MOF DERIVED HIERARCHICALLY POROUS NANOSTRUCTURES AS A ROBUST ELECTROCATALYST FOR EFFICIENT WATER OXIDATION

Abstract

Fossil fuels the predominant source of energy today, are a significant to the release of carbon dioxide into the atmosphere causing greenhouse effect and global warming. Alternative and renewable energy sources must be identified immediately in order to address these issues. Water, a readily available renewable energy resource, is an appealing option in this regard owing to its ability to produce H2 (HER) and O2 (OER) through water splitting mechanism. Utilizing distinct catalysts for the anodic and cathodic reactions in electrochemical water splitting reaction will escalate the cost of the electrolytic cell. Therefore, research and development efforts to develop novel electrocatalysts with dual functions for OER and HER is imperative and highly desirable. Metal Organic Framework (MOFs) and their derivatives are the well-known candidates having porous 3D -framework structures with high surface areas that have been rapidly developed in the last decade as an efficient electrocatalyst for water splitting. Among others, Zeolitic Imidazolate framework (ZIFs) are sub class of MOFs which have robust electrocatalytic properties are less explored for water splitting application. In this study we have prepared three different polymorphs of ZIF-7 porous framework material and their composite with zeolite both having sodalite topology. The three phases of ZIF-7 are described as ZIF-7-I, ZIF-7(I+III) and ZIF-7-III with a space group of rhombohedral(R-3), triclinic(P-1) and monoclinic(C2/c) respectively. ZIF-7 (I) is prepared from Zinc nitrate as metal nodes and benzimidazole as a linker, that crystallizes in the rhombohedral conformation with a lattice parameter of a=22.989[A ֯] b=22.989[A] c=15.763[A]. We further found that on addition of 1.25wt% of zeolite to pure ZIF-7(I) phase there triggers a phase transformation to ZIF(III) polymorph and we ended up into a combination of ZIF-7 (I+III) mixture. When the concentration of Zeolite increases to 5wt% into pure ZIF-7-I it is completely transformed into ZIF-7-III phase that crystallizes in the space group monoclinic (C2/c) with a lattice parameter of a= 16.106A b=19.511A c=16.126A. ZIF-7-I is further irradiated with 500 keV (Cu++) ions at dosages of 1 × 1014 ions cm-2that causes yet another stimulating phase transformation from ZIF-7-I to a less symmetric ZIF-7-II polymorph that crystallizes in the space group triclinic (P-1) with a lattice parameter of a=23.984[A ֯] b=21.354[A] c=16.349A. All the synthesized materials were characterized utilizing XRD, FTIR, SEM, xiii EDX, Raman spectroscopy, XPS, and TGA analysis. The crystallinity, phase purity and phase transformation from ZIF-7-I(rhombohedral) with a space group of R-3 to ZIF-7-III (monoclinic) with a space group of C2/c are confirmed by XRD. The average crystallite size of ZIF-7-I was found to be 15.687nm. The FTIR spectra of different polymorphs of ZIF-7 samples display the characteristics vibrational and stretching modes along with the formation of major bond at 426cm-1 attributed to Zn-N stretching modes that affirm the connectivity of metal nodes of Zn with benzimidazole linkers a vital part for the formation of porous framework. SEM images coupled with EDX revealed surface composition, morphology, and topography information of samples, confirming its crystallinity and phase purity and also tells the average particle size that was 61.5nm for pure ZIF-7-I. XPS for evaluation of elemental composition. The major elements that were present are C, N, O, Zn2p1/2 and Zn2p3/2 at binding energies of 285eV,399eV, 532eV, 1044eVand 1021eV respectively. Raman spectroscopy identify the major mode at 154cm-1 of Zn Thermogravimetric Analysis for thermal stability evaluation reveals that ZIF-7-I was thermally stable up to 540 C. A suite of electrochemical techniques, including EIS to probe charge transfer resistance, LSV to determine the value of overpotential at 10 mAcm-2, Tafel slope to determine the kinetics, and chronoamperometry to determine the material's stability. For ZIF-7(I+III) an overpotential of 290mV at 10mA cm-2 for the OER was reached. ZIF-7(I+III) electrocatalyst required only a cell voltage of 1.52V for water splitting and it performed very well among all catalysts. The high activity of ZIF-7(I+III) was further evidenced by a narrower semi-circle in Nyquist plot. Furthermore, higher stability was offered when tested with continual production of oxygen for 15 hours. The overpotential required for OER of pure ZIF-7-I was 414mV at 10 mAcm-2 with a Tafel slope of 66.98mV/dec and a wide semi-circle for EIS revealing good catalytic activity. The overpotential of combination of ZIF-7 (I+III) was 290mV with a Tafel slope of 59.21mV/dec and a small semi-circle for EIS showing the best catalytic activity. The overpotential for ZIF-7-III was 387mV with a Tafel slope of 48.49mV/dec and a wide semi-circle for EIS showing better catalytic activity. The comparative study of different polymorph both structurally and electrochemically shows that the best electrocatalyst for OER was the combination of two ZIF-7(I+III).