Hybrid supercapacitors, with their enhanced energy density (and power density levels similar to regular or symmetric supercapacitors) have been in development in recent years. Turns out, the most widely studied configuration of hybrid supercapacitors has a dual carbon electrode configuration, with graphite being the preferred option to replace the capacitive activated carbon at the negative electrode of the symmetric supercapacitor (while keeping the high surface area activated carbon electrode at the positive end). These hybrid supercapacitors (also called lithium-ion capacitors (LIC)) can thus deliver higher specific capacitance compared to existing symmetric electric double-layer capacitors and pseudo-capacitors. This improved performance is attributed not only to the replacement of the negative electrode from a double-layer capacitive storage mechanism to a Li-ion intercalation mechanism but also to advances in the carbon materials (activated carbon and graphite) used for both electrodes. However, with increasing desire to withstand high-power peaks in various applications, focus has shifted to replacing graphite with materials capable of higher capacity and charge/discharge rate-capability (since the graphite electrode and not the capacitive activated carbon electrode, is the rate limiter). So-called, hard carbons have shown promise as alternatives to graphite.
Hard carbons have similar densities as graphite but have higher Li-ion intercalation capacity – resulting in higher mAh/g anode materials and also fast-charging capabilities. Various precursors have been evaluated for these hard carbons and include petroleum pitch, coconut shell, and furfuryl alcohol.
On the other hand, for the activated-carbon electrode, advances in performance have focused on increasing the specific surface area to improve specific capacitance (Farad/g) and investigating various natural sources as precursors. Recently, furfuryl-alcohol precursor based activated carbons has been reported to show excellent capacitive characteristics. This development is an exciting opportunity since both activated and hard carbon can be made from the same precursor – furfuryl alcohol – utilizing the same manufacturing infrastructure as used by the conventional activated carbon industry today. Furfuryl alcohol derived carbons tend to have a higher raw material cost than the competing precursors, but their increased performance can more than overcome that in $/performance terms.
To that end, the development of alternative furfuryl-alcohol precursor based hard carbons and activated carbons in the lithium-ion capacitor industry would be highly valued. Recently, a group of researchers from the CIC Energigune in Spain: Dr. María Arnaiz and Dr. Jon Ajuria, in collaboration with Dr. Vinod Nair and Dr. Shantanu Mitra at Farad Power Inc. in California, USA, demonstrated a simple process to synthesize both carbons from furfuryl alcohol precursors. Their approach differed from previous attempts that used furfuryl alcohol as a precursor for carbons, in its choice of additives and processing parameters to manage and influence the polymerization process (of the furfuryl alcohol) so as to result in the desired characteristics of the activated and hard carbons obtained. Not only was the polymerization process different for the activated carbon and hard carbon, but the subsequent carbonization and calcination/activation steps were also tailored differently for the two carbons. They have recently published the work in the research journal, Electrochimica Acta.
The team then focused on constructing a Li-ion capacitor using activated carbon and hard carbon derived from furfuryl alcohol and measuring the performance of the anode, cathode, and device under various conditions. The authors reported that the hard carbons were capable of delivering a stable capacity of ~400 mAh/g vs. Li+/Li at C/10, with excellent capacity retention of 50% at 10C and 25% at 50C. In addition, the LICs showed considerably higher energy density over its electric double layer capacitor counterpart, delivering a maximum energy density (based on the total electrode active mass weight) of 150 Wh/kg at a power density of 150 W/kg, with a 66% retention of the initial energy at the highly demanding 10,000 W/kg power peak point, these numbers are based on the electrode active mass weight only, without considering the weight of the other passive cell components. Most interestingly, the anode when tested at different C-rates, showed a capacity of 25 mAh/g at a C-rate of 100. This type of performance (fast charging capability) is not possible with graphite anode materials.
In summary, the study described the synthesis of hard carbons and activated carbons – using different processing conditions – from the same furfuryl alcohol precursor material. Overall, the researchers successfully established that furfuryl-alcohol based carbons are a viable option for energy storage applications. Furthermore, the use of a renewable resource like furfuryl alcohol (which is extracted from agricultural waste like corn cob and sugarcane) makes these carbons an attractive alternative to the current energy storage carbons (e.g., mined or artificially synthesized graphite).
María Arnaiz, Vinod Nair, Shantanu Mitra, Jon Ajuria. Furfuryl alcohol derived high-end carbons for ultrafast dual carbon lithium-ion capacitors. Electrochimica Acta, volume 304 (2019) page 437- 446.Go To Electrochimica Acta