Proton-exchange membrane fuel cell

Proton-exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells, are a type of fuel cell being developed mainly for transport applications, as well as for stationary fuel-cell applications and portable fuel-cell applications. Their distinguishing features include lower temperature/pressure ranges and a special proton-conducting polymer electrolyte membrane. PEMFCs generate electricity and operate on the opposite principle to PEM electrolysis, which consumes electricity. They are a leading candidate to replace the aging alkaline fuel-cell technology, which was used in the Space Shuttle.

Science

PEMFCs are built out of membrane electrode assemblies which include the electrodes, electrolyte, catalyst, and gas diffusion layers. An ink of catalyst, carbon, and electrode are sprayed or painted onto the solid electrolyte and carbon paper is hot pressed on either side to protect the inside of the cell and also act as electrodes. The pivotal part of the cell is the triple phase boundary where the electrolyte, catalyst, and reactants mix and thus where the cell reactions actually occur. Importantly, the membrane must not be electrically conductive so the half reactions do not mix. Operating temperatures above 100 °C are desired so the water byproduct becomes steam and water management becomes less critical in cell design.

Reactions

A proton exchange membrane fuel cell transforms the chemical energy liberated during the electrochemical reaction of hydrogen and oxygen to electrical energy, as opposed to the direct combustion of hydrogen and oxygen gases to produce thermal energy.
A stream of hydrogen is delivered to the anode side of the MEA. At the anode side it is catalytically split into protons and electrons. This oxidation half-cell reaction or hydrogen oxidation reaction is represented by:
At the anode:
The newly formed protons permeate through the polymer electrolyte membrane to the cathode side. The electrons travel along an external load circuit to the cathode side of the MEA, thus creating the current output of the fuel cell.
Meanwhile, a stream of oxygen is delivered to the cathode side of the MEA. At the cathode side oxygen molecules react with the protons permeating through the polymer electrolyte membrane and the electrons arriving through the external circuit to form water molecules. This reduction half-cell reaction or oxygen reduction reaction is represented by:
At the cathode:
Overall reaction:
The reversible reaction is expressed in the equation and shows the reincorporation of the hydrogen protons and electrons together with the oxygen molecule and the formation of one water molecule. The potentials in each case are given with respect to the standard hydrogen electrode.

Polymer electrolyte membrane

To function, the membrane must conduct hydrogen ions but not electrons as this would in effect "short circuit" the fuel cell. The membrane must also not allow either gas to pass to the other side of the cell, a problem known as gas crossover. Finally, the membrane must be resistant to the reducing environment at the cathode as well as the harsh oxidative environment at the anode.
Splitting of the hydrogen molecule is relatively easy by using a platinum catalyst. Unfortunately however, splitting the oxygen molecule is more difficult, and this causes significant electric losses. An appropriate catalyst material for this process has not been discovered, and platinum is the best option.

Strengths

1. Easy sealing
PEMFCs have a thin, polymeric membrane as the electrolyte. This membrane is located in between the anode and cathode catalyst layers and allows the passage of protons to pass to the cathode while restricting the passage of electrons. The better the connection of the polymeric membrane to the catalyst layer, the lower the interface resistance. Compared to liquid electrolytes, a polymeric membrane has a much lower chance of leakage . The membrane is commonly made of materials such as perfluorosulfonic acid, which minimize gas crossover and short circuiting of the fuel cell. A disadvantage of fluor containing polymers is the fact that during production PFAS products are formed. PFAS, the so-called forever chemicals, are highly toxic. Newer polymers such as the recently patented SPX3 are fluor free and therefore do not carry the PFAS risk.
2. Low Operating Temperature
Under extreme sub-freezing conditions, the water produced by fuel cells can freeze in porous layers and flow channels. This freezing water can block gas and fuel transport as well as cover catalyst reaction sites, resulting in a loss of output power and a start-up failure of the fuel cell.
However, the low operating temperature of a PEM fuel cell allows it to reach a suitable temperature with less heating compared to other types of fuel cells. With this approach, PEM fuel cells have been shown to be capable of cold start processes from −20°C.
3. Light mass and high power density
PEM fuel cells have been shown to be capable of high power densities up to 39.7 kW/kg, compared to 2.5 kW/kg for solid oxide fuel cells. Due to this high power density, much research is being done on potential applications in transportation as well as wearable technology.

Weaknesses

Fuel Cells based on PEM still have many issues:
1. Water management
Water management is crucial to performance: if water is evaporated too slowly, it will flood the membrane and the accumulation of water inside of field flow plate will impede the flow of oxygen into the fuel cell, but if water evaporates too fast, the membrane will dry and the resistance across it increases. Both cases will cause damage to stability and power output. Water management is a very difficult subject in PEM systems, primarily because water in the membrane is attracted toward the cathode of the cell through polarization.
A wide variety of solutions for managing the water exist including integration of an electroosmotic pump.
Another innovative method to resolve the water recirculation problem is the 3D fine mesh flow field design used in the Toyota Mirai, 2014. Conventional design of FC stack recirculates water from the air outlet to the air inlet through a humidifier with a straight channel and porous metal flow fields.The flow field is a structure made up of a rib and channels. However, the rib partially covers the gas diffusion layer and the resultant gas-transport distance is longer than the inter-channel distance. Furthermore, the contact pressure between the GDL and the rib also compresses the GDL, making its thickness non-uniform across the rib and channel. The large width and non-uniform thickness of the rib will increase potential for water vapor to accumulate and the oxygen will be compromised. As a result, oxygen will be impeded to diffuse into catalyst layer, leading to nonuniform power generation in the FC.
This new design enabled the first FC stack functions without a humidifying system meanwhile overcoming water recirculation issues and achieving high power output stability. The 3D micro lattice allows more pathways for gas flow; therefore, it promotes airflow toward membrane electrode and gas diffusion layer assembly and promotes O2 diffusion to the catalyst layer. Unlike conventional flow fields, the 3D micro-lattices in the complex field, which act as baffles and induce frequent micro-scale interfacial flux between the GDL and flow-fields. Due to this repeating micro-scale convective flow, oxygen transport to catalyst layer and liquid water removal from GDL is significantly enhanced. The generated water is quickly drawn out through the flow field, preventing accumulation within the pores. As a result, the power generation from this flow field is uniform across the cross-section and self-humidification is enabled.
2. Vulnerability of the Catalyst
The platinum catalyst on the membrane is easily poisoned by carbon monoxide, which is often present in product gases formed by methane reforming. This generally necessitates the use of the water gas shift reaction to eliminate CO from product gases and form more hydrogen. Additionally, the membrane is sensitive to the presences of metal ions, which may impair proton conduction mechanisms and can be introduced by corrosion of metallic bipolar plates, metallic components in the fuel cell system or from contaminants in the fuel/oxidant.
PEM systems that use reformed methanol were proposed, as in Daimler Chrysler Necar 5; reforming methanol, i.e. making it react to obtain hydrogen, is however a very complicated process, that also requires purification from the carbon monoxide the reaction produces. A platinum-ruthenium catalyst is necessary as some carbon monoxide will unavoidably reach the membrane. The level should not exceed 10 parts per million. Furthermore, the start-up times of such a reformer reactor are of about half an hour. Alternatively, methanol, and some other biofuels can be fed to a PEM fuel cell directly without being reformed, thus making a direct methanol fuel cell. These devices operate with limited success.
3. Limitation of Operating Temperature
The most commonly used membrane is Nafion by Chemours, which relies on liquid water humidification of the membrane to transport protons. This implies that it is not feasible to use temperatures above 80 to 90 °C, since the membrane would dry. Other, more recent membrane types, based on polybenzimidazole or phosphoric acid, can reach up to 220 °C without using any water management : higher temperature allow for better efficiencies, power densities, ease of cooling, reduced sensitivity to carbon monoxide poisoning and better controllability ; however, these recent types are not as common. PBI can be doped with phosphoric or sulfuric acid and the conductivity scales with amount of doping and temperature. At high temperatures, it is difficult to keep Nafion hydrated, but this acid doped material does not use water as a medium for proton conduction. It also exhibits better mechanical properties, higher strength, than Nafion and is cheaper. However, acid leaching is a considerable issue and processing, mixing with catalyst to form ink, has proved tricky. Aromatic polymers, such as PEEK, are far cheaper than Teflon and their polar character leads to hydration that is less temperature dependent than Nafion. However, PEEK is far less ionically conductive than Nafion and thus is a less favorable electrolyte choice. Recently, protic ionic liquids and protic organic ionic plastic crystals have been shown as promising alternative electrolyte materials for high temperature PEMFCs.