It is widely accepted that hydrogel surfaces are slippery, and have low friction, but dynamic applied stresses alter the hydrogel composition at the interface as water is displaced. The induced osmotic imbalance of compressed hydrogel which cannot swell to equilibrium should drive the resistance to slip against it. This paper demonstrates the driving role of poroelasticity in the friction of hydrogel-glass interfaces, specifically how poroelastic relaxation of hydrogels increases adhesion. We translate the work of adhesion into an effective surface energy density that increases with the duration of applied pressure from 10 to 50 mJ m, as measured by micro-indentation. A model of static friction coefficient is derived from an area-based rules of mixture for the surface energies, and predicts the friction coefficient changes upon initiation of slip. For kinetic friction, the competition between duration of contact and relaxation time is quantified by a contacting Péclet number, Pe. A single length parameter on the scale of micrometers fits these two models to experimental micro-friction data. These models predict how short durations of applied pressure and faster sliding speeds, do not disrupt interfacial hydration; this prevailing water maintains low friction. At low speeds where interface drainage dominates, the osmotic suction works against slip for higher friction. The prediction of friction coefficients after adhesion characterization by micro-indentation makes use of the interplay between poroelasticity, adhesion, and friction. This approach provides a starting point for prediction of, and design for, hydrogel interfacial friction.

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Highlights New colloid-inspired fabrication with calendering improved electrode utilization. FDI system used automatic water recirculation with calibrated valve timing. Water recirculation with dense electrodes improved salt removal. System removed 90% of salt from 100 mM NaCl brackish water. System can remove 80% of salt with thermodynamic energy efficiency of 80%.

The rate of ionic conduction through the electrolyte of porous electrodes is determined in part by the tortuosity, a factor describing the effective length an ion must travel through the microstructure's pores. To facilitate ionic conduction and adsorption into the electric double-layers of capacitive electrodes, we show that macroscopic pores can be added to reduce the effective tortuosity by providing more direct paths to capacitive interfaces. We show experimental and simulated results of fabricating and testing electrodes that are machined to include macro-pores aligned normal to current collectors. Through the reduction of tortuosity, these "bi-tortuous" electrodes surpass unpatterned electrodes in effective ionic conductivity and capacitance. The degree of improvement is dependent on the electrodes' thickness and charging rate. Potable water demand together with agricultural and industrial needs are likely to drive the desalination of seawater, wastewater, and brackish water resources, and capacitive deionization (CDI) is one potential technology for these purposes.1 In CDI anions and cations are removed from feedwater solution by way of capacitive adsorption into electric double-layers (EDLs). The rate of salt removal in CDI ultimately influences the cost of water production, and, hence, achieving high salt removal rates is critical to making economical CDI devices. Salt removal rate typically increases with the current density used 2 because one electron is transferred for every monovalent ion adsorbed when co-ion adsorption is negligible. Thick electrodes in CDI have the potential to increase areal capacitance and enable the treatment of concentrated salt water, but such electrodes typically suffer from cracking during fabrication, increased ionic resistance, and energy consumption that typically restricts thickness to less than 350 μm.3 While alternative strategies can be used to eliminate these effects (including flow-electrode architectures 4 and intercalation-based electrodes 5-7 ) strategies to simultaneously increase salt removal rate with conventional EDL-based electrodes are desirable.In porous electrodes, including both EDL-and intercalation-based, microstructure is known to influence internal resistance, capacitance, and energy efficiency. In particular, the tortuosity τ of a porous electrode material is the ratio of the microscopic path length that an ion takes within pores normalized by the Cartesian distance between the endpoints of the path. [8][9][10] In general, porous electrodes exhibit tortuosity exceeding unity as a result of the random arrangement of impenetrable solid matrix comprising the electrode. Aside from its geometric interpretation, tortuosity impacts electrode charging dynamics by way of the effective ionic conductivity κ ef f = κ 0 ε/τ and the effective ionic diffusivity D ef f = D 0 ε/τ, where κ 0 and D 0 are the bulk values of ionic conductivity and diffusivity, and ε is porosity. 8,[11][12][13][14] The reduction of the effective transport property relative to its corres...

Capacitive deionization (CDI) with electric double layers is an electrochemical desalination technology in which porous carbon electrodes are polarized to reversibly store ions. Planar composite CDI electrodes exhibit poor energetic performance due the resistance associated with salt depletion and tortuous diffusion in the macroporous structure. In this work, we investigate the impact of bi-tortuosity on desalination performance by etching macroporous patterns along the length of activated carbon porous electrodes in a flow-by CDI architecture. Capacitive electrodes were also coated with thin asymmetrically charged polyelectrolytes to improve ion-selectivity while maintaining the bitortuous macroporous channels. Under constant current operation, the equivalent circuit resistance in CDI cells operating with bi-tortuous electrodes was approximately 2.2 times less than a control cell with unpatterned electrodes, leading to significant increases in working capacitance (20–22 to 26.7–27.8 F g −1 ), round-trip efficiency (52–71 to 71–80%), and charge efficiency (33–59 to 35–67%). Improvements in these key performance indicators also translated to enhanced salt adsorption capacity, rate, and most importantly, the thermodynamic efficiency of salt separation (1.0–2.0 to 2.2–4.1%). These findings demonstrate that the use of bi-tortuous electrodes is a novel approach of reducing impedance to ionic flux in CDI.

and aerospace engineering, university of florida, gainesville fl, usa; b Department of Mechanical science & engineering, university of illinois at urbana-champaign, urbana il, usa; c J. crayton Pruitt family Department of biomedical engineering, university of florida, gainesville fl, usa; d institute for cell engineering and regenerative Medicine, university of florida, gainesville fl, usa;

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Due to the growing population and human-made climate change spurred by the use of fossil fuels, the urgent need for energy-efficient desalination has motivated research into electrochemical desalination as a technology to remove salt ions from water. Prussian Blue analogues (PBAs) are a promising class of redox-active intercalation compounds with high capacity and cycle life used in sodium-ion batteries. While their functionality was previously shown to enable desalination in symmetric cation intercalation desalination (CID) cell architectures [S. Porada, A. Shrivastava, P. Bukowska, P. M. Biesheuvel, and K. C. Smith, Electrochim. Acta 255, 369 (2017)], they have low electronic conductivity which limits their charge and discharge rates while causing high energy consumption due to ohmic losses [A. Shrivastava and K. C. Smith, J. Electrochem. Soc. 165, A1777 (2018)]. A common technique to increase electronic conductivity in battery electrodes is the inclusion of additives such as carbon black, however increasing conductive particle content reduces active particle loading and overall salt absorption capacity. To overcome these limitations, the effects of volume fraction of either C45 or Ketjen Black conductive additive on electrode transport properties were studied, and we demonstrated that orders of magnitude improvement in electronic conductivity could be achieved using the smaller Ketjen Black particles [E. R. Reale, A. Shrivastava, and K. C. Smith, Water Res. 165, 114995 (2019)]. Through this discovery, the low electronic conductivity of PBA electrodes was overcome, and new higher-conductivity electrodes were successfully fabricated and used in a flow-through cation intercalation desalination cell built with a custom-designed 3D printed flow field. The new CID cell functioned using a PBA anode and cathode separated by an anion exchange membrane to transport salt into one half of the cell while desalinating the other, and displayed energy consumption an order of magnitude lower than that of other electrochemical desalination technologies while removing similar quantities of salt from an influent feed. For a salt concentration of 100 mM, a salinity comparable to brackish water found in areas such as estuaries and aquifers, approximately 25% of influent salt was removed from one feed and transferred into the other. Further improvements can be made through the use of intercalation materials with higher volumetric storage capacity and increased mobility for sodium, which were both limiting factors when using PBAs which lowered utilization and prevented higher salt removal. Early tests with such electrodes have shown their electronic conductivity to be several times higher than PBA electrodes, further reducing energy consumption. Another modification to the cell is the use of an in-house designed recirculation system to increase salt removal over multiple passes rather than single-pass flow. The goal of our research is to develop CID technology into a form of energy-efficient desalination capable of surpassing reverse osmosis in salt removed for a given energy input.