Evaluating the Validity of a Hydrogen Mapping Method Based on Laser-induced Breakdown Spectroscopy

For the development and maintenance of materials and infrastructures that enable the realization of “hydrogen society”, an analytical technique capable of determining hydrogen distribution is highly desirable. Recently, we have developed a method using laser-induced breakdown spectroscopy (LIBS) for the quantitative determination of the three-dimensional hydrogen distribution; however, the validity of the method was not completely confirmed. Herein, we demonstrate the validity of the hydrogen mapping method using LIBS by comparing the hydrogen mapping and a surface photographic image of an electrodeposited Ni(OH)2 cathode that was subjected to charge-discharge reaction cycles. The hydrogen mapping was obtained by measuring the hydrogen emission line intensities at 656.28 nm under a 3000-Pa helium atmosphere for the cathode after the third charge, and a pristine Ni(OH)2 cathode and a nickel plate as reference samples for 2.15 and 0 mass% of hydrogen, respectively. The photographs of the pristine Ni(OH)2 cathode and a cathode once charged fully, which primarily comprised NiOOH, showed black and gray, respectively. In the hydrogen mapping of the cathode after the third charge, dark gray areas showed the hydrogen concentrations above 2.15 mass%, corresponding to the hydrogen concentration of Ni(OH)2, whereas gray areas showed the hydrogen concentrations between 1.09 mass% (corresponding to the hydrogen concentration of NiOOH) and 2.15 mass%. The experimental results demonstrate that the hydrogen distribution measured using LIBS corresponded to the distribution of Ni(OH)2 and NiOOH estimated according to their photographic colors, indicating the validity of the LIBS-based method for hydrogen mapping.