Fuel-cell and Battery Technology Research Areas

Solid-oxide fuel cells
The higher temperature (typically above 700 °C) of solid oxide fuel cells allows for efficient use of hydrocarbon fuels. At these temperatures the waste heat from the fuel cell can be effectively integrated with the fuel processor. Energy is required to convert the hydrocarbon fuel to hydrogen and carbon dioxide. The good temperature match between the solid oxide fuel cell and the reformer leads to systems with overall high efficiency. What’s more, the additional waste heat is of high quality and can be used for other heat and power applications, such as driving an absorption chiller or feeding a low-pressure turbine.

These high temperatures, on the other hand, create challenges for the design and reliability of SOFC systems. At temperatures above 800 °C, materials for cell construction are very limited; therefore research focuses on development of electrode materials and electrolytes that can operate at lower temperatures, say 500 °C. There is also interest in the design, modeling, fabrication, and characterization of functionally graded electrodes that effectively combine gas, electron, and ionic transport. Various in-situ characterization techniques, including simultaneous FTIR/Raman (SERS & TERS) spectroscopy, impedance spectroscopy, and mass spectrometry are used to investigate the detailed mechanisms of surface reactions. Because most fossil fuels contain sulfur, a further area of research is in identifying sulfur tolerant anode materials.

Hybrid power systems
Power systems in general, and electrochemical systems for energy conversion and storage in particular, are moving more and more toward hybrid systems. The success of the hybrid electric vehicle Priusâ is a prime example. This trend extends from the smallest devices built on a “chip” to transportation vehicles to large-scale power generation from gasified coal. It should be clear based on the needs for carbon-free power, that renewable energy use and carbon sequestration must grow dramatically to limit atmospheric CO2 levels. One undesirable characteristic of most renewable energy sources is their intermittency. Power output from solar, hydroelectric, and wind systems are affected by diurnal and seasonal variations as well as the local weather. A second factor is that power generation is in general not coincident with power demand. This mismatch combined with the discontinuous nature of power generation suggests that considerable overcapacity is needed and/or an efficient and cost-effective means of energy storage and transportation are required to meet the power demands. Hybrid systems are one means to address this need. At the conceptual design stage, we seek to understand the best ways to integrate these systems together to more effectively match power demand and power generation.

Fuel processing
Despite the long-term upward trend and recent large fluctuations in prices for oil and natural gas, fossil fuels are still relatively abundant and cheap compared to alternatives. This situation is likely to continue for many years. In virtually all respects, hydrocarbon fuels are superior to hydrogen—the notable exception is carbon dioxide emissions. Energy security is enhanced by diversifying fuel sources, and technologies that allow natural gas, coal, oil, to be used more flexibly are desirable. Therefore, separations and chemical transformations of fuels, such as gas to liquids, coal gasification, and reforming, will be increasingly important.

 For the most part, hydrocarbon fuels must be chemically transformed into a stream that is hydrogen rich and containing CO and CO2 before being supplied to the fuel cell. This fuel reformation is usually done in one of three types of reactors: steam reformer, partial oxidation reformer, or an autothermal reformer. The reformation reaction is endothermic and thus energy is required. Close coupling with the fuel-cell system is essential to achieve high system efficiencies. While reforming on a large scale is relatively mature, the processes for chemical transformation for distributed generation need further development and innovative solutions.

Microscale fuel cells
Whereas most direct methanol systems are targeted at power levels of about 10 W for portable devices such as mobile phones and PDA’s, our primary focus is on very small (~mW) fuel cells operating on methanol or other simple fuels. These have potential applications for remote sensors and microelectromechanical systems (MEMS). These devices often have a duty cycle with short on-times (communicating for instance) followed by long stand-by periods. Micro-fuel cells in a hybrid system with careful attention to power management are suitable for these applications. The micro-fuel cell systems build on materials and processes presently used for electronic devices and can be placed on the substrate used for the electronic circuit. Therefore the potential for low cost is good.

Battery Materials and Rapid Charging
In contrast to fuel cells and electrolyzers, batteries already are an indispensable part of everyday life and ubiquitous. Batteries share two characteristics with the fuel cell/electrolyzer. First, both are electrochemical devices that store and convert energy. Thus, much of the underlying physics and chemistry, and many of the design principles, are shared. Second, each readily converts between chemical and electrical energy. Since electrons and hydrogen are the only carbon-free energy carriers, they are expected to play a larger and larger role in providing society’s power needs.

Efforts today are centered on finding new electrolytes and improving materials for positive and negative electrodes of advanced batteries, such as lithium. Significantly improving the energy density of these batteries requires use of metallic lithium for the negative electrode, and therefore overcoming challenges with dendrites and safety concerns.

A further interest is in integrating batteries into hybrid electric systems. Rapid charging and discharging of secondary batteries is critical in improving the operability of these systems. Detailed understanding of the limiting phenomena and development of novel control algorithms is of particular interest.