Fuel Cell Technology Dr Usha Raghavan Head, Information Technology

VPM’s Polytechnic,Thane

Introduction: Fuel Cells are electrochemical devices capable of converting the chemical energy from the fuel to electrical energy through a chemical reaction with oxygen or an oxidizing agent. The energy conversion in fuel cell is an electrochemical energy conversion. Hence it does not involve any type of mechanical movement for the process leading to a higher lifetime and efficiency. It is silent in operation. In conventional approaches, chemical energy of the fuel is converted initially into heat energy and finally into useful forms. This reduces the efficiency of the system. In Fuel cells, the chemical energy available in the fuel is converted into electricity and eliminates the conversion into heat and hence provides higher efficiency. Heat is generated from fuel cell and can be treated as a by-product of the conversion process and this can be used for other purposes.

What is a fuel cell?

In principle, a fuel cell operates like a battery. Unlike a battery, a fuel cell does not discharge. A fuel cell consists of two electrodes sandwiched around an electrolyte. Oxygen passes over one electrode and hydrogen over the other, generating electricity, water and heat. Hydrogen fuel is fed into the "anode" of the fuel cell. Oxygen (or air) enters the fuel cell through the cathode. Assisted by a catalyst, the hydrogen atom splits into a proton and an electron that take different paths to the cathode. The proton passes through the electrolyte. The electrons create a separate current that can be utilized before they return to the cathode, to be reunited with the hydrogen ion and oxygen into a molecule of water.

In a typical fuel cell, hydrogen fuel or hydrogen rich fuel and oxygen oxidant are combined in the presence of a catalyst to generate electricity by the oxidation – reduction reaction. It involves oxidation half reaction at the anode and reduction half reaction at the cathode and the specific half reactions vary depending on the type of fuel cell. Fuel cells have been developed by changing the type of electrolyte used

.Types of fuel Cells

Phosphoric Acid fuel cell (PAFC)

Phosphoric acid fuel cells use liquid phosphoric acid as the electrolyte and operate at about 450°F. One of the main advantages to this type of fuel cell, besides the nearly 85% cogeneration efficiency, is that it can use impure hydrogen as fuel. PAFCs can tolerate a CO concentration of about 1.5 percent, which broadens the choice of fuels they can use. PAFCs generate electricity at more than 40% efficiency - and nearly 85% of the steam this fuel cell produces is used for cogeneration.

Proton Exchange Membrane fuel cell (PEM)

PEM fuel cells use thin, solid, organic polymer as the electrolyte and operate at relatively low temperatures (175°F). They have an efficiency of 25-30 % .PEMs have high power density, can vary their output quickly to meet shifts in power demand, and are suited for applications, such as in automobiles, where quick startup is required.This type of fuel cell is sensitive to fuel impurities. Cell outputs generally range from 50 watts to 75 kW.

The High Temperature –PEM fuel cells operate at high temperatures (250°F - 390°F) than PEM fuel cells. The Membrane Electrode Assemblies consist of proton conductive polymer or a polymer doped with a proton conductive compound.HT-PEM fuel cells have been proven to tolerate up to 3% CO.

Molten Carbonate fuel cell (MCFC)

Molten carbonate fuel cells use an electrolyte composed of a molten carbonate salt mixture (Carbonates of Lithium, sodium or potassium) suspended in a porous, chemically inert matrix, and operate at high temperatures - approximately 1,200ºF.They require carbon dioxide and oxygen to be delivered to the cathode. They are used for large scale power generation. MCFC have an efficiency of 45-55 %. Cell output ranges from 10 kW – 10 MW.

Solid Oxide fuel cell (SOFC)

Solid oxide fuel cells use a hard, non-porous ceramic compound as the electrolyte, and operate at very high temperatures - around 1800°F. They are used for small to large-scale power generation. SOFC have an efficiency of 45 -60 %. Cell output ranges from 1 kW – 10 MW. They are suitable for auxiliary power units used in vehicles to power the electronic circuits.

Alkaline fuel cell (AFC)

Alkaline fuel cells use potassium hydroxide as the electrolyte and operate at 160°F.They can achieve power generating efficiencies of 70 %. They are susceptible to carbon contamination. Hence they require pure hydrogen and oxygen. They are used in space vehicles and submarines.

Direct Methanol fuel cell (DMFC)

Like PEM cells, they use polymer membranes as electrolytes. It typically operates at a temperature between 120-190°F. It has an efficiency of 20- 40 %.Cell output ranges from 3 kW – 250kW. It can be used for powering mobiles and laptops. The anode catalyst draws hydrogen from liquid methanol.

Microbial fuel cell (MFC)

Microbial fuel cells use the catalytic reaction of microorganisms such as bacteria to convert organic material into fuel. Enclosed in oxygen-free anodes, the organic compounds are oxidized by the bacteria or other microbes. The electrons are pulled from the compound and conducted into a circuit with the help of an inorganic mediator. MFCs operate at low temperatures, such as 20-40 ºC, and have 50% efficiency. These cells are suitable for small scale applications.

Protonic Ceramic fuel cell (PCFC) - This fuel cell is based on a ceramic electrolyte material that exhibits high protonic conductivity at elevated temperatures (700º C).The high operating temperature is necessary to achieve very high electrical fuel efficiency with hydrocarbon fuels. PCFCs have a solid electrolyte so the membrane cannot dry out as with PEM fuel cells, or liquid can't leak out as with PAFCs. Fuel Cell Components

Physical components play a very significant role in the performance of the fuel cell. The main physical components are Electrolyte/ Membrane, Electrode, Gas diffusion layer, Sealant, Bipolar plate/ interconnect, End plate/ collector.

The membrane plays a vital role in the electrical performance of the fuel cell. Some of the properties of the membrane/ electrolyte that influence the performance are ionic conductivity, Gas permeability, modulus of elasticity of the membrane, thickness.

Electrode is the bridge between the electronic conductor and ionic conductor. The electrodes must have the property of the membrane and should be a good electron conductor. The catalyst used on the electrode, surface area, porosity, and hydrophobicity are crucial properties of the electrodes that can affect fuel cell performance.

Gas diffusion layer provides mechanical support to the electrodes. Porosity of this layer should be high to allow gas flow. To avoid water accumulation in the diffusion layer, the hydrophobicity should be high. Also the electronic conductivity must be high to eliminate resistive losses.

Sealant reduces the operational cost by reducing the wastage of fuel and provides safety. The thickness of gasket has to match with the thickness of the diffusion layer.

Bipolar plate acts as an internal current collector in fuel cell stack. Electrical conductivity, thermal conductivity, Porosity, density and hardness of the bipolar plate are some parameters that influence the performance of the fuel cell.

End plates are used at both the ends of the fuel cell stack to hold the stack.

Benefits of fuel cell

A fuel cell power plant creates very less pollution compared to the pollution caused by conventional combustion generating systems. Fuel cell vehicles operating on hydrogen stored on-board produce zero pollution. Fuel cell power generation systems today achieve 40% to 50% fuel- to-electricity efficiency utilizing hydrocarbon fuels. More efficient systems are under development. They are highly reliable and provide flexibility of the fuel that is being used. Fuel cells are scalable. Fuel cells can be stacked until the desired power output is reached. They are light in weight and hence can be used in portable equipments.

Fuel cell systems must be cost effective and perform at par with the traditional power technologies. These challenges can be overcome by identifying and developing new materials