Mn ch3coo 2 4h2o

The spinel pre-catalysts were synthesized by a modified conventional sol-gel combustion method. Metal acetates Co(CH₃COO)₂·4H₂O (99.999% trace metals basis, Sigma-Aldrich), Zn(CH₃COO)₂·2H₂O, Mn(CH₃COO)₂·4H₂O, Ni(CH₃COO)₂·4H₂O, Fe(NO₃)₃·9H₂O (Sigma-Aldrich), and LiNO₃ (Sigma-Aldrich) with a specific molar ratio were first mixed in dilyted nitric acid solution through vigorous stirring. Citric acid (ACS reagent, ≥99.5%, Sigma-Aldrich) acting as chelating agent, was the added in the mixture. Under the condition of constant stirring at 90 °C for several hours, the mixture solution converts into viscous gel. The gel was then decomposed in air at 170 °C (with a heating rate of 10 °C min⁻¹ to 170 °C) for 12 hours to remove the remaining water and ground thoroughly. After applying a further heat treatment in air at 400 °C for six hours, phase pure spinel oxide powders were obtained.

All chemicals (H₂SO₄, H₂O₂, Mn(CH₃COO)₂·4H₂O, polyethylene oxide (PEO, M_W = 10⁶), Na₂SO₄, NaH₂PO₄·2H₂O and Na₂HPO₄·H₂O) and commercial manganese oxides (Mn₂O₃, Mn₃O₄, and activated porous MnO₂) were purchased from Sigma-Aldrich (Munich, Germany).

LiNi_0.5Mn_1.5O₄ was prepared
using a modified co-precipitation pathway.^{43 (link),44 (link)} LiCH₃COO·2H₂O (99%, Sigma-Aldrich), Ni(CH₃COO)₂·4H₂O (99%, Sigma-Aldrich),
and Mn(CH₃COO)₂·4H₂O (99%, Sigma-Aldrich)
were dissolved with a Li/Ni/Mn molar ratio of 1.1:0.5:1.5 in a water/ethanol
mixture to get solution A (water/ethanol 1:5 v/v). Furthermore, H₂C₂O₄·2H₂O (99%, Aldrich)
was dissolved in an identical hydro-alcoholic solution (B). This latter
solution (B) was dropwise added to solution A with stirring of the
mixture and then kept at ambient temperature for 12 h to precipitate
the metal oxalates. Afterward, the precipitate was treated for 12
h at 80 °C under constant stirring to evaporate water and ethanol.
The precipitate was annealed for 6 h at 500 °C in a dry air flow
to obtain an oxide powder (heating ramp, 5 °C min^–1). This powder was ground in a mortar, pressed into pellets, and
calcined for 12 h at 800 °C in a dry air flow to obtain LiNi_0.5Mn_1.5O₄ (heating ramp, 5 °C min^–1).

α‐MnO₂ nanorods were synthesized through a hydrothermal method. KMnO₄ (3.75 g, 99%, Sigma‐Aldrich) and MnSO₄·H₂O (0.21 g, 99%, Sigma‐Aldrich) were dissolved in deionized water (240 mL), and the mixture was then loaded into a Teflon‐lined autoclave (300 mL) and heated at 180 °C for 14 hours. The obtained products were filtered, washed thoroughly using DI water, and dried at 80 °C for 12 hours. β‐MnO₂ nanorods were also obtained using the same method as α‐MnO₂ nanorods, but Mn(CH₃COO)₂·4H₂O (7.59 g, 99.99%, Sigma‐Aldrich) and (NH₄)₂S₂O₈ (7.06 g, 97%, Sigma‐Aldrich) were used as precursors for β‐MnO₂ nanorods.

Bare and doped samples were synthesized through hydrothermal route. As concerns the synthesis of pure MnO₂, the appropriate amount of manganese acetate salt precursor (32 mmol Mn(CH₃COO)₂ × 4H₂O, purchased by Sigma-Aldrich, St. Louis, MO, USA; 99.99% purity) was dissolved in 30 cm³ of 5% ethanol–water solution, under vigorous stirring at 80 °C. Then, 32 mmol of the oxidizing agent (NH₄)₂S₂O₈ (purchased by Sigma-Aldrich, St. Louis, MO, USA; 98% reagent grade), dissolved in other 30 cm³ of 5% ethanol-water, were added dropwise to the previous solution. The system was kept under stirring for 7 h. Subsequently, it was cooled down at room temperature and left to settle for a night. After the reaction was completed, the resulting brownish-black solid product was centrifuged and washed several times with MilliQ water, until the pH became neutral. Then, the product was dried in oven at 60 °C for about 24 h.
An analogous procedure was adopted for the Co-/Fe-doped MnO₂ powders, adding the dopant precursors (Co(NO₃)₂ or Fe(NO₃)₂, respectively) in order to have dopant/manganese molar ratios equal to 2% and 5%, for each doping agent.
All samples were named as nx_MnO₂, where n is the doping percentage (2% or 5%) while x stands for Co or Fe.