Synthesis and catalysis of copper sulfide/carbon nanodots for oxygen reduction in direct methanol fuel cells

► Cu2−xS (Cu2S and Cu9S5)/carbon dots (C dots) were prepared. ► The Cu2−xS/C dot electrode is active for oxygen reduction reaction in acidic media. ► The onset potential for the Cu2−xS/C dot electrode is 0.92 V (vs. Ag/AgCl). ► The Cu2−xS/C dot electrode is durable and cost-effective. ► The Cu2−xS/C dot is potential catalyst for direct methanol fuel cells.