An RDE and RRDE study into the electrodeposition of manganese dioxide

Electrodeposition of manganese dioxide has been examined using a combination of rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) voltammetry, with the goal of developing an understanding of the electrodeposition mechanism. Experimental variables that have been examined include electrolyte composition (combined changes within the ranges 0.1-1.0 M MnSO₄ and 0.1-5.0 M H₂SO₄), rotation rate (1000-4000 rpm) and temperature (22-98 °C). Voltammetric data (current peak instead of sigmoidal response, and non-proportional current-concentration data) indicates that already deposited manganese dioxide is a poorer catalytic surface compared to Pt. The overall electrodeposition process revolves on the formation of a Mn(III) intermediate, and whether it is soluble for extended periods of time, as in concentrated H₂SO₄ (>1.0 M), or whether it hydrolyzes rapidly to precipitate as a solid Mn(III) species (e.g. MnOOH) as in more dilute H₂SO₄ solutions (<1.0 M). In the more concentrated acid electrolytes most of the Mn(III) was lost to the bulk electrolyte through convection, with what little manganese dioxide that was formed resulting from chemical disproportionation. However, in dilute acid electrolytes, evidence suggests that the solid hydrolysis product underwent solid state oxidation to manganese dioxide. Activation energies extracted from temperature studies supports the different mechanism under different acid concentrations. Experiments examining the effect of rotation rate also indicate that the overall electrodeposition process is not mass transport limited.     

EQCM study of the electrodeposition of manganese in the presence of ammonium thiocyanate in chloride-based acidic solutions

The influence of ammonium thiocyanate (NH₄SCN) on the mechanism of manganese electrodeposition from a chloride-based acidic solution was investigated by cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). The EQCM data were represented as plots dΔm dt⁻¹ versus E, known as massograms. Because massograms are not affected by interference from the hydrogen evolution reaction, they clearly show the manganese reduction and oxidation processes. By comparing the voltammograms with their corresponding massograms, it was possible to differentiate mass changes due to faradaic processes from those due to non-faradaic processes. Morphology, chemical composition and structure of the manganese deposits formed in different potential ranges were analyzed by scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), and X-ray diffraction (XRD). The results showed that in the absence of NH₄SCN, Mn(OH)_2(s) is formed in the potential range −1.1 to −0.9 V due to the hydrogen evolution reaction in this region. At more cathodic potentials, the deposition of β-manganese and the inclusion of Mn(OH)_2(s) into the deposit occur; both of these species underwent dissolution by non-faradaic processes during the anodic scan. In the presence of NH₄SCN, the formation of α- and γ-manganese was observed. When the potential was ≤−1.8 V and [NH₄SCN] exceeded 0.3 M, the α-manganese phase was favored.     

Application of Combinatorial Methodologies to the Synthesis and Characterization of Electrolytic Manganese Dioxide

A combinatorial method has been used to investigate the effects of anodic current density, and Mn(II) and H₂SO₄ concentrations on the electrochemical synthesis and characterization of electrolytic manganese dioxide (EMD). The combinatorial method involved rapid parallel and series electrochemical deposition of EMD from electrolytes with various Mn(II)(0.15–1.82 M) and H₂SO₄(0.05–0.51 M) concentrations, at various anodic current densities (25–100 A m^–2), onto individual 1 mm² titanium electrodes, in an overall array consisting of 64 electrodes. Electrode characterization was then by average plating voltage (recorded during deposition), and open circuit voltage and chronoamperometric discharge in 9 M KOH. The applicability and benefit of the method was demonstrated by identifying the conditions of 0.59 M Mn(II), 0.17 M H₂SO₄ and 62.5 A m^–2 anodic current density as leading to the best performing EMD. These are comparable with existing knowledge regarding the synthesis and electrochemical performance of EMD, demonstrating clearly the capabilities of the combinatorial method, and providing a starting point for future experimentation. An added benefit of the method in this work was the considerable time saved during experimentation.     

The Redox Chemistry of Manganese(III) and -(IV) Complexes

The oxidation-reduction thermodynamics for the manganese(III), -(IV), and -(II) ions, and their various complexes, are reviewed for both aqueous and aprotic media. In aqueous solutions the reduction potential for the manganese(III)/(II) couple has values that range from +1.51 V vs. NHE (hydrate at pH 0) to −0.95 V (glucarate complex at pH 13.5). The Mn(IV)/(III) couple has values that range from +1.0 V (solid Mn^IVO₃ at pH 0) to −0.04 V (tris gluconate complex at pH 13.5). With anhydrous media the propensity for the Mn(III) ion to disproportionate to solid Mn^IVO₂ and Mn(II) ion is avoided. For aprotic systems the range of redox potentials for various manganese complexes is from +2.01 V and +1.30 for the Mn(IV)/(III) and Mn(III)/(II) couples (bis terpyridyl tri-N-oxide complex in MeCN), respectively, to −0.96 V for the Mn(IV)/(III) couple (tris 3,5,-di-tert-butylcatecholate complex in Me₂SO). The redox reactions between manganese complexes and dioxygen species (O₂, O₂, and H₂O₂) also are reviewed.     

Studies of the reduction mechanism of selenium dioxide and its impact on the microstructure of manganese electrodeposit

The influence of selenium dioxide (SeO₂) on the microstructure and electrodeposition of manganese coatings obtained from a sulfate based neutral solution was investigated by material characterization methods and electrochemical techniques. The crystal structure and surface morphology of these coatings were studied by scanning electron microscopy (SEM) and powder X-ray diffraction spectroscopy (XRD), respectively. The SEM and XRD data showed that SeO₂ could effectively accelerate phase transformation, and facilitate leveled and fine grain growth. The electrochemical results indicated that SeO₂ could inhibit hydrogen evolution reaction and promote manganese deposition. The action of selenium dioxide in manganese deposition was found to be a reduction and adsorption mechanism. The process could be explained as following: First, Se (IV) was reduced to Se (0), and part of Se (0) future reduce to selenide, which then combined with the remainder Se (0) forming a complicate compound (multi-selenium ions).     

Influence of manganese(II), cobalt(II), and nickel(II) additives in electrolyte on performance of graphite anode for lithium-ion batteries

Manganese dissolution into an electrolyte from the spinel LiMn₂O₄ in the lithium-ion cell has been recently investigated. In order to study the influence of the dissolved manganese species on the lithium intercalation/deintercalation into a natural graphite electrode, the electrochemical behavior of graphite was investigated in 1 mol dm⁻³ LiClO₄ electrolyte solution containing a small amount of Mn(II) by the addition of manganese(II) perchlorate. During the charging process, Mn(II) ions were firstly electroreduced on the electrode around 1.0 V versus Li/Li⁺ followed by irreversible decomposition of the electrolyte and lithium intercalation into the graphite. By microscopic observation of the graphite surface, manganese deposition was confirmed after the charge/discharge test. Due to the manganese deposition, the reversible capacity of the graphite electrode was drastically decreased. Furthermore, the cyclability of the anode was degraded with the amount of the manganese additive increasing. We compared these results with those of the cobalt(II) and nickel(II) additives by dissolving the corresponding perchlorates. Furthermore, we discussed the influence in practical cells based on the consideration of electrochemistry of the deposited metals.     

Study of adsorption of citrate on Pt by CV and EQCM

Citrates are frequently used as additives in the electrodeposition of metals such as manganese in order to improve deposit characteristics and process efficiency. Nevertheless, few studies have been reported on the mechanism of citrate adsorption. This paper presents the results of a study of citrate adsorption onto platinum from solutions of various citrate concentrations in 0.1 M HClO₄. Simultaneous observations with an electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry (CV) indicate that with increasing concentrations, up to 10⁻² M, the fractional surface coverage by citrate rises to 13% in the region of hydrogen UPD, and the anodic formation of PtO is inhibited and reduced by as much as 43%. Combination of EQCM with cathodic linear-sweep voltammetry shows the interactions of citrate with platinum oxidation and hydrogen UPD. Thus, this approach reveals changes in additive adsorption with potential and indicates in particular the influence of the additive on the adsorption and desorption of hydrogen as a step in understanding metal deposition.     

Efficient epoxidation of alkenes with sodium periodate catalyzed by manganese porphyrins in ionic liquid: Investigation of catalyst reusability

In the present work, efficient epoxidation of alkenes with sodium periodate catalyzed by tetraphenylporphyrinatomanganese(III) chloride, Mn(TPP)Cl, and octabromotetraphenylporphyrinatomanganese(III) chloride, Mn(Br₈TPP)Cl, using (1-n-butyl-3-methylimidazolium tetrafluoroborate), [bmim][BF₄], as ionic liquid is reported. The effect of substituted bromines on the catalytic activity and reusability of manganese porphyrins was also investigated.     

Electrocatalytic activity of manganese oxides prepared by thermal decomposition for oxygen reduction

The oxygen reduction reaction (ORR) was studied in KOH electrolyte on manganese oxides supported on Vulcan carbon (Mn_yO_x/C). The oxides were prepared by thermal decomposition of manganese nitrate at different conditions. The oxides were characterized by X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The electrochemical studies were conducted using cyclic voltammetry (CV) and steady state polarization measurements carried out with a thin layer rotating ring/disk electrode. XRD results showed that the manganese oxide prepared at 200 °C in air is formed by a major phase of β-MnO₂ and the polarization curves indicated the highest activity for this material. In situ XANES evidenced the occurrence of a redox process involving Mn(II)/Mn(III) and Mn(III)/Mn(IV) in the range of potentials of the CV measurements. The electrocalytic activity was related to the occurrence of a mediation process involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen and by a disproportionation reaction of the HO₂⁻ species in the Mn_yO_x sites. In situ XANES results showed that the Mn(IV) species is MnO₂ and the Mn(III) is most likely MnOOH.     

Manganese(III) mediated synthesis of A2B Mn(III) corroles: A new general and green synthetic approach and characterization

A new general and green synthetic protocol for the synthesis of manganese(III) metallocorroles has been developed from substituted aryl aldehydes and 5-(4-nitrophenyl)dipyrromethane using manganese salt as template. This is the first report in the synthesis of corroles: the formation of direct CC bond through metal initiation. This method allows higher working concentrations than those previously reported. The new A₂B manganese(III) metallocorroles were synthesized in good yield for different applications. The single crystal X-ray structure of 10-(3,4,5-fluorophenyl)-5,15-bis(4-nitrophenyl)manganese(III) corrole is also reported and shows that manganese atom is situating atop on macrocyclic plane.