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Letter
The Experimental Measurement of Local and Bulk Oxygen Transport Resistances in the Catalyst Layer of Proton Exchange Membrane Fuel Cells Chao Wang, Xiaojing Cheng, Jiabin Lu, Shuiyun Shen, Xiaohui Yan, Jiewei Yin, Guanghua Wei, and Junliang Zhang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02580 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 11, 2017
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The Experimental Measurement of Local and Bulk Oxygen Transport Resistances in the Catalyst Layer of Proton Exchange Membrane Fuel Cells Chao Wang,† Xiaojing Cheng,† Jiabin Lu,† Shuiyun Shen,† Xiaohui Yan,† Jiewei Yin,† Guanghua Wei,‡ Junliang Zhang*†1 †
Institute of Fuel Cells, School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Rd., Shanghai, China
‡
SJTU-ParisTech Elite Institute of Technology, Shanghai Jiao Tong University, 800 Dongchuan Rd., Shanghai, China
Abstract Remarkable progress has been made in reducing the cathodic Pt loading of PEMFCs, however, a huge performance loss appears at high current densities, indicating the existence of a large oxygen transport resistance associated with the ultralow Pt-loading catalyst layer. To reduce the Pt loading without sacrificing cell performance, it is essential to illuminate the oxygen transport mechanism in the catalyst layer. Towards this goal, an experimental approach to measure the oxygen transport resistance in catalyst layers is proposed and realized for the first time in this study. The measurement approach involves a dual-layer catalyst layer design, which consists of a dummy catalyst layer and a practical catalyst layer, followed by changing the thickness of dummy layer to quantify the local and bulk resistances via limiting current measurements combined with linear extrapolation. The experimental results clearly reveal that the local resistance dominates the total resistance value in the catalyst layer.
* Corresponding Author:
[email protected] 1
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Proton exchange membrane fuel cells (PEMFCs) have been paid extraordinary attentions for their potential applications as electrochemical power sources. For a low commercial cost, tremendous efforts are being made to lower the Pt loading used in membrane electrode assemblies (MEAs) of PEMFCs. Recently, the required Pt loading in MEAs has been successfully reduced via the development of highly active Pt alloy catalysts or core-shell catalysts,1 however, another issue of severe performance loss at high current densities arises associated with the ultralow Pt loading catalyst layer (CL), indicating the existence of an unexpected large oxygen transport resistance within the CL. The oxygen transport resistance within the CL consists of two parts, i.e., bulk resistance resulted from molecular/Knudsen diffusion in the micro-pores2-4 and local resistance through the thin ionomer films to Pt surface.5-7 To clarify the transport process for reducing Pt loading and maintaining the high performance, various research groups quantitatively evaluated the transport resistance in CL with different methods.8-10 For instance, Iden et al calculated the gas transport resistance in catalyst layers based on gas crossover rate through Nafion membrane.11 Baker et al. studied the
bulk
transport
resistances
under different
pressures
to
separate
the
pressure-dependent and -independent resistances, which corresponds to molecule diffusion and Knudsen diffusion, respectively.12 Greszler et al developed a transmission line model to calculate the local resistance based on the tested total resistance in cells.13 Nonoyama et al. separated total transport resistances into molecular diffusion resistance, Knudsen diffusion resistance and local resistance within the cells, in which a uniformly distributed transmission-line model and ex-situ oxygen diffusion properties of ionomer film were utilized.14 Moreover, the effects of equivalent weight of ionomer15 and relative-humidity16-17 on the local transport resistance were also studied. Although progress has been made in understanding the transport resistance in CLs, the local and bulk transport resistances were obtained by the combination of experimental and
numerical approaches previously,
which require several
ideal assumptions and ex-situ tested physical properties of related materials. To 2
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directly measure the local and bulk oxygen transport resistances in CL, we propose a new experimental approach in this study. We design a MEA with dual-layer cathode catalyst layer, which consists of a dummy catalyst layer (DCL) and a real catalyst layer. Through varying the thickness of DCL, the bulk diffusion property of catalyst layer and the local resistance can be quantified via limiting current measurements combined with linear extrapolation. Moreover, the experimental results show that local resistance plays a dominant role in catalyst layers with low Pt loading. The total oxygen transport resistance ( ) in a cell consists of the resistances in different components: = + + (1)
where , and indicate the resistances in catalyst layer, gas diffusion layer (GDL) and flow field, respectively. The total transport resistance of oxygen, , is measured by the limiting current density method based on the combination of Fick’s law and Faraday’s law:
= − =
(2)
(3) 4
where is the molar flux of oxygen and
is the effective diffusion
coefficient of oxygen. and denote the transport length and Faraday constant, respectively. The current density was proportional to the oxygen concentration gradient from flow channel to catalyst surface shown as following equation: − , = 4
!
(4)
where is the oxygen concentration in the channels, ,
!
concentration on the Pt surface. At the limiting current density, ,
is the oxygen !
value can be
considered as zero and in cathode can be calculated as: = 4
− ,
!"
= 4 ∗
1 $%&
(5)
As oxygen concentration is calculated from the dry mole fraction of oxygen (( ) in N2-O2 mixture via gas state equation, is found to be in inverse proportion to the limiting current density with a constant ( 3
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=
4 ∗ ( ∗ ) 1 ∗ (6) * $%&
where ) is the partial pressure of oxygen in channels, is the universal gas constant and * is the absolute temperature of cells. Therefore, can be calculated by Eq. 6 with certain ( and ) at limiting current density. As shown in Eq. 1, and should be quantified before the investigation of CL resistance, . Oxygen is uniformly distributed in channels, but blocked by the ribs. This blockage is denoted as the transport resistance in the flow field ( ), determined by the flow field geometry. According to Baker’s results12, of the parallel flow field in this study is calculated to be 0.258 s cm-1. The transport resistance in GDL, , derives from the resistance within carbon fiber diffusion media (DM) and microporous layer (MPL), which have different transport properties. DM owns large pore size, in which molecular diffusion is dominant; in contrast, the pore size of MPL is much smaller, thus Knudsen diffusion should be taken into account. In this study, the whole resistance of GDL was directly measured by changing the GDL thickness (i.e. different numbers of GDLs are used). Meanwhile, the gaskets were added with the increase in layer number to keep a constant compression ratio of GDL. With several layers of GDL used, the total resistance can be expressed as = 0 ∗ 1 + + (7)
where 0 is the resistance of a single GDL and n is the number of GDLs. In the case of limiting current method (Eq. 6), $%& is in inverse proportion to n as 1 $%&
=
* * ∗ ( + ) 0 ∗ 1 + (8) 4 ∗ ( ∗ ) 4 ∗ ( ∗ )
The value of 1/$%& is linear to n, therefore 0 can be obtained based on the slope.
4
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Figure 1 (a) An example of GDL resistance tested by limiting current density method in 4% O2/N2-O2 mixture; (b) the test results of 0 Figure 1(a) shows an example of 1/$%& results with different GDL numbers in 4% O2/N2-O2 mixture. According to Eq. 8, the resistance of a single GDL, i.e. 0 , can be calculated by the plot slope, which has been given in Figure 1(b). Several tests were performed to ensure the test accuracy and the average 0 value is 0.409 s cm-1. The transport resistance in CL, , consists of the local resistance near Pt surface (567$ ) and the bulk resistance in the porous structure of CL (8!$9 ). The former part comes from the transport process in the vicinity of Pt surface, such as oxygen dissolution and diffusion in ionomer. The latter part derives from molecular diffusion and Knudsen diffusion in the pores of CL. A DCL was added between CL and GDL to measure the bulk resistance as shown in Figure S1. DCL has the same structure as the CL, but the carbon support without Pt particles was used instead of Pt/C catalyst. The resistance in DCL includes bulk resistance only since no electrochemical reaction happens within it. can be further expressed as following equation which exhibits a linear relationship with the diffusion length of O2 in the DCL: = 08!$9 ∗ + + + (9)
where 08!$9 is defined as the bulk resistance per unit thickness, is the 5
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thickness of DCL. Sharing the same porous structure, DCL and CL also have the same 08!$9 value: 08!$9 =
8!$9 = (10)
where is the effective thickness of CL which denotes the average diffusion length of oxygen molecules in CL. Thus, Eq. 9 can be expressed as following equation to separate 567$ in the cell:
= 08!$9 ∗
