Energy of Hydrogen

Archive for the ‘Fuel cells’ Category

         As I noted before chemical reactions in fuel cells take place on special porous electrodes (anode and cathode) activated by palladium (or other platinum group metals), where chemical energy of hydrogen and oxygen is efficiently converted into electricity. Hydrogen is oxidized on the anode and oxygen  is reduced on the cathode. Catalyst on the anode speeds up the oxidation of hydrogen molecules into hydrogen ions (Н+) and electrons. Hydrogen ions (protons) pass through the membrane to the cathode where the catalyst stimulates the formation of water out of protons, electrons and oxygen. Free electrons are conducted through the external circuit to produce direct current for various applications. Interruption of the circuit brings fuel cell to a standstill.

         Voltage in a separate fuel cell doesn’t exceed 1,1V. To achieve the required voltage fuel cells are consequently combined in stacks, and this stacks are connected in parallel to reach the required capacity. Every fuel cell produces heat as a by-product but one can use co-generation to recapture it.
          There are several types of fuel cells. They are usually differentiated by the type of fuel used, operating pressure and temperature, area of application. In the most wide-spread classification they are distinguished by the type of electrolyte material used as a medium for the internal transfer of ions (protons). The type of electrolyte determines the operating temperature on which the type of catalyst depends. The choice of fuel and oxidant for any fuel cell depends on their electrochemical activity (that is, the speed of electrode reaction), cost, and easiness of fuel and oxidant delivery and removal of reaction by-products. The main source of fuel for fuel cells is hydrogen, but fuel conversion process allows recovering hydrogen from other materials like methanol, natural gas, oil, etc.

• Alkaline Electrolyte Fuel Cells. The electrolyte in this fuel cell is concentrated (85 wt.) potassium hydroxide (KOH) in high temperature cells (~250ºC), or less concentrated (35-50 wt.) KOH for lower temperature (<120ºC) operation. In mid-1960s they were used for the Buran and Shuttle space vehicles. However, they have had relatively little success in terrestrial applications due to the high cost of producing high purity fuel and oxidiser streams, plus corrosion problems. Typical efficiency is 60%.
• Proton Exchange Membrane Fuel Cells. The electrolyte in this fuel cell is a solid polymer membrane (thin plastic film) that is an excellent ion (proton) conductor. High current density in these cells means low weight, volume and cost. Solid electrolyte makes easier the process of sealing in the fuel cells production, reduces corrosion and provides longer service life. Low operating temperature (below 100˚C) facilitates start-up and reaction to power requirements. These fuel cells are ideal for transport vehicles and small-scale stationary applications.
• Phosphoric Acid Electrolyte Fuel Cells. The electrolyte in this fuel cell is 100% concentrated phosphoric acid retained in a matrix which is usually silicon carbide. These fuel cells were the first to reach commercialization. Applications: stationary power plants in houses, hotels, hospitals, airports. Their efficiency exceeds 40% and may reach 85% when the by-product steam is.
• Molten Carbonate Electrolyte Fuel Cells. The electrolyte in this fuel cell is usually a combination of alkali carbonates, such as Na and K, which is retained in a ceramic matrix of LiAlO2. The fuel cell operates at about 600 to 700ºC thus allowing to use fuel directly, without any additional processing, and Ni may be used as a catalyst. These fuel cells offer higher electrical efficiencies than phosphoric acid fuel cells at around 60% plus the possibility of cogeneration (water heating) which makes overall efficiencies of 80% feasible. Reaction to any changes in the power requirement is slow; this is why they are suitable for applications where high power is needed constantly. At present there are numerous demonstration plants in the U.S.A. and Japan. One of American plants has a capacity of 1.8 MW.
• Solid Oxide Electrolyte Fuel Cells. The electrolyte in this fuel cell is a solid, nonporous metal oxide, usually Y2O3-stabilised ZrO2. Cells operate at 650 to 1000ºC where efficient conduction of anode seeking oxygen ions takes place. Operating temperatures are high enough to allow internal reforming and promote rapid kinetics with non precious materials. They are suitable for use in stationary power plants of large and very large scale. Overall efficiency is about 60%.

Most of recent solutions provided effective and the cleanest way to produce power with a whole number of primary energy sources is to use hydrogen and fuel cells. Hydrogen isn’t primary energy source unlike coal and natural gas. It is only energy carrier. The most effective conversion of hydrogen to energy provides with using fuel cells.

  A fuel cell works by catalysis, separating the component electrons and protons of the reactant fuel, and forcing the electrons to travel though a circuit, hence converting them to electrical power. The catalyst is typically comprised of a platinum group metal or alloy. Another catalytic process takes the electrons back in, combining them with the protons and the oxidant to form waste products (typically simple compounds like water and carbon dioxide).

In other words a fuel cell is an electrochemical energy conversion device. A fuel cell converts the chemicals hydrogen and oxygen into water, and in the process it produces electricity. The other electrochemical device that we are all familiar with is the battery. A battery has all of its chemicals stored inside, and it converts those chemicals into electricity too. This means that a battery eventually “goes dead” and you either throw it away or recharge it. With a fuel cell, chemicals constantly flow into the cell so it never goes dead — as long as there is a flow of chemicals into the cell, the electricity flows out of the cell. Most fuel cells in use today use hydrogen and oxygen as the chemicals.

The principle of the fuel cell was discovered by German scientist Christian Friedrich Schonbein in 1838. Based on this, the first fuel cell was developed by Welsh scientist Sir William Robert Grove in 1839.