Understanding the role of cathode structure and property on water management and electrochemical performance of a PEM fuel cell

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Understanding the role of cathode structure and property on water management and electrochemical performance of a PEM fuel cell Aidan Li a,*, Siew Hwa Chan b a

Horizon Fuel Cell Technologies, No. 237 Pandan Loop, #08-03 Westech Building, Singapore 128424, Singapore School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

b

article info

abstract

Article history:

Water management is vital for the successful development of PEM fuel cells. Water should

Received 8 January 2013

be carefully balanced within a PEM fuel cell to meet the conflicting requirements of

Received in revised form

membrane hydration and cathode anti-flooding. In order to understand the key factors

29 June 2013

that can improve water management and fuel cell performance, the cathodes with

Accepted 29 June 2013

different structures and properties are prepared and tested in this study. The experimental

Available online 1 August 2013

results show that even though no micro-porous layer (MPL) is placed between the cathode catalyst layer (CCL) and macro-porous substrate (MaPS), a hydrophobic CCL is effective to

Keywords:

prevent cathode flooding and keep membrane hydrated. The impedance study and the

Hydrophobicity

analysis of the polarization curves indicate that the optimized hydrophobic micro-porous

Micro-porous layer

structure in the MPL or the hydrophobic CCL could be mainly responsible for the improved

Cathode catalyst layer

water management in PEM fuel cells, which functions as a watershed to provide wicking of

Water management

liquid water to the MaPS and increase the membrane hydration by enhancing the backdiffusion of water from the cathode side to the anode side through the membrane. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

One of the most important components in PEM fuel cells is cathode, which dominates the fuel cell performance due to sluggish oxygen reduction reaction and slow two-phase mass transport of oxygen gas and liquid water. The cathode is generally composed of a cathode catalyst layer (CCL) and a gas diffusion layer (GDL), of which the CCL is the location for oxygen reduction reaction and water generation, while the GDL conducts electrons between the CCL and current collector and transports oxygen and water to and from the catalyst layer. In literature, there are two GDL configurations that have been

commonly studied, i.e. single-layer GDL and dual-layer GDL [1,2]. The single-layer GDL has macro-pores in its structure, allowing the effective transport of oxygen gas to the catalyst layer. However, it gives rise to large contact resistance with the catalyst layer and poor physical support to the catalyst particles. Furthermore, this configuration was reported to be unfavorable for water management in the fuel cell [3,4]. The dual-layer GDL is named because a thin micro-porous layer (MPL) is placed between the CCL and conventional macroporous substrate (MaPS). The MPL can provide good physical support for the catalyst layer and minimize the electrical contact resistance [5,6]. In addition, the MPL has been known

* Corresponding author. Tel.: þ65 96726905. E-mail address: [email protected] (A. Li). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.06.130

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to play a key role in effectively removing liquid water from the cathode [7,8]. The coarse pore structure of the backing layer allows the growth of large liquid water droplet, while the hydrophobic fine pore structure in MPL can limit their growth and prevent the stable droplet forming [9e13]. It was also reported that the MPL could create a pressure barrier for liquid water at the CCL to keep the membrane better hydrated [6,14]. Although the importance of GDL to prevent cathode flooding has been extensively studied [15,16], the role of the CCL on fuel cell water balance has never been investigated in depth, either experimentally or numerically. Traditionally, the CCL was considered as an ultra-thin layer to shorten the distance of the gas diffusion and to facilitate water transport. Most cell and stack models treated the CCL as an infinitesimally thin interface without structural resolution when dealing with the flooding problem because of the complexity of CCL [17]. Only few efforts have been put to investigate the effects of CCL properties on the PEM fuel cell performance [18,19]. The structure-based model revealed that the CCL is a critical fuel cell component in view of flooding that could give rise to limiting current behavior [20]. The CCL can act as a watershed in fuel cells to regulate the balance between liquid water and vapor. Jiao and Zhou [21] numerically showed that different wettability in both CCL and GDL could affect liquid water flow patterns significantly, thus influencing the performance of the cell. In our previous studies, it has been demonstrated that using hydrophobic dimethyl silicone oil (DSO) to modify CCL can substantially improve the fuel cell performance with single-layer GDL [22]. However, the roles of the cathode structure and property on the water management and fuel cell performance are still not well understood. The present study focuses on addressing following three questions: (1) what are the desired cathode structure and property to improve fuel cell performance? (2) how do they affect the fuel cell performance? and (3) what are the main factors for the performance improvement? To answer these questions, four different types of cathodes, e.g. normal CCL on MaPS, DSO-loaded CCL on MaPS, normal CCL on MPL and DSO-loaded CCL on MPL, are designed and prepared. Their physical properties and electrochemical behaviors are compared and discussed to systematically understand the mechanism of the water management in the cathode using various characterization techniques, such as SEM, porosimetry analysis, electrochemical impedance spectrometry analysis and steady-state polarization.

2.

Experimental

The carbon paper, TGP-H-090, was used as electrode backing layer. The PTFE-treated carbon papers were directly applied as the single-layer GDL and the backing layer of the dual-layer GDL. The MPL was prepared on the teflonated backing layer based on the procedure reported by Han et al. [1]. The loading of carbon (Vulcan-72) was ca. 1 mg/cm2 and the PTFE content was around 33% in the MPL. The carbon-supported 50 wt.% Pt/ C (Johnson Matthey) and 5% Nafion solution were employed to make the CCL. An appropriate amount of dimethyl silicone oil (DSO, Shin-Etsu, Japan) was emulsified into a mixture of

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catalyst and Nafion by a highly intensive ultrasonic machine (Vcx 750, Sonics, USA) to make the DSO-loaded CCL. The resulting amount of Pt and Nafion loading in CCL were 0.35 mg/cm2 and 0.56 mg/cm2 for all electrodes, respectively. For the DSO-loaded CCL, the content of DSO was 0.5 mg/cm2. A commercial Nafion 112 membrane (EW 1100, Dupont) was employed in the membrane electrode assembly (MEA). The morphologies of the carbon paper and CCL were observed under scanning electron microscopy (SEM) (JSM5600, JEOL, Japan). Pore size and distribution of all the electrodes were measured by a mercury intrusion porosimetry (Autopore 9500, Micromeritics Inc.). Cumulative pore volume as a function of pore diameter was determined from the mercury intrusion data, i.e. the volume of mercury penetrating the pores versus the applied pressure. Impedance spectra were recorded at frequencies range from 0.01 Hz to 10 kHz with AC amplitude of 10 mV and a superimposed DC bias. The fuel cell polarization curves were measured under humidified H2/dry air and dry H2/dry air conditions at room temperature. Hydrogen and air were fed into the anode and cathode, respectively, at a constant flow rate with stoichiometry of 2.1 calculated at 1 A/cm2.

3.

Results and discussion

3.1.

Physical characterization of the cathodes

Fig. 1 shows the top views of the PTFE-treated MaPS, MPL, normal CCL and DSO-loaded CCL. After treated with PTFE (wt 10%), a number of pores in TGP-H-090 carbon paper were blocked by thin PTFE films near the surface (Fig. 1a), which made the carbon paper more hydrophobic [23]. Compared with the surface of the PTFE-treated MaPS, the MPL has finer pores and more uniform pore size distribution, which is believed to provide a better contact and support for the catalyst and facilitate the water removal (Fig. 1b). The morphologies of the normal CCL and DSO-loaded CCL are significantly different from the MPL in the pore size and the shape of the pores (Fig. 1c and d). The pore shape in the normal CCL is irregular and the pore size around 0.5 mm is larger than that of the MPL. In the presence of DSO in the CCL, the DSO-loaded CCL presents much lower compactness and larger pore size, most of which are in the range of 0.5e3 mm. The porosity and pore size distribution are very important parameters for the cathode design, since they significantly affect the transport of O2/H2O within the cathode. Fig. 2a and b shows the cumulative pore volume vs. pore size for the cathodes with normal CCL on MPL, DSO-loaded CCL on MPL, normal CCL on MaPS, and DSO-loaded CCL on MaPS. It is clear that, for the four cathodes, the cumulative pore volumes increase sharply for the pores with diameters between 10 mm and 100 mm, whereas for the pores with diameter below 10 mm, the cumulative pore volumes increase gradually and then remain rather constant. This implies that most of the pores are located in the macro-porous substrate (MaPS) and only small portion of the pores are in the catalyst layer. The results also clearly show that the cathodes with DSO-loaded CCL have slightly higher cumulative pore volume than the cathodes with normal CCL.

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Fig. 1 e Top views of (a) PTFE-treated MaPS, (b) MPL, (c) normal CCL and (d) DSO-loaded CCL.

The differential pore volumes (dV/dlogdp) for the cathodes with normal CCL on MPL, DSO-loaded CCL on MPL, normal CCL on MaPS, and DSO-loaded CCL on MaPS are illustrated in Fig. 2c and d. As compared with Fig. 2c, a small peak in the pore sizes range from 0.03 mm to 0.3 mm appears in the cathodes with normal CCL on MPL and DSO-loaded CCL on MPL, which is contributed by the MPL (Fig. 2d). Since the MPL is composed of a mixture of carbon black (Vulcan XC-72) and PTFE, the macropore size is primarily determined by the size of carbon particle aggregates. For Vulcan XC-72, the primary particle size is about 5e10 nm. These primary particles form aggregates of sub-micron size as illustrated in the pore size distribution curve. Water may not be able to form stable droplets inside such small and hydrophobic pores [10]. Therefore, the permeability of liquid water could be reduced by MPL, which results in a higher liquid water pressure profile within MPL to push more water back to the anode through the membrane [7,24]. The fluctuation of the differential volume in the cathode with DSO-loaded CCL on MaPS suggests that the addition of DSO in the CCL could create more random size pores in the range of 0.004e5 mm. This random size pores created by the addition of DSO are occupied by the liquid hydrophobic DSO in the actual operation of the PEM fuel cell. With the similar function as the MPL, the hydrophobicity created by the DSO in the CCL can significantly increase the capability of repelling water out of the CCL.

3.2. Effect of cathode structure and property on fuel cell performance The MPL has been believed to be a key component in cathode to improve the water management in PEM fuel cells [4]. However, our previous studies showed that even though no MPL is applied between the CCL and macro-porous substrate, a hydrophobic CCL can dramatically prevent the cathode flooding [25]. In order to better understand the effect of cathode structure and property on the water management in PEM fuel cells, the electrochemical perfor mances of the fuel cells with four different types of cathodes, e.g. normal CCL on MaPS, normal CCL on MPL, DSOloaded CCL on MaPS, and DSO-loaded CCL on MPL, were investigated under two different operation conditions, e.g. humidified H2/dry air and dry H2/dry air. Fig. 3 shows the IeV curves of the fuel cells with different types of cathodes tested in humidified H2/dry air. It is found that the fuel cell with normal CCL on MaPS has poorest electrochemical performance among the four fuel cells. The limiting current density of the fuel cell with normal CCL on MPL reaches w900 mA/cm2 which is more than two times higher than that of the fuel cell with normal CCL on MaPS of w400 mA/ cm2. This is consistent with the reported function of MPL to provide wicking of liquid water into the MaPS, thus preventing cathode flooding. However, if a hydrophobic CCL

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Fig. 2 e (a, b) The cumulative volume and (c, d) pore size distribution of the normal CCL on MPL, DSO-loaded CCL on MPL, normal CCL on MaPS, and DSO-loaded CCL on MaPS.

modulated by DSO is directly applied onto the MaPS, the fuel cell performance is comparable with that of the fuel cell with normal CCL on MPL. This is because the hydrophobic CCL functions as a watershed similar to the MPL. The fuel cell with DSO-loaded CCL on MaPS is found to have slightly poorer performance in the low current density region, but it surpasses the fuel cell with normal CCL on MPL at current densities higher than 600 mA/cm2 (or at cell voltages lower than 0.5 V). Furthermore, when a hydrophobic DSO-loaded CCL was applied on MPL, it is not surprised to see a further improvement in the fuel cell performance in the high current density region as compared with the fuel cell with DSO-loaded CCL on MaPS with the combined effect of MPL and hydrophobic CCL on preventing cathode flooding. The main factor for the improved water management by the MPL is believed to be the hydrophobic micro-porosity, which prevents the formation of stable water droplet in the hydrophobic micron size pores inside of the MPL [13,26,27]. However, it is still under debating whether the MPL helps to reduce water saturation in the adjacent CCL by providing wicking of liquid water to the MaPS [10,28] or to increase the membrane hydration by enhancing back-diffusion of water from the cathode to

anode [5,24,29]. To further understand the mechanism by which the MPL and hydrophobic CCL improve the fuel cell performance, the polarization curves of the fuel cells with four types of cathodes in dry H2/dry air were obtained and compared in Fig. 4. Under such operation condition, the membrane dehydration instead of cathode flooding limits the fuel cell performance, especially when the fuel cells operate at low current densities. For the fuel cell with normal CCL on MaPS, the membrane dehydration fatally affects the fuel cell performance and almost no power is generated from the fuel cell. For the fuel cell with normal CCL on MPL, the current density drops quickly at the beginning of the IeV sweep due to the high ohmic resistance of the dry membrane. However, when the cell voltage reaches 0.6 V, there is an obvious increase in the current density. Similar behavior is also observed in the IeV curve of the fuel cell with DSO-loaded CCL on MaPS. This phenomenon confirms the capability of MPL and hydrophobic CCL to enhance the backdiffusion of liquid water from the cathode side to the anode side through the membrane, thus hydrating the membrane and subsequently decreasing ohmic resistance of the membrane. Moreover, the fuel cell with DSO-loaded CCL on MaPS is found to have much better performance than that with normal CCL on MPL at cell voltages below 0.5 V (or current densities higher than

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Fig. 3 e Comparisons of IeV polarization curves of fuel cells with different types of cathodes under humidified H2/dry air condition at room temperature.

400 mA/cm2). This indicates that the hydrophobic CCL is more effective than the MPL to increase the membrane hydration. When a hydrophobic CCL is applied on the MPL, the performance of the fuel cell with DSO-loaded CCL on MPL drops significantly at the beginning of the IeV sweep, but gradually recovers back to almost normal at high current density. This could be explained by the poor water retention ability of the cathode with DSO-loaded CCL on MPL caused by the over repelling of water out of the CCL to MaPS. Based on the experimental results shown in Figs. 3 and 4, one should bear in mind that the hydrophobic level of the CCL is crucial and must be carefully controlled to balance the conflicting requirements of membrane hydration and cathode anti-flooding.

3.3. Analysis of the role of MPL and hydrophobic CCL on water management

concentration polarization. In order to identify the physical and electrochemical factors that may affect the fuel cell water management and performance, electrochemical impedance spectroscopy (EIS) was adopted to characterize the fuel cells with different cathodes. Fig. 5 shows the impedance spectra of the fuel cell with normal CCL on MPL obtained at various cell voltages under the humidified H2/dry air condition at room temperature. At voltages above 0.6 V, the impedance spectra show only a depressed arc over the whole frequency range. However, when the cell voltages are below 0.6 V, a second arc appears in the low frequency range. The high and low frequency arcs were reported to be related to the charge-transfer process and mass-transport process, respectively [30]. Thus, the ohmic resistance (Rohm), charge-transfer resistance (Rcharge-transfer) and mass-transport resistance (Rmass-transport) can be obtained by fitting the Nyquist plots with an equivalent circuit of Rohm[RchCPEmass-transport], arge-transfer, CPEcharge-transfer][Rmass-transport, where CPE is the constant phase element. Except for the ohmic resistance, the other two components in the circuit account only for the cathode due to the negligible overpotential loss of the anode. In order to understand the structure-related performance difference, the same method was also applied to characterize the other fuel cells with different types of cathodes and the resistances of the fuel cells with different types of cathodes as a function of cell voltage were compared in Fig. 6. It is found from Fig. 6a that, at low cell voltage (e.g. high current density), similar ohmic resistances are observed for the fuel cells with normal CCL on MPL, normal CCL on MaPS and DSO-loaded CCL on MaPS due to the good hydration of the membrane by the humidified hydrogen and the generated water. However, at a high output voltage such as 0.8 V, because of low output current, the generated water is not sufficient to fully hydrate the membrane. As a result, higher ohmic resistances are expected at higher cell voltages. Among the fuel cells, the fuel cells with normal CCL on MPL and DSO-loaded CCL on MaPS show much lower ohmic resistances than the fuel cell with normal CCL on MaPS at high cell voltages, confirming the function of MPL and hydrophobic CCL to create a pressure barrier to push the

In PEM fuel cells, the polarization losses mainly originate from three sources: ohmic polarization, activation polarization and

Fig. 4 e Comparisons of IeV polarization curves of fuel cells with different types of cathodes under dry H2/dry air condition at room temperature.

Fig. 5 e Impedance spectra obtained at various values of cell voltages for fuel cell with normal CCL on MPL under humidified H2/dry air condition at room temperature.

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(a)

(b)

(c) Fig. 6 e (a) The ohmic resistance, (b) the charge-transfer resistance, and (c) the mass-transport resistance of the fuel cells with different types of cathodes at various cell voltages.

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generated water to the membrane [5,26]. The highest ohmic resistance of the fuel cell with normal CCL on MaPS could be caused by the poor water retention capacity of the normal CCL at the membrane/CCL interface. Fig. 6b illustrates the charge-transfer resistances of the fuel cells with different types of cathodes at various voltages. It is obvious from the results that the charge-transfer resistance decreases in the order of DSO-loaded CCL on MaPS, normal CCL on MaPS and normal CCL on MPL at cell voltage of 0.8 V. However, at cell voltages below 0.6 V, the decreasing order of the charge-transfer resistance changes to normal CCL on MaPS, normal CCL on MPL and DSO-loaded CCL on MaPS. The effective charge-transfer resistance is associated with the rate of the interfacial oxygen reduction process, the protonic conductivity and the oxygen permeability limitations within the CCL. At cell voltage of 0.8 V, the low level of water retention in the hydrophobic DSO-loaded CCL limits the protonic conductivity, which impedes the oxygen reduction. However, with the increase of the current density, the hydrophobic CCL is found to be the most effective way to balance the electrolyte hydration within CCL and cathode anti-flooding, achieving lowest charge-transfer resistance at cell voltages below 0.6 V. The mass-transport resistances of the fuel cells with different types of cathodes are summarized in Fig. 6c. Severe flooding was observed in the fuel cell with normal CCL on MaPS at cell voltages below 0.6 V (shown in Fig. 6c), which is consistent with the appearance of a limiting current density at cell voltage of 0.5 V in Fig. 3. In addition, even though flooding phenomena are not obvious for the fuel cells with normal CCL on MPL and DSO-loaded CCL on MaPS at high current densities (e.g. low cell voltages), the mass-transport resistance of the fuel cell with normal CCL on MPL is found to be slightly higher than that of the fuel cell with DSO-loaded CCL on MaPS, which could be due to the additional diffusion resistance of oxygen through MPL [28]. Besides the impedance analysis of the fuel cells with different types of cathodes, the method proposed by Williams et al. [31] was also adopted to further understand the effect of the cathode structure and property on the concentration polarization within the cathode. Two assumptions should be made before the analysis of the polarization curves: (i) The ORR kinetics follows a first order of oxygen concentration in the ButlereVolmer equation, which has been verified in phosphoric acid fuel cells [31]; and (ii) the anode polarization is negligible. The later assumption is valid since the exchange current density for the hydrogen oxidation reaction has several orders of magnitude higher than that of the sluggish ORR and pure hydrogen has a much faster diffusion rate than oxygen. To determine the concentration polarization in the fuel cells, the polarization curves were firstly corrected with the ohmic loss of the fuel cells shown in Fig. 6a. In the low current density region of 1e100 mA/cm2, the ohm-corrected polarization curves can be considered as activation control with negligible concentration loss, which is expressed as follows   h i ¼ io exp ohmcorrected bk

(1)

where bk is the kinetic Tafel slope in the absence of oxygen transport limitation in the CCL in the low current density region.

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However, the concentration polarization gradually becomes significant with the increase of current density. As a result, a modified Tafel equation is applied to analyze the ohm-corrected polarization curves in the intermediate and high current density region. ikin ¼

  i h ¼ i0o exp ohmcorrected 1  i=ilim bmt

(2)

where ilim is the limiting current density, ikin the kinetic current density corrected for ilim, hohm-corrected the ohm-corrected overpotential, and bmt the empirical Tafel slope in the intermediate and high current density which characterizes the oxygen reduction kinetics in the presence of oxygen transport limitation in the catalyst CCL [32]. By comparing the values of bk and bmt, one can understand the concentration polarization of the fuel cells within the CCL. The fuel cell with DSO-loaded CCL on MaPS was taken as an example to demonstrate the ohm-corrected polarization curves in terms of measured current density i and kinetic current density ikin in Fig. 7. The value of bmt according to the slope of the straight line on the semilog plot at the zone with ikin > 1000 mA/cm2 was 99 mV/ dec. The kinetic Tafel slope bk was 58 mV/dec which was fitted to the ohm-corrected polarization curve in the range of around 1 mA/cm2 to 100 mA/cm2 in which oxygen reduction is mainly controlled by the activation polarization. The values of empirical Tafel slope, bmt, and kinetic Tafel slope, bk, for the fuel cells with normal CCL on MPL, normal CCL on MaPS, and DSO-loaded CCL on MPL were obtained by the same data analysis manner and summarized in Fig. 8. It is found that the values of bmt was approximately much higher than the kinetic Tafel slope bk, indicating that the oxygen transport limitation in CCL significantly affected the fuel cell polarization in the intermediate and high current region [19e22]. It is obvious in Fig. 8 that the value of bmt for the fuel

Fig. 8 e Comparison of the kinetic Tafel slope bk and the empirical Tafel slope bmt for the fuel cells with four different types of cathodes.

cell with normal CCL on MaPS is around 20 mV/dec higher than these for the fuel cells with other three types of cathodes, which could be caused by the serious flooding in its CCL in the intermediate and high current density region.

4.

Conclusions

Four different types of cathodes, e.g. normal CCL on MaPS, DSO-loaded CCL on MaPS, normal CCL on MPL and DSOloaded CCL on MPL, were prepared in this study to understand the effect of cathode structure and property on the water management and electrochemical performance of a PEM fuel cell. Their porous structures were characterized with mercury porosimetry and their electrochemical behaviors were studied by an electrochemical analyzer. The experimental results showed that the fuel cells with normal CCL on MPL and DSO-loaded CCL on MaPS had superior electrochemical performance in both humidified and dry feed conditions, demonstrating the strong capability of MPL and hydrophobic CCL to simultaneously prevent cathode flooding and keep membrane hydrated because of their hydrophobic micro-porous structure. Furthermore, it was found that the fuel cell with DSO-loaded CCL on MaPS exhibited better performance than that with normal CCL on MPL in both humidified and dry feed conditions, which could be attributed to the lower oxygen diffusion resistance and more effectiveness to increase the membrane hydration of the cathode with hydrophobic CCL on MaPS.

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Fig. 7 e Data analysis of ohm-corrected polarization curves of the fuel cell with DSO-loaded CCL on MaPS presented in terms of the measured current density and the kinetic current density ikin.

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