(Co, Mn)-Doped NiSe2-diethylenetriamine (dien) nanosheets and (Co, Mn, Sn)-doped NiSe2 nanowires for high performance supercapacitors: compositional/morphological evolution and (Co, Mn)-induced electron transfer

Abstract

A series of MSe₂-dien (M = metal(II) ion and dien = diethylenetriamine) were grown on Ni foam (NF) based on Co(II)/Mn(II) salts with different molar ratios. It was found that the Co-free sample exhibited hollow tubes built by numerous interconnected nanowires, whereas nanosheets were observed in the Co-involved samples. The formation of nanosheets is associated with Co(II), which is due to the fact that Co(II) promotes the metal selenide nanosheet to grow along its (01) facet (thickness direction). Furthermore, the formation and compositional/morphological evolution of the samples were investigated. Among them, (Co, Mn)-NiSe₂-dien/NF (2 : 1-Co/Mn sample) showed the largest specific capacity of 288.6 mA h g⁻¹ at 1 A g⁻¹ with a retention of 69% at 10 A g⁻¹ (198.6 mA h g⁻¹), which is associated with its ultrathin nanosheet arrays and the co-doping of (Co, Mn) into NiSe₂-dien, leading to the redistribution of electron densities around the Ni and Se centers. XPS and density functional theory (DFT) calculations proved the electron transfer from NiSe₂-dien to the adsorbed OH⁻ ions from the electrolyte solution, which can facilitate the redox reaction between active sites and electrolyte ions to enhance the electrochemical performance. A hybrid supercapacitor, (Co, Mn)-NiSe₂-dien/NF//activated carbon, was fabricated, which displayed an energy density of 50.9 W h kg⁻¹ at a power density of 447.3 W kg⁻¹ and good cycling stability with 84% capacity retention after 10 000 charge–discharge cycles. Furthermore, (Co, Mn)-doped NiSe₂-dien nanosheets could be transformed into (Co, Mn, Sn)-doped NiSe₂ nanowire arrays after immersion in SnCl₂ alcoholic solution due to cation exchange and the Kirkendall effect, and the obtained sample exhibited a decent areal capacity of 0.267 mA h cm⁻² at 5 mA cm⁻².