Sustainable Photoelectrochemical Direct Lignin Upcycling via Ni-Co Phthalocyanine Catalysts Integrated With Organic Semiconductors

Abstract

Sustainable lignin valorization through solar energy conversion remains challenging due to the inherent resistance of lignin in photoelectrochemical (PEC) oxidations. Herein, an integrated PEC system combining metal phthalocyanine catalysts with organic semiconductors is presented to achieve direct Kraft lignin oxidation with an unprecedented production rate and stability. An optimized Ni-Co phthalocyanine/organic-semiconductor photoanode achieves a photocurrent density of 13.4 mA cm-2 and maintains stable operation for 28 h, representing the highest performance reported for direct lignin oxidation. This system efficiently suppresses the oxygen evolution reaction, directing 80.6% of the total current toward lignin oxidation. Reaction environment modulation allows precise control of the reaction pathway, with a vanillic acid production ratio reaching 76.1% and a production rate of 7.43 mu mol cm-2 h-1 under 1 M NaOH. Density functional theory calculations clarify that lignin preferentially adsorbs parallel to the phthalocyanine surface, forming pi-pi interactions that facilitate beta-O-4 bond cleavage, the key step for generating valuable aromatic monomers. Furthermore, the central metal tuning modulates the energy barrier with cobalt-centered phthalocyanines exhibiting the lowest barrier and highest activity. These insights provide a molecular-level design principle to overcome the resistance of lignin and establish a cornerstone for sustainable lignocellulosic biomass upcycling.image