Facile synthesis of novel carbon dots@metal organic framework composite for remarkable and highly sustained oxygen evolution reaction

Highlights

• Ni-based MOF was successfully synthesized by solvothermal method.

• CDs@MOF composite presented lower onset potential and overpotential for OER as compared to other MOF based electrocatalysts.

• The composite showed outstanding stability for about 40 hrs and up to 1,000 CV cycles.

• These results are comparable with the state of the art electrocatalysts i.e. RuO₂ and IrO₂.

Abstract

Craving to search sources of energy is constantly compelling the mankind to find a source that is renewable or has enormous reservoirs and at the same time it poses no hazardous effect to mankind. Today’s energy deficient world has been seeking for an electrocatalyst which has a simple and facile synthesis strategy with marvellous catalytic performance and has outstandingly sustained stability for the highly lethargic Oxygen Evolution Reaction (OER). Here, a Ni-based mixed linker Metal Organic Framework (MOF) was synthesized using 1,3,5-benzenetricarboxylic acid (BTC) and 4,4`-bipyridine (bpy) as organic linkers. Carbon Dots (CDs) were incorporated into MOF during synthesis to impart synergistic effect to the as-synthesized composite. All the synthesized samples i.e CDs, MOF and CDs@MOF were characterized by various techniques to confirm the structural, compositional, morphological and textural analysis. The synthesized composite possessed rods like morphology with uniform distribution of CDs which offered superb stability for almost 40 hrs, large electrochemical active surface area (ECSA), least charge transfer resistance (R_ct), high turnover frequency (TOF), reasonably low onset potential, low overpotential and a comparably low Tafel slope. The electrocatalyst owed its great catalytic efficiency and substantial stability due to the synergistic effect between CDs and MOF. This work is an effort to exploit the fabulous potential of MOF based nano-composite towards water splitting.

Introduction

Humanity on earth has been facing three huge challenges for the last few decades i.e. energy crises, rapidly deteriorating environmental condition and resultantly the economy issues. Interestingly, all these problems are interrelated to one another. Majority of human race consume fossil fuels to fulfil their energy needs. Initially the consumers were ignorant of the scarcity of the fossil fuels and also of their perilous effect on the environment. Aggravation in scarcity led to unparalleled inflation and unabated use caused unsurpassed environmental degradation. Scientists suggested the solution in probing an alternate source of energy that must be sustainable and have no hazardous effects on environment in any way [1], [2], [3], [4], [5], [6]. Solar, Tidal, Wind, and Hydro energies were considered and harnessed to provide the humanity with affluent and inexpensive energy but none of these was devoid of drawbacks. So, researchers diverted their attention towards chemical energy and came to know that H₂ is the cleanest and purest form of energy and has the potential to surpass all the contemporary sources in terms of efficiency and availability. Exploring various sources of H₂ was the next level of research [7], [8], [9], [10], [11]. During the course of research, it was found that H₂ obtained from water splitting has various advantages like raw material i.e. H₂O was almost 71% on earth. It means it is matchlessly in abundance than anything else. Secondly, the H₂ produced from that procedure is uncontaminated and the cleanest. Thirdly, it is eco-friendly, as the only by-product formed during its consumption is water again. All these interpretations compelled the researchers to divert their attentions towards this novel and dynamic avenue [12], [13], [14], [15].

Basically, the water oxidation comprised of two half-cell reactions i.e. Oxygen Evolution Reaction (OER) and Hydrogen Evolution Reaction (HER). Thermodynamic and Kinetic studies of these reactions revealed that these are slothful reactions and require a high potential of 1.23 V to proceed. A wide variety of catalysts such as metal oxides [16], [17], metal selenides [18], metal tellurides [7], metal sulphides [19], metal phosphides [20], MOF etc. were employed to overcome the energy barriers of that reaction. It was discovered that Pt and oxides of Ir and Ru showed greater catalytic activity to overcome the energy obstacles of HER and OER, respectively. But problem continued to exist, as these catalysts belonged to noble metals which were not cost effective. Their sources were inadequate and they were the most precious among the metals. These factors prohibited their use on commercial scale. Under these circumstances, an expedition to fabricate an earth abundant, eco-friendly, robust, cost effective catalyst, which not only have the potential to replace these noble metal catalysts but also have the tendency to surpass their catalytic activity, is on the way [21], [22], [23], [24].

Carbon is a royal element and exists in various allotropic forms like diamond, graphite etc. It also has huge variety of nano-materials which include carbon dots, carbon nano-fibres, nano-tubes, nano-diamonds, fullerene and graphene etc. Carbon dots are spherical in shape, having size< 10 nm [25]. Carbon nano-fibers have cylindrical structures with graphene layers arranged as stacked cones, cups or plates [26]. When these graphene layers are wrapped into perfect cylinders, carbon nano-fibers are called carbon nano-tubes [27]. Carbon nano-diamonds are truncated octahedral architectures ranging from 2 to 8 nm in diameter. In fullerene, carbon atoms are connected via single or double bonds to form a closed or semi-closed structure with fused rings of five to seven atoms [28]. Graphene consists of a single layer of carbon atoms arranged in a two-dimensional honeycomb lattice [29]. All these materials have been exploited for wide range of explorations and splendid results have been exhibited by them [30]. Among all, carbon dots (CDs) represent an innovative and diversified class of carbon materials with nano-size range and unusual fluorescent and catalytic features [31]. These have established their unique characteristics which are far superior and magnificent than their counterparts. They are extraordinarily biocompatible and non-toxic materials which have been exclusively employed in bio-imaging probes [32]. These wonderful and impressive merits have expanded the scope of CDs beyond mere bio-imaging probes to other dynamic fields of biomedicines including bio-sensing, drug delivery, gene delivery [33], as excellent theranostic agent [34], for carrying anti-cancer drugs [35] and in sensing many biological and non-biological molecules [36]. CDs have also played crucial part to demonstrate magnificent bacterial imaging, antibacterial activity [37] and in catalysis [38], [39].

Metal Organic Frameworks (MOFs) constitute a unique arena of materials with wide range of applications. Metal ions and organic linkers are the leading constituent ingredients of these promising materials. MOFs are widely probed due to their highly ordered structures, sumptuous porosities, diversified surface, resultantly they possess very distinctive and dynamic properties [40], [41]. Amazingly, their characteristics are tuneable and can be sculpted to the desired one, either by careful selection of the materials or by vigilant handling of the technique [42], [43]. Versatile MOFs have found their efficacy in broad array of executions that include adsorption [44], sensors [45], gas storage [46], gas separation [47], drugs loading [48], photocatalytic reduction of CO₂ [49], electrochemical applications [50], [51], [52] and in searching the alternative source of energy [53]. MOFs can be used in their pristine form in variety of fields, as template for the growth of various attractive materials and also as precursors for the synthesis of numerous vital substances whose synthesis would have been difficult otherwise. The tuneable porosity and rigid framework of MOFs also allow other stable materials like metal oxides, metal chalcogenide etc., to be incorporated into them to have combined and boosted activity [54]. Recently, many researchers have successfully tried calcinations with MOFs to get metals embedded into the carbon structures and have employed them in a variety of applications with tremendous and loftier results [55], [56], [57], [58].

In this study, we have fabricated a novel MOF comprising of two ligands i.e. 1,3,5-benzenetricarboxylic acid (BTC) and 4,4`-bipyridine (bpy) and nickel nitrate hexahydrate [Ni (NO₃)₂•6H₂O] as a Ni source. CDs were incorporated into the MOF to get the composite. It was revealed that our target material was found to exhibit brilliant OER activity in alkaline medium due to the synergistic effect of CDs@MOF. CDs based MOF composite loaded on NF showed an onset potential of 1.50 V vs RHE, the overpotential 320 mV at current density of 10 mV cm⁻². Further, the as-prepared composite offered low resistance in the charge transfer and dominated with marvellous stability for 1,000 CV cycles and almost negligible current decay for continuous 40 hrs during chronoamperometric analysis.