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High-temperature PEM fuel cells: Hotter, simpler, cheaper
FEATURE
High-temperature PEM fuel cells: Hotter, simpler, cheaper By Vicki P. McConnell, North American Correspondent The myriad benefits from high-temperature proton-exchange membrane fuel cells (HTPEMFCs), which can operate between 120 and 180°C, suggest this technology could be a commercialization game changer. The essence of developing HTPEMFCs that operate beyond the temperature range of the majority of proton-exchange membrane systems – which is at 80–90°C – is threefold. They offer more efficient use of thermal energy or byproduct heat created within the electrochemical reaction; reduction of balance-of-plant (BOP) components for cooling, water management and purification; and when using reformate fuels rather than pure hydrogen, increased tolerance to carbon monoxide (CO) and sulfur, which can poison the membrane in lowertemperature PEM systems. The resulting triad of benefits – simpler yet compact and robust design running independent of humidification, high fuel flexibility, and lower cost – spell significant competitive advantages for commercial PEMFC systems. Worldwide, many membrane-electrode assembly (MEA) suppliers, bipolar plate molders, PEMFC stack manufacturers and system integrators are working to optimize HTPEMFC systems, and answer these questions: what is required of stack components to operate in this higher-temperature regime, and what are the next steps in development?
Emory De Castro, Executive Vice President, says that, ‘to the best of our knowledge, we’re the only MEA company that begins with base materials such as platinum, cloth or paper, and polymer monomers, and fabricates a complete five-layer MEA.’ Specific Celtec products include Celtec-P 1000 (for backup power systems and auxiliary power units or APUs in the power range from 250 W to 10 kW); Celtec-P 2000 (for combined heat and power or CHP systems in the 750 W to 10 kW range); and Celtec-P 3000 (for micro fuel cells in portable applications that require a power range of 10–100 W). To date, verified lifetime for Celtec MEAs is 20 000 hours with less than 6 μV/h degradation rate. The materials technology origins for Celtec HTPEMFC MEAs come out of 15 years of development, including R&D within the company’s former subsidiary, PEMEAS GmbH in Germany. ‘There were several key discoveries made along the path to technology commercialization of this MEA,’ says De Castro. ‘First, in the fabrication of the membrane (or acidic electrolyte),
which initially used a preformed, dry polybenzimidazole (PBI) membrane subsequently infused with phosphoric acid. We have since patented a process to polymerize PBI membrane material in the presence of highly concentrated phosphoric acid. This process allows us to create membranes with high loadings of acid, uniform acid distribution, and overall better proton conduction.’ The second key discovery regarding Celtec MEA technology involves a multilayered gas diffusion electrode (GDE) that is specially tuned for use with the PBI/phosphoric acid membrane. BASF’s design team used these discoveries to create an MEA with a unique electrode architecture that conducts protons in the absence of water, capitalizes on the phosphoric acid behavior, and resists poisoning from reformate fuels.
Reformation and refinement With their tolerance to high concentrations of CO and sulfur, PBI MEAs make reformate fuel opportune, and can handle the heat generated by reformers in a HTPEMFC system. Independent research by the US Naval Research Laboratory in Washington, DC tested PBI membranes (and platinum catalysts on PBI MEAs) under various conditions where
The crucial MEA component The only commercial MEA designed specifically for 120–180°C PEM fuel cell stacks is the Celtec®-P MEA product line, available from BASF Fuel Cell Inc in Somerset, New Jersey.
BASF Fuel Cell offers the only commercial MEA designed specifically for high-temperature PEM fuel cell stacks. The five-layer Celtec®-P MEA comprises platinum, graphite cloth or paper, and polymer monomers.
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Fuel Cells Bulletin
The Celtec®-P MEA product line from BASF Fuel Cell uses polymerized PBI in the presence of highly concentrated phosphoric acid to create membranes with high protonation at low water content.
December 2009
FEATURE
Plug Power is utilizing BASF’s PBI-based MEA in its GenSys® Blue 5 kW combined heat and power unit. These systems are currently undergoing field trials and certifications to support widespread availability by 2012.
reformate fuel impurities could cause poisoning and reduction of performance. The agency reports achieving anode catalyst tolerance on a PBI membrane of 10 ppm of hydrogen sulfide (H2S) and 3% CO at 160°C. Furthermore, a reversal of the poisoning effect from 1 ppm H2S on Pt catalyst on a PBI MEA is possible when the PEMFC system is exposed to neat air. The agency calls for more complete studies, but from those already conducted, suggests ‘the PBI functional groups possibly play a role in the expulsion of charged sulfur species.’[1] BASF’s De Castro observes that ‘high operating temperature is certainly a unique characteristic that gives PEM fuel cell builders and OEMs new features and possibilities, and opportunities to reduce system cost significantly.’ He adds that ‘the use of the PBI membrane for the first time extends the market for proven phosphoric acid to around the 100 kW scale, and all the way down to 25 W systems, with an inherently lower-cost manufacturing system. We believe the ability to operate a PEMFC dry at over 120°C presents a game changer in fuel cell capability.’ This May, BASF Fuel Cell Inc opened a new Celtec production facility within its 55 000 ft2 (5100 m2) New Jersey site, equipped with advanced production and automation equipment. ‘For now, we’re focused on reliably producing high quantities of Celtec MEA for our customers,’ says De Castro. ‘We expect future improvements in our MEA product line to enable increased start-stop stability in HTPEMFCs, reduced platinum catalyst loading, and lifetime of 40 000 hours under specific run conditions. We have numerous in-house programs to improve our product and provide more flexible MEA manufacturing options.’
December 2009
Testing positive in the field Since market introduction in 2005, BASF’s Celtec MEAs have been widely tested, and several companies are implementing the innovative MEA technology in their energy generation products. At Latham, New York-based Plug Power, Rick Cutright, Director of Marketing and Strategy for the Continuous Power Division, verifies that BASF’s PBI-based MEA operates at 160–180°C in the company’s GenSys® Blue combined heat and power (CHP) unit, offering 5 kW of power for residential and small commercial applications. These systems utilize a fifth-generation design, and are currently undergoing field trials and certifications to support widespread availability by 2012. Plug Power’s Cutright says that ‘reforming technology has come a long way; the complicated reformers of 10 years ago have evolved to simple, cost-effective units that are easily integrated into a HTPEMFC system.’ He believes that ‘maybe the fuel cell industry moved away from reformation too fast. Bringing biomethane into the existing natural gas infrastructure is a renewable path that doesn’t require the massive investments of a new hydrogen infrastructure.’ He does admit that the power density of HTPEMFC systems may not be ideal for automotive applications today, where size and weight are critical to performance, but they offer a great solution for stationary power systems. In terms of the introduction of commercial GenSys Blue units, Cutright has confidence that BASF’s MEA will provide the five-year, 40 000 h durability required. ‘Working with BASF, we have tested many variants of the Celtec MEA, and are comfortable that this
product will meet the CHP system demand to produce high-quality usable heat, hot water and electricity for residential and light commercial power generation.’ Cutright knows exactly how long Plug Power has been working on and field testing HTPEMFC technology, because ‘I interviewed for a job at Plug Power about 10 years ago, on the very day they tested their first high-temperature PEM cell.’ He adds that ‘our initial focus with HTPEMFCs was on CO tolerance, but we have since been able to capitalize on the multiple strengths of high-temperature systems. The high-quality waste heat enables the technology to play in many CHP markets where the combination of electrical and thermal performance can yield energy savings of 25 to 35%, and a carbon footprint reduction to match.’ Cutright cites another benefit of hightemperature PEMFC operation. ‘Typically, the cleaner an energy system is, the more water it requires or consumes. With GenSys Blue, we have an energy system that is water-neutral, solving today’s energy problems without making tomorrow’s water problems worse – so in our view, this technology is greener at the core.’
Kinetics and conduction ‘Higher-temperature operation increases the electrochemical reaction kinetics in a PEMFC,’ says Per Sune Koustrup, Sales Manager for Serenergy A/S of Hobro, Denmark. ‘This means better proton conduction and improved performance.’ Serenergy designs and manufactures its air-cooled Serenus PEMFC stacks for OEM integrators, and utilizes BASF’s Celtec MEAs in the stacks. Koustrup recounts that Mads Bang, Serenergy CTO, and Anders Risum Korsgaard, CEO, began working with the BASF MEA in 2004 while in postgraduate research at Aalborg University. They have continued to work closely with BASF since Serenergy’s founding in 2006.
Serenergy designs and manufactures air-cooled HTPEMFC stacks for OEM integrators, and utilizes BASF’s Celtec MEAs in the stacks. Serenergy believes that HTPEMFCs – such as this Serenus 390 unit – combined with reformer systems are a realistic commercial option.
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FEATURE
EnerFuel Inc is evaluating an extended-range electric vehicle (EREV) powered by a hybrid HTPEMFC/ lithium-ion (Li-ion) battery system.
In Bang’s opinion, ‘we consider the technology base of BASF fuel cell processes one of the best in the fuel cell business, with strong leadership in HTPEMFCs.’ Serenergy’s humidification-free HTPEMFC stacks, and a proprietary system controller, have been tested in 300 W to 25 kW applications, including a hybrid vehicle, small passenger ferry, and the Antares piloted aircraft in a collaboration with the German Aerospace Centre (DLR) – the German equivalent to NASA in the US. The fuel cell system in the Antares aircraft has an average efficiency of 52%. The low-pressure system utilizes a low-cost, mass-produced air blower as its only moving part. Simplifying the BOP, as noted above, boosts overall system efficiency by reducing parasitic losses. With HTPEMFCs making reformer systems a realistic option, Bang relays that ‘our integrator customers are testing both autothermal and steam reformers with our stacks, which have demonstrated CO tolerance from 0.2 to 3% from fuels such a methanol, ethanol, natural gas, diesel and JP8.’ Koustrup suggests that the HTPEMFC/ reformer start-up time issue can be addressed in several ways. ‘Preheating time can vary greatly depending on the method used – from minutes to hours – but we believe that HTPEMFC should be integrated in hybrid systems where batteries supply initial power requirements.’ When it comes to power density, Koustrup believes ‘one has to look at the complete system and not just at the stack.’ The ratios of weight to wattage should be considered; Koustrup cites a recent competitive product introduction that delivers 1.2 kW from a 22 kg (48 lb) system. ‘Our Serenus weight ratio is 140 W/kg, compared to that product at 55 W/kg,’ he observes. Koustrup goes on to say that ‘we expect to deliver a 10 kW system, including tank and 14
Fuel Cells Bulletin
reformer, in less than a year. The system will provide 50 kWh per fueling, which is comparable to the energy content in the Tesla Roadster battery. That battery weighs approximately 450 kg, which is 340 kg more than our fuel cell system. If 100 kWh of energy is required, the weight will be less than 140 kg.’ Koustrup continues: ‘Batteries will still be needed, but by using our fuel cell system in combination with batteries, one can greatly reduce overall vehicle weight; this decreases energy consumption per kilometer. Other benefits include faster acceleration and refueling, and heat utilization for cabin heating and cooling.’ ‘We are very excited about this possible game changer for the automotive industry,’ says Anders Korsgaard, CEO of Serenergy.
EV range extender Concerns about global warming and the burgeoning global energy demand are key factors in the recent rapid spate of emerging electric vehicles (EVs), with range offering a key competitive advantage. Enter the extendedrange electric vehicle, or EREV, and a hybrid HTPEMFC/lithium-ion (Li-ion) battery power system for these EREVs from EnerFuel Inc. The West Palm Beach, Florida company is a wholly owned subsidiary of Ener1 Inc, a Li-ion battery producer headquartered in New York City, with a manufacturing plant, EnerDel, based in Indianapolis, Indiana. Daniel Betts, EnerFuel’s Director of Business Affairs, cites the benefits of the fuel cell in this hybrid system. ‘The cathode output is in the form of steam, so the need to humidify the inlet air and manage liquid water that is so challenging in lower-temperature PEMFCs is eliminated. This in turn allows for simplification of the stack and total system design. A
second advantage comes from EnerFuel’s aircooled HTPEMFC design, which improves heat rejection to the environment and eliminates radiator and coolant from the BOP.’ EnerFuel has been developing proprietary HTPEMFC stacks and systems since 2006. Over the past year, a 3 kW PEMFC prototype EREV equipped with the hybrid HTPEMFC/ battery power system has been under evaluation at its West Palm Beach facility. The overall fuel cell system weighs 80 kg (176 lb) including BOP and storage tank, and can deliver 20 kWh of energy to the battery per charge. With the fuel cell, the vehicle, an AC Propulsion E-Box, had a 50% increase in typical driving range on a 1.2 kg, 5000 psi (350 bar) hydrogen tank. Betts further reports that the stack has demonstrated operating efficiency of up to 42% (including power conditioning). ‘Hybridization with batteries allows a HTPEMFC to operate at discrete power outputs, reducing complexity in overall system control, since the battery covers the transient load. Also, we can optimize the fuel cell power output to approximate average vehicle power requirements, instead of maximum power,’ Betts explains. ‘This will result in less cost, weight and size for the HTPEMFC system, and less cost and weight for the battery, by lowering the required amount of battery energy storage.’ EnerFuel’s strategic roadmap for its hybrid power system in range-extender products sets a target of product qualification by 2012, with incorporation into an OEM vehicle the following year.
Plates take a new route Conductive bipolar plates for HTPEMFC stacks are reflecting differences in design to handle the hotter, drier operating environment and deliver specific performance. James Lewis, Director of Sales and Marketing for Southampton, UK plate producer, Bac2, reports testing plates to 200°C. The plates are compression-molded from Bac2’s proprietary ElectroPhen® material, a composite with a phenolic-like polymer matrix filled with large-diameter graphite flake. The company’s solvent and catalyst expertise further enhances the successful manufacture of these high-temperature bipolar plates. Noting that flow-field channels molded into bipolar plates are proprietary to each customer, Lewis observes generally that HTPEMFC channels ‘may route the air across the plate in a way that compensates for the large difference in ambient air and operating temperature. Designs are accommodating expansion of inlet air supplied at low differential pressure.’ Post-mold temperature cycling performed by Bac2 to fully cure and quality test its high-
December 2009
FEATURE temperature plates occurs at 220–250°C. The company has single-cell tests up and running, as well as the ElectroPhen plates in multi-cell HTPEMFC stacks. Also compression-molding composite plates for HTPEMFCs is Entegris Inc of Chaska, Minnesota. Managing Director of Fuel Cells and Energy, Owen Hopkins, describes plate differences for higher-temperature operation as ‘somewhat larger, with a web thickness to 0.50 mm (0.020 in) and with a phenolic matrix to handle the heat.’ Plate material formulations are developed internally through the Entegris Specialty Materials Division. ‘Carbon black loading is approximately 80% for these plates, with conductivity at about 40 S/cm and temperature stability past 220°C in cell stack operation,’ says Hopkins. Entegris purchased Poco Graphite in 2007, and Hopkins reports that ‘we are working with the Poco team to improve the conductivity of our bipolar plate material for HTPEMFC stacks.’ Poco graphites range from highly dense grades that are impervious to water, to grades with up to 50% porosity. ‘We can make plates that are hydrophilic or hydrophobic, depending on customer requirements,’ says Hopkins. ‘Positive feedback from customers for our Poco AXF-5Q pyrolitic-coated industrial grade material convinces us that this material offers a highend solution to customers who need increased conductivity in bipolar plates and want to reduce the number of cells in their stack.’
The academic connection University research remains a vitally important incubator for advancing fuel cell technology, certainly including HTPEMFCs. A case in point: Case Western Reserve University (CWRU) in Cleveland, Ohio built and tested a HTPEMFC in 1995, patented the PBI-phosphoric acid electrolyte membrane in 1996, and has been working with the European fuel cell industry for the past five years specifically on HTPEMFC membrane R&D. According to Robert Savinell, head of the CWRU Advanced Power Institute and former director of the university’s Yeager Center for Electrochemical Sciences, ‘issues involving membrane and catalyst behavior in HTPEMFCs are under investigation within multiple research projects at CWRU.’ Savinell observes that ‘the stability or distribution of free phosphoric acid in PBI MEAs between the electrolyte and electrode phase is not fully understood, so we have research focused on gaining this understanding. The
December 2009
acid blocks Pt catalyst, preventing oxygen absorption and thus hindering electrochemical oxygen reduction, and can dissolve the catalyst into the electrolyte and lose activity. We’re examining approaches to modifying the Pt or Pt support to compensate for this acid behavior.’ He indicates this might involve nanotube or graphene support materials. Savinell is of the same opinion as BASF’s De Castro: HTPEMFCs represent a game changer for fuel cell commercialization. ‘Getting rid of the water for conducting protons and understanding fully the way that the electrolyte and catalyst interact will change the performance landscape for PEM technology at high temperature,’ notes Savinell. CWRU is part of the High Temperature Membrane Working Group within the US Department of Energy’s (DOE) $19 million High Temperature, Low Relative Humidity Membrane Program. The Florida Solar Energy Center at the University of Central Florida is the R&D lead for this Working Group, and the program has funded 12 projects to advance membrane durability and extend shelf life while reducing cost. The research emphasis for this Working Group is first to develop and evaluate new polymeric electrolyte phosphotungstic acid composite membranes. In addition, the Working Group is forming standardized experimental methodologies to characterize mechanical, mass transport and surface properties of membranes and MEAs fabricated by group members. Other academic participants on this working group include Arizona State University, Clemson University in South Carolina, Colorado School of Mines, Pennsylvania State University, University of Tennessee, and Virginia Tech. Other Working Group members include Argonne National Laboratory, Jet Propulsion Laboratory, Los Alamos National Laboratory, Lawrence Berkeley National Laboratory and Oak Ridge National Laboratory. Among industry partners are FuelCell Energy, General Electric, Giner Electrochemical Systems, Arkema, and 3M.
An alternative approach FCB talked with Dr Eric Funkenbusch, Program Director for 3M Fuel Cell Components (St. Paul, Minnesota) and Dr Steve Hamrock about the company’s development of perfluorinated sulfonic acid (PFSA) ionomeric membranes and MEAs, relative to the high-temperature category for PEMFCs. Funkenbusch suggests that 3M ‘has some of the building blocks for high-temperature MEAs, but the current level we’ve commercialized is
based on the prevailing need of our customers and partners, which is 80°C to 120°C.’ Hamrock notes that ‘we have been working with customers such as Plug Power, testing a membrane made with PBI chemistry in their liquefied petroleum gas (LPG) reformate GenSys PEM systems. This membrane chemistry delivers high proton conduction without the need for humidification beyond what is naturally produced at this temperature.’ Both attest to 3M having successfully run PFSA MEAs to over 10 000 h in PEMFC stacks under accelerated testing for automotive application. Funkenbusch points to 3M’s concentration over the past nine years on MEA manufacturing technology that uses a continuous rolled goods process and exclusive equipment built in-house. The supplier has production capacity for MEAs in the millions. Other promising research areas at 3M include nanostructured thin-film electrocatalysts that stabilize Pt and could accelerate kinetics, as well as analysis of catalyst supports, which can degrade under higher-temperature operation. Membrane nanomaterials are also in the picture, in particular to enhance stability and durability.
Just a matter of time The material universe is never in stasis, with new discoveries occurring daily. Near press time, Nature Chemistry published a research paper by a team of researchers from the University of Calgary and the National Research Council of Canada’s Steacie Institute of Molecular Sciences.[2] The team has discovered Na3(2,4,6-trihydroxy-1,3,5benzenetrisulfonate), also referred to as E-PCMOF2, which is a proton-conducting crystalline metal organic framework (MOF). Although the behavior of MOFs has been studied previously, this has not been in terms of using guest molecules as ‘scaffolding’ to help control the MOF functions. E-PCMOF2 has so far been tested in both membrane and MEA form, and demonstrated temperature resistance to 150°C. The research team includes doctoral chemistry students Jeff Hurd and George Shimizu from the University of Calgary, and researchers Christopher Ratcliffe and Igor Moudrakovski from the NRC.
References 1. Y. Garsany et al.: Comparison of the sulfur poisoning of PBI and Nafion PEMFC cathodes, Electrochemical and Solid State Letters 12(9) B138–140 (September 2009) [DOI: 10.1149/1.3168516].
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FEATURE 2. J.A. Hurd et al.: Anhydrous proton conduction at 150°C in a crystalline metal–organic framework, Nature Chemistry 1 705–710 (18 October 2009) [DOI: 10.1038/ nchem.402].
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Vicki P. McConnell, Word Warrior, has been writing about the technology and business development of the fuel cell industry for the last decade of her 30-plus year career as a technical journalist, and is based in Denver, Colorado.
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BASF Fuel Cell Inc: www.basf-fuelcell.com Plug Power: www.plugpower.com Serenergy A/S: www.serenergy.dk EnerFuel Inc: www.ener1.com Bac2: www.bac2.co.uk
automotive PEMFC
Patents MOSFET switching for improved PEMFC/energy storage system Assignee: Daimler, Germany Inventor: W. Walter Patent number: US 7514164 Published: 7 April 2009 (Filed: 6 Jan. 2005)
Fluid recovery and reuse in PEMFC system, and undesirable gas purged
Assignee: General Motors, USA Inventors: J.D. Rainville et al. Patent number: US 7514171 Published: 7 April 2009 (Filed: 9 June 2006)
Polymer electrolytes crosslinked by e-beam process, for PEMFC MEAs Assignee: 3M, USA Inventors: M.A. Yandrasits et al. Patent number: US 7514481 Published: 7 April 2009 (Filed: 29 Jan. 2007)
Assignee: Plug Power, USA Inventor: M.M. Walsh Patent number: US 7514165 Published: 7 April 2009 (Filed: 5 May 2003)
Improved measurement accuracy of current detector in automotive PEMFC system
Reduction of SOFC anodes to extend stack lifetime
Assignees: Toyota Motor Corp., Japan and Denso Corporation, Japan Inventors: M. Shige et al. Patent number: US 7517599 Published: 14 April 2009 (Filed: 13 April 2004)
Assignee: Bloom Energy Corp., USA Inventors: D. Hickey et al. Patent number: US 7514166 Published: 7 April 2009 (Filed: 1 April 2005)
Low-cost gas-liquid separator for use in DMFC portable devices Assignee: Panasonic Corporation, Japan Inventors: K. Sone et al. Patent number: US 7514168 Published: 7 April 2009 (Filed: 19 July 2004)
PEM or DMFC with unreacted air exhausted through stack, as cooling Assignee: Samsung SDI Co, Korea Inventors: S.-J. An et al. Patent number: US 7514170 Published: 7 April 2009 (Filed: 26 Jan. 2005)
Cathode humidity control during high to low power transients in 16
Fuel Cells Bulletin
Multiple pressure regime control to minimize RH excursions during transients in automotive PEMFC system Assignee: General Motors, USA Inventors: D.A. Arthur et al. Patent number: US 7517600 Published: 14 April 2009 (Filed: 1 June 2006)
Lower-cost, thinner SOFC with cell performance maintained Assignee: Dai Nippon Printing Co, Japan Inventors: K. Yoshikata et al. Patent number: US 7517601 Published: 14 April 2009 (Filed: 9 Dec. 2003)
Compact, thin, trapezoidal SOFC stack with excellent sealing Assignee: Honda Motor Co, Japan Inventor: H. Homma
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Entegris: www.entegrisfuelcells.com Case Western Reserve University, Advanced Power Institute: http://energy.case.edu/fuel_cells. shtml University of Central Florida, Florida Solar Energy Center: www.fscec.ucf.edu 3M Fuel Cell Components: www.3m.com/fuelcells
Patent number: US 7517602 Published: 14 April 2009 (Filed: 23 Dec. 2004)
Stressed thin-film membrane islands in micro SOFCs that survive repeated thermal cycling Assignee: Lilliputian Systems, USA Inventors: S.B. Schaevitz et al. Patent number: US 7517603 Published: 14 April 2009 (Filed: 23 Sep. 2004)
High-temperature (up to 150°C) PEMFC electrolyte membrane with acidic polymer Assignee: 3M, USA Inventors: S.J. Hamrock et al. Patent number: US 7517604 Published: 14 April 2009 (Filed: 19 Sep. 2005)
Planar SOFC separator with gas fed to entire current collector area Assignees: Mitsubishi Materials Corp., Japan and Kansai Electric Power Co, Japan Inventors: N. Komada et al. Patent number: US 7517605 Published: 14 April 2009 (Filed: 6 Dec. 2006)
Uniform, spherical metal-carbon composite powders, e.g. for PEMFC catalysts Assignee: Cabot Corporation, USA Inventors: T.T. Kodas et al. Patent number: US 7517606 Published: 14 April 2009 (Filed: 31 July 2002)
Light-based reactive deposition for production of PEM, MCFC and SOFC Assignee: NanoGram Corporation, USA Inventors: C.R. Horne et al. Patent number: US 7521097 Published: 21 April 2009 (Filed: 27 May 2004)
December 2009
High-temperature PEM fuel cells: Hotter, simpler, cheaper By Vicki P. McConnell, North American Correspondent The myriad benefits from high-temperature proton-exchange membrane fuel cells (HTPEMFCs), which can operate between 120 and 180°C, suggest this technology could be a commercialization game changer. The essence of developing HTPEMFCs that operate beyond the temperature range of the majority of proton-exchange membrane systems – which is at 80–90°C – is threefold. They offer more efficient use of thermal energy or byproduct heat created within the electrochemical reaction; reduction of balance-of-plant (BOP) components for cooling, water management and purification; and when using reformate fuels rather than pure hydrogen, increased tolerance to carbon monoxide (CO) and sulfur, which can poison the membrane in lowertemperature PEM systems. The resulting triad of benefits – simpler yet compact and robust design running independent of humidification, high fuel flexibility, and lower cost – spell significant competitive advantages for commercial PEMFC systems. Worldwide, many membrane-electrode assembly (MEA) suppliers, bipolar plate molders, PEMFC stack manufacturers and system integrators are working to optimize HTPEMFC systems, and answer these questions: what is required of stack components to operate in this higher-temperature regime, and what are the next steps in development?
Emory De Castro, Executive Vice President, says that, ‘to the best of our knowledge, we’re the only MEA company that begins with base materials such as platinum, cloth or paper, and polymer monomers, and fabricates a complete five-layer MEA.’ Specific Celtec products include Celtec-P 1000 (for backup power systems and auxiliary power units or APUs in the power range from 250 W to 10 kW); Celtec-P 2000 (for combined heat and power or CHP systems in the 750 W to 10 kW range); and Celtec-P 3000 (for micro fuel cells in portable applications that require a power range of 10–100 W). To date, verified lifetime for Celtec MEAs is 20 000 hours with less than 6 μV/h degradation rate. The materials technology origins for Celtec HTPEMFC MEAs come out of 15 years of development, including R&D within the company’s former subsidiary, PEMEAS GmbH in Germany. ‘There were several key discoveries made along the path to technology commercialization of this MEA,’ says De Castro. ‘First, in the fabrication of the membrane (or acidic electrolyte),
which initially used a preformed, dry polybenzimidazole (PBI) membrane subsequently infused with phosphoric acid. We have since patented a process to polymerize PBI membrane material in the presence of highly concentrated phosphoric acid. This process allows us to create membranes with high loadings of acid, uniform acid distribution, and overall better proton conduction.’ The second key discovery regarding Celtec MEA technology involves a multilayered gas diffusion electrode (GDE) that is specially tuned for use with the PBI/phosphoric acid membrane. BASF’s design team used these discoveries to create an MEA with a unique electrode architecture that conducts protons in the absence of water, capitalizes on the phosphoric acid behavior, and resists poisoning from reformate fuels.
Reformation and refinement With their tolerance to high concentrations of CO and sulfur, PBI MEAs make reformate fuel opportune, and can handle the heat generated by reformers in a HTPEMFC system. Independent research by the US Naval Research Laboratory in Washington, DC tested PBI membranes (and platinum catalysts on PBI MEAs) under various conditions where
The crucial MEA component The only commercial MEA designed specifically for 120–180°C PEM fuel cell stacks is the Celtec®-P MEA product line, available from BASF Fuel Cell Inc in Somerset, New Jersey.
BASF Fuel Cell offers the only commercial MEA designed specifically for high-temperature PEM fuel cell stacks. The five-layer Celtec®-P MEA comprises platinum, graphite cloth or paper, and polymer monomers.
12
Fuel Cells Bulletin
The Celtec®-P MEA product line from BASF Fuel Cell uses polymerized PBI in the presence of highly concentrated phosphoric acid to create membranes with high protonation at low water content.
December 2009
FEATURE
Plug Power is utilizing BASF’s PBI-based MEA in its GenSys® Blue 5 kW combined heat and power unit. These systems are currently undergoing field trials and certifications to support widespread availability by 2012.
reformate fuel impurities could cause poisoning and reduction of performance. The agency reports achieving anode catalyst tolerance on a PBI membrane of 10 ppm of hydrogen sulfide (H2S) and 3% CO at 160°C. Furthermore, a reversal of the poisoning effect from 1 ppm H2S on Pt catalyst on a PBI MEA is possible when the PEMFC system is exposed to neat air. The agency calls for more complete studies, but from those already conducted, suggests ‘the PBI functional groups possibly play a role in the expulsion of charged sulfur species.’[1] BASF’s De Castro observes that ‘high operating temperature is certainly a unique characteristic that gives PEM fuel cell builders and OEMs new features and possibilities, and opportunities to reduce system cost significantly.’ He adds that ‘the use of the PBI membrane for the first time extends the market for proven phosphoric acid to around the 100 kW scale, and all the way down to 25 W systems, with an inherently lower-cost manufacturing system. We believe the ability to operate a PEMFC dry at over 120°C presents a game changer in fuel cell capability.’ This May, BASF Fuel Cell Inc opened a new Celtec production facility within its 55 000 ft2 (5100 m2) New Jersey site, equipped with advanced production and automation equipment. ‘For now, we’re focused on reliably producing high quantities of Celtec MEA for our customers,’ says De Castro. ‘We expect future improvements in our MEA product line to enable increased start-stop stability in HTPEMFCs, reduced platinum catalyst loading, and lifetime of 40 000 hours under specific run conditions. We have numerous in-house programs to improve our product and provide more flexible MEA manufacturing options.’
December 2009
Testing positive in the field Since market introduction in 2005, BASF’s Celtec MEAs have been widely tested, and several companies are implementing the innovative MEA technology in their energy generation products. At Latham, New York-based Plug Power, Rick Cutright, Director of Marketing and Strategy for the Continuous Power Division, verifies that BASF’s PBI-based MEA operates at 160–180°C in the company’s GenSys® Blue combined heat and power (CHP) unit, offering 5 kW of power for residential and small commercial applications. These systems utilize a fifth-generation design, and are currently undergoing field trials and certifications to support widespread availability by 2012. Plug Power’s Cutright says that ‘reforming technology has come a long way; the complicated reformers of 10 years ago have evolved to simple, cost-effective units that are easily integrated into a HTPEMFC system.’ He believes that ‘maybe the fuel cell industry moved away from reformation too fast. Bringing biomethane into the existing natural gas infrastructure is a renewable path that doesn’t require the massive investments of a new hydrogen infrastructure.’ He does admit that the power density of HTPEMFC systems may not be ideal for automotive applications today, where size and weight are critical to performance, but they offer a great solution for stationary power systems. In terms of the introduction of commercial GenSys Blue units, Cutright has confidence that BASF’s MEA will provide the five-year, 40 000 h durability required. ‘Working with BASF, we have tested many variants of the Celtec MEA, and are comfortable that this
product will meet the CHP system demand to produce high-quality usable heat, hot water and electricity for residential and light commercial power generation.’ Cutright knows exactly how long Plug Power has been working on and field testing HTPEMFC technology, because ‘I interviewed for a job at Plug Power about 10 years ago, on the very day they tested their first high-temperature PEM cell.’ He adds that ‘our initial focus with HTPEMFCs was on CO tolerance, but we have since been able to capitalize on the multiple strengths of high-temperature systems. The high-quality waste heat enables the technology to play in many CHP markets where the combination of electrical and thermal performance can yield energy savings of 25 to 35%, and a carbon footprint reduction to match.’ Cutright cites another benefit of hightemperature PEMFC operation. ‘Typically, the cleaner an energy system is, the more water it requires or consumes. With GenSys Blue, we have an energy system that is water-neutral, solving today’s energy problems without making tomorrow’s water problems worse – so in our view, this technology is greener at the core.’
Kinetics and conduction ‘Higher-temperature operation increases the electrochemical reaction kinetics in a PEMFC,’ says Per Sune Koustrup, Sales Manager for Serenergy A/S of Hobro, Denmark. ‘This means better proton conduction and improved performance.’ Serenergy designs and manufactures its air-cooled Serenus PEMFC stacks for OEM integrators, and utilizes BASF’s Celtec MEAs in the stacks. Koustrup recounts that Mads Bang, Serenergy CTO, and Anders Risum Korsgaard, CEO, began working with the BASF MEA in 2004 while in postgraduate research at Aalborg University. They have continued to work closely with BASF since Serenergy’s founding in 2006.
Serenergy designs and manufactures air-cooled HTPEMFC stacks for OEM integrators, and utilizes BASF’s Celtec MEAs in the stacks. Serenergy believes that HTPEMFCs – such as this Serenus 390 unit – combined with reformer systems are a realistic commercial option.
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EnerFuel Inc is evaluating an extended-range electric vehicle (EREV) powered by a hybrid HTPEMFC/ lithium-ion (Li-ion) battery system.
In Bang’s opinion, ‘we consider the technology base of BASF fuel cell processes one of the best in the fuel cell business, with strong leadership in HTPEMFCs.’ Serenergy’s humidification-free HTPEMFC stacks, and a proprietary system controller, have been tested in 300 W to 25 kW applications, including a hybrid vehicle, small passenger ferry, and the Antares piloted aircraft in a collaboration with the German Aerospace Centre (DLR) – the German equivalent to NASA in the US. The fuel cell system in the Antares aircraft has an average efficiency of 52%. The low-pressure system utilizes a low-cost, mass-produced air blower as its only moving part. Simplifying the BOP, as noted above, boosts overall system efficiency by reducing parasitic losses. With HTPEMFCs making reformer systems a realistic option, Bang relays that ‘our integrator customers are testing both autothermal and steam reformers with our stacks, which have demonstrated CO tolerance from 0.2 to 3% from fuels such a methanol, ethanol, natural gas, diesel and JP8.’ Koustrup suggests that the HTPEMFC/ reformer start-up time issue can be addressed in several ways. ‘Preheating time can vary greatly depending on the method used – from minutes to hours – but we believe that HTPEMFC should be integrated in hybrid systems where batteries supply initial power requirements.’ When it comes to power density, Koustrup believes ‘one has to look at the complete system and not just at the stack.’ The ratios of weight to wattage should be considered; Koustrup cites a recent competitive product introduction that delivers 1.2 kW from a 22 kg (48 lb) system. ‘Our Serenus weight ratio is 140 W/kg, compared to that product at 55 W/kg,’ he observes. Koustrup goes on to say that ‘we expect to deliver a 10 kW system, including tank and 14
Fuel Cells Bulletin
reformer, in less than a year. The system will provide 50 kWh per fueling, which is comparable to the energy content in the Tesla Roadster battery. That battery weighs approximately 450 kg, which is 340 kg more than our fuel cell system. If 100 kWh of energy is required, the weight will be less than 140 kg.’ Koustrup continues: ‘Batteries will still be needed, but by using our fuel cell system in combination with batteries, one can greatly reduce overall vehicle weight; this decreases energy consumption per kilometer. Other benefits include faster acceleration and refueling, and heat utilization for cabin heating and cooling.’ ‘We are very excited about this possible game changer for the automotive industry,’ says Anders Korsgaard, CEO of Serenergy.
EV range extender Concerns about global warming and the burgeoning global energy demand are key factors in the recent rapid spate of emerging electric vehicles (EVs), with range offering a key competitive advantage. Enter the extendedrange electric vehicle, or EREV, and a hybrid HTPEMFC/lithium-ion (Li-ion) battery power system for these EREVs from EnerFuel Inc. The West Palm Beach, Florida company is a wholly owned subsidiary of Ener1 Inc, a Li-ion battery producer headquartered in New York City, with a manufacturing plant, EnerDel, based in Indianapolis, Indiana. Daniel Betts, EnerFuel’s Director of Business Affairs, cites the benefits of the fuel cell in this hybrid system. ‘The cathode output is in the form of steam, so the need to humidify the inlet air and manage liquid water that is so challenging in lower-temperature PEMFCs is eliminated. This in turn allows for simplification of the stack and total system design. A
second advantage comes from EnerFuel’s aircooled HTPEMFC design, which improves heat rejection to the environment and eliminates radiator and coolant from the BOP.’ EnerFuel has been developing proprietary HTPEMFC stacks and systems since 2006. Over the past year, a 3 kW PEMFC prototype EREV equipped with the hybrid HTPEMFC/ battery power system has been under evaluation at its West Palm Beach facility. The overall fuel cell system weighs 80 kg (176 lb) including BOP and storage tank, and can deliver 20 kWh of energy to the battery per charge. With the fuel cell, the vehicle, an AC Propulsion E-Box, had a 50% increase in typical driving range on a 1.2 kg, 5000 psi (350 bar) hydrogen tank. Betts further reports that the stack has demonstrated operating efficiency of up to 42% (including power conditioning). ‘Hybridization with batteries allows a HTPEMFC to operate at discrete power outputs, reducing complexity in overall system control, since the battery covers the transient load. Also, we can optimize the fuel cell power output to approximate average vehicle power requirements, instead of maximum power,’ Betts explains. ‘This will result in less cost, weight and size for the HTPEMFC system, and less cost and weight for the battery, by lowering the required amount of battery energy storage.’ EnerFuel’s strategic roadmap for its hybrid power system in range-extender products sets a target of product qualification by 2012, with incorporation into an OEM vehicle the following year.
Plates take a new route Conductive bipolar plates for HTPEMFC stacks are reflecting differences in design to handle the hotter, drier operating environment and deliver specific performance. James Lewis, Director of Sales and Marketing for Southampton, UK plate producer, Bac2, reports testing plates to 200°C. The plates are compression-molded from Bac2’s proprietary ElectroPhen® material, a composite with a phenolic-like polymer matrix filled with large-diameter graphite flake. The company’s solvent and catalyst expertise further enhances the successful manufacture of these high-temperature bipolar plates. Noting that flow-field channels molded into bipolar plates are proprietary to each customer, Lewis observes generally that HTPEMFC channels ‘may route the air across the plate in a way that compensates for the large difference in ambient air and operating temperature. Designs are accommodating expansion of inlet air supplied at low differential pressure.’ Post-mold temperature cycling performed by Bac2 to fully cure and quality test its high-
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FEATURE temperature plates occurs at 220–250°C. The company has single-cell tests up and running, as well as the ElectroPhen plates in multi-cell HTPEMFC stacks. Also compression-molding composite plates for HTPEMFCs is Entegris Inc of Chaska, Minnesota. Managing Director of Fuel Cells and Energy, Owen Hopkins, describes plate differences for higher-temperature operation as ‘somewhat larger, with a web thickness to 0.50 mm (0.020 in) and with a phenolic matrix to handle the heat.’ Plate material formulations are developed internally through the Entegris Specialty Materials Division. ‘Carbon black loading is approximately 80% for these plates, with conductivity at about 40 S/cm and temperature stability past 220°C in cell stack operation,’ says Hopkins. Entegris purchased Poco Graphite in 2007, and Hopkins reports that ‘we are working with the Poco team to improve the conductivity of our bipolar plate material for HTPEMFC stacks.’ Poco graphites range from highly dense grades that are impervious to water, to grades with up to 50% porosity. ‘We can make plates that are hydrophilic or hydrophobic, depending on customer requirements,’ says Hopkins. ‘Positive feedback from customers for our Poco AXF-5Q pyrolitic-coated industrial grade material convinces us that this material offers a highend solution to customers who need increased conductivity in bipolar plates and want to reduce the number of cells in their stack.’
The academic connection University research remains a vitally important incubator for advancing fuel cell technology, certainly including HTPEMFCs. A case in point: Case Western Reserve University (CWRU) in Cleveland, Ohio built and tested a HTPEMFC in 1995, patented the PBI-phosphoric acid electrolyte membrane in 1996, and has been working with the European fuel cell industry for the past five years specifically on HTPEMFC membrane R&D. According to Robert Savinell, head of the CWRU Advanced Power Institute and former director of the university’s Yeager Center for Electrochemical Sciences, ‘issues involving membrane and catalyst behavior in HTPEMFCs are under investigation within multiple research projects at CWRU.’ Savinell observes that ‘the stability or distribution of free phosphoric acid in PBI MEAs between the electrolyte and electrode phase is not fully understood, so we have research focused on gaining this understanding. The
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acid blocks Pt catalyst, preventing oxygen absorption and thus hindering electrochemical oxygen reduction, and can dissolve the catalyst into the electrolyte and lose activity. We’re examining approaches to modifying the Pt or Pt support to compensate for this acid behavior.’ He indicates this might involve nanotube or graphene support materials. Savinell is of the same opinion as BASF’s De Castro: HTPEMFCs represent a game changer for fuel cell commercialization. ‘Getting rid of the water for conducting protons and understanding fully the way that the electrolyte and catalyst interact will change the performance landscape for PEM technology at high temperature,’ notes Savinell. CWRU is part of the High Temperature Membrane Working Group within the US Department of Energy’s (DOE) $19 million High Temperature, Low Relative Humidity Membrane Program. The Florida Solar Energy Center at the University of Central Florida is the R&D lead for this Working Group, and the program has funded 12 projects to advance membrane durability and extend shelf life while reducing cost. The research emphasis for this Working Group is first to develop and evaluate new polymeric electrolyte phosphotungstic acid composite membranes. In addition, the Working Group is forming standardized experimental methodologies to characterize mechanical, mass transport and surface properties of membranes and MEAs fabricated by group members. Other academic participants on this working group include Arizona State University, Clemson University in South Carolina, Colorado School of Mines, Pennsylvania State University, University of Tennessee, and Virginia Tech. Other Working Group members include Argonne National Laboratory, Jet Propulsion Laboratory, Los Alamos National Laboratory, Lawrence Berkeley National Laboratory and Oak Ridge National Laboratory. Among industry partners are FuelCell Energy, General Electric, Giner Electrochemical Systems, Arkema, and 3M.
An alternative approach FCB talked with Dr Eric Funkenbusch, Program Director for 3M Fuel Cell Components (St. Paul, Minnesota) and Dr Steve Hamrock about the company’s development of perfluorinated sulfonic acid (PFSA) ionomeric membranes and MEAs, relative to the high-temperature category for PEMFCs. Funkenbusch suggests that 3M ‘has some of the building blocks for high-temperature MEAs, but the current level we’ve commercialized is
based on the prevailing need of our customers and partners, which is 80°C to 120°C.’ Hamrock notes that ‘we have been working with customers such as Plug Power, testing a membrane made with PBI chemistry in their liquefied petroleum gas (LPG) reformate GenSys PEM systems. This membrane chemistry delivers high proton conduction without the need for humidification beyond what is naturally produced at this temperature.’ Both attest to 3M having successfully run PFSA MEAs to over 10 000 h in PEMFC stacks under accelerated testing for automotive application. Funkenbusch points to 3M’s concentration over the past nine years on MEA manufacturing technology that uses a continuous rolled goods process and exclusive equipment built in-house. The supplier has production capacity for MEAs in the millions. Other promising research areas at 3M include nanostructured thin-film electrocatalysts that stabilize Pt and could accelerate kinetics, as well as analysis of catalyst supports, which can degrade under higher-temperature operation. Membrane nanomaterials are also in the picture, in particular to enhance stability and durability.
Just a matter of time The material universe is never in stasis, with new discoveries occurring daily. Near press time, Nature Chemistry published a research paper by a team of researchers from the University of Calgary and the National Research Council of Canada’s Steacie Institute of Molecular Sciences.[2] The team has discovered Na3(2,4,6-trihydroxy-1,3,5benzenetrisulfonate), also referred to as E-PCMOF2, which is a proton-conducting crystalline metal organic framework (MOF). Although the behavior of MOFs has been studied previously, this has not been in terms of using guest molecules as ‘scaffolding’ to help control the MOF functions. E-PCMOF2 has so far been tested in both membrane and MEA form, and demonstrated temperature resistance to 150°C. The research team includes doctoral chemistry students Jeff Hurd and George Shimizu from the University of Calgary, and researchers Christopher Ratcliffe and Igor Moudrakovski from the NRC.
References 1. Y. Garsany et al.: Comparison of the sulfur poisoning of PBI and Nafion PEMFC cathodes, Electrochemical and Solid State Letters 12(9) B138–140 (September 2009) [DOI: 10.1149/1.3168516].
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FEATURE 2. J.A. Hurd et al.: Anhydrous proton conduction at 150°C in a crystalline metal–organic framework, Nature Chemistry 1 705–710 (18 October 2009) [DOI: 10.1038/ nchem.402].
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Vicki P. McConnell, Word Warrior, has been writing about the technology and business development of the fuel cell industry for the last decade of her 30-plus year career as a technical journalist, and is based in Denver, Colorado.
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BASF Fuel Cell Inc: www.basf-fuelcell.com Plug Power: www.plugpower.com Serenergy A/S: www.serenergy.dk EnerFuel Inc: www.ener1.com Bac2: www.bac2.co.uk
automotive PEMFC
Patents MOSFET switching for improved PEMFC/energy storage system Assignee: Daimler, Germany Inventor: W. Walter Patent number: US 7514164 Published: 7 April 2009 (Filed: 6 Jan. 2005)
Fluid recovery and reuse in PEMFC system, and undesirable gas purged
Assignee: General Motors, USA Inventors: J.D. Rainville et al. Patent number: US 7514171 Published: 7 April 2009 (Filed: 9 June 2006)
Polymer electrolytes crosslinked by e-beam process, for PEMFC MEAs Assignee: 3M, USA Inventors: M.A. Yandrasits et al. Patent number: US 7514481 Published: 7 April 2009 (Filed: 29 Jan. 2007)
Assignee: Plug Power, USA Inventor: M.M. Walsh Patent number: US 7514165 Published: 7 April 2009 (Filed: 5 May 2003)
Improved measurement accuracy of current detector in automotive PEMFC system
Reduction of SOFC anodes to extend stack lifetime
Assignees: Toyota Motor Corp., Japan and Denso Corporation, Japan Inventors: M. Shige et al. Patent number: US 7517599 Published: 14 April 2009 (Filed: 13 April 2004)
Assignee: Bloom Energy Corp., USA Inventors: D. Hickey et al. Patent number: US 7514166 Published: 7 April 2009 (Filed: 1 April 2005)
Low-cost gas-liquid separator for use in DMFC portable devices Assignee: Panasonic Corporation, Japan Inventors: K. Sone et al. Patent number: US 7514168 Published: 7 April 2009 (Filed: 19 July 2004)
PEM or DMFC with unreacted air exhausted through stack, as cooling Assignee: Samsung SDI Co, Korea Inventors: S.-J. An et al. Patent number: US 7514170 Published: 7 April 2009 (Filed: 26 Jan. 2005)
Cathode humidity control during high to low power transients in 16
Fuel Cells Bulletin
Multiple pressure regime control to minimize RH excursions during transients in automotive PEMFC system Assignee: General Motors, USA Inventors: D.A. Arthur et al. Patent number: US 7517600 Published: 14 April 2009 (Filed: 1 June 2006)
Lower-cost, thinner SOFC with cell performance maintained Assignee: Dai Nippon Printing Co, Japan Inventors: K. Yoshikata et al. Patent number: US 7517601 Published: 14 April 2009 (Filed: 9 Dec. 2003)
Compact, thin, trapezoidal SOFC stack with excellent sealing Assignee: Honda Motor Co, Japan Inventor: H. Homma
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Entegris: www.entegrisfuelcells.com Case Western Reserve University, Advanced Power Institute: http://energy.case.edu/fuel_cells. shtml University of Central Florida, Florida Solar Energy Center: www.fscec.ucf.edu 3M Fuel Cell Components: www.3m.com/fuelcells
Patent number: US 7517602 Published: 14 April 2009 (Filed: 23 Dec. 2004)
Stressed thin-film membrane islands in micro SOFCs that survive repeated thermal cycling Assignee: Lilliputian Systems, USA Inventors: S.B. Schaevitz et al. Patent number: US 7517603 Published: 14 April 2009 (Filed: 23 Sep. 2004)
High-temperature (up to 150°C) PEMFC electrolyte membrane with acidic polymer Assignee: 3M, USA Inventors: S.J. Hamrock et al. Patent number: US 7517604 Published: 14 April 2009 (Filed: 19 Sep. 2005)
Planar SOFC separator with gas fed to entire current collector area Assignees: Mitsubishi Materials Corp., Japan and Kansai Electric Power Co, Japan Inventors: N. Komada et al. Patent number: US 7517605 Published: 14 April 2009 (Filed: 6 Dec. 2006)
Uniform, spherical metal-carbon composite powders, e.g. for PEMFC catalysts Assignee: Cabot Corporation, USA Inventors: T.T. Kodas et al. Patent number: US 7517606 Published: 14 April 2009 (Filed: 31 July 2002)
Light-based reactive deposition for production of PEM, MCFC and SOFC Assignee: NanoGram Corporation, USA Inventors: C.R. Horne et al. Patent number: US 7521097 Published: 21 April 2009 (Filed: 27 May 2004)
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