Multifunctional (Fe 0.5 Ni 0.5 )S 2 nanocrystal catalysts with high catalytic activities for reduction of I 3 − and electrochemical water splitting

It is important to synthesize environmentally friendly transition metal sulfide catalysts with superior performance for electrochemical water splitting and reduction of I3− in dye-sensitized solar cells (DSSCs). In this work, cubic (Fe0.5Ni0.5)S2 nanocrysals with a monodispersed size of ca. 10 nm were successfully prepared via a hot-injection reaction with a Schlenk line system. Because of the FeNi-oleate complex as transition metal source, no other by-products were produced. The (Fe0.5Ni0.5)S2 nanocrystal/carbon black composite with different nanocrystal contents was control fabricated. Because the composite possessed two merits, more catalytically active sites of nanocrystals and fast electron transfer of carbon black, it is a promising catalyst for electrochemical water splitting and reduction of I3− in DSSCs. As the counter electrode catalysts for reduction of I3−, DSSCs based on the composite with 57% (Fe0.5Ni0.5)S2 nanocrystal contents have a high power conversion efficiency of 6.71%, which was comparable to Pt-based DSSCs (7.05%). The electrochemical measurement showed that (Fe0.5Ni0.5)S2 nanocrystals had a good catalytic activity for reduction of I3−. As the catalytic electrode for hydrogen evolution reaction (HER), the composite electrode with 57% (Fe0.5Ni0.5)S2 nanocrystal contents displayed an overpotential of 250 mV to reach the current density of 10 mA cm−2 in alkaline solution. It retained good HER activity for 1000-cycle measurements. The density functional theory showed the free energy of hydrogen adsorbed on the Ni site near S defects was − 0.12 eV, which was smaller than that of the Fe site near S defects. So, the Ni site near S defects of (Fe0.5Ni0.5)S2 was the main catalytically active site for HER. Also, the (Fe0.5Ni0.5)S2 nanocrystals displayed good electrocatalytic activity for oxygen evolution reaction. This type of double metal sulfide with monodispersed size paves the way for new insight into earth-abundance catalysts for water splitting and I3− reduction.