Nanostructured Electrode Enabling Fast and Fully Reversible MnO₂-to-Mn²⁺ Conversion in Mild Buffered Aqueous Electrolytes

Abstract

On account of their low-cost, earth abundance, eco-sustainability, and high theoretical charge storage capacity, MnO 2 cathodes have attracted a renewed interest in the development of rechargeable aqueous batteries. However, they currently suffer from limited gravimetric capacities when operating under the preferred mild aqueous conditions, which leads to lower performance as compared to similar devices operating in strongly acidic or basic conditions. Here, we demonstrate how to overcome this limitation by combining a well-defined 3D nanostructured conductive electrode, which ensures an efficient reversible MnO 2-to-Mn 2+ conversion reaction, with a mild acid buffered electrolyte (pH 5). A reversible gravimetric capacity of 560 mA•h•g-1 (close to the maximal theoretical capacity of 574 mA•h•g-1 estimated from the MnO 2 average oxidation state of 3.86) was obtained over rates ranging from 1 to 10 A•g-1. The rate capability was also remarkable, demonstrating a capacity retention of 435 mA•h•g-1 at a rate of 110 A•g-1. These good performances have been attributed to optimal regulation of the mass transport and electronic transfer between the three process actors, i.e. the 3D conductive scaffold, the MnO 2 active material filling it, and the soluble species involved in the reversible conversion reaction. Additionally, the high reversibility and cycling stability of this conversion reaction is demonstrated over 900 cycles with a Coulombic efficiency > 99.4 % at a rate of 44 A•g-1. Besides these good performances, also demonstrated in a Zn/MnO 2 cell configuration, we discuss the key parameters governing the efficiency of the MnO 2-to-Mn 2+ conversion. Overall, the present study provides a comprehensive framework for the rational design and optimization of MnO 2 cathodes involved in rechargeable mild aqueous batteries.

Towards a high MnO2 loading and gravimetric capacity from proton-coupled Mn4+/Mn2+ reactions using a 3D free-standing conducting scaffold

“…The gravimetric capacity of the cathode is thus as high as 338 mA•h/g MnO2+CNF (equivalent to 534 mA•h/g MnO2 ). By using CNFs, we thus significantly improved the m MnO2 /m carbon ratio and so areal/gravimetric capacity as compared to the early work performed by Mateos et al 15 on model GLAD-ITO electrodes in the same buffered aqueous electrolyte (Table 1). The present results also compare favorably with those reported up-to-date for MnO 2 -cathodes based on commercial carbon/graphite felts (cloths) and operating through a similar conversion mechanism (see Table 1 for selected data from publications providing detailed information on the commercial carbon substrate).…”

“…14 This issue was recently solved using transparent 3D ITO electrodes as a conductive mesoporous scaffold for the electrodeposition-electrodissolution of MnO 2 , leading to a significant improvement of the reversible gravimetric capacity up to 560 mA•h/g MnO2 . 15 However, while such model ITO-based electrodes are of great interest for fundamental quantitative studies, they do not allow to switch to operational batteries. For such purpose, a conductive scaffold combining high specific surface electroactive area and an easy as well as low-cost preparation is highly desirable, which can be notably achieved with electrospun carbon nanofibers (CNFs).…”

The Role of Al3+‐Based Aqueous Electrolytes in the Charge Storage Mechanism of MnOx Cathodes

“…MnO x cathodes were prepared as previously described [19,20] by anodic electrodeposition of the active material (in a buffered pH 5 electrolyte containing 0.1 m MnCl 2 ) into the pore structure of two different types of 3D conductive scaffolds (see Experimental Section in Supporting Information), i.e., 3D transparent electrodes made of a 1 µm-thick nanostructured ITO film deposited by glancing angle deposition (GLAD) over a flat commercial ITO-coated glass substrate [21] and 3D assemblies of self-standing electrospun carbon nanofibers (i.e., 360 µm-thick CNFs with an average fiber diameter of 127 ± 47 nm). [20] The main interest of the MnO x -loaded 3D transparent ITO electrodes is that they offer the possibility to quantitatively monitor, by in operando UV-vis spectroelectrochemistry, the fate of MnO x during galvanostatic cycles (see Scheme 1 depicting the three-electrode setup).…”

“…[20] The main interest of the MnO x -loaded 3D transparent ITO electrodes is that they offer the possibility to quantitatively monitor, by in operando UV-vis spectroelectrochemistry, the fate of MnO x during galvanostatic cycles (see Scheme 1 depicting the three-electrode setup). [19] For such a purpose, the GLAD-ITO electrodes were loaded by systematically applying a deposited charge of 100 mC cm −2 . This results in the deposition of 48 ± 1 µg cm −2 of MnO x within the GLAD-ITO film mesoporosity.…”

Evidence of Bulk Proton Insertion in Nanostructured Anatase and Amorphous TiO2 Electrodes

“…7 This concept of proton insertion via a weak acid also applies to multivalent aquo-metal complexes (e.g., [Zn(H2O)6] 2+ , [Al(H2O)6] 3+ , …), which generally possess an intrinsic weak acidity through their coordinated water molecules. 5,[8][9][10] To date, most of the knowledge on the bulk insertion of protons in metal oxides comes from studies on nickel hydroxide materials. This is mainly because they are abundantly used as cathodic active materials in alkaline nickel-metal hydride (NiMH) or nickel-zinc aqueous batteries.…”