Abstract <p>Operation of a single cell of redox-flow hydrogen–halogenate power source converting the energy of the oxidation reaction of gaseous hydrogen by sodium chlorate in aqueous sulfuric-acid solution into electric energy with the use of a following membrane–electrode assembly: (–) H₂, Pt–C//PEM//NaClO₃, C (+) is studied. A load combined operation regime (which includes stages of potentiostatic and galvanostatic control) is applied, in order to take into account specific features of the chlorate electroreduction half-reaction, i.e., its redox-mediator autocatalytic mechanism (EC-autocat). For aqueous electrolytes containing various sulfuric acid contents, the system parameters determining the power and efficiency of the hydrogen–chlorate power source are found: the Faradaic and energy efficiencies, the average discharge power, and the time necessary for the reaching of the steady-state mode. The hydrogen–chlorate cell under study is found to function most efficiently with 5 M sulfuric-acid electrolyte: it reached the current density 0.25&#xa0;A/cm² within 1.5 min; it can convert chemical energy into electric one with the 55%-efficiency at the average specific discharge power of 0.23 W/cm².</p>