Browsing by Author "Uzundal, Can Berk"

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Aksaray'da hurdacılık ve dökümcülük ekonomiye etkileri
(Bilkent University, 2016) Tuğrul, Ömer Faruk; Akalan, Melih; Tığ, Emre; Uzundal, Can Berk; Çörtü, Mahmut Yiğit
Open Access
Analysis of errors in zero-free-parameter modeling approach to predict the voltage of electrochemical energy storage systems under arbitrary load
(Electrochemical Society, 2017) Ulgut, Burak; Uzundal, Can Berk; Özdemir, Elif
In a recently published article (J. Electrochem. Soc. 164 (2017) A1274-A1280), we described a new method to predict the voltage response of electrochemical energy storage systems during arbitrary load profiles. Our work shows that the impedance spectrum can be employed in the frequency domain in order to ultimately calculate the time domain behavior of the electrochemical energy storage system. The big advantage of this method is the fact that there are no free parameters and fits throughout. The present work deals with the sources of error in the above-mentioned prediction approach and looks for the effects of the various sources of error. The current analysis concludes that two big contributors to the overall error are the inaccuracies in the DC part of the prediction and the non-linearities that are not modeled by a linear impedance spectrum. Discussions are also made regarding ways to improve the performance of the modeling approach the most and where future work is going to be looking to improve.
Open Access
Coarse-grained electrostatic model including ion-pairing equilibrium that explains DC and AC X-ray photoelectron spectroscopy measurements on ionic liquids
(American Chemical Society, 2019) Uzundal, Can Berk; Aydoğan-Göktürk, Pınar; Süzer, Şefik; Ülgüt, Burak
The dynamics of the electrochemical double layer in ionic liquids can be experimentally probed by a number of experimental techniques. Earlier, we reported on the results of an X-ray photoelectron spectroscopic investigation under applied square-wave bias of two distinct frequencies. Our studies had revealed counterintuitive properties involving the physical and temporal progression of the effect of the electrochemical double layer that could not be modeled with conventional equivalent-circuit approaches. Herein, we present a new coarse-grained modeling methodology that accounts for particle diffusion, migration, and ion-association equilibrium. Our model is computationally efficient enough to be used to predict and match the results at extended time scales and distances of the experiment. Modeling efforts predict that a crucial component of the behavior is controlled by the ion-pairing equilibrium, an often overlooked aspect of ionic liquids.
Open Access
Electrochemical investigations of ion transport in unconventional environments
(2018-07) Uzundal, Can Berk
Ion transport is a critical process that occurs routinely in all electrochemical devices. Especially the new generation batteries extensively utilized in portable devices, Li-Ion batteries, operate simply through Li+ transport. Although, a multitude of similar energy storage and conversion devices are prevalent in both the industry and the electrochemical literature, these devices are not the only technologies that ion transport is critical. Other technologies such as electrochromic devices, organic electronics and next generation display panels also involve electrochemical processes that are inherently dependent on or limited by ion transport. Conventional electrochemical methods enable interrogation of the various interfaces these devices possess however limited information on the behavior of the concentrated-like media investigated in this thesis compared to the well documented behavior of the dilute media, limits their applicability. In this thesis we not only offer investigations on possible esoteric materials that have unique ion transport properties that show emergent behavior in the systems they are employed at through fundamental electrochemical studies but also we outline the development of new electroanalytical tools to better interrogate ion transport in variety of electrochemical systems. The attempts at understanding and explaining the ion transport in various media and to leverage its benefits towards better devices is outlined. Electrochemical noise measurements for Li-Ion batteries is shown as a potential tool for ion transport interrogation, while the ion transport behavior of Liquid Crystals and Ionic Liquids are investigated. A new electroanalytical tool to separate counter ion diffusion from electron transfer is also shown. Finally, an electrochemical method to visualize corrosion that utilizes ion transport is outlined in this thesis.
Open Access
Linear and nonlinear electrochemical impedance spectroscopy studies of Li/SOCl2 batteries
(Electrochemical Society Inc., 2019) Zabara, Mohammed Ahmed; Uzundal, Can Berk; Ülgüt, Burak
Performing accurate, linear and stable Electrochemical Impedance Spectroscopy of Li/SOCl2 based batteries (and more generally, all primary batteries) is challenging due to the lack of a (well-defined) charging reaction. The difficulty stems from irreversible operation chemistry (compromising stable operation) and the inconsistent anode passivation of the cell. The very scarce literature examples lack proper measurement protocol and accurate EIS data for Li/SOCl2 that can be modeled. In this study, we demonstrate how these challenges can be overcome by performing Galvanostatic-EIS in discharge mode and investigate the details of how experimental parameters influence not only the measurement, but also the cell itself. We present linear and stable data that is compatible with the Kramers-Kronig relations in frequency ranges as wide as 10 kHz to 1 mHz for fully charged to fully discharged cells. Moreover, we utilize Harmonic Analysis to study the nonlinearities in the measurement and further show that the passivation of the anode is a major cause for the nonlinearities.
Open Access
A method for voltage noise measurement and ıts application to primary batteries
(ECS, 2018) Ülgüt, Burak; Uzundal, Can Berk
Though proven effective in localized corrosion studies, electrochemical noise measurements in batteries with Lithium based chemistries suffer from lack of well–defined measurement and analysis methods. The high capacitance electrodes made out of highly porous materials requires noise measurements to be extremely precise since the small charge due to stochastic events leading to electrochemical noise leads to very small voltage changes due to the large capacitance. Typically, the required precision is achieved by high gain after the offset is corrected. In this article, we are introducing a new offset correction scheme that mitigates the negative effects of electronic offset reduction methods. Using this new offset correction scheme we report the measurement of the otherwise elusive voltage noise of primary Li batteries.
Open Access
Performance modeling of unmanaged hybrid battery/supercapacitor energy storage systems
(Elsevier, 2021-09-13) Zabara, Mohammed Ahmed; Uzundal, Can Berk; Ülgüt, Burak
Unmanaged hybrid battery/supercapacitor energy storage systems possess higher performance with lower cost and complexity compared to not only individual cells, but also electronically managed hybrid systems. Achieving full performance requires the understanding of the power distribution and predicting their best combinations. In this work, a semi-empirical modeling methodology is presented that can predict the current distribution and the voltage response of battery/supercapacitor hybrid systems under arbitrary charge/discharge profiles. Results are presented for the assessment of hybrid systems under real life scenarios. The key strength of the presented method is that it is free of any parametrization, fits or subjective inputs. The modeling methodology is validated with experimental measurements for two different Li-ion battery chemistries, namely Lithium Iron Phosphate and Lithium Vanadium Pentoxide, connected in parallel to wide range of supercapacitors. Finally, we outline several design rules for hybrid storage systems for different use cases.
Open Access
Synthesis and water oxidation electrocatalytic and electrochromic behaviours of mesoporous nickel oxide thin film electrodes
(Royal Society of Chemistry, 2019) Amirzhanova, Assel; Karakaya, Irmak; Uzundal, Can Berk; Karaoğlu, Gözde; Karadaş, Ferdi; Ülgüt, Burak; Dağ, Ömer
Nickel oxide (NiO) thin film with a high surface area is an important material in electrochemical applications. Spin coating of a preformed clear solution of ethanol, nickel nitrate, and surfactants (C12H25(OCH2CH2)10OH, C12E10 and C16H33N(CH3)3Br, CTAB) over a substrate produces a lyotropic liquid crystalline phase that is calcined into a transparent mesoporous NiO (m-NiO) thin film with a uniform film morphology and high porosity and surface area. m-NiO, coated over FTO glass, can be used as an electrode in an electrochromic device and electrocatalytic water oxidation processes with excellent efficiency (Tafel slope of 62 mV dec−1 and a low overpotential of 0.200 V at 1 mA cm−2 current density). Repetitive cyclic voltammetry and chronopotentiometry measurements convert the NiO pore walls into a NiO/Ni(OH)2 core–shell structure. In the water oxidation process, various Ni2+ surface species on the pore-walls are oxidized to Ni3+ and further to Ni4+ electrocatalytic active species. In an electrochromic device, the oxidation of the Ni2+ species to Ni3+ species reversibly contributes to the electrochromic behaviour, but if the electrochromic device is run at more positive potentials (such as those beyond water oxidation potentials), then electrochromic switching takes place between the Ni(OH)2 and NiOOH surface species.
Open Access
Uneven discharge of metallic lithium causes increased voltage noise in Li/MnO2 primary batteries upon shorting
(IOP Publishing, 2020) Karaoğlu, Gözde; Uzundal, Can Berk; Burak, Ülgüt
The voltage noise of primary batteries with Li/MnO2 chemistry increases when the battery suffers a short circuit. This noise level is measurable only after a proper offsetting scheme that involves a battery that has undergone the same history is employed. In the current work, we are showing that the increased voltage noise is due to the metallic lithium anode and the heterogeneous discharge thereof. Periods of short circuit causes the lithium to get depleted from points of lowest resistance, which eventually causes uneven depletion and the current collector to be partially exposed. This is similar in nature to pitting corrosion of metals and their protective films, a phenomenon which is also routinely characterized by electrochemical noise signals.
Open Access
X-ray photoelectron spectroscopy with electrical modulation can be used to probe electrical properties of liquids and their interfaces at different stages
(American Chemical Society, 2019) Uzundal, Can Berk; Şahin, Özgür; Aydoğan-Göktürk, Pınar; Wu, H.; Mugele, F.; Ülgüt, Burak; Süzer, Şefik
Operando X-ray photoelectron spectroscopy (o-XPS) has been used to record the binding energy shifts in the C 1s peak of a pristine poly(ethylene glycol) (PEG) liquid drop in an electrowetting on dielectric (EWOD) geometry and after exposing it to several high-voltage breakdown processes. This was achieved by recording XPS data while the samples were subjected to 10 V dc and ac (square-wave modulation) actuations to extract electrical information related to the liquid and its interface with the dielectric. Through analysis of the XPS data under ac actuation, a critical frequency of 170 Hz is extracted for the pristine PEG, which is translated to a resistance value of 14 MΩ for the liquid and a capacitance value of 60 pF for the dielectric, by the help of simulations using an equivalent circuit model and also by XPS analyses of a mimicking device under similar conditions. The same measurements yield an increased value of 23 MΩ for the resistance of the liquid after the breakdown by assuming that the capacitance of the dielectric stays constant. In addition, an asymmetry in polarity dependence is observed with respect to both the onset of the breakdown voltage and also the leakage behavior of the deteriorated (PEG + dielectric) system such that deviations are more pronounced at positive voltages. Both dc and ac behaviors of the postbreakdown system can also be simulated, but only by introducing an additional element, a diode or a polarity- and magnitude-dependent voltage source (VCVS), which might be attributed to negative charge accumulation at the interface. Measurements for a liquid mixture of PEG with 8% ionic liquid yields an almost 2 orders of magnitude smaller resistance for the drop as a result of the enhanced conductivity by the ions. Coupled with modeling, XPS measurements under dc and ac modulations enable probing unique electrochemical properties of liquid/solid interfaces.