Journal Article electrocatalysis iridium water splitting oxygen evolution reaction composite materials Research Support, Non-U.S. Gov't Research Support, U.S. Gov't, Non-P.H.S. Review active site active sites alloying composite material electronic structures energy energy conversion fuel cell hydrazine electrooxidation large current density layered material metal-free catalyst nitrogen and oxygen co-doped mesoporous carbon open framework structures oxygen evolution reaction, electrocatalysis polypyrrole porous materials solid solution