Design of an Advanced Membrane Electrode Assembly Employing a

Dec 2, 2015 - The membrane electrolyte assembly (MEA) designed in this study utilizes a double-layered cathode: an inner catalyst layer prepared by a ...
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Design of an Advanced Membrane Electrode Assembly Employing a Double-Layered Cathode for a PEM Fuel Cell GyeongHee Kim,†,‡ KwangSup Eom,†,‡ MinJoong Kim,§ Sung Jong Yoo,‡ Jong Hyun Jang,‡ Hyoung-Juhn Kim,‡ and EunAe Cho*,§ ‡

Fuel Cell Research Center, Korea Institute of Science and Technology, 14gil-5 Hwarang-ro Sungbuk-gu, Seoul 136-791, Republic of Korea § Department of Materials Science & Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro Yuseong-gu, Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: The membrane electrolyte assembly (MEA) designed in this study utilizes a double-layered cathode: an inner catalyst layer prepared by a conventional decal transfer method and an outer catalyst layer directly coated on a gas diffusion layer. The double-layered structure was used to improve the interfacial contact between the catalyst layer and membrane, to increase catalyst utilization and to modify the removal of product water from the cathode. Based on a series of MEAs with double-layered cathodes with an overall Pt loading fixed at 0.4 mg cm−2 and different ratios of inner-to-outer Pt loading, the MEA with an inner layer of 0.3 mg Pt cm−2 and an outer layer of 0.1 mg Pt cm−2 exhibited the best performance. This performance was better than that of the conventional single-layered electrode by 13.5% at a current density of 1.4 A cm−2. KEYWORDS: polymer electrolyte membrane fuel cell, membrane electrolyte assembly, double-layered electrode, decal transfer method, catalyst-coated gas diffusion layer, water removal

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subsequent hot pressing can improve interfacial contact between the electrodes and the membrane but destroy the pore structures of catalyst layers and gas diffusion layers.2 By contrast, in the CCM method, the gas diffusion layers retain their pore structures, and catalyst particles are not lost while the interfacial resistance is relatively high and the membrane is swollen during catalyst coating.3−5 Much effort has been devoted to producing high-performance MEAs by modifying the above-mentioned MEA fabrication methods.4−8 The decal transfer method, which involves coating a Pt/C catalyst layer on a decal film and then transferring the prepared catalyst layer to a membrane by hot pressing, has been reported to combine the advantages of the CCG and CCM methods. The hot pressing ensures good interfacial contact between the catalyst layer and the membrane

olymer electrolyte membrane fuel cells (PEMFCs) are among the most attractive future power generation systems for stationary and automotive applications because of their high energy efficiency, the absence of pollutants, and their low operating temperature of 60−80 °C. The core component of the PEMFC is a membrane electrode assembly (MEA) composed of a sandwich structure employing Pt/C catalyst electrodes on both sides of a membrane. Because the electrochemical reactions of fuel (hydrogen) and oxidant (oxygen) and the transport of the reactants and product water occur in the MEA, the performance of a PEMFC strongly depends on the structure and fabrication process of the MEA.1−9 Representative MEA fabrication methods include the socalled “‘catalyst-coated gas diffusion layer (CCG)” and “catalyst-coated membrane (CCM)” methods, in which a catalyst layer is coated on a gas diffusion layer and membrane, respectively.1−8 In the CCG method, catalyst particles can penetrate into the highly porous gas diffusion layers, and the © XXXX American Chemical Society

Received: August 9, 2015 Accepted: December 2, 2015

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DOI: 10.1021/acsami.5b07346 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces without destroying the pore structures of the gas diffusion layer. Coating the catalyst ink onto a decal film instead of onto a membrane prevents swelling of the membrane. However, the pore structure of the electrodes prepared by the decal transfer method can be destroyed during hot pressing, resulting in an increase in mass transport resistance. Water management in MEAs plays a critical role in determining cell performance, especially in the high current density range.10−15 The effective removal of product water from the cathode is necessary to prevent the well-known flooding phenomenon,15−20 which impedes the transport of reactant gases to the catalytic active sites and sharply decreases the cell performance.15−20 Lu et al.17 modified the water removal process by adding a microporous layer to the top surface of the gas diffusion layer (GDL) to take advantage of the capillary force. Qi et al.18 and Eom et al.15 improved cell performance by adding hydrophobic polytetrafluoroethylene to the surface of the gas diffusion layers. In this work, to develop high-performance MEAs and thus improve the interfacial contact between the electrodes and the membrane, increase catalyst utilization, and modify the removal of water from the cathode, we designed a new MEA with a double-layered cathode comprising an inner catalyst layer prepared by the conventional decal transfer method and an outer catalyst layer directly coated on gas diffusion media. Five types of MEAs were designed as depicted in Figure 1. Total Pt

Figure 2. Cross-sectional SEM images of (a) a transferred CCM electrode (0.4/0.0) and (b) a CCG electrode (0.0/0.4). (c) Crosssectional EPMA image of a CCG electrode (0.0/0.4) showing Pt intrusion behavior into the GDL interior.

employing an inner/outer layer of 0.3/0.1 mg cm−2 Pt loading exhibited the best performance. With decreasing Pt loading in the decal layer from 0.4 to 0.3, 0.2, 0.1 and 0.0, the cell voltage at 1 A cm−2 was measured to be 0.585, 0.591, 0.557, 0.537, and 0 V, and at a current density of 1.4 A cm−2, the cell voltage was measured to be 0.37, 0.42, 0.403, 0.317, and 0 V. To estimate the electrochemically active surface area (EAS) and resistances, we performed CV and EIS. The EAS for the prepared cathodes was calculated with the Coulombic charge for hydrogen desorption (QH) from the CV curves21−23 shown in Figure 3b. In accordance with the cell performance in Figure 3a, the double-layered cathode with (inner 0.3/outer 0.1) Pt loading exhibited the highest hydrogen desorption area observed at approximately 100 mV, implying that the cathode with (inner 0.3/outer 0.1) is the most electrochemically active. As the inner Pt loading decreased from 0.4 to 0.3, 0.2, 0.1, and 0.0, the EAS value was calculated to be 66.9, 72.3 59.6, 56.4, and 40.9 m2 g−1. In the Nyquist plots shown in Figure 3c, the high-frequency intersection of the x-axis frequency and the diameter of the semicircle correspond to the ohmic resistance (Rohm) and charge-transfer resistance (Rct), respectively.21−23 All the prepared cathodes exposed to hot pressing have similar ohmic resistance (Rohm) values between 0.12−0.15 Ohm cm2: 0.149, 0.141, 0.124, and 0.116 Ohm cm2 as the inner Pt loading decreased from 0.4 to 0.1. By contrast, the single-layered CCG had an ohmic resistance of 0.246 Ohm cm2, approximately two times higher than that of the other 4 types of MEAs. These results can be attributed to the higher interfacial resistance between the electrode and the membrane for the single-layered CCG electrode and clearly show that the electrodes prepared by hot-pressing have good interfacial contact between the electrode and the membrane. By contrast, the double-layered electrode with the inner 0.3/outer 0.1 Pt loading exhibited the lowest charge transfer resistance; as the Pt loading in the inner decal layer decreased from 0.4 to 0.0, the charge transfer resistance (Rct) was measured to be 0.833, 0.722, 0.966, 1.014, and 7.044 Ohm cm2, in good agreement with the cell performance and EAS results. Figure 3d shows the performance and power density of the best performing double-layered (0.3/0.1) cathodes compared

Figure 1. Illustrations of membrane electrode assemblies (MEAs) with single-layered and double-layered electrodes as the cathodes. The Pt loading ratios on each surface of the GDL and membrane are denoted as (Pt loading on membrane/Pt loading on GDL).

loading of cathode was fixed to 0.4 mg cm−2 Pt. Pt loadings in the inner/outer layer were 0.4/0.0 (a single-layered decal electrode), 0.3/0.1, 0.2/0.2, 0.1/0.3, and 0.0/0.4 (a singlelayered CCG electrode) mg cm−2 Pt. Pt loading of each anode was 0.25 mg cm−2 Pt. Figure 2 shows cross-sectional SEM images of single-layered electrodes fabricated by the decal and CCG methods, revealing that both electrodes were coated uniformly on a membrane and a GDL, respectively. However, the thickness of the electrodes was 11.5 ± 0.5 and 10.5 ± 0.5 μm, respectively, for the decal and CCG electrode, implying that the CCG method produced a thinner catalyst layer than the decal transfer method by about 1.0 μm, although the CCG electrode was not hot pressed. These results could be attributed to penetration of Pt/C particles into the microporous layer (MPL) and GDL, as clearly demonstrated in the cross-sectional EPMA image in Figure 2c. Figure 3a−c show i−V curves, CV curves, and EIS Nyquist plots for the single cells employing the prepared MEAs. The operating temperature was 65 °C, and the RH of hydrogen and air was 100%.21−23 As shown in Figure 3a, the single cell B

DOI: 10.1021/acsami.5b07346 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. Electrochemical performances of the single PEM fuel cells employing various MEAs employing single-layered and double-layered electrodes in cathodes (0.4/0.0, 0.3/0.1, 0.2/0.2, 0.1/0.3, and 0.0/0.4 mg Pt cm−2 on membrane and GDL); (a) Current density−voltage (i−V) curve, (b) cyclic voltammetry (CV), (c) electrochemical impedance spectroscopy (EIS). Figure 3(d) shows i−V curves and power densities for single-layered (0.4/0.0) and double-layered (0.3/0.1) electrodes.

Figure 4. (a) Concentration overpotential (ηconc) of the single PEMFCs employing single-layered CCM (0.4/0.0) and CCG (0.0/0.4) and doublelayered electrode of (0.3/0.1), (0.2/0.2), and (0.1/0.3) as a function of current density. SEM surface images of (b) a transferred CCM electrode (from the 0.4/0.0 electrode) and (c) a CCG electrode (from the 0.0/0.4 electrode).

electrode.3,4 However, at high current densities, the performance of the single-layered electrode decayed significantly, possibly because of the mass transport resistance of the hotpressed electrode (0.142 V and 0.227 W cm−2 at 1.6 A cm−2). It should be noted that the double-layered (0.3/0.1) electrode

with the conventional single-layered decal electrode (0.4/0.0). The single-layered electrode showed excellent cell performance because of the large three-phase interface caused by strong adhesion between the catalyst layer and membrane, which led to low interfacial resistance and high utilization of Pt in the C

DOI: 10.1021/acsami.5b07346 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

ACS Applied Materials & Interfaces

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designed in this work exhibited improved performance over the conventional decal transfer electrode, especially in the high current range (0.293 V and 0.469 W cm−2 at 1.6 A cm−2). To investigate effects of the double-layered cathode on the overpotentials of MEAs in relation to the best performance of the double-layered electrode (0.3/0.1), we calculated activation overpotential (ηact), ohmic overpotential (ηohm), and concentration overpotential (ηconc) using the following known equation24 ηconc(i) = Eeq − E(i) − ηact(i) − ηohm

Letter

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07346. Experimental section, further details of MEA fabrication methods used in this work, and the activation overpotential and ohmic loss (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

(1)

Author Contributions

where Eeq is the thermodynamic equilibrium potential (1.23 V). Rohm obtained from the Nyqusit plot in Figure 3c was used to calculate ηohm. As demonstrated in Figure 4a, in terms of concentration overpotential, 0.2/0.2 was the optimal combination possibly due to the capillary force for water removal. With increasing or decreasing outer layer from 0.2 mg Pt/cm2, concentration overpotential decreased. SEM surface images of the CCG and decal electrode in Figure 4b, c show that the CCG electrode exhibited larger pores, whereas the CCM electrode exhibited a compact agglomeration of particles with relatively small pores. Therefore, it is considered that the smaller pores of inside catalyst layer (CCM) allow the produced liquid water to flow out by the capillary force, and the bigger pores of outside (CCG) provide enough volume to accommodate and exhaust the accumulated water. It should be noted that the difference in concentration overpotential between 0.2/0.2 and 0.3/0.1 was very small. On the other hand, in the point of activation overpotential (Figure S2a), 0.3/0.1 has the lowest value in accordance with the highest electrochemical active surface area and the lowest charge transfer resistance. In addition to water removal, the double-layered structure is beneficial to formation of triple phase boundary. Ohmic resistance (Figure S2b) was almost same for the hotpressed MEAs. Only the 0.0/0.4 exhibited higher ohmic resistance than the others, implying that ohmic resistance is mainly dependent on the interfacial contact resistance between the electrode and the membrane and can be significantly reduced by hot pressing process. As a results, the 0.3/0.1 combination could have the best performance (higher than the conventional CCM by 13.5% at 1.4 A cm−2) among the tested 5 designs of MEA. In summary, to improve the performance of PEMFCs, we designed a double-layered cathode composed of an inner decal layer and an outer CCG layer. The double-layered electrode with the inner 0.3 and outer 0.1 mg cm−2 Pt loading exhibits the best performance, higher than the conventional singlelayered decal electrode by 13.5% in power, particularly at a high current density (1.4 A cm−2). These results can be attributed to the improved mass transport in the electrode; the CCG layer with larger pores and high porosity can provide an effective pathway for water removal. In addition, the low ohmic and charge transfer resistance of the double-layered electrode confirm that interfacial contact between the electrode and the membrane is good for the double-layered electrode because the inner layer was hot pressed with the membrane.

†

G.K. and K.E. contributed equally to this work as co-first authors

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20143030031340), and the Technology Innovation Program Of the Korea Evaluation Institute of Industrial Technology (KEIT), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (10052823).

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DOI: 10.1021/acsami.5b07346 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX