Atomistic-to-Device Understanding of Oxygen Reduction Using Redox-Active Lanthanide-Coupled Carbon Nitride...

Efficient oxygen reduction at the cathode remains a critical bottleneck in advancing bio-electrochemical energy conversion, necessitating integrated experimental and atomistic-level understanding. 2D polymeric nanomaterials offer stable, nitrogen-rich frameworks as sustainable alternatives to platinum catalysts. However, their poor conductivity and low active-site density hinder oxygen reduction reactions (ORR), create an inefficient two-electron pathway, and limit the use of bio(electrochemical) devices for power generation. Lanthanide incorporation, owing to their unique redox and electronic properties, offers a promising route to overcome these shortcomings. In this investigation, lanthanides (RE) were incorporated into graphitic carbon nitride nanoparticles (g-C₃N₄ NPs) via one-pot synthesis, resulting in structural and electronic modifications confirmed by experiments and simulations, thereby enhancing their suitability for bio(electrochemical) systems. High-resolution TEM (HR-TEM) shows lattice distortion and nanosheet corrugation after lanthanide incorporation. X-ray photoelectron spectroscopy (XPS) confirms mixed RE³⁺/RE⁴⁺ states and valence-band modulation, which agrees well with density of states (DOS) calculations indicating Ce/Gd 4f-π hybridization near the Fermi level. Density functional theory (DFT) charge-density and adsorption analyses reveal that lanthanide sites stabilize key ORR intermediates and lower the reaction overpotential, favoring a four-electron pathway. This is consistent with the experimentally observed electron transfer number of n ≈ 3.9. Electrochemical tests show improved ORR activity for Gd-g-C₃N₄ NPs, with an onset potential of 0.81 V vs RHE and performance approaching Pt/C. When used as a microbial fuel cell cathode, Gd-g-C₃N₄ NPs deliver higher power output and COD removal, along with low peroxide yield, good stability, and strong methanol tolerance. When Gd-g-C₃N₄ NPs are used in microbial fuel cells (MFCs) as cathode catalysts, they achieve a peak power density of 447 mW m^-2 and 83% chemical oxygen demand (COD) removal, outperforming both pristine and Ce-based variants. The catalyst also demonstrated a low peroxide yield (< 5%), excellent stability, and superior methanol tolerance compared with Pt/C. This investigation offers mechanistic insights from integrated experimental and computational approaches to advance efficient electrocatalysts for power generation in MFCs and clean energy and water systems.