Elsevier

Electrochimica Acta

Volume 265, 1 March 2018, Pages 336-347
Electrochimica Acta

MOF-derived nickel and cobalt metal nanoparticles in a N-doped coral shaped carbon matrix of coconut leaf sheath origin for high performance supercapacitors and OER catalysis

https://doi.org/10.1016/j.electacta.2018.01.210Get rights and content

Abstract

Coconut leaf sheath-derived nitrogen doped carbon framework is developed and incorporated with nickel and cobalt metal nanoparticles in the carbon matrix by a facile process of growing ZIF-67 metal organic framework particles on the graphitised carbon, followed by annealing it in inert atmosphere. Various parameters are modified to obtain three different samples. These samples are tested for high performance supercapacitors and oxygen evolution reaction (OER) catalysts. The optimised sample NiCosingle bondC-1 gave a high specific capacity of 308 mAh g−1 at a current density of 1 A g−1 in a 2 M KOH electrolyte. An asymmetric supercapacitor assembly prepared from NiCosingle bondC-1 as the positive electrode and the nitrogen-doped carbon as the negative electrode, exhibited an energy density of up to 31.8 Wh Kg−1 for a high power density of 6.2 kW kg−1 over a potential window of 0–1.55 V. Two of our best samples were also tested for OER, giving good water oxidation kinetics, revealed by their lower Tafel slopes of around 107 mV and a low over potential (η) of around 420 mV at a current density of 10 mA cm−2. Hence, this work opens great avenues for biomass-derived materials for high performance supercapacitors and catalysis.

Introduction

Biomass energy is believed to be a major solution to our energy crisis, which is looming into the near future. The increasing population and the industrialisation that has flowed into all nooks and corners of our planet is causing an alarming depletion of our non-conventional sources of energy like petroleum, fossil fuels etc. The high demand for energy is also a major concern that needs to be addressed from more sustainable and non-polluting sources of energy like biomass, solar energy, wind energy etc. Interestingly, biomass is an abundant source of energy, primarily coming from the solar energy that reaches our planet earth in abundance [1]. Recent studies show that biomass is a very important topic of study for researchers in the energy sector all over the world. Carbonised cellulose-nanofibrils were used by Zolin et al. as a current collector and conducting substrate for a Li-ion cell structure, while cellulose-based materials are put to use for dye-sensitized solar cells (DSSC) [[2], [3], [4]]. Seaweeds were used to produce biopolymeric electrolytes for DSSC and in another similar study, hydrogels made of water and cellulose derivatives were used for a novel sustainable solar cell [5,6]. There are also new studies which make use of fuel cells for completely converting carbohydrates to electricity [7]. While finding sources of energy, it is equally important to find sources of energy storage systems as well, which can cater to the needs of our mankind in all wakes of life, ranging from usage for day to day activities to use in desolate places, for the population is ever increasing, that even the most desolate places are occupied nowadays for space. Batteries and supercapacitors are major energy storage devices, which are available in the market today [[8], [9], [10]]. While the batteries have been available as major players with large energy densities, supercapacitors are believed to be the saviours of our future with their high energy and power densities, that will enable us to meet the growing energy requirements, in time as short as minutes and seconds [11]. While many materials have been researched upon their capacities, durability, cost-effectiveness and shelf life, there is an increasing urgency for efficient energy storage systems, that makes this area one of mounting importance [1,9]. The major materials could be mostly divided as those which act as electric double layer capacitors (EDLC) and the pseudocapacitors based on the difference in the principles of energy storage [12,13]. While carbon based materials mostly constitute the EDLCs, transition metal oxides and conducting polymers are the commonly used materials in pseudocapacitors [14,15]. Research has showed that a good blend of both EDLCs and pseudocapacitors is much more beneficial than using the materials individually, as there is a synergy of their performances and nullifies many of their disadvantages when used alone [[16], [17], [18]]. Our study has succeeded in incorporating nickel (Ni) and cobalt (Co) metal nanoparticles in a nitrogen-doped carbon matrix which is obtained from coconut leaf sheath, an abundant source of biomass in Southeast Asia. Ni and Co oxides/hydroxides have been an intriguing topic of research and have been proved to be very promising candidates for use in supercapacitors, by virtue of their high charge storage properties and battery type behaviour [[19], [20], [21], [22]]. However, Ni and Co nanoparticles have been hardly studied for use in supercapacitors. One major reason is their ready oxidation in air and also under high pH conditions and their tendency for agglomeration owing to the large surface energies of metallic nanoparticles [23]. The Ni and Co nanoparticles, when introduced in the aqueous alkaline electrolyte, undergo surface oxidation to give Ni and Co oxides and their subsequent redox reactions lead to the high charge storage capacity [24,25]. Recent studies by Wu et al. with nickel nanoparticles in a graphitic framework grown from a Ni organic framework yielded a specific capacitance of 886 F g−1at 1 A g−1 in a 1 M KOH electrolyte [25]. Liu et al. also made a negative electrode using nickel doped activated carbon microspheres, giving a specific capacitance of 361 F g−1in 6 M KOH in a potential window of 0 to −1 V [26]. Biomass-derived N doped carbon was recently loaded with nickel-cobalt double layered hydroxides which yielded a specific capacitance of 1949.5 F g−1 at 1 A g−1 in a 6 M KOH solution [27]. ZIFs or Zeolite Imidazole Frameworks are a class of metal organic frameworks (MOF) which are an upcoming class of materials and have properties which are valuable for a systematic approach for the synthesis of their corresponding metal oxides [[28], [29], [30]]. ZIF-67 is a linked network of 2-methyl imidazolate anions and cobalt cations with a sodalite zeolite-type topology [28,31]. Our study provides a facile way to surround the ZIF 67-derived Ni and Co nanoparticles in a nitrogen doped carbon matrix, of coconut leaf sheath origin, so that it forms a protective matrix and prevents the ready oxidation of the particles [23]. Our best sample, NiCosingle bondC-1 yielded a high specific capacity of 308 mAh g−1 at a current density of 1 A g−1 in a 2 M KOH electrolyte for a voltage window of 0–0.45 V. An asymmetric supercapacitor, which was further assembled from NiCosingle bondC-1 (positive electrode) and the nitrogen doped carbon from the coconut leaf sheath (negative electrode) gave a very impressive performance with a high energy density of 31.8 Wh Kg−1 for a high power density of 6.2 kW kg−1 over a potential window of 0–1.55 V. Oxygen evolution reactions (OER) are very crucial in solar cells, water splitting etc and use expensive catalysts like ruthenium, iridium etc [32,33]. Many catalysts have been synthesised and tested for OER catalysis, using metallic oxides of iron, nickel, cobalt, copper etc and also carbon based materials like graphene and carbon nanotubes (CNT) [[34], [35], [36]]. Two of our best samples, NiCosingle bondC-1 and NiCosingle bondC-3, were also tested as catalysts for OER to show the multiple applications they could be used for and performed well, describing their very good water oxidation kinetics and thus highlighting their use for OER. These studies are validated by a low overpotential [η] of around 420 mV at a current density of 10 mA cm−2 and a low Tafel slope of 107 mV. The study highlights the use of Ni and Co nanoparticles in a biomass derived carbon framework for multiple applications. Our work will come a long way in minimising and efficiently handling bio-waste for useful energy applications.

Section snippets

Chemicals

The purchased chemicals were all analytical grade and were used without further purifications/treatments.

Synthesis of nitrogen doped carbon from coconut leaf sheath

The coconut leaf sheath (CLS) was well dried and the inner part of the fiber was cut into small pieces. The pieces were dipped overnight in a 0.5 M urea solution (de-ionized (D.I) water as the solvent) with magnetic stirring and then annealed in a tube furnace, first in a N2 atmosphere for 3 h at 700 °C and then activated in a CO2 atmosphere for an hour at 800 °C, both at a heating rate of

XRD measurements

XRD measurements revealed the presence of nickel and cobalt metal nanoparticles as shown in Fig. 1a. Though the cobalt and metal nanoparticle peaks are overlapping, the very sharp and well-formed peaks reveal the presence of highly crystalline pure nickel and cobalt nanoparticles, also reiterated by XPS analysis. The peaks at around 44.5°, 51.6°, and 76.2° are the major peaks corresponding to metallic fcc nickel which coincide with the planes [1 1 1], [2 0 0] and [1 6 1], respectively (JCPDS

Conclusion

We successfully synthesised a nitrogen doped coral shaped carbon framework with nickel and cobalt nanoparticles incorporated in it. Detailed optimisation and characterisation of the samples were performed and its efficient use in supercapacitors and as an OER catalyst was demonstrated. Our samples were prepared under three different synthesis procedures, mostly differing on their carbon framework and the different temperatures they were treated in. The optimised sample NiCosingle bondC-1 showed an

Acknowledgements

This work is supported by the Academic Research Fund (RG17/16) provided by the Ministry of Education in Singapore. We also thank Dr P.R Ashalatha and Mrs Praseetha, India, for providing us with the coconut leaf sheath raw materials.

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