From Water Oxidation to Reduction: Transformation from Ni x Co 3−x O 4 Nanowires to NiCo/NiCoO x Heterostructures Xiaodong Yan, † KeXue Li, § Lu Lyu, ‡ Fang Song, ∥ Jun He, ‡ Dongmei Niu, ‡ Lei Liu, § Xile Hu, ∥ and Xiaobo Chen* ,† † Department of Chemistry, University of Missouri−Kansas City, Kansas City 64110, Missouri United States ‡ School of Physics and Electronics, Hunan Key Laboratory for Super-microstructure and Ultrafast Process, Central South University, 932 South Lushan Road, Changsha, Hunan 410083, China § State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun, 130033, China ∥ Ecole Polytechnique Fe ́ de ́ rale de Lausanne (EPFL), Institute of Chemical Sciences and Engineering, EPFL-SB-ISIC-LSCI, bch 3350, Lausanne CH 1015, Switzerland * S Supporting Information ABSTRACT: A homologous Ni−Co based nanowire catalyst pair, composed of Ni x Co 3−x O 4 nanowires and NiCo/NiCoO x nanohybrid, is developed for efficient overall water splitting. Ni x Co 3−x O 4 nanowires are found as a highly active oxygen evolution reaction (OER) catalyst, and they are converted into a highly active hydrogen evolution reaction (HER) catalyst through hydrogenation treatment as NiCo/NiCoO x heteronanostructures. An OER current density of 10 mA cm −2 is obtained with the Ni x Co 3−x O 4 nanowires under an overpotential of 337 mV in 1.0 M KOH, and an HER current density of 10 mA cm −2 is obtained with the NiCo/NiCoO x heteronanostructures at an overpotential of 155 mV. When integrated in an electrolyzer, these catalysts demonstrate a stable performance in water splitting. KEYWORDS: nickel cobalt oxide, nanowires, metal/metal oxide heterostructures, hydrogen evolution reaction, oxygen evolution reaction 1. INTRODUCTION Water splitting through photocatalysis and electrolysis has attracted huge attention, 1−4 as hydrogen is a highly desirable energy carrier for future clean and renewable energy supply. Over the past several years, water splitting through photo- catalysis has made great breakthrough, especially owing to the discovery of various black titanium dioxide nanomaterials through hydrogenation treatment. 1,5,6 Although remarkably enhanced hydrogen generation rate was observed in black titanium dioxide, 1 hydrogen production through photocatalysis is still far from practical applications due to its low efficiency. On the other hand, sustainable hydrogen production on a large scale can be achieved by water electrolysis using electricity from solar and wind energy. 7,8 The key to water splitting through electrolysis is the electrocatalysts. The state-of-the-art catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are platinum and noble metal oxides (e.g., IrO 2 and RuO 2 ), respectively. However, their scarcity and high cost largely restrict their widespread applications. 9−13 Therefore, exploring the earth-abundant, low-cost electrocatalysts with high activity toward HER and/or OER is of significant importance. Over the past decade, earth-abundant transition metals (especially Fe, Co, and Ni) and their derivatives have attracted tremendous attention. The discovery of new compounds contributed greatly to the development of earth-abundant, low-cost electrocatalysts. For example, transition metal phosphides 9,14−17 and transition metal layered double hydrox- ides 12,13,18−20 presented high catalytic activity for HER and OER, respectively. Another efficient way to achieve high- activity catalysts is to modify the structure of the existing materials. For instance, metal/metal oxide/carbon composites synthesized through carbon thermal reduction have been reported to possess much higher activity toward HER than the pristine metal/carbon composites. 21,22 Another example is electrochemical tuning, which can effectively tune the electronic structure of the materials for a better catalytic activity. 11,23−26 Recently, modification of metal oxides through hydrogenation treatment opens a new avenue to tune the catalytic activity of the metal oxide materials. 3,27,28 For example, Co/Co 3 O 4 hybrid Received: November 6, 2015 Accepted: January 19, 2016 Published: January 19, 2016 Research Article www.acsami.org © 2016 American Chemical Society 3208 DOI: 10.1021/acsami.5b10724 ACS Appl. Mater. Interfaces 2016, 8, 3208−3214