Papermaking waste turned into a gel for harvesting heat and sunlight

Low-grade heat is widely available from industrial processes, cooling operations and everyday activities, but efficiently converting these relatively small temperature differences into electricity remains challenging. At the same time, many thermogalvanic systems rely on redox couples containing metal-based components, creating additional considerations for cost, resource availability and environmental sustainability. In a study published in the Journal of Bioresources and Bioproducts, researchers developed a thermogalvanic system based on lignosulfonate (LS), a major component of papermaking black liquor. By combining LS with ammonium persulfate (APS), the researchers established a redox system involving lignin-derived quinone/hydroquinone species. Under a temperature gradient, these reversible reactions generate a thermovoltage. The optimized liquid electrolyte achieved a Seebeck coefficient of 2.92 mV/K. The researchers then incorporated the electrolyte into a poly(vinyl alcohol) matrix through freeze-thaw gelation, producing a quasi-solid LSAK/PVA thermogalvanic gel. At an optimized lignosulfonate concentration, the gel reached a Seebeck coefficient of 4.3 mV/K and a maximum output power density of 976 μW/( m²·K²). An 11-unit series-connected device generated approximately 1.56 V under a 40 K temperature difference and delivered 1.75 μW of power, sufficient to operate small electronic devices such as a timer and calculator.

The study also demonstrated that the material could directly utilize the heat released during black liquor cooling. Heated black liquor generated an open-circuit voltage of about 45 mV during cooling, illustrating the possibility of using the waste stream as both a source of heat and a source of redox-active biomass material. Beyond waste-heat harvesting, the lignin-based gel was tested under simulated sunlight and outdoor conditions. Under an irradiation intensity of 1 kW/m², the gel developed a temperature difference of approximately 25 K and generated an open-circuit voltage of about 0.1 V. A five-cycle test showed stable voltage output, while an outdoor device produced a thermovoltage of 0.33 V after 30 minutes of light exposure. The results point to a material strategy in which a papermaking by-product serves not only as a feedstock for functional materials but also as an active component in energy conversion. The authors suggest that lignin-based thermogalvanic materials could provide a pathway for recovering otherwise underused low-grade thermal energy while adding value to industrial biomass waste.

See the article:

DOI

https://doi.org/10.1016/j.jobab.2026.100304

Original Source URL

https://www.sciencedirect.com/science/article/pii/S2369969826000769

Journal

Journal of Bioresources and Bioproducts