We report our studies to compare energy consumption of a CDI cell in constant voltage (CV) and constant current (CC) operations, with a focus on understanding the underlying physics of consumption patterns. The comparison is conducted under conditions that the CV and CC operations result in the same amounts of input charge and within identical charging phase durations. We present two electrical circuit models to simulate energy consumption in charging phase: one is a simple RC circuit model, and the other a transmission line circuit model. We built and tested a CDI cell to validate the transmission line model, and performed a series of experiments to compare CV versus CC operation under the condition of equal applied charge and charging duration. The experiments show that CC mode consumes energy at 33.8 kJ per mole of ions removed, which is only 28% of CV mode energy consumption (120.6 kJ/mole), but achieves similar level of salt removals. Together, the models and experiment support our major conclusion that CC is more energy efficient than CV for equal charge and charging duration. The models also suggest that the lower energy consumption of CC in charging is due to its lower resistive dissipation.

Capacitive deionization (CDI) is a promising desalination technology, which operates at low pressure, low temperature, requires little infrastructure, and has the potential to consume less energy for brackish water desalination. However, CDI devices consume significantly more energy than the theoretical thermodynamic minimum, and this is at least partly due to resistive power dissipation. We here report our efforts to characterize electric resistances in a CDI system, with a focus on the resistance associated with the contact between current collectors and porous electrodes. We present an equivalent circuit model to describe resistive components in a CDI cell. We propose measurable figures of merit to characterize cell resistance. We also show that contact pressure between porous electrodes and current collectors can significantly reduce contact resistance. Lastly, we propose and test an alternative electrical contact configuration which uses a pore-filling conductive adhesive (silver epoxy) and achieves significant reductions in contact resistance.

We demonstrate a novel assay for physicochemical extraction and isotachophoresis-based purification of 16S rRNA from whole human blood infected with Pseudomonas putida. This on-chip assay is unique in that the extraction can be automated using isotachophoresis in a simple device with no moving parts, it protects RNA from degradation when isolating from ribonuclease-rich matrices (such as blood), and produces a purified total nucleic acid sample that is compatible with enzymatic amplification assays. We show that the purified RNA is compatible with reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and demonstrate a clinically relevant sensitivity of 0.03 bacteria per nanoliter using RT-qPCR.

We report on our efforts to create an on-chip system to simultaneously purify and fractionate nucleic acids and proteins from complex samples using isotachophoresis (ITP). We have developed this technique to simultaneously extract extracellular DNA and proteins from human blood serum samples and deliver these to two separate output reservoirs on a chip. The purified DNA is compatible with quantitative polymerase chain reaction (qPCR), and proteins can be extracted so as to exclude albumin, the most abundant protein in serum. We describe significant remaining challenges in making this bidirectional method a robust and efficient technique. These challenges include managing channel surface adsorption of proteins, identifying the cause of observed reductions in low molecular weight proteins, and dealing with nonspecific binding of proteins and DNA.

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.