Sustainable utilization of lignite through mild depolymerization and electro-oxidation for oxygenates and green hydrogen co-production

Beyond its traditional role as a fuel, lignite represents an underutilized carbonaceous feedstock, calling for innovative and sustainable conversion pathways. To unlock its potential while addressing the inherent challenges posed by its high moisture and oxygen content, this study presents an integrated strategy by combining mild depolymerization with electro-oxidation (EO) in an aqueous solution for the co-production of oxygenated chemicals and green hydrogen. Meanwhile, a highly active NiCoOOH-CuO/CF anode was fabricated to selective EO of lignite and its depolymerized products in alkaline media. Experimental results show that the integration of mild depolymerization with EO significantly improves the yield of humic acid (HA) compared to the direct EO of lignite. Specifically, the integration of alkali depolymerization (AD) with EO achieved HA yields of 70.53 %, 61.14 % and 54.61 % for Baoqing (BQ), Mile (ML) and Zaotong (ZT) lignites, respectively. This approach not only lowers the required operational potential required for EO but also improves the selectivity toward target products. The resulting HA could be further converted into fulvic acid (FA) via EO with over 80 % selectivity and exceeding 97 % H2 Faraday efficiency. The NiCoOOH-CuO/CF anode exhibited exceptional catalytic activity, requiring only 1.52 V vs. RHE to achieve 200 mA/cm2 during HA electrolysis. Importantly, the integrated process attained a total carbon recovery exceeding 90 % with negligible CO2 emissions, substantially outperforming traditional chemical oxidation routes. This study provides a novel and sustainable pathway for the simultaneous production of high-value oxygenates and green hydrogen, demonstrating significant potential for resource-efficient and low-emission utilization of lignite.