The safe and efficient storage of hydrogen is a critical hurdle to widespread adoption of this clean energy source. While metal hydrides offer high storage capacity, they are expensive and heavy. Carbon materials, being lightweight and cost-effective, offer a promising alternative, but they face challenges in achieving strong enough hydrogen-carbon interactions for optimal release under working conditions. In a study published in the International Journal of Hydrogen Energy in 2024, Giovanni-Mondragón et al. addressed this by transforming disposable diaper waste—specifically, the superabsorbent polymer (SAP)—into a nanostructured, high-surface-area activated carbonActivated carbon is a form of carbon that has been processed to create a vast network of tiny pores, increasing its surface area significantly. This extensive surface area makes activated carbon exceptionally effective at trapping and holding impurities, like a molecular sponge. It is commonly More (AC) for electrochemical hydrogen storage. This process not only valorizes an abundant, environmentally problematic waste material but also creates a high-performance electrode.

The researchers used the Box-Behnken experimental design to optimize the chemical activation conditions for the carbonized SAP (SAPC) precursor. The analysis of variance (ANOVA) determined that the activation temperature and the KOH:SAPC molar ratio were the most significant factors influencing both the carbon’s textural characteristics and its hydrogen storage capacity. The optimal material, designated R9, was produced at 700∘C, 1 hour, and a KOH:SAPC ratio of 2.5. This sample exhibited an impressively high BET surface area of 2555 m2/g (or 2552 m2/g as mentioned in the conclusion) and a substantial total micropore volume of 0.880 cm3/g (0.871 cm3/g in conclusion). This abundance of micropores and high surface area are critical factors for achieving high hydrogen uptake

The best electrode material (R9) demonstrated an exceptionally high specific capacity of 1878 mAh/g, which corresponds to a gravimetric hydrogen storage capacity of 6.60 wt% H2​. This result ranks among the best reported for activated carbon materials andsurpasses the U.S. Department of Energy (DOE) target of 5.6 wt% H2​ for onboard vehicle applications. Galvanostatic Charge/Discharge (GCD) curves exhibited a nearly linear and symmetrical behavior, which indicates good reversibility and minimal solution resistance. Crucially, the GCD tests revealed that higher current densities of 1 and 2 A/g could achieve a faster release of stored hydrogen in approximately 3 to 5 minutes, thus meeting a key DOE standard for charging/discharging times. Cyclic Voltammetry (CV) analysis confirmed that the primary mechanism for hydrogen storage is the electric double layer (EDLC) formation, which facilitates charge accumulation at the carbon’s surface.

The overall findings, supported by a statistically significant model with an R2 value of 0.9460, strongly confirm the feasibility of using this waste-derived carbon for efficient and sustainable electrochemical hydrogen storage.

Source: Giovanni-Mondragón, C., Lobato-Peralta, D. R., Okolie, J. A., Arias, D. M., Orugba, H. O., Sebastian, P. J., & Okoye, P. U. (2024). Electrochemical hydrogen storage in high surface area microporous carbon from disposable diaper waste. International Journal of Hydrogen Energy, 50, 1369–1380.