In this work, we demonstrate the power of a simple top-down electrochemical erosion approach to obtain Pt nanoparticle with controlled shapes and sizes (in the range from ~ 2 to ~ 10 nm). Carbon supported nanoparticles with narrow size distributions have been synthesized by applying an alternating voltage to macroscopic bulk platinum structures, such as disks or wires. Without using any surfactants, the size and shape of the particles can be changed by adjusting simple parameters such as the applied potential, frequency and electrolyte composition. For instance, application of a sinusoidal AC voltage with lower frequencies results in cubic nanoparticles; whereas higher frequencies lead to predominantly spherical nanoparticles. On the other hand, the amplitude of the sinusoidal signal was found to affect the particle size; the lower the amplitude of the applied AC signal, the smaller the resulting particle size. Pt/C catalysts prepared by this approach showed 0.76 A/mg mass activity towards the oxygen reduction reaction which is ~ 2 times higher than the state-of-the-art commercial Pt/C catalyst (0.42 A/mg) from Tanaka. In addition to this, we discussed the mechanistic insights about the nanoparticle formation pathways.