Electrochemical performance of nanosized MnO2 synthesized by redox route using biological reducing agents

Abstract

For more than a century, manganese dioxides (MDOs) have been used as electrochemically active materials in energy storage and conversion applications. To reduce the cost and hazardous impact of synthesis, a redox method was used to prepare MnO2 using extracts of green (GT-MnO2) and black (BT-MnO2) tea as biological reducing agents. Polyphenols present in the extracts of tea not only reduce Mn 7+ ions but also act as "capping agents" to stabilize and prevent the produced MnO2 nanoparticles from degradation and aggregation. Elemental, structural properties and morphology of nanosized α-KxMnO2 were studied by thermogravimetric analysis, X-ray powder diffraction and Raman spectroscopy, which revealed the different crystallinity between GT-MnO2 and BT-MnO2 due to the strength of the antioxidant species. Electrochemical investigations highlight the beneficial presence of K + ions in (2×2) tunnels; with a higher concentration, α-K0.135MnO2 exhibits a discharge specific capacity of 155 mAh g-1 after 20 cycles at C/26 rate.

Synthesis of High Surface Area α-KyMnO2 Nanoneedles Using Extract of Broccoli as Bioactive Reducing Agent and Application in Lithium Battery

“…The combination of XRD data and HRTEM image reveals that NNs are composed of a few aggregated crystallites. This result is comparable with those obtained from α-KyMnO2 nanoneedles prepared by the mild synthetic method using the extract of tea as reducing agent, in which XRD patterns are dominated by the (211) Bragg line and the crystallite size is found to be ~4 nm [27]. Figure 3b shows the N2 adsorption desorption isotherm and Barrett-Joyner-Halenda (BJH) poresize distribution for as-prepared α-KyMnO2.…”

“…Using different DTG behaviors of the literature, it was found that there is an almost linear relationship between the transition temperature and the content of K + ions in α-K x Mn 8 O 16 . According to Figure 2 shown in [27], we can estimate a concentration of potassium (i.e., x = 0.035), which is close to the value obtained from ICP measurement. Thus, the amount of electrochemically inactive K + ions (1.45 wt.%) is confirmed to be much smaller than that of Mn (52.82 wt.%), and does not obviously affect the specific capacity of the electrode.…”

“…Although numerous works published in the literature neglect this aspect. Several articles considered the presence of potassium as the combined outcome of the preparation of cryptomelane or birnessite MnO 2 phases [27,[38][39][40]. As a result, mixed +4 and +3 oxidation states of Mn cations could be induced in the α-MnO 2 (Mn 8 O 16 ) lattice (i.e., KMn IV 7 Mn III O 16 ).…”

“…The preparation of α-MnO 2 nanostructured particles has been realized by different techniques, including co-precipitation [16], comproportionation [21], hydrothermal [22], sonochemical reduction [23], micro-emulsion route [24], simple redox reaction [25] and reflux method [26]. Green synthesis is also one of these approaches using extracts of plants as reducing agents taking into account the presence of flavonoids and polyphenols [27][28][29]. Phenolic compounds are characterized to have at least one aromatic ring attached to one or more hydroxyl groups, with various arrangements of their carbon atoms.…”

“…The higher the number of hydroxyl groups, the higher the antioxidant activity [30]. Previously, we reported successful preparations of nanostructured MnO 2 via biological reducing agents such as extract of green and black tea [27], citrus peel [31], and extract of orange peel [25]. Broccoli is a kind of vegetable similar to cauliflower and cabbage, all of them are rich in natural antioxidants, antimicrobial and anticancer activities [32].…”