Hierarchical free-standing networks of MnCo 2 S 4 as efficient Electrocatalyst for oxygen evolution reaction Harsharaj S. Jadhav, Animesh Roy, Gaurav M. Thorat, Wook-Jin Chung, Jeong Gil Seo* Department of Energy Science and Technology, Energy and Environment Fusion Technology Center, Myongji University, Nam-dong, Cheoin-gu, Yongin-si, Republic of Korea A R T I C L E I N F O Article history: Received 27 September 2018 Received in revised form 28 November 2018 Accepted 1 December 2018 Available online xxx Keywords: Electrocatalyst Oxygen evolution reaction Electrodeposition Ion-exchange Free-standing A B S T R A C T The development of highly efficient, stable and cost-effective electrocatalyst for oxygen evolution reaction (OER) is critical. Herein, we report growth of MnCo 2 S 4 flakes on SS-mesh using two-step strategy, and used as an efficient, highly active and stable electrocatalyst for OER under alkaline condition. The free-standing electrocatalyst delivers exceptional stability of 100 h and activity for OER with overpotential of 290 mV at a current density of 10 mA cm 2 in 1 M KOH. The enhanced electrocatalytic performance was supported experimentally by electrochemical impedance spectra and measurement of the electrochemically active surface area. The high electrochemical active surface area and electrical conductivity of MnCo 2 S 4 flakes played an essential role in their high electrocatalytic performance. © 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. Introduction In the recent years, electrochemical water splitting plays important role in the several energy conversion and storage technologies such as hydrogen/oxygen production, regenerative fuel cells, and metal-air batteries, etc [1,2]. In electrochemical water splitting which involves hydrogen evolution reaction (HER) at cathode and oxygen evolution reaction (OER) at anode offers a large-scale production of high purity of hydrogen [3–6]. In this processes, the kinetic bottleneck of water splitting for practical applications is the OER, due to sluggish kinetics derived from the multistep proton-coupled electron transfer process results in high overpotential [7,8]. Therefore, high performance catalysts are required to reduce the overpotential and accelerate the OER. So far, platinum (Pt), iridium oxides (IrO 2 ) and ruthenium oxides (RuO 2 ) have been used as the most active OER electrocatalyst because of fast reaction rate with lower overpotential. However, their scarcity, high cost and stability hinders large scale application. Therefore, it is desirable to develop highly active, durable and cost-effective OER electrocatalyst. Accordingly, several transition metal based oxides [9,10], 2D- layered double hydroxides (LDH) [11], chalcogenides [12,13], phosphides [9], and nitrides [14] have been investigated exten- sively as OER electrocatalyst on various supports. It is well known that bimetallic sulfide materials have higher electronic conductiv- ity than corresponding bimetallic oxides, which offers enhance- ment in electrochemical performance. Recently, transition metal sulfides have been studied as catalyst for different applications such as secondary batteries, supercapacitors, metal-air batteries, dye sensitized solar cells, and water splitting. Especially, Co-based bimetallic sulfides (MCo 2 S 4 , where M = Ni, Zn, Cu and Mn) exhibit the superior performance because of excellent electrical and optical properties [15,16]. Among these MnCo 2 S 4 have been widely studied as an electrode material for supercapacitors but not extensively studied as electrocatalyst for water splitting [17–19]. Recently, Zhang et al. reported that MnCo 2 S 4 nanowire array grown on Ti mesh acts as an efficient catalyst for OER [20]. Herein, we have reported facile, cost-effective and low- temperature synthesis of MnCo 2 S 4 nanoflakes on stainless steel (SS) mesh (MnCo 2 S 4 /SS) by electrodeposition method followed by sulfidation of MnCo-precursor flakes by ion-exchange method. The synthesized electrocatalyst exhibits superior activity, acceptable overpotential with long-term stability of 100 h when used as electrocatalyst for OER. Experimental section Chemicals Mangnese nitrate tetrahydrate [Mn(NO 3 ) 2 4H 2 O(99%)] was purchased from sigma-Aldrich. Cobalt nitrate hexahydrate [Co (NO 3 ) 2 6H 2 O] and sodium sulphide hydrate (Na 2 SxH 2 O) were * Corresponding author. E-mail address: jgseo@mju.ac.kr (J.G. Seo). https://doi.org/10.1016/j.jiec.2018.12.002 1226-086X/© 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx G Model JIEC 4295 No. of Pages 8 Please cite this article in press as: H.S. Jadhav, et al., Hierarchical free-standing networks of MnCo 2 S 4 as efficient Electrocatalyst for oxygen evolution reaction, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.12.002 Contents lists available at ScienceDirect Journal of Industrial and Engineering Chemistry journal homepa ge: www.elsev ier.com/locate/jie c