Electrodeposition of zinc at current densities close to the mass transport limit produces needle-like dendrites. Suppressing dendrites is of technological interest to applications in sacrificial corrosion protection coatings and flow batteries. In the present work, we report the use of polyethylene glycol (PEG, M.W. = 200) as an effective electrolyte additive to suppress dendrites during zinc electrodeposition from halide-based electrolytes. Dendrite growth rate is measured as a function of the PEG concentration using in situ optical microscopy, which shows that the dendrite suppression efficacy due to PEG increases with PEG concentration. Polarization experiments on a rotating disk electrode provide system parameters, i.e., the exchange current density and the cathodic transfer coefficient, which confirm that PEG suppresses the zinc electrodeposition kinetics. The kinetic parameters are incorporated into a simple electrochemical model for activation-controlled dendrite propagation. The model predicts an order of magnitude reduction in the zinc dendrite growth rates in the presence of high concentration of PEG (10000 ppm), consistent with experimental findings.
Polymer hydrogels synthesized by chemical crosslinking of acrylate or acrylamide monomers can absorb more than 100 times their weight in water. However, such gels are usually fragile and rupture when stretched to moderate strains (∼50%). Many strategies have been developed to create tougher gels, including double-networking, incorporation of nanoparticles as cross-linkers, etc., but these strategies typically retard the water absorbency of the gel. Here, we present a new approach that gives rise to superabsorbent hydrogels having superior mechanical properties. The key to our approach is the self-cross-linking ability of N,Ndimethylacrylamide (DMAA). That is, we conduct a free-radical polymerization of DMAA (along with an ionic comonomer such as sodium acrylate) but without any multifunctional monomers. A hydrogel still forms due to interchain covalent bonds between the growing linear polymer chains. Gels formed by this route can be stretched up to 1350% strain in the unswollen state. The same gels are also superabsorbent and can imbibe up to 3000 times their weight in water (which is believed to be a record). Even in the swollen state, these gels can be stretched up to strains ∼400% before rupture, which substantially exceeds that of conventional superabsorbent gels. The superior properties of DMAAbased gels are attributed to a more uniform distribution of cross-links within their networks.
We describe a new approach to creating hybrid polymer hydrogels that comprise two different gel types (gel 1 and gel 2) juxtaposed in predetermined zones or patterns and with the unique properties of each gel being retained. The key to our approach is to ensure that the viscosities of the pregel mixtures are high when they are brought into contact and subsequently polymerized; this limits the diffusion at gel/gel interfaces. The final gel appears as a single, homogeneous, transparent material with smooth, robust interfaces between the dissimilar zones. However, its hybrid nature is revealed by specific tests. In one example, we use the same monomer, N,Ndimethylacrylamide, for gels 1 and 2, but gel 1 is cross-linked by a chemical cross-linker while gel 2 is cross-linked by laponite nanoparticles. In this case, when the hybrid gel is immersed in a mixed solution of cationic and anionic dyes, gel 2 selectively absorbs the cationic dye due to the strong affinity of the nanoparticles in it for cationic species. Additionally, a mechanical test on the above hybrid shows that the gel 2 region is stronger and much more extensible than the gel 1 region. We also make use of the fact that the gel 2 (high-laponite) regions are optically birefringent relative to gel 1 (low or no laponite). This allows us to embed a pattern ("message") of gel 2 within gel 1. While the hybrid gel appears featureless under white light, the hidden "message" becomes visible when the gel is viewed under crossed polarizers. The overall approach described here can be extended in myriad ways for the generation of gels possessing new and unique properties.
The diffusion and adsorption properties of an electrolyte additive polyethylenimine (PEI), employed during zinc (Zn) electrodeposition, are characterized via additive injection experiments implemented on a rotating disc electrode (RDE). Under galvanostatic Zn electrodeposition on a RDE, the transient response of the electrode potential upon PEI injection into the electrolyte is measured. The electrode potential response is indicative of the gradual adsorption of PEI on the electrode surface and thus provides an adsorption time constant. The measured adsorption time constant is analyzed using a transient diffusion-adsorption model, which leads to the determination of the PEI diffusion coefficient (DPEI) and its surface-limited adsorption rate constant (kads). Analysis confirms that PEI adsorption during Zn electrodeposition is limited by its slow diffusion and not by the fast kinetics of surface adsorption. The additive injection method implemented herein provides a convenient metrology for characterizing the diffusion and adsorption properties of polymeric additives used in numerous electrodeposition systems, while providing insights into the rate-limiting processes that govern additive adsorption during electrodeposition.
Electrodeposition of metals such as lithium (Li) and zinc (Zn) is central to charging of advanced energy storage devices including lithium-metal batteries and zinc-halogen flow batteries. However, a key technological hurdle facing the development of high energy density rechargeable metal anodes is the formation of dendritic morphology during battery charging and cycling [1,2]. In the present talk, key mechanisms underlying the development of dendritic morphology during Li and Zn electrodeposition will be discussed. Mechanistic effects associated with electrolyte components (additives and solvents) will be elucidated through in situ ‘live’ monitoring of the dendrite evolution process [3]. Performance of a variety of additives (polymers, sulfides, carbonates) and solvents (aqueous and organic) in the Li and Zn system will be discussed. In situ dendrite propagation studies will be complemented with electroanalytical studies such as polarization measurements on a rotating disc electrode. As shown in attached Figs. 1 and 2, polarization measurements provide valuable insights into the modulation of surface electrochemical kinetics by additives and solvents. Knowledge of surface polarization then enables quantitative modeling of the dendrite growth propensity and the additives-assisted dendrite suppression efficacy [4,5]. A combination of theory and experiment allows development of a comprehensive framework for characterizing the surface chemistry responsible for the dendritic morphology. Based on this framework, quantitative guidelines for designing chemistries to suppress dendrite formation will be presented. References: M. Skyllas-Kazacos et al., J. Electrochem. Soc., 158, R55 (2011). K. Xu, Chem. Rev., 104, 4303 (2004). S. Banik and R. Akolkar, J. Electrochem. Soc., 160 (11) D519 (2013). R. Akolkar, J. Power Sources, 232, 23 (2013). R. Akolkar, J. Power Sources, 246, 84 (2014).
Rechargeable zinc-based battery systems hold great promise for future energy storage devices due to their low cost and high theoretical energy density.1 However, the development of portable or stationary rechargeable zinc batteries has been hindered by several critical issues. The key issue has been the evolution of dendritic zinc electrodeposit morphology during battery charging,2,3leading to battery capacity fade and cell shorting. In this work, we focus on the use of organic electrolyte additives to suppress zinc dendrite growth. We examine both acidic and alkaline electrolytes, which are relevant to acidic zinc-halogen flow batteries and alkaline zinc-metal batteries, respectively. To study zinc dendrite propagation, we employ in-situ microscopy during growth of the electrodeposit.4 To study the mechanism by which organic additives modulate the dendrite propagation rate, we utilize classical polarization measurements on a rotating disc electrode. For acidic zinc electrolytes, we studied numerous additives (Figure 1) and their dendrite suppression efficacy. We observed that strongly polarizing additives suppress zinc dendrites more than weak polarizers. For zinc electrodeposition from an alkaline electrolyte, we identified that polyethylenimine (PEI) enables near-complete dendrite suppression,5even at low concentrations (~50 ppm, see Figure 2). Figure 2 also shows electrochemical polarization measurements on a rotating disk electrode, which confirm the polarizing effect of the PEI. The aforementioned characterization techniques, together with electrochemical quartz crystal microgravimetry, ex-situscanning electron microscopy, and various ‘additive-injection’ tests, provide insights into the mechanism by which additives suppress dendritic growth of zinc. Additives are believed to adsorb onto the zinc surface and suppress zinc electrodeposition kinetics locally, thereby inhibiting activation-controlled dendrite propagation. In the talk, we will develop the mechanistic understanding further and provide guidelines for judicious selection of the additive chemistry for dendrite-free zinc electrodeposition in acidic and alkaline systems. References: [1]: Y. Li and H. Dai, Chem. Soc. Rev., 43, 5143 (2014). [2]: K. Wang, P. Pei, Z. Ma, H. Xu, P. Li, X. Wang, J. Power Sources, 271, 65 (2014). [3]: C. P. de Leon, A. Frias-Ferrer, J. Gonzalez-Garcia, D. A. Szanto, and F. C. Walsh, J. Power Sources, 160, 716 (2006). [4]: S. J. Banik and R. Akolkar, J. Electrochem. Soc., 160, D519 (2013). [5]: S. J. Banik and R. Akolkar, Manuscript submitted to Electrochimica Acta, October 2014. Figure 1
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