Noncovalent Immobilization of Pentamethylcyclopentadienyl Iridium Complexes on Ordered Mesoporous Carbon for Electrocatalytic Water Oxidation

Abstract

The attachment of molecular catalysts to conductive supports for the preparation of solid-state anodes is important for the development of devices for electrocatalytic water oxidation. The preparation and characterization of three molecular cyclopentadienyl iridium(III) complexes, Cp*Ir(1-pyrenyl(2-pyridyl)ethanolate-κO,κN)Cl (1) (Cp* = pentamethylcyclopentadienyl), Cp*Ir(diphenyl(2-pyridyl)methanolate-κO,κN)Cl (2), and [Cp*Ir(4-(1-pyrenyl)-2,2'-bipyridine)Cl]Cl (3), as precursors for electrochemical water oxidation catalysts, are reported. These complexes contain aromatic groups that can be attached via noncovalent π-stacking to ordered mesoporous carbon (OMC). The resulting iridium-based OMC materials (Ir-1, Ir-2, and Ir-3) were tested for electrocatalytic water oxidation leading to turnover frequencies (TOFs) of 0.9-1.6 s^-1 at an overpotential of 300 mV under acidic conditions. The stability of the materials is demonstrated by electrochemical cycling and X-ray absorption spectroscopy analysis before and after catalysis. Theoretical studies on the interactions between the molecular complexes and the OMC support provide insight onto the noncovalent binding and are in agreement with the experimental loadings.

Keywords: electrocatalysis; iridium molecular complexes; noncovalent immobilization; ordered mesoporous carbon; water oxidation.

Scheme 1

Structures of molecular iridium complexes previously studied as catalyst precursors for electrochemical water oxidation. A,B) the study by Schley et al.^{[ 29 ]}; C) the study by Thomsen et al.^{[ 28 ]}; D) the study by Abril et al.,^{[ 27 ]} E) the study by van Dijk et al.^{[ 26 ]}; F) the study by Olivares et al.^{[ 25 ]}

Scheme 2

Schematic of supported iridium molecular complexes for water oxidation. a) the study by deKrafft et al. ^{[ 35 ]}; b) the study by Nieto et al. and Sánchez‐Page et al.^{[ 36} , ^{37 ]}; c) the study by Joya et al.^{[ 38 ]}; d) the study by Sheehan et al.^{[ 41 ]}; e) this work.

Scheme 3

a) Synthesis of Cp*Ir(1‐pyrenyl(2‐pyridyl)ethanolate‐κO,κN)Cl (1) and Cp*Ir(diphenyl(2‐pyridyl)methanolate‐κO,κN)Cl (2). b) Synthesis of [Cp*Ir(4‐(1‐pyrenyl)‐2,2′‐bipyridine)Cl]Cl (3).

Figure 1

a) Oak Ridge Thermal Ellipsoid Plot (ORTEP) drawing of crystal structure of Cp*Ir(1‐pyrenyl(2‐pyridyl)ethanolate‐κO,κN)Cl (1) with ellipsoids shown at 50% probability. Hydrogen atoms and solvent molecules have been omitted for clarity. Selected bond lengths (Å) and angles (°) for 1: Ir1—O1 2.0571(16), Ir1—N1 2.079(2), Ir1—Cl1 2.4508(6), Ir—Cp*(centroid) 1.7706(12), O1—Ir1—N1 77.83(7); b) ORTEP drawing of crystal structure of [Cp*Ir(4‐(1‐pyrenyl)‐2,2′‐bipyridine)Cl]Cl (3) with ellipsoids shown at 50% probability. Hydrogen atoms, solvent molecules, and counterions have been omitted for clarity. Selected bond lengths (Å) and angles (°) for 3: Ir1—N1 2.077(9), Ir1—N2 2.103(8), Ir1—Cl1 2.405(3), Ir—Cp*(centroid) 1.788(8), N1—Ir1—N2 76.4(3).

Figure 2

a) TEM image of Fe₃O₄ nanoparticles; b) TEM image of OMC annealed at 900 °C in forming gas (5% H₂ in N₂); c) Schematic illustration of loading process of molecular complex and OMC under sonication.

Figure 3

a) UV–vis absorption spectra of Ir molecular complexes before and after absorbed on OMC; b) TEM image of complex 2 loaded on OMC (Ir‐2); c) LSV plot of Ir‐1, Ir‐2, Ir‐3, and pristine OMC; d) Stability test of Ir‐2 catalyst with continuous LSV scans.

Figure 4

PBE—D3‐predicted binding of the Ir complexes (numbered) to a periodic sheet of 6 × 6 graphene (dark gray atoms). Periodic images extend infinitely in the x and y directions. A vacuum was placed above the Ir complexes to inhibit interaction of the periodic images in the z direction.

Figure 5

UFF‐predicted binding for four of each Ir complex (numbered) on the LDAC surface (brown atoms).^{[ 56 ]} Given the variation of the carbon surface, four molecules were used to sample the different local topologies present. The average per molecule binding energy was calculated simply by dividing the total system binding energy by 4. Hydrogen atoms omitted for clarity.

Figure 6

a) TEM image of Ir‐2 after stability tests of 1000 LSV scans (0.3–1.63 V versus RHE); b) XPS spectra of complex 2 and Ir‐2 after electrochemical conditions; c) EXAFS analysis of complex 2 and Ir‐2 for before and after electrochemical conditions.