Learn everything about fuel cells in this detailed Electrochemistry Lecture 17 (English). Understand the working principle, types of fuel cells, electrochemical reactions, efficiency, and applications in energy systems. Perfect for CSS aspirants, chemistry students, and anyone studying electrochemistry or physical chemistry. Watch the full lecture for step-by-step explanations, examples, and exam preparation tips.
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00:01Hello everyone. Welcome to this exciting lecture series on electrochemistry.
00:10In this video, we are going to study about fuel cells.
00:15Fuel cells are a fascinating type of electrochemical cells that convert chemical energy contained in a fuel directly into the electrical energy.
00:25And it does so without combustion. This direct conversion allows for efficient and clean energy production.
00:36Various fuels can be used in the fuel cells, including hydrogen fuel, carbon dioxide, methane, propane and methanol.
00:46Each of these fuels can serve as a source of electrons that generate electricity through redox reactions inside the fuel cell.
00:56A key feature of the fuel cell is that the fuel supply is continuous.
01:01The system is designed to keep feeding the fuel as the reaction proceeds.
01:06Similarly, the reaction products are continuously removed to maintain optimal conditions for ongoing electricity generation.
01:14Fuel cells come in many varieties with different designs and applications suited to various fuels and operating environments.
01:24However, among these, the hydrogen fuels are the most important fuel cells that are most commonly used.
01:32Understanding a fuel cell is basically essential as they represent a promiseable technology for clean and sustainable energy, especially in transportation and portable power applications.
01:46So, here is a general diagram for the fuel cell, which is for the hydrogen and oxygen fuel cells.
01:54We will study about it in the coming slides.
01:58So now, before studying the working of the fuel cell, let us explore some of the types of the fuel cell.
02:08So, in this slide, we will explore one of the most common and diversified fuel cells called the polymer electrolyte membrane fuel cells, which are widely used in transportation and power generation.
02:21The polymer electrolyte membrane fuel cells, or PEMFCs, is one of the most popular types of fuel cells today.
02:31You might hear it called the proton exchange membrane fuel cells also because the key feature is the polymer membrane that allows protons to pass through.
02:40These cells typically operate at relatively low temperatures ranging from 50 degrees centigrade to 100 degrees centigrade.
02:49This makes them quick to start and ideal for applications like vehicles where you need rapid response time.
02:57The electrolyte is a special polymer that conducts protons which are the hydrogen ions but it is impermeable for gases like oxygen and oxygen.
03:10This selectivity is crucial for the cell function and continuous power generation or the hydrogen fuel generation.
03:21Structurally, a PEM fuel cell consists of several parts.
03:27For example, bipolar plates that help distribute gases and conduct electricity,
03:33a catalyst that facilitates the electrochemical reactions,
03:36electrodes where reactions take place,
03:38and the polymer membrane acting like an electrolyte.
03:43These cells are specially valued for their environmentally benign nature.
03:49That is the main by-product is the water,
03:53which makes them an excellent choice for transportation applications such as fuel cell vehicles.
03:58They are also used in fixed power generation units and portable power devices due to their efficiency and relatively low operating temperature.
04:09Next, we will discuss the phosphoric acid fuel cell known for its higher operating temperature and unique electrolyte properties.
04:20The Phosphoric acid fuel cell, which are also called the PAFC, uses Phosphoric acid as its electrolyte.
04:30This acid effectively conducts hydrogen ions, which are the H plus ions, which is essential for the fuel cell's electrochemical reactions.
04:37These cells typically operate at high temperature, around 150 to 200 degrees centigrade,
04:46which helps improve tolerance to impurities in the fuel but also demands more robust material for its operation.
04:55Unlike the polymer membrane, Phosphoric acid does not conduct electrons.
05:00This means electrons generated at the anode must travel through an external circuit to reach the cathode,
05:08which creates an electric current that power devices.
05:12One drawback of the cell is that the acidic nature of the electrolyte causes corrosion or oxidation of the cell components over time.
05:21This reduces the lifespan of these cells and necessitates the use of corrosion-resistant materials
05:27and continuous maintenance of the cell.
05:31Despite these challenges, these cells are reliable and have been used in stationary power generation applications,
05:38including combined heat and power systems.
05:44Now, let us explore solid acid fuel cells, which use unique solid acid electrolyte
05:51and exhibit interesting temperature-dependent conductivity behavior.
05:55So, we see that the solid acid fuel cells, also called SAFC fuel cells, they use solid acids as their electrolyte material,
06:05which is quite different from the liquid or polymer electrolyte in other fuel cells.
06:10At low temperature, these solid acids maintain an organized molecular structure that limits ion movement,
06:20meaning conductivity is relatively low.
06:22However, when the temperature rises, a phase transition or shift occurs in the molecular arrangement,
06:30which leads to a dramatic increase in the proton conductivity.
06:35This phase change allows the fuel cell to operate efficiently at intermediate or you can say at some higher temperatures.
06:42Two common examples of these solid acids are the cesium hydrogen sulfate and cesium dihydrogen phosphate.
06:52These compounds are favored for their proton conductivity after the phase change.
06:57Let us look at the alkaline fuel cells, historically important for the space missions and known for their efficiency and clean by-products.
07:11Alkaline fuel cells uses an aqueous alkaline electrolyte, typically potassium hydroxide,
07:22which saturates a porous mixture matrix that physically separates the electrode but allows ion transport.
07:29One of the key historical notes is that these cells were the major source of power for the Apollo space program,
07:39providing reliable and clean energy during the space missions.
07:43These cells operate at relatively low temperatures, which help in material longevity and reduces start-up times.
07:55These cells are very efficient and produce electricity, heat and power as by-products, making them environmentally friendly.
08:04Due to alkaline environment, these cells are highly selective for oxygen reduction, which contributes to their high efficiency.
08:14However, they are sensitive to carbon dioxide contamination, which can affect performance.
08:21So, pure oxygen and hydrogen supplies are needed for these cells.
08:25Let us explore molten carbonate fuel cells, which operate at high temperature and can use a variety of carbon-based fuel efficiently.
08:38Molten carbon fuel cells employ a lithium-potassium carbonate salt mixture as their electrolyte.
08:46When heated to high temperature around 650 degrees centigrade, the salt melts and becomes conductive, which allows carbonate ions to migrate between the electrodes.
09:02Operating at high temperature gives these cells certain advantages.
09:07They include high efficiency and the ability to internally reform hydrocarbon fuels like natural gas and biogas, which make them versatile for the real world applications.
09:22However, at high temperature, due to high temperature and the corrosive nature of the carbonate electrolyte,
09:31it means that the anode and cathode material must be carefully selected to withstand corrosion over time.
09:41These fuel cells are promising for stationary power generation, especially where waste heat can be used effectively, improving overall energy utilization.
09:52So, these cells can run on carbon-based fuel cells like natural gas and biogas also.
09:59Moving next to the types of fuel cells, we will now study the solid oxide fuel cells, which are commonly known as the SOFCs.
10:13These cells use a solid, non-poreous ceramic electrolyte, which means that instead of a liquid or gel electrolyte like in some fuel cells,
10:23these cells use a hard ceramic material that allows oxygen to move through it.
10:30This makes the cell very robust and suitable for high temperature operations, typically between 5000C to 1000C.
10:39The electrolyte's role here is to transport negative ions, which are the O- ions or the oxygen ions from the cathode to anode,
10:50which facilitates the electrochemical reactions needed for the electricity generation.
10:54Because of the high operating temperature and efficient transport, these cells achieve an efficiency of about 50% to 60%, which is quite high compared to the other fuel cells.
11:07At the anode, oxygen molecules gain electrons to form oxygen ions and these are the reduced.
11:18So, basically, at anode, a reduction happens.
11:21At the cathode, hydrogen gas reacts with the oxygen to form water.
11:26Okay.
11:27So, we see that the two electrons are released here and these two electrons move from cathode to anode through an external circuit producing the electricity.
11:39The overall reaction is that hydrogen reacts with half molecule of oxygen to produce water.
11:47So, it also produces water along with releasing energy as the electrical power.
11:57These cells are used in the specialized applications like satellites and space capsules where reliable high efficiency power is required under harsh conditions.
12:08They are also commonly used in large scale industrial power plants due to their ability to generate high power efficiently,
12:15making them ideal for fixed and stationary power generation.
12:24Okay.
12:26Another type of the fuel cell is the Zinc Air Fuel Cell or ZAFCs.
12:32These cells represent an innovative approach to clean energy.
12:38It was originally developed in the United States with the goal of powering vehicles,
12:44offering a greener substitute to traditional fuel cell engines.
12:50The electrolyte used in this type of cell is an aqueous molecule solution, most commonly potassium hydroxide.
12:59This facilitates the ionic movement necessary for the redox reaction at the electrode.
13:04At anode, zinc undergoes oxidation by reacting with the hydroxide ions to form zinc hydroxide.
13:13Okay.
13:15And it is relative to electrons.
13:17So, you can see the equation here.
13:20At cathode, oxygen from the air reacts with water and electrons to form, you can say, the four hydroxide ions.
13:31These electrons actually travel through the external circuit from anode to cathode.
13:40So, this reaction is what gives the zinc air fuel cell its name.
13:45Because from the air, we are having the oxygen molecule.
13:50The overall reaction shows that the zinc and oxygen combining with water to form zinc hydroxide.
13:58Okay.
13:59So, this is the overall reaction.
14:01This overall reaction is what drives the generation of electrical energy in the cell.
14:06These cells are attractive for the vehicular applications because of their high energy density, a relatively low cost, and the abundance of zinc as a resource.
14:21Moreover, they emit no harmful gases, making them a sustainable option.
14:25These fuel cells are also lightweight which make them suitable for electric vehicles and portable devices where space and weight are important factors.
14:36Next, we will study about the direct methanol fuel cells or the DMFCs.
14:46This is a unique variant of proton exchange membrane fuel cell that directly uses methanol as the fuel instead of hydrogen.
14:59One of the biggest advantages of this type of fuel cell is the convenience in handling methanol.
15:05Being a liquid at room temperature, methanol A is easier to store, transport and refuel than the hydrogen gas.
15:14Because we know that the hydrogen gas requires high pressure tanks or the cryogenic storages.
15:20The electrolyte in the DMFCs is a polymer membrane which is similar to the polymer electrolyte membrane's fuel cells.
15:29Which allows the proton which are that plus sign to pass through while forcing electrons to travel through an external circuit thereby generating electric current.
15:40At anode, methanol reacts with water to produce protons, carbon dioxide and the electrons.
15:50This reaction shows that the fuel itself methanol is oxidized.
15:54At cathode, oxygen reacts with the proton and electron to form water.
16:01Ok, so by reacting 3 by 2 molecules of the oxygen, we get the 3 molecules of water.
16:09The overall balanced equation becomes this one.
16:13CH3OH plus 3 by 2O2.
16:16They give rise to CO2 gas and water.
16:18While the emission of CO2 is downside from a climate perspective, these cells are still cleaner than the combustion engines.
16:29And because they operate at low to moderate temperature, which is around 60 to 130 degree centigrade, they can be used in portable electronic devices, laptops, military applications and small electric vehicles.
16:43The simplicity, low weight and liquid fuel compatibility make these cells a very attractive fuel cell type for the mobile and off-grid energy needs.
16:59Now, let us talk about the working of a fuel cell.
17:04Here, we will study about the alkaline fuel cell where we see that water will produce oxygen gas and the hydrogen gas.
17:17These types of fuel cells were utilized in the Apollo space program and it had two purposes.
17:23First, it was to act as a fuel source of fuel.
17:30Also, it was to act as a supply for the drinking water.
17:34The water vapor produced from the cell when condensed was fit for the human consumption.
17:40The fuel cell worked by transferring hydrogen and oxygen through the carbon electrodes into a concentrated sodium hydroxide solution.
17:50Let us now study about the reactions which are involved in the process.
18:01So, the reaction at the cathode is that oxygen is reacting with water along with the four electrolytes to produce four hydroxide ions.
18:11So, here is at the right side, we have the cathode which basically has the negative charge.
18:17So, oxygen from the air because air coming in from here.
18:21So, from air, oxygen is obtaining four electrons along with water molecules to form four hydroxide ions.
18:31The unreacted gases leave out from this bottom.
18:35So, this is the cathode and this will be the anode.
18:39And at the center, we have the electrolyte.
18:43At anode which is on the left side, we see that the two hydrogen atoms, they react with four hydroxide ions which are formed on the cathode.
18:53They further form the four water molecules and the four electrons.
18:57The four electrons that are generated here, they travel from a node through the external circuit to the cathode.
19:07The net cell reaction will be 2H2 plus O2 that gives rise to two water molecules.
19:14So, we see that using hydrogen and oxygen from the air, we can actually produce water along with producing electricity.
19:24So, from the left side or on the cathode side, we have the air coming in which have basically the oxygen.
19:32And from the left side, we have the fuel coming in.
19:36Fuel is basically our H2 molecule.
19:39So, hydrogen fuel is used at the anode to produce water molecules and cathode at the right side to produce hydroxyl molecule.
19:47And the overall net reaction is given here.
19:50So, the electrolyte function is that it moves basically hydroxyl lines from the cathode to the anode.
20:00So, that is the basic function of the electrolyte.
20:03But we see that the electrons released or this electrochemical process is a slow response rate process.
20:11Because this is not a very fast process.
20:12So, because this is not a very fast reaction, we have to modify this cell a bit.
20:24So, to solve this problem, we use a catalyst such as platinum or palladium, which actually increases the rate for these cathodic and anodic reactions.
20:34Okay.
20:35So, when the speeds are increased, this slower process becomes a little more efficient.
20:43Next, we see that we don't just put the platinum as it is into the electrolyte solution.
20:55Before being inserted into the electrodes, the catalyst is finally separated to maximize the effective surface area.
21:05This gives the chemical reactions more sites to occur, which speed up the things significantly.
21:10Okay.
21:11So, the catalyst used is in the powdered form in these fuel cells, which are embedded in the electrodes.
21:21When we compare fuel cells with thermal power plants, the difference in efficiency is very clear.
21:27We see that the fuel cells have a 70% efficiency in the generation of electricity, whereas the thermal power plants have an efficiency of 40%.
21:40That is nearly the double performance of the fuel cells than the thermal power plants.
21:46But we see that, why there is such a big gap in this energy generation or the efficiency?
21:52Well, in thermal power plants, the process to produce electricity is indirect.
21:59The creation of electric current requires the conversion of water into steam and the use of that steam to move turbines.
22:07Due to this turbine, we generate electricity.
22:12So, in this process, a lot of energy is lost along the way.
22:17Fuel cells, on the other hand, are much more elegant in their approach.
22:21They provide a platform for converting chemical energy into electrical energy directly, without all those extra steps that are involved in these thermal power plants.
22:34And that is the secret to their high efficiency.
22:41After having an insight into the working of the fuel cell and the different type of the fuel cell, let us go through the advantages of the fuel cells.
22:49Fuel cells are emerging as a promising alternative source of electrical energy, and they come with a set of important advantages over galvanic cells and traditional electricity generation methods, such as combustion of fuel cells or the nuclear reactions.
23:07Okay.
23:08So, talking about the advantages of fuel cells, let us start with the efficiency.
23:13One of the most significant advantages of the fuel cells is their high efficiency.
23:18Unlike conventional power plants that first convert chemical energy into heat, then into the mechanical energy by moving turbines, and finally into the electricity.
23:28Fuel cells can transfer the energy of fuel cells directly into the electrical energy, without going through so much steps.
23:40Okay.
23:41So, we see that we can use fuel in the thermal power plants such as the hydrogen, methane, methanol, carbon fuels, or nuclear reactors.
23:54Okay.
23:55So, because of this direct conversion, fuel cells should theoretically be 100% efficient, though in practice, the efficiencies are of 60 to 70%, which is still far superior to the superior or the traditional methods.
24:14Okay.
24:15By contrast, the traditional approaches like the burning of methanol, methane, methanol, some hydrogen fuel, carbon-based fuel, are using the nuclear reactors.
24:29They typically result in only 40% of the energy.
24:33This lower efficiency is due to energy loss at each convergence stage, especially as heat during the steam and the turbine steps.
24:44Okay.
24:45To understand the fuel cell efficiency from a thermodynamic point of view, we use the following formula, which is the N is equal to delta G divided by A multiplied by 1000.
24:57Here in this equation, delta is the heat of combustion.
25:01Okay.
25:02So, this is basically the delta H and the delta H is the work done from the fuel cell.
25:08This is basically the eta, which shows the efficiency of the process.
25:15So, delta D basically represents the Gibbs free energy, or the maximum useful work that we can obtain from a chemical reaction.
25:25So, in essence, this equation shows us how much of the chemical energy is effectively converted into the usable electrical energy.
25:36Let us now talk about two more important advantages of the fuel cell, which is the pollution-free working and their ability to provide continuous supply of energy.
25:51Starting with the pollution, fuel cells are very clean in how they operate.
25:56In fact, the byproducts produced by a fuel cell do not pollute the environment.
26:03A perfect example of this is the hydrogen oxygen fuel cell that we have studied in the previous slide.
26:10It generates just water as a byproduct.
26:13Okay.
26:14So, in this cell, there is no smoke, there are no harmful gases, no carbon emissions, just water.
26:21This makes fuel cells especially attractive as we look forward to the greener energy alternative in the fight against climate change.
26:30Now, moving to the continuous supply of energy.
26:33One of the best thing about the fuel cell is that they can provide energy indefinitely as long as the fuels are supplied into them.
26:43Unlike batteries or the interrelational cells which start to lose voltage or current as they discharge over time,
26:51fuel cells do not experience this kind of drop-off.
26:54Their voltage and power output remain steady and variable.
26:59This makes fuel cells extremely useful for their applications where consistent long-term power is needed like in space missions, remote stations or electric vehicles.
27:13Now, we will talk about some of the limitations of the fuel cell which explain why their widespread use still faces some of the greater challenges.
27:23First of all, handling the gaseous fuels is quite tough.
27:28Gases like hydrogen and oxygen don't simply sit safely at the room temperature.
27:33Instead, fuel gas must be stored as a liquid inside the special cylinder at very low temperature and under high pressure.
27:42This storage requirement means the equipment becomes bulky, complex and expensive.
27:48Because of these strict conditions, the cost of the fuel cell system increases significantly.
27:57The rise in cost comes from both the need for specialized storage tanks and the practical issues that come with handling such fuels safely.
28:07Another major contributor to cost is the use of expensive catalysts like platinum, platinum, palladium or the silver,
28:17which are essential for speeding up the chemical reactions at the electrodes.
28:22These catalysts are effective but their high prices add to the overall cost of the fuel cell.
28:32Lastly, the electrolytes used inside fuel cells are highly caustic substances.
28:39This means they can be very corrosive and require careful handling and durable materials,
28:45which introduces additional practical problems when designing and maintaining fuel cells.
28:51So, while fuel cells offer many benefits, these challenges which are handling gaseous fuels,
28:58high cost from storage and catalysts and caustic nature of the electrolyte must be addressed
29:04to make them more practical and affordable for widespread usage.
29:08So, that is all for today's video.
29:14Thank you very much for listening to the lecture.
29:16Thank you for listening to the lecture.
29:17Thank you for listening to the lecture.
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