A highly sensitive amperometric hydrazine monitoring sensor offering an ultrawide dynamic range of 5 μM to 1 M in alkaline media (e. g., 1 M KOH) was developed via co-electrodepositing iridium-nickel alloy nanoparticles (NPs) functionalized with multi-walled carbon nanotubes (IrÀ NiÀ MWCNTs) on a disposable screen-printed carbon electrode. The synergistic interaction of MWCNTs with IrÀ Ni alloy NPs resulted in enlarged active surface area, rapid electron transfer, and alkaline media stability with an onset potential of À 0.12 V (vs. Ag/AgCl) toward hydrazine oxidation. A limit of detection for hydrazine was 0.81 μM with guaranteed reproducibility, repeatability, and storage stability alongside a superb selectivity toward ethanolamine, urea, dopamine, NaBH 4 , NH 4 OH, NaNO 2 , and Na 2 CO 3 . The sensor was finally applied to on-site monitoring of the carbonfree hydrazine concentration at the anode and cathode of a hydrazine fuel cell, providing more insight into the hydrazine oxidation process during cell operation.