I only found sources (like this ) giving impulse density of hydrazine used as a bipropellant in conjunction with various oxidizers, but I was unable to find any that would give me density impulse of monopropellant hydrazine... does anyone know?
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Well, the density of hydrazine is $1.021 \frac{kg}{L}$, and its $I_{sp}$ is $220 s$ as a mono propellant. (Both values from the Wiki page).
That is a total of $224.6 \frac{kg \cdot s}{L}$, using the same units as your source.