| 1. |
- Ribadeneyra, Maria, et al.
(author)
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Lignin-derived electrospun freestanding carbons as alternative electrodes for redox flow batteries
- 2020
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In: Carbon. - : Elsevier Ltd. - 0008-6223 .- 1873-3891. ; 157, s. 847-856
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Journal article (peer-reviewed)abstract
- Redox flow batteries represent a remarkable alternative for grid-scale energy storage. They commonly employ carbon felts or carbon papers, which suffer from low activity towards the redox reactions involved, leading to poor performance. Here we propose the use of electrospun freestanding carbon materials derived from lignin as alternative sustainable electrodes for all-vanadium flow batteries. The lignin-derived carbon electrospun mats exhibited a higher activity towards the VO2 +/VO2+ reaction than commercial carbon papers when tested in a three-electrode electrochemical cell (or half-cell), which we attribute to the higher surface area and higher amount of oxygen functional groups at the surface. The electrospun carbon electrodes also showed performance comparable to commercial carbon papers, when tested in a full cell configuration. The modification of the surface chemistry with the addition of phosphorous produced different effect in both samples, which needs further investigation. This work demonstrates for the first time the application of sustainably produced electrospun lignin-derived carbon electrodes in a redox flow cell, with comparable performance to commercial materials and establishes the great potential of biomass-derived carbons in energy devices.
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| 2. |
- Schlee, Philipp, et al.
(author)
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Free-standing supercapacitors from kraft lignin nanofibers with remarkable volumetric energy density
- 2019
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In: Chemical Science. - : Royal Society of Chemistry. - 2041-6539. ; 10:10, s. 2980-2988
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Journal article (peer-reviewed)abstract
- We have discovered a very simple method to address the challenge associated with the low volumetric energy density of free-standing carbon nanofiber electrodes for supercapacitors by electrospinning Kraft lignin in the presence of an oxidizing salt (NaNO3) and subsequent carbonization in a reducing atmosphere. The presence of the oxidative salt decreases the diameter of the resulting carbon nanofibers doubling their packing density from 0.51 to 1.03 mg cm−2 and hence doubling the volumetric energy density. At the same time, the oxidative NaNO3 salt eletrospun and carbonized together with lignin dissolved in NaOH acts as a template to increase the microporosity, thus contributing to a good gravimetric energy density. By simply adjusting the process parameters (amount of oxidizing/reducing agent), the gravimetric and volumetric energy density of the resulting lignin free-standing carbon nanofiber electrodes can be carefully tailored to fit specific power to energy demands. The areal capacitance increased from 147 mF cm−2 in the absence of NaNO3 to 350 mF cm−2 with NaNO3 translating into a volumetric energy density increase from 949 μW h cm−3 without NaNO3 to 2245 μW h cm−3 with NaNO3. Meanwhile, the gravimetric capacitance also increased from 151 F g−1 without to 192 F g−1 with NaNO3.
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| 3. |
- Schlee, Philipp, et al.
(author)
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From waste to wealth : From kraft lignin to free-standing supercapacitors
- 2019
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In: Carbon. - : Elsevier BV. - 0008-6223 .- 1873-3891. ; 145, s. 470-480
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Journal article (peer-reviewed)abstract
- Pure eucalyptus Kraft lignin derived carbon fiber mats were produced based on a model workflow. It covers the preparation and characterization of the lignin precursor and the carbon materials and its testing in the final application (supercapacitor). Sequential solvent extraction was employed to produce a eucalyptus Kraft lignin precursor which could be electrospun into lignin fibers without any additives. The fiber formation from low molecular weight lignin is assigned to strong intermolecular interactions via hydrogen bonding and π-π-stacking between individual lignin macromolecules which gives rise to association complexes in the electrospinning solution. By stabilization in air, carbonization in N2 and an activation step in CO2, free-standing microporous carbon fiber mats could be produced. These fiber mats possess mainly basic oxygen functional groups which proved to be beneficial when tested as free-standing electrodes in symmetric supercapacitors. Consequently, the CO2-activated fiber mats showed a high specific gravimetric capacitance of 155 F/g at 0.1 A/g, excellent rate capability with 113 F/g at 250 A/g and good capacitance retention of 94% after 6000 cycles when tested in 6 M KOH electrolyte. Therefore, we conclude that lignin itself is a promising precursor to produce microporous, oxygen functionalized carbon fibers serving as free-standing electrodes in aqueous supercapacitors.
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| 4. |
- Schlee, Philipp, et al.
(author)
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Hardwood : versus softwood Kraft lignin-precursor-product relationships in the manufacture of porous carbon nanofibers for supercapacitors
- 2020
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In: Journal of Materials Chemistry A. - : Royal Society of Chemistry. - 2050-7488 .- 2050-7496. ; 8:44, s. 23543-23554
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Journal article (peer-reviewed)abstract
- The process of stabilization is essential in the production of carbon fibers from lignins. During stabilization, the initially thermoplastic lignin polymer is converted to a thermoset polymer allowing for high-temperature treatment without a change in shape. In this work, hardwood (HKL) and softwood (SKL) Kraft lignins were stabilized in air at temperatures between 190 and 340 °C before carbonization at 800 °C in a nitrogen atmosphere. Due to the differences in side-chain linkages, functional groups and molar mass, the lignins exhibit different structural changes upon stabilization and hence develop different porosities upon carbonization. Both lignins undergo major crosslinking reactions in the side chains at low temperatures and degradation reactions at high temperatures during stabilization. Crosslinking gives rise to narrow pore size distributions with mainly (sub-) nanometer pores, whereas degradation reactions lead to a more open pore structure with additional mesoporosity (>2 nm). When both types of reactions take place simultaneously, highly accessible (sub-) nanoporosity can be effectively created, which boosts the performance of supercapacitors operating in 6 M KOH(aq). This effect terminates when the crosslinking reactions cease and mainly degradation reactions take place, which occurs in HKL at 340 °C. SKL shows both a lower degree of crosslinking and degradation and hence develops less specific surface area. The optimum performance in an aqueous alkaline supercapacitor is achieved with HKL stabilized at 310 °C. It shows a specific gravimetric capacitance of 164 F g-1 at 0.1 A g-1 and 119 F g-1 at 250 A g-1 with a capacitance retention of more than 90% after 10 000 cycles.
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