**4.1 Chemistry and electrochemistry of MnO2**

Manganese oxide (MnO2) is a two-dimensional (2D) layered semiconducting material. The outermost electronic configuration of Mn+2 is 3d5 4s0 , where the d orbital is in the half filled (unsaturated) state, which makes it more susceptible to loss or gain of electrons; this is the triggering property in the process of catalysis. So, it can behave as a good catalyst especially in the area of electrocatalysis. The major advantage with MnO2 is the multivalent nature of manganese and its greater structural flexibility, due to which it can exist in more than 20 crystallographic forms like α-, β-, γ-, δ-, ε-, λ-types, amorphous MnO2 (AMO) and others [30, 31]. All these crystallographic forms are formed by the basic unit MnO6, where oxygen atoms are present at the top of eight surface body angles and manganese is located at the center. These crystal structures diverge from one another by way of linkage of their basic octahedral MnO6 unit, which results in each crystal structure to have distinct properties. Along with many forms of crystal structures, MnO2 is associated with many other advantages such as their capability to form many polymorphs, natural abundance, environmental compatibility, low cost and non-toxicity. There are many polymorphs of MnO2: tunnel (1D), layered (2D) and spinel (3D) structures, prepared by dedication and great efforts. Nevertheless, most of the polymorphs of MnO2 have an open tunnel structure, which can accommodate any of the small guest ions such as K<sup>+</sup> , Ag+ , Na<sup>+</sup> , Mg2+ etc. Moreover, in MnO2, Mn centers are in mixed oxidation states (+3 and +4), where the charge neutrality is maintained by the assist of guest cations entrapped in the tunnels. The bifunctional activity of MnO2 strongly depends on their crystal structure and morphology as well as their intrinsic properties. The bifunctional activity of MnO2 with respect to the crystal structure follows the order α- > AMO > β- > δ-MnO2 [32]. The superior OER activity of α-MnO2 is attributed to the presence of mixed oxidation states of manganese (average oxidation state = 3.7), their capability to exist with enormous amounts of di-μ-oxo bridges as protonation sites and their suitable tunnel sizes (0.46 0.46 nm<sup>2</sup> ) affording high accessibility to electrolytes

(reactant) as well as efficient charge transport. The excellent ORR activity (4 e transfer) of the same crystal structure is due to its expedient O2 adsorption capability and enriched amount of higher oxidation state (+3, +4) of manganese on the surface, whereas other crystal structures follow 2 e transfer reduction pathways. Amorphous MnO2 is the next better bifunctional catalyst because of the presence of excess oxygen defects and randomness. Besides, α-MnO2 contains 2x2 tunnel structures along the caxis made of double chain of the basic MnO6 unit. The higher activity of the α-form is also supported by this tunnel and layered structures, having more number of edges and corner sharing of the MnO6 unit. In addition, it is worth noting that the α-MnO2 possesses higher activity per cost than Pt/C [33].

Although there is extensive information about MnO2, the key factors influencing the OER and ORR activities are not well defined due to the lack of straightforward structure-related electrocatalytic activities. Many of the crystal structures of MnO2 are still poorly understood. However, MnO2 with various metal valences usually revealed different morphologies, which complicates the underlying relationship between metal valence and activity.
