A mini-review of noble-metal-free electrocatalysts for overall water splitting in non-alkaline electrolytes

Development of noble -metal -free materials with remarkable electrocatalytic water -splitting performance in acidic or neutral media has sparked considerable attention in recent years. Herein, we review the latest research on design and fabrication of precious -metal -free catalytic materials for overall water electrolysis in non -alkaline environment, especially highlighting several optimizing approaches to enhance the catalytic behavior and to realize effective bifunctional electrocatalysts. All these involved noble -metal -free electrocatalysts are classi fied into transition -metal oxides (TMOs), transition -metal nitrides (TMNs), transition -metal carbides (TMCs), transition -metal phosphides (TMPs), transition -metal chalcogenides, metal complexes, and metal -free carbons, as shown in the main part. Besides, the paper also offers an introduction of the fundamental electrochemistry of water splitting before entering the subject, as well as a prospective discussion on mechanism understanding, novel catalysts fabrication, and standardized performance measurements/evaluation in the last section.