Tuning of adsorbent properties - oxidative hydrophilization of activated carbon monoliths for heat storage applications

In this work, methods and processes for the production of hydrophilized and hydrothermally stable activated carbon honeycomb structures based on cost-efficient raw materials for the use in heat storage applications are presented. As a reference, activated carbon (AC) was hydrophilized using nitric acid. In a second step, AC monoliths with a channel substructure were prepared by an extrusion process and consequently oxidized using either nitric acid or ozone gas in order to restore microporosity and create hydrophilic groups on the inner surface of the material. The influence of the shaping and hydrophilization procedures towards substrate chemistry, pore structures, equilibrium water adsorption capacity, thermophysical properties such as heat capacity and conductivity as well as the multi-cycle ad-/desorption stability are comprehensively discussed. The successful hydrophilization by the nitric acid treatment was demonstrated for a powder AC sample. After 4 h of treatment a water adsorption capacity of 0.28 gg−1 at relative pressures p/p0 = 0.4 was observed, which represents a multiplication of capacity by 50 or 70 times compared to the raw material. In case of the AC monoliths, a capacity of 0.12 gg−1 at p/p0 = 0.4 is achieved by nitric acid treatment compared to 0.07 gg−1 for the ozone treatment.