Photothermal conversion and geochemical characterization of sulfur-rich lignite for non-conventional energy applications

Lignite has emerged as a critical material in contemporary energy portfolios, particularly in electricity generation. However, this work explores lignite's potential beyond conventional uses, exploring on its energy conversion applications. In a pioneering move, lignite samples have been directly utilized as materials for photothermal conversion applications, along with a description of their geochemical features, such as sulfur compositions, forms, and microscopic characteristics. Among the twenty lignite samples that were geologically studied, two sulfur-rich samples (CS-1 and CS-2) were chosen to explore their photothermal conversion performance. The powder-XRD diffraction patterns of CS-1 and CS-2 reveal the presence of the hexagonal phase of carbon C₁ with kaolinite (Al₂Si₂O₅(OH)₄). Their XPS spectra indicate that both organic and metal-bonded sulfide moieties are present in the lignite samples. CS-1 and CS-2 were implemented under 1 Sun irradiation, and it was found that CS-2 exhibits superior light-absorbing properties, resulting in enhanced water evaporation rates. In addition, the photothermal imaging also shows a temperature increase to 58.2 °C within 10 min for lignite-coated membranes, compared to 31 °C for the blank under similar conditions. These findings can be leveraged to explore lignite's untapped potential in various technological domains, propelling the global transition towards cleaner energy solutions.