One stone two birds: Vanadium doping as dual roles in self-reduced Pt clusters and accelerated water splitting

doi:10.1016/j.jechem.2021.08.061

Journal of Energy Chemistry

Volume 66, March 2022, Pages 493-501

https://doi.org/10.1016/j.jechem.2021.08.061 Get rights and content

Abstract

Integrating active Pt clusters into transition-metal oxides with water-dissociation ability is effective to prepare a bifunctional electrocatalyst for water splitting in alkaline. However, the additional utilization of a reductant and/or the operation at the elevating temperature causes the over-growth and agglomeration of Pt clusters, thus losing the high catalytic performance. Herein, we report that V dopant not only favors self-reducing Pt clusters on NiFe layered double hydroxide (LDH) (Pt/NiFeV) at room temperature, but also regulates interfacial charge redistribution to enhance the water-splitting performance. Experimental and theoretical studies reveal that V dopant into NiFe LDH triggers more electrons to transfer to adjacent Fe atoms, thus leading to a higher reducing ability compared to that without V-doping. When used as water-splitting electrocatalyst, V doping promotes electron loss of Pt clusters in Pt/NiFeV, optimizing the free energy of hydrogen adsorption and proton recombination kinetics at the cathode. Meanwhile, it also moves the d-band center of Ni away from the Fermi level to optimize the adsorption of *OH intermediates and facilitate the desorption of oxygen molecules at the anode. Thereby, Pt/NiFeV exhibits much higher bifunctional performance than V-free Pt/NiFe LDH toward both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). This work can spark inspiration of designing other bifunctional electrocatalysts for energy conversion and storage.

Graphical abstract

Vanadium dopant into NiFe LDH nanosheets can not only favor mild self-reduction of Pt ions into clusters, but also boost the water-splitting kinetics of the resulting Pt/NiFeV LDH nanocomposites via regulating the interfacial electron redistribution. This work can spark inspiration of designing other bifunctional electrocatalysts toward efficient energy conversion and storage.

Introduction

Electrocatalytic water splitting is a promising strategy to produce high-purity hydrogen gas, which is also an effective way to alleviate energy crisis and environment pollution [1], [2]. The reaction kinetics of hydrogen evolution reaction (HER) in alkaline medium is approximately two or three orders lower than that in acid medium, which is related to the supernumerary water dissociation step in alkaline [3]. Platinum (Pt)-based materials have been regarded as the state-of-the-art electrocatalysts for the HER, but inefficient water dissociation definitely triggers a significant increase of overpotential in alkaline medium. Recently, transition metal oxide-based electrocatalysts have been explored to break up HO–H bond, but their HER performance is still unsatisfactory due to the sluggish rate in the combination of adsorbed protons to H₂ [4]. Therefore, rationally coupling of highly active Pt clusters with more exposed surface atoms and transition-metal oxides with good water dissociation ability is regarded as a considerably potential way to solve these problems [5], [6]. The resultant nanocomposites could be used as bifunctional electrocatalysts to accelerate water splitting process in alkaline for both HER at the cathode and the oxygen evolution reaction (OER) at the anode [7], [8]. However, Pt clusters with high surface energy usually tend to aggregate or grow into large nanoparticles, thus resulting in the under-expected catalytic performance. So, it is highly desirable to search for a facile method to synthesize ultrafine Pt clusters without agglomeration on an oxide support.

Among many oxide-based substrates, layered double hydroxide (LDH) is a two-dimensional layered structure, which consists of the cations hydrotalcite laminate balanced with the exchangeable anions [9]. Among LDH compounds, NiFe LDH has attracted much attention due to the outstanding OER performance, but the unsatisfactory HER performance hinders its practical application as bifunctional electrocatalysts for both HER and OER [10]. It has been reported that Ni sites in LDH exhibited an extremely negative Gibbs free energy when absorbing hydrogen-containing intermediates [11], and thus the active sites were blocked. Furthermore, NiFe LDH has a wide bandgap between valence band and conduction band [9], exhibiting a poor electrical conductivity. Previous studies confirmed that the doping of the heterogeneous elements (e.g., Co, Mn and V) is feasible to regulate the electronic structure of NiFe LDH [9], [12], thus effectively improving the performance of NiFe LDH.

Vanadium (V) is regarded as a promising candidate for dopant due to its earth-abundant reserve and multiple valence effect on the electronic structure [13]. The five-valence configuration ensures a high energy of 3d electron orbital and makes it feasible to turn the properties of electro-acceptor and donator [14]. In addition, doping V into NiFe LDH can narrow the bandgap between valence band and conduction band, and enhance the intensity of density of states (DOS) near the Fermi level [9], which is extremely important to promote OER process. V doping into ultrathin porous NiFe LDH was also reported by Dinh et al, which displays a modest overpotential of 231 mV for OER and 125 mV for HER in 1 mol L⁻¹ KOH electrolyte, exhibiting an efficient bifunctional activity for overall water splitting [15]. Furthermore, V as a dopant may introduce crystalline dislocations and defects based on its variable valence and different atomic radius. And it can also increase the number of active sites and possibility of loading other metals. In recent years, many studies focused on the electronic structure effect of V doping on the LDH electrocatalyst performance [16]. However, it has yet been reported the effect of V doping into NiFe LDH on the post-loading of active metal clusters, and the influence on the catalytic activity of the resultant nanocomposite.

Herein, we report that V-doped NiFe LDH (NiFeV LDH) nanosheets can be used as a self-modulated reductant for synthesizing well-dispersed Pt clusters in aqueous solution at room temperature. Moreover, the V doping-induced coupling effect of Pt clusters and NiFeV LDH boosts the reaction kinetics in water splitting. Arising from the low electronegativity, V doping modifies the chemical environment of adjacent Fe atoms by transferring more charges from V to Fe. Thus, NiFeV LDH is endowed with a higher reducing ability than NiFe LDH without V doping, triggering the mild reduction of Pt ions at the Fe sites on the surface of NiFeV LDHs as illustrated in Fig. 1(a). The resultant Pt/NiFeV demonstrates excellent bifunctional performance for HER and OER toward alkaline water splitting, remarkably superior to the V-free Pt/NiFe LDH (Pt/NiFe). Toward water splitting, theoretical calculations reveal that V doping adjusts electron distribution at the interface of Pt clusters and NiFeV LDH and downshifts d-band center of Ni. Therefore, compared to Pt/NiFe, the adsorbed water molecule at the Ni site in Pt/NiFeV is thermodynamically easier to dissociate into H* and *OH intermediates for the subsequent HER and OER, while the adjacent Pt clusters with deficient electrons have optimal adsorption energy for the produced H* and favorably finish the combination of two protons to form H₂ molecule (Fig. 1b). Meanwhile, the Ni sites with lowered d-band center exhibit optimized adsorption of *OH intermediate and boost the combination with OH⁻ into an oxygen molecule at the anode. This self-modulated synthesis strategy opens a new avenue for the design of efficient bifunctional electrocatalysts in energy conversion and storage.

Section snippets

Materials

Nickel (II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O), iron (III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O) and potassium hydroxide (KOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. Vanadium (III) chloride (VCl₃) and chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O) were purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD.

Pretreatment of Ni foams

Ni foams (about 1 × 1 cm²) were carefully cleaned with dilute hydrochloric acid (100 mL 0.1 mol L⁻¹ HCl) in an ultrasound bath for 15 min to remove the NiO_x layer. And

Results and discussion

An interfacial-fitting structure induced by metal doping is an effective strategy to modulate the chemical environment for advanced catalysis [23]. DFT calculations were firstly performed to reveal the effect of V doping on local environment of NiFe LDH. The theoretical results show that the Fe–O bond length in NiFe LDH is significantly extended by 0.21 Å after V doping (Fig. 1c), whereas the increase of Ni–O bond length is only 0.16 Å (Fig. S2). This suggests that V doping exerts more

Conclusions

In summary, we reported the double-kill role of V doping in the synthesis and electronic modulation at the interface of Pt/NiFeV, which not only modifies the chemical environment to make self-reduction of Pt⁴⁺ cations come true, but also optimizes the electron interaction between the formed Pt clusters and NiFeV LDH. The outstanding performance of alkaline water splitting is attributed to an interfacial-fitting structure induced by V doping to accelerate the kinetics of HER and OER. DFT

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors are grateful to the financial support from the Science and Technology Commission of Shanghai Municipality (19ZR1479500) and the National Natural Science Foundation of China (52072389). J.W. thanks the Program of Shanghai Academic Research Leader (20XD1424300) for financial support.

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¹: These authors contributed equally to this work.

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One stone two birds: Vanadium doping as dual roles in self-reduced Pt clusters and accelerated water splitting

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Pretreatment of Ni foams

Results and discussion

Conclusions

Declaration of Competing Interest

Acknowledgments

Mater. Today Phys.

Nano Energy

Nano Energy

Chem. Eng. J.

Comput. Mater. Sci.

Energy Environ. Sci.

Adv. Mater.

Science

Angew. Chem. Int. Ed.

Nano Lett.

Nat. Commun.

Adv. Energy Mater.

Adv. Energy Mater.

Adv. Energy Mater.

Nat. Commun.

Small

Nat. Commun.

Nat. Commun.