Academia.eduAcademia.edu

Figure 1 – uploaded by Dario Dekel

See full PDFdownloadDownload figure
[a] ETFE = ethylene tetrafluoroethylene, PTFE =polytetrafluoroethylene. [b] Anion content in the AEM was measured by ion exchange to Cl” and titratio based on the Warder method. [c] RH = relative humidity. Table 1. Summary of OH -form AEM neutralization to HCO, /CO,*.

Table 1 [a] ETFE = ethylene tetrafluoroethylene, PTFE =polytetrafluoroethylene. [b] Anion content in the AEM was measured by ion exchange to Cl” and titratio based on the Warder method. [c] RH = relative humidity. Table 1. Summary of OH -form AEM neutralization to HCO, /CO,*.

Related Figures (16)

WiHHaM £. WIUSTAIN FeECeIVved MIs FN.U. in chemical engineering from the Illi- nois Institute of Technology in 2006. He then moved to Georgia Tech for his postdoctoral studies. He now runs the Laboratory for Electrocata- lysts and Fuels as a Full Professor at the University of South Carolina. His group focuses on the development of novel catalysts for electrochemi- cal synthesis, acidic and alkaline fuel cells and electrolyzers, lithium-ion and aqueous batteries, and so forth. He has been the recipient of several awards, including the Illinois Institute of Technology Young Alumnus Award, U.S. Department of Energy Early Career Award, and Fulbright Scholar Fellowship.
WiHHaM £. WIUSTAIN FeECeIVved MIs FN.U. in chemical engineering from the Illi- nois Institute of Technology in 2006. He then moved to Georgia Tech for his postdoctoral studies. He now runs the Laboratory for Electrocata- lysts and Fuels as a Full Professor at the University of South Carolina. His group focuses on the development of novel catalysts for electrochemi- cal synthesis, acidic and alkaline fuel cells and electrolyzers, lithium-ion and aqueous batteries, and so forth. He has been the recipient of several awards, including the Illinois Institute of Technology Young Alumnus Award, U.S. Department of Energy Early Career Award, and Fulbright Scholar Fellowship.
Figure 1. Schematic representation of electrode reactions and ion and water transport in an A) AEMFC and B) proton-exchange membrane fuel cell (PEMFC). Reprinted with permission from Ref. [14].
Figure 1. Schematic representation of electrode reactions and ion and water transport in an A) AEMFC and B) proton-exchange membrane fuel cell (PEMFC). Reprinted with permission from Ref. [14].
Figure 2. The change in alkaline anion content if an OH -form Tokuyama A201 AEM was exposed to air (wet conditions); #: OH”, e: CO,”, a: HCO;. Adapted from Ref. [17] with permission of the Electrochemical Society. Identical carbonation behavior was also found in a model developed by Myles et al.?" They divided the carbonation pro- cess into two regions: hydroxide depletion [corresponding to Equations (1) and (2)], followed by bicarbonate accumulation [corresponding to the backward reaction in Equation (2)], until
Figure 2. The change in alkaline anion content if an OH -form Tokuyama A201 AEM was exposed to air (wet conditions); #: OH”, e: CO,”, a: HCO;. Adapted from Ref. [17] with permission of the Electrochemical Society. Identical carbonation behavior was also found in a model developed by Myles et al.?" They divided the carbonation pro- cess into two regions: hydroxide depletion [corresponding to Equations (1) and (2)], followed by bicarbonate accumulation [corresponding to the backward reaction in Equation (2)], until
Figure 3. Conductivity of AEM (A201, Tokuyama Co.) at 50°C, 95% RH, as a function of the equivalent fraction of carbonate ion species (CO, + HCO, ) to quaternary ammonium base. Adapted from Ref. [10] Copyright 2013, with permission from Elsevier. Figure 3 shows AEM conductivity as a function of the rela- tive fraction of functional groups occupied by CO,” and HCO; , as measured by Suzuki et al." As the fraction of car- bonate and bicarbonate species in the AEM increases, the con- ductivity decreases, which is to be expected from the expres-
Figure 3. Conductivity of AEM (A201, Tokuyama Co.) at 50°C, 95% RH, as a function of the equivalent fraction of carbonate ion species (CO, + HCO, ) to quaternary ammonium base. Adapted from Ref. [10] Copyright 2013, with permission from Elsevier. Figure 3 shows AEM conductivity as a function of the rela- tive fraction of functional groups occupied by CO,” and HCO; , as measured by Suzuki et al." As the fraction of car- bonate and bicarbonate species in the AEM increases, the con- ductivity decreases, which is to be expected from the expres-
[a] ATMPP =aminated trimethyl poly(phenylene), QAPSF = quaternary ammonium polysulfone, PSF = poly(arylene ether sulfone), PSU = polysulfone, TMA trimethylamine, DABCO = 1,4-diazabicyclo[2.2.2]octane, PPO = poly(p-phenylene oxide), PS= polystyrene, BTMA=benzyltrimethylammonium, HB-QPVBC hyperbranched quaternized poly[4-(chloromethyl)styrene]. [b] Exchanged for mixed HCO;-/CO,;” form. [c] Equilibrated with air containing 38% CC [d] Equilibrated with pure O). [e] In water. [f] 95% RH. Table 2. AEM ionic conductivities in different anion forms.
[a] ATMPP =aminated trimethyl poly(phenylene), QAPSF = quaternary ammonium polysulfone, PSF = poly(arylene ether sulfone), PSU = polysulfone, TMA trimethylamine, DABCO = 1,4-diazabicyclo[2.2.2]octane, PPO = poly(p-phenylene oxide), PS= polystyrene, BTMA=benzyltrimethylammonium, HB-QPVBC hyperbranched quaternized poly[4-(chloromethyl)styrene]. [b] Exchanged for mixed HCO;-/CO,;” form. [c] Equilibrated with air containing 38% CC [d] Equilibrated with pure O). [e] In water. [f] 95% RH. Table 2. AEM ionic conductivities in different anion forms.
Figure 4. Change in OH -form AEM (methylated poly(hexamethyl-p-terphe- nylbenzimidazolium) (HMT-PMBI)) conductivity with time from the beginning of exposure to ambient air containing CO,. Adapted from Ref. [9] with per- mission from The Royal Society of Chemistry.
Figure 4. Change in OH -form AEM (methylated poly(hexamethyl-p-terphe- nylbenzimidazolium) (HMT-PMBI)) conductivity with time from the beginning of exposure to ambient air containing CO,. Adapted from Ref. [9] with per- mission from The Royal Society of Chemistry.
Figure 5. Conductivity data reported in the literature for AEMs in the three different anion forms. Adapted from Refs. [9, 10,17, 22, 29,37, 38, 40-49].
Figure 5. Conductivity data reported in the literature for AEMs in the three different anion forms. Adapted from Refs. [9, 10,17, 22, 29,37, 38, 40-49].
Figure 6. Effects of CO, concentration in the cathode inlet gas on polariza- tion curves of the AEMFCs at the anode and cathode (the difference in volt- age compared to CO,-free gas is displayed). Adapted from Ref. [60]. The observation of decreasing ORR reaction rate with in- creased carbonation was also observed in reacting environ- ments more similar to those of operating AEMFCs. For exam- ple, Gunasekara et al. studied the effect of carbonation on the ORR at the interface with a membrane, as opposed to the tra- ditional tests in aqueous solutions; a special electrochemical cell was constructed, which consisted of flat-tip microelectro- des pressed against an AEM.°*" Oxygen gas was supplied to the cell through a porous glass filter. Although the exchange current density with a carbonated AEM was slightly lower than that in the case of an AEM with OH” only (carbonate-free), overall, the carbonates had little effect on the intrinsic ORR ki- netics.5*! However, the study also reported a significant de- crease in the O, diffusion coefficient (one order of magnitude lower) with a CO,?--form AEM relative to an AEM in the OH™ form, which greatly affected the electrochemical reaction rate. This small effect of CO, on the intrinsic ORR kinetics was con-
Figure 6. Effects of CO, concentration in the cathode inlet gas on polariza- tion curves of the AEMFCs at the anode and cathode (the difference in volt- age compared to CO,-free gas is displayed). Adapted from Ref. [60]. The observation of decreasing ORR reaction rate with in- creased carbonation was also observed in reacting environ- ments more similar to those of operating AEMFCs. For exam- ple, Gunasekara et al. studied the effect of carbonation on the ORR at the interface with a membrane, as opposed to the tra- ditional tests in aqueous solutions; a special electrochemical cell was constructed, which consisted of flat-tip microelectro- des pressed against an AEM.°*" Oxygen gas was supplied to the cell through a porous glass filter. Although the exchange current density with a carbonated AEM was slightly lower than that in the case of an AEM with OH” only (carbonate-free), overall, the carbonates had little effect on the intrinsic ORR ki- netics.5*! However, the study also reported a significant de- crease in the O, diffusion coefficient (one order of magnitude lower) with a CO,?--form AEM relative to an AEM in the OH™ form, which greatly affected the electrochemical reaction rate. This small effect of CO, on the intrinsic ORR kinetics was con-
[a] Extracted from polarization curves. [b] At 200 mAcm~. [c] At 100 mAcm~*. [d] At 4 mAcm~®. [e] Volume ratio of 1:2. Table 3. Effect of CO, on AEMFC performance.
[a] Extracted from polarization curves. [b] At 200 mAcm~. [c] At 100 mAcm~*. [d] At 4 mAcm~®. [e] Volume ratio of 1:2. Table 3. Effect of CO, on AEMFC performance.
Figure 7. Effect of CO, on fuel cell performance: a) Fuel cell polarization curves for a comparison of ambient air with clean air/pure O,. Data adapted from Refs. [9, 11, 12,71,72]. b) Immediate effect of switching (at t=0) from CO,-free air to ambient air in the cathode of a fuel cell operating at constant current density. The data shown was normalized by the initial power density or voltage. Data adapted from Refs. [11,73]. to Suzuki et al.,"" an AEMFC operated with H, and clean air with an AEM in the OH™ form could have a cell voltage up to
Figure 7. Effect of CO, on fuel cell performance: a) Fuel cell polarization curves for a comparison of ambient air with clean air/pure O,. Data adapted from Refs. [9, 11, 12,71,72]. b) Immediate effect of switching (at t=0) from CO,-free air to ambient air in the cathode of a fuel cell operating at constant current density. The data shown was normalized by the initial power density or voltage. Data adapted from Refs. [11,73]. to Suzuki et al.,"" an AEMFC operated with H, and clean air with an AEM in the OH™ form could have a cell voltage up to
Figure 8. Schematic representation of AEMFC current density on the self-purging process in the AEM. Left: at low current density, OH” is slowly formed in the cathode, but quickly converted into HCO; /CO,”-. Right: at high current density, OH” surpasses the carbonation process and HCO; /CO,?- are released at the anode as CO,. The self-purging mechanism is the process by which HCO; / CO,” can be removed from the AEMFC under high current conditions, which may help to improve cell performance, as demonstrated in several studies.®'*'*'””! The process may be explained as follows: at relatively high current densities, a large amount of OH anions are formed as a result of the high rate of ORR in the cathode [Eq. (7)]. The existing charge, which includes HCO,” and CO,”-, is transported through the AEM to the anode electrode. It is not known whether these ions react directly with H, [Eqs. (10) and (11)] or whether OH” remains the active anion [Eq. (9)] and under hydroxide consumption the equilibrium is shifted in the reverse direction of Equa- tions (1) and (2). As discussed above, anode studies alone sug- gest that it is the latter. In either case, the result is that CO, gas is readily released to the anode and purged from the system. This concept is schematically shown in Figure 8. At in- creased current densities, the rate of OH production in the cathode overcomes the rate of CO, absorption into the mem- brane, and therefore, the overall result is the increase in OH™ concentration at the expense of HCO,~/CO,”- concentration. It
Figure 8. Schematic representation of AEMFC current density on the self-purging process in the AEM. Left: at low current density, OH” is slowly formed in the cathode, but quickly converted into HCO; /CO,”-. Right: at high current density, OH” surpasses the carbonation process and HCO; /CO,?- are released at the anode as CO,. The self-purging mechanism is the process by which HCO; / CO,” can be removed from the AEMFC under high current conditions, which may help to improve cell performance, as demonstrated in several studies.®'*'*'””! The process may be explained as follows: at relatively high current densities, a large amount of OH anions are formed as a result of the high rate of ORR in the cathode [Eq. (7)]. The existing charge, which includes HCO,” and CO,”-, is transported through the AEM to the anode electrode. It is not known whether these ions react directly with H, [Eqs. (10) and (11)] or whether OH” remains the active anion [Eq. (9)] and under hydroxide consumption the equilibrium is shifted in the reverse direction of Equa- tions (1) and (2). As discussed above, anode studies alone sug- gest that it is the latter. In either case, the result is that CO, gas is readily released to the anode and purged from the system. This concept is schematically shown in Figure 8. At in- creased current densities, the rate of OH production in the cathode overcomes the rate of CO, absorption into the mem- brane, and therefore, the overall result is the increase in OH™ concentration at the expense of HCO,~/CO,”- concentration. It
Figure 9. CO, emission from the anode exhaust of an AEMFC operated with ambient air during a stepwise increase in current density. Adapted from Ref. [72] Copyright 2009 with permission from the Electrochemical Society.
Figure 9. CO, emission from the anode exhaust of an AEMFC operated with ambient air during a stepwise increase in current density. Adapted from Ref. [72] Copyright 2009 with permission from the Electrochemical Society.
Table 4. Summary of computational models for AEM properties.
Table 4. Summary of computational models for AEM properties.
Figure 10. Modeling results for the effect of operating conditions on AEM ex situ conductivity: a) Model predictions by Grew et al.” for Tokuyama A201 AEM ionic conductivity in equilibrium with gas containing 0 (pure O,; blue line), 1 (green line), and 385 ppm CO, (red line) (Copyright 2011 with permission from the Electrochemical Society). b) Model predictions for AEM conductivity at different hydration levels by Kiss et al.?7) The hydroxide (blue line) and mixed (combined carbonate and bicarbonate; red line) forms are plotted (Copyright 2013 with permission from the Electrochemical Society).
Figure 10. Modeling results for the effect of operating conditions on AEM ex situ conductivity: a) Model predictions by Grew et al.” for Tokuyama A201 AEM ionic conductivity in equilibrium with gas containing 0 (pure O,; blue line), 1 (green line), and 385 ppm CO, (red line) (Copyright 2011 with permission from the Electrochemical Society). b) Model predictions for AEM conductivity at different hydration levels by Kiss et al.?7) The hydroxide (blue line) and mixed (combined carbonate and bicarbonate; red line) forms are plotted (Copyright 2013 with permission from the Electrochemical Society).
Figure 11. Results of the computational model for profiles of equivalent anion ratios of a) OH~, b) CO,?-, and c) HCO,” across the membrane be- tween the anode (0 um) and cathode (28 um) for different current densities (100, 500, 1000, and 1500 mAcm~”). Reprinted with permission from Ref. [67].
Figure 11. Results of the computational model for profiles of equivalent anion ratios of a) OH~, b) CO,?-, and c) HCO,” across the membrane be- tween the anode (0 um) and cathode (28 um) for different current densities (100, 500, 1000, and 1500 mAcm~”). Reprinted with permission from Ref. [67].
Related topics:
CO2 captureAnion Exchange Membraneanion exchange membrane fuel cells
580 California St., Suite 400
San Francisco, CA, 94104
© 2025 Academia. All rights reserved