Chapter 7. Highly efficient CO 2 utilization via aqueous zinc- or aluminum-CO 2 systems for hydrogen
7.4 Results and Discussion
7.4.4 Full-cell performance and comparison to other works
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Figure 7.12 Zn- and Al-CO2 systems performance. Full cell tests were conducted in three-electrode configuration using Ag/AgCl reference electrode. The polarization I-V profiles were measured under CO2-saturated 1 M KOH for various catalysts for a, Zn-CO2 system and b, Al-CO2 system. c, Comparison of maximum power density and corresponding current density for various metal-CO2
cells. d, Chronopotentiometric reduction profiles at 5 mA cm-2 in CO2-saturated 1 M KOH for Zn-CO2
system (above) and Al-CO2 system (below). e, The in-operando qualitative GC profiles of outlet CO2
feed gas before and during discharging at 100 mA under CO2-saturated 1 M KOH (above) and
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seawater (below). The enlarged areas near 12 min indicating the amount of converted CO2 is shown as the insets.
Figure 7.13 The polarization I-V profiles obtained at the various catalyst loading density of 1, 2, and 3 mg cm-2 for a, Pt/C, b, PBSCF, c, FL-graphene, d, KB.
The polarization discharge curves for Al-CO2 system are presented in Figure 7.12b. The Al- CO2 system has a slightly higher OCV (1.3 V) than that of the Zn-CO2 system (the oxidation of Al was observed at -1.9 V vs. Ag/AgCl). As similarly observed in the Zn-CO2 system, the oxidation I-V profiles of Al anode are also achieved reproducibly for all catalysts. For the Al-CO2 system, the maximum current densities achieved were 87.3, 305.3 and 532.1 mA cm-2 for FL-graphene, PBSCF, and Pt/C, respectively. Surprisingly, the highest electrochemical performance was obtained at a maximum power density of 125.4 mW cm-2 at 254.3 mA cm-2 for Pt/C catalyst, far superior to the best performance reported for the metal-CO2 systems; i.e., Zn-CO2 cell based on CO2-HCOOH interconversion of 5.5 mW cm-2 at 11 mA cm-2.21 For direct comparison, the maximum power density (Pmax) and the corresponding current density (Jcorr) of various metal-CO2 cells in the literatures12-21 were presented in Figure 7.12c. The performances of both Zn- or Al-CO2 systems are clearly better
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than those of the conventional aprotic metal-CO2 systems.12-20 Interestingly, it has been observed that the performances of our systems are comparable to those of Zn- or Al-air cells which have been actively studied as promising representatives of metal-air batteries.27-33
To investigate the stability and durability of this system, the chronopotentiometric profiles on a mechanically rechargeable Zn/Al-CO2 system were examined at current density of 5 mA cm-2 (Figure 7.12d). For the Zn-CO2 system, as similarly observed in the polarization profiles of Figure 7.12a, Pt/C, PBSCF, and FL-graphene presents 0.84, 0.54, and 0.30 V, respectively, for 10 hours. For the Al- CO2 system tested under the same conditions, the discharge voltages were slightly higher: 1.11, 0.83, and 0.63 V for Pt/C, PBSCF, and FL-graphene, respectively. The discharge profiles measured at high current densities (10 and 50 mA cm-2) more than 50 hours for Zn/Al-CO2 systems using Pt/C are shown in Figure 7.14.
Figure 7.14 Discharge profiles measured at the current density of 10 and 50 mA cm-2 for a, Zn-CO2
and b, Al-CO2 cell using Pt/C catalyst. The discharge profile of Al-CO2 cell in this figure was measured by using a low purity Al plate (Al plate, alloy 6061, Alfa-aesar Co.) rather than the high purity Al foil (99.99 %) because a self corrosion rate of thin Al foil is too fast.
It is notable that all profiles have shown in plateau without significant degradation, indicating a stable generation of a gas phase H2 and electricity. In other words, there is no clogging or physical damage on the electrode during continuous operation as examined from scanning electron microscopy images of Pt/C, PBSCF, and FL-graphene (Figure 7.15-7.17), unlike existing aprotic metal-CO2 cells which have solid products, such as Li2CO3(s) or Na2CO3(s) on electrodes during discharge.12-20 Also, the discharge profiles were further investigated in CO2 dissolved seawater electrolyte, an environment similar to the natural condition for removing atmospheric CO2 (Figure 7.18). The performance profiles measured in CO2-saturated seawater for Zn- and Al-CO2 systems were obtained at 0.77 and 1.00 V for Pt/C, respectively, indicating also active and stable performance. Consequently, this
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finding confirmed that the dissolution of CO2 led to a favorable HER environment in both KOH solution and seawater.
Figure 7.15 SEM images of Pt/C catalyst loaded carbon paper electrode before and after tests in Zn- and Al-CO2 systems. a, and b, SEM images of Pt/C electrode before tests. c, Energy dispersive X-ray spectroscopy (EDX) image of the electrode before tests. Corresponding elements mapping images for d, carbon e, oxygen and f, platinum. g, and h, SEM images of Pt/C electrode after tests. i, EDX image of the electrode before tests. Corresponding elements mapping images for j, carbon k, oxygen and l, platinum.
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Figure 7.16 SEM images of PBSCF loaded electrode. a, SEM image of PBSCF catalyst electrosprayed carbon paper electrode examined before a test b, after electrochemical test. c, and d, enlarged images of a, and b, presenting PBSCF loaded carbon fiber tissue of carbon paper electrode.
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Figure 7.17 SEM images of FL-graphene loaded electrode. a, SEM image of FL-graphene catalyst electrosprayed carbon paper electrode examined before a test, b, after electrochemical test. c, and d, enlarged images of a, and b, presenting FL-graphene loaded carbon fiber tissue of carbon paper electrode.
Figure 7.18 Chronopotentiometric discharge profiles of Zn- and Al-CO2 systems measured under CO2
saturated seawater.
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Figure 7.19 Gas chromatography (GC) profiles of generated gas during discharge process. The gas obtained during cathodic reaction proceeded in a, CO2-saturated 1 M KOH, b, CO2-saturated seawater.
Figure 7.20 a, Theoretical H2 generation rate at current of 100 mA. b, Theoretical CO2 conversion rate at current of 100 mA.