Electrochemical Oxidation of Mn∕MnO Films: Mechanism of Porous Film Growth

Abstract

The electrochemical oxidation of dense Mn films is shown to occur through the following sequence of oxides: Mn to MnO to Mn 3 O 4 to Mn 2 O 3 to MnO 2 . X-ray photoelectron spectroscopy analysis reveals that at each stage, the manganese exists as a hydrated compound. Complete oxidation was shown to be necessary to maintain high capacitance values when utilized as a redox supercapacitor. Partial oxidation was shown to reduce the amount of MnO 2 present in the film and the relative surface area of the film, thus having a negative effect on the capacitance. Efforts to improve the hydration of the films oxidized to 0.9 V using a dual current oxidation proved unsuccessful.Electrochemical capacitors are used primarily for applications that require a high power output and a high cycle capacity. 1,2 A redox pseudocapacitor makes use of both reversible redox reactions and double-layer charging in order to store charge. Traditionally, ruthenium oxide has been the material of choice for pseudocapacitors as it exhibits specific capacitance values of up to 750 F/g. However, raw material costs are prohibitive to large-scale commercial production. 2 Manganese oxides provide a lower cost, lower toxicity replacement for ruthenium oxides in capacitor applications. 3 Typically, MnO 2 films have been created using chemical reactions or electrochemical deposition and often result in an amorphous hydrated MnO 2 product or a weakly crystalline hydrated MnO 2 product. [3][4][5][6][7][8] Broughton and Brett developed a procedure making use of physical vapor deposition of a manganese layer followed by an electrochemical oxidation in Na 2 SO 4 in order to produce a capacitive film. 9 Preliminary studies indicated that the as-deposited layer was a twophase mixture of Mn and MnO. 10 The electrochemical oxidation step was identified as the crucial step for providing a capacitive film. 11 The oxidation process produced a highly porous, hydrated, amorphous MnO 2 film. 11 One intermediate step was also studied, in which the porous surface layer was determined to be hydrated Mn 3 O 4 , and confirmed that the bulk substructure oxidizes from Mn/MnO to Mn 3 O 4 . The purpose of this work is to complete the study of the electrochemical oxidation process in this system in order to identify the mechanism by which the oxidation occurs. ExperimentalThe manganese films were fabricated in a sputter deposition system using a 50 mm diameter Mn target and a target to substrate distance of 220 mm. The base pressure of the system was 4 ϫ 10 −5 Pa and the pressure in the chamber during deposition was approximately 0.8-0.93 Pa. The sputtering gas was argon, fed in at a rate of 6 ϫ 10 −4 m 3 /h, and sputtering was carried out under constant power operation at 200 W. The manganese was deposited onto a Si substrate coated with a standard Ti/Pt metallization layer. The Ti layer thickness was approximately 25 nm and the Pt layer was 300 nm. The deposited manganese layer thickness for all samples was 50 nm.Electrochemical oxidation was done in 1 M Na 2 SO 4 electrolyte...

Effects of temperatures and cations of electrolyte on the capacitive characteristics of the manganese oxide deposited by hydrothermal electrochemical method

“…The manganese oxidation states were determined by calculating the multiplet splitting width of Mn 3s orbit peaks from the XPS spectra shown in Fig. 3 [26][27][28]. The peaks of XPS spectra shifted slightly to the left with increasing temperature.…”

“…The multiplet splitting width of manganese oxide deposited at various temperatures was calculated; the results are listed in Table 1. The peak separation ( E) of Mn 3s spectra decreased from 5.50 to 5.34 eV when the temperature was increased from 60 to 150 • C; [26][27][28]. The amount of hydration was 24.87% at 60 • C, and declined rapidly to 3.71% after the temperature was increased to 150 • C. The relative amounts of hydration are summarized in Table 1.…”

Cycle stability of birnessite manganese dioxide for electrochemical capacitors

“…This is significant because this process was shown to be irreversible, at least in alkaline electrolytes [33][34][35], and also via chemical reduction [32]. Furthermore, it also becomes apparent that in previous studies where birnessite has been used [40][41][42] as the active material, the faradaic contribution to the capacitance is small since if it were to continue to greater depths of discharge, the irreversible formation of Mn 3 O 4 would be the result.…”

Efficient Electrolyte Additives of Phosphate, Carbonate, and Borate to Improve Redox Capacitor Performance of Manganese Oxide Electrodes

“…It was previously reported that a Mn 3 O 4 thin film electrode was changed into electrochemically active materials, which show pseudo-capacitive behavior delivering 300 F g −1 after successive potential cycling. 10 Broughton et al 8,11 reported the transformation from MnO, Mn 3 O 4 , Mn 2 O 3 finally into manganese dioxide, and the phase conversion from Mn 3 O 4 to Na y MnO 2 · nH 2 O was also described in literature. 12, 13 Hu and coworkers demonstrated the original and unique preparation routes of Mn 3 O 4 electrodes having highly ordered structure and succeeded to attain advanced capacitor properties.…”