PEM Fuel Cell Technology
Proton exchange membrane (PEM) fuel cells operate at relatively low temperatures (<80ºC or <175ºF), offer quick start-up times, and require only hydrogen and oxygen to operate. Protonex’ patented design and manufacturing processes for PEM systems provide significant technical and cost advantages compared to competing solutions. Protonex fuel cell stacks utilize a unique mould-in-place seating approach that provides much greater repeatability and robustness in assembly and performance than traditional approaches. Our fuel cells are also driven by balance-of-plant subsystems based on commercially available, proven components and materials.
The Fuel Cell System
The core of a fuel cell system is the fuel cell stack. A Protonex fuel cell stack consists of two primary components: 1) the bipolar plate (cathode and anode), and 2) the membrane electrode assembly (MEA). A single cell of the fuel cell stack is created by stacking a cathode bipolar plate on top of a MEA on top of an anode bipolar plate as shown in Figure 1; this stack up is repeated as necessary to adjust the electrical output characteristics of the fuel cell. The MEA consists of two porous, catalyst-coated electrodes (cathode and anode) that are layered upon either side of an electrolytic membrane. The bipolar plates are electrically conductive and facilitate the supply of oxygen (cathode) and hydrogen (anode) to the MEA via integral flow passages.
As shown in Figure 1, on the anode side of the cell, platinum catalyst within the MEA’s electrode layer separates the hydrogen’s negatively charged electrons from positively charged ions (protons). The protons move through the membrane to toward the cathode. The electrons from the anode side cannot pass through the membrane and as a result are forced around the membrane through an external electrical load before returning to the cathode side of the cell; the resultant flow of electrons is a useful electrical current. At the cathode side of the cell, the catalyst within the MEA’s electrode layer facilitates the re-combination of the protons and electrons along with supplied oxygen to produce water and heat.
Figure 1. Fuel cell operational schematic indicating fundamental electrochemical reaction and typical cell MEA stack up.
In most fuel cell applications pure gaseous hydrogen will not be available for fuel cell operation either due to logistical constraints or weight and volume limitations (the latter is especially true for portable power systems). In certain applications, such as space-based and underwater systems, gaseous oxygen will also not be available. For this reason, higher density hydrogen and oxygen storage options are required to improve the system level performance characteristics of the PEM fuel cell.
Many high density supply options involve the chemical transformation of a reactant into hydrogen or oxygen gas and other byproducts not consumed by the fuel cell. Since PEM fuel cell MEA performance can degrade quickly when supplied with even small amounts of certain byproducts, such as carbon monoxide (CO), alternative reactant supply systems must include filtration/purification subsystems to reduce harmful byproduct levels. Technical maturity, weight, and/or volume of purification hardware may end up prohibiting the practical use of a particular reactant, especially in portable systems. For example, it is very difficult to reduce the CO produced through the reformation of hydrocarbon fuels, such as propane and kerosene, to levels appropriate for PEM operation in portable systems.
Protonex has developed several alternative hydrogen supply options for fielded, portable PEM fuel cell systems; these include metal hydride, chemical hydride, and methanol-based systems. Protonex continues to identify and evaluate hydrogen and oxygen supply options in the context of a wide range of PEM fuel cell applications; ultimately, the reactant choice will be dependent on the system size and application.
Protonex currently supports the following hydrogen supply options for PEM systems:
- Compressed gaseous hydrogen
- Cryogenic liquid hydrogen
- Must be vaporized and heated en route to fuel cell
- Metal hydrides
- Hydrogen gas desorbed from hydride material
- Chemical hydrides
- Hydrogen gas produced using decomposition reactor
- Protonex has developed sodium borohydride-based chemical hydride systems for unmanned aerial vehicle (UAV) and soldier-worn applications
- Hydrogen gas produced using reforming reactor
Protonex currently supports the following oxygen supply options for PEM systems:
- Compressed or ambient (pressurized via fan/blower)
- Compressed gaseous oxygen
- Cryogenic liquid oxygen
- Vaporized and heated en route to fuel cell
- Hydrogen peroxide
- Oxygen gas produced using decomposition reactor
Fuel Cell Subsystems (Balance of Plant)
Protonex PEM fuel cell systems typically contain three major subsystems: 1) the reactant conditioning system, 2) the thermal management system, and 3) the electronic control system. These subsystems are typically referred to as the fuel cell balance of plant (BOP).
The reactant conditioning system is designed to transform stored reactants into a form suitable for consumption within the fuel cell stack. For example, a methanol-based conditioning system facilitates the conversion of methanol into hydrogen gas through the use of a reforming reactor. A typical fuel cell reactant conditioning system may contain a supply tank, a pump, control valves, a reactor, a filtration/purification system, and/or a humidifier.
The thermal management system is designed to regulate fuel cell stack, and potentially reactant subsystem component, thermal characteristics. The fuel cell produces thermal energy at a level roughly equivalent to that produced electrically (1 W heat for 1 W of electrical power) and this heat must be dissipated to prevent stack overheating and performance degradation. A fuel cell thermal management system will typically contain pumps, control valves, an accumulator, and a heat exchanger to dissipate heat to the environment. Protonex fuel cell thermal management systems are capable of operation using a range of compatible fluids.
The control electronics system is designed to regulate and control subsystem operation and stack electrical output. A Protonex control electronics subsystem will typically manage the operation of the reactant conditioning and thermal management subsystems as a function of electrical output requirements. The control electronics subsystem also incorporates load regulation, typically via a DC/DC converter, and output buffering capabilities, typically via a rechargeable battery, to manage system electrical output.
The Protonex Solution
While fueling and reliability remain issues on the path to widespread commercial success, the foremost hurdle facing fuel cell developers is the high cost of production. Protonex has developed innovative designs and proprietary manufacturing processes for volume production of PEM fuel cell stacks; for this reason, we are able to produce reliable, high performance PEM fuel cell stacks at low cost. The key to Protonex’ manufacturing approach is our patented adhesive bonded stack technology, illustrated by comparison to traditional approaches in Figure 2. The bonded stack approach reduces component (bipolar plates and MEA) complexity and lowers part count providing a simple construction approach that enables fast build cycles and the ability to automate. A major benefit of the approach is that it provides for component vendor flexibility; more specifically, the approach is MEA supplier independent. From a functional standpoint, by removing the need for a compression gasket the adhesive bonded design eliminates exterior leakage paths making the stack highly durable.
Figure 2. Traditional stack manufacturing versus Protonex proprietary stack manufacturing.
Protonex System Design and Integration
Protonex’ expertise encompasses the design and manufacture of complete fuel cell systems as well as fuel cell stacks. Advanced stacks are only part of the technical challenge in the scale manufacture of fuel cell systems. Selection, design, assembly, and integration of balance of plant components and fuel subsystems greatly affect critical system parameters, including size, weight, performance, reliability, durability, and overall cost. Reactants and BOP components typically account for the majority of the weight of the total fuel cell system, and Protonex has established beneficial relationships with well-known suppliers to obtain quality off-the-shelf components at low cost. The careful selection of subsystems and components has allowed Protonex to develop and demonstrate a system manufacturing approach based on modular design and assembly of balance of plant components.