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Innovative Fuel Cell Stack Developments
Polymer electrolyte fuel cell technology is widely considered the most suitable for transportation and portable power applications. These types of fuel cells have low operating temperatures, high power densities, and high energy-conversion efficiencies.
Electrochemical fuel cells cleanly and efficiently convert the chemical energy of hydrogenated fuels directly into electrical energy. Like a battery, a fuel cell consists of two electrodes separated by an electrolyte made of a thin polymeric membrane. But unlike a battery, a fuel cell does not need recharging. It will continue to produce electricity as long as fuel flows through it.
The research areas below represent the range of polymer electrolyte fuel cell technology under development by Los Alamos scientists and engineers.
Direct Methanol Fuel Cells ..... Air-Breathing Stacks ..... Adiabatic Stacks
Direct Methanol Fuel Cells
Polymer electrolyte-based direct methanol fuel cells (DMFCs) are a topic of extensive research at Los Alamos, as lightweight, portable power sources. Methanol is used because of its high power density, safety, low cost, ease of handling and distribution, and high electrochemical activity. Using a liquid fuel instead of a gaseous fuel also simplifies the system design.
The Technology
In a DMFC, methanol solutions in water are fed into the anode as fuel. This allows for a substantial system simplification relative to reformate-based fuel cells and a higher energy density than that presently available with hydrogen-based systems. However, at present DMFCs require much higher platinum loadings than either hydrogen or reformate-based systems.
Background
Beginning in 1993, Los Alamos fuel cell researchers have worked to improve the performance of direct methanol fuel cells.
At the start of the project, research focused on improving anode performance, developing reliable diagnostics and screening membranes with reduced cross-over. Subsequently, the focus shifted to optimizing Nafion-based systems through various operational scenarios. Incremental improvements were made in membrane/electrode assembly (MEA) performance. These MEAs were incorporated into stacks for 80 W systems. Current research has several thrusts, all focused toward the development of stacks, yet including more aggressive attempts to identify and implement game-changing new components.
Accomplishments
Recent accomplishments include the development of improved membranes with lower cross-over, the successful deployment of MEAs based on these new polymers in fuel cells, the demonstration of stacks and (with Ball Aerospace) systems based on LANL stacks, the development of methanol sensors for use in systems and the development of lightweight high-performance stack hardware. Electrocatalytic advances have improved overall cell performance and efficiency. Some steps toward reducing catalyst loadings, thereby lowering cost, have also been taken.
Research Objectives
- Improved membranes with higher selectivity (ratio of conductivity to cross-over rate)
- Anode improvement: improved methanol oxidation catalysis
- Cathode performance improvement: improved methanol tolerant oxygen reduction reaction (ORR) catalysts, improved air electrodes for lower loading
- Implement new activity related to high temperature membranes
- Increase membrane/electrode assembly (MEA) durability
- Establish reliable MEA fabrication approaches
- Improve understanding of factors influencing electroosmosis
Research Approach
- Develop advanced polymer electrolyte membranes with low methanol permeability (or cross-over from anode to cathode) and high protonic conductivity. Collaboration with Virginia Tech has led to promising new materials. Extensive work on understanding influence of polymer composition and morphology on transport parameters.
- Improved anode activity through improved electrocatalysts and electrode structure. Work with several manufacturers is leading to advancements in catalysts. To advance the development of catalysts, we are implementing various approaches to catalyst nanoparticle production in-house.
- Improved cathode performance with less catalyst involves work on new catalysts and electrode structures.
- Improved MEA durability includes extensive diagnostic studies of electrode performance over time, identification of longevity limiting factors, and high throughput testing of new catalyst layer compositions.
- Improved methods for rapid production of MEAs on lab scale are being developed.
For information on how to license this technology, please contact Karl Jonietz in our Technology Transfer Office.
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Air-Breathing Stacks
To provide a fuel cell system for low-voltage portable power applications, Los Alamos researchers developed simple, reliable, “air-breathing” fuel cell stacks. They operate without peripheral fans for cooling or reactant flow, or the attendant electronics and controls, effectively avoiding the need for active humidification, active cooling, or cathode air pressurization. Air-breathing stacks have already been used in flashlights, remote-controlled cars, and laptop computers.
The Technology
The air-breathing stacks are stable and self-regulating, relying on diffusion-limited oxygen access to maintain a positive water balance in the cells. Oxygen in the air is allowed to diffuse into the stack from the periphery of the flow-field plates (the source of the stacks’ name,) as water diffuses out. The water produced serves as a necessary hydrant to the polymer electrolyte membrane while the surplus is lost through evaporation.
The configuration of the stack is based on circular flow-field plates and an annular hydrogen feed manifold, which are anchored by a single tie-bolt extending through the stack’s central axis.
“Air-Breather” Fuel Cell Stack Configuration
With this geometry, the hydrogen flows out from the central manifold while the oxygen diffuses inward from the periphery. The result is a system that allows for efficient cooling through conduction, maximized air access, and minimized diffusion path lengths for both hydrogen and oxygen.
The system’s radial symmetry provides for a simplified part fabrication process, and the single tie-bolt reduces the stack’s footprint, overall size, and weight. Fuel can be supplied from a pressurized source through a low-pressure regulator, or even a hydrogen-filled balloon.
For information on how to license this technology, please contact Karl Jonietz in our Technology Transfer Office.
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Adiabatic Stacks
Adiabatic fuel cell stacks have attracted industry attention for their simple design, low cost, and reliability. Operating at near-ambient pressure, their efficiency and net power density make them competitive with more complex pressurized systems.
The simplicity of adiabatic stacks is their most attractive feature and is accomplished primarily through two technological elements. First is the direct humidification of the fuel cell membrane electrode assemblies (MEAs) with liquid water, and the second is operation of the fuel cell stack at very-near-ambient pressure.
Direct MEA humidification is made possible through the introduction of an anode-wicking backing that conveys liquid water from the anode flow-field plenum through the nominally hydrophobic gas diffusion layer directly to the membrane throughout the active area.
Because even modest pressure can result in high compression power requirements, near-ambient pressure operation is critical to the stack’s efficiency. In conventional systems humidification modules, internal manifolding, and two-phase flows in the cathode channels create high-pressure drops that necessitate air inlet pressurization, but the direct humidification system avoids these pressure drops and allows the inlet pressure to be kept to about six inches of water.
During the normal operation of this well-humidified fuel cell stack with a dry, ambient temperature cathode air inlet, the airstream becomes heated and saturated with water vapor as it passes through the cells. This effect provides in situ evaporative cooling of the stack, eliminating the need for separate cooling systems or in-stack cooling plates. The non-isothermal stack operation and evaporative cooling result in an “adiabatic” stack.
The simplicity of the adiabatic system is easy to appreciate when compared to the conventional system, with its extensive flow and control elements.
A simple plastic condenser is used to recover surplus water and works effectively even in Los Alamos’ high desert climate. The single-step heat exchange process allows higher temperature differentials in the condenser than could be attained in a radiator, and may prove to be a general improvement over more conventional approaches using radiators and coolants.
For information on how to license this technology, please contact Karl Jonietz in our Technology Transfer Office.
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