Redesigning the Electrode Architecture to Balance Rate and Capacity in Aqueous Electrochemical Capacitors
Authors: Sassin Megan Bourg , Wallace Jean Marie , Long Jeffrey W. , Rolison Debra R. 
Published: February 2020

Electrochemical capacitors (ECs) are a well-established class of energy-storage devices that address the critical performance gap between the high specific energy of batteries and the high specific power of conventional electrostatic capacitors, and enable many emerging technologies that have challenging energy/power requirements. Commercially available ECs are still largely based on a symmetric device configuration in which charge is stored in the electrochemical double-layer formed at the interface between the high-surface-area carbon-based electrode and a nonaqueous electrolyte. The reliance on charge storage in the electrochemical double-layer ultimately limits device-level specific energy (3-6 W h kg–1), hindering the use of symmetric ECs in a broader application space. Advances in nanoscale carbons and high-voltage electrolytes may offer some improvement in specific energy, but symmetric carbon–carbon ECs will still fall short for applications requiring higher specific energy (10–25 W h kg–1).
The next evolution in ECs aims to address the energy/power performance gap that exists between symmetric carbon–carbon ECs (high power/moderate energy) and batteries (low power/high energy). Specific energy can be increased through the incorporation of materials, such as particular metal oxides (RuO2, MnOx, FeOx), that store charge via rapid and reversible faradaic reactions (pseudocapacitance) in aqueous electrolytes, augmenting double-layer capacitance.
[i]
In order to fully utilize charge-storage enhancements offered by pseudocapacitance-supporting metal oxides, while maintaining specific power comparable to symmetric carbon–carbon ECs, one must redesign the electrode architecture to facilitate electron and ion transport to the active oxide.
We have developed self-limiting redox deposition protocols[ii],[iii],[iv] to incorporate conformal, nanoscale metal oxide (MnOx or FeOx) coatings throughout the macroscopic thickness (100–300 μm) of fiber-paper-supported carbon nanofoams.[v] The pseudocapacitance of the incorporated metal oxide increases the charge-storage capacity of the nanofoam by factors of 2–10, while the conformal, nanoscale nature of the metal oxide ensures that the frequency response of the underlying carbon nanofoam is retained. The high surface-to-volume ratio of the metal oxide-coated carbon nanofoam yields footprint-normalized capacitances as high as 7.5 F cm–2, which is much higher than the footprint-normalized capacitances (~mF cm–2) of powder-composite electrode architectures composed of similar materials.
[vi]

En route to high-performance aqueous asymmetric ECs that offer optimal combinations of energy and power, we use the highly tunable pore–solid architecture of carbon nanofoams to examine the impact of the pore structure (size/distribution) and thickness (100–300 µm) of the resulting MnOx-coated electrode on rate-dependent capacitance and time response, as measured for symmetric MnOx–carbon nanofoam ECs with concentrated near-neutral pH aqueous electrolytes.[vii] Devices comprising carbon nanofoam papers with 10–20-nm mesopores support high MnOx loadings (60 wt.%) and device-level capacitance (30 F gT
–1), but this capacitance is not accessible at high rates (> 5 mV s–1) due to inadequate electrolyte transport via the mesoporous network. Increasing the average pore size (100–200 nm) facilitates operation at higher rates (50 mV s–1), but at the expense of much lower device capacitance (13 F gT
–1), which is due to lower MnOx loadings (41 wt.%). Carbon nanofoams comprising interconnecting networks of meso- and macropores offer an optimal blend of energy and rate performance, delivering 33 F gT
–1 at 5 mV s–1 and 23 F gT
–1 at 50 mV s–1.
In addition to optimizing the electrode architectures, we are also exploring more practical issues, including the down-selection of separators that support high-rate operation of aqueous-based ECs that use concentrated near-neutral pH electrolytes and electrolyte compositions that support operation at low temperatures (< –20°C).

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