Our proposed ion acceleration scheme, micronozzle acceleration (MNA), generates proton beams with extremely high kinetic energies on the giga-electron-volt (GeV) order. The underlying physics and performance of MNA are studied with two-dimensional particle-in-cell simulations. In MNA targets, a micron-sized hydrogen rod is embedded inside a hollow micronozzle. Subsequent illumination of the target along the symmetric axis by an ultraintense ultrashort laser pulse forms a strong electrostatic field with a long lifetime and an extensive space around the downstream tail of the nozzle. The electric field significantly amplifies the kinetic energies of the accelerated protons, and $$\gtrsim$$ GeV protons are generated at an applied laser intensity of $$10^{22}$$ W/ $$\hbox {cm}^2$$ .