Lignin-Derived Fe-Loaded Graphene-Like Carbon as an Efficient Electrocatalyst for Nitrate-to-Ammonia Conversion

Abstract

We developed a sustainable route for the synthesis of iron-loaded graphene-like carbon catalysts from lignin, a waste byproduct from the paper industry. The catalysts were prepared by pyrolyzing Fe(II)-impregnated alkaline lignin at 1280 °C under an argon atmosphere. Structural characterization revealed the coexistence of α-Fe, γ-Fe, magnetite (Fe3O4), and pyrrhotite (Fe1–xS) within a conductive graphene-like matrix. The electrocatalytic nitrate reduction reaction performance and reaction mechanism were investigated using an H-type electrolytic cell integrated with a rotating disk electrode. The optimized catalyst, AL-Fe0.25, exhibited a substantially lower Tafel slope (136 mV dec–1) than bulk iron species, indicating accelerated electron transfer kinetics during the initial rate-determining step. Benefiting from these kinetics, AL-Fe0.25 exhibited a Faradaic efficiency of 80.8% for ammonia production and an ammonia yield rate of 9.27 mg h–1 mgcat–1 at –0.75 V vs reversible hydrogen electrode. This performance was attributed to the cooperative interactions among the multiple Fe phases embedded in the conductive biomass-derived carbon framework. Long-term durability tests (12 h) showed a gradual decline in ammonia selectivity, which was ascribed to the corrosion of the graphene-like carbon framework induced by nitrate or reaction intermediates.