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Abstract
The electrochemical reactions of multi-phase metal hydride (MH) alloys were studied using a series of Laves phase-related body-centered-cubic (BCC) Ti15.6Zr2.1V43Cr11.2Mn6.9Co1.4Ni18.5Al0.3X (X = V, B, Mg, Y, Zr, Nb, Mo, La, and Nd) alloys. These alloys are composed of BCC (major), TiNi (major), C14 (minor), and Ti2Ni (minor) phases. The BCC phase was found to be responsible for the visible equilibrium pressure plateau between 0.1 MPa and 1 MPa. The plateaus belonging to the other phases occurred below 0.005 MPa. Due to the synergetic effects of other non-BCC phases, the body-centered-tetragonal (BCT) intermediate step is skipped and the face-centered-cubic (FCC) hydride phase is formed directly. During hydrogenation in both gaseous phase and electrochemistry, the non-BCC phases were first charged to completion, followed by charging of the BCC phase. In the multi-phase system, the side with a higher work function along the grain boundary is believed to be the first region that becomes hydrogenated and will not be fully dehydrided after 8 h in vacuum at 300 °C. While there is a large step at approximately 50% of the maximum hydrogen storage for the equilibrium pressure measured in gaseous phase, the charge/discharge curves measured electrochemically are very smooth, indicating a synergetic effect between BCC and non-BCC phases in the presence of voltage and charge non-neutrality. Compared to the non-BCC phases, the C14 phase benefits while the TiNi phase deteriorates the high-rate dischargeability (HRD) of the alloys. These synergetic effects are explained by the preoccupied hydrogen sites on the side of the hydrogen storage phase near the grain boundary.
Highlights
The synergetic effects in multi-phase metal hydride (MH) alloys refer to the presence of microsegregated secondary phases occurring in the melted alloys that effectively provide beneficial effects [1]
The synergetic effects in the gaseous phase hydrogen storage can improve the storage capacity and reversibility, and are characterized by a continuous transition in the pressure-concentration-temperature (PCT) isotherm, from the plateau pressure corresponding to the phase with a stronger metal–hydrogen (M–H) bond strength to the phase with a weaker M–H
Transmission electron microscopy (TEM) studies have demonstrated the interface between the main C14 phase and other secondary phases (C15, Zr7 Ni10, etc.) are clean [3,4], and strong crystallographic orientation alignment can be established by electron beam back-scattering diffraction pattern studies [5,6]
Summary
The synergetic effects in multi-phase metal hydride (MH) alloys refer to the presence of microsegregated secondary phases occurring in the melted alloys that effectively provide beneficial effects [1]. Transmission electron microscopy (TEM) studies have demonstrated the interface between the main C14 phase and other secondary phases (C15, Zr7 Ni10 , etc.) are clean [3,4], and strong crystallographic orientation alignment can be established by electron beam back-scattering diffraction pattern studies [5,6]. Some synergetic effects in the electrochemical environment are similar
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