High voltage supercapacitor has attracted great attentions. In this study, low-cost, environmentally friendly “water in salt (WIS)” electrolyte was introduced to replace the expensive and toxic of organic and ionic electrolyte. Very recently, potassium acetate (CH₃CO₂K) electrolyte was found to be able to replace the LiTFSI in LiB. Moreover, CH₃CO₂K is safer and cheaper than LiTFSI-based WIS electrolytes. Therefore, in this study, we open up new opportunities of using acetate based WIS electrolyte in the supercapacitor. We investigated several acetate based WIS, such as CH₃CO₂K, CH₃CO₂Na, CH₃CO₂Li, and its combination. The result shows that mixing two WIS leads to wider potential window up to 2.2V. We investigated the performance of CH₃CO₂K-CH₃CO₂Li WIS in layered iron vanadate nanocomposite. The synergistic effect of unique properties of layered iron vanadate nanocomposite and bi-WIS resulting in high energy density of 53 Wh kg^-1 and power density of 10 kW kg^-1. This environmentally friendly electrolyte is very promising and expands the scope of aqueous electrolyte for next generation of supercapacitor.