Hello there everybody! We are Team A
Tyler Emery – I wish to go to graduate school for Organic Chemistry
David Blihar – I am aspiring to be a neurosurgeon
Andrew Lange – I am aspiring to be a surgeon for the Military
Good luck to all of you and what you wish to accomplish in your life times.
Our first project will be fuel cells.
Our outline for the project will follow the format below:
1) Engineering details – how the hydrogen fuel cell is made
2) Thermodynamics principles – how the cell behaves according to thermodynamic laws and how much energy the hydrogen cell could produce
3) Best efficiency based on thermodynamics – whether or not the hydrogen fuel cell behaves under the carnot cycle and why or why not
4) Environmental Impacts – what are the environmental impacts of hydrogen and the hydrogen fuel cells
5) Personal view points – how we feel about using hydrogen fuel cells
We will work as a team on this project and determine who presents which part of the outline as time goes on.
To open our presentation of a hydrogen fuel cell, the general purpose of a hydrogen fuel cell engine will be examined. The path that hydrogen takes through a fuel cell will be outlined. Lastly, the engineering of a hydrogen fuel cell engine will be discussed.
A Hydrogen Fuel Cell is a power cell that reacts hydrogen gas with oxygen gas from the air to produce electrical work, water, and heat. In the fuel cell the heat can be recycled to increase the efficiency of the cell.
In a fuel cell, an engine blower blows in air to a stack of super thin hydrogen cells. The fuel cells are fed hydrogen gas from a fuel tank. The hydrogen enters the stack of hydrogen fuel cells through the ends. Each cell contains a channeled plate usually made of carbon. The carbon conducts electricity and increases the speed of electron transfer through the cells.
As hydrogen enters through the carbon channels, the gas encounters a chemically treated paper that conducts the gas to electricity. The hydrogen gas travels from the paper to a membrane that splits the gas into proton and electrons. Oxygen gas from the air enters the membrane the same way the hydrogen does.
The oxygen coming into the membrane reacts with the free protons to produce water. The water is drawn out of the cell by a pump. The electrons that were split from the hydrogen gas travel to the end of the stack of fuel cells to electrical wires.
A hydrogen fuel cell is made by taking the carbon channeled plate and stacking the chemically treated paper onto it, followed by the membrane, then another chemically treated paper, and lastly another carbon channeled plate. A hydrogen fuel cell engine is made by stacking cells onto one another like this until over a hundred cells are stacked together.
The paper and membrane contain a catalyst like platinum or tungsten sulfide. The catalysts split the hydrogen gas into protons and electrons. The catalysts also serve as the reaction site where drawn in oxygen gas reacts with the generated protons to form water.
The stacked fuel cells are compressed together using a hydraulic press in order to increase the efficiency of the current. The compression also forces the rubber seal of each fuel cell around each other that forms a sealed container. Engineers run nitrogen gas through each hydrogen fuel cell stack after the compression to make sure there are no leaks.
After the cells are compressed and secured together with reinforced steel rods, an electrical circuit board is installed to measure the voltage of each cell. The electrical circuit board is connected to the fuel cells by a silver containing adhesive to increase conductivity. The fuel cell engine has a hydrogen line, air line (for reaction), and water line for cooling.
In this section of the presentation, the thermodynamics of the hydrogen fuel cell will be examined. The hydrogen fuel cell generates electrical work so in order to look at the thermodynamics, the efficiency, and compare a fuel cell to a Carnot cycle the electrical work must be converted to a Gibbs free energy representation. To do this, we will start with classical work and electrical work.
Classical work is defined as the negative external pressure applied over a change in volume gives joules of work. Moreover, dw = -Pext(ΔV) where Pext is the external pressure in bar, ΔV is the change in volume in L, and a bar * L is 100 joules.
For the electrical work of a fuel cell, the work is defined as the potential of the system multiplied by the current of the change over time gives joules of work. Moreover, dw = E*I*dt, where E is the electrochemical potential of a chemical reaction in volts, I is the current of the electrons from the reaction in amps, and dt is the change in time in seconds. This equation for electrical work was derived using the unit relationships of (Volt)(Ampere)(second) = (Volt)(Coulomb) = joule.
Electrical work can be converted to Gibbs free energy by using the ideas of the first law of thermodynamics and the second law of thermodynamics. The first law states that the total energy of a system is the sum of energy lost in the form of heat plus energy in the form of work. Moreover, dU =dq + dw, where U is the total energy, q is the heat transfer, and w is the work done. Fuel cells operate under constant pressure so dw=-Pext(ΔV)=0. Therefore, dU=(dq)p and dq=dH using state function identities. So dH can be substituted back into the first law equation dH = dq – dw. In this equation work (dw) becomes negative to denote the flow of energy in the system. Moreover, if heat energy is positive then work energy has to be negative.
Continuing with the idea of electrical work, the potential multiplied by the current multiplied by the change in time equals the potential multiplied by the number of moles of electrons multiplied by Faraday’s constant. Moreover, E*I*dt = E*n*F or I*dt = n*F, where n is the number of moles of electrons and F is Faradays constant (96,485.3365 C/mol*e-). Therefore, dw = n*F*E by means of substitution. This electrical work equation can be substituted into the altered first law equation, dH = dq – n*F*E.
The second law of thermodynamics states that the enthalpy of a reversible system equals the entropy of the system multipled by the temperature. Moreover, the temperature dependent entropy is defined as TdS = dqrev, where T is the temperature of the system in Kelvin, dS is the change in entropy of the system in joules per Kelvin, and dqrev is the enthalpy of the reversible system in joules. So TdS can be substituted into the altered first law equation for q and, dH = TdS – n*F*E.
The fuel cell is assumed then to operate reversibly because of the constant pressure. So using the above equation for a fuel cell and the ideas of Gibbs free energy, the equation of Gibbs free energy can be derived. Knowing that dH = TdS – n*F*E and that the Gibbs free energy for an electrical system is dG = n*F*E = electrical work:
dH – TdS = dG = n*F*E
Therefore, the Gibbs free energy is derived dG = dH – TdS, where dG is the maximum amount of energy available to do work under perfect conditions in joules, dH is the thermal energy in reactants in joules, and TdS is the temperature dependent amount of heat produced by a fuel cell in joules.
In the following sections, we use this Gibbs free energy of a fuel cell idea above to show the efficiency of a fuel cell, that a fuel cell is not a Carnot cycle engine, and that a fuel cell is much more efficient than a Carnot cycle.
Efficiency of a hydrogen fuel cell deviates from traditional Carnot cycle efficiency because it relies on electrical work and not pressure volume work. The electrical work for a hydrogen fuel cell comes from the hydrogen fuel cell reaction:
2H2 (l) + O2 (g) –> 2H2O (g)
The energy source for a fuel cell is a liquid hydrogen fuel that is kept under high pressure so it will remain in the liquid phase. When the diatomic hydrogen enters the fuel cell it is stripped of its electrons via a catalytic reaction and the remaining protons travel through a guiding proton membrane. The potential difference caused by removing the electrons is what supplies the energy, not a combustion of materials. The hydrogen fuel cell process can be easily explained with the following graphics.
The hydrogen fuel enters the fuel cell and reacts at the surface of the catalysis.
The catalysis separates the diatomic hydrogen into protons and electrons. The protons travel through the membrane and the electrons travel through an electrolyte solution to create a potential difference.
The potential difference generated by the electron flow creates energy, in this case the energy is in the form of light.
Once the protons and electrons travel through the cell they recombine and are oxidized by the oxygen in the air.
The final product from the hydrogen fuel cell reaction is water in the form of exhaust. The reason water is the product is because there is no combustion that occurs, just a transfer of electrons aided by a catalysis.
Because there is no combustion occurring in a hydrogen fuel cell the efficiency does not follow the 1-(Tc/Th) Carnot cycle model, rather it is limited by the chemical reaction. The biggest factor affecting the chemical reaction is the catalysis and its ability to strip electrons from hydrogen. The best known catalysis for reactions involving hydrogen is platinum but the drawback is platinum is a precious metal and very expensive. The reason platinum is good at removing electrons from hydrogen is because it contains a high number of active sites where hydrogen can bind. The active sites on the surface of the catalysis is where the chemistry happens and where hydrogen loses its electrons, more active sites means a more efficient catalysis for the reaction. A new tungsten sulfide catalysis is being developed that will reduce the cost of the catalysis required for the fuel cell reactions, bringing the hydrogen fuel cell closer to reality. Compare the following SEM images of a platinum catalysis and the newly developed tungsten sulfide catalysis.
SEM image of platinum.
SEM image of tungsten sulfide.
The noticeable difference between the two images is that platinum contains far more active sites than tungsten sulfide. Because the tungsten sulfide contains fewer active sites it performs experimentally as a less efficient catalysis when compared to platinum. The great balance in the production of hydrogen fuel cells is cost versus efficiency with the biggest factor depending on the type of catalysis. Hydrogen fuel cells have produced efficiency numbers in the 60-80% range, compared to a combustion engine efficiency of approximately 25%. With the development of newer and cheaper catalysis hydrogen fuel cells will continue offering promising potentials.
The environmental impacts associated with hydrogen fuel cells are not straight forward and present complex implications. On one hand the only byproduct of the fuel cell reaction is water and on the other the current method for creating hydrogen gas relies on hydrocarbons, which is counter productive. There are several methods for generating hydrogen gas and a few will be listed with comments on their pros and cons.
Electrolysis of water: H2O (l) + current (amp) –> H2 (g) + O2 (g)
One method is the electrolysis of water to produce hydrogen gas. This method involves passing a direct current through liquid water to decompose the water molecule into gaseous hydrogen and oxygen. Although this is not the main method used to create hydrogen presently it is a method that could be a completely renewable cycle. By using hydro or wind power to generate the current needed for electrolysis this method has the potential to be completely fossil fuel free. One possible downside to this method is it would be hard to scale this process up to a size large enough to produce a substantial amount of hydrogen based on a currently only from hydro or wind power.
Steam reforming: CH4 + H2O ⇌ CO + 3 H2
The currently used method to produce hydrogen gas on a large scale is steam reforming with hydrocarbons such as methane. This method has the capacity to produce hydrogen on the scale required for large consumption but also requires hydrocarbons and produces green house gases in the process. Using a process that produces green house gases is a counter productive model when the ultimate goal is a cleaner and renewable energy source.
The hydrogen fuel cell faces some challenges on its path to becoming a viable energy source. Regardless of the method used to produce hydrogen fuel no method is 100% efficient at trapping and storing the hydrogen produced. Some scientific models suggest that 20% of the hydrogen produced is loss to the atmosphere during production and that lost hydrogen has the potential of contributing to the destruction of the ozone layer as much as any other green house gas. Because of this reason and the large reliance the United States industrial sector has on hydrocarbons, it would be a long road for the integration of the hydrogen fuel cell as a primary energy source in the near future.
My view point on hydrogen fuel cells is that they are the energy of the future. The fall backs of the fuel cell membrane catalyst price, the hydrogen volatility, and hydrogen explosiveness are all able to be overcome by making fuel cells mass producible. Catalysts can be made in cheaper fasions as the product of fuel cells gets more available and develops a financed backing that’s beyond research funding. The hydrogen fuel tanks are leak proof and designed to have a pressure that makes explosion and explosive decompression unlikely.
Even in light and the dangers of hydrogen, the fuel cells are so much more attractive of an energy source because they do not produce any waste. The generation of hydrogen also does not cause waste amount of damage to the planet because fossil fuels are not needed or extracted from the planet. I believe that we should put a lot more efforts into making hydrogen fuel cells more available to the public.
Hydrogen fuel technology has a great potential for providing green energy on a large scale but will have to overcome a few hurtles. My personal definition of clean is what justifies my classification of hydrogen as a green fuel source. I define hydrogen fuel as being clean because the reaction does not emit nitrogen oxides or carbon oxides in the process of producing the usable energy. The process of producing the actual hydrogen fuel via a clean method is one of the hurdles hydrogen fuel technology is facing. Another large hurdle is constructing a cheap and efficient catalyst for the hydrogen fuel reaction. Hydrogen fuel will not convert human habits of burning fossil fuels overnight but rather it is an alternative that promotes brainstorming. Only when people start thinking of energy in a way other than the classical concept of combusting finite resources will the world accept the host of alternatives available. The research and development of alternative energies has the potential of discovering completely new energy sources during the process. A shift in human habits is what is required to life the dependence of fossil fuels from the Earth.