A flow reactor system for providing on-demand H.sub.2 evolution at pressure from a liquid organic hydrogen carrier and/or blends thereof includes a reactor that includes a reaction vessel having an inlet and outlet. The inlet is configured to introduce reactants into the reaction vessel, and the outlet is configured to release reaction products. The reaction vessel is configured to hold therein a catalyst system capable of catalyzing the evolution of molecular hydrogen from a liquid organic hydrogen carrier. Advantageously, the reaction vessel is configured to operate at pressures greater than or equal to 50 psig (e.g., from about 50 psig to about 10500 psig. The flow reactor system also includes a source of preheated liquid organic hydrogen carrier in fluid communication with the reactor and a purification system in fluid communication with the outlet that provides purified molecular hydrogen gas for on-demand applications.