Faradaic Reactive MoS2-Carbon Frameworks for Ultrahigh-Energy-Density Electrochemical Capacitors

The demand for high-performance energy storage devices has grown at an astonishing pace for various applications including portable electronic gadgets and electric automobiles. Among diverse electrochemical energy devices, pseudocapacitors may be one of the promising candidates to fulfill the requirements of high power (e.g., &gt; 3000 W/kg) and energy (e.g., &gt; 50 Wh/kg) densities.¹ The pseudocapacitors can demonstrate not only large capacitive energy storage but also fast rechargeability and long cyclic stability. Despite the excellent characteristics, one major drawback of the pseudocapacitors has overshadowed their practical use. It is their low energy density compared to that of the ion batteries (over 100 Wh/kg).<br/><br/>To overcome the limitation, one may improve the energy storage mechanisms: surface-controlled electrical double-layer capacitance (SDC) and diffusion-controlled faradaic pseudocapacitance (DFC).² The SDC-driven mechanism is highly affected by the specific surface area of the electrodes since it is determined by the extent of accumulated charges at the electrode. On the other hand, the energy storage by the FPC process may originate from the charge transfer between the electrode and the ions via faradaic reaction. Therefore, the energy density of a pseudocapacitor may be boosted by enhancing the surface area and the faradaic reactivity of the electrode.<br/><br/>In this work, we introduce a ternary composite electrode made of carbon nanotubes (CNT), zeolitic imidazolate frameworks (ZIF), and molybdenum disulfide (MoS2) to improve SDC and DFC simultaneously. The hybrid electrode demonstrates an ultrahigh energy density that can rival those of lithium-ion batteries, while maintaining the excellent properties of capacitors. This was made happen by exploiting the distinct benefits of the heteromaterials and engineering the morphology of the composite. The percolated CNT provide a robust scaffold with high conductivity for a quick charge transfer.³ The carbon networks are coated with porous ZIF structures, allowing a fast ion diffusion and offering a large surface area. In the CNT-ZIF-MoS2 composite, the MoS2 layer from topochemical synthesis presents densely-packed petal-like morphologies. The MoS2 structure was developed to bear many sulfur vacancies which may serve as active sites for faradaic reaction. The hierarchical pseudocapacitor electrode thus increase not only the specific surface area for SPC-based storage but also the considerable faradaic reactivity to boost electric currents and capacitance.<br/><br/>Our ternary pseudocapacitor demonstrates an ultrahigh energy density of over 100 Wh/kg which may be comparable to the performance of typical ion batteries. The composite electrode also demonstrates a quick charging/discharging behavior with a maximum power density of well over 10 kW/kg. Additionally, it shows a highly stable cyclic behavior over 25,000 cycles. The new approach presented in this work may open new possibilities for developing future electrochemical capacitors.<br/><br/><b>References</b><br/>1. Choudhary, R. B.<i>, et al.</i>, <i>Renewable and Sustainable Energy Reviews </i>2021, <b>145</b>, 110854<br/>2. Chen, J.<i>, et al.</i>, <i>Advanced Energy Materials </i>2021, <b>11</b>, 2003311<br/>3. Wan, L.<i>, et al.</i>, <i>Carbon </i>2017, <b>121</b>, 330