Ni²⁺/Co²⁺ doped Au-Fe₇S₈ nanoplatelets with exceptionally high oxygen evolution reaction activity

Highlights

• The ultrafast and single step ion diffusion method is developed to produce Ni and Co heavily doped 2D Au-Fe₇S₈ nanoplatelets.

• Ni²⁺ and Co²⁺ co-doped Fe₇S₈ NPLs show exceptionally high OER activities.

• Hexagonal morphology and location of Au seed was retained in the pristine Au-Fe₇S₈ NPLs before and after OER testing.

Abstract

To overcome the limited potency of energy devices such as alkaline water electrolyzers, the construction of active materials with dramatically enhanced oxygen evolution reaction (OER) performance is of great importance. Herein we developed an ion diffusion-induced doping strategy that is capable of producing Ni²⁺/Co²⁺ doped two-dimensional (2D) Au-Fe₇S₈ nanoplatelets (NPLs) with exceptionally high OER activity outperforming the benchmark RuO₂ catalyst. The co-existence of Co and Ni in Au-Fe₇S₈ NPLs led to the lowest OER overpotential of 243 mV at 10 mA cm^-2 and fast kinetics with a Tafel slope of 43 mV dec^-1. Density functional theory (DFT) calculations demonstrated that Ni²⁺/Co²⁺ doping improves the binding of OOH species on the {001} surfaces of Au-Fe₇S₈ NPLs and lowers the Gibbs free energy of the OER process, which are beneficial to outstanding OER activity of the nanoplatelets.

Introduction

Electrocatalytic water splitting is an efficient way to convert the renewable electrical energy into chemical fuel in the form of hydrogen and oxygen by means of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [1], [2], [3]. This process, which embodies two half reactions occurring at the cathode and anode respectively, holds special significance due to its ability to generate renewable energy for energy conversion and storage devices. The OER process, as compared to the HER, is restrained to a much greater extent by the slow kinetics, leading to a thermodynamic “up-hill” [4], [5], [6], [7]. In this regard, the access to stable and efficient electrocatalysts which can boost the kinetics and reduce the overpotential is of utmost importance. In light of the aforementioned issues, the formation, understanding and exploration of unique catalysts with well-defined structure and supreme performance in OER have become a hot pursuit [8], [9], [10], [11], [12].

As emerging nanostructures with superior conductivity, transition metal chalcogenides have been in the spot-light from last decade for energy-related reactions due to the relatively low activation energy for the transfer of electron between cations [13], [14], [15], [16], [17], [18], [19]. Besides this, the presence of cations with multiple valencies imparts transition metal sulfides with desirable electrochemical features towards OER [20], [21]. This has been mainly ascribed to the ease in their compositional tuning for regulating the electronic and structural properties by means of doping and substitution. The elements of nickle and cobalt are just next to iron and they have similar radii while locating in the same row of the periodic table of elements. Doping Ni²⁺ and/or Co²⁺ could not only increase the composition diversity of iron compounds, but also improve their catalytic activities through modifying the electronic structures of the host materials. A number of recent reports have suggested the significantly enhanced OER performance by employing binary and ternary nickel-iron-cobalt (Ni-Fe-Co) based electrocatalysts. For instance, loading of Fe₃O₄ into the Co₉S₈ NPs to produce Fe₃O₄@Co₉S₈/rGO-2 [22] heterostructure leads to the increase in OER performance with an overpotential of 340 mV. This was credited to the ease in breaking the Co-O bond by virtue of electron transfer from Fe to Co₉S₈. Similarly, Zhan et al [23]. recently reported nanoarrays of sea-urchin like Fe doped FeNiCoP structure with an overpotential of 259 mV for OER, ascribed to synergistic effect and fast electron kinetics. Other examples include NiFeS₂ [24], CoFeSP/CNT [25], P-(Ni,Fe)₃S₂/NF [26] nanostructures and so on. In spite of their improved performance, some of these reported structures lag behind in terms of well-defined morphology while others in novel integration of Fe, Co and Ni chalcogenides in one system. Some additional challenges involve complex synthesis approach, big dimensions and obscurity around underpinning the exact mechanism for improved OER performance in Fe, Co and Ni hybrid catalyst [27].

Multiple methods are available for the synthesis of hybridized structures of Fe, Co and Ni sulfides with an impressive variety in shapes and sizes on the grounds of their adjacent position in the periodic table [10], [27], [28], [29], [30], [31]. Among other doping methods, post-synthetic ion diffusion stands out the most [32]. Chen et al [33]. have reported this mechanism as a two-step process which begins with surface adsorption followed by lattice incorporation. By using pre-formed host nanocrystals (NCs) all of the follow-up change in its properties can be entirely assigned to the dopant atoms which would be otherwise very complicated, if introduced during synthesis, to differentiate between the influence of dopant and its precursor on the host NCs [34]. It is noteworthy that the synergistic interaction of dopant atoms with the host material can switch p-type carriers of Fe into n-type by slightly altered electronic properties of Co [7], [33], [34], [35], [36], [37] and introduce sub-energy bands which in effect reduces the bandgap and henceforth overpotential [8], [32], [38], [39], [40]. In addition to the aforementioned reason, morphology and dimension of the final structure is also considered as one of the major factors in lowering the onset potentials for electrocatalytic water oxidation because of its direct link with stability, large surface area and availability of enough space for the diffusion of electroactive species [29], [37], [38], [39], [40].

However, as mentioned above, the intrinsic and extrinsic catalytic mechanisms in regards to the involved active sites are still under debate and are not well understood [8], [9], [25], [26], [28], [29], [30]. For instance, several studies claim that in Fe-Ni based catalysts, Fe^III acts as a Lewis acid and facilitates the formation of Ni^IV and Co^III which in turn act as the active sites [37], [41]. Other studies provided different perspectives that Ni/Co merely acts as conductive support to Fe active sites during OER or Fe and Ni/Co works collectively to boost the overall catalytic performance which essentially means that Fe^IV is as equally contributing as Ni^IV towards the OER process [37], [41], [42], [43]. Further studies to elucidate how the addition of Ni/Co can enhance the performance of the catalysts and to reveal the underpinned mechanisms are highly desirable.

Semiconductor nanoplatelets are a type of intriguing material that have attracted tremendous attention. They have giant oscillation strength and the charge carriers of nanoplatelets are generally confined along their thickness direction [44], thus providing an ideal model to couple the experimental and theoretical investigations on structure–performance relationship of nanomaterials. Nanoplatelets with such a unique shape offer more access to the active sites anchored on their surfaces, therefore maximizing the number of catalytic sites and areal density. Intrigued by these rationales, herein, we have selected two-dimensional (2D) Au-Fe₇S₈ nanoplatelets (NPLs) [45] as a host material which has valence states of both Fe^II and Fe^III for the synthesis of bi and trimetallic systems i.e. Ni²⁺or Co²⁺ monodoped and Ni²⁺/Co²⁺ co-doped systems and used Ni-doped, Co-doped, and Ni/Co-codoped Au-Fe₇S₈ nanoplatelets as the electrocatalysts for OER (Scheme 1). This particular form of iron sulfide, Fe₇S₈, offers an advantage of providing mixed valence states and crystal defects which are believed to originate from sulfur vacancies [8]. A facile approach of ion diffusion was developed to prepare Co and Ni doped Au-Fe₇S₈ NPLs while still retaining their 2D hexagonal morphology (Scheme 1). To investigate the effect of multimetals towards electrocatalysis, Co and Ni dopants were introduced both individually and collectively to develop three different heterostructures, i.e., Au-Co/Fe₇S₈, Au-Ni/Fe₇S₈ and Au-NiCoFe₇S₈. The as-prepared NPLs exhibited drastically improved and remarkable OER activity that outperformed the benchmark RuO₂ catalyst. Such substantial improvement in electrocatalytic performance is attributed to the modified electronic structure as demonstrated by density functional theory (DFT) calculations where Co and Ni doping induces lowering of the Gibbs-free energy at an active site of Fe (001). The interaction between Fe, Co and Ni constituent metals, i.e., the synergy between active centres and ease in ion diffusion leading to gas release, is an additional contributing factor.