Improving the Charge Storage of α-MoO₃ Electrodes in Zinc-Ion Batteries via Chemical Incorporation of Dopamine-Derived Carbon

Orthorhombic molybdenum trioxide (α-MoO₃) is widely studied as an active electrode material for electrochemical energy storage devices, owing to its layered structure, which offers interlayer sites for cation intercalation, and high Mo oxidation state, which generates potential for multiple electron transfer during reversible intercalation of electrochemically cycled cations. However, one significant impediment to the electrochemical properties of α-MoO₃ is its low electrical conductivity, which prevents rapid charge transfer during electrochemical cycling and thus limits charge storage, particularly at high rates of operation. Thus, the attractive strategy to achieve advances in α-MoO₃ electrochemistry is the incorporation of an electrically conductive species to form an intimate heterointerface between the constituting materials.<br/><br/>In this work, a novel MoO₃-based electrode material containing carbon derived from chemically incorporated dopamine molecules shows an increased capacitance compared to reference α-MoO₃ electrodes in a 5M ZnCl₂ aqueous electrolyte. The material was synthesized <i>via </i>a hydrogen peroxide-initiated sol-gel reaction that entailed the oxidization of Mo powder in a dopamine hydrochloride (Dopa HCl) solution. A (Dopa)_xMoO_y powder precursor was isolated from the metastable gel through freeze-drying, and subsequent hydrothermal treatment (HT) of this precursor at 180°C produced the new MoO₃ material with carbonized Dopa molecules, denoted as HT-MoO₃/C. The reference α-MoO₃ electrodes (α-MoO₃-ref) were synthesized similarly, but in the absence of Dopa HCl 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 confirm the presence of carbon in its structure. SEM images show a uniform nanobelt morphology with fragmentation due to interactions between interlayer Dopa and MoO₃ layers under the conditions of hydrothermal treatment. By means of four-point probe conductivity measurements, the electronic conductivity of rolled films containing 80 wt% HT-MoO₃/C, 15 wt% graphene nanoplatelets (GNPs), and 5 wt% PTFE was determined to be 5.9 × 10^-1 S/cm, compared to 2.9 × 10^-6 S/cm measured for α-MoO₃-ref films containing identical ratios of (GNPs) and PTFE, thus confirming improved electron transport in the material synthesized via integration with the dopamine-derived carbon.<br/><br/>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 vs. 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. Furthermore, HT-MoO₃/C retained greater capacitance compared to α-MoO₃-ref at each of the increasing sweep rates when cycled from 1 mV/s 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 α-MoO₃ showed electrochemical activity at negative potentials vs. Ag/AgCl. The HT-MoO₃/C electrode exhibited a capacitance of 347.6 F/g (178 mAh/g) on the second cycle when cycled from -0.85–1.00V vs. Ag/AgCl at 2 mV/s in 5M ZnCl₂ electrolyte. The increased charge storage of HT-MoO₃/C compared to α-MoO₃-ref is attributable to facilitated electron transfer in HT-MoO₃/C due to the presence of a tight molybdenum trioxide/carbon heterointerface in its structure as a result of the bottom-up sol-gel approach adopted in its synthesis.<br/>This work demonstrates a new strategy to improve the electrochemical performance of transition metal oxide electrodes for energy storage systems. Integration of oxides with carbon through wet chemistry synthesis approaches that involve carbonization of organic molecules can be used to create pathways for electron transport, leading to improved charge transfer and energy storage properties.