For comparison sake, 6.6 gallons of gasoline in a light vehicle that gets 35 miles per gallon would have a range of 231 miles. At the current national average retail gasoline price of $2.42/gallon, the fuel cost per mile is ($2.42/35 miles) = $0.069/mile.
Of course the attraction of a vehicle powered by hydrogen is that there are no carbon emissions from the tailpipe. And, depending on the energy source used to produce the potassium borohydride, it has the potential to be an overall (nearly) zero carbon transportation option.
There are multiple classes of hydrogen storage options that can store more hydrogen than potassium borohydride. Just among the borohydrides, potassium borohydride can store 7.4 weight percent hydrogen, while lithium borohydride can store 18.3 weight percent. But lithium borohydride would not work in their system, as it reacts immediately on contact with water.
In conclusion, there are no huge technical problems that would prevent such a system from working as advertised. Range may be a bigger concern, but the upside is another potential transportation option that doesn’t require fossil fuels.