Research ArticleMATERIALS SCIENCE

Electrochemically mediated gating membrane with dynamically controllable gas transport

See allHide authors and affiliations

Science Advances  16 Oct 2020:
Vol. 6, no. 42, eabc1741
DOI: 10.1126/sciadv.abc1741
  • Fig. 1 Schematics showing the working principles of an electrochemically mediated gas gating membrane.

    (A) Gating membrane in its open state, where the AAO membrane with ordered one-directional pores allows rapid mass transfer to the gas-liquid interface. The top surface of the membrane is rendered electrically conductive via physical deposition of a gold thin film. (B) Gating membrane in its closed state. Electrodeposition of a dense metallic nanocrystal layer on the conductive membrane surface can afford an effective gas barrier to reduce the membrane permeability. (C and D) Operational modes of the electrochemically mediated CO2 concentrator demonstrated in this work, which is constructed by integrating an electrochemical CO2 capture cell with two gating membranes. Quinone molecules serve as the redox-active sorbents for CO2 capture/release, which can form stable CO2 adducts when being reduced and release CO2 when being oxidized. M denotes an electrochemically deposited metal coating, which is Zn in this study. Mn+ denotes the corresponding cation of M, which is Zn2+ in this study. (C) For CO2 capture, the gate is open toward a dilute feed stream, and the quinone electrode is being reduced to absorb CO2. (D) For CO2 release, the gate polarity switches to open toward the product stream, and the quinone electrode is electrochemically oxidized to release pure CO2.

  • Fig. 2 Design rationale for the electrochemically mediated gas gating membrane.

    SEM images of (A) pristine and (B) Au-coated AAO membrane. Photo images of (C) pristine and (D) Au-coated AAO membrane. The AAO is peripherally bonded to an annular polypropylene supporting ring for ease of handling. Photo credit: Yayuan Liu, Massachusetts Institute of Technology. (E) CV of 1.0 M Zn(NO3)2 in dimethyl sulfoxide (red curve) and 5 mM anthraquinone in dimethyl sulfoxide with 0.1 M tetrabutylammonium hexafluorophosphate supporting salt under CO2 atmosphere (black curve). The scan rate was 50 mV/s. The inset shows the reversible reaction between anthraquinone and CO2. (F) Voltage profiles of Zn deposition on Au-coated aluminum foil in 0.5 M ZnTFSI2 PC electrolyte with different amounts of EG additive. The deposition was carried out at a current density of 0.5 mA cm−2, and slurry-coated Zn powder was used as the counter electrode to minimize the polarization contribution from the counter electrode. For better visual comparison, the profiles are shifted vertically. The inset shows the photo image of deposition without EG and with 10% EG as additive. Photo credit: Yayuan Liu, Massachusetts Institute of Technology. (G and H) Top-view and (I) side-view SEM images of Zn deposition on AAO obtained in 0.5 M ZnTFSI2 PC electrolyte with 10% EG. The deposition was carried out at a current density of 3 mA cm−2 for 10 min.

  • Fig. 3 Ex situ testing of the electrochemically mediated gating membrane.

    (A) Photo images showing the liquid-state diffusion tests at different time points for pristine AAO and 0.5 mAh cm−2 Zn gated AAO. Photo credit: Yayuan Liu, Massachusetts Institute of Technology. (B) Crossover dye concentration in the permeate chamber as a function of time for pristine AAO, and AAO gated with 0.1, 0.3, and 0.5 mAh cm−2 Zn. (C) Schematic illustrating the setup for ex situ gas-phase testing of the electrochemically mediated gating membrane. Membranes with different amounts of Zn deposited were mounted on brass gaskets, and the concentration of CO2 permeated through the membranes was determined using an in-line CO2 detector. Photo credit: Yayuan Liu, Massachusetts Institute of Technology. (D) CO2 concentrations in the N2 sweep stream under different gas flow conditions for pristine AAO, and AAO gated with 0.1, 0.3, and 0.5 mAh cm−2 Zn. The flow rates of the 15% CO2 (balance N2) feed stream and the N2 sweep stream are indicated in the plot.

  • Fig. 4 In situ testing of the electrochemically mediated gas gating membrane.

    (A) Schematic illustrating the setup for in situ testing of the electrochemically mediated gas gating membrane. A stainless steel mesh with little impedance to gas flow was selected as the counter electrode. The testing used a “zero-gap” design, where both the membrane and the counter electrode were in close contact with the electrolyte-imbibed separator with no air gap in between, such that CO2 crossover can only occur via dissolution-diffusion through the electrolyte. (B) Permeated CO2 concentration in response to reversible gating (2 sccm N2 as the carrier gas). The cycling protocol was 10-min deposition/dissolution at 3 mA cm−2, with 10-min rest in between. (C) Detailed gating response of a single deposition/dissolution cycle marked with a dashed line in (B). The time lag between the onset of deposition/dissolution and the CO2 signal response was due to system dispersion and the headspace volume of the testing device. (D) Zn cycling CE during in situ testing. (E) Gating responses under different Zn cycling current densities (5 sccm carrier gas). The deposition capacity was kept constant (0.5 mAh cm−2).

  • Fig. 5 Electrochemical properties of the redox-active quinone-based CO2 sorbent.

    (A) CO2 capture-release voltage profiles of PAQ electrodes with different mass loadings and cycling rates. 1C = 260 mA g−1 (capture/release in 60 min). The electrolyte was 0.1 M LiTFSI + 0.5 M ZnTFSI2 in PC with 10% EG. (B) Cycling stability of PAQ with a mass loading of 0.75 mg cm−2 at 2C rate (0.4 mA cm−2). The CO2 capacity is calculated assuming 100% capture efficiency. Each capture/release step was 30 min following a constant current (CC)–constant voltage (CV) protocol. Inset shows the SEM image of the PAQ electrode. (C) Diffusion model for limiting current density calculation. (D) Steady-state CO2 concentration at the depth of the PAQ electrode as a function of current density under different conditions. (E) CO2 permeation in response to electrochemical cycling of the CO2 cell, and the corresponding voltage profiles. Same device as gating membrane testing was used. (F) Detailed CO2 permeation behavior during one capture-release cycle marked with a dashed line in (E), compared to the CO2 permeation response predicted by COMSOL simulation. During CC capture, dissolved CO2 was being consumed at a constant rate, resulting in a continuous decrease in permeation. When the cutoff voltage for CC capture was reached, CV capture started with decaying current density, resulting in a gradual restoration in CO2 permeation. The subsequent CC release generated CO2 at a constant rate, leading to a sharp concentration spike and the release rate decayed during CV release until permeation returned to its background value (~4500 ppm). The simulation predictions match closely with the experimental results.

  • Fig. 6 High-efficiency CO2 separation by integrating the CO2 cell with two gas gating membranes.

    (A) Cross-sectional schematic of the integrated CO2 separation system. Gating membranes were mounted on brass gaskets. PAQ and LFP electrodes were sandwiched between the gating membranes. The components were separated by electrolyte-imbibed polypropylene separators and confined with rubber gaskets without headspace. CO2 flowed on one side of the device, and N2 flowed on the other side to carry permeated CO2 to the detector. (B) The gating cell is defined as closed when the system is facing the feed stream to capture CO2, and as open when the system is facing the N2 sweep stream to release CO2. (C) CO2 permeation during the operation of the integrated system, and the corresponding voltage profiles. The system started with a closed gate, and the CO2 cell was electrochemically reduced to capture CO2 (30-min CC-CV capture, red segment). Once the capture capacity was reached, the gating cell switched polarity and electrolyte outgassing caused CO2 permeation to the sweep stream (Zn cycling at 3 mA cm−2 for 10 min, teal segment). The permeation increased first followed by a gradual decrease, because CO2 in the electrolyte cannot be replenished from the feed side (gray segment). The captured CO2 was then released from the CO2 cell, resulting in a pronounced concentration spike (30-min CC-CV release, blue segment). The gate was finally closed by switching the gating cell polarity again to complete the cycle, with crossover concentration returned to zero (yellow segment). Inset shows the COMSOL simulation of the system response. (D) Continuous cycling of the integrated system with 20% CO2 as the feed. Contribution from the CO2 cell is marked in blue, and contribution from the physical solubility of the electrolyte is marked in teal.

Supplementary Materials

  • Supplementary Materials

    Electrochemically mediated gating membrane with dynamically controllable gas transport

    Yayuan Liu, Chun-Man Chow, Katherine R. Phillips, Miao Wang, Sahag Voskian, T. Alan Hatton

    Download Supplement

    This PDF file includes:

    • Figs. S1 to S21
    • Supplementary Methods S1 to S3
    • References

    Files in this Data Supplement:

Stay Connected to Science Advances

Navigate This Article