The quest is on for high power and energy density power sources at ever smaller sizes for applications ranging from on-chip sensors to insertable pharma-delivery to flying microrobots to more mundane applications powering PDAs and computers. To date, batteries have powered most all small-sized devices, supplying micro-Watts to hundred Watts as needed. But everyone wants their devices to run longer and with more features in smaller packages, which means for the same size that more energy needs to be stored (the energy density determines how long they run) and the rate that the energy can be delivered (power) needs to be higher.
In a cruel twist of irony for batteries, as the power draw increases, the energy density decreases, so a careful tradeoff is made between power and energy with battery powered devices. Nanotechnology is increasing both the power and energy coming from power sources, by making better use of energy sources (such as lithium ion in batteries), and perhaps more importantly, by allowing new fuels to be used that inherently have higher energy densities.
Table 1 shows energy density per unit mass and volume for different fuels, or energy sources. Note that most fuels have an order of magnitude more energy density than lithium ion, which is now the premier battery fuel.
While nanotechnology is being used to construct better anodes and cathodes for lithium ion batteries, the redox potential does not have as much available energy as other fuels. The problem is that, while lithium ion (and zinc air) batteries can actually deliver the energy at the small scale, the other higher density fuels still need robust and efficient ways to convert that energy to power.
It is not a given that these fuels can deliver power at the very small scale, or that they can exceed the performance of lithium ion batteries. Can nanotechnology meet changes in power range, deliver energy densities greater than a kilo-Watt-hr/liter, operate over wide range of ambient conditions (temperature, pressure, and humidity), so that higher energy potential fuels can be realistically used?
Fuel cells have often been touted as a next generation power source that can deliver both high power and energy density (in part because they do not have to carry the oxidizer on-board, using the oxygen in air). However, microscale fuel cells are fraught with problems. Unlike batteries that carry both redox reactants whose products remained within the battery, and which do not need ancillary devices (save for the container and electrodes), fuel cells need a means to supply the fuel, oxygen, exhaust the products, and control the hydration level throughout the device.
Fuel cells also need a means to control the fuel and oxygen delivery with changes in electrical load, which often use elaborate mechanical and electrical control systems. Therefore, it is difficult for many fuel cells to handle huge changes in load. The main problems for microscale fuel cells, therefore, are how to supply fuel with high energy density, without using large ancillary systems that consume significant amounts of power, and for the fuel cell to respond to large changes in electrical load, in varying ambient temperatures and humidities.
In spite of these challenges, proton exchange membrane (PEM) micro-fuel cells have now reached less than 10 microliters in total volume1, as shown in Figure 1, including the fuel, PEM, and ancillary systems, with instantaneous peak power density of 360 W/l and an energy density over 250 W•hr/l, and are headed to instantaneous power density higher than 1000 W/l and an energy density above 500 W•hr/l.
Figure 1. A complete nanoenabled 10 nanoliter fuel cell.
These fuel cells run on metal hydrides, and have a dynamic range of over three orders of magnitude of operation from micro-Watts (steady-state) to milli-Watts (steady-state), with 10 mW instantaneous peak power. Nanostructured metal hydrides will react nearly instantaneously with water in any form to produce hydrogen gas, which supplies the fuel cell with its high energy density2,3,4.
However, to achieve this dynamic range, energy, and power densities, the membrane electrode assembly, which is comprised of nanopore membranes (shown in Figure 2), nanocatalysts and current collecctors, and a nanoliter mechanical control system all have to be designed and optimized to maximize fuel storage, without using parasitic power5,6.
Figure 2. Cartoon of the PEM on the left side, and an SEM image of the nanopores within the silicon on the right side. The nanopores are functionalized with sulfonate groups to allow hydration with water with deprotonated walls to enhance proton transport within the pores.
In this way, nano-chemical-electrical-mechanical systems can help pave a new path towards high energy and power density sources for a wide range of current, and most excitingly emerging applications that would not be possible without new power sources.
1. Moghaddam, S., E. Pengwang, K. Y. Lin, R. I. Masel, M. A. Shannon, "Millimeter-Scale Fuel Cell with On-Board Fuel and Passive Control System," Journal of Microelectromechanical Systems 17:6, 1388-1395, 2008.
2. Zhu, L., D. Kim, H. Kim, R. I. Masel, and M. A. Shannon, "Hydrogen Generation from Hydrides in Millimeter Scale Reactors for Micro Proton Exchange Membrane Fuel Cell Applications," Journal of Power Sources 185:2, 1334-1339, 2008.
3. Zhu, L., K. Y. Lin, R. D. Morgan, V. V. Swaminathan, H. S. Kim, B. Gurau, D. Kim, B. Bae, R. I. Masel, and M. A. Shannon, "Integrated Micro-Power Source Based on a Micro-Silicon Fuel Cell, a MEMS Hydrogen Generator," Journal of Power Sources 185:2, 1305-1310, 2008.
4. Zhu, L; V. Swaminathan; B. Gurau; R.I. Masel; and M. A. Shannon, "An Onboard Hydrogen Generation Method Based on Hydrides and Water Recovery for Micro-Fuel Cells," Journal of Power Sources 192, 556-561, 2009.
5. Moghaddam, S., E. Pengwang, R. I. Masel, and M. A. Shannon, "A Self-regulating Hydrogen Generator for Micro Fuel Cells," Journal of Power Sources (2008) 185:1, 445-450, 2008.
6. Moghaddam, S.; E. Pengwang, Y-B. Jiang, A. R. Garcia, D. J. Burnett, J. Brinker, R. I. Masel, And M. A. Shannon, "Nanoengineering a Next Generation Proton Exchange Membrane for Fuel Cells," Nature Nanotechnology (under review 2009).
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