Abstract:

Asymmetric manganese cluster, the active center of photosystem II (PSII) in nature, is hydrogen-bonded to surrounding amino acid residues and water molecules. This phenomenon is of great inspiration significance for developing and studying artificial Mn-based oxygen evolution reaction (OER) catalysts. Herein, we prepared manganese phosphate nanosheets through intercalation of ethylenediamine ions and water molecules ((EDAI)(H₂O)MnPi) using a simple co-precipitation method. (EDAI)(H₂O)MnPi is also hydrogen-bonded to interlayer ethylenediamine ions and water molecules, forming a hydrogen-bonding network. The morphology of the (EDAI)(H₂O)MnPi sample was characterized by scanning electron microscopy (SEM) and transmission electron microscopy. The thickness of (EDAI)(H₂O)MnPi was characterized by atomic force microscopy. The composition and structure of (EDAI)(H₂O)MnPi were characterized by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy. For control studies, manganese phosphate (EDAI)MnPi and (H₂O)MnPi samples were also synthesized. The structure and morphology of (EDAI)MnPi and (H₂O)MnPi samples were characterized by XRD and SEM. The difference between (EDAI)(H₂O)MnPi, (EDAI)MnPi, and (H₂O)MnPi were further characterized by thermal gravimetric analysis and derivative thermogravimetric analysis. Electrocatalytic properties of the (EDAI)(H₂O)MnPi, (EDAI)MnPi, and (H₂O)MnPi for OER were studied in 0.05 mol∙L⁻¹ pH = 7 phosphate buffered saline solution, through linear sweep voltammetry, electrochemical impedance spectroscopy, and controlled potential electrolysis (CPE) test. The electrochemical surface area (ECSA) analyses of (EDAI)(H₂O)MnPi, (EDAI)MnPi, and (H₂O)MnPi samples were recorded by charging currents in the non-Faradaic potential region at different scan rates. Considering the different ECSAs of different materials, the water oxidation activities of three materials were normalized by ECSA. Compared with counterparts of (EDAI)MnPi (610 mV) and (H₂O)MnPi (580 mV), manganese phosphate nanosheets (EDAI)(H₂O)MnPi exhibited a lower overpotential of 520 mV when driving a current density of 1 mA∙cm⁻² in neutral conditions. The CPE experiment revealed that (EDAI)(H₂O)MnPi remained active for at least 10 h. Manganese phosphate nanosheets containing a rich, extensive, and continuous hydrogen bond network exhibited improved OER performance in neutral conditions. The hydrogen-bonding network in manganese phosphate nanosheets has similar functions to the hydrogen-bonding network in PSII, which could accelerate the transfer rate of protons and facilitate electrocatalytic water oxidation. This study may provide guidance for the design of water oxidation catalysts with rich hydrogen bond network.

Key words: Electrocatalysis, Oxygen evolution reaction, Water oxidation, Manganese phosphate, Hydrogen bond