Browsing by Subject "Water oxidation"

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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
Cobalt borophosphate on nickel foam as an electrocatalyst for water splitting
(Elsevier BV, 2022-06-10) Ülker, E.; Akbari, Sina Sadigh; Karadas, Ferdi
One of the most critical steps in the transition to carbon-free energy systems is sustainable hydrogen evolution from water. In this research, a cobalt borophosphate crystalline compound consisting of phosphate and borate anions was synthesized with a solid-state reaction. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and X-ray Photoelectron (XPS) was employed to investigate the structure, composition, and morphology of Co3BPO7. Electrocatalytic performances of the catalyst towards oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) have been investigated on nickel foam (NF) electrode in 1.0 M KOH (pH 13.6) by linear sweep voltammetry, chronopotentiometry, cyclic voltammetry, and electrochemical impedance spectroscopy. For OER, the catalyst exhibits an overpotential of 230 mV at 10 mA cm−2 with a Tafel slope of 130 mV dec−1, which is comparable to that of the benchmark RuO2 electrocatalyst, and 220 mV overpotential for a current density of 10 mA cm−2 with a Tafel slope of 147 mV dec−1 for HER process. Long-term chronoamperometry and multiple cyclic voltammetric experiments indicate the catalyst is stable throughout both HER and OER processes. Electrochemical experiments and characterization studies performed on the pristine and post-catalytic electrode indicate that the catalyst is robust under alkaline electrocatalytic conditions (pH 13.6).
Open Access
Design of multifunctional prussian blue analogues for solar driven water oxidation
(2021-07) Ghobadi, Türkan Gamze Ulusoy
The development of earth-abundant, robust, and low-cost photoanodes for water oxidation is one of the most critical steps in ‘artificial leaf’. A promising approach in this field is to build dye-sensitized photoanodes by coupling a molecular photosensitizer (PS) with a water oxidation catalyst (WOC) on a proper semiconductor (SC) for efficient charge separation. All dye-sensitized photoanodes reported in the literature consist of either a ruthenium photosensitizer, a ruthenium water oxidation catalyst, or both. We aim to overcome this critical challenge by developing a new family of organic- or iron-based donor-acceptor chromophores incorporated in a Prussian blue (PB) structure, which are coated on proper semiconductors. Our studies within this context could be divided into three sections: (i) PB based photocatalytic water oxidation: In this work-package, an entirely precious metal-free chromophore-donor-water oxidation catalyst triad system is developed. The synthesis involves the coordination of a porphyrin derivative to a bridging Fe(CN)5 group, which is then reacted with cobalt ions to prepare a covalently linked chromophore-Prussian blue analogue (CoFe(CN)5–Ligand) assembly. Light-driven water oxidation studies in the presence of an electron scavenger indicate that the triad is active and maintains a steady activity for at least 3 hours. Transient absorption experiments and computational studies reveal that the Fe(CN)5 group is more than just a linker. It takes part in electron donation and co-operates with porphyrin in the charge separation process. (ii) PB based photoelectrochemical water oxidation: Here, we move one step forward and design a ruthenium-free water oxidation photoanode by the sensitization of titanium dioxide (TiO2) nanowires with a PB-organic chromophore structure. A phenazine-based organic group, Janus Green B (JG), is chosen as the chromophore since it has a broad absorption response in the visible and near-infrared ranges. The resulting multifunctional PB modified TiO2 electrode demonstrates a low-cost and easy-to-construct photoanode, which exhibits a remarkable excited-state lifetime in the order of nanoseconds and an extended light absorption capacity of up to 500 nm. Moreover, the photoanode retains its structural integrity and photoelectrochemical activity for at least 2 hours. Despite all the above-mentioned improvements, the performance of the cell, [CoFe–JG]/TiO2, is relatively poor due to improper band energy alignment between the chromophore and the semiconductor. In a follow-up study, we tune the chromophore and the semiconductor to achieve a proper band energy alignment, and thus, to improve the performance. Another phenazine-based molecule, Safranin O (SF), is utilized as the organic photosensitizer. Moreover, a visible-light absorbing semiconductor, WO3, is used to utilize the solar spectrum completely. [CoFe–SF]/WO3 exhibits a record photocurrent density of 1.3 mA/cm2 at 1.23 VRHE, demonstrating that proper modification of components in PB based dye-sensitized photoanodes could pave the way for the development of high-performance water splitting cells. (iii) Iron chromophore based photoelectrochemical water oxidation: In this section, the iron site that has been previously utilized as a relay is promoted to an iron chromophore. Five cyanide ligands are coordinated to the iron site to destabilize the metal-centered states. At the same time, an electron-deficient cationic pyridinium group occupies the remaining coordination sphere of the octahedral iron site to facilitate the metal-to-ligand charge transfer (MLCT) process. This iron complex is coated initially on TiO2 nanowires and then reacted with cobalt ions to produce a CoFe PB (CoFe(CN)5-L) layer on the electrode surface. In this photoanode, the excited-state lifetime of the iron chromophore exceeds 1 ns, which demonstrates the first example of an iron-sensitized water oxidation cell in the literature. Overall, this thesis presents an alternative perspective to realize high performance, low-cost, stable, and robust dye-sensitized water oxidation systems. The impact of the acquired knowledge in this thesis is also discussed to define the current status, challenges, and future of PB based water oxidation systems.
Open Access
Enhancing oxygen evolution catalytic performance of nickel borate with cobalt dopingand carbon nanotubes
(Wiley, 2023-02-16) Enez, S.; Karani Konuksever, V.; Samuei, S.; Karadaş, Ferdi; Ülker, E.
Open Access
Investigation of the ideal composition of metal hexacyanocobaltates with high water oxidation catalytic activity
(TÜBİTAK, 2019) Karadaş, Ferdi
The electrocatalytic activities of Prussian blue analogues (PBAs) have recently received much attention due to their robustness and efficiency. Considering that PBAs with hexacyanocobaltate building block stand forward among other PBAs, a systematic study on a family of metal hexacyanocobaltates is presented in this study. Metal hexacyanocobaltates (M = Co, Mn, Ni, and Fe) were prepared, characterized, and electrochemical studies were performed. A series of mixed-metal cobalt-iron hexacyanocobaltates has also been studied to determine the ideal composition of a metal hexacyanocobaltate for electrocatalytic water oxidation process. The overall study clearly indicates that cobalt hexacyanocobaltate exhibits the highest electrocatalytic activity among all.
Open Access
Molten salt assisted self-assembly: synthesis of mesoporous LiCoO2 and LiMn2O4 thin films and investigation of electrocatalytic water oxidation performance of lithium cobaltate
(Wiley-VCH Verlag, 2018) Saat, G.; Balci, F. M.; Alsaç, E. P.; Karadas, F.; Dağ, Ömer
Mesoporous thin films of transition metal lithiates (TML) belong to an important group of materials for the advancement of electrochemical systems. This study demonstrates a simple one pot method to synthesize the first examples of mesoporous LiCoO2 and LiMn2O4 thin films. Molten salt assisted self-assembly can be used to establish an easy route to produce mesoporous TML thin films. The salts (LiNO3 and [Co(H2O)6](NO3)2 or [Mn(H2O)4](NO3)2) and two surfactants (10-lauryl ether and cethyltrimethylammonium bromide (CTAB) or cethyltrimethylammonium nitrate (CTAN)) form stable liquid crystalline mesophases. The charged surfactant is needed for the assembly of the necessary amount of salt in the hydrophilic domains of the mesophase, which produces stable metal lithiate pore-walls upon calcination. The films have a large pore size with a high surface area that can be increased up to 82 m2 g−1. The method described can be adopted to synthesize other metal oxides and metal lithiates. The mesoporous thin films of LiCoO2 show promising performance as water oxidation catalysts under pH 7 and 14 conditions. The electrodes, prepared using CTAN as the cosurfactant, display the lowest overpotentials in the literature among other LiCoO2 systems, as low as 376 mV at 10 mA cm-2 and 282 mV at 1 mA cm-2.
Open Access
A Noble-Metal-Free Heterogeneous Photosensitizer-Relay Catalyst Triad That Catalyzes Water Oxidation under Visible Light
(Wiley-VCH Verlag, 2018) Ulusoy-Ghobadi, Türkan Gamze; Yıldız, E. A.; Büyüktemiz, M.; Akbari, S. Sadigh; Topkaya, D.; İşçi, Ü.; Dede, Y.; Yağlioğlu, H. G.; Karadaş, Ferdi
An entirely earth-abundant chromophore-relay water oxidation catalyst triad system, which is robust and efficient at neutral pH, is presented. The synthesis involves the coordination of a porphyrin derivative to a bridging Fe(CN)5 group, which is then reacted with Co ions to prepare a covalently linked chromophore-Prussian blue analogue assembly. Light-driven water oxidation studies in the presence of an electron scavenger indicate that the triad is active and it maintains a steady activity for at least three hours. Transient absorption experiments and computational studies reveal that the Fe(CN)5 group is more than a linker as it takes part in electron-transfer and co-operates with porphyrin in the charge separation process.
Open Access
One-dimensional copper (II) coordination polymer as an electrocatalyst for water oxidation
(Wiley-VCH Verlag, 2017) Mishra, R.; Ülker, E.; Karadas, F.
Although cobalt-based heterogeneous catalysts are the central focus in water oxidation research, interest in copper-based water oxidation catalysts has been growing thanks the great abundance of copper and its biological relevance. Several copper oxides have recently been reported to be active catalysts for water oxidation. In this study, a heterogeneous copper-based water oxidation catalyst that is not an oxide has been reported for the first time. Single-crystal XRD studies indicate that the compound is a one-dimensional coordination compound incorporating copper paddle-wheel units connected through phosphine dioxide ligands. The catalyst exhibits an onset potential of 372 mV at pH 10.2, whereas an overpotential of only 563 mV is required to produce a current density of 1 mA cm−2. In addition to cyclic voltammetric and chronoamperometric studies, an investigation into the effect of pH on the catalytic activity and the robustness of the catalyst using long-term bulk electrolysis (12 h) is presented.
Open Access
Precious metal‐free photocatalytic water oxidation by a layered double hydroxide‐Prussian blue analogue hybrid assembly
(Wiley-VCH Verlag, 2020-11) Akbari, Sina Sadigh; Karadaş, Ferdi
The development of earth‐abundant photocatalytic assemblies has been one of the bottlenecks for the advancement of scalable water splitting cells. In this study, a ZnCr layered double hydroxide and a CoFe Prussian blue analogue are combined to afford an earth‐abundant photocatalytic assembly involving a visible light‐absorbing semiconductor (SC) and a water oxidation catalyst (WOC). Compared to bare ZnCr‐LDH, the SC‐WOC hybrid assembly exhibits a threefold enhancement in photocatalytic activity, which is maintained for 6 h under photocatalytic conditions at pH 7. The band energy diagram was extracted from optical and electrochemical studies to elucidate the origin of the enhanced photocatalytic performance. This study marks a straightforward pathway to develop low‐cost and precious metal‐free assemblies for visible light‐driven water oxidation.
Open Access
The pursuit of an ideal coordination environment of the catalytic site for water splitting
(2022-07) Ahmad, Aliyu Aremu
The construction of catalysts from cheap materials and exquisite tuning of the coordination environment of the active site is pivotal to the development of a highly active sustainable water-splitting catalyst. Although recent years have seen tremendous growth in the application of Prussian Blue Analogues (PBAs) as non-noble catalysts for water splitting, the effect of the structural coordination of the active sites on the activity of a Prussian blue (PB) catalyst is yet to be explored. Herein, using two simple synthetic strategies, we show that manipulating the coordination environment of the catalytic sites affects the morphology, electronic properties, and eventually the catalytic activity of PBAs. Moreover, this study mimics natural photosynthesis by using solar light as an energy source. First, we demonstrate that the water oxidation activity and stability of a Co–Fe PBA can be tuned by coordinating bidentate capping ligands to the catalytic cobalt sites. Structural characterization studies reveal that the ligand decorated structures are of relatively lower dimensionality and they retained their network structures even after photocatalysis. Photocatalytic water oxidation studies indicate that coordination of one equivalent ligand group to the catalytic cobalt sites (CoL–Fe) results in an enhancement of about 50 times in upper-bound turnover frequency (TOF), while coordination of two equivalent ligand groups to the catalytic cobalt sites (CoL2–Fe) lead to an inactivity, which is attributed to the lack of coordination of water molecules to the catalytic sites. In addition, computational studies support experimental observation by showing that bidentate pyridyl groups enhance the susceptibility of the rate-determining Co(IV)-oxo species to the nucleophilic water attack during the critical O−O bond formation. We found in the second study that the replacement of [Fe(CN)6]3− unit with a square planar [Ni(CN)4]2− building block drastically changes the electronic environment and catalytic properties by converting the PB structure from 3D to a 2D layered structure, and we utilized it for the first time for photocatalytic hydrogen evolution reaction. We synthesized a 2D cyanide-coordination compound [Co–Ni] and performed a complete structural and morphological characterization that fully supports our synthetic claim. Relying on its exposed facets, layered morphology, and abundant surface-active sites, [Co–Ni] can efficiently convert water and sunlight to H2 in the presence of a ruthenium photosensitizer with an optimal evolution rate of 30,029 μmol g−1 h −1, greatly exceeding that of 3D PBA frameworks and top-ranked catalysts operating under the same condition. Furthermore, [Co–Ni] retains its structural integrity throughout a 6-hour photocatalytic cycle, which is confirmed by XPS, XRD and Infrared analysis. Overall, these two strategies signify the importance of the coordination environment of the active sites in exploiting structure/morphology and optimizing the activity of the catalyst.
Open Access
Pushing the limits in photosensitizer-catalyst interaction via a short cyanide bridge for water oxidation
(Cell Press, 2021-02-24) Ghobadi, Türkan Gamze Ulusoy; Ghobadi, Amir; Demirtaş, M.; Phul, Ruby; Yıldız, E. A.; Yağlıoğlu, H. G.; Durgun, Engin; Özbay, Ekmel; Karadaş, Ferdi
The realization of high-performance, precious-metal-free, stable, and robust photoanodes for water oxidation is one of the bottlenecks for dye-sensitized water splitting. Herein, we integrate an organic photosensitizer, which absorbs visible light above 500 nm, with a Prussian blue (PB) network to sensitize a visible-light-absorbing semiconductor, WO3. Through comprehensive steady-state and ultrafast transient absorption studies, we show that the coupling of a photosensitizer to a catalyst through a short cyanide bridging group in a PB structure generates appropriate energy levels for an efficient charge transfer from the photosensitizer to the visible-light-absorbing semiconductor. The photoanode retains its structural integrity and high photoelectrochemical activity for at least 2 h of solar irradiation under mildly acidic conditions (pH 3), which reaches around 1.30 mA/cm2 at 1.23 VRHE. This work provides a simple recipe with a toolbox that can be extended to a variety of organic photosensitizers and semiconductors.
Open Access
A robust, precious‐metal‐free dye‐sensitized photoanode for water oxidation: A nanosecond‐long excited‐state lifetime through a Prussian blue analogue
(Wiley-VCH Verlag, 2020-02) Ulusoy-Ghobadi, T. Gamze; Ghobadi, Amir; Büyüktemiz, M.; Akhüseyin Yıldız, E.; Yıldız, D. B.; Yağlıoğlu, H. G.; Dede, Y.; Özbay, Ekmel
Herein, we establish a simple synthetic strategy affording a heterogeneous, precious metal‐free, dye‐sensitized photoelectrode for water oxidation, which incorporates a Prussian blue (PB) structure for the sensitization of TiO2 and water oxidation catalysis. Our approach involves the use of a Fe(CN)5 bridging group not only as a cyanide precursor for the formation of a PB‐type structure but also as an electron shuttle between an organic chromophore and the catalytic center. The resulting hetero‐functional PB‐modified TiO2 electrode demonstrates a low‐cost and easy‐to‐construct photoanode, which exhibits favorable electron transfers with a remarkable excited state lifetime on the order of nanoseconds and an extended light absorption capacity of up to 500 nm. Our approach paves the way for a new family of precious metal‐free robust dye‐sensitized photoelectrodes for water oxidation, in which a variety of common organic chromophores can be employed in conjunction with CoFe PB structures.
Open Access
Tuning the electronic properties of prussian blue analogues for efficient water oxidation electrocatalysis: experimental and computational studies
(Wiley-VCH Verlag, 2018) Alsaç, Elif Pınar; Ülker, E.; Nune, Satya Vijaya Kumar; Dede, Y.; Karadaş, Ferdi
Although several Prussian Blue analogues (PBAs) have been investigated as water oxidation catalysts, the field lacks a comprehensive study that focuses on the design of the ideal PBA for this purpose. Here, members of a series of PBAs with different cyanide precursors have been investigated to study the effect of hexacyanometal groups on their electrocatalytic water oxidation activities. Cyclic voltammetric, chronoamperometric, and chronopotentiometric measurements have revealed a close relationship between the electron density of electroactive cobalt sites and electrocatalytic activity, which has also been confirmed by infrared and XPS studies. Furthermore, pH-dependent cyclic voltammetry and computational studies have been performed to gain insight into the catalytic mechanism and electronic structure of cyanide-based systems to identify possible intermediates and to assign the rate-determining step of the target process.
Open Access
Water oxidation electrocatalysis with a cobalt ‐ borate ‐ based hybrid system under neutral conditions
(Wiley-VCH Verlag, 2018) Turhan, Emine A.; Nune, Satya Vijaya Kumar; Ülker, E.; Şahin, U.; Dede, Y.; Karadaş, Ferdi
The development of new water oxidation electrocatalysts that are both stable and efficient, particularly in neutral conditions, holds great promise for overall water splitting. In this study, the electrocatalytic water oxidation performance of a new cobalt-based catalyst, Co3(BO3)2, with a Kotoite-type crystal structure is investigated under neutral conditions. The catalyst is also hybridized with CNTs to enhance its electrocatalytic properties. A remarkable increase in catalytic current along with a significant shift in the onset overpotential is observed in Co3(BO3)2@CNT. Additionally, CNT addition also greatly influences the surface concentration of the catalyst: 12.7 nmol cm−2 for Co3(BO3)2@CNT compared with 3.9 nmol cm−2 for Co3(BO3)2. Co3(BO3)2@CNT demands overpotentials of 303 and 487 mV to attain current densities of 1 and 10 mA cm−2, respectively, at pH 7. Electrochemical and characterization studies performed over varying pH conditions reveal that the catalyst retains its stability over a pH range of 3-14. Multi-reference quantum chemical calculations are performed to study the nature of the active cobalt sites and the effect of boron atoms on the activity of the cobalt ions.