**5.1 Recent exploration in MnO2**

Manganese oxides are one of the most widely investigated materials in bifunctional electrocatalysis and currently employed in commercial Zn air and alkaline batteries. The inspiration from the biological catalyst CaMn4Ox acts as an oxygen evolving centre in Photosystem II; Gorlin et al. developed a new catalyst α-Mn2O3 where Mn is in the +3 oxidation state, showing excellent bifunctional activity almost similar to the activities of precious catalysts like Pt, Ir and Ru [41]. To improve the activity of the catalyst, it is essential to understand the changes on the surface of the material during the process of OER and ORR. With this mind in their another study, a powerful in situ X-ray absorption spectroscopy technique was used to analyze the chemical nature of the surface during the process of OER and ORR [41]. Such an attempt reveals that the exposure of catalyst in ORR (0.7 V) and OER (1.8 V) potential causes the distortion of the Mn3 II,III,III O4 phase and maximum (80%) oxidation of the catalytic surface to form a mixed MnIII,IV oxide. They also confirmed that the observed result is irrespective of the film thickness of the catalyst in both OER and ORR potential. At OER potential, MnIII,IV oxide is more dominant on the catalytic surface due to the electrochemical oxidation, indicating that it is the phase responsible for the observed OER activity rather than the Mn3 II,III,III O4 phase [42]. Such an oxidation state changes upon the exposure to OER and ORR potential on α-Mn2O3, observed by the same group with the aid of ex situ X-ray photoelectron spectroscopy technique in their earlier studies [43, 44] and by Marcel Risch via in situ soft X-ray absorption spectroscopy study [45].

Moreover, the effect of surface manganese valence of MnO2 on ORR activity is studied by Tang et al. [12]; from their observation, it has been understood that the oxidation state of the catalytic surface plays a crucial role in facilitating bi-functional catalysis. The main reason for the activity of manganese oxides is the presence of surface Mn+3 ion having Mn-O-Mn bonds, which are found to be more active for OER and are not found in the species having Mn+2/Mn4+. These labile Mn-O bonds allow the formation of surface Mn-OH2 species and favor the cleavage of Mn-O2 bonds, which increases the overall activity of the catalyst. Besides, in its electronic configuration, Mn(III) having one eg <sup>1</sup> electron leads to the lattice distortion owing to the Jahn-Teller effect and contributing to the structural flexibility, promoting the catalytic activity of the material [46, 47].

The importance of surface facets on bifunctional activity has been discussed in another work [48], where different nanostructures of MnO such as nano-flowers, nanoparticles and nano-polypods are prepared with the exposure of different crystal facets. The detailed investigation of this study demonstrated that the maximum exposure of (100) facets of MnO nano-polypods largely promote the electrocatalytic activity in both OER and ORR compared to others facets. Therefore, it has been understood that (100) is the superior active facet of MnO2 for both OER and ORR.

As mentioned earlier, MnO2 can exist in large numbers of crystallographic forms, so it is essential to address the suitable crystal structure for encouraging electrolysis.

To resolve this issue, Meng et al. examined the influence of crystallographic structures of MnO2 such as α-, β-, δ-MnO2 and amorphous (AMO) MnO2 on bifunctional activity. During the formation of various crystal structures of MnO2, changes in morphologies such as nanoflakes, nanowires and nanoparticles were also obtained. The obtained results reveal that the bifunctional activity of MnO2 is strongly dependent on the structures and follows the order α > AMO > β > δ-MnO2 [32]. There is another study related to the crystal structure influence in water oxidation, which has been investigated under various conditions. In this study, the authors have prepared nine various MnOx-based catalysts, that is, α2-, β-, δ2- and γ-MnO2, Mn3O4, Mn2O3, L-MnO2, γ-MnOOH, and R-MnO2 and their water oxidation efficiency were analyzed and α-MnO2 was found in alkali media as a superior one among all catalysts [49]. These studies confirm the influence of crystallographic orientation on catalytic activity, which is majorly controlled by the morphology of the catalyst. Moreover, Debart et al. explained the highest charge storage capacity, that is, 3000 mA g<sup>1</sup> of α-MnO2, among other crystal structures such as β-MnO2, γ-MnO2, λ-MnO2, Mn2O3 and Mn3O4 through the study of rechargeable lithium-air batteries [50]. They have also showed the surpassed catalytic activity of α- and β-MnO2 nanowire compared to the bulk MnO2, which is mainly due to the higher surface area and crystal structure. This further confirms the importance of the crystal structure and the nanoscale morphology of the electrocatalyst in bifunctional activity. Our group result also supports the above-stated information that α-MnO2 nanowire turns out as the more preferable morphology and the crystal structure of MnO2 for superior bifunctional activity over the other forms [37].

Another strategy to enhance ORR activity of MnO2 is introduction of native oxygen defects without employment of any foreign additives, where the effort is devoted to identify the effect of oxygen nonstoichiometric on ORR activity in alkaline electrolytes. The study has introduced oxygen vacancy into the thermodynamically stable high purity rutile β-MnO2 by a simple heat treatment method in argon atmosphere. And it is found that the oxygen defect bearing MnO2 requires more positive overpotential, yields lower amount of peroxide in ORR and also facilitates the kinetics of OER [51].

In MnO2, especially when we intended to tune metal centre valences (oxidation state), it lead to unavoidable changes in morphology along with changes in the oxidation state. Such circumstances usually add further complications to understanding of the actual relation between the metal valence and electrochemical activity. Although it is reported that MnOOH species is superiorly ORR active among manganese oxides [52], this study did not include the influence of morphology of the catalyst on activity. The influence of morphology on ORR activity is explained by two familiar examples, where nanoparticles and nanowires are the morphology. In MnO2 nanoparticles, the activity increases in the order β- < λ- < γ- < α- ≈ δ-MnO2 [53], whereas in MnO2 nanowires, the activity follows the order λ- < β- < α-MnO2 [54]. These results provide strong evidence for the influence of nanoscale morphology on ORR activity.

Although manganese oxide is a superior non-precious bifunctional catalyst, still it is unable to surpass the activity of precious metal catalysts due to its poor conductivity. The well-known approach to improve the conductivity of the material is decoration of metal oxides over carbon-based substrates. However, in the case of bifunctional electrocatalysis, the carbon substrate is not a preferable one due to the peroxide production and self-oxidation on OER condition. To resolve this issue, Ng et al. developed manganese oxide on a stainless-steel substrate (MnOx-SS) through electrodeposition followed by calcination at 480°C. Calcination causes the phase conversion of MnOx to Mn2O3 on stainless steel. Further, this MnOx-SS material

#### *A Perspective on the Recent Amelioration of Co3O4 and MnO2 Bifunctional… DOI: http://dx.doi.org/10.5772/intechopen.109922*

employed as an O2 electrode in unitized regenerative fuel produces round-trip efficiencies of 42–45% at 20 mA cm<sup>2</sup> over 10 cycles that is excellent catalytic activity and durability in both OER and ORR even compared to the precious catalyst Pt/C-SS. The interaction between the stainless-steel substrate and catalyst significantly enhances the catalytic activity due to synergetic effect and prevents the agglomeration of catalysts by providing an appropriate platform [55].

The next approach to improve the activity is expanding the surface area and active sites by introducing porosity. The activity of the catalyst is directly proportional to the surface area due to the rise in the density of active sites. But there are many ways to introduce porosity, like the template-assisted method, surfactant-assisted micelle/ inverse micelle sol-gel method, etc. However, each method is associated with its own merits and demerits. In the case of template-assisted process, although uniformity in the pore size and stability was achieved, the template had to be etched post synthesis, which added complexity to the method and was time consuming. However, although the surfactant-assisted method is a simple and cost-effective approach, there is no assurance for uniformity in the pore size.

In this view, Kuo et al. developed a highly effective mesoporous MnOx material with a crystal structure of Mn2O3 via a simple inverse micelle template approach. This adopted methodology has the advantage of being a single-step synthesis, does not require any post-synthesis treatment, can be scalable and most importantly controls the formation of Mn2O3 with enriched Mn3+. Then, the prepared mesoporous Mn2O3 material demonstrated superior catalyst property in both electrochemical water oxidation and photochemical processes with the highest TOF value [56].

Some other attempts have also been made to tune the Mn oxidation state, that is, the introduction of foreign elements into manganese oxide. It was found that the incorporation of foreign elements like gold nanoparticles [57, 58], Ca2+ [59] and cesium [60] into manganese oxide promotes the formation of Mn+3 ions, which significantly enhances the ORR activity.

Recently, Kang et al. have reported the influence of the interlayer distance of MnO2 on OER activity by systematically varying the interlayer distance with the aid of intercalation of dissimilar-sized alkaline cations such as Li<sup>+</sup> , Na<sup>+</sup> , K<sup>+</sup> , Rb<sup>+</sup> and Cs<sup>+</sup> between the layers and the observed interlayer distance values are 0.5, 0.6, 0.9, 1.05 and 1.5 nm, respectively. At the end, they concluded that the Cs<sup>+</sup> ion intercalated MnO2 possesses a larger interlayer distance along with the accommodation of larger quantities of water molecules as a result of superiority toward OER activity, whereas Li<sup>+</sup> ion intercalated MnO2 is inactive for OER due to their smaller interlayer distance and dehydrated structure [61]. In contrast to the aforementioned report, Kosasang et al. reported that the Li<sup>+</sup> intercalated MnO2 is a superior candidate for both OER and ORR, where they intercalate the alkali cations such as Li, Na, K, Rb and Cs between the layers of MnO2 and their bifunctional activity was examined. They have further ensured the observed result by the DFT calculations [62].

From the above revealed mechanistic insights, it is understood that developing an efficient method to facilitate both ORR and OER on a single active site is not an easy task due to the drastic difference in their overpotential as well as the rate-determining step (RDS). In fact, an efficient ORR active site naturally renders a poor OER activity and vice versa, making it challenging to maintain a proper balance for reversible OER/ ORR catalysis on a single electrode surface. Due to this reason, typically employed bifunctional catalysts are a combination of precious metals and their oxides like Pt, Ru/RuO2 and Ir/IrO2, where ORR and OER reactions are taken care of by pure metal and metal oxides, respectively.

Therefore, a probable way to simultaneously catalyze both OER and ORR is to design bifunctional catalysts inherited with different active sites for ORR and OER separately. That can be achieved by the deliberate engineering of transition metal oxides, particularly their oxidation state, crystal structure, exposure of crystal plane and morphology are the efficient routes to develop an efficient inexpensive bifunctional electrocatalyst for oxygen electrode reactions.
