the second reduction wave. The peak current density is linearly proportional to ν1/2.
Figure 8
Figure 8
UV-vis spectroelectrochemical measurements of Zr(dcphOH-NDI)@FTO using 0.8 M KPF6/DMF as the supporting electrolyte (top). Photos of Zr(dcphOH-NDI)@FTO thin film electrodes (bottom) at various applied potentials (a) 0 V, (b) −1.26 V, and (c) −1.76 V vs. Fc+/0
See this image and copyright information in PMC

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