**1. Introduction**

Consumption and production of energy is the sign of industrial growth and progress of any country, as energy develops everything, and around 85% of our energy commitments depend upon fossil fuels [1,2]. However, energy resources such as fossil fuels reduce speedily due to escalating life standards and growing populations. In addition, the economic growth of developed countries, industrial civilization and modern lifestyles rely on the energy withdrawal from gas and oil supplies [3,4]. For intermittent energy generation technologies to strengthen their foothold, energy storage solutions need to become better performing and economically viable [5].

Recently, fuel cells have been considered to be a promising energy resource in contrast to other substitutes that convert chemical energy into electrical energy during a catalytic reaction [6,7]. Various types of fuel cells are available among these; the polymer electrolyte membrane fuel cell (PEMFC) and alkaline fuel cell (AFC) have the advantage of having smaller size, light weight and astonishing power density [8]. Consequently, they can be utilized for stationary and portable applications. They can perform work constantly at low temperature and give high current densities [9]. Cost and stability, however, are the two main factors that delay the commercialization of fuel cells at a large scale.

Since the key cost is because of extreme and ineffective utilization of platinum based electro catalysts, Pt electrodes present the ideal catalytic activity for ORR (oxygen reduction reaction), thus serving as a standard electrode for all the catalysts prepared up until

**Citation:** Salahuddin, U.; Iqbal, N.; Noor, T.; Hanif, S.; Ejaz, H.; Zaman, N.; Ahmed, S. ZIF-67 Derived MnO2 Doped Electrocatalyst for Oxygen Reduction Reaction. *Catalysts* **2021**, *11*, 92. https://doi.org/ 10.3390/catal11010092

Received: 9 December 2020 Accepted: 5 January 2021 Published: 12 January 2021

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now [10]. Time is needed to prepare various new non-noble metal catalysts which have generated a lot of attraction because of their vastly effectual catalytic properties [11,12].

Multiple techniques have been tested to address the catalysis, including the use of microspheres, nanoparticles, perovskites, etc., of which metal organic frameworks are also part [13–17]. Recently, metal organic frameworks (made up of organic ligands and Inorganic metal ions) have been the subject of significant attention in the field of electrochemistry because they have a variety of structures with large surface area, large pore volume, high porosity and tunable pore size, and are being tested as an economically viable substitutes for noble metal nano-composites [18,19]. Wang et. al. designed a carbon matrix with nitrogen phosphorous doping using Cu-MOF, showing an extraordinary performance as an electro-catalyst for hydrogen evolution reaction (HER) and ORR [20].

In addition, the nano sized pores present in the metal organic frameworks, when turned into porous carbons, make the access to gues<sup>t</sup> molecules much easier, thus increasing the likelihood of an active site being available [21]. Moreover, nano carbons formed from metal organic frameworks are formed as sheets, nanotubes and multiple other forms which can act as high-performance nonmetal catalysts. Besides, for improving their mechanical strength and conductivity, they are transformed conventionally into NPC (nitrogen doped nanoporous carbon), which has shown outstanding performance in the electrochemical field. Gai et al. modify an electrode by NPC prepared from ZIF-8 for the detection of uric acid, ascorbic acid and dopamine. Rizvi et.al. reported Cu-MOF Derived Cu@AC electrocatalyst for ORR in PEMFC. The composite Cu@AC (1:1) shows the peak current density of 2.11 mA cm<sup>−</sup><sup>2</sup> in 0.1 M KOH at a potential of 0.9 V with a scan rate of 50 mV s<sup>−</sup>1, which shows superior activity compared to commercial grade Pt/C, having a peak current density of 1.37 mA cm<sup>−</sup><sup>2</sup> at a potential of 0.86 V [10]. Moreover, bimetallic MOFs have been utilized to boost the catalyst electrocatalytic performance [22,23]. Yoon el al. reported new bimetallic 2D MOFs (Co*x*Ni*y*-CATs) for electrochemical reduction of oxygen; the two metal ions, i.e., Co2+ and Ni2+, are rationally controlled in Co*x*Ni*y*-CATs (a bimetallic catalyst) for efficient performance in the oxygen reduction reaction (ORR) [24].

Electro catalysts derived from metal organic frameworks, i.e., ZIFs (Zeolitic imidazole frameworks), which are rich in transition metals, i.e., Zn+2, CO+<sup>2</sup> and nitrogen and carbon and preparation of Zeolitic frameworks, which are single-site solid catalysts with effective and uniform catalytic activity, can be accomplished via the use of metal organic frameworks [25,26]. The metal organic framework is a nitrogen and carbon precursor with a transition metal and is heat treated at 800–1000 ◦C to form nitrogen doped electro catalyst [27]. ZIFs have been utilized in the production of ORR catalysts where a metal– nitrogen–carbon structure is formed when ZIF-67 is pyrolyzed in the presence of iron carrier, which showed effective electrochemical activity owing partially to the increased surface area provided to the active metals [28,29]. ZIF-67 can also provide the basis for creating tunable structures owing to the ordered arrangemen<sup>t</sup> of atoms in the framework. The formation of nanocrystals of carbon decorated with cobalt catalyst has been reported with the ability to catalyze ORR and to perform this function in symmetry. Cobalt containing ZIF-67 based catalysts can also perform catalysis under special preparatory conditions [30,31]. This ability to create uniform crystals can also be utilized in conjunction with the flexibility of carbon materials for ORR [32,33].

Besides cobalt and nickel, manganese has also been shown to perform catalytic activity pertaining to oxygen reduction, which has sparked interest in utilizing this capability in ZIF-67 based carbon electrodes [34,35]. Work with ZIF-67 involving the use of magnesium oxides has yielded remarkable enhancement in the catalytic capability of carbon-based electrodes. In work utilizing Mn3O4 and Co3O4, aimed at catalyzing water splitting reactions and oxygen reduction, the reversible overpotentials were reported to be better than those shown by electrodes containing noble metals like platinum and ruthenium [36,37].

In this paper, we have followed a novel approach to recommend a new material for ORR reaction in fuel cells. ZIF-67 derived nanoporous carbon was modified with MnO2 particles using a simple hydrothermal process to enhance its ORR performance. Both ZIF-67 derived nanoporous carbon and the modified sample were tested through cyclic voltammetry to analyze the difference in their individual performance.

#### **2. Results and Discussion**

*2.1. Characterization of Prepared Catalyst*

The morphology of synthesized catalysts such as ZIF-67, ZIF derived carbon nanotubes (ZCNT) and Manganese oxide doped ZCNT (ZCNT-M) was analyzed by scanning electron microscopy as illustrated in Figure 1a–c. The rhombic dodecahedron shaped nano crystal of the ZIF-67 is well preserved, as shown in Figure 1a.

(**a**) 

(**c**)

From Figure 1b,c, it was observed that after pyrolysis of ZIF-67, carbon nanotubes (CNTs) are visible in SEM images of ZCNT and ZCNT-M. Moreover, at high magnification, the SEM image shows that the obtained ZIF-67 surface was smooth, and their dodecahedron-shaped crystals were closely affixed to the CNTs. In addition, the surface pop and shacks of ZIF-67 nanoparticles more evidently approve that nanoparticles of ZIF-67 were in situ grown on CNTs' surfaces.

The EDS analysis of prepared catalysts such as ZCNT and ZCNT-M shows the presence of manganese, cobalt, oxygen and carbon without any impurity. Table 1 shows the weight percentages of the following element. ZCNT has the maximum carbon percentage while other samples such as ZCNT-M have comparatively lower percentages of carbon, as the unstable organic groups have evaporated after heating, which also reduces the carbon percentage. Moreover, after Mn loading, the relative wt. % of carbon decreases in ZCNT-M, correspondingly.


**Table 1.** EDS outcome of ZCNT, ZCNT-M.

Moreover, EDS elemental mapping images, i.e., Figure 2a,b, illustrate that the uniform loading of Mn in the sample and the elemental composition match well with the expected ratio of elemental weight and atomic %.

**Figure 2.** Elemental Mapping of ZCNT-M. (**a**) map showing C, O, Mn & Co distribution, (**b**) mapshowing Mn distribution.

Figure 3a illustrates the XRD pattern of ZIF-67. The presence of characteristic peaks indicates the successful synthesis of material, i.e., 7.2◦ (011), 10.4◦ (002), 12.7◦ (112), 14.7◦ (022), 16.4◦ (013), 18◦ (222), 22.1◦ (114), 26.5◦ (134), 29.6◦ (044), 31.3◦ (244), 32.5◦ (235), and 43.1◦ (100) [38].

Figure 3b illustrates the XRD pattern of prepared ZCNT. The peak at 26.3 (002) confirms the presence of graphitized CNTs and other peaks at 44.36◦, 51.67◦ and 75.98◦ correspond to Co (111), Co (200) and Co (220) [39].

Figure 3c illustrates the XRD pattern of prepared ZCNT-M sample. The presence of characteristic peaks corresponds to cobalt carbide (JCPDS card number 43-1144) [40], cobalt oxide (JCPDS card number 43-1003) [41], manganese oxide (JCPDS card number 44-0141) [42] and cobalt manganese oxide (JCPDS card number 32-0297) [43].

**Figure 3.** XRD plot for (**a**) ZIF-67 (**b**) ZCNT (**c**) ZCNT-M.
