Harvesting battery materials from waste soft drinks

Hierarchical carbon structures also show promise for CO₂ capture

Approximately 181 million tonnes of sugar (chemical name: sucrose) were produced worldwide in 2021/22. Typically extracted and refined from sugarcane or sugar beet, sucrose adds sweetness to countless foods and beverages. In addition, thanks to its abundance and relatively low cost, sucrose is viewed as a carbon source by the chemical sector, playing a small-but-growing role in meeting global demand for the element. A team led by the University of Newcastle (Australia) say that sucrose can also be up-converted into hierarchical carbon structures suitable for high-tech applications, using one-pot synthesis. And, as they explain in the latest issue of Carbon [DOI: 10.1016/j.carbon.2023.118085], the feedstock can be a waste material.

In this study, they focused on synthesising carbon structures that could be applied in battery technologies and as a gas storage medium. The carbon feedstock was expired – and sucrose-rich – Coca-Cola®, which was mixed with a small quantity of sulfuric acid. They used KIT-6, a commercially-available mesoporous silica molecular sieve, to act as the hard template. It was filled with the feedstock mixture, along with varying quantities of ZnCl₂ (0.5 g, 0.75 g, 1.25 g and 3.0 g) which acted as the activating reagent. After incubation, the pore-filling and heating steps were repeated. The samples were then carbonized at 900 °C, before undergoing acid treatment to entirely remove the template, and overnight drying. The resulting activated mesoporous carbon materials were labelled AMCK-0.5, AMCK-0.75, AMCK-1.25, AMCK-2.0 and AMCK-3.0, reflecting the quantity of zinc chloride used in their synthesis. A mesoporous sample made without zinc chloride was also synthesised, and labelled MCK.

The authors carried out detailed chemical and physical characterisation of each sample. Techniques included powder X-ray diffraction (XRD), micro-Raman spectroscopy, surface area and porosity analysis, field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). In addition, they obtained near-edge X-ray absorption fine structure (NEXAFS) of the samples using the Soft X-ray Spectroscopy Beamline at the Singapore Synchrotron facility.

XRD analysis confirmed that the materials are partially amorphous. It also showed that the highly ordered structure of KIT-6 was successfully replicated in all AMCK samples, excluding AMCK-3.0. Micro-Raman analysis was used to further investigate the graphitic nature of the AMCK materials. Porosity analysis found that the AMCK-0.5, -0.75 and -1.25 samples displayed a highly ordered structure with uniform mesopores. The surface area of the same three samples was measured at 1565, 1732 and 2003 m² g⁻¹, respectively. Interestingly, the surface area of the AMCK-1.25 sample was seven times larger than that of MCK. The authors say that these results suggest that an “…appropriate amount of ZnCl₂ can carefully activate the mesoporous carbon materials without collapsing its ordered mesoporous nature,” whereas increasing the activating reagent beyond an optimal value “…can easily demolish the highly ordered structure.” Electron microscopy analysis and NEXAFS further highlighted the impact of excess ZnCl₂ on material porosity. The AMCK-0.5, -0.75 and -1.25 samples displayed well-connected pore arrays, whereas AMCK-3.0 exhibited partially broken pore arrays and “remarkably depressed peaks of carboxylic groups.”

Electrochemical measurements were carried out to investigate the performance of the AMCK samples toward Li⁺ ion and Na⁺ ion storage. Electrodes were fabricated by mixing with 80 wt% of AMCK material with binders and spreading them onto copper substrates. These were assembled into a half-cell configuration with a thin polypropylene membrane separator and a lithium- or sodium-based electrolyte (LIB or SIB, respectively). When tested in an LIB, AMCK-1.25 exhibited the highest discharge capacities at a wide range of current densities. The same was true when samples were tested in an SIB – AMCK-1.25 performed optimally.

And finally, the authors used a high-pressure volumetric analyzer to investigate the CO₂ gas adsorption capability of their AMCK samples. This analysis was carried out at pressure range of 1 – 30 bars at 0 °C, and it found that AMCK-1.25 had the highest CO₂ adsorption, which is attributed to its high surface area. This sample was then analysed at two other temperatures (10 °C and 25 °C). They found that while adsorption capacity is the highest at 0 °C, it still acted as an effective carbon dioxide adsorber at elevated temperatures.

The cost of synthesising hierarchical carbon from soft drink waste was estimated to be approximately $105 for 1 g of carbon, which, the authors write, “… is less expensive than highly ordered mesoporous carbon (which costs $210 for 1 g of CMK-3). This demonstrates the economic advantages of utilizing expired soft drink as a carbon precursor in industrial fabrication processes, which could potentially replace the widely used carbon precursor, e.g., sucrose.”

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Stalin Joseph, Gurwinder Singh, Jang Mee Lee, Xiaojiang Yu, Mark BH. Breese, Sujanya Maria Ruban, Suresh Kumar Bhargava, Jiabao Yi, Ajayan Vinu. “Hierarchical carbon structures from soft drink for multi-functional energy applications of Li-ion battery, Na-ion battery and CO2 capture,” Carbon, Volume 210 (June 2023), 118085. DOI: 10.1016/j.carbon.2023.118085