Polymer Electrolyte Membrane Fuel Cells
The Polymer electrolyte membrane fuel cells (PEM) —also called proton exchange membrane fuel cells—deliver high-power density and offer the advantages of low weight and volume, compared with other fuel cells. Polymer Electrolyte Membrane fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum catalyst.
They need only hydrogen, oxygen from the air, and water to operate and do not require corrosive fluids like some fuel cells. They are typically fueled with pure hydrogen supplied from storage tanks or on-board reformers.
The Concept of Fuel Cell Technology
The anode (negative electrode) receives the hydrogen and the cathode (positive electrode) collects the oxygen. Fuel cell technology is twice as efficient as combustion in turning carbon fuel to energy. Hydrogen, the simplest chemical element (one proton and one electron), is plentiful and exceptionally clean as a fuel. Hydrogen makes up 90 percent of the universe and is the third most abundant element on the earth’s surface. Such a wealth of fuel would provide an almost unlimited pool of clean energy at relatively low cost. But there is a hitch.
With most fuels, hydrogen is bonded to other substances and “unleashing” the gas takes energy. In terms of net calorific value (NCV), hydrogen is more costly to produce than gasoline. Some say that hydrogen is nearly energy neutral, meaning that it takes as much energy to produce as it delivers at the end destination.
History of Fuel Cell
Sir William Grove, a Welsh judge and gentleman scientist, developed the fuel cell concept in 1839, but the invention never took off. This was during the development of the internal combustion engine that showed promising results. It was not until the 1960’s that the fuel cell was put to practical use during the Gemini space program. NASA preferred this clean power source to nuclear or solar power. The alkaline fuel cell system that was chosen generated electricity and produced drinking water for the astronauts.
High material costs made the fuel cell prohibitive for commercial use. The fuel cell core (stack) is expensive and has a limited life span. Burning fossil fuel in a combustion engine is the simplest and most effective means to harness energy, but it pollutes.
High cost did not discourage the late Karl Kordesch, the co-inventor of the alkaline battery, from converting his car to an alkaline fuel cell in the early 1970’s. He mounted the hydrogen tank on the roof and placed the fuel cell and backup batteries in the trunk. According to Kordesch, there was enough room for four people and a dog. He drove his car for many years in Ohio, USA, but the only problem, Kordesch told me in person, was that the car did not pass inspections because it had not tail pipe.
Polymer Electrolyte Membrane Fuel Cells (PEM) Technology:
Polymer electrolyte membrane fuel cells operate at relatively low temperatures, around 80°C (176°F). Low-temperature operation allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. However, it requires that a noble-metal catalyst (typically platinum) be used to separate the hydrogen’s electrons and protons, adding to system cost.
The platinum catalyst is also extremely sensitive to CO poisoning, making it necessary to employ an additional reactor to reduce CO in the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel. This also adds cost. Developers are currently exploring platinum/ruthenium catalysts that are more resistant to CO.
Fuel Cell Membranes
In order for a PEM fuel cell to operate, a Proton Exchange Membrane is needed that will carry the hydrogen ions, proton, from the anode to the cathode without passing the electrons that were removed from the hydrogen atoms. These polymer membranes that conduct proton through the membrane but are reasonably impermeable to the gases, serve as solid electrolytes (vs. liquid electrolyte) for variety of electrochemical applications, and are commonly known as Proton Exchange Membrane and/or Polymer Electrolyte Membranes (PEM).
For PEM fuel cell and electrolyzer applications, a polymer electrolyte membrane is sandwiched between an anode electrode and a cathode electrode. During electrochemical reaction, oxidation reaction at the anode generates protons and electrons; reduction reaction at the cathode combines protons and electrons with oxidants to generate water. To complete the electrochemical reaction, the proton exchange membrane plays a critical role that conducts protons from anode to cathode through the membrane. The proton exchange membrane also performs as a separator for separating anode and cathode reactants in fuel cells and electrolyzers.
Types of Fuel Cell Membrane
- Cation Exchange Membrane (CEM)
- Anion Exchange Membrane (AEM)
- Bipolar Membrane
Whats is the Difference Between Cation and Anion Exchange Membranes?
Cation Exchange Membranes (CEM) are usually comprised of a fluorinated polymer with sulfonic acid sites and have excellent ionic conductivity and thermal/chemical durability. Currently manufactured Anion Exchange Membranes (AEMs) can utilize various alkaline stable polymeric materials as the host material and come with various functional sites that conducts OH- or any other anionic species. Thermal/chemical durability of AEMs are in general lower compared to its CEM counterparts.
Polymer Electrolyte Membrane Fuel Cells (PEM) Fuel Cell Applications:
PEM fuel cells are used primarily for transportation applications and some stationary applications. Due to their fast startup time, low sensitivity to orientation, and favorable power-to-weight ratio, PEM fuel cells are particularly suitable for use in passenger vehicles, such as cars and buses.
Polymer Electrolyte Membrane Fuel Cells (PEM) Disadvantages
PEM Fuel cell with methanol reformer-CO resistant proton exchange membrane fuel cell system-onboard fuel cell processor-higher density liquid fuels
A significant barrier to using these fuel cells in vehicles is hydrogen storage. Most fuel cell vehicles (FCVs) powered by pure hydrogen must store the hydrogen on-board as a compressed gas in pressurized tanks. Due to the low-energy density of hydrogen, it is difficult to store enough hydrogen on-board to allow vehicles to travel the same distance as gasoline-powered vehicles before refueling, typically 300–400 miles.
Higher-density liquid fuels, such as methanol, ethanol, natural gas, liquefied petroleum gas, and gasoline, can be used for fuel, but the vehicles must have an on-board fuel processor to reform the methanol to hydrogen. This requirement increases costs and maintenance. The reformer also releases carbon dioxide (a greenhouse gas), though less than that emitted from current gasoline-powered engines.
Portable Fuel Cells of the Future
Portable fuel cells have gained attention and the most promising development is the direct methanol fuel cell. This small unit is inexpensive to manufacture, convenient to use and does not require pressurized hydrogen gas. The Methanol fuel cell has good electro-chemical performance and refilling is done by squirting in liquid or replacing the cartridge. This enables continued operation without downtime.
Manufactures admit that a direct battery replacement by the fuel cell is years away. To bridge the gap, the micro fuel cell serves as a charger to provide continuous operation for the onboard battery. Furthermore, methanol is toxic and flammable, and there are limitations to how much fuel passengers can carry on an aircraft. In 2008 the Department of Transportation issued a ruling to permit passengers and crew to carry an approved fuel cell with an installed methanol cartridge and up to two additional spare cartridges of 200 ml.
