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In this paper, the effect of changes in the thermal conductivity of porous electrodes in three coordinate directions on the capability of proton exchange membrane fuel cells is investigated on the basis of current density versus voltammetry curves, and the temperature distribution and water-carrying capacity distribution of the membrane. The results show that when the cell discharge voltage of the PEMFC is 0.3 V, the thermal conductivity in the Z-direction of the porous electrode has a greater effect on the performance of the PEMFC than in the other directions, with the thermal conductivity in the X- and Y-directions of the porous electrode having less than a 5% effect on the performance of the PEMFC, which can therefore be neglected. When the thermal conductivity of the porous electrode in the Z-direction of the PEMFC is 500 W/(m·K) and 1000 W/(m·K), the performance of the PEMFC is improved by 5.78% and 5.87%, respectively, and when the thermal conductivity of the porous electrode in the X-direction of the PEMFC is 500 W/(m·K) and 1000 W/(m·K), the performance of the PEMFC is improved by 2.09% and 2.89%, and the PEMFC performance is improved by 1.51% and 2.00% when the Y-direction thermal conductivity of the porous electrode of the PEMFC is 500 W/(m·K) and 1000 W/(m·K), respectively. The improvement in performance decreases with increasing thermal conductivity, because the thickness of the porous electrode is too thin. Since the side of the model is set to adiabatic heat exchange conditions, while the top and bottom surfaces are set to natural convection heat exchange conditions, the Z-direction thermal conductivity of the porous electrode plays the most important role in the temperature distribution of the PEMFC. The Z-direction thermal conductivity of the porous electrode causes the temperature distribution of the PEMFC assembly to be more uniform, and the Z-direction thermal conductivity of the porous electrode also causes the area of the high-water-content region on the proton exchange membrane to significantly increase.

Chapter 1 contains an overview of the background of this thesis. Porous carbon electrodes and their applications are introduced, followed by a discussion of the Electric Double Layer (EDL) in porous carbon electrodes. Then, experimental work on the thermodynamics of the EDL in porous electrodes is presented. This Chapter ends with a brief description of the electrochemical calorimetry setup. In Chapter 2, the electrochemical calorimetry setup is examined in more detail. The cell contained an aqueous electrolyte solution and three electrodes. The potential of the working electrode (WE) was applied with respect to the reference electrode (RE), and the current was measured between WE and counter electrode (CE). Behind the WE, a heat flux sensor (HFS) measured the heat flow coming from the WE during the (dis)charging of the EDL. The heat was calibrated on the Joule heat produced in a full charge-discharge cycle. We us the setup to measure the heat of (dis)charging the WE as a function of the potential applied to the electrode. Chapter 3 describes how the setup was used to determine the potential-dependent internal energy of porous carbon electrodes in aqueous solutions of different salts. Changes were calculated from the (electrical) work performed on the system and the heat released or absorbed by the system. The energy changes consist of two contributions, which respectively scaled linearly and quadratically with applied potential. The linear contribution was ascribed to the attraction of ions to the surface of the pores, given by the number of adsorbed ions times a fixed attraction energy per ion. The quadratic contribution was slightly smaller than expected for a parallel plate capacitor of the same capacitance, which we interpreted in terms of the average electric potential experienced by ions inside the pores. Chapter 4 demonstrated that the formula found in Chapter 3 can also explain the time-dependent heat production. The charging rate was varied by changing the amount of time taken to build up the applied potential linearly until the final potential was reached. Using four constant parameters, both the reversible and irreversible heat production rates could be explained. We verified that the reversible heat was the same regardless of the charging rate. Agreement between model and experiment was only semi-quantitative, which we attributed to a time dependence of the electric current more complicated than mono-exponential decay. In Chapter 5 we analyzed the time dependence of the current for the (dis)charging of porous carbon electrodes in 1M solutions. The rate of exponential decay of the current was initially determined by the RC time and later by a distribution of time constants, which we attributed to polydispersity of the pores. The late-time current decay was slower at cathodic than at anodic potentials, but we argued that this was not because of the different diffusion coefficients of the cations and anions, which were nearly identical for KCl. Instead, the potential-dependent dynamics were discussed in terms of the different capacitance of the electrode in the cathodic and anodic ranges, resulting from specific ion adsorption.

Chapter 1 contains an overview of the background of this thesis. Porous carbon electrodes and their applications are introduced, followed by a discussion of the Electric Double Layer (EDL) in porous carbon electrodes. Then, experimental work on the thermodynamics of the EDL in porous electrodes is presented. This Chapter ends with a brief description of the electrochemical calorimetry setup. In Chapter 2, the electrochemical calorimetry setup is examined in more detail. The cell contained an aqueous electrolyte solution and three electrodes. The potential of the working electrode (WE) was applied with respect to the reference electrode (RE), and the current was measured between WE and counter electrode (CE). Behind the WE, a heat flux sensor (HFS) measured the heat flow coming from the WE during the (dis)charging of the EDL. The heat was calibrated on the Joule heat produced in a full charge-discharge cycle. We us the setup to measure the heat of (dis)charging the WE as a function of the potential applied to the electrode. Chapter 3 describes how the setup was used to determine the potential-dependent internal energy of porous carbon electrodes in aqueous solutions of different salts. Changes were calculated from the (electrical) work performed on the system and the heat released or absorbed by the system. The energy changes consist of two contributions, which respectively scaled linearly and quadratically with applied potential. The linear contribution was ascribed to the attraction of ions to the surface of the pores, given by the number of adsorbed ions times a fixed attraction energy per ion. The quadratic contribution was slightly smaller than expected for a parallel plate capacitor of the same capacitance, which we interpreted in terms of the average electric potential experienced by ions inside the pores. Chapter 4 demonstrated that the formula found in Chapter 3 can also explain the time-dependent heat production. The charging rate was varied by changing the amount of time taken to build up the applied potential linearly until the final potential was reached. Using four constant parameters, both the reversible and irreversible heat production rates could be explained. We verified that the reversible heat was the same regardless of the charging rate. Agreement between model and experiment was only semi-quantitative, which we attributed to a time dependence of the electric current more complicated than mono-exponential decay. In Chapter 5 we analyzed the time dependence of the current for the (dis)charging of porous carbon electrodes in 1M solutions. The rate of exponential decay of the current was initially determined by the RC time and later by a distribution of time constants, which we attributed to polydispersity of the pores. The late-time current decay was slower at cathodic than at anodic potentials, but we argued that this was not because of the different diffusion coefficients of the cations and anions, which were nearly identical for KCl. Instead, the potential-dependent dynamics were discussed in terms of the different capacitance of the electrode in the cathodic and anodic ranges, resulting from specific ion adsorption.

Chapter 1 contains an overview of the background of this thesis. Porous carbon electrodes and their applications are introduced, followed by a discussion of the Electric Double Layer (EDL) in porous carbon electrodes. Then, experimental work on the thermodynamics of the EDL in porous electrodes is presented. This Chapter ends with a brief description of the electrochemical calorimetry setup. In Chapter 2, the electrochemical calorimetry setup is examined in more detail. The cell contained an aqueous electrolyte solution and three electrodes. The potential of the working electrode (WE) was applied with respect to the reference electrode (RE), and the current was measured between WE and counter electrode (CE). Behind the WE, a heat flux sensor (HFS) measured the heat flow coming from the WE during the (dis)charging of the EDL. The heat was calibrated on the Joule heat produced in a full charge-discharge cycle. We us the setup to measure the heat of (dis)charging the WE as a function of the potential applied to the electrode. Chapter 3 describes how the setup was used to determine the potential-dependent internal energy of porous carbon electrodes in aqueous solutions of different salts. Changes were calculated from the (electrical) work performed on the system and the heat released or absorbed by the system. The energy changes consist of two contributions, which respectively scaled linearly and quadratically with applied potential. The linear contribution was ascribed to the attraction of ions to the surface of the pores, given by the number of adsorbed ions times a fixed attraction energy per ion. The quadratic contribution was slightly smaller than expected for a parallel plate capacitor of the same capacitance, which we interpreted in terms of the average electric potential experienced by ions inside the pores. Chapter 4 demonstrated that the formula found in Chapter 3 can also explain the time-dependent heat production. The charging rate was varied by changing the amount of time taken to build up the applied potential linearly until the final potential was reached. Using four constant parameters, both the reversible and irreversible heat production rates could be explained. We verified that the reversible heat was the same regardless of the charging rate. Agreement between model and experiment was only semi-quantitative, which we attributed to a time dependence of the electric current more complicated than mono-exponential decay. In Chapter 5 we analyzed the time dependence of the current for the (dis)charging of porous carbon electrodes in 1M solutions. The rate of exponential decay of the current was initially determined by the RC time and later by a distribution of time constants, which we attributed to polydispersity of the pores. The late-time current decay was slower at cathodic than at anodic potentials, but we argued that this was not because of the different diffusion coefficients of the cations and anions, which were nearly identical for KCl. Instead, the potential-dependent dynamics were discussed in terms of the different capacitance of the electrode in the cathodic and anodic ranges, resulting from specific ion adsorption.

Microfluidic fuel cells, electrolyzers, and redox flow batteries utilize laminar flow channels to provide reactants, remove products and avoid their crossover. These devices often also employ porous flow-through electrodes as they offer a high surface area for the reaction and excellent mass transfer. The geometrical features of these electrodes and flow channels strongly influence energy efficiency. We derive explicit analytical relations for the optimal flow channel width and porous electrode volumetric surface area from the perspective of energy efficiency. These expressions are verified using a two-dimensional tertiary current distribution and porous electrode flow model in COMSOL and are shown to be able to predict optimal parameters in commonly used flow-through and interdigitated flow fields. The obtained analytical models can dramatically shorten modelling time and expedite the industrial design process. The optimal channel width and pore sizes we obtain, in the order of 100 microns and 1 micron respectively, are much smaller than those often used. This shows that there is a significant room for improvement of energy efficiency in flow cells that can sustain the resulting pressure drop.

Microfluidic fuel cells, electrolyzers, and redox flow batteries utilize laminar flow channels to provide reactants, remove products and avoid their crossover. These devices often also employ porous flow-through electrodes as they offer a high surface area for the reaction and excellent mass transfer. The geometrical features of these electrodes and flow channels strongly influence energy efficiency. We derive explicit analytical relations for the optimal flow channel width and porous electrode volumetric surface area from the perspective of energy efficiency. These expressions are verified using a two-dimensional tertiary current distribution and porous electrode flow model in COMSOL and are shown to be able to predict optimal parameters in commonly used flow-through and interdigitated flow fields. The obtained analytical models can dramatically shorten modelling time and expedite the industrial design process. The optimal channel width and pore sizes we obtain, in the order of 100 microns and 1 micron respectively, are much smaller than those often used. This shows that there is a significant room for improvement of energy efficiency in flow cells that can sustain the resulting pressure drop.

Compressing porous carbon electrodes is a common approach to improve flow battery performance, but the resulting impact on electrode structure, fluid dynamics, and cell performance is not well understood. Herein, microtomographic imaging, load cell testing, and flow cell diagnostics are employed to characterize how compression-induced changes impact pressure drop, polarization, and mass-transfer scaling. Five different compressions are tested, spanning ranges typically used in literature, for AvCarb 1071 cloth (0%, 9%, 20%, 25%, 32%) and Freudenberg H23 paper (0%, 8%, 12%, 17%, 22%). It is found that the two electrode structures have distinct responses to compression, resulting in differing optimal conditions identified for each material; specifically, the Freudenberg H23 exhibits lower combined ohmic, charge-transfer, and mass-transport values at 8% compression, resulting in improved electrochemical performance across all compressive values, as compared to the optimal AvCarb 1071 compression (20%). Overall, Freudenberg H23 exhibits a greater sensitivity to compression with peak electrochemical activity correlating with increased permeability, whereas AvCarb 1071 is insensitive to compressive loads but produces lower electrochemical performance. Herein, the trade-offs of mechanical robustness on fluid-dynamic and electrochemical performance between the two electrodes are demonstrated by the aforementioned findings, suggesting each could be used for specific operating environments.

Redox flow batteries are a promising technological option for large-scale energy storage, but their deployment is hampered by suboptimal performance and elevated costs. The reactor performance is determined by the mass, charge, momentum, and heat transport rates coupled with electrochemical reactions. Understanding, measuring, and simulating these metrics is thus necessary for device optimization. In this chapter, we first describe the main transport mechanisms and cell overpotentials relevant to flow batteries with a focus on single cells representative of individual units in a stack. Then, we critically review some experimental methods, including bulk and locally resolved diagnostics, which enable determination of cell overpotentials. Finally, we briefly describe the most promising modeling approaches. Our goal is to provide students and scientists with the fundamental guiding principles of transport phenomena and cell overpotentials that can be leveraged to design advanced materials and reactor architectures with enhanced performance.

Porous materials are frequently used as e.g. electrodes or porous transport layers in various types of electrolyzers. A better understanding of the bubble dynamics on porous electrodes is especially important to optimize new electrolyzer designs like membraneless electrolyzers. The developed 3-electrode cell was optimized with regard to the analysis of the bubble nucleation, growth and detachment on the applied working electrode. Noteworthy in this regard is the 2-dimensional optical measurement system to characterize the bubble dynamics from the side and top. The cell provides a platform to run parametric studies in alkaline electrolytes. The present data set compares three different expanded nickel metals at applied current densities of |j|= 10, 20, 50, 100 or 200 mA/cm² and flow rates of ̇V̇ = 0 or 5 ml/min. As electrolyte 1 M KOH is used. An overview of all performed experiments and the main parameters (current density j and flow rate V̇) is given in the file Overview of all performed experiments.pdf. The data is analyzed as described in the corresponding journal publication Bubble size distribution and electrode coverage at porous nickel electrodes in a novel 3-electrode flow-through cell. For each parameter set 3 data sets are given to ensure statistical confidence. Each data set, stored as .hdf5-file, is structured in groups as follows: Electrochemical Measurement Data Sideview Raw Images Topview Raw Images Results Detected Bubbles Electrode Coverage In the attributes assigned to the groups in the .hdf5-file all relevant metadata is given, including the experimental parameters, used devices and characteristics of the mounted WE. In addition to exemplary data sets for all three electrodes, the CAD files of the experimental setup used and sample videos of the raw images are also provided within this data publication. The remaining data sets of all measurements performed can be made available upon request.

Sodium-nickel-chloride batteries have a proven track record for backup power applications, but also show great potential for large-scale stationary electricity storage currently dominated by lithium-ion batteries. While lithium-ion cells rely on critical cobalt and lithium, sodium-nickel-chloride batteries are based on abundant, non-critical sodium chloride, nickel, and alumina, and are thus ideally suited for large-scale deployment. However, to be competitive with lithium-ion batteries, the charge and discharge rate capabilities of sodium-nickel-chloride batteries need to be improved and cell production cost needs to be reduced. In this PhD thesis, rate limiting processes in state-of-the-art sodium-metal-chloride cells are assessed and measures are proposed to improve their performance and reduce cost. On the negative electrode, plating and stripping of liquid sodium metal from a ceramic Na-ß"-alumina electrolyte is investigated at 250 °C in a home-built, specifically-designed high-temperature electrochemical cell. Operating the negative electrode above the melting temperature of sodium eliminates mass transport limitations at the sodium/Na-ß"-alumina interface and enables stable cycling at unprecedentedly high current densities above 1000 mA/cm2 without sodium metal dendrite formation. On the positive electrode, the chlorination and de-chlorination conversion reactions of nickel and iron in sodium tetrachloroaluminate electrolyte are investigated at 300 °C employing planar model electrodes. Analysis of cyclic voltammetry data reveals a diffusion limitation during oxidation with a diffusion coefficient of 2.3·10-3 cm2/s for the nickel electrode and an upper bound of 1.2·10-5 cm2/s for the iron electrode. Post-mortem analysis of the electrodes shows that chlorination of the nickel electrode proceeds via uniform oxidation of nickel and the formation of NiCl2 platelets on the surface of the electrode. In contrast, the chlorination of the iron electrodes proceeds via stress-induced cracking, resulting in non-uniform iron oxidation and the pulverization of the iron electrode. For enhanced rate capability, it is thus important to improve metal-ion diffusion in the tetrachloroaluminate electrolyte, preferably by enhancing metal-ion mobility. A complementary approach to improve the rate capability of sodium-nickel-chloride batteries is to modify the microstructure of the positive electrode. By moving from planar model electrodes to high-capacity porous electrodes, it is shown how microstructure design can improve sodium ion transport across the porous positive electrode. By these measures, the nominal discharge current density could be increased from 70 to 150 mA/cm2 demonstrating the potential for further improvements in rate capability. To reduce production cost of sodium-nickel-chloride batteries, the replacement of tubular Na-ß"-alumina electrolytes by planar Na-ß"-alumina electrolyte discs is investigated. Despite their potential for lower cost, the use of planar discs results in severely higher mechanical stress on the electrolyte. A practical solution to mitigate the gas pressure difference is to seal the cell below atmospheric gas pressure allowing implementation of planar electrolyte discs. Insights into the transport and conversion processes at the electrodes presented in this PhD thesis will contribute to enable competitive high-power sodium-nickel-chloride batteries for stationary electricity storage.

Redox flow batteries (RFBs) are emerging as viable options for grid-scale energy storage, but their elevated costs hamper commercialization. Enhancing the porous carbon electrode performance to improve power density and reduce system costs is an effective strategy toward widespread deployment; however, the porous carbon electrode must satisfy multiple contradictory roles, including providing high surface area, low pressure drop, and facile mass transport, thus motivating electrode engineering efforts. In this work, we systematically explore the non-solvent induced phase separation (NIPS) technique as a platform to synthesize a family of distinct microstructures for use in RFBs. Flow cell studies in commercially relevant redox pairs (i.e., Fe2+/3+, V2+/3+, and V4+/5+) are performed, revealing diverse performance profiles, synthesis-structure-performance relationships, and opportunities for high-power electrode materials. We anticipate that, with further refinement and customization, NIPS electrodes can broadly benefit electrode engineering efforts for electrochemical energy storage and conversion applications.

[ES] Hoy en día aún existen ciertos problemas para lograr la generación de hidrógeno que provenga de energías renovables, conocido como Hidrógeno Verde. Actualmente se busca mejorar los métodos de producción de hidrógeno para lograr la “descarbonización”. Se pretende perfeccionar dichos métodos para lograr una mejora climática global donde poder dejar de depender de los combustibles fósiles, así como del hidrógeno proveniente de estos. La incorporación de nanopartículas de metales nobles a los electrodos de níquel en los electrolizadores alcalinos se cree que puede mejorar la eficiencia de los mismos. El presente documento recoge los resultados y conclusiones obtenidas de la caracterización de un electrolizador para la obtención de hidrógeno usando 5 configuraciones diferentes de electrodos: ánodo de níquel liso - cátodo de níquel liso, ánodo de níquel liso - cátodo de níquel poroso , ánodo de níquel liso - cátodo de níquel poroso con incrustación de oro, ánodo de níquel poroso con incrustación de oro - cátodo de níquel poroso con incrustación de oro y ánodo de níquel poroso - cátodo de níquel poroso. El método de caracterización fue mediante electrolisis alcalina del agua, para la producción de hidrógeno, empleando un Voltámetro de Hoffman, a 3 densidades de corriente, 20, 50 y 100 mA cm-2 y 3 temperaturas, 30, 50 y 800C., [EN] Nowadays there are still certain problems to achieve the generation of hydrogen that comes from renewable energies, known as Green Hydrogen. Currently, it is seeking to improve hydrogen production methods to achieve "decarbonization". It is intended to perfect these methods to achieve a global climate improvement where we can stop depending on fossil fuels, as well as the hydrogen from these. The incorporation of noble metal nanoparticles to nickel electrodes in alkaline electrolyzers is believed to improve their efficiency. This document collects the results and conclusions obtained from the characterization of an electrolyzer for obtaining hydrogen using 5 different electrode configurations: smooth nickel anode - smooth nickel cathode, smooth nickel anode - rough nickel cathode, nickel anode smooth - rough nickel cathode with gold inlay, rough nickel anode with gold inlay - rough nickel cathode with gold inlay and rough nickel anode - rough nickel cathode. The characterization method was by alkaline electrolysis of water, for the production of hydrogen, using a Hoffman Voltameter, at 3 current densities, 20, 50 and 100 mA cm-2 and 3 temperatures, 30, 50 and 800C.

Fuel-cell catalyst layers (CLs) are porous electrodes that are fabricated from CL inks: colloidal dispersions of catalyst particles and ion-conducting polymer (ionomer), dispersed in solvent(s). The complex interactions between the ink components ultimately dictate CL microstructure and electrochemical performance. To control the CL formation process and optimize fuel-cell performance, knowledge of these ink interactions is vital. In this review, we analyze data from ink-focused papers to elucidate how ink parameters (solvent type, ionomer-to-carbon ratio, etc.) impact ink interactions and CL performance. We then discuss these results in the context of the current understanding of two critical ink interactions: ionomer/solvent and ionomer/catalyst particle interactions.

The solid-state spherical diffusion equation with flux boundary conditions is a standard problem in lithium-ion battery simulations. If finite difference schemes are applied, many nodes across a discretized battery electrode become necessary, in order to reach a good approximation of solution. Such a grid-based approach can be appropriately avoided by implementing analytical methods which reduce the computational load. The pseudo-steady-state (PSS) method is an exact analytical solution method, which provides accurate solid-state concentrations at all current densities. The popularization of the PSS method, in the existing form of expression, is however constrained by a solution convergence problem. In this short communication, a modified PSS (MPSS) expression is presented which provides uniformly convergent solutions at all times. To minimize computational runtime, a fast MPPS (FMPPS) expression is further developed, which is shown to be faster by approximately three orders of magnitude and has a constant time complexity. Using the FMPSS method, uniformly convergent exact solutions are obtained for the solid-state diffusion problem in spherical active particles.

Selective laser sintering (SLS) 3D printing was utilized to fabricate highly porous carbonous electrodes. The electrodes were prepared by using a mixture of fine graphite powder and either polyamide-12, polystyrene, or polyurethane polymer powder as SLS printing material. During the printing process the graphite powder was dispersed uniformly on the supporting polymer matrix. Graphite’s concentration in the mixture was varied between 5 and 40 wt % to find the correlation between the carbon content and conductivity. The graphite concentration, polymer matrix, and printing conditions all had an impact on the final conductivity. Due to the SLS printing technique, all the 3D printed electrodes were highly porous. By using polyurethane as the supporting matrix it was possible to produce flexible electrodes in which the conductivity is sensitive to pressure and mechanical stress. Physical properties such as graphite distribution, attachment, and the overall porosity of the printed electrodes were studied using scanning electron microscopy (SEM), helium ion microscopy (HIM), and X-ray tomography. The results show that the combination of chemical design of the printing material and the utilization of SLS 3D printing enables fabrication of highly customizable electrodes with desired chemical, physical, mechanical, and flow-through properties. peerReviewed

[ES] En la actualidad, existe una dependencia elevada de los combustibles fósiles. El uso de dichos combustibles conlleva diversos problemas como la contaminación y el agotamiento de recursos. Para disminuir la dependencia de los combustibles fósiles se ha comenzado a confiar en las fuentes de energía renovables, donde el hidrógeno puede actuar de vector energético. El hidrógeno puede almacenar grandes cantidades de energía para que se pueda disponer de ella posteriormente. El método más extendido y sencillo para obtener hidrógeno a través de fuentes de energía renovable es la electrólisis alcalina del agua. El problema que presenta este método es el consumo energético que requiere y su baja eficiencia. Para conseguir solventar estos problemas se estudia el desarrollo de nuevos electrodos. El aumento del área superficial específica a través de la porosidad de los electrodos, así como la presencia en el electrodo de materiales nobles que catalizan la reacción de evolución de hidrógeno permiten mejorar la eficiencia y el consumo energético. El presente trabajo final de grado ha sido realizado con la finalidad de estudiar la mejora de la electrólisis alcalina del agua empleando como cátodo electrodos de níquel poroso dopados con nanopartículas de plata y paladio. Lo que se pretende conseguir con el uso de estos electrodos catódicos es la disminución del sobrepotencial y por consiguiente la mejora en el rendimiento energético. Se estudiaron cuatro electrodos: un electrodo de níquel liso; un electrodo de níquel poroso; un electrodo de níquel poroso con nanopartículas de paladio depositadas bajo tratamiento térmico (EPdTT); un electrodo de níquel poroso con nanopartículas de paladio depositadas bajo tratamiento térmico y nanopartículas de plata electrodepositadas (E-PdTT_AgEDP). Para el estudio se han realizado barridos de potencial y ensayos galvanostáticos a diversas intensidades. Además, los experimentos se realizaron a diferentes temperaturas para ver el efecto de ésta sobre la reacción de evolución de hidrógeno. Tras el análisis de los resultados obtenidos, se ha podido corroborar que el aumento de temperatura facilita la reacción de evolución de hidrógeno. Además, se ha podido demostrar que tanto el electrodo E-PdTT como el electrodo E-PdTT_AgEDP ofrecen buenas propiedades electrocatalíticas. Pero el electrodo E-PdTT_AgEDP es el que más destaca al reducir el sobrepotencial, disminuir el consumo energético, aumentar el rendimiento farádico y aumentar el rendimiento energético máximo., [CA] En l’actualitat, existeix una dependència elevada dels combustibles fòssils. L’ús d’aquests combustibles comporta diversos problemes com la contaminació i l’esgotament dels recursos. Per a reduir la dependència dels combustibles fòssils s’ha començat a confiar en les fonts d’energia renovables, on l’hidrogen pot actuar com a vector energètic. L’hidrogen pot emmagatzemar grans quantitats d’energia perquè es puga disposar d’ella posteriorment. El mètode més estès i senzill per a l’obtenció d’hidrogen a través de fonts d’energia renovable és l’electròlisi alcalina de l’aigua. El problema que presenta aquest mètode és el consum energètic que requereix i la seua baixa eficiència. Per a aconseguir resoldre aquests problemes s’estudia el desenvolupament de nous elèctrodes. L’augment de l’àrea superficial específica a través de la porositat dels elèctrodes, així com la presència en l’elèctrode de materials nobles que catalitzen la reacció d’evolució d’hidrogen permeten millorar l’eficiència i el consum energètic. El present treball de fi de grau ha sigut realitzat amb la finalitat d’estudiar la millora de l’electròlisi alcalina de l’aigua emprant com a càtode elèctrodes de níquel porós dopats amb nanopartícules de plata i pal·ladi. Lo que es pretén aconseguir amb l’ús d’aquests elèctrodes catòdics és la disminució del sobrepotencial i per consegüent la millora en el rendiment energètic. Es van estudiar quatre elèctrodes: un elèctrode de níquel llis; un elèctrode de níquel porós; un elèctrode de níquel porós amb nanopartícules de pal·ladi depositades baix tractament tèrmic (E-PdTT); i un elèctrode de níquel porós amb nanopartícules de pal·ladi depositades baix tractament tèrmic i nanopartícules de plata electrodepositades (E-PdTT_AgEDP). Per a l’estudi s’han realitzat corbes de descàrrega d’hidrogen i assajos galvanostàtics a diverses intensitats. A més, els experiments es van realitzar a diferents temperatures per a vorer l’efecte d’aquesta sobre la reacció d’evolució d’hidrogen. Després de l’anàlisi dels resultats obtinguts, s’ha pogut corroborar que l’augment de temperatura facilita la reacció d’evolució d’hidrogen. A més, s’ha pogut demostrar que tant l’elètrode E-PdTT com l’elèctrode E-PdTT_AgEDP ofereixen bones propietats electrocatalítiques. Però l’elèctrode EPdTT_AgEDP és el que més destaca al reduir el sobrepotencial, disminuir el consum energètic, augmentar el rendiment faràdic i augmentar el rendiment energètic màxim., [EN] Nowadays, there is a high dependence on fossil fuels. The use of these fuels entails problems such as contamination and resource depletion. To reduce the dependence on fossil fuels, a reliance on renewable sources has begun, where hydrogen can act as an energy carrier. Hydrogen can store large amounts of energy so that it can be used later. The most extended and simple method to obtain hydrogen through renewable sources of energy is the alkaline water electrolysis. The energetic consumption that this method needs and its low efficiency are the main problems. The development of new electrodes is studied in order to solve these problems. The increase of the specific surface area through porosity of the electrodes, as well as the presence in the electrode of noble materials that catalyse the hydrogen evolution reaction, enable the improvement of the efficiency and the energetic consumption. This work has been done to study the improvement of the alkaline water electrolysis using porous nickel electrode doped with palladium and silver nanoparticles as cathodes. What is intended to achieve with the use of these cathodic electrodes is the reduction of overpotential and, therefore, the improvement of the energetic efficiency. Four electrodes have been studied: a smooth nickel electrode; a porous nickel electrode; a porous nickel electrode with palladium nanoparticles deposited by a heat treatment (E-PdTT); and a porous nickel electrode with palladium nanoparticles deposited by a heat treatment and silver nanoparticles electrodeposited (E-PdTT_AgEDP). For the study of the electrodes, potential sweeps and galvanostatic tests at various currents have been done. Furthermore, the experiments have been done at different temperatures to see the effect of the temperature over the hydrogen evolution reaction. After the analysis of the obtained results, it has been corroborated that the increase of the temperature facilitates the hydrogen evolution reaction. In addition, it has been proved that both the E-PdTT electrode and the E-PdTT_AgEDP electrode offer good electrocatalytic properties. But the EPdTT_AgEDP electrode is the one that stands out the most due to the reduction of overpotential, the reduction of energetic consumption, the increase of faradic efficiency and the increase of the maximum energetic efficiency.

The challenge of optimizing the pore size distribution of porous electrodes for different electrolytes is encountered in supercapacitors, lithium-ion capacitors and hybridized battery-supercapacitor devices. A volume-averaged continuum model of ion transport, taking into account the pore size distribution, is employed for the design of porous electrodes for electrochemical double-layer capacitors (EDLCs) in this study. After validation against experimental data, computer simulations investigate two types of porous electrodes, an activated carbon coating and an activated carbon fabric, and three electrolytes: 1.5 M TEABF4 in acetonitrile (AN), 1.5 M TEABF4 in propylene carbonate (PC), and 1 M LiPF6 in ethylene carbonate:ethyl methyl carbonate (EC:EMC) 1:1 v/v. The design exercise concluded that it is important that the porous electrode has a large specific area in terms of micropores larger than the largest desolvated ion, to achieve high specific capacity, and a good proportion of mesopores larger than the largest solvated ion to ensure fast ion transport and accessibility of the micropores.

Porous carbonaceous electrodes are performance-defining components in redox flow batteries (RFBs), where their properties impact the efficiency, cost, and durability of the system. The overarching challenge is to simultaneously fulfill multiple seemingly contradictory requirements-i.e., high surface area, low pressure drop, and facile mass transport-without sacrificing scalability or manufacturability. Here, non-solvent induced phase separation (NIPS) is proposed as a versatile method to synthesize tunable porous structures suitable for use as RFB electrodes. The variation of the relative concentration of scaffold-forming polyacrylonitrile to pore-forming poly(vinylpyrrolidone) is demonstrated to result in electrodes with distinct microstructure and porosity. Tomographic microscopy, porosimetry, and spectroscopy are used to characterize the 3D structure and surface chemistry. Flow cell studies with two common redox species (i.e., all-vanadium and Fe2+/3+ ) reveal that the novel electrodes can outperform traditional carbon fiber electrodes. It is posited that the bimodal porous structure, with interconnected large (>50 µm) macrovoids in the through-plane direction and smaller (

Diese Arbeit pr��sentiert einen analytischen und numerischen Modellansatz zur Vorhersage der effektiven W��rmeleitf��higkeit por��ser Elektrodenbeschichtungen als Funktion von Mikrostrukturparametern. Beide Modelle ber��cksichtigen die morphologischen Parameter und die thermischen Bulkeigenschaften der fundamentalen Batteriezellkomponenten. Die Ergebnisse beider Modelle wurden erfolgreich gegeneinander verifiziert und durch Literaturdaten sowie eigene experimentelle Messungen validiert.

Galvanic displacement reactions between a solid oxide and a dissolved metal cation are interesting processes for the preparation of oxide nanoparticles or electrocatalytic layers consisting of secondary mixed oxides. Their mechanism is not yet fully clarified. The composition of the secondary mixed oxides and their growth rate depend on many physical and chemical variables. In the present study, we have focused on the effects of the concentration of the ions released from the sacrificial oxide to the solution, when they are intentionally added to reaction media. We have studied the displacement of sacrificial PbO2 by either Mn2+ or Co2+ cations in acetate solution that contained variable concentrations of Pb2+, using electrochemical methods, SEM-EDS and XPS. The evolution of the open circuit potential of the systems was monitored during the reactions. We have found that, for both divalent cations, increasing concentrations of Pb2+ ions in the acetate solutions caused the formation of mixed oxides richer in Pb. Effects on growth rate and equilibrium potential were different for Mn2+ and Co2+.

The porous structure of the electrodes in redox flow batteries (RFBs) plays a critical role in their performance. We develop a framework for understanding the coupled transport and reaction processes in electrodes by combining lattice Boltzmann modelling (LBM) with experimental measurement of electrochemical performance and X-ray computed tomography (CT). 3D pore-scale LBM simulations of a non-aqueous RFB are conducted on the detailed 3D microstructure of three different electrodes (Freudenberg paper, SGL paper and carbon cloth) obtained using X-ray CT. The flow of electrolyte and species within the porous structure as well as electrochemical reactions at the interface between the carbon fibers of the electrode and the liquid electrolyte are solved by a lattice Boltzmann approach. The simulated electrochemical performances are compared against the experimental measurements with excellent agreement, indicating the validity of the LBM simulations for predicting the RFB performance. Electrodes featuring one single dominant peak (i.e., Freudenberg paper and carbon cloth) show better electrochemical performance than the electrode with multiple dominant peaks over a wide pore size distribution (i.e., SGL paper), whilst the presence of a small fraction of large pores is beneficial for pressure drop. This framework is useful to design electrodes with optimal microstructures for RFB applications.

Thin-film lithium-ion batteries (TFBs) are promising components for powering rigid and mechanically flexible electronic devices. The production of such all-solid-state power units is yet complex and cost-intensive, and the performance is to be improved. Because binders are often absent in TFBs, it is a key issue to retain the mechanical stability and reversible discharge capacities upon mechanical bending. One approach to reduce contact losses upon cell-bending maybe the fabrication of electrodes with controlled residual porosities. The remaining voids could buffer occurring volume changes during cycling of typical cathode materials. To systematically approach the fabrication of long-living high-performance flexible TFBs, this work addresses the relationship between different residual porosities of pure TFB electrodes and their electrochemical performance in flat TFB systems. Crystalline and phase-pure Li4Ti5O12 (LTO) thin-film electrodes were deposited in situ with the flame spray pyrolysis technique and compressed at two different pressures. Zero-strain LTO has been chosen as a model material because it enabled us to understand the sole impact of porosity on electrochemical performances without interfering volume changes. Our results showed that denser LTO particle networks were accompanied by a larger number of LTO particle contacts. Also, the LTO contact densities to the neighboring interfaces in a TFB cell have increased. The improved electrode contacting led to reduced electrical sheet resistivities and significantly increased practicable capacities. Future projects will require the substitution of LTO with high-voltage cathode materials to maximize energy densities. Cycling under statically and dynamically bent condition will answer whether tuned residual porosities are useful for the production of long-living flexible TFBs.

An algorithm for the simulation and evaluation of cyclic voltammetry (CV) at macroporous electrodes such as felts, foams, and layered structures is presented. By considering 1D, 2D, and 3D arrays of electrode sheets, cylindrical microelectrodes, hollow‐cylindrical microelectrodes, and hollow‐spherical microelectrodes the internal diffusion domains of the macroporous structures are approximated. A universal algorithm providing the time‐dependent surface concentrations of the electrochemically active species, required for simulating cyclic voltammetry responses of the individual planar, cylindrical, and spherical microelectrodes, is presented as well. An essential ingredient of the algorithm, which is based on Laplace integral transformation techniques, is the use of a modified Talbot contour for the inverse Laplace transformation. It is demonstrated that first‐order homogeneous chemical kinetics preceding and/or following the electrochemical reaction and electrochemically active species with non‐equal diffusion coefficients can be included in all diffusion models as well. The proposed theory is supported by experimental data acquired for a reference reaction, the oxidation of [Fe(CN)6]4− at platinum electrodes as well as for a technically relevant reaction, the oxidation of VO2 + at carbon felt electrodes. Based on our calculation strategy, we provide a powerful open source tool for simulating and evaluating CV data implemented into a Python graphical user interface (GUI)., Simulating CV: The theory of cyclic voltammetry (CV) at macroporous electrodes is developed by considering random 1D, 2D and 3D arrays of microelectrodes. A universal algorithm for simulating the CV responses of the individual planar, cylindrical and spherical microelectrodes, is presented as well. Experimental data support the proposed model. Finally, an innovative strategy for CV data evaluation is provided and implemented into an open source software tool.

Investigating the electrokinetic performance of novel electrode materials by means of diffusional cyclic voltammetry has emerged to the standard approach in electrochemistry. The straightforward implementation of the method in a three-electrode compartment provides scientists with a feasible ex-situ technique for assessing reaction kinetics in terms of potential-dependent redox currents. Providing that well-defined diffusion conditions are complied, i.e. the experiments are conducted at planar electrodes in a semi-infinite diffusion domain, characteristic features such as the separation, symmetry and magnitude of the redox peaks can be related unambiguously to the electrode kinetics. However, as soon as non-planar electrodes or electrodes with finite diffusion domains are employed an equivocation between the measured redox peaks and the intrinsic electrode kinetics emerges. Consequently, a quantitative interpretation of cyclic voltammetry data becomes exceptionally arduous. In particular porous structures like felts and foams, predominantly utilized as electrode materials in the field of battery research, exhibit an intimidating ambiguity of the polarographic current signal. Therefore, the majority of experimentalists restrict themself to a qualitative interpretation of cyclic voltammetry data in terms of arbitrarily chosen onset-potentials. Scientists who are still targeting to quantify the electrode kinetics usually aim to exploit alternative techniques such as electrochemical impedance spectroscopy. However, from a theoretical perspective this approach is not capable of solving the dilemma either since the experiments are subjected to the same diffusion complication, examined with a different potential perturbation only. Consequently, developing a theory of cyclic voltammetry for porous electrodes is inevitable to permit a quantitative analysis of experimental results. This thesis consists of the cumulative work on the theory of cyclic voltammetry at macroporous electrodes with emphasis on felt-like structures. It is demonstrated that linking the high sensitivity of cyclic voltammetry with a sophisticated mathematical diffusion model allows for an electrochemical and morphological characterization of porous electrodes simultaneously, promoting the so-called „electrochemists spectroscopy “ to the next level. All theoretical concepts are supported by experimental data acquired for the electrochemical redox-reactions of vanadium(II)/ vanadium(III) and oxovanadium(IV)/ dioxovanadium(V), relevant in the field of vanadium redox-flow battery research. In a first approximation, porous electrodes are treated as random arrays of microelectrodes in a finite diffusion space with a statistically fluctuating size. A systematic investigation of simulated and experimentally acquired cyclic voltammetry data for both, porous and non-porous electrodes, draws an enlightening picture on the complex interplay of electrode porosity and reaction kinetics. With this knowledge, precise values for the heterogeneous rate constant of the oxovanadium(IV)/ dioxovanadium(V) redox reaction are obtained. These values usually scatter over orders of magnitude in the recent literature, most likely due to an inconsequent interpretation of data. In another study, a strategy for real-space simulation of cyclic voltammetry at carbon felt electrodes is presented. For this purpose, in-situ micro X-ray computed tomography is exploited to construct a template of the three-dimensional diffusion domain inside a porous electrode. This renders any statistical assumptions obsolete. To perform the simulations, two self-reliant computational methods, namely digital simulation and convolutive modeling, are combined. The resulting method offers significant advantages with respect to computation time, programming effort and mathematical complexity. Since effects of electrochemical double-layer charging, nonlinear contributions of ohmic resistances, coupled chemical reactions and limited electron transfer kinetics can be accounted for readily, the novel approach covers an extraordinarily wide range of electrochemical situations. The exceptional endowment of simulating polarographic experiments at porous electrodes was finally implemented into an open source program named „Polarographica “. This software provides the experimentalists community with a straightforward way of interpreting cyclic voltammetry data of porous electrodes in terms of a fitting routine. Since many other electroanalytical techniques are supported in the environment of Polarographica as well, it will eventually lead to a more decent interpretation of cyclic voltammetry data, based on mathematical models instead of ambiguous current peaks and arbitrarily chosen onset-potentials.

Electrochemical impedance spectroscopy (EIS) technique is used for characterization of single cell symmetric capacitors having different mass loadings of activated carbon (AC). Relevant values of charge storage capacitance (CT) and internal resistance (ESR) were evaluated by the single frequency and multi-frequency analyses of measured impedance spectra. Curve fittings were based on the non-ideal R-C model that takes into account the parasitic inductance, contributions from electrode materials/contacts and the effects of AC porosity. Higher CT and lower ESR values were obtained not only for the cell with higher mass of AC, but also using the single vs. multi-frequency approach. Lower CT and higher values of ESR that are generally obtained using the multi-frequency method and curve fittings should be related to the not ideal capacitive response of porous AC material and too high frequency chosen in applying the single frequency analysis.

Journal of power sources 509, 230345 (2021). doi:10.1016/j.jpowsour.2021.230345, Published by Elsevier, New York, NY [u.a.]

The lack of uniformity in the experimental conditions used for testing carbon supercapacitors by different authors highlights the need for standardized procedures to evaluate their performance. This study demonstrates the usefulness of a three-electrode arrangement in the investigation of electrochemical processes on both electrodes of double-layer carbon/carbon supercapacitors in a T-type sandwich-designed cell by performing simultaneous electrochemical measurements of positive and negative electrodes in six different acetonitrile-containing electrolytes. The testing employed electrochemical impedance spectroscopy, cyclic voltammetry, and constant current charging/discharging measurements. The results demonstrated that the impedance measurements were highly sensitive to the experimental arrangement and that the impedance data were distorted due to geometrical and electrical asymmetry within the cell. Such asymmetries did not affect cyclic voltammetry and constant current charging/discharging measurements, and the results proved valuable in the simultaneous investigations of both electrodes in supercapacitors. The study detected different behavior of anion and cation adsorption in the porous structure of carbon electrodes. While quaternary ammonium cations exhibit constant and even slightly increasing specific capacitances during successive cycling, specific capacitances of BF4− and PF6− anions exponentially decrease. Irreversible adsorption of anions inside the positive electrode explained such behavior. Overall, the results show the viability of a three-electrode arrangement in the investigation of electrochemical processes on both electrodes in supercapacitors and batteries. The results confirm that tetraethylammonium tetrafluoroborate is a superior electrolyte for carbon/carbon supercapacitors, while lithium tetrafluoroborate does not have adequate properties in acetonitrile for use in electrochemical power sources.

Here, inspired by mechanoreceptors in the human body, a self‐sensing ionic soft actuator is developed that precisely senses the bending motions during actuating utilizing a 3D graphene mesh electrode. The graphene mesh electrode has the permeability of mobile ions inside the ionic exchangeable polymer and shows low electrical resistance of 6.25 Ω Sq−1, maintaining high electrical conductivity in large bending deformations of 180°. In this sensing system, the graphene woven mesh is embedded inside ionic polymer membrane to interact with mobile ions and to trace their movements. The migration of mobile ions inside the membrane induces an electrical signal on the mesh and provides the information regarding ion distribution, which is proven to be highly correlated with the bending deformation of the actuator. Using this integrated self‐sensing system, the responses of an ionic actuator to various input stimulations are precisely estimated for both direct current and alternating current inputs. Even though the generated displacement is extremely small around 300 µm at very low driving voltage of 0.1 V, high level accuracy (96%) of estimated deformations could be achieved using the self‐sensing actuator system., A graphene mesh electrode having high ion permeability and electrical conductivity is adopted as a self‐sensing ionic soft actuator inspired by mechanoreceptors in the human body. The graphene mesh electrode provides electrical signals sensitive to ion migration inside the polymer electrolyte without blocking their movements and is used to predict the bending deflection of the actuator.

[ES] La producción de hidrógeno como método de almacenamiento de energía está cobrando importancia sobretodo en combinación con las energías renovables. El hidrógeno es generado a partir de agua y energía eléctrica, por tanto es interesante disminuir la cantidad de energía empleada para generar un gramo de hidrógeno. Esto se intenta conseguir con el desarrollo de nuevos materiales de electrodos que permitan disminuir dicho consumo energético. En el proyecto que se propone se utilizarán materiales porosos que proporcionan una gran superficie de reacción en combinación con nanopartículas de Pd que aportan actividad catalítica. Se estudiará el efecto que tienen las nanopartículas de paladio sobre el rendimiento y el consumo energético de la producción de hidrógeno a partir de la electrólisis alcalina del agua a varias temperaturas., [EN] Hydrogen production as an energy carried is gaining importance if combined with renewable energy. Hydrogen is generated from water and electrical energy through water electrolysis, so it is interesting to reduce the amount of energy used to generate a gram of hydrogen. This is possible with the development of new electrode materials that allow to diminish this energetic consumption. In the proposed project porous materials that provide a large reaction surface in combination with Pd nanoparticles that provide catalytic activity will be used. The effect of palladium nanoparticles on the performance and energy consumption for hydrogen production from alkaline water electrolysis at different temperatures will be studied.

International audience; This article reports on a procedure to predict the optimal thickness of cylindrical porous electrodes operating a single redox reaction. This is obtained from a macroscopic model for the coupled diffusion-reaction process that is first validated with voltammetry experiments of the H2O2/H2O reductionreaction carried out with a series of porous electrodes elaborated in this work. An analytical solution to this model is developed in the steady regime and for electrodes featuring a thickness to mean radius ratio small enough compared to unity. An analytical expression of the optimal electrode thickness is derived corresponding to the crossover value of two asymptotic regimes characterizing the dependence of the volume currentdensity produced by the electrode upon its thickness. The predictive tool of the optimal thickness is general, regardless of the porous microstructure. The case of the electrodes used in the reported experiments illustrates that the optimal thickness is not intrinsic to the microsctructure characterized by the size of the representative volume, its specific area and effective diffusion coefficient. It also depends on the operating conditions reflected in the kinetic number, Ki, and the thickness of the diffusion layer surrounding the electrode. The dependence of the optimal thickness on these two parameters is quite significant in a range of very small values of Ki but remains quasi constant beyond a threshold value.

Using electrodes or catalytic layers that are porous increases the reactive surface area but also the distance that ions and electrons have to travel. Thicker electrodes, through their larger surface area, reduce the activation overpotential but increase the ohmic losses. There will therefore be an electrode thickness for which the voltage losses are minimal, corresponding to a maximum energy efficiency. Simple approximate relations are derived here for the value of this optimal thickness, for both Tafel and linearised Butler-Volmer kinetics. We additionally optimise the power density of Galvanic cells, the capacity of battery electrodes, and the porosity of both particulate and foam-like electrodes. For this analysis we introduce an intuitive new definition of the electrode effectiveness factor. An accurate explicit current-voltage expression, including the transition from linear to Tafel kinetics and from a single to a doubled Tafel slope, is obtained. The present analysis is limited to a configuration where ions and electrons enter and leave at opposite sides of the electrode, as in most stacks, and applies only when mass transfer effects can be neglected. These results can nonetheless be useful for optimization of various electrochemical devices including fuel cells, batteries, flow batteries, electrochemical reactors, and electrolysers.

For hydrogen to be used as an energy vector in the transition to renewable energy, the water electrolysis process must first become more efficient in order to be competitive with the SMR. This current work focuses on intensifying the electrolysis of alkaline water using 3‐D electrodes, forced electrolyte flow and pulsed electrical power. 3‐D electrodes, having a much larger active surface area than 2‐D electrodes for the same projected surface, reduce the overpotential of the electrochemical reaction. But under natural convection, gas bubbles may remain trapped inside the macro‐porous 3‐D structure, which counterbalances this intrinsic advantage. Implementing a forced electrolyte flow through the electrodes is therefore necessary in order to take full advantage of their benefits. A third way to intensify the process is to use pulsed electrical power.

Alcohol electrolysis using polymeric membrane electrolytes is a promising route for storing excess renewable energy in hydrogen, alternative to the thermodynamically limited water electrolysis. By properly choosing the ionic agent (i.e. H+ or OH−) and the catalyst support, and by tuning the catalyst structure, we developed membrane-electrode-assemblies which are suitable for cost-effective and efficient alcohol electrolysis. Novel porous electrodes were prepared by Atomic Layer Deposition (ALD) of Pt on a TiO2-Ti web of microfibers and were interfaced to polymeric membranes with either H+ or OH− conductivity. Our results suggest that alcohol electrolysis is more efficient using OH− conducting membranes under appropriate operation conditions (high pH in anolyte solution). ALD enables better catalyst utilization while it appears that the TiO2-Ti substrate is an ideal alternative to the conventional carbon-based diffusion layers, due to its open structure. Overall, by using our developmental anodes instead of commercial porous electrodes, the performance of the alcohol electrolyser (normalized per mass of Pt) can be increased up to ∼30 times.

The galvanic displacement reaction between sacrificial PbO2 layers and Fe(II) ions has been carried out in mildly acid acetate solutions. The effect of experimental variables, like morphology of the PbO2 layers, solution pH, temperature and concentration of Fe(II) ions, has been investigated by combining electrochemical methods, SEM-EDS, XRD and XPS. The reaction between PbO2 and Fe(II) ions produced secondary dual layers, consisting of an inner amorphous FeOOH film and an outer crystalline γ-FeOOH deposit, respectively. This peculiar behavior was in contrast with analogous reactions involving other transition metal ions, which yielded only continuous amorphous secondary oxide layers. PbO2/FeOOH layers were tested as anodes for the oxygen evolution reaction in basic media.

Au nanoparticles (Au-NPs) were successfully synthesized and incorporated into the surface of a macroporous Ni electrode fabricated via galvanostatic electrodeposition at high current densities in order to produce hydrogen by means of alkaline water electrolysis. The developed electrodes were morphologically characterized by means of confocal laser scanning and field emission scanning electron microscopes. The electrocatalytic behaviour towards the hydrogen evolution reaction was studied by Tafel polarization curves and electrochemical impedance spectroscopy. It was clear that enlarging the real surface area of an electrode its catalytic activity was greatly enhanced. This improvement was further increased when Au-NPs were added to the macroporous Ni surface. In this case, the improvement was not only caused by enlarging the surface area but also by an improvement in the intrinsic catalytic activity of the alloy, as it was shown by the exchange current densities values, calculated from the real surface area., The authors acknowledge the support of Generalitat Valenciana (PROMETEO/2010/023) and Universidad Politecnica de Valencia (PAID-06-10-2227).

Detailed insight into electrochemical reaction mechanisms and rate limiting steps is crucial for targeted optimization of solid oxide fuel cell (SOFC) electrodes, especially for new materials and processing techniques, such as Ni/Gd-doped ceria (GDC) cermet anodes in metal-supported cells. Here, we present a comprehensive model that describes the impedance of porous cermet electrodes according to a transmission line circuit. We exemplify the validity of the model on electrolyte-supported symmetrical model cells with two equal Ni/Ce0.9Gd0.1O1.95-δ anodes. These anodes exhibit a remarkably low polarization resistance of less than 0.1 Ωcm2 at 750 °C and OCV, and metal-supported cells with equally prepared anodes achieve excellent power density of >2 W/cm2 at 700 °C. With the transmission line impedance model, it is possible to separate and quantify the individual contributions to the polarization resistance, such as oxygen ion transport across the YSZ-GDC interface, ionic conductivity within the porous anode, oxygen exchange at the GDC surface and gas phase diffusion. Furthermore, we show that the fitted parameters consistently scale with variation of electrode geometry, temperature and atmosphere. Since the fitted parameters are representative for materials properties, we can also relate our results to model studies on the ion conductivity, oxygen stoichiometry and surface catalytic properties of Gd-doped ceria and obtain very good quantitative agreement. With this detailed insight into reaction mechanisms, we can explain the excellent performance of the anode as a combination of materials properties of GDC and the unusual microstructure that is a consequence of the reductive sintering procedure, which is required for anodes in metal-supported cells.

Ionic electroactuator beams are promising systems for artificial muscles in microrobotics. Here a theory is developed to investigate one promising class of such systems, which employs flexible volume-filling electrodes impregnated with polymer–electrolyte. The theory provides analytical formulae for the equilibrium beam curvature as a function of voltage and structure-related operational parameters. It predicts a possible enhancement of beam curvature by orders of magnitude over that of flat electrodes. Volume-filling electrodes thus constitute one of the 'strongest' architectures for voltage-induced movement. Approximate expressions for the dynamics of tandem pore charging and beam deflection are developed to determine the maximum pore length that still warrants a sufficiently fast response time (up to 1 s). Upper bounds on applied voltage and response time constrain the maximal device thickness and curvature, and therefore, the resulting work such a device can perform.

[EN] Silver nanoparticles were electrodeposited into the surface of a macroporous Ni electrode. The developed electrodes were characterized morphologically by confocal laser scanning microscopy and field emission scanning electron microscopy. The activity of the developed electrodes towards the hydrogen evolution reaction was assessed by pseudo-steady-state polarization curves and electrochemical impedance spectroscopy in alkaline solutions at different temperatures. The incorporation of the silver nanoparticles on the surface of the electrode improved the catalytic activity towards the hydrogen evolution reaction due to an improvement in the electrochemically active surface area rather than in the intrinsic catalytic activity., This work was partially supported by the Fondo FONSEC-SRE Mexico-Argentina en Nanotecnologia (CONACyT-ANPCYT), Projects 0191145-PICT2012-3081. Ramiro Medina wishes to thank to the CONACYT for the postgraduate grant (304413 + Beca mixta).

The purpose of this work is to develop a one-dimensional mathematical model for predicting the cell performance of a direct formic acid fuel cell and compare this with experimental results. The predicted model can be applied to direct formic acid fuel cells operated with different formic acid concentrations, temperatures, and with various electrolytes. Tafel kinetics at the electrodes, thermodynamic equations for formic acid solutions, and the mass-transport parameters of the reactants are used to predict the effective diffusion coefficients of the reactants (oxygen and formic acid) in the porous gas diffusion layers and the associated limiting current densities to ensure the accuracy of the model. This model allows us to estimate fuel cell polarization curves for a wide range of operating conditions. Furthermore, the model is validated with experimental results from operating at 1–5 M of formic acid feed at 30–80 °C, and with Nafion-117 and silane-crosslinked sulfonated poly(styrene-ethylene/butylene-styrene) (sSEBS) membrane electrolytes reinforced in porous polytetrafluoroethylene (PTFE). The cell potential and power densities of experimental outcomes in direct formic acid fuel cells can be adequately predicted using the developed model.

The future development of electric vehicles is mostly dependent of improvements in battery performances. In support of the actual research of new materials having higher performances in terms of energy, power, durability and cost, it is necessary to develop modeling tools. The models are helpful to simulate integration of the battery in the powertrain and crucial for the battery management system, to improve either direct (e.g. preventing overcharges and thermal runaway) and indirect (e.g. state of charge indicators) safety. However, the battery models could be used to understand its physical phenomena and chemical reactions to improve the battery design according with vehicles requirements and reduce the testing phases. One of the most common model describing the porous electrodes of lithium-ion batteries is revisited. Many variants available in the literature are inspired by the works of prof. J Newman and his research group from UC Berkeley. Yet, relatively few works, to the best of our knowledge, analyze in detail its predictive capability. In the present work, to investigate this model, all the physical quantities are set in a dimensionless form, as commonly used in fluid mechanics: the parameters that act in the same or the opposite ways are regrouped, and the total number of simulation parameter is greatly reduced. In a second phase, the influence of the parameter is discussed, and interpreted with the support of the limit cases. The analysis of the discharge voltage and concentration gradients is based on galvanostatic and pulse/relaxation current profiles and compared with tested commercial LGC cells. The simulations are performed with the software Comsol® and the post-processing with Matlab®. Moreover, in this research, the parameters from the literatures are discussed to understand how accurate are the techniques used to parametrize and feed the inputs of the model. Then, our work shows that the electrode isotherms shapes have a significant influence on the accuracy of the evaluation of the states of charges in a complete cell. Finally, the protocols to characterize the performance of commercial cells at different C-rates are improved to guarantee the reproducibility., Le développement futur des véhicules électriques est lié à l’amélioration des performances des batteries qu’ils contiennent. Parallèlement aux recherches sur les nouveaux matériaux ayant des performances supérieures en termes d'énergie, de puissance, de durabilité et de coût, il est nécessaire développer des outils de modélisation pour : (i) simuler l'intégration de la batterie dans la chaine de traction et (ii) pour le système de gestion de la batterie, afin d'améliorer la sécurité et la durabilité. Soit de façon directe (par exemple, la prévention de surcharge ou de l’emballement thermique) soit de façon indirecte (par exemple, les indicateurs de l’état de charge). Les modèles de batterie pourraient aussi être utilisés pour comprendre les phénomènes physiques et les réactions chimiques afin d'améliorer la conception des batteries en fonction des besoins de l’utilisateur et de réduire la durée des phases de test. Dans ce manuscrit, un des modèles les plus communs décrivant les électrodes poreuses des batteries au lithium-ion est revisité. De nombreuses variantes dans la littérature s’inspirent directement du travail mené par le professeur J. Newman et son équipe de chercheurs à l’UC Berkeley. Pourtant relativement peu d’études analysent en détail les capacités prédictives de ce modèle. Dans ce travail, pour étudier ce modèle, toutes les grandeurs physiques sont définies sous une forme adimensionnelle, comme on l'utilise couramment dans la mécanique des fluides : les paramètres qui agissent de manière identique ou opposée sont regroupés et le nombre total de paramètres du modèle est considérablement réduit. Cette étude contient une description critique de la littérature incluant le référencement des paramètres du modèle développé par le groupe de Newman et les techniques utilisées pour les mesurer, ainsi que l’écriture du modèle dans un format adimensionnel pour réduire le nombre de paramètres. Une partie expérimentale décrit les modifications de protocoles mis en œuvre pour améliorer la reproductibilité des essais. Les études effectuées sur le modèle concernent d’une part l’identification des états de lithiation dans la cellule avec un attention particulière sur la précision obtenue, et enfin une prospection numérique pour examiner l’influence de chaque paramètre sur les réponses de la batterie en décharge galvanostatique puis en mode impulsion et relaxation.

International audience; Using redox enzymes as biocatalysts in fuel cells is an attractive strategy for sustainable energy production. Once hydrogenase for H2 oxidation and bilirubin oxidase (BOD) for O2 reduction have been wired on electrodes, the enzymatic fuel cell (EFC) thus built is expected to provide sufficient energy to power small electronic devices, while overcoming the issues associated with scarcity, price and inhibition of platinum based catalysts. Despite recent improvements, these biodevices suffer from moderate power output and low stability. In this work, we demonstrate how substrate diffusion and enzyme distribution in the bioelectrodes control EFC performance. A new EFC was built by immobilizing two thermostable enzymes in hierarchical carbon felt modified by carbon nanotubes. This device displayed very high power and stability, producing 15.8 mW h of energy after 17 h of continuous operation. Despite the large available electrode porosity, mass transfer was shown to limit the performance. To determine the optimal geometry of the EFC, a numerical model was established, based on a finite element method (FEM). This model allowed an optimal electrode thickness of less than 100 μm to be determined, with a porosity of 60%. Thanks to very efficient enzyme wiring and high enzyme loading, non-catalytic signals for both enzymes were detected and quantified, enabling the electroactive enzyme distribution in the porous electrode to be fully determined for the first time. High total turnover numbers, approaching 107 for BOD and 108 for hydrogenase, were found, as was an impressive massic activity of 1 A mg−1 with respect to the mass of the electroactive enzyme molecules. This strategy, relying on stable enzymes and mesoporous materials, and the model set up may constitute the basis for a larger panel of bioelectrodes and EFCs.

© 2017 American Chemical Society. An electrochemical technique is developed to activate graphene oxide (GO) at relatively low temperature and assemble it into porous electrodes. The activation process is carried out in molten KOH by switching the polarity between 2 symmetrical GO electrodes. The electrochemically activated graphene (ECAG) showed a specific surface area as high as 2170 m2 g-1 and nanometer-sized pore created at a temperature as low as 450 °C. The ECAG electrode shows a significant enhancement in the electrochemical activity and thus improved electrochemical performance when being used as electrodes in supercapacitors and capacitive deionization (CDI) cells. A specific capacitance of 275 F g-1 is obtained in 6 M KOH electrolyte, and 189 F g-1 in 1 M NaCl electrolyte, which maintains 95% after 5000 cycles. The desalination capacity of the electrodes was evaluated by a batch mode electrosorption experiment. The ECAG electrode was able to remove 14.25 mg of salts per gram of the active materials and satisfy a high adsorption rate of 2.01 mg g-1 min-1. The low energy consumption of the CDI system is demonstrated by its high charge efficiency, which is estimated to be 0.83.

Controlling the interface between biological tissues and electrodes remains an important challenge for the development of implantable devices in terms of electroactivity, biocompatibility, and long-term stability. To engineer such a biocompatible interface a low molecular weight gel (LMWG) based on a glycosylated nucleoside fluorocarbon amphiphile (GNF) was employed for the first time to wrap gold electrodes via a noncovalent anchoring strategy, that is, self-assembly of GNF at the electrode surface. Scanning electron microscopy (SEM) studies indicate that the gold surface is coated with the GNF hydrogels. Electrochemical measurements using cyclic voltammetry (CV) clearly show that the electrode properties are not affected by the presence of the hydrogel. This coating layer of 1 to 2 μm does not significantly slow down the mass transport through the hydrogel. Voltammetry experiments with gel coated macroporous enzyme electrodes reveal that during continuous use their current is improved by 100% compared to the noncoated electrode. This demonstrates that the supramolecular hydrogel dramatically increases the stability of the bioelectrochemical interface. Therefore, such hybrid electrodes are promising candidates that will both offer the biocompatibility and stability needed for the development of more efficient biosensors and biofuel cells.

Pore structures have a major impact on the transport and electrical properties of electrochemical devices, such as batteries and electric double-layer capacitors (EDLCs). In this work we are concerned with the prediction of the electrical conductivity, ion diffusivity and volumetric capacitance of EDLC electrodes, manufactured from hierarchically porous carbons. To investigate the dependence of the effective properties on the pore structures, we use a structurally resolved parametric model of a random medium. Our approach starts from 3D FIB-SEM imaging, combined with automatic segmentation. Then, a random set model is fitted to the segmented structures and the effective transport properties are predicted using full field simulations by iterations of FFT on 3D pore space images and calculations based on the geometric properties of the structure model. A parameter study of the model is used to investigate the sensitivity of the effective conductivity and diffusivity to changes in the model parameters. Finally, we investigate the volumetric capacitance of the EDLC electrodes with a geometric model, make a comparison with experimental measurements and do a parameter study to suggest improved microstructures. Fil: Prill, Torben. Fraunhofer ITWM; Alemania Fil: Jeulin, Dominique. Fraunhofer ITWM; Alemania Fil: Willot, François. Fraunhofer ITWM; Alemania Fil: Balach, Juan Manuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; Argentina. Universidad Nacional de Río Cuarto. Facultad de Ciencias Exactas Fisicoquímicas y Naturales. Departamento de Química; Argentina Fil: Soldera, Flavio. Universitat Saarland; Alemania

Pt-based structured catalysts for the low-temperature combustion of methanol have been prepared by electrochemical methods, using Fe-Cr-Al alloy (Fecralloy) foam supports. Among the strategies investi- gated, pulsed electrodeposition of Pt nanoparticles from an H2PtCl6 solution, followed by cathodic elec- trodeposition of CeO2 thin films from a nitrate bath was the most successful. Pt loading and surface area were measured by ICP-MS and cyclic voltammetry, respectively. Although the presence of a CeO2 film decreased the Pt surface area accessible to electrolyte it enhanced the performance of the catalysts towards methanol combustion, without affecting the activation energy of the process, due to the forma- tion of additional active sites along the interface of CeO2-coated Pt nanoparticles

The effects of cobalt additive on the positive electrode surface of nickel alkaline batteries are investigated. Electrode surface modifications by electroless cobalt deposits were made at different immersion times. The performance of nickel hydroxide electrodes was studied by optical techniques, such as scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX) and electrochemical methods as cyclic voltammetry, charge–discharge curves and electrochemical impedance spectroscopy (EIS). According to these results, electroless cobalt deposits obtained with 5 min of immersion time in the electroless-bath exhibit a better electrode performance. Fil: Ortiz, Mariela Gisela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; Argentina. Universidad Tecnologica Nacional. Facultad Regional La Plata; Argentina Fil: Castro, Elida Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; Argentina Fil: Real, Silvia Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Universidad Nacional de La Plata; Argentina. Universidad Tecnologica Nacional. Facultad Regional La Plata; Argentina

In article number https://doi.org/10.1002/advs.201901711, Il‐Kwon Oh and co‐workers propose a new sensing mechanism for measuring the actuation displacement of ionic actuators using a graphene mesh. Inspired by the mechanoreceptor in the human body, a graphene mesh is embedded inside the actuator membrane to record the change of electrical potential due to ion movement during actuation. This signal is then correlated with the movement of the actuator which also caused by ion migration inside the membrane.

The use of microbial fuel cells (MFCs) for wastewater treatment fits in a circular economy context, as they can produce electricity by the removal of organic matter in the wastewater. Activated carbon (AC) granules are an attractive electrode material for bioanodes in MFCs, as they are cheap and provide electroactive bacteria with a large surface area for attachment. The characterization of biofilm growth on AC granules, however, is challenging due to their high roughness and three-dimensional structure. In this research, we show that 3D magnetic resonance imaging (MRI) can be used to visualize biofilm distribution and determine its volume on irregular-shaped single AC granules in a non-destructive way, while being combined with electrochemical and biomass analyses. Ten AC granules with electroactive biofilm (i.e. granular bioanodes) were collected at different growth stages (3 to 21 days after microbial inoculation) from a multi-anode MFC and T1-weighted 3D-MRI experiments were performed for three-dimensional biofilm visualization. With time, a more homogeneous biofilm distribution and an increased biofilm thickness could be observed in the 3D-MRI images. Biofilm volumes varied from 0.4 μL (day 4) to 2 μL (day 21) and were linearly correlated (R2 = 0.9) to the total produced electric charge and total nitrogen content of the granular bioanodes, with values of 66.4 C μL−1 and 17 μg N μL−1, respectively. In future, in situ MRI measurements could be used to monitor biofilm growth and distribution on AC granules.

Flexible pressure sensors are crucial components for the next generation wearable devices to monitor human physiological conditions. In this paper, we present a novel resistive pressure sensor based on hybrid composites made from carbon nanotube (CNT) for the conductive coating layer and polydimethylsiloxane (PDMS) elastomers as the substrate. The high sensitivity of these sensors is attributed to the change of contact resistance caused by the variation of the contact areas between the wavy film and the electrodes. Porous electrodes were designed to increase the roughness of the interfaces, thus further enhancing the pressure sensitivity. The developed device was verified through a series of tests, and the sensor exhibited a high sensitivity of 2.05 kPa-1 under a low pressure of 35.6 Pa.