An analytical approach that can rapidly determine a wide range of hydrogen concentration in solid-state materials has been recently demanded to contribute to the hydrogen economy. This study presents a method for estimating hydrogen concentrations ranging from 0.2 to 7.6 mass % via laser-induced breakdown spectroscopy (LIBS) in a few seconds, with an improvement in the upper limit of determination (7.6 mass %) by approximately 1.3 times compared with a previous work (5.7 mass %). This extension of the determinable concentration range was achieved by measuring the emission intensity at 656.28 nm from the sample in a helium atmosphere at 3000 Pa under focused laser irradiation and by reducing the water residues in both the sample and gas line of the LIBS system. The as-determined hydrogen concentrations in magnesium hydride (MgH 2 ) samples agreed well with those estimated through inert gas fusion/gas chromatography. The calibration curve for LIBS analysis was acquired by measuring the emission intensity at 656.28 nm of standard Mg/MgH 2 mixtures containing various hydrogen concentrations (0, 0.1, 0.2, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and 7.6 mass %). Results indicated that the proposed LIBS-based method is applicable to the rapid quantitative analysis of hydrogen in hydrogencontaining material of MgH 2 .

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.Keywords Hydrogen mapping; Laser-induced breakdown spectroscopy; Nickel-metal hydride battery cathode 400 4.0). You are free to copy and redistribute articles in any medium 401 or format and also free to remix, transform, and build upon articles 402 for any purpose (including a commercial use) as long as you give 403 appropriate credit to the original source and provide a link to the 404 Creative Commons (CC) license. If you modify the material, you 405 must indicate changes in a proper way.406

A three-dimensional quantitative hydrogen distribution and state-of-charge distribution in a Ni–MH battery cathode can be acquired using laser-induced breakdown spectroscopy.