Advancement of Platinum (Pt)-Free (Non-Pt Precious Metals) and/or Metal-Free (Non-Precious-Metals) Electrocatalysts in Energy Applications: A Review and Perspectives

The recent advances in platinum-free and/or metal-free electrocatalysts for electrochemical redox reactions at the electrodes of fuel cells in particular and, broadly speaking, energy devices were thoroughly reviewed. This comprehensive review focuses on nonplatinum precious metal (Pt-free) and/or nonprecious metal (metal-free) electrocatalysts regarding the electrochemical redox reactions at the electrodes of the energy devices. The recent development of electrocatalysts governing the electrochemical redox reaction mechanisms at the electrodes of the fuel cells/energy devices using density functional theory (DFT) methods was overviewed. The understanding of the following were overviewed theoretically in detail: the site of intermediates (oxygen, hydrogen, water, hydrogen peroxide, etc.); adsorption electrocatalysis via oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), as well as bifunctional electrocatalysis such as ORR/OER and OER/HER; trifunctional electrocatalysis (ORR, OER, HER); and multifunction catalysis. The critical issues, challenges, and perspectives on the further development of less-expensive and highly efficient (active and stable) electrocatalysts were provided.