A Co/metal–organic-framework bifunctional electrocatalyst: The effect of the surface cobalt oxidation state on oxygen evolution/reduction reactions in an alkaline electrolyte
Introduction
The large-scale electric energy demand for electric vehicles and portable electronics is driving the development of batteries with high energy densities, such as lithium-ion batteries, lithium–sulfur batteries and metal–air batteries. Among these batteries, the lithium–air battery attracts an increasing attention because of its extremely high theoretical energy density of ∼11,140 W h/kg, comparable to that of gasoline (∼11,860 W h/kg) [1]. As the core electrode reactions in a rechargeable lithium–air battery, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the cathode dominate the performance of the battery. However, the ORR and OER are kinetically sluggish, resulting in the large overpotential with respect to the standard potential and thus lower the round-trip efficiency and life cycle of the battery [2]. Therefore, the electrocatalysts, notably those bifunctional electrocatalysts that unify the catalytic activities of both OER and ORR, are urgently needed for rechargeable lithium–air batteries, because they can promote ORR and OER in the cathode during the discharging and charging processes, respectively. These electrocatalysts thus decrease the over-potential and improve the battery performance.
Up to date, several electrocatalysts containing precious metals such as mesoporous Pt nanorods [3], Pt nanoparticles @ carbon [4], Ru–Ir nano-alloy [5], trimetallic Pt–Pd–Ru nanodendrites [6], IrO2/Nb0.05Ti0.95O2 [7], PtAu nanoparticles [8], Ir@Pt nanodendrites [9], and Ag nano-powders [10], have been developed. These electrocatalysts exhibit high ORR and/or OER activities. However, the high cost and scarcity of the precious metals limit the widespread commercial applications in lithium–air batteries. Hence, a bifunctional electrocatalysts with a low cost and easy availability must be developed. Recently, non-precious-metal electrocatalysts, such as earth-abundant metal oxides [11], [12], [13], [14], [15], [16], [17], [18], carbon-based nano-materials [19], [20], [21], [22], [23], [24], [25], and nanocomposite electrocatalysts [21], [26], [27], [28], [29], [30], [31], [32], have been intensively investigated because of their low cost and electrocatalytic potential in OER and/or ORR. Among these electrocatalysts, Co-based electrocatalysts such as Co3O4, CoO and MnCo2O4 have attracted substantial interest because they have shown excellent bifunctional catalytic activities in both OER and ORR [31], [33], [34], [35]. Nevertheless, developing that Co-based bifunctional electrocatalysts that satisfies the requirements in practical applications for lithium–air batteries remains challenging. Additional effort is required to rationally design and fabricate the Co-based electrocatalysts with highly efficient bifunctional activities. More recently, several reports have revealed that the OER or ORR activities of the Co-based electrocatalysts are related with the oxidation states of cobalt (CoII, CoIII and CoIV) at the surfaces of the electrocatalysts. For instance, the work of Xiao et al. showed that the surface CoII are the active sites for ORR, resulting in the high ORR activity for Co3O4 nano-octahedrons anchored on graphene sheets and wrapped by {111} surfaces with a high surface CoII density [36]. The results derived from Zhu et al. demonstrated that the ORR activities depended on the CoII in C–CoxFe3−xO4 nanoparticles [37]. Tan et al. revealed that the Co species with a high oxidation state on the surface of cobaltite spinels promoted them to catalyze the OER processes efficiently [38]. Rosen et al. showed that the Co4O4 cubane in the spinel-structured Co3O4 (CoIII at octahedral sites) was the active site for OER [39]. In addition, the abundant surface CoIV active sites of the ultrathin CoSe2 nanobelts were responsible for the excellent OER activities of the Mn3O4/CoSe2 nanocomposites [30]. These reported results assist the design and preparation the bifunctional electrocatalysts more facilely. However, to the best of our knowledge, the detailed effects of the oxidation states of cobalt on the bifunctional activities of both OER and ORR have not been fully explored.
In our previous work, we developed an α-MnO2/MIL-101(Cr) composite electrocatalyst. Compared to the pristine α-MnO2, the composite electrocatalyst exhibited superior bifunctional activities for both OER and ORR. These improved activities were attributed to the enhancement of MIL-101(Cr) with a high stability, high porosity and open framework, indicating that MIL-101(Cr) can be used as an excellent electrocatalyst support [40]. In addition, the large pore size of MIL-101(Cr) affords the space for accommodating guest species such as Keggin anions [41] and metal nanoparticles [42]. In this study, we employ MIL-101(Cr) as the support to design and synthesize a Co-based bifunctional electrocatalyst as illustrated in Scheme 1. We tuned the surface CoIII and CoII contents using KBH4 and H2O2 as the reducing agent and oxidant, respectively, during the impregnation processes. Meanwhile, we applied the as-prepared electrocatalysts as a specific case to investigate the effects of the surface CoIII and CoII on the activities of the OER and ORR and the overall bifunctional activities. This protocol will be helpful for preparing efficient bifunctional electrocatalysts based on MOFs via facilely incorporation of highly active species by post-synthetic treatments. In addition, it will also extend the applications of MOFs-based materials in electrocatalysis and energy storage and conversion.
Section snippets
Electrocatalyst preparations
The MIL-101(Cr) employed in this work was synthesized using a method described elsewhere with modifications [41]. The detailed synthesis procedure is shown in the Supplementary Data. In a typical procedure for preparation of the electrocatalysts, 0.25 g of the as-synthesized MIL-101(Cr) was dispersed into 50 mL anhydrous ethanol and vigorously stirred for 30 min. In total, 0.12 g Co(NO3)2·6H2O (Shanghai Lingfeng Chemical Reagent CO. LTD, 99%) were then dissolved in the dispersion when stirring
Structural characterizations
MIL-101(Cr) is considered to be a stable MOF due to its novel structure and strong interaction between the hard ions (i.e., Cr3+) and the carboxylate liners [43], [44]. Hence, it was used as the catalyst support in the previous work [40], as well as in this work. To investigate the stability in the used electrolyte, the as-synthesized MIL-101(Cr) was soaked in 0.1 M KOH for 72 h. The XRD patterns of the MIL-101 (Cr) before and after soaking in 0.1 M KOH (Fig. S1) exhibit that the crystallinity
Conclusions
We have reported a bifunctional electrocatalyst based on MIL-101(Cr) and cobalt that was synthesized via an oxidation or reduction treatment during an impregnation procedure. The as-prepared electrocatalysts maintain abundant micropores and preserve the primary morphology of the pre-synthesized MIL-101(Cr), on which the Co species with the various CoIII and CoII contents are well-dispersed at the surfaces of the electrocatalysts. The investigation of the relationship between the
Acknowledgments
This work is supported by the National Natural Science Foundation of China (Project No. 21276018) and the Natural Science Foundation of Jiangsu Province of China (Project No. BK2012596).
References (64)
- et al.
Lithium-oxygen batteries-limiting factors that affect performance
J Power Sources
(2011) - et al.
A novel Rh-Ir electrocatalyst for the oxygen reduction reaction and the hydrogen and methanol oxidation reactions
Int J Hydrog Energy
(2014) - et al.
IrO2/Nb-TiO2 electrocatalyst for oxygen evolution reaction in acidic medium
Int J Hydrog Energy
(2014) - et al.
One-pot synthesis of Ir@Pt nanodendrites as highly active bifunctional electrocatalysts for oxygen reduction and oxygen evolution in acidic medium
Electrochem Commun
(2012) - et al.
Bifunctional electrocatalyst for oxygen/air electrodes
Energy Convers Manage
(2014) - et al.
Development of shape-engineered α-MnO2 materials as bi-functional catalysts for oxygen evolution reaction and oxygen reduction reaction in alkaline medium
Int J Hydrog Energy
(2014) - et al.
Electrochemical study of Ba0.5Sr0.5Co0.8Fe0.2O3 perovskite as bifunctional catalyst in alkaline media
Int J Hydrog Energy
(2013) - et al.
Novel FexCr2−x(MoO4)3 electrocatalysts for oxygen evolution reaction
Int J Hydrog Energy
(2012) - et al.
Large-area manganese oxide nanorod arrays as efficient electrocatalysts for oxygen evolution reaction
Int J Hydrog Energy
(2012) - et al.
Facile synthesis of graphene/N-doped carbon nanowire composites as an effective electrocatalysts for the oxygen reduction reaction
Int J Hydrog Energy
(2015)
Electrocatalytic performance of Ni modified MnOx/C composites toward oxygen reduction reaction and their application in Zn-air battery
Int J Hydrog Energy
Carbon supported MnOx-Co3O4 as cathode catalyst for oxygen reduction reaction in alkaline media
Int J Hydrog Energy
Enhanced electrocatalytic activity of oxygen reduction by cobalt-porphyrin functionalized with graphene oxide in an alkaline solution
Int J Hydrog Energy
Pyrolyzed cobalt porphyrin-modified carbon nanomaterial as an active catalyst for electrocatalytic water oxidation
Int J Hydrog Energy
Insight the effect of surface Co cations on the electrocatalytic oxygen evolution properties of cobaltite spinels
Electrochim Acta
Hydrothermal synthesis of α-MnO2/MIL-101(Cr) composite and its bifunctional electrocatalytic activity for oxygen reduction/evolution reactions
Catal Commun
Highly stable Ti-Co-Phen/C electrocatalysts as the cathode for proton exchange membrane fuel cells
Int J Hydrog Energy
Layer-pillared zinc (II) metal-organic framework built from 4,4′-azo(bis) pyridine and 1,4-BDC
Micropor Mesopor Mater
Synthesis and adsorption performance of MIL-101(Cr)/graphite oxide composites with high capacities of n-hexane
Chem Eng J
Preparation and characterization of Co-Ru/TiO2/MWCNTs electrocatalysts in PEM hydrogen electrolyzer
Int J Hydrog Energy
Preparation, characterization and bifunctional catalytic properties of MOF(Fe/Co) electrocatalyst for oxygen reduction/evolution reactions in alkaline electrolyte
Int J Hydrog Energy
On the efficacy of electrocatalysis in nonaqueous Li-O2 batteries
J Am Chem Soc
Electrochemical synthesis of one-dimensional mesoporous Pt nanorods using the assembly of surfactant micelles in confined space
Angew Chem Int Ed
Preparation of a platinum electrocatalyst by coaxial pulse arc plasma deposition
Sci Technol Adv Mater
One-step synthesis of trimetallic Pt-Pd-Ru nanodendrites as highly active electrocataltsts
RSC Adv
Platinum-gold nanoparticles: a highly active bifunctional electrocatalysts for rechargeable lithium-air batteries
J Am Chem Soc
Design principles for oxygen-reduction activity on perovskite oxide electrocatalysts for fuel cells and metal-air batteries
Nat Chem
Benchmarking heterogeneous electrocatalysts for oxygen evolution reaction
J Am Chem Soc
Role of the morphology and surface planes on the catalytic activity of spinel LiMn1.5Ni0.5O4 for oxygen evolution reaction
ACS Catal
Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction
Chem Soc Rev
Nanostructured nonprecious metal electrocatalysts for oxygen reduction reaction
Acc Chem Res
Active and stable carbon nanotubes/nanoparticle composite electrocatalyst for oxygen reduction
Nat Commun
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