Studies in molybdenum/manganese content in the dual body-centered-cubic phases metal hydride alloys

• Mo-addition to a body-centered-cubic alloy was studied.
• Grain boundary between two body-centered-cubic phases is critical for hydrogen storage.
• Pure body-centered-cubic phase shows an electrochemical capacity of 639 mAh g−1.
• Surface passivation from electrolyte was improved by higher concentration of KOH.

A series of TiVCrMn-based body-centered-cubic (BCC) alloys with design compositions of Ti40V30Cr15Mn15−xMox, where x = 0, 2, 4, 6, 8, 10, and 12, were prepared by an arc melter. Their microstructures were characterized and compared to the hydrogen storage properties in both gaseous phase and electrochemistry. X-ray diffraction results indicate the alloys are dominated by either one (x = 0 and 2) or two BCC phases (x = 4 to 12). As the Mo-content in the alloy increases, much of the first BCC phase is replaced by the second BCC phase with a stronger metal–hydrogen bond, lattice constant of the first BCC phase increases while that of the second BCC phase remains about the same, and both gaseous and electrochemical hydrogen storage capacities and plateau pressure increases and then decreases. The boundary between the two BCC phases is found to be crucial for the hydrogen storage properties. Alloy with a composition of Ti40V30Cr15Mn11Mo4 shows a good balance among various electrochemical properties, and alloys with higher Mo-contents (8 at% for example) achieved discharge capacity up to 639 mAh g−1 at 4 mA g−1 but are less stable in 30% KOH due to cracking and surface oxidation. Using higher concentration of KOH at 50% as the electrolyte is able to improve 8 at% Mo-containing alloy by nearly 60% by reducing the adverse effect from passivation.

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