Real-Time Visualization of Hydrogen Distribution in Metals Using Polyaniline: An Ultrasensitive Hydrogenochromic Sensor

Hydrogen is one of the key elements for the economy based on renewable and sustainable resources. Because a very low concentration of hydrogen in metallic structural materials often causes hydrogen embrittlement and decreases their mechanical property, there is a great interest in the development of an inexpensive and versatile sensor for hydrogen in metals with high sensitivity and high spatial resolution. Here, real-time visualization of atomic state hydrogen in a metal has been successfully achieved using a hydrogenochromic sensor consisting of a conventionally fabricated polyaniline layer and an interfacial Ni layer plated on the metal surface. The color change of the polyaniline layer due to its hydrogenation reaction can visualize the distribution of atomic state hydrogen in metals. Because the Ni interfacial layer functions as a catalyst for the hydrogenation reaction, the high sensitivity of ≈2 µmol m^–2 and high spatial resolution of hundreds of micrometers are realized. This study opens a gate to a general approach for visualizing atomic state hydrogen in metals using polymers. In addition to the easy fabrication process and simple analytical method, the high sensitivity and high spatial resolution make the hydrogenochromic sensor promising for studying hydrogen behavior in metals.