P,N-doped nickel–carbon nanofibers derived from ammonium phosphate for efficient glycerol electrooxidation and renewable power generation from biodiesel waste

Abstract

Nickel electrocatalysts present an economic alternative to noble metals for alcohol oxidation in alkaline fuel cells. Here, P,N-doped Ni-containing carbon nanofibers (Ni-CNFs) were synthesized by electrospinning of nickel acetate solutions with ammonium phosphate followed by thermal stabilization and calcination. Phosphate and nitrogen dopants regulated the electronic structure and surface chemistry of the nanofibers, which promoted the formation of active NiOOH species for catalysis. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the formation of a metallic nickel (Ni⁰)/graphitic carbon nanofiber (CNF) hybrid with nanoscale dispersion of nickel domains, while X-ray photoelectron spectroscopy (XPS) verified the coexistence of Ni⁰/Ni²⁺ species together with phosphate (P⁵⁺–O) and nitrogen–carbon (N–C) functionalities, which collectively promote redox reversibility. Electrochemical activation in 1.0 M KOH yielded a high electrochemical surface area (∼40 600 cm² g⁻¹) and distinct Ni(II)/Ni(III) redox transitions. The optimized 1 wt% DAP-Ni-CNF electrode achieved a glycerol oxidation current density of ∼140 mA cm⁻² in 0.25 M glycerol with an apparent activation energy of ∼15 kJ mol⁻¹, which points towards fast charge transfer through a NiOOH-mediated pathway. In direct glycerol fuel cell tests, the same electrode achieved a peak power density of ∼200 mW m⁻² at 1.0 M glycerol. Notably, this study demonstrates that glycerol that is collected as a waste by-product from biodiesel production can be utilized as a viable new renewable energy source, valorizing low-value residues to clean electricity. The developed P,N-doped Ni-CNFs thus provide a sustainable route for glycerol valorization and circular bioenergy generation through alkaline fuel cell technology.