Today, there are only two ways to power remote sensors and similar devices that require little power over years of unattended use. For devices with lifetimes of less than 10 years, the solution is expensive, bulky lithium batteries. For longer lifetimes, the answer is batteries that draw energy from radioactive isotopes.

While experimenting with making metallic nanowires at the University of Wisconsin, Madison, Lux hit upon a way to build three-dimensional electrodes porous enough for nano fuel cells. By making a nanowire alloy of two metals, he found it was possible to remove the atoms of one metal in the alloy, leaving behind a densely porous 3-D structure that increased the surface area of the electrode by orders of magnitude.

Three-dimensionality is key, he said. "Our goal is a is a nano fuel cell on a chip, but to do that you need 3-D, because the usual planar structures inside chips do not provide enough surface area," said Lux, who performed the work with University of Puerto Rico doctoral candidate Karien Rodriguez. "We estimated that you needed to increase the surface area of electrodes about 10,000 times to achieve enough power density.

"The trick we used was to to adapt our technique for making copper-platinum [alloy] nano- wires. By removing the copper atoms from the alloy we basically destroy the nanowire, but what's left behind is a highly porous three-dimensional electrode," said Lux

Lux and Rodriguez discovered the best way to make porous 3-D platinum electrodes: soak copper-platinum alloy nanowires in nitric acid, removing their copper. Later, they found, they could create nano fuel cells by merely laying them out lithographically so their anode and cathode electrodes protruded from the same side, with a liquid electrolyte reservoir that bent to chemically connect them. With concept proven, Lux is trying to replace the liquid electrolyte with a solid-state version, enabling future remote sensor chips to potentially integrate all the components but fuel for arrays of on-chip fuel cells.

"What we already did is make arrays of copper- platinum nanowires, remove the copper, then sandwich them together in pairs in a liquid electrolyte," said Lux. "Now I am working with a group [at Lawrence Berkeley National Labs] that thinks we can replace the liquid electrolyte with a solid oxide."

Lux predicts that future applications will gang thousands of the nano fuel cells to provide milliwatts of power from an external reservoir of hydrogen fuel that could essentially be any size--potentially powering remote sensors for tens of years unattended.

Membrane transfer
Today, polymer electrolyte membrane (PEM) fuel cells channel hydrogen fuel through an intake manifold to the anodes, which are arrayed on one side of the cell. A second manifold simultaneously channels air to the cathode on the other side of the cell. A platinum catalyst at the anode splits the hydrogen into positive ions (protons) and electrons. The PEM membrane allows the protons to pass through it to the cathode, with the electrons forced to travel through the circuit they are to power before reaching the cathode. There, they enable the oxygen to combine with the hydrogen, forming water as exhaust.

Lux's approach was to instead fold the cells, so that both electrodes protrude from one side, and then stack them side-by-side. In this way the nano fuel cells could share a common reservoir of hydrogen and oxygen, while eliminating the bulky manifolds that PEM cells need to supply fuel and remove waste. Folding also enabled the researchers to demonstrate that the nano fuel cells can be easily wired in series or in parallel.

"In most designs, the cells are stacked so that the anode of one is wired to the cathode of the next cell, which gives you a series connection," Lux said. "But with our nano fuel cell design, you basically have all the cells sharing the same air and fuel reservoirs, which are outside the fuel cell array. So you can easily wire them in parallel or series."

The resulting cylindrical nano fuel cells measured only 200 nm in diameter. In the demonstration, Lux and Rodriguez stacked 109 of these cells next to one another to achieve a power density of 1 mW/cm2--a far cry of PEM's 500 mW/ cm2, but in a much smaller and longer-lived device.

Next, Lux plans on performing heat treatments to further increase the porosity of his fuel cell's electrodes, in hopes of edging its power density closer to the coveted "one water per centimeter square" goal, which would enable easy commercialization. Lux claims the nano fuel cell also offers long-term fail-safe features: Because thousands of cells will have to be ganged together, losing a few will have no effect on performance, he said.