What role does carbonized tannic acid play in energy storage composites?

Abstract

Transformation of tannic acid (TA), a cheap, abundant and environmentally friendly (by)product, upon carbonization at various temperatures was examined as it is extensively being used in energy storing devices. In addition of reviling what is happening with TA upon carbonization, a step further has been taken to scrutinize the role of carbonized TA (CTA) playing in energy storage composites. Increasing the carbonization temperature from 500 °C to 800 °C led to a nine orders of magnitude increase in conductivity, from 9·10–9 S cm−1 to 6 S cm−1, respectively. Concurrently, transformation from largely mesoporous and low surface area (≈10 m2 g−1, at 500 °C) to dominantly microporous materials with respectable surface area (≈292 m2 g−1, at 800 °C) is evidenced, while surface oxygen content dropped from 18.2 to 3.5 at.% in the same temperature range. Capacitance values are determined to see how it might contribute to the overall capacitance of composites. It was found that capacitance is greatly affected by these transformations and range from 18 F g−1 to 38 F g−1 when in pristine carbonized condition, to 75 F g−1 when mixed with conducting Vulcan XC72. Decoupling electric double layer and diffusion limited capacitance (Cdiff) indicated that later is predominant and can be associated with transformations of various surface oxygen groups with linear relationship found between Cdiff and surface area occupied by oxygen per gram of sample. Presented results suggest that carbonized TA can contribute significantly to capacitance and conductivity as a part of a composite electrode in energy storing devices and its contribution to overall capacitance cannot be neglected. Alternatively, carbonized TA, pristine or activated, can be viewed as a cheap and abundant material for green supercapacitors.