Turning Polluted Biomass Waste into Sustainable Carbon-Based Catalysts for Hydrogen Production via Water Electrolysis

Abstract

The development of highly efficient, effective, and low-cost carbon-based catalysts for hydrogen production through water electrolysis represents a significant challenge in sustainable energy conversion. In this work, carbon materials derived from biomass waste, specifically a metal-polluted vegetal species () from a former mining location, were used. Biomass was subjected to hydrothermal carbonization, producing hydrochar. The influence of both thermal and chemical post-treatment was studied in relation to hydrogen production efficiency. The thermal treatment was conducted at 300, 500, and 1000 °C, while the chemical precursors used were KOH and H₃PO₄. Additionally, these waste-derived carbon materials were compared with carbon Vulcan XC-72, a common reference material in these processes originated from fossil sources. Several electrochemical techniques were employed to evaluate and identify the most suitable sample for the hydrogen evolution reaction (HER). Additionally, physicochemical characterization analyses were conducted to gain a comprehensive understanding of the morphology, composition, and surface structure of the biomass-derived carbon materials, as well as to establish correlations with their electrochemical behavior toward the HER. The sample that demonstrated the most favorable performance was the one chemically activated with KOH, which exhibited an outstanding Tafel slope (147 mV/dec) and a low overpotential at 10 mA/cm² (-550 mV vs RHE) surpassing even the commercial Vulcan XC-72 sample. Furthermore, the chronoamperometry test showed a very stable performance for this sample. These results demonstrate that plant biomass waste containing metals presents a viable alternative to carbon blacks, commonly used as electrocatalysts for hydrogen production, also providing an efficient and sustainable method to valorize these wastes.

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High-resolution SEM images for (a) 130/hydro, (b) 130/300, (c) 130/500, (d) 130/1000, (e) 130/600 KOH, (f) 130/600 H₃PO₄.

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(a) XRD patterns, (b) RAMAN spectra and (c) FT-IR spectra of contaminated biomass catalysts.

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Core level high-resolution C 1s (a), O 1s (b) and N 1s (c) XPS spectra of contaminated biomass catalysts.

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(a) LSV curves for HER using a potential ramp from −800 to −1500 mV vs Hg/HgO, using a scan rate of 5 mV/s of treated biomass catalysts and CVulcan; (b) Tafel curves for 130/500, 130/1000, 130/600 KOH, 130/600 H₃PO₄ and CVulcan; (c) onset potential for 130/1000, 130/600 KOH, 130/600 H₃PO₄ and CVulcan at −2 mA/cm² and (d) overpotential to generate a current density of 10 mA/cm for 130/1000 and 130/600 KOH.

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CV plots recorded at scan rates ranging from 5 to 100 mV/s with a 0.1 M NaOH + 1 M Na₂SO₄ solution for (a) CVulcan and (b) 130/600 KOH. (c) Current density vs scan rate determined from CVs of the contaminated biomass catalysts and CVulcan with double layer capacitance C _dl in each electrode. (d) ECSA values in cm².

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(a) Nyquist plots of the contaminated biomass catalysts and CVulcan with the equivalent circuit and (b) the same figure enlarged for better visibility.

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(a) CA stability studies for contaminated biomass catalysts and CVulcan during 1 h with a 0.1 M NaOH + 1 M Na₂SO₄ solution at −1400 mV vs Hg/HgO; (b) degradation percentage of each electrode, and (c) CA stability studies for 130/600 KOH during 24 h with a 0.1 M NaOH + 1 M Na₂SO₄ solution at −1400 mV vs Hg/HgO.