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Safe and effective operation of fuel cells requires
the use of a wide variety of chemical sensors
to measure the quality and composition of the
fuel and to sense the release of any harmful species
into the air. Two such sensors are being developed
by NexTech Materials Ltd. to detect hydrogen in
air and sulfur species in the fuel stream.
One of the greatest concerns as we drive towards
the hydrogen economy is the safe storage, transfer,
and operations of this explosive fuel – there
is a real danger of explosion with concentrations
as low as 4% in air, the lower explosion limit
or LEL. Thus hydrogen vehicles and fueling stations
will require significant numbers of hydrogen sensors.
It is estimated that each hydrogen-powered vehicle
will use between 10 and 20 such sensors. Current
technologies tend to be cross-sensitive to other
chemicals (e.g. sulfur) or combustibles (e.g.
carbon monoxide or natural gas), or they are relatively
expensive.
NexTech has recently developed a sensor based
on its nanoscale oxide technology. The sensor
shows a resistive response to low levels (down
to 0.1%) of hydrogen in air – well below
the LEL. Chemical resistor devices have an advantage
in that the sensor is readily integrated into
monitoring systems using relatively simple signal
processing. NexTech has demonstrated technology
that is repeatable and quantitative (see Figure
1), with virtually no cross-sensitivity to carbon
monoxide (CO) (see Figure 2).
While hydrogen generated from renewable resources
such as wind or solar is the ideal future solution
to our energy needs, fuel cells will be operated
from infrastructure fuels for many years to come.
Many applications, such as military and aerospace,
will require the direct use of diesel, gasoline
or jet fuel. Others will use natural gas either
directly (e.g. stationary) or indirectly as reformed
hydrogen. These fuels all contain some level of
sulfur species, typically organo-sulfurs, either
naturally or as a safety additive. Sulfur is a
potent poison for fuel cell anodes and catalysts
and must be managed and sensed, often to extremely
low levels. Fuel cell manufacturers have expressed
the need to sense concentrations as low as 10
parts per billion (ppb). Current sulfur sensing
technology requires oxygen, which is not available
in the fuel stream.
A novel set of chemi-resistive materials for sensing
hydrogen sulfide in fuel streams has been identified.
During sulfur removal and reforming steps of fuel
processing much of the organo-sulfurs present
are converted to hydrogen sulfide and thus this
is the compound that is most likely to slip into
the fuel stream. Various materials have demonstrated
responses to hydrogen sulfide in syngas or pure
humidified hydrogen at levels ranging from 25
ppb to 10 ppm. Figure 3 shows that the sensor
has sufficient sensitivity to detect sulfur at
the requested 10 ppb.
It should be noted that the actual sensor response
times are much faster than the apparent response
times shown so far. The slow responses are artifacts
of the sensor test configuration. Tests have recently
been performed on a redesigned test fixture that
has significantly reduced dead-space. As can be
seen in Figure 4, time to achieve 90% full scale
response (t90) is less than one minute, and is
more closely reflective of the actual sensor.
Future work for sensors includes further testing
on cross-sensitivity and assessments of sensor
lifetimes. NexTech is seeking partners to evaluate
prototypes, and to co-develop the package or integrate
into their fuel cell and environmental monitoring
technologies. For further information, please
contact Chris Holt or Jon Foreman at 614-842-6606
or email: sales@nextechmaterials.com.
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