Optimized Charge Storage in Aza-Based Covalent Organic Frameworks by Tuning Electrolyte Proton Activity

Proton activity in electrolytes plays a crucial rolein decidingthe electrochemical performance of aqueous batteries. On the one hand,it can influence the capacity and rate performance of host materialsbecause of the high redox activity of protons. On the other hand,it can also cause a severe hydrogen evolution reaction (HER) whenthe protons are aggregated near the electrode/electrolyte interface.The HER dramatically limits the potential window and the cycling stabilityof the electrodes. Therefore, it is critical to clarify the impactof electrolyte proton activity on the battery macro-electrochemicalperformance. In this work, using an aza-based covalent organic framework(COF) as a representative host material, we studied the effect ofelectrolyte proton activity on the potential window, storage capacity,rate performance, and cycle stability in various electrolytes. A tradeoffrelationship between proton redox reactions and the HER in the COFhost is revealed by utilizing various in situ and ex situ characterizations.Moreover, the origin of proton activity in near-neutral electrolytesis discussed in detail and is confirmed to be related to the hydratedwater molecules in the first solvation shell. A detailed analysisof the charge storage process in the COFs is presented. These understandingscan be of importance for utilizing the electrolyte proton activityto build high-energy aqueous batteries.