H+ insertion/Zn4(OH)6SO4 precipitation is the dominant charge-storage mechanism of crystalline ZnMn2O4@CNF in ZnSO4 (aq), despite specific Zn2+ lattice sites.

We recently demonstrated that advanced multifunctional 3D MnOx@carbon nanofoam (MnOx@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-MnOx@CNF and mild-pH Zn2+-containing electrolyte results in a complex charge-storage mechanism in which a H+ inserts into the birnessite-MnOx during discharge, resulting in an increase in the local pH that triggers precipitation of Zn4(OH)6SO4·5H2O, 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, MnOx 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 MnOx polymorphs in such 3D multifunctional electrode designs, we developed an “in-place” conversion to generate nanocrystalline ZnMn2O4 spinel from the birnessite-MnOx coating on the CNF. Crystallization is achieved by heating at relatively mild temperatures (300°C), such that the nanoscale morphology of the original MnOx 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 ZnMn2O4@CNF in 1 M ZnSO4 and found that it is even more complex than the charge-storage mechanism of birnessite-MnOx@CNF. Discharge of ZnMn2O4@CNF proceeds via two steps, the first occurring by Zn2+ insertion into the spinel and the second by H+ insertion accompanied by Zn4(OH)6SO4·5H2O 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 MnOx@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 MnOx@carbon electrode nanoarchitectures for rechargeable zinc-ion cells. MRS Communications 9, 99–106 (2019).

Abstract Fundamental charge‐transfer dynamics for technologically relevant carbons, such as disordered “hard” carbons, can be studied by designing planar mimics. We fabricate thin films of “pyrolytic carbon” (pyC) by decomposition of benzene at 1000 °C and examine the physical and electrochemical properties of the native pyC film, as well as variants after heteroatom doping, plasma oxidation, and metal nanoparticle modification. The pyC films are optically reflective at the macroscale, relatively planar (1–3 nm RMS roughness by atomic force microscopy) and disordered (via Raman scattering). Thiophenyl‐doped pyC films (0.6–1.7 atom % sulfur) suitably mimic the disorder, chemical state (X‐ray photoelectron spectroscopy), and work function (Kelvin probe) of a workhorse carbon black, Vulcan XC‐72. Using ferri/ferrocyanide as a redox probe, we find that oxygen functionalities enhance the heterogeneous electron‐transfer rate constant up to 3×, while high levels of sulfur dopants decrease the rate 2×. We also explore thiophenyl‐directed adsorption of Au nanoparticles and show that hydrogen evolution at 0.1 mA cm−2 occurs ∼95 mV more positive at <1 % Au@pyC∼S than at pyC∼S.

Inspired by recent breakthroughs with Zr(OH)(4)-type materials that decompose chemical warfare agents (CWAs), we explore aerogel forms of zirconium oxyhydroxide (ZrOxHy) as reactive sorbents for dimethyl methylphosphonate (DMMP), a common simulant for organo-phosphorous CWAs. Zirconia gels were synthesized using propylene oxide and aqueous ZrCl4, followed by supercritical-CO2 extraction to yield monolithic ZrOxHy aerogels. Subsequent calcination at temperatures >= 350 degrees C removes organic byproducts of the epoxide-based synthesis while preserving high specific surface area (up to 234 m(2) g(-1)) and the aerogel-like, cocontinuous pore-solid structure. Infrared (IR) spectroscopy and H-1 nuclear magnetic resonance spectroscopy confirm that ZrOxHy aerogels retain a high concentration of surface hydroxyls, even when heated to 600 degrees C and converted to nanocrystalline cubic/monoclinic ZrO2. We used in situ IR spectroscopy to probe the interactions of DMMP with a series of ZrOxHy aerogel powders, showing that DMMP rapidly decomposes by reacting with the hydroxyl-rich aerogel to form surface-bound Zr-OCH3 and bridging O-P-O species. We compare these results to those of related zirconia-type materials and discuss prospects and advantages of ZrOxHy aerogels for CWA mitigation.