Cobalt tetraphosphate as an efficient bifunctional electrocatalyst for hybrid sodium-air batteries

doi:10.1016/j.nanoen.2021.106485

Nano Energy

Volume 89, Part B, November 2021, 106485

https://doi.org/10.1016/j.nanoen.2021.106485 Get rights and content

Highlights

•
First report of productive bi-functional activity (Oxygen evolution and oxygen reduction) on this Co-based material.
•
Utilization of Ascorbic Acid as fuel resulted in an in-situ carbon coating, which is crucial for electronic conductivity.
•
K₂Co(PO₃)₄ was tested for hybrid Na-air battery, which produces a stable charge-discharge profile with satisfactory round trip efficiency.
•
DFT calculations revealed the half-metallic nature of Co, which is beneficial for the catalytic activity.

Abstract

Economic and efficient bifunctional electrocatalysts are pivotal in realization of rechargeable (hybrid) metal-air batteries. It is ideal to employ noble-metal free bifunctional electrocatalysts that are not selective towards oxygen evolution and reduction (OER and ORR) activities. This work unveils cobalt-based tetraphosphate K₂Co(PO₃)₄ as an economic bifunctional electrocatalyst acting as cathode for rechargeable hybrid sodium-air batteries. Autocombustion route led to the development of homogeneous, carbon-coated, spherical K₂Co(PO₃)₄ nanoparticles enabling active site exposure to incoming guest molecules (O₂, OH^-). This monoclinic compound exhibited superior oxygen evolution activity with low overpotential (ca. 0.32 V) surpassing the commercial RuO₂ catalyst. Tetraphosphate K₂Co(PO₃)₄ was successfully implemented in hybrid Na-air batteries delivering reversible cycling with roundtrip efficiency over 70%. DFT study revealed this catalytic activity stem from the most active and stable surface (001) and half-metallic nature of Co in K₂Co(PO₃)₄. Cobalt tetraphosphates can be harnessed to design low cost electrocatalysts for hybrid sodium-air batteries.

Graphical Abstract

Introduction

Energy has emerged as the paramount issue in the modern world with net energy consumption poised to cross over 40 Terawatt by 2050. While a meagre 2% energy arises from renewable sources, fossil fuels cater the terawatt-scale energy generation inevitably leading to CO₂ emission and detrimental environmental hazards causing geopolitical instability [1], [2]. It has triggered research effort on sustainable energy production, storage and (mobile) delivery with minimal pollution. For efficient storage and delivery, lithium-ion batteries (LIBs) have emerged as key players with ideal mix of energy/power density, cycle life and device economy. With the LIBs approaching their theoretical capacity, metal-air batteries (MABs) are widely being explored vying for superior performance. The merits of MABs are: (i) high theoretical energy density, (ii) environmentally benign by using oxygen as active cathode material, and (iii) hybrid Na-air battery system forming soluble discharge product that enhances the cycle life [3], [4]. Nonetheless, MABs suffer from issues such as formation of insoluble products leading to incomplete discharge process restricting their final performance [5], [6]. It can be circumvented by hybrid metal-air and seawater batteries with reversible performance [7]. (Hybrid) metal-air battery operation involves electrochemical reactions like oxygen evolution and oxygen reduction (OER/ORR) activity [8], [9], [10]. They are inherently sluggish due to the formation and cleavage of strong covalent (i.e. O double bond O and O-H) bonds. Thus, practical MABs need efficient bifunctional electrocatalysts to realize their full potential. While noble metal catalysts such as Pt/C, RuO₂ and IrO₂ exhibit excellent electrocatalytic activity with minimal overpotential, they are very expensive. It motivates the search for low-cost transition metal-based bifunctional electrocatalysts for hybrid metal-air batteries that are stable under oxidative environments [11], [12], [13].

In this quest, new avenues to design bifunctional electrocatalysts have been realized by exploiting various advanced electrocatalysts, including 3D transition metal (Ru, Rh, Ir) nanoparticles on a carbon support, Ni-MnO complex, spinel, Ni-Ni₃S₂, Co-P_x, Co-O_x, cobalt boride, transition metal nitrides, oxides, perovskites and polyanionic insertion materials (e.g. phosphates) [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. They offer superior catalytic activity along with stabilization of active centers by the neighboring groups such as (pyro)phosphate in the crystal structure. To achieve full potential, the binding energy of an incoming species with an active site of catalyst should be moderate. If the adsorbed species binds either very strongly or weakly, no reactions can occur. This principle is known as the ‘Sabatier Principle’. Various earth-abundant 3d transition metals (Mn, Fe, Co, Ni) and their corresponding metal oxides have shown reasonable bifunctional electrocatalytic activity. The different oxidation states of these transition elements help in the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) electrocatalytic activity during the water splitting process. Particularly, Co and Ni-based perovskites and oxides yield reasonable bifunctional activity in alkaline and neutral conditions [27], [28]. Electrocatalysts consisting of 3d transition metals and PO₄-based polyanionic groups can be harnessed for bifunctional activity [29], [30]. Polyanionic compounds offer structural stability due to strong covalent bonds and also can attain various geometries like octahedral, tetrahedral, trigonal planar and their distorted forms rendering versatile coordination to the electrocatalysts. This geometric diversity helps in stabilizing intermediate species containing the catalytic metal center during the redox process [30]. Some such Co-based catalysts are LiCoO₂, LiCoPO₄, NaCoPO_4, Li₂CoP₂O₇, Na₂CoP₂O₇, NaFe₂Co(PO₄)_3, NaCo(PO₃)₃ etc [27], [28], [30], [31], [32], [33], [34]. For example, Na₂CoP₂O₇ pyrophosphate with distorted tetrahedral symmetry exhibits high oxygen evolution activity in neutral conditions along with robust structural stability and metaphosphate NaCo(PO₃)₃ has activity similar to the RuO₂ benchmark catalyst. Phosphate units present in the framework help stabilize the active electrophilic Co site.

Inspired by the bifunctional activity of Co-based insertion materials, here we have explored cobalt tetraphosphate K₂Co(PO₃)₄ synergizing experimental and computational tools. To demonstrate a real-time application, the K₂Co(PO₃)₄ bifunctional electrocatalyst was used as an air cathode in hybrid sodium-air batteries showing good battery performance. The utilization of Co redox followed by stabilization of active center by the phosphate framework enhance the activity, thereby working as an air cathode. Prepared by scalable economic combustion synthesis route, this tetraphosphate assumes homogeneous nanoscale morphology. The hybrid battery assembled using K₂Co(PO₃)₄ exhibited both high-power and energy density along with reasonable roundtrip efficiency, performing better than conventional sodium-air battery. Constituting earth-abundant potassium and phosphorus, the K₂Co(PO₃)₄ tetraphosphate works as bifunctional electrocatalyst and hybrid sodium-air batteries, suitable for mid-to-large scale grid storage applications.

Section snippets

Synthesis of K₂Co(PO₃)₄ tetraphosphate

The end product K₂Co(PO₃)₄ was synthesized by solution combustion method using the precursors: 2 moles of potassium nitrate KNO₃, 1 mole of cobalt nitrate hexahydrate Co(NO₃)₂.6H₂O, 4 moles of ammonium dihydrogen orthophosphate (NH₄H₂PO₄) and 1 mole of either ascorbic acid or urea (fuel) (with purity of 99%). These precursors were dissolved in 15 mL of distilled water by steady stirring (for 30 min) to form a homogeneous solution. This reaction mixture was heated at 150 °C to dehydrate excess

Results and discussion

The target material K₂Co(PO₃)₄ was prepared by solution combustion method utilizing nitrate/phosphate-based precursors as ‘oxidants’ and ascorbic acid (or urea) as ‘fuel’. Involving a self-sustained combustion reaction, this reaction produces exothermic heat so as to rapidly increase the local temperature as high as 600–900 °C. It leads to quick reaction completion and formation of final product in short duration with restricted grain growth/ Ostwald ripening. The exothermicity of combustion

Conclusions

To summarize, nanostructured K₂Co(PO₃)₄ tetraphosphate was found to be a novel economic bifunctional electrocatalyst. Scalable auto combustion synthesis led to homogeneous carbon coated nanoparticles (~100 nm) with spherical morphology involving half-metallic nature of Co species, which enhances the overall catalytic activity. While this air stable K₂Co(PO₃)₄ phosphate has shown reasonable ORR activity, it exhibits excellent OER activity superior to that of benchmark RuO₂ catalysts with good

CRediT authorship contribution statement

Chinnasamy Murugesan: Conceptualization, Methodology, Experiments, Data curation, Writing − original draft preparation. Sathiya Priya Panjalingam: Data curation. Shubham Lochab: Methodology, Experiments, Data curation, Writing − original draft preparation. Rajeev Kumar Rai: Data curation, TEM experiments. XiaoFeng Zhao: Data curation, DFT calculations. Deobrat Singh: Data curation, DFT calculations and write-up. Rajeev Ahuja: Supervision of DFT calculations and write-up. Prabeer Barpanda:

Declaration of Competing Interest

There are no conflicts to declare.

Acknowledgments

We acknowledge the financial support from the Technology Mission Division (Department of Science and Technology, Government of India) under Materials for Energy Storage (MES-2018) program (DST/TMD/MES/2K18/207). D.S. and R.A. thanks Olle Engkvists stiftelse (198–0390), Carl Tryggers Stiftelse for Vetenskaplig Forskning (CTS: 18:4) and Swedish Research Council (VR-2016–06014 and VR-2020-04410) for financial support. SNIC and HPC2N are acknowledged for providing the computing facilities. We thank

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¹: These authors contributed equally to this work.

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Full paperCobalt tetraphosphate as an efficient bifunctional electrocatalyst for hybrid sodium-air batteries

Highlights

Abstract

Graphical Abstract

Introduction

Section snippets

Synthesis of K2Co(PO3)4 tetraphosphate

Results and discussion

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgments

Matter

Prog. Energy Combust. Sci.

J. Power Sources

Matter

Carbon

Nat. Commun.

J. Power Sources

Comput. Mater. Sci.

Carbon

Future global energy prosperity: the terawatt challenge

MRS Bull.

Powering the planet: chemical challenges in solar energy utilization

Proc. Natl. Acad. Sci. U. S. A.

Can hybrid Na-Air batteries outperform nonaqueous Na-O2 batteries?

Adv. Sci. (Weinh.)

Lithium−air battery: promise and challenges

J. Phys. Chem. Lett.

Sodium–oxygen batteries: a new class of metal–air batteries

J. Mater. Chem. A

Advanced perspective on the synchronized bifunctional activities of P2-type materials to implement an interconnected voltage profile for seawater batteries

J. Mater. Chem. A

A rechargeable room-temperature sodium superoxide (NaO2) battery

Nat. Mater.

A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions

Nat. Nanotechnol.

Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction

Chem. Soc. Rev.

Hypofractionated postmastectomy radiation therapy is safe and effective: first results from a prospective phase II trial

J. Clin. Oncol.

Electronic modulation of non‐van der Waals 2D electrocatalysts for efficient energy conversion

Adv. Mater.

Transition metal nitrides for electrochemical energy applications

Chem. Soc. Rev.

Metallenes as functional materials in electrocatalysis

Chem. Soc. Rev.

Low-cost rapid template-free synthesis of nanoscale zinc spinels for energy storage and electrocatalytic applications

ACS Appl. Energy Mater.

The design of zinc-substituted cobalt (pyro)phosphates as efficient bifunctional electrocatalysts for zinc-air batteries

Chem. Commun. (Camb.)

Highly efficient oxygen reduction reaction activity of N‐doped carbon–cobalt boride heterointerfaces

Adv. Energy Mater.

Modulation of single atomic Co and Fe sites on hollow carbon nanospheres as oxygen electrodes for rechargeable Zn–air batteries

Small Methods

Superior oxygen electrocatalysis on nickel indium thiospinels for rechargeable Zn–air batteries

ACS Mater. Lett.

Exploring indium-based ternary thiospinel as conceivable high-potential air-cathode for rechargeable Zn-air batteries

Adv. Energy Mater.

Boosting bifunctional oxygen electrocatalysis with 3D graphene aerogel-supported Ni/MnO particles

Adv. Mater.

Earth-abundant alkali iron phosphates (AFePO4) as efficient electrocatalysts for the oxygen reduction reaction in alkaline solution

ChemCatChem

Full paper
Cobalt tetraphosphate as an efficient bifunctional electrocatalyst for hybrid sodium-air batteries

Synthesis of K₂Co(PO₃)₄ tetraphosphate