Trimetallic Oxide Electrocatalyst for Enhanced Redox Activity in Zinc–Air Batteries Evaluated by In Situ Analysis

Abstract Researchers are investigating innovative composite materials for renewable energy and energy storage systems. The major goals of this studies are i) to develop a low‐cost and stable trimetallic oxide catalyst and ii) to change the electrical environment of the active sites through site‐selective Mo substitution. The effect of Mo on NiCoMoO4 is elucidated using both in situ X‐ray absorption spectroscopy and X‐ray diffraction analysis. Also, density functional theory strategies show that NiCoMoO4 has extraordinary catalytic redox activity because of the high adsorption energy of the Mo atom on the active crystal plane. Further, it is demonstrated that hierarchical nanoflower structures of NiCoMoO4 on reduced graphene oxide can be employed as a powerful bifunctional electrocatalyst for oxygen reduction/evolution reactions in alkaline solutions, providing a small overpotential difference of 0.75 V. Also, Zn–air batteries based on the developed bifunctional electrocatalyst exhibit outstanding cycling stability and a high‐power density of 125.1 mW cm−2. This work encourages the use of Zn–air batteries in practical applications and provides an interesting concept for designing a bifunctional electrocatalyst.


Figures 1 to 25
Tables 1 to 7 References (2)   Where, J, J L and J k are measured current density, diffusion-limiting current density and kineticlimiting current density, respectively.F= Faraday constant (F = 96485 C mol −1 ) ω= Angular velocity for RDE (rad s -1 ) D O Oxygen diffusion co-efficient 1.9 × 10 −5 cm 2 s −1 C O =Saturated oxygen concentration of 1.2 × 10 −3 mol L -1 in 0.1 M KOH n = number of electron transfer during the ORR ν kinetic viscosity of the electrolyte (0.01 cm 2 s -1 ) All the measured potential vs Ag/AgCl (E Ag/AgCl ) were convert in to potential vs standard RHE (E RHE ) by using following universal Nernst equation (3).
Where E Ag/AgCl = 0.1976 at 25 C and pH of 0.1M (ORR measurement) and 0.1M KOH (OER measurement) 2.4.3.For OER measurements.3 mg of electrocatalyst and 5% Nafion were combined with 30 L of a 1:1 isopropyl alcohol/DI water solution, sonicated for 60 minutes, and the result was a homogenous ink.The generated ink was applied to a carbon paper surface area of about 1 cm 2 , and it was allowed to dry for 12 hours at 60 °C in a drying oven.Furthermore, the active ingredients were injected at a density of around 3 mg cm -2 .Similar processes were used to create catalyst inks made of commercialized IrO 2 (99.9%;Sigma-Aldrich) with Pt/C (20 wt%) that have been coated on carbon paper to comparative testing.The OER electrochemical performance was evaluated using a new three-electrode cell design with a working electrode, reference electrode, and counter electrode were using catalyst-coated carbon paper, Ag/AgCl and a graphite rod.
Experimental OER polarization curves were recorded at a constant scanning rate of 1 mV s -1 .
Using a resonant frequency of 0.01 to 106 Hz and a potential magnitude of 5 mV, electrochemical impedance spectroscopy (EIS) was carried out on the synthesized electrocatalysts.In the non-Faradic region of the CV curve, a double-layer capacitance (C dl ) values of the generated catalyst were evaluated to use a scanning rate of 10 to 100 mV s -1 .The middle of the potential was defined as the disparity between the densities of the cathodic and anodic currents.The slope of current density with respect to scan rate was twofold as steep as that of C dl .

Turnover Frequency Calculations (TOF)
. TOF, which is used to compare the intrinsic activity of several catalysts, is defined as the frequency of the reaction per active site.Typically, the equation: is used to determine the TOF value for OER.

TOF (5)
where A is the electrode's geometric area, j is the current density following iR compensation, n is the molar number of active sites, η is the Faradic efficiency, F is the Faraday constant.In our work, Mo, Ni, and Co were taken to be the active sites for NCMO@rGO and NCO@rGO catalysts, and the number n was estimated using total loading mass using the equation: (6)  where m is the loading mass, NA is Avogadros constant, and Mw is the molar mass of the catalysts.

Zinc-air Battery
Test.The catalytic inks NCMO@rGO and NCO@rGO were deposited on carbon paper foam, the amount of catalyst was maintained at 3 mg cm -2 .An air cathode, 0.25 mm thick (Alfa Aesar, UK) zinc foil, as well as an electrolyte of 6 M KOH and 0.2 M Zn (CH 3 COO) 2 were used to build a zinc-air battery.Durability testing for extensive chargedischarge cycles were looked into using a Gamry 600 electrochemical workstation.An air cathode electrode constructed of Pt-C (20 wt%) and IrO 2 with a weight ratio of 1:1 was also created using a similar procedure.
Using Pt-C + IrO 2 , NCO@rGO, and NCMO@rGO as the air cathode, equations 7 and 8 were used to find the power density (mW cm -2 ) and specific capacity (mAh g -1 ) using zinc-air batteries. [2]wer density (mW cm -2 ) = Voltage × current density (7)   Specific capacity (mAh g -1 ) = current × service hours/weight of consumed Zn (8) 2.5.Computational method.The electrocatalyst was further thoroughly evaluated by DFT calculations using the Vienna Ab initio Simulation Package(VASP) [3], [4] based on the information given by XRD results.Grimme's DFT-D3 functional and the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional were used with a semiempirical GGA type theory. [5]Ion-electron interactions were studied using the projector augmented wave method (PAW), which was employed in VASP. [6]A vacuum spacing of 15 Å was applied for NCMO and NCO to avoid interaction between adjacent atoms.Plane-wave basis set 400 eV was fixed as cut-off energy.The structures were relaxed totally until the Hellman-Feynman forces were lower than 0.02 eV/Å.For energy and DOS calculations the script "VASPKIT" in the VASP program generates a KPOINT file to calculate the k-point mesh. [7]rmation energy (E F ) is defined as in eq. 9 below: ∑ where E*, n i , and E i denote the energy of each crystal structure, number of elements in the crystal, and energy of each constituent element, respectively.
The ORR process can be decomposed into the following elementary steps: where * represents the active site on the surfaces of the catalysts.
For each elementary step of ORR, the free energy has been calculated according to the method proposed by Nørskov et al.The free energy change from initial state to final state of the reaction is defined as: ΔG = ΔE + ΔE ZPE -TΔS + ΔG U + ΔG pH , where ΔE represents the reaction energy difference of reactant and product, which can be directly calculated from DFT computations.
ΔE ZPE and ΔS are the changes in zero-point energies and entropy at room temperature (T = 298.15K), which can be computed from the vibrational frequencies.ΔG pH is the correction on the pH in the electrolyte, which can be determined by ΔG pH = k B Tln10 × pH.Based on previous theoretical studies, [8], [9], [10] the value of pH in this work was assumed to be zero for acidic medium.The Gibbs free energy of O 2 (G O2) will be obtained by G O2 = G H2O  2G H2 + 4.92 eV, because the DFT method fails to accurately describe the highspin ground state of the O 2 molecule.The free energy change for the four elementary ORR steps can be obtained as: Therefore, the overpotential (η) that evaluates the performance of OER, and ORR is applied according to the following equations: where 1.23 represents the equilibrium potential.Based on previous reports, a lower η value on a given catalyst suggests a less energy input for ORR, thus demonstrating its higher ORR catalytic activity.ICP-OEC analysis of NCO@rGO and NCMO@rGO catalysts, respectively.Co 3 O 4 @NiCo 2 O 4 0.81 1.65 0.84 [18]   NiO/NiCo 2 O 4 0.73 1.587 0.85 [19]   MnCo 2 O 4 /CNF 0.83 1.63 0.79 [20]   CoNiMn@500* 0.65 1.72 1.07 [21]   Co@Co 3 O 4 /NC-2 0.74 1.64 0.90 [22]   NiCo 2 O 4 0.72 1.75 1.03 [23]   CoO@Co 3 O 4 /NSG 0.79 1.69 0.90 [24]   PCO/Co 3 O 4 NCs 0.72 1.60 0.88 [25]

Figure S7 .
Figure S7.In-situ XANES spectra pre-edge peak at Ni K-edge (a) and (b) Co K-edge of

Figure S8 .
Figure S8.(a-h) The fitting results of the Co K-edge EXAFS spectra at OCP and -0.80 V applied

Figure S9 .
Figure S9.(a-h) The fitting results of the Ni K-edge EXAFS spectra at OCP and -0.80 V applied

Figure S10 .
Figure S10.(a-c) Typical experimental set-up for an in situ XAS experiment on an ORR

Figure S13 .
Figure S13.For ORR study: (a, b) CV curves with different scan rates, (c) double-layer

Figure S15 .
Figure S15.For OER study: (a-c) before and after stability CV curves, (d) After cyclic stability

Figure S16 .
Figure S16.(a) Bulk structure of NCO and, (b-e) NCMO at different configurations of the

Figure S17 .
Figure S17.The optimized bulk structure, the catalyst was cleaved at the 311 planes with

Figure S22 .
Figure S22.(a) Divided Test Cell for In-Situ XRD Analysis of Battery Electrode with Beryllium (Be) sheet, (b) charge-discharge curves of the Zn-air batteries at current density 10 mA cm -2 , (c)

Figure S25 .
Figure S25.Before and after Zn-air battery charge-discharge stability, XPS peak tables of (a, b)

Table S1 .
The details of the fitting of the EXAFS spectra of Co K-edge, showing the bond distance.

Table S2 .
The details of the fitting of the EXAFS spectra of Ni K-edge, showing the bond distance.Waller factor (a measure of thermal and static disorder in absorber-scatterer distances); E 0 , inner potential correction; R-factor, indicating the goodness of the fit.Table S3 Calculated formation energies of NiCo 2 O 4 and different configuration of NiCoMoO 4.

Table S4
Calculated formation energies of NiCo 2 O 4 and NiCoMoO 4 at 311 planes.

Table S5
The calculated Gibbs free energy (∆G) of the catalysts

Table S6 .
Summary of the electrochemical performance of various bifunctional spinel catalysts as reported by other researchers.E 1/2 is half-wave potential for ORR and E i=10 is the potential for OER at current density of 10 mA cm -2 .The △E (△E= E i=10 -E 1/2 ) serves as a metric to evaluate

Table S7 .
The performance comparison of reported aqueous Zn-Air batteries for spinel-based catalysts.