Room temperature hydrogen storage enhancement in copper-doped zeolitic imidazolate frameworks with trioctylamine

Abstract

Physisorption materials provide an opportunity for safe, reversible and cost-effective hydrogen storage under near-ambient conditions. In this work, zeolitic imidazolate frameworks (ZIF-8 and ZIF-8-T) and copper-doped ZIFs (CuZIF-8 and CuZIF-8-T) were efficiently synthesized for hydrogen storage at room temperature using trioctylamine (TOA) as a structure directing agent. The as-synthesized adsorbents were subjected to material characterization and found to exhibit rhombic dodecahedral shapes with particle sizes ranging from 100 to 550 nm and suitable crystalline parameters. X-ray photoelectron spectroscopy revealed a higher concentration of nitrogen–metal(s) functional groups for the Cu doped TOA modulated sample (CuZIF-8-T), suggesting the maximum number of organic ligands coordinated to zinc or copper during the framework formation. As a result, CuZIF-8-T showed a greater specific surface area of 1973.7 m² g⁻¹ and micropore volume of 0.72 cm³ g⁻¹, measured by N₂ adsorption at 77 K, indicating the formation of more adsorption sites compared to the other samples. Collectively, these features enhanced the hydrogen storage capacity (0.70 wt%) of this adsorbent, followed by ZIF-8-T, CuZIF-8 and ZIF-8 at 25 °C and 100 bar. Finally, the repeatability test of CuZIF-8-T revealed only a 3% reduction in hydrogen adsorption with 100% reversibility even after ten adsorption/desorption cycles. Overall, the present findings demonstrate a simple and cost-effective synthesis approach to modify ZIF-8 materials for enhanced physisorption based hydrogen storage properties at room temperature.