Morphological and Electronic Dual Regulation of Cobalt-Nickel Bimetal Phosphide Heterostructures Inducing High Water-Splitting Performance.

ABSTRACT

Electrocatalytic water splitting (EWS) is a key technology for generating clean and sustainable hydrogen, which can store abundant energy but is impeded by the insufficient efficiency of the anode and cathode catalyst. Designing and constructing non-noble metal composite bifunctional electrocatalysts for promoting both the cathodic hydrogen evolution (HER) and anodic oxygen evolution reactions (OER) is clearly of great importance for EWS. Thus, the chemical composition and morphology of cobalt-nickel bimetal phosphide (Ni, Co)2P nanoparticles (NPs) encapsulated in nitrogen-doped carbon nanotube hollow microspheres (NCNHMs) can regulate the redox-active sites and enhance the electron transfer, resulting in superior splitting efficiency. Contributing to the synergistic effects between highly active Co-Ni bimetal phosphide NPs and NCNHMs, the obtained Co-Ni bimetal phosphide/NCNHMs display remarkable electrochemical performance for water splitting compared with Ni2P/NCNHMs. Therefore, the Ni1.4Co0.6P/NCNHMs catalysts achieved through a nitriding-phosphidation strategy derived from a hollow Ni1.4-Co0.6-based metal organic framework (MOF) exhibit superior HER catalytic activity (87.9 mV at 10 mA cm-2 tested in 0.5 M H2SO4 and 64.4 mV at 10 mA cm-2 tested in 1 M KOH) and OER catalytic activity (320.0 mV at 10 mA cm-2 tested in 1 M KOH). The Ni1.4Co0.6P/NCNHMs deliver excellent water-splitting catalytic activity (1.55 V at 10 mA cm-2 tested in 1 M KOH), which is competitive with that of current non-noble metal electrocatalysts. Density functional theory (DFT) simulations and related experimental results suggest that the electron transfer from Co doping and coating with NCNHMs improves the electronic states, which would enhance the binding strength with H-bonds and then promote the electrocatalytic activity.