High Energy Density Hybrid Pseudocapacitors
Authors: Xing Weibing 
Published: February 2020

Supercapacitors, also called electric double-layer capacitors (EDLC) or ultracapacitors, are of great interest for power applications due to their high power capability (~ 10 kW/kg) and long cycle life (> 10,000 cycles) [1]. However, supercapacitors have much smaller energy densities (< 10 Wh/kg) [2] than their battery counterparts (e.g., ~ 200 Wh/kg for Li-ion batteries) [3]. Historically, research efforts have been focused on incorporating nanostructured transition metal oxides into supercapacitors to increase specific capacitance and energy via Faradaic redox reactions (pseudocapacitance) [4]. Examples of these activities include electrochemically oxidized RuO2 on platelet carbon nanofiber [5], hydrothermal Co3O4 on carbon fiber papers [6], Co3O4/MnO2 nanowires [7], electrodeposition of MnO2 on carbon nanotube (CNT)-textile [8], wet chemistry of MnO2 in CNT [9], hydrothermal Ni(OH)2 on graphene [10], electrochemical anodization of NiO-TiO2 nanotubes [11], and solution synthesis of SnO2-graphenen composites [12]. More recently, atomic layer deposition (ALD) has been used to deposit V2O5 on CNT [13] and amorphous TiO2 on graphene [14]. While these studies have shown improved performance to various degrees, they were invariantly performed in aqueous electrolyte based supercapacitor cells which were limited in their energy density due to low electrochemical window of water (~ 1.23V) [15].
Here we present a novel approach utilizing ALD coated metal oxide on activated carbons (AC) in a non-aqueous electrolyte based, hybrid pseudocapacitor cell configuration analogous to that of lithium ion capacitors [16] where a faradaic intercalation anode (e.g., Li4Ti5O12 or graphite active materials) is paired with a non-faradaic cathode (e.g., AC active material). Our approach utilizes the ALD coated metal oxide on AC as the cathode to store charge and energy both electrostatically (non-faradaic) and electrochemically (faradaic). We will show that this hybrid pseudocapacitor approach leads to a significant improvement in energy density due to pseudocapacitive contributions from the ALD enabled metal oxides on AC cathodes.
Figure 1 shows the first discharge voltage vs. specific capacity of an ALD metal oxide on AC based and AC control hybrid pseudocapacitor cells. The first discharge ALD metal oxide-based cells have higher initial voltages, or, fresh cell open circuit voltages (OCV’s) than the AC control cells. It was observed that the increased metal oxide loading (wt%) corresponded to higher OCV. ALD metal oxide-based cells delivered higher first discharge specific capacities than the AC control cells. These observations suggest the intended ALD metal oxide coatings on AC.
Figure 2 shows formation discharge voltage vs. specific capacity of an ALD metal oxide on AC based and AC control hybrid pseudocapacitor cells. The ALD metal oxide-based, asymmetric hybrid pseudocapacitor cells showed distinctly different discharge voltage profiles than the AC control cells (without ALD metal oxide coating). The ALD metal oxide-based pseudocapacitor cells delivered higher specific capacity, capacitance and energy. The improved specific energy of the ALD metal oxide-based hybrid pseudocapacitor cells is due to higher specific capacity and higher average discharge voltage.
The first round of an ALD metal oxide coating on AC displays an optimal metal oxide loading range (~20 wt%) for maximum specific capacity and energy improvement. The increased specific capacity, capacitance and energy of ALD metal oxide-based pseudocapacitor cells are attributed to pseudocapacitive contributions from the ALD coated metal oxide on AC cathode. A cell level specific energy improvement of ≥ 20% is projected in ALD metal oxide-based asymmetric hybrid pseudocapacitor cells.
We will show strategies to achieve high rate capability and long-term cycleability of the ALD oxide-based, high energy density hybrid pseudocapacitor cells.

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Figure 1