Gradient design of Pt/C ratio and Nafion content in cathode catalyst layer of PEMFCs
Introduction
Proton exchange membrane fuel cell (PEMFC) exhibits a multitude of merits including high energy density, high efficiency and environmentally friendliness, thus is attracting intensified attention in stationary and transportation applications [1]. Membrane electrode assembly (MEA), consisting of gas diffusion layer (GDL), catalyst layer (CL) and proton exchange membrane, is the core component of PEMFC. Reduction in the MEA's cost, one of the most urgent issues in commercialization of fuel cell vehicles, can be accomplished by improving the Pt utilization which at present is generally restricted by impeded mass transfer processes [2] and sluggish kinetics of oxygen reduction reaction (ORR) [3]. Given today's electrocatalysis in PEMFCs, tailoring the structure of the MEA to facilitate the mass transport and extend the triple-phase boundary is practically significant [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. The approaches include introducing pore former [4], optimizing ionomer content [5], [6] and fabricating a multi-layer structured CL [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. The multi-layer structured cathode has been proved to be an effective method to reduce the mass transfer loss and improve Pt utilization in the cathode. The key parameters for the multi-layer structured CL are the gradients of Nafion ionomer content [7], [8], [9], [10], [11] and Pt loading.
It has been reported that PEMFCs performance can be improved through fabricating multi-layer structured CL with graded distribution of Nafion ionomer. Specifically, Nafion ionomer content at CL/GDL interface should be lower compared to that at CL/membrane interface in cathode [7], [8], [9], [10], [11]. The reason can be attributed to the facilitated oxygen supply and water removal process in cathode without sacrificing the proton conductivity [7], [8], [9].
Regarding Pt loading in the cathode, in-plane [12], [13] and through-plane [14], [15], [16] gradients have been studied. The reactant gas concentration and current density will gradually decrease from the gas inlet to outlet, thus leading to a non-uniform utilization of the active area, which may have negative effects on the PEMFC lifetime. The current density uniformity and therefore the PEMFC performance can be improved through introducing an in-plane Pt gradient by increasing the Pt loading from cathode inlet to outlet [12], [13]. In terms of through-plane gradient, increasing the Pt loading from the outer side (near the gas diffusion layer, GDL) to inner side (near the PEM) is beneficial to the performance [14], [15], [16] [17]. Furthermore, the PEMFC performance can be significantly improved through combined optimization of both Pt and Nafion ionomer distribution in the cathode CL [15], [16]. Moreover, hydrophilic gradient catalyst layer with different kind of catalyst support improves water management of PEMFC in low-humidity conditions and at high current density [18]. In some cases the hydrophilic gradient design improved Pt utilization (above 50%), lowered CL/gas diffusion layer interfacial resistance and decreased mass transportation, thus yielding an output power density up to 0.76 W cm-2 with cathode Pt loading as low as 0.28 mg cm-2 [19].
In this paper, a double catalyst layer (DCL) cathode with both gradient Pt/C ratio and Nafion content is designed. The performance of DCL with different Pt/C ratio gradient direction and Nafion gradient span is compared to that of a single catalyst layer (SCL) in the cathode MEAs. Enhanced performance has been obtained by optimizing the gradient of Pt and Nafion in the DCL. We also investigated the effects of Pt/C ratio gradient and Nafion gradient under low humidity. With optimal gradient cathode the poor performance of the MEA under low humidity improved significantly due to the better catalysts utilization, mass transfer and water management.
Section snippets
MEA fabrication
In this study, the catalyst slurry was prepared by disperse Pt/C catalyst Johnson Matthey (JM) in isopropanol with 5 wt% Nafion solution (DuPont, USA). Nafion 211 (DuPont, USA) was used as the membrane. Then, the “heat-setting” ultrasonic sprayed approach was employed for MEA fabrication. 60 wt% Pt/C and 24.5 wt% Nafion content catalyst slurry were sprayed on the membrane for anode and the Pt loading was 0.12 mg cm−2. The cathode Pt loading was 0.2 mg cm−2 for both DCL and SCL. 60 wt% Pt/C was
Study of polarization curves
The MEAs polarization curves at 80 RH% and 60 °C were shown in Fig. 2. Noticeable differences can be found between MEAs with various Pt/C and Nafion loading profiles, especially in high current density region. The MEA 6 with gradient Pt/C ratio (70/40) and Nafion loading (33/23) outpaces other MEAs in the examined voltage range. The MEA 3 with gradient Pt/C ratio (70/40) but uniform Nafion loading (24.5/24.5) closely follows beneath the MEA 6. The MEA 4 with uniform Pt/C ratio (60/60) but
Conclusion
A DCL cathode with Pt/C ratio and Nafion gradient was fabricated to enable gradient design in this study. The new gradient cathode with appropriate Pt/C ratio gradient direction (70 wt% inner side and 40 wt% outer side) and Nafion gradient span (33 wt% inner side and 23 wt% outer side) improved both Pt utilization and mass transfer process, thus significant performance improvement was achieved. Especially under low humidity, the performance of appropriate gradient MEA was 135.7% higher than the
Acknowledgement
This work is financially supported by the National Science Foundation of China (NSFC) (Grant Nos. U1462112, 21573122), This work is financially supported by the National Key Research and Development Program of China (Program No. 2016YFB0101200 (2016YFB0101208)).
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