No 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.

This wort compares lhc clcclrodl.'JlOSition behavior of lhin lilms of mangaOOliC dioxide produced from solutions of four commonly available Mn(ll) sails; namely, sulfate, chloride, nltrale and acetate, as well as lhcir rorrcsponding acids. The dl.-posilion behavior is invcsligalcd in lcrms of the masses of the lhin films produced, calculated by inlcgralion of cllargc cumml durillll deposition, lllld by the clcclrochcmit:al quarti c.:ryslal micruhalam::c (li.QCM). Generally, ii wus shown that EQCM masses dcvialcd from lhc mass expected !furn current integration, wilh im.TCast."S lllllging rnm1 14-40%, dl-')lCnding on the anioo. The must notable dcvialiuns from Ibis trend were lhc high-add (0.1 M) and low-Mn(ll) (0.01-0.001 M) solutions wht.'fC MnC..12 and Ht.1 were used. 'Ibis dilfcrenu: is diM:usst.'