Preparation, characterization, and investigation of H2 storage capacities of Pruussian Blue analogues

Abstract

A convenient and efficient hydrogen storage system is a research challenge since hydrogen storage is one of the most critical steps in hydrogen economy. The media where hydrogen molecule is stored is important not only because of safety but also to generate maximum energy and consume minimum energy during the adsorption-desorption process. Porous coordination polymers have been considered as convenient hydrogen sorbents among other hydrogen storage systems due to their structural properties. Research in this field focused also on porous Prussian Blue Analogues incorporating [Fe(CN)6] 3- and [Fe(CN)5NO]3- building blocks due to their robustness at extreme conditions (e.g. high temperature and high pressure),

easy preparation, and versatilities. The project discussed herein focuses on the preparation of five new Prussian Blue analogues incorporating [Fe(CN)5NH3] 3- and [Fe(CN)5pyrazine]3- building blocks. It is aimed to improve the porosity and increase the number of polar functional groups on the pore surfaces by substituting one of the cyanide groups with nitrogen donor ammonia or pyrazine group. M3[Fe(CN)5NH3]2 and M3[Fe(CN)5pz]2 type of Prussian Blue analogues with M= Cu2+, Ni2+, Co2+ were prepared and characterized by FTTR, XRD and TGA techniques. The effects of the ligands bound to central cation(Fe2+) and the type of the outer cation in the framework on the CN- stretching mode in IR spectrum have been studied. Cu3[Fe(CN)5NH3]2 , Ni3[Fe(CN)5NH3]2,and Co3[Fe(CN)5NH3]2 compounds have been shown to adopt Fm3mtype structure similar to classic Prussian Blue analogues. Cu3[Fe(CN)5pz]2 compound has exhibited four diffraction angles which belong to Fm3m space group and two additional peaks when Ni3[Fe(CN)5pz]2 has exhibited relatively weak diffraction pattern belonging to Fm3m space group. Thermal stabilities have been discussed and framework changes were observed with increasing temperature. TGA, IR, and XRD techniques have been performed to investigate the structural changes in target materials at different degassing temperatures. The adsorption profiles of nitrogen and hydrogen have been investigated at two different temperatures based on TGA results. Surface areas and hydrogen uptake capacities of five new Prussian Blue analogues have been studied. Co3[Fe(CN)5NH3]2 showed the highest surface area(243.2 m2 /g) while Ni3[Fe(CN)5NH3]2 showed the highest hydrogen uptake capacity(1.79 wt % H2) at 95°C among the studied materials. Cu3[Fe(CN)5pz]2 and Ni3[Fe(CN)5pz]2 compounds were observed to exhibit no hydrogen uptake. It is also observed that surface areas of Co3[Fe(CN)5NH3]2 and Ni3[Fe(CN)5NH3]2 compounds increased, surface areas of other three compounds decreased when they are degassed at 225°C. Hydrogen storage capacity is also increased from 1.07 wt % to 1.85 wt % significantly for Co3[Fe(CN)5NH3]2 by increasing the degassing temperature.