Building a Better Fuel Cell Using Microfluidics
By Ernie Tretkoff
A novel microfluidic fuel cell uses laminar flow to operate without a solid membrane separating fuel and oxidant, making possible efficient alkaline fuel cells that could provide cheap and effective power for small electronic devices. Paul Kenis of the University of Illinois at Urbana-Champaign described the new design at the APS March Meeting in Los Angeles.
On the macroscale, when two streams come together, turbulence causes them to mix, such as when two rivers merge or you pour cream into your coffee, Kenis explained. But on the microscale, fluids can flow without turbulence, so several thin streams can flow down the same narrow channel without mixing, creating an arrangement that looks like Aquafresh toothpaste, said Kenis.
Kenis and his colleagues took advantage of this laminar flow to design a more efficient fuel cell. A typical fuel cell consists of two electrodes, a fuel source, an oxidant, and a membrane separating the fuel and oxidant. Reactions at the anode strip protons and electrons from hydrogen atoms in the fuel. The protons pass through the membrane to the cathode, where they combine with oxygen gas to form water, while the electrons travel through an external circuit, providing current to an electronic device. Most fuel cells now use a polymer electrolyte membrane to separate the fuel and the oxidant.
The new fuel cell design does away with the membrane. Instead, it consists of a Y-shaped channel in which two tiny liquid streams, one fuel and one oxidant, merge and continue to flow in parallel without mixing in a millimeter-wide channel between two catalyst-covered electrodes.
This configuration has few parts and a simple, elegant design, said Kenis. His said his group’s tests indicate that the novel device could perform better than the standard membrane-based fuel cells, which have several significant problems. For instance, the membrane tends to be a very expensive component. Membranes can sometimes allow fuel to cross over to the wrong side, degrading the performance of the cell.
Also, although alkaline fuel cells would outperform acidic ones, membrane-based fuel cells don’t work well with alkaline chemistry, for several reasons. Most membranes, which permit protons to pass in acidic fuel cells, are not permeable to the larger hydroxide ions which would, in an alkaline cell, take the place of the protons. Also, alkaline reactions produce carbonates, which tend to clog the membrane. But in the new microfluidic fuel cell, hydroxide ions can freely diffuse through the boundary between the fuel and oxidant, and the steady flow just washes the carbonates away, so they don’t clog the device.
(Actually, alkaline fuel cells with membranes are used by the space program, but they require exceptionally pure hydrogen as the fuel to avoid clogging the membrane, and so they are prohibitively expensive for commercial applications, said Kenis.)
The new fuel cell is small to take advantage of microfluidic properties, and could not simply be scaled up to make larger fuel cells. "Since the membraneless fuel cell is based on a phenomenon that only occurs at the microscale, we can’t just scale up to larger dimensions," said Kenis. However, many of the tiny fuel cells could be linked together into arrays to produce more power.
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