Manganese oxides (MnOx) are a well-established class of active materials for electrochemical energy-storage technologies ranging from primary alkaline cells to rechargeable Li-ion batteries. More recently, the use of manganese oxides has extended to aqueous-electrolyte electrochemical capacitors (ECs) in which nanostructured forms of MnOx exhibit pseudocapacitive charge-storage behavior that can be tapped for pulse-power applications. The ability of MnOx to alternately express battery-like and capacitor-like functionality offers intriguing prospects to design electrode materials and corresponding devices that deliver both high energy content and rapid charge/discharge response. We are exploring such opportunities with electrode architectures comprising nanoscale MnOx coatings affixed to porous carbon frameworks [1,2,3]. The battery- and capacitor-like character of these materials can be tuned by varying such factors as the oxide crystal structure (layered birnessite-MnOx vs. cubic spinel LiMn₂O₄) and the composition of the contacting electrolyte (mixtures of Na⁺, Li⁺, and/or Zn²⁺) [4]. To deconvolve the complex electrochemical response of such systems, we apply a suite of electroanalytical methods that are based on voltammetry and impedance. The 3D projection of Bode-plot parameters has proven particularly useful in mapping frequency-dependent capacitance contributions onto the potential scale, revealing mechanisms that deliver/store charge at high rates. In parallel with investigations of macroscale electrode architectures, we also examine simplified 2D MnOx//carbon interfaces where surface-sensitive characterization methods (X-ray photoelectron spectroscopy, scanning-probe microscopy) provide insights on the impact of the carbon substrate on charge-transfer kinetics, charge-storage mechanisms, and stability.

1. E. Fischer, K.A. Pettigrew, D.R. Rolison, R.M. Stroud, and J.W. Long, Nano Letters 2007, 7, 281–286.
2. W. Long, M.B. Sassin, A.E. Fischer, and D.R. Rolison, J. Phys. Chem. C 2009, 113, 17595–17598.
3. B. Sassin, S.G. Greenbaum, P.E. Stallworth, A.N. Mansour, B.P. Hahn, K.A. Pettigrew, D.R. Rolison, and J.W. Long, J. Mater. Chem. A 2013, 1, 2431–2440.
4. S. Ko, M.B. Sassin, J.F. Parker, D.R. Rolison, and J.W. Long, Sustainable Energy Fuels, 2018, 2, 626–636.