Inhibiting hydrogen evolution reaction by adjusting electrophilicity of quaternary ammonium salts for aluminum-air battery

Aluminum-air battery (AAB), as a kind of chemical power supply, exhibited great application potential owing to its high energy density, low cost and high safety. However, its further development was restricted by severe hydrogen evolution reaction (HER). To address this issue, hybrid additives including of ZnO and quaternary ammonium salts (QAS, denoted as C1, C4, C8, C12 and C16, respectively, depending on the length of single alkyl group) were introduced into the electrolyte in this work. The results showed that the capacity density was improved with the increasing length of single alkyl group as well as its discharge voltage. The addition of QAS regulated the electrochemical environment on Al anode surface, where "physical-hydrophobic electrochemical interface" was created by the non-polar group due to its electrophilicity. Simultaneously, polar group could form strong H-bonds with H2O molecules and reduce the activity of free water, thus constructing "chemical-hydrophobic electrochemical interface". Meanwhile, QAS also promoted the uniform growth of Zn-based film, improving its structural integrity, as a barrier against H2O. As a result, HER was effectively inhibited to improve the capacity density up to 2470 mAh/gAl, corresponding to anodic utilization efficiency of 82.9 %. This work not only revealed the effect of electrophilicity on electrochemical environment around Al anode as well as the inhibition of HER, but also provide a guidance for the selection rule of organic additives to inhibit side reaction in aqueous batteries.