The material is the same than in the RO membranes so you need a preservative and a biocide: "To prevent microbial growth, the membranes can be stored in a 0.5% solution of formaldehyde.  Alternatively, the membranes can be stored in a 1.0% solution of sodium metabisulfite (SMBS).  To maintain efficacy, the SMBS solution should be replaced monthly.  Or the membranes can be stored in UPDI water that is replaced weekly."(The cite is from a trader of the material but is correct, you can alway make a verification.) I hope it could be useful.

Thanks you