(617b) In Situ Hydrogen Utilization in An Internal Reforming Methanol Fuel Cell

Though H₂ is being considered as the optimum fuel for PEM fuel cells, a number of reasons render its use difficult, especially in mobile or portable applications. In addition, the lack of a worldwide distribution network with respect to well-established fossil fuel network favors the transition to the real hydrogen economy via a near term solution of on-site (stationary) or on-board (mobile) H₂ production from reforming of various liquid fuels such as gasoline, natural gas, alcohols, etc. The complexity of the balance of plant of a fuel cell-fuel processor unit challenges the design and development of compact and user friendly fuel cell. Depending on the starting fuel different configurations may be applied. The ideal system would be that of a pure H₂-fuelled PEMFC. However, such a system is accompanied with several drawbacks. On the other hand, a voluminous fuel processor with heat exchangers and complex BoP components is required in the case of an alternative liquid fuel as hydrogen carrier. However, the step of enrichment of the hydrogen-stream (i.e. WGS reactor) can be avoided in the case of using methanol as a starting fuel. The direct and efficient processing of methanol has been considered as a potential alternative due to the favorable properties of methanol. The desired power system can be fueled with a renewable, lightweight fuel, such as methanol and will provide uninterrupted power for extended periods at a considerable weight advantage over other standard fuel cell or battery solutions. Future emerging applications include both military and commercial applications such as portable power supply in fires, earthquakes and more, mobile computing, electric/electronic power supply for soldiers, broadband communications, remote monitoring devices.

Recent advances in the design and development of materials, such as polymer electrolyte membranes (e.g. ADVENT TPS^® high-temperature polymer electrolyte), electrocatalysts and methanol reforming catalysts, allow the operation of PEMFCs at temperatures in the range 200-220^oC. There are several technological and commercial reasons for operating PEMFC at high temperatures:

• Rates of electrochemical kinetics are enhanced
• Water management and cooling is simplified
• Useful waste heat can be recovered
• Lower quality reformed hydrogen containing up to 2% CO may be used as the fuel.

An attractive configuration which would take advantage of the HT-PEMFC features would be the incorporation of fuel reformer inside the fuel cell stack. This requires the development of highly active and selective methanol reforming catalysts, which will be able to operate at temperatures as low as possible with minimal CO formation. Taking advantage of the most recent achievements on the development of reformed hydrogen fuel cells, we propose a novel portable power system based on the incorporation of a methanol reforming catalyst into the fuel cell stack either in separate compartments or directly adjoined with the anode electrodes of the stack, so that methanol reforming takes place inside the fuel cell stack (Internal Reforming).

The proposed fuel cell comprises a high-temperature, membrane electrode assembly (HT-MEA), being able to operate at temperatures of 200-220^oC and novel reforming catalysts based on copper – manganese spinel oxides and/or PdZn-based catalysts supported on copper foam or carbon paper, that showed comparative activity at 200-220 ^oC with commercial Cu/ZnO/Al₂O₃ catalysts. The MEA has been developed by ADVENT technologies and the polymer electrolyte is based on crosslinked aromatic polyethers with pyridine units.  The MEA has proven operation on a long term basis at 210^oC with minimal H₃PO₄ loss for 500 h. The anode is directly adjoined with a methanol reforming catalyst, which will provide the required concentration of H₂. The reformer can be placed in the stack in two different ways: (i) inside the anode compartment adjacent to the Pt-based electrocatalyst, (ii) in between the bipolar plates.

The reforming catalyst is deposited on the surface of conductive structures (i.e. metallic foams or carbon papers) and could be coated by a carbon paste, which efficiently conducts the current out of the MEA to the current collector. If the reforming catalytic bed has a monolithic structure or it is conductive itself it can also operate as a current collector. Finally, current collectors on both sides are directly adjoined with bipolar plates (stainless steel, graphite or graphite composites) that surround the unit cell. The Internal Reforming Methanol Fuel Cell can be supplied with a methanol fuel (mixed with water in appropriate ratios), which is catalytically steam reformed to a H₂-rich gas mixture (it also contains carbon dioxide and water), which together with air supplied on the cathode side drive the electrocatalytic operation of HT-MEA. The outlet stream of the fuel cell contains water and carbon dioxide. The overall design allows for efficient heat management, since the “waste” heat produced by the fuel cell is in-situ utilized to drive the endothermic reforming reaction.

A single-cell of a high-temperature, polymer electrolyte fuel cell incorporating ADVENT TPS^® phosphoric acid doped copolymer and a methanol reforming catalyst in the anode has been already constructed and tested at 200^oC demonstrating the functionality of the described unit. Specific targets for improvement of the efficiency have been also identified. These are the activity of the reforming catalyst and the thermal stability of the membrane for operation above 200^oC.  The employed single cell has the following features: (i) triple serpentine flow fields arrangement, (ii) Poco^® graphite bipolar plates, (iii) copper current collectors, (iv) electrode area of 8 or 22 cm² with a platinum loading of 1.5 mg Pt cm^-2, 2 g H₃PO₄/g Pt (v) copolymer TPS^® electrolyte membrane, thickness 180 μm, 190 wt. % H₃PO₄ doping level. The membrane was sandwiched between the anodic (Cu-Mn spinel oxide supported on metallic copper foam and placed adjacent to the Pt/C anode electrocatalyst) and Pt/C cathodic gas diffusion electrodes. Vaporized methanol and water mixtures were supplied to the anode compartment, where the reforming catalyst is directly adjoined with the anode electrode. The cell temperature was set at 200°C. Electrocatalytic studies with the proposed configuration confirmed the promoting effect of H⁺ pumping through the fuel cell membrane on methanol conversion. Methanol is being reformed inside the anode compartment of the fuel cell at 200°C producing H₂, which is readily oxidized at the anode to produce electricity. The IRMFC (foam of 10 mm thickness) operated efficiently for more than 72 h at 200°C with a current density of 263 mA cm^-2 at 500 mV, when 20% CH₃OH/30% H₂O/He (anode feed) and pure O₂ (cathode feed) were supplied. Its open circuit voltage was 990 mV. It was interestingly observed that due to H₂ utilization/depletion at the anode the reforming reaction rate was enhanced even up to 20%. The obtained polarization curves and hydrogen production rates at 200^oC under fuel cell operation and under OCV conditions showed that significant improvement is achieved by decreasing the reformer thickness from 10 mm to 3 mm. Similar current densities were obtained in both configurations. At 200^oC, H₂ utilization/depletion at the anode was facilitated in the case of a foam reformer with 3 mm thickness and resulted in a much higher enhancement of methanol reforming reaction rate as compared with a reformer of 10 mm thickness. In any case, there is enough hydrogen supply for efficient fuel cell operation at 600 mV with 0.2 A cm^-2 for λ_Η2 (H₂ fed to anode electrode/H₂ reacted) = 1.2. It should be noted though that the methanol conversion obtained with the thinner foam catalyst was lower (75% under fuel cell operation). The activity of current methanol reforming catalysts at the temperature of 200^oC is clearly a constraint for further development of the IRMFC concept. There are several advantages of integrating the reformer into the fuel cell:

• Simplification of design. The reformer is placed into the stack, eliminating the need for a separate fuel processor and heat exchangers thus contributing significantly to the stability of the system.    
• Increased overall efficiency. In contrast to a conventional PEM fuel cell fed by reformed fuel, the internal reforming fuel cell does not require burning of fuel to drive the reforming reaction because it uses directly the excess heat created by the fuel cell instead. In addition, a heat exchanger is not needed because the reforming catalyst is in direct contact with the heat source.
• Minimization of system weight and volume. Elimination of the WGS and PROX reactors results in a significant decrease in the weight and volume of the whole system.
• The option of in-situ extraction of H₂ from the reforming catalyst via its oxidation by the anode electrocatalyst, enhances the reforming reaction kinetics and results in smaller mass of catalyst (a common kinetic aspect of methanol reforming catalysts is H₂ inhibition).