Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom One-step construction of porous Ni/Co metal/oxide nanocubes for highly efficient oxygen evolution Binling Chen a , Zhuxian Yang a , Qijian Niu a,b , Hong Chang a , Guiping Ma a,b , Yanqiu Zhu a , Yongde Xia a, ⁎ a College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, UK b State Key Laboratory of Chemical Resource Engineering, Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, PR China ARTICLE INFO Keywords: Oxygen evolution reaction Electrocatalyst Ni/Co metal oxide Nanocubes: Prussian blue analogue ABSTRACT A one-step method to synthesize porous Ni/Co metal/oxide as highly efficient oxygen evolution reaction (OER) catalysts is presented. Using Ni-Co Prussian blue analogue as both the precursor and the template, a simple oxidation in air at low temperatures results in the formation of Ni/Co metal/oxide nanocubes with fine particles. The optimized porous NiCoO x -400 exhibits excellent OER activity with a low onset of 1.40 V (vs. RHE), an overpotential of 280 mV at 10 mA/cm 2 and a small Tafel slope of 74 mV/decade in 0.1 M KOH solution, sig- nificantly superior to those of the state-of-the-art noble-metal OER catalyst IrO 2 /C. This approach offers the potential to generate highly active and low-cost OER catalysts on a large scale. 1. Introduction Sustainable energy conversion and electrochemical energy storage techniques, such as water splitting, metal-air batteries and fuel cells, have been widely considered as next generation sustainable energy technologies that may ease the global energy crisis, due to their ad- vantages of low environmental impact and excellent conversion effi- ciency and capacity [1–3]. Among these techniques, oxygen evolution reaction (OER) is one of the key electrode half reactions but it turns out to be the bottleneck for practical applications due to its sluggish ki- netics of this reaction. Currently, noble metal Ir- and Ru-based mate- rials are benchmarked OER catalysts which are, however, too expensive to allow their wide applications [4–6]. As a result, it is imperative to develop highly active and cost effective OER catalysts. Among a variety of potential OER catalysts, earth-abundant Ni/Co oxides have drawn attention due to the large number of active centres and low cost. Single Ni and Co based transition metal oxides are re- garded as low cost OER catalysts [7–9], but more and more researchers found that bimetal oxides may over-perform single metal oxides [10–12]. Ni/Co oxide can be generally chemically formed from their inorganic salts precursors such as metal carbonates, hydroxides, and so on [13–15]. Recently, it is reported that metallic Ni/Co or its combi- nation with their oxides are excellent OER catalysts [16–18]. Metal-organic frameworks (MOFs) are a typical type of porous crystalline materials with easily tailorable structures. Early efforts have been devoted to exploring MOF-derived materials for electrochemical applications [19–24], and their performance can be significantly af- fected by the control of their morphology, topology and chemical composition [25–27]. As a special class of MOFs, Prussian blue analo- gues (PBA) are in the formula of M II 3 [M III (CN) 6 ] 2 ·nH 2 O (where M stands for Fe, Ni, Co, etc.) and have been explored as versatile precursors to generate different metal hydroxides, sulfides or oxides with tunable nanostructures for various energy related applications [28–30]. How- ever, additional chemicals such as NaOH [28] and Na 2 S[29] or multi- step annealing treatments [30] were generally required to produce the target materials. Herein, for the first time we report a simple one-step technique for the synthesis of Ni/Co oxide based nanocubes derived from PBA, and we further demonstrate that the resultant NiCoO x nanocubes are pro- mising OER electrocatalysts with excellent electrochemical activity and stability. 2. Experimental 2.1. Synthesis of Ni-Co Prussian blue analogue nanocubes Ni-Co Prussian blue analogue (PBA) nanocubes were synthesized by a modified literature method [29]. Typically, 1.35 mmol sodium citrate and 0.9 mmol nickel nitrate were dissolved in 30 mL deionized water. 0.6 mmol potassium hexacyanocobaltate was dissolved in another https://doi.org/10.1016/j.elecom.2018.07.016 Received 4 June 2018; Received in revised form 27 June 2018; Accepted 12 July 2018 ⁎ Corresponding author. E-mail address: Y.Xia@exeter.ac.uk (Y. Xia). Electrochemistry Communications 93 (2018) 191–196 1388-2481/ © 2018 Elsevier B.V. All rights reserved. T