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PEMFC And The Future Of The Hydrogen Energy

The environmental problems that come from the utilization of fossil fuels are a great reason for the future implementation, in large scale, of different energetic matrices. Hydrogen is one of the most promising options, being the theme of intense scientific investigation geared towards a "Hydrogen Economy", a world scenario which consists of having hydrogen and electricity as its protagonists.

The hydrogen gas can be obtained through different processes, among some one can point out the electrolysis and the natural gas reformation, with the last one being responsible for over 48% of such worldwide production. The reformation of natural gas has, as an advantage, its low cost, but the hydrogen produced by that process has high levels of carbon monoxide contamination, which is a limiting factor for some applications.

The most efficient and cleanest form of extracting electrical energy from hydrogen is through fuel cells (FCs). Differently from internal combustion machines, FCs don't have their efficiency limited by the Carnot Cycle, but by the thermodynamics of the electrochemical reactions. In hydrogen's case:

H2(g) + ½ O2(g) -> H2O(l)     DG°(298K) = -237 kJ/mol, DH°(298K) = -286 kJ/mol


The efficiency, which is the result of dividing 237/286, is of 83% in reversible conditions (in real applications, the efficiency depends on the electrical current density of the FC), where H2O is the only compound that is generated in the process. Because of such characteristics, this technology is suitable for implementation in the transportation sector. In 2004, Toyota had developed a vehicle with a hydrogen fuel cell in conjunction with a nickel-metal hydride battery, and years before that, other automotive companies had built prototypes that followed similar ideas. Many nations, such as Iceland, Canada, Japan and Germany have already expressed their support for a transition towards a hydrogen economy based on the implementation of FCs in the automotive sector.

The Proton Exchange Membrane Fuel Cells were initially developed by the General Electrical Company in the USA and implemented as an energy source in Project Gemini, the second space exploration project from the National Aeronautics and Space Administration (NASA). This type of FC works with low temperatures (around 80oC). Because of that, it requires electrocatalysers with noble metals to accelerate the electrochemical reactions (this is their biggest limiting factor for their global implementation). The basic structure of the PEMFCs consists of two gas diffusion electrodes (GDEs) separated by an electrolytic membrane, combining to create a Membrane-Electrode Assembly (MEA). The GDEs are normally made of a noble metal dispersed in the form of nano-particles over a porous conductive substrate. The electrolytic membrane, which is currently used in PEMFCs, is made of Nafion, a sulfonated tetrafluoroethylene based fluoropolymer-copolymer. This membrane allows the operation of the fuel cell with a maximum current density of 1 Acm-2.

Reference:


LARMINIE, James; DICKS, Andrew. Fuel Cell Systems Explained. 2. ed. West Sussex, England: Wiley & Sons Ltd., 2003. 418 p.
 





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