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Fuel Cells

Fuel cells are a special type of galvanic cells. They can be fueled by solid, liquid, or gaseous fuel. The electrochemical oxidation of the fuel is coupled to energy gain, which is captured in form of electricity – as opposed to heat during chemical oxidation. Hence, fuel cells are direct energy converters with high efficiency. Most fuel cells achieve an energy conversion efficiency of 70-90%. If the conversion is 100%, no waste heat is produced. This ideal case of energy conversion is called ‘cold combustion’ which has been demonstrated for the first time in 1955 by Justi & Winsel. The fuel for this process is hydrogen gas, H2. It enters a porous nickel tube (gas diffusion electrode) where it is dissociated into protons and electrons according to:

H2 → 2 H+ + 2 e

Hydrogen fuel (H2) and oxygen (O2) are pumped into a fuel cell where two electrodes and the electrolyte fuse them to water.

During desorption, each H atom releases a proton (H+) and an electron (e). The electron is discharged onto the electrode, called anode, and the proton into the electrolyte. As a result of the dissociation process, the anode becomes negatively charged. On the second electrode, called cathode, oxygen gas, O2, is then charged with the electron and converted into O2− ions. The cathode becomes positively charged. Both electrodes are submerged in electrolytes which is in most cases a potassium hydroxide, KOH, solution of water. In the electrolyte, cations (H+) and anions (O2−) form water by chemical fusion. Theoretically, the efficiency is 92% accompanied by little waste heat – as opposed to normal combustion where heat of ~3,000ºC is produced.

2 H2 + O2 → H2O

Unlike heat power generators, fuel cells achieve high direct energy conversion efficiency because they avoid the additional step of heat generation. Besides shortcutting heat generation, fuel cells operate without mechanical parts and emit no noise, flue gas, or radioactivity, which puts them in focus of future developments. Due to their high energy efficiency and the high energy density of hydrogen, fuel cells are ideal for electric vehicles. In space flight, fuel cells were first used during Apollo Program between 1968 and 1972, in the Skylab Project 1973, the Apollo-Soyus Program, the Space Shuttle Program, and on board the International Space Station. There, they provide the electrical power for tools and water treatment. One benefit is that the final product of cold combustion in fuel cells is that water is the final product which is used by astronauts on their missions.

There are various types of fuel cells but all have in common that they consist of electrodes for fuel and O2 activation, and electrolytic conductors between these electrodes. Recent variations of fuel cells include methane fuel cells and microbial fuel cells. Due to the high activation energy of methane, methane fuel cells usually operate at high temperature using solid electrolytes. Microbial fuel cells, use microbes as anodic catalyst and organic matter in water as fuel. This makes them ideal for wastewater treatment.

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Corrosion is the chemical attack on a material leading eventually to its destruction if not stopped. It is caused by electrolytes, gases, solutions, or smelt. Corrosion occurs in different forms depending on the material under corrosive attack and the attacking agent. On metals, iron, for instance, its most visible manifestation is aerial or localized rust, such as needle holes in the surface. Crystalline corrosion of metals follows grain boundaries on surfaces. Corrosion is highly accelerated if the corroding material is in electrolytical contact with a more noble material. If this electrolytical contact is a liquid or humid substance, then corrosion is further accelerated. The reason is that the corroding material acts as anode (local cell) of a galvanic cell. Mechanical strain can accelerate corrosion as well.

A simple galvanic cell. The metal on the left side acts as anode and is dissolved into metal ions (M+). On the cathode water is transformed to hydrogen gas.

Corrosion protection is accomplished by coating the vulnerable material with corrosion resistant dense films. Such coating can be other metals such as zinc or chrome, as well as glazing, for example enamel. Protective paint is a wide-spread measure and is accomplished by adding pigments such as red (minium) or white lead, or organic substances. Tight plastic wrap is used as well. Iron is protected through transformation into stainless steel by adding chrome, nickel, etc.

The sacrificial anode is not a dissolving metal but soil or sewer organics. Microbes destroy these organics and produce CO2

If the material is exposed to water permanently, cathodic protection is frequently employed. To accomplish cathodic protection, the vulnerable material is connected to sacrificial anodes such as rods or plates that dissolve over time. Alternatively, directed current is used in many applications. Our patent pending solution provides a microbial anode that uses organic matter in soil or sewer as sacrificial anode. Instead of dissolving the metal, organic matter is degraded by microbes.

If a potentiostat is added to the galvanic cell, cathodic protection can be tailored to the protected material or the organics.

Besides metals, natural (wood, silk) and artificial polymers (plastics, rubber) can corrode as well. Softwood is generally more resistant than hardwood. Weak acids usually do no harm to wood. However, corrosion protection of wood is accomplished by painting or soaking it using protective agents. Artificial polymers rarely corrode as quickly as metals and if they do, a protective agent is mixed into the polymer formula at the time of its synthesis.