Colloidal Ni₂P Nanocrystals Encapsulated in Heteroatom-Doped Graphene Nanosheets: A Synergy of 0D@2D Heterostructure Toward Overall Water Splitting

Abstract

Transition-metal phosphide (TMP) nanostructures have been extensively studied for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, phase-controlled synthesis of colloidal Ni2P nanocrystals (NCs) or related heterostructures remains challenging and their use as bifunctional electrocatalysts in overall water splitting (OWS) is not systematically studied. Herein, zero-dimensional (0D) colloidal Ni2P NCs are synthesized using a robust solution-phase method and encapsulated in two-dimensional (2D) N- and S-doped graphene (NSG) nanosheets via facile ex situ sonication to form a 0D@2D Ni2P@NSG heterostructure. The interaction between surface functionalities of Ni2P NCs and defective NSG via strong van der Waals force provides a robust sheath to Ni2P NCs when encapsulated in NSG nanosheets, further enhancing the specific surface area and active site exposure. Density functional theory calculations indicate that the dual interaction of N and S dopants with Ni2P benefits the synergistic effect of optimized water and hydrogen free energy adsorption. As a result, Ni2P@NSG electrocatalysts manifest high catalytic activity toward HER and OER, and a two-electrode alkaline electrolyzer assembled by Ni2P@NSG as both an anode and a cathode requires only 1.572 V to reach a current density of 10 mA/cm2.

Recent Progresses in Engineering of Ni and Co based Phosphides for Effective Electrocatalytic Water Splitting

“…The elemental mapping results the presence of elements such as Ni, P, C, N and S confirms the presence of Ni 2 P over NSG (Figure 16g). The catalyst shows high efficiency in both HER and OER as well as in the TWS by keeping Ni 2 P@NSG as both anode and cathode shows the overall cell voltage as 1.57 V at 10 mA cm −2 [63] . The strong interaction of 0D Ni 2 P and the 2D NSG changes the electronic configuration via transferring electron from Ni 2 P. Also, the binding energy change on the intermediate formed with the Ni 2 P@NSG and the heteroatom doping further alters the adsorption energy of the intermediated in the process of OER and HER [9,59] .…”

“…Suryawanshi et al . demonstrated the phase controlled Ni 2 P nanocrystals as an active electrocatalyst for total water splitting in alkaline medium [63] . The colloidal solution of 0D Ni 2 P was synthesised via solution‐phase method and following that the bare graphene was oxidised and hydrothermal treated at 160 °C for 12 h which forms 2D N, S‐doped graphene (NSG) (Figure 15).…”

“…The strong interaction of 0D Ni 2 P and the 2D NSG changes the electronic configuration via transferring electron from Ni 2 P. Also, the binding energy change on the intermediate formed with the Ni 2 P@NSG and the heteroatom doping further alters the adsorption energy of the intermediated in the process of OER and HER [9,59] . The detail of the studies with various nickel phosphides describes the efficiency and durability of the catalyst for TWS application [63] . Following NiP, the transition metal phosphide such as CoP was also studied widely for alkaline water electrolysis and was detailed with the synthesis methods.…”

“…During HER and OER mechanistic steps, the presence of metal phosphides showed enhanced charge transfer with high electrical conductivity of phosphides compared to their own oxide counterparts [16,55–61] . Hence, it is highly desired to visualize the importance of Ni and Co based phosphides towards electrocatalytic water splitting with such high improvements towards AWS [1,62–65] . Figure 2 shows the schematic representation of metal phosphides that can be used as both anode and cathode (bi‐functional catalyst) towards water splitting.…”

Electrocatalytic Properties of Pulse-Reverse Electrodeposited Nickel Phosphide for Hydrogen Evolution Reaction

“…The relatively higher positive charges of Ni δ+ and the stronger ensemble effect of phosphorous with increasing contents of phosphorous in Ni-P compounds have been reported to improve the catalytic activities especially for HER due to the easier desorption of H 2 ( Pan et al, 2015b ; Menezes et al, 2017 ). The Ni-P electrocatalysts for water splitting have been fabricated by various approaches, such as colloidal synthesis, phosphorylation, physical vapor deposition, chemical vapor deposition, plasma spraying method, and electrodeposition ( Wu and Duh, 2003 ; Mahalingam et al, 2007 ; Panneerselvam et al, 2008 ; Wu and Wu, 2013 ; Cai et al, 2015 ; Kornienko et al, 2017 ; Hasannaeimi and Mukherjee, 2019 ; Kim et al, 2019 ; Suryawanshi et al, 2020 ). The electrodeposition technique can be performed to deposit homogeneous Ni-P electrocatalysts in low temperature and atmospheric pressure without high-cost facilities.…”