Molybdenum Oxide/Dopamine-Derived Carbon Electrodes with Enhanced Electrochemical Activity in Energy Storage Systems

Abstract

Herein, a versatile sol-gel reaction produces new electrode materials consisting of tightly integrated molybdenum oxide and carbon derived from chemically incorporated dopamine molecules, and the materials show enhanced electrochemical activity in nonaqueous Li-ion and aqueous Zn-ion energy storage systems. The novel synthesis entails the oxidation of molybdenum in aqueous solutions of excess and limited dopamine (Dopa) hydrochloride via a hydrogen peroxide-initiated process.The transformation of molybdenum under the condition of Dopa excess (Mo:Dopa molar ratio of 1:1) resulted in the formation of a metastable precipitate of polydopamine (PDopa) spheres encapsulated by Dopa-preintercalated molybdenum oxide, (Dopa)xMoOy@PDopa. Hydrothermal treatment (HT) of (Dopa)xMoOy@PDopa precursor was concomitant with concurrent Dopa carbonization and molybdenum reduction processes, resulting in a formation of spherical matrices of Dopa-derived carbon decorated by MoO2 nanoplatelets (HT-MoO2/C), as determined through FTIR spectroscopy, Raman spectroscopy, and SEM imaging. Annealing (An) of HT-MoO2/C at 600°C under argon atmosphere (AnHT-MoO2/C) led not only to improvements in MoO2 crystallinity, but also to an increased oxidation state of molybdenum and a facilitated interaction between molybdenum-based and Dopa-derived components, resulting in an intimate MoO2/C heterointerface. Consequently, while both HT-MoO2/C and AnHT-MoO2/C showed reversible intercalation-type behavior when evaluated as electrodes versus Li/Li+ in nonaqueous lithium-ion cells, AnHT-MoO2/C demonstrated higher capacities, enhanced capacity retention, better rate capability, and lower charge transfer resistance. The AnHT-MoO2/C electrode showed an initial specific capacity of 260 mAh/g and 67% capacity retention after 50 cycles at 10 mA/g, compared to an initial specific capacity of 235 mAh/g and 47% capacity retention shown by HT-MoO2/C at the same current density. Furthermore, in rate capability experiments, HT-MoO2/C and AnHT-MoO2/C delivered specific capacities of 93 mAh/g and 120 mAh/g respectively at 100 mA/g.When molybdenum was transformed in the presence of Dopa deficit (Mo:Dopa molar ratio of 5:1), a (Dopa)xMoOy powder precursor was isolated, and subsequent hydrothermal treatment of this precursor produced an MoO3 material with carbonized Dopa molecules, HT-MoO3/C. Reference α-MoO3 electrodes (α-MoO3-ref) were synthesized similarly but in the absence of Dopa molecules in the initial sol-gel reaction. The appearance of characteristic D and G bands in the Raman spectra and distinct vibrational modes in the FTIR spectra of HT-MoO3/C confirmed the presence of carbon in its structure. SEM images showed a uniform nanobelt morphology with fragmentation due to interactions between interlayer Dopa and MoO3 layers under the conditions of hydrothermal treatment. HT-MoO3/C delivered a second-cycle capacitance of 141.4 F/g when cycled at 2 mV/s in a -0.25–0.70 V versus Ag/AgCl potential window in 5M ZnCl2 electrolyte, while α-MoO3-ref delivered a nearly two-fold smaller second-cycle capacitance of 76.1 F/g under the same conditions. HT-MoO3/C also showed increased capacitance compared to α-MoO3-ref when cycled at increasing sweep rates up to 20 mV/s. The superior performance of HT-MoO3/C prompted a study of the electrode in an expanded potential window, based on previous reports in which MoO3 showed electrochemical activity at negative potentials versus Ag/AgCl. The HT-MoO3/C electrode exhibited a capacitance of 347.6 F/g on the second cycle when cycled between -0.85–1.00V versus Ag/AgCl at 2 mV/s in 5M ZnCl2 electrolyte.This work demonstrates a new strategy to improve the electrochemical performance of transition metal oxide electrodes for next-generation energy storage systems. Integration of oxides with carbon through the wet chemistry synthesis approaches that involve carbonization of organic molecules can be used to control oxide crystal phase and heterointerfaces leading to improved charge transfer and energy storage properties.