Investigations toward a Non‐Aqueous Hybrid Redox‐Flow Battery with a Manganese‐Based Anolyte and Catholyte

Abstract

The importance of electrical energystorage systems (EES), for a successful integration of intermittent renewable energy sources into the electrical grid is beyond dispute. [1][2][3][4] For mobile applications, lithium ion batteries (LIBs) with high energy density prevail. [2,5,6] Although many efforts focus on alternative chemical systems (e.g., multivalent Al, Mg, and Ca), it is hard to imagine that LIBs will disappear in the near future. [4,7] Yet, for large scale stationary EES, there is no such prevailing technology. Although other alternatives, like pumped hydro or fuel cells are available, batteries are amongst the most promising technologies for this purpose. [1,3,8] With the energy density being slightly less relevant, other redox active materials could be employed in large scale EES, i.e., redox-flow batteries (RFBs) with their very long cycle life and decoupled capacity, power and energy output. [2,9,10] Their benchmark is the allvanadium redox-flow battery (VRFB) with V II /V III and V IV / V V redox couples, [2] as well as a 15 000-20 000 charge/discharge cycles lifetime and an acceptable energy density of 25-35 Wh L −1 installed in up to 60 MWh capacity EES. [2,6] However, probably due to the high cost, a commercial breakthrough still has to come. [2,6,9] Considering abundancy and cost, few elements are suitable as redox active material in sustainable batteries. [4,11] Manganese is one of them and, therefore, finds application in LIB-cathode active materials (e.g., LiMn 2 O 4 or Li[Ni 0.8 Co 0.1 Mn 0.1 ]O 2 ) or in cathode materials of primary batteries (MnO 2 ). [5,12] However, to the best of our knowledge, only one battery system exclusively using manganese compounds at both electrodes is described, [13] i.e., Mn(acac) 3 acetonitrile (MeCN) solutions with the Mn II /Mn III couple at the negative and the Mn III /Mn IV couple at the positive electrode. Yet, with E cell of 1.1 V the system does not exploit the large potential range advantage of a non-aqueous electrolyte. Already the aqueous standard potential difference ΔE 0 of Mn 0 /Mn II and Mn II /Mn III redox couples amounts to an impressive 2.69 V. In addition, the Mn volumetric specific capacity is 7034.7 Ah L −1 (two-electron-process). It clearly exceeds that of zinc (5853.8 Ah L −1 ), which in the zinc A new all-Manganese flow battery (all-MFB) as a non-aqueous hybrid redox-flow battery is reported. The discharged active material [Cat] 2 [Mn II Cl 4 ] (Cat = organic cation) utilized in both half-cells supports a long cycle life. The reversible oxidation of [Mn II Cl 4 ] 2− to [Mn III Cl 5 ] 2− at the positive electrode and manganese metal deposition from [Mn II Cl 4 ] 2− at the negative electrode give a cell voltage of 2.59 V. Suitable electrolytes are prepared and optimized, followed by a characterization in static battery cells and in a pumped flow-cell. Several electrode materials, solvents, and membranes are tested for their feasibility in the all-MFB. An electrolyte consisting of [EMP] 2 [MnCl 4 ] and some solvent γ-butyrolactone is cycle...

Interface Engineering to Improve the Rate Performance and Stability of the Mn-Cathode Electrode for Aqueous Zinc-Ion Batteries

“…High-efficiency electrical energy storage systems are important for modern society. − Although lithium-ion batteries dominate the market for rechargeable high-energy batteries, its application in large-scale energy storage systems is still restricted due to the high manufacturing cost. Particularly, the use of highly toxic and combustible organic electrolytes may result in dangerous reactions between electrodes and electrolytes. − Rechargeable aqueous metal-ion batteries, especially the recently developed zinc-ion batteries (ZIBs), are a candidate with great potential for large-scale energy storage systems, attributed to their ease of handing, environmental benignity, low potential, and safety. − Similar to the traditional commercial alkaline zinc–manganese batteries, manganese-based oxides are regarded as ideal electrode materials for ZIBs because of their relatively high capacity and energy density, natural abundance, and low toxicity. ,− Therefore, many Mn-based electrodes have been successfully fabricated for ZIBs through different methods.…”

Elucidating Instabilities Contributing to Capacity Fade in Bipyridine‐Based Materials for Non‐aqueous Flow Batteries

“…Electroactive species for non‐aqueous redox flow batteries have spanned the gamut, from small organic molecules, to metal coordination complexes (MCCs), to large metal‐based clusters, to hybrid systems [6–9] . A recent review by Palmer et al .…”

“…[3][4][5] Electroactive species for non-aqueous redox flow batteries have spanned the gamut, from small organic molecules, to metal coordination complexes (MCCs), to large metal-based clusters, to hybrid systems. [6][7][8][9] A recent review by Palmer et al summarizes the variety of metal-based charge carriers used in nonaqueous flow batteries in the past 5 years. [10] In our previous work, we optimized, modified, and evaluated the performance of symmetric and asymmetric iron bipyridine (bpy) based nonaqueous flow batteries originally published by Mun et al [11][12][13] By varying the electronics of bpy ligands, we tuned the redox potentials of the catholyte and anolyte with the goal of increasing operating voltage in symmetric iron bpy systems.…”

A Hexacyanomanganate Negolyte for Aqueous Redox Flow Batteries

“…This strategy has been adopted in several RFB systems, as evidenced by earlier reports. 29−31 Although this two-electron reaction has been controversial due to the presence of an unstable Mn 3+ 35 However, solid/liquid redox reactions involving Mn 0 (s) and Mn IV (s) states tend to be complicated because of the presence of intermediate steps, resulting in slow kinetics and poor capacity reversibility due to nonuniform coverage of solid Mn or MnO 2 layers on the electrode. 36,37 Therefore, the development of effective and stable Mn-based redox materials for RFBs remains an important research area.…”

“…This strategy has been adopted in several RFB systems, as evidenced by earlier reports. − Although this two-electron reaction has been controversial due to the presence of an unstable Mn 3+ intermediate state, the complexation effect from acetate (CH 3 COO – ) ligands allowed for the bypass of the unstable Mn 3+ state during the Mn 2+ (aq)/Mn IV O 2 (s) reaction, demonstrating the strategic role of ligand selection in overcoming inherent reaction challenges. − Mn 0 (s)/Mn II (aq) reactions, which are generated by the deposition of zerovalent manganese, have been considered as anode redox reactions in RFBs. Schmucker et al presented Mn 0 /[Mn II Cl 4 ] 2– redox materials (−1.18 V vs SHE) consisting of an all-manganese-based flow battery coupled with [Mn II /Cl 4 ] 2– /[Mn III Cl 5 ] 2– . However, solid/liquid redox reactions involving Mn 0 (s) and Mn IV (s) states tend to be complicated because of the presence of intermediate steps, resulting in slow kinetics and poor capacity reversibility due to nonuniform coverage of solid Mn or MnO 2 layers on the electrode. , Therefore, the development of effective and stable Mn-based redox materials for RFBs remains an important research area.…”