**3.4 Mixed metal engineering**

As stated in the earlier section, an esteemed electrocatalyst must be effectual in its inherent properties that are intrinsic activity, density of active sites, conductivity, surface area, wettability and stability. But practically, it is rare that a single material has all these mandatory properties for effective catalysis especially for bifunctional (ORR/OER) catalysis; hence doping the foreign metal into the lattice of the host material is a fruitful way to implant additional properties which were absent or inadequate in the host material. Among the various properties, electrical conductivity and catalytic activity are the most essential properties of the non-noble catalyst that are mainly considered inadequate and need to be improved as far as possible. These two properties could be significantly improved by two tactics: one is the doping of an appropriate metal into the host material and another is the formation of a core-shelltype hybrid structure, where the core part must be highly conductive and the shell part should be capable of catalyzing the oxygen electrode reactions [28, 29]. The enriched activity of the resultant hybrid catalyst is mainly due to the synergistic effect of both the guest and host materials. Hence, the hybrid catalyst has the advantage to utilize certain properties from one part and remaining properties from other parts of the catalyst to exhibit the improved performance of the oxygen electrode reaction. Therefore, it has been understood that mixed metal engineering is a profitable way for selective embedding of certain properties into the electrocatalyst for boosted oxygen electrocatalysis.
