Numerous transition metal-carbon composite catalysts (M = V. Zn, Ni, Sn, Ce, Ba, Fe, Cu) have been synthesized and tested for electroreduction of O-2 to H2O2, The activity and selectivity of all synthesized catalysts for electrosynthesis of H2O2 were determined by the rotating ring-disk electrode method in acidic and neutral electrolytes. The Co-based catalysts in general showed the highest activity towards H2O2 formation. Experiments with different loading contents of Co showed that the activation overpotential losses of oxygen reduction to H2O2 reduces as loading increases to about 4 wt% Co. Addition of Co beyond this level did not seem to impact the overpotential losses. The cobalt-based catalysts, were spray-coated onto 120 mu m thick Toray (R) graphite substrates, and were studied in bulk electrolysis cells for up to 100 h at potentiostatic conditions (0.25 V vs. RHE) in pH 0, 3. and 7 electrolytes. At (25 degrees C and 1 bar) with a catalysts loading of about 1 mg cm(geometric)(-2) and using dissolved O-2 in 0.5 M H2SO4, typical H2O2 electrosynthesis rates of about 5 mu mol h(-1)cm(geometric)(-2) were reached with current efficiencies of about 85 +/- 5% at 0.25 V (vs. RHE). (C) 2011 Elsevier Ltd. All rights reserved.