Interface-engineered nickel oxyhydroxide on carbon nanofibers for efficient urea oxidation and wastewater-to-energy conversion

RESEARCH ARTICLE Interface-engineered nickel oxyhydroxide on carbon nanofibers for efficient urea oxidation and wastewater-to-energy conversion Nasser A. M. Barakat 1,2*, Ahmed Saadawi1, Osama M. Erfan3, Ibrahim Mustafa4, Gaber Edris 4, Ayman Yousef 5* 1 Chemical Engineering Department, Faculty of Engineering, Minia University, El-Minia, Egypt, 2 Chemical Engineering Department, Faculty of Engineering, Atatürk University, Erzurum, Turkey, 3 Department of Mechanical Engineering, College of Engineering, Qassim University, Buraydah, Saudi Arabia, 4 Chemical Engineering Department, Faculty of Engineering, King Abdulaziz University, Jeddah, Saudi Arabia, 5 Department of Chemical Engineering, College of Engineering and Computer Sciences, Jazan University, Jazan, Saudi Arabia * (NAMB); (AY) Abstract OPEN ACCESS Citation: Barakat NAM, Saadawi A, Erfan OM, Mustafa I, Edris G, Yousef A (2026) Interfaceengineered nickel oxyhydroxide on carbon nanofibers for efficient urea oxidation and wastewater-to-energy conversion. PLoS One 21(6): e0347020. https://doi.org/10.1371/ journal.pone.0347020 Editor: Zafar Ghouri, Texas A&M University at Qatar, QATAR Received: March 25, 2026 Accepted: April 20, 2026 Published: June 4, 2026 Peer Review History: PLOS recognizes the benefits of transparency in the peer review process; therefore, we enable the publication of all of the content of peer review and author responses alongside final, published articles. The editorial history of this article is available here: https://doi.org/10.1371/journal. pone.0347020 Copyright: © 2026 Barakat et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, Urea is a common nitrogenous pollutant in agricultural and industrial wastewaters, and its electrochemical oxidation in alkaline media enables simultaneous detoxification and energy recovery. Herein, we report an interface-engineering strategy based on P,N-modified nickel–carbon nanofibers prepared via a scalable electrospinning– carbonization process using ammonium phosphate as a multifunctional precursor. During thermal treatment, phosphorus and nitrogen heteroatoms are incorporated into the carbon framework while regulating the surface chemistry of embedded nickel nanoparticles, promoting the formation of a NiOOH-ready interfacial layer. Structural analyses (XRD, SEM/TEM) confirm the formation of metallic Ni domains uniformly dispersed within turbostratic carbon nanofibers. XPS reveals the coexistence of Ni0/ Ni+2/Ni+3 species along with phosphate- and nitrogen-derived functionalities that enhance electronic modulation and redox reversibility. In 1.0 M KOH, the optimized composition exhibits a Ni+2/Ni+3 redox charge of ~12.5 mC.cm−2. For urea electrooxidation (0.5 M urea), the catalyst delivers a maximum current density of ~115 mA.cm−2 with a low onset potential of 0.37 V (vs Ag/AgCl). The apparent activation energy is 9.82 kJ.mol−1, indicating a kinetically favorable NiOOH-mediated pathway. Chronoamperometry shows stable operation with ~70–80% current retention over 1000 s. When implemented as an anode in a membrane-less direct urea fuel cell, the material achieves a peak power density of ~1.1 W.m−2 at 35 °C, demonstrating practical wastewater-to-energy feasibility. This work highlights how dual heteroatom modification of carbon nanofibers can regulate nickel interfacial chemistry, enabling efficient urea remediation coupled with renewable power generation. PLOS One | https://doi.org/10.1371/journal.pone.0347020 June 4, 2026 1 / 25 and reproduction in any medium, provided the original author and source are credited. Data availability statement: All relevant data are within the manuscript. Funding: This work was supported by the Deanship of Graduate Studies and Scientific Research, Saudi Arabia (project number JU-202505362-DGSSR-ORA-2025 to AY). Competing interests: The authors have declared that no competing interests exist. 1. Introduction Carbon nanofibers (CNFs) have emerged as a versatile platform for (electro)chemical energy technologies because their high axial aspect ratio combines the advantages of nanoparticles (large surface area) with those of macroscopic fibers (continuous electron pathways and mechanical integrity) [1,2]. The one-dimensional architecture fosters rapid in-plane charge transport, provides abundant external surface for reactant access, and forms self-supporting porous mats that can be used directly as binder-free electrodes [3]. Additional benefits include chemical/thermal stability, facile integration into textiles or gas-diffusion layers, and straightforward postfunctionalization. Owing to these attributes, CNFs are widely exploited as supports for functional electrocatalysts—for example, for alcohol/urea oxidation, water splitting, oxygen reduction, and CO2 conversion—where the support dictates dispersion of the active phase, electron/ion percolation, and durability under bias [4,5]. Heteroatom modification further upgrades the catalytic competence of carbon. Phosphorus (P) doping introduces electron-rich P–C/P–O moieties that perturb the π network, polarize adjacent carbon atoms, and create Lewis-basic sites [6]. These effects enhance wettability, increase defect density, and strengthen metal–support interactions (e.g., P–O–M linkages), which are advantageous for anchoring transition-metal nanoparticles and for interfacial reactions that require OH⁻ activation. P-modified carbons have delivered notable performance in oxygen electrocatalysis, metal–air batteries, and small-molecule oxidation [7,8]. Nitrogen (N) doping—in pyridinic, pyrrolic, or graphitic forms—typically raises the electronic conductivity of carbon, alters charge density at neighboring carbons, and can act as an intrinsic catalytic site or a strong metal-anchoring motif. N-doped CNFs therefore excel as supports for Fe/Co/Ni-based catalysts, enabling improved dispersion, electron transfer, and resistance to sintering/corrosion in alkaline media [9,10]. Dual P&N modification is particularly appealing because it combines the electronic enrichment of N with the interfacial polarity and coordination chemistry of P [11,12]. The two dopants cooperatively tune work function, local charge distribution, and hydrophilicity, while furnishing a dense population of edge/defect sites and robust P–O–M/ N–M coordination. Such synergy is expected to facilitate OH⁻ supply and stabilize metal nanoparticles under turnover—both pivotal for alkaline electrooxidation [13]. Electrospinning provides an attractive alternative: it is low-cost, scalable, and high-throughput, producing continuous polymer nanofibers that are readily carbonized into conductive CNFs. Crucially, molecular precursors, metal salts, and heteroatom sources can be co-dissolved in the spinning dope, enabling one-pot control over composition and morphology [14]. The resulting mats are mechanically robust and (...truncated)