Abstract
Platinum cluster formations have been investigated as a way to reduce the amount of Pt at the cathode of polymer electrolyte membrane fuel cells. One, two, and three layers of Pt (0.05 mg/cm2) sputtered directly on microporous layers of gas diffusion layers with and without interfacial carbon-Nafion layers and carbon-polytetrafluoroethylene (CPTFE) layers have been used as a cathode. Comparison with experimental results had showed that the best performance was obtained with three layers of Pt sputtered on carbon-Nafion containing 34.8 wt.% of Nafion and sputtered carbon-polytetrafluoroethylene containing 16.9 wt.% of polytetrafluoroethylene. High limiting current densities (>1.1 A/cm2) have been reached with cathode Pt loading as low as 0.05 mg/cm2. SEM imagery and cyclic voltammetry characterization have been performed to consolidate this study. High Pt utilization can be showed by this method. The factor influencing Pt utilisation in the oxygen reduction reaction is intrinsically related to Pt clusters formation and helps in enhancing the PEMFC performance with low Pt loading.
1. Introduction
Several researches have been done in the latest decade to reduce the amount of Pt in polymer electrolyte membrane fuel cells (PEMFCs) principally in the cathode to reduce its weight, volume, and cost [1–8]. The oxygen reduction reaction (ORR) occurring at the cathode of a PEMFC cell is catalyzed by high loadings of Pt to reduce significantly the overpotential loss. The activation overpotential is the principal loss, and its magnitude depends on the reaction kinetic parameters. More precisely, it depends on the size of the exchange current density [9]. Therefore, improving reaction kinetic performance is done by increasing the exchange current density, which depends on reactant concentration, activation barrier, reaction sites, and temperature. Those factors hinder large-scale commercialization and have motivated intense research to low loading of Pt, more active, inexpensive, and stable. Promising directions include metal alloys, notably Pt- and Pd-based inorganic compounds such as chalcogenides and organic compounds such as transition metal macro-cycles, have been investigated [10–13]. In addition of, the complexity of the catalyst layer has permitted the studies about coating methods, water management, Nafion content, and carbon supported Pt [6, 14–19]. It is obvious that the approach of Pt alloying with nonnoble transition metals can create some active alloy catalysts for catalytic activity enhancement towards ORR [12]. However, there are many concerns about the long-term stability of these Pt-alloy catalysts due to leaching issues of the metals under the cathode-operating environment [12]. A better utilization of low loading pure Pt must be fully investigated as it has the highest activity of elemental metals in order to improve catalyst layer performance.
Natarajan and Hamelin [20] have shown an increase in membrane electrode assembly (MEA) performance with three sputtered layers of Pt at the anode (total loading of 0.05 mg/cm2) as compared with a standard single layer of Pt MEA. In the present paper, we have investigated the Pt cluster formation to reduce the Pt loading at the cathode of a PEMFC while retaining high performances using a magnetron sputter technique. Different configurations of the active layer based on the distribution of the Pt sputtered on carbon-Nafion (CN) and carbon-polytetrafluoroethylene (CPTFE) dry inks are used to fabricate MEAs. The MEA performance is analysed by its current voltage characteristic. Microscopic visualization of the electrodes as well as cyclic voltammetry measurements is carried out to get the structural relationship, the cluster formation, and the distribution of Pt.
2. Experimental Protocol
2.1. Electrode Preparation and Electrochemical Characterization Procedure
A carbon-Nafion (CPTFE, resp.) mixture was prepared by adding carbon powder to a Nafion solution. A 10 wt.% Nafion (1 wt.% PTFE, resp.) solution (DuPont Inc.) was used as a proton conductive agent and a binder for the catalyst layer. The ratio of Nafion (PTFE, resp.) solution in carbon at the cathode was controlled to 1 : 4. A ratio 1 : 1 of Nafion solution in carbon was prepared for the anode. The mixture was magnetically stirred for at least 24 h. The ink is then applied on a 5 cm2 teflonized carbon paper (SGL, Sigracet) using the brushing method and dried for approximately 2 h in order to achieve a total carbon loading of 0.5 mg/cm². After drying, multiple layers of sputtered Pt were applied onto the substrate surface to reach a total loading of 0.05 mg/cm². The multilayer sputtering consisted of sputtered Pt on CN or CPTFE (Table 1). The electrodes obtained were used as cathodes. Anode electrodes with a Pt loading of 0.05 mg/cm² sputtered directly on the microporous layer (MPL) of carbon papers were used with all samples, except for samples EA2 and PA2 that have a CN ration of 1 : 1. Nafion 212 (DuPont) was used as the membrane. The MEA obtained was placed between two Furon gaskets. A hydraulic hot press was used to stick the MEA and the Furon under a 1000 psi pressure at 130°C during 2 min after being preheated during 1 min under atmospheric pressure. All assemblies were inserted between two graphite plates with serpentine flow fields and then placed in a single cell test fixture from Fuel Cell Technologies. Humidified hydrogen (80%) and air (80%) was fed into the cell at 200 sccm at the anode and 360 sccm at the cathode. The experiments were carried out at a cell temperature of 80°C. A standard MEA with a Pt loading of 0.2 mg/cm² for both of the anode and the cathode was also prepared for comparison.
2.2. Cluster Formation and Performance Analysis
In order to permit the formation of Pt clusters, multiple layers of Pt were sputtered on the surface of the substrates using Automatic Sputter Coater model NSC-3000 from Nano-Master [21]. Argon is first introduced into the chamber in order to provide an environment in which plasma could be ignited and maintained. When the plasma is ignited, positive ions strike the target cathode, which then released Pt atoms by means of momentum transfer. These Pt atoms form a vapor that condenses into a thin film on the substrate surface (located on the platen). This procedure is repeated one, two, or three times to get one, two, or three layers, respectively separated or not by CPTFE or CN layers until a total amount of Pt (0.05 mg/cm2) is reached for each sample. Sputtering takes place at 10−4 Torr where a better uniformity is achieved.
Scanning electron microscopy (SEM) was performed to analyse the morphology and the elemental compositions of the Pt clusters. Finally, cyclic voltammetry was performed on a 3 mm glassy carbon working electrode for different configurations of sputtered Pt in order to compare their active surface area.
3. Results and Analysis
3.1. Polarization Curves
Figure 1 shows the polarization curves for MEA1, MEA2, and MEA3 with the same low Pt loading. In the low current density region where activation losses are dominant, little difference can be seen between them. When comparing the cell voltages in the ohmic region, a visible difference can be seen between the three MEAs. The ohmic resistance increases with the number of sputtered layers showing that the distribution of Pt has an influence on electron and proton accessibility to reaction sites. This can be due to contact resistance between layers. In the concentration region, it is easy to see that MEA3 has a higher limiting current density (1.422 A/cm2) then MEA1 (1.017 A/cm2) and MEA2 (1.079 A/cm2). The maximum power (0.5517 W/cm2) was reached with MEA3 and is almost two times higher than the one with MEA1 (0.3272 W/cm2) with the same Pt loading. This gain of performance obtained with MEA3 needs to be pointed out here for that experimental result. This improvement may be due to an increase of surface reaction reached by the reactants at higher current densities caused by a higher porosity of the active layer. The pore volume has been calculated by BET analysis from nitrogen adsorption curves. Although this technique is not the most accurate for the scale of the average pore width (70 nm), it shows an augmentation of the pore volume for multiple Pt layers. The isolated Pt particles observed in an agglomerate catalyst that do not participate in electrochemical reactions maybe reduced due to the stacking of Pt before linking them with Nafion. Thicker active layers coupled to uniform dispersions of Pt clusters may also be the cause of the better polarization curve obtained with the three layers configuration. The presence of MPL, CN layers, layer interfaces, and the low Pt loading may prevent significant accumulation of liquid water at the reaction sites and does not impose severe mass transport limitations, either by blocking the gas pores or by covering up the reactive surface area. The diffusion of reactant to the reaction sites is noticeably improved with this multilayered electrode configuration.
Experiments were performed using the performance enhancements of Pt cluster formation shown in Figure 1 to compare the use of CN and CPTFE layers in between the Pt layers (Figures 2 and 3). The cathodes of EA1 and PA1, with a 34.8 wt.% of Nafion and 16.9 wt.% of PTFE, respectively, show best performances (Figures 2 and 3) than those of EA2 and PA2 with a CN ratio of 1 : 1 at the anodes (EA2 and PA2). It is important to point out that the reduction of the mass of Pt in the active layer results in an increase of its porosity. This increase in porosity enhances mass transport by increasing the effective oxygen diffusion coefficient and increasing the area available for oxygen dissolution. Thus, the Pt clusters formed at the cathode reaction sites are well supplied in oxygen, resulting in a high power density and high limiting current density (>1.2 A/cm2). The limiting current density achieved by EA1 is higher than that of PA1 due to the higher proton conductivity of Nafion. This ability promotes good proton conductivity in the cathode reaction sites leading to better performances. However, the comparison between the maximum power densities reached with MEA, EA, and PA series with standard MEAs, with a Pt loading of 0.2 mg/cm², reveals that the maximum power reached by standard MEA is not too much different from that of EA and PA series, as shown in Figure 4.
3.2. Microstructure Characterization Using SEM
SEM images of Figure 5 show the difference between the studied configurations of MEA1, MEA2, and MEA3. We can distinguish the formation of Pt clusters. The Pt clusters are significantly larger in Figures 5(c) and 5(e) than those in Figure 5(a). Remember that the distributions showed in Figures 5(a), 5(c), and 5(e) are formed by sputtering multiple layers of Pt for a total loading of 0.05 mg/cm2 (one layer of 0.05 mg/cm², two layers of 0.025 mg/cm² each, and two layers of 0.015 mg/cm² each plus a third layer of 0.02 mg/cm²). The gain in performance of these cluster configurations can be explained by the formation of larger Pt clusters with increased active surface area (as will be seen in Section 3.3). In Figure 5(c), the Pt is dispersed in the GDE that resulted from less cluster formation, resulting in isolated sites leading to a low active surface for the same amount of Pt.
(a)
(b)
(c)
(d)
(e)
(f)
3.3. Cyclic Voltammetry Measurement
To get more insight into the experimental results for the different performances observed between one, two, and three catalyst layers, cyclic voltammetry (CV) was performed using glassy carbon working electrode. The same total amount of Pt (0.05 mg/cm²) was sputtered on all glassy carbon electrodes for one, two and three layer configurations. The voltammograms are shown in Figures 6, 7, and 8 for one, two, and three layers of Pt. The electrochemical active surface (EAS) was calculated using the following equation:
where (mC/cm²) is the mean of charges exchanged during hydrogen adsorption and desorption, takes the numerical value 0.21 mC/cm² and represents the charge required to oxidize a monolayer of hydrogen, and is the Pt loading (mg/cm2).
The analysis of the results shows that the three layers of Pt have a greater electrochemical active surface (EAS) that of one and two layers as presented in Table 2. Those results reflect very well the difference in performance observed between MEA1, MEA2, and MEA3 (Figure 1). Thus, the effective areas for the ORR and HOR are influenced by the manner in which the Pt is distributed on the electrode.
4. Conclusion
The electrochemical characterization of MEA with one, two, and three layers of Pt (total loading of 0.05 mg/cm2) at the cathode showed that the best polarization curve performance was achieved with three layers. High limiting current density could be achieved using Pt cluster formation (>1.1 A/cm2) on CN and CPTFE layers between the Pt layers. Higher Pt utilization is responsible for this gain in performance due to the reduction of isolated sites. The mass transport has been enhanced due to the increase of limiting current density. The results also show that that the use of carbon-Nafion is better than the use of carbon-PTFE due to the decrease of ohmic loss, suggesting a better proton conductivity of Nafion than that of the PTFE solution. The maximum power reached by the use of CN or PTFE is similar to that obtained from the standard MEA.
Acknowledgment
The authors would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for their financial aid.