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Carbon Nanoarchitectures with Conformal, Nanoscopic MnO2 Coatings as High-Performance 3D Electrode Structures for Energy Storage

We recently demonstrated a simple but highly effective strategy to incorporate nanoscopic, conformal coatings of electroactive manganese oxides (here denoted as “MnO2”) throughout the ultraporous structure of preformed carbon nanofoam substrates. Manganese oxides, primarily in the form of birnessite-MnO2 nanofilaments, are generated directly on the surface of the nanofoam via a redox reaction with aqueous permanganate. Although there are many synthetic conditions that affect the redox deposition process, our preliminary studies indicate that the initial pH of the permanganate is the most important in determining the morphology of the resulting MnO2 coating. For example, deposition from neutral-pH permanganate solutions produces MnO2 coatings with a self-limited growth of 10– 20 nm that permeate the macroscopic thickness (170 μm) of the carbon nanofoam. Nitrogen-sorption measurements and scanning electron microscopy confirm that the wellconnected pore network of the nanofoam is retained following the MnO2 deposition process. By contrast, redox deposition from acidic permanganate solutions creates micrometers-thick MnO2 crusts at the boundary of the nanofoam electrode, with minimal MnO2 incorporation within the electrode interior. Multifunctional MnO2–carbon nanofoam structures with homogeneous distribution of nanoscale MnO2 throughout the carbon substrate are attractive electrode architectures for electrochemical capacitor applications. The carbon nanoarchitecture serves an important function as a three-dimensional current collector, shuttling electrons to and from the MnO2 coating, and also providing the physical pore–solid structure of the composite electrode. The faradaic pseudocapacitance of the MnO2 coating improves the energy-storage capacity with respect to what can be achieved solely by doublelayer capacitance at the unmodified carbon nanofoam surface. For example, the specific capacitance of carbon nanofoam structures in mild aqueous electrolytes (1 M Na2SO4) can be increased from 50 F g for an unmodified nanofoam, up to 130 F g for a MnO2-coated nanofoam with a manganese oxide weight loading of ~ 56%. Capacitance enhancements are even more significant when normalized to either the volume or geometric footprint of the nanofoam structure, which is not altered by MnO2 deposition. For the same conditions described above, the volumetric capacitance of the carbon nanofoam increases from 20 F cm to 156 F cm after MnO2 deposition, while the geometric area-normalized capacitance increases from 0.3 F cm to 2.6 F cm. Rapid charge–discharge of the electrode is facilitated by the ultrathin nature of the MnO2 coating, which minimizes solid-state transport distance for ions through the MnO2 phase, and also by the high-quality pore network of the carbon nanofoam, which provides effective infiltration and transport of ions and solvent throughout the 3D electrode. Although these initial results are very promising, we anticipate further performance improvements by varying such parameters as: (i) the MnO2 mass loading; (ii) the crystal structure of the MnO2 phase, templated by supporting cations in the permanganate deposition solution; (iii) the pore structure and density of the carbon nanofoam, or alternatively carbon nanotube/nanofiber assemblies; and (iv) thermal treatments to balance the degree of order (crystallinity) and disorder in the deposited MnO2 phases. We are also investigating how the composition and pH of the electrolytes used with such composite electrode structures affect the redox mechanisms and charge-storage properties of the MnO2 phase. For example, the electrochemical response of MnO2 in mild aqueous electrolytes mimics a capacitance mechanism (see Figure 1, top panel), although XPS measurements confirm that a Mn redox reaction is responsible for the additional charge storage observed. This same MnO2–carbon electrode structure exhibits a very different electrochemical response in 9 M KOH, where distinct and reasonably reversible redox peaks are observed, indicating a different charge-storage mechanism. A more thorough understanding of the charge-storage mechanisms of nanoscopic MnO2 phases will be critical for further optimization of the electrochemical performance of such composite electrode structures.