Hierarchically Porous Nickel Sulfide Multifunctional Superstructures

DOI: 10.1002/aenm.201502333 Herein we report the rational design and synthesis of new hierarchically multifunctional porous nickel sulfi de superstructures (h-NiS x ) through template-free electrodeposition of porous metallic Ni microsphere arrays on a nickel foam followed by low-temperature sulfurization. The hierarchically macroporous structure of h-NiS x with 3D confi guration offers large interfacial area, reduced ionic diffusion distance, convenient charge separation, and gas transport. Moreover, the penetrated complementary macropores and mesopores provide highly effective accessibility of active sites. [ 3c , 12 ] With such a desirable nanoarchitecture, our h-NiS x exhibited outstanding catalytic performance and robust stability for both HER and OER in an alkaline electrolyte (1.0 M KOH) as well as high specifi c areal capacitance. For instance, the overpotentials of h-NiS x to afford a current density of 10 mA cm −2 are as small as −60 mV for HER and 180 mV for OER in 1.0 M KOH, respectively. Furthermore, the h-NiS x -based alkaline water electrolyzer approached 10 and 20 mA cm −2 at cell voltages of 1.47 and 1.53 V, respectively, along with robust durability. These values placed our h-NiS x in a position better than almost all of the recently reported nonprecious electrocatalysts for complete water splitting. In addition, a supercapacitor electrode based on h-NiS x yielded an ultrahigh areal capacitance of 6104 mF cm −2 at 10 mV s −1 and excellent cycling stability. Overall, our h-NiS x represents a superior multifunctional nanomaterial for overall water splitting (both HER and OER) electrocatalysis and electrochemical capacitive storage. It should be noted that our synthetic strategy is potentially generalizable for many other materials to achieve similar hierarchically porous superstructures. For the synthesis of h-NiS x , hierarchically metallic Ni microsphere arrays with open space were fi rstly electrodeposited on commercial 3D porous nickel foam ( Scheme 1 a; see the Supporting Information for more details). The evolved H 2 bubbles during the electrodeposition might function as templates for the formation of porous Ni microsphere arrays. X-ray diffraction (XRD) pattern and scanning electron microscopy (SEM) images of the resulting hierarchically metallic Ni microsphere arrays/Ni foam (named as h-Ni) confi rmed the formation of metallic Ni and 3D microsphere arrays with a continuous size distribution ranging from 500 nm to 3 μm, respectively (Figures S1a and S2, Supporting Information). High-magnifi cation SEM image of h-Ni revealed an open porous structure composed of stacked Ni microspheres with smooth surface (Figure S1b, Supporting Information), in sharp contrast to that of the pristine Ni foam (Figure S1a, Supporting Information). The resulting h-Ni was then subjected to low-temperature sulfurization to obtain the fi nal 3D hierarchically porous h-NiS x superstructures (Scheme 1 b). Upon sulfurization, nickel microspheres were transformed to NiS x , which contained a mixture of NiS and NiS 2 , as revealed by the XRD patterns (Figure S1a, Supporting Information). Inductively coupled plasma atomic emission The ever increasing global energy needs associated with environmental issues resulting from unsustainable fossil fuel consumption have prompted intense research interest in renewable energy utilization. [ 1 ] Conversion and storage of intermittent renewable energy (e.g., solar, wind, etc.) in chemical forms have been considered as an attractive approach to tackle the energy challenge. [ 2 ] Therefore, renewable energy-driven water splitting, consisting of H 2 and O 2 evolution reactions (HER and OER, respectively), has been widely advocated as an ideal strategy to transform renewable energy to chemical fuel, H 2 . [ 3a,b ] In addition, H 2 is an extremely important chemical commodity for many global-scale industrial processes, such as petroleum refi ning and ammonium synthesis. Another appealing strategy is to store renewable energy via electrical charges in supercapacitors. Owing to the advantages of these two strategies for renewable energy conversion and storage, such as high effi ciency, environmental friendliness, large power intensity, and long lifetime, a substantial amount of efforts has been devoted to developing competent materials for water splitting and supercapacitor applications. [ 3c-h ] In spite of recent progress in both areas, large-scale commercialization still faces numerous challenges, such as the high price and scarcity of water splitting electrocatalysts (the state-of-the-art catalysts are RuO 2 and IrO 2 for OER and Pt for HER) [ 4 ] and the mediocre energy density of supercapacitors. [ 5 ] Thus, it is imperative to develop novel nanostructured materials consisting of solely earth abundant elements while possessing high performance for both applications. [ 6 ]