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Development of Nanoporous Carbons from Renewable Lignin Feedstocks for Sustainable Applications

Bristol student theses: Doctoral ThesisDoctor of Philosophy (PhD)

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Abstract

Activated carbons are widely used adsorbents in sustainable applications, including hydrogen storage and supercapacitance. The performance of activated carbons in these applications is highly dependent on the structural properties of the material including, the pore size distribution, surface area, and pore volume. Lignin, a naturally occurring macromolecule, has received increasing interest as a promising feedstock for nanoporous carbons. Lignins extracted from different feedstocks differ in the ratio of aromatic monomers S, G, and H. On carbonisation, only this aromatic backbone remains. The primary aim of this thesis was to ascertain whether the aromatic structure of lignin affected the porosity of activated carbons, and if the pore size could be tuned for specific applications.

An in-depth characterisation of four organosolv-extracted lignins from different
feedstocks was performed. Lignins were divided into two groups based on their aromatic unit ratio: high S/G and low S/G. Evolved low molecular weight volatiles coalesced into large bubbles, forming a foam. The aromatic ratio affected the mechanical strength of foams; low S/G ratio lignins produced more stable monoliths. These carbon foams have potentially exciting future applications as binder-free monoliths for supercapacitor electrodes. Lignin carbon powders exhibited reasonable hydrogen uptake (1.8 wt.% at 77 K and 1 bar) due to intrinsic microporosity and capacitance values of 60 - 80 F g-1 (organic and aqueous electrolytes) with scope for further optimisation, aided by an experimental design protocol produced within this work. No correlation was found between the aromatic lignin structure and activated carbon porosity, determined by gas sorption and small-angle scattering. This lack of correlation was attributed to volatiles coalescing rather than forming internal porosity in the charring stage. These findings are significant for the industrial production of activated carbon powders, implying that an organosolv lignin from any feedstock can produce a microporous carbon with a high surface area (>1000 mg-1) and reasonable performance in sustainable applications.

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Original languageEnglish
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Award date19 Mar 2019

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