Pyrolysis Creates Electron Storage Capacity of Black Carbon (Biochar) from Lignocellulosic Biomass
Danhui Xin, Nepu Saha, M. Toufiq Reza, Jeffrey M. Hudson, Pei C. Chiu
Abstract
Black carbon such as biochar has been shown to possess electron storage capacity (ESC), which enables black carbon to store and reversibly exchange electrons with its surroundings. However, the origin of black carbon ESC remains unknown to date. To answer this question, we measured the ESC of cellulose, lignin, hemicellulose, and biochars prepared from these biopolymers, their mixture, and a pinewood, through pyrolysis at 350–650 °C. Mediated electrochemical analysis (MEA) with ABTS and diquat as mediators and chemical redox titration (CRT) with O2 and titanium(III) citrate were used to quantify ESC. MEA could measure lignin ESC but significantly underestimated biochar ESC; CRT could measure the total reversible ESC of biochars but not that of lignin. Lignin was the only biopolymer that possessed ESC, which was largely destroyed during pyrolysis at 350 °C. After pyrolysis at ≥450 °C, the three biopolymers, their mixture, and pinewood all yielded biochars that possessed a highly reversible ESC of 1–2 mmol e–/g. This indicates that pyrolysis created reversible ESC of biochar from lignocellulosic biomass, presumably by converting oxygen-containing moieties of the biopolymers into (hydro)quinones in biochar. The implications of our findings for biogeochemistry, climate, contaminant fate, and engineering applications are discussed.