Novel biomass materials suitable for various applications need to be developed in order to establish a biobased raw material platform within the bioeconomy. These biobased materials must be able to compete with conventional fossil fuel-based materials, both from a technological and economic point of view. Researchers at the University of Hohenheim are working on the development of conductive carbon materials from biomass with the long-term goal of making the substitution of fossil electrode materials in high-capacitance energy storage systems ready for market.
The future belongs to electromobility – this is not only politically but also ecologically desirable, in view of dwindling fossil resources and the changing climate. The German government estimates that around one million electric vehicles will be on German roads by 2020, and six times as many by 2030.1 The VDI status report “Zukunft des Autos” (Future of the Car) claims that “the success of electric engines stands and falls with the ability to store energy”.2 Energy storage systems not only need to be long-lived and powerful. First and foremost, they must be able to store very large amounts of energy (high-capacitance storage devices). The so-called secondary batteries (i.e. rechargeable batteries) currently on the market lack both – their lifetime is limited to a few thousand cycles due to the chemical decomposition processes as the battery is charged and discharged and because the storage and release of energy is a very slow process.12 Supercapacitors, also known as electric double-layer capacitors (EDLC – see information box) do not have these problems and therefore represent a promising addition to existing energy storage devices. The raw materials for producing the components, however, still come from fossil sources.
At the same time, approximately 1.5 million tonnes of biomass (2015) are generated in Baden-Württemberg every year.3 A variety of conversion processes can be used to produce high-quality materials for storing energy in supercapacitors. As economies are shifting towards bioeconomies, the idea is to replace materials of fossil origin with biobased equivalents from regional biomass. But what kind of materials are we actually talking about? We need to take a much more detailed look at the technology of supercapacitors in order to answer this question.
Information box - supercapacitors
Design: Supercapacitors consist of positively and negatively charged electrodes (cathodes, anodes) that are immersed in an electrolyte solution of good electrical conductivity. The electrolyte solution contains dissociated salts in the form of positive and negative ions (cations and anions) and solvent molecules with dipole character, i.e. possessing a plus and a minus pole. Due to their polarity, the solvent molecules arrange themselves like a shell around the ions, thus forming solvated positive and negative ions.
Storage mechanism: When the supercapacitor is charged, i.e. voltage is applied to the electrodes, then the solvent dipoles and the solvated ions of the electrolyte form several layers on the electrode surface, resulting in so-called Helmholtz and Stern double layers, named after the scientists who discovered them, in which the energy is stored electrostatically, i.e. in an electric field (double-layer capacitance).
As the name electric double-layer capacitors (EDLC) suggests, electrochemical storage mechanisms always play a role in these devices. Depending on the nature of the electrodes, the process leads to a more or less pronounced desolvation of individual charge carriers. If the electrode contains metal oxides, some charge carriers will lose their solvate shell consisting of solvent molecules due to strong adhesive forces and adhere to the electrode surface. This results in the reversible transfer of electrons without chemical binding, i.e. reversible redox reactions take place and energy is stored electrochemically (pseudocapacitance).11
The storage mechanism of EDLCs does not depend on irreversible chemical reactions or decomposition processes. Charging and discharging processes therefore take very little time. The electrode material is not attacked chemically, resulting in high specific power densities of > 1 kW/kg and lifetimes of up to 500,000 cycles. These properties make EDLCs interesting for applications in which power peaks need to be intercepted or large amounts of energy supplied in (milli) seconds. Examples of this are found not only in the renewable energy field (photovoltaics, wind energy), but also in the mobility sector. Super capacitors are therefore used in regenerative braking systems and hybrid vehicles, especially for public transport.7
The amount of energy that can be stored in supercapacitors in the form of charge (i.e. total capacitance as the sum of double-layer capacitance and pseudocapacitance) depends on the number of ions that adhere to the electrode. In practical terms this means that the larger the electrode surface, the more ions can adhere to it and the higher capacitances can be achieved. Highly porous carbon materials (fossil activated charcoal) have therefore proven to be suitable materials for producing electrodes.9 This is also due to diverse nanostructures (e.g. graphs, nanotubes). It is likely that fossil carbon will become scarce and expensive in the future. Alternative carbon sources and the development of biobased carbon materials with high capacitance values are essential. This is mainly because it has been shown that under certain conditions it is possible to achieve higher capacitance values with biobased electrodes than with conventional materials.12
The University of Hohenheim’s Department of Conversion Technology and Life Cycle Assessment of Renewable Resources uses agricultural waste as the starting material for producing highly porous hydrocarbons. The method used is called hydrothermal carbonisation (HTC), and is simply the carbonisation of wet biomass under pressure and at temperatures of between 180 and 250 ˚C. Various chemical processes convert individual biomass components (lignin, cellulose, hemicellulose) into carbon nanostructures. The properties of these carbon nanostructures are similar to those of brown coal.
An additional activation step, in which chemical and physical processes increase the pore volume of the coals, produces activated carbons (charcoal) with specific surfaces of up to 3,000 m2 / g. Activated charcol with different pore volumes, pore size distributions or surface characteristics can be produced with different starting materials, temperatures, duration of carbonisation and activation processes. The last step involves testing the charcoal for its suitability as electrode material. For this purpose, electrical conductivity measurements, resistance measurements and complex cyclovoltammetric measurements are performed. The latter, for example, provide information on the proportion of double-layer capacitance and pseudocapacitance in relation to the total capacitance.8-11 This information is of major importance because pseudocapacitance can be increased relatively quickly by modifying the carbon materials.
This applies, for example, to composite materials made from charcoal and various metal oxides or heteroatoms, such as nitrogen. The inorganic components and heteroatoms of the carbon structure have good pseudocapacitative properties, so that more redox reactions take place and pseudocapacitance increases considerably. It is unclear which combination of starting biomass, process parameters and metal or heteroatoms generates the highest capacitance values and thus produces the most promising material for replacing current "state-of-the-art" materials.
If the Hohenheim researchers manage to develop a competitive material on the laboratory scale, they will then need to see whether it can be brought to market maturity. It would either need to be cheaper than conventional materials or have better capacitance values. Prof. Dr. Andrea Kruse, head of department at the University of Hohenheim, is more focused on the latter and is convinced that biogenic materials will conquer the market despite higher production costs: "The goal must be to produce a better product. It helps if clients feel better when they purchase these products, as is the case with Fairtrade coffee.” The biggest challenge is financing large-scale tests on an industrial scale, because "a pilot plant costs several million euros, depending on size. We need to find investors who do not mind not knowing when and how much they can earn with the product. This is often the death knell for a new process and the development of an innovative product.” However, Prof. Kruse is convinced that market forces will compel companies to go bio when the price of fossil resources increases.
In view of the cheap and freely available starting materials in the form of biomass, the comparatively low-energy HTC method and the ambitious research goals of the Hohenheim researchers, it is perfectly probable that biobased electrode materials will end up replacing their fossil predecessors in the future.
1 Die Bundesregierung (2011): Regierungsprogramm Elektromobilität. Online at: https://www.bmbf.de/files/programm_elektromobilitaet.pdf
2 Kaiser, O.; Eickenbusch, H.; Grimm, V.; Zweck, A. (2008): Die Zukunft des Autos. VDI Technologiezentrum. Online at: https://www.vdi.de/fileadmin/vdi_de/redakteur/dps_bilder/SK/2008/Studie_Zukunft-des-Autos.pdf
3 Baden-Württemberg Statistics Office (2015): Abfallbilanz 2015 – Ressourcen aus unserer kommunalen Kreislaufwirtschaft. Online at: https://um.baden-wuerttemberg.de/fileadmin/redaktion/m-um/intern/Dateien/Dokumente/2_Presse_und_Service/Publikationen/Umwelt/Abfallbilanz_2015.pdf
4 Enock, T. K., King, C. K., Pogrebnoi, A., Abeid, Y., & Jande, C. (2017). Status of Biomass Derived Carbon Materials for Supercapacitor Application, 2017, 1–41
5 Gao, Z., Zhang, Y., Song, N., & Li, X. (2016). Biomass-derived renewable carbon materials for electrochemical energy storage. Materials Research Letters, 3831(November), 1–20. https://doi.org/10.1080/21663831.2016.1250834
6 Glasner, C., Deerberg, G., & Lyko, H. (2011). Hydrothermale Carbonisierung: Ein Überblick. Chemie-Ingenieur-Technik, 83(11), 1932–1943. https://doi.org/10.1002/cite.201100053
7 Kotz, R., Carlen, M., Kötz, R., Kötz, R., Carlen, M., Carlen, M. (2000). Principles and applications of electrochemical capacitors. Electrochim. Acta, 45, 2483–2498. https://doi.org/10.1016/S0013-4686(00)00354-6
8 Lee, K. K., Hao, W., Gustafsson, M., Tai, C.-W., Morin, D., Björkman, E., Hedin, N. (2016). Tailored activated carbons for supercapacitors derived from hydrothermally carbonized sugars by chemical activation. RSC Adv., 6(112), 110629–110641. https://doi.org/10.1039/C6RA24398C
9 Pandolfo, A. G., & Hollenkamp, A. F. (2006). Carbon properties and their role in supercapacitors. Journal of Power Sources, 157(1), 11–27. https://doi.org/10.1016/j.jpowsour.2006.02.065
10 Qu, D., & Shi, H. (1998). Studies of activated cabons used in double-layer supercapacitors. Journal of Power Sources, 109(2), 403–411
11 Sharma, P., & Bhatti, T. S. (2010). A review on electrochemical double-layer capacitors. Energy Conversion and Management, 51(12), 2901–2912. https://doi.org/10.1016/j.enconman.2010.06.031
12 Titirici, M.-M., White, R., Falco, C., & Sevilla, M. (2012). Black perspectives for a green future : hydrothermal carbons for environment protection and energy storage. Energy and Environmental Science, 5(6796), 6796–6822. https://doi.org/10.1039/c2ee21166a
13 Titirici, M.M.; Funke, A.; Kruse, A. (2015): Hydrothermal Carbonization of Biomass. Pandey, A. [Hrsg.] Recent Advances in Thermochemical Conversion of Biomass. Amsterdam [i.a.]: Elsevier, 2015