Investigation on the reaction area of PEMFC at different position in multiple catalyst layer
Introduction
Proton exchange membrane fuel cell (PEMFC) has a merit of high energy density, high efficiency and environmentally friendliness and attracts an intensified attention in stationary and transportation applications [[1], [2], [3]]. However, its endurance and performance still need to be improved before mass production and the cost of PEMFC system should also be decreased at the same time [4]. Catalyst layer (CL) is a critical part in PEMFC MEA, where the electrochemical reaction happens and the chemical energy is transformed into electrical energy. Due to uneven distribution of water and reactant, there is high overpotential caused by diffusion and activation process in some area of CL. Many researchers have designed a gradient CL to improve the overall performance and endurance of the single cell through redistributing the ingredient in CL, such as Nafion resin, Pt loading and catalyst support, so that an appropriate distribution of porosity, hydrophobicity and Pt was realized in a so called gradient MEA [5]. Compared with traditional uniform MEA, gradient MEA has lower Pt loading and presents a better performance due to a well-designed distribution of Pt, porosity and hydrophobicity. Besides, it is much closer to actual production and the technology is much more mature in comparison to ordered MEA.
Gradient MEA usually refers to a gradient design of Pt ration in catalyst, Nation content and porosity in CL [6], a higher Pt ratio and Nafion content near PEM and a higher porosity at GDL side are preferred. Considering the complexity of production process, gradient MEAs were constructed as dual layer [[7], [8], [9], [10]] or a triple layers [11]. Experimental data showed that gradient MEAs with multiple layer presented a much better performance than MEAs with uniform CL at higher current. As for the endurance of PEMFC, performance degeneration caused by Pt dissolving during operation could also be mitigated through gradient design of Pt loading and size of Pt particle [12,13]. Gradient MEA with more layers was commonly studied through numerical simulation and similar conclusions are reached [14].
Besides experiments, mechanisms how gradient design improve the performance have also been studied by other researchers through numerical computation. Simulation results of oxygen concentration have shown that the area next to PEM has the lowest oxygen concentration [15,16], Similar conclusions about reactant concentration distribution have also been also achieved by many other researchers [[17], [18], [19]]. Ramin Roshandel [17] suggested that higher Pt loading should be used in CL where the reaction rate is the most rapid. According to the result of simulation using aggregate model, the performance of single cell can be improved through increasing the Pt loading near PEM. Simulation results of Omeiri [18] showed a higher reaction rate near PEM (a higher fraction of total current near the PEM).
In a gradient MEA, Pt and Nafion are redistributed to adapt the ununiform distribution of reactant and reaction rate to improve the performance of single cell. Thus, the distribution of oxygen concentration and current in cathode is important to gradient design of MEA. Many researchers have studied the distribution of reactant, current and overpotential through simulataion, but the conclusions are still not consistent with each other. In this work, a special CL structure with multiple layer was presented to study the performance of reaction area at different area of CL in through plane direction. The reaction area was set at different position in cathode CL and the process of electrochemical reaction was studied through EIS, CV and polarization curve, the structure of multiple CL has been shown in Fig. 1.
Section snippets
MEA fabrication
Shin [20] introduced a MEA with multiple CL in the study of the resistance and porosity of CL fabricated by catalyst ink with different IC (ionomer carbon) ratio and solvent composition. In this work, a newly designed multiple layer CL was introduced to study the reaction condition in the different area of CL. The multiple cathode layer consisted of three layers, layer X was next to the PEM and only responsible for proton transport, layer Z laid next to GDL and only affected air diffusion. Both
Polarization
The SEM and EDS of MCL samples' cross section are shown in Fig. 3 and Fig. 4. Fig. 3 shows that the total thickness of each sample's cathode layer (12, 40 and 60 Passes) is around 19 μm, and each layer's thickness of three samples has been listed in Table 4. The value in Table 4 showed the proportion of each layer (X, Y and Z) in whole cathode layer and was calculated according to the equivalent run times in Table 2. Since each layer was fabricated in the same process and used the same ink,
Conclusions
A special multiple catalyst layer was introduced into this work, in which the reaction area could move from PEM to GDL. The polarization curve and EIS of reaction area at different position in cathode CL was studied.
The reaction area near GDL showed much better performance (800 mA cm-2 @ 0.6 V) than the reaction area near PEM (530 mA cm-2 @ 0.6 V). Given the whole CL is equipotential, most of the current should be provided by the reaction area near the GDL in an ordinary MEA since the sample
Acknowledgment
This work was financially supported by National Natural Science Foundation of China (grant No.21776222), National Key Research and Development Program of China (2017YFB0102803).
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