As the concern for finding alternative energy to power our industries grows, recent developments in fuel cell technology have made powering smaller and larger scale operations possible. Although the most prominent fuel cell in the media has been the hydrogen fuel cell, there are astonishingly many different types of this fuel cell which operate under different conditions for different purposes. Let’s take a closer at three of the most promising upcoming fuel cell types!
Fuel Cell Overview
Many of us have heard of or probably used an automobile that runs on a fuel cell, but rarely do we ever hear about how they work. Quite simply, a fuel cell is a type of voltaic cell that runs on the movement of electrons in one direction and the movement of ions in another direction, usually across a salt bridge that exists in the form of a membrane electrolyte. The reaction that occurs in a fuel cell is electrochemical in nature, and the class of reactions that occur are called redox reactions (reduction and oxidation). While reduction absorbs electrons on the other side of the cell, oxidation creates electrons that flow across the cell.1 Electricity is essentially defined to be the flow of electrons, so the electron flow created by reduction and oxidation reactions at cathode and anode respectively generate electricity from the fuel cell.
https://web.archive.org/web/20160404090950if_/http://www.youtube.com/embed/Dcomfck7xDgSo far then, a fuel cell sounds identical to a standard battery and it surprisingly is just a kind of battery that is fed with a fuel source instead of containing fuel within itself.2 There is always some type of input and output in a fuel cell system—the output is the reduced end product and the input is the oxidized starting material. For example, in a hydrogen fuel cell, the inputs are both hydrogen and oxygen. The hydrogen is oxidized to create H+ ions and electrons that move across a membrane and external circuit respectively while the oxygen is reduced by receiving electrons from the hydrogen to create H2O as a final product.3 Hydrogen, however, will not always readily oxidize at the anode and a catalyst is needed to assist the oxidation reaction in going to completion. Platinum is a popular catalyst, especially for Polymer Electrolyte Membrane (PEM) fuel cells, but platinum is one of the most expensive metals—going as high as $1550 USD per ounce at the end of August 2012.4 There are hopes for a future hydrogen economy as new research into cheaper metallic catalysts is reducing the cost of using fuel cells. For example, scientists at Lawrence Berkeley Labshave managed to come up with a molybdenum – oxo complex that might work suitably as a catalyst for a much reduced cost.5
1) PEM Fuel Cells
PEM (Polymer Electrolyte Membrane) is another name for a hydrogen fuel cell, where the name comes from membrane in the fuel cell that separates the cathode and anode. PEM fuel cells are essentially fueled with pure, highly pressurized stored hydrogen and possess several operational advantages. The operating temperature of the unit itself is fairly low—60 to 80 degrees Celsius with a high power density.7 PEM fuel cells are conceptually the easiest to grasp due to their relative simplicity. As a summary, the input is hydrogen gas, which is fed into the anode of the fuel cell where it is oxidized to generate electrons that flow in an external circuit across the fuel cell.8
One of the oxidized products, H+ protons, flow across a polymer electrolyte membrane that acts as a proton exchange membrane9 which is reduced by combining with oxygen to produce water as an end product. The initial oxidation step is assisted with a platinum catalyst that is coated along the anode of the fuel cell. Unfortunately this reaction in a single fuel cell can only produce 0.7 volts unassisted.10 Therefore, several cells must be stacked together to produce reasonable voltage. A way around this problem is bipolar plates,11 in which each plate acts as an anode and cathode that are made of graphite, carbon composites, or lightweight non-corroding metals. It is often difficult to come about pure hydrogen, so often a storage tank or reformer is used with vehicles using hydrogen fuel cells. Simply put, a reformer can generate hydrogen for the fuel cell from ethanol or gasoline. However, this process results in lower fuel efficiency as the reformer will produce heat and non-hydrogen byproduct gases and thus this is not the preferred mechanism for a fuel cell.
Non-Automobile Applications for PEM Fuel Cells: Although the use of hydrogen PEM fuel cells has largely been proposed for future automobiles, there are other applications that his type of fuel cell can serve.
- Communications Towers – companies such as ReliOn and Altergy provide fuel systems as primary or back up power units for telecom switch nodes that are stand alone, durable and long lasting. These fuel cells can provide between 1 – 5 Kw of power in a stacked arrangement and offer a good solution to remote telecommunications management.13
- Survival Portables – Many portable fuel cell units are being made for military and survival applications for emergency equipment and surveillance systems.14
- Warehousing – Interestingly, there are more than 3000 fuel cell forklifts in deployment for warehouse operations across the United States. The use of such fuel cells has been a great success, and major warehousing companies such as Walmart and Sysco are increasing their use of fuel celled forklifts. While the average battery powered forklift can sustain a mere 6 hours, hydrogen or methanol fuel cell forklifts can manage up to 12 – 14 hours in operation –with zero emissions ! Additionally, fuel cells can be used in frigid temperatures, the type of environment characteristic of packing perishables, in which a battery unit cannot sustain due to significant power drains.15
2) Solid Oxide Fuel Cells
As a fuel cell, the basic idea for solid oxide variants are still the same as a PEM fuel cell but the mechanism of operation is slightly different. The inputs and outputs are essentially the same as that of a hydrogen fuel cell and the same half reactions occur at the anode and cathode for oxidation and reduction with the same end product of water. The difference in nomenclature is completely derived from the type of electrolyte used in the fuel cell that is sandwiched between the cathode and anode. For PEM hydrogen fuel cells, the electrolyte was a polymer electrolyte membrane,16 which allowed for hydrogen protons (H+) to pass from the anode to the cathode. Solid oxide fuel cells use a nonporous, ceramic compound as an electrolyte, and usually these ceramics are metallic oxides of calcium or zirconium.17 Additionally, the migratory ion between the anode and cathode include oxygen ions18 as opposed to solely H+ ions. Another difference is in the temperature of operation and efficiency. First, these fuel cells operate at extremely high temperatures, usually around 1000 Celsius.19 Efficiency of the cell is also at 60%20, giving the cell an output of up to 100 Kw.21 Second, the high temperature of operation removes the need for a reformer in the fuel cell.22 There are currently two different designs for solid oxide fuel cells, which deviates from the exclusive planar arrangement needed for PEM fuel cells. Not only can a planar fuel cell be made using the solid oxide electrolyte, but the fuel cell can be shaped into a tubular design for specific operational parameters.23
Current Applications for Solid Oxide Fuel Cells: The high efficiency and power output of a solid oxide fuel cell makes it an ideal powering source for large scale industrial operations. Some examples include:
- Powering Buildings and Factories – A company known as Bloom energy has already begun selling fuel cell power units to companies such as Google, Bank of America, and Wal-mart. The idea is that Bloom’s solid oxide fuel cells would power corporate offices at about 8 to 10 cents a kilowatt hour—a fraction of the current cost !24
- Vehicle Power Units – Although the high operating temperature is a major concern with using solid oxide fuel cells in automobiles, many auto manufacturers are investing up to 4.5 billion in this type of fuel cell technology due to its versatility (no fuel reformer needed) and high energy output combined with zero emissions .25
3) Molten Carbonate Fuel Cell
Once again, from a basic standpoint, a molten carbonate fuel cell is a hydrogen fuel cell in disguise. Hydrogen is still the main input fuel and water is the output. The main difference, however, is the electrolyte and the operating temperature of such a fuel cell. The electrolyte is a carbonate salt mixture deposited on a ceramic lithium aluminum oxide matrix.27 Since the average operational temperature reaches 650° Celsius, the carbonate actually melts during operational temperatures and conducts ions between the anode and cathode.28 Molten carbonate fuel cells, however, operate at such high temperatures that carbon and oxygen ionize from the electrolyte and travel across the cell as carbonate ions that recombine to form carbon dioxide. But before you get worried, this carbon dioxide is channeled back into the fuel cell and I never released as an end product. Due to high operational temperatures, these fuel cells do not require a reformer to generate purer hydrogen for the fuel cell and can actually accept natural gas or steam for conversion into hydrogen .The news gets even better—the fuel cell also does not require expensive catalysts to operate due in part to high operating temperatures. These cells are highly efficient as well, granting up to 85% in conversion efficiency.29 While operating temperature is a great boon towards the technology, it is also a major source of concern. At such high temperatures, it is not uncommon for electrolytes to corrode. Scientists are currently researching materials that are able to avoid this key hindrance.30
Applications for Molten Carbonate Fuel Cells:
- Waste Gas Reforming – Excess natural gas or steam from factories and power plants can be fed into molten carbonate fuel cells to produce electrical power.31
- Large Scale Industrial Operation – like a solid oxide fuel cell, a molten carbonate fuel cell is capable of handling the relatively large power loads needed by industry. A company known as FuelCell energy is developing molten carbonate fuel cells and has installed 250 kW units in over 50 locations so far.32
- Shipping –Molten carbonate cells have been proposed to be used in backup power systems for seafaring ships. Although the system will not be capable of driving the motors, fuel cells can be used to power control systems, communication, lighting, and main auxiliary systems.33
4) Experimental Designs
Regenerative Fuel Cells: There is a good deal of research going into a type of closed loop fuel cell that is self-contained and does not require a fuel input. Instead, water will be electrolyzed into hydrogen and oxygen through electricity generated by solar panels. The water end product is then re-circulated back to the electrolyzer to repeat the process.34 Additionally, hydrogen fuel is marginally cheaper than storing electricity in batteries in high quantities.35 Therefore, not only can this type of fuel cell continuously generate power, but when combined with a hydrogen storage tank, it can also store energy when needed.
Microbial Fuel Cells: A radical new experimental fuel cell hopes to use enzymatic oxidation reactions in micro-organisms to generate electrons for the fuel cell with the help of organic mediator chemicals. The anode itself will be coasted with a nutritional medium to form a bio-film that transfers electrons to the anode. Furthermore, nano components on the bio film will facilitate electron flow between the micro-organism and the anode.36 These fuel cells are projected to operate between 68 to 104 degrees Fahrenheit with up to 50% efficiency, but with low wattage for powering smaller medical devices.37 For more on microbial fuel cell development, check out this article: Microbial Fuel Cell !
