We report experimental observations on the temporal dynamics in the electrocatalytic. oxidation of formic acid (HCOOH) on a polycrystalline Pt electrode modified by deposition of bismuth. Bismuth modification significantly enhanced the current density of HCOOH oxidation, since it suppressed the poisoning branch and increased the apparent direct oxidation rate. Impedance spectroscopy and the galvanostatic scan in HCOOH oxidation on Bi/Pt exhibited a hidden negative differential resistance and a Hopf bifurcation. The electrocatalytic oxidation of HCOOH on Bi/Pt spontaneously underwent transitions from homogeneous catalytic activity to spatiotemporally inhomogeneous distributions of the interfacial electrode potential, in the form of traveling pulses of the interfacial potential.