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Effects of N-functional groups on the electron transfer kinetics of VO2+/VO2+ at carbon: Decoupling morphology from chemical effects using model systems

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... 12,13 Although the mechanism is not fully understood, it is believed that the electron-donor properties of nitrogen induce faradaic reactions through its lone electron pair, which increase the pseudocapacitance, 14 therefore adding to the capacitance produced from the electrochemically accessible surface area (ECSA). 13,15 Heteroatoms also improve the charge transfer kinetics of common aqueous redox couples on carbon surfaces. Recent studies, both at an experimental and computational level, suggest that adding heteroatoms on the carbon surface increases the number of active adsorption sites due to charge delocalisation, enhancing then the electron transfer kinetics of the V(III)/V(II) pair, 16 which increases the exchange current density of both V(III)/V(II) and V(IV)/V(V) couples in all-vanadium redox flow batteries (VRFB). ...
... Recent studies, both at an experimental and computational level, suggest that adding heteroatoms on the carbon surface increases the number of active adsorption sites due to charge delocalisation, enhancing then the electron transfer kinetics of the V(III)/V(II) pair, 16 which increases the exchange current density of both V(III)/V(II) and V(IV)/V(V) couples in all-vanadium redox flow batteries (VRFB). 15 Biobased carbon materials, i.e., derived from biomass that is already rich in heteroatoms, make great candidates for newgeneration electrodes. 14,15,17 The use of oxygen-and nitrogendoped biobased carbons, such as hydrochars and biochars, coupled to aqueous electrolytes are a promising avenue to develop low-cost and high-performance devices. ...
... 15 Biobased carbon materials, i.e., derived from biomass that is already rich in heteroatoms, make great candidates for newgeneration electrodes. 14,15,17 The use of oxygen-and nitrogendoped biobased carbons, such as hydrochars and biochars, coupled to aqueous electrolytes are a promising avenue to develop low-cost and high-performance devices. Hydrochar and biochar are carbon-rich solid materials predominantly produced from hydrothermal carbonisation (HTC) or pyrolysis of biomass, respectively. ...
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Low-cost and high performance electrodes are critical to advance electrochemical energy storage devices that decouple energy supply from demand. At their core, carbon is ubiquitously employed given its availability, chemical and electrochemical stability, electrical conductivity, and affordable cost. However, due to their relative inertness, carbonaceous electrodes suffer from limited wettability and kinetic activity with aqueous electrolytes. A common approach is to introduce heteroatoms, either through post-processing (thermal/acid activation) or by employing different precursors. Specifically, biobased carbons like hydrochar and biochar are rich in heteroatoms that are naturally incorporated through the production process into the electrode structure. However, achieving a fundamental understanding of the interactions between metal ions and carbon surfaces has proven elusive, leading researchers to rely on empirical approaches for heteroatom doping of carbons. To achieve a better understanding of the fundamental mechanisms, we performed density functional theory calculations of a commonly employed iron redox couple, Fe(iii) and Fe(ii). We investigated binding mechanisms in graphitic carbon model systems with different surface features, and explored the effect of nitrogen doping and surface topology on the binding energy, as well as the effect of ions' spin multiplicity in the carbon-metal coordination mechanisms. Our results suggest that the interactions of Fe(iii) and Fe(ii) ions with the nitrogen-doped carbon electrodes not only depend on the surface curvature or the nitrogen content and functionality, but also on the spin multiplicity of the metal ion. Iron ions always evolve into an open-shell electronic structure with a high number of unpaired electrons to increase their coordination sphere with the graphitic surface. We hope that our findings can assist the development of fit-for-purpose heteroatom-doped carbon electrodes with a tailored nanostructure for electrochemical devices utilizing the Fe(iii)/Fe(ii) redox couple.
... Figure 3c shows the best-fit of the N 1s spectrum which was obtained using three components attributed to graphitic-N (400.9 eV), pyrrolic-N (400.0 eV) and pyridinic-N (398.0 eV). [79,80] The relative contributions of these three functionalities are reported in Table 1 and indicate that the majority of the nitrogen is present in the form of pyrrolic/pyridinic groups. It was not possible to resolve any contributions at 397 eV expected for Fe-N species; [75] this is likely due to their relative contributions being much smaller than those arising from Nfunctionalities in the carbon scaffold. ...
... An interesting aspect of the Fe@C : N electrocatalyst architecture is that the rich library of surface functionalization strategies applicable to carbon nanomaterials is available to fine tune the binding strengths of substrate and intermediates, as previously demonstrated in our group for other organic and inorganic redox processes. [55,80,95] ...
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Carbon porous materials containing nitrogen functionalities and encapsulated iron‐based active sites have been suggested as electrocatalysts for energy conversion, however their applications to the hydrogenation of organic substrates via electrocatalytic hydrogenation (ECH) remain unexplored. Herein, we report on a Fe@C:N material synthesized with an adapted annealing procedure and tested as electrocatalyst for the hydrogenation of benzaldehyde. Using different concentrations of the organic, and electrolysis coupled to gas chromatography experiments, we demonstrate that it is possible to use such architectures for the ECH of unsaturated organics. Potential control experiments show that ECH faradaic efficiencies >70 % are possible in acid electrolytes, while maintaining selectivity for the alcohol over the pinacol dimerization product. Estimates of product formation rates and turnover frequency (TOF) values suggest that these carbon‐encapsulated architectures can achieve competitive performance in acid electrolytes relative to both base and precious metal electrodes.
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Chapter
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Redox flow batteries are a promising electrochemical technology for energy-intensive grid storage applications, but further cost reductions are needed for universal adoption. As porous electrodes are responsible for functions within the flow cell that impact charge transfer, ohmics, and mass transport, improvements in electrode materials and design may yield significant performance and economic benefits. This mini-review summarizes recent developments in the design and characterization of porous electrodes with a focus on understanding and controlling both the microstructure and surface chemistry, which broadly align with mass transport and reaction kinetics. Key opportunities and challenges in the science and engineering of these materials are also presented with the goal of engaging the broader community and accelerating progress towards chemistry-specific flow battery electrodes.
Article
Metal-free nitrogenated amorphous carbon electrodes were synthesised via dc plasma magnetron sputtering and post-deposition annealing at different temperatures. The electrocatalytic activity of the electrodes towards the oxygen reduction reaction (ORR) was studied as a function of pH using cyclic voltammetry with a rotating disk electrode. The trends in onset potential were correlated to the carbon nanostructure and chemical composition of the electrodes as determined via Raman spectroscopy and X-ray photoelectron spectroscopy analysis. Results suggest that: 1) the ORR activity in acidic conditions is strongly correlated to the concentration of pyridinic nitrogen sites. 2) At high pH, the presence of graphitic nitrogen sites and a graphitized carbon scaffold are the strongest predictors of high ORR onsets, while pyridinic nitrogen site density does not correlate to ORR activity. An inversion region where pyridine-mediated activity competes with graphitic-N mediated activity is identified in the pH region close to the value of pKa of the pyridinium cation. The onset of the ORR is therefore determined by the activity of different sites as a function of pH and evidence for distinct reduction reaction pathways emerges from these results.
Article
Vanadium redox flow battery(VRFB) is one of the most promising large- scale energy storage system; however, a wide spread VRFB development is still limited by the poor electrochemical activity of graphite electrodes and a poor understanding of redox reactions occurring at electrode /electrolyte interface. In this work, DFT was performed to study the first solvation shell structure of all vanadium ions and to investigate the reactivity of modified graphite electrodes toward the V2+/V3+ redox species.The results suggest that the presence of oxygen and nitrogen functionalities at the electrode edges provides more active sites for adsorption of the V2+/V3+ redox couple, and therefore improve electron transfer kinetics. These results have been experimentally validated by means of Cyclic Voltammetry and Electrochemical Impedance Spectroscopy with carbon black electrode having different density of oxygen and nitrogen-containing surface groups
Article
Vanadium redox flow battery (VRFB) is one of the most promising large-scale energy storage system; however, a widespread VRFB development is still limited by the poor electrochemical activity of graphite electrodes and a poor understanding of redox reactions occurring at electrode/electrolyte interface. In this work, DFT was performed to study the first solvation shell structure of all vanadium ions and to investigate the reactivity of modified graphite electrodes toward the V2+/V3+ redox species. The results suggest that the presence of oxygen and nitrogen functionalities at the electrode edges provides more active sites for adsorption of the V2+/V3+ redox couple, and therefore improve electron transfer kinetics. These results have been experimentally validated by means of Cyclic Voltammetry and Electrochemical. Impedance Spectroscopy with carbon black electrode having different density of oxygen and nitrogen-containing surface groups.
Article
In order to quantitatively investigate the kinetic performance and the pore size distribution of carbon felt electrodes for the application in vanadium redox flow batteries, the theory of cyclic voltammetry (CV) is derived for a random network of cylindrical microelectrodes on the base of convolutive modeling. In this context we present an algorithm based on the use of a modified Talbot contour for inverse Laplace transformation, providing the mass transfer functions required for the calculation of the CV responses in external cylindrical finite diffusion space. First order homogenous chemical kinetics preceding and/or following the electrochemical reactions are implemented in this algorithm as well. The VO2+ oxidation is investigated as model reaction at pristine and electrochemically aged commercial carbon felt electrodes. A fit of simulated data to experimental data clearly shows that an electrochemical aging predominantly affects the kinetics of the electron transfer reaction and that internal electrode surfaces and pore size distributions remain constant. The estimated pore size distributions are in excellent agreement with porosimetry measurements, validating our theory and providing a new strategy to determine electrode porosities and electrode kinetics simultaneously via CV.
Article
Abnormal levels of the neurotransmitter dopamine have been linked to a variety of neurochemical disorders including depression and Parkinson's disease. Dopamine concentrations are often quantified electrochemically using biosensors prepared from carbon electrode materials such as carbon paste or glassy carbon. The charge transfer kinetics of dopamine is highly sensitive to carbon surface termination, including the presence of certain oxygen functional groups and adsorption sites. However, the nature of the binding sites and the effects of surface oxidation on the voltammetry of dopamine are both poorly understood. In this work the electrochemical response of dopamine at glassy carbon model surfaces was investigated to understand the effects of altering both the carbon nanostructure and oxygen surface chemistry on dopamine charge transfer kinetics and adsorption. Glassy carbon electrodes with low oxygen content and a high degree of surface graphitisation were prepared via thermal annealing at 900 °C, whilst highly oxidised glassy carbon electrodes were obtained through electrochemical anodisation at 1.8 V vs Ag/AgCl. The carbon surface structure and composition in each case was studied via X-Ray Photoelectron Spectroscopy. Voltammetry in solutions of dopamine at acidic pH confirmed that both annealing and anodisation treatments result in carbon surfaces with rapid charge transfer kinetics. However, dopamine adsorption occurs only at the low-oxygen, highly-graphitized carbon surface. Density functional theory studies on graphene model surfaces reveal that this behaviour is due to non-covalent interactions between the π-system of dopamine and the basal sites in the annealed surface. Simulations also show that the introduction of oxygen moieties disrupt these interactions and inhibit dopamine adsorption, in agreement with experiments. The results clarify the role of oxygen moieties and basal plane sites in facilitating both the adsorption of and charge transfer to DA at carbon electrodes.
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Vanadium redox flow batteries (VRFBs) have been highlighted for use in energy storage systems. In spite of the many studies on the redox reaction of vanadium ions, the mechanisms for positive and negative electrode reaction are under debate. In this work, we conduct an impedance analysis for positive and negative symmetric cells with untreated and heat-treated carbon felt (CF) electrodes to identify the reaction mechanisms. The negative electrode reaction (V²⁺/V³⁺) is highly dependent on the heat treatment and reaction temperature, which is a feature of an inner-sphere mechanism, whereas the positive electrode reaction (VO2⁺/VO²⁺) reaction is rather insensitive to the heat treatment and reaction temperature, suggesting an outer-sphere mechanism. An atomistic molecular dynamics simulation suggests that the different mechanisms are quite feasible considering the difference in the structure of the hydration shell for the vanadium ions. The deeper understanding of the reaction mechanism and its influence on cell performance will be helpful to advance VRFBs.
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An issue that limits the large-scale application of vanadium redox flow batteries (VRFBs) is their low power density, which is associated with the slow reaction kinetics of vanadium redox couples. To enhance the activities of the electrode toward vanadium redox couples, modifying carbon electrode surfaces with heteroatom doping is an effective strategy. In this work, we investigate the catalytic activity of nitrogen (N), boron (B) and phosphorus (P) doped graphite electrodes for VRFBs via density functional theory calculations. A layer of graphene is adopted to represent the surface of a graphite electrode. It is found that water adsorption is stronger with hydrogenated pyridinic N-doped and pyrrolic N-doped graphene than that of graphitic N-, B- and P-doped graphene, while the density of state of all the modified graphene remains metallic features. These results indicate good wettability and electronic conductivity of heteroatom doped graphite electrodes for VRFBs. To further evaluate their catalytic activity towards the V²⁺/V³⁺ redox reaction, the metrics of energy difference between inner-sphere and outer-sphere adsorption modes for V(H2O)6²⁺ and V(H2O)6³⁺ are considered. An interesting finding is that for the P-doped graphene surface, the catalytic activity for both V²⁺ and V³⁺ ions can be significantly improved, suggesting a promising method for developing carbon electrodes for VRFBs.
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Sluggish vanadium reaction rates on the porous carbon electrodes typically used in redox flow batteries have prompted research into pretreatment strategies, most notably thermal oxidation, to improve performance. While effective, these approaches have nuanced and complex effects on electrode characteristics hampering the development of explicit structure-function relations that enable quantitative correlation between specific properties and overall electrochemical performance. Here, we seek to resolve these relationships through rigorous analysis of thermally pretreated SGL 29AA carbon paper electrodes using a suite of electrochemical, microscopic, and spectroscopic techniques and culminating in full cell testing. We systematically vary pretreatment temperature, from 400 to 500 °C, while holding pretreatment time constant at 30 h, and evaluate changes in the physical, chemical, and electrochemical properties of the electrodes. We find that several different parameters contribute to observed performance, including hydrophilicity, microstructure, electrochemical surface area, and surface chemistry, and, it is important to note that not all of these properties improve with increasing pretreatment temperature. Consequently, while the best overall performance is achieved with a 475 °C pretreatment, this enhancement is achieved from a balance, rather than a maximization, of critical properties. A deeper understanding of the role each property plays in battery performance is the first step towards developing targeted pretreatment strategies that may enable transformative performance improvements.
Article
N-doped carbon nanomaterials have received increased attention from electrochemists due to their applications in the metal-free electrocatalysis of important redox processes. In this work, a series of graphitized undoped and nitrogen-doped carbon electrodes prepared by thermal annealing of sputtered amorphous carbon films were prepared and characterized using a combination of X-ray photoelectron spectroscopy and Raman spectroscopy. Adsorption of the surface-sensitive redox probe dopamine at each electrode surface was then studied using cyclic voltammetry and the results correlated to the physico-chemical characterisation. Results indicate that dopamine adsorption is influenced by both the nitrogen surface chemistry and the degree of graphitization of the carbon scaffold. N-doping, with predominantly graphitic-N sites, was found to increase adsorption of dopamine more than 6 fold on carbon surfaces when the introduction of N atoms did not result in substantial alterations to the sp² network. However, when an identical type and level of N-doping is accompanied by a significant increase in disorder in the carbon scaffold, adsorption is limited to levels comparable to those of nitrogen-free carbon. Density functional theory studies of dopamine adsorption on graphene and N-doped graphene model surfaces showed that dopamine interacts via π-stacking at the graphene surface. The Gibbs free energy of adsorption on N-doped graphenes were estimated at 12-13 kcal mol⁻¹, and found to be approximately twice that of undoped graphenes. Results suggest that chemical changes resulting from N-doping enhance adsorption; however, high coverage values depend on the availability of sites for π-stacking. Therefore, the structurally disruptive effects of N-incorporation can significantly depress the dopamine response by limiting the availability of basal sites, ultimately dominating the overall electrochemical response of the carbon electrode.
Article
Voltammetric waves under five different mass-transport regimes (macroelectrode, microdisc, micro-hemisphere, micro-hemicylinder and single microband) for an irreversible one-electron transfer process were simulated and analysed to find the appropriate Tafel region for accurate analysis. The transfer coefficient was found to deviate significantly from its true value as a function of potential in all cases due to the influence of mass-transport. If and how a simple analytical mass-transport correction in which the current is corrected for the change in the reactant concentration at the surface can be used to improve the measurement of transfer coefficient was investigated. It is shown that this correction is only rigorously valid for a uniformly accessible microelectrode under a true steady-state condition. This translates to hemispherical electrodes only of the set of five considered. The fraction of the current used in Tafel analysis (Tafel region) can be increased to around 50% for quasi-steady state regimes (hemicylindrical and single band electrodes) with this analytical correction but it completely failed in linear diffusion regimes (macroelectrodes). In the latter case an improved empirical correction is suggested.
Article
With the consideration of understanding the interplays between the electrolyte and electrode for the VO2⁺/VO²⁺ redox reactions at various electrodes and developing high-activity electrode materials for an all-vanadium redox flow battery, the reduction reaction kinetics of vanadium(V) ions on a platinum (Pt) electrode in comparison with that on carbon electrodes is investigated by steady-state potentiodynamic polarization and impedance spectroscopy measurements in sulfuric acid solutions with various pH and vanadium concentrations. No abnormal increase is observed neither in the cathodic Tafel slope nor the charge transfer resistance (Rct) at a transition potential (EK) on the Pt electrode. However, an abrupt change in the Tafel slope and Rct of vanadium(V) ions is observed on carbon electrodes. The above results indicate that the reduction reaction kinetics of vanadium(V) ions in acidic solutions on the Pt electrode show some unusual difference with that on carbon electrodes, and this is the first study to report such a new phenomenon. Additionally, possible mechanism pathways for the reduction reaction of vanadium(V) ions in acidic solutions are proposed for the above-mentioned carbon and Pt electrodes in this work.
Article
Highly catalytic electrodes play a vital role in exploiting the capability of vanadium redox flow batteries (VRFBs) but suffer from a tedious synthesis process and ambiguous interaction mechanisms for catalytic sites. Herein, a facile pyrolysis of urea was applied to prepare graphitic carbon nitride-modified graphite felt (GF@CN), and by the virtue of a density functional theory-assisted calculation, the electron-rich pyridinic nitrogen atom of CN granules is demonstrated as the adsorption center for redox species and plays the key role in improving the performance of VRFBs, with 800 cycles and an energy efficiency of 75% at 150 mA cm2. Such experimental and computational collaborative investigations guide a realizable and cost-effective solution for other high-power flow batteries.
Article
Developing high-performance electrodes with high operating current density and long-term cycling stability is crucial to the widespread application of vanadium redox flow batteries (VRFBs). In this work, boron-doped graphite felt electrodes are designed, fabricated and tested for VRFBs. First-principles study firstly demonstrates that the boron-doped carbon surface possesses highly active and stabilized reaction sites. Based on this finding, we fabricate boron-doped graphite felt electrodes for VRFBs. Testing results show that the batteries with boron-doped graphite felt electrodes achieve energy efficiencies of 87.40% and 82.52% at the current densities of 160 and 240 mA cm⁻², which are 15.63% and 19.50% higher than those with the original electrodes. In addition, the batteries can also be operated at high current densities of 320 and 400 mA cm⁻² with energy efficiencies of 77.97% and 73.63%, among the highest performances in the open literature. More excitingly, the VRFBs with the boron-doped graphite felt electrodes exhibit excellent stability during long-term cycling tests. The batteries can be stably cycled for more than 2000 cycles at 240 mA cm⁻² with ultra-low capacity and efficiency decay rates of only 0.028% and 0.0002% per cycle. In addition, after refreshing the electrolytes, the performances of the batteries are nearly recovered regardless of the evitable decay of the membrane. All these results suggest that the highly efficient and ultra-stable boron-doped graphite felts are promising electrodes for VRFBs.
Article
The doping types of graphene sheets are generally tuned by different dopants with either 3 or 5 valence electrons. As a 5-valence-electrons element, however, nitrogen dopants in graphene sheets have several substitutional geometries. So far, their distinct effects on electronic properties predicted by theoretical calculations have not been well identified. Here, we demonstrate that the doping types of graphene can be tuned by N monoelement under proper growth conditions using chemical vapor deposition (CVD), characterized by combining scanning tunneling microscopy/spectroscopy, x-ray/ultraviolet photoelectron spectroscopy, Hall effect measurement, Raman spectroscopy, and density functional theory calculations. We find that a relatively low partial pressure of CH4 (mixing with NH3) can lead to the growth of dominant pyridinic N substitutions in graphene, in contrast with the growth of dominant graphitic N substitutions under a higher partial pressure of CH4. Our results unambiguously confirm that the pyridinic N leads to the p-type doping, and the graphitic N leads to the n-type doping. Interestingly, we also find that the pyridinic N and the graphitic N are preferentially separated in different domains. Our findings shed light on continuously tuning the doping level of graphene monolayers by using N monoelement, which can be much convenient for growth of functional structures in graphene sheets.
Article
The redox reaction of vanadium ions (V2+↔3+↔4+↔5+) on well-developed nitrogen-doped ordered mesoporous carbon (NOMC) was extensively investigated in different electrolyte solutions by electrochemical methods. It is found that both the electronic structure modulated by nitrogen doping and the enriched electrochemically active functional groups on NOMC favor the three electrochemical transitions between the adjacent couples, viz. V2+↔3+↔4+↔5+, as compared with Vulcan XC72 carbon black. Salient findings are as follows. First, the transition of V3+↔2+ is the same on the two distinctly different carbons, which indicates that this reaction is an outer-sphere charge transfer reaction. The concomitant hydrogen evolution reaction makes NOMC unsuitable to be used as a negative electrode material in flow batteries. Second, the transition of V5+↔4+ shows a quasi-reversible behavior, indicating that NOMC can be used as a positive electrode material. Simulation of cyclic voltammogram (CV) reveals that the standard rate constant and the adsorption equilibrium constant are (7.0 ± 0.9)*10⁻³ cm s⁻¹ and 0.70 ± 0.09 (both V⁴⁺ and V⁵⁺), respectively. Third, the transition of V4+↔3+ is recognized in the CV curve, which proceeds in a quasi-reversible reaction. The preceding adsorption of the symmetrical ions (V³⁺) is found to play a key role in determining the kinetics. Finally, for the two latter transitions, the content of dopant nitrogen yields a negligible effect on the electrochemical activity, excluding the possibility of its direct involvement in electrocatalysis. The above findings not only reveal the applicability of the nitrogen-doped carbon to be used as an electrode material in flow batteries, but also offer an in-depth understanding of the reaction mechanism of vanadium redox couples.
Article
There has been growing interest in the performance of vanadium redox flow batteries (VRFBs) depending on the electrolyte temperature and flow rate. In this work, we have devised a single-cell test system with four reservoirs which can effectively control the temperature and flow rate of VRFB to investigate electrochemical properties during discharging in VRFB. The temperature has been set between 278 K and 318 K for the electrolytes composed of 1600 mol/m³ V³⁺/V⁴⁺ with 4000 mol/m³ H2SO4, while the flow rate of the electrolytes is in the range of 10–100 mL/min. The exchange current density extracted by Tafel theory is expressed by Arrhenius-like equation and ranges between 38.83 and 49.07 A/m². Meanwhile, the electron transfer coefficient increases from 0.31 to 0.51 with increased temperature and flow rate. The area-specific resistance is found to decrease with increased temperature at the rate of 20.3 mΩ cm²/K. With these, the proposed analytical method successfully predicts the obtained experimental data with excellent accuracy. Our study offers the fundamental understandings of electrochemical properties of VRFB as well as can be applied to evaluate the VRFB energy storage system at the early conceptual design even without prototypes.
Article
In this work we present a combination of mathematical modeling and experimental kinetic characterization of carbon felt electrodes in positive electrolyte (PE) and negative electrolyte (NE) of vanadium redox-flow batteries. The mathematical model is applied to check for homogeneous transfer current density within a radially connected carbon electrode intensively flown through by the electrolyte. Transfer current homogeneity depends mainly on electrolyte conductivity, electrode thickness and charge transfer coefficient. The transfer current inhomogeneity of the investigated electrode samples is below 5%, when single layers of carbon electrodes (415 μm thickness at 89% porosity and 210 μm thickness at 75% porosity) are applied and high electrolyte conductivities (>800 mS∙cm⁻¹) are maintained by low vanadium concentrations (<15 mM). Experimental results show charge transfer coefficients for high overpotentials of 0.26 ± 0.03 for the reduction reaction in NE and 0.37 ± 0.04 for the oxidation reaction in NE. The charge transfer coefficients of the PE are 0.13 ± 0.02 for reduction and 0.30 ± 0.04 for the oxidation reaction. Application of the Butler-Volmer equation to describe the polarization behavior shows adequate agreement with the experimental results at states of charge between 25% and 75%. The rate constants for the PE reaction are nearly two times higher than for the NE reaction.
Article
Various testing methods, such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), have been employed to evaluate the electrode performance of vanadium flow battery (VFB). Due to the variations in the testing devices and characterization parameters in the literature, a number of reported results are uncomparable and even sometimes contradictionary. In this report, a reliable device is proposed for electrochemical characterization of electrode materials, and the parameter selection was demonstrated to be critical for achieving reliable evaluation and reducing the effects of side reactions. As for the structure of graphite felt electrode, volume current density is proposed to replace the area current density as the main evaluation indicator. Furthermore, the effect of side reactions is discussed and the upper limit voltage of 1.60 V is found to be suitable in charge process of VFB since selecting this value can greatly protect the active sites on electrode and avoid the capacity fading caused by side reactions.
Article
The modification of carbon materials via the incorporation of nitrogen has received much attention in recent years due to their performance as electrodes in applications ranging from electroanalysis to electrocatalysis for energy storage technologies. In this work we synthesized nitrogen-incorporated amorphous carbon thin film electrodes (a-C:N) with different degrees of nitrogenation via magnetron sputtering. Electrodes were characterized using a combination of spectroscopic and electrochemical methods, including X-ray photoelectron spectroscopy, ellipsometry, voltammetry and impedance spectroscopy. Results indicate that low levels of nitrogenation yield carbon materials with narrow optical gaps and semimetallic character. These materials displayed fast electron-transfer kinetics to hexammine ruthenium(II)/(III), an outer-sphere redox couple that is sensitive to electronic properties near the Fermi level in the electrode material. Increasing levels of nitrogenation first decrease the metallic character of the electrodes and increase the impedance to charge transfer and, ultimately, yield materials with optical and electrochemical properties consistent with disordered cluster aggregates rather than amorphous solids. A positive correlation was found between the resistance to charge transfer and the optical gap when using the outer sphere redox couple. Interestingly, the use of ferrocyanide as a surface-sensitive redox probe resulted in a monotonic increase of the impedance to charge transfer vs. nitrogen content. This result suggests that surface chemical effects can dominate the electrochemical response, even when nitrogenation results in enhanced metallic character in carbon electrodes.
Article
Porous carbon materials are of tremendous importance for electrochemical energy storage. Their low cost, wide potential window and high surface area make them ideal electrodes for many applications. The activity of the electrode towards a certain reaction is given by both the available wetted surface area and the electron transfer constant k0. The present study investigates which electrochemical methods are suitable to determine k0 on porous carbon electrodes. For this purpose, we investigate the ferric/ferrous redox couple on a porous carbon nanotube electrode as model system. We show that results from cyclic voltammetry (CV) can yield an apparent catalytic effect and elucidate its origin. Chronoamperometry and electrochemical impedance spectroscopy are shown to produce consistent values for the exchange current density I0, which can then be normalized to k0. Limitations of both methods in terms of k0 and diffusion constants are discussed.
Article
The morphology, surface composition, wettability and the kinetic parameters of the electrochemically oxidized graphite electrodes obtained under different anodic polarization conditions have been examined by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), contact angle measurements, steady-state polarization and cyclic voltammetry (CV) tests, with an attempt to investigate the inherent correlation between the physicochemical properties and the kinetic characteristics for carbon electrodes used in an all-vanadium redox flow battery (VRFB). When the anodic polarization potential raises up to 1.8 V vs. SCE, the anodic corrosion of the graphite might happen and a large number of oxygen-containing functional groups generate. The VO2+/VO2+ redox reaction can be facilitated and the reaction reversibility tends to become better with the increasing anodic potential, possibly owing to the increased surface oxides and the resulting improved wettability of the electrode. Based on this, a real reaction kinetic equation for the oxidation of VO2+ has been obtained on the electrode polarized at 1.8 V vs. SCE and it can be also well used to predict the polarization behavior of the oxidized electrode in vanadium (IV) acidic solutions.
Article
ABSTRACT: Vanadium flow batteries are a promising system for stationary energy storage. One of their shortcomings is a low power density caused by slow kinetics of the redox reactions. To alleviate this drawback, many studies tried to catalyze the redox reactions. However, up to now, there is no consensus in the literature on which of the two half-cell reactions, the V2+/V3+ or the VO2+/VO2+ reaction, features the slower electron transfer. The present study is the first showing that reaction rates for the halfcells are of the same order of magnitude with their respective rate constants depending on the composition of the electrode material. The surface functional groups hydroxyl, carbonyl, and carboxyl on carbon increase the wetted surface area, catalyze the V2+/V3+ redox reaction, but impede the VO2+/VO2 + redox reaction. This complex situation was unraveled by using a newly developed procedure based on electrochemical impedance spectroscopy. Reaction mechanisms based on these results are discussed.
Article
As demand for renewable energy storage increases, flow batteries such as those based on the resilient vanadium redox reactants and inexpensive carbon electrode materials have become more widespread. Thus far, however, there are many conflicting results in the literature regarding the reaction kinetics of the V2+/V3+ and VO2+/VO2+ redox couples and various methods to enhance these reactions. The present work demonstrates how the misinterpretation of cyclic voltammetry for porous carbon materials may account for many of these inconsistencies. Several oxidation treatments investigated here are observed to have a significant effect on the cyclic voltammogram of the V2+/V3+ redox couple reactions. Using electrochemical impedance spectroscopy and a recently developed analytical flow cell technique, these changes are shown to be due almost entirely to the effective wetting of the porous carbon paper rather than any electroactivation effect of surface functional groups on the intrinsic kinetics of the V2+/V3+ reaction. A similar methodology is recommended for any future assessment of electrode treatments.
Article
The right kind of dopant The oxygen reduction reaction is an important step in fuel cells and other electrochemical processes but is still largely dependent on precious metal-containing catalysts. Recently explored alternatives include carbon materials that are doped with different, preferably non-precious metal, atoms. Guo et al. studied model graphite catalysts to try to understand the role of nitrogen doping and to elucidate the active catalytic sites. A nitrogen atom bound to two carbons formed an active catalyst site with an activity rivaling that of N-doped graphene catalysts. Science , this issue p. 361