Deep Eutectic Mn(NO₃)₂−H₂O Binary System as a Safe, Cost‐Effective, and Efficient Electrolyte for Supercapacitor Applications

Abstract

Deep eutectic solvents (DESs) have proved to be an effective substitute for conventional electrolytes for energy storage applications because of their unique properties and ability to adapt them to specific applications. In this regard, this study examines an easy-to-produce, widely available and cheap manganese nitrate concentrated electrolyte that holds DES traits. The electrolyte (i. e., manganese nitrate; (Mn(NO 3 ) 2 • 6H 2 O with a weight composition of 42 %) revealed its eutectic behaviour by benchmarking the eutectic distance (53 °C), thermal expansion coefficient (3.9 × 10 À 4 K), activation energy for conductivity (~2.45 kJ mol À 1 ), and viscosity (14 kJ mol À 1 ). What's more, it was successfully incorporated into a twoelectrode symmetric supercapacitor comprising activated carbon electrodes and demonstrated specific capacitance retention of 93 % (170 F g À 1 ) through 2000 consecutive galvanostatic charge-discharge cycles at 25 and À 20 °C.

Characterization of Manganese Acetate Hydrate Solutions and Their Potential Use for Energy Storage Applications

“…The competition between such interactions leads to an inflexion point at ~40 °C. The r H values in the temperature range of 30 to 50 °C are similar to the ones reported for the manganese nitrate salt [22] and at lower temperatures (10-25 °C) twice as large. In water and at 25 °C, the r H for Mn 2 + -H 2 O values are of the same order of magnitude as the ones reported in aqueous solution for alkali cations, such as lithium, sodium and potassium.…”

“…As a comparison, the diagram of the Mn(NO 3 ) 2 -ice mixture (black curve in Figure 1b) is also included. The eutectic depth (ΔT, the ideality line connecting the melting temperatures of water and the tetrahydrate salt and the eutectic temperature) reaches 93 °C, and is greater than its counterpart Mn(NO 3 ) 2 -H 2 O (ΔT = 65 °C), [22] suggesting a stronger amplitude of interactions between species (anion-H 2 O) in the eutectic mixture. [28] All electrolytes reveal cold crystallization temperatures (T cc ) suggesting that the molecules contain a rigid core and flexible fragments (i. e., acetate anions) [29] , [30] which in turn, increase the degree of freedom.…”

“…[59] In the The Mn(NO 3 ) 2 -H 2 O system from ref. [22] is displayed here for comparison purposes. case of Mn(Ac) 2 , σ max attains 35 mS cm À 1 at a much lower salt concentration, i. e., 0.8 mol L À 1 (Figure 6a).…”

“…To this end, building on the recent breakthrough, where, for the first time, another manganese-based salt (Mn(NO 3 ) 2 , manganese nitrate) was presented as an electrolyte in AC/Mn(NO 3 ) 2 / AC supercapacitors, [22] we present the utilization of Mn-(CH 3 COO) 2 salt for a similar supercapacitor type. This salt serves a dual purpose by facilitating in-situ MnO 2 deposition on the cathode and acting as an electrolyte for a similar-type AC// MnO 2 supercapacitor.…”

“…(a) Thermograms of the solutions ofTable1(S1-S6). (b) Eutectic diagram of the aqueous acetate solution constructed with the aid of Ref [22]. incorporating the position of the six electrolytes used in this study (S1-S6).…”