A method of keeping an aqueous solution of sodium borate liquid at a storage temperature, in which method, in order to pass said solution from an initial temperature to the storage temperature, said aqueous solution of sodium borate is subjected to heat treatment comprising at least one cooling or heating operation at a speed lying in the range 1° C./min to 100° C./min, to reach a holding temperature lying in the range −50° C. to +200° C., followed by holding the holding temperature for a time lying in the range 1 s to 100 h, followed by cooling or heating at a speed lying in the range 1° C./min to 100° C./min. Use of the method in a method of supplying hydrogen for a fuel cell.