06213 – Hydrogen Fuel Cell Test Station
Technical Data Package
Feb. 24th, 2006
Group Members: Sean Ashman, Chad Byler, Dennis Farley, Brian Holzberger, Shan Hu, Corey Reynolds, Dan Upton, Steve Yang
Introduction
The goal of this project is to create an apparatus capable of testing the performance of hydrogen fuel cells for the NanoPower Research Laboratory (NPRL) located within the science department of Rochester Institute of Technology. The hydrogen fuel cell has recently become a popular topic of discussion as oil production is disrupted by military conflict abroad and natural disasters domestically. How a fuel cell works, however, is not general knowledge. The first task of this project was then to discover how a fuel cell works. Second, then was to realize our customer, the NanoPower Research Lab (NPRL), is testing an experimental fuel cell setup, that is not precisely like a typical cell. The NPRL manufactures their own nanotube catalyst layer internally, a critical component of the hydrogen proton exchange membrane (PEM) fuel cell. It is the performance of this nanotube catalyst layer compared to more traditional catalyst layers used in fuel cells that they wish to test. The customer is also interesting in that they are an expert in carbon nanotubes but are new to fuel cell research. Consequentially they are not sure of the best practices or the standard test procedures used in other fuel cell labs. The customer’s original need statement was, “If you can duplicate the capabilities of the test cell in the Mechanical Engineering Lab, you’ve succeeded.” However, after an answer of $1,500 for our budget, but an answer of $29,950 for the TVN Systems RU-2100 that sits in the Mechanical Engineering test lab, we realized what the project was. The needs of our customer NPRL can be divided into three general categories, the Mechanical Assembly section, the Interface and Control section and the Hydrogen and Oxygen Gas Supply section. The primary need relating to the Mechanical Assembly subsection is the assembly of the fuel cell itself. The mechanical assembly must supply enough compressive force to contain the various layers that make up the fuel cell and the hydrogen and oxygen gas being supplied to the fuel cell. The assembly must also be able to heat the fuel cell to a temperature up to 80°C, and maintain desired levels of testing temperatures. The principal need under the Interface and Control section deals with the environmental parameters within the system. The environmental parameters pressure, temperature and humidity must all be monitored and recorded. This data also needs to be put into a format compatible with Labview, as this was the customer’s software package of choice. The chief need of the Hydrogen and Oxygen Supply section is to be able to supply Hydrogen and Oxygen at selectable pressures and humidities to the system.
All those viewing this project, must have at least a passing familiarity with the operation of a proton exchange membrane (PEM) fuel cell, so here it is.
Basic Operation Diagram of a Proton Exchange Membrane
The proton exchange membrane fuel cell uses one of the simplest reactions of any fuel cell. Within a standard fuel cell, there are four basic components; the anode, the cathode, the proton exchange membrane and the catalyst. The anode is the negative post of the fuel cell. It performs several functions. It conducts the electrons that are freed from the hydrogen molecules so that they can be used in an external circuit and it has channels etched into it that disperse the hydrogen gas equally over the surface of the catalyst. The cathode is the positive post of the fuel cell. Like the anode, the cathode also has channels etched into it, but these channels distribute the oxygen to the surface of the catalyst instead of the hydrogen. The cathode also conducts the electrons back from the external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water. The proton exchange membrane (PEM) is a specially treated material that only conducts positively charged ions. The PEM blocks electrons. The catalyst is a special material that facilitates the reaction of oxygen and hydrogen. It is usually made of platinum powder very thinly coated onto carbon paper or cloth. The catalyst is rough and porous so that the maximum surface area of the platinum can be exposed to the hydrogen or oxygen. The platinum-coated side of the catalyst faces the PEM. The NPRL utilized nanotubes in the construction of their catalyst layers. On the anode side, Hydrogen gas is forced through the catalyst by pressure. When an H2 molecule comes in contact with the platinum on the catalyst, it splits into two H+ ions and two electrons. The electrons are conducted through the anode, where they make their way through the external circuit (doing useful work) and return to the cathode side of the fuel cell. While all this is happening on the anode side, the cathode side of the fuel cell is also busy. Oxygen gas is being forced through the catalyst, where it forms two oxygen atoms. Each of these atoms has a strong negative charge. This negative charge attracts the two H+ ions through the membrane, where they combine with an oxygen atom and two of the electrons from the external circuit to form a water molecule H2O.
This technical report divides the material covered during the winter quarter of 2006 into the following topics.
• Layering of Fuel Cell
• Mechanical Assembly Process
• Heating of the Fuel Cell
• Humidification of gas
• Exhaust/Back Pressure Control
• Electrical Sensors and Power Supply
• Heating elements
• Layout of program logic
• Data Acquisition
• Budget
Gas Diffusion Plate/Electrode Assembly
The testing of the PEM fuel cell requires that a good electrical connection be made to the carbon nano-tube catalyst. The difficulty in this is that in order for the cell to work, H2 and O2 need to be able to come in contact with the nano-tube layers. This requires that the electrode also have means for the gases to pass through them as well as provide electrical contact to the nano-tube layers. The customer also expressed interest in being able to change the reaction area of the nano-tube layers. This required either adjustable electrodes or multiple electrodes. Initially it was discussed to use an electrode that would essentially be a washer. This would allow a fast, easy, cheap, and accurate method for producing an electrode that would be able to handle the current generated by the fuel cell, control the reaction area of the fuel cell, and maximize the gas contact with the nano-tube layers. After discussing this design idea with the customer, we were informed that due to the nature of the nano-tube technology being used, the electrical resistance in the nano-tube layer was higher than that of conventional electrode layers. This required us to find a method that would maximize both the electrode contact and the gas contact with the nano-tube over the same area.
The solution came in the form of a mesh. It would provide a good electrical contact over a large area as well as allow gas to pass easily through and reducing low concentration of the gas on the layers (area where the electrode would block gas from contacting the catalyst layer). Using a mesh type electrode provided the surface area needed by both the electrical aspect as well as the fluid aspect of the project, but the mechanical pressure exerted by the electrode to provide good electrical contact needed to be addressed.
Multiple ideas for creating a good positive pressure on the fuel cell stack were discussed. Some included springs that would provide constant pressure on the nano-tube layers, others simply used mechanical pressure from a power screw or other device to maintain constant pressure. Knowing that the plates were required to be under pressure to create a tight seal to the nano-tube layers but not make contact to the PEM fostered a solution that utilized a raised portion on the electrode that would be the contact surface for electric current to flow as well as have a mesh designed into it so as to allow gas to flow through as is seen in the picture below.
Now that a design for the electrode was decided, a method of mounting the electrode to the clamp assembly needed to be assessed. Since the clamp assembly is primarily metal, an electrically insulating material would have to be inserted between the mechanical press and the electrode. This insulator would also have to allow gas to flow through so it could go through the electrode and it had to withstand the pressure of the mechanical press. Because of these conditions, a composite material would be used as it provided good electrical insulation and could withstand the compression loads of the mechanical press. The insulator would also allow for quick tube fittings to be installed that would carry the gases to and from the fuel cell.
The meshed electrode is mounted to the insulator with ¼-20 flat head cap screws. A short rod that is threaded on the inside and outside is inserted through the back side of the insulator and in conjuncture with a wire lead allows the current to be carried to the terminals of the electrical sensors. A thin self adhesive seal is attached to one electrode of a stack to help seal the gases from escaping.
As the fuel cell stack is intended to be assembled off from the mechanical press but be easily mounted to the mechanical assembly, a method of assembling the two sides of the fuel cell stack together was needed. Short rods with spring loaded balls were used. They allow both alignment of the sides of the stack and the temporary assembly before the stack is inserted into the press.
Mechanical Assembly
One of the problems that is encountered by the customer is that the fuel cell is assembled by hand and held together with 4 bolts. When the cell is bolted together there is no way to easily achieve a repeatable pressure. The process is time consuming and can possibly lead to hazardous situations if the assembly is not bolted together properly. If it is assembled improperly Hydrogen or Oxygen can escape out of the assembly and present a fire hazard.
The system designed must be able to provide a repeatable mechanical pressure applied to the assembly. The system must also prevent possible misalignments that allow gas to escape and to seal the system to prevent this outward flow. The assembly should also be able to quickly and easily assemble the fuel cell.
Based on these requirements we were able to come up with an initial mechanical assembly design. This mechanical assembly included a rotating backing plate that allowed the fuel cell to be assembled on a horizontal surface that then locked the fuel cell into position, using pegs that replaced the 4 bolts, and allowed it to be rotated to a vertical position. Once in a vertical position the assembly can be compressed using another plate attached to a power screw for compression. The compression plate is guided by a machined slot that prevents rotation.
The power screw used needs to be able to handle a max internal fuel cell pressure of 60 PSI, including a factor of safety of 2 that is 120 PSI that needs to be handled. The initial fuel cell design had internal fuel cell dimensions of one and 5/8 inch square. Using these dimensions and power screw specifications for a ½”-10 power screw we can calculate the torque required to compress the fuel cell.
Internal Pressure / 120 / psiSurface Area / 2.64 / in^2
Max Force / 316.8 / lb
tan(l) must be less than the coefficient of friction in order to be positive locking
Screw Type: 1/2" - 10
Lead / L= / 0.1 / in
Root Dia. / Dr= / 0.45 / in
tan(l)=L/(p*Dr)
tan(l)= / 0.071
m = / 0.3 / Coefficient of friction steel to steel
tan(l) < m, therefore the screw is self locking
T=F*Dr/2*((L+p*m*Dr)/(p*Dr-m*L)
T = / 26.999 / Lb/in
T = / 2.25 / Lb/ft
This means that it will take 2 and ¼ lb/ft of torque to compress the system in a worst case scenario. This is a relatively small torque and means that this is a good power screw for our system if there is enough space to keep it in. The design also requires the power screw to be self locking and using the screw geometry we are well under the friction limit.
There are other advantages that this fuel cell design provides that will allow us to better control the fuel cell environmental conditions. The sponsor wanted to have a way to control the temperature in a range of 20oC to 80oC. In order to do this the backing plates can have water jackets machined into them. This is achieved by moving the 4 pegs to the corners of the plates. This provides adequate clearance to drill 3 holes in the plate, such that it will allow the flow of water. One of the holes will be plugged to prevent flow out.