Exploring the potential of a potassium 4-piperidinolate/4-pyridinolate pair for reversible hydrogen storage

Abstract

Hydrogen storage remains a major challenge for the widespread adoption of hydrogen as a clean and sustainable energy carrier. Chemical hydrogen storage in organic materials offers a feasible solution for safe, reversible, and high-capacity storage. However, the dehydrogenation process typically requires high temperatures due to its endothermic nature. In this study, we present a potassium 4-piperidinolate/4-pyridinolate pair (4-K-pip/4-K-pyr), which has a hydrogen storage capacity of 4.3 wt%, developed using a molecular engineering strategy that combines the effects of a heteroatom and alkali metal substitution within a single molecule to optimize dehydrogenation thermodynamics. Density functional theory (DFT) calculations indicate that this system has a favorable enthalpy change of dehydrogenation (ΔH_d) of 35.15 kJ per mol-H₂. Results from solid-state and aqueous solution experiments demonstrate the system's ability to achieve reversible hydrogen storage under moderate conditions, and most importantly, with excellent stability over multiple cycles in aqueous solution using a single catalyst. A comparison of potassium 4-piperidinolate with other hydrogen-rich compounds that have closely related structures shows that its superior hydrogen desorption performance can be attributed to its favorable ΔH_d, which arises from an effective synergy between the effects of ring nitrogen and potassium as a strong electron-donating substituent. Notably, favorable dehydrogenation thermodynamics, simple synthesis, air stability, and low material cost position the 4-K-pip/4-K-pyr system as a promising candidate for practical hydrogen storage.