Atomic Layer Deposition of Metal Oxides on Activated Carbons for High Energy Density and High Performance Supercapacitors
Authors: Xing Weibing 
Published: February 2020

Nanostructured ceramic oxides have shown to increase specific capacitance of supercapacitors via Faradaic redox reactions [1]. Many research and development efforts have focused on transition metal oxides as alternatives to expensive hydrous RuO2. Examples of these activities include hydrothermal Co3O4 on carbon fiber papers [2], Co3O4/MnO2 nanowires [3], electrodeposition of MnO2 on carbon nanotube (CNT)-textile [4], wet chemistry of MnO2 in CNT [5], hydrothermal Ni(OH)2 on graphene [6], electrochemical anodization of NiO-TiO2 nanotubes [7], and solution synthesis of SnO2-graphenen composites [8]. More recently, atomic layer deposition (ALD) has been used to deposit V2O5 on CNT [9] and amorphous TiO2 on graphene [10]. While these studies have shown improved performance to various degrees, they either involve the use of expensive materials, such as CNT or graphene, or processes that are not easy to implement for large scale production.
Here we present an alternative and promising method to prepare high energy density and extremely high performance active materials for supercapacitors (or pseudocapacitors) by direct ALD oxide coatings onto high surface area, activated carbons (AC).
Figure 1 shows discharge voltage profiles of cells containing ALD oxide coated electrodes, overlaid with a control cell without oxide coating. The cells were built in symmetric configuration with a conventional Li-ion electrolyte. The cells were cycled between 0 to 2.5V under a constant current of 0.15A/g at room temperature. Both NiO and V2O5 coated cells delivered a specific capacitance of 114 F/g (based on single electrode active weight), representing > 50% increase over the control cell (75 F/g). The enhanced specific capacitance of the ALD oxide coated cells is a result of pseudocapacitive contribution from the ALD oxides.
Figure 2 shows rate capability test of the symmetric supercapacitor cells. The discharge capacitance is normalized to that of a low current density (0.15A/g). The ALD oxide coated cells showed excellent rate capability, with > 87% and 92% capacitance retention for V2O5 and NiO coated cells, respectively, when discharged at 15 A/g current. Both oxide coated cells showed superior rate capability performance to that of the control cell. The remarkable rate capability demonstrated by the ALD oxide coated cells is attributed to nano-sized oxide coatings afforded by ALD, which results in facile surface redox reactions.
Figure 3 shows cycle life test where the symmetric supercapacitor cells were cycled between 0 to 2.5V at a constant current density of 1.5 A/g. The ALD NiO cell showed enhanced specific capacitance while maintaining extremely stable, long cycle performance with > 90% capacitance retention after 30,000 cycles. Such extremely stable cycle life suggests that the nano-structured oxides enabled by ALD are stress/stain free during charge and discharge cycles involving Faradaic charge transfer (redox) reactions.
We will show that ALD enabled oxide coatings on AC allow for high operating voltages, leading to increased capacitance and energy density while demonstrating superior electrochemical performance stability to samples without ALD oxide coating.
Our results in this study suggest pseudocapacitive oxide coating via ALD on AC is a promising material processing methodology for supercapacitor energy storage devices with significantly enhanced specific capacitance, energy density and electrochemical stability while maintaining exceptionally high rate capability and extremely stable cycle life. Such methodology is scalable economically when high throughput and high volume ALD processes are implemented.

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Figure 1