PEM Hydrogen Fuel Cell — How They Work and Modeling it in MatLab
Proton-exchange membrane fuel cells (PEM) are becoming popular in the world of hydrogen fuel cells due to their high power density and minimal corrosion. They fall under the category of hydrogen fuel cells which uses the chemical energy of hydrogen to produce electricity. While PEM fuel cells are rising in popularity, PEM fuel cells have actually been around for the last sixty years. To give a bit of context, PEM fuel cells were invented during the 1960s by William Thomas Grubb at General Electrics. Then another GE engineer named Leonard Niedrach proposed the use of a platinum catalyst to help in the performance of the fuel cell. The Grubb-Niedrach Cell was then used in the Gemini fuel cell and Apollo space missions for NASA. The fuel cell provided electric power and helped maintain water filtration while onboard the space shuttle. The US wasn’t the only one looking into PEM Fuel cells as the Soviet Union was using these fuel cells in the military specifically for making a submarine and in space crafts. Ever since the seventies, PEM fuel cells have been researched to be used in cars due to the demand for a green alternative and the fact it is the favored hydrogen fuel cell amongst many researchers.
How Energy is Produced in a PEM Fuel Cell
Hydrogen fuel cells in general work similarly to a battery; there is a cathode[1], anode[2], and an electrochemical reaction[3]. In the fuel cell, positive hydrogen ions are used as carrier ions, atoms that carry electric charges, that move from the anode to the cathode. Here are the 5 basic steps of how a fuel cell produces electricity:
- At the anode (+), there are hydrogen molecules which are then split apart of their protons and electrons when they come in contact with a catalyst.
- The protons then migrate through the electrolyte to the cathode (-). Flow plates assist in this transfer. The electrons remain behind and thereby give the anode a negative charge, creating a voltage difference[4] between the anode and the cathode.
- Electrons are then forced to move through a circuit, with this flow of electrons generating electricity.
- The protons and electrons move to the cathode. At the cathode, there are oxygen molecules. When the subatomic particles come into contact with oxygen, this produces water molecules and H2O. Heat is also produced as a byproduct.
- The electricity created is then converted to a usable form of energy for the car through the power converter. The H2O molecules that are produced as a byproduct of the reaction are discarded through the tailpipe.
This process utilizes several different mechanisms and materials to convert hydrogen and oxygen into electricity and water. The following picture shows a simulation that showcases these several components. In the next few sections, I will explain each part at the same pace as the flow of energy.
Hydrogen Source:
The hydrogen source acts as a chamber of hydrogen for the fuel cell reaction. There is technically no naturally occurring hydrogen source. This means the hydrogen source must come from filtering it out from another compound. The most common way of producing hydrogen is through reforming hydrocarbons, a compound of hydrogen and carbon. Reforming is a processing technique in which the molecular structure of a hydrocarbon is rearranged to alter its properties (Britannica). This converts the hydrocarbon into a mixture of H2 and CO.
There are two different methods of storing hydrogen in a fuel cell. Hydrogen can be stored as a gas in high-pressure tanks. It can also be stored as a liquid in tanks that are set at cryogenic temperatures. In this simulation, hydrogen is stored as a gas at 70 MPa.
Oxygen Source:
The oxygen source provides a steady stream of oxygen to the anode and is stored as a gas in a tank. The oxygen comes straight from the air outside.
Recirculation:
The recirculator receives any excess hydrogen gas that wasn’t consumed by the reaction in the fuel cell and reintroduces it back into the stack. This makes sure that no hydrogen is wasted in the process, therefore reducing costs.
Anode and Cathode Humidifier:
Humidity is crucial to the PEM fuel cell. It affects the conductivity of the membrane which as a result affects the efficiency of the fuel cell and cost. If the membrane of the fuel cell is not humidified enough, the conductivity decreases which as a result leads to more energy being consumed during the proton transportation phenomenon. Humidity also provides the water to transport protons through the membrane.
The purpose of the anode and cathode humidifiers is to provide heat and humidity to either the oxygen or hydrogen source that is about to travel to either the anode or cathode. The addition of water vapor increases the volume of gas flowing through the system, therefore, increasing the kinetic energy of the gas particles. The humidifier also helps in regulating the temperature of the fuel and oxidant. It adds moisture to the oxidant so that the temperature is above the temperature of the fuel cell.
Membrane Electrode Assembly (MEA)
This is the core component of the fuel cell that is responsible for producing the electrochemical reaction. The MEA is made up of several different components.
The first component is the bipolar plate. The bipolar plate separates the reactant gasses and distributes the gasses over the active area. It also removes the unreacted gasses and water from the MEA to be recycled. Next, is the gas diffusion layer. This layer is 100–300m thick and is porous to allow for the distribution of the gas through the surface area of the membrane.
The gas diffusion layer also assists in water management and allows the needed amount of water to be fueled for the membrane. This layer acts as the electrical conductor that transports electrons to and/or from the catalyst layer.
This leads us to the catalyst layer. The catalyst layer is responsible for the electrochemical reaction. The electrochemical reaction gives a proton a jolt of energy similar to how you may feel after drinking coffee, to allow for the proton to flow through the membrane from the anode to the cathode. This layer’s main purpose is to maximize the number of reactions while minimizing the obstacles preventing the reaction. This layer maximizes the active surface area per unit mass of the electrocatalyst happening per layer.
This finally brings us to the membrane layer which is what makes this PEM fuel cell special. A PEM fuel cell has a solid polymer membrane as its electrolyte. This membrane is an electronic insulator, but an excellent conductor of protons. The membrane layer connects the anode and cathode. It acts as a freeway allowing protons to travel from the anode to the cathode but it stops the electrons from going. The electrons build up and are forced to travel through another pathway, the circuit, therefore producing electricity. The flow of electrons is the very definition of electricity.
Coolant
The coolant’s main job is to ensure the system doesn’t overheat by absorbing the heat of the different components. It is basically like the big snow plows that clear out any excess snow on the road that doesn’t melt from the salt. The coolant makes sure that the water vapor that helps transport protons doesn’t get exhausted when some of the water vapor is transformed into liquid water that is released through the exhaust.
Anode and Cathode Exhaust
The anode and cathode exhaust’s main responsibility is to regulate the fuel cell stack by maintaining the pressure of the system. It also is responsible for sending an excess gas that was not used in the reaction to the recirculator, so it can be sent back to the MEA to undergo the electrochemical reaction process again.
PEM Fuel Cells Pros and Cons
PEM fuel cells provide several advantages which make them favored by researchers.
- PEM fuel cells are small and lightweight due to their high power density.
- PEM fuel cells can quickly vary their power output based on demand.
- PEM fuel cells operate at low temperatures ranging from 60–80 °C. This allows them to warm up, start, and respond to demand changes quickly.
- PEM fuel cells have minimal corrosion because they utilize pure hydrogen for their fuel.
- PEM fuel cells are easy to handle, seal, and assemble.
PEM fuel cells’ current biggest disadvantage is their cost. They utilize a platinum catalyst which is considered the gold standard of catalysts. This is because it can bind the molecules together like no other catalyst.
Analyzing The Simulation
In the simulation I utilized, I focused on modeling two different graphs: the fuel cell i-v curve and the temperature in the fuel cell and coolant. I focused on these two graphs as they relate to the efficiency and performance of the system.
- Fuel Cell i-v Curve
This is also known as the polarization (the ability of a cation[5] to distort an anion[6]) curve. It showcases the voltage output of a fuel cell for a specific current density, and the amount of electric current per unit cross-section area. The curve shape is created by measuring the different voltages with a potentiostat/galvanostat. It is obtained by slowly increasing the load of the current that is given by potentiostat/potentiostat, then using the device to measure the fuel cell voltage output.
The overall shape of the curve is almost like an x=-y^{3} graph. The power increases until the voltage output hits its maximum, then it decreases as the current reaches its maximum due to loss of power because of resistance.
There are three different regions of the fuel cell polarization curve based on the current density.
- Low Power Density: There is an initial drop in cell potential because of activation polarization. Activation polarization happens at the beginning as overcoming the threshold energy barrier. It is the overpotential, the difference between the theoretical cell voltage and the actual voltage. The actual voltage tends to be higher than the theoretical cell voltage.
- Moderate Current Densities: This is when the cell potential decreases linearly as the current increases. This decrease is caused due to ohmic loss, the resistance of a conductor that causes energy to be lost to heat.
- High Current Density: The relationship between the cell potential drop, a drop in the difference of cell potential between two halves of the cells, and the current density no longer is linear. This creates a more defined concentration polarization, the polarization of the fuel cell in the electrolyte due to the current that is being passed through.
- The temperature in the Fuel Cell and Coolant
The first graph shows the temperature of the fuel stack in different locations. The optimal version of the graph is to have a distinct line parallel to the x-axis but different components end up heating the fuel (Ex: The fuel is warmed going through the recirculator; the oxygen is warmed going through the compressor.).
The second graph shows the coolant pump mass flow rate at each second. This graph illustrates the varying demand for coolant which match the different processes that are happening at the same time.
Conclusion
Furthermore, PEM fuel cells have the capability in the future to meet the demand for clean energy in the future. There have been several companies investing in this technology. Ex:
- GKN Aerospace has created the first liquid hydrogen fuel system on an aircraft that utilizes PEM fuel cells.
- Shell and its partners have received 5 million euros to create a tanker that runs on PEM fuel cells.
- Ballard Power Systems has created the first hydrogen-powered ferry. Onboard, it has two 200-kilowatt PEM fuel cells to power the ferry.
- General Motors and Nel ASA have collaborated to research an electrolyzer for PEM fuel cells.
PEM fuel cells have a lot of promise for the green energy industry but these fuel cells are still in the testing phase. These fuel cells will need a significant reduction in cost in the catalyst before we see PEM fuel cells taking over the industry.
[1]cathode — In the conventional current flow, positive charges leave this electrode.
[2]anode — In the conventional current flow model, negative charges are accepted by this electrode.
[3]electrochemical reaction — the transfer of electrons between a solid and a liquid substance to create an electric current (https://www.britannica.com/science/electrochemical-reaction)
[4]voltage difference — the difference in electrical potential energy between two places in a circuit. It is the work required, per unit charge, to move a charge from point A to B
[5]cation — positively charged ion
[6]anion — negatively charged ion
Sources: