I am not a tax expert,but i have been doing some looking around on the web and it seems as there might be a few possibilities first:

http://www.nationalvapor.com/f8911.pdf

secondly

http://www.irs.gov/pub/irs-pdf/f8849s3.pdf

Not sure if anyone else has looked into this very deeply , but it seems to me that if you can get a credit for BIO fuel they should allow one for hydrogen ( the 8911 actually does mention it i believe).Any thoughts or ideas ?