Editorial Feature

Transparent, Flexible Hydrogen Sensors using Palladium Nanowires

Researchers from the Korea Advanced Institute of Science and Technology (KAIST) have developed a hydrogen sensor that can measure hydrogen gas concentrations in the range 100-900 ppm using hollow Pd-Ag allow nanowires.

They used the nanowires to create transparent, bendable hydrogen sensing layers for use in applications such as wearable sensors, windows or pipes. The details of the hydrogen sensors were published in the September 2017 edition of ACS Applied Materials and Interfaces.

Figure 1. Flexible, transparent hydrogen sensors could provide the basis for wearable hydrogen sensor technologies in applications like safety goggles or hats.

Hydrogen is widely used in the chemical industry. However, hydrogen is a highly flammable gas that can cause explosions. The Hindenburg disaster of 1937, in which an airship using hydrogen fuel caught fire and killed 35 people, was generally accepted to be caused by a hydrogen leak combined with a spark. The Hindenburg disaster highlights the potential danger of hydrogen leaks.

To prevent hydrogen explosions, it is important to be able to detect hydrogen leaks quickly and accurately before hydrogen concentrations reach explosive levels. The US Department of Energy has therefore issued guidelines stating that hydrogen sensors must be able to detect 1% concentrations of hydrogen in the air in less than 60 seconds to allow for an adequate response.

Sensors that can be used in applications such as smart applications like windows, pipes and wearable sensors could form the basis of the next generation of hydrogen sensors. However, such sensors must still provide accurate and rapid hydrogen sensing capabilities, combined with additional properties such as transparency and flexibility.

Palladium-based alloys have attracted attention over recent years as promising hydrogen sensing materials due to their reliability and sensing speed. However, palladium alloys often have reduced hydrogen responses compared to pure palladium, resulting in reduced hydrogen sensitivity.

It has been suggested that nanostructured layers of palladium alloys with high surface areas and high porosity may help to overcome the limitations of palladium alloy sensors, but accurately manipulating the size, porosity and morphology of palladium alloy sensing layers to provide the required properties has so far proven to be challenging.

The team led by Professor Il-Doo Kim of the KAIST and Professor Reginald M. Penner of the University of California-Irvine, synthesized hollow Pd-Ag allow nanowires with enhanced hydrogen sensing properties. The Pd-Ag alloy nanowires were created using a galvanic replacement reaction induced by electrochemical potential differences between the two metals, in which the active Pd metal was introduced to an otherwise inactive Ag nanowire.

The technique allowed the team to create nanostructured palladium alloys with high porosity and high surface areas, which were able to detect hydrogen at low concentrations in the range 100-900 pm.  

The team also synthesized Pd-Ag alloy nanowires in a grid with perpendicular Au nanowires on a transparent polyamide substrate, which provided transparent and flexible hydrogen sensing layers. Although the transparent, flexible sensors did not demonstrate such high sensitivity or speed as the Pd-Ag alloy nanowires alone, the sensors were able to detect 900 ppm H2 both when bent and when flat.

Although further work is needed to increase their sensitivity and detection speeds, these flexible and transparent sensors could provide the basis for the next generation of hydrogen sensors in smart applications.

Transparent sensors could be used in smart windows, while flexible sensors could be used in wearable applications such as safety goggles and hats, or applied to winding objects such as gas lines to detect hydrogen leaks.

Image Credit:

Anatoly Menzhiliy/ Shutterstock.com

References:

Jang J-S, Qiao S, Choi S-J, Jha G, Ogata AF, Koo W-T, Kim D-H, Kim I-D, Penner RM, ‘Hollow Pd-Ag Composite Nanowires for Fast Responding and Transparent Hydrogen Sensors’ ACS Applied Materials & Interfaces, DOI: 10.1021/acsami.7b10908, 2017.

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