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| Fig. 1: PEM Fuel Cell Overview. (Source: EERE, courtesy of the U.S. Department of Energy) |
The United States - and the whole world - continues to rely on fossil fuels as their main source of energy. The US needs to push technological innovations in renewable energy because environmental concerns have raised, particularly in the western countries. The atmospheric CO2 reached a globally averaged concentration of 387.2ppm in 2009. [1] There are two ways to reduce carbon emission: 1) by economizing one or more alternative fuels, such as solar and wind, or 2) by capturing the CO2 itself and transforming it to another useful substance.
Catalysis essential for accelerating and directing chemical reactions. It is widely applied in almost all processes across all industries, and is one of the key factors to creating an environmentally friendly process for the conversion of fossil fuel as well as alternative energy sources. Most of the reactions required for energy-related reactions have already been studied. The technology to produce the reactions is hardly the issue - rather, the rate of the reaction and the cost has been the bottleneck over the past few decades. Finding a cost-efficient catalyst for a certain reaction can drastically increase the production of the reaction as well as lower the cost.
PEM fuel cell, a type of hydrogen fuel cell is one of the best future alternative energy sources because it has relatively low operating temperature, high power density, quick response, and pollution-free operation. [2] However, its relatively low power output compared to that of its price has prevented it from many practical applications.
Nanoparticles such as gold and platinum that are platelet shaped and have direct contact to the substrate are known to possess catalytic capabilities. [3] Furthermore, it has been shown that platelet shaped gold (Au) and palladium (Pd) nanoparticles could be synthesized through the two-phase method and spread on an air/water interface. [4]
There are two reactions that need to be catalyzed, which are shown as follows:
Figure 1 gives a schematic of how the fuel cell operates. On the input end, we need to accelerate the process of splitting hydrogen into protons and electrons, also known as increasing catalytic activity. At the output end, CO is formed, which severely poisons the electrode. As a result, we also need to catalyze the reaction that combines CO with H2O to form H2 and CO2, also known as reducing poisoning.
On increasing catalytic activity, it has been shown that decreasing the size of the particles result in an increase in fuel cell performance due to the increase surface area to volume ratio. In addition, one study shows that high-index facets of platinum nanoparticles results in higher oxygen reduction than normal platinum nanoparticles. [5] On the reducing poisoning side, one study reported that cube-shaped platinum nanoparticles with (100) faces resulted in a higher power output than randomly faceted platinum nanoparticles of similar size. [6] This technology is currently still in the research phase, because gold and palladium are very expensive materials. As a result, it is still impractical to mass produce this technology for industrial purposes.
© Chun-Kai Kao. The author grants permission to copy, distribute, and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] P. Friedlingstein et al., "Update on CO2 Emissions," Nature Geoscience 3, 811 (2010).
[2] A. Saccà et al., "Structural and Electrochemical Investigation on Re-Cast Na-ion Membranes for Polymer Electrolyte Fuel Cells (PEFCs) Application," J. Membrane Sci. 278, 105 (2006).
[3] A. Cho, "Connecting the Dots to Custom Catalysts," Science 299, 1684 (2003).
[4] Y. Sun et al., "Characterization of Palladium Nanoparticles by Using X-ray Reflectivity, EXAFS, and Electron Microscopy," Langmuir 22, 807 (2006).
[5] N. Tian et al., "Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity," Science 316, 732 (2007).
[6] C. Wang et al., "A General Approach to the Size- and Shape-Controlled Synthesis of Platinum Nanoparticles and Their Catalytic Reduction of Oxygen," Angew. Chemie Intl. Ed. 47, 3558 (2008).