Chapter Conclusions and Recommendations 6.1 Conclusions In this thesis an efficient fabrication method has been developed to synthesize integrated Pt/CNT/carbon paper-based electrodes for high efficiency PEMFCs. This fabrication method consisted of a combined process of in situ growth of CNTs onto carbon paper and direct sputter-deposition of Pt catalysts onto CNTs. It was found that a dense CNT layer was formed on the carbon paper after growth via the thermal CVD process. The CNT layer showed extremely high porosity and roughness to provide a tremendously enlarged surface area for Pt catalysts compared to that of a pristine carbon paper. In addition, Raman characterization demonstrated that the CNTs synthesized were multi-walled carbon nanotubes with a large amount of disordered carbon on surface. This is a favorable characteristic for effective Pt deposition. Pt catalysts were sputter-deposited subsequently by radio-frequency magnetron sputtering technique, which allowed direct deposition of Pt catalysts onto the electrode-electrolyte interface. TEM micrographs indicated that the sputterdeposited Pt catalysts were nanosized Pt particles well-dispersed on the CNT surface. Furthermore, it was found that the Pt nanoparticles were mostly in the pure metal state according to XPS deconvolution results. In contrast, the Pt catalysts synthesized by wet chemical methods in previous studies revealed a high proportion of Pt oxides where a reduction process was required after Pt deposition. A systematic study on the CNT growth and Pt sputter-deposition processes was conducted to optimize the combined fabrication method. The CNTs grown by CVD technique showed tunable morphologies by altering the flow rate of carbon feedstock 151   gas. It was found that an increase in the gas flow rate yielded a higher density and smaller diameter of the in situ grown CNTs, as well as an immensely larger surface area. Enhanced surface area could have increased the distribution area of Pt catalysts; however, a very dense structure of the CNT layer obtained at high gas flow rate may have hindered the penetration depth of the sputter-deposited Pt particles. The Pt distribution area was thus balanced by the density and surface area of the CNT support layer. An optimum gas flow rate for improving Pt distribution area was found to be 20 sccm, from which a maximum power density of 680 mW cm-2 was obtained based on a 0.04 mgPt cm-1 catalyzed Pt/CNT-cathode. In addition, different sputtering input powers were investigated on their influence on electrode performance. Results indicated that decreasing sputtering input power enhanced the cell performance of the Pt/CNT-based electrode by improving Pt dispersion on the CNT surface. However, a low sputtering power required a greatly prolonged sputtering process for the desired Pt loading thus it is much less efficient. The optimized sputtering input power was 50 W and the corresponding sputtering time was 240 s to reach a total Pt loading of 0.04 mg cm-1. To validate the electrochemical activity and stability of the Pt/CNT catalyst, a series of in situ PEMFC tests were performed in a single cell test system. When compared with two commercial Pt/VXC72R-based electrodes, the Pt/CNT-based electrode showed a notable improvement in polarization performance with an ultra low Pt loading of 0.04 mg cm-2 at both the anode and cathode. In situ CV confirmed that the Pt/CNT catalyst had a higher active Pt surface area compared to that of the commercial Pt/VXC72R catalyst. The results are in good agreement with previous studies where the Pt/CNT catalysts were extensively studied in ex situ tests in 152   simulated PEMFC environments. Moreover, EIS studies revealed that the integrated Pt/CNT catalyst layer greatly reduced the charge transfer resistance and mass transport resistance of the electrode, leading to a significantly enhanced polarization performance. The reduced charge transfer resistance and mass transport resistance are mainly due to the effective interaction between the Pt catalyst and CNT support. Furthermore, several accelerated degradation tests (ADT) were carried out to evaluate the electrochemical stability of the Pt/CNT catalyst under real fuel cell conditions. It was found that the Pt/CNT catalyst was visibly more stable than the Pt/VXC72R catalysts in all these ADT tests. This improvement can be attributed to the higher graphitization of the CNT support than the carbon black VXC72R. The ADT results also suggest that the in situ ADT method by means of potential cycling between 0.6 V and 1.8 V is able to expedite the durability evaluation of carbon-supported Pt composite catalysts in an acceptable time domain. The main contribution of this study is that the combined fabrication method can provide an efficient solution to the synthesis of Pt/CNT catalysts with reliable catalytic performance for PEMFC applications. The Pt/CNT catalyst prepared by the combined method has demonstrated a remarkable improvement in the electrochemical activity and stability of the Pt/CNT catalyst under real fuel cell conditions. It has provided a substantial support to previous studies on Pt/CNT catalysts where the electrochemical tests were mostly performed in various simulated PEMFC environments. Unlike other studies also using in situ grown CNTs as Pt catalyst support, this combined fabrication method shows a major advantage that the CNT layer grown by the CVD technique is extremely dense and porous to serve as both a gas diffusion layer and catalyst layer excluding an additional gas diffusion layer or a 153   surface oxidation pre-treatment prior to Pt deposition. Owing to the direct deposition of Pt nanoparticles onto electrode-electrolyte interface by the sputtering technique, the combined fabrication method has another key advantage that the highly localized Pt catalysts considerably enhance the Pt utilization to obtain low Pt-contained electrodes with high catalytic efficiency. In addition, the sputtering technique provides a great potential for the mass production of high efficiency PEMFC electrodes. However, a drawback of the sputtering technique lies in the fact that the penetration depth of the sputtered Pt particles is constrained in a few microns at the electrode surface thus limiting the overall Pt surface area. Therefore, cell performance showed only slight improvement when higher Pt loadings were sputter-deposited onto the electrode. The Pt utilization was diminished with increasing Pt loading over 0.04 mg cm-1. This technique is thus more suitable for fabricating low Pt loading electrode with high Pt utilization for PEMFC applications. 6.2 Recommendations for Future Research The in situ grown CNT layer has exhibited to be an effective support for the sputter-deposited Pt catalyst; therefore, developing novel bimetallic or trimetallic composite catalysts that are deposited on the CNT layer by means of sputtering can be considered as future research directions to further enhance Pt utilization and reduce Pt content to further lower the economic cost of PEMFCs. Previously, there were a large number of studies that investigated the electrochemical activity of various Pt-based bimetallic alloys using bulk Pt-alloy materials [1-8]. Ross [1, 2] and Landsman [3] reported improved ORR activity in phosphoric acid fuel cells from various Pt3M (M denotes the first-row transition metal, such as Ti, V, Cr, Fe, Ni and Co) alloys, with an ordered fcc structure of the AuCu3 type, with regard to the crystal structure of 154   platinum. Thus Pt3M catalysts were widely fabricated and evaluated in various simulated PEMFC environments for PEMFC applications. Jalan et al. [4] investigated the Pt-alloys supported on carbon black also in phosphoric acid fuel cells and found that the enhancement of Pt catalytic activity in Pt-based alloys could be attributed to the lattice contraction by alloying Pt with a base-metal with a smaller lattice constant. In view of this result, they claimed that the shortening of the Pt interatomic distance was favorable in the catalysis of the ORR. Their assumption was then supported by Appleby and Mukerjee’s studies where smaller Pt-Pt bond distances were observed to yield more active sites and enhance the dissociative adsorption of oxygen [5, 6]. Furthermore, Glass et al. [7] claimed that this enhancement only occurred in highly dispersed Pt alloy catalysts but bulk alloys. Later Paffet et al. [8] proposed that this phenomenon was associated with the surface roughening effect that the total Pt surface area was increased by Pt enrichment on surface due to the dissolution of basemetals into the acidic PEMFC electrolyte. In previous studies Pt-based alloys were obtained mostly by two synthesis methods where carbon black was usually used as the catalyst support. In the first method, simultaneous deposition of Pt and a base-metal on the carbon support was achieved by concurrent reduction of the two metal precursors [2-5]. In the second synthesis method, a base-metal was first chemically reduced and deposited onto a carbon black supported Pt catalyst, and the carbon black supported bimetallic catalyst was then alloyed into a Pt-based alloy catalyst by annealing it at an elevated temperature [6-8]. These synthesis methods are basically wet-chemical processes where difficulties were found in the control of the Pt dispersion on carbon support as well as the compositional homogeneity of Pt and base-metals [2-8]. Moreover, 155   although it was reported that several non-noble transition metals, such as Fe, Ni, Co, are promising alloying elements to improve the electrochemical activity of Pt catalyst in simulated fuel cell environments [9, 10], this improvement has not been verified by in situ fuel cell tests, probably due to the difficulties in synthesizing nanosized Ptalloy catalysts through conventional metallurgy processes. As proposed for future research plans, the multi-metal composite catalysts can be obtained by co-sputtering different metal sources concurrently onto the in situ grown CNT support. Particularly, the co-sputtering technique is able to deposit Pt-alloy nanoparticles onto the in situ grown CNT layer as demonstrated for the Pt catalysts in this study. In addition, it is possible to adjust the composition of different metal elements by controlling their respective sputtering rate. Therefore, the combination of CVD and sputter-deposition techniques can provide a versatile fabrication method to synthesize a variety of Pt/CNT-based electrodes for future PEMFC research and applications. These Pt/CNTbased electrodes containing Pt-based alloy catalysts with much reduced Pt content can considerably enhance the cost-effectiveness of PEMFCs and thus boost their commercial viability. 156   References [1] P. N. Ross, Proceedings of the National Fuel Cell Seminar, San Diego, CA, 14 (1980). [2] P. N. Ross, EPRI Final Report, Project 1200-5, March 1980. [3] D. A. Landsman, and F. J. Luczak, U.S. Patent 4,316,944 (1982). [4] V. Jalan, and E. J. Taylor, J. Electrochem. Soc., 130, 2299 (1983). [5] A. J. Appleby, Energy, 11, 13 (1986). [6] S. Mukerjee, and S. Srinivasan, J. Electroanal. Chem., 357, 201 (1993). [7] J. T. Glass, G. L. Cahen, and G. E. Stoner, J. Electrochem. Soc., 134, 58 (1987). [8] M. T. Paffet, G. J. Beery, and S. Gottesfeld, J. Electrochem. Soc., 135, 1431 (1988). [9] U. A. Paulus, A. Wokaun, and G. G. Scherer, J. Phys. Chem. B, 106, 4181 (2002). [10] V. Stamenkovic, T. J. Schmidt, P. N. Ross, and N. M. Markovic, J. Phys. Chem. B, 106, 11970 (2002). 157   .  Chapter 6 Conclusions and Recommendations 6. 1 Conclusions In this thesis an efficient fabrication method has been developed to synthesize integrated Pt/CNT /carbon paper -based electrodes for high. This technique is thus more suitable for fabricating low Pt loading electrode with high Pt utilization for PEMFC applications. 6. 2 Recommendations for Future Research The in situ grown. of a pristine carbon paper. In addition, Raman characterization demonstrated that the CNTs synthesized were multi-walled carbon nanotubes with a large amount of disordered carbon on surface.