*2.2. Electrochemical Analyses*

Eco Chemie Autolab PGSTAT 302 potentiostat/galvanostat (Utrech, The Netherlands) was used to perform cyclic voltammetry measurements and GPES software 4.9 was used to run the experiments on the equipment. A three-electrode system was used in which Ag/AgCl was used as the reference electrode, platinum wire as the counter electrode, while glassy carbon (7.065 mm2) was used as the working electrode.

Moreover, for the modification of working electrode (GCE) ink is deposited on its surface and ink is prepared by adding ZCNT and ZCNT-M catalyst in 100 μL ethanol with 20 μL Nafion (5 wt %) as the binding and conducting agen<sup>t</sup> to form the catalyst ink, which was later deposited (20 μL) on the glassy carbon electrode and allowed to dry. All the prepared composites were tested for different techniques, i.e., cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy (EIS) in 0.1 M KOH (an electrolyte) by using the same method of preparing ink. In addition, ZCNT and ZCNT-M samples were tested under inert, dissolved oxygen and oxygen rich conditions. A frequency range of 10 to 40 kHz with a scan rate amplitude of 50 mVs−<sup>1</sup> was used for electrochemical impedance spectroscopy under potentiostatic mode. Chronoamperometry was also performed for 3600 s.

#### *2.3. Electrochemical Evaluation of Prepared Catalsyts*

At first, ZIF-67 derived CNTs (ZCNT) and ZIF-67 derived CNTs/MnO2 (ZCNT-M) were compared. To ensure that dissolved oxygen is the only analyte present within the KOH solution and there are no other analyte species to react with the electrode, the solution was purged with argon gas for 2–3 min and then the response was recorded and compared with oxygen dissolved solution. Figure 4 shows that in the presence of oxygen, the prepared catalysts' reduction current was noticeably increased, which may be attributed to the presence of continuously regenerated reaction centers that might lead to current value amplification during the reduction process [44].

**Figure 4.** ZCNT-M Performance under dissolved O2 and Ar purged environment.

With an optimal flow rate of oxygen gas and at a scan rate of 50 mV/s, cyclic voltammetry of prepared catalysts was performed; in this study, the electrochemical activity of ZCNT was compared with ZCNT-M to obtain the values of peak current density, onset potential and peak potentials for oxygen reduction reactions, as illustrated in Figure 5. From the figure, it can be observed that ORR performance of ZCNT-M is much better than ZCNT due to the addition of MnO2, and it has markedly increased the current densities up to 6.56 mA/cm<sup>2</sup> for ORR with an ORR onset potential (V vs. RHE) of 1.02 V, which is comparable with that of Pt/C (1.01 V), illustrating that there is a current density of 5.02 mA/cm<sup>2</sup> [45]. In comparison to ZCNT-M, commercial MnO2 shows remarkably low current density with 0.25 mA/cm<sup>2</sup> and with onset potential (V vs. RHE) of 0.96 V, as reported by Huang et al. and Chhetri et al. [46,47]. The increased current densities of ZCNT-M can be attributed to good catalytic ORR activity of MnO2; as with the ZIF-67 derived CNTs (ZCNT), it enhances the surface area and conductivity of prepared catalysts to a significant level. Thus, as-prepared ZCNT-M composite was used as an efficient nonprecious cathodic electrocatalyst with preferable ORR stability, enhanced electron-transport performance, and elevated antitoxic property in alkaline media for ORR.

Figure 6 illustrates the effect of scan rate on the current density of the prepared sample. All the tests were executed with a diverse range of scan rate values such as 5 mV/s, 15 mV/s, 25 mV/s and 50 mV/s in an alkaline media (0.1 M KOH). Prepared ink composition (i.e., 3 mg per catalyst) remained the same in all the experiments. The current density of ZCNT-M in 0.1M KOH for ORR increased gradually because of electroactive species' easy access to the surface of the electrode in the lesson time period [5]; also, at high scan rates, non-electrolytic species were not able to be reduced or oxidized into products. Consequently, only electroactive products were liable for high current density values, this remarkable response of catalyst is linked to the improved extent of reaction. Moreover, a slight shift in peaks was observed, which indicated a slight irreversibility during reaction.

**Figure 5.** Cylic voltammograms of ZCNT and ZCNT-M for ORR.

**Figure 6.** Cyclic voltammograms at different scan rates for ZCNT-M.

Furthermore, linear sweep voltammetry was performed with oxygen purging to study the effect of increasing analyte concentration on current density, as shown in Figure 7. Pure oxygen was purged through the electrolyte for one, two and three minutes, respectively, before performing the linear cyclic voltammetry experiment. A continuous supply of oxygen was maintained during the experimental run. Oxygen purging showed a marked increase in the current density relative to the dissolved oxygen case for ORR. The graph below shows that peak current densities increased as the amount of oxygen present in the electrolyte was increased; however, the current density decreased beyond two minutes of oxygen purging. A possible explanation for this decrease is the saturation of the electrolyte with analyte along with a decrease in oxygen diffusion to the electrode surface.

**Figure 7.** Current densities at different oxygen purging durations for ZCNT-M.

Furthermore, the kinetics of ORR reactions were found to be diffusion controlled. A plot between the square root of scan rate and peak current density was made for ZCNT-M, as shown in Figure 8.

**Figure 8.** Scan rate vs. peak current density for ZCNT-M.

The figure illustrates that the square roots of scan rates and current densities have a linear relationship, while this linear plot is relative to D1/2 to obtain the slope value. Moreover, diffusion coefficients were calculated using the Randles-Sevcik Equation (1) [48].

$$Ip = 0.4463nFAC \sqrt{\frac{nFvD}{RT}}\tag{1}$$

where *D* is the diffusion coefficient, *A* is the active surface area (cm2), *C* is the molar bulk concentration of 0.1 M KOH, *v* is the scan rate (V s<sup>−</sup>1), and n is the number of electrons transferred.

The diffusion coefficients for ORR of ZCNT-M are calculated as DORR = 6.6 × 10−<sup>4</sup> cm2/s. These results support that a diffusion-controlled mechanism is followed by electrocatalytic oxygen evaluation reaction and oxidation reduction reaction.

Finally, to analyze the trend of overpotential with the current density, Tafel plots (Figure 9) were made which were then used to calculate the exchange current densities value of 1.49 × 10−<sup>3</sup> A/cm2. Firstly, overpotential is calculated by using the formula such as E—Eo [19]. In order to comprehend the reaction kinetic performance, Tafel slopes were calculated by using the subsequent Equation (2).

$$\left| \text{d} \mathbf{n} / \text{d} \, \text{ln} \, \left| \text{j} \right| = -\text{RT} / \text{\textdegree } \text{cnF} \tag{2}$$

where α was calculated by using Equation (3) [42]:

$$\text{Ep} - \text{Ep}/2 = 1.857 \,\text{RT}/\text{\alpha}\text{F} \tag{3}$$

**Figure 9.** Tafel plots for ZCNT-M.

For the oxygen reduction reaction, n was deemed to be 4. The Tafel slope value for ZCNT-M is calculated and obtained in the range of 165–200 mV/dec, and the value of slope is determined such that if >118 mV/dec, then the rate determining steps are ascribed via (i) ongoing chemical oxidation, (ii) the resulting chemical combination and (iii) the transfer of electrons occurring via an oxide layer. In order to elude the confusion, the outcomes collected from the Tafel slopes will be referred as "cathodic quantities" and the mechanisms for ORR can be established precisely by these approaches [17]. The outcomes are in accord with the literature, where the first C–H bond breaking in ORR occurs because of the low potential region along with the rate determining step through the first electron transfer, while in the high potential region, the increase in slope values is because of poisonous intermediate species having less exposure [10].

To understand the activity of the modified electrode in a better way, electrochemical impedance spectroscopy was performed using the same three electrode systems in 0.1 M KOH solution under the potentiostatic mode. The Nyquist plot below in Figure 10 represents two regions, presenting an idea regarding solution resistance (Rs) and charge transfer resistance (Rct); the small semicircle clearly shows that the charge transfer resistance for ZCNT-M is lower in comparison to ZCNT. Moreover, corresponding low Rct and Rs values are liable for higher catalyst activity as well [5,49]. The decreased value of resistance in ZCNT-M can be attributed in good catalytic ORR activity of MnO2, as it improves the surface area and conductivity of prepared catalysts to a significant level. Similarly, the high electronic and ionic conductivity of ZCNT-M may possibly be responsible for the straight line. This significant reduction in charge transfer resistance in ZCNT-M clearly favors ORR reactions in ZCNT-M as compared to ZCNT.

**Figure 10.** Electrochemical impedance spectroscopy (EIS) plot for ZCNT-M and ZCNT.

The prepared catalysts' stability was determined via the chronoamperometry technique, a key parameter to accomplish the practical application of synthesized samples. The stability test of ZCNT-M was carried out in 0.1 M KOH solution at a potential of 0.1 V for 3600 s in the similar three electrode setup and subsequent electrolyte. At the start, the current dropped substantially very quickly, which can be justified with the following reasons: (a) adsorption of reaction intermediate to the electrode surface; (b) blockage of active site due to evolved oxygen accumulation on the surface of the electrode [10] and (c) flake off material caused by extreme bubbling, but later it adopted a fairly stable trend for the remainder of the hour, as shown in the Figure 11 below. Moreover, Figure 12 shows the plot between the current and square root of current and describes a linear trend over time.

**Figure 11.** Chronoamperometric plot for ZCNT-M.

**Figure 12.** Chronoamperometric plot between current and square root of current for ZCNT-M.

Figure 13 shows the mechanism of ORR in basic media (KOH); it describes the complex reaction pathway by which reductive splitting of the oxygen O–O bond occurs on the catalyst adsorbed surface. Here, k1 epitomizes the direct reduction of O2 to OH− ion without any intermediate formation.

**Figure 13.** Oxygen reduction reactions (ORR) mechanism on the surface of electrode.

In addition, the k2 is a comprehensive rate constant for the adsorbed peroxide formation, and might implicate other rate constants that are associated to both the disproportionation reaction and intermediate formation of the adsorbed super oxide; besides, k3 is the rate constant for peroxide reduction, k4 refers to the catalytic decaying of adsorbed peroxide on the electrode surface, and k5 represents rate constants for peroxide desorption and adsorption processes [50].
