Jean Marie Wallace

The right kind of interior for multifunctional electrode architectures: carbon nanofoam papers with aperiodic submicrometre pore networks interconnected in 3D

Carbon nanoarchitectures are versatile platforms for advanced electrode structures in which the carbon edifice serves multiple simultaneous functions: a massively parallel 3-D current collector with an interpenetrating structural flow field that facilitates the efficient transport of electrons, ions, and molecules throughout the structure for further functionalization or high-performance electrochemical operation. We fabricate carbon nanofoam papers by infiltrating commercially available low-density carbon fiber papers with phenolic resin. The polymer-filled paper is ambiently dried and then pyrolyzed to create lightweight, mechanically flexible, and electronically conductive sheets of ultraporous carbon with an electronic conductivity characteristic of the paper support (20–200 S cm−1) rather than RF-derived carbon (typically 0.1–1 S cm−1). The resulting composites comprise nanoscopic carbon walls that are co-continuous with an aperiodic, 3-D interconnected network of mesopores (2 to 50 nm) and macropores (50 nm to 2 µm). Macropores sized at 100–300 nm have not been adequately explored in the literature and offer ample headspace to modify internal carbon walls, thereby introducing new functionality without occluding the interconnected void volume of the nanofoam. Increasing the viscosity of the polymer sol and matching the surface energetics of the carbon fibers and aqueous sol is necessary to avoid forming a standard carbon aerogel pore–solid structure, where the pores are sized in the micropore (<2 nm) and mesopore range. Carbon nanofoam papers can be scaled in x, y, and z and are device-ready electrode structures that do not require conductive additives or polymeric binders for electrode fabrication. This one class of nanofoams serves as a high-surface-area scaffold that can be segued by appropriate modification into multifunctional nanoarchitectures that improve the performance of electrochemical capacitors, lithium-ion batteries, metal–air batteries, fuel cells, and ultrafiltration.

Manganese Oxide Nanoarchitectures as Broad-Spectrum Sorbents for Toxic Gases

We demonstrate that sol-gel-derived manganese oxide (MnOx) nanoarchitectures exhibit broad-spectrum filtration activity for three chemically diverse toxic gases: NH3, SO2, and H2S. Manganese oxides are synthesized via the reaction of NaMnO4 and fumaric acid to form monolithic gels of disordered, mixed-valent Na-MnOx; incorporated Na(+) is readily exchanged for H(+) by subsequent acid rinsing to form a more crystalline H-MnOx phase. For both Na-MnOx and H-MnOx forms, controlled pore-fluid removal yields either densified, yet still mesoporous, xerogels or low-density aerogels (prepared by drying from supercritical CO2). The performance of these MnOx nanoarchitectures as filtration media is assessed using dynamic-challenge microbreakthrough protocols. We observe technologically relevant sorption capacities under both dry conditions and wet (80% relative humidity) for each of the three toxic industrial chemicals investigated. The Na-MnOx xerogels and aerogels provide optimal performance with the aerogel exhibiting maximum sorption capacities of 39, 200, and 680 mg g(-1) for NH3, SO2, and H2S, respectively. Postbreakthrough characterization using X-ray photoelectron spectroscopy (XPS) and diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) confirms that NH3 is captured and partially protonated within the MnOx structure, while SO2 undergoes oxidation by the redox-active oxide to form adsorbed sulfate at the MnOx surface. Hydrogen sulfide is also oxidized to form a combination of sulfate and sulfur/polysulfide products, concomitant with a decrease in the average Mn oxidation state from 3.43 to 2.94 and generation of a MnOOH phase.

Routes to 3D conformal solid-state dielectric polymers: electrodeposition versus initiated chemical vapor deposition

We show that two distinct methods, electropolymerization and initiated chemical vapour deposition (iCVD), can be adapted to generate ultrathin polymers (30–50 nm thick) at three dimensionally (3D) porous conductive substrates comprising ∼300 μm-thick carbon-coated silica fiber paper (C@SiO2). We selected 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (“V3D3”) as a common monomer amenable to polymerization by either approach. Electroanalytical and electrical measurements confirm that all carbon surfaces are passivated with electronically insulating poly(V3D3) coatings.

Crystal engineering in 3D: converting nanoscale lamellar manganese oxide to cubic spinel while affixed to a carbon architecture

By applying differential pair distribution function (DPDF) analyses to the energy-storage relevant MnOx/carbon system—but in a 3D architectural rather than powder-composite configuration—we can remove contributions of the carbon nanofoam paper scaffold and quantify the multiphasic oxide speciation as the nanoscale, disordered MnOx grafted to the carbon walls (MnOx@CNF) structurally rearranges in situ from disordered birnessite AMnOx (A = Na+; Li+) to tetragonal Mn3O4 to spinel LiMn2O4. The first reaction step involves topotactic exchange of interlayer Na+ by Li+ in solution followed by thermal treatments to crystal engineer the ∼10 nm-thick 2D layered oxide throughout the macroscale nanofoam paper into a cubic phase. The oxide remains affixed to the walls of the nanofoam throughout the phase transformations. The DPDF fits are improved by retention of one plane of birnessite-like oxide after conversion to spinel. We support the DPDF-derived assignments by X-ray photoelectron spectroscopy and Raman spectroscopy, the latter of which tracks how crystal engineering the oxide affects the disorder of the carbon substrate. We further benchmark MnOx@CNF with nonaqueous electrochemical measurements versus lithium as the oxide converts from X-ray-amorphous birnessite to interlayer-registered LiMnOx to spinel. The lamellar AMnOx displays pseudocapacitive electrochemical behavior, with a doubling of specific capacitance for the interlayer-registered LiMnOx, while the spinel LiMn2O4@CNF displays a faradaic electrochemical response characteristic of Li-ion insertion. Our results highlight the need for holistic understanding when crystal engineering an (atomistic) charge-storing phase within the (architectural) structure of practical electrodes.

Cytochrome c Stabilization and Immobilization in Aerogels

Sol-gel-derived aerogels are three-dimensional, nanoscale materials that combine large surface area with high porosity. These traits make them useful for any rate-critical chemical process, particularly sensing or electrochemical applications, once physical or chemical moieties are incorporated into the gels to add their functionality to the ultraporous scaffold. Incorporating biomolecules into aerogels, other than such rugged species as lipases or cellulose, has been challenging due to the inability of most biomolecules to remain structurally intact within the gels during the necessary supercritical fluid (SCF) processing. However, the heme protein cytochrome c (cyt.c) forms self-organized superstructures around gold (or silver) nanoparticles in buffer that can be encapsulated into wet gels as the sol undergoes gelation. The guest-host wet gel can then be processed to form composite aerogels in which cyt.c retains its characteristic visible absorption. The gold (or silver) nanoparticle-nucleated superstructures protect the majority of the protein from the harsh physicochemical conditions necessary to form an aerogel. The Au~cyt.c superstructures exhibit rapid gas-phase recognition of nitric oxide (NO) within the bioaerogel matrix, as facilitated by the high-quality pore structure of the aerogel, while remaining viable for weeks at room temperature. More recently, careful control of synthetic parameters (e.g., buffer concentration, protein concentration, SCF extraction rate) have allowed for the preparation of cyt.c-silica aerogels, sans nucleating nanoparticles; these bioaerogels also exhibit rapid gas-phase sensing while retaining protein structural stability.