Honors & Awards
Stanford School of Engineering Fellowship Award, Stanford University (2013-2015)
Stanford Graduate Fellowship Award, Stanford University (2014-2018)
Juan Santiago, Doctoral Dissertation Advisor (AC)
Michael Stadermann, Yatian Qu, Juan G Santiago, Ali Hemmatifar. "United States Patent 9,758,392 Phased charging and discharging in capacitive desalination", Lawrence Livermore National Security, Leland Stanford Junior University, Jan 9, 0180
Self similarities in desalination dynamics and performance using capacitive deionization.
2018; 140: 323–34
Charge transfer and mass transport are two underlying mechanisms which are coupled in desalination dynamics using capacitive deionization (CDI). We developed simple reduced-order models based on a mixed reactor volume principle which capture the coupled dynamics of CDI operation using closed-form semi-analytical and analytical solutions. We use the models to identify and explore self-similarities in the dynamics among flow rate, current, and voltage for CDI cell operation including both charging and discharging cycles. The similarity approach identifies the specific combination of cell (e.g. capacitance, resistance) and operational parameters (e.g. flow rate, current) which determine a unique effluent dynamic response. We here demonstrate self-similarity using a conventional flow between CDI (fbCDI) architecture, and we hypothesize that our similarity approach has potential application to a wide range of designs. We performed an experimental study of these dynamics and used well-controlled experiments of CDI cell operation to validate and explore limits of the model. For experiments, we used a CDI cell with five electrode pairs and a standard flow between (electrodes) architecture. Guided by the model, we performed a series of experiments that demonstrate natural response of the CDI system. We also identify cell parameters and operation conditions which lead to self-similar dynamics under a constant current forcing function and perform a series of experiments by varying flowrate, currents, and voltage thresholds to demonstrate self-similarity. Based on this study, we hypothesize that the average differential electric double layer (EDL) efficiency (a measure of ion adsorption rate to EDL charging rate) is mainly dependent on user-defined voltage thresholds, whereas flow efficiency (measure of how well desalinated water is recovered from inside the cell) depends on cell volumes flowed during charging, which is determined by flowrate, current and voltage thresholds. Results of experiments strongly support this hypothesis. Results show that cycle efficiency and salt removal for a given flowrate and current are maximum when average EDL and flow efficiencies are approximately equal. We further explored a range of CC operations with varying flowrates, currents, and voltage thresholds using our similarity variables to highlight trade-offs among salt removal, energy, and throughput performance.
View details for DOI 10.1016/j.watres.2018.04.042
View details for PubMedID 29734040
Thermodynamics of Ion Separation by Electrosorption.
Environmental science & technology
We present a simple, top-down approach for the calculation of minimum energy consumption of electrosorptive ion separation using variational form of the (Gibbs) free energy. We focus and expand on the case of electrostatic capacitive deionization (CDI). The theoretical framework is independent of details of the double-layer charge distribution and is applicable to any thermodynamically consistent model, such as the Gouy-Chapman-Stern and modified Donnan models. We demonstrate that, under certain assumptions, the minimum required electric work energy is indeed equivalent to the free energy of separation. Using the theory, we define the thermodynamic efficiency of CDI. We show that the thermodynamic efficiency of current experimental CDI systems is currently very low, around 1% for most existing systems. We applied this knowledge and constructed and operated a CDI cell to show that judicious selection of the materials, geometry, and process parameters can lead to a 9% thermodynamic efficiency and 4.6 kT per removed ion energy cost. This relatively high thermodynamic efficiency is, to our knowledge, by far the highest thermodynamic efficiency ever demonstrated for traditional CDI. We hypothesize that efficiency can be further improved by further reduction of CDI cell series resistances and optimization of operational parameters.
View details for DOI 10.1021/acs.est.8b02959
View details for PubMedID 30141621
Self-Cleaning Porous Surfaces for Dry Condensation
ACS APPLIED MATERIALS & INTERFACES
2018; 10 (31): 26759–64
Efficient water removal from a cool surface during condensation is critically important to the enhancement of a variety of heat transfer applications. Previous work has focused on the fabrication of superhydrophobic surfaces which promote water droplets and removal via droplet shedding or jumping. Here, we report a novel strategy with a droplet self-cleaning surface which spontaneously transports all of the droplets from the condensation surface to the back side. We fabricate the self-cleaning surface by simply tailoring the wettability of the two sides of a porous membrane and demonstrate that the hydrophobic side is effective in clearing off droplets of a wide range of diameters. Even during rapid impingement of droplets smaller than 10 μm, this surface remains dry. We further demonstrate a "dry condensation" process wherein a surface undergoing rapid condensation is maintained free of droplets. This minimizes the essential thermal resistance of the process, and we estimate a twofold increase in condensation rate compared with a simple copper surface under the same conditions. Our method is tailorable, extendable to a wide range of materials and geometries, and shows great potential for a broad range of condensation processes.
View details for DOI 10.1021/acsami.8b07261
View details for Web of Science ID 000441477800104
View details for PubMedID 30059209
- Adsorption and capacitive regeneration of nitrate using inverted capacitive deionization with surfactant functionalized carbon electrodes SEPARATION AND PURIFICATION TECHNOLOGY 2018; 194: 410–15
Charging and Transport Dynamics of a Flow-Through Electrode Capacitive Deionization System
JOURNAL OF PHYSICAL CHEMISTRY B
2018; 122 (1): 240–49
We present a study of the interplay among electric charging rate, capacitance, salt removal, and mass transport in "flow-through electrode" capacitive deionization (CDI) systems. We develop two models describing coupled transport and electro-adsorption/desorption which capture salt removal dynamics. The first model is a simplified, unsteady zero-dimensional volume-averaged model which identifies dimensionless parameters and figures of merits associated with cell performance. The second model is a higher fidelity area-averaged model which captures both spatial and temporal responses of charging. We further conducted an experimental study of these dynamics and considered two salt transport regimes: (1) advection-limited regime and (2) dispersion-limited regime. We use these data to validate models. The study shows that, in the advection-limited regime, differential charge efficiency determines the salt adsorption at the early stage of the deionization process. Subsequently, charging transitions to a quasi-steady state where salt removal rate is proportional to applied current scaled by the inlet flow rate. In the dispersion-dominated regime, differential charge efficiency, cell volume, and diffusion rates govern adsorption dynamics and flow rate has little effect. In both regimes, the interplay among mass transport rate, differential charge efficiency, cell capacitance, and (electric) charging current governs salt removal in flow-through electrode CDI.
View details for DOI 10.1021/acs.jpcb.7b09168
View details for Web of Science ID 000422814000024
View details for PubMedID 29292999
Equilibria model for pH variations and ion adsorption in capacitive deionization electrodes
Ion adsorption and equilibrium between electrolyte and microstructure of porous electrodes are at the heart of capacitive deionization (CDI) research. Surface functional groups are among the factors which fundamentally affect adsorption characteristics of the material and hence CDI system performance in general. Current CDI-based models for surface charge are mainly based on a fixed (constant) charge density, and do not treat acid-base equilibria of electrode microstructure including so-called micropores. We here expand current models by coupling the modified Donnan (mD) model with weak electrolyte acid-base equilibria theory. In our model, surface charge density can vary based on equilibrium constants (pK's) of individual surface groups as well as micropore and electrolyte pH environments. In this initial paper, we consider this equilibrium in the absence of Faradaic reactions. The model shows the preferential adsorption of cations versus anions to surfaces with respectively acidic or basic surface functional groups. We introduce a new parameter we term "chemical charge efficiency" to quantify efficiency of salt removal due to surface functional groups. We validate our model using well controlled titration experiments for an activated carbon cloth (ACC), and quantify initial and final pH of solution after adding the ACC sample. We also leverage inductively coupled plasma mass spectrometry (ICP-MS) and ion chromatography (IC) to quantify the final background concentrations of individual ionic species. Our results show a very good agreement between experiments and model. The model is extendable to a wide variety of porous electrode systems and CDI systems with applied potential.
View details for DOI 10.1016/j.watres.2017.05.036
- Adsorption and capacitive regeneration of nitrate using inverted capacitive deionization with surfactant functionalized carbon electrodes Separation and Purification Technology 2017
Energy breakdown in capacitive deionization.
2016; 104: 303-311
We explored the energy loss mechanisms in capacitive deionization (CDI). We hypothesize that resistive and parasitic losses are two main sources of energy losses. We measured contribution from each loss mechanism in water desalination with constant current (CC) charge/discharge cycling. Resistive energy loss is expected to dominate in high current charging cases, as it increases approximately linearly with current for fixed charge transfer (resistive power loss scales as square of current and charging time scales as inverse of current). On the other hand, parasitic loss is dominant in low current cases, as the electrodes spend more time at higher voltages. We built a CDI cell with five electrode pairs and standard flow between architecture. We performed a series of experiments with various cycling currents and cut-off voltages (voltage at which current is reversed) and studied these energy losses. To this end, we measured series resistance of the cell (contact resistances, resistance of wires, and resistance of solution in spacers) during charging and discharging from voltage response of a small amplitude AC current signal added to the underlying cycling current. We performed a separate set of experiments to quantify parasitic (or leakage) current of the cell versus cell voltage. We then used these data to estimate parasitic losses under the assumption that leakage current is primarily voltage (and not current) dependent. Our results confirmed that resistive and parasitic losses respectively dominate in the limit of high and low currents. We also measured salt adsorption and report energy-normalized adsorbed salt (ENAS, energy loss per ion removed) and average salt adsorption rate (ASAR). We show a clear tradeoff between ASAR and ENAS and show that balancing these losses leads to optimal energy efficiency.
View details for DOI 10.1016/j.watres.2016.08.020
View details for PubMedID 27565115
- Two-Dimensional Porous Electrode Model for Capacitive Deionization JOURNAL OF PHYSICAL CHEMISTRY C 2015; 119 (44): 24681-24694
- Continuous size-based focusing and bifurcating microparticle streams using a negative dielectrophoretic system MICROFLUIDICS AND NANOFLUIDICS 2013; 14 (1-2): 265-276