Tailoring Non-Polar Groups of Quaternary Ammonium Salts for Inhibiting Hydrogen Evolution Reaction of Aluminum-Air Battery

Aluminum-air battery (AAB) with alkaline-based aqueous electrolyte have attracted intensive research interests due to high-capacity density, low cost, and high safety. However, severe hydrogen evolution reaction (HER) of Al anode in alkaline electrolyte extremely restricts its large-scale application. In this work, ZnO with a group of quaternary ammonium salts (QAS, denoted as C1, C2, C4, C6, and C8 depending on the length of non-polar group) are introduced into electrolytes to inhibit HER. It is revealing that capacity density initially increases with C1, C2, or C4, up to 2564 mAh gAl-1 with anodic utilization rate of 86.0%, and then follows by a decline (C6 and C8). The addition of QAS creates "physical-hydrophobic interface" by non-polar group owing to its electrophilic property and constructs "chemical-hydrophobic interface" with polar group by reducing water activity. QAS also promotes the uniform growth of Zn-based film, as a barrier against H2O. Thus, HER is effectively inhibited to improve the capacity density. With further increasing non-polar group length (C6 or C8), the bonding status of the film deteriorates with enhanced H2O activity, leading to the recurrence of HER. This work explores the effect of non-polar groups on inhibiting HER and opens the door to stable anodes for alkaline-based batteries. The quaternary ammonium salt (QAS) creates "physical-hydrophobic interface" by non-polar group owing to electrophilic property, and constructs "chemical-hydrophobic interface" with polar group by reducing water activity. QAS also promotes the uniform Zn-based film as a barrier against H2O. Thus, hydrogen evolution reaction (HER) is inhibited. However, excessive non-polarity of QAS (C6, C8) decreases the bonding of film and HER recovers. image