Finally, with the MnPS 3 @PEI system, we find a result that fulfills both criteria: It is stable for long time in solution (as indicated by AFM) and has a high catalytic performance. The reason for this interesting result may be related to the structure of PEI, much less dense and packed than the PVP one, allowing the access of H 3 O + ions to the layers. At the same time, it is tight enough for preventing the restacking of MnPS 3 layers with time. In fact, PEI capping agent has been used previously for catalysis-related applications of nanomaterials. In order to compare our results with the ones obtained in the literature, all previous measurements have been performed in acidic media and using a platinum wire as the counter electrode. However, knowing that some platinum contamination may happen with this system, 29 we have repeated the measurements using a graphite rod instead of platinum as a counter electrode, as some works suggest. The overpotential values are much higher with this configuration, but the tendency among the four systems is the same, being the best catalyst MnPS 3 @H 2 O, followed by MnPS 3 @PEI, MnPS 3 @PVP, and bulk MnPS 3 (ESI Fig S13). This confirms the higher availability of electrocatalytic active sites in bare flakes and in those covered with PEI, in comparison with those covered with PVP.