Influence of nickel doping on MnO2 nanoflowers as electrocatalyst for oxygen reduction reaction

Doping is promising strategy for the alteration of nanomaterials to enhance their optical, electrical, and catalytic activities. The development of electrocatalysts for oxygen reduction reactions (ORR) with excellent activity, low cost and durability is essential for the large-scale utilization of energy storage devices such as batteries. In this study, MnO2 and Ni-doped MnO2 nanowires were prepared through a simple co-perception technique. The influence of nickel concentration on electrochemical performance was studied using linear sweep voltammetry and cyclic voltammetry. The morphological, thermal, structural, and optical properties of MnO2 and Ni-doped MnO2 nanowires were examined by SEM, ICP-OES, FT-IR, XRD, UV–Vis, BET and TGA/DTA. Morphological analyses showed that pure MnO2 and Ni-doped MnO2 had flower-like and nanowire structures, respectively. The XRD study confirmed the phase transformation from ε to α and β phases of MnO2 due to the dopant. It was also noted from the XRD studies that the crystallite sizes of pure MnO2 and Ni-doped MnO2 were in the range of 2.25–6.6 nm. The band gaps of MnO2 and 0.125 M Ni-doped MnO2 nanoparticles were estimated to be 2.78 and 1.74 eV, correspondingly, which can be seen from UV–Vis. FTIR spectroscopy was used to determine the presence of functional groups and M–O bonds (M = Mn, Ni). The TGA/TDA examination showed that Ni-doping in MnO2 led to an improvement in its thermal properties. The cyclic voltammetry results exhibited that Ni-doped MnO2 nanowires have remarkable catalytic performance for ORR in 0.1 M KOH alkaline conditions. This work contributes to the facile preparation of highly active and durable catalysts with improved catalytic performance mainly due to the predominance of nickel. MnO2 and Ni-doped MnO2 nanowires were synthesized via a facile co-perception approach. Nickel doping in MnO2 induces the formation of wire-like nanostructures. Nickel doping enhances the electrochemical activity and thermal stability of MnO2 nanoflowers. The addition of nickel into MnO2 promoted the catalytic activity for oxygen reduction reaction. A higher catalytic activity was achieved in 0.125 M Ni-MnO2 nanowires. MnO2 and Ni-doped MnO2 nanowires were synthesized via a facile co-perception approach. Nickel doping in MnO2 induces the formation of wire-like nanostructures. Nickel doping enhances the electrochemical activity and thermal stability of MnO2 nanoflowers. The addition of nickel into MnO2 promoted the catalytic activity for oxygen reduction reaction. A higher catalytic activity was achieved in 0.125 M Ni-MnO2 nanowires.

• The addition of nickel into MnO 2 promoted the catalytic activity for oxygen reduction reaction. • A higher catalytic activity was achieved in 0.125 M Ni-MnO 2 nanowires.

Introduction
Globally, the focus on low-carbon emissions and the concept of sustainability has notably proposed the development of sustainable energy sources [1,2]. Electrochemical energy storage tools such as metal-air batteries (MABs) and fuel cells are considered to play a significant impact in the transformation to a sustainable prospect [3][4][5]. Moreover, one of the main problems that restrict the application of lithium-ion batteries for electric vehicles is its low specific energy density [6,7]. Recently, special attrition is given to the performance enhancement of MABs due to their extraordinary ideal energy density and low cost, mainly in lithium-air and zinc-air batteries (ZABs) [8][9][10]. Hence, ZABs have a low cost, comparatively higher energy density, and excellent safety [11,12]. However, the fabrication with effective air electrodes is found to be the critical problem associated with ZABs [13,14]. The challenges of air electrodes can be solved by preparing active, efficient, stable and durable oxygen catalysts for the ORR [15]. The ORR is a vital cathodic reaction in fuel cells and MABs [16,17]. Precious metals such as Platinum (Pt) and Pt-based alloys are the most effective ORR electrode substances [18,19]. However, the application of Pt and its derivatives as ORR catalysts is limited because of their poor stability, limited access, and high price [20, into MnO 2 such as Iron (Fe), Nickel (Ni) [38], Chromium (Cr), Cobalt (Co) [39], Copper (Cu) [40], Molybdenum (Mo) [41], Aluminum (Al) [42], and Vanadium (V) [43]. Those results exhibited that doping improves the overall performance of MnO 2 NPs [44]. One on hand different approaches have been investigated for the synthesis of MnO 2 NPs such as emulsion, sol-gel, co-precipitation, thermal decomposition [45], simple reduction, and hydrothermal approach [46,47]. However, those approaches are expensive, needs sophisticated instruments, and relatively longer preparation time [48,49]. Among these, the co-precipitation approach is advantageous due to easy, fast, cost-effective, and easy to control during preparation [50,51]. Many studies have been investigated on optical, electrical, and magnetic characteristics of Ni doped MnO 2 nanoparticles [52]. However, only a few studies have been reported about the structural, morphological, thermal, and electrochemical characteristics of Ni-doped MnO 2 NPs by defect engineering at ambient conditions. Herein, we examined the influence of nickel doping on structural, morphological, thermal, and electrochemical characteristics of MnO 2 NPs prepared by a facile co-precipitation approach [53,54].

Materials
All chemicals and reagents were used without any refinement and of analytical grade. Ethanol, Vulcan XC 72 carbon powder, and isopropanol were obtained from India (Alpha Chemika

Characterization
The functional groups were studied using Fourier transform infrared spectrometer (FT-IR, FT-IR 6660 (JASCO MODEL)) in the wavenumber range of 4000-400 cm −1 , in a standard KBr pellet method. The crystal structures of the synthesized nanoparticles were examined using X-ray diffraction (MAXima_X XRD-7000, SHIMADZU) using radiation of Cu-K α . XRD analysis at room temperature in theta-2 theta range between 10° and 80°. It was continuously scanned at a step size of 0.02° with a counting time of 0.30 s per step with a scan speed of 4.0 (deg/min) operating at 30 mA and 40 kV with a divergence slit of: 1.0 (°), scatter slit: 1.0 (°), receiving slit: 0.30 (mm). Scanning electron microscopy (SEM, INSPECT F50) was used to examine the morphological properties of as-prepared NPs. A thermal property study was performed using the TGA/ DTA study. Ultraviolet-visible spectrophotometer (UV-Vis, Lambda 35 (PerkinElmer)) in the wavelength range of 200-800 nm was conducted to analyze the optical characteristics of prepared NPs. Brunauer-Emmett-Teller (BET, Quanta chrome Instruments version 11.0) was used to determine the specific surface areas of as-prepared nanoparticles. After dissolving the synthesized materials in HCl and HNO 3 aqueous solution, the composition of Ni and Mn in the Ni-MnO 2 product was analyzed via inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin-Elmer 800). Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were done for electrochemical analysis using CHI760E electrochemical workstation.

Preparation of working electrode
To examine the electrochemical characteristics of the developed MnO 2 and Ni-doped MnO 2 NPs, the working electrode was fabricated as follows: MnO 2 and Ni-doped MnO 2 nanoparticle samples were combined with carbon powder at a mass ratio of 30:70 to ensure adequate electronic conductivity. Then, 5 mg of MnO 2 and Ni-doped MnO 2 catalysts (dispersed in 1 mL mixed solution of water/ isopropanol, 2.5:1 v/v) was sonicated for 30 min to form a uniform ink. Then 5 µL of the catalyst solution was pipetted onto a glassy carbon electrode (GCE) of 3 mm diameter and dried at room temperature.

Electrochemical characterization
Electrochemical analyses were accomplished using a computer-controlled potentiostat (CHI760E electrochemical workstation) equipped with a three-electrode conventional cell. The electrocatalytic characteristics were analyzed by LSV and CV. CV was estimated in a 0.1 M KOH electrolyte using platinum coil and Ag/AgCl as a counter and reference electrode, respectively. All potentials stated in this study were referenced to the Ag/AgCl electrode. For CV analyses, the working electrode was examined between + 0.1 to − 0.7 V at a sweep rate of 5, 10, 20, 50, and 100 mV s −1 at room temperature via saturating the electrolyte through O 2 for 30 min. LSV analysis was done at a scan rate of 5 mV s −1 in + 0.1 to − 0.7 V (vs. Ag/AgCl) of potential range.

Structure analysis
The crystalline size, structure, and phase purity, of the MnO 2 , and Ni-MnO 2 samples were examined using XRD. The XRD patterns of the MnO 2 and Ni-MnO 2 nanoparticles are shown in Fig. 2. Figure

Scherrer technique
The average crystallite sizes of pure MnO 2 and Ni-MnO 2 NPs at different doping level were determined by using the Scherrer Eq. (1) [57].
where λ, K, D, θ, and β are wavelength of the x-ray sources, Scherer constant, crystallites size, peak position and full width at the half maximum intensity (FWHM), respectively [2].  Table 1). The dislocation density and strain of the MnO 2 and Ni-MnO 2 NPs are other vital determinants that can be calculated from the XRD analysis. The dislocation density (δ) values were estimated using Eq. (2).
where D and δ are crystallites size and dislocation density, respectively. Hence, the addition of dopant increases the dislocation density of the NPs.

Williamson-Hall method
A Williamson-Hall plot was drawn to determine the lattice strain and effective crystallite size using Eq. (3) [7]: where K, β, D, λ and ε are shape factor, full width hall maximum, crystallite size, wavelength, and microstrain, respectively. Therefore, the graph was plotted within "4sinθ" on x-axes and "βcosθ" on the y-axis, and a straight line was obtained. The intercept of a straight line is the crystallite size whereas slope provides the value of the microstrain as presented in Fig (Table 1). It was noticed that both methods gave very close crystallite size.

Morphology analysis
The morphologies of MnO 2 and Ni-doped MnO 2 were studied using SEM. Figure 4 displays the SEM image of MnO 2 NPs, which have a flower-like structure and a diameter of 4.7 nm. This flower-like structure is composed of different self-assembled irregular nanosheets. On the other hand, 0.025 M Ni-doped MnO 2 has agglomerated small nanoneedles of porous morphology with an average particle size of 7.13 nm (Fig. 5). This suggested that, the addition of nickel affected the surface morphology of MnO 2 , as shown in Fig. 5. Moreover, 0.125 M Ni-doped MnO 2 consisted of nanowires with an average particle size of 7.07 nm, which implies the key role of Ni +2 is regulating the nanostructures of MnO 2 (Fig. 6). The average diameter of as-prepared nanowires were analysed via particle size distribution histogram computed from SEM image via Image J software as shown in Fig. 7a-c. Generally, when Ni 2+ were

ICP-OES analysis
Elemental compositions studies of the as-prepared samples were examined by ICP-OES to confirm the amount of Mn and Ni loaded in Ni-doped MnO 2 nanoparticles. ICP-OES analysis were conducted via dissolving the as-prepared nanoparticle in a concentrated acidic solution (HCl:HNO 3, 3:1 v/v by volume). Table 2

Surface area analysis
BET analysis techniques have been utilized to determine the textural properties such as specific surface areas, pore volume and pore size of MnO 2 and Ni-MnO 2 nanoparticles.

FT-IR analysis
FT-IR spectroscopy was accomplished within 400 to 4000 cm −1 ranges of the electromagnetic spectrum.

UV-visible analysis
To study optical characteristics of the as-prepared samples, UV-Vis analysis were performed between 200 and 800 nm. Figure 9a illustrates UV-visible spectrum of MnO 2 and Nidoped MnO 2 NPs. It was observed that MnO 2 showed absorption peaks at 318 nm [40]. Furthermore, the characteristics absorption bands at 378 nm corresponded to Ni-doped MnO 2 nanoparticles sample. The optical band gap of MnO 2 and Ni-doped MnO 2 sample were computed using Tauc relationship (Eq. 1) [60].
where α, hν, and A are the absorption coefficient, photon energy and constant. Figure 9b shows the energy band gap values of MnO 2 and Ni-MnO 2 NPs computed by extrapolating the linear part of these plots of (αhν) 2 axis to (hν) axis. The results showed that 2.78 and 1.74 eV were band gap energy of MnO 2 and Ni-doped MnO 2 NPs, respectively. UV-Vis investigation showed that Ni-doped MnO 2 NPs with enhanced electronic conductivity.

Thermal analysis
TGA and DTA analysis were investigated to study the thermal and phase evolution properties of pure MnO 2 and Ni-MnO 2 NPs. The as-prepared NPs were heated in the range of 25-950 °C with 10 °C min −1 heating rate in air atmosphere. Figure 10  Moreover, in the DTA curve of pure MnO 2 the two sharp exothermic and endothermic peaks that arise at 588 °C and 815 °C, respectively that might be due to a variation in crystallinity or possibly due to a phase transition in the NPs. A final weight of 6.91 mg was obtained from the initial weight of 8 mg which corresponds to 13.6% weight loss when heated to 950 °C (Fig. 10a). Similarly, the 0.125 M Ni-MnO 2 TGA curve, shows the first weight loss of 0.55 mg approximately at 385 °C, and the corresponding endothermic peak at 117 °C may be due to loss of physically adsorbed water (Fig. 10b) respectively, which might be due to a variation in crystallinity or possibly due to a phase transition in the NPs. Hence, we observe that the study was begun with 8 mg of catalyst, and after complete heating up to 950 °C, the catalyst remained was 7.01 mg. Thus, the percentage weight loss for our analysis up to 950 °C, is 12.36%. Generally, in our analysis weight loss of 13.63% and 12.36% is associated to MnO 2 and Ni-MnO 2 NPs, correspondingly. Hence, our analysis showed that Ni-MnO 2 NPs with improved thermally stability. The TGA and DTA result showed similarities with the previously reported results. However, slight change in the peak positions were observed at different stages, which may be due to the use of different reagents in the synthesis process [61].

CV study
The catalytic activity of un-doped MnO 2 and Ni-MnO 2 electrocatalysts were analyzed using CV in O 2 -saturated KOH (0.1 M) solution (Fig. 11). Figure 11a which exhibits Ag/AgCl), the current of the ORR at the electrode with a catalyst 0.125 M Ni-MnO 2 is 38.56 µA, which is higher than that of the electrode with pure MnO 2 (32.17 µA). The ORR peak current of Ni-MnO 2 catalyst is bigger than that of pure MnO 2 catalyst [24]. The findings are Our work in agreement with previously reported results [44,62]. It can be said that the as-prepared MnO 2 and Ni-doped MnO 2 electrocatalysts are to be an ideal air electrode material for metal-air batteries and fuel cells. Figure 11b, c illustrates the CV illustration of un-doped MnO 2 and Ni-MnO 2 nanowires at numerous scan rates. The effect of scan rate in between 5 and 100 mV/s was studied at pure MnO 2 and 0.125 M Ni-MnO 2 modified electrode. Figure 11b, c shows the largest variation between the four ORR curves where a negative peak potential shifts with increase of scan rate are observed, which is detected when the oxidation reaction is irreversible. Hence, the ORR activities of MnO 2 and Ni-MnO 2 catalysts are also dependent on the scan rates. The result showed that excellent ORR catalytic activity can be obtained at lower scan rate [63,64]. Figure 12 shows the ORR activity of pure MnO 2 , and 0.125 M Ni-MnO 2 modified electrode before saturation and after oxygen saturation. The result showed that un-saturated pure MnO 2 with better catalytic performance but poor current intensity. Moreover, un-saturated 0.125 M Ni-MnO 2 modified electrode showed poor catalytic performance and current intensity. It is notably seen that nickel doped (0.1 M Ni-MnO 2 and 0.125 M Ni-MnO 2 ) electrode displayed excellent reduction peak potential, which confirms a higher electrocatalytic activity to ORR due to the synergistic effect of nickel and MnO 2 .

LSV analysis
ORR activity of the un-doped MnO 2 and Ni-MnO 2 (0.125 M) NPs were evaluated using linear scanning voltammetry (LSV). Figure 13 shows the LSV profile of MnO 2 and Ni doped MnO 2 (0.125 M) nanoparticles in O 2 -saturated KOH (0.1 M) alkaline media at a scan rate of 5 mV s −1 from + 0.1 to − 0.7 V (vs. Ag/AgCl). As it can be seen in Fig. 13 [65].

CV scans rate analysis
CV analyses were conducted by changing the scan rate between 5 and 100 mV/s (Fig. 14a). Scan rate analysis can reveal whether the electrocatalytic activity of Ni-doped MnO 2 NPs on ORR is controlled by diffusion, adsorption and even these two processes. The plot of cathode peak current (i pc ) with the scan rate (v) 1/2 and the square root of the scan rate (v) are shown in Fig. 14b, c. It is observed that a linear relationship between the cathode peak current versus v 1/2 and v values (Fig. 14b, c). The linear relationship of peak current and v 1/2 shows a diffusion control process. Moreover, the peak current is also linearly related to v, which means that there is adsorbed electroactive species. These result show that the ORR on the Ni-MnO 2 /GCE is a combination of kinetic and diffusion control reactions, involving the adsorption of O 2 on the surface of the electrode [66]. The involved numbers of electrons (n) in the catalytic reaction of irreversible CV behaviour have been calculated via Laviron equation. The linear relationship among logarithmic sweep rate and peak potential is shown in Fig. 14d. The logarithm of the reduction potential and the scan rate is in the range of 5-100 mV/s as shown in Eq. (5).  where E p/2 is the potential when the current is at half the peak value. Therefore, the computed value of α is 0.62. Hence, the number of electrons transferred (n) during the reduction process of Ni-MnO 2 /GCE surface oxygen is computed to be 3.47. The finding shows that Ni-doped MnO 2 NPs are an active electrocatalyst with high catalytic performance towards ORR. Table 4 displays the comparison the electrochemical characteristics of recently studied MnO 2 based electrocatalysts. The surface area of the as-prepared Ni-MnO 2 nanoparticles in this study is 124.4 m 2 g −1 which is in close agreement those reported for Ni-α-MnO 2 (52.3 m 2 g −1 ) and Cu-α-MnO 2 Nanowire (83.8 m 2 g −1 ). Similarly, the number of transferred electron is slightly greater than that of Cu-α-MnO 2 nanowires (n = 3.2-3.3) and Ni-MnO 2 nanoneedles (n = 2.33), which show that the enhancement of the electrochemical performance of Ni-MnO 2 nanoparticles was observed in this study.

Conclusions
Pure MnO 2 and Ni-MnO 2 electrocatalysts were prepared using a co-precipitation approach. The synthesized MnO 2 and Ni-doped MnO 2 electrocatalysts were analyzed using SEM, ICP-OES, FT-IR, XRD, UV-Vis, BET and TGA/DTA. Moreover, the electrochemical behaviors of the prepared catalysts were studied using LSV and CV. The characterization results demonstrated that highly active, cost-effective, and thermally stable electrode materials were achieved through simple co-preciptation approach. The XRD investigation showed that the doping of nickel promotes the formation of α and β-MnO 2 NPs. The tetragonal and hexagonal crystal arrangement of α, β, and ε-MnO 2 was verified via XRD investigation. The XRD examinations confirmed that the crystallite sizes of MnO 2 and Ni-doped MnO 2 were in the range of 2.25-6.6 nm. The morphological analyses showed that un-doped MnO 2 and 0.125 M Ni-MnO 2 have flower-like and nanowire structures with the estimated bandgap energy of 2.78 and 1.74 eV, respectively. It was also inferred that the thermal stability of the synthesized material was improved due to the incorporation of Ni on the MnO 2 framework. The Ni-doped MnO 2 NPs showed higher catalytic performance, as confirmed by LSV and CV analysis. Overall, Ni-doped MnO 2 NPs exhibited excellent catalytic activities, which might be because of enhanced electronic conductivity in the doping process. Hence, the addition of Ni dopants had influenced the physical characteristics of MnO 2 NPs and affected the catalytic performance of the pure MnO 2 NPs.