Browsing by Subject "Coordination chemistry"

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Open Access
Catalysis with engineered Prussian blue analogues under external bias, light, and magnetic field
(2022-07) Oglou, Ramadan Chalil
The design of robust and feasible catalysts is one of the main concerns towards a carbon emission-free world. Prussian blue analogues (PBAs), the most well-known family of cyanide-based compounds, offer diversity and facile tunability of the structural components to achieve robust catalysts with high selectivities and reproducibilities. Herein this thesis, the catalytic performances of CoFe PBAs have been investigated for glucose and water oxidation processes. The structure of the Prussian blue (PB) framework has been engineered to tune the morphological and electronic properties for enhanced catalytic activity. In this regard, my thesis could be divided into three sections: (i) Electrocatalytic glucose oxidation: In the first part of this study, a CoFe PB modified fluorine-doped tin oxide (FTO) electrode, which is prepared via an electrodeposition method, was investigated as a non-enzymatic glucose sensor under neutral conditions. The electrode exhibits a linear detection of glucose in the 0.1 − 8.2 mM range with a detection limit of 67 μM, and a sensitivity of 18.69 mA mM−1 cm−2. Its stability is confirmed with both electrochemical experiments and characterization studies performed on the pristine and post-mortem electrodes. We also conducted a comprehensive electrochemical analysis to elucidate the identity of the active site and the glucose oxidation mechanism on the PB surface. In the second part, a series of PB modified carbon cloth (CC) electrodes were prepared with different cyanoferrate groups. A sensitivity as high as 145.43 μA mM−1 cm−2 in a 0.1 – 6.5 mM concentration range is achieved with a response time below 2 s under physiological pH. The electrodes exhibit a superior selectivity of glucose in the presence of interfering agents, including sucrose, lactose, sodium chloride, ascorbic acid, and uric acid. The electrodes also show outstanding long-term stability over 15 days. Furthermore, we performed comprehensive electrochemical and characterization studies to elucidate the role of the cyanoferrate group on the morphologic and electronic properties of non-enzymatic glucose sensors. (ii) Photocatalytic water oxidation: We present a simple and easy-to-scale synthetic method to plug common organic photosensitizers into a cyanide-based network structure for the development of photosensitizer-water oxidation catalyst (PS−WOC) dyad assemblies for the photocatalytic water oxidation process. Three photosensitizers, one of which absorbs red light similar to P680 in photosystem II, were utilized to harvest different regions of the solar spectrum. Photosensitizers are covalently coordinated to CoFe PB structures to prepare PS-WOC dyads. All dyads exhibit steady water oxidation catalytic activities throughout a 6 h photocatalytic experiment. Our results demonstrate that the covalent coordination between the PS and WOC groups enhances not only the photocatalytic activity but also the robustness of the organic PS group. We find that the photocatalytic activity of these “plug and play” dyads relies on several structural and electronic parameters, including the position of the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the PS with respect to the HOMO level of the catalytic site, the intensity and wavelength of the absorption band of the PS, and the number of catalytic sites. (iii) Intermetallic charge transfer induced electrocatalysis: We report a novel route to enhance the sluggish kinetics of oxygen evolution reaction (OER) by manipulating the intermetallic charge transfer (IMCT) of PBAs. It is found that CoFe PBAs with dissimilar charge transfer abilities reveal a positive response for OER under external stimuli such as magnetic field and light illumination, in which the magnitude of enhancement can be correlated to the intensity of metal-to-metal charge transfer (MMCT) profiles rather than the catalytic activity. An enhancement of almost 57% for OER activity is observed under a 1 h light irradiation for the CoFe PBA that exhibits the strongest IMCT nature. Several control experiments are conducted correlating the direct relation of IMCT and external stimuli induced activity involving –electrochemical experiments at varying pH conditions. Overall, this thesis indicates that CoFe PBAs could be engineered to design robust catalysts for oxidation reactions. Furthermore, they could also be fine-tuned to develop catalytic assemblies, which are responsive to applied bias, magnetic field, and light irradiation. Given the previous efforts in employing PBAs for catalytic applications, this thesis pushes the limits one step forward and brings a new level to this challenge.
Open Access
Electrocatalytic water splitting with Prussian blue analogues under external stimuli
(2023-09) Ahmad, Waqar
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.