Anion-exchange Membrane Fuel Cells with Improved CO2

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Anion-exchange Membrane Fuel Cells with Improved CO-tolerance: Impact of Chemically Induced Bicarbonate Ion Consumption Yu Katayama, Kosuke Yamauchi, Kohei Hayashi, Takeou Okanishi, Hiroki Muroyama, Toshiaki Matsui, Yuuki Kikkawa, Takayuki Negishi, Shin Watanabe, Takenori Isomura, and Koichi Eguchi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09877 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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ACS Applied Materials & Interfaces

Anion-exchange membrane fuel cells with improved CO2-tolerance: impact of chemically induced bicarbonate ion consumption

Yu Katayama1, Kosuke Yamauchi1, Kohei Hayashi1, Takeou Okanishi1, Hiroki Muroyama1, Toshiaki Matsui1, Yuuki Kikkawa2, Takayuki Negishi2, Shin Watanabe2, Takenori Isomura2 and Koichi Eguchi1*

1

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto

University, Kyoto 615-8510, Japan 2

Corporate Development Department, Tokuyama Corporation, Tsukuba, Ibaraki, 300-4247,

Japan

KEYWORDS: Anion exchange membrane fuel cell, Hydrogen carrier, Ammonia, Carbon dioxide, in situ ATR-IR spectroscopy

ACS Paragon Plus Environment

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ABSTRACT

Over the last few decades, owing to the significant development of anion exchange membranes, increasing efforts have been devoted the realization of anion exchange membrane fuel cells (AEMFCs) that operate with the supply of hydrogen generated on-site. In this paper, ammonia was selected as a hydrogen source, following which the effect of conceivable impurities, unreacted NH3 and atmospheric CO2, on the performance of AEMFCs was established. As expected, we show that these impurities worsen the performance of AEMFCs significantly. Furthermore, with the help of in situ attenuated total reflection infrared (ATR-IR) spectroscopy, it was revealed that the degradation of the cell performance was primarily due to the inhibition of the hydrogen oxidation reaction (HOR). This is attributed to the active site occupation by COrelated adspecies derived from (bi)carbonate adspecies. Interestingly, this degradation in the HOR activity is suppressed in the presence of both NH3 and HCO3− due to the bicarbonate ion consumption reaction induced by the existence of NH3. Further analysis using in situ ATR-IR and electrochemical methods revealed that the poisonous CO-related adspecies were completely removed under NH3−HCO3− conditions, accompanied by the improvement in HOR activity. Finally, a fuel cell test was conducted by using the practical AEMFC with the supply of NH3contained H2 gas to the anode and ambient air to the cathode. The result confirmed the validity of this positive effect of NH3−HCO3− co-existence on CO2-tolerence of AEMFCs. The cell performance achieved nearly 95% of that without any impurity in the fuels. These results clearly show the impact of the chemically induced bicarbonate ion consumption reaction on the realization of highly CO2-tolerent AEMFCs.


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TEXT 1. Introduction Fuel cells offer a viable route to store large amounts of energy in chemical bonds.1 While fuel cell technologies have developed significantly over the past few years, high costs and low efficiencies still impede their widespread application. In an alkaline anion exchange membrane fuel cell, gaseous hydrogen is oxidized at the cathode, and gaseous oxygen is reduced at the cathode to form hydroxyl ions. Both these processes suffer from several drawbacks relating to the purity and chemical composition of the feed gas. Hydrogen has been considered as the most typical fuel for fuel cells, but its physical properties, such as flammability, have limited its applicability in practical fuel cells. In order to overcome this problem, a variety of hydrogen carriers have been proposed as alternative fuels 2 3. Among the hydrogen carriers, NH3 is one of the promising candidates due to its superior characteristics; low production cost, ease in liquefaction at ambient temperatures, high volumetric energy density, and no carbon content

4 5

. However, there are still some challenges

associated with its practical use in fuel cells. For instance, in polymer electrolyte fuel cells, which operate in acidic conditions, NH3 causes serious deterioration of cell performance through its neutralization reaction6 7. On the other contrary, NH3 can be electrochemically oxidized in alkaline environments, which enables the use NH3 as an alternative fuel for anion exchange membrane fuel cells (AEMFCs) 8. Furthermore, the oxygen reduction reaction (ORR), which occurs at the cathode in fuel cell systems, proceeds with much lower overpotential in basic conditions9 10 11 12. Thus the sluggish kinetics of the NH3 oxidation reaction significantly lowers the terminal voltage of direct NH3-fueled AEMFC

13 14

. In addition, poisonous species formed

during the reaction cause serious deterioration in the activity of anode catalyst14. Although these


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disadvantages of the direct NH3-fueled AEMFC have been steadily improved by designing the anode electrocatalyst15

16 17 18 19 20 21

, another potential approach is to combine the NH3

decomposition reactor with AEMFC and supply NH3-derived H2 to the anode instead of pure NH3. Catalysts for NH3 decomposition have been extensively studied due to an increasing interest in on-site generation of COx-free hydrogen22

23 24 25

. Previous reports suggested that the

NH3 conversion is sensitive to the reaction temperature, and its value nearly reaches 100% at relatively high temperature (ca. 600 °C) but falls to 40–60 % at ca. 500°C22 23. For the utilization of AEMFC combined with the NH3 decomposition system, therefore, it is crucial to study the effect of NH3 residue to the fuel cell performance. For the cathode reaction, there have been several studies regarding the impurity tolerance26 9 27

. One of the most detrimental impurities at the cathode side is CO2, which has shown to cause

increases in the ohmic resistance and electrode overpotential when the AEMFC is operated in air. The following reactions proceed at the cathode28 29: O2 + 2H2O + 4e− → 4OH−

(1)

OH− + CO2 → HCO3−

(2)

HCO3− + OH− ↔ CO32− + H2O

(3)

Here, Reaction (1) is the typical four electron ORR in alkaline conditions. Reactions (2) and (3) represent the thermodynamic equilibrium between hydroxide, bicarbonate and carbonate ions in aqueous media. As reported in a previous study, the main cause of the degradation of AEMFC is the formation of carbonate/bicarbonate ions (CO32−/HCO3−) in the electrolyte membrane28. Inaba et al. reported that the replacement of carbonate/bicarbonate ions with OH− ion is more difficult


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and/or slower at the anode than at the cathode even at high current densities30. In other words, carbonate/bicarbonate ions migrate toward the anode under a current loading condition, resulting in the condensation of these anions in the vicinity of anode. This causes a significant increase in the overpotential of the anode reaction, which is generally the hydrogen oxidation reaction (HOR) 30 31. Although the replacement of cathode gas from pure O2 to an ambient air is attractive in terms of cost and simplification of the fuel cell system, no strategy has been presented to address the problem regarding to the formation of carbonate/bicarbonate ions. In this study, then, the effect of conceivable impurities, especially unreacted NH3 and atmospheric CO2, in AEMFC equipped with the NH3 decomposition system is first considered. It has been reported that the reduction in the activity of HOR in the presence of CO2-derived anion species causes serious degradation in the fuel cell performance30

31

. To understand the

fundamental degradation mechanism, in situ observation of the Pt catalyst surface during the electrochemical reaction was performed by using attenuated total reflection infrared spectroscopy (ATR-IR). Furthermore, the effect of co-existence of NH3 and HCO3− was studied extensively using both in situ ATR-IR and electrochemical methods. Finally, we conducted device-level performance tests on AEMFC with the supply of NH3-contained H2 gas to the anode and ambient air to the cathode in order to establish the effect of NH3 and HCO3− co-existence.


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2. Experimental 2.1. Electrocatalyst Preparation For in situ ATR-IR measurements, the Pt working electrode, composed of a thin (ca. 50 nm) Pt film, was deposited on the total reflecting plane of a hemispherical Si ATR prism (radius 22 mm) by an electroless deposition method32. First, the base plane of the Si prism was given a hydrophilic treatment by contacting with 40% NH4F solution for a minute. Then, palladium seeds were deposited on the base plane with 1% HF–1 mM PdCl2 for 5 min at room temperature. After rinsing with water, platinum electroless deposition was carried out by contacting with the Pt plating solution at 50 °C for ca. 12 min. The Pt plating solution was prepared by mixing LECTROLESS Pt 100 basic solution (30 mL, Electroplating Engineering of Japan Ltd), LECTROLESS Pt 100 reducing solution (0.6 mL), 28% NH3 solution, and ultrapure water. Osawa et al. reported the morphology of a chemically deposited Pt electrode fabricated in the similar way, which had the characteristic surface structure to provide the surface enhancement effect33.

2.2. Fuel cell operation The basic properties of the anion exchange membrane (A201, Tokuyama) used in this study have been described elsewhere14. A commercially available Pt/C catalyst (Tanaka Kikinzoku Kogyo, TEC10E50E, 46.1 wt% Pt on Ketjen Black) was used in the anode and the cathode in all cases. A catalyst slurry was prepared by mixing catalyst with the anion exchange ionomer solution (AS-4, Tokuyama) to obtain the weight ratio of the polymer content of 0.8 with respect to carbon for both anode and cathode. The slurry was directly screen-printed on the membrane.


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The geometric electrode area was 1.0 cm2 or 5.0 cm2, and the Pt loadings were set at 0.5 mg cm−2 for both electrodes. A membrane electrode assembly (MEA) was constructed by sandwiching the catalyst-coated membrane between two microporous gas diffusion layers (24BC, SGL). The MEA was mounted in a single-cell holder composed of two carbon separator plates with ribbed single serpentine flow channels. The cells were operated at 50°C and ambient pressure under high humidity conditions (95% or 100% relative humidity (RH) for both electrodes). Humidified H2 or (100–x)% H2–x% NH3 (x = 0, 0.5, 1, 3, 5, 7, 10, 15, 20) was fed to the anode, and 100% O2, 20% O2–80% N2, or an ambient air (20.94% O2–78.09% N2–0.93% Ar–0.03% CO2) was supplied to the cathode. The flow rate for all of the feed gases was 100 mL min−1, unless otherwise noted. The obtained current-voltage (I–V) characteristics were measured at a scan rate of 10 mV s−1 using a Solartron 1470E potentiostat. The galvanostatic measurements were conducted using a Solartron 1470E potentiostat. All results for the galvanostatic measurements shown were collected after holding the current for 15 min to make sure the cell voltage stabilizes before measurement. The impedance spectra were measured at the open circuit condition using a Solartron 1260A impedance analyzer.

2.3. In situ ATR-IR Measurement Details of in situ ATR-IR spectroscopy are described elsewhere 34 35 36 37. A Pt-deposited Si prism was used for in situ ATR-IR measurement. The prism was mounted in a spectroelectrochemical three-electrode cell with an Ag/AgCl reference electrode and a platinum wire counter electrode. The cell was then placed in a self-made reflection optics setup at an incident angle of 70° as shown in the literature32. A Fourier transform infrared spectrometer equipped


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with a MCT detector (NICOLET8700, Thermo Fisher Scientific) was employed for in situ ATRIR measurements. The optical path was fully replaced with N2 gas. The electrolyte solutions were prepared by mixing 28wt% NH3 solution (Wako Pure Chemical) with KOH (Sigma– Aldrich, >85 wt%) or KHCO3 (Sigma–Aldrich, 99.7 wt%) and ultrapure water. After deoxygenation of the electrolyte solution by purging Ar, the prism surface was cleaned by cycling the potential between 0.05 and 0.90 V vs. reversible hydrogen electrode (RHE). For the electrochemical measurements, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were conducted at room temperature by using HSV-110 (Hokuto Denko). All potentials in this paper have been converted to the potential versus the reversible hydrogen electrode (RHE). The electrochemical surface area (ECSA) of the Pt electrocatalysts was calculated from cyclic voltammograms recorded in the Ar purged KOH electrolyte by integrating the charge in the hydrogen adsorption/desorption region, in the range of 0.05–0.45 V vs. RHE. The current density was normalized by ECSA (denoted as A cm–2). All spectra are shown in absorbance units defined as log(I0/I), where I0 and I represent the spectra at reference and sample potentials, respectively. The reference spectrum of I0 was measured at 0.05 V in the blank KOH solution unless otherwise noted. The ATR-IR spectra below 1200 cm−1 had poor S/N due to strong IR absorption by the Si prism and thus the results above 1200 cm−1 are reported.

2.4. Electrochemical measurements All electrochemical measurements were conducted in a conventional three-electrode cell. A Pt disk electrode (geometric area: 0.196 cm2) was used as a working electrode. The platinum wire and reversible hydrogen electrode (RHE) were used as counter and reference electrodes,


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respectively. After the electrochemical cell was purged for 30 min with Ar and then for 2 h with H2, polarization curves for hydrogen oxidation reaction (HOR) were obtained using a HSV-110 potentiostat in a H2-saturated 1 M KOH, 1 M KOH–1 M NH3, 1 M KHCO3, and 1 M KHCO3–1 M NH3 solutions with the rotation rate of 800 r.p.m. The ECSA was determined in the same way as described in section 2.3.

3. Results and discussion 3.1. Effects of NH3 contamination and atmospheric CO2 on AEMFC performance In order to study the effect of NH3 contamination on AEMFC performance, I–V curves were measured at 50 ºC, while supplying pure O2 and H2–NH3 (NH3: 0.5%–15%) to the cathode and anode, respectively (Figure 1). As can be seen here, the addition of NH3 to the anode gas slightly changed the I-V response. The overall trend is fairly predictable - the cell performance deteriorated with an increase in the NH3 content of the anode fuel. This can partially be attributed to the decrease in the hydrogen partial pressure with the increasing amount of NH3 in the fuel. However, the OCV for 85% H2–15% NH3 system was ca. 0.1 V lower than that of 85% H2–15% Ar system, indicating the undesirable impact of NH3 on cell performance is not purely from the decrease in hydrogen partial pressure but also from additional factor, such as poisoning of the catalyst surface. (see Figures S1 and S2, Tables S1 and S2). Here, we selected Ar as a dilution gas since Ar is considered as an inert gas that can be used to change the partial pressure


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of hydrogen without any influence on the activity of the electrocatalyst. Note that the value of OCV was considerably higher than that of direct NH3-fueled AEMFCs (cf. OCV of ca. 0.45 V for 100% NH3 (RH100%), see Table S1) 14 17 31, indicating that at this condition, the influence of the NH3 oxidation reaction at the anode catalyst can be considered to be negligible. A closer look at Figure 1, however, shows that the current density was slightly improved when the small amount (less than 7%) of NH3 was contained in the fuel. This rather subtle, but interesting behavior can be explained by the positive effect of NH3 contamination on the conductivity of AEM. Figure 2 shows the impedance spectra of AEMFC under the open circuit condition with the supply of pure O2 and H2–NH3 (NH3: 0.5%–15%) to the cathode and anode, respectively. The ohmic resistance of the cell, which can be derived from the value of x-intercept, slightly decreased with the increment of NH3 content in the anode fuel. One possible explanation for this behavior is that the addition of NH3 into the anode gas increased the basicity of the AEM, which gave rise to an increment of OH– content in the AEM. Unfortunately, when the concentration of NH3 in the fuel became larger than 7%, the negative effect from NH3 outweighed the improvement in the ohmic resistance and the overall cell performance deteriorated (see Figure S1). In the terminal voltage range of ca. 0.65 - 0.8 V (ohmic polarization region), all curves showed almost the same slope; in the NH3-contained cases, the slope was slightly gradual as compared to the case of 100% H2, due to the improvement in the ohmic resistance of AEM (For detail, see Figures S1 and S2). Below the terminal voltage of ca. 0.65 V, however, the slope of NH3-contained system became steeper, indicating the inhibition of the gas diffusion by NH3 existence. Figure 3 shows the effect of presence of CO2 in the cathode gas with the anode gas composition fixed at 100% H2. The terminal voltage at 400 mA cm–2 decreased by ca. 18% when


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the gas composition was changed from 100% O2 to 20% O2–80% N2, which can be explained by the difference in the oxygen partial pressure. However, although the difference in oxygen partial pressure between 20% O2–80% N2 and ambient air (20.94% O2–78.09% N2–0.93% Ar–0.03% CO2) is considerably small, the difference in terminal voltage under those two conditions, was significantly large; the terminal voltage observed under the ambient air fell to ca. 40% of that in 20% O2–80% N2. This notable reduction in AEMFC performance was induced by the existence of CO2 at the cathode gas, which was known to increase the ohmic resistance as well as the electrode overpotential

28

. Recent research has revealed that this negative impact of CO2 on the

electrode reaction mainly occurs at the anode side of the cell, where the hydrogen oxidation reaction (HOR) proceeds28 30 38 39 40.

3.2. Effects of NH3 and CO2-derived species on HOR activity In order to further investigate the effect of NH3 and CO2 on the HOR, both electrochemical and spectroscopic experiments were conducted under a simplified aqueous electrolyte system. Throughout the experiments, KHCO3 and KOH electrolytes were used to simulate the system with and without CO2, respectively. Polarization curves for the HOR were obtained by using the rotating Pt-disk electrode in various H2-saturated electrolytes. As can be seen in Figure 4, polarization curves in 1 M KOH and 1 M KOH–1 M NH3 were almost the same, indicating no negative effect of NH3 on the HOR. On the other hand, the x-intercept of the polarization curve in 1 M KHCO3 is positively shifted ca. 0.025 V, which clearly indicated the increase in the overpotential of HOR due to the presence of CO2-related species. It should be mentioned that the shape of the polarization curve highly depends on the pH of the electrolyte41. In this case, the pH


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of 1 M KOH and 1 M KOH–1 M NH3 electrolytes was ca. 14, whereas 1 M KHCO3 electrolyte has a pH of ca. 8. Therefore, the influence of the pH on obtained polarization curves cannot be ignored when a potential value at a certain current density is used as a metric to compare the electrocatalytic activity. In this study, we selected the x-intercept of the polarization curve to avoid the influence of the pH. In order to gain further mechanistic insight into the HOR suppression in CO2-contained electrolyte, in situ ATR-IR technique was used in order to observe adsorbed species on the Pt surface during the electrochemical reaction. In situ IR spectra shown in Figure 5 were collected during the linear sweep voltammetry from 0.05 V to 0.9 V in 1 M KHCO3 (The LSV result is shown in Figure S3). In the spectra, three potential dependent peaks were observed at 1491–1545 cm–1, 1731–1794 cm–1, and 1961–2001 cm–1. According to the previous studies, peaks at 1491– 1545 cm–1 can be assigned to the O-C-O stretching vibration of (bi)carbonate species42

43

. The

large shift of the wavenumber with potential is possibly related to the stark effect, the dipoledipole interaction, the change in the adsorption mode, and the change in the molecular structure44 45 46

. Two peaks observed at 1731–1794 cm–1 and 1961–2001 cm–1 seem to agree with the

characteristic C≡O stretching mode of CO adspecies47 48. However, the peaks from CO adspecies for the same Pt surface always appeared ca. 40 cm–1 higher in 1 M KOH as compared to those observed in 1 M KHCO3, indicating a slight difference in the strength of C≡O bonding for these two cases (see Figure S4). At this point, the exact cause of this change in wavenumber is not known definitively. It could be because of the difference in adsorption site and/or surrounding environment, e.g. interaction with water through hydrogen. Here, we emphasize that this adsorbed species (hereafter denoted as CO-related adspecies) are expected to have a similar molecular structure to CO adspecies, which can be observed under CO-gas saturated conditions


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(see Figures S4 and S5). For Pt-based electrocatalysts, CO adspecies is known to behave as strong poisonous species8 49 50. The comparison of the band intensity with the spectra obtained in CO-saturated 1 M KOH and 1 M KHCO3 revealed that the estimated maximum coverage of the CO-related adspecies formed in 1 M KHCO3 was nearly 0.9 (for detail, see Figures S5). The COrelated adspecies observed in 1 M KHCO3 cannot be removed completely from the Pt surface until the potential reaches 0.7 V vs. RHE, implying that the HOR on the Pt electrocatalyst proceeded under the influence of these adspecies. Figure 6 shows in situ ATR-IR spectra obtained under the open circuit condition immediately after the cyclic voltammetry from 0.3 V to 0.9 V. It can be seen that the intensities of CO-related bands gradually increase with a time at the open circuit state, indicating that the CO-related adspecies form through a catalytic process as well as an electrocatalytic process. Furthermore, the peak intensity of (bi)carbonate adspecies decreased when the CO-related adspecies started to form on the surface. This indicates the CO-related adspecies is produced partially from the (bi)carbonate adspecies through a catalytic process. This result agrees well with the observation previously reported by Aldaz et al., which confirmed the formation of CO-related adspecies through the dissociation of (bi)carbonate species51. In addition, there is also a possibility that the electrochemical reduction of (bi)carbonate species is triggered by the application of potential to the electrode. However, the formation of the CO-related adspecies through the electrochemical reduction process is reported to start at the potential lower than ca. 0.10 V vs. RHE in 0.1 M KHCO3 solution51 52. In the case of our experimental conditions, therefore, the influence of this electrochemical reduction process can be considered to be negligible. Following this reaction mechanism for the formation of CO-related adspecies, we conclude that the reduction in the


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concentration of (bi)carbonate adspecies is the key to suppressing the negative impact of CO2 on the HOR.

3.3. Impact of co-existence of NH3 and CO2-derived species When NH3 and bicarbonate ions coexist in the electrolyte, the following chemical reaction is expected53 31: HCO3− + NH3 ⇌ H2NCO2− + H2O

(4)

This reaction consumes a part of bicarbonate species. Therefore, the addition of NH3 into the system contributes to improve the CO2 tolerance of the Pt electrocatalyst. Furthermore, as confirmed by Figure 4, the existence of NH3 did not inhibit the HOR. The trend in onset potential for the HOR shown in Figure 7 clearly supports our hypothesis that the overpotential of the HOR was significantly improved by the co-existence of NH3 and bicarbonate ion. This positive effect of the co-existence of NH3 and bicarbonate ion was further confirmed by using in situ ATR-IR. The spectra shown in Figure 8 were collected in the same manner as in Figure 5 in 1 M KHCO3–1 M NH3. The result clearly demonstrated the positive impact of the existence of NH3, which led to the complete removal of the CO-related adspecies from the Pt surface. In Figure 8, two new peaks emerged at ca. 1380 cm–1 and ca. 1480 cm–1, which can be ascribed to the C-N stretching and O-C-O stretching of the H2NCO2 adspecies, respectively54 56

55

. It is important to note that according to the HOR polarization curve, the adsorption of

H2NCO2 does not prevent the HOR on the Pt electrocatalyst, possibly due to its relatively weak adsorption and/or low surface coverage. The positive impact of NH3-existance is also observed


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under the open circuit condition where the formation of the CO-related adspecies was suppressed completely (Figure 9). As expected, no (bi)carbonate peak was observed in Figure 9, indicating NH3 additive sufficiently consumed the bicarbonate ion which is considered to be the starting material for the formation of the CO-related adspecies. The strong and immediate influence of reaction (4) on surface adspecies is established in Figure 10. In Figure 10, the reference spectrum was measured under the open circuit condition after holding the Pt surface at OCV for 10 min in 1 M KHCO3. After 10 seconds of ATR-IR measurement in 1 M KHCO3, NH3 was introduced into the solution to elucidate the effect of reaction (4) on the composition of surface adsorbed species. The result is rather straightforward; three bands at 1510 cm–1, 1794 cm–1, and 1998 cm–1 which are attributed to either (bi)carbonate and CO-related adspecies, were observed with negative instensities, suggesting the consumption and/or removal of these adspecies from the Pt surface via the reaction (4). In contrast, the formation of H2NCO2 adspecies through the reaction (4) is validated by the appearance of two positive-going bands at 1345 cm–1 and 1450 cm–1.

3.4. Improvement in CO2 tolerance of AEMFC The results so far clearly establish the potential of NH3 addition for the improvement in CO2tolerance of Pt electrocatalyst under aqueous conditions. Finally, in this section, the effect of NH3 addition to the anode gas was investigated using the practical AEMFC system with the supply of various NH3-containing H2 gases to the anode and an ambient air (20.94% O2–78.09% N2–0.93% Ar–0.03% CO2) to the cathode. Figure 11 shows the terminal voltage of AEMFC at current density of 400 mA cm–2 collected from galvanostatic curves after 15 min duration of current holding with feeding NH3-containing H2 fuels. As shown in Figure 11, even the supply of


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1% of NH3 into the anode gas significantly improved the performance of the cell, which agrees well with the conclusions, derived using an aqueous system. The activity enhancement by NH3 addition lasted until the NH3 content reached 15%, which is different from the trend observed without the influence from CO2 in Figure 1. This trend indicates that the enhancement is not only due to the increase in the conductivity, but also due to the improvement in the HOR activity. Note that even in 99% H2–1% NH3, there is enough amount of NH3 to capture and convert the bicarbonate ion into H2NCO2 ion. The complex structure inside the AEMFC, however, may prevent the NH3 to react sufficiently with bicarbonate ion, leading to the local maximal enhancement at an NH3 content of 15%. Above NH3 content of 15%, however, the negative effect of NH3, e.g. decrease in hydrogen partial pressure, may begin to govern the overall activity. The maximum terminal voltage observed under the supply of ambient air at 400 mA cm–2 was ca. 0.41 V, which was almost equivalent to the value observed under the supply of 20% O2–80% N2 into the cathode, ca. 0.44 V (see Figure 3). Therefore, by combining the results shown in Figures 3 with Figure 11, it is successfully demonstrated that our system design strategy to improve the CO2-tolerance using a simplified aqueous system is applicable to practical AEMFCs.


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4. Conclusions

In this work, the effect of NH3 and CO2 on the performance of AEMFC was studied using a range of techniques. By combining the results of in situ ATR-IR spectroscopy and fuel cell tests, it was revealed that the inhibition of the HOR by the formation of CO-related adspecies on the electrocatalyst was the main cause of the negative effect of CO2 on AEMFC performance. The CO-related adspecies were able to form from (bi)carbonate adspecies through catalytic reaction. Under the co-existence of NH3 and HCO3−, the (bi)carbonate adspecies were consumed through the chemical reaction with NH3, and in turn, the poisonous CO-related adspecies were completely removed from the electrocatalyst. The expected positive effect of this bicarbonate ion consumption reaction on the HOR was validated by the improvement in the onset potential of the HOR in 1 M KHCO3−1 M NH3. Finally, in order to substantiate the effect of co-existance of NH3 and HCO3−, fuel cell tests was conducted using a practical AEMFC. Under the supply of 85% H2−15% NH3 to the anode and ambient air to the cathode, the cell performance was significantly improved by a factor of ca. 1.6 compared to a cell without the addition of NH3. Furthermore, the given performance achieved nearly 95% of that obtained without the supply of CO2 in the system. These results clearly show the potential use of the chemically induced bicarbonate ion consumption reaction for the realization of AEMFCs with high CO2-tolerence. The approach proposed here is not only a fundamentally new one to improve the critical disadvantage of AEMFCs, it also sheds light on the advantage of combining AEMFCs with NH3 decomposition reactors. Since NH3 is one of the promising hydrogen carriers available today, our approach is a practical and efficient way to accelerate the realization of the AEMFC system.


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FIGURE CAPTIONS Figure 1 I-V curves of AEMFC. Anode gas: (100–x)% H2–x% NH3 (x = 0, 0.5, 1, 7, 15, 100) (RH 100 %), Cathode gas: 100 % O2 (RH 100 %), Operating temperature: 50 ºC. Scan rate: 10 mV s–1. Figure 2 Impedance spectra of AEMFC under the open circuit state. Operating temperature: 50ºC, Anode gas: (100–x)% H2–x% NH3 (x = 0, 0.5, 1, 7, 15) (RH 100 %), Cathode gas: 100 % O2 (RH 100 %). Figure 3 Terminal voltage of AEMFC at current density of 400 mA cm–2. Data were obtained from corresponding galvanostatic curves of AEMFC after 15 min of current holding. Anode gas: 100% H2 (RH 95 %), Cathode gas: 100 % O2, 20% O2–80% N2, and an ambient air (20.94% O2– 78.09% N2–0.93% Ar–0.03% CO2) (RH 95 %), Operating temperature: 50 ºC. Blue line corresponds to the oxygen content in cathode gas. Figure 4 Polarization curves for the hydrogen oxidation reaction in H2-saturated 1 M KOH, 1 M KOH–1 M NH3, and 1 M KHCO3 on Pt disk electrode. The rotation rate for all measurements was 800 r.p.m.; a sweep rate of 10 mV s–1 was used in all experiments. Figure 5 Time-resolved IR spectra of the Pt surface during the linear sweep voltammetry (LSV) in 1 M KHCO3. Brown lines at 1961-2001 cm-1 and 1731-1794 cm-1 correspond to the peak wavenumber for the C≡O stretching mode of CO-related adspecies, blue line at 1648 cm-1 correspond to the peak wavenumber for the H-O-H bending mode of bulk water, and orange line at 1491–1545 cm–1 correspond to the peak wavenumber for the O-C-O stretching mode of (bi)carbonate adspecies. Figure 6 Time-resolved IR spectra of the Pt surface at the open circuit state for 60 sec immediately after the cyclic voltammetry from 0.3 V to 0.9 in 1 M KHCO3. Brown lines at 19581978 cm-1 and 1737-1774 cm-1 correspond to the peak wavenumber for the C≡O stretching mode of CO-related adspecies, blue line at 1648 cm-1 correspond to the peak wavenumber for the H-O-H bending mode of bulk water, and orange line at ca. 1481 cm–1 correspond to the peak wavenumber for the O-C-O stretching mode of (bi)carbonate adspecies. Figure 7 Trend in onset potential for the hydrogen oxidation reaction (HOR) under various conditions. The onset potential was determined by analyzing the potential obtained at the point of oxidation current reached 10 µA cm–2 in HOR polarization curves. The rotation rate for all measurements was 800 r.p.m.; a sweep rate of 10 mV s–1 was used in all experiments. Figure 8 Time-resolved IR spectra of the Pt surface during the linear sweep voltammetry in 1 M KHCO3–1 M NH3. Brown lines at 1961-2001 cm-1 and 1731-1794 cm-1 correspond to the expected peak wavenumber for the C≡O stretching mode of CO-related adspecies, blue line at 1658 cm-1 correspond to the peak wavenumber for the H-O-H bending mode of bulk water, and red lines at ca. 1380 cm–1 and ca. 1480 cm–1 corresponds to the C-N stretching and O-C-O stretching of the H2NCO2 adspecies, respectively.


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Figure 9 Time-resolved IR spectra of the Pt surface at the open circuit state for 120 sec immediately after the cyclic voltammetry from 0.3 V to 0.9 in 1 M KHCO3–1 M NH3. Brown lines at 1958-1978 cm-1 and 1737-1774 cm-1 correspond to the expected peak wavenumber for the C≡O stretching mode of CO-related adspecies, blue line at 1652 cm-1 correspond to the peak wavenumber for the H-O-H bending mode of bulk water, and red lines at ca. 1360 cm–1 and ca. 1480 cm–1 corresponds to the C-N stretching and O-C-O stretching of the H2NCO2 adspecies, respectively. Figure 10 Time-resolved IR spectra of the Pt surface at OCV. NH3 was added to the 1 M KHCO3 solution soon after starting the measurement. Brown lines at 1998 cm-1 and 1794 cm-1 correspond to the peak wavenumber for the C≡O stretching mode of CO-related adspecies, orange line at 1510 cm-1 correspond to the peak wavenumber for the O-C-O stretching mode of (bi)carbonate adspecies, and red lines at ca. 1360 cm–1 and ca. 1430 cm–1 corresponds to the C-N stretching and O-C-O stretching of the H2NCO2 adspecies, respectively. Figure 11 Terminal voltage of AEMFC at current density of 400 mA cm–2. Data were collected from corresponding galvanostatic curves of AEMFC after 15 min of current holding. Anode gas: (100–x)% H2–x% NH3 (x = 0, 1, 3, 5, 10, 15, 20) (RH 95 %), Cathode gas: ambient air (20.94% O2–78.09% N2–0.93% Ar–0.03% CO2) (RH 95 %), Operating temperature: 50 ºC. Red line corresponds to the ammonia content in anode gas.

ASSOCIATED CONTENT Supporting Information Available: Current-voltage curves and summary of open circuit voltage of AEMFC with different anode fuels, liner sweep voltammograms during ATR-IR measurement, in situ ATR-IR spectra during CO bubbling in 1 M KOH, and estimated coverage of the CO-related adspecies observed in 1 M KHCO3. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author


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*Corresponding author. Tel.: +81-75-383-2519; fax: +81-75-383-2520 E-mail address: [email protected] (K. Eguchi).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by Council for Science, Technology and Innovation (CSTI), Crossministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST). Y. K. is supported by Grant-in-Aid for JSPS Research Fellow. We thank Tokuyama Corporation for the supply of anion exchange membrane (A201) and anion exchange ionomer (AS-4). The authors would like to acknowledge Reshma R. Rao for her insightful comments and suggestions.

REFERENCES

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53. Wang, X.; Conway, W.; Fernandes, D.; Lawrance, G.; Burns, R.; Puxty, G.; Maeder, M., Kinetics of the Reversible Reaction of CO2(aq) with Ammonia in Aqueous Solution. J. Phys. Chem. A 2011, 115 (24), 6405-12. 54. Khanna, R. K.; Moore, M. H., Carbamic Acid: Molecular Structure and IR Spectra. Spectrochim. Acta A 1999, 55, 961-967. 55. Jamroz, M. H.; Dobrowolski, J. C.; Borowiak, M. A., Theoretical IR Spectra of the 2:1 Ammonia–Carbon Dioxide System. Vibr. Spectroscopy 2000, 22, 157-161. 56. Park, H.; Jung, Y. M.; You, J. K.; Hong, W. H.; Kim, J., Analysis of the CO2 and NH3 Reaction in an Aqueous Solution by 2D IR COS: Formation of Bicarbonate and Carbamate. J. Phys. Chem. A 2008, 112, 6558-6562.


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1.0 100 % H2 99.5% H2-0.5% NH3 Terminal voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


99% H2-1% NH3

0.8

93% H2-7% NH3 85% H2-15% NH3 100% NH3

0.6

0.4

0.2 0.0

0.1

0.2

0.3

0.4

0.5

-2 GEO

Current density / A cm

Fig. 1 Y. Katayama et al.



-0.10

-0.05

Z'' /

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.00

100% H2 99.5% H2-0.5% NH3 99% H2-1% NH3 93% H2-7% NH3

0.05 0.00

85% H2-15% NH3

0.05

0.10

0.15

0.20

0.25

Z' /



0.6

100

0.5

Terminal voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

0.4 60 0.3 40 0.2

Oxygen content / %

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20

0.1 0.0

0




100 80

Current density / A cm-2 ECSA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 40 20 0 -20

1 M KOH 1 M KOH-1 M NH3

-40 -60

1 M KHCO3

-80 -100 -0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Potential / V vs. RHE



Page 29 of 36

H2Obulk E / V vs. RHE

(Bi)carbonate adspecies

Si

0.002 abs.

0.9 0.8 0.7

0.6

Absorbance

Potential scanning direction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5 0.4 0.3

0.2 0.05 1961–2001 cm–1 1731–1794 cm–1 2200

2000

1800

1600

Wavenumber / cm

-1

1400

1200




1737–1774 cm–1

1958–1978 cm–1

(Bi)carbonate H2Obulk adspecies Si

60 s

Absorbance

Elapsed time

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0s

0.002 abs. 2200

2000

1800

1600

Wavenumber / cm

-1

1400

1200



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0 5

Onset potential / mV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 15 20 25 30 35 40




H2Obulk

E / V vs. RHE

H2NCO2,ad H2NCO2,ad

0.001 abs.

0.9

Si

0.8

0.7 0.6

Absorbance

Potential scanning direction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.5 0.4 0.3

0.2 0.05

1961–2001 cm–1 1731–1794 cm–1 2200

2000

1800

1600

Wavenumber / cm

-1

1400

1200



Page 33 of 36

H2Obulk

H2NCO2,ad H2NCO2,ad

1737–1774 cm–1 1958–1978 cm–1 120 s

Si

Absorbance

Elapsed time

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0s

0.001 abs. 2200

2000

1800

1600

Wavenumber / cm

-1

1400

1200




(Bi)carbonate adspecies H2NCO2,ad H2NCO2,ad

1794 cm–1

1998 cm–1 Si

0s

Absorbance

NH3

KHCO3

10 s

KHCO3–NH3

300 s

Elapsed time

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2200

0.001 abs. 2000

1800

1600

Wavenumber / cm

-1

1400

1200



Page 35 of 36

0.42

25

0.40

0.36 15

0.34 0.32

10

0.30

Ammonia content / %

20

0.38

Terminal voltage / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.28 5

0.26

0.00

0




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26


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Anion-exchange Membrane Fuel Cells with Improved CO2

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