Hydrolysis of metal borohydrides in the presence of CO2 has not been studied so far, although carbon dioxide contained in air is known to accelerate hydrogen generation. KBH4 hydrolysis promoted by CO2 gas put through an aqueous solution was studied by time-resolved ATR-FTIR spectroscopy, showing a transformation of BH4− into B4O5(OH)42−, and a drastically accelerated hydrogen production which can be completed within minutes. This process can be used to produce hydrogen on-board from exhaust gases (CO2 and H2O). We found a new intermediate, K9[B4O5(OH)4]3(CO3)(BH4)·7H2O, forming upon hydrolysis on air via a slow adsorption of the atmospheric CO2. The same intermediate can be crystallized from partly hydrolyzed solutions of KBH4 + CO2, but not from the fully reacted sample saturated with CO2. This phase was studied by single-crystal and powder X-ray diffraction, DSC, TGA, Raman, IR and elemental analysis, all data are fully consistent with the presence of the three different anions and of the crystallized water molecules. Its crystal structure is hexagonal, space group P-62c, with lattice parameters a = 11.2551(4), c = 17.1508(8) Å. Formation of the intermediate produces 16 mol of H2 per mole of adsorbed CO2 and thus is very efficient both gravimetrically and volumetrically. It allows also for an elimination of carbon dioxide from exhaust gases.