Browsing by Subject "Oxygen evolution reaction"

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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.
Open Access
Fabricaiton, characterization, and electrolysis of mesoporous CaFe2O4 thin film electrodes
(2023-06) Raza, Hamid Ali
Transition metal ferrites have attracted the attention of many scientists because of their low cost, high earth abundance, low band gap, and biocompatibility. They can be prepared in different morphologies, and because of this, they may have a high surface area and excellent electrochemical and photoelectrochemical properties. In this thesis, we have prepared mesoporous calcium iron oxide (CFO) thin films using the molten-salt assisted self assembly (MASA) method and analyzed its electrochemical applications for oxygen evolution reaction (OER). The clear and homogeneous aqueous solution of metal salts (Calcium nitrate tetrahydrate, iron (III) nitrate nonahydrate) and surfactants (cethyltrimethyl ammonium bromid, CTAB, and C12H25(OCH2CH2)10OH, C12EO10) were coated on microscope glass slides by various coating techniques to obtain mesophases. Later on, the mesophases and their aging process were analyzed by the small-angle XRD measurements, ATR-FTIR and POM. Diffraction lines between 1 and 5°, 2θ, indicate the formation of ordered lyotropic liquid crystalline mesophases. These mesophases were subjected to calcination at various temperatures, and the powders obtained were further characterized by wide-angle XRD measurements, SEM, EDX, TEM, XPS, ATR-FTIR, and N2-adsorption and desorption techniques. The calcium iron oxide in highly crystalline form are prepared at 800 °C, having thin film morphology. Interestingly, we are able to retain the porous structure even at such a high temperature. The amorphous phase contains calcium carbonate as a side product that was confirmed by ATR-FTIR, XRD and XPS data. The maximum surface area of mesoporous material is 145 m2/g, while water being used as a solvent. Similarly, we prepared the same materials using different precursors (chlorides) and solvent (ethanol) to see the effect of counter anion and solvent on the porosity, self-assembly, morphology, and electrocatalytic performance of the material in the OER. We observed that while using chloride precursors, the material was quite crystalline even at low calcination temperature, i.e., 300 °C. Iron oxide forms at low temperatures and with the increase in temperature, it finally transforms to calcium iron oxide. But in this case, the materials are not as porous and display a surface area of only 5 m2/g at 300 °C. Similarly, we also characterized these materials using the above-mentioned techniques. While using ethanol as a solvent, keeping nitrate precursors the same, and using two different mole ratios of calcium and iron (2:4, 3:6), we also tried to elucidate the effect of solvent on morphology and catalytic properties of materials. In this case, we observed that the surface area did not drop immediately (as in the case of water) but gradually. The maximum surface area, obtained are almost similar to the material prepared by water as a solvent. All the solutions mentioned above (prepared by using different precursors, solvent, and mole ratios) are coated (by dip-coating) on the graphite rod to determine the catalytic activities by various electrochemical experiments (cyclic Voltammetry (CV), chronopotentiometry (CP), and chronoamperometry (CA)). Electrodes are quite stable in all cases, even in harsh conditions (CP at 100 mA for 2 h). Also, enhanced activity may be because of reduced resistance and increased conductivity with the usage. In all cases, the minimum Tafel slopes are almost similar, and vary between 47 and 83 mV/dec. The overpotentials at various current densities are 260 mV for 1 mA/cm2, about 450 mV for 10 mA/cm2, and about 700 mV for 100 mA/cm2. Additionally, effect of the coated material's thickness on the electrocatalyst's activity is also investigated. It has been found that by decreasing the amount of coated material (by diluting up to 100 times), there is no change in the activity of the material. Finally, our results indicate that these types of energy material's (CFO) performance depends on the surface's characteristics rather than the coating material's thickness or the pores' size. Also, we found it unnecessary to waste a large amount of metal salts to fabricate these materials; OER performance is similar regardless of coating thickness. Therefore, the surface reaction is the primary factor in electrode activity, with pore shape being the critical characteristic.
Open Access
Investigating the effect of catalysts in sodium-oxygen batteries
(2017-11) Tovini, Mohammad Fathi
The unique electrochemical and chemical features of sodium oxygen (Na-O2) batteries distinguish them from the lithium-oxygen (Li-O2) batteries. NaO2, which is the main discharge product, is unstable in the cell environment and its dissolution in the electrolyte triggers side products formation and charging potential increment. In the rst part of this thesis, RuO2 nanoparticles (NPs) dispersed on carbon nanotubes (CNTs) are used as a catalyst for Na-O2 batteries to elucidate the e ect of catalyst on this complex electrochemical system. RuO2/CNT contributes to the formation of a poorly crystalline and coating like NaO2 structure during oxygen reduction reaction (ORR) which is drastically di erent from the conventional micron sized cubic NaO2 crystals deposited on CNT. Our ndings demonstrate a competition among NaO2 and side products decompositions for RuO2/CNT during oxygen evolution reaction (OER). We believe that this is due to the lower stability of coating like NaO2 because of its non-crystalline nature and high electrode/electrolyte contact area. Although RuO2/CNT catalyzes the decomposition of side products at a lower potential (3.66 V) compared to CNT (4.03 V), it cannot actively contribute to the main electrochemical reaction of the cell during OER (NaO2→ Na+ + O2 + e{ ) due to the fast chemical decomposition of lm NaO2 to side products. Even though the long term e ect of RuO2 catalyst during cycling and resting tests seems to be positive in terms of lower overpotential, no bene ts of catalyst is observed for stability and e ciency of the cell for the rst cycles. Therefore, tuning the morphology and crystallinity of NaO2 by catalyst is detrimental for Na-O2 cell performance and it should be taken into account for the future applications. In the second part of this thesis, a 3D RuO2/Mn2O3/carbon nano ber (CNF) composite has been prepared as a bi-functional electrocatalyst towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in Na-O2 batteries. RuO2/Mn2O3/CNF exhibited higher speci c capacity (9352 mAh.gcarbon -1) than CNF (1395 mAh.gcarbon -1), Mn2O3/CNF (3108 mAh.gcarbon -1) and RuO2/CNF (4859 mAh.gcarbon -1), which is believed to be due to its higher active surface area than its counterparts and its unique morphology. Taking the bene t of RuO2 and Mn2O3 synergistic e ect, the decomposition of inevitable side products at the end of charge occurs at 3.838 V vs. Na/Na+ by using RuO2/Mn2O3/CNF, which is 388 mV more cathodic compared with CNF.
Open Access
Mesoporous MnCo2O4 NiCo2O4 and ZnCo2O4 thin-film electrodes as electrocatalysts for the oxygen evolution reaction in alkaline solutions
(American Chemical Society, 2021-03-22) Amirzhanova, Assel; Akmanşen, Nesibe; Karakaya, Irmak; Dağ, Ömer
The oxygen evolution reaction (OER) is the bottleneck of the electrochemical water-splitting process, where the use of porous metal oxide electrodes is beneficial. In this work, we introduce a one-pot synthesis method to fabricate a series of mesoporous metal cobaltite (m-MCo2O4, M = Mn, Ni, and Zn) electrodes for the OER. The method involves preparation and coating of a homogeneous clear solution of all ingredients (metal salts and surfactants) over a fluorine-doped tin oxide surface as a thin lyotropic liquid crystalline film and calcination (as low as 250 °C) to obtain a 400 nm thick crystalline m-MCo2O4 electrode with a spinel structure. Mesophases and m-MCo2O4 films are characterized using structural and electrochemical techniques. All electrodes are stable during the electrochemical test in 1 M KOH aqueous solution and perform at as low as 204 mV overpotential at 1 mA/cm2 current density; the m-MnCo2O4 electrode works at current densities of 1, 10, and 100 mA/cm2 at 227, 300, and 383 mV overpotentials after compensating the IR drop, respectively. The Tafel slope is 60 mV/dec for the m-NiCo2O4 and m-ZnCo2O4 electrodes, but it gradually increases to 85 mV/dec in the m-MnCo2O4 electrode by thermal treatment, indicating a change in the OER mechanism.
Open Access
Nanoarchitectonics of mesoporous CaFe2O4 thin-film electrodes from salt-surfactant lyotropic liquid crystalline mesophases and their OER performance
(American Chemical Society, 2023-09-05) Raza, Hamid Ali; Karakaya, Irmak; Dağ, Ömer
Metal oxides of earth-abundant elements (such as Ca and Fe) are highly important for fabricating active electrodes for various electrochemical applications (such as electrocatalysis and photo-electrocatalysis). Here, we employed a molten-salt-assisted self-assembly process to fabricate CaFe2O4 thin-film electrodes on graphite rods. The roles of precursor type (nitrates and chlorides) and solvent (water and ethanol) have been addressed in the fabrication of the electrodes that are tested in oxygen evolution reaction (OER) in alkali media. The mesophases have an unusual orthorhombic structure that is likely transformed from a well-known 3D hexagonal phase by an elongation along the b-axis caused by the hydrolysis and condensation of the Fe(III) species in the lyotropic liquid crystalline media. Four sets of mesoporous electrodes with a high surface area are fabricated using nitrate and chloride precursors in aqueous media and nitrates in ethanol. The electrodes, fabricated from the chloride precursors, are not as porous as nitrates, but they display better performance in the OER. The electrodes, fabricated from ethanol solutions, outperform, are more robust, and display as low as 250, 342, and 642 mV overpotentials at 1, 10, and 100 mA/cm2 current densities with a Tafel slope of around 60 mV/dec. The electrode thickness has no role in the electrode performance and can be prepared as thin as tens of nanometers with good stability and OER performance.
Open Access
Nanohybrid structured RuO2/Mn2O3/CNF as a catalyst for Na-O2 batteries
(Institute of Physics Publishing, 2018) Tovini, M. F.; Patil, B.; Koz, C.; Uyar, Tamer; Yılmaz, E.
A 3D RuO2/Mn2O3/carbon nanofiber (CNF) composite has been prepared in this study by a facile two step microwave synthesis, as a bi-functional electrocatalyst towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). RuO2 nanoparticles with the mean size of 1.57 nm are uniformly distributed on Mn2O3 nano-rods grown on electrospun CNFs. The electrocatalytic activity of the composites are investigated towards ORR/OER under alkaline condition. The ternary RuO2/Mn2O3/CNF composite showed superior ORR activity in terms of onset potential (0.95 V versus RHE) and Tafel slope (121 mV dec-1) compared to its RuO2/CNF and Mn2O3/CNF counterparts. In the case of OER, the RuO2/Mn2O3/CNF exhibited 0.34 V over-potential value measured at 10 mA cm-2 and 52 mV dec-1 Tafel slope which are lower than those of the other synthesized samples and as compared to state of the art RuO2 and IrO x type materials. RuO2/Mn2O3/CNF also exhibited higher specific capacity (9352 mAh ) than CNF (1395 mAh ), Mn2O3/CNF (3108 mAh ) and RuO2/CNF (4859 mAh g carbon -1) as the cathode material in Na-O2 battery, which indicates the validity of the results in non-aqueous medium. Taking the benefit of RuO2 and Mn2O3 synergistic effect, the decomposition of inevitable side products at the end of charge occurs at 3.838 V versus Na/Na+ by using RuO2/Mn2O3/CNF, which is 388 mV more cathodic compared with CNF.
Open Access
Synthesis & characterization of mesoporous zinc cobaltite thin films and its electrochemical application for OER
(2021-07) Kalaycı, Nesibe Akmanşen
Transition metal cobaltite materials were widely used as electrode material due to their excellent electrochemical performance, flexibility to be prepared with different morphologies and, high surface area. In this thesis, mesoporous zinc cobaltite thin films were synthesized in cubic spinel structure via molten-salt assisted surfactant assembly (MASA) method with a high surface area and its electro-catalytic performance in oxygen evolution reaction (OER) was analyzed. Clear and homogenous aqueous solution of surfactants (P123 and CTAB), zinc nitrate hexahydrate and cobalt(II) nitrate hexahydrate (as precursors) are coated on glass substrate to obtain mesophases, thereafter mesophases are calcined to synthesize mesoporous zinc cobaltite (denoted as m-ZnCo) as powder. m-ZnCo-60 (with a total salt/P123 ratio of 60) samples were synthesized with a smooth film morphology and maximum surface area of 102 m2/g. The mesophases with different compositions were analyzed using X-Ray Diffraction (XRD) technique. The line(s) between 1.5 and 2°, 2θ, in the XRD pattern is an indication for the formation of ordered lyotropic liquid crystalline mesophase. Aging of the mesophase was monitored via XRD and POM techniques to establish its stability. The stable mesophases were used to synthesize m-ZnCo film and powder samples. The powder samples were collected after calcination process and characterized by XRD, N2 adsorption-desorption, SEM, HR-TEM techniques. The precursor solutions were spin coated on half of 1cm x 2cm size FTO glasses, then calcined and used in three-electrode system as working electrodes. The electrocatalytic performance of the materials was analyzed by cyclic voltammetry (CV), chronopotentiometry (CP), and chronoamperometry (CA) experiments for oxygen evolution reaction (OER). All electrodes were stable up to 100 mA/cm2 current density and displayed minimum Tafel slope value of 41 mV/dec. Mesoporous zinc cobaltite materials were also synthesized through precursor solutions without CTAB. Removing CTAB from the synthesis results films with rougher surface and reduced crystallinity. Same techniques were also employed for characterization. The prepared electrodes of non-CTAB samples exhibited a lower Tafel slope of 40 mV/dec and overpotential of 256 mV at 1mA/cm2 current density. In addition, silica templated mesoporous zinc cobaltite was synthesized by adding TMOS to the precursor solution of ZnCo-60 to increase the surface area, the calcined samples were denoted m-ZnCo-60-S20-300 (S20 is represents 20 TMOS/P123 mole ratio). The m-ZnCo-60-S20-300 sample has the highest specific surface area of 215 m2/g. However, despite having higher surface area due to high resistance of silica material, silicated samples exhibited higher overpotential values.
Open Access
Synthesis, characterization, and electrochemical properties of mesoporous spinel LiMn2-xMxO4 (M = Mn, Fe, Co, Ni, AND Cu) thin films
(2024-05) Durukan, Irmak Karakaya
Mesoporous LiMn2-xMxO4 electrodes are promising candidates for efficient oxygen evolution (OER) electrocatalysis. In this study, mesoporous LiMn2-xMxO4 (where M is Mn, Fe, Co, Ni, and Cu) thin films have been fabricated by employing molten-salt assisted self-assembly (MASA) method on fluorine doped tin oxide (FTO) surface. The electrodes are characterized according to their structure, morphology, and thicknesses using various characterization techniques. The electrochemical properties of the films are comprehensively investigated under acidic, alkaline, neutral, and non-aqueous solutions. The manganese oxide-based electrodes undergo Mn(III) and Mn(VI) disproportionation reactions. Here, we have extensively investigated these disproportionation reactions by post-characterization techniques after electrochemical experiments using LiMn2-xMxO4 (M is Fe, Co, Ni, and Cu and x is 0, 0.1, 0.3, 0.4, and 0.67) and Mn3O4 electrodes. The LiMn2O4 thin films are found to be more stable in OER compared to Mn3O4. Lithium de-intercalation of the LiMn2O4 films produces a λ-MnO2 phase robust against Mn(VI) disproportionation. The electrochemical degradation rates are investigated using the LiMn2O4 electrodes, fabricated at various spin rates (from 2000 and 10000 rpm). The film thicknesses are between 150 and 500 nm. The LiMn2O4 electrode at 5000 rpm is more resistant to physical degradation during electrochemical tests. Charge capacity values of the thin films are determined by electrochemical experiments in LiNO3 electrolyte and found to be between 136 and 273 mC/cm2 for the films, then these values are used to calculate their approximate weights (between 30 and 60 μg/cm2). The annealing temperature for the LiMn2O4 thin films is also optimized for a stable OER. The LiMn2O4 film, fabricated at 5000 rpm spin rate and annealed at 300 oC, is found to be a more robust and efficient electrode with a 60 mV/dec Tafel slope and 812 mV overpotential at 10 mA/cm2 current density. The same fabrication parameters are used for the other mesoporous LiMn2-xMxO4 thin films. The LiMn2-xMxO4 thin films are used to collect their N2-adsorption-desorption isotherms. The isotherms display type IV hysteresis, characteristic of mesoporous materials. BET surface areas are estimated as 98, 99, 116, 112, and 75 m2/g for the LiMn2O4, LiMn1.7Fe0.3O4, LiMn1.7Co0.3O4, LiMn1.7Ni0.3O4 and LiMn1.7Cu0.3O4, films, respectively. Moreover, the LiMn2-xMxO4 electrodes (fabricated at 5000 rpm spin rate and 300 oC annealing temperature) are investigated for lithium de-intercalation/intercalation behavior in 1 M LiNO3 solution. Then, the same electrodes are used to collect 300 CVs, CAs, and CPs in 1 M KOH solution to evaluate electrochemical behaviors. From these measurements, the origin of phase separation and bearing lower oxidation states of the nickel and copper at higher x values are identified in the spinel structure. The Mn(VI) disproportionation reaction on the LiMn2-xMxO4 electrodes is investigated by CV cycling experiments in 1 M KOH. The LiMn2O4, LiMn2-xFexO4, and LiMn2-xCuxO4 electrodes undergo fast degradation compared to LiMn2-xCoxO4 and LiMn2-xNixO4 through the dispersion of [MnO4]- and [FeO4]2- ions and dissolution of the CuO phase formed in the electrodes during OER. The LiMn1.7M0.3O4 thin films on FTO are used in OER electrocatalysis and the overpotential values at 10 mA/cm2 are evaluated as 645, 686, and 657 mV for the LiMn1.7Fe0.3O4, LiMn1.7Co0.3O4, LiMn1.7Ni0.3O4 electrodes, respectively. The exact compositions are also coated on graphite substrates and their overpotential values are also evaluated as 629, 462, 440, and 532 mV at 10 mA/cm2 for the LiMn2O4, LiMn1.7Fe0.3O4, LiMn1.7Co0.3O4, LiMn1.7Ni0.3O4 electrodes, respectively. The LiMn1.7Co0.3O4 on graphite and LiMn1.7Ni0.3O4 on FTO electrodes are found to be the most robust and efficient electrodes at a 50 mA/cm2 current density.