Polyaniline-zeolitic imidazolate framework composite nanofibers for hydrogen gas sensing application

Abstract

The quest for renewable, sustainable and environmentally compatible energy sources have been on-going for decades. Green technology such as hydrogen fuel cell has attained much attention as an alternative energy carrier to carbon-based fuels owing to its renewability and cleanliness. However, hydrogen gas feed to the fuel cell can easily be ignited if its concentration is above 4 wt.% at room temperature. Thus, hydrogen safety mechanisms such as hydrogen sensors are vital to guarantee people‘s safety in the hydrogen infrastructure. Sensors based on metals and metal oxides have been widely applied for hydrogen gas detection. Nonetheless, these materials are only sensitive to hydrogen gas at elevated temperatures (˃ 100 °C) and they also possess low surface area (< 20 m2/g). Hence in this work, we present polyaniline (PANI) doped with cobalt-based zeolitic benzimidazolate framework (CoZIF) and zinc-ZIF to fabricate (PANI-CoZIF and PANIZnZIF) composite nanofibers as effective electrocatalysts for hydrogen gas sensing application. The composites were synthesised through chemical oxidative polymerisation of aniline monomer in the presence of 3.6 wt.% CoZIF and ZnZIF, respectively. The structural properties of the synthesised materials were studied using Ultraviolet visible (UV-vis), X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman spectroscopy and simultaneous thermal analysis (STA). FTIR, Raman and XRD studies showed successful synthesis of CoZIF, ZnZIF and their composites. Furthermore, the studies indicated the co-existence of both CoZIF and ZnZIF in the PANI matrix upon composites formation, indicated by reduction in crystalline size, decrease in band gap and increase in thermal stability. as compared to the neat PANI. Morphological characteristics of the prepared samples were investigated usingscanning electron microscopy (SEM) and transmission electron microscopy (TEM) coupled with both energy dispersive spectroscopy and X-ray (EDS/EDX). PANICoZIF revealed the grafting of CoZIF on to the surface of PANI matrix while PANI-ZnZIF composite showed that PANI is wrapping the cube nanofiber-like structures of ZnZIF also supported by selected area electron diffraction (SAED). Cyclic voltammetry (CV), Tafel analysis and turn over frequencies (TOFs) were performed to study the electrochemical performance of the synthesised materials through hydrogen evolution reaction (HER) for gas sensing. Both composites presented drastic enhancement in the catalytic H2 evolution at 0.033 mol.L-1 H2SO4 with the Tafel slope of 160 mV/dec and exchange current density of 3.98 A.m-2 for PANI-CoZIF composite, while the Tafel slope and exchange current density for PANIZnZIF composite were 246 mV/dec and 5.01 A.m-2, respectively. Moreover, the TOFs of the PANI-CoZIF composite (0,117 mol H2.s-1) was higher as compared to neat PANI (0.040 mol H2.s-1). The TOF values for PANI and PANI-ZnZIF composite were 0.04 and 0.45 mol H2.s-1, respectively. In addition, the chronoamperometric (CA) results exhibited the significant improvement in the electrochemical hydrogen sensing ability of PANI-CoZIF and PANI-ZnZIF composites with higher current response and sensitivity values of 12 and 10.8 µA.mmol.L-1 H2, respectively. The composites exhibited faster steady state response time of 5 s for PANI-CoZIF composite and 4 s for PANI-ZnZIF composite accompanied by lower detection limit (5.27 µmol.L-1) as compared to the neat PANI matrix. The high electrochemical current response is due to extraordinary specific surface area, more accessible active sites available for the electrolyte provided by CoZIF and ZnZIF and high conductivity supplied by PANI. These results proved that the PANI-CoZIF and PANI- ZnZIF composites are suitable electrocatalytic materials for hydrogen gas sensing application through HER in acidic medium. These results further suggest that the safety of people in mining sectors and other industries can be addressed through simple electrocatalytic gas sensing systems.