Challenges and perspectives of hydrogen evolution-free aqueous Zn-Ion batteries

Rechargeable aqueous Zn-ion batteries (ZIBs) featuring the advantages of high safety, low cost, environmental friendliness, and satisfactory energy density have been considered as one of the most promising candidates for next-generation energy storage systems. However, Zn anode issues of dendrite growth, hydrogen evolution reaction (HER), and side reaction can not only shorten the lifespan of ZIBs, but also even raise the safety risks, both of which largely hinder their large-scale practical applications. Although great efforts have recently been devoted to suppressing Zn dendrites and side reactions for stable Zn metal anode, inadequate attentions are received in regard to the side of HER, which is derived from the thermodynamic instability of Zn metal in aqueous electrolytes, triggering cell expansion and safety hazards of ZIBs. In this review, the fundamental principle of HER on the surface of Zn anode in neutral or mild acidic aqueous electrolyte is firstly revealed, together with the interconnections between HER, side reaction, and dendrite growth. The currently existing strategies for suppressing HER on the surface of Zn anode are then comprehensively summarized. Finally, perspectives on the modification and innovation directions of future Zn anode are presented for the large-scale application of ZIBs.