The Nectar fuel cell-powered USB charger converts butane into electricity
Burning fuel results in a good deal of heat. While this is a useful
property, small amounts of heat are very difficult to efficiently
convert into electrical energy. This is where fuel cells enter the
picture. In the simple example shown below, a solid oxide fuel cell
(SOFC) similar to that in the Nectar takes in hydrogen and oxygen, and
produces electricity and water. Where does the electricity come from?
Let's track the reactions taking place in the fuel cell.
We provide the fuel cell with hydrogen gas on the anode side and
oxygen (usually the oxygen in air) on the cathode side. In the cathode,
the oxygen molecule splits apart and doubly charged negative oxygen ions
are formed. The electrons needed to form the oxygen ions come from the
anode of the cell through an electrical load. Those electrons are
released in the anode by the reaction of the hydrogen fuel with the
oxygen ions to form water.
The role of the electrolyte here is key as it allows the oxygen ions
to easily pass between the cathode and the anode, but blocks electrons
from passing. This forces these electrons to pass through the external
electrical load, where their energy can do work. In the end, the
properties of the electrolyte allow a fuel cell to generate electricity.
A typical electrolyte for an SOFC is ceramic, a common example of which
is yttrium-stabilized zirconia (YSZ).
Fuel cells typically provide less than a volt of electric potential. In the basic reaction for an H
2O
2
fuel cell, an oxygen ion, a hydrogen molecule, and two electrons react
to form a molecule of water. The overall energy from burning a single
molecule of hydrogen to form water divided by the charge of a pair of
electrons is the theoretical maximum voltage of the fuel cell, which is
about 1.23 eV in this case. However, in practice and under load the
actual voltage is 0.7 to 0.8 V owing to a number of inefficiencies
dwelling within a practical fuel cell.
In developing the Nectar, the team at Lilliputian Systems decided to
use the highest energy density fuels, reaction conditions that give the
highest possible system efficiency, and to make a fuel cell of large
energy density, so that the Nectar beats the performance of a battery in
the real world. They chose butane as a fuel, a SOFC design, and to
minimize system size by using MEMS manufacturing techniques. None of
these selections lead toward simple engineering problems.
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