Efficient Electrolyte Additives of Phosphate, Carbonate, and Borate to Improve Redox Capacitor Performance of Manganese Oxide Electrodes

Abstract

For a faradaic electrochemical capacitor, birnessite-based manganese dioxide electrode, prepared by the electrochemical activation of ball-milled Mn 3 O 4 composite electrodes as we recently reported, delivers 190 F g −1 in a Na 2 SO 4 aqueous solution in the potential range of −0.1 and 0.9 V vs. Ag/AgCl at a current rate of 1.0 A g −1 . However, the specific capacitance of birnessite gradually decreases during successive charge and discharge cycle, and the capacity decrease is accompanied by the gradual loss of net weight of manganese oxide because of manganese dissolution from electrode into electrolyte solution. By adding and dissolving small amounts of Na 2 HPO 4 , NaHCO 3 , and Na 2 B 4 O 7 into the Na 2 SO 4 solution, the capacitance of birnessite increases to 200-230 F g −1 with much improved cyclability over >1000 cycles, resulting from suppression of the manganese dissolution because of pH buffer action by the additives. The improvement of cyclability and capacitance is due to the formation of protection layer on the birnessite and the optimized pH buffer capacity of aqueous electrolyte.Manganese-based oxides are in widespread investigation and development for electrochemical devices, because of low cost and environmentally benign of manganese. Recently, Whitacre's research group developed aqueous cells, consisting of activated carbon / Na 2 SO 4 aq. / MnO 2 , for large format energy storage devices for stationary use. 1 Electrochemical capacitors (supercapacitors) are attractive energy storage devices because of their high-power capability compared to conventional secondary batteries. 2-5 However, their energy density is normally lower than that of lithium-ion batteries. In order to incrementally improve both the energy density and satisfactory cyclability of aqueous supercapacitors, transition metal oxides are applied as electrode materials for advanced supercapacitors to utilize both their redox activity near the surface of electrode active materials and electric double layer capacitance at the electrode / electrolyte interface. Among transition metal oxides, cost-and environment-friendly manganese(IV) oxides exhibit a high specific capacitance due to utilization of both its electric double layer and Mn(III)/Mn(IV) redox couple. 6-9

A review of electrolyte materials and compositions for electrochemical supercapacitors

“…Various factors of the neutral electrolytes, such as pH, the types of cation and anion species, 117 salt concentrations, [118][119][120] additives, 121,122 and solution temperatures, have been found to have an influence on the ES performance. Various factors of the neutral electrolytes, such as pH, the types of cation and anion species, 117 salt concentrations, [118][119][120] additives, 121,122 and solution temperatures, have been found to have an influence on the ES performance.…”

“…121 For example, Komaba et al 121 found that upon addition of a small amount of Na 2 HPO 4 , NaHCO 3 , or Na 2 B 4 O 7 into the Na 2 SO 4 electrolyte, the specific capacitance of the birnessite electrode could be increased from 190 F g À1 to 200-230 F g À1 at 1.0 A g À1 , and at the same time the cyclability was also significantly improved (41000 cycles). 121 For example, Komaba et al 121 found that upon addition of a small amount of Na 2 HPO 4 , NaHCO 3 , or Na 2 B 4 O 7 into the Na 2 SO 4 electrolyte, the specific capacitance of the birnessite electrode could be increased from 190 F g À1 to 200-230 F g À1 at 1.0 A g À1 , and at the same time the cyclability was also significantly improved (41000 cycles).…”

“…Various factors of the neutral electrolytes, such as pH, the types of cation and anion species, 117 salt concentrations, [118][119][120] additives, 121,122 and solution temperatures, have been found to have an influence on the ES performance. Regarding the cation species, various alkaline metal or alkaline-earth metal cations have different ionic sizes and hydrated ion sizes, and thus have different diffusion coefficients and ionic conductivities, which are expected to have strong influences on both the specific capacitance and ESR of the ESs.…”

“…In an effort to improve the ES performance, other strategies such as adding additives into the neutral electrolytes have also been explored. 121 For example, Komaba et al 121 found that upon addition of a small amount of Na 2 HPO 4 , NaHCO 3 , or Na 2 B 4 O 7 into the Na 2 SO 4 electrolyte, the specific capacitance of the birnessite electrode could be increased from 190 F g À1 to 200-230 F g À1 at 1.0 A g À1 , and at the same time the cyclability was also significantly improved (41000 cycles). This was attributed to the formation of a protection layer on the birnessite, which could inhibit Mn dissolution due to the optimized pH buffer provided by the additives (Fig.…”

Enhanced Desalination Performance Using Phosphate Buffer-Mediated Redox Reactions of Manganese Oxide Electrodes in a Multichannel System

“…Since negatively charged buffer ions (e.g., phosphate, carbonate, and borate) can create an insoluble layer on the surface of MnO 2 combined with Mn 2+ , it is likely that regulating both H + and Mn 2+ contributes to the improvement. 32 To determine how the solution pH was influenced by the redox reaction of MnO 2 (i.e., H + insertion/deinsertion), we measured the effluent pH from the anode (oxidation) and cathode (reduction) compartments of a 2-channel flow cell divided by a CEM. The anode effluent pH decreased from 8 to 4 in the absence of phosphate (Figure 2a), which was attributed to H + deinsertion upon oxidation (Figure 2b).…”