<p>Functionalized cellulose nanofibril membranes have emerged as promising candidates for their use as electrolytes in proton exchange membrane fuel cells (PEMFCs). However, knowledge on this application is still limited, and topics like the effect of the chemical characteristics of the nanomaterial and their possible interaction with additives and crosslinkers have remained unclear. For this reason, this study focuses on investigating the influence of lignin in the nanomaterial, the type of functionalization of the nanofibrils, and the use of Kymene™ as a crosslinker, on the physicochemical and transport properties of membranes, as well as their performance in a laboratory PEMFC. For this, sulfated lignocellulose nanofibrils (LCNF–OSO₃⁻), sulfonated lignocellulose nanofibrils (LCNF–SO₃⁻), and sulfated cellulose nanofibrils (CNF–OSO₃⁻) were produced and used to fabricate membranes via solvent casting. Additionally, Kymene™ resin was added. The characterization of nanomaterials showed that lignin hinders the sulfation of nanofibrils and the cleavage of cellulose. The type of functionalization affected the fibril-Kymene™ reactivity in the membranes. On the one hand, the LCNF–SO₃⁻ primarily interacted with Kymene™ via hydrogen bonds, which enhanced the water absorption capacity and swelling ratio of the membrane. On the other hand, the sulfated nanofibrils exhibited the highest reactivity with Kymene™, linked mostly via covalent bonds. Finally, the LCNF–OSO₃⁻ membranes crosslinked with Kymene™ exhibited superior proton conductivity (4.3 mS/cm at 40&#xa0;°C) and performance in laboratory PEMFC (2.87 mW/cm² at 23&#xa0;°C), showing that lignin-containing nanofibrils and sulfate groups enhance the membrane properties for its application as an electrolyte.</p>