Fuel Cells & RENEWABLE ENERGY

 

FUEL CELL          BATTERY &  FUEL CELL       GREEN ALGAE           BIO OIL

      BIOELECTROCHEMICAL CELL                  Back to Material Science

Types of fuel cells

Polymer Electrolyte Membrane (PEM)         Phosphoric Acid        Direct Methanol      

Alkaline       Molten Carbonate      Electrochemical reactions of the Fuel Cell types

 

What is a Fuel Cell

A fuel cell works similar to a battery. In a battery there are two electrodes, which are separated by an electrolyte. At least one of the electrodes is generally made of a solid metal. This metal is converted to another chemical compound during the production of electricity in the battery. The energy that the battery can produce in one cycle is limited by the amount of this solid metal that can be converted. In the fuel cell an electrode that is not consumed and a fuel that continuously replenishes the fuel cell replace the solid metal. This fuel reacts with an oxidant such as oxygen from the other electrode. A fuel cell can produce electricity as long as more fuel and oxidant are pumped through it. At the anode the hydrogen molecules give up electrons and form hydrogen ions, a process which is made possible by the platinum catalyst. The electrons travel to the cathode through an external circuit, producing electrical current. The proton exchange membrane allows protons to flow through, but stops electrons from passing through it. As a result, while the electrons flow through an external circuit, the hydrogen ions flow directly through the proton exchange membrane to the cathode, where they combine with oxygen molecules and the electrons to form water.

 

Anode: 2H2 --> 4H+ + 4e-

       Cathode: 4e- + 4H+ + O2 --> 2H2O

           Overall: 2H2 + O2 --> 2H2O

 

The ideal available electrical work (assuming no losses by heat) from the electrons flowing through the circuit is

Wmax= -n F E

where n is the number of equivalents, or electrons per molecule of fuel, F is the Faraday (96,493 Coulombs per equivalent) and E is  the thermodynamic reversible voltage of the reaction (1.229 for this reaction) Thus, Wmax  comes out as 548 W.h. So theoretically 548 W.h of enrgy can be obtained out of a liter tank of hydrogen.

BATTERY &  FUEL CELL

       The fuel cell is an electrical energy producer. It takes a fuel ,say methanol and leads into the oxidizing anode and at the counter-cathode , oxygen in air is reduced. The free energy of oxidation directly comes out as electrical energy. So fuel cell may be called electrochemical electricity producer.

       Batteries, on the other hand, must have electricity produced elsewhere ( during  charging by  an external electricity produced by  fossil fuels in thermal power). The battery receives this electricity which drives a reaction on each of the two electrodes , up a free energy gradient for the overall reaction. The "charged" battery can then deliver the stored electricity to release this energy downhill in a spontaneous reaction.

 

   

 

So batteries store electricity and fuel cell produce it. Fuel cells produce electricity with no polluting effluents whatsoever. But with batteries , the problems of pollution are there from the combustion of coal or oil used to make electricity with which to charge them. Fuel cells can give uninterrupted energy as long as fuel is there. Batteries give energy during discharging and the cycles of charging and discharging are limited to a life period.

BIOELECTROCHEMICAL FUEL CELL 

A bioelectrochemical fuel cell is a device that realizes the direct conversion of biochemical energy into electricity. The driving force of  bioelectrochemical fuel cells is the redox reaction of substrate by microorganisms. There are two types of what can be called a microbial fuel cell. The first type utilizes electroactive metabolites, e.g., hydrogen, converted by microbial metabolism or enzyme reaction in a chemical fuel cell, and the other type utilizes mediators as electron transporters from a certain metabolic pathway of the microorganism or enzyme directly to electrodes

 

General characteristics of chemical and biological fuel cell

 

Chemical Fuel Cell 

Biological Fuel Cell

Catalyst

Noble Metals 

Microorganism / enzyme

pH

Acidic Solution (pH<1)

Neutral Solution pH 7.0-9.0

Temperature 

over 200 ° C

Room Temperature 22-25 ° C

Electrolyte

Phosphoric-acid

Phosphate Solution 

Capacity

High

Low

Efficiency

40 - 60 %

over 40 %

Fuel Type

Natural gas, H2, etc.

Any Carbohydrates and hydrocarbons

 

 

 

ALGAE

 

Algae, group of structurally simple plant-like organisms found in the form of pond scum, seaweeds, red tide, blue-green discolouration of aquarium walls, the green coating on trees  possess an enzyme Called HYDROGENASE , capable of splitting water into hydrogen by photosynthesis.

12H+ + 2Xreduced ® 6 H2 + 2Xoxidized

The electron carrier, X, is thought to be ferredoxin. Since ferredoxin is reduced with water as an electron donor by the photochemical reaction, green alga are theoretically water-splitting microorganisms.

Light absorption by the photosynthetic apparatus is essential for the generation of hydrogen gas because light energy facilitates the oxidation of water molecules, the release of electrons and protons as explained in the following sequence of reaction

NAD ( Green algae) + CO2 + H2O + hn (light energy ) ® Carbohydrates + NADH2 + O2

NADH2 + 2Fd ( Ferredoxin) ® NAD + 2FdH

Bio Oil

 

Types of fuel cells

Fuel cells are classified primarily by the kind of electrolyte they employ. This determines the kind of chemical reactions that take place in the cell, the kind of catalysts required, the temperature range in which the cell operates, the fuel required, and other factors. These characteristics, in turn, affect the applications for which these cells are most suitable

Polymer Electrolyte Membrane 

Diagram: How a Polymer Electrolyte Membrane (PEM) fuel cell works. A PEM fuel cell consists of a polymer electrolyte membrane sandwiched between an anode (negatively charged electrode) and a cathode (positively charged electrode). The processes that take place in the fuel cell are as follows: 1. Hydrogen fuel is channeled through field flow plates to the anode on one side of the fuel cell, while oxygen from the air is channeled to the cathode on the other side of the cell.  2. At the anode, a platinum catalyst causes the hydrogen to split into positive hydrogen ions (protons) and negatively charged electrons.  3. The Polymer Electrolyte Membrane (PEM) allows only the positively charged ions to pass through it to the cathode.  The negatively charged electrons must travel along an external circuit to the cathode, creating an electrical current.  4. At the cathode, the electrons and positively charged hydrogen ions combine with oxygen to form water, which flows out of the cell.

Polymer electrolyte membrane (PEM) fuel cells—also called proton exchange membrane fuel cells—deliver high power density and offer the advantages of low weight and volume, compared to other fuel cells. PEM 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 onboard reformers.

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. 

Phosphoric Acid 

Phosphoric acid fuel cells use liquid phosphoric acid as an electrolyte—the acid is contained in a Teflon-bonded silicon carbide matrix—and porous carbon electrodes containing a platinum catalyst. The chemical reactions that take place in the cell are shown in the diagram to the right. 

The phosphoric acid fuel cell (PAFC) is considered the "first generation" of modern fuel cells. It is one of the most mature cell types and the first to be used commercially, with over 200 units currently in use. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses

Direct Methanol 

Most fuel cells are powered by hydrogen, which can be fed to the fuel cell system directly or can be generated within the fuel cell system by reforming hydrogen-rich fuels such as methanol, ethanol, and hydrocarbon fuels. Direct methanol fuel cells (DMFCs), however, are powered by pure methanol, which is mixed with steam and fed directly to the fuel cell anode.
Direct methanol fuel cells do not have many of the fuel storage problems typical of some fuel cells since methanol has a higher energy density than hydrogen—though less than gasoline or diesel fuel. Methanol is also easier to transport and supply to the public using our current infrastructure since it is a liquid, like gasoline. 

Alkaline 

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the U.S. space program to produce electrical energy and water onboard spacecraft. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100ºC and 250ºC (212ºF and 482ºF). However, more-recent AFC designs operate at lower temperatures of roughly 23ºC to 70ºC (74ºF to 158ºF). 

AFCs are high-performance fuel cells due to the rate at which chemical reactions take place in the cell. They are also very efficient, reaching efficiencies of 60 percent in space applications. 

The disadvantage of this fuel cell type is that it is easily poisoned by carbon dioxide (CO2). In fact, even the small amount of CO2 in the air can affect the cell's operation, making it necessary to purify both the hydrogen and oxygen used in the cell. This purification process is costly. Susceptibility to poisoning also affects the cell's lifetime (the amount of time before it must be replaced), further adding to cost. 

Molten Carbonate 

Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide (LiAlO2) matrix. Since they operate at extremely high temperatures of 650ºC (roughly 1,200ºF) and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs.

Unlike alkaline, phosphoric acid, and polymer electrolyte membrane fuel cells, MCFCs don't require an external reformer to convert more energy-dense fuels to hydrogen. Due to the high temperatures at which they operate, these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming, which also reduces cost. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that increase cell life without decreasing performance. 

 

Solid Oxide 

Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte. Since the electrolyte is a solid, the cells do not have to be constructed in the plate-like configuration typical of other fuel cell types. SOFCs are expected to be around 50-60 percent efficient at converting fuel to electricity. In applications designed to capture and utilize the system's waste heat (co-generation), overall fuel use efficiencies could top 80-85 percent.
Solid oxide fuel cells operate at very high temperatures—around 1,000ºC (1,830ºF). High temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system. 

Electrochemical reactions of the Fuel Cell types