Mechanochemical synthesis and application of mixed-metal copper–ruthenium HKUST-1 metal–organic frameworks in the electrocatalytic oxygen evolution reaction†

Abstract

Novel electrode materials for electrocatalytic hydrogen generation are investigated for increasing the activity of expensive noble-metal components. Here various mixed-metal copper–ruthenium combinations of the metal–organic framework (MOF) HKUST-1 (HKUST = Hong Kong University of Science and Technology, with the formula [Cu₃(BTC)₂(H₂O)₃]_n (BTC = benzene-1,3,5-tricarboxylate)) as Cu_xRu-BTC were synthesized through a mechanochemical method. This mechanochemical method allowed for gram-scale synthesis of the mixed-metal MOFs in a one-hour synthesis time. Characterization through powder X-ray diffraction (PXRD), N₂-adsorption, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and Fourier transform infrared (FT-IR) spectroscopy confirmed the formation of a MOF with the HKUST-1 topology, albeit with lower porosity compared to neat HKUST-1. The synthesized MOFs were tested as precursor materials for catalysts for the oxygen evolution reaction (OER) and performed comparably to the industry standard ruthenium oxide (RuO₂). An overpotential (η) of 314 mV (RuO₂η = 312 mV), a Tafel slope (b) of 55 mV dec⁻¹ (RuO₂b = 47 mV dec⁻¹) was achieved which in combination with a charge-transfer resistance (R_CT) of 13.6 Ω (RuO₂R_CT = 52.8 Ω) and a faradaic efficiency (FE) of 70% (RuO₂ FE = 66%) supports the derived catalyst from Cu₁₀Ru-BTC with an intimate mixture of copper and ruthenium at the nanoscale to be effective for the OER having lower ruthenium content than RuO₂. All derived catalysts from the Cu_xRu-BTC samples and RuO₂ showed good stability in a chronopotentiometric measurement over 12 h.