Nanocoral CuCo2S4 thiospinels: Oxygen evolution reaction via redox interaction of metal ions

doi:10.1016/j.electacta.2020.137701

Electrochimica Acta

Volume 370, 20 February 2021, 137701

https://doi.org/10.1016/j.electacta.2020.137701 Get rights and content

Abstract

Understanding the electrocatalytic activity and redox mechanism in mixed metal sulfides has steered the present work. Mixed metal sulfide, CuCo₂S₄, synthesised by hydrothermal method, reveals the presence of highly crystalline nanocoral sheets of CuCo₂S₄ with mixed oxidation states of Co and Cu ions. Electrochemical measurements show superior oxygen evolution reaction (OER) activity and high exchange current density over CuCo₂S₄ in comparison with pristine (Co₃S₄ and Cu₂S) and sintered CuCo₂S₄. In 0.5 M KOH, the nanocoral CuCo₂S₄ catalyst and its sintered form show a current density of 10 mA cm⁻² at overpotential of 368 mV and 511 mV, respectively. A close inspection of the electrocatalytic performance in correlation with XPS spectra reveals the synergistic effect of Co and Cu ions through a redox interaction. Density Functional Theory calculations also indicate strong dependence of redox activity on redox couples as accompanied with changes in the partial charges of different constituent ions in the catalyst upon adsorption of hydroxyl ions which is a key step in OER. The study not only details sulfide-based catalytic compound for OER but also provides conclusive insights into the surface redox processes thereby providing an alternative direction to the search on novel OER catalysts.

Graphical abstract

Introduction

In the present scenario, when the world is moving towards developing sustainable energy resources forced by the exhaustion of fossil fuels and increased greenhouse gas emissions, demand is growing for various energy storage/conversion techniques such as supercapacitors, electrolyzers and fuel-cell based devices. Spinels have already captured this contemporary area with applications as electrode materials in metal ion batteries, and as catalysts facilitating NO_x reduction, CO₂ reduction, hydrocarbon oxidation, CO oxidation, oxygen evolution reaction (OER), oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), methane combustion, urea and NH₃ oxidation to name a few [1,2].

Spinels as electrocatalysts have the potential to compete with catalysts of simple metal oxides, perovskites or precious metals due to their cost effectiveness, low temperature synthesis, thermal stability and eco-friendliness [1]. Various spinel oxide categories like MCo₂O₄, MMn₂O₄, and MFe₂O₄ (M= Ru, Zn, Ni, Mn, Fe, Cu) provide excellent OER/ORR properties. Among these spinels, cobalt oxides deserve a special mention for their catalytic activity in alkaline media. They are perceived as fundamental normal spinel oxides in which electrochemical activity can be upgraded by tuning morphology, composition and designing hybrids with graphene or by developing micro/nanostructures [3], [4], [5], [6], [7].

Apart from oxides, simple transition metal sulfides like MoS_2, MnS, CoS, Co₃S₄, CuS, Ni₃S₄ have been extensively used for electrocatalysis and supercapacitor applications because of their reverse redox reactions, good conductivity, high specific capacitance and density [8], [9], [10], [11], [12], [13], [14], [15], [16]. In transition metal sulfides, bimetallic sulfides like CuCo₂S₄, NiCo₂S₄ have additional advantages of multiple oxidation states with different coordinating active sites which can improve their overall electrochemical performance [17], [18], [19], [20], [21]. The thiospinel CuCo₂S₄ has been identified as the mineral carrollite having a basic cubic structure comprising of eight FCC unit cells with Cu ions in the tetrahedral sites and Co ions occupying the octahedral sites. Recently, there has been a surge in CuCo₂S₄ for OER and supercapacitor applications [22], [23], [24], [25], [26]. Structural variations of CuCo₂S₄ like nanosheets and nanotubes offer large surface areas unravelling the role of morphology in improved catalytic activity [27], [28], [29]. Presence of high spin Co³⁺ ions in octahedral sites and their lesser probability of populating the inert tetrahedral sites make this material an interesting electrochemical material.

In this paper, nanocoral structures of CuCo₂S₄ are synthesised using hydrothermal method, and then subjected to evaluation of their electrocatalytic OER performance. Hydrothermal synthesis is credited to be a straightforward and accurate synthesis approach with aqueous solutions under high temperature and high pressure. The focus of this article is in correlating the electrochemical studies with XPS and to investigate the reaction mechanism. Our analysis throws light on the fact that Cu in tetrahedral sites plays a decisive role in the electrochemical activity other than Co ions. A combination of electrochemical, spectroscopic and computational electronic structure calculations put us in a position to probe details of accompanying surface process and decide the role of chemical environment and redox couples in the electrocatalytic material governing the activity of the electrocatalyst reported in this study.

Section snippets

Experimental details

Synthesis of CuCo₂S₄ was carried out using hydrothermal method. 0.2 mol of Co (NO₃)₂•6H₂O and 0.1 mol of Cu (NO₃)₂•3H₂O were dissolved in 30 mL of distilled water. The solution was stirred continuously for 10 min followed by the addition of 0.4 mol thiourea (CH₄N₂S). 2 mL ethylene glycol was added to this pink coloured solution and kept under vigorous stirring for 15 min. The solution was then transferred to a 60 mL capacity Teflon-lined stainless autoclave and kept at 180 °C for 18 h. The

Structure and morphology

X-ray diffraction patterns of both the samples (inset (a) in Fig. 1) showed major diffraction peaks at 2θ values corresponding to planes (220), (311), (400), (422), (511), (440) and matched well with CuCo₂S₄ [ICDD data card no. 42–1450]. It is also observed that on heating, crystallinity of sample increased, as is evident from the sharper and more intense diffraction peaks in heated sample. Observed peaks at higher 2θ values around 65° and 68° corresponding to (533) and (444) planes start to

Conclusions

In conclusion, CuCo₂S₄ nanocoral structures’ comprising layered nanosheets was synthesised using hydrothermal method. The structure and morphology of the synthesised sample was different from its heated sample (at 250 °C for 6 h). The difference in morphology was reflected in electrochemical studies with a variation of 143 mV in overpotential. Both samples gave a better electrocatalytic activity than their end sulphides; Co₃S₄ and Cu₂S proving the role of ionic interaction for higher activity.

ORCID

Sudhanshu Sharma: 0000–0002–5217–9941

Manu Sharma: 0000–0002–9948–8020

Parag A Deshpande: 0000–0002–2445–5873

Silvia Irusta: 0000–0002–2966–9088

N Sethulakshmi: 0000–0001–6530–4476

Subramanian Nellaiappan: 0000–0002–9318–2477

Authors’ contributions

SN and NS have carried out the synthesis of nanocoral structures of catalysts with the assistance of useful suggestions from SS and MS. SN and NS have carried out characterizations and electrochemical analysis. SI has carried out the XPS measurements and has assisted in XPS analysis. PP and PAD have carried out the computational part of the work. MS, SS and PAD has helped in correlating the results towards the manuscript completion. All authors gave approval for the final draft for submission.

Appendix: associated contents

Supplementary information includes complete XPS spectra of Cu 2p and Co 2p before and after OER, and Cyclic voltammogram of CuCo₂S₄, heated sample and individual sulfides Co₃S₄ and Cu₂S.

Declaration of Competing Interest

The authors declare no competing financial interest.

Acknowledgments

N.S. acknowledges DST Women in Science (WoS-A) program (SR/WOS-A/PM-35/2017(G)) for granting project. S.S. and S.N. gratefully acknowledge DST-SERB sponsored research project (EMR/2016/000806) for funding and fellowship. Authors are thankful to IIT Gandhinagar for CIF facility and VIT, Vellore for HR-TEM measurements

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¹: Both the authors contributed equally to this work.

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Nanocoral CuCo2S4 thiospinels: Oxygen evolution reaction via redox interaction of metal ions

Abstract

Graphical abstract

Introduction

Section snippets

Experimental details

Structure and morphology

Conclusions

ORCID

Authors’ contributions

Appendix: associated contents

Declaration of Competing Interest

Acknowledgments

J. Catal.

J. Power Sources

Electrochim. Acta

Nano Energy

Int. J. Hydrog. Energy

Mater. Chem. Phys.

Nano Energy

J. Alloys Compd.

J. Phys. Chem. Solids

Electrochim. Acta

J. Ind. Eng. Chem.

J. Power Sources

J. Catal.

J. Colloid Interface Sci.

Spinels: controlled preparation, oxygen reduction/evolution reaction application, and beyond

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Facile synthesis of CuCo2S4 as a novel electrode material for ultrahigh supercapacitor performance

Chem. Commun.

Nanocoral CuCo₂S₄ thiospinels: Oxygen evolution reaction via redox interaction of metal ions

Pt-Doped and Pt-Supported La_1–xSr_xCoO₃: comparative Activity of Pt⁴⁺ and Pt⁰ toward the CO poisoning effect in formic acid and methanol electro-oxidation

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Nanoporous CuCo₂S₄ microspheres: a novel positive electrode for high-performance hybrid energy storage devices

Wearable high-performance supercapacitors based on silver-sputtered textiles with FeCo₂S₄–NiCo₂S₄ composite nanotube-built multitripod architectures as advanced flexible electrodes

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