Fuel Cells

History:

Although fuel cells seem like new technology, they have actually been known to science since the early 1900’s and even a curiosity in the 1800’s.  William Nicholson and Anthony Carlisle first documented the process of using electricity to break water into hydrogen and oxygen in 1800.  The first known demonstration of this process was then created by William Robert Grove in 1839 that later become known as the “Grove Cell.” From looking at Nicholson and Carlisle’s notes, Grove’s idea was to then recombine hydrogen and oxygen to create water in a series circuit.  He did so by placing platinum electrodes in both oxygen and hydrogen that was also then submerged in a sulfuric acid solution (shown below). The sealed containers that contained both water and the gasses would rise in water level as the current flowed.

After Grove, there were many separate teams running fuel cell experiments.  In 1893, Friedrich Wilhelm Ostwald, one of the founders of physical chemistry, provided a lot of information for the components and interconnecting roles of these components within fuel cells such as: electrodes, electrolyte, oxidizing/reducing agents, anions, and cations. In 1896, William Jacques who was an electrical engineer and chemist constructed a carbon battery by injecting air into an alkali electrolyte to react with a carbon electrode.

 

 

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Basic Construction/concept:

Fuel (Hydrogen the vast majority of the time) enters on the anode side of the device and is ionized, usually by use of a catalyst or heat. An oxidizer (oxygen from the air most of the time) enters on the cathode side of the device and “pulls” the electrons towards it. The fuel and oxidizer then mix and react in the electrolyte that lies between the anode and the cathode. This produces heat and byproducts, in the case of H2 and O2 the only byproduct is water. This byproduct is then exhausted from the device and the reaction continues creating a constant current from the anode to the cathode, allowing a DC electrical device to be powered. Often times the water being produced from the device is recycled to prevent the electrolyte from drying out. The heat created can also be used for heating systems, or can be used to produce steam and will increase the overall efficiency of the fuel cell.

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Applications:

The use of fuel cells can be categorized into three broad areas: portable power generation, stationary power generation, and power for transportation.

  • Transportation: will provide power to drive something forward either directly or indirectly
  • For vehicles such as cars and vans, use hydrogen fuel cells and are beginning to plan for mass production within the next few years
  • Portable: will provide power to products that are designed to move, they will either be built into the equipment or charge it up.
  • These will replace or increase battery technology
  • Stationary: units that will provide electricity or maybe even heat
  • CHP – combined heat and power – take advantage of the fact that fuel cells can also generate heat when producing electricity, such as hot water, will make these very efficient
  • UPS – uninterruptible power systems – can provide a guarantee of power even in the case of a power outage
  • Large stationary units are being created to try and replace the grid

Materials/reactions

Oxidizer always required (usually O2 from atmosphere, rarely other oxidizers.)

 Fuel Cell Body

Depending on the type of cell it may need to be made out of more exotic materials that have higher melting points. For example, aluminum cannot be used in the fuel stack for a solid oxide fuel cell because it would melt under the operating temperature, this means that different materials are needed, such as titanium. Steel could be used but it will corrode in many environments and is more prone to thermal stress cracking than titanium. In extremely rare applications tungsten or iridium alloys can be used when thermal stability and strength are crucial.

Catalysts

If operating under high temperatures the catalyst doesn’t need to be made out of precious metals because of the availability of ambient energy due to the heat, but if the operating temperature of the cell is low precious metals such as platinum will need to be used as a catalyst or the reaction will not take place at a useful rate.

Electrodes

Material must be conductive and corrosion resistant

Carbon is used in many applications because it is cheap, meets the criteria and is readily available, it however tends not to be as durable as other materials.

Platinum can be used due to its excellent corrosion resistance, malleability in case of bumps and shock, and its high melting point of about 1700 degrees C.

Fuels

Hydrogen

-Hydrogen is the most common due to it only byproduct being water after reacting with O2

2H2 + O2 —> 2H2O

Hard to store

Most abundant element in the universe however relatively rare on earth

Can be made from water through electrolysis but this takes large amounts of energy

Burning of hydrogen releases heat due to molecular bond reconfiguration

We will be focusing mainly on this fuel type 

Biofuels

Methanol

  • Methanol is common because of its high Hydrogen to Carbon ratio and can be used in its pure form
  • Can be produced from carbon monoxide making it some what eco friendly and can also be made in small quantities through fermentation and distillation, making it somewhat renewable.

Production of Methanol: CO+ 2H2 —-> CH3OH with a delta H of -91 kJ/mol

Burning of Methanol: CH3CH2OH + 3O2 —>3H2O + 2CO2

 

Methane

  • Methane can also be used and is easy to obtain from decomposing matter, common in landfills. This makes it an excellent choice of fuel except for the release of greenhouse gasses

Burning of Methane: 3CH4 + 6O2 —>3CO2+ 6H2O

 5 types of fuel cells

Phosphoric Acid Fuel Cells

Phosphoric acid is used as the electrolyte

Fairly tolerant of poisoning compounds making them easy to run on hydrogen created from other fuel sources.

Low efficiency without utilizing heat regeneration for further power generation

Molten Carbonate Fuel Cells

Fairly high running temperature (around 650 degrees C) meaning that they do not require a precious metal catalyst, making them relatively cheap.

Can be run on non-hydrogen fuel sources,including biofuels without modification of the fuel

Solid Oxide Fuel Cells

Extremely promising because they can tolerate high levels of “poisoning compounds”, which means non-hydrogen fuels, including biofuels, can be used

Operate at temperatures between around 900-1000 degrees C, this eliminates the need for a precious metal catalyst, keeping it cheap.

Use a non-porous ceramic electrolyte

Alkaline Fuel Cells

Usually uses KOH as an electrolyte

The type in use on the ISS

Uses no precious metal catalysts, keeping them relatively inexpensive

Very sensitive to CO2 poisoning, causing a major decrease in efficiency.

Proton exchange membrane Fuel Cells

Require extremely expensive platinum electrodes to run

 

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Thermodynamics Behind Fuel Cells

Understanding fuel cell performance requires a knowledge of kinetics and thermodynamics. Thermodynamics places upper limits on the maximum electrical potential that can be generated in a reaction. Thus, thermodynamics yields the theoretical boundaries of what is possible with a fuel cell; it gives the “ideal case.” Any fuel cell will perform at or below its thermodynamic limit. \

A fuel cells purpose is to produce internal energy from a source that can be converted into a more efficient form of energy .The maximum amount of energy that can be produced is dependent on what it was produced from, either heat or work.

The term “reversible” when talking about the thermodynamics of fuel cells, implies equilibrium. A reversible fuel cell voltage is the voltage produced by a fuel cell at equilibrium. A process is reversible when an infinitesimal reversal in the driving force causes it to reverse direction; such a system is always at equilibrium. Equations relating to reversible fuel cell voltages only apply to equilibrium conditions. As soon as current is drawn from a fuel cell, equilibrium is lost and reversible fuel cell voltage equations no longer apply.

The maximum heat energy that can be extracted from a fuel is given by the fuel’s enthalpy of reaction (at a constant pressure).

The differential expression for enthalpy is:

dH = TdS + V dp

For a constant-pressure process (dp = 0) it’s derived to

dH = T dS

dH is the same as the heat transferred (dQ) in a reversible process. Enthalpy can be thought of as a measure of the heat potential of a system under constant-pressure. For a constant-pressure reaction, the enthalpy change is the amount of heat that could be created by the reaction. Expressing dH in terms of dU at constant pressure equates to:

dH = T dS = dU + dW

Heat evolved by a reaction is because of the changes in the internal energy of the system, after accounting for any energy gone towards work. The internal energy change of the system is due to the reconfiguration of chemical bonds. The burning of hydrogen releases heat due to molecular bonding reconfiguration. The product water rests at a lower internal energy than that of the initial hydrogen and oxygen reactants. After accounting for the energy that goes toward work, the rest of the internal energy is transformed into heat during the reaction. The enthalpy change associated with a combustion reaction is called the heat of combustion. The name heat of combustion indicates the close tie between enthalpy and heat potential for constant-pressure chemical reactions.

Enthalpy is associated with the reconfiguration of chemical bonds during a reaction, they can be calculated by considering the bond enthalpy differences between the reactants and products. The amount of heat energy that a substance can absorb changes with temperature. It follows that a substance’s formation enthalpy also changes with temperature. The variation of enthalpy with temperature is described by a substance’s heat capacity:

hf=ho+ToTCp(T)dT

where Δhf is the formation enthalpy of the substance at an arbitrary temperature T, Δho is the reference formation enthalpy of the substance at T0 = 298.15 K, and cp(T) is the constant-pressure heat capacity of the substance (which itself may be a function of temperature). If phase changes occur along the path between T0 and T, extra caution must be taken to make sure that the enthalpy changes associated with these phase changes are also included. In a similar manner, the entropy of a substance also varies with temperature. Again, this variation is described by the substance’s heat capacity:

S=So+ToTCp(T)TdT

In a perfect world, we could harness all of the enthalpy released by a chemical reaction to do useful work. Unfortunately, thermodynamics tells us that this is not possible. Only a portion of the energy evolved by a chemical reaction can be converted into useful work. For electrochemical systems (i.e., fuel cells), the Gibbs free energy gives the maximum amount of energy that is available to do electrical work.

Gibbs free energy can be considered to be the net energy required to create a system and make room for it minus the energy received from the environment due to spontaneous heat transfer. Thus, G represents the energy is transferred to create a system. (The environment also transferred some energy via heat, but G subtracts this contribution out.) If G represents the net energy you had to transfer to create the system, then G should also represent the maximum energy that you could ever get back out of the system. In other words, the Gibbs free energy represents the exploitable energy potential, or work potential, of the system.

Since the Gibbs free energy is the key to the work potential of a reaction, it is necessary to calculate Δgrxn and to also calculate ΔHrxn and ΔSrxn values. Recalling how G is defined, it is apparent that G already contains H, since G = U + PV − TS and H = U + PV. Gibbs free energy is defined as

G = H − TS  

Differentiating this expression gives

dG = dH − TdS − SdT

Holding temperature constant (isothermal process, dT = 0) and writing this relationship in terms of molar quantities give

ΔG = ΔH − T ΔS

Thus, for an isothermal reaction, we can compute ΔG in terms of ΔH and ΔS. The isothermal reaction assumption means that temperature is constant during the reaction.

Now that there’s an understanding for ΔG,  the work potential of a fuel cell can be derived. For fuel cells,  we are specifically interested in electrical work. Again Gibbs free energy is defined as

dG = dU − T dS − SdT + pdV + V dp

dU based on the first law of thermodynamics can be represented into the above equation.However, the work term  must be represented in dU to include both mechanical  and electrical work:

dU = T dS − dW

= T dS − (pdV + dWelec)

which yields:

dG = −SdT + V dp − dWelec

For constant-temperature, constant-pressure process (dT, dp = 0):

dG = −dWelec

The maximum electrical work that can be done by the  system  in a constant-temperature, constant-pressure process is given by the negative of the Gibbs free-energy difference for the process.  Thee constant-temperature, constant-pressure assumption used here is not really as restrictive as it seems. The only limitation is that the temperature and pressure do not vary during the reaction process. Since fuel cells usually operate at constant temperature and pressure, this assumption is reasonable. It is important to realize that the expression derived above is valid for different values of temperature and pressure as long as these values are not changing during the reaction. We could apply this equation for T = 200 K and p = 1 atm or just as validly for T = 400 K and p = 5 atm.

The potential of a system to perform electrical work is measured by voltage also called electrical potential. The electrical work done by moving a charge Q, measured in coulombs, through an electrical potential difference E in volts is

Welec = EQ

If the charge is assumed to be carried by electrons, then

Q = nF

Where n is number of moles of electrons transferred and F is Faraday’s constant. Combining these equations we derive:

ΔG = −nFE

Thus, the Gibbs free energy sets the magnitude of the reversible voltage for an electrochemical reaction. For example, in a hydrogen–oxygen fuel cell, the reaction

H2 + (½)O2 ⇌ H2O

has a Gibbs free-energy change of –237 kJ/mol under standard-state conditions for liquid water product. The reversible voltage generated by a hydrogen–oxygen fuel cell under standard-state conditions is thus

Eo = −(ΔGorxn/nF)

= −(−237, 000 J∕mol/(2 mol e−∕mol reactant)(96, 485 C∕mol)

= +1.23 V

EO is the standard-state reversible voltage and ΔGorxn is the standard-state free-energy change for the reaction. At STP, thermodynamics dictates that the highest voltage attainable from a H2–O2 fuel cell is 1.23 V. If something like 8Volts is needed, this is not possible. The chemistry of the fuel cell sets the reversible cell voltage. By picking a different fuel cell , it could establish a different reversible cell voltage. However, most feasible fuel cell reactions have reversible cell voltages in the range of 0.8–1.5 V. In order to achieve 8 Volts from fuel cells, it would be necessary to stack several cells together in series.

Fuel Cells

The Basics:

  • Fuel cells are an electric source of chemically produced energy using hydrogen or some other fuel source to hopefully produce the most efficient and environmentally friendly electricity.
  •  When hydrogen is the fuel source only water, heat, and electricity are possible outcome products
  • Electricity produced varies from enough to power a phone (on the small end) all the way to powering an entire manufacturing plant.
  • More beneficial than combustion engines (which is what most of the bigger power sources use, for example vehicles and company buildings)
  • Higher efficiency (60%)

Lower emissions – less pollution (because hydrogen fuel cells only produce water as a byproduct)

How they work:

  • similar to batteries, but don’t die or need to be charged (continuous cycle
  •      consists of two electrodes, the anode (negative) contains the hydrogen, or other fuel source and the cathode (positive) with contains oxygen.  The two electrodes are on either side of an electrolyte.
  •      electrolyte: chemical substance that produces an electric current resulting when the substance is dissolved in water or heated causing dissociation of charged particals.

Alkyl:

  •      KOH and water (electrolyte)
  •      operating temp. 150-200
  •      70% efficiency
  •      output: 300 W-5kW
  •      diagram (pictured above)

Phosphoric Acid (PAFC):

  •      phosphoric acid (electrolyte)
  •      operating temp: 150-200
  •      40%-80% efficiency depending on circumstance
  •      output: 200kW-11MW
  •      tolerates small amount of CO2 (expands the amount of different fuel sources able to be used)

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Fuel Cell Efficiency:

Unlike heat engines that can use the Carnot efficiency limit, fuel cells are not subject the limit. There is, however, the calorific value which measures the difference in the electrical energy produced with the heat and produced by burning the fuel. One will have to be careful though when calculating the calorific value, or ΔHf, because there are two different values of ΔHf that could be used; one where the product water stays as a gas or the other were the product waters condenses back into a liquid. Because of this the terms HHV, higher heating value, and LHV, lower heating value, have been used for calculating the maximum efficiency.

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Another way to find the cell efficiency is to look at the operating voltage of a fuel cell. This is called the fuel cell stack efficiency and this is commonly taken to mean the actual efficiency of the electrochemical reaction. For the fuel cell stack, all we would have to do to find the efficiency is to take the calculated voltage of the cell, V, and divide it by its theoretical maximum, :

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In addition to the fuel cell stack efficiency, there is also something called the fuel cell system efficiency. This type of efficiency relates to the overall performance of a fuel cell power plant. A practical fuel cell system requires additional equipment to regulate the gas and fluid streams, provide lubrication, operate auxiliary equipment, manages the electrical output and controls the process and this will reduce the total efficiency of the system from its theoretical ideal.

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“Three points can be noted from looking at this graph:

  1.      Although the graph and table would suggest that lower temperatures are better, the voltage losses are nearly always less at higher temperatures. So, in practice, fuel cell voltages are usually higher at higher temperatures.
  2.      The waste heat from the higher-temperature cells is more useful than that from lower-temperature cells.
  3.      Fuel cells do NOT always have a higher efficiency limit than heat engines.”

Fuel Cell Waste:

Energy is released whenever a fuel reacts chemically with the oxygen in air. The electrical energy can be used to do useful work directly while the heat is either wasted or used for other purposes. Instead of being “wasted” by release into the environment, sometimes waste heat can be utilized by another process. Waste heat is low temperature heat contains very little capacity to do work so the heat is qualified as waste heat and rejected to the environment. In some cases it is possible to use waste heat, for instance in heating homes by cogeneration. However, by slowing the release of the waste heat, these systems always entail a reduction of efficiency for the primary user of the heat energy.

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