Mechanistic insights into hydrothermal carbon formation: from biomass to pyrolyzed carbons with enhanced interparticle connectivity for energy-related applications

Abstract

Hydrothermal carbonization (HTC) is widely regarded as a sustainable route for converting biomass into carbon materials; however, the formation mechanisms of structurally diverse biomass feedstocks remain insufficiently understood. This work systematically investigates HTC reaction pathways of both solid and liquid products using wheat straw (second-generation biomass), wet corn gluten feed (first-generation by-product), and sugars as model systems under varying pH, temperature, and residence time, while directly linking structural evolution of the solids to their electrochemical applicability. A key novelty is the comprehensive, quantitative analysis of liquid filtrates by 1D- and 2D-NMR spectroscopy, enabling access to possible green platform chemicals (levulinic acid, 3-hydroxypyridine, furfurals) as well as considerations regarding toxicology for downstream processing. 3-Hydroxypyridine derivatives form the main N-containing molecular motif in the filtrate, likely formed via Strecker degradation of amino acids with C5-sugar-derived diketones—confirmed through model reactions of C5 and C6 sugars with glycine. Overall, HTC as a pretreatment before pyrolysis increases the carbon content, sp²/sp³ ratio, and bulk conductivity due to increased interparticle connectivity of sugar-derived hydrochar, while reducing alkali contaminants (<0.1 wt%) compared to direct pyrolysis of the biomass. As a proof of concept, selected pyrolyzed carbons are decorated with Pt nanoparticles and exhibit oxygen reduction reaction activity, electrochemical surface area, and kinetic currents approaching those of commercial Pt/C catalysts on a rotating disk electrode, indicating their potential as catalyst supports while highlighting the surface area–conductivity trade-off that limits their applicability.