Electrocatalytic water splitting with Prussian blue analogues under external stimuli

Abstract

The development of long-lasting and efficient catalysts for water splitting is crucial for the advancement of a carbon emission-free world. A well-known class of compounds called Prussian blue analogues (PBAs) offers several advantages such as high stability, diversity, and simple synthesis for the development of sustainable water-splitting devices. This thesis investigates the construction of PBA-based overall water-splitting electrolytic cells assisted with external stimuli. Alsac et al. investigated the oxygen evolution reaction (OER) efficiency of various PBAs and concluded that Co-Co exhibits the best performance as an OER catalyst among the Co-M PBAs. Ahmad et al. studied the hydrogen evolution reaction (HER) performance of various PBAs and observed that Co-Ni stands out in performance. Furthermore, Chalil Oglou et al. elucidated the effect of the magnetic field on the OER catalytic activity of Co-Fe PBA electrodeposited on the surface of the FTO. His findings unveiled an enhanced catalytic activity under the influence of a magnetic field. To further explore these concepts, we aim to move one step ahead and combine all these studies to investigate overall water splitting (OWS) under the influence of magnetic field and solar light irradiation. In this thesis, [Co-Co] was used for the OER reaction, while [Co-Ni] was utilized for the HER reaction. Both electrodes were prepared involving a two-step electrodeposition method and comprehensively characterized with SEM, EDAX, P-XRD, XPS, and ATR-FTIR. SEM images unveiled threat-like and needle-like grown particles with uniform sizes of 1-2 µm for [Co-Co] and [Co-Ni] formed on the fluorine-doped tin oxide (FTO) electrode respectively. The oxidation states of the pristine and post-catalytic electrodes and the stability during the electrocatalytic process were confirmed with XPS and FTIR studies. The electrochemical characterization of these catalysts was thoroughly investigated with linear sweep voltammetry (LSV), chronoamperometry (CA), and cyclic voltammetry (CV) profiles. The electrochemical performance was investigated in three chapters; OER, HER, and overall water splitting under magnetic and solar light irradiation. (i) OER performance of FTO/[Co-Co] was evaluated with LSV, which shows prominent enhancement peaks under the influence of external stimuli. Under the influence of the magnetic field, it illustrated an enhancement of 11.9% with an overpotential of 949 mV, while in the presence of solar light, it showed an augmentation of 10.7% with an overpotential of 949 mV. CA profiles, recorded under magnetic field showed that there is a direct relation between magnetic field strength and the enhancement in the current density. On the contrary, an opposite trend is observed with the CA profiles under solar light irradiation, which suggests that the origin of the enhancement under the magnetic field is different from the one under solar light irradiation. (ii) Similar to OER studies, HER activity of FTO/[Co-Ni] was investigated under the effect of solar light irradiation and magnetic field. The LSV profile showed enhancement only in the case of solar light, while no significant enhancement was observed under the magnetic field, contrary to the previous studies. Similar to OER, the CA profiles of FTO/[Co-Ni] illustrated the opposite trend with respect to overpotential applied. In the case of HER, CA under a magnetic field showed a small enhancement (1.4%) with an overpotential of 300 mV, which was attributed to the magnetohydrodynamic effect. (iii) Two and three-electrode systems were used to conduct the investigation into overall water splitting. To achieve a current density of 1 mA/cm2 in the two-electrode having FTO/[Co-Co] on the working/working sense electrode (W/WS) and FTO/[Co-Ni] on the counter/reference electrode R/C configuration, the system required an overpotential of roughly 1013 mV. The subsequent analysis of each electrode's unique voltage contributions helped explain this observation. OER takes around 1.3 V while it is 0.6 V for the HER side. On the other hand, in the three-electrode configuration, the working electrode was FTO/[Co-Co], the counter electrode was FTO/[Co-Ni], and the reference electrode was Ag/AgCl. The observed profile notably showed significant improvement seen when solar light and magnetic fields were present. Overall, this study indicates that there is still plenty of room for enhancement in catalysis, with slight modification in reaction conditions from another perspective i.e., external stimulus. This thesis takes a progressive step by raising the bar and adding a new dimension to the challenge of using PBAs in catalytic applications, building on earlier efforts.