Megan B. Sassin

Electroless deposition of conformal nanoscale iron oxide on carbon nanoarchitectures for electrochemical charge storage.

We describe a simple self-limiting electroless deposition process whereby conformal, nanoscale iron oxide (FeO(x)) coatings are generated at the interior and exterior surfaces of macroscopically thick ( approximately 90 microm) carbon nanofoam paper substrates via redox reaction with aqueous K(2)FeO(4). The resulting FeO(x)-carbon nanofoams are characterized as device-ready electrode structures for aqueous electrochemical capacitors and they demonstrate a 3-to-7 fold increase in charge-storage capacity relative to the native carbon nanofoam when cycled in a mild aqueous electrolyte (2.5 M Li(2)SO(4)), yielding mass-, volume-, and footprint-normalized capacitances of 84 F g(-1), 121 F cm(-3), and 0.85 F cm(-2), respectively, even at modest FeO(x) loadings (27 wt %). The additional charge-storage capacity arises from faradaic pseudocapacitance of the FeO(x) coating, delivering specific capacitance >300 F g(-1) normalized to the content of FeO(x) as FeOOH, as verified by electrochemical measurements and in situ X-ray absorption spectroscopy. The additional capacitance is electrochemically addressable within tens of seconds, a time scale of relevance for high-rate electrochemical charge storage. We also demonstrate that the addition of borate to buffer the Li(2)SO(4) electrolyte effectively suppresses the electrochemical dissolution of the FeO(x) coating, resulting in <20% capacitance fade over 1000 consecutive cycles.

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.

Achieving electrochemical capacitor functionality from nanoscale LiMn2O4 coatings on 3-D carbon nanoarchitectures

Conformal nanoscale coatings of Na+-birnessite manganese oxide (MnOx) produced via redox reaction between aqueous permanganate (NaMnO4·H2O) and the carbon surfaces of fiber-paper-supported carbon nanofoams are converted to LiMn2O4 spinel through topotactic exchange of Na+ for Li+ in the as-deposited lamellar birnessite, followed by mild thermal treatments to complete the transformation to LiMn2O4. The evolution of the birnessite-to-spinel conversion is verified with X-ray diffraction, solid-state nuclear magnetic resonance, X-ray absorption spectroscopy, electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. The mild conditions used to convert birnessite to spinel ensure that the conformal nanoscale nature of the oxide coating is retained throughout the macroscopically thick (170 μm) carbon nanofoam substrate during the conversion process. The architecture of the LiMn2O4–carbon nanofoam facilitates rapid ion/electron transport, enabling the LiMn2O4 to insert and extract Li+ from aqueous electrolytes at scan rates as high as 25 mV s−1, and with a relaxation time of 37 s as derived from electrochemical impedance. This architectural expression of nanoscale LiMn2O4 delivers full theoretical capacity (148 mA h g−1) at 2 mV s−1.

Designing high-performance electrochemical energy-storage nanoarchitectures to balance rate and capacity

The impressive specific capacitance and high-rate performance reported for many nanometric charge-storing films on planar substrates cannot impact a technology space beyond microdevices unless such performance translates into a macroscale form factor. In this report, we explore how the nanoscale-to-macroscale properties of the electrode architecture (pore size/distribution, void volume, thickness) define energy and power performance when scaled to technologically relevant dimensions. Our test bed is a device-ready electrode architecture in which scalable, manufacturable carbon nanofoam papers with tunable pore sizes (5-200 nm) and thickness (100-300 μm) are painted with ~10 nm coatings of manganese oxide (MnOx). The quantity of capacitance and the rate at which it is delivered for four different MnOx-C variants was assessed by fabricating symmetric electrochemical capacitors using a concentrated aqueous electrolyte. Carbon nanofoam papers containing primarily 10-20 nm mesopores support high MnOx loadings (60 wt%) and device-level capacitance (30 F g(-1)), but the small mesoporous network hinders electrolyte transport and the low void volume restricts the quantity of charge-compensating ions within the electrode, making the full capacitance only accessible at slow rates (5 mV s(-1)). Carbon nanofoam papers with macropores (100-200 nm) facilitate high rate operation (50 mV s(-1)), but deliver significantly lower device capacitance (13 F g(-1)) as a result of lower MnOx loadings (41 wt%). Devices comprising MnOx-carbon nanofoams with interconnecting networks of meso- and macropores balance capacitance and rate performance, delivering 33 F g(-1) at 5 mV s(-1) and 23 F g(-1) at 50 mV s(-1). The use of carbon nanofoam papers with size-tunable pore structures and thickness provides the opportunity to engineer the electrode architecture to deliver scalable quantities of capacitance (F cm(-2)) in tens of seconds with a single device.

Fabrication Method for Laboratory-Scale High-Performance Membrane Electrode Assemblies for Fuel Cells

Custom catalyst-coated membranes (CCMs) and membrane electrode assemblies (MEAs) are necessary for the evaluation of advanced electrocatalysts, gas diffusion media (GDM), ionomers, polymer electrolyte membranes (PEMs), and electrode structures designed for use in next-generation fuel cells, electrolyzers, or flow batteries. This Feature provides a reliable and reproducible fabrication protocol for laboratory scale (10 cm2) fuel cells based on ultrasonic spray deposition of a standard Pt/carbon electrocatalyst directly onto a perfluorosulfonic acid PEM.

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.

Combining battery-like and pseudocapacitive charge storage in 3D MnOx@carbon electrode architectures for zinc-ion cells

We demonstrate that electrodes comprising nanoscale, birnessite-type manganese oxide affixed to carbon nanofoam paper (MnOx@CNF) exhibit two distinct charge-storage mechanisms—battery-like Zn2+ insertion/de-insertion and pseudocapacitance—when electrochemically cycled in aqueous electrolytes that include both Na+ and Zn2+ salts. When the mixed-electrolyte composition is 0.75 M Na2SO4 + 0.25 M ZnSO4 (i.e., “6[Na+] : 1[Zn2+]”), the MnOx@CNF electrode delivers high specific capacity at low rates, approaching theoretical capacity for Zn2+ insertion/de-insertion at MnOx. At high rates (>10C) the Na+-supported pseudocapacitance mechanism maintains charge-storage capacity well above that observed with electrolytes that contain only ZnSO4. Impedance analysis was performed to discriminate between these distinct charge-storage mechanisms by visualizing the frequency- and potential-dependent capacitance as 3D Bode plots. In the 6[Na+] : 1[Zn2+] electrolyte, the potential-independent pseudocapacitance is augmented by reversible Zn2+-based redox processes between 1.4 and 1.8 V vs. Zn/Zn2+. Galvanostatic testing with two-electrode zinc-ion cells that pair MnOx@CNF with a zinc foil negative electrode proves the practical performance advantages of combining pseudocapacitance and Zn2+-insertion mechanisms: higher energy efficiency and greater specific power in the 6[Na+] : 1[Zn2+] electrolyte compared to 1 M ZnSO4.

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.

3D architectures are not just for microbatteries anymore

Building battery architectures with functional interfaces that are interpenetrated in three dimensions opens the door to major gains in performance as compared to conventional 2-D battery designs, particularly with respect to the battery footprint. We are developing 3-D solid-state Li-ion batteries that are sequentially assembled from interpenetrating and tricontinuous networks of anode, cathode, and electrolyte/separator materials. We use fiberpaper- supported carbon nanofoams as a massively parallel, conductive, ultraporous base platform within which to create the 3-D cell. The components required for battery operation are incorporated into the x,y,z-scalable papers and include nanoscale coatings of metal oxides that serve as Li-ion-insertion electrodes and ultrathin, electroninsulating/ Li-ion conducting polymer coatings that serve as the electrolyte/separator.