Speaker
Description
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 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 [${A_2B_4}$] 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.
However, ${AB_n}$ binaries do not show good enough 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 $\mathrm{Sm_2Mn_xNi_{7-x}}$ 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 discuss this point, DFT calculation was performed, and compared to anomalous diffraction results obtained on the DiffAbs beamline (SOLEIL synchrotron). For thermodynamic properties, larger and flatter isotherm curves were obtained for Mn substituted compounds with x>0.3. A reversible capacity >1.4 wt.% was 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${^{5}}$.