Electrocatalytic Hydrogen Evolution from a Cobaloxime-Based Metal-Organic Framework Thin Film

Abstract

Molecular hydrogen evolution catalysts (HECs) are synthetically tunable and often exhibit high activity, but they are also hampered by stability concerns and practical limitations associated with their use in the homogeneous phase. Their incorporation as integral linker units in metal-organic frameworks (MOFs) can remedy these shortcomings. Moreover, the extended three-dimensional structure of MOFs gives rise to high catalyst loadings per geometric surface area. Herein, we report a new MOF that exclusively consists of cobaloximes, a widely studied HEC, that act as metallo-linkers between hexanuclear zirconium clusters. When grown on conducting substrates and under applied reductive potential, the cobaloxime linkers promote electron transport through the film as well as function as molecular HECs. The obtained turnover numbers are orders of magnitude higher than those of any other comparable cobaloxime system, and the molecular integrity of the cobaloxime catalysts is maintained for at least 18 h of electrocatalysis. Being one of the very few hydrogen evolving electrocatalytic MOFs based on a redox-active metallo-linker, this work explores uncharted terrain for greater catalyst diversity and charge transport pathways.

Figure 1

(A) Structure of the cobaloxime linker in UU-100(Co) and (B) structural model of UU-100(Co) MOF viewed along [001].

Figure 2

(A) Pawley fit (red) of powder X-ray diffraction pattern (λ = 1.5418 Å) for UU-100(Co) against the experimental PXRD pattern (black), showing good agreement factor (weighted-profile R factor R_wp = 0.0770 and unweighted-profile R factor R_p = 0.0585 after convergence). (B) SEM image of UU-100(Co) with corresponding energy dispersive X-ray spectroscopy elemental maps of Zr Lα, Co Kα, Cl Kα, and O Kα. The scale bar in the EDX maps represents 1 μm. (C) HRTEM image of UU-100(Co) along [110] that shows lattice fringes with d₀₀₁= 19.1 Å representing the packing of Zr clusters (dark features). (D) Fourier transform of the image showing the 00l reflections. (E) Enlarged HRTEM image of the region marked by a blue square in C; two Zr cluster columns are marked by red circles. (F) N₂ sorption isotherm at 77 K (closed and open circles denote adsorption and desorption, respectively) and (G) DFT pore size distribution of UU-100(Co).

Figure 3

(A) PXRD of UU-100(Co) thin films (λ = 1.5418 Å). SEM images of (B) UU-100(Co)|FTO and (C) UU-100(Co)|GC. Panel (B) shows the rod-shaped UU-100(Co) crystals on a bare FTO surface, while the glassy carbon surface in (C) is coated with the smaller particles of UU-100(Co).

Figure 4

(A) Cyclic voltammograms of UU-100(Co)|FTO electrodes at different scan rates in DMF containing 0.1 M LiClO₄ (10, 20, 30, 40, 60, 80, and 100 mV s^–1); inset shows the linear dependency of the peak current (Co^II/Co^I couple) on the scan rate (ν) at scan rates under 100 mV s^–1. (B) UV–vis spectroelectrochemical data on UU-100(Co)|FTO thin-film electrodes, showing the steady-state relative absorbance at different applied potentials. (C) Optical transmittance kinetic curve of the UU-100(Co) thin-films measured at 520 and 670 nm by switching the potential from −0.05 to −1.5 V (electrode held at the potential for 60 s). (D) XPS core-level spectra of UU-100(Co)|FTO films showing Co 2p region before (black) and after (red) electrochromic switching tests. Blue and purple filled peaks represent the peak fits corresponding to Co 2p_1/2 and 2p_3/2, respectively. (E) Photographs of the UU-100(Co)|FTO thin films at −0.05 and −1.5 V (vs Fc^+/0).

Figure 5

(A) Cyclic voltammograms of UU-100(Co)|GC at different scan rates (0.2, 0.16, 0.14, 0.12, 0.1, 0.8, 0.06, 0.05, and 0.025 V s^–1) in DMF. (B) Linear sweep voltammograms of UU-100(Co)|GC (red) and blank GC (black) electrodes at pH 4 recorded at 20 mV s^–1 (dashed line shows the potential applied for electrolyses experiments). (C) Tafel plot derived from the LSV; black line shows the linear fit of the data in the low-overpotential region.

Figure 6

(A) Controlled potential electrolysis using UU-100(Co)|GC electrode at −0.45 V vs RHE in acetate buffer at pH 4. The black trace represents geometric current density, and the red circles denote faradaic efficiency for H₂ evolution. (B) PXRD patterns of the as-synthesized electrodes (black), after solvent exchange with acetone for 24 h (blue), and after 5 h of electrocatalysis (red) demonstrate that the MOF retains its crystalline structure after electrolysis (see Figures S6 and S7 for PXRD of solvent exchanged UU-100(Co)). (C) SEM images of the UU-100(Co)|GC after electrolysis and (D) corresponding EDX line scan showing retention of rodlike morphology and uniform distribution of Zr and Co in the MOF crystal.