Skip to main content

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.
 





Labels:

Comments

Post a Comment

Popular posts from this blog

Photovoltaics: Band Diagram

In the previous post we discussed silicon, which is the most used material in photovoltaics. In this post, we introduce the band diagram, for which we will use silicon as an example. We will start our discussion of the band diagram with the Bohr model of the silicon atom. In semiconductor materials the outer shell of the atom, which is called the valence shell, is not completely filled. The outer shell of silicon contains 4 out of the possible 8 electrons, which we call valence electrons. As we discussed in the previous post, each silicon atom in a crystalline structure is bonded to four other silicon atoms. The bonds between the silicon atoms are called covalent bonds. These bonds actually consist of two valence electrons that are shared by two silicon atoms. All valence electrons are fixed in the lattice, forming covalent bonds, and are therefore immobile. However, at a temperature above absolute zero, thermal energy is supplied to these miconductor and some of the vale...
Post a Comment
Read more

Heat Management in Fuel Cells

For a fuel cell to run efficiently, there needs to be proper control of its temperature and heat generation. Some fuel cells work well in room temperature, but others require temperatures as high as 1000 ºC, and any value outside of the accepted range results in lowered efficiency of the device. Higher temperatures lead to faster kinetics and voltage, and lower temperatures cause shorter warm-up times, lower thermodynamical stresses and retardation of corrosion and other temperature-dependent processes. For fuel cells, higher temperatures also mean greater vaporization of the liquid water and, as a result, more of the waste heat becomes the latent vaporization heat.  The temperature profile in a fuel cell is ever-changing, even when the flow rate of the gases is constant. That happens because of the transfer of heat and phase change of some reactants. The accurate prediction of the temperature and heat distribution is essential to determine temperature-dependent parameters a...
Post a Comment
Read more

Fuel Cell Charge Transport

Conduction is the process which dictates the transport of charges through the fuel cell layers, except for the membrane. That means that the lack of adequate contact between the diffusion layer, bipolar plates and cooling plates is the cause of most of the ohmic losses outside of the membrane. There is where most of the overall ohmic loss occur. To help solving that problem, either the membrane needs to be made thinner, or its material needs to be more conductive. History and tests given that, have told us that making the membrane thinner is easier than the alternative. The challenge with making the membrane material more conductive balancing that property with its thermal and chemical stability.  The image below describes the relationship between membrane thickness and local conductivity: Source: SPIEGEL, Colleen.  PEM Fuel Cell Modeling and Simulation Using MATLAB ® .  Burlington, MA, USA: Academic Press, 2008. 440 p.
Post a Comment
Read more