The role of doping in tannin-derived carbon materials for electrochemical applications.
Authors:   , Pinto Burgos Oscar 
Published: November 2024

There is a growing and urgent demand for eco-friendly, high-power devices, especially in portable applications like portable electronics or hybrid-electric/electric vehicles (H/EVs). Electrochemical energy storage systems are highly sought after in this regard. The main technologies currently available are Li-ion batteries and electrochemical capacitors (ECs), each offering distinct yet often complementary performance characteristics. Meanwhile, the former stores the energy mainly through chemical pathways, the latter stores the charge via physical process where the electric double layer is a key parameter. The ECs' storage process is rapid, highly reversible, and exhibits minimal effects on device performance over numerous charge/discharge cycles, making them advantageous in the long term. However, ECs face the drawback of low specific energy, prompting scientific efforts to enhance this value without compromising power.
One promising approach to achieve this enhancement is through heteroatom doping of carbon materials. In this context, tannins, a polyphenolic compound frequently derived from tree bark, serve as a suitable precursor due to their auto-condensation reactions, leading to a well-connected porosity network and high reactivity, enabling the incorporation of functionalities.
In this context, this doctoral research focuses on the study of the relationship between pore size distribution, surface chemistry, and electrochemical performance of tannins-derived carbon materials as electrodes in electrochemical capacitors, with an emphasis on surface modification with oxygen, nitrogen and/or boron. Advanced characterization techniques such as adsorption-desorption isotherms, and X-ray photoelectron spectroscopy (XPS) were employed to analyze the structure and chemistry of the carbon materials. Additionally, electrochemical tests were conducted to evaluate their energy storage capacity, power density and overall electrochemical performance.
The research establishes tannins-derived carbon materials as effective electrodes for electrochemical capacitors. Chemically activated tannins-derived carbon materials exhibit a linear correlation between capacitance and specific surface area, optimizing electrochemical performance with energy densities reaching 4.4 W∙h∙kg-1 at 1.1 kW∙kg-1.
The hydrothermal carbonization doping method successfully incorporates nitrogen or boron into tannins-derived carbon materials, introducing significant alterations to tailored chemical and morphological modifications. Without doping agents, hydrothermal carbonization leads to carboxylic acid-dominated surface chemistry, limiting electric double layer formation. Boron incorporation moderately enhances electrochemical performance. Meanwhile, nitrogen doping significantly improves surface chemistry and textural properties after CO2 activation, enhancing electrochemical performance. The top-performing material achieves remarkable energy density of 6.0 W∙h∙kg-1 at 1.3 kW∙kg-1, retaining nearly 96% of initial energy storage after 30,000 cycles.