Quantifying and Elucidating the effect of CO2 on the Thermodynamics, Kinetics and Charge Transport of AEMFCs

Exposing operating AEMFCs to CO2 leads to performance-robbing overpotentials, linked to fundamental thermodynamics, transport and kinetics – the impact of which can be reduced through careful systems design and selection of operating conditions.


List of tables
.  Nafion, which is a typical PEM, has a structure of a copolymer that consists of a Teflon backbone, which gives the backbone a hydrophobic character, and sulfonic acid groups (HSO3 -) grafted into backbone, which give the head groups a hydrophilic character. These two tend to phase separate and the hydrophilic domains provide the ability for the membrane to be ionically conductive and uptake the water needed to hydrate the polymer.  It has been proposed that the use of an anion exchange membrane (AEM) in place of liquid KOHcreating so-called anion exchange membrane fuel cells (AEMFCs)can eliminate the effect of carbonation because there is no possibility for precipitation to occur.
The carbonate anions should be freely transported through the AEM when they are formed.
In AEMFCs, Figure 1.1b, hydrogen reacts with hydroxide anions in the electrolyte to create water and electrons (Equation 1.1). The electrons move through the external circuit to cathode where they react with oxygen and water to create the OH -(Equation 1.2). The latter is supplied through the electrolyte to the anode by migration.
Researchers have aimed to prepare AEMs with high anion (hydroxide) conductivity, long-term stability in alkaline media at fuel cell operating temperature, robust mechanical properties for overcoming in-use pressure differences, as well as proper water uptake and swelling, which facilitate ion transport within the electrodes and membrane. Early development of AEMFCs was hindered by AEMs with very poor alkaline stability 11 24 Kiss et al. 25 developed a mathematical model for ion transport in AEMs and calculated that the ion-membrane diffusion coefficient of OHwas up to 1.3 times higher than that of the CO3 2coefficient and 1.5 times higher than that of HCO3 -. With the transport of carbonate from cathode to anode, carbonate accumulates in the anode, resulting in a lower pH. The pH gradient reduces the cell voltage, approximated as 70 mV per pH unit over most of the pH range. 26 This thermodynamic effect from carbonation can result in a severe reduction in the operating cell voltage, with carbonate-related overpotentials as high as 400 mV 27 . Rheinhardt et al. 28 also proposed an electrochemical capture or release device of CO2 by controlling pH: an increase in pH at the cathode helps uptake CO2, trapped CO2 (as carbonate species) move towards the anode by electromigration, and regenerated CO2 will come out of the acidic anode.
Several experimental groups have also investigated the CO2 poisoning issue in AEMFCs. Suzuki et al. found that the flux of CO2 in the anode exhaust of AEMFCs increased with CO2 concentration in the cathode and with cell current density. 29 They found that the increasing of Ohmic resistance was noticeable by the supply of CO2 to cathode but not obvious when CO2 was fed to the anode. Kimura and Yamazaki 24 measured an increase in ex-situ conductivity of an AEM exposed to CO2. Shiau et al. 30 found by modeling that as the current increases, the flux of CO2 from the anode outlet increases, which means that less CO2 might be accumulated in the AEM; suggesting that carbonate buildup will minimize the purging effect. Fukuta et al. 31 believed almost all CO3 2was released at the higher current density by self-purging, and small pH change caused big difference in anode catalytic activity. However, the extent to which self-purging could be used to decarbonate active cells has not been confirmed experimentally and the carbonation dynamics are poorly understood. Watanabe et al. 32 found that the ion ratio (OH -, CO3 2and HCO3 -) depended on current density.
The literature work discussed above suggests that there are many parameters that will affect the carbonation of AEMFCs and influence their behavior during operation. polyethylene)-BTMA-based radiation-grafted AEM 38 . The LDPE-BTMA AEM is more chemically and mechanically stable at elevated temperatures than its ETFE-BTMA counterpart and was used when investigating the influence of elevated temperature on CO2related overpotential losses.
After soaking for 1 h, excess KOH was removed from the GDEs and AEMs before cell assembly. The GDEs and AEMs were pressed together in the cell to form the membrane electrode assembly (MEA) with no prior hot pressing. The MEAs were loaded into 5 cm 2 Scribner hardware between two single pass serpentine flow graphite plates. An 850e Scribner Fuel Cell Test Station was used to control the gas stream dew points, cell temperature, gas flowrates and the operating current density.
Before CO2 measurements were made, all cells underwent a break-in procedure. First, the cell was brought to its operating temperature under N2 flow on both sides of the cell at 100% relative humidity (RH). Then, the feed gases were switched to Ultra High Purity H2 and O2 (Airgas) at the anode and cathode, respectively. Then, the cell was operated chronoamperometrically stepwise from 0.7 V to 0.3 V (0.1 V steps, held for a minimum of 30 min at each step) as the reacting gas dew points were optimized per our standard procedure 35 . The optimized reacting gas dew points were very repeatable from cell-tocell, typically 52 o C at the anode and 54 o C at the cathode for an AEMFC operating at 60 o C.
Following the optimization of the reacting gas dew points, the cells were operated galvanostatically at the current density of interest (0.2, 0.5, 1.0 or 2.0 A cm -2 ) and allowed to equilibrate for at least 30 min before CO2 exposure was initiated. Multiple cells (no less than three) were constructed and tested for each measurement.

AEMFC carbon dioxide measurements
Following the break-in procedure and 30 min equilibration, the cell current was held constant and CO2 was parametrically added to the Ultra High Purity O2 cathode stream.
CO2 was added to O2 instead of air in order to simplify observations and isolate the effects of CO2 on performance, since air has additional O2 mass transport impact (e.g. N2 dilution) during cell operation, which is largely eliminated by utilizing O2 as the reacting gas. The flowrate for O2 and H2 in all experiments was 1 L min -1 . CO2 cathode concentrations as low as 2 ppm and as high as 3200 ppm were tested. Typically, after CO2 addition the cell was operated for 30 min, which was much longer than the time required to reach quasisteady-state operation (typically < 5 min, though lower CO2 concentrations took longer to reach steady-state). After 30 min operation at constant current, CO2 was removed from the gas stream and the cell was allowed to decarbonate for an initial 30 min. After this, the cell was further decarbonated through self-purging by one of two approaches: i) the cell was allowed to operate at the same current density until the voltage reached its pre-CO2 level and no CO2 emission was measured at the anode (shown in Figure 2.1); or ii) more typically, to reduce the time between CO2 trials, the cell potential was pulsed down to 0.1 V for 1 min (Figure 2.2), after which no CO2 emission was measured in the anode stream.
When CO2 was fed to the cathode, the concentration of CO2 being emitted from the anode and cathode were both constantly monitored in real time using a PP Systems WMA-5 nondispersive infrared CO2 gas analyzer (a water trap was placed in-line before the WMA-5 in order to preserve the unit and its calibration).
A second set of experiments were done where CO2 at concentrations between 2 and 400 ppm was added to the anode instead of the cathode. This was meant to simulate two possible scenarios: i) CO2 accumulation in the anode; and ii) CO2 exposure at the anode from the oxidation of carbonaceous fuels (through reforming or direct alcohol oxidation).
When CO2 was fed to the anode, the concentration of CO2 being emitted from the anode and cathode was constantly monitored in real time using the WMA-5. The cathode data is not shown since CO2 concentration was always below the detection limit during operation (though a very small amount of CO2 was observed in the cathode exhaust when the cell current was turned off due to diffusion across the AEM, which is shown).
The third set of experiments investigated the effects of temperature on CO2-related voltage losses. CO2 was fed separately to both the cathode and anode at 400 ppm. The cell setup and operation were identical to the previous description with one exception: the AEM

Results and discussions
In a typical analysis of fuel cell performance, it is often assumed that the cell voltage (Vcell) can be represented by Equation 3.1: where VOCV is the open-circuit voltage, i is the cell current, R is the Ohmic resistance to ion transport, Rct is the charge transfer resistance and Rmt is the mass transport resistance.
In PEMFCs, it is typically assumed that Rct is dominated by the oxygen reduction reaction (ORR), but this is likely a poor assumption in AEMFCs where the kinetics for the hydrogen oxidation reaction (HOR) are slower in alkaline vs. acid electrolyte and the HOR overpotential can be significant 39 . Therefore, discussion regarding charge transfer resistance should take into consideration both the ORR and HOR, which can be denoted as RctORR and RctHOR, respectively. PEMFCs also assume that Rmt is dominated by oxygen diffusion, which is likely to hold in AEMFCs as well (can be denoted as RmtORR), though this can often be neglected with high stoichiometry pure O2 flows). However, the presence of CO2 and carbonate anions complicates this type of analysis.
The electrochemical production of hydroxide anions in the presence of CO2 and their subsequent equilibrium reactions were summarized in Equations 1.2 -1.4. It should be noted here that OH -/CO3 2-/HCO3equilibrium constants exist such that OHand HCO3can never exist together in large quantities. However, CO3 2can exist in high concentrations with either OHor HCO3 -. During cell operation at practical current densities, a significant amount of OHis produced and CO2 is purged from the cell. Therefore, the two ions that dominate under operating conditions are OHand CO3 2-, which has been confirmed through theoretical modeling 33 . For this reason, the remainder of the discussion in this work will only consider the presence of "carbonate" as CO3 2-, although it is recognized that bicarbonate is often present in highly carbonated AEMs and AEMFCs before significant levels of electrochemical ORR have occurred at the cathode.
After their formation at the cathode, the CO3 2anions are transported through the AEM to the anode by migration, resulting in the "carbonation" of the AEM and the catalyst layer ionomers (Figure 3.1). This carbonation reduces the AEM conductivity since CO3 2has a lower intrinsic mobility than OH -40-42 , which increases the area-specific resistance (ASR) relative to OH --only operation (ASR). However, this effect should not be overstated as it is only able to account for a small fraction of the performance loss when CO2 is added to the cathode stream. Definitive experimental evidence will be presented below to support this. Less discussed, though thoughtfully pointed out and modeled by a few studies in the literature 33,43,44 , migration is not the only mass transport event that influences the location and distribution of CO3 2-; diffusion also plays a role. The interplay between migration and diffusion results in carbonate concentration profiles that impact performance in two primary ways beyond Ohmic considerations, one pH-based (Nernstian) and the other electrocatalytic. has shown evidence that at high anode overpotentials that CO2 is quickly removed from operating AEMFCssuggesting that carbonate may directly react with H2 at those overpotentials to produce water and CO2 thereby significantly accelerating decarbonization (also supported by data on slide 17 in Ref. 46 ). However, the long timescales needed to completely decarbonate AEMFCs at typical operating current and higher cell voltages (lower anode overpotentials), such as Figure 3.1, strongly suggests that such direct reaction does not appreciably occur at conditions of practical interest. Hence, it can be assumed in this work that essentially the entirety of the steady-state electrochemical current is generated through OH --based HOR and ORR reactions (Equations 1.1 and 1.2, respectively). Therefore, when CO3 2anions carry charge through the AEM, the balance of reacting OHthat is no longer supplied by migration (due to CO3 2carbonate conduction) must be compensated for by diffusion, which is an intrinsically slower process.
Therefore, CO3 2in the anode effectively shuts off catalyst sites with high local CO3 2concentration due to reduced access to OHionsincreasing the effective current density on OHaccessible anode catalysts. This means that although the presence of carbonate species does not negatively impact the intrinsic HOR electrocatalysis 47 , the high CO3 2concentration in the anode does cause an increase in the kinetic resistance, inducing polarization losses that lower the operating cell voltage (denoted as RctHOR).
These new resistances lead to a more complex equation for the operating cell voltage, though one that is insightful for the analysis of AEMFCs that have been carbonated: The assignment of all of the new kinetic overpotential to the anode is supported by experimental work by Matsui et al. 48 who found, using a three-electrode AEMFC configuration with a reversible hydrogen reference electrode, that the cathode overpotential was hardly changed by the presence of CO2, while the overpotential of the anode increased considerably.
The above-discussed behavior of carbonated AEMFCs is very similar to cationcontaminated PEMFCs 49-52 , though some critical differences do exist. Most important, in this case the "contaminant", CO3 2-, is continuously created at the cathode, moved to the anode, and removed from the anode gas stream. Similar processes do not exist for cationcontaminated PEMFCs with the exception of the NH3/NH4 + couple 53 . For CO2 containing AEMFCs, CO3 2can be removed during operation by introducing a CO2-free oxidant, activating a "self-purging" mechanism, which has been discussed in Chapter 1 27  In order to minimize the effect of CO2 and carbonation on operating AEMFCs, it is important for the field to better understand how CO2 uptake, membrane carbonation, and CO2 release occur. There are both transient and steady-state concerns with little experimental data to provide insight or support modeling validation. The results presented here quantify the uptake and release rates of CO2, quantify the amount of CO2 within the MEA under different steady-state conditions, and provide data as to the performance and high frequency resistance of AEMFCs under specific CO2 conditions. This first of its kind data provides significant insight into the performance losses and ultimate potential of AEMFCs when exposed to CO2. This work provides direct evidence regarding the extent to which the CO2 fed to the cathode becomes integrated into the AEMFC, directly correlates carbonation with AEMFC performance, and provides critical data needed to validate modeling efforts that try to quantify rates of CO2 uptake and release, as well as the negative effects of CO2 on performance.  54 , it was clear during the experiment that the AEM and AEI were being converted to the carbonate form since the amount of CO2 leaving the cathode was far below the 400 ppm feed, Figure 3.2b, especially over the first 300 s.
After the CO2 was added to the cathode at OCV, the concentration initially rose from zero to ca. 130 ppm as two things were occurring: absorption of CO2 into the AEM and ionomer and the increase in the CO2 partial pressure in the gas stream (the humidifier and cell lag in the CO2 concentration is denoted as "blank" in Figure 3.2b -determined in a cell containing a Teflon membrane, which does not uptake CO2 and form CO3 2anions).
Comparing the "blank" and 0.0 A cm -2 (black dotted line) plots in Figure 3.2b, it was clear that there was rapid CO2 uptake into the AEM because the concentration of CO2 leaving the AEM-containing cell was always lower than with the "blank". By 600 s, the concentration of CO2 in the cathode rose to the inlet concentration, suggesting that the AEM was extensively carbonated after 10 min, which is in good agreement with previous studies on AEM carbonation in the presence of gas-phase CO2 27,55,56 .
When CO2 was added to the cathode of a fully broken-in cell operating at a constant current density, the cell response was very different. In all cases (from 0.2 A cm -2 to 2.0 A cm -2 ), after a brief time lag, the cell operating voltage precipitously declined, the ASR increased, and CO2 was emitted at the anode; this is shown in Figures 3.2a and b. What changed with current density were the magnitude and timing of these phenomena. At the highest current density that was tested, 2.0 A cm -2 , it took approximately 31 s for CO2 to be measured in the anode stream (from the time that the reacting gas CO2 concentration increased). It took another 96 s after CO2 was initially measured in the anode gas before a quasi-steady-state was achieved. When the current was halved to 1.0 A cm -2 , the time for Hence, the quantity of CO2 that has been taken up into the cell by the AEM and AEI is the integrated area between these two plots.        cannot exist with significant quantities of OHand CO3 2due to the equilibrium constraints.
If it is assumed that the ionomeric materials are completely carbonated, then the average charge per CO2, , can be found by: Here,  = 1.79, meaning that 79% of the charge groups are in the CO3 2form and 21% are in the HCO3form at open circuit.
From the transient flux data, the amount of carbonate in the system at steady-state, as well as the degree of carbonation, could be calculated (Table 3. that is entering the cell when no uptake occurs ("blank"). It also provides the CO2 flux that is leaving from both the anode and cathode with time until the cell reaches steady state.
From this data, the three curves ("blank" vs. time, anode exhaust vs. time, and cathode exhaust vs. time) can be integrated and the total number of moles of CO2 (NCO2), and hence CO3 2-, can be calculated by: 2 = ∫ "blank"(t) dt − ∫ "anode exhaust"(t) dt − ∫ "cathode exhaust"(t) dt From here, the DOC was also calculated from the equation above.
As expected, there was a greater amount of CO3 2present in the system with higher concentrations of CO2 in the cathode stream. It was also found that the total amount of Another interesting point in the dataset where it would be informative to know how much carbonate was in the system is after the CO2 was removed from the cathode and the cell has reached the new quasi steady state. Figure 3.11a showed a set of typical carbonation + decarbonation experiments, where the cell was exposed to a known amount of CO2 for 30 min and then the CO2 was removed from the cathode gas stream. Cell decarbonation happened in 2 stages. The first stage was when the cell was operated for 30 min at the same current density. The second stage occurred after this 30 min of operation, when the cell voltage was pulsed down to 0.1 V. Here, the CO2 coming out of the anode came out in a large slug that quickly decayed over 1-2 minutes. After that, the cell voltage was allowed to come back to steady state at the initial operating current and "complete" decarbonation was assumed if the steady state voltage was equal to the pre-CO2 exposed operating voltage. However, one interesting observation was that a plot of the total carbonate in the system vs. the change in the ASR (Figure 3.9) did not yield a single straight line for all conditions, but there were trends as a function of current density and CO2 concentration. To understand this, it should be noted that the HFR measurement by the fuel cell test station is only measuring the two closest points separated by the ionomer; in other words, it is essentially a measurement of the membrane resistance. Therefore, the fact that the ASR is lower at a higher current density, even under conditions where the total amount of CO3 2in the cell is nearly identical to a lower current density, suggests that there is less carbonate in the AEM and more carbonate in the anode electrode as the current density is increased.
In summary, there were seven interesting observations when CO2 was fed to the AEMFC cathode: 1) the CO2 concentration leaving the cathode was only very modestly affected by the current density ( the steady-state ASR increased with decreasing current density (Figure 3.2a); 6) increasing current density decreased the amount of CO3 2present in the system at steady-state (Table   3.1); and 7) even at the highest current density and lowest CO2 concentration (2.0 A cm -2 and 100 ppm, respectively) the CO2-related overpotential was significant (167 mV), and the CO2-related overpotential at 2.0 A cm -2 and 400 ppm CO2 was even higher (259 mV).
Combined, these observations suggest that: i) CO3 2formation at the cathode is very rapid (likely in quasi-equilibrium, which will be discussed more later); ii) initially CO3 2-accumulates in the membrane and release is slow until a critical concentration is reached; and iii) higher current densities increase the amount of CO3 2in the anode electrode.
To further study the dynamics of CO2 uptake and CO3 2formation in the AEMFC system, as well as to simulate CO2 that would build up in the anode or could be formed as an oxidative product of an alcohol fuel, CO2 was also directly fed to the anode. For comparison sake, the CO2 concentration in the anode H2 reacting gas was also 400 ppm.
The results of these experiments are shown in Figures 3.2d and e. The first thing that should be noted is that while current was flowing, no measurable CO2 was ever found leaving the cathode, which can be attributed to the high net anionic flux relative to typical diffusion rates. Simply, CO3 2cannot diffuse and accumulate to a critical concentration at the cathode faster than migration pushes it to the anode under the conditions tested. Therefore, Figure 3.2d only shows the CO2 concentration of the anode effluent and Figure   3.2e only shows the anode CO2 flux. Like the cathode, there was approximately a 45 s lag between the time that CO2 was turned on and its measurement (Figure 3.3). In this set of experiments, the dynamic CO2 concentration in the effluent (before steady-state) increased with increasing current density, suggesting lower CO2 uptake and CO3 2formation at higher currents. Also, the overall voltage decrease and ASR increase were both lower (but only slightly so) when CO2 was fed to the anode vs. the cathode, most likely because of reduced carbonation stemming from the direction of ion transport.

Deconvolution of carbonate-related losses in operating AEMFCs
Though As discussed earlier, the introduction of CO2 to operating AEMFCs initiates an interesting series of dynamic events that, in concert, lead to reduced steady-state performance through three mechanisms: increasing the Ohmic resistance (ASR), increasing the anode charge transfer resistance (RctHOR) and increasing the thermodynamic anode potential (VNernst). The challenge here is to find a systematic way to use the CO2 exposure and removal data in Figure 3.10a, 3.11 to quantify the contribution of each of these resistances to the total CO2-related overpotential. The general approach to extracting these three losses from the data was consistent regardless of the experiment. A representative description for 400 ppm CO2 at 1.0 A cm -2 is given here for illustrative purposes, and then the summary of all the calculated parameters is shown in Figures 3.10b d.    suggest that the "self-purging" mechanism has a relatively modest effect in decarbonating the cell, and reducing CO2-related voltage losses to an acceptable level during operation on ambient air will be a significant challenge, and may not be possible at all.
The first CO2-related loss that was calculated was ASR. The ASR as a function of time is shown in Figure 3.2a, and under this operating condition, ASR was 25 m cm 2 .
Assuming this ASR resulted in proportional Ohmic losses, at 1 A/cm 2 this would result in an Ohmic loss of 25 mV. For completeness, it is acknowledged that the measured ASR values do not yield the exact potential drop related to ion movement through the AEM due to the influence of diffusion 49,50 . However, the value measured here does give an accurate measure of average anion mobility and is presented here as an overestimation of the maximum Ohmic resistance that could be attributed to carbonation which remains a small percentage of total overpotential loss (<10%). Perhaps what is most important is that this observation clearly shows that the ASR change caused by the emergence and transport of CO3 2through the AEM represents a very small portion of the overall CO2 overpotential.
For the AEMFC operating at 1.0 A cm -2 with 400 ppm CO2 in the cathode, at minimum, 255 mV of the CO2-related loss remains to be accounted for. The next stage of the deconvolution comes when CO2 is removed from the cathode stream. Experimentally, a rapid increase in the cell potential was observed, to ca. 0.54 V, though the potential never exactly levels off to reach a new steady-state. That is because the only way that a true steady-state can be re-achieved is for all of the CO3 2to be removed, either by waiting for many hours ( Conducting the same analysis over the entire range of current densities and CO2 concentrations can yield values for the total CO2-related overpotential, ASR, RctHOR, and VNernst as well as the Ohmic voltage loss (VOhmic) and the CO2-related kinetic polarization (VctHOR) at every condition. All of these values are given in Table 3.2.
Performing the data deconvolution over such a wide range of current densities and cathode CO2 concentrations yielded some very revealing trends and important insight into the behavior of carbonated AEMFCs. Not too surprisingly, the total CO2-related overpotential was increased with decreasing current density and increasing CO2 concentration in the cathode (Table 3.2). However, understanding why this happened requires digging into the trends in ASR, RctHOR, and VNernst more extensively. Figure 3.10b presents the ASR values at all conditions. As the concentration of CO2 in the cathode reacting gas was decreased, there less of a negative impact on the ASR. This makes sense from the transient and steady-state experimental results (Table 3.1) which showed that the total amount of CO3 2in the AEMFC was lower at lower CO2 concentration in the cathode and increased current density. As discussed earlier, the overall trends in the ASR with current density and cathode CO2 concentration (Figure 3.11) led to the conclusion that increasing the current density shifts the CO3 2concentration gradient toward the anode electrode. Hence, with increasing current density, relatively less and less CO3 2is present in the AEM (though the total CO3 2flux is higher, Figure 3.2c), resulting in a lower ASR.
The fact that the concentration gradient shifts toward the anode with current density might lead to the assumption that VNernst (Figure 3.10c) should also increase with current density. However, there are two counter points that require discussion. First, the total quantity of carbonate in the cell is decreasing with increasing current density, which alone might limit the achievable value for VNernst, particularly at high currents. Second, the anode potential is measured at the outermost portion of the anode at the gas diffusion layer, which is likely the point of the highest CO3 2concentration, as illustrated in Figure 3.1, and it is possible for that one specific location to be close to saturation over a wide range of conditions. VNernst appeared to decrease with increasing current density, though the values at current densities ≤ 1.0 A cm -2 were very similar. The assertion that the outermost portion of the anode can be close to saturation was supported by the magnitude of VNernst at the lower current densities, ~165 mV. The effective alkalinity of AEMFC cathode is between pH 13 -14. It is also known that CO3 2is overwhelmingly the dominant carbon-based charge carrier and this can only happen in water at pH values > 11. Therefore, the maximum pH shift that could possibly be expected at the anode in an operating cell would be 3, resulting in a VNernst,max of 177 mV. The only data point in Figure 3.10c where VNernst is markedly lower is at very high current, 2.0 A cm -2 , where VNernst is ~125 mV.
This lower value can be explained by either the lower overall carbonate concentration in the cell and anode at higher currents, and/or the development of a mixed potential throughout the anode because at high current density there is a significant number of OHions being released throughout the anode as CO2 is evolved through the reverse of Equations 1.3-1.4, though the root cause for this behavior will likely need to be teased out through computational modeling. It is also noteworthy that the VNernst was completely unaffected by the cathode CO2 concentration, which gives additional support to the arguments above.
One area where the higher carbonate concentration in the anode did have a major impact on the AEMFC behavior is in RctHOR, Figure 3.10d. At higher overall carbonate content, increased cathode CO2 concentration and/or lower current density, RctHOR was also higher, sharply increasing over the entire scale of tested concentrations. At a constant CO3 2concentration (same ppm CO2 in the cathode stream), RctHOR actually decreased with increasing current, even though the total amount of carbonate in the anode electrode was higher at higher current. This observation yields important insight into the location of carbonate in electrodes, suggesting that higher current densities compress the volume occupied by carbonates to the outermost portion of the anode, which effectively allows more catalyst sites to have easy access to reacting OH -.

AEMFC response at low CO2 concentrations
A practical interpretation of the experiments shown in Figure 3.10 is that the polarization losses from AEMFC carbonation are significant at all current densities and near-ambient CO2 concentrations, and that AEMFCs will likely require pre-scrubbing of CO2 from the operating air. Additionally, the dynamics of CO2 uptake (fast) and release (slow) mean that even if CO2 could be quickly removed from the anode stream to avoid significant accumulation, losses would still be high. One sensible approach to reducing CO2-related overpotential is to lower the cathode inlet concentration, which is particularly intriguing for stationary implementations of AEMFCs where the volume and weight of a CO2 scrubber is less of a concern than it is for mobile or transportation applications. Figure   3.12a explores the response of an AEMFC operating at 1 A cm -2 with 5 -50 ppm CO2 in the cathode reacting gas. Though the voltage loss was less than at higher concentrations, even down to 5 -10 ppm CO2 in the cathode the CO2-related polarization was significant, approximately 140 mV. 3.4 Influence of temperature on CO2-related polarization losses at 400 ppm Figure 3.12 showed that simply removing a portion of the CO2 in ambient air will not be sufficient to eliminate the CO2-related losses in operating AEMFCs. In fact, the above work demonstrated that even at 5 ppm CO2 significant performance losses occurred.
Therefore, it is important for researchers to identify other fundamental and operational properties of the system that can be manipulated to reduce the AEMFC sensitivity to CO2.
One pathway to reducing the amount of carbonate accumulated in the system is to increase the cell operating temperature. Increasing temperature would have several positive impacts on carbonate: i) CO2 has lower solubility in water as the temperature is increased; 57 ii) the kinetics for CO2 release (reverse of Equations 1.3 and 1.4) at the anode will improve; iii) the mass transport rate of evolved gaseous CO2 from the anode will increase; and iv) the intrinsic kinetics for the ORR and HOR will improve. following the introduction of 400 ppm CO2 to the cathode and anode. Regardless of where the CO2 was introduced, increasing the temperature simultaneously decreased the total CO2 overpotential and the ASR (Figures 3.13ac). This experimental result is in stark contrast to recent modeling results that suggested increasing the cell temperature would not have a beneficial effect on AEMFC operation 33 . One possible explanation for the increased performance is that less CO2 was apparently taken up into the system. Figure 3.13b shows that the concentration of CO2 being emitted from the anode side of the cell decreased with increasing temperature. At steady-state, this means that less CO2 was absorbed at the cathode. Figure 3.13d shows that when CO2 was fed to the anode, increasing the temperature resulted in lower CO2 uptake at that electrode as well, which is shown by the increasing concentration of CO2 in the anode effluent. It should also be noted in Figures   3.13b and d that the values trend upward with increasing current density due to the consumption of the fuel and oxidant gases. Positively, the improved performance at elevated temperatures suggests that increasing temperature is indeed one possible mechanism to improve the CO2 tolerance of operating AEMFCs; however, the CO2-related overpotential is still too high for many practical applications. A combination of lower CO2 concentration, more modest air stoichiometry, and elevated temperature can further reduce the total CO2 overpotential. For instance, it was observed that an AEMFC operating at 1 A cm -2 and 80 o C with 10 ppm CO2 fed to the cathode (the same LDPE-BTMA membrane) had a total CO2 overpotential of only 90 mV.
Deconvoluted data for AEMFCs operating at different temperatures but at a constant current of 1 A cm -2 and constant cathode CO2 concentration of 400 ppm to find ASR, VNernst and RctHOR can be found in Table 3.3. As expected, the ASR generally decreased with increasing temperature due to the lower quantity of carbonates that were taken up into the membrane. However, the ASR value only varied slightly with increasing temperature, which meant that a similar portion of CO3 2anions were carrying the charge through the AEM, supported by the results of accelerated decarbonation experiments at 0.1 V as Table   3.4, which led RctHOR to be fairly constant with temperature as well. Therefore, the primary impact of an overall reduced number of CO3 2anions in the AEM was that the carbonate accumulation in the anode (and hence the concentration gradient across the cell) was less severe with increased temperature. As a result, VNernst was the most dependent on temperature, decreasing by nearly 50% from 60 -80 °C. Impact of temperature on the total CO2-related overpotential, HFR and anode CO2 exhaust with 400 ppm CO2 fed to the cathode at multiple current densities. Total CO2 overpotential (solid lines) and ASR (dashed lines) when CO2 was fed to the a) cathode and c) anode. CO2 concentration in the anode effluent when CO2 was fed to the b) cathode and d) anode. An LDPE-BTMA AEM (IEC = 2.5 mmol g -1 ) was used in these experiments. In summary, with regards to temperature, it is possible that even higher temperatures (> 90 °C) may help, though no AEMs are currently readily available with stability above 80 °C in highly alkaline media that also have acceptable conductivity and water transport properties, though there is promising work ongoing in this area 58 . What this really points to is that improving the CO2 tolerance of AEMFCs will require a combination of approaches to achieve success, at least some of which are not known today and will be particularly challenging for dynamic operation.

Influence of flowrate on AEMFC performance with 400 ppm cathode CO2
Research about the effect of reacting gases flowrate is rare. Gerhardt et al. 59 modeled the carbonation behavior along the flow channel as changing flow type and flow rate, and found that an optimum flow rate existed to balance oxygen transport loss with CO2 related performance loss.
The effect of oxygen gas flowrate on the behavior of a carbonated AEMFC is shown in Figure 3.14. As shown in Figure 3.14a, the CO2 overpotential increased (approximately linearly) with the increasing O2 flow rate, showing that the total dosing of the cell by CO2 plays an important role in carbonation. Hence, the carbonate concentration inside of operating cells is clearly increased with increased oxidant flowrate. release is the accumulation of carbonate in the cell, which eventually leads to the thermodynamic shift from CO3 2to HCO3and then CO2 as the concentration in the anode increases. Also interesting, Figure 3.14 b shows that AEM-like devices can also act as CO2-separators that also simultaneously generate, not consume, power. Figure 3.14d shows the deconvolution of the CO2-related polarization losses with changing cathode flowrates.
It was found that lowering the oxygen flowrate did not appreciably impact the Nernst loss.
What this shows is that the outermost part of the anode remains nearly saturated at all flowrate. The most severe increase is in the charge transfer resistance. The kinetic resistance increases with increasing flowrate at the same Nernst loss showing that the overall carbonate content of the anode is higher in this case (Table 3.5); hence at higher flowrate, more of the anode is "shut off" by carbonationleading to higher voltage losses.
Interestingly, when both flowrates are lowered equally, the dosing and removal track very well shown as Figure 3.15. Lower flowrates have higher overpotentials, even when the amount of carbonate in the cell is similar at steady state, suggesting the carbonation is slower than the removal rate at higher flowrtae. Also seen in Table 3.6, the rate of carbonate "decomposition" to CO2 (the "removal amount") is a function of anode flowrate (removing gas flowrate). As flowrates increase, fuel cell system takes in more CO2 (hence the RctHOR goes up) but there is less carbonate in the anode due to an increase in the rate of CO2 removal. The Nernstian voltage loss is increasing as decreasing of both flowrates.
The effect of hydrogen gas flowrate on the behavior of a carbonated AEMFC is shown in Figure 3. 16. In general, the anode flowrate did not have a severe an impact on carbonation as the cathode flowrate. Figure 3.16a shows that the CO2 overpotential increased with the decreasing H2 flowrate. This was not due to a significant increase in the amount of carbonate in the membrane, as evidenced by the similar HFR for all cases and the total cell carbonation being similar (Table 3.7). This suggests that the main reason for increased polarization is increased carbonate concentration in the anode, particularly right at the anode/GDL interface, which is evidenced by larger Nerstian losses at lower flowrates.
An additional observation was that as the anode flowrate was decreased, the anode exhaust concentration increased. In fact, it was possible for the concentration of CO2 in the exhaust to be significantly higher than the cathode feed, showing that these devices can also be CO2 concentrators. Next, the effect of water on carbonation was studied by increasing the dew points of the anode and cathode reacting gases. The results are shown in Figure 3.17. As the dew points for the reacting gases fed to the anode and cathode were increased, the hydration level of the AEMFC also increases. This led to a reduced effect of CO2 poisoning on cell performance, meaning that the magnitude of the CO2 overpotential and HFR decreased as the dew points were increased, Figure 3.17a. Interestingly, as the dew points were increased, the concentration of CO2 in the anode exhaust decreased while the concentration leaving the cathode increased, Figure 3.17b. This suggests that increasing the amount of free water in the cell prevents CO2 uptake in the cathode. As the amount of free water increases, the degree of carbonation of the polymer decreases, though the total amount of carbonate in the cells is approximately the same. This suggests that some of the CO2/carbonate is actually present in the liquid water phase and not in the polymer. However, there is a        Figure 3.17. Therefore, it is important to find ways to operate AEMFCs at high hydration levels and avoid electrode flooding.

Comparison between model prediction and experimental data
In the literature, there have been a few theoretical models proposed that aim to capture the dynamics of AEMFC carbonation. For instance, Shiau et al. 30 rightfully predicted that as the current increases, the flux of CO2 from the anode outlet increases, which means that less CO2 might be accumulated in the AEM; suggesting that carbonate buildup will minimize the purging effect. Setzler et al. 60 simulated that a dynamic pH gradient profile through the MEA would occur when 400 ppm CO2 fed to cathode. It showed a steep pH gradient from 9.5-13 crossing AEMFC, and the gradient flattens when low current density was applied. It was only when the anode reaches the lowest pH's that CO2 is evolved into the anode, explaining the lag in time from the feed of CO2 to its detection in Figures 3.2-3.7. Also, increasing cathode or reducing anode flowrate was predicted to increase the level of cell carbonation, in agreement with this thesis. One prediction that has not been yet validated experimentally is AEMFC behavior at very low concentration; Setzler found that even 0.1 PPM CO2 results in 10 mV performance loss at steady state, 3 ppm CO2 caused ca. 50mV loss which is close to our experimental results. Finally, Gerhardt et al. 59 suggested two main reasons for voltage lossthe thermodynamic and anode kinetic overpotentials which were verified by our experimental data. Some models are not consistent with our experiment data. For instance, Krewer et al. 33 modeled that increasing cell temperature does not have a beneficial effect on the carbonation process which we had the opposite conclusion. However, one limitation of nearly all computation models regarding CO2 in the literature is that they have not been experimentally validated. It is suggested that the modeling and experimental groups combine forces, which can yield new insights into this issue that is critically important for AEMFCs.

Chapter 4
Conclusions Even in highly performing AEMFCs, the addition of CO2 has a severe negative impact, where the cell operating voltage is generally decreased by 200 -500 mV depending on the reaction conditions. Lower CO2 concentration in the reacting gas, higher current density and higher operating temperature all reduce the voltage penalty, but none have been shown be able to sufficiently minimize the CO2 impact. This experimental work, the first of its kind to systematically investigate carbonation and to deconvolute the root causes for performance decline, has provided new insight into the dynamics of CO2 and CO3 2in operating AEMFCs.
The formation of carbonates in the AEMFC occurs very quickly and in quasiequilibrium with the reacting gas in the cathode. Decarbonation of the cell does not occur through direct electrochemical reaction under typical operating conditionsand is hence very slow; however, it is likely that carbonates do directly react with H2 in the anode at very low voltages/very high anode potentials, which can allow for rapid cell decarbonation by pulsing away from typical operating conditions (e.g. 0.1 V or short-circuiting the cell).
by diffusion toward the anode reacting gas. Therefore, decarbonation during operation by the so-called "self-purging" mechanism is slow, taking several hours even after only transient exposure to CO2. Hence, "self-purging" cannot be relied upon to decarbonate a real system efficiently. Also, although pulsing to low operating voltages can be used for decarbonation, it most likely cannot be practically applied to engineered fuel cell stacks where some individual cells would experience negative voltages. The dominating loss in operating AEMFCs in the presence of CO2 is not due to an increase in the Ohmic resistance from electrolyte carbonation. The dominating mechanism for voltage loss is accumulation of carbonate anions in the anode, which results in two performance-robbing mechanisms: 1) a Nernstian thermodynamic shift in the anode potential from a decrease in the anode pH with carbonates; and 2) an increase in charge transfer resistance due to a lack of availability of reacting OHanions. The CO2 concentration in the cathode and the current density are both determining factors for the quantity of CO3 2in the system, and the current density appears to play a primary role in dictating the CO3 2location and distribution. The HOR charge transfer resistance increases markedly with both increased CO2 concentration and lower current density. Increasing the cell operating temperature appears to have almost no effect on the charge transfer resistance, but a significant effect on the Nernstian loss, meaning that the total CO2-related overpotential can be reduced by increasing the temperatureor better yet, through a combination of higher current density, lower CO2 concentration and higher operating temperature.
With constant 400 ppm CO2 fed to cathode stream, CO2 overpotential linearly increases with increasing O2 flowrate, while decreasing with H2 flow rate. The carbonation degree of the fuel cell is clearly increased with increased oxidant flowrate. It provides evidence that AEM-like devices can also act as CO2 separators that simultaneously generate power.
CO2 concentration in the anode exhaust increases with decreasing H2 flowrate, though the