Copper Bipyridyl Redox Mediators for High Performance Dye-Sensitized Solar Cells

Photovoltaic (PV) technologies attract a lot of attention as a result of the increasing energy demand and environmental concerns stemming from the conventional energy resources. Accordingly, the power conversion efficiency (PCE) values of various solar energy harvesting units continue to increase, targeting the Shockley-Queisser limit. However, the commercialization and integration of these photovoltaic devices in various applications depend on several criterions such as cost, ease of fabrication and reliability of the materials employed. Regarding their cost-effective and multifarious manufacturing possibilities, and short energy-payback times we can foresee dye-sensitized solar cells (DSCs) as proper candidates to become widespread power-supply devices. In a DSC system, the open circuit potential (VOC) is determined as the difference between the quasi-Fermi level of the semiconductor and redox potential of the electrolyte. By employing redox mediators with more positive redox potentials, the dye regeneration overpotentials can be reduced and VOC outputs can be increased. During the course of this PhD study, we investigated Cu(II)/Cu(I) coordination complexes with bipyridine ligands holding methyl groups on the 6,6 positions as redox mediators in DSCs. As revealed by DFT calculations, steric hindrance of the methyl groups provided proper geometries (tetrahedral for Cu(I) and distorted tetragonal for Cu(II)) for minimizing the reorganization energy for electron transfer. Thus, successful dye regeneration even with 0.1eV driving force became feasible without compromising photocurrent densities. We achieved high photovoltages of over 1.0 V and PCEs over 10%, using the organic Y123 dye under 1000 Wm-2 AM1.5G illumination by the series of copper complexes. After this initial study, we concentrated on the complications associated with the soft nature and the instability of the coordination sphere of copper species. Firstly, we showed that the Cu(II) species synthesized by CuCl2 precursor exhibit different electrochemical behaviors in comparison to Cu(I) counterparts. Hence, we proposed three procedures that leads to neat Cu(II) species: chemical oxidation of Cu(I), electrochemical oxidation and changing our precursor to Cu(TFSI)2. Secondly, we studied the effect of the 4-tertbutyl pyridine and similar base additives in the electrolyte medium. With bases, the coordination sphere and the geometry of the complexes were different in comparison to the ones presumed without a base. We studied the effects of this occurrence to the dye regeneration and charge recombination reaction rates in detail in reference to the Marcus Theory. We demonstrated [Cu(tmby)2]2+/1+(tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine), as an effective hole transport material (HTM) in solid-state DSCs (ssDSCs). With this HTM, we circumvent the pore filling problems occurring in conventional ssDSCs. Therefore, we could use thicker TiO2 films to achieve better light harvesting efficiencies. Furthermore, for the same redox mediator we demonstrated that the DSCs show remarkable PCE values at ambient lighting. Under illumination from a fluorescent light tube (1000 lux) we achieved PCE values of 28.9%. All our findings indicate that the DSCs employing copper complexes can be utilized as power sources for low capacity electronics (such as portable devices and wireless sensor networks) and open up a new industrialization path for this scientifically well-established technology

Electrochemical Properties of Cu(II/I)-Based Redox Mediators for Dye-Sensitized Solar Cells

Three Cu(II/I)-phenanthroline and Cu(II/I)-bipyridine redox mediators are studied on various electrodes and in variety of electrolyte solutions using cyclic voltammetry and impedance spectroscopy on symmetrical dummy cells. Graphene-based catalysts provide comparably high activity to PEDOT, and both catalysts outperform the activity of platinum. The charge-transfer kinetics and the diffusion rate significantly slowdown in the presence 4-tert-butylpyridine. This effect is specific only for Cu-mediators (is missing for Co-mediators), and is ascribed to a sensitivity of the coordination sphere of the Cu(II)-species to structural and substitutional changes. The 'Zombie Cells' made from symmetrical PEDOT/PEDOT devices exhibit enhanced charge-transfer rate and enhanced diffusion resistance. Electrochemically clean Cu(II)-bipyridine species are prepared, for the first time, by electrochemical oxidation of the parent Cu(I) complexes. Our preparative electrolysis brings numerous advantages over the standard chemical syntheses of the Cu(II)-bipyridine complexes. The superior performance of electrochemically grown clean Cu(II)-bipyridine complex is demonstrated on practical dye-sensitized solar cells. (C) 2016 Elsevier Ltd. All rights reserved

Dye-sensitized solar cells for efficient power generation under ambient lighting

Solar cells that operate efficiently under indoor lighting are of great practical interest as they can serve as electric power sources for portable electronics and devices for wireless sensor networks or the Internet of Things. Here, we demonstrate a dye-sensitized solar cell (DSC) that achieves very high power-conversion efficiencies (PCEs) under ambient light conditions. Our photosystem combines two judiciously designed sensitizers, coded D35 and XY1, with the copper complex Cu(II/I)(tmby) as a redox shuttle (tmby, 4,4′,6,6′-tetramethyl-2,2′-bipyridine), and features a high open-circuit photovoltage of 1.1 V. The DSC achieves an external quantum efficiency for photocurrent generation that exceeds 90% across the whole visible domain from 400 to 650 nm, and achieves power outputs of 15.6 and 88.5 mW cm–2 at 200 and 1,000 lux, respectively, under illumination from a model Osram 930 warm-white fluorescent light tube. This translates into a PCE of 28.9%

Novel Blue Organic Dye for Dye-Sensitized Solar Cells Achieving High Efficiency in Cobalt-Based Electrolytes and by Co-Sensitization

Blue and green dyes as well as NIR-absorbing dyes have attracted great interest because of their excellent ability of absorbing the incident photons in the red and near-infrared range region. A novel blue D-pi-A dye (Dyenamo Blue), based on the diketopyrrolopyrrole (DPP)-core, has been designed and synthesized. Assembled with the cobalt bipyridine-based electrolytes, the device with Dyenamo Blue achieved a satisfying efficiency of 7.3% under one sun (AM1.5 G). The co-sensitization strategy was further applied on this blue organic dye together with a red D-pi-A dye (D35). The successful co-sensitization outperformed a panchromatic light absorption and improved the photocurrent density; this in addition to the open-circuit potential result in an efficiency of 8.7%. The extended absorption of the sensitization and the slower recombination reaction between the blue dye and TiO2 surface inhibited by the additional red sensitizer could be the two main reasons for the higher performance. In conclusion, from the results, the highly efficient cobalt-based DSSCs could be achieved with the co-sensitization between red and blue D-pi-A organic dyes with a proper design, which showed us the possibility of applying this strategy for future high-performance solar cells

Magnetic resonance imaging based kidney volume assessment for risk stratification in pediatric autosomal dominant polycystic kidney disease

IntroductionIn the pediatric context, most children with autosomal dominant polycystic kidney disease (ADPKD) maintain a normal glomerular filtration rate (GFR) despite underlying structural kidney damage, highlighting the critical need for early intervention and predictive markers. Due to the inverse relationship between kidney volume and kidney function, risk assessments have been presented on the basis of kidney volume. The aim of this study was to use magnetic resonance imaging (MRI)-based kidney volume assessment for risk stratification in pediatric ADPKD and to investigate clinical and genetic differences among risk groups.MethodsThis multicenter, cross-sectional, and case-control study included 75 genetically confirmed pediatric ADPKD patients (5–18 years) and 27 controls. Kidney function was assessed by eGFR calculated from serum creatinine and cystatin C using the CKiD-U25 equation. Blood pressure was assessed by both office and 24-hour ambulatory measurements. Kidney volume was calculated from MRI using the stereological method. Total kidney volume was adjusted for the height (htTKV). Patients were stratified from A to E classes according to the Leuven Imaging Classification (LIC) using MRI-derived htTKV.ResultsMedian (Q1-Q3) age of the patients was 6.0 (2.0–10.0) years, 56% were male. There were no differences in sex, age, height-SDS, or GFR between the patient and control groups. Of the patients, 89% had PKD1 and 11% had PKD2 mutations. Non-missense mutations were 73% in PKD1 and 75% in PKD2. Twenty patients (27%) had hypertension based on ABPM. Median htTKV of the patients was significantly higher than controls (141 vs. 117 ml/m, p = 0.0003). LIC stratification revealed Classes A (38.7%), B (28%), C (24%), and D + E (9.3%). All children in class D + E and 94% in class C had PKD1 variants. Class D + E patients had significantly higher blood pressure values and hypertension compared to other classes (p > 0.05 for all).DiscussionThis study distinguishes itself by using MRI-based measurements of kidney volume to stratify pediatric ADPKD patients into specific risk groups. It is important to note that PKD1 mutation and elevated blood pressure were higher in the high-risk groups stratified by age and kidney volume. Our results need to be confirmed in further studies

Development and modeling for process control purposes in PEMs

To maintain suitable operating conditions, polymer electrolyte membrane (PEM) fuel cell stacks require additional equipment and control systems. Fuel supply, power and thermal management, purge strategy and individual cell voltage control must be in place and operate reliably for a fuel cell system to achieve similar levels of performance as conventional energy generators. System design, auxiliary equipment selection and selection of control strategies have effects on fuel cell efficiency, durability and reliability. In this study we report on our efforts to develop the piping and instrumentation diagram of a 3 kW PEM fuel cell, including the control instrumentation. A semi-empirical model was put together to understand dynamic system behavior for purpose of evaluating possible operating scenarios, in an effort to have useful insight into the system during the equipment selection stage. The model complexity was reduced by ignoring the spatial variations and assuming isothermal stack operation. The stack, cooling system, humidifier, compressor, inlet and outlet manifold were modeled and integrated to formulate a comprehensive prototype model. This model was subsequently used to generate predictions for the responses of the compressor, humidifier, humidification of the stack, power and heat generation for a multitude of dynamic changes in load. With the predictive capability enabled by the model, equipment and algorithm selections can be made in a more directed fashion, reducing the initial design and development costs by delivering a hardware configuration that is close to an ideal one with minimal iterations. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

Model based temperature controller development for water cooled PEM fuel cell systems

PEM (proton exchange membrane) fuel cell operation necessitates thermal management to satisfy the requirements of safe and efficient operation by keeping the temperature within a certain range independent of varying load conditions. Heat generation within the fuel cell changes according to the power delivered from the stack, requiring a dynamic control system to remove this excess heat and maintain the desired stack temperature. In this study, a closed loop water circulation strategy is considered and evaluated for cooling a 3 kW PEM fuel cell. The cooling system consists of a water circulation pump and a radiator coupled to a fan, integrated with the fuel cell stack. A first principles based model is developed for the integrated cooling system through an energy balance containing the relevant terms. A sequence of dynamic tests is performed on the cooling system to identify the parameters appearing in the model developed. The resulting semi-empirical model is used to evaluate possible control strategies managing the cooling loop. Three specific strategies are analyzed and the performances of these controllers are evaluated in terms of stack temperature, integral time weighted absolute error (ITAE) and the parasitic energy requirements. Minimizing fan usage with an on/off controller while keeping the pump voltage as a continuously manipulated variable through a feedback PI (proportional integral) controller delivers the best results. The MATLAB-SIMULINI (R) platform is used in the development and implementation of the models and controllers. In our strategy, the characterization of the cooling loop is physically de-coupled from the development of the fuel cell stack - allowing for the evaluation of candidate equipment and algorithms prior to the fuel cell stack being available, which is often the case during prototype development. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

Does long-term talc exposure have a carcinogenic effect the female genital system of rats? An experimental pilot study

WOS: 000271197900010PubMed ID: 19301023Objective In several studies, the prolonged exposure to talc has been associated with development of ovarian cancer. However, some studies have advocated contrary views. The present study aims to investigate histopathological changes and whether long-term talc exposure is associated with potential carcinogenic effects on the female genital organs of rats. Materials and methods The present study was conducted at Dumlupinar University Medical Faculty and a total of 28 Sprague-Dawley rats were included. The experimental animals were allocated into four groups having seven rats each. Groups 1 and 2 served as controls, where the rats in Group 1 did not receive any intervention and Group 2 received intravaginal saline. Groups 3 and 4 received intravaginal or perineal talc application, respectively. Talc was applied for 3 months on a daily basis. Histopathological changes in the peritoneum and female genital system were evaluated. For statistical analyses, Fisher's exact test was carried out using SPSS. Findings In both the groups exposed to talc (Groups 3 and 4), evidence of foreign body reaction and infection, along with an increase in inflammatory cells, were found in all the genital tissues. Genital infection was observed in 12 rats in the study group and 2 rats in the control group. Neoplastic change was not found. However, there was an increase in the number of follicles in animals exposed to talc. No peritoneal change was observed. In the groups not exposed to talc, similar infectious findings were found, but there was a statistically significant difference between the groups (Groups 1 and 2 vs. Groups 3 and 4, P > 0.05). Neoplastic change was also not observed in these groups. Four groups were compared in terms of neoplastic effects and infections. In Groups 1, 5 rats were normal, two developed vulvovaginitis and endometritis with overinfection (in both ovaries), and one developed salpingitis (in both fallopian tubes), that is, infection was found in a total of two rats. In Group 2, only one experimental animal had endometritis. All the animals in Groups 3 and 4 developed infections. Conclusions Talc has unfavorable effects on the female genital system. However, this effect is in the form of foreign body reaction and infection, rather than being neoplastic

Does preoperative neutrophil-lymphocyte ratio indicate postoperative morbidity after repair of tetralogy of Fallot?

WOS: 000385271000004Background: In this study, we aimed to investigate the impact of preoperative neutrophil-lymphocyte ratio on postoperative morbidity after repair of tetralogy of Fallot. Methods: In this retrospective study, 51 patients (30 males, 21 females; median age 2 years; 1-3. quartiles 1.19 to 3.65 years) who underwent tetralogy of Fallot repair between August 2011 and October 2013 were included. The study population was divided into two groups: group 1 including patients without morbidity and group 2 including patients with morbidity. Results: No mortality was observed in the study population. There was no significant difference in the median of preoperative neutrophil-lymphocyte ratio between the groups [0.87 (0.68-1.29) in group 1 vs. 0.75 (0.44-1.10) in group 2; p= 0.094]. Vasoactive inotrope score, duration of intubation, and mediastinal drainage index were significantly higher in group 2 (p= 0.014, p= 0.001 and p= 0.003, respectively). While preoperative and postoperative C-reactive protein were positively correlated with the corresponding neutrophil-lymphocyte ratio (p< 0.01, p< 0.01, respectively), preoperative neutrophil-lymphocyte ratio was not correlated with postoperative C-reactive protein (p= 0.701). Conclusion: Our study results showed that preoperative neutrophillymphocyte ratio can be a good predictor of instantaneous inflammatory status, although there is no significant relationship between preoperative neutrophil-lymphocyte ratio and morbidity after complete repair of tetralogy of Fallot