University of California, Irvine chemists have built the first practical nanoscale hydrogen sensor that can be used to detect dangerous levels of the explosive gas in devices such as fuel cells and internal combustion engines.
Reginald M. Penner, professor of chemistry, and his team built the sensors using nanowires made from palladium, a malleable platinum-like metallic element with high electrical conductivity.
The sensors were found to respond faster and be more selective for hydrogen than conventional sensors while needing significantly less power for operation.
‘Hydrogen is a clean energy source, but before hydrogen can replace hydrocarbons as a fuel, we need better ways to store it and to control its use in motors and fuel cells,’ Penner said. ‘Reliable, cheap, compact and safe hydrogen sensors are needed for monitoring ambient air for leaked gas. This nanowire-based sensor meets these needs.’
‘The long, uniform metal nanowires we are making are ideal for new types of devices like these sensors,’ Penner added.
The sensor consists of up to 100 palladium nanowires of between 100 to 500 microns in length contacted on each end by strips of silver. In tests, Penner’s group applied a small voltage to the sensor and then exposed it to hydrogen gas. When exposed to the gas, the wires swelled by as much as 3 percent in size. When the gas was removed, the wires then shrunk to their regular size, but in doing this, atomic breaks formed in the wires, making them non-conductive.
After these breaks had formed, the sensor was again exposed to air containing hydrogen gas. As the wires swelled, breaks in individual wires began to seal, making those wires conductive.
At lower concentrations of hydrogen, a few wires became conductive but when the chemists increased the concentration of hydrogen, an increasing number of wires became conductive, and more electricity flowed through the device.
Because of this, the researchers found that they could measure the percentage of hydrogen in the ambient air through the amount of electricity being used by the sensor – the greater the current, the higher the concentration of hydrogen.
The sensor functioned when the ambient air contained as much as a 10 percent concentration of hydrogen, at which point the breaks in all the palladium wires sealed.
This operating capacity is crucial, Penner said, because concentrations of hydrogen above 4 percent are explosive. In addition, the Penner group found that when compared to conventional sensors, their nanowire-based sensor responded faster, required much less power (less than 100 nanowatts) and was less sensitive to other gases such as carbon monoxide and methane.
In building this sensor, Penner and his team created nanowires using step-edge decoration. In this method, palladium ions are electrochemically transformed into atoms of palladium metal on a piece of graphite, and rudimentary wires begin to grow when these palladium atoms collected at step-edges, which are nanometer-height defects on the graphite surface that are similar to the steps on a staircase.
The Penner group first used step-edge decoration to make molybdenum nanowires. They discovered that this method allowed them to grow very long wires – up to a millimetre in length – featuring the strength and conductivity required for use in microelectronic devices, such as their sensors.
Patents are pending on Penner’s method of synthesising long, metallic nanowires and on the application of these wires in a hydrogen sensor.