Células de combustível de membrana de troca de prótons
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Fonte: Laboratórios de Margaret Workman e Kimberly Frye – Universidade Depaul
Os Estados Unidos consomem uma grande quantidade de energia – a taxa atual é de cerca de 97,5 quadrilhões de BTUs anualmente. A grande maioria (90%) dessa energia vem de fontes de combustível não renováveis. Essa energia é utilizada para eletricidade (39%), transporte (28%), indústria (22%) e uso residencial/comercial (11%). Como o mundo tem um fornecimento limitado dessas fontes não renováveis, os Estados Unidos (entre outros) estão expandindo o uso de fontes de energia renovável para atender às necessidades energéticas futuras. Uma dessas fontes é o hidrogênio.
O hidrogênio é considerado uma fonte potencial de combustível renovável, porque atende a muitos critérios importantes: está disponível no mercado interno, tem poucos poluentes nocivos, é eficiente em energia e é fácil de aproveitar. Embora o hidrogênio seja o elemento mais abundante no universo, ele só é encontrado em forma composta na Terra. Por exemplo, é combinado com oxigênio na água como H2O. Para ser útil como combustível, ele precisa estar na forma de gás H2. Portanto, se o hidrogênio deve ser usado como combustível para carros ou outros eletrônicos, o H2 precisa ser feito primeiro. Assim, o hidrogênio é frequentemente chamado de “porta-energia” em vez de “combustível”.
Atualmente, a maneira mais popular de fazer gás H2 é a partir de combustíveis fósseis, através da reforma a vapor de hidrocarbonetos ou gaseificação de carvão. Isso não reduz a dependência de combustíveis fósseis e é intensivo em energia. Um método menos utilizado é por eletrólise da água. Isso também requer uma fonte de energia, mas pode ser uma fonte renovável, como energia eólica ou solar. Na eletrólise, a água (H2O) é dividida em suas partes componentes, gás hidrogênio (H2) e gás oxigênio (O2), através de uma reação eletroquímica. O gás hidrogênio feito através do processo de eletrólise pode então ser usado em uma célula de combustível de Membrana de Troca de Prótons (PEM), gerando uma corrente elétrica. Esta corrente elétrica pode ser usada para alimentar motores, luzes e outros dispositivos elétricos.
Principles
Procedure
Results
During the electrolysis procedure, hydrogen and oxygen gas are generated once the solar panel is connected and exposed to sunlight. It takes approximately 10 min to generate enough H2 gas to fill the inner cylinder (Table 1). Note that there is twice as much H2 generated as O2, as seen in the balanced equation:
2 H2O(l) → 2 H2(g) + O2(g)
Once the H2 gas is generated and the tubing is connected to the fuel cell, the fuel cell generates electricity and causes the fan to spin. This lasts approximately 10 min on a full cylinder of H2 gas.
| Time (s) | Hydrogen Generated (mL) | Oxygen Generated (mL) |
| 0 | 0 | 0 |
| 30 | 4 | 2 |
| 60 | 8 | 4 |
| 90 | 10 | 6 |
| 120 | 12 | 6 |
| 150 | 14 | 6 |
| 180 | 14 | 8 |
| 210 | 16 | 8 |
| 240 | 18 | 8 |
| 270 | 20 | 10 |
| 300 | 22 | 10 |
| 330 | 22 | 10 |
| 360 | 24 | 12 |
| 390 | 24 | 12 |
| 420 | 26 | 12 |
| 450 | 26 | 14 |
| 480 | 28 | 14 |
| 510 | 28 | 14 |
| 540 | 28 | 14 |
| 570 | 30 | 16 |
| 600 | 30 | 16 |
Table 1: Time Required for Generating Different Hydrogen and Oxygen Quantities
Applications and Summary
Hydrogen is a flexible fuel. It can be produced on-site in small quantities for local use or in large quantities at a centralized facility. The hydrogen can then be used to produce electricity with only water as a byproduct (provided a renewable source of energy, like a wind turbine, was used to generate the hydrogen gas). For example, in Boulder, Colorado, the Wind2H2 project has wind turbines and solar panels connected to electrolyzers that produce hydrogen gas from water and then stores it to be used in their hydrogen fueling station.
This process can also be used to make cars run on hydrogen gas (H2) instead of fossil fuels. If a PEM fuel cell is installed in a car, electricity can be used to make the motor run. The only exhaust would be water (H2O). From an air pollution perspective, this is advantageous. There are many prototype fuel cell cars being developed by major car manufacturers. Due to the amount of space currently required to store the compressed hydrogen tanks on a vehicle, hydrogen fuel cells are mainly seen on buses. Fuel cell buses can be found in several countries around the world. There are some technological issues that need to be addressed before fuel cell cars are a viable alternative to internal combustion engine cars including providing more infrastructure, reducing costs, and an increased use of renewable energy sources when making H2 gas.
In addition, hydrogen fuel cells can be used in place of batteries for things like video cameras and radios. An example is the UPP device, which is a portable power pack based on hydrogen fuel cell technology that can be used to charge USB compatible devices.
내레이션 대본
Fuel cells are devices that transform chemical energy to electrical energy, and are frequently used as a clean, alternative energy source.
Although gasoline is still the primary fuel source for vehicles in the US, alternative fuel sources have been explored in recent decades in order to decrease dependence on fossil fuels, and generate cleaner sources of power.
Hydrogen fuel cells utilize clean hydrogen as fuel, and produce only water as waste. Though they are often compared to batteries, fuel cells are more similar to automobile engines, as they cannot store energy and require a constant source of fuel in order to produce energy. As a result, a significant amount of hydrogen is needed for constant fuel cell operation.
This video will introduce laboratory-scale electrolysis of water to produce hydrogen gas, followed by the operation of a small-scale hydrogen fuel cell.
Hydrogen is the most abundant element in the universe. On Earth, it is primarily found in compounds with other elements. Therefore, in order to use elemental hydrogen as a fuel, it must be refined from other compounds. Most hydrogen gas is produced through the energy-intensive methane reforming process, which isolates hydrogen from methane gas. However, this process is extremely energy intensive, utilizes fossil fuels, and results in significant quantities of waste gases. This contributes to climate change, and also poisons fuel cells and diminishes operability.
The electrolysis of water is an alternative method for producing clean hydrogen gas, meaning hydrogen that is free of contaminant gases. In electrolysis, water is split into hydrogen and oxygen gas, using an electric current. To do this, an electrical power source is connected to two electrodes, which are made of an inert metal. The electrodes are then placed into the water, and electrical current applied. For small-scale electrolysis, a battery or small solar panel can be used to generate enough current to split water. However in large-scale applications, higher energy-density sources are required.
The electrolysis reaction is an oxidation-reduction, or redox, reaction. There are twice as many hydrogen molecules produced as oxygen molecules, according to the balanced chemical reaction. The hydrogen gas generated from this electrochemical reaction can be collected and stored for use as fuel in a fuel cell. A proton exchange membrane, or PEM, fuel cell transforms chemical energy, or hydrogen gas, to electrical energy. As with electrolysis, the PEM fuel cell employs a redox reaction. Hydrogen gas is delivered to the anode of the fuel cell assembly, where it is oxidized to form protons and electrons.
The positively charged protons migrate across the proton exchange membrane, to the cathode. However, the negatively charged electrons are unable to permeate the membrane. The electrons travel through an external circuit, providing electrical current. Oxygen gas is delivered to the cathode of the fuel cell assembly, where the reduction reaction occurs. There, the oxygen reacts with the protons and electrons that were generated at the anode, to form water. The water is then removed from the fuel cell as waste.
Now that the basics of fuel cell operation have been explained, let’s look at this process in the laboratory.
To begin the procedure, setup the electrolyzer and the two gas collection cylinders. Fill the outer containers with distilled water to the zero mark. Place the gas collection cylinders in the outer containers.
Next, connect the electrolyzer to the gas collection cylinders using tubing. Connect a solar panel to the electrolyzer using jumper wires. Place the solar panel in direct sunlight in order to power the production of hydrogen gas. If there is not enough natural light, simulate sunlight using a lamp.
Hydrogen and oxygen gas will begin entering the inner gas collection cylinders. Monitor the volume of each gas produced in 30-s intervals, using the scale marked on the outer cylinder.
When the inner cylinder is completely full of hydrogen gas, bubbles will emerge from the inner cylinder, eventually reaching the surface. At this point, disconnect the solar panel from the electrolyzer and close the cincher on the hydrogen gas tube, so none of the hydrogen gas escapes. Note there is twice as much hydrogen gas produced as oxygen gas, as predicted in the balanced chemical equation.
To begin fuel cell operation, set the fuel cell on the bench top. Disconnect the hydrogen gas tubing from the electrolyzer and connect it to the fuel cell. The oxygen required is collected from the air.
Connect the fuel cell to a fan or LED light in order to visualize power generation. Release the cinch on the hydrogen gas tube to enable gas flow to the fuel cell. If the fan does not begin spinning, press the purge valve on the fuel cell to encourage gas flow.
The fan will continue to spin until all of the hydrogen gas is consumed.
There are many different types of fuel cells that are being developed as clean energy solutions. Here we present three emerging technologies.
Solid oxide fuel cells, or SOFC’s, are another type of fuel cell, which operate similarly to a PEM fuel cell, except the permeable membrane is replaced with a solid oxide. As with PEM fuel cells, operability of SOFC’s decrease upon exposure to contaminant gases containing sulfur and carbon. In this example, SOFC electrodes were fabricated, and then exposed to typical operating environments at high temperature in the presence of sulfur and carbon contaminated fuel.
Electrode surface poisoning was studied using electrochemistry and Raman spectroscopy. The results showed that current was diminished upon sulfur poisoning, but that recovery was possible. Atomic force microscopy studies elucidated the morphology of carbon deposits, which may lead to further development to prevent this poisoning.
A microbial fuel cell derives electrical current from bacteria found in nature. In this example, bacteria acquired from wastewater treatment plants were grown, and used to culture biofilms. A three electrode electrochemical cell was set up, in order to culture bacteria on the surface of an electrode. The biofilm was grown electrochemically in several growth cycles.
The resulting biofilm was then tested for extracellular electron transfer electrochemically. The electrochemical results were then used to understand electron transfer and the potential application of the biofilm to microbial fuel cells.
Electrolysis requires energy to break water into hydrogen and oxygen. This process is energy intensive on the large scale, but can be operated on the small scale using a solar cell.
An alternative energy source for electrolysis is wind power. In the laboratory, electrolysis can be powered with a bench-scale wind turbine. In this demonstration, the wind turbine was powered using simulated wind generated by a tabletop fan.
You’ve just watched JoVE’s introduction to the PEM fuel cell. You should now understand the basic operation of a PEM fuel cell and the generation of hydrogen gas via electrolysis. Thanks for watching!
