Integrating the multifunction necessary for electrochemical power into energy- and size-scalable ultraporous nanoarchitectures
Debra R. Rolison
1Surface Chemistry Branch, Code 6170, Naval Research Laboratory, Washington, DC 20375
Designing high-performance energy-storage devices that combine nanometric feature size with well-wired transport paths and that bridge to the macroscale requires an architectural perspective. We use aerogel-like carbon nanofoam papers as our architectural test-bed because they provide a low cost and scalable nanocomposite that offers an optimal balance of critical architectural features: (1) open, 3D interconnected macropores sized at 20-to-250 nm co-continuous with (2) ~20-nm pore walls of a size that reduces dead weight and volume yet retains mechanical strength and flexibility without compromising electronic conductivity. Charge-storage or catalytic functionality can then be imparted to internal carbon walls simply by transporting reactants within the 3D “plumbing” of the macroporous foam. Self-limiting modification strategies allow us to incorporate conformal, nanoscopic functional “paints” of metal(Mn, Ti, Ru, Fe)oxides or polymer (redox-active or electron insulating) or specifically adsorb metal nanoparticles (Pt, Au, Pd, Ag) throughout the macroscopic thickness (0.07 to 0.3 millimeter) of carbon nanofoam papers. For instance, painting the carbon walls with 10-nm MnOx increases the mass-, geometric-, and volume-normalized capacitance (2- to 10-fold) relative to the native carbon nanofoam without significantly altering its high-rate character. The oxide-modified paper is now a multifunctional electrode structure that can be used in an aqueous asymmetric electrochemical capacitor or as an air cathode in a Zn/air cell to electrocatalyze oxygen reduction and provide pulse power. Our redesign of electrode structures using modified carbon nanofoam papers has catalyzed breakthroughs in our work within a broad range of multifunctional energy storage and conversion, including asymmetric electrochemical capacitors, air cathodes for metal–air batteries, 3-D batteries, and semi-fuel cells.