Sulfur‑Driven Reconstruction of Ni Foams into NiOOH for Lignin Electro‑Depolymerization

The transition toward renewable carbon platforms has intensified interest in lignin, a complex biopolymer whose aromatic richness remains largely underexploited due to its structural heterogeneity. Developing catalytic systems capable of activating lignin under mild and sustainable conditions is therefore a central challenge in biomass valorization. Electrochemical oxidation has emerged as a promising strategy, enabling selective C–C bond cleavage through the generation of high‑valent metal‑oxide species. However, its efficiency critically depends on electrodes that can form and stabilize these oxidizing phases in situ.

In this work, we introduce a sulfur‑modified nickel foam (S/NF) that undergoes self‑reconstruction during electrolysis, yielding a highly active NiOOH surface for lignin depolymerization. Operando Raman spectroscopy reveals that sulfur incorporation promotes the early formation of NiOOH vibrational signatures at substantially lower potentials. Complementary depth‑profile XPS confirms the emergence of a core–shell structure, where a reconstructed NiOOH overlayer encapsulates a Ni₃S₂/Ni core. This architecture accelerates Ni²⁺/Ni³⁺ redox kinetics and increases the electrochemically active surface area by a factor of 2.2.

These enhancements translate into superior catalytic performance. S/NF achieves almost totally conversion (92 %)of the lignin model dimer veratryl‑glycerol with 62% selectivity toward veratric acid, demonstrating efficient C–C bond cleavage to carboxylic products. Beyond model substrates, S/NF effectively depolymerizes pine lignin, producing ~1.5 wt% of vanillin and vanillic acid in an H‑cell, as validated by GC‑MS, HSQC‑NMR, and GPC analyses.

Overall, this study positions sulfur‑assisted reconstruction as a powerful design principle for generating highly active NiOOH surfaces tailored for the electrochemical valorization of lignin and other bio‑derived aromatic streams.