Journal Article oxygen reduction reaction fuel cells M–N–C Research Support, Non-U.S. Gov't Research Support, U.S. Gov't, Non-P.H.S. Review carbon-based metal-free electrocatalysts conjugated microporous polymers covalent organic frameworks hydrogen storage hydrogenation metal organic frameworks metal-free electrocatalysts metal-nitrogen-carbon multidoping nitrogen doping nonprecious metal catalysts single‐atom alloy single‐atom catalyst stability storage stability supercapacitor