Proton Exchange Membrane (PEM) Fuel Cells - Conversion Of Hydrocarbon Fuels by NexTech Materials

Topics Covered

Background
Fuel Processors
     Synthesis Gas Conversion
     Water Gas Shift (WGS) Reaction
     Clean Up Stage
Pyrophoric Copper Based Catalysts
Water Gas Shift Catalysts
Applications
Benefits

Background

In order to use hydrocarbon fuels in proton-exchange-membrane (PEM) fuel cell systems, fuel (e.g., gasoline or natural gas) must first be converted into a hydrogen-rich gas with little or no carbon monoxide (CO). Fuel processors based on multiple catalytic stages are being developed for this application.

Synthesis Gas Conversion

In the first stage the hydrocarbon fuel is converted (reformed) to a synthesis gas (hydrogen and carbon monoxide) with a CO content of about 10-15 mol%.

Water Gas Shift (WGS) Reaction

The second stage is the water gas shift (WGS) reaction, which converts carbon monoxide to carbon dioxide while increasing the hydrogen content (CO + H₂O → CO₂+ H₂). This reaction is generally performed in the temperature range of 250 to 400°C.

After the WGS stage, the CO content in the gas is between 0.5 and 1 mol%.

Clean Up Stage

The final CO clean-up stage in a fuel processor involves either preferential oxidation of CO to CO₂ by addition of air or collection of hydrogen using a hydrogen permeable membrane.

In traditional WGS fuel processors the reaction is carried out with two catalytic stages: relatively inactive iron-chrome catalysts running at higher temperatures in the first and active but temperature-limited copper-based catalysts in the second.

Pyrophoric Copper Based Catalysts

A critical issue is that copper-based catalysts are pyrophoric and must be protected from air and condensed water during start-up and shut-down of the fuel processor. No single catalyst material has been available that provides the desired attributes of high activity and stability over the entire temperature range of interest - until now!

Water Gas Shift Catalysts

Our WGS catalysts, based on nano-particle ceria-based mixed oxide supports with highly dispersed precious metals, provide a technically viable and cost-effective alternative for WGS catalysts used in fuel processors. Our catalysts are highly active above 250°C and are non-pyrophoric. Furthermore, these catalysts can be washcoated onto monolithic (honeycomb or foamed ceramic) supports so that small size can be maintained and precious metal utilization maximized.

Applications

• Monolithic WGS reactors for PEM fuel cell systems
• Micro-channel WGS reactors for PEM fuel cell systems
• WGS membrane reactors for PEM fuel cell systems

Benefits

• Pt/ceria catalysts are non-pyrophoric and provide high activity at temperatures above 250ºC (see Figures 1 and 2) for excellent start up, operating and shut down performance
• Pt/ceria catalysts can be regenerated by annealing in air (see Figure 3) for long life cycles
• Catalysts can be deposited onto monoliths, using low-cost, high-volume methods that are well established for automotive catalytic converters (see Figure 5)
• Less than six grams of precious metal will be required for a WGS monolith sized for a 50-kilowatt fuel processor based upon kinetics models (see Figure 6).

Figure 1. CO conversion versus time at 250ºC for a Pt/ceria WGS catalyst (0.10 grams catalyst). Input gas flow was 251 cc/min (the gas composition is shown in the figure).

Figure 2. CO conversion versus time at 250ºC, demonstrating the effect of air annealing at 350ºC on the regeneration of performance (conditions were provided in Figure 1).

Figure 3. Conversion versus temperature for a Pt/ceria WGS, after 100 hours on stream. The dashed line shows equilibrium for this gas composition (conditions shown in Figure 1).

Figure 4. Arrhenius plots derived from low-temperature WGS data in Figure 3. The reaction rate constant (K) was calculated based on a plug flow model using reaction rate orders shown on the figure.

Figure 5. WGS performance data on obtained on one of NexTech's WGS monoliths, tested with an inlet CO content of 10 percent and a space velocity of 20,000 hr^-1 (data courtesy of Süd-Chemie, Inc.).

Figure 6. Effect of WGS operating temperature on the weight and cost of Pt/ceria catalysts required for achieving equilibrium for the gas composition shown (estimated on a per kilowatt basis).

Source: NexTech Materials

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