W Doping in Ni12P5 as a Platform to Enhance Overall Electrochemical Water Splitting.

Sirshendu Ghosh,Alevtina Neyman,Ronen Bar-Ziv,Shaylee Kolatkar,Maya Bar-Sadan,Sunil R Kadam,Andrey N Enyashin,Chanderpratap Singh

doi:10.1021/acsami.1c16755

Abstract

Bifunctional electrocatalysts for efficient hydrogen generation from water splitting must overcome both the sluggish water dissociation step of the alkaline hydrogen evolution half-reaction (HER) and the kinetic barrier of the anodic oxygen evolution half-reaction (OER). Nickel phosphides are a promising catalysts family and are known to develop a thin active layer of oxidized Ni in an alkaline medium. Here, Ni12P5 was recognized as a suitable platform for the electrochemical production of γ-NiOOH—a particularly active phase—because of its matching crystallographic structure. The incorporation of tungsten by doping produces additional surface roughness, increases the electrochemical surface area (ESCA), and reduces the energy barrier for electron-coupled water dissociation (the Volmer step for the formation of Hads). When serving as both the anode and cathode, the 15% W-Ni12P5 catalyst provides an overall water splitting current density of 10 mA cm–2 at a cell voltage of only 1.73 V with good durability, making it a promising bifunctional catalyst for practical water electrolysis.

Highlights

Hydrogen is considered an ideal renewable energy carrier: it is environmentally benign and has a high gravimetric energy density.[1]
Transition-metal phosphides have attracted great interest for their low cost, facile synthesis, and impressive catalytic activity.[6−8] In an alkaline medium, surface Ni atoms bond with oxygen and hydroxyl groups and are transformed into the active NiOOH under a positive bias as a result of the oxidation of Ni2+ to Ni3+.9−11 Previous studies established that NiOOH is the dominant active species in the oxygen evolution reaction (OER).[9−11] A further boost in catalytic activity can be achieved by heteroatom doping of NiOOH, for example, with Fe.[12,13]
W doping in NiCoP and Ni(OH)x/Co(OH)x showed high electrocatalytic HER activity in alkaline and neutral media attributed to the W capability to accelerate water dissociation as well as to facilitate Had recombination.[25]

Summary

Introduction

Hydrogen is considered an ideal renewable energy carrier: it is environmentally benign and has a high gravimetric energy density.[1]. The need persists for alternative catalysts with lower overpotential and improved stability. Transition-metal phosphides have attracted great interest for their low cost, facile synthesis, and impressive catalytic activity.[6−8] In an alkaline medium, surface Ni atoms bond with oxygen and hydroxyl groups and are transformed into the active NiOOH under a positive bias as a result of the oxidation of Ni2+ to Ni3+.9−11 Previous studies established that NiOOH is the dominant active species in the OER.[9−11] A further boost in catalytic activity can be achieved by heteroatom doping of NiOOH, for example, with Fe.[12,13] Another pathway to enhanced activity consists of forming Ni vacancies in Ni(OH)[2] to reduce the formation energy of the active NiOOH.[14] doping with W is a promising route to boost the activity of the catalyst by enhancing its interaction with intermediates during the reactions.[9,15,16] Heteroatom doping in transition-metal phosphides (such as V,17−19 Mo20,21 Mn,[22] Fe, and W24) is a promising way to improve the electrocatalytic water splitting activity by manipulation of the electronic structure. The high affinity of W for water makes it advantageous for alkaline water splitting in particular since the water adsorption and dissociation is a prime step.[25,26] W doping in NiCoP and Ni(OH)x/Co(OH)x showed high electrocatalytic HER activity in alkaline and neutral media attributed to the W capability to accelerate water dissociation as well as to facilitate Had recombination.[25]

Methods

Results

Conclusion