This chapter shows the growth of hierarchical morphologies of complex metal oxides directly onto the substrate. Herein, it reports unique hollow-cuboidal MnCo2O4 (h-MCO) morphology that offers insights into the efficient charge-transfers and surface kinetics for oxygen evolution reaction. The h-MCO coupled nickel phosphate under alkaline conditions outperforms the benchmark catalyst RuO2.High stability, low cost, and green synthesis procedure mark h- MCO/NiPi as one of the futuristic catalysts to be used in industrial applications.
A. K. Shah, et al., Chem. Commun., 2021, 57, 8027.
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Diffusion of electrolyte
Hollow cuboidal MnCo
2O
4-NiPi O
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5.1 INTRODUCTION
The electrocatalytic water splitting technique is gaining popularity due to its great potential to fulfill future green and sustainable energy demands.1 Material containing noble metals such as RuO2 and IrO2 show excellent results for oxygen evolution reaction (OER) in alkaline and acidic conditions2,3. However, due to high cost and scarcity, commercial implementation of these materials becomes economically inviable.
Different strategies, such as doping and morphological tuning, are attempted to modify the existing materials such as metal sulphides, metal oxides, metal phosphides to improve the OER.4-6 However, they underperform when compared to the precious metal counterparts reported.7
MnCo2O4, a stablespinel oxide, is gaining popularity in OER, due to its low cost, eco-friendliness, tunable morphological features, and high stability in alkaline medium.
Previous reports show the use of Mn and Co based catalysts for OER. However, binary metal oxides such as MnCo2O4 show better conductivity than unitary MnO2 and Co3O4
apart from their exceptional stability under alkaline conditions.8 To date, the use of spinel MnCo2O4 showing OER activity is sparse in the literature.9-11 One of the major drawbacks of MnCo2O4 is its high charge transfer resistance at the electrode-electrolyte interface, which affects its OER activity. The above-mentioned drawbacks can be addressed through morphological and surface modification of MnCo2O4. Popular metal phosphates such as Ni3(PO4)2 (NiPi) and Co3(PO4)2 (CoPi) are used as catalysts in electrochemical water oxidation.12, 13 However NiPi is preferred compared to CoPi due to its stability in harsh pH conditions and rapid redox process (Ni2+ ↔ Ni3+) which helps in achieving higher reaction kinetics.13 Phosphate group in NiPi also facilitates the adsorption, thereby enhancing OER activity of MnCo2O4 composite.14
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90 Part of this chapter has been published in Chem. Commun., 2021,57, 8027
Recent efforts towards increasing the number of active sites in MnCo2O4 areby its morphological tuning. The MnCo2O4 with proper morphology can become an excellent candidate for electrocatalytic water splitting.15,16 However, reported literature on MnCo2O4 mainly focuses on synthesizing it in powderform through the hydrothermal procedure, which was later drop-casted over glassy carbon.16,17 Recent reports on the synthesis of MnCo2O4 over nickel foam and titanium mesh reported good efficacy, but due to its high price and issues of reusability under harsh alkaline conditions, are not deemed suitable for commercial implementation.10,18
In the present manuscript, we intended to overcome the substrate deposition issues by growing the hollow cuboidal structures of MnCo2O4 directly onto the FTO substrate.
We have utilized a green and simple synthetic route involving hydrothermal conditions to grow hollow cuboidal structures using urea and NH4F to achieve high-quality and stable films for OER performance. Further, to overcome the poor charge transfer kinetics of h-MCO, we have electrodeposited nickel phosphate over the h-MCO surface.
Surface modification of h-MCO improves the overpotential required for OER activity apart from improving the charge transfer kinetics and turnover frequency in h- MCO/NiPi. As synthesized h-MCO/NiPi anode displays excellent long-term stability in alkaline conditions and shows good OER activity for its use in large-scale application to meet the future energy demand.
5.2 EXPERIMENTAL SECTION
5.2.1 Synthesis and Fabrication Protocol of h-MCO
Step-by-step fabrication of h-MCO/NiPi electrode is shown in Scheme 5.1. The h- MCO was synthesized over FTO through a hydrothermal route.18 Mn(NO3)2.4H2O (2 mmol), Co(NO3)2.6H2O (4 mmol), urea (1.44 g), and NH4F (0.37 g), were dissolved in 70 ml deionized water, and the solution was kept under stirring for 2 h. FTO in a teflon vessel was kept facing
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Chapter 5 MnCo2O4/NiPi up in a 100 ml teflon-lined autoclave. The area of FTO exposed to the solution was controlled by putting polyimide tape over the FTO which can sustain very high temperature and pH. Only 1 x 1 cm2 of FTO was kept exposed to the solution. The above solution mixture was then transferred into the autoclave and kept in an oven at 150 °C for 24 h. The pink-colored film was formed over FTO and was washed with deionized water and ethanol. After drying at 100
°C, it was transferred to a muffle furnace and calcined at 450 °C for 3 h to form black-colored hallow cuboidal MnCo2O4.
5.2.2 Deposition of NiPi Over h-MCO (h-MCO/NiPi)
NiPi layer was deposited over h-MCO through the electrodeposition technique.19 In a typical synthetic procedure Ni(NO3)2.6H2O (3 mmol) and NaH2PO4.H2O (2 mmol) were dissolved in 300 ml 1:1 (v/v) water and ethanol mixture solution and the solution was kept stirring for 1 h. The three electrode cell (Ag/AgCl as a reference, Pt as a counter, and FTO/ h- MCO as a working electrode) was used for deposition of NiPi layer over h-MCO. Cyclic voltammetry was performed at the scan rate of 10 mV/sec within the voltage range of -1.2 V to 0.4 V for different number cycles. The sample was then washed with deionized water followed by drying overnight at 100 °C.
Scheme 5.1 Step-by-step procedure for the green and cost-effective fabrication of h-MCO/NiPi electrode directly over FTO.
Hydrothermal
@150 0C for 24 h
F T O Calcination @ 450 0C for 3h
F T O Electrodeposition
FTO
MnCo(OH)X Mn(NO3)2+ Co(NO3)2
Urea +NH4F
MnCo2O4/NiPi
MnCo2O4
FTO
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92 Part of this chapter has been published in Chem. Commun., 2021,57, 8027
5.3 RESULTS AND DISCUSSION
5.3.1 Powder X-Ray Diffraction (PXRD) Analysis
To confirm the formation and purity of the respective catalysts powder X-ray diffraction (PXRD) analysis was done. Figure 5.1 shows the PXRD pattern of h-MCO, NiPi, and h-MCO/NiPi electrodes.20,21 Diffraction peaks of h-MCO were indexed to the corresponding (hkl) planes, confirming the formation of h-MCO, with no detectable impurity (JCPDS Card No - 00-023-1237). PXRD of NiPi, shows a broad peak in the range, 2Ɵ (15-40) confirming the formation and amorphous NiPi. In the PXRD pattern of h-MCO/NiPi, all the peaks of h-MCO were indexed. Due to the amorphous nature of NiPi, the diffraction peak of