Porous ZnO–NiO composite micropolyhedrons have been successfully synthesized by calcination of mixed oxalate (Zn_0.9Ni_0.1(C₂O₄)₂·nH₂O) precursors in air. The oxalate precursor micropolyhedrons were synthesized by a mild chemical precipitation method without any template or surfactant, and found to have a relatively low decomposition temperature. We have successfully explored the application of the resulting porous ZnO–NiO composite micropolyhedrons as electrochemical capacitors. Electrochemical study shows that the obtained ZnO–NiO composites under different conditions have different electrochemical supercapacitor properties in 3.0 or 1.0 M KOH solutions. The porous ZnO–NiO micropolyhedron material (P1) obtained by calcination of the oxalate precursor at 400 °C has a large specific capacitance 649.0 F g⁻¹ in 3.0 M KOH solution and could maintain 99.1% of this value after 400 cycles at 5.8 A g⁻¹. Even at a high current density of 58.0 A g⁻¹, the specific capacitance of P1 is 395.2 F g⁻¹.