Abstract: Designing electrodes and catalytic platforms as architectures in which the entire volume of the porous “reactor” is wired continuously in three dimensions for electron, ion, and molecular transport expands the reactive electrochemical and catalytic turf beyond the limited footprint imposed by a two-dimensional cross-section or a single three-phase boundary per supported nanoparticle. Amplifying the electron/ion/molecularly wired interfacial area by hundreds of square centimeters per cross-sectional square centimeter converts redox reactions that lose morphological control at high local current density into more uniformly reactive events that experience low local current density. Similar distributed arrangement of the reacting species in architected catalytic platforms imparts resilience of the supported catalyst. Aperiodic architectures such as foams and sponges effectively distribute the available reactive, electron/ion/molecularly wired interfaces while maintaining a co-continuous mapping of void and solid to facilitate ingress/egress of reactants and products. Examples from our work with electrode architectures show the power of controlling energy-storage reactions locally by distributing them within electron-wired high-surface interiors. The arrangement ensures that per area current remains low throughout the volume of the electrode, yet the electrified area sums to provide device-relevant current. The intimate interfacial contact we achieve for copper nanoparticles supported on ceria aerogels yields active, selective, and stable architected catalysts for preferential oxidation of carbon monoxide in hydrogen feedstreams.

Abstract: Transition-metal oxides that exhibit “pseudo-capacitance” are challenging high-surface-area carbons as the active material of choice for next-generation electrochemical capacitors (ECs). 1 The electrochemical performance of such EC-relevant metal oxides as MnO x has been extensively reported, 2 yet the underlying mechanisms responsible for pseudocapacitance are still a subject of debate in the scientific literature. Spectroscopic techniques have been used to verify reversible oxidation-state cycling of metal sites in these oxides (Mn(III/IV) in MnO x ; Fe(II/III) in FeO x ), 3,4 but less is known about the participation of charge-compensating cations from the contacting electrolyte. In the case of mild aqueous electrolytes, redox reactions that give rise to pseudocapacitance may be coupled with insertion/ deinsertion of cations from the electrolyte salt (e.g., Li + , Na + , K + ), protons that are available from the H 2 O solvent, or combinations thereof. Electrochemical quartz crystal microbalance (EQCM) techniques provide information on changes in electrode mass during electrochemical cycling that can address questions regarding ion insertion. 5 Until recently the use of EQCM has been limited to well-defined thin-film electrodes, but advancements in EQCM methods now permit analysis of a wider range of substrates, including powder-composite electrodes of EC relevance. 6,7 In this paper, we will describe the application of EQCM analysis to a class of electrode architectures comprising carbon nanofoam papers 8 whose exterior and interior surfaces are exhaustively coated with nanometers-thick metal oxides that exhibit pseudocapacitance. 4 ,9,10,11 Carbon nanofoam papers are freestanding, device-ready electrode architectures of controllable macroscale dimensions (1–10 2 cm 2 in area; 100–500 mm thickness) in which nanoscale dimensions of pore and solid can also be readily adjusted over a wide range (10 nm to several mm). The addition of a nanoscale metal-oxide coating to nanofoam substrates amplifies charge-storage capacity via pseudocapacitance, while the high-frequency response inherent to the nanofoam architecture is retained by ensuring that the metal oxide coating is conformal and self-limiting and that the pore network stays open in three dimensions. For EQCM analysis of these nanofoam-based electrode architectures, we attach disk-shaped pieces of native or oxide-coated carbon nanofoam paper to Au-coated quartz crystals using a carbon-based conductive epoxy. Voltammetric cycling of the resulting electrodes in an EQCM cell yields the expected pseudocapacitance response in terms of both shape and magnitude, demonstrating effective electrical contact to the nanofoam paper. Coupled to the electrochemical response, we monitor mass changes during electrochemical cycling while also varying such parameters as aqueous electrolyte composition (Li + vs . Na + vs . K + ), average pore size of the nanofoam (10–200 nm), and the type of metal-oxide coating (MnO x vs. FeO x ). Preliminary EQCM experiments show that “depletion” effects 6 are observed in the presence of Li + -containing electrolytes and for MnO x -coated nanofoams with pores 〈 40 nm, in which the pseudocapacitance mechanism switches from Li + to NO 3 – insertion/deinsertion over the most positive regions of the active potential window of MnO x . The EQCM-observed depletion effect corresponds with the slowest time response of this series of MnO x –carbon nanofoams, as previously reported. 12 We compare EQCM results obtained from nanofoam-based electrode architectures with those for thin-film versions of the respective metal oxides. Monitoring mass changes during electrochemical cycling will also allow us determine the conditions under which deleterious dissolution reactions may occur. References 1. J.W. Long, D. Bélanger, T. Brousse, W. Sugimoto, M.B. Sassin, and O. Crosnier, MRS Bull. 7 , 513 (2011). 2. D. Bélanger, T. Brousse, and J.W. Long, ECS Interface 17(1) , 49 (2008). 3. J.K. Chang, M.T. Lee, and W.T. Tsai , J. Power Sources 166 , 490 (2007). 4. M.B. Sassin, A.N. Mansour, K.A. Pettigrew, D.R. Rolison, and J.W. Long, ACS Nano 4 , 4505 (2010). 5. D.A. Buttry and M.D. Ward, Chem. Rev . 92 , 1355 (1992). 6. M.D. Levi, G. Salitra, N. Levy, D. Aurbach, and J. Maier, Nature Mater. 8 , 872 (2009). 7. See http://www.gamry.com/application-notes/characterization-of-a-supercapacitor-using-an-electrochemical-quartz-crystal-microbalance/. 8. J.C. Lytle, J.M. Wallace, M.B. Sassin, A.J. Barrow, J.W. Long, J.L. Dysart, C.H. Renninger, M.P. Saunders, N.L. Brandell, and D.R. Rolison, Energy Environ. Sci. 4 , 1913 (2011). 9. A.E. Fischer, K.A. Pettigrew, R.M. Stroud, D.R. Rolison, and J.W. Long, Nano Lett . 7 , 281 (2007). 10. A.E. Fischer, M.P. Saunders, K.A. Pettigrew, D.R. Rolison, and J.W. Long, J. Electrochem. Soc . 1 55 , A246 (2008). 11. M.B. Sassin, C.N. Chervin, D.R. Rolison, and J.W. Long, Acc. Chem. Res. 46 , 1062 (2013). 12. M.B. Sassin, C.P. Hoag, B.T. Willis, N.W. Kucko, D.R. Rolison, and J.W. Long, Nanoscale 5 , 1649 (2013).

Abstract: Electrochemical capacitors (ECs) comprising high-surface-area carbon electrodes and nonaqueous electrolytes offer many attractive performance attributes including long cycle life and the ability to operate at low temperatures (–40⁰C), yet their energy density is ultimately limited by reliance on the double-layer charge-storage mechanism. Asymmetric EC configurations that include pseudocapacitive materials (e.g., transition metal oxides) offer the opportunity to increase energy density while also using safer and cheaper aqueous electrolytes. We have shown that with the appropriate electrode structure and electrolyte additives, the additional charge stored via pseudocapacitance is delivered at EC-like rates over 1000 cycles with minimal capacitance fade. To further prove the practical feasibility of such aqueous-electrolyte ECs, we turned our attention to low-temperature performance, focusing on aqueous electrolyte compositions containing lithium or sodium cations and sulfate or nitrate anions. We characterized pertinent bulk properties (e.g., freezing point and ionic conductivity) of down-selected electrolytes and found that the anion dominates the freezing point, with sulfate-based electrolytes having a freezing point of –45 o C, while nitrate-based electrolytes have a freezing point of –35 o C. The ionic conductivities ranged from 63 to 136 mS cm -1 at 20 o C, conductive enough to support high-rate operation. To assess the impact of temperature on performance as a function of electrolyte composition, we fabricated symmetric pouch cells comprised of MnO x @carbon nanofoam electrodes, evaluating the resulting devices via cyclic voltammetry and electrochemical impedance spectroscopy over a wide temperature range (25⁰C to –45⁰C). All ECs exhibited a gradual decrease in capacitance as the temperature decreased; however, capacitance was recovered upon warming to 25°C for all electrolyte compositions. We found that ECs with Li 2 SO 4 -based electrolytes (neat or mixed composition) were operational at temperatures as low as –35°C, revealing that these electrolytes are competitive with the nonaqueous electrolytes used in conventional double layer capacitors.

Abstract: Electrochemical capacitors, ECs, should provide high performance over the broadest range of operating temperatures which is primarily determined by the properties of the electrolyte (freezing and boiling points, ionic conductivity and salt solubility as a function of temperature, flash point for flammable solvents). In the case of mild-pH aqueous electrolytes, where flammability is a non-issue, the sub-ambient temperature performance is the most critical factor. En route to demonstrating the performance of aqueous-electrolyte ECs at low temperature, we have characterized both the bulk thermal properties of single and multi-component electrolytes and their behavior when used in EC pouch cells. The ionic conductivities of bulk electrolytes with a fixed cation concentration of 5 M for single-component (e.g. Li 2 SO 4 , NaNO 3 ) and multi-component electrolytes (e.g. Li 2 SO 4 + NaNO 3 ) ranged from 63 to 136 mS cm -1 at 20 o C. The freezing points of these electrolytes were measured with differential scanning calorimetry. It was found that the anion dominates the freezing-point behavior, with sulfate-based electrolytes supporting freezing points down to –45 o C and nitrate-based electrolytes down to –35 o C. To assess the impact of temperature on performance as a function of electrolyte composition, we fabricated symmetric pouch cells comprising MnO x -carbon nanofoam electrodes and tested them over the temperature range of 25 o C to –45 o C in an environmental chamber. All ECs exhibited a gradual decrease in capacitance as the temperature decreased; however, upon re-warming to 25°C, the capacitance was recovered, indicating that freezing does not cause significant degradation to electrode or separator components. Aqueous electrolytes comprising mixtures of Li 2 SO 4 and NaNO 3 yielded the best combination of depressed freezing point and ionic conductivity among the compositions evaluated in EC pouch cells.

Abstract: As the energy and power demands of current and next-generation devices continually strain the capabilities of present power sources, there is a push to develop materials with high capacity and electrode architectures to facilitate delivering that capacity at high rates. En route to addressing this energy/power issue, we have shown that with the appropriate ultraporous electrode architecture, it is possible to extract full theoretical capacity from LiMn 2 O 4 , a nominal battery material, in as little as 18 seconds.[i] While representing a significant advancement at the single-electrode level, the implementation of these advanced electrode materials in practical devices has been hampered by the lack of suitable negative electrode materials with sufficient capacity to balance that of the LiMn 2 O 4 . A conducting polymer-based negative electrode would be ideal to pair with the LiMn 2 O 4 in terms rate capability, but issues with solution processability have typically limited these materials to thin film-based electrodes, which will not provide adequate capacity for the LiMn 2 O 4 . Recently, the Reynolds group at Georgia Tech has developed synthetic protocols to overcome this roadblock for ProDOT-biEDOT-based polymers.[ii],[iii] This advancement makes it possible to incorporate such conducting polymers as thin, conformal coatings on the interior and exterior surfaces of conductive, porous carbon nanofoam-based electrodes. The nanoscale nature of the coating coupled with the intrinsically fast charge-discharge of the polymer supports high rate and the 3D projection of the conducting polymer via the macroscopically-thick carbon nanofoam provides high area-normalized capacitance to balance that of the LiMn 2 O 4 . [i]. Sassin, M.B., Long, J.W., Greenbaum, S.G., Stallworth, P.E., Mansour, A.N., Hahn, B.P., and Pettigrew, K.A., “Achieving electrochemical capacitor functionality from a traditional battery material: Conformal, nanoscale LiMn2O4 coatings on 3-D, device-ready carbon nanoarchitectures,” J. Mater. Chem . 1 , 2431 (2013). [ii]. Ponder Jr., J. F.; Österholm, A. M.; Reynolds, J. R. “Designing a Soluble PEDOT Analogue without Surfactants or Dispersants, ” Macromolecules 49 , 2106 (2016). [iii]. Österholm, A. M.; Ponder Jr., J. F.; Kerszulis, J. A.; Reynolds, J. R., “Solution Processed PEDOT Analogues in Electrochemical Supercapacitors, ” ACS Appl. Mater. Interfaces 8 , 13492 (2016).

Abstract: Manganese oxides (MnO x ) 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 MnO x exhibit pseudocapacitive charge-storage behavior that can be tapped for pulse-power applications. The ability of MnO x 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 MnO x 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-MnO x vs. cubic spinel LiMn 2 O 4 ) and the composition of the contacting electrolyte (mixtures of Na + , Li + , and/or Zn 2+ ) [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 MnO x //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. E. Fischer, K.A. Pettigrew, D.R. Rolison, R.M. Stroud, and J.W. Long, Nano Letters 2007 , 7 , 281–286. W. Long, M.B. Sassin, A.E. Fischer, and D.R. Rolison, J. Phys. Chem. C 2009 , 113 , 17595–17598. 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. S. Ko, M.B. Sassin, J.F. Parker, D.R. Rolison, and J.W. Long, Sustainable Energy Fuels , 2018 , 2 , 626–636.

Abstract: Metal–air batteries have the highest specific (packaged) energy among common battery chemistries (over 400 Whkg –1 for Zn–air) with the added benefits conferred by aqueous electrolytes of safety and affordability. But air-dependent batteries are power-limited by catalytic and mass-transfer restrictions associated with reduction of molecular oxygen at the air cathode—an electrode that must support multiple tasks: O 2 transport, ion transport, electron conduction, and electrocatalytic reactivity. To address this power limitation, we have redesigned the air cathode using an architectural and nanoscopic perspective that marries complementary characteristics of batteries and electrochemical capacitors. We demonstrate that air cathodes comprising fiber paper–supported carbon nanofoams modified with conformal nanoscale coatings of Na + -birnessite-type MnO x (10–20 nm thick) exhibit dual functionality: ( i ) oxygen reduction to sustain long-term energy delivery and ( ii ) faradaic pseudocapacitance associated with Mn(III/IV) redox to provide intermittent, farads-worth discharge pulses over 10s of seconds [1,2]. Manganese (III) sites in the post-pulsed oxide spontaneously recharge to Mn(IV) in the presence of oxygen from air, making the oxide ready and available for subsequent pulse-power discharge. [1] Redesigning air cathodes for metal–air batteries using MnO x -functionalized carbon nanofoam architectures. C.N. Chervin, J.W. Long, N.L. Brandell, J.M. Wallace, and D.R. Rolison, J. Power Sources 2012 , 207 , 191–199. [2] Dual-function air cathode nanoarchitectures for metal–air batteries with air-independent pulse power capability. J.W. Long, C.N. Chervin, N.W. Kucko, E.S. Nelson, and D.R. Rolison, Adv. Energy Mater. 2013 , 3 , 584–588.

Abstract: Transition-metal oxides that exhibit “pseudocapacitance” are promising alternatives to high-surface-area carbons as charge-storing materials in next-generation electrochemical capacitors (ECs). Hydrous ruthenium oxides remain the state-of-the-art for pseudocapacitive materials due to their fortuitous combination of high electronic and ionic conductivity. Lower-cost alternatives are being vigorously pursued, yet the low electronic conductivity of most other metal oxides of interest (e.g., MnO x ) necessitates that they be thoughtfully incorporated with a conductive carbon support. We have developed an electrode design in which pseudocapacitive oxides, such as MnO x and FeO x , are applied as nanoscale coatings onto ultraporous carbon nanofoam substrates that define the macroscale-to-nanoscale structure of the resulting electrode architecture [1,2]. In addition to their practical advantages for device fabrication, these nanofoam paper-based materials have also provided a designer platform with which to investigate the interplay of pore structure and electrochemical performance [3] and for in situ analysis of the mechanisms responsible for pseudocapacitance [4]. In the background of these studies, we have accumulated evidence that the physicochemical nature of the carbon–metal oxide interface can have a significant impact on electrochemical performance. While continuing to develop and transition our 3D electrode architectures, we are refocusing our research efforts on investigating fundamental charge-transfer properties at nanoscale carbon–metal oxide interfaces. Shifting from the characterization complexities of the nanofoam-based architectures, we use planar pyrolytic-carbon substrates to mimic the surface properties of 3D carbon, but in forms that are more readily characterized by conventional surface spectroscopy and scanning-probe microcopy techniques. We apply nanoscale pseuodocapacitive oxides to these planar carbon films using redox-deposition protocols previously demonstrated at 3D carbons [5], and explore how the physical, chemical, and electronic structure of the resulting carbon–metal oxide interface impacts electrochemical properties. Lessons learned from these model interfaces are readily translated to improved performance in practical 3D electrode architectures. 1. Fischer, A.E.; Saunders, M.P.; Pettigrew, K.A.; Rolison, D.R.; Long, J.W. J. Electrochem. Soc . 2008 , 155 , A246. 2. Sassin, M.B.; Mansour, A.N.; Pettigrew, K.A.; Rolison, D.R.; Long, J.W. ACS Nano 2009 , 4 , 4505. 3. Sassin, M.B.; Hoag, C.P.; Willis, B.T.; Kucko, N.W.; Rolison, D.R.; Long, J.W. Nanoscale , 2013 , 5 , 1649. 4. Beasley, C.A.; Sassin, M.B.; Long, J.W. J. Electrochem. Soc. 2015 , 162 , A5060. 5. Sassin, M.B. Chervin, C.N.; Rolison, D.R.; Long, J.W. Acc. Chem. Res. 2013 , 46 , 1062.

Abstract: Hard (non-graphitizable) carbon has emerged as a prospective charge-storage material for negative electrodes in sodium-ion batteries, yet reported electrochemical performance across this broad category of carbons varies widely. Such discrepancies arise in part due to the multiple distinct sodiation reactions that are possible with disordered carbon electrodes, whereby sodium ions can be stored in defect sites, graphitic layers, and/or micropores depending on the pore–solid architecture, surface chemistry, and solid-state structure of a particular carbon. To address both fundamental questions and practical application in Na-ion batteries, we are investigating carbon nanofoam papers (CNFPs), synthesized by infiltrating the voids of carbon-fiber paper with resorcinol–formaldehyde (R–F) formulations to form porous polymer nanofoam that is subsequently converted to the conductive carbon analog via pyrolysis at 1000°C [1]. When tested as free-standing electrode architectures in Na-ion cells, CNFPs deliver specific capacity 〉 300 mAh g –1 at a 1C rate and 〉 250 mAh g –1 at 10C, with a first-cycle Coulombic efficiency near 85% under galvanostatic operation. The galvanostatic intermittent titration technique (GITT) confirms that Na-ion diffusion is facile in the defect-mediated charge-storage regime. We attribute these favorable properties to the high defect concentration in the disordered R–F-derived carbon domains, the 3D-interconnected porosity within the carbon nanofoam, and the absence of otherwise-necessary binder and conductive additives that are commonly used in conventional powder-composite electrodes. Our results demonstrate the utility of CNFPs as device-ready, self-supported electrodes and advance the design of related carbon materials for next-generation Na-ion batteries. [1] J.C. Lytle, J.M. Wallace, M.B. Sassin, A.J. Barrow, J.W. Long, J.L. Dysart, C.H. Renninger, M.P. Saunders, N.L. Brandell, and D.R. Rolison, Energy Environ. Sci. , 4 (2011) 1913–1925.

Abstract: We recently demonstrated that advanced multifunctional 3D MnO x @carbon nanofoam (MnO x @CNF) electrodes cycled in mixed-salt aqueous electrolytes [1] extend the performance advantages of rechargeable “zinc-ion” batteries, an emerging energy-storage technology with inherent cost and safety benefits. [2] , [3] The combination of the multifunctional birnessite-MnO x @CNF and mild-pH Zn 2+ -containing electrolyte results in a complex charge-storage mechanism in which a H + inserts into the birnessite-MnO x during discharge, resulting in an increase in the local pH that triggers precipitation of Zn 4 (OH) 6 SO 4 ·5H 2 O, while Na-ions support pseudocapacitance reactions. [4] This dual charge-storage mechanism yields high capacity at low rates (308 mA h g -1 at 1C, MnO x theoretical capacity) and pulse power at high rates (100 mA h g -1 at 20C) via pseudocapacitance; these mechanisms are reversible over hundreds of cycles, attributed in part to the through-connected pore structure of the CNF. In order to explore the efficacy of other MnO x polymorphs in such 3D multifunctional electrode designs, we developed an “in-place” conversion to generate nanocrystalline ZnMn 2 O 4 spinel from the birnessite-MnO x coating on the CNF. Crystallization is achieved by heating at relatively mild temperatures (300°C), such that the nanoscale morphology of the original MnO x coating and through-connected pore structure of the underlying CNF are maintained. We used a suite of ex-situ characterization methods (SEM/EDS, XRD, XPS) to elucidate the charge-storage reaction of ZnMn 2 O 4 @CNF in 1 M ZnSO 4 and found that it is even more complex than the charge-storage mechanism of birnessite-MnO x @CNF. Discharge of ZnMn 2 O 4 @CNF proceeds via two steps, the first occurring by Zn 2+ insertion into the spinel and the second by H + insertion accompanied by Zn 4 (OH) 6 SO 4 ·5H 2 O precipitation; the reaction reverses upon recharge. We will discuss the implications of these mechanisms for such performance characteristics as rate capability and cycle life in their ultimate application as positive electrodes in next-generation zinc-ion batteries. [1] . J.S. Ko, M.B. Sassin, J.F. Parker, D.R. Rolison, and J.W. Long: Combining battery-like and pseudocapacitive charge storage in 3D MnO x @carbon electrode architectures for zinc-ion cells. Sustainable Energy Fuels 2 , 626–636 (2018). [2] . B. Tang, L. Shan, S. Liang, and J. Zhou: Issues and opportunities facing aqueous zinc-ion batteries. Energy Environ. Sci . 12 , 3288–3304 (2019). [3] . L. E. Blanc , D. Kundu , and L. F. Nazar : Scientific Challenges for the Implementation of Zn-Ion Batteries. Joule 4 , 771–799 (2020). [4] . J.S. Ko, M.D. Donakowski, M.B. Sassin, J.F. Parker, D.R. Rolison, and J.W. Long: Deciphering charge-storage mechanisms in 3D MnO x @carbon electrode nanoarchitectures for rechargeable zinc-ion cells. MRS Communications 9 , 99–106 (2019).