Transient Distribution of Water in an Anion Exchange Membrane Fuel Cell Monitored by Operando Coherent Anti-Stokes Raman Scattering Spectroscopy

Abstract

The hydration condition of an anion exchange membrane (AEM) in an operating fuel cell significantly affects its performance as well as its lifespan. In this paper, an in-house build coherent anti-Stokes Raman scattering (CARS) vibrational spectroscopy is used to establish the hydration of an AEM in an anion exchange membrane fuel cell (AEMFC) while it generates power. During steady-state operation, water on the anode side increased with current density. On the cathode side and at the center of the membrane, water initially decreased with current density and then started increasing at a slower pace than on the anode side. A deconvolution of the OH peak in the recorded CARS spectra into nine species revealed that only the H-bonded water species underwent variation. The rest of the species experienced a negligible change. A transient study revealed that maximum disturbance to the water distribution was achieved after 5 s of applying a current jump. The distribution of water became stable within 20 s after applying the current jump. The response to the current jump on the anode side was opposite to that on the cathode. These results open the way for a widespread dynamic study of water distribution in different AEMFCs. The technique could also be directly used to evaluate the dynamic degradation of AEMs.

1. Introduction

Polymer electrolyte membrane fuel cells (PEFCs) are among the most preferred clean energy conversion devices for portable applications such as automobiles. In addition to zero emissions, they enable rapid refueling, silent operation, low operation temperatures (60–120 °C),^1,2 among other advantages. Low operating temperature is essential as it reduces the vehicle’s startup time. The two main PEFCs suitable for automobile application are the proton exchange membrane fuel cell (PEMFC) and the anion exchange membrane fuel cells (AEMFC). PEMFCs are more developed and are already at the commercial stage in automobiles.^3–7 AEMFCs are considered the fuel cell of the future because, unlike PEMFCs, they do not depend on Pt-based catalysts, whose global supply is rapidly depleting and generally unreliable. Nevertheless, AEMFCs still lag behind the PEMFCs due to their inadequate stability.^8–10

Factors affecting AEMFC stability can be roughly divided into structural and operational.^9,11 Structural factors include the type of catalyst and membrane, the composition of the gas diffusion layer (GDL), the membrane-GDL interface, and the design of flow channels.⁹ Operation factors include cell temperature, relative humidity (RH), flow rates, type of flow, purity of the gases, current density, and, most notably, water management.^9,10,12 Deng et al. studied the effect of different operational parameters on AEMFC performance.¹³ They observed that insufficient water in the cathode was the main contributor to losses in performance, which resulted in low limiting current density.¹³ Reshetenko et al. found out that the RH at the cathode in their AEMFC decreased faster than in typical PEMFCs, reducing anion conductivity and, thus, performance.¹⁴ Machado et al. observed that in an AEMFC, the reduction of the RH of inlet gas from 90 % to 80 % had more adverse effects on the cell output than the reduction from 100 % to 90 %.¹² Otsuji et al. evaluated the performance hysteresis phenomena of AEMFCs using a Fe–N–C cathode catalyst with QPAF-4 ionomer.¹⁵ From the analysis of polarization curves and Tafel slopes, they concluded that the differences in hysteresis resulted from differences in water distribution on the cathode.¹⁵ They later used thin hydrophilized membranes and a hydrophobic Fe–N–C cathode catalyst to mitigate this problem.¹⁶ Cho et al. observed, in the high current region, a better performance for GDLs with a large penetration thickness of microporous layer (MPL) because of the balanced gradient of capillary pressure, which enhanced water distribution.¹⁷ Similar observations on water have been made elsewhere.^18,19 Wrubel et al. studied the self-purging in AEMs to improve the longevity of the cells, a concept whereby increased current density aids in releasing CO₂ in the AEMFC.²⁰ They observed that self-purging occurred through an electrochemical mechanism involving bicarbonate and carbonate ions, which require water.²⁰ One of the significant factors determining the performance and lifespan of any PEFC is, therefore, the water transport dynamics within the membrane.

In real-life applications, the operation of a fuel cell demands subjection to both steady-state loads and transient loads.^19,21–23 In the case of automobile applications, steady loading is expected when the vehicle is cruising or running auxiliary features such as onboard computers while going downhill or stationary. Transient loading, on the other hand, is expected whenever the vehicle is accelerating from a stop, overtaking other cars, changing from level roads to a steep uphill road, and also at the instance when auxiliary systems such as air conditioners/heaters are switched ON. Since the performance requirement for the fuel cell is considerably different during these two situations,^19,24 it is essential to study both of them. In the event of a power deficit during the transient response in a fuel cell car, a battery bank is used to supply the power shortage to maintain satisfactory performance.²⁵ However, it is desirable to understand the cell dynamics during this transient period to prevent possible cell damage and to speed up the re-establishment of predictable steady-state performance.

The transient response is a complex function of the ion conductivity of the membrane, membrane thickness, porosity of the gas diffusion layer, temperature, and water transport.^13,18,26 This is commonly evaluated by applying a current or voltage jump.^18,27 In this paper, a current jump will be used because the constant current mode is the most widely reported mode for polarization curve measurement in the AEMFC publications.^{15,16,28–34} A current jump can result in a voltage overshoot or undershoot, depending on the concentration of the reactants.¹⁹ Verma and Pitchumani studied the effect of operation parameters on the transient response of a PEMFC using both experimental and numerical techniques.¹⁸ They concluded that voltage undershoot during the current jump results from a jump in the water transported across the membrane by electro-osmotic drag.¹⁸ Huang et al. studied the dynamic response of a proton exchange fuel cell to a step current²¹ and noted that dehydration of the membrane could result in significant discrepancies in the dynamic performance of the cell. Insufficient hydration resulted in larger voltage undershoots.²¹ Structurally, it has been observed that higher Pt loading results in better transient response.^21,35 This is, however, beyond the scope of this paper.

The actual duration of transient behavior is mainly dictated by the time required to effect change in the cell hydration and the time for stabilization of reactant concentration at the reaction sites.¹⁹ The transport of water as one of the reactants is determined by the back-diffusion, diffusion of water from/into the fuel stream, flow channel orientation/design, gas flow rate, hydrophilicity, cell vibrations, and back pressure.^36,37 For instance, the hydrophilic nature of the membrane helps seal water inside the membrane, slowing down dehydration.³⁸ Wang et al. studied the dynamic response of a PEMFC under mechanical vibration on an automobile vibrating platform.³⁹ They observed that the 20-Hz mechanical vibrations increased the time required to establish steady-state performance from 20 to 50 s.³⁹ Further investigation using a transparent cell revealed that the difference in transient response resulted from the coalescence of water droplets in the flow channels into bigger droplets, thus hindering fuel access to the active sites.³⁹ Therefore, water distribution is expected to play a significant role in both the steady state as well as transient response of fuel cells.

To understand the differences in water distribution on the anode and cathode side of an AEMFC under steady and transient response, it is essential to look at the corresponding electrochemical reactions. This can be summarized into Eqs. 1, 2, and 3.

At the cathode

  

\begin{equation} \frac{1}{2}\text{O$_{2}$} + \text{H$_{2}$O} + \text{2e$^{-}$} \to \text{2OH$^{-}$} \end{equation} (1)

At the anode

  

\begin{equation} \text{H$_{2}$} + \text{2OH$^{-}$} \to \text{2H$_{2}$O} + \text{2e$^{-}$} \end{equation} (2)

Net cell reaction

  

\begin{equation} \text{H$_{2}$} + \frac{1}{2}\text{O$_{2}$} \to \text{H$_{2}$O} \end{equation} (3)

In typical AEMFCs, humidified O₂/air is supplied to the cathode and humidified H₂ to the anode. At the cathode, O₂ is reduced to OH⁻ ions in the presence of water and a suitable catalyst. The resultant OH⁻ ions move to the anode across the membrane, bringing along additional water molecules by electro-osmotic drag.^10,40 At the anode side, the OH⁻ coming from the cathode reacts with H₂ fuel to form H₂O and releases electrons to the external circuit (Eq. 2), where electric current then flows. Unlike in a PEMFC, the humidification present in the cathode gas of an AEMFC is used both as a reactant and a hydrant for the electrolyte membrane. Although water is generated at the anode, as seen in Eq. 2, the anode gas still needs to be humidified to replenish water lost by evaporation and back diffusion across the membrane.⁴¹

Several techniques are available for water distribution study, including X-ray diffraction,⁴² three-dimensional model simulation,^37,43 Raman spectroscopy,^44–47 coherent anti-Stokes Raman scattering (CARS) spectroscopy,^48–51 neutron scattering,^52–54 and neutron imaging.^55,56 In this paper, CARS was specifically selected because of its excellent time resolution compared to the other techniques.^51,57,58 Figures S1 and S2 in Supporting Information show the schematic of the CARS system and the setup for CARS measurement, respectively. In this paper, the exposure time for obtaining each spectrum was 0.2 s with a depth accuracy of 0.9 µm, as previously reported.⁴⁹

The membrane selected in this dynamic study of water distribution in AEMs was QPAF-4 (Fig. 1).⁵⁹ QPAF-4 as an AEM has been extensively studied in the recent past,^{15,16,59–62} and therefore, forms a good starting point for transient study of AEMFCs. Several innovative AEMs have also been reported,^11,63–69 which will be looked at in the future.

Figure 1.

Chemical structure of QPAF-4.

Our recent papers^70,71 discussed the OH speciation in AEMs using both Raman and CARS spectroscopy. In this paper, reference will be made to the discussed species, which are based on donor-acceptor relationships in liquid water.^72–75 Briefly, the species expected in the QPAF-4 membrane during power generation are: (1) OH H-bonded directly to OH⁻ around 2880 cm⁻¹, (2) C–H in the ammonium group around 2942 cm⁻¹, (3) C–H on the benzene ring around 3091 cm⁻¹, (4) single donor double acceptor H-bonded water (DAA) around 3163 cm⁻¹, (5) double donor double acceptor (DDAA) H-bonded water around 3187 cm⁻¹, (6) single donor single acceptor (DA) water around 3348 cm⁻¹, (7) double donor single acceptor (DDA) H-bonded water around 3470 cm⁻¹ (8) non-H-bonded water around 3510 cm⁻¹ and (9) OH⁻ around 3725 cm⁻¹.⁷⁰ It must, however, be added that the OH peaks are expected to undergo some displacements in confined spaces such as polymer membranes.⁷⁶ Also, since OH⁻ is known to have a strong effect on H-bonding networks,⁷⁷ further shifts in peaks are expected as the ratio of OH⁻ to water changes. For instance, in the alkaline solution data reported,⁷⁸ peak shifts of up to 150 cm⁻¹ are observed in the same alkaline with changing concentration. Detailed information on changes in peak parameters such as full width at half maximum (FWHM), amplitude, and peak position in the QPAF-4 with changing hydration conditions is available in our previous publication.⁷¹

The main objective of this paper was to establish the practicality of using CARS spectroscopy in the study of both transient and steady-state water distribution in AEMFCs. Previously, transient water distribution in AEMFCs using vibrational spectroscopy has never been carried out, to the best of our knowledge. To make such measurements possible, we recently developed a new 785 nm femto second CARS.⁷¹ The CARS system was tested on an AEMFC without power generation.⁷¹ The use of a longer wavelength (785 nm instead of the commonly used 532 nm) in the new system considerably reduced the heating effect of the laser beam on samples. Additionally, using femtosecond mode (from previous 1.2 ps to 100 fs) made it possible to use lower average power but with greater peak power by a factor of 12, thus eliminating laser damage while retaining high spectral resolution.⁷¹ As part of the same continuing project, in this paper, we test the system by measuring water content at different depths during steady state and transient power generation in an AEMFC. This success is expected to open the way for qualitative comparative studies of water distribution in new and existing AEMs. Such information would allow for more rapid advancement in performance and, eventually, a drop in the production cost of AEMFCs. This could open the way for a possible fuel cell-powered future, especially in the heavy commercial vehicles industry.

2. Experimental Technique

2.1 Cell preparation

A QPAF-4 membrane of thickness 30 µm and ion exchange capacity 2.0 meq g⁻¹ was solution cast as reported.⁵⁹ The membrane was cut to a size of 4 cm × 4 cm. Catalyst ink for both the cathode and anode GDL was prepared using a Pt catalyst supported on carbon black (TEC10E50E, Tanaka Kikinzoku Kogyo, K. K.) using the method reported by Otsuji et al.¹⁶ The pulse-swirl-spray (PSS, Nordson Co., Ltd.) technique was then used to apply the ink onto the microporous layer of the GDLs in around ten spray cycles. As GDLs, W1S1010 from Cetech Co., Ltd. was used for the anode, and 29BC from SGL Carbon Group Co. was used for the cathode. The Pt loading was 0.2 mg cm⁻² for both GDLs. The GDEs were cut to 2.0 cm × 2.0 cm. The GDEs and the QPAF-4 membrane were immersed in 1 M KOH for 48 hours to ion exchange to the hydroxide form.¹⁶ The cell was then assembled, as shown in Fig. 2.

Figure 2.

The assembly of the cell. Both the anode and cathode have parallel flow channels of square cross-sections of 1 mm in size.

During assembly, a pinhole of 0.5 mm was punched at the center of the anode for optical access to the membrane.^{44,46,48–51,70} A Pt foil of the same diameter size as the pinhole was then placed between the QPAF-4 film and the GDE on the cathode side for the efficient reflection of the CARS light. A transparent quartz window 16 mm × 19 mm in area and 200 µm in thickness was mounted at the anode end plate to irradiate laser lights and detect the CARS signal. Heat pressing was not used to assemble the MEA.

2.2 Water measurement during power generation

The summary of the cell operation parameters is shown in Table 1. Pure O₂ and H₂ were used as the cathode and anode gases, respectively. Water distribution across the membrane was measured at intervals of 5 µm. Measurement was done along membrane thickness, as shown in Fig. S1. The mentioned 785 nm CARS spectroscopy system was used to carry out the measurements. As reported in our previous paper,⁴⁹ the spatial resolution of the beam was a focal depth of 0.9 µm and a spot of diameter 4 µm.

Table 1. Cell Operation Parameters.

Number.	Parameter	Value
1	Cell temperature	60 °C
2	Relative humidity	90 %
3	Anode gas (H2) flow rate	100 ml min−1
4	Cathode gas (O2) flow rate	100 ml min−1
5	Flow type	Counterflow
6	Back pressure	50 kPa

2.3 Transient measurement

The current density was first stabilized at 0.01 A cm⁻² for two hours; the current was then changed to a higher current density value in a single step. Water, ohmic resistance, and cell voltage measurements were recorded simultaneously every second for 10 s before and 20 s after the jump. The measurements were repeated at intervals of 5 µm in the membrane.

3. Results and Discussion

3.1 Water distribution during steady state power generation

Figure 3 shows the spectra at different depths during power generation at a current density of 0.01 A cm⁻². The measurement positions in the membrane are illustrated in Fig. S3. Similar spectra were recorded at other current density values. The area ratio A_OH/A_Aromatic was computed for each spectrum and converted to the number of water molecules per ammonia group (λ) using a calibration curve obtained from mass water uptake measurement as reported in our earlier papers.^49–51,71

Figure 3.

Spectra across the membrane at 0.01 A cm⁻².

The area A_OH was obtained by taking the total area under the fitting curve, as shown in Fig. 4 and subtracting the area of P2 and P3, which are not related to OH. The area A_aromatic was obtained from aromatic peak, as shown in Fig. 3 by fitting a single curve and establishing the area under the fitted curve. Curve fitting and calculation of the area under the fitting curve were done using the Multipeak fitting package in Igor Pro 8 software. Figure S4 shows the calibration graph used. The resulting λ plot is shown in Fig. 4.

Figure 4.

λ at different locations in the membrane at different current densities.

Under the open-circuit-voltage (OCV) condition, water was evenly distributed across the membrane. The water content in the membrane increased from the cathode side to the anode side during power generation. On the anode side, the hydration number increased steadily above the value observed at the OCV. However, the overall rise at the anode was less than 8 % of the water in the membrane. At the center of the membrane, the water dropped slightly with increasing current density (up to 0.05 A cm⁻²), then increased sluggishly with increasing current density. The amount of water approached the initial amount in the membrane as the current density approached 0.25 A cm⁻². At the cathode interface, the quantity of water dropped sharply as the current density increased from 0 to 0.1 A cm⁻². From 0.1 A cm⁻² upwards, the water started to rise. The water on the cathode side remained significantly lower than the water observed in the membrane at the OCV. The spectra measured at the cathode (0 µm depth) and anode (30 µm depth) were all deconvoluted into nine peaks.^70,71 Figure 5 shows the deconvolution of the OH peak at 15 µm depth and 0.01 A cm⁻². The peaks at (1) 2841, (2) 2942, (3) 3091, (4) 3042, (5) 3230, (6) 3393, (7) 3546, (8) 3615 and (9) 3687 cm⁻¹ are assigned to (1) OH H-bonded directly to OH⁻, (2) C–H in the quaternary ammonium group, (3) C–H on the benzene ring, (4) DAA H-bonded water, (5) DDAA water, (6) DA water, (7) DDA water, (8) non-H-bonded water and (9) OH⁻, respectively. The λ value from the different OH peaks on the cathode and anode were then evaluated. Figure 6 shows the resulting water species distribution plots at the anode and cathode. Figure 6a shows that the total water increased with the current density on the anode side, as previously mentioned. However, only the H-bonded species (DAA, DDAA, DA, and DDA hydrated hydroxide) increased with the current density. The non-H-bonded species (OH⁻ and isolated water) remained roughly constant. On the cathode side (Fig. 6b), the total water decreased with increasing current density up to 0.1 A cm⁻² before it increased. Again, only the H-bonded species increased with current density.

Figure 5.

The deconvoluted OH peak at 15 µm depth and 0.01 A cm⁻².

Figure 6.

Change in the OH species in the membrane. (a) on the anode side, (b) on the cathode side.

Assuming a linear relationship (which is not the case), the average change in λ on the anode side with increasing current density is +6.7 water molecules per 1 A cm⁻² increase in current density. On the cathode, the average linear change in λ is −6.8 water molecules per 1 A cm⁻² increase in current density. A pictorial representation of the models of OH species is shown in Fig. S5.

3.2 Dynamic water distribution

Figure 7 shows the resistance, voltage, λ, and proposed water transport mechanism for a current jump ten times the initial value (from 0.01 to 0.1 A cm⁻²) at a gas flow rate of 100 ml min⁻¹. During the first 10 s, where a constant current density was maintained, the cell voltage and resistance were roughly constant at around 0.89 V and 0.28 Ω cm², respectively, as seen in Fig. 7a. Water distribution across the membrane in this interval remains almost uniform with time, as shown in Fig. 7b. The anode shows more water than the cathode side, as expected from Eq. 1.

Figure 7.

Transient response to 0.01 to 0.1 A cm⁻² current jump. (a) Voltage and resistance (b) Water distribution (c) Water distribution during a current jump from and changes in resistance and voltage. The size of the arrow indicates the quantity of water moving.

Upon implementation of the current jump at time 0 s, membrane resistance, cell voltage, and water distribution immediately responded. The membrane resistance dropped instantly from an average of 0.28 to an average of 0.05 Ω cm². This drop is attributed to the increased hydroxide movement from the cathode to the anode as an enabler of the set current density. In the case of a PEM, such a drop in resistance is an indication of increased λ in both the membrane and ionomer, promoting proton movement.^79,80 The cell voltage dropped from an average of 0.89 to 0.62 V. No overshoot was observed. While studying PEMFCs, Jia et al. reported that the degree of overshoot was significantly reduced with increasing relative humidity of the anode.⁸¹ Therefore, in the case of AEMFCs, the degree of overshoot would similarly be reduced if the cathode RH is maintained close to 100 %. In this paper, a fixed RH of 90 % was used. Thus, not much overshoot was expected.

The water distribution response to the current jump depended on the position in the membrane. On the anode side, the amount of water increased sharply within the first 4 s, then dropped steadily. At 20 s, the amount of water was almost stable but slightly higher than the value during the duration before 0 s (under the steady-state current of 0.01 A cm⁻²). At the center of the membrane (15 µm depth), the amount of water increased slightly but then recovered within 10 s. At the cathode, the water dropped between 0 and 4 s; it, however, took longer to recover than the water on the anode side. At 0.01 A cm⁻², the water on the anode was 140 % of the water at the cathode. 5 s after the current jump to 0.1 A cm⁻², the water on the anode was 210 % of the water at the cathode. 20 s after the jump, the water was partially stable, with the anode water being 160 % of that on the cathode side. By comparison, during steady-state water measurement at 0.1 A cm⁻², the water on the anode was approximately 150 % of that on the cathode. Although at time 20 s the water in the membrane was almost stabilized, it was not yet at the final value. The water is expected to take time to change from 160 % to a value close to 150 %. A recent experiment by X-ray scattering showed that the QPAF-4 membrane could take up to 7.5 hours to reach its final hydration condition.⁶² Nishiyama et al. carried out the same experiment on Nafion with a current jump from 0.1 to 1.0 A cm⁻².⁵⁰ They reported a maximum disturbance in water distribution after 4 s of applying the current jump, where the cathode water was 150 % of that at the anode.⁵⁰ This shows that a current jump in AEMFCs (even if ten times smaller than that applied in PEMFC) has a larger influence on water distribution than in PEMFC.

The water transport mechanism can be modeled as shown in Fig. 7c. During the 10 s of steady state power generation at 0.01 A cm⁻², the water distribution was uniform with time because of the stabilized rate of water movement by diffusion, back-diffusion, and electro-osmotic drag (hydroxide-induced water movement). Implementing the current jump at time zero led to increased movement of OH⁻ ions from the cathode to the anode. The increased hydroxide movement had three immediate effects: (1) The membrane’s ohmic resistance dropped, (2) the Electro-osmotic drag of water increased from the cathode to the anode by the moving OH⁻ ions. A similar electro-osmotic drag by protons is reported during the current jump in PEMFCs.¹⁸ (3) The cell voltage dropped due to reduced membrane resistance. For the next 4 s, the increased water transportation from the cathode to the anode reduced the water on the cathode side and increased the water on the anode side. At 4 s, the water on the cathode was reduced to its lowest, decreasing the amount of water transported to the anode. As a result, the anode water reached its peak, and the cathode side reached maximum dehydration. Back diffusion from the anode to the cathode also increased slightly because of the steeper concentration gradient in water between the anode and cathode. From 5 s onwards, the extra water on the anode diffused into the GDE, leading to a drop in the anode hydration peak. At the same time, additional water diffused into the membrane from the GDE, leading to a gentle rise in the cathode water. Because of all these occurrences, the water distribution stabilizes considerably in around 15 s.

3.3 Comparison of the water distribution in the AEM with the one in PEM

Water distribution in PEMFCs while generating power has been done in the past using both Raman⁴⁶ and CARS^48–50 spectroscopy. Hara et al. carried out in situ confocal micro-Raman spectroscopy on a PEMFC during power generation.⁴⁶ They observed that the λ value increased with current density and gas utilization during power generation.⁴⁶ They concluded that the water distribution in the membrane was a balance of the back-diffusing water produced at the cathode, electro-osmotic drag, and the exit of water from the cell via the GDL.⁴⁶ While the water in the PEMFC increased at all depths with increasing current density, this was not the case for the AEMFC. In the AEMFC, water increase with increasing current density only occurred at the anode side. At the cathode, the increase depended mainly on the level of back diffusion from the anode side and diffusion from the GDL. Nishiyama et al. carried out steady water measurements in a PEMFC using CARS.⁴⁹ They observed that the number of H-bonded species increased with increasing current density while the protonated water species remained unchanged.⁴⁹ This observation on OH species is identical to the observation made for AEMFCs on the anode side. All the H-bonded species on the anode side increased with increasing current density while OH⁻ and C–H remained constant. On the cathode, only the H-bonded species changed again. The water change transitioned from a decrease in H-bonded species to an increase as the current density increased beyond some current density values that vary with depth. Nishiyama et al. studied transient response in a PEMFC using CARS with a current jump of 0.1 to 1 A cm⁻².⁴ They observed that λ at the membrane surface of the cathode side initially overshot and reached equilibrium after 7 s accompanied with a simultaneous voltage undershoot before a slight recovery.⁵⁰ Compared to the AEMFC, the hydration overshoot on the anode side of the AEMFC disappeared much faster than in the overshoot on the PEMFC cathode. This rapid recovery could be because a small current jump was used in the case of AEMFC. However, as mentioned earlier, the difference in hydration on the anode from the hydration on the cathode because of the current jump was more severe in the AEMFC than in the PEMFC. From the three papers, it can be concluded that the distribution of water in the AEMFC and PEMFC, although following the same principle of water transport mechanism and change of OH species, the resulting concentration gradient is very different. The AEM is more prone to both flooding as well as dehydration.

4. Conclusions

We demonstrated that CARS spectroscopy was suitable for the transient and operando studies of AEMFCs. For QPAF-4, the OH peak was deconvoluted into nine peaks. An increase in current density in AEMs steadily increased water on the anode side. At the middle of the membrane and cathode side, the amount of water dropped first before increasing sluggishly. During the transient response, the maximum change in the water distribution occurred within the first 5 s of applying the current-density jump. At this point, the water on the anode can be up to 220 % of the water on the cathode. Only the H-bonded species changed whenever the amount of water in the membrane changed; the non-H-bonded species remained constant. An adequate water management strategy for AEMFCs must aim to even out the imbalance in hydration to prevent possible dehydration on the cathode. Because of the extreme hydration and dehydration during transient response, the AEM membrane will likely experience maximum physical strain during this period.

Acknowledgment

This work was supported by Grant-in-Aid for Scientific Research (Nos. 19K12632, 21H02044, 22K12673, and 23H2058) and Data Creation and Utilization Type Material Research and Development Project (JPMXP1122712807) from MEXT, by ECCEED’30 and ECCEED_GDL Projects from the New Energy and Industrial Technology Development Organization (NEDO), and by Yanmar Resource Circulation Support Organization.

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.24893214.

• 1) The hydration condition of an anion exchange membrane (AEM) in an operating fuel cell significantly affects its performance as well as its lifespan. In this paper, an in-house build coherent anti-Stokes Raman scattering (CARS) vibrational spectroscopy is used to establish the hydration of an AEM in an anion exchange membrane fuel cell (AEMFC) while it generates power. During steady-state operation, water on the anode side increased with current density. On the cathode side and at the center of the membrane, water initially decreased with current density and then started increasing at a slower pace than on the anode side. A deconvolution of the OH peak in the recorded CARS spectra into nine species revealed that only the H-bonded water species underwent variation. The rest of the species experienced a negligible change. A transient study revealed that maximum disturbance to the water distribution was achieved after 5 s of applying a current jump. The distribution of water became stable within 20 s after applying the current jump. The response to the current jump on the anode side was opposite to that on the cathode. These results open the way for a widespread dynamic study of water distribution in different AEMFCs. The technique could also be directly used to evaluate the dynamic degradation of AEMs.

CRediT Authorship Contribution Statement

Solomon Wekesa Wakolo: Data curation (Lead), Formal analysis (Lead), Investigation (Lead), Methodology (Equal), Software (Lead), Writing – original draft (Lead), Writing – review & editing (Lead)

Kenji Miyatake: Resources (Lead), Writing – review & editing (Lead)

Junji Inukai: Conceptualization (Lead), Funding acquisition (Lead), Investigation (Lead), Methodology (Equal), Project administration (Lead), Resources (Lead), Supervision (Lead), Validation (Lead), Writing – original draft (Equal), Writing – review & editing (Lead)

Conflict of Interest

The named authors have no conflict of interest, financial or otherwise.

Funding

MEXT: 19K12632

MEXT: 21H02044

MEXT: 22K12673

MEXT: 23H2058

MEXT: JPMXP 1122712807

New Energy and Industrial Technology Development Organization: ECCEED’30

NEDO: ECCEED_DGL

Yanmar Resource Circulation Support Organization: fund

Footnotes

S. W. Wakolo: ECSJ Student Member

K. Miyatake and J. Inukai: ECSJ Active Members

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