Week 2 Challenge

1. Fuel Cell:

A fuel cell can be defined as an electrochemical cell that generates electrical energy from fuel via an electrochemical reaction. Fuel cells require a continuous input of fuel and an oxidizing agent in-order to sustain the reactions that generate the electricity. Therefore, these cells can constantly generate electricity until the supply of fuel and oxygen is cut off. Despite being invented in the year 1838, fuel cells began commercial use only a century later when they were used by NASA to power space capsules and satellites. Today, these devices are used as the primary or secondary source of power for many facilities including industries, commercial buildings and residential buildings. A fuel cell is similar to electrochemical cells, which consists of a cathode, an anode and an electrolyte. In these cells, the electrolyte enables the movement of the protons.

Working of Fuel Cell:

The reaction between hydrogen and oxygen can be used to generate electricity via a fuel cell. Such a cell was used in the Apollo space programme and it served two different purposes- It was used as a fuel source as well as a source of drinking water as the water vapour produced from the cell, when condensed was fit for human consumption. The working of this fuel cell involved the passing of hydrogen and oxygen into a concentrated solution of sodium hydroxide via carbon electrodes. The cell reaction can be written as follows:

Cathode Reaction: O2 + 2H2O +4e- ------------> 4OH-

Anode Reaction: 2H2 + 4OH- ----------> 4H2O + 4e-

Net cell reaction: 2H2 + O2 ------------> 2H2O

However, the reaction rate of this electrochemical reaction is quite low. This issue is overcome with the help of a catalyst such as Platinum or Palladium. In-order to increase the effective surface area, the catalyst is finely divided before being incorporated into the electrodes. A block diagram of this fuel cell is provided below in figure (1).

The efficiency of the fuel cell described above in the generation of electricity generally approximates to 70% whereas thermal power plants have an efficiency of 40%. This substantial difference in efficiency is because the generation of electric current in a thermal power plant involves the conversion of water into steam and the usage of this steam to rotate a turbine. Fuel cells, however after a platform for the direct conversion of chemical energy into electrical energy.

Working of Fuel Cell:

Fuel cell systems are a clean, efficient, reliable and quiet source of power. Fuel cells do not need to be periodically recharged like batteries, but instead continue to produce electricity as long as a fuel source is provided. A fuel cell is composed of an anode, cathode and an electrolyte membrane. A typical fuel cell works by passing hydrogen through the anode of a fuel cell and oxygen through the cathode. At the anode site, a catalyst splits the hydrogen molecules into electrons and protons. The protons pass through the porous electrolyte membrane, while the electrons are forced through a circuit, generating an electric current and excess heat. At the cathode, the protons, electrons and oxygen combine to produce water molecules. As there are no moving parts, fuel cells operate silently and with extremely high reliability. Figure (2) shows the working of fuel cell.

Due to their chemistry, fuel cells are very clean. Fuel cells that use pure hydrogen are completely carbon-free, with their only byproducts being electricity, heat and water. Some types of fuel cell systems are capable of using hydrocarbon fuels like natural gas, biogas, methanol and others. Because fuel cells generate electricity through chemistry rather than combustion, they can achieve much higher efficiencies than traditional energy production methods such as steam turbines and internal combustion engines. To push the efficiency even higher, a fuel cell can be coupled with a combined heat and power system that uses the cell's waste heat for heating or cooling applications. Fuel cells are also scalable. This means that individual fuel cells can be joined with one another to form stacks. In turn, these stacks can be combined into larger systems. Fuel cell systems vary greatly in size and power, from combustible engine replacements for electric vehicles to large-scale, multi-megawatt installations providing electricity directly to the utility grid.

Types of Fuel Cell:

Fuel cells are classified primarily by the kind of electrolyte they employ. This classification determines the kind of electrochemical 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. There are several type of fuel cells currently under development. The following are the types of fuel cells:

a) Polymer electrolyte membrane fuel cells.

b) Direct methanol fuel cells.

c) Alkaline fuel cells.

d) Phosphoric acid fuel cells.

e) Molten carbonate fuel cells.

f) Solid oxide fuel cells.

Polymer electrolyte membrane fuel cells:

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 with other fuel cells. PEM fuel cells use a solid polymer as an electrolyte and porous carbon electrodes containing a platinum or platinum alloy catalyst. They only need hydrogen, oxygen from air and water to operate. They are typically fueled with pure hydrogen supplied from storage tanks or reforms. PEM fuel cells operate at low temperatures, which allows them to start quickly and results in less wear on system components which means better durability. The platinum catalyst is the expensive material in this type of fuel cell system. Also, platinum catalyst are extremely sensitive to carbon monoxide poisoning. So, additional reactor to reduce carbon monoxide in the fuel gas is required if the hydrogen is derived from a hydrocarbon fuel.

Direct methanol fuel cells:

Direct methanol fuel cells are powered by pure methanol which is usually mixed with water 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 cell systems because methanol has a higher energy density than hydrogen though less than gasoline or diesel fuel. Methanol is also easier to transport and supply to public using current infrastructure as it is a liquid like gasoline.

Alkaline fuel cells:

Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed and they were the first type widely used in the US space program to produce electrical energy and water on-board 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. These fuel cells are closely related to conventional PEM fuel cells, except that they use an alkaline membrane instead of an acid membrane. The high performance of AFCs is due to the rate at which electrochemical reactions take place in the cell. They have also demonstrated efficiencies above 60% in space applications. A key challenge A key challenge for this fuel cell type is that it is susceptible to poisoning by carbondioxide (CO2). Alkaline cells with liquid electrolytes can be run in a recirculating mode, which allows for electrolyte regeneration to reduce the effects of carbonate formation. Electrolyte recirculation introduces issues with shunt currents. The liquid electrolyte systems also suffer from additional concerns including wettability, increased corrosion, and difficulties handling differential pressures. Alkaline membrane fuel cells (AMFCs) address these concerns and have lower susceptibility to CO2 poisoning than liquid-electrolyte AFCs do. However, CO2 still affects performance, and performance and durability of the AMFCs still lag that of PEMFCs. AMFCs are being considered for applications in the W to kW scale. Challenges for AMFCs include tolerance to carbon dioxide, membrane conductivity and durability, higher temperature operation, water management, power density, and anode electrocatalysis.

Phosphoric acid fuel cells:

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 PAFC (Phosphorous Acid Fuel Cell) 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. This type of fuel cell is typically used for stationary power generation. PAFCs are more tolerant of impurities in fossil fuels that have been reformed into hydrogen than PEM cells. PAFCs are more than 85% efficient when used for the co-generation of electricity and heat but they are less efficient at generating electricity alone (37%-42%). PAFC efficiency is only slightly more than that of combustion-based power plants, which typically operate at around 33% efficiency. PAFCs are also less powerful than other fuel cells, given the same weight and volume. As a result, these fuel cells are typically large and heavy. PAFCs are also expensive. They require much higher loadings of expensive platinum catalyst than other types of fuel cells do, which raises the cost.

Molten carbonate fuel cells:

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 aluminium oxide matrix because they operate at high temperatures non-porous metals can be used as catalysts at the anode and cathode reducing costs. Improved efficiency is another reason MCFCs offer significant cost reductions over phosphoric acid fuel cells. Molten carbonate fuel cells when coupled with a turbine can reach efficiencies approaching 65%, considerably higher than the 37%-42% efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be over 85%. Unlike alkaline, phosphoric acid, and PEM fuel cells, MCFCs do not require an external reformer to convert fuels such as natural gas and biogas to hydrogen. At the high temperatures at which MCFCs operate, methane and other light hydrocarbons in these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming, which also reduces cost. The primary disadvantage of current MCFC technology is durability. 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 double cell life from the current 40,000 hours (~5 years) without decreasing performance.

Solid oxide fuel cells:

Solid oxide fuel cells use a hard, non-porous ceramic compound as the electrolyte. SOFCs (Solid Oxide Fuel Cell) are around 60% efficient at converting fuel to electricity. Overall fuel use efficiencies could top 85%. SOFCs operate at very high temperatures—as high as 1,000°C. 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. SOFCs are the most sulfur-resistant fuel cell type; they can tolerate several orders of magnitude more sulfur than other cell types can. Also, SOFCs are not poisoned by carbon monoxide. This property allows SOFCs to use natural gas, biogas, and gases made from coal. High temperature operation has disadvantages. It results in slow startup and requires significant thermal shielding to retain heat and protect personnel.

The table below shows the comparison of different types of fuel cells:

Fuel Cell Type	Common Electrolyte	Operating Temperature	Typical Stack Size	Electrical Efficiency	Applications	Advantages	Challenges
Polymer Electrolyte Membrane (PEM)	Perfluorosulfonic Acid	<120 Deg C	<1kW	60%: Direct H2 40%: Reformed fuel	Backup Power, Portable Power, Distributed Generation, Transportation, Specialty vehicles	1. Solid electrolyte reduces corrosion and electrolyte management problems. 2. Low temperature startup. 3. Quick start up and load following.	1. Expensive catalysts. 2. Sensitive to fuel impurities.
Alkaline (AFC)	Aqueous potassium hydroxide soaked in a porous matrix or alkaline polymer membrane	<100 Deg C	1-100kW	60%	Military, Space, Backup Power, Transportation	1. Wider range of stable materials allows lower cost components. 2. Low temperature. 3. Quick startup.	1. Sensitive to CO2 in fuel and air. 2. Electrolyte Management (aqueous). 3. Electrolyte conductivity (Polymer).
Phosphoric Acid (PAFC)	Phosphoric acid soaked in a porous matrix or imbibed in a polymer membrane	150 Deg C-200 Deg C	5-400kW, 100kW module (liquid PAFC) <10kW (Polymer membrane)	40%	Distributed generation	1. Suitable for CHP. 2. Increased tolerance to fuel impurities.	1. Expensive catalysts. 2. Long start-up time. 3. Sulfur sensitivity.
Molten Carbonate (MCFC)	Molten Lithium, sodium, and/or potassium carbonates soaked in a porous matrix	600 Deg C-700 Deg C	300kW-3MW, 300kW module	50%	Electric Utility, Distributed generation.	1. High Efficiency. 2. Fuel Flexibility. 3. Suitable for CHP. 4. Hybrid/gas turbine cycle.	1. High temperature corrosion and breakdown of cell components. 2. Long Start up time. 3.Low power density.
Solid Oxide (SOFC)	Yttria stabilized zirconia	500 Deg C-1000 Deg C	1kW-2MW	60%	Auxiliary power, Electric Utility, Distributed generation.	1. High Efficiency. 2. Fuel Flexibility. 3. Solid Electrolyte. 4. Suitable for CHP. 5. Hybrid/ gas turbine cycle.	1. High Temperature Corrosion and breakdown of cell components. 2. Long startup time. 3. Limited number of shutdowns.

2. Energy Density vs Power Density Plot for various batteries:

Energy density is the amount of energy in a given mass (or volume) and power density is the amount of power in a given mass. The distinction between the two is similar to the difference between energy and power. If a system has a high energy density then it is able to store a lot of energy in a small amount of mass. A higher energy density does not necessarily mean a high power density. An object with a high energy density, but low power density can perform work for a relatively long period of time.

If a system has a high power density, that it can output large amounts of energy based on its mass. For example, a tiny capacitor may have the same power output as a large battery. However, since the capacitor is so much smaller, it has a higher power density. Since they release their energy quickly, high power density systems can also recharge quickly.

Figure (3) and figure (4) below is the energy density vs power density plot of various battery types like Lithium-ion, Lead Acid, Nickel cadnium, Ultracapacitor and fuel cell.

In figure (3) and figure (4), the power density of electrostatic capacitors is high with a low energy density. Fuel cells have the highest energy density with a low power density. The best option is electrochemical capacitors with better values of power and energy densities. The Ragone plot is very useful for estimating energy storage performance but ignores the critical features such as cycle life, cost estimation and safety for comparison between supercapacitors and batteries. For a better comparison and understanding of energy storage technology, it is important to consider safety, cost estimation, and cycle life. In supercapacitors, carbon nano-tubes, carbon nanofibers, and graphene are used. Carbon nanofibers are toxic and graphene is bad for humans and the environment. So, better electrode materials are required for the improvement of supercapacitors. The main difference in batteries and supercapacitors is the storage formula, energy storage amount, power restrictions, and life. The cyclic life of a supercapacitor is 1 million to 30,000 h, i.e., extraordinarily higher than the batteries, which are 500 h, and the recharging time for the supercapacitor is remarkably low, i.e., 1–10 s vs. battery 10–60 min. One of the most attractive features of a supercapacitor is thermal stability, which is tuned by a wise choice of materials, including electrolytes. In supercapacitors, the potential energy deposits in the electric field, whereas batteries store potential energy in the form of chemical energy.