Water hyacinth-derived biochars - from invasive biomass to active Pt-free alkaline oxygen reduction reaction...

Electrochemical energy conversion technologies are central to sustainable power generation, yet sluggish oxygen reduction reaction (ORR) kinetics remain a key limitation in alkaline fuel cells. Although Pt-based electrocatalysts are highly active, their cost and scarcity motivate the development of sustainable, carbon-based alternatives. Here, biochars derived from different parts of an abundant invasive plant, water hyacinth (WH), namely bulbs, wood and leaves, were prepared, screened, and evaluated as ORR electrocatalysts. Among them, bulb-derived biochar (WHB) showed the highest potential, and nitrogen doping produced the most pronounced enhancement. N-doped WHB exhibited an onset potential of 0.85 V, a limiting current density of -3.19 mA·cm^-2, an electron transfer number of [Formula: see text] = 3.16 and a Tafel slope of 57 mV·dec^-1, approaching Pt/C in activity metrics. XPS and Raman analyses linked these improvements to favorable nitrogen speciation-particularly graphitic N-and increased structural disorder. In parallel, layer-by-layer (LbL) hybridization with multi-walled carbon nanotubes (MWNTs) provided complementary insight into structure-performance relationships. Hybridization improved ORR activity when the base material was pristine WHB (among the nanocomposites, 3:1 WHB:N-MWNT performed best, with E_onset = 0.81 V, j_L = -3.37 mA·cm^-2, [Formula: see text] = 3.12, and TS = 76 mV·dec^-1), consistent with effective integration of porous biochar and conductive nanotube pathways. In contrast, adding MWNTs to already highly active N-WHB reduced performance, likely due to dilution of catalytic sites and disruption of active interfaces. Overall, this study demonstrates invasive biomass as a viable feedstock for high-performance, Pt-free ORR catalysts and highlights when chemical tuning versus hybrid assembly is most beneficial for catalyst design.