Mass Transport Properties of Manganese Dioxide Phases for Use in Electrochemical Capacitors: Structural Effects on Solid State Diffusion

Abstract

The solid state mass transport characteristics of various manganese dioxide phases has been examined with a focus on their use in electrochemical capacitors. The phases examined included γ-MnO 2 (electrolytic manganese dioxide), β-MnO 2 (Pyrolusite), Ramsdellite, δ-MnO 2 (Birnessite), α-MnO 2 (Cryptomelane) and λ-MnO 2. Diffusion within each phase was examined using electrochemical impedance spectroscopy (EIS) and step potential electrochemical spectroscopy (SPECS). A √ D (where A is surface area and D is the diffusion coefficient) decreases with depth of discharge, and is also affected by the phase of manganese dioxide studied, with γ-MnO 2 exhibiting the highest A √ D value. Overall, values of A √ D varied between 3 × 10 −8-2 × 10 −10 m 3 /s 1/2 /g, which is comparable with literature data. These results also provide information on the kinetics of lattice expansion and contraction which occurring during cycling. High surface area phases such as γ-MnO 2 , Ramsdellite and Cryptomelane, showed significant hysteresis in lattice contraction which is attributed to the diffusion of protons through surface domains. Low surface area phases (Pyrolusite and λ-MnO 2) did not display this hysteresis, suggesting that proton diffusion occurs predominantly in the bulk of the material. No direct correlation between mass transport and specific capacitance is observed, suggesting that other material properties contribute to specific capacitance.

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

“…The second one involves the insertion of cations into the bulk manganese oxide particles which is not limited to only the surface in contact with the electrolyte because of mass transport control. 30 Thus, the specific surface area and the kind of cation species, C + , their concentration, and pH (in the case of aqueous solution) must affect the electrochemical behavior. As described by Chu 23 and Toupin et al, 31 the faradaic process is mainly attributed to H + insertion into amorphous or poorly crystallized manganese dioxide with insignificant insertion of alkali cation (Na + ) and H 3 O + ions, whereas highly crystalline layered structure of manganese dioxide is capable of insertion / deinsertion of sodium ions.…”

A Step Potential Electrochemical Spectroscopy Analysis of Electrochemical Capacitor Electrode Performance

“…This method has previously been used to examine the diffusion characteristics of battery cathode materials, particularly with a focus on examining the quasi-equilibrium discharge and charge characteristics, as well as the kinetics of diffusion through the active material structure as a function of applied potential and state of charge [17][18][19][20]. More recently, the SPECS method used in this work has been applied to manganese dioxide electrodes and successfully differentiated between the charge storage contribution from faradaic and non-faradaic processes [21].…”

Separating the Faradaic and Non-Faradaic Contributions to the Total Capacitance for Different Manganese Dioxide Phases

“…This process is diffusion limited and as such, would be contributing to the diffusion limited current. However, the transport of small ions through bulk electrolyte is many orders of magnitude faster than the solid state diffusion of protons through manganese dioxide, 14 and therefore has negligible contribution to the diffusional current compared to solid state diffusion. However, the diffusion of aqueous ions through micro-pores is not as fast as through bulk electrolyte, and may therefore be a non-negligible contributing process.…”

“…Evidence suggests that the rate at which protons and electrons can diffuse through the material affects the magnitude of pseudo-capacitive charge storage since this process enables access to more of the manganese dioxide structure 12,13 Previous work has shown that different manganese dioxide crystal structures have different rates of proton diffusion at different depths of discharge, which has been attributed to the different tunnel structures either facilitating or hindering proton diffusion. 14 However, it has been difficult to determine an exact relationship between proton diffusion and overall performance due to the effects of double layer capacitance. Manganese dioxide phases all have different specific surface areas, meaning the proportion of capacitance attributed to double layer charging is likely to vary significantly.…”