Regulating Hollow Carbon Cage Supported NiCo Alloy Nanoparticles for Efficient Electrocatalytic Hydrogen Evolution Reaction

Abstract

The NiCo alloy is one of the most promising alternatives to the noble-metal electrocatalysts for the hydrogen evolution reaction (HER); however, its performance is largely restricted by insufficient active sites and low surface area. Here, we fabricated a hierarchical hollow carbon cage supported NiCo alloy (denoted as HC NiCo/C) and a bulk NiCo alloy (denoted as NiCo) by reduction of a partially ZIF-67 etched ZIF-67@NiCo-LDH (LDH = layered double hydroxide) precursor and a fully ZIF-67 etched NiCo-LDH precursor, respectively. The as-prepared HC NiCo/C, in which the Ni29Co71 alloy nanocrystals with an average 6 nm size were encapsulated in graphitic carbon layers, provided a vastly increased electrochemically active surface area (ca. 13 times than the NiCo) and abundant catalytic active sites, which resulted in a higher HER performance with an overpotential of 99 mV than the 198 mV for NiCo at 10 mA cm–2. Detailed experimental results suggested that only the HC NiCo/C possessed the active alloy surface composed of unsaturated Ni0 and Co0 atoms, and both the metal–support interaction and alloying effect influenced the electronic structure of Co and Ni in HC NiCo/C, whereas the NiCo exhibited pure Ni surface. Theoretical calculations further revealed the Ni29Co71 alloy surface in HC NiCo/C possessed the appropriate adsorption energy of the intermediate state (adsorbed H*). This work provided new insight into the construction of the stable small-sized bimetallic alloy nanocatalysts by regulating the reduction precursors.

Convenient Synthesis of NiCo Alloy Nanoparticles Encapsulated by N-Doped Porous Carbon for the Oxygen Evolution Reaction

“…The XRD pattern of the Ni@NC electrode material shows distinct diffraction peaks at 44.5, 51.6, and 76.3°, which could be indexed successfully to the (111), (200), and (220), respectively, which matched standard cards for the Ni fcc structure (JCPDS 04–0850) . When Co ions are added, the peak of the sample is obviously shifted to a lower angle, between the peak positions of Ni and Co (JCPDS 15–0806) metals. , The three peaks belonging to the NiCo alloy phase are located at 44.3, 51.6, and 76.1° corresponding to the (111), (200), and (220) planes, respectively . This indicates the successful synthesis of NiCo alloy .…”

“…18,19 The three peaks belonging to the NiCo alloy phase are located at 44.3, 51.6, and 76.1°c orresponding to the (111), (200), and (220) planes, respectively. 18 This indicates the successful synthesis of NiCo alloy. 20 The wide diffraction peak at 26.4°is attributed to the (002) plane of graphitic carbon.…”

“…17 When Co ions are added, the peak of the sample is obviously shifted to a lower angle, between the peak positions of Ni and Co (JCPDS 15−0806) metals. 18,19 The three peaks belonging to the NiCo alloy phase are located at 44.3, 51.6, and 76.1°c orresponding to the (111), (200), and (220) planes, respectively. 18 This indicates the successful synthesis of NiCo alloy.…”

General synthesis of carbon-guarded cobalt-based nanospheres for oxygen reduction electrocatalysis

“…When Ni/Co microspheres and NiCo 2 O 4 NPs are heated with melamine, the Ni/Co alloy is formed due to the reduction effect of melamine's pyrolysis products. 32,33 The Raman spectra (Fig. 2b) show that Ni/Co@NC NPs (1.72) have a larger I D /I G value than that of Ni/Co@NC (1.21), implying the presence of more carbon defects that act as active sites for ORR.…”

“…2d and e) show that Ni/Co@NC NPs have a negative shift in binding energies of Ni 2p 3/2 and Co 2p 3/2 peaks compared to Ni/Co@NC, which indicates that electrons tend to transfer to Ni and form a relatively electron-rich state of Ni. 32 In Ni/Co@NC NPs, the thinner carbon shell reduces electron transfer from the Ni/Co alloy core to the carbon shell, thereby enhancing oxygen adsorption and promoting ORR. 23 It should be noted that +2 and +3 valence states of Co and Ni can be detected in Ni/Co@NC NPs and Ni/Co@NC, which may be attributed to the oxidation of Ni and Co in the carbon shell.…”