Coral-like carbon-wrapped NiCo alloys derived by emulsion aggregation strategy for efficient oxygen evolution reaction

Abstract

The transition metal-based catalysts have great potential to boost the electrocatalytic reactions due to their flexible electronic configuration and low cost. This work developed a facile emulsion aggregation strategy to synthesize coral-like carbon-wrapped NiCo alloy (Co_0.5Ni_0.5/rGO) with high oxygen evolution reaction (OER) activity. The effect of alloy composition and GO content on the OER activity was evaluated in the 1 mol L⁻¹ KOH solution. The OER mechanism of the Co_0.5Ni_0.5/rGO catalyst was disclosed by X-ray photoelectron spectra (XPS) and synchrotron radiation X-ray absorption spectra (XAS). The emulsion containing amphipathic graphene oxide (GO) and hydrophobic nickel/cobalt complexes induces the formation of the carbon-wrapped nanostructure. The coral-like Co_0.5Ni_0.5/rGO catalyst exhibits the low overpotential of 288 mV at the current density of 10 mA cm⁻² and good durability, both of which are superior to the standard RuO₂. The synergistic effect between nickel and cobalt effectively regulates the electronic structure and OER activity of the alloy catalysts. Moreover, the interaction between NiCo alloys and carbon shells can reduce the interfacial resistance.

Introduction

The exhaustive exploitation of fossil fuels and the increasingly serious environmental pollution address the urgency to develop sustainable energy systems. Electrochemical water splitting and metal-air batteries are widely considered as promising strategies to convert and store sustainable energy due to their nature of clean, high energy density, and renewability [1], [2]. However, as compared with the relatively fast cathodic hydrogen evolution or Zn reduction reaction, the oxygen evolution reaction (OER) with a four-electron transfer process is kinetically sluggish in both water splitting and metal-air batteries [3]. The Ru or Ir based noble metals are regarded as the efficient catalysts to boost the OER process, while these materials are expensive and scarce. Therefore, it is highly desirable to design inexpensive and high-active substitutable catalysts.

During the past decades, a series of earth-abundant transition metal materials have attracted considerable attention.[4], [5]. Among them, Co- or Ni-based catalysts including the oxides, hydroxides, sulfides, selenides, and phosphides have been widely studied to facilitate the oxygen evolution reaction [6], [7]. Because nanoalloys can combine the advantages of the desired elements, previous work has indicated that the synergistic effect of different elements in alloy catalysts can modify the physicochemical properties toward a better catalysis activity [8]. For instance, Yu et al. reported that NiCo alloys prepared via a one-pot pyrolysis route show higher OER activities than the individual metal hybrids, which highlighting a synergy between the Ni and Co components [9]. A similar phenomenon can be observed in the OER process of NiIn₂S₄/CNFs, which outperforming those of monometallic Ni or In sulfides [10]. However, the agglomeration of nanoparticles partly restricts their activities in the electrochemical process. It has been demonstrated that the carbonaceous materials can afford a protective shell to inhibit the aggregation of nanoparticles and promote the stability of catalysts under the harsh alkaline condition [11]. Bai et al. found that the RGO-Ni_xCo_100–x nanocomposites prepared by a co-reduction process enhance the catalytic activity toward the reduction of 4-nitrophenol as compared with bare Ni_xCo_100–x alloy [12]. Therefore, the carbon-composited NiCo alloy catalysts potentially change the intrinsic catalytic activity of the hybrids owing to their interplay [13].

Various strategies have been developed to construct the alloy/carbon composites, such as chemical vapor deposition, solvothermal, pyrolysis, and so on. Especially, the hierarchical carbon hybrids are presumed because these nanostructures can improve the contact of catalysts with electrolytes and increase the electrochemically active surface area and conductivity [14]. For example, NiCo₂S₄ nanocrystals anchored on nitrogen-doped carbon nanotubes were prepared by nucleation-crystallization strategy and exhibited extraordinary activity for ORR/OER due to the synergy between NiCo₂S₄ and carbon nanotubes [15]. Hou et al. synthesized a unique 3D hybrid of N-doped NiCo alloys encapsulated carbon nanotubes through simultaneous pyrolysis of metal salts and carbonized potassium citrate, which exhibits an excellent OER activity even beyond the standard Ir/C catalyst [16]. The two-dimensional graphene oxide (GO) sheets are regarded as the outstanding carbon support because the diverse functional groups or defects can effectively anchor other active metal atoms. Geng et al. developed a hydrothermal approach to preparing NiFe nanoalloy coated with reduced graphene oxide nanosheets, which presenting a high OER activity and stability because GO sheets well disperse the alloy nanoparticles and expose more active sites [17]. Moreover, the lamellar GO can potentially tune the size, morphology, and the active sites of the alloy/carbon composites. Self-limiting growth behavior of two-dimensional palladium between graphene oxide layers was disclosed due to the strong interaction of Pd with the confining GO sheets [18], [19], and the exfoliated Pd nanosheets show high electrocatalysis efficiency. The GO-based three-dimensional structure can be further constructed by template-assisted, self-assembly or sol–gel methods to increase mass/ion transport channels and accessible surface area [20], [21].

Herein, an emulsion aggregation strategy has been developed to construct carbon-wrapped NiCo alloy catalysts by simultaneous reduction of nickel and cobalt complexes from the mixed solution, which is composed of amphipathic graphene oxide (GO) and hydrophobic complexes of Ni(II) or Co(II) with bis(2-ethylhexyl) phosphate (HDEHP). The coral-like carbon-wrapped NiCo alloys are obtained via the annealing treatment. The morphologies of the resulting materials are verified by SEM and TEM. The effects of alloy compositions and the added amount of GO on the OER activity of catalysts are compared. And the interaction within the alloy and with the carbon layer is disclosed by the XPS and XAFS analysis.