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.

Immobilization of “Capping Arene” Cobalt(II) Complexes on Ordered Mesoporous Carbon for Electrocatalytic Water Oxidation

“…The three-dimensional Fe 3 O 4 nanoparticle face-center cubic (fcc) superlattices were produced through slow evaporation of hexane solvent under ambient conditions, which were then annealed at 500 °C in nitrogen gas to carbonize the oleic acid surfactant and subsequently washed in hydrochloric acid at 120 °C to remove the Fe 3 O 4 template (Figure S5, Supporting Information). The resulting OMC material, after a further treatment at 900 °C in forming gas (5% H 2 in N 2 ) to enhance the electrical conductivity and degree of graphitization as we reported before, shows an ordered structure with a pore size of ∼7.5 nm and a wall thickness of ∼2.5 nm (Figure a).…”

“…The Co complex-loaded OMC was generated by sonicating the mixture of the molecular complex, OMC, and isopropanol (Figure b,c and Figure S6, Supporting Information). We anticipated that the Co complex attachment onto the OMC would be enhanced by π–π stacking interaction, similar to our previous work on Ir complex-loaded OMC . Because of the porous structure and large surface area with facilitated diffusion from sonication, the OMC material should enable a high loading of the molecular Co complexes.…”

“…Recently, research efforts have focused on the preparation of supported electrocatalysts by anchoring molecular catalysts directly to an electrode or to a carbon support using covalent and noncovalent attachment methods. − Several examples have been reported for the synthesis of cobalt-heterogenized catalysts for water oxidation via van der Waals interactions between a cobalt complex with long alkyl chains and fluorine-doped tin oxide (FTO) or carbon electrodes (see, for example, g in Scheme ). − Supported Co II phthalocyanine complexes are efficient molecular water oxidation electrocatalysts at pH 7 when physiosorbed onto FTO ( h , Scheme ) or at basic pH (1 M KOH) when π–π-stacked onto carbon nanotubes .…”

Outer-coordination sphere in multi-H+/multi-e–molecular electrocatalysis

“…Although a library of impressive homogeneous electrocatalysts bearing outer coordination sphere(s) for multi-H + and multi-etransfer catalysis has been documented, practical energy devices to address global energy challenges demand heterogeneous electrocatalysts. Therefore, the immobilization of molecular electrocatalysts on solid electrodes is an emerging trend in heterogeneous electrocatalysis (Brunner et al, 2020;Geer et al, 2021;Sinha et al, 2020;Zhang and Warren, 2020). The major challenge in developing these systems is optimizing the synthetic strategies for immobilizing molecules on the electrode surface.…”

In Situ Confinement of Ultrasmall Metal Nanoparticles in Short Mesochannels for Durable Electrocatalytic Nitrate Reduction with High Efficiency and Selectivity

“…In contrast, mesoporous carbon materials possess high surface areas, large pore sizes, excellent electrical conductivity, good corrosion resistance, and low cost, and thus have been regarded as the most promising catalyst supports for widespread applications. [32][33][34][35] Generally, the metal nanoparticle catalysts are loaded in mesoporous carbon matrix by the post-impregnation or co-assembly processes, but which easily result in the pore blockage, uneven dispersion, and aggregation of nanoparticles. Moreover, the increased long-cycling stability is typically achieved at the expense of decreased catalytic activities.…”