Electro-oxidative upgrading of lignite alkali-soluble derivatives for clean production of fulvic acids using NiCo-LDH@NiC2O4/NF anode†

Abstract

Utilizing renewable electricity and cost-effective carbon resources to facilitate water electrolysis for hydrogen production represents a promising and efficient technology for energy storage and conversion. Anode valorization via selective oxidation can produce valuable chemicals, offering a cleaner alternative to chemical oxidation. In this study, to upgrade lignite alkali-soluble derivatives (ASD), a novel NiCo-LDH@NiC₂O₄/NF electrode was developed using a continuous electrochemical deposition method. When employed as the anode, this electrode achieved an 85% conversion of mandelic acid (MA) to benzoic acid with over 95% selectivity at a constant potential of 1.5 V (vs. RHE) in a 1.0 M KOH solution over 1 h. Concurrently, 75% of ASD was successfully upgraded to fulvic acid (FA) with a selectivity exceeding 90%. Compared to NiCo-LDH/NF and NiC₂O₄/NF electrodes, the NiCo-LDH@NiC₂O₄/NF demonstrated superior electro-catalytic activity and stability. NiCo-LDH is the main active phase, while the pre-deposited NiC₂O₄ significantly enhances both its catalytic activity and stability. The degradation of MA involves the cleavage of the ArC(OH)–C bond due to OH activation, whereas the depolymerization of ASD into FA is primarily attributed to the ring-opening oxidation of aromatic nuclei. The present work provides a green and efficient electro-oxidation strategy, coupled with a novel non-precious metal electrocatalyst, to produce coal-based aromatic carboxylic acids, marking a significant step towards more sustainable and environmentally friendly coal-based chemical manufacturing.