Review of Degradation in PEM Fuel Cells
Abstract
According to the EPA, the transportation industry is responsible for 27% of emissions in the US. Majority of those emissions come from the burning of fossil fuels in combustion engines. Proton-Exchange Membrane fuel cells (PEM) are a clean alternative that utilize an electrochemical reaction between hydrogen and oxygen to produce electricity and the byproduct of water. One of the biggest barriers preventing PEM fuel cells from being implemented in cars today is their short life span compared to traditional combustion engines. The short life span is caused by the degradation (thermal, chemical, and physical) or durability in the fuel cell. The two main components that are affected by degradation includes bipolar plates and the catalyst layer which are two layers a part of the membrane electrode assembly. One thing to note is that these layers both face major problems in degradation and are the most costly components for PEM fuel cells. The materials that are being looked at to improve the durability of these components are mainly carbon-based due to its high electric conductivity properties and durability.
Table of Contents:
2. Degradation
2A. Definition
2B.Types of Degradation
2B1. Mechanical
2B2. Chemical
2B3. Thermal
2C. Affects
3. Bipolar Plates Degradation
3A. Function
3B. Current Makeup
3B1. Non-porous graphite
3B2. Metals
3B3. Composites
3C. Advancements
4. Catalyst Layer Degradation
4A. Function
4B. Current Makeup
4B1. Catalyst Layer
4B2. Supports
4C. Advancements
4C1. Carbon Nanotubes
4C2. Mesoporous Carbon
4C3. Conductive Diamonds
4C4. Other Advancements
5. Discussion & Conclusion
6. Sources
1. Introduction:
According to the EPA, the transportation industry is responsible for 27% of emissions in the US. A large portion of those emissions come from gas-powered vehicles including cars, light trucks, and heavy-duty trucks. A single-passenger vehicle emits about 4.6 metric tons of carbon dioxide per year (EPA). To put it in perspective, that is about the average weight of one elephant. That is a significant amount of emissions coming from one car. There has been a massive emphasis on the transportation industry to reduce its emissions with the growing problem of global warming. Because of this emphasis, proton exchange membrane (PEM) fuel cells have been receiving a lot of attention as an alternative.
PEM fuel cells fall under the umbrella of hydrogen fuel cells where oxygen and hydrogen are used to create electricity and the by-product of water. Hydrogen fuel cells in general work similarly to a battery as it utilizes an electrochemical reaction. In the fuel cell, positive hydrogen ions are used as carrier ions, atoms that carry electric charges, that move from the anode (positively charged electrode) to the cathode (negatively charged electrode). 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.
What makes PEM fuel cells unique is the solid polymer membrane it utilizes as an electrolyte to connect the anode and cathode. It facilitates the transfer of carrier ions across the MEA (membrane electrode assembly). (To read more about the different components of PEM fuel cells and how they work, read this article: https://medium.com/@syona-gupta/pem-fuel-cell-it-powered-space-missions-but-now-might-power-our-buildings-and-vehicles-b250e7e763a9)
PEM fuel cells are not a new technology but they are still some gaps in the technology preventing PEM fuel cells from being widely implemented. One of the gaps is degradation. Degradation directly affects the longevity of PEM fuel cells and currently makes it not economically sensible to place a PEM fuel cell in a car. There is an emphasis put by the US government to further research into materials that are resistant to degradation and are relatively cheap to produce.
2. Degradation
2A. Definition
The degradation of PEM fuel cells refers to the durability of the fuel cell. The goal when creating a PEM fuel cell is to strive to have a longer life span, allowing the fuel cell to be used several times. Long-term durability is needed in fuel cells as they are constantly being turned on and off the repeated action can place strain on the fuel cell which can cause long-term damage and reduce efficiency. Degradation, if not mitigated, can result to fuel cell failure.
2B. Types
There are three main categories of degradation: mechanical, chemical, and thermal. The next three sections explain these concepts.
2B1. Mechanical Degradation
Mechanical degradation is the breakdown of molecules due to mechanical stress[1]. One example is mechanical degradation can cause the membrane to swell bringing on numerous stresses. Some examples of mechanical stress include compression, tension, shear, torsion, and bending. Mechanical degradation can look like perforation[2], cracks, tears, and pinholes in the membrane. If mechanical degradation should occur, this can result in both reactants traveling to reverse electrodes, therefore, inducing an exothermic reaction that ultimately speeds up the rate of degradation.
Mechanical degradation can be brought upon by numerous amount of factors such as sloppy assembly, the humidity and temperature cycle, and varying voltage levels. Mechanical degradation can also be brought upon debris from other components that can travel to places it shouldn’t be therefore causing harm to the fuel cell.
2B2. Chemical Degradation
Chemical degradation is the result of chemical reactions that can remove chemicals/molecules that are essential to the fuel cell. Chemical degradation can be influenced by ions[3] present in areas that it isn’t supposed to be. Chemical degradation is also affected by the highly acidic environment within the fuel cell. This can ultimately negatively impact some of the mechanical properties of the fuel cell. Some examples of chemical degradation include the production of hydrogen peroxides and carbon corrosion.
In the membrane, chemical reactions in the anode and cathode catalyst layers can produce hydrogen peroxide which decomposes into hydroperoxide radicals when there are iron ions present. These radicals can then attack other molecules present resulting in the wearing down of the fuel cell.
Carbon corrosion is the deterioration of carbon to form an oxide. This can occur in the catalyst layer. Carbon corrosion tends to occur when there is a high potential in the cathode during the on/off cycles of the fuel cell. This results in the changing of the structure of the electrode, loss of catalyst particles, and a decrease in the gas able to travel through the layer. Carbon corrosion can be prevented by the corrosion resistance in the bipolar layer.
2B3. Thermal Degradation
Temperature and humidity play a crucial role in the conductivity, gas impermeability, and mechanical strength of hydrogen fuel cells. Temperatures between 60 °C to 80 °C and high humidity are ideal conditions for fuel cells. These conditions allow for rapid startup and stable performance of the fuel cell. Temperature degradation occurs when the temperature or humidity does not fit the range causing a change in properties. Thermal degradation is caused due to poor water management and the freezing/thawing of the system.
2C. Affects
Degradation can cause membrane failure and the wearing down of the membrane, therefore, preventing the fuel cell from producing electricity. Degradation is one of the gaps in fuel cell technology that is preventing fuel cells from being implemented widely. To mitigate degradation, a lot of the time, it can drive up the costs of the materials due to the need for high-quality materials. The two main problem areas affecting degradation are bipolar plates and the catalyst layers, both layers within the membrane electrode assembly.
3. Bipolar Plates Degradation
3A. Function
The bipolar plate is the outer layer within the membrane electrode assembly (MEA). It is like the bread in a sandwich as it is the outer layer that provides several different purposes to the fuel cell. The bipolar plates are responsible for distribution, separation, and management within the fuel cell.
- Because the bipolar plate is on the outside of the MEA, it is the first thing that the reactants come in contact with. The bipolar plate is responsible for acting as a pathway and distributing the fuel and oxidant within the cell.
- The bipolar plate also takes a role in water management. As mentioned earlier, humidity plays a crucial role in the electrochemical reaction. The bipolar plates are responsible for making sure there isn’t too much excess water within the MEA, therefore, preventing the MEA from functioning.
- The bipolar plate is also responsible for the heat management of the fuel cell stack and making sure the temperature stays within the needed range.
- The bipolar plate is responsible for carrying the current outside of the stack as it is the outer layer. The current is sent from the plates away from the cell.
- Lastly, the bipolar plates act as a physical separation barrier between each fuel cell stack.
Down below is a figure of the physical and chemical properties that are crucial to the functions of the bipolar plates.
3B. Current Makeup
Bipolar plates can be made up of several different materials. The three different categories of materials include non-porous graphite, metals, and composites.
3B1. Non-porous graphite
Non-porous graphite is the most commonly used material to create bipolar plates due to the fact it has excellent chemical stability to survive the highly acidic fuel cell environment. The graphite used can either be natural or synthetic. This is the gold standard of bipolar plates. It has a very low electrical resistance, therefore, allowing for the highest electrochemical power output. Some issues with this material include the high cost, low mechanical strength, and the need for expensive machinery to form the flow channels.
3B2. Metals
A metallic layer is a close second to a material utilized for a bipolar layer. Metals have good mechanical stability, electrical conductivity, and thermal conductivity and can be easily stamped. Although, temperatures of around 80◦C, may lead to corrosion in the metallic layer; because this temperature is in the ideal range for PEM fuel cells, research needs to be done to improve this layer. Metallic layers can either be noncoated or coated.
The only metal that can be considered for non-coated metallic layers is stainless steel. This is because of its relatively high strength, high chemical stability, low gas permeability, a wide range of alloy choices, applicability to mass production, and low cost. Stainless steel is considered to be good at preventing oxidization because of the thin oxide layer on the surface of stainless steel which acts as a barrier. One of the main concerns for non-coated metallic bipolar layers is the intensity of corrosion that occurs.
Coated metallic bipolar layers are the other option considered. The metal layer is coated with a protective coating layer that is a thin conductive layer that prevents corrosion. The reason why the applied layer is thin is to prevent any formation of micro-pores and micro-cracks in coatings that may form when the layer is heated and cooled which may cause expansion and degradation. Currently, there hasn’t been a coating created for the metallic layer that prevents pinholes to form in the layer.
3B3. Composites
Composites is a material that is made up of two or more materials with different physical or chemical properties. Composite bipolar plates are a broad category encompassing both metal and carbon-based composites. On the metal-composite side, this includes porous graphite, polycarbonate plastic, and stainless steel. On the carbon-based composite side, this includes thermoplastic (polypropylene, polyethylene, polyvinylidene fluoride) and thermosetting resins (phenolics, epoxies, and vinyl esters). Polymer–graphite composites are a promising material due to their high performance and are being produced as bipolar layers by big companies such as DuPont. But it is important to note that the conductivity of composite layers is still below the targets set in place by the U.S. Department of Energy.
3C. Advancements
Metallic bipolar plates are the leading material for bipolar plates due to their high corrosion resistance, low toxic ion emission, high electric conductivity, satisfying formability, and low cost. The current research for these plates has been focusing on the type of coating for the metallic plates to prevent corrosion.
The first option is carbon films. Carbon films have become popular due to their lower cost and better performance. They have excellent corrosion resistance and electrical conductivity when exposed to a similar environment to PEM fuel cells. The carbon film’s structure allows for their performance to improve along with the addition of doping the carbon to increase stability.
Another option includes Transition-metal carbide (TMC) coatings. TMC coatings have high corrosion resistance and low interfacial contact resistance. One of the major concerns for this coating is the energy consumption, complexity of equipment, and manufacturing inefficiency due to the high deposition and heat treatment temperatures required.
Other types of coatings that are currently being looked into are conductive polymer coatings, metal nitride films, and noble metals. Noble metals refer to metals such as gold that have a high cost but have a very good electrochemical performance and stability. The current front runners in the research for the coating include doped carbon, metal nitride, and noble metal films.
4. Catalyst Layer Degradation
4A. Function
The catalyst layer houses the electrochemical reactions of the fuel cell. When a reactant travels through the catalyst layer, the protons and electrons are separated. The purpose of the catalyst layer is to optimize the amount of the reaction that occurs while utilizing the least amount of catalyst for the desired power output. The catalyst layer also maximizes the active surface area per unit mass while minimizing any obstacles for the reactants to travel throughout the cell.
4B. Current Makeup
The catalyst layer is compromised of a porous mixture of the catalyst, catalyst supports, and an ionomer.
4B1. Catalyst
The catalyst is responsible for the reaction. The most common and widely used material for a catalyst is platinum nanoparticles. Platinum is used as a catalyst due to its stability, binding interaction, poisoning resistance, and selectivity. Stability is essential to prevent the catalyst particle from dissolving at low pHs or high electrode potentials. Binding interaction refers to the strength with which the catalyst absorbs the reactant molecule. The catalyst must use enough strength to break the bond but still be able to release the catalyst. Poisoning resistance refers to the ability of the catalyst to resist degradation by being in contact with impurities that may come through the system. Lastly, selectivity is the ability of the catalyst to create the necessary products while minimizing the amount of byproduct produced. While platinum is able to fit all these categories, it is considered one of the blocking points for implementation for this tech due to its high costs.
4B2. Catalyst Supports
The durability of the catalyst is influenced by the supports used. The catalyst supports are responsible for anchoring the catalyst particles to provide stability. Supports are used to increase the active surface areas by helping the catalyst particles to remain highly dispersed. The typical material that is used for catalyst supports is carbon black.
There are five requirements for catalyst supports: high specific surface area, low combustive reactivity under both dry and humid air conditions at low temperatures (150 C or less), high electrochemical stability, high conductivity, and the ability to easily recover the used catalyst particles. These requirements are similar to the requirements for a catalyst as they are put through similar conditions.
4B3. Ionomer
An ionomer is basically what it sounds like, an ion that contains a polymer. It gives the catalyst electrode structure. These materials are special as they are self-healing due to their unique structure. The leading ionomer utilized in PEM fuel cells is a material called Nafion©. It helps improve the proton conductivity in the layer, extend the reaction zone, and increase the active surface areas.
4C. Advancements
Advancements in the catalyst layer have been focused on the catalyst supports to improve the catalytic activity and durability. Some examples include carbon nanotubes, mesoporous carbon, conductive diamonds, conductive oxides, and carbides.
4C1. Carbon Nanotubes
Carbon nanotubes are an emerging technology in material sciences. Carbon Nanotubes are a type of nanomaterial known for their strength. To break down the name of a carbon nanotube, the carbon part of the word signals the fact that this material is made up of carbon atoms arranged in a hexagonal pattern called aromatic rings. This hexagonal pattern forms graphene molecules. Several graphene molecules (aka the carbon hexagonal atoms) are lined up together in a lattice formation to form a tube-shaped molecule, hinting at the second part of the name carbon nanotubes. Due to it being a nanomaterial, carbon nanotubes are small in size, being one micron long and having a diameter of nanometers.
Carbon nanotubes are proposed for catalyst supports due to their structure and properties which grants properties of high electrical conductivity and higher catalytic activity. Compared to the standard carbon black, carbon nanotubes have fewer impurities so it isn’t as likely to poison the platinum catalyst. In addition, the carbon nanotubes’ structure prevents the development of deep cracks that will lose catalytic activity. Carbon nanotubes still have gaps that are preventing them from being implemented. There is a need for more studies to be done on the structural effect on CNTs as they act as catalyst supports and improvements on the catalyst performance as carbon invites defect sites.
4C2. Mesoporous Carbon
Mesoporous carbon is a nanomaterial that has pores with a diameter between 2 to 50 nanometers. Mesoporous carbon is synthesized by nanocasting mesoporous silica in templates. The advantage of mesoporous carbon is that it can uniformly and highly disperse catalytic metals, have high electrical conductivity, and enhance the transfer of reactants.
4C3. Conductive Diamonds
Conductive diamonds are basically diamonds that are doped with another element such as boron. While expensive, these diamonds are excellent electrodes as they have a wide potential window, very low background current, and high stability. This is due to their microstructures allowing for no microstructural changes when exposed to the fuel cell environment. The current gaps with the conductive diamonds include low conductivity, low surface area, and poor dispersion of catalyst.
4C4. Other Material Advancements
Other materials that are being researched for catalyst support include conductive oxides and carbides. Conductive oxides are nanoparticles that have an increasing effect on the activity of the catalyst and the durability of the catalyst. These oxides do still struggle with corrosion but the surface can be modified to minimize this. Carbides are compounds made up of carbon and a less electronegative element. Carbides have high chemical flexibility and their surface allows for the chemical composition and the catalytic properties to easily be modified during synthesis or post-treatment. One example of carbide is tungsten carbides; they are promising as they resemble the noble metal platinum but they still need to be investigated further.
5. Discussion & Conclusion
Overall, PEM fuel cell degradation is one of the biggest barriers to the implementation of PEM fuel cells in cars. There are three types of degradation: thermal, chemical, and mechanical. These types of degradation tend to be intensified due to the harsh environment of PEM fuel cells. The two components of the fuel cell that are most affected by this problem are bipolar plates and the catalyst layer. These components also end up making up the majority of the costs of the fuel cell. Degradation plays hand-in-hand with the problem of cost as higher-quality materials tend to cost more. The most promising materials for bipolar plates is stainless steel metal plates with either a doped carbon, metal nitride, or noble metal film coating. The most promising materials for the catalyst layer include the use of carbon nanotubes due to their structure making them less prone to cracks and composition making them less prone to oxidization. PEM fuel cells have come a long way from the 1960s but there is still room for improvement to allow for these fuel cells to replace the traditional combustion engine.
6. Sources
Degradation Mechanisms in Automotive Fuel Cell Systems
Bipolar plates for PEM fuel cells: A review
Novel catalyst support materials for PEM fuel cells: Current status and future prospects
https://iopscience.iop.org/article/10.1149/2.F07053IF