To determine the room-temperature partial electronic conductivity of the MgB2Se4 spinels, electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) measurements were performed. Following the investigation of MgSc2Se4 in our earlier work, we assembled symmetrical Au|MgB2Se4|Au pouch cells, containing a spinel pellet with vapordeposited Au layers as ion-blocking electrodes. In the Nyquist plots (Figure ), produced from the EIS measurements, depressed semicircles for all samples were observed, as already obtained for MgSc2Se4, where the low-frequency tail is missing due to the use of imperfect ion-blocking electrodes. These similarities to MgSc2Se4 indicate that the three new spinels seem to be mixed conductors as well. Thus, the electronic conductivity σel can be calculated analogous from the electronic resistance Rel, observed at the intercept of the Nyquist plot with the real axis at low frequency, by applying Equation 1 (experimental section). Interestingly, compared to MgSc2Se4 (σel = 3.4x10 -8 S cm -1 ), up to two orders of magnitude higher electronic conductivity for the new MgB2Se4 spinels [σel(MgTm2Se4) = 3.0x10 -7 S cm -1 , σel(MgEr2Se4) = 2.7x10 -6 S cm -1 , σel(MgY2Se4) = 4.7x10 -6 S cm -1 ] were calculated, although a smaller direct band gap is predicted for MgSc2Se4 (EB = 1.727 eV) than for MgY2Se4 (EB = 2.020 eV) . Similar electrochemical results have also been obtained by the dc polarization measurement shown in Figure Interestingly, considering the phase composition obtained by the Rietveld refinements (Figure ), a clear trend of increasing electronic conductivity with the amount of binary impurity phases (B2Se3, BSe3, etc.) emerges. While MgSc2Se4 and MgTm2Se4 contain mainly impurities of MgSe (wide-band gap semiconductor) and Tm2O2Se, respectively, which are probably less electron-conducting phases, the other spinels are contaminated with considerable amounts of electronically conducting Er2Se3 (σel = 1.3x10 -1 S cm -1 ) or Y2Se3/YSe (σel = 2.0x10 -1 S cm -1 /3.3x10 -1 S cm -1 ) . This demonstrates that these impurities may have a large influence on the electronic conductivity of the investigated materials, possibly even more than point defects in the spinel phase itself, since small traces of electronically conductive impurities cannot be ruled out for all four spinels. Kundu et al. suggest that these impurities may occur as nano-scaled free-electron containing inclusions, inducing electron transport between each other by jumping/tunneling through the low-electron conducting spinel matrix, described as a Berthelot-type conductivity. Nevertheless, to make the spinels applicable, their contribution to self-discharge in electrochemical cells must be reduced either by minimizing the electronic conductivity due to optimization of the synthesis and the purity of precursors or by using electron-blocking interlayers.