Fuel Cell

• Building a Better MEA - Asymtek Fuel Cell Brochure (PDF 500 KB)

Fuel cells are a form of energy generation. That is, they generate electricity by employing an electrochemical reaction. The outputs from these electrochemical reactions are 1, Electricity, 2. Heat and 3. Water. (See animated Cathode picture to the right)

The way the fuel cell makes energy is by using hydrogen (in gas form) and oxygen (normally air), then passing the gas over a catalyst (Pt for anode, Pt+Ru for cathode), and getting the hydrogen electrons to travel an external path while the hydrogen protons pass through the electrolyte membrane.

What makes up a fuel cell

Fuel cells have basic components. In it’s simplistic form, usually it has an anode, cathode, and electrolyte membrane, gas diffusion layer, and a backing plate. These components make up is known as a “stack”, and the stacks are added together (much like a battery) to get the required voltage or power requirements.

Catalyst Inks

In PEMs and DMFCs, the electro-chemical reaction is between Hydrogen (H 2) and Oxygen (O 2). To cause this reaction, a catalyst is used, and (normally) it is Platinum (Pt 78). The Pt (being expensive and rare) is sintered onto a carbon (C 6) particle, then put into solution (usually some solvent like IPA or water). Along with these materials, prefluorosulfonic polymer is included. This material is used to adhere the carbon/Pt to the substrate after the solvent vehicle has evaporated.

The catalyst inks have a low viscosity. The reason for a low viscosity is, to get the best results form the electro-chemical reaction, usually several very thin layers of the ink are coated onto the substrate. This ink solution is then brushed, sprayed, or silk-screened (in some applications), and jet-dispensed onto the electrolyte membrane and/or onto the gas diffusion layer.

The electrolyte membrane is a plastic film (similar to Mylar). This material is DuPont Nafion, The Nafion if perflourosulfonic material with a polymer. The polymer hold the sulfonic acid, while the perfluoron material makes it hydrophobic. The sulfonic acid is the active material, allowing the protons to pass while making the electrons move to the external circuit.

The Nafion material may be in liquid form, and applied to other materials to form the membrane.

The Gas Diffusion Layer is a paper material, usually a carbon cloth, and is coated to prevent moisture absorption.

How to apply the catalyst

The catalysts can be applied by brush, silk-screen, spray, or inkjet, however the goal is to achieve a controlled film thickness and a certain density per centimeter 2. These technologies give some idea of the state of the art to which the developers have investigated or to the limit of achievable results.

Brushing (simple, straight forward) gives quick results to cover an area, but fail to get the desired density or film thickness.

The Silk-screen process works well for large area coverage, and high volume manufacturing. The process requires screens for every different shape or thicknesses, thus change-over becomes time consuming and costly.

Spray coating is the most widely used because it’s inexpensive (airbrushes or small spray guns can be purchased at the local auto parts store), easy to implement (operator training), and cleanup/change over is simple.

Jet dispensing offers the most versatility. Inkjet style dispensing can be versitle for programming different shapes, patterns or designs. Inkjets offer simple cleanup and change over, as well.

The desired film thickness and density of the catalyst ink relate back to the efficiency of the membrane and how well it makes energy. The density is mostly responsible for the use of spray technology.

The catalyst (in powder form) is mixed with some very low viscosity solvent, that becomes the vehicle. Spraying (now that the material is in a form that can be atomized) allows for even distribution of the material, and the quick evaporation of the vehicle solvent. Because the carbon/platinum particles are small, it takes several passes (with the spray gun) to build up the density, and thus the film thickness.

To help control the density, a scale can be employed to help measure the flow of material through the dispensing technology, onto the membrane. By measuring the amount of material (flowing from the dispensing technology) for a given amount of time, a rudimentary calculation made to control the volume going onto the membrane (usually controlling the traverse velocity of the dispensing technology).

Specific catalyst needs

In applications where spray or inkjet technology is employed, most catalyst inks tend to be thin, low viscosity fluids. Where brush and screen print technologies are used, the inks are usually thicker. The different viscosities present different challenges for controlling key parameters.

The thin viscosities cause the platinum particles to fall out of suspension, thus changing the density. To help, agitating the material keeps the particles in suspension and aids the proper density. Whereas thicker viscosity materials (used for brush or silk screenprint applications) require a formulation that does not dry out, however this process utilizes an oven to dry or cure the materials (once applied). To help with the peculiarities of the materials (particle sedimentation, fume exhaust, membrane fixturing), the platform and software need to have additional options or features that point to these material qualities.

Other applications

Applications for dispensing on fuel cell components are varied (depending upon the type of fuel cell). The PEM and DMFC require the catalysts on a membrane, however there are form-in-place gasket applications, sealing, and bonding that require investigation.

Links

http://www.fuelcells.org/

http://w1.cabot-corp.com/controller.jsp?entry=market&N=23+4294967098+1000

http://en.wikipedia.org/wiki/Fuel_cell

Asymtek at the Hannover Messe (Fuel Cell Trade Show)

Dispensing Systems

• S-910 Spectrum™ Scalable Dispensing Solutions for PCB Assembly and Microelectronics Packaging
• S-920 Spectrum™ Scalable Dispensing Solutions for Microelectronics Packaging
• X-1010 Axiom™ SMT Dispenser
• X-1020 Axiom™ Semiconductor Dispenser
• S-820 Spectrum™ Precision Batch Semiconductor Dispenser
• D-580 DispenseMate® Benchtop Dispensing System

Pumps & Valves

• DJ-9000 DispenseJet® The Latest Jet Technology
• DV-7000 Heli-Flow® Pump
• DV-8000 Heli-Flow® Pump

Fuel Cell Information Request Form

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