Electrochemical capacitors, also called supercapacitors, are of great interest as high power electrochemical energy storage devices to complement batteries in either stationary or mobile electrical energy storage. Their fast discharge capability combined with very long cycle-life (beyond millions of cycles) makes them particularly suitable for public electrical transportation where the breaking energy can be easily recovered at high charging rate. They find also applications in aerospace industry thanks to their wide operational temperature range or in decentralised stationary storage to provide power boosts on the electrical distribution network.
Despite all these features that batteries do not have, the energy density achieved by commercial supercapacitors stays relatively low (5 Wh/kg of electrode material compared to 160 Wh/kg for Li-ion batteries). This limits their use in energy harvesting applications. Therefore, efforts are focused now on increasing the energy density of supercapacitors, whilst retaining their high power and long cycle-life.
The charge storage mechanism of electrochemical capacitors can be differentiated in:
i) electrochemical double-layer capacitance, which stores energy by electrostatic adsorption of electrolyte ions on the surface of electrically conductive porous electrodes.
ii) pseudo-capacitance, where the energy is stored through reversible redox reactions at the electrode/electrolyte interface.
Porous carbon materials are the main candidate for supercapacitors thanks to their good electrical conductivity, low environmental impact and large surface area. They also show high versatility with regards to porosity development and surface chemistry. Most carbon materials behave as electrochemical double-layer capacitors, with their large surface area providing available surface for the ions to absorb. Nevertheless, many carbons possess surface functional groups, such as nitrogen and oxygen groups. They combine several advantages: they can increase the wettability of the electrodes, modify the electrical conductivity but most importantly contribute to an additional pseudo-capacitance on the top of the double-layer capacitance. Commercial porous carbons are currently made by steam activation of bio-resources such as coconut shells, however, their porous structure is not tuned specifically for high capacitance due to the precursor’s heterogeneity.
In our group we synthesize well defined porous carbon materials with tailored porosity, functionality and electronic conductivity based on low-cost and renewable bio-waste precursors. Amongst the various carbon materials used for supercapacitive applications, our group has developed flexible and free-standing carbon nanofiber electrodes made from at least 90% lignin. Lignins are macromolecules present in the plant cells and constitute 5-40% of the biomass on earth. They provide rigidity to plant steams and protection against microbiological attack. Currently extracted from the wood at a rate of millions of tons per year by the paper industry, they can also be extracted from biomass via other milder processes such as organosolv processes. We choose to mostly use lignins as bio-precursors for its high carbon content compared to carbohydrates providing high carbon yields after pyrolysis.
Our lignin-based nanofiber electrodes are electrospun in a home-made electro-spinner which allows us to control humidity and temperature, which facilitates the spinning of fibres by tuning the evaporation rate. After the electrospinning, the nanofiber mat is carbonised at high temperatures, yielding porous carbon nanofibers with various crystallinity and surface functionalities. We can control the pore size and the oxygen functionalities by simply changing the pyrolysis temperature or the oxidant concentration. The nanofibers can also be compressed into a non-flexible dense electrode in order to increase the volumetric performances of the supercapacitive device.
We also developed lignin-based meso-porous materials produced by soft-templating using the Evaporation Induced Self Assembly (EISA) method, which allows the control of the lyotropic liquid phase.
Finally, we test our materials in two-electrodes and 3 electrodes cells, using various electrolytes and we try to correlate the materials properties with its electrochemical performances. We typically characterise our materials via surface gas adsorption, X-Rays tomography, Small Angle X-Rays Diffractions, X-Rays photoelectron spectroscopy, Transmission Electron Microscopy, Temperature Programmed Desorption or Raman spectroscopy.