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 classified 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.