Abstract
Structure, gaseous phase hydrogen storage, and electrochemical properties of a series of TiVCrMn-based body-centered-cubic (BCC) alloys with different partial substitutions for Mn with covalent elements (B and Si), transition metals (Ni, Zr, Nb, and Mo), and rare earth element (La) were investigated. Although the influences from substitutions on structure and gaseous phase storage properties were minor, influences on electrochemical discharge capacity were significant. The first cycle capacity ranged from 16 mAh·g−1 (Si-substituted) to 247 mAh·g−1 (Mo-substituted). Severe alloy passivation in 30% KOH electrolyte was observed, and an original capacity close to 500 mAh·g−1 could possibly be achieved by Mo-substituted alloy if a non-corrosive electrolyte was employed. Surface coating of Nafion to the Mo-substituted alloy was able to increase the first cycle capacity to 408 mAh·g−1, but the degradation rate in mAh·g−1·cycle−1 was still similar to that of standard testing. Electrochemical capacity was found to be closely related to BCC phase unit cell volume and width of the an extra small pressure plateau at around 0.3 MPa on the 30 °C pressure-concentration-temperature (PCT) desorption isotherm. Judging from its high electrochemical discharge capacity, Mo was the most beneficial substitution in BCC alloys for Ni/metal hydride (MH) battery application.
Highlights
Among all metal hydride (MH) alloy families, body-centered-cubic (BCC) solid solution alloy has the highest reversible hydrogen storage at ambient temperature
Its gaseous phase hydrogen storage capacity is very high, few electrochemical studies have been performed on the pure BCC phase MH alloy due to its strong metal-hydrogen bonding and low surface reaction activity [2,3,4,5]
We focus on continuing the work on Ti40V30Cr15Mn15 alloy with an electrochemical study performed at room temperature and an examination of substitution effects from covalent elements, transition metals, and rare-earth elements on structure, gaseous phase, and electrochemical properties
Summary
Among all metal hydride (MH) alloy families, body-centered-cubic (BCC) solid solution alloy has the highest reversible hydrogen storage at ambient temperature. Its gaseous phase hydrogen storage capacity is very high (up to 4.0 wt%, equivalent to 1072 mAh·g−1 [1]), few electrochemical studies have been performed on the pure BCC phase MH alloy due to its strong metal-hydrogen bonding and low surface reaction activity [2,3,4,5] Inoue and his coworker reported a TiV3.4Ni0.6 alloy achieving. Mori and Iba improved both the capacity and cycle stability by adding Y, lanthanoids, Pd, or Pt into a TiCrVNi BCC alloy and reached 462 mAh·g−1 [4] Yu and his coworker reported a Ti40V30Cr15Mn15 alloy with an initial capacity of 814 mAh·g−1 measured with a rate of 10 mA·g−1 at 80 °C; degradation was high due to surface cracking, preferential leaching of V into the KOH electrolyte, and formation of TiOx on the surface that further blocks electrochemical reaction [5]. One or more secondary phases, such as C14, C15, and/or B2, with a high grain boundary density was introduced to improve the absorption kinetics [6], facilitate formation due to its brittleness [7,8,9], and increase the surface catalytic activity [10,11,12,13,14,15]
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