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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.
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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.
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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.
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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.
References
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).
Copyright AZoNano.com, Professor Mark A. Shannon, (University of
Illinois at Urbana-Champaign)