Eric N. Guyes,
Anastasia Simanovski and
Matthew E. Suss*
Faculty of Mechanical Engineering, Technion – Israel Institute of Technology, Technion City 1, Haifa 32000, Israel. E-mail: mesuss@technion.ac.il
First published on 13th April 2017
Capacitive deionization (CDI) is a fast-emerging water desalination technology in which an applied potential of ∼1 V across a pair of porous carbon electrodes removes salt from the feedwater via electrosorption. In flow-through electrode (FTE) CDI cell architecture, feedwater is pumped through the macropores of the porous electrodes, enabling highly compact cells and rapid salt removal. However, FTE CDI requires specialized electrode materials possessing large macropores with diameters above 1 μm in order to allow for reasonable feed pressures. We herein present and demonstrate a low-cost, simple technique for laser perforating CDI electrodes, which delivers highly permeable electrodes from carbon materials with pore structures otherwise unsuitable for flow-through operation. We show that laser cutting channels with ∼200 μm aperture diameters through a commercial CDI electrode material with sub-micron macropores increases the electrode hydraulic permeability by approximately five orders of magnitude, thereby greatly reducing the feed pressure. Furthermore, we demonstrate that key performance metrics such as the cell's salt adsorption capacity and gravimetric capacitance are largely unaffected by the laser perforation technique.
Another CDI cell architecture is the flow-through electrode (FTE) architecture,14–20 where the feedwater flows directly through electrodes' macropores (see Fig. 1A). In FTE cells, the electric field and flow directions are parallel, allowing for convenient optimization of flow and ionic resistances,19 one-dimensional system modeling,20 and improved desalination rates and more compact cells.20 FTE cells also have drawbacks relative to FB cells, such as an observed enhanced rate of anode corrosion, though this can be mitigated by de-oxygenating the feedwater.17 Additionally, FTE cells can require greater feed pressures than FB cells.19 To maintain acceptably low feed pressure, which should be at least one order of magnitude under the feed osmotic pressure to prevent excessive pump energy requirements, FTE cells typically require use of specialized electrodes consisting of relatively large macropores. For example, Suss et al. utilized hierarchical carbon aerogel monolith electrodes with macropore diameters of roughly 2 μm,19 while Cohen et al. used woven carbon cloth electrodes with order 100 μm gaps between fibers as the flow channels.15 The requirement of large macropores significantly limits the number of CDI electrode materials that can be utilized as FTEs.
Here, we demonstrate and study laser perforating an array of flow channels into CDI electrodes in order to improve electrode hydraulic permeability and reduce feed pressure. Laser-perforated electrodes have been utilized in flow battery and fuel cell systems for the purposes of improving power density, cell stability, and cell performance,21–25 and it is predicted that perforated graphite anodes can increase ion transport rates in Li-ion batteries.26 However, laser perforations have not yet been explored as a technique for reducing pressure requirements in FTE CDI cells, and the effect of the perforations and laser-cutting process on desalination performance has not been previously measured. We here show that laser perforating commercial CDI electrodes, which would ordinarily not be appropriate for FTE CDI as they contain mostly sub-micron macropores, renders them suitable for use as FTEs. Specifically, we show that laser cutting dramatically increases the hydraulic permeability of these electrodes by roughly five orders of magnitude. Further, we show that laser perforating the electrodes does not affect key performance metrics such as electrode gravimetric capacitance and salt adsorption capacity (SAC).
In order to produce the laser-perforated electrodes, we cut a 21 × 21 array of channels in the electrode material with a CO2 laser (VLS 4.60, United Laser Systems, Scottsdale, AZ, USA) (see Fig. 1C and D). We designed the perforation cross section to be circular with a 200 μm diameter, which was approximately the smallest diameter attainable by our laser system. The channel density was chosen to provide close spacing between perforations while minimizing the mass loss of the electrode due to ablation to ∼5%. At the electrode surface farthest from the laser head during cutting, the resulting laser cut holes were roughly circular in cross section and with ∼200 μm diameter (see Fig. 1D and S1†). However, the perforations on the top surface were larger and exhibited a more elliptical shape, demonstrating that the laser holes are not perfectly cylindrical through the electrode (see Fig. S1†). From mass measurements of 21 perforated electrode samples and 22 unperforated samples, perforation removed an average of 6.1% of the electrode mass.
For CDI system feed pressure measurements, a pump drew deionized water from a bulk reservoir and introduced it into the CDI cell via semi-rigid polyethylene tubing. We removed the separator from the CDI cell in order to avoid adding an extra hydraulic resistance to the system. A pressure sensor (PX419 series, Omega Engineering, Stamford, CT, USA) was connected to the tubing approximately 10 cm upstream of the cell inlet. The sensor pressure without water flow was slightly offset from zero and was recorded before each experiment to provide a baseline pressure. For experiments using the as-received (unperforated) electrodes, we utilized a peristaltic pump (Masterflex 0755130, Cole-Parmer) that was able to withstand the high flow pressures. We measured the baseline pressure at zero flow and then increased the flow rate in increments of 0.1 mL min−1. The actual flow rates were determined to be ±2% of the set flow rates via effluent mass measurements. Pumping at each flow rate continued until the pressure no longer changed in time, except for small pressure oscillations of ∼5 kPa caused by the peristaltic pumping action. Each pressure was computed as a time average to account for the effect of the oscillations. High flow pressures due to flowing through the unperforated electrode material, which approached the maximum allowable pressure of our peristaltic pump, restricted the maximum flow rate to 1.0 mL min−1. For pressure measurements with perforated electrodes, we utilized a syringe pump with ±2% maximum flow rate error (Fusion 720, Chemyx, Stafford, TX, USA) that provides a constant flow rate without the aforementioned oscillations and is suited for the low pressures observed with these electrodes. A large-diameter cylindrical container filled with DI water served as the outlet reservoir, and the CDI cell outlet line rested slightly below the water's surface in order to minimize pressure oscillations caused by droplet formation. The large reservoir diameter ensured that the water level remained approximately constant over the course of an experiment. We increased the flow rate from 0 to 5.0 mL min−1 in increments of 0.5 mL min−1. Each flow rate was maintained until steady pressure measurements were achieved.
For desalination experiments, the peristaltic pump drew feedwater (NaCl 5.5 mM concentration) from a glass bulk reservoir and fed it into the CDI cell via the semi-rigid polyethylene tubing. The pump flow rate was set as 1.0 mL min−1 and the actual flow rate was determined from effluent mass measurements. The cell effluent entered a conductivity cell (5-ring, Metrohm, Inc., Switzerland) that contained a custom-milled insert to reduce the internal sensor volume, and then returned to the bulk reservoir. The bulk reservoir volume (∼2 L) was significantly larger than the volume of the remainder of the setup (∼20 mL) in order to maintain constant concentration in the reservoir throughout the experiments. During the charging half-cycle (30 min) of the desalination experiments, a voltage source (SourceMeter 2400, Keithley Instruments, Inc., Solon, OH, USA) applied a voltage of 1.0 V to the cell, while during the discharging half-cycle (30 min) it maintained a voltage of 0 V.
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Fig. 2 Plot of pressure versus pump flow rate for the as-received unperforated electrodes (circle markers) and the laser-perforated electrodes (triangle markers). Theory lines were obtained from eqn (1) for unperforated electrodes, and with eqn (2) for the perforated electrodes. Error bars represent the 95% confidence interval calculated from at least three realizations. |
In order to relate our feed pressure measurements to the electrodes' permeability, we developed models of the electrodes' hydraulic permeability, κ, and the expected pressure drop across the electrodes, ΔP. Flow through a porous medium can be described using Darcy's law as
![]() | (1) |
Rearranging eqn (1) with the given expression for κ yields the following equation for the pressure drop across both perforated electrodes,
![]() | (2) |
Here the velocity Ux is replaced by Q/A, the flow rate divided by the cross-sectional electrode area available for flow (2.4 cm2). We used eqn (2) to calculate the pressure drop across two perforated electrodes. Furthermore, as the pressure drop expected across the perforated electrodes is on the order of the pressure drop anticipated across other cell and system components, such as the reservoir channels and flow tubing, we also included these other components in a predictive model for feed pressure (see Section SI2 for further details). The model results for feed pressure for the system with perforated electrodes can be seen in Fig. 2 (dashed line), which only slightly under-predicts the measured data.
While laser perforation can significantly improve electrode hydraulic permeability, it is also important to evaluate the effect of the laser cutting process on electrode desalination performance. CDI electrode performance is primarily evaluated via the metric of maximum salt adsorption capacity (mSAC), which is the mass of salt that a pair of CDI electrodes can store upon reaching equilibrium after being charged with a constant voltage.2,33 The latter metric is obtained by integrating the cell's effluent concentration versus time curve during the desalination half cycle.2 The measured cell effluent concentration over full charge/discharge cycles are shown in Fig. 3A and B for the CDI cell with either unperforated or perforated electrodes. Ten full cycles are shown using a charge voltage of 1.0 V and discharge voltage of 0 V. The first few cycles show significant changes in the height of the peaks and depths of the valleys in the effluent concentration profile, where such changes are expected and widely observed as the CDI cell reaches its limit cycle.2 Past the fourth cycle, the cell demonstrates approximately uniform conductivity profiles for the remaining cycles. In Fig. 3C, we show the maximum salt adsorption capacity (mSAC) obtained from integrating the concentration versus time curves of Fig. 3A and B, for all curves past the fourth cycle. We also show the measured gravimetric capacitance of the electrodes, obtained from integrating the cell's current response to an applied voltage (see Fig. S3† for current vs. time plots). As can be seen, the cell's mSAC and gravimetric capacitance are nearly identical when using perforated or unperforated electrodes, demonstrating that the laser perforation process does not appreciably impact the remaining electrode material's charge or salt storage capability. The SEM images provide further evidence for this result, in which no apparent heat damage is observed to the remaining electrode material surrounding the laser perforations, aside from the apparent melted binder on the bottom side of the electrode (see Fig. S1†).
While mSAC measures the equilibrium salt storage in CDI electrodes, the metric of average salt adsorption rate (ASAR) quantifies the salt electrosorption kinetics. ASAR is defined as the salt adsorption capacity divided by the full cycle time, and can be affected by both electrode and cell parameters (e.g. by contact resistance and separator parameters).2,27 In the 10th cycle, ASAR is nearly identical between the cell with unperforated electrodes and perforated electrodes at 0.047 and 0.045 mg g−1 min−1, respectively. However, the ASAR averaged over a cycle reaching equilibrium does not give the total picture of the salt electrosorption kinetics. When comparing cell effluent versus time curves between the perforated and unperforated cells, we can see subtle differences in the dynamics (see Fig. S5† for a direct comparison). Generally, the features in the unperforated electrode case appear sharper than those in the perforated case, and a brief inversion peak is observed immediately upon cell charging only in the unperforated case. The latter observation is likely due to the introduction of an additional transport process in the system with perforated electrodes. In this system, salt must transport out of the laser-cut channels and into the ion-depleted porous electrode material during cell charging to effect desalination of the feed stream, and this transport is likely via molecular diffusion. In the perforated electrodes, the half-distance L between perforations is ∼500 μm, and the diffusion time over this distance, τD = L2/D, is roughly two minutes (where the mean ion diffusivity D is 1.6 × 10−5 cm s−1 for 1–10 mM NaCl solutions34). This additional timescale is small relative to the half-cycle time of 30 minutes in our experiments, but not insignificant. Thus, the introduced diffusion process may serve to slightly dampen the effluent concentration vs. time signal of the perforated case compared to the unperforated case. A future strategy to optimize the performance of laser-perforated FTE CDI electrodes may be to created denser channel arrays in order to further reduce this extra diffusion time.
To further delve into the presence of inversion peaks at the beginning of each charge step in Fig. 3A, we note that they appear at every cycle except the first despite the use of fresh electrodes for this experiment. We hypothesize that this electrode material possessed an initially high concentration of carboxyl groups in the micropores, which can cause inversion features due to enhanced co-ion expulsion from anode micropores.16,35,36 Such features are typically seen only after extended electrode cycling,37 but here are seen immediately upon use of the electrode. The low mSAC measured for this electrode of only ∼3 mg g−1 and a low calculated charge efficiency of ∼40% are also signs of excessive co-ion expulsion.2,27 In Fig. 3B, the inversion peak is not observed during charging for any of the 10 cycles. We attribute the latter observation to the aforementioned additional diffusion timescale in the perforated electrodes that may dampen a sharp inversion peak. The extensive co-ion expulsion in the perforated electrode cell is instead detected indirectly via this cell's low mSAC of ∼3 mg g−1 and charge efficiency of ∼40%, nearly identical to that of the unperforated electrode cell.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra00459a |
This journal is © The Royal Society of Chemistry 2017 |