Analyzing effects of algal feedstock, catalytic enhancement, and low-temperature oxidation on the observed carbon and bulk conductivity of biochars via pyrolysis

Abstract

Conductive carbon fillers for composites, electrodes, and energy storage are conventionally sourced through mining or synthesized from fossil fuels and require a myriad of environmentally detrimental steps before becoming useful in the global effort to efficiently store energy and conduct electricity. Algal biomass is a renewable, carbon-sinking alternative feedstock for carbon materials, but algal biochar remains understudied as a conductive material. Existing work has focused on porosity and electrolyte ion transport relevant to capacitive applications, and direct measurements of bulk DC conductivity are close to absent from the literature. This thesis addresses that gap by fabricating biochar from three algal feedstocks, the protein-rich microalga Limnospira platensis (Sp.), the carbohydrate-rich red macroalga Gracilaria gigas (Gr.), and a 1:1 blend of the two, carried through a full 3 × 2 × 3 factorial design varying thermal atmosphere (nitrogen-only versus a low-temperature oxidative hold) and catalyst (none, iron (III) chloride, or tannic acid) at a pyrolysis temperature of 900°C under inert gas (nitrogen). Resulting chars were characterized for mass yield, structural ordering by Raman spectroscopy (ID/IG), bulk conductivity by two-point compression measurement, as well as morphology and elemental composition by SEM/EDS.The studied feedstocks exhibited a consistent increase in char yields from Spto SpGr blend to Gr (27.04, 29.27, and 31.81 wt%, respectively) but almost no main effect on ID/IG; instead, feedstock type governed the width of range across processing conditions with Sp spanning an ID/IG range of 1.37 (1.09-2.46) against 0.58 (1.36-1.94) for Gr. The two single feedstocks have similar conductivity, 38.92 S/m for Sp and 39.48 S/m for Gr, but the 1:1 blend fell far below the additive prediction in every matched condition, by a mean of 6.72 S/m, consistent with resistance at interparticle contacts rather than a deficit in carbon quantity or order. Catalyst effects were conditional on feedstock chemistry: iron (III) chloride raised yield and lowered ID/IG in most conditions but reversed on protein-rich Sp under the nitrogen-only cycle, and neither catalyst raised mean conductivity above the uncatalyzed chars, 39.76 S/m against 36.56 S/m for tannic acid and 34.56 S/m for iron. The oxidative hold left mean ID/IG essentially unchanged but reduced its variability across feedstocks by roughly a third and raised conductivity in most feedstock-catalyst combinations, including the study's highest value, 48.00 S/m for iron-catalyzed Sp. Across all eighteen conditions, ID/IG and bulk conductivity showed no statistically distinguishable correlation, indicating that structural ordering alone is not a reliable predictor of bulk electron transport. Algal biochar produced under these conditions conducts at 25.69 to 48.00 S/m, well below commodity conductive carbons, and closing that gap will require attention to interparticle contact resistance rather than catalyst selection alone. Nonetheless, these findings represent a first baseline for algal biochar as a conductive material, established here using whole, unprocessed feedstocks. Preprocessing to reduce non-carbon content and morphology control to improve interparticle contact are possible next steps. This work underscores the potential of algal biomass as a renewable, carbon-sinking route to conductive carbon materials for composites, electrodes, and energy storage.