Bipolar Plates CNC Machining for Fuel Cell Applications

Due to its very low electrical resistivity of 5 to 15 milliohm-cm, graphite composites enable current collection with minimal losses of under 50millivoltst per plate. They also have good corrosion resistance to the PEM environment and hydrogen permeation barrier properties. Their thermal conductivity is between 10 to 100 watts per meter-Kelvin, making them good for heat removal, and there is design flexibility for complex flow patterns. For stainless steel 316L, it is much cheaper due to the ability to use stamping, which achieves cycle times of 5 to 15 seconds, which also means higher volume rates. He also mentions that titanium alloys have the highest corrosion resistance and that he can achieve hydrogen compatibility and resistance. Lastly, carbon-coated aluminum is good for lightweight construction with adequate conductivity and corrosion protection.

Bipolar plates are CNC milled using high-speed CNC milling machines on graphite composites for the creation of flow field channels with a depth of 0.5 to 1.5 millimeters, widths of 1 to 3 millimeters, and a tolerance of ±0.003 inches. Diamond tools are used to achieve a surface finish of Ra<1.6 microns. In precision stamping, metal bipolar plates are formed in progressive dies where channel depth is controlled to an accuracy of ±0.050 millimeters. Laser welding of stamped metal half-plates creates solid hermetic passageways for coolant flow, and graphite can have complex flow patterns created with EDM machining. The graphite is milled down with surface grinding to achieve a flatness of 0.005 inches. PVD adds protective coatings of nitride or carbon with a thickness of 0.1-0.5 microns, and through-hole drilling for manifold ports provides a positional accuracy of ±0.005 inches.

The tolerances achieved include channel depth to ±0.003 inches to maintain control of the pressure drop and variability (<5 percent) across the active area. The channel width is controlled to ±0.005 inches for uniform distribution of the reactants. The overall plate dimensions are ±0.008 inches for assembly of the stack. The plates are flat to 0.005 inches for the avoidance of gas leakage, and the surface finish Ra is <1.6 microns for contact resistance to be <15 milliohm-cm² on the contact surfaces. Manifold hole positions are controlled to ±0.005 inches for the integrity of the seal.

For prototyping, Zintilon provides rapid prototyping and low-volume production of fuel cell stacks. This includes single-cell tests that analyze voltage-current performance and electrochemical impedance spectroscopy. We also produce prototype vehicles and demonstration systems that range from 100 to 5000 bipolar plates. For commercial fuel cell vehicles, we produce plates on an annual basis in the tens to hundreds of thousands range. This includes full-dimensional inspection using optical measurement systems, contact resistance testing achieving below 15 milliohm-cm² at 1.4 MPa compression, pressure drop testing validating below 5 kPa at operating flow rates, and corrosion testing per DOE protocols in a simulated fuel cell environment. We also certify materials for various tests, including electrical conductivity, hydrogen permeation rate, and corrosion testing.

Yes. All the components are fully manufactured within an ISO 9001 system as an entire quality management system with full material traceability. They meet the fuel cell standards with traceability, analysis of material dimensions as per design, and dimensions verification as per the design specification. They also comply with the standards of SAE J2719 for hydrogen fuel systems, ISO 23828 for fuel cell data exchange, DOE technical targets for bipolar plate performance, and ISO 14687 for hydrogen fuel quality. Moreover, they meet the performance targets of power density 0.3 to 0.6 watts per square centimeter, over 5000 hours, and cost targets below $3 per kilowatt for automotive applications.

Finishes comprise precision machining to Ra under 1.6 microns on graphite to reducing contact resistance of 10 to 15 milliohm-cm² at 1.4 MPa compression, titanium nitride (TiN) coated stainless steel achieving contact resistance under 20 milliohm-cm² with corrosion protection to metal ion contamination under 0.1 ppm, diamond-like carbon (DLC) coated with the highest corrosion resistance and contact resistance under 15 milliohm-cm², gold plated on contact areas to obtain minimum resistance under 10 milliohm-cm², and unique treatments such as hydrophobic coating on flow channels for easy water removal, graphitization heat treatment for enhanced electrical conductivity, and resin impregnation to seal porous graphite composites.

Standard graphite composite bipolar plates with machined flow fields will take 8–14 business days to complete, including machining and surface treatments. Stamped metal plates will take 10–16 weeks, including the time to create the tooling for your stamped metal plates. Prototype plates for single-cell testing can be completed in 6–10 days, which allows for fast flow field optimization and performance validation.

Absolutely! We focus on very lightweight plates with thicknesses under 0.5 mm, achieving stack power densities over 4kw/liter. We also build large plates with active areas over 1000 cm². We design for high temperatures too, with phosphoric acid fuel cells operating up to 200 degrees C. We improve axial flow field design to reduce pressure drop, improve water management, and overall increase field-oro flow flexibility designed specialized three-channel configurations to separate hydrogen, oxygen, and coolant flows. We engineered gradient plates with varying degrees of porosity to improve mass transport, and with embedded sensor plates to monitor current and temperature distributions. We also do segmented designs to scale the stack up from 1kw to 150kw.

Channels machined to within ±0.003 inches in depth and ±0.005 inches in width guarantee that there will be no more than 5 percent variation in current distribution during the electrochemical reaction between the reactants. This prevents reactivation gradient formation in poorly supplied zones identified with current densities between 0.8 to 1.2 amperes per square centimeter. The channel surfaces are smooth and are outlined within a roughness average (Ra) of 1.6 microns, and as a result, there is a pressure drop of 3 to 7 kPa. This allows air to be used in excess of 2 to 3 times the stoichiometric requirement, thus lowering the parasitic power of the compressor to 10 to 20 percent of the stack output. Contact surfaces of the plates within 0.005 inches guarantee uniform contact pressure between 1 to 1.5 MPa on the membrane electrode assembly, thus achieving a contact resistance of 15 milliohm-cm². Even a 5 milliohm-cm² increase will result in a 50 millivolt drop, and in turn, will reduce stack efficiency from 60 to 58 percent. A land-to-channel ratio of 1:1 provides the best combination of electrical conduction and reactant access. Quality protective coatings that prevent corrosion and maintain metal ion contamination below 0.1 ppm, with excess iron above 1 ppm. Properly designed water management channels remove flooding product water that obstructs reactant access, reducing performance by 40 percent. Thermal conductivity of 10 to 100 watts per meter-Kelvin prevents active area temperature disparity of ±5°C.
When fuel cells are made correctly, they will work well. These fuel cells will support hydrogen systems with a single-cell voltage of 0.6 to 0.7 volts, range from 0.6 to 1.2 amperes per square centimeter in current density, and a stack power density of 2 to 4 kilowatts per liter. They also have a specific power of 1 to 2 kilowatts per kilogram, and durability of more than 5000 hours in fuel cell electric vehicles. The backup power systems have 1 to 10 kilowatts, portable generators are 100 to 1000 watts, and material handling equipment is 10 to 30 kilowatts. This serves the automotive, stationary power, aerospace, and distributed energy applications.