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

Abstract

Hydrothermal carbonization (HTC) offers a sustainable pathway to transform biomass residues into functional carbon materials. This work systematically explores HTC reaction mechanisms using wheat straw (second-generation biomass), wet corn gluten feed (first-generation by-product), and sugars (model system) under varying pH, temperature, and reaction time. Comprehensive analysis of hydrochar and liquid filtrates, including 1D- and 2D-NMR spectroscopy, unravel the conversion of cellulose, hemicellulose, starch, lignin, proteins, and inorganics and highlight the influence of acidic conditions and prolonged residence times on sugar leaching, spherical particle formation, and levulinic acid generation. NMR analytics reveal 3-hydroxypyridine as the dominant N-containing filtrate product, most likely originating from Strecker degradation of amino acids with C5-sugar-derived diketones—confirmed through model reactions of C5 and C6 sugars with glycine. HTC pretreatment enables sugar- and protein-rich biomass to form N-containing carbons with increased bulk conductivity through effective hydrolysis and repolymerization into interconnected spheres, though the resulting smooth morphology comes at the cost of a low BET surface area. In contrast, lignocellulosic biomass such as wheat straw retains parts of its cell-wall structure through solid–solid transformation, producing higher BET surface areas but with lower conductivity and persistent silicon impurities. HTC pretreatment increases the carbon content, sp2/sp3 ratio, conductivity and interparticle connectivity compared to direct pyrolysis of biomass, while effectively reducing alkali contaminants (<0.1 wt%). As proof of concept, selected HTC-derived amorphous carbons were decorated with Pt nanoparticles and exhibited comparable oxygen reduction reaction activity, electrochemical surface area, and kinetic currents to commercial Pt/C catalysts on a rotating disk electrode, indicating their potential as supports while highlighting the surface area–conductivity trade-off that limits their applicability.