EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

One issue regarding the use of renewable energies is related to storage. Within this frame, intermetallic compounds are key materials for energy transition as they form reversible metallic hydrides (${M}$H) that can be used to store reversibly large amounts of gaseous hydrogen in practical conditions or as anodes in Ni-${M}$H or Li-ion batteries${^{1,2}}$. Their general formula ${AB_n}$ (A = Rare Earth, Mg; B = Ni, Co, Mn; 2<n<5) can be described as stacking structures of [${AB_5}$] and [${AB_2}$] sub-units along the c crystallographic axis. They provide larger hydrogen sorption capacity than the commonly used ${AB_5}$-type alloys and better reversibility than ${AB_2}$-type alloys.

${AB_n}$ binaries do not show good practical properties for applications. Upon hydrogenation, they exhibit multiplateau behavior and poor reversibility, attributed to H-induced amorphization${^{3,4}}$. These drawbacks can be overcome by chemical substitutions on the A and/or the B sites. The present work focuses on structural, thermodynamic and corrosion properties of the pseudo-binary Sm${_2}$Mn${_x}$Ni${_{7-x}}$ system (0≤x<0.5) by means of X-ray diffraction, chemical analysis, scanning electron microscopy, thermogravimetric analysis and magnetic measurements. Unusual lattice parameter variation with Mn content is observed and could be interpreted by the formation of vacancies and preferred Mn occupation within the Ni sublattice. To clarify this point, first principle calculations were performed and compared to anomalous diffraction results obtained on the DiffAbs beamline (SOLEIL synchrotron). For thermodynamic properties, larger and flatter isotherm curves are obtained for Mn substituted compounds with ${x}$>0.3 for which a reversible capacity >1.4 wt.% is reached. Regarding corrosion, the binary compound corrodes in alkaline medium to form rare earth hydroxide and nanoporous nickel. As for the Mn-substituted compounds, a new corrosion product is formed in addition to those above mentioned, as manganese initiates a sacrificial anode mechanism taking place at the early corrosion stage${^{5}}$.