Electrodeposited amorphous nickel–iron phosphide and sulfide derived films for electrocatalytic oxygen evolution

Highlights

• One-step electrodeposition of Ni–Fe sulfide and phosphide films on Ni foam.

• The Ni–Fe–S shows the highest OER activity, with 0.23 V overpotential at 0.05 A cm⁻².

• The electron interaction tunes the hydroxyl adsorption energy and accelerates OER.

• Nickel phosphide exhibits higher intrinsic OER activities than nickel sulfide.

Abstract

The preparation of inexpensive and efficient electrocatalysts for oxygen evolution reaction (OER) is crucial in the widespread application of water electrolyzers. A simple one-step aqueous electrodeposition method is utilized to prepare amorphous nickel-iron sulfide (Ni–Fe–S) and phosphide (Ni–Fe–P) films on Ni foam. The deposited films are highly porous, and can convert to active electrocatalysts for OER. In 1 M KOH, the Ni–Fe–S shows the highest OER activity, and requires only 230 mV overpotential to reach 0.05 A cm⁻² OER current densities. The Fe–Ni–S also sustains the 30 h 0.05 A cm⁻² galvanostatic OER test. Ex-situ characterizations show that sulfur in the Fe–Ni–S is oxidized and leached into the solution during OER, and that (oxy)hydroxide layer is formed at the surface. The adsorption energy of the hydroxyl group, an OER intermediate, is tuned by the electron interaction between the Ni and Fe, and the Ni–Fe–S exhibits the optimum hydroxyl group adsorption energy and the most facile OER kinetics. Also, higher intrinsic OER activity is observed for the electrodeposited amorphous nickel phosphide-derived film than the amorphous nickel sulfide-derived film.

Introduction

Hydrogen production by electrolysis of water is regarded as one of the most promising technologies, in which electricity generated from renewable energy sources can be utilized [1]. Water electrolysis consists of two half-reactions, the cathodic hydrogen evolution reaction and the anodic oxygen evolution reaction (OER) [2,3]. The OER (4 OH⁻ → H₂O + O₂ + 4 e⁻) requires consecutive transfer of four electrons, and is kinetically sluggish. Therefore, active electrocatalysts are required to reduce the overpotential and to accelerate the OER [4,5]. Nickel-based phosphides and sulfides have recently been discovered as active pre-catalysts for OER, owing to their high conductivity and abundant electrochemically active sites [6,7]. Spontaneous conversion of the surface phosphides and sulfides to oxides and (oxy)hydroxides is widely observed under OER conditions [8], and compared to as-synthesized oxides or (oxy)hydroxides, this conversion generates oxides and (oxy)hydroxides with higher amount of electrochemically active sites. Also, the existing steric and electronic interactions between the surface oxide or (oxy)hydroxide layer with the core phosphide or sulfide can enhance the OER activity. These advantages lead to significant efforts on optimizing the OER activity of the phosphide and sulfide-based materials.

Incorporation of heteroatoms to the nickel phosphides and sulfides is an effective approach to dramatically enhance the OER activity [6,7,9]. Metallic (e.g. Fe [10], Co [11], Mo [12]) and non-metallic elements (e.g. O [13], N [14]) incorporated nickel phosphides and sulfides have been synthesized and exhibit good OER activity. For example, Yuan et al. synthesized Fe–Ni₃S₂ on FeNi alloy foil, which required 282 mV overpotential to reach 10 mA cm⁻² in 1 M KOH [15]. Wang et al. obtained Fe_0.5Co–P catalyst by phosphidizing MOF precursors, which can drive 10 mA cm⁻² at the overpotential of 260 mV in 1 M KOH [16]. Marquez-Montes et al. reported electrodeposited Ni–S–P–O films on Ni foam, with the overpotential of 259 mV at 10 mA cm⁻² in 1 M KOH [13]. The binding energies of the OER intermediates are modulated through the electron interaction between the incorporated heteroatoms with Ni, therefore the OER kinetics is accelerated. Also, the heteroatoms may act as the active site or accelerate the kinetics through bifunction mechanism [17,18]. Besides, morphological control on the nickel phosphides and sulfides is an effective route in enhancing the number of electrochemically active sites and in facilitating mass transport [19].

Controlled synthesis of nickel-based phosphides and sulfides under mild conditions is highly desirable. Electrodeposition offers a facile route under ambient temperature in synthesizing nickel-based phosphides and sulfides, in comparison to the calcination [2,20], solvothermal [21], or physical/chemical vapor deposition methods [22]. Also, electrodeposited films feature strong interaction to the substrate, and the “blocking effect” caused by the polymer binder (to bind the nanoparticle electrocatalysts to the conductive substrate) is avoided [23]. In addition, the morphology, composition, and thickness of the deposited film can easily be tuned by changing the electrolyte and the electrodeposition method. Nickel-based phosphides and sulfides have been electrodeposited on TiO₂ nanotubes [24], Ni foam [13,25,26], Cu foil [23,27], Ti sheets [28] and carbon based substrates [29], and the resulting electrodes are evaluated towards hydrogen evolution reaction and/or OER. The adopted electrodeposition methods in most cases are potentiostatic or galvanostatic. During the electrodeposition, the nucleation and growth rates strongly depend on the applied potential. Therefore, unlike potentiostatic or galvanostatic deposition, the potentiodynamic deposition, e. g. cyclic voltammetry in a wide potential range, offers the possibility to prepare sulfide or phosphide film with unique structures, as the rate of nucleation and growth is changed significantly during the deposition. Also, a comparative study on the nickel-based phosphides and sulfides that are synthesized using similar electrodeposition method could aid the understanding of the structure-activity relationship. Therefore, we electrodeposited nickel-iron phosphide and sulfide films on Ni foam from aqueous solutions using cyclic voltammetry at room temperature. These films can form active and stable electrocatalysts during OER in alkaline solutions.